Encyclopedia of Polymer Applications, 3 Volume Set 9781351019415, 1351019414

Table of contents :
Content: Cover
Half Title
Series Page
Title Page
Copyright Page
Brief Contents
Dedication
Encyclopedia of Polymer Applications
Contents
Contributors
Preface
About the Editor
Volume I
Actuators: Buckypaper Composite
Agriculture: Natural Polymers for Controlled Release of Fertilizers
Agriculture: Polymers in Crop Production Mulch and Fertilizer
Agriculture: Polymers in Crop Production Pesticides
Agriculture: Polymers in Crop Protection
Agriculture: Super Absorbent Functional Polymers
Alginate
Automotive Applications: Polylactic Acid and Biocomposite Parts Automotive Applications: Polymers inAutomotive Applications: Reinforced Material Components
Aviation: Polymer Composites, Processes, and Properties
Aviation: Thermoplastic and Thermoset Polymers in
Biomaterials: Soft Tissue Injuries
Biomedicine: Cellulose in
Biomedicine: Cross-Linked Polymers Applications
Biomedicine: Polymers with Silsesquioxane Units
Biosensing Devices: Conjugated Polymer Based Scaffolds
Catalysts: Polymers in Biodiesel Production
Chemotherapy: Polycaprolactone (PCL)-Based Drug Delivery System
Chitosan: Antimicrobial and Edible Coatings Chitosan: Biodegradable Food PackagingChitosan: Tissue Engineering and Wound Dressing Applications
Communication: Polymers in
Composites and Nanocomposites: Thermoplastic Polymers for Additive Manufacturing
Composites: Biobased for Materials Applications
Composites: Conductive Elastomer CNT-Based
Composites: Environment-Friendly Nanocomposites
Composites: Natural Fibers Reinforced
Composites: Polylactic Acid-Based Blends
Conducting Polymers: Applications
Conducting Polymers: Electrospun Materials
Construction: Rigid Bio-based Polyurethane Foams for Sandwich Panels Corrosion Inhibitor: Polymeric DesiccantCorrosion Protection: Natural Polymer in
Cosmetics: Active Polymers
Cosmetics: Polymers in
Cosmetics: Polymers in Delivery of Actives
Display: Photorefractive Polymers in
Drug Delivery: Biodegradable Polymers in
Drug Delivery: Microencapsulation Techniques for
Drug Delivery: PLGA-PEG-Based Bioerodable Nanoparticles
Drug Delivery: Polymeric Conjugates for Dietary Phytochemicals
Drug Delivery: Polymeric Micelles
Drug Delivery: Polymers and Polymeric Membranes
Drug Delivery: Polysaccharide-Based Composites Drug Delivery: Stimuli-Responsive Polymeric Prodrugs for CancerDrug Delivery: Trends and Future Prospects
Volume II
Electronics: Nonvolatile Memory Technologies
Electronics: Polymer-Graphene Composites
Energy: Conducting Polymers and Conjugated Porous Polymers
Energy: Polymer Electrolytes for Lithium Ion Batteries
Energy: Polymer Supercapacitors
Energy: Polymer-Functionalized Graphene
Energy: Polymers in Harvesting, Conversion, and Storage
Energy: Polymers in the Active Layer of Solar Cells
Environmental Applications: Biopolymer Sorbents for Heavy Metal Removal

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Encyclopedia of

Polymer Applications Edited by Munmaya Mishra, PhD

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Brief Contents

Composites: Natural Fibers Reinforced . . . . . . . . Composites: Polylactic Acid-Based Blends . . . . . . Conducting Polymers: Applications . . . . . . . . . . . Conducting Polymers: Electrospun Materials . . . Construction: Rigid Bio-based Polyurethane Foams for Sandwich Panels . . . . . . . . . . . . . . . . Corrosion Inhibitor: Polymeric Desiccant . . . . . . Corrosion Protection: Natural Polymer in . . . . . . Cosmetics: Active Polymers . . . . . . . . . . . . . . . . . . Cosmetics: Polymers in . . . . . . . . . . . . . . . . . . . . . Cosmetics: Polymers in Delivery of Actives . . . . Display: Photorefractive Polymers in . . . . . . . . . . Drug Delivery: Biodegradable Polymers in . . . . . Drug Delivery: Microencapsulation Techniques for . . . . . . . . . . . . . . . . . . . . . . . . . . Drug Delivery: PLGA-PEG-Based Bioerodable Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . Drug Delivery: Polymeric Conjugates for Dietary Phytochemicals . . . . . . . . . . . . . . . . . . . Drug Delivery: Polymeric Micelles . . . . . . . . . . . . Drug Delivery: Polymers and Polymeric Membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Drug Delivery: Polysaccharide-Based Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . Drug Delivery: Stimuli-Responsive Polymeric Prodrugs for Cancer . . . . . . . . . . . . . . . . . . . . . Drug Delivery: Trends and Future Prospects . . .

Volume I Actuators: Buckypaper Composite . . . . . . . . . . . . 1 Agriculture: Natural Polymers for Controlled Release of Fertilizers . . . . . . . . . . . . . . . . . . . . . 19 Agriculture: Polymers in Crop Production Mulch and Fertilizer . . . . . . . . . . . . . . . . . . . . . 28 Agriculture: Polymers in Crop Production Pesticides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 Agriculture: Polymers in Crop Protection . . . . . . 67 Agriculture: Super Absorbent Functional Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 Alginate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 Automotive Applications: Polylactic Acid and Biocomposite Parts . . . . . . . . . . . . . . . . . . . . . . . 147 Automotive Applications: Polymers in . . . . . . . . . 165 Automotive Applications: Reinforced Material Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179 Aviation: Polymer Composites, Processes, and Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195 Aviation: Thermoplastic and Thermoset Polymers in . . . . . . . . . . . . . . . . . . . . . . . . . . 216 Biomaterials: Soft Tissue Injuries . . . . . . . . . . . . . 224 Biomedicine: Cellulose in . . . . . . . . . . . . . . . . . . . . 240 Biomedicine: Cross-Linked Polymers Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262 Biomedicine: Polymers with Silsesquioxane Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285 Biosensing Devices: Conjugated Polymer Based Scaffolds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 360 Catalysts: Polymers in Biodiesel Production . . . . 387 Chemotherapy: Polycaprolactone (PCL)-Based Drug Delivery System . . . . . . . . . . . . . . . . . . . . 399 Chitosan: Antimicrobial and Edible Coatings . . . 415 Chitosan: Biodegradable Food Packaging . . . . . . 425 Chitosan: Tissue Engineering and Wound Dressing Applications . . . . . . . . . . . . . . . . . . . . 442 Communication: Polymers in . . . . . . . . . . . . . . . . 467 Composites and Nanocomposites: Thermoplastic Polymers for Additive Manufacturing . . . . . . . 486 Composites: Biobased for Materials Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . 501 Composites: Conductive Elastomer CNT-Based . . 515 Composites: Environment-Friendly Nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . 530

543 571 591 602 624 639 651 705 722 743 757 770 783 803 817 829 855 870 883 899

Volume II Electronics: Nonvolatile Memory Technologies . . . 913 Electronics: Polymer–Graphene Composites . . . 932 Energy: Conducting Polymers and Conjugated Porous Polymers . . . . . . . . . . . . . . . . . . . . . . . . . 961 Energy: Polymer Electrolytes for Lithium Ion Batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 972 Energy: Polymer Supercapacitors . . . . . . . . . . . . . 990 Energy: Polymer-Functionalized Graphene . . . . 1009 Energy: Polymers in Harvesting, Conversion, and Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1035 Energy: Polymers in the Active Layer of Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1049 Environmental Applications: Biopolymer Sorbents for Heavy Metal Removal . . . . . . . . . 1066 v

vi

Brief Contents

Volume II (cont’d.) Environmental Applications: Hydrogels . . . . . . . 1087 Environmental Applications: Polymers in . . . . . . 1106 Fire Protection: Flame-Retardant Additives and Fillers for Polymers . . . . . . . . . . . . . . . . . . . . . . 1122 Fire Protection: Flame-Retardant Epoxy Resins in . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1139 Fire Protection: Flame-Retardant Polymers in . . . . 1258 Food Packaging: Cellulose Nanocrystals in . . . . . 1273 Food Packaging: Edible Products . . . . . . . . . . . . . 1304 Food Packaging: Natural and Synthetic Biopolymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1325 Food Packaging: Polyhydroxyalcanoates (PHA) Containing Antimicrobial Additives . . . . . . . . 1343 Food Packaging: Polymer Composites . . . . . . . . . 1359 Food Packaging: Polymers as Packaging Materials in Food Supply Chains . . . . . . . . . . . 1374 Food Packaging: Starch and Non-Starch Blend Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1398 Food Packaging: Starch-Based Bionanocomposites . . . . . . . . . . . . . . . . . . . . . . . 1416 Food: Polymers in . . . . . . . . . . . . . . . . . . . . . . . . . . 1432 Footwear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1444 Fuel Cell: Cellulose-Based Polyelectrolyte Proton Exchange Membranes . . . . . . . . . . . . . . 1470 Fuel Cell: Polymeric Membrane . . . . . . . . . . . . . . 1499 Furniture: Eco-Friendly Polymer Composites Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1517 Furniture: Polymers in . . . . . . . . . . . . . . . . . . . . . . 1548 Healthcare: Polymer as Vital Materials . . . . . . . . 1561 Household Goods: Polymers in . . . . . . . . . . . . . . . 1580 Insulators: Polymers for High-Voltage Outdoor Use . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1596 Laboratory Applications: Polymers in . . . . . . . . . 1611 Marine Applications . . . . . . . . . . . . . . . . . . . . . . . . 1629 Medicines: Polymers for . . . . . . . . . . . . . . . . . . . . . 1679 Membrane: Preparation by Nonsolvent-Induced Phase Inversion . . . . . . . . . . . . . . . . . . . . . . . . . . 1692 Membranes: Graft Modification of Polymers for . . . . . . . . . . . . . . . . . . . . . . . . . . 1712 Membranes: Hemodialysis Applications . . . . . . . 1729 Microcapsules Polymeric: Self-Healing Smart Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1773 Microencapsulation: Phase Change Material in Textile and Building Construction . . . . . . . . . . 1789 Microneedles Arrays: Based on Natural Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1800 Nanocellulose: Environmental and Engineering Applications of . . . . . . . . . . . . . . . . . . . . . . . . . . 1813

Nanocellulose: Health Care Applications . . . . . . . Nanocomposites: Polyamide/Polyhedral Oligomeric Silsesquioxane . . . . . . . . . . . . . . . . . Nanocomposites: POSS-Based . . . . . . . . . . . . . . . Orthopedic Implants: Applications of Bioabsorbable Polymers . . . . . . . . . . . . . . . . . .

1829 1853 1876 1886

Volume III Oxo-Biodegradable Polymers . . . . . . . . . . . . . . . . 1907 Packaging, Active, and Intelligent: Polymer Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1958 Packaging: Food Waste Reduction . . . . . . . . . . . . 1990 Packaging: Polyhydroxyalkanoates (PHAs) in . . . 2010 Packaging: Polymer–Metal-Based Micro- and Nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . 2021 Petroleum Production: Polymers in . . . . . . . . . . . 2041 Poly(3-hydroxybutyrate): Applications . . . . . . . . 2061 Polymers and Polymeric Membranes . . . . . . . . . . 2077 Polypyrrole: Properties and Application . . . . . . . 2092 Polyurethanes: Biobased . . . . . . . . . . . . . . . . . . . . 2104 Porous Polymers: Gas Separation and Storage . . . . 2120 Rapid Prototyping of Microfluidic Devices in Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2146 Rapid Prototyping of Polymeric Biomaterials . . . 2163 Recycling: Converting Waste Plastics to Polyolefin Wax . . . . . . . . . . . . . . . . . . . . . . . . . . 2179 Recycling: Polyurethane Foam Wastes . . . . . . . . . 2195 Regenerative Medicine: Natural Polymers . . . . . . 2214 Resorbable Embolics . . . . . . . . . . . . . . . . . . . . . . . . 2238 Semiconducting Polymers in Photovoltaics . . . . . 2254 Sensors: Advanced Aptasensors Design . . . . . . . . 2271 Sensors: Ion-Sensing Polymers . . . . . . . . . . . . . . . 2291 Sensors: Natural Polymeric Composites . . . . . . . 2306 Sensors: Polymers in Sensing . . . . . . . . . . . . . . . . 2328 Sensors: Zeolite–Polymer Composites for Gas Sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2345 Separation: Poly(N-isopropylacrylamide) in . . . . . 2367 Separation: Polymers in . . . . . . . . . . . . . . . . . . . . . 2380 Smart Polymers: Lightweight Composites and Foams Tailored with Magnetic Field . . . . . . . . 2398 Smart Polymers: Molecularly Imprinted Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2426 Spin Coated Films of Polymeric Materials . . . . . . 2441 Starch: Renewable Source for Thermoplastic . . . 2461 Textile: Fiber Forming Polymers . . . . . . . . . . . . . 2490

Brief Contents vii

Textile: Flame Retardancy Through Surface Engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Textile: Polymer-Based Materials . . . . . . . . . . . . . Textile: Stimuli-Responsive Polymers in . . . . . . . Textile: Substrates Modification by Novel Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Textiles: Novel Polymers . . . . . . . . . . . . . . . . . . . . Textiles: Polymers and Fibers . . . . . . . . . . . . . . . . Thermogelling Polymers: Biomedical Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tissue Engineering: PolyhydroxyalkanoateBased Materials and Composites . . . . . . . . . . . Tissue Engineering: Polymeric Dermal Filler . . . . Tissue Engineering: Polymeric Scaffolds for MSC-Based Cartilage . . . . . . . . . . . . . . . . . . . .

2507 2522 2545 2562 2580 2592 2627 2652 2676 2683

Tissue Engineering: Polymers in Soft Tissue Cartilage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tissue Engineering: Thermoplastics for Scaffold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Value-Added Products from Natural Fibers Reinforced Composites . . . . . . . . . . . . . . . . . . . Veterinary Medicine: Polymers in . . . . . . . . . . . . Water Treatment: Metal and Metal Oxide Asymmetric Poly(Ether)Sulfone Nanocomposite Membranes . . . . . . . . . . . . . . . Water Treatment: Polymers for Coagulation and Flocculation . . . . . . . . . . . . . . . . . . . . . . . . . Wholly Aromatic Polyamide–hydrazides . . . . . . .

2704 2719 2767 2780

2795 2810 2825

Dedication

To my family Also, to those who made and will make a difference through polymer innovations for improving the quality of life!

ix

Encyclopedia of Polymer Applications Editor-in-Chief Munmaya Mishra, PhD c/o Altria Research Center, Richmond, Virginia, U.S.A.

xi

Contributors

Azila Abdul-Aziz  /  Department of Bioprocess and Polymer Engineering, University of Technology, Malaysia, Skudai, Malaysia Mariani Abdul Hamid  /  Department of Bioprocess and Polymer Engineering, University of Technology, Malaysia, Skudai, Malaysia Obaid-Ur-Rahman Abid  /  Department of Chemistry, Hazara University, Mansehra, Pakistan Jiji Abraham  /  International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, India Nidal Abu-Zahra  /  Materials Science and Engineering Department, University of Wisconsin–Milwaukee, Milwaukee, Wisconsin, U.S.A. Totan Adak  /  Crop Protection Division, Indian Council of Agricultural Research (ICAR), National Rice Research Institute, Cuttack, India Blessing Aderibigbe  /  Department of Chemistry, University of Fort Hare, Fort Hare, South Africa I.A. Aderibigbe  /  Department of Chemical and Metallurgical Engineering, Tshwane University of Technology, Pretoria, South Africa Garima Agrawal  /  Department of Polymer and Process Engineering, Indian Institute of Technology Roorkee, Saharanpur Campus, Saharanpur, India Rahul Agrawal  /  Department of Biochemical Engineering and Biotechnology, Indian Institute of Technology Delhi, New Delhi, India Seema Agrawal  /  University School of Basic and Applied Sciences, GGS Indraprastha University, New Delhi, India Zahed Ahmadi  /  Department of Chemistry, Amirkabir University of Technology, Tehran, Iran Dheeraj Ahuja  /  Dr. S.S. Bhatnagar University Institute of Chemical Engineering & Technology, Panjab University, Chandigarh, India Asif Ali  /  Department of Polymer and Process Engineering, Indian Institute of Technology Roorkee, Saharanpur, India Kazi Asraf Ali  /  Dr. B.C. Roy College of Pharmacy and Allied Health Science, Durgapur, India Abd Karim Alias  /  University of Science, Malaysia, Pulau Pinang, Malaysia Nahal Aliheidari  /  Advanced Composites Laboratory, School of Mechanical and Materials Engineering, Washington State University, Tri-Cities, Washington, U.S.A. Elif Alyamaç  /  Petroleum Engineering, Izmir Katip Celebi University, Izmir, Turkey Amir Ameli  /  Advanced Composites Laboratory, School of Mechanical and Materials Engineering, Washington State University, Tri-Cities, Washington, U.S.A. Sandro Campos Amicos  /  Post-Graduate Program in Mining, Metallurgical and Materials Engineering, Federal University of Rio Grande do Sul, Porto Alegre, Brazil Priscila Anadão  /  Department of Metallurgical and Materials Engineering, School of Engineering, University of São Paulo, São Paulo, Brazil Arfat Anis  /  Department of Chemical Engineering, King Saud University, Riyadh, Saudi Arabia xiii

xiv Contributors

Balaprasad Ankamwar  /  Bio-Inspired Materials Research Laboratory, Department of Chemistry, Savitribai Phule Pune University, Ganeshkhind, India Seema Ansari  /  Centre for Materials for the Electronics Technology, Thrissur, India Sridhar Aparna  /  Department of Chemical Engineering, Birla Institute of Technology and Science-Pilani, Hyderabad Campus, Hyderabad, India Diana Araújo  /  UCIBIO-REQUIMTE, Department of Chemistry, Faculty of Science and Technology (FCT), New Univerity of Lisbon, Caparica, Portugal Zeenat Arif  /  Department of Chemical Engineering and Technology, Indian Institute of Technology (Banaras Hindu University), Varanasi, India Mohammed Arif P  /  International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, India Fazilah Ariffin  /  University of Science, Malaysia, Pulau Pinang, Malaysia Divya Arora  /  Quality Control, Quality Assurance & CMC Division, Council of Scientific & Industrial Research (CSIR), Indian Institute of Integrative Medicine, Jammu, India; Academy of Scientific and Innovative Research (AcSIR), Jammu, India Burhan Ates  /  Department of Chemistry, Faculty of Arts and Sciences, Inonu University, Malatya, Turkey I. Azreen  /  Chemical Engineering Programme, Faculty of Engineering, Universiti Malaysia Sabah, Kota Kinabalu, Malaysia Samal Babanzadeh  /  Iran Polymer and Petrochemical Institute, Tehran, Iran Nabila Badar  /  Department of Chemistry, Hazara University, Mansehra, Pakistan Fatemeh Bagheri  /  Tarbiat Modares University, Tehran, Iran Laxmi P. Bagri  /  Bose Memorial Research Laboratory, Department of Chemistry, Government Autonomous Science College, Jabalpur, India Anil Kumar Bajpai  /  Bose Memorial Research Laboratory, Department of Chemistry, Government Autonomous Science College, Jabalpur, India Saleheen Bano  /  Department of Polymer and Process Engineering, Indian Institute of Technology Roorkee, Saharanpur, India Eugen Barbu  /  School of Pharmacy and Biomedical Sciences, University of Portsmouth, Portsmouth, U.K. Mandira Barman  /  Division of Soil Science and Agricultural Chemistry, Indian Council of Agricultural Research (ICAR), National Rice Research Institute, Cuttack, India Rocío Barreiro  /  Department of Analytical Chemistry, Nutrition and Bromatology, Faculty of Veterinary Science, University of Santiago de Compostela, Lugo, Spain Pallavi Bassi  /  Department of Pharmaceutical Sciences and Drug Research, Punjabi University, Patiala, Patiala, India Anish Benny  /  Department of Electrical and Electronics Engineering, Amal Jyothi College of Engineering, Kottayam, India Tanumoy Bera  /  Horticultural Science Department, University of Florida, Gainesville, Florida, U.S.A. Muzzaffar Ahmad Bhat  /  School of Studies in Chemistry, Jiwaji University, Gwalior, India Rinkesh Bhatt  /  Department of Physics, Global Engineering College, Jabalpur, India A. Bhattacharya  /  Membrane Science and Separation Technology Division, Council of Scientific & Industrial Research-Central Salt and Marine Chemicals Research Institute (CSIR-CSMCRI), Gujarat, India Rupa Bhattacharyya  /  Narula Institute of Technology, Kolkata, India M. V. Bhavya  /  JSS University, Mysuru, India

Contributors xv

Anjali Bishnoi  /  Department of Polymer & Rubber Technology, Shroff S. R. Rotary Institute of Chemical Technology, Ankleshwar, India; Department of Chemistry, Indian Institute of Technology, New Delhi, India Shanta Biswas  /  Department of Applied Chemistry and Chemical Engineering, Faculty of Engineering and Technology, University of Dhaka, Dhaka, Bangladesh Pierre-Alexandre Blanche  /  College of Optical Sciences, University of Arizona, Tucson, Arizona, U.S.A. A. P. Bonartsev  /  Faculty of Biology, M.V. Lomonosov Moscow State University, Moscow, Russia I. Bordianu-Antochi  /  Petru Poni Institute of Macromolecular Chemistry, Iași, Romania K.C.S. Brabes  /  Faculty of Engineering, Federal University of Grande Dourados, Dourados, Brazil J. I. Cadiz-Miranda  /  Applied Biotechnology Research Group, Faculty of Science and Technology, University of Westminster, London, U.K. Alberto Cepeda  /  Department of Analytical Chemistry, Nutrition and Bromatology Faculty of Veterinary Science, Universidade de Santiago de Compostela, Lugo, Spain Debanjana Chakraborty  /  Indian Institute of Chemical Biology, Kolkata, India Aparna R. Chakravarti  /  Biointel Laboratory, Department of Chemical and Petroleum Engineering, School of Engineering, University of Kansas, Lawrence, Kansas, U.S.A. Pousali Chal  /  Polymer Science Unit, Indian Association for the Cultivation of Science, Kolkata, India Vineethkrishna Chandrasekar  /  Invictus Oncology Pvt. Ltd., New Delhi, India Dibyendu Chatterjee  /  Crop Production Division, Indian Council of Agricultural Research (ICAR), National Rice Research Institute, Cuttack, India Narendra Pal Singh Chauhan  /  Department of Chemistry, Bhopal Nobles University, Udaipur, India Emo Chiellini  /  Inter University National Consortium of Materials Science and Technology (INSTM), Florence, Italy Yahya E. Choonara  /  Wits Advanced Drug Delivery Platform Research Unit, Department of Pharmacy and Pharmacology, School of Therapeutic Sciences, Faculty of Health Sciences, University of the Witwatersrand, Johannesburg, South Africa Barbiee Choudhary  /  Amity Institute of Biotechnology, Amity University, Noida, India Benjamin Church  /  Materials Science and Engineering Department, University of Wisconsin–Milwaukee, Milwaukee, Wisconsin, U.S.A. Stefania Cometa  /  Jaber Innovation S.R.L., Rome, Italy Patrícia Concórdio-Reis  /  UCIBIO-REQUIMTE, Department of Chemistry, Faculty of Science and Technology (FCT), New University of Lisbon, Caparica, Portugal Andrea Corti  /  Department of Chemistry and Industrial Chemistry, University of Pisa, Pisa, Italy C. Cotofana  /  Petru Poni Institute of Macromolecular Chemistry, Iași, Romania Pierre-Jean Cottinet  /  INSA Lyon, Lyon, France M. D’Auria  /  Institute for Polymers, Composites and Biomaterials, National Research Council, Portici, Italy Pulak Datta  /  Department of Chemical Engineering, Birla Institute of Technology, Mesra, India D. Davino  /  Institute for Polymers, Composites and Biomaterials, National Research Council, Portici, Italy

xvi Contributors

Mriganka De  /  University of Idaho, United States Department of Agriculture (USDA), Agricultural Research Service (ARS), Aberdeen, Idaho, U.S.A. Rafael de Avila Delucis  /  Post-Graduate Program in Mining, Metallurgical and Materials Engineering, Federal Universtiy of Rio Grande do Sul, Porto Alegre, Brazil Farha Deeba  /  Department of Polymer and Process Engineering, Indian Institute of Technology Roorkee, Saharanpur, India Joshua DeGraff  /  Florida State University, Tallahassee, Florida, U.S.A. Z.D. Dexter  /  Chemical Engineering Programme, Faculty of Engineering, University Malaysia Sabah, Kota Kinabalu, Malaysia; Sustainable Palm Oil Research Unit, University Malaysia Sabah, Kota Kinabalu, Malaysia Arindam Dey  /  Department of Chemical Engineering, Institute of Technology, Kharagpur, India Prodyut Dhar  /  Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati, India Mónica Díaz-Bao  /  Department of Analytical Chemistry, Nutrition and Bromatology, Faculty of Veterinary Science, University of Santiago de Compostela, Lugo, Spain Randy Donelson  /  Radiology, University of Minnesota, Minneapolis, Minnesota, U.S.A. Shengzhi Dub  /  Department of Electrical Engineering, Faculty of Engineering and the Built Environment, Tshwane University of Technology, Pretoria, South Africa Ravindra Dhar Dubey  /  Invictus Oncology Pvt. Ltd., New Delhi, India; Formulation and Drug Delivery Division, Indian Institute of Integrative Medicine (CSIR), Jammu, India Ubong M. Eduok  /  Department of Chemistry, King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia Liju Elias  /  National Institute of Technology, Karnataka, India Fatma Erdoğan  /  Materials Science and Engineering Graduate Program, Ege University, Izmir, Turkey Mohamadreza Baghaban Eslaminejad  /  Department of Stem Cells and Developmental Biology, Cell Science Research Center, Royan Institute for Stem Cell Biology and Technology, Academic Center for Education, Culture and Research (ACER), Tehran, Iran Xiaoguang Fan  /  College of Engineering, Shenyang Agricultural University, Shenyang, People’s Republic of China Frederico Castelo Ferreira  /  Massachusetts Institute of Technology Portugal Program, Porto Salvo, Portugal; Department of Bioengineering and IBB-Institute for Bioengineering and Bioscience, University of Lisbon, Lisbon, Portugal Lígia Figueiredo  /  Mechanical Engineering Institute (IDMEC), Instituto Superior Técnico, University of Lisbon, Lisbon, Portugal; Massachusetts Institute of Technology Portugal Program, Porto Salvo, Portugal Filomena Freitas  /  UCIBIO-REQUIMTE, Department of Chemistry, Faculty of Science and Technology (FCT), New University of Lisbon, Caparica, Portugal Apurv Gaidhani  /  Institute of Chemical Technology, Mumbai, India Somenath Ganguly  /  Department of Chemical Engineering, Institute of Technology, Kharagpur, India José A. Garde-Belza  /  Packaging Technologies Department, AINIA, Paterna, Spain Anshul Gautampurkar  /  Department of Polymer and Surface Engineering, Institute of Chemical Technology, Mumbai, India Gibin George  /  Jyothi Engineering College, Thrissur, India; Fayetteville State University, Fayetteville, North Carolina, U.S.A.

Contributors xvii

Soney C. George  /  Centre for Nanoscience and Nanotechnology, Amal Jyothi College of Engineering, Kottayam, India Saee Gharpure  /  Bio-Inspired Materials Research Laboratory, Department of Chemistry, Savitribai Phule Pune University, Ganeshkhind, India Mazaher Gholipourmalekabadi  /  Cellular and Molecular Research Center, Iran University of Medical Sciences, Tehran, Iran; Department of Tissue Engineering & RegenerativeMedicine, Faculty of Advanced Technologies in Medicine, Iran University of Medical Sciences, Tehran, Iran Luis Gil  /  Packaging Technologies Department, AINIA, Paterna, Spain Vikas V. Gite  /  Department of Polymer Chemistry, School of Chemical Sciences, North Maharashtra University, Jalgaon, India Saraswathy Gnanasundaram  /  Shoe and Product Design Centre, Council of Scientific and Industrial Research (CSIR), Central Leather Research Institute, Chennai, India Behnam Gohari  /  Materials Science and Engineering Department, University of Wisconsin–Milwaukee, Milwaukee, Wisconsin, U.S.A. Vincent G. Gomes  /  School of Chemical & Biomolecular Engineering, University of Sydney, Sydney, Australia Deepu A. Gopakumar  /  International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, India; Laboratory of Engineering Materials Brittany (LIMATB), Dupuy Research Institute of Lome (IRDL), University of South Brittany, Lorient, France Rekha Gorai  /  Department of Chemical Engineering, Institute of Technology, Kharagpur, India Mershen Govender  /  Wits Advanced Drug Delivery Platform Research Unit, Department of Pharmacy and Pharmacology, School of Therapeutic Sciences, Faculty of Health Sciences, University of the Witwatersrand, Johannesburg, South Africa Ivo Grabchev  /  Faculty of Medicine, Sofia University, Sofia, Bulgaria Yves Grohens  /  Laboratory of Engineering Materials Brittany (LIMATB), Dupuy Research Institute of Lome (IRDL), University of South Brittany, Lorient, France Xinyi Guan  /  School of Materials, University of Manchester, Manchester, U.K. M. K. Gupta  /  Department of Mechanical Engineering, Motilal Nehru National Institute of Technology Allahabad, Allahabad, India D. V. Gowda  /  JSS University, Mysuru, India Nooshin Haghighipour  /  National Cell Bank of Iran, Pasteur Institute of Iran, Tehran, Iran Jaydeep Haldar  /  Division of Vegetable Protection, Indian Council of Agricultural Research (ICAR), Indian Institute of Vegetable Research, Varanasi, India Papia Haque  /  Department of Applied Chemistry and Chemical Engineering, Faculty of Engineering and Technology, University of Dhaka, Dhaka, Bangladesh Tina Harifi  /  Amirkabir University of Technology, Tehran, Iran Rosnani Hasham  /  Department of Bioprocess and Polymer Engineering, University of Technology, Malaysia, Skudai, Malaysia Mohammad Mahbubul Hassan  /  Food & Bio-based Products Group, AgResearch Limited, Lincoln, New Zealand Siddaramaiah Hatna  /  JSS Science and Technology University, Mysuru, India Rajashekaraiah Hemanth  /  Department of Mechanical Engineering, NIE Institute of Technology, Mysuru, India

xviii Contributors

A. B. Hemavathi  /  Department of Polymer Science and Technology, Sri Jayachamarajendra College of Engineering, Mysuru, India Hooman Hosseini  /  Department of Chemical & Biochemical Engineering, Missouri University of Science and Technology, Rolla, Missouri, U.S.A. Samaneh Hosseini  /  Department of Stem Cells and Developmental Biology, Cell Science Research Center, Royan Institute for Stem Cell Biology and Technology, Royan Institute for Stem Cell Biology and Technology, Academic Center for Education, Culture and Research (ACECR), Tehran, Iran Sunaina Indermun  /  Wits Advanced Drug Delivery Platform Research Unit, Department of Pharmacy and Pharmacology, School of Therapeutic Sciences, Faculty of Health Sciences, University of the Witwatersrand, Johannesburg, South Africa Mohammad R. Ishak  /  Department of Aerospace Engineering, University Putra Malaysia, Serdang, Malaysia Md. Minhajul Islam  /  Department of Applied Chemistry and Chemical Engineering, Faculty of Engineering and Technology, University of Dhaka, Dhaka, Bangladesh Md. Sazedul Islam  /  Department of Applied Chemistry and Chemical Engineering, Faculty of Engineering and Technology, University of Dhaka, Dhaka, Bangladesh Anusha Iyangar  /  Department of Chemical Engineering, Birla Institute of Technology and Science, Pilani, Hyderabad, India Sundeep Jaglan  /  Quality Control, Quality Assurance & CMC Division, Council of Scientific & Industrial Research (CSIR), Indian Institute of Integrative Medicine, Jammu, India; Academy of Scientific and Innovative Research (AcSIR), Jammu, India Jemy James  /  International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, India; University of South Brittany, Lorient, France Sougata Jana  /  Department of Pharmaceutics, Gupta College of Technological Sciences, Asansol, India Subrata Jana  /  Department of Chemistry, V.E.C., Sarguja University, Ambikapur, India Nirmala Kumari Jangid  /  Department of Chemistry, Bhopal Nobles University, Udaipur, India Mohammad Jawaid  /  Department of Structure and Material, Faculty of Mechanical Engineering, Technical University of Malaysia, Durian Tunggal, Malaysia Duraisamy Jeyakumar  /  Functional Materials Division, Council of Scientific and Industrial Research (CSIR), Central Electrochemical Research Institute, Karaikudi, India Mingliang Jin  /  Guangdong Provincial Key Laboratory of Optical Information Materials and Technology & Institute of Electronic Paper Displays, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou, China; National Center for International Research on Green Optoelectronics, South China Normal University, Guangzhou, China Blessy Joseph  /  International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, India Nirav Joshi  /  São Carlos Institute of Physics, University of São Paulo, São Paulo, Brazil Emad Jowshan  /  Department of Polymer Engineering, Amirkabir University of Technology, Tehran, Iran Ridhwan Jumaidina  /  Department of Mechanical and Manufacturing Engineering, University Putra Malaysia, Serdang, Malaysia; Department of Structure and Material, Faculty of Mechanical Engineering, Technical University of Malaysia, Durian Tunggal, Malaysia

Contributors xix

Nandakumar Kalarikkal  /  International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, India Noel Jacob Kaleekkal  /  Department of Chemical Engineering, School of Chemical and Biotechnology, SASTRA University, Tanjore, India Hanisah Kamilah  /  University of Science, Malaysia, Pulau Pinang, Malaysia Pradnya P. Kanekar  /  Department of Biotechnology, Modern College of Arts, Science and Commerce, Pune, India Sagar P. Kanekar  /  Department of Biotechnology, Modern College of Arts, Science and Commerce, Pune, India Mayuree Kanlayavattanakul  /  School of Cosmetic Science, Mae Fah Lung University, Chiang Rai, Thailand Vimal Katiyar  /  Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati, India Anupreet Kaur  /  Basic and Applied Sciences, Punjabi University, Patiala, Patiala, India Gurpreet Kaur  /  Department of Pharmaceutical Sciences and Drug Research, Punjabi University, Patiala, Patiala, India Navjeet Kaur  /  Department of Chemistry, Bhopal Nobles University, Udaipur, India Ayesha Kausar  /  School of Natural Sciences, National University of Sciences and Technology (NUST), Islamabad, Pakistan Anupama Kaushik  /  Dr. S.S. Bhatnagar University Institute of Chemical Engineering & Technology, Panjab University, Chandigarh, India Thomas Kerr-Phillips  /  Polymer Electronics Research Centre, School of Chemical Sciences, The University of Auckland, Auckland, New Zealand; The MacDiarmid Institute for Advanced Materials and Nanotechnology, Wellington, New Zealand Melis Kesika  /  Department of Chemistry, Middle East Technical University, Ankara, Turkey Randa E. Khalifa  /  Polymer Materials Research Department, Advanced Technology and New Materials Research Institute, Alexandria, Egypt Reza Khalili  /  School of Chemical Engineering, College of Engineering, University of Tehran, Tehran, Iran Khosrow Khodabakhshi  /  Paint and Surface Coating Group, Faculty of Processing, Iran Polymer and Petrochemical Institute, Tehran, Iran Karan Khubdikar  /  Department of Chemical Engineering, Birla Institute of Technology and Science, Pilani, Hyderabad, India Anirudh Krishnamurthy  /  Department of Chemical & Biochemical Engineering, Missouri University of Science and Technology, Rolla, Missouri, U.S.A. Madhumita Kulkarni  /  Department of Polymer and Surface Engineering, Institute of Chemical Technology, Mumbai, India Snehal O. Kulkarni  /  Department of Biotechnology, Modern College of Arts, Science and Commerce, Pune, India Anurag Kulshreshtha  /  Department of Paper Technology, Indian Institute of Technology Roorkee, Saharanpur, India Bijender Kumar  /  Department of Polymer and Process Engineering, Indian Institute of Technology Roorkee, Saharanpur, India Bipin Kumar  /  Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Kowloon, Hong Kong Jitendra Kumar  /  Indian Council of Agricultural Research (ICAR), Directorate of Medicinal & Aromatic Plants Research, Gujarat, India

xx Contributors

Pradeep Kumar  /  Wits Advanced Drug Delivery Platform Research Unit, Department of Pharmacy and Pharmacology, School of Therapeutic Sciences, Faculty of Health Sciences, University of the Witwatersrand, Johannesburg, South Africa Sanjay Kumar  /  Department of Electronics and Communication Engineering, Birla Institute of Technology, Mesra, India Sunil Kumar  /  Department of Polymer & Rubber Technology, Shroff S. R. Rotary Institute of Chemical Technology, Ankleshwar, India; Centre for Polymer Science & Engineering, Indian Institute of Technology, New Delhi, India Nithin Kundachira Subramani  /  National Institute of Engineering, Mysuru, India Youngmin Lee  /  Department of Chemical Engineering, Pennsylvania State University, State College, Pennsylvania, U.S.A. Poh Lee-Shang  /  University of Science, Malaysia, Pulau Pinang, Malaysia Thabiso Letseka  /  Chemistry and Chemical Technology, National University of Lesotho, Roma, Lesotho Richard Liang  /  Florida State University, Tallahassee, Florida, U.S.A. Xuqing Liu  /  School of Materials, University of Manchester, Manchester, U.K. L. R. Lizarraga-Valderrama  /  Applied Biotechnology Research Group, Faculty of Science and Technology, University of Westminster, London, U.K. Nattaya Lourith  /  School of Cosmetic Science, Mae Fah Lung University, Chiang Rai, Thailand Washington Luiz Esteves Magalhães  /  Embrapa Florestas, Colombo, Brazil Zhiping Luo  /  Fayetteville State University, Fayetteville, North Carolina, U.S.A. Mahendra Mahajan  /  Department of Polymer Chemistry, School of Chemical Sciences, North Maharashtra University, Jalgaon, India P.A. Mahanwar  /  Department of Polymer and Surface Engineering, Institute of Chemical Technology, Mumbai, India Prakash Mahanwar  /  Department of Polymer and Surface Engineering, Institute of Chemical Technology, Mumbai, India Hamid Mahdavi  /  Novel Drug Delivery Systems and Biomaterial Department, Science Faculty, Iran Polymer and Petrochemical Institute, Tehran, Iran Pankaj M. Maheriya  /  Department of Pharmaceutical Research and Development, Ajanta Pharma Limited, Mumbai, India Kaiser Mahmood  /  University of Science, Malaysia, Pulau Pinang, Malaysia Arindam Maity  /  Dr. B. C. Roy College of Pharmacy and Allied Health Science, Durgapur, India Shiekh Abdul Majid  /  School of Studies in Chemistry, Jiwaji University, Gwalior, India Giulio Malucelli  /  Department of Applied Science and Technology, Polytechnic University of Turin, Alessandria, Italy Paramita Manna  /  Membrane Science and Separation Technology Division, Council of Scientific & Industrial Research-Central Salt and Marine Chemicals Research Institute (CSIR-CSMCRI), Gujarat, India Hanna J. Maria  /  International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, India Maria de Fatima V. Marques  /  Institute of Macromolecules Professor Eloisa Mano, Federal University of Rio de Janeiro (UFRJ), Rio de Janeiro, Brazil S. M. Martelli  /  Faculty of Engineering, Federal University of Grande Dourados, Dourados, Brazil Shahram Mehdipour-Ataei  /  Iran Polymer and Petrochemical Institute, Tehran, Iran

Contributors xxi

Rajeev Mehta  /  Chemical Engineering Department, Thapar Institute of Engineering and Technology University, Patiala, India Romil Mehta  /  Membrane Science and Separation Technology Division, Council of Scientific & Industrial Research-Central Salt and Marine Chemicals Research Institute (CSIR-CSMCRI), Gujarat, India Tsuyoshi Michinobu  /  Department of Materials Science and Engineering, Tokyo Institute of Technology, Tokyo, Japan Muzzaffar Ahmad Mir  /  School of Studies in Chemistry, Jiwaji University, Gwalior, India Rui Miranda Guedes  /  Department of Mechanical Engineering (DEMec), Faculty of Engineering, University of Porto (FEUP), Porto, Portugal, Laboratory of Optics and Experimental Mechanics, Institute of Science and Innovation in Mechanical and Industrial Engineering (INEGI), Porto, Portugal; LABIOMEP, Porto Biomechanics Laboratory, University of Porto, Porto, Portugal Hamid Mirzadeh  /  Department of Polymer Engineering and Color Technology, Amirkabir University of Technology, Tehran, Iran Abhilasha Mishra  /  Graphic Era University, Dehradoon, India P.K. Mishra  /  Department of Chemical Engineering and Technology, Indian Institute of Technology (Banaras Hindu University), Varanasi, India Swati Mishra  /  Papé Family Pediatric Research Institute, Department of Pediatrics, Oregon Health & Science University, Portland, Oregon, U.S.A. Rebone Moerane  /  Department of Production Animal Studies, Faculty of Veterinary Science, University of Pretoria, Pretoria, South Africa Nadia A. Mohamed  /  Department of Chemistry, Faculty of Science, Cairo University, Giza, Egypt Abdorreza Mohammadi Nafchi  /  University of Science, Malaysia, Pulau Pinang, Malaysia Smita Mohanty  /  Central Institute of Plastics Engineering and Technology (CIPET), Chennai, India; Laboratory for Advanced Research in Polymeric Materials (LARPM), Bhubaneswar, India Somesh Mohapatra  /  Metallurgical and Materials Engineering Department, Indian Institute of Technology, Roorkee, India Davod Mohebbi-Kalhori  /  Department of Chemical Engineering, Faculty of Engineering, University of Sistan and Baluchestan, Zahedan, Iran Mohamed S. Mohy Eldin  /  Polymer Materials Research Department, Advanced Technology and New Materials Research Institute, Alexandria, Egypt Majid Montazer  /  Amirkabir University of Technology, Tehran, Iran Masoud Mozafari  /  Bioengineering Research Group, Nanotechnology and Advanced Materials Department, Materials and Energy Research Center (MERC), Tehran, Iran Chethana Mudenur  /  Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati, India M. N. Muralidharan  /  Centre for Materials for the Electronics Technology, Thrissur, India Jitendra B. Naik  /  Department of Pharmaceutical Technology, University Institute of Chemical Technology, North Maharashtra University, Jalgaon, India Arun K. Nandi  /  Polymer Science Unit, Indian Association for the Cultivation of Science, Kolkata, India Sumit Nandi  /  Narula Institute of Technology, Kolkata, India Shilpa Narang  /  Chemistry Department, Multani Mal Modi College, Patiala, India

xxii Contributors

Anudeep Kumar Narula  /  University School of Basic and Applied Sciences, GGS Indraprastha University, New Delhi, India Sanjay K. Nayak  /  Central Institute of Plastics Engineering and Technology (CIPET), Chennai, India; Laboratory for Advanced Research in Polymeric Materials (LARPM), Bhubaneswar, India Suraj K. Nayak  /  Department of Biotechnology and Medical Engineering, National Institute of Technology, Rourkela, India Yuvraj Singh Negi  /  Department of Polymer and Process Engineering, Indian Institute of Technology Roorkee, Saharanpur, India Dorota Neugebauer  /  Department of Physical Chemistry and Technology of Polymers, Faculty of Chemistry, Silesian University of Technology, Gliwice, Poland Mir Mohammad Alavi Nikje  /  Department of Chemistry, Faculty of Science, Imam Khomeini International University, Qazvin, Iran K. Nithin  /  Department of Chemistry, The National Institute of Engineering, Mysuru, India P. Nombula  /  Department of Chemical and Metallurgical Engineering, Tshwane University of Technology, Pretoria, South Africa Olufunke Nwosu  /  Production Animal Studies, University of Pretoria, Pretoria, South Africa A. Oancea  /  Petru Poni Institute of Macromolecular Chemistry, Iași, Romania I.B. Obot  /  Centre of Research Excellence in Corrosion, Research Institute, King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia M. Olaru  /  Petru Poni Institute of Macromolecular Chemistry, Iași, Romania Nuno Guitian Oliveira  /  Mechanical Engineering Institute (IDMEC), Instituto Superior Técnico, University of Lisbon, Lisbon, Portugal; Massachusetts Institute of Technology Portugal Program, Porto Salvo, Portugal Gbenga Folorunso Oluyemi  /  School of Engineering, Robert Gordon University, Aberdeen, U.K. Tamer M. Omer  /  Polymer Materials Research Department, Advanced Technology and New Materials Research Institute, Alexandria, Egypt Ismail Ozdemir  /  Catalysis Research and Application Center, Inönü University, Malatya, Turkey and Department of Chemistry, Inonu University, Malatya, Turkey Avinash R. Pai  /  International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, India Kunal Pal  /  Department of Biotechnology and Medical Engineering, National Institute of Technology, Rourkela, India Ning Pan  /  Division of Textiles, Biological & Agricultural Engineering, University of California, Davis, Davis, California, U.S.A. Shweta Paroha  /  School of Pharmaceutical Sciences, Siksha O Anushandhan University, Bhubaneswar, India Leonor Pascual-Ramírez  /  Packaging Technologies Department, AINIA, Paterna, Spain Shresthaa Patel  /  School of Biotechnology, KIIT University, Bhubaneswar, India Anurag Patil  /  Department of Chemical Engineering, Birla Institute of Technology and Science, Pilani, Hyderabad Campus, Hyderabad, India Suprio R. Paul  /  Amity Institute of Biotechnology, Amity University, Noida, India Inge-Marie Petzer  /  Department of Production Animal Studies, Faculty of Veterinary Science, University of Pretoria, Pretoria, South Africa

Contributors xxiii

Cesar Liberato Petzhold  /  Institute of Chemistry, Federal University of Rio Grande do Sul, Porto Alegre, Brazil Pietro Picuno  /  School of Agricultural, Forestry, Food and Environmental Sciences (SAFE), University of Basilicata, Potenza, Italy Viness Pillay  /  Wits Advanced Drug Delivery Platform Research Unit, Department of Pharmacy and Pharmacology, School of Therapeutic Sciences, Faculty of Health Sciences, University of the Witwatersrand, Johannesburg, South Africa Luís F.V. Pinto  /  Altakitin S.A., Lisbon, Portugal; Materials Research Center, hosted by the New University of Lisbon/Institute of Nanostructures, Nanomodelling and Nanofabrication (CENIMAT/I3N), Department of Materials Science, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Lisboa, Portugal Baharak Pooladian  /  Department of Chemistry, Faculty of Science, Imam Khomeini International University, Qazvin, Iran A.P.I. Popoola  /  Department of Chemical and Metallurgical Engineering, Tshwane University of Technology, Pretoria, South Africa Yasir Beeran Pottathara  /  International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, India Sukanya Pradhan  /  Central Institute of Plastics Engineering and Technology (CIPET), Chennai, India Vipul D. Prajapati  /  Department of Pharmaceutics, SSR College of Pharmacy, Silvassa, India Sivadasu Praveen  /  JSS University, Mysuru, India Ruchir Priyadarshi  /  Department of Polymer and Process Engineering, Indian Institute of Technology Roorkee, Saharanpur, India Kalyani Prusty  /  Department of Chemistry, Veer Surendra Sai University of Technology, Sambalpur, India Vikas Pruthi  /  Department of Biotechnology, Indian Institute of Technology Roorkee, Roorkee, India Doddipatla Purnima  /  Department of Chemical Engineering, Birla Institute of Technology and Science, Pilani, Hyderabad Campus, Hyderabad, India Ahmad Rabiee  /  Polymer Science Department, Iran Polymer and Petrochemical Institute, Tehran, Iran Md. Shirajur Rahman  /  Department of Applied Chemistry and Chemical Engineering, Faculty of Engineering and Technology, University of Dhaka, Dhaka, Bangladesh Mohammed Mizanur Rahman  /  Department of Applied Chemistry and Chemical Engineering, Faculty of Engineering and Technology, University of Dhaka, Dhaka, Bangladesh Sandeep Rai  /  Department of Polymer & Rubber Technology, Shroff S. R. Rotary Institute of Chemical Technology, Ankleshwar, India; GRP Limited, Bharuch, India Mohan Ramesh  /  Functional Materials Division, Council of Scientific and Industrial Research (CSIR),Central Electrochemical Research Institute, Karaikudi, India V.S. Rana  /  Division of Agricultural Chemicals, Indian Council of Agricultural Research (ICAR), Indian Agricultural Research Institute, New Delhi, India Mohan Ranganathan  /  Council of Scientific and Industrial Research (CSIR), Chennai, India Suraya Abdul Rashid  /  Materials Processing and Technology Laboratory (MPTL), Institute of Advanced Technology, University Putra Malaysia, Serdang, Malaysia Bharatraj Singh Rathore  /  Department of Chemistry, PAHER University, Udaipur, India

xxiv Contributors

C. R. Rech  /  Faculty of Exact Sciences and Technology, Federal University of Grande Dourados, Dourados, Brazil Patricia Regal  /  Department of Analytical Chemistry, Nutrition and Bromatology, Faculty of Veterinary Science , Universidade de Santiago de Compostela, Lugo, Spain Fateme Rezaei  /  Department of Chemical & Biochemical Engineering, Missouri University of Science and Technology, Rolla, Missouri, U.S.A. Ufana Riaz  /  Materials Research Laboratory, Department of Chemistry, Jamia Millia Islamia (A Central University), New Delhi, India Nuno Ribeiro  /  Orthopedics and Traumatology Service, Lusíadas Hospital, Lisboa, Portugal Alexandra Rodrigues  /  Mechanical Engineering Institute (IDMEC), Instituto Superior Técnico, Instituto Superior Técnico, University of Lisbon, Lisbon, Portugal; Research Group on Modeling and Optimization of Multifunctional Systems (GI-MOSM), Lisbon Higher Institute of Engineering (ISEL), ADEM, Lisbon Higher Institute of Engineering, Lisbon, Portugal G. Roshan Deen  /  Soft Materials Laboratory, Natural Sciences and Science Education, National Institute of Education, Nanyang Technological University, Nanyang, Singapore Ali A. Rownaghi  /  Department of Chemical & Biochemical Engineering, Missouri University of Science and Technology, Rolla, Missouri, U.S.A. I. Roy  /  Applied Biotechnology Research Group, Faculty of Science and Technology, University of Westminster, London, U.K. Kyle Russell  /  Mechanical Engineering Department, University of Wisconsin–Milwaukee, Milwaukee, Wisconsin, U.S.A. K. T. Sabu  /  Rajiv Gandhi Institute of Technology, Kottayam, India E. R. Sadiku  /  Department of Polymer Technology, Tshwane University of Technology, Pretoria, South Africa Amir Reza Sadrolhosseini  /  Functional Devices Laboratory (FDL), Institute of Advanced Technology, University Putra Malaysia, Serdang, Malaysia and Materials Processing and Technology Laboratory (MPTL), Institute of Advanced Technology, University Putra Malaysia, Serdang, Malaysia Mohammad Reza Saeb  /  Department of Resin and Additives, Institute for Color Science and Technology, Tehran, Iran Ajoy Saha  /  Indian Council of Agricultural Research (ICAR), Directorate of Medicinal & Aromatic Plants Research, Gujarat, India Japar Saharie  /  Department of Mechanical and Manufacturing Engineering, University Putra Malaysia, Serdang, Malaysia Rajesh Kumar Sahoo  /  Buxi Jagabandhu Bidyadhara Junior College, Bhubaneswar, India Anshuman K. Sahu  /  Department of Mechanical Engineering, National Institute of Technology, Rourkela, India Rajesh Saini  /  Bose Memorial Research Laboratory, Department of Chemistry, Government Autonomous Science College, Jabalpur, India Asit Baran Samui  /  Institute of Chemical Technology, Mumbai, India Ankit Saneja  /  Formulation and Drug Delivery Division, Indian Institute of Integrative Medicine (CSIR), Jammu, India Veeman Sannasi  /  Functional Materials Division, Council of Scientific and Industrial Research (CSIR),Central Electrochemical Research Institute, Karaikudi, India

Contributors xxv

Bianca P. Santos  /  Institute of Macromolecules Professor Eloisa Mano, Federal University of Rio de Janeiro (UFRJ), Rio de Janeiro, Brazil Oshin Sapra  /  Chemical Engineering Department, Indian Institute of Technology, Roorkee, India Salit M. Sapuana  /  Department of Mechanical and Manufacturing Engineering, University Putra Malaysia, Serdang, Malaysia; Laboratory of Biocomposite Technology, Institute of Tropical Forestry and Forest Products, University Putra Malaysia, Serdang, Malaysia Dhruba Jyoti Sarkar  /  Division of Agricultural Chemicals, ICAR-Indian Agricultural Research Institute, New Delhi, India Gautam Sarkhel  /  Department of Chemical Engineering, Birla Institute of Technology, Mesra, India Sauraj  /  Department of Polymer and Process Engineering, Indian Institute of Technology Roorkee, Saharanpur, India S. Sentilkumar  /  Oren Hydrocarbons Private Limited, Chennai, India M. Özgür Seydibeyoğlu  /  Materials Science and Engineering, Izmir Katip Celebi University, Izmir, Turkey Suhaidi Shafie  /  Functional Devices Laboratory (FDL), Institute of Advanced Technology, University Putra Malaysia, Serdang, Malaysia Md. Shahruzzaman  /  Department of Applied Chemistry and Chemical Engineering, Faculty of Engineering and Technology, University of Dhaka, Dhaka, Bangladesh Najam Akhter Shakil  /  Division of Agricultural Chemicals, ICAR-Indian Agricultural Research Institute, New Delhi, India Mohammad Amin Shamekhi  /  Novel Drug Delivery Systems and Biomaterial Department, Science Faculty, Iran Polymer and Petrochemical Institute, Tehran, Iran; Department of Polymer Engineering, Islamic Azad University, Sarvestan Branch, Sarvestan, Iran Arnab Shit  /  Polymer Science Unit, Indian Association for the Cultivation of Science, Kolkata, India Lingling Shui  /  Guangdong Provincial Key Laboratory of Optical Information Materials and Technology & Institute of Electronic Paper Displays, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou, China; National Center for International Research on Green Optoelectronics, South China Normal University, Guangzhou, China Chow Wen Shyang  /  School of Materials and Mineral Resources Engineering, Engineering Campus, University of Science, Malaysia, Nibong Tebal, Malaysia H. Siddaramaiah  /  Department of Polymer Science and Technology, Sri Jayachamarajendra College of Engineering, Mysuru, India B. Simionescu  /  Costin D. Nenitescu Centre of Organic Chemistry, Bucharest, Romania Anurag Singh  /  Department of Mechanical Engineering (DEMec), Faculty of Engineering, University of Porto (FEUP), Porto, Portugal Sauraj Singh  /  Department of Polymer and Process Engineering, Indian Institute of Technology Roorkee, Saharanpur, India Vinay K. Singh  /  Department of Biotechnology and Medical Engineering, National Institute of Technology, Rourkela, India K. S. Sisanth  /  International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, India

xxvi Contributors

Lakshmipriya Somasekharan  /  International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, India L. Sorrentino  /  Institute for Polymers, Composites and Biomaterials, National Research Council, Portici, Italy Rungsinee Sothornvit  /  Department of Food Engineering, Faculty of Engineering at Kamphaeng Saen, Center of Advanced Studies in Industrial Technology, Kasetsart University, Nakhon Pathom, Thailand Saniye Soylemez  /  Department of Chemistry, Middle East Technical University, Ankara, Turkey Ananthakrishnan Srinivasan  /  Department of Physics, University of South Africa, Florida Campus, Johannesburg, South Africa; Department of Physics, Indian Institute of Technology Guwahati, Guwahati, India Desislava Staneva  /  University of Chemical Technology and Metallurgy, Sofia, Bulgaria Seemadri Subhadarshini  /  Department of Biotechnology and Medical Engineering, National Institute of Technology, Rourkela, India Syazana Sulaiman  /  University of Science, Malaysia, Pulau Pinang, Malaysia Panuwat Suppakul  /  Department of Packaging and Materials Technology, Faculty of Agro-Industry, Kasetsart University, Bangkok, Thailand Bheemappa Suresha  /  Department of Mechanical Engineering, The National Institute of Engineering, Mysuru, India Sarat K. Swain  /  Department of Chemistry, Veer Surendra Sai University of Technology, Sambalpur, India Zahra Tabatabaei-Yazdi  /  Iran Polymer and Petrochemical Institute, Tehran, Iran Tamer M. Tamer  /  Polymer Materials Research Department, Advanced Technology and New Materials Research Institute, Alexandria, Egypt Divya Thakur  /  Department of Pharmaceutical Sciences and Drug Research, Punjabi University, Patiala, Patiala, India Leboli Thamae  /  Chemistry and Chemical Technology, National University of Lesotho, Roma, Lesotho Thimothy Molefi Thamae  /  Chemistry and Chemical Technology, National University of Lesotho, Roma, Lesotho C. Thomas  /  Applied Biotechnology Research Group, Faculty of Science and Technology, University of Westminster, London, U.K. Sabu Thomas  /  International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, India Radha Tomar  /  School of Studies in Chemistry, Jiwaji University, Gwalior, India Levent Toppare  /  Department of Chemistry, Middle East Technical University, Ankara, Turkey, Department of Biotechnology, Middle East Technical University, Ankara, Turkey; Department of Polymer Science and Technology, Middle East Technical University, Ankara, Turkey; The Center for Solar Energy Research and Application (GUNAM), Middle East Technical University, Ankara, Turkey Cristiana A.V. Torres  /  UCIBIO-REQUIMTE, Department of Chemistry, Faculty of Science and Technology (FCT), New University of Lisbon, Caparica, Portugal Sergio Torres-Giner  /  A. Schulman, Inc., Almazora, Spain Jadranka Travas-Sejdic  /  Polymer Electronics Research Centre, School of Chemical Sciences, The University of Auckland, Auckland, New Zealand; The MacDiarmid Institute for Advanced Materials and Nanotechnology, Wellington, New Zealand Eliane Trovatti  /  University of Araraquara, Araraquara, Brazil

Contributors xxvii

Ahmet Ulu  /  Department of Chemistry, Faculty of Arts and Sciences, Inonu University, Malatya, Turkey Oznur Dogan Ulu  /  Catalysis Research and Application Center, Inönü University, Malatya, Turkey Saviour A. Umoren  /  Centre of Research Excellence in Corrosion, Research Institute, King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia S.N. Upadhyay  /  Department of Chemical Engineering and Technology, Indian Institute of Technology (Banaras Hindu University), Varanasi, India C. Ursu  /  Petru Poni Institute of Macromolecular Chemistry, Iași, Romania Sreedevi Vallabhapurapu  /  Department of School of Computing, University of South Africa, Johannesburg, South Africa Anurakshee Verma  /  Materials Research Laboratory, Department of Chemistry, Jamia Millia Islamia, New Delhi, India Namrata Verma  /  Department of Chemical Engineering, Birla Institute of Technology and Science, Pilani, Hyderabad Campus, Hyderabad, India Mrunal R. Waghulde  /  Department of Pharmaceutical Technology, University Institute of Chemical Technology, North Maharashtra University, Jalgaon, India Qing Wang  /  Department of Material Science and Engineering, Pennsylvania State University, State College, Pennsylvania, U.S.A. Yang Wang  /  Department of Materials Science and Engineering, Tokyo Institute of Technology, Tokyo, Japan Lihui Weng  /  Department of Radiology, University of Minnesota, Minneapolis, Minnesota, U.S.A. Hélio Wiebeck  /  Department of Metallurgical and Materials Engineering, School of Engineering, University of São Paulo, São Paulo, Brazil Zhibin Yan  /  Guangdong Provincial Key Laboratory of Optical Information Materials and Technology & Institute of Electronic Paper Displays, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou, China; National Center for International Research on Green Optoelectronics, South China Normal University, Guangzhou, China Guang Yang  /  Department of Materials Science and Engineering, Pennsylvania State University, State College, Pennsylvania, U.S.A. Lei Yang  /  School of Environmental and Biological Engineering, Liaoning Shihua University, Fushun, People’s Republic of China Hüsnügül Yilmaz Atay  /  Department of Material Science and Engineering, İzmir Katip Çelebi University, Çiğli İzmir, Turkey Hongjun Yu  /  University of Manchester, Manchester, U.K. A.Y. Zahrim  /  Chemical Engineering Programme, Faculty of Engineering, University Malaysia Sabah, Kota Kinabalu, Malaysia, Water Research Unit, University Malaysia Sabah, Kota Kinabalu, Malaysia; Sustainable Palm Oil Research Unit, Universiti Malaysia Sabah, Kota Kinabalu, Malaysia; Energy Research Unit, Universiti Malaysia Sabah, Kota Kinabalu, Malaysia Payam Zarrintaj  /  School of Chemical Engineering, College of Engineering, University of Tehran, Tehran, Iran Jing Zhang  /  School of Environmental and Biological Engineering, Liaoning Shihua University, Fushun, People’s Republic of China

Contents

Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiii Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxxv About the Editor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxxvii

Volume I Actuators: Buckypaper Composite  /  Joshua DeGraff, Richard Liang, and Pierre-Jean Cottinet . . Agriculture: Natural Polymers for Controlled Release of Fertilizers  /  Cristiana A.V. Torres and Filomena Freitas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Agriculture: Polymers in Crop Production Mulch and Fertilizer  /  Dhruba Jyoti Sarkar, Mandira Barman, Tanumoy Bera, Mriganka De, and Dibyendu Chatterjee . . . . . . . . . . . . . . . . . . . . . . . . . . Agriculture: Polymers in Crop Production Pesticides  /  Dhruba Jyoti Sarkar, Najam Akhter Shakil, V. S. Rana, Jitendra Kumar, Ajoy Saha, Totan Adak, and Jaydeep Haldar . . . . . . . . . . . . . Agriculture: Polymers in Crop Protection  /  Pietro Picuno . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Agriculture: Super Absorbent Functional Polymers  /  Bijender Kumar, Farha Deeba, Ruchir Priyadarshi, Sauraj, and Yuvraj Singh Negi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Alginate  /  Pankaj M. Maheriya and Vipul D. Prajapati . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Automotive Applications: Polylactic Acid and Biocomposite Parts  /  Khosrow Khodabakhshi . . . Automotive Applications: Polymers in   /  I.A. Aderibigbe, E.R. Sadiku, and A.P.I. Popoola . . . . . . Automotive Applications: Reinforced Material Components  /  Bheemappa Suresha, Rajashekaraiah Hemanth, and Kundachira Subramani Nithin . . . . . . . . . . . . . . . . . . . . . . . . . . . . Aviation: Polymer Composites, Processes, and Properties  /  I.A. Aderibigbe, P. Nombula, and A.P.I. Popoola . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Aviation: Thermoplastic and Thermoset Polymers in  /  M. Özgür Seydibeyoğlu, Fatma Erdoğan, and Elif Alyamaç . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biomaterials: Soft Tissue Injuries  /  Anurag Singh and Rui Miranda Guedes . . . . . . . . . . . . . . . . . . Biomedicine: Cellulose in  /  Eliane Trovatti and Eugen Barbu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biomedicine: Cross-Linked Polymers Applications  /  B. A. Aderibigbe . . . . . . . . . . . . . . . . . . . . . . Biomedicine: Polymers with Silsesquioxane Units  /  B. Simionescu, A. Oancea, I. BordianuAntochi, C. Ursu, C. Cotofana, and M. Olaru . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biosensing Devices: Conjugated Polymer Based Scaffolds  /  Saniye Soylemez, Melis Kesika, and Levent Toppare . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Catalysts: Polymers in Biodiesel Production  /  Farha Deeba, Bijender Kumar, Sauraj, Ruchir Priyadarshi, Yuvraj S. Negi, and Vikas Pruthi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chemotherapy: Polycaprolactone (PCL)-Based Drug Delivery System  /  Somesh Mohapatra, Vineethkrishna Chandrasekar, Aparna R. Chakravarti, Shweta Paroha, Oshin Sapra, Ankit Saneja, and Ravindra Dhar Dubey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chitosan: Antimicrobial and Edible Coatings  /  Anjali Bishnoi, Sandeep Rai, Nirav Joshi, and Sunil Kumar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chitosan: Biodegradable Food Packaging  /  Ruchir Priyadarshi, Sauraj Singh, Saleheen Bano, Asif Ali, and Yuvraj Singh Negi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chitosan: Tissue Engineering and Wound Dressing Applications  /  Mohammad Amin Shamekhi, Hamid Mahdavi, Hamid Mirzadeh, Ahmad Rabiee, Davod Mohebbi-Kalhori, Nooshin Haghighipour, and Mohamadreza Baghaban Eslaminejad . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Communication: Polymers in  /  Pulak Datta, Gautam Sarkhel, and Sanjay Kumar . . . . . . . . . . . . . xxix

1 19 28 48 67 93 111 147 165 179 195 216 224 240 262 285 360 387

399 415 425

442 467

xxx Contents

Volume I (cont’d.) Composites and Nanocomposites: Thermoplastic Polymers for Additive Manufacturing  /  Nahal Aliheidari and Amir Ameli . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Composites: Biobased for Materials Applications  /  Thimothy Molefi Thamae, Thabiso Letseka, and Leboli Thamae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Composites: Conductive Elastomer CNT-Based  /  Jiji Abraham, Mohammed Arif P., Nandakumar Kalarikkal, Sabu Thomas, and Soney C. George . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Composites: Environment-Friendly Nanocomposites  /  Sukanya Pradhan, Smita Mohanty, and Sanjay K. Nayak . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Composites: Natural Fibers Reinforced  /  M. K. Gupta . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Composites: Polylactic Acid-Based Blends  /  Payam Zarrintaj, Reza Khalili, Emad Jowshan, Zahed Ahmadi, Mohammad Reza Saeb, and Masoud Mozafari . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conducting Polymers: Applications  /  Anupreet Kaur . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conducting Polymers: Electrospun Materials  /  Thomas Kerr-Phillips and Jadranka Travas-Sejdic . . Construction: Rigid Bio-based Polyurethane Foams for Sandwich Panels  /  Rafael de Avila Delucis, Sandro Campos Amicos, Washington Luiz Esteves Magalhães, and Cesar Liberato Petzhold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Corrosion Inhibitor: Polymeric Desiccant  /  Madhumita Kulkarni and P.A. Mahanwar . . . . . . . . . Corrosion Protection: Natural Polymer in  /  I. B. Obot, Saviour A. Umoren, and Ubong M. Eduok . . . Cosmetics: Active Polymers  /  Mayuree Kanlayavattanakul and Nattaya Lourith . . . . . . . . . . . . . . . Cosmetics: Polymers in  /  M. V. Bhavya, D. V. Gowda, Sivadasu Praveen, Nithin Kundachira Subramani, and Siddaramaiah Hatna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cosmetics: Polymers in Delivery of Actives   /  Azila Abdul-Aziz, Rosnani Hasham, and Mariani Abdul Hamid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Display: Photorefractive Polymers in   /  Pierre-Alexandre Blanche . . . . . . . . . . . . . . . . . . . . . . . . . . Drug Delivery: Biodegradable Polymers in  /  Sauraj Singh, Ruchir Priyadarshi, Bijender Kumar, Saleheen Bano, Farha Deeba, Asif Ali, and Yuvraj Singh Negi . . . . . . . . . . . . . . . . . . . . . . . . . . . . Drug Delivery: Microencapsulation Techniques for  /  Mrunal R. Waghulde and Jitendra B. Naik . . . Drug Delivery: PLGA-PEG-Based Bioerodable Nanoparticles  /  Arindam Maity, Kazi Asraf Ali, Debanjana Chakraborty, and Sougata Jana . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Drug Delivery: Polymeric Conjugates for Dietary Phytochemicals  /  Divya Arora and Sundeep Jaglan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Drug Delivery: Polymeric Micelles  /  Dorota Neugebauer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Drug Delivery: Polymers and Polymeric Membranes  /  Shilpa Narang, Zeenat Arif, P.K. Mishra, S.N. Upadhyay, and Rajeev Mehta . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Drug Delivery: Polysaccharide-Based Composites  /  Sougata Jana and Subrata Jana . . . . . . . . . . Drug Delivery: Stimuli-Responsive Polymeric Prodrugs for Cancer  /  Sauraj, Ruchir Priyadarshi, Farha Deeba, Bijender Kumar, and Yuvraj Singh Negi . . . . . . . . . . . . . . . . . . . . . . . . Drug Delivery: Trends and Future Prospects  /  Ufana Riaz and Anurakshee Verma . . . . . . . . . . . .

486 501 515 530 543 571 591 602

624 639 651 705 722 743 757 770 783 803 817 829 855 870 883 899

Volume II Electronics: Nonvolatile Memory Technologies  /  Sreedevi Vallabhapurapu, Shengzhi Dub, and Ananthakrishnan Srinivasan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electronics: Polymer–Graphene Composites  /  Seema Ansari and M.N. Muralidharan . . . . . . . . . Energy: Conducting Polymers and Conjugated Porous Polymers  /  Narendra Pal Singh Chauhan, Nirmala Kumari Jangid, Navjeet Kaur, and Masoud Mozafari . . . . . . . . . . . . . . . . . . . . Energy: Polymer Electrolytes for Lithium Ion Batteries  /  Guang Yang, Qing Wang, and Youngmin Lee . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

913 932 961 972

Contents xxxi

Energy: Polymer Supercapacitors  /  Anish Benny and Soney C. George . . . . . . . . . . . . . . . . . . . . . . Energy: Polymer-Functionalized Graphene  /  Arun K. Nandi, Pousali Chal, and Arnab Shit . . . . . Energy: Polymers in Harvesting, Conversion, and Storage  /  Shahram Mehdipour-Ataei and Zahra Tabatabaei-Yazdi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Energy: Polymers in the Active Layer of Solar Cells  /  Bianca P. Santos and Maria de Fatima V. Marques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Environmental Applications: Biopolymer Sorbents for Heavy Metal Removal  /  Patrícia Concórdio-Reis and Filomena Freitas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Environmental Applications: Hydrogels  /  G. Roshan Deen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Environmental Applications: Polymers in  /  Chow Wen Shyang . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fire Protection: Flame-Retardant Additives and Fillers for Polymers  /  Hüsnügül Yilmaz Atay . . Fire Protection: Flame-Retardant Epoxy Resins in  /  Seema Agrawal and Anudeep Kumar Narula . . Fire Protection: Flame-Retardant Polymers in  /  Ayesha Kausar . . . . . . . . . . . . . . . . . . . . . . . . . . . Food Packaging: Cellulose Nanocrystals in  /  Prodyut Dhar, Chethana Mudenur, and Vimal Katiyar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Food Packaging: Edible Products  /  Rungsinee Sothornvit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Food Packaging: Natural and Synthetic Biopolymers  /  Ruchir Priyadarshi, Bijender Kumar, Farha Deeba, Yuvraj Singh Negi, and Anurag Kulshreshtha . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Food Packaging: Polyhydroxyalcanoates (PHA) Containing Antimicrobial Additives   /  C. R. Rech, K. C. S. Brabes, and S. M. Martelli . . . . . . . . . . . . . . . . . . . . . . . . . . . Food Packaging: Polymer Composites  /  Garima Agrawal and Rahul Agrawal . . . . . . . . . . . . . . . . Food Packaging: Polymers as Packaging Materials in Food Supply Chains  /  A. B. Hemavathi and H. Siddaramaiah . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Food Packaging: Starch and Non-Starch Blend Films  /  Hanisah Kamilah, Kaiser Mahmood, Poh Lee-Shang, Syazana Sulaiman, Abdorreza Mohammadi Nafchi, Ariffin Fazilah, and Alias A. Karim . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Food Packaging: Starch-Based Bionanocomposites  /  Kalyani Prusty and Sarat K. Swain . . . . . . . Food: Polymers in  /  Vincent G. Gomes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Footwear  /  Saraswathy Gnanasundaram and Mohan Ranganathan . . . . . . . . . . . . . . . . . . . . . . . . . . Fuel Cell: Cellulose-Based Polyelectrolyte Proton Exchange Membranes  /  Randa E. Khalifa, Tamer M. Tamer, Ahmed M. Omer, and Mohamed S. Mohy Eldin . . . . . . . . . . . . . . . . . . . . . . . . . . Fuel Cell: Polymeric Membrane  /  Saleheen Bano, Ruchir Priyadarshi, Sauraj, Bijender Kumar, and Yuvraj Singh Negi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Furniture: Eco-Friendly Polymer Composites Applications  /  Md. Shahruzzaman, Shanta Biswas, Md. Minhajul Islam, Md. Sazedul Islam, Md. Shirajur Rahman, Papia Haque, and Mohammed Mizanur Rahman . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Furniture: Polymers in  /  Gibin George, Liju Elias, and Zhiping Luo . . . . . . . . . . . . . . . . . . . . . . . . Healthcare: Polymer as Vital Materials  /  Abhilasha Mishra, Rajesh Saini, and Anil Kumar Bajpai . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Household Goods: Polymers in  /  Rupa Bhattacharyya and Sumit Nandi . . . . . . . . . . . . . . . . . . . . . Insulators: Polymers for High-Voltage Outdoor Use  /  Anshul Gautampurkar and Prakash Mahanwar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Laboratory Applications: Polymers in  /  Oznur Dogan Ulu and Ismail Ozdemir . . . . . . . . . . . . . . . Marine Applications  /  Asit Baran Samui . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Medicines: Polymers for  /  Narendra Pal Singh Chauhan, Nirmala Kumari Jangid, Navjeet Kaur, Bharatraj Singh Rathore, Mazaher Gholipourmalekabadi, and Masoud Mozafari . . . . . . . . . . . . . Membrane: Preparation by Nonsolvent-Induced Phase Inversion  /  Priscila Anadão and Hélio Wiebeck . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Membranes: Graft Modification of Polymers for  /  Romil Mehta, Paramita Manna, and A. Bhattacharya . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

990 1009 1035 1049 1066 1087 1106 1122 1139 1258 1273 1304 1325 1343 1359 1374

1398 1416 1432 1444 1470 1499

1517 1548 1561 1580 1596 1611 1629 1679 1692 1712

xxxii Contents

Volume II (cont’d.) Membranes: Hemodialysis Applications  /  S. Sentilkumar and Noel Jacob Kaleekkal . . . . . . . . . . . Microcapsules Polymeric: Self-Healing Smart Coatings  /  Mahendra Mahajan and Vikas V. Gite . . . Microencapsulation: Phase Change Material in Textile and Building Construction  /  Arindam Dey, Rekha Gorai, and Somenath Ganguly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Microneedles Arrays: Based on Natural Polymers  /  Diana Araújo and Filomena Freitas . . . . . . . Nanocellulose: Environmental and Engineering Applications of  /  Deepu A. Gopakumar, Avinash R. Pai, K.S. Sisanth, Sabu Thomas, K.T. Sabu, Abdul Khalil H.P.S., and Yves Grohens . . . . . . . . Nanocellulose: Health Care Applications  /  Blessy Joseph, Hanna J. Maria, Sabu Thomas, Nandakumar Kalarikkal, Deepu A. Gopakumar, Jemy James, Yves Grohens, and Abdul Khalil H.P.S. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanocomposites: Polyamide/Polyhedral Oligomeric Silsesquioxane  /  Ayesha Kausar, Nabila Badar, and Obaid-Ur-Rahman Abid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanocomposites: POSS-Based  /  Lakshmipriya Somasekharan, Nandakumar Kalarikkal, and Sabu Thomas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Orthopedic Implants: Applications of Bioabsorbable Polymers  /  Lígia Figueiredo, Nuno Guitian Oliveira, Frederico Castelo Ferreira, Luís F.V. Pinto, Nuno Ribeiro, and Alexandra Rodrigues . .

1729 1773 1789 1800 1813

1829 1853 1876 1886

Volume III Oxo-Biodegradable Polymers  /  Emo Chiellini, Stefania Cometa, and Andrea Corti . . . . . . . . . . . . Packaging, Active, and Intelligent: Polymer Applications  /  Panuwat Suppakul . . . . . . . . . . . . . . . Packaging: Food Waste Reduction  /  Sergio Torres-Giner, Luis Gil, Leonor Pascual-Ramírez, and José A. Garde-Belza . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Packaging: Polyhydroxyalkanoates (PHAs) in  /  Pradnya P. Kanekar, Snehal O. Kulkarni, and Sagar P. Kanekar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Packaging: Polymer–Metal-Based Micro- and Nanocomposites  /  Rajesh Kumar Sahoo . . . . . . . . Petroleum Production: Polymers in   /  Gbenga Folorunso Oluyemi . . . . . . . . . . . . . . . . . . . . . . . . . Poly(3-hydroxybutyrate): Applications  /  A. P. Bonartsev . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polymers and Polymeric Membranes  /  Zeenat Arif, P.K. Mishra, S.N. Upadhyay, and Rajeev Mehta . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polypyrrole: Properties and Application  /  Amir Reza Sadrolhosseini, Suraya Abdul Rashid, and Suhaidi Shafie . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polyurethanes: Biobased  /  Dheeraj Ahuja and Anupama Kaushik . . . . . . . . . . . . . . . . . . . . . . . . . . . Porous Polymers: Gas Separation and Storage  /  Fateme Rezaei, Hooman Hosseini, Anirudh Krishnamurthy, and Ali A. Rownaghi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Rapid Prototyping of Microfluidic Devices in Polymers  /  Zhibin Yan, Mingliang Jin, and Lingling Shui . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Rapid Prototyping of Polymeric Biomaterials  /  Swati Mishra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Recycling: Converting Waste Plastics to Polyolefin Wax  /  Apurv Gaidhani and Prakash Mahanwar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Recycling: Polyurethane Foam Wastes  /  Mir Mohammad Alavi Nikje and Baharak Pooladian . . . Regenerative Medicine: Natural Polymers  /  Suraj K. Nayak, Seemadri Subhadarshini, Vinay K. Singh, Kunal Pal, Shresthaa Patel, Suprio R. Paul, Barbiee Choudhary, Anshuman K. Sahu, and Arfat Anis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Resorbable Embolics  /  Lihui Weng and Randy Donelson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Semiconducting Polymers in Photovoltaics  /  Veeman Sannasi, Mohan Ramesh, and Duraisamy Jeyakumar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1907 1958 1990 2010 2021 2041 2061 2077 2092 2104 2120 2146 2163 2179 2195

2214 2238 2254

Contents xxxiii

Sensors: Advanced Aptasensors Design  /  Rinkesh Bhatt, Laxmi P. Bagri, Rajesh Saini, and Anil Kumar Bajpai . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sensors: Ion-Sensing Polymers  /  Tsuyoshi Michinobu and Yang Wang . . . . . . . . . . . . . . . . . . . . . . . Sensors: Natural Polymeric Composites  /  Ahmet Ulu and Burhan Ates . . . . . . . . . . . . . . . . . . . . . . Sensors: Polymers in Sensing  /  Balaprasad Ankamwar and Saee Gharpure . . . . . . . . . . . . . . . . . . Sensors: Zeolite–Polymer Composites for Gas Sensing  /  Muzzaffar Ahmad Mir, Muzzaffar Ahmad Bhat, Shiekh Abdul Majid, and Radha Tomar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Separation: Poly(N-isopropylacrylamide) in  /  Lei Yang, Jing Zhang, and Xiaoguang Fan . . . . . . . Separation: Polymers in  /  Shahram Mehdipour-Ataei and Samal Babanzadeh . . . . . . . . . . . . . . . . . Smart Polymers: Lightweight Composites and Foams Tailored with Magnetic Field  /  L. Sorrentino, M. D’Auria, and D. Davino . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Smart Polymers: Molecularly Imprinted Polymers  /  Mónica Díaz-Bao, Rocío Barreiro, Alberto Cepeda, and Patricia Regal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Spin Coated Films of Polymeric Materials  /  Ayesha Kausar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Starch: Renewable Source for Thermoplastic  /  Ridhwan Jumaidina, Salit M. Sapuana, Mohammad Jawaid, Mohammad R. Ishak, and Japar Saharie . . . . . . . . . . . . . . . . . . . . . . . . . . . . Textile: Fiber-Forming Polymers  /  Mohammad Mahbubul Hassan . . . . . . . . . . . . . . . . . . . . . . . . . Textile: Flame Retardancy Through Surface Engineering  /  Giulio Malucelli . . . . . . . . . . . . . . . . Textile: Polymer-Based Materials  /  Ayesha Kausar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Textile: Stimuli-Responsive Polymers in  /  Desislava Staneva and Ivo Grabchev . . . . . . . . . . . . . . . Textile: Substrates Modification by Novel Polymers  /  Majid Montazer and Tina Harifi . . . . . . . . . Textiles: Novel Polymers  /  Bipin Kumar and Ning Pan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Textiles: Polymers and Fibers  /  Xuqing Liu, Xinyi Guan, and Hongjun Yu . . . . . . . . . . . . . . . . . . . . Thermogelling Polymers: Biomedical Applications  /  Gurpreet Kaur, Divya Thakur, and Pallavi Bassi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tissue Engineering: Polyhydroxyalkanoate-Based Materials and Composites  /  L. R. LizarragaValderrama, C. Thomas, J. I. Cadiz-Miranda, and I. Roy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tissue Engineering: Polymeric Dermal Filler  /  Nattaya Lourith and Mayuree Kanlayavattanakul . . . Tissue Engineering: Polymeric Scaffolds for MSC-Based Cartilage  /  Samaneh Hosseini, Mohamadreza Baghaban Eslaminejad, Fatemeh Bagheri, and Mohammad Amin Shamekhi . . . . . Tissue Engineering: Polymers in Soft Tissue Cartilage  /  Sunaina Indermun, Mershen Govender, Pradeep Kumar, Yahya E. Choonara, and Viness Pillay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tissue Engineering: Thermoplastics for Scaffold  /  Doddipatla Purnima, Sridhar Aparna, Namrata Verma, Karan Khubdikar, Anurag Patil, and Anusha Iyangar . . . . . . . . . . . . . . . . . . . . . Value-Added Products from Natural Fibers Reinforced Composites  /  Yasir Beeran Pottathara, Sabu Thomas, Deepu Ambika Gopakumar, and Yves Grohens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Veterinary Medicine: Polymers in  /  Olufunke Nwosu, Blessing Aderibigbe, Inge-Marie Petzer, and Rebone Moerane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Water Treatment: Metal and Metal Oxide Asymmetric Poly(Ether)Sulfone Nanocomposite Membranes  /  Behnam Gohari, Benjamin Church, Nidal Abu-Zahra, and Kyle Russell . . . . . . Water Treatment: Polymers for Coagulation and Flocculation  /  A.Y. Zahrim, Z.D. Dexter, and I. Azreen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Wholly Aromatic Polyamide–hydrazides  /  Nadia A. Mohamed . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2271 2291 2306 2328 2345 2367 2380 2398 2426 2441 2461 2490 2507 2522 2545 2562 2580 2592 2627 2652 2676 2683 2704 2719 2767 2780 2795 2810 2825

Preface

Undoubtedly, the applications of polymers are rapidly evolving. Technology is continually changing and quickly advancing as polymers are needed to solve a variety of day-to-day challenges leading to improvements in the quality of life. As a result, there is a clear need for a source of technical information that has broad coverage, is written at a level comprehensible to nonexperts, and has the ability to be updated on a regular basis. With these considerations in mind, the decision to publish the Encyclopedia of Polymer Applications (EPA) was apparent. My hope was to present the material in such a manner that it conveys important overviews to help stimulate further advancements in all areas of polymers. Since the field is advancing at a rapid pace, the encyclopedia will be published online (in addition to this print edition) and will be updated in a timely manner (as new research unfolds). This will continue to provide the researchers with the latest advances and keep the encyclopedia at the forefront of this important field well into the future. The EPA is a dynamic and somewhat comprehensive multivolume reference on the broad subject area of polymer applications, which will enable readers to have targeted knowledge in this evolving arena. I have had the privilege of working with experts worldwide in preparing the EPA with its vast entries in this first edition spanning almost the entire field of polymer applications. This groundbreaking work includes multiple articles on many subjects and offers a broad-based perspective on a plethora of topics in a variety of applications. The encyclopedia caters to engineers, scientists (polymer scientists, materials scientists, biomedical engineers, macromolecular chemists), researchers, and students, as well as general readers in academia, industry, research institutions, etc. I hope that the EPA will serve as an important and continual resource for polymer innovators whose work will make a difference in improving the quality of life. I feel honored to undertake this important and challenging endeavor of developing the EPA that will cater to the needs of many who are working in the field. I express my sincere gratitude and appreciation to the authors for their excellent professionalism and dedicated work. Needless to say, an encyclopedia of this nature would never exist if the expert authors had not devoted their valuable time for preparing the authoritative entries. I thank the entire management of encyclopedia program of Taylor & Francis Group (T&F–CRC Press) and particularly to Ms. Barbara (Glunn) Knott who made this possible. I gratefully acknowledge Ms. Stephanie DeRosa, T&F Encyclopedia Program, for her invaluable help during the development of this encyclopedia. Her hard work and professionalism made this a wonderful experience for all parties involved. I take the opportunity to express my appreciation to my family, and particularly, I acknowledge my wife Bidu Mishra for her encouragement, sacrifice, and support, during weekends, early mornings, and holidays spent on this encyclopedia. Without their help and support, this project would have never been started nor completed. Munmaya K. Mishra, Editor-in-Chief

xxxv

About the Editor

Munmaya Mishra, PhD, is a polymer scientist who has worked in the industry for more than 30 years. He has been engaged in research, management, technology innovations, and product development. He currently holds research and technical management responsibility at Altria Research Center and serves as the editor-in-chief of three renowned polymer journals of Taylor & Francis. He has contributed immensely to multiple aspects of polymer applications, including encapsulation and controlled release technologies. He has authored and coauthored hundreds of scientific articles, is the author or editor of eight books, and holds more than 60 U.S. patents, more than 50 U.S. patent-pending applications, and more than 100 world patents. He has received numerous recognitions and awards. He is the editor-in-chief of the recently published 11-volume Encyclopedia of Biomedical Polymers and Polymeric Biomaterials. He founded the International Society of Biomedical Polymers and Polymeric Biomaterials as well as a scientific meeting titled “Advanced Polymers via Macromolecular Engineering,” which has gained international recognition and is under the sponsorship of the International Union of Pure and Applied Chemistry.

xxxvii

Drug Delivery (cont.)–Trends

Display–Drug Delivery

Construction– Cosmetics

Composites– Conducting

Biosensing– Communication

Biomaterials– Biomedicine

Alginate–Aviation

Encyclopedia of Polymer Applications Actuators– Agriculture

Actuators– Agriculture

Actuators: Buckypaper Composite Joshua DeGraff and Richard Liang Florida State University, Tallahassee, Florida, U.S.A.

Pierre-Jean Cottinet National Institute of Applied Sciences of Lyon, Lyon, France

Abstract Recent advances in flexible electronics, micro-robotics, and wearable technology create new demands to integrate lightweight, open-air actuators powered by polymer electrolyte membrane (PEM) thin films. PEMs are electroactive and can generate large deformations in response to small voltages (≤4 V). Their lightweight and flexibility are comparable to biological muscles, which are attractive for flexible devices that perform delicate tasks like surgery and specimen collection. Conventional PEM actuators usually utilize platinum (Pt) electrodes that perform well. However, commercialization is limited by high costs (about $100,000/kg) and complex, chemical deposition manufacturing methods. We have successfully proven carbon nanotube thin film or buckypaper (BP) to be a promising flexible electrode material. As BP’s sole constituent, carbon nanotubes offer an extraordinary blend of mechanical and electrical properties. BP’s high surface area, flexibility, low profile, and lightweight make it a viable replacement to metallic electrode films. This research will introduce a scalable, laminate manufacturing process for BP composite actuators. This entry will review the BP composite actuator’s working mechanisms; establish property–performance ­relationships; and illustrate its potential as a soft, bioinspired device for a wide range of applications. Keywords: Bio-mimetics; Buckypaper; Carbon nanotubes; Electroactive polymers; Flexible technology; Ionic liquid; Low voltage actuation; Nafion.

INTRODUCTION As the size of devices and mechanical systems continues to shrink in medical, aerospace, and micro-­robotic applications, smart actuation materials continue to attract researchers. [1–4] The ability of a single, soft, and lifelike material to move and remember its movements also gives way to new applications in flexible and wearable electronics. [5] Micro-sized robots must possess the mobility and dexterity to adapt to unpredictable situations in surveillance and disaster relief. Biological organisms are brilliant at this. Advanced research studies using high-speed cameras reveal that organisms adapt to their environment by morphing their body’s geometry. [6,7] By morphing their bodies and/or extremities, many small organisms can maintain their agility in air, water, and various terrains. A bird can vary its velocity to sustain flight in unsteady aerodynamic conditions, an insect can overcome its weight to hover through windy environments, and a jellyfish can easily propel itself through the seas’ currents. [6–9] These organisms provide a status quo for researchers to model and mimic biological movements for the bioinspired actuation needed in small electromechanical systems that require both small size and flexibility. Encyclopedia of Polymer Applications, First Edition DOI: 10.1201/9781351019422-140000035 Copyright © 2018 by Taylor & Francis. All rights reserved.

Smart polymeric materials can generate lifelike movements, and, depending on the material, a variety of external stimuli, including chemical, optical, and thermal energies, can invoke actuation.[10] Electroactive polymer (EAP) actuators are the most attractive actuation materials due to their controllability, low power consumption, and aptitude for sensing and adaptability.[11] Ionic polymer–metal composite (IPMC) actuators are laminar EAP composites that have especially performed well as bioinspired actuators. They possess attractive qualities such as softness, silent operation, a fairly simple structure that allows easy miniaturization, controllability, and the ability to function in liquid environments.[12] An IPMC is essentially a bimorph structure consisting of a polymer electrolyte membrane (PEM) sandwiched between two chemically deposited electrodes. Figure 1 is a basic schematic of an IPMC. IPMC manufacturing typically involves repetitive cycles of adsorption and reduction of platinum particulates into and onto a Nafion PEM.[11–13] Later, a proper reducing agent such as LiBH4 or NaBH4 is introduced to metalize the polymer. The process requires a suitable metallic salt ­complex, such as Pt(NH3) 4HCl, that can easily be reduced. The principles of the electrode composition process involve the ion exchange of platinum cations into the PEM inner surface prior to reducing with a chemical reduction agent

1

2

Actuators: Buckypaper Composite

Actuators– Agriculture

Mesoporous platinum electrode Electrode/electrolyte interface Polymer electrolyte membrane

Fig. 1  Illustration of the ionic polymer–metal composite actuator structure

Fig. 2  SEM cross-sectional image of an IPMC

such as LiBH4 or NaBH4. The method results in the metalizing of Nafion’s inner surface and requires three to five cycles of adsorption and reduction to establish a complete particulate coating that consolidates deep into the polymer. As seen in the scanning electron microscopy (SEM) images in Fig. 2, the particulates penetrate Nafion’s surface more than 20-μm deep.[14] The electrode/electrolyte interface appears seamless; however, the manufacturing process is expensive (about $100,000/kg) and complex. The process is also rather wasteful, as IPMCs suffer from poor particle utilization (~20%). Aside from these limitations, they have performed exceptionally well and have made the way for PEM-driven, artificial muscle technology. IPMCs are fracture tolerant and can generate relatively large mechanical strains. They require low amounts of power and operating voltage (2%/s). SWCNTs ­obviously exhibit great performance in open-air actuation, but their high cost limits commercialization. However, given their single-layer structure, chemical modifications also often diminish their electrical properties.[21] Thus, MWCNTs have recently attracted more attention from researchers. Their structure is similar to SWCNTs; however, their having multiple walls allows them to maintain high conductivity even after chemical modifications. More importantly, their cost is between 0.01% and 10% less than the cost of SWCNTs.[21] Multiwalled carbon nanotube buckypaper (MWCNT-BP) is a promising, freestanding electrode film given its porosity, flexibility, electrical conductivity (204 S/cm), thickness (t < 10 μm), and lightweight (5 g/m2).[1,4,26] Its high surface area (390 m2 /g) promotes a seamless electrode/electrolyte interface. Figure 4 displays an image of MWCNT-BP in addition to SEM images of the film’s microstructure. The buckypaper composite actuator (BCA) presented here is a thin, nanocomposite film with a laminate structure of MWCNT-BP/Nafion/MWCNT-BP. A hot-pressing procedure is used to consolidate the materials into a single film. BP’s piezoresistivity also gives the BCA the ability

to sense its actuation strain.[27] This is interesting because it incites the establishment of ­closed-loop actuation on a single device. NAFION’S STRUCTURAL MORPHOLOGY AND ELECTROACTIVITY Nafion’s morphology and property have been studied extensively for several decades.[28] There have historically been debates about the structural details, but experimental results conclude that Nafion contains three distinct phases.[28–33] Figure 5 illustrates the amphiphilic polymer structure along with notable mechanisms. The first of the three phases include hydrophobic backbones made of highly ordered polytetrafluoroethylene. These Teflon-like backbones influence good mechanical properties, high processing temperature (Tm ≈ 230°C), and chemical resistance. The second phase is an amorphous fluorocarbon region. The third phase includes perfluorinated vinyl PTFE (Teflon-like) backbones: Mechanical properties High processing temperature Hydrophobicity

[(CFCF2)a(CF2CF2)b] (OCF2CFO)x(CF2)ySO3–H+ CF3

Transport cation: Mechanism behind Nafion performance

n

Sulfonate anion: Hydrophillicity (attracts water and H+ ions)

Fig. 5  Nafion’s polymer chain structure and the roles of its major constituents

4

Actuators: Buckypaper Composite

Actuators– Agriculture

ester side chains, which are capped with sulfonate (SO3−) exchange sites. The exchange sites give Nafion the unique ability to absorb water, ILs, saturated vapor, and mobile cations.[1,2,34–41] Nafion intrinsically contains hydrogen (H+) ions bonded to the exchange sites. Illustration of Ionic Clusters Through small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction methods, Gierke et al. established a fundamental model to explain Nafion’s morphology. The model is illustrated in Figs. 6 and 7, which presents the morphology under no electric potential as a cluster network of SO3− exchange sites that are interconnected by tiny micro-channels.[29] Gierke’s SAXS results reveal that fully hydrated Nafion membranes possess SO3− clusters with an average diameter of 4 nm long with 1 nm-wide micro-channels. When a dry membrane absorbs water via vapor or bulk water, the clusters coalesce together forming larger clusters saturated with water and cations. This evenly swells the polymer. When a PEM actuator is charged, the ions and water molecules diffuse towards the cathode. The clusters towards the anode coalesce and absorb water, while the cathode’s clusters shrivel holding much less water. This cluster coalescence is depicted in Fig. 6. Recent progress in molecular dynamics has provide accurate illustrations and mathematical models that explain the water gradient within Nafion.[42–46] Orfino et al. concludes that measuring both the [H2O]/[SO3−] ratio and the concentration of cations [H+] provides an accurate indication of ionic conductivity.[43,47] Figure 7 presents the

overall cluster network of a BP composite actuator subject to zero potential. The hydration levels are at equilibrium, and the actuator is at rest. SEM and transmission electron microscopy (TEM) studies have further proven the size of the anionic clusters. Ceynowa et al. distinguished the SO3− clusters via SEM by doping Nafion with lead. The lead provides ­electron-density contrast in the microscopy images.[32] Xue et al. stained Nafion’s side chains with ruthenium tetroxide (RuO4) for contrast in TEM images.[31] Both studies also confirm spherical clusters with diameters that range between 2.5 and 5 nm.[31,32] The methods reveal different sizes for the anionic clusters; however, each study does reveal the same coalescence behavior. It is also clear that water is a key component to Nafion’s performance in fuel cells, actuators, and sensor applications.[13,43,45,46,48–51] Nafion’s Actuation Mechanism Nafion produces low power, zero emission, and compliant actuation via an electrochemical mechanism like biological muscles.[1,2,34–41] Under a small electric field, the ions and water molecules diffuse towards the negatively charged electrode. The diffusion creates a gradient of elastic pressure. The water-saturated regions of the membrane inflate, while the unsaturated regions deflate; this causes Nafion to deform. Park et al. performed microscopy experiments using Nafion doped with ethidium bromide (EmBr).[52] The Em+ mobile cation fluoresces under ultraviolet light; thus, cation diffusion can be visually mapped

Cluster coalescence from increased hydration

Increased hydration from cation diffusion

Dehydrated clusters shrink

Sulfonated anion

Hydrated cation

Fig. 6  Illustration of cluster coalescence. The sulfonated anions cluster together when they are hydrated. During actuation (constant ion diffusion), localized clusters grow and shrink in relation to their respective hydration levels

Actuators– Agriculture

Actuators: Buckypaper Composite 5

Water cluster Sulfonated anion

Hydrated cation

Buckypaper

Fig. 7  Illustration of T.D. Gierke’s cluster-network model. Hydrated cations can diffuse from electrode to electrode through the clusters and micro-channels

with a high-speed camera. The microscopy images reveal a gradient of fluorescent light that strongly correlates with the PEM-­generated displacement. The images also clearly show water diffusion towards the region of high light intensity. Recent research has focused on the need for modeling Nafion’s dynamic nature with respect to hydration levels. The membrane’s water content strongly influences its mechanical and electrical properties. Since environmental conditions are difficult to control, Nafion’s properties are often predicted via molecular dynamic simulations.[45,53–56]

30% decrease in strain generation, whereas water-swollen actuators stop working after 3000 cycles in the same conditions.[64] ILs are stable ionic compounds that do not require dissolution. They also encompass large cations that improve actuation response via ion exchange. ILs can withstand higher voltages (>4 V) before molecular breakdowns than water, and their low vapor pressure prevents evaporation in open air making them viable candidates to replace water as Nafion’s diluent.[64] Replacing Water with IL

Water’s Role in Actuation Hydration has a significant effect on Nafion’s physical structure, mechanical properties, and electroactivity.[43,49,54,57–59] Even small changes in hydration have drastic effects on ionic conductivity.[59,60] Water molecules dissociate the ­cations from the SO3− exchange sites, and if water content is low, the actuator will require high activation energies. Water is essentially served as a lubricant. Must et al. presented experimental results on the effects of humidity on a IL-swollen IPMCs.[15] As relative humidity approaches 20%, electrical conductivity increases by more than 50%. Completely hydrating the membrane increases conductivity by more than 90%.[15] Water is abundant, inexpensive, and nontoxic; however, it has become a key obstacle in manufacturing open-air actuators because of its instability. Water undergoes hydrolysis at low voltages (>1.23 V), and its volatility makes it susceptible to evaporation even in open air.[43,61,62] Bar-Cohen et al. fabricated IPMCs with a silicon-based coating that was able to prolong the actuators’ lifetime; however, the barrier coating added stiffness to the membrane reducing strain generation.[61,63] A few years later, Bennett et al. demonstrated that IL-swollen actuators can operate in open air for more than 4 months with a

IL’s thermal and electrochemical stabilities have eliminated the need for barrier coatings and climate-controlled environments in PEM actuation.[43,49,57–59] The success of ILs as diluents and cation donors over the last decade has stimulated researchers to explore the mechanisms of IL-swollen actuators. Ion exchange is the process of swapping the H+ cations within Nafion with large cations from an ionic salt solution. Figure 8 summarizes popular dopant cations in terms of their respective ionic radii and critical uptakes. The critical uptake of each cation denotes the amount of IL required to dissociate all the exchanged cations from the sulfonate exchange sites. Larger cations can dissociate themselves from Nafion’s fixed anions easier than hydrogen. According to Coulomb’s law of electrostatic energy, the binding energy between the charges diminishes as the cation’s ionic radius increases. Fourier transform infrared spectroscopy (FTIR) provides a means to investigate the binding associations among the ions in a Nafion membrane.[65] Bennett et al. measured the mobility of various ions subject to increasing IL uptake by using Lowry et al.’s FTIR studies on water-swollen Nafion membranes.[65,66] Their experimental results support the concept that larger cations experience low electrostatic binding energy making them easier to mobilize.

6

Actuators: Buckypaper Composite

Critical uptake (mol IL/mol SO 3–)

Actuators– Agriculture

0.75

Na+ Li+

0.65

K+

0.55

EMI+

Cs+

0.45

TEA+

0.35 0.25

0

0.5

1

1.5 2 Atomic radius (nm)

2.5

3

3.5

Fig. 8  Graph summarizes popular dopant ions and their respective critical uptakes

CARBONACEOUS ELECTRODES

Vertically Aligned SWCNTs Composite Electrode

Bucky Gel Electrode: SWCNTs and IL

Liu et al. exploited SWCNTs anisotropic nature by fabricating vertically aligned SWCNT (Va-SWCNT) composite electrodes.[67] In this work, Va-SWCNTs were prepared via a modified CVD method. A Nafion suspension is infiltrated into the densified array of Va-CNTs by soaking the BP in a Nafion/dimethylformamide (DMF) solution and slowly evaporating the DMF solvent, which has a high boiling temperature (>150°C) and evaporates very slowly. After 1 week, the final composite electrode was obtained, embedded in epoxy, and measured to be 12 μm thick.[67] After ultrasonic spraying of a thin Nafion layer onto an IL-swollen Nafion PEM, the Va-SWCNT composite electrodes were pressed onto the IL-swollen PEM without heat. The final actuator structure was 49 μm thick and exhibited high strains (>8% strain under 4 V) with a considerably strain rate (>10%/s). The group concludes that continuously aligned, highly packed SWCNTs create a unique morphology that enhances electroactive performance in three distinct ways: by creating direct and continuous pathways for fast ion diffusion, ­establishing continuous CNT percolation pathways for ­electrical conduction, and tailoring Young’s modulus to enhance actuation strain.[67] Figure 9 illustrates the performance enhancements of vertically aligned SWCNT composite electrodes compared to randomly oriented SWCNT composite electrode. Randomly aligned CNTs present tortuous pathways that obstruct ion diffusion, whereas Va-SWCNTs promote fast ion diffusion through the composite electrode with diffusion paths. SWCNTs exhibit great performance in open air, but their high cost limits their application. The actuator generated large strains; however, its 49 μm thickness limits the ability to exert sufficient blocking force (2%/s).[24] Asaka attributes these results to more accumulated charges in the electrode layer during actuation due to the strong interaction between the cations and SWCNTs.[24]

Cations diffuse more freely though vertically-aligned carbon nanotube networks

Fig. 9  Difference in morphology in SWCNT composites comprised of randomly oriented CNTs and vertically aligned CNTs. ­Vertically aligned CNTs have much less tortuous pathways for ion diffusion

MWCNT-BP Electrode Liang et al. developed an open-air actuator using an IL-swollen Nafion PEM and polymer-free, MWCNT-BP electrodes.[1,2,4,26] The BP fabrication procedure is documented in detail by Smalley[68] and Wang;[69] however, Liang et al. modified the process by adding surfactant (Triton-X) to the aqueous CNT suspension to improve CNT dispersion. The BP films are fabricated by filtering the CNT suspension through a 0.45 μm mesh onto a polycarbonate substrate.[5] Figure 10 displays this process flow. The films are repeatedly washed with distilled water and annealed at 850° under argon gas for 4 h to remove ­impurities and residual surfactant. The produced MWCNT-BP films are 10–20 μm thin, possess an areal density of 10 g/m 2, and exhibit a modulus and electrical conductivity of 3.7 GPa and 205 S/m, respectively. The films were then coated with nine drops of Nafion solution and hot pressed to a commercial Nafion membrane following ion exchange. The MWCNT-BP actuator is promising because it performs well for more than 20 h; it can generate up to 15 mm of displacement under low electric fields (700%). However, obtaining graphene’s theoretical potential has been difficult, as graphene platelets tend to restack. The restacking of graphene platelets greatly decreases the ­surface area, and, thus, surface ­modifications is important. Chen et al. developed hybrid electrodes by combining reduced graphene oxides (RGOs) with MWCNTs to prevent restacking and improve electrical conductivity (from 45 to 135 S/cm).[20] Although the highly porous structure of RGO-based electrodes promotes high capacitance, their low conductivity has been a limitation. As reported by Oh et al., the defect sites on RGO greatly limit their conductivity.[71] This could be compensated by anchoring silver (Ag) metal particles to these defect sites. The graphene/Ag composite electrode exhibited a high electrical conductivity of 900 S/cm, which is higher than both the RGO and RGO/MWCNT electrodes.[21,71] It is also interesting to note that the RGO/Ag hybrid actuators exhibited better stability than pure Ag-based actuators, because the wrapping of the metallic nanoparticles with RGO prevents them from being corroded. Recently, Choi et al. functionalized RGOs with various loadings (0, 2, 5, 10, 15, 30, and 70 wt %) of Nafion and revealed that a 5 wt% Nafion loading promotes both good mechanical and electrical properties. [27] High loadings of Nafion insulated the RGO film; conductivity decreased from 1200 S/cm to less than 600 S/cm. Though the actuation studies were not conducted, the functionalized RGO/Nafion composite exhibited an improved capacitance of 119 from 62.3 F/g. Its excellent ability to charge is attractive; however, the lack of actuation tests is likely a result of the durability issues revealed in previous research. RGO/Nafion composites have great potential in flexible energy storage, but cyclic actuation reveals the durability issues. [39] Chen et al. recently reviewed CNT and graphene-based actuator technology.[21] CHARACTERIZING THE BP COMPOSITE ACTUATOR Dynamic Mechanical Analysis The dynamic mechanical analysis (DMA) is an important characterization method in understanding the actuator’s material response to stresses under various temperatures. Young’s modulus can be tested using the tensile test configuration. For the BP composite actuator, tensile tests are performed in ambient conditions subject to a controlled force ramp of 3 N/min. Young’s modulus influences the actuator’s flexibility.

Actuators: Buckypaper Composite

The BCA’s viscoelastic properties can also be measured by using the same tensile test configuration; however, the samples are subject to a constant and cyclic strain of 1% at a frequency of 1 Hz. The furnace is heated at a ramp rate of 5°C. The TA Instruments Universal Analysis Package plots the storage and loss moduli and determines the tangent loss (tanδ) used to analyze the actuator’s mechanical response to cyclic actuation. Viscoelasticity is an ­important property in processing Nafion. Electrochemical Impedance Spectroscopy Electrochemical impedance spectroscopy (EIS) experiments on the Versastat III potentiostat can produce cyclic voltammetry (CV) and Nyquist plots to analyze the actuator’s ionic properties. CV experiments plot the induced current versus the applied voltage, I(V), at a designated scan rate (mV/s) and voltage range. Since the BP composite actuator is essentially two conductive BP layers separated by a dielectric Nafion electrolyte, the induced current can be expressed as a parallel-plate capacitor IC (V ) = C

dV dt

(1)

where I(V) denotes the capacitive current, C denotes the BCA’s capacitance, and dV/dt denotes the derivative of voltage with respect to time. Current literature lacks a standard method for capacitance calculations, so a method is presented here. Figure 11 presents an example of a CV graph with four plots. The first plot, denoted by the angled oval, is the original CV curve, I(V), which encompasses both the resistive and capacitive currents. To determine capacitive current, I(V), the resistive current must be subtracted. By fitting the curve with a linear regression model, denoted in Eq. 2 by f(V) and plotted in Fig. 11 by the straight line, one can subtract the resistive current. Figure 11 presents IC (V) as a horizontal oval: IC (V ) = IC + R (V ) − f (V ) (2) With the capacitive current and derivative of voltage known, one can now solve for capacitance, C, in Eq. 1. The specific capacitance of the BCA in Fig. 11 is 10.6 F/g. The Nyquist plot, as shown in Fig. 12, can reveal the actuator’s impedance behavior in a range of frequencies (ω) by plotting the imaginary component (Z″) versus the real component (Z′) of the impedance vector (Z). [72] This is a well-established method by Cole and Cole that measures the electrical losses due to the membrane’s resistance. [73] The Nyquist plot for a “lossy” dielectric like Nafion is typically in the form of a semicircle, which intercepts the axis of the real values at two critical points. These denote the actuator’s impedance behavior at two

Actuators: Buckypaper Composite 9

Actuators– Agriculture

Graph of cyclic voltammetry at 120mV/s 3

Current (mA)

2 1 0 –1 –2 Current (resistive + capacitive) Linear fitting (loss) Capacitive current

–3

Capacitance fitting

–0.5

–0.4

–0.3

–0.2

–0.1

0

0.1

0.2

0.3

0.4

0.5

Voltage (V)

Fig. 11  Plot of CV of a BCA

Impedance (Imaginary(Ω))

70 y = 9.9x – 231 R2 = 0.994

60 50 40 30 20

Equivalent series resistance = 23.4 Ω

10 0

0

5

10

15

20

25

30

35

Impedance (real (Ω))

Fig. 12  Nyquist plot for a BCA

Property–Performance Modeling of Actuation

respective frequencies, one very high and one very low. The high-frequency response determines the actuator’s ESR (R), which can be also used to measure ionic conductivity (σ). [73] This follows the basic nature of an RC circuit, which will be discussed in the Nafion Adhesive Casting section. The following equation expresses ionic conductivity:

σ=

L Rwt

by a fixture fitted with copper foil electrodes. The clamp is mounted onto a breadboard (ThorLabs) within a Plexiglas enclosure to prevent air flow disturbance. The breadboard is  mounted onto an IsoPlate Passive Isolation System to dampen unwanted vibrations. Both direct current and AC potentials are generated between the electrodes via a function generator (Agilent 2310A). The actuator’s deflection in response to the potentials is measured by a MicroTrak II laser displacement sensor. The sensor is interfaced with a computer making the performance analysis over long durations easy. Figure 13 provides a high-level schematic of the actuation test.

(3)

where L, w, and t denote the actuator’s length, width, and thickness, respectively. The BCA exhibits an ionic ­conductivity of 0.22 S/m. Actuation Tests BP composite actuators are tested using a cantilevered configuration, in which one end of the actuator is clamped

Strain and Force Generation Figure 14 illustrates both free displacement and blocking force with two schematics. Figure 14a shows the free displacement of an actuator of length L. If a rigid structure impedes actuation as in Fig. 14b, the actuator will exert what is known as blocking force on the structure. Alici’s tri-layer model, which was modified by Asaka to model carbon-based electrodes, is based on four main assumptions. [18] First, the Young’s moduli and Poisson’s ratio of both layers remain unchanged throughout the deformation. Second, the electrode’s deformation in response to voltage is negligible with respect to Nafion’s thickness. Third, the rate of ions entering and leaving the electrode layers during charging/discharging processes is ­constant along the actuator length. Lastly, strain is symmetric about the neutral axis throughout the entire actuator, and force ­generation is a direct result of the straining actuator. [18]

10

Actuators: Buckypaper Composite

Actuators– Agriculture

Function generator

Computer

Buckypaper composite actuator

MTI instrument

Laser displacement sensor

Fig. 13  Illustration of stain generation measurement

(a) y(x)

(b) P

Rigid load-cell Counter-force P direction

yf

y(x)

– 3V

F

yi

P

+ BCA force direction

L Top view

Fig. 14  Illustrations of (a) free displacement generation and (b) blocking force

The measured displacement can be transformed into the curvature (1/R) as follows: 1 2δ = R L2 + δ 2

(4)

where δ and L denote displacement and free length, respectively. The radius of curvature can be used to calculate strain (ε) as follows:

Both the Young’s modulus and moment of inertia of the electrodes and electrolyte contribute to the global rigidity. More specifically, the moment of inertia for BP and Nafion can be expressed as follows: 3

IBP

3

 t  t   2 w   Nafion  + tBP  2 w  Nafion  2 2      = − 3 3

(7)

3

ε=

EI

(

REBP wtBP tBP + tNafion

)



(5) INafion

where EI actuator rigidity, EBP is BP Young’s modulus, w is actuator width, tBP is BP thickness, and tNafion is Nafion thickness, respectively. As a bimorph structure, rigidity (EI) can be expressed as follows: EI = EBP IBP + ENafion INafion

(6)

t  2 w  Nafion   2  = 3

(8)

Once strain is determined via Eq. 4, blocking force can be found via Eq. 8, which expresses blocking force as having a linear relation with strain. It is also important to note that thickness has a significant influence on the moment of inertia, which in turn influences rigidity.[74,75] This has negative effects on strain and positive effects on

blocking force; further research must optimize rigidity via ­dimensional control: F=

(

EBP wε tBP tBP + tNafion L

)

(9)

Frequency-Dependent Equivalent Circuit PEM actuators generally have a strong frequency dependence. As the input signal’s frequency increases, the PEM actuators’ strain rate increases; however, there is a drastic reduction in strain generation. This may be due to the reduction of the allotted time for the cations to diffuse back and forth through the polymer. In turn, researchers have modeled nanocomposite actuators’ electrochemical behavior via EIS. EIS can be used to build equivalent circuit models (ECMs) that quantitatively describe the frequency-dependent behavior and provide analysis about the porous electrode and its capacitive properties.[76–80] Early ECMs were quite complex, as researchers considered factors such as electrode’s pore shape, chemical redox reactions at the pore wall, pore size distribution, and frequency dispersion of the electrical conductivity.[21] After Baughman et al. developed an ECM of their liquid electrolyte-based actuator, Asaka et al. further developed a vital and accurate frequency-dependent model for open-air carbon electrode/PEM actuators containing IL.[19,81] The group attributes the model’s accuracy to the separation of the porous electrode into three impedance components, as shown in Fig. 15: the distributed impedance of the electrolyte in the pore1, χ1; that of the electrode2, χ2; and that across the pore wall3, χ3. Figure 15a illustrates the model’s components, and Fig. 15b provides the model’s equivalent circuit diagram. Asaka further simplified the model for specific capacitance calculation resulting in the diagram in Fig. 15c. Here, the double-layer capacitance (C) lies in series between ionic resistance from the PEM (R1) and the electrode’s resistance (Rel). Specific capacitance (F/g) is subject to the mass of the electrode. (a)

(b)

χi

χi

χi

χi χiii χii

χiii

χiii

χiii

χii

χii

(c) R

C

To achieve high efficiency, the actuator must convert a large percentage of electrical energy into mechanical work. Cottinet et al. provided a detailed and convincing model to explain the energy losses (i.e., electrical and mechanical losses) endured throughout actuation. The total efficiency of the actuator’s energy conversions is as follows:

ηtotal = ηelectrical ⋅ ηtransduction ⋅ ηmechanical

Rel

Fig. 15 Asaka’s ECM of carbon-based electrode/PEM ­actuators: (a) schematic of equivalent circuit components, (b) equivalent circuit diagram, and (c) final equivalent circuit for capacitance calculations

(10)

where 𝜂electric is the electrical energy conversion, 𝜂transduction is the electrical-to-mechanical conversion, and 𝜂mechanic is the mechanical conversion. The efficiency of the electrical energy conversion is expressed as follows:

ηelectrical =

V ⋅ Icharge V ⋅ Itotal



(11)

where V is the applied voltage, Itotal is the applied current, and Icharge voltage-induced charge. Cottinet’s group realized that most of the energy loss occurs in the transduction or electrical-to-mechanical conversion. The transduction efficiency is expressed as follows:

ηtransduction =

0.5 ⋅Y ⋅ ε 2 ⋅ f ⋅ Vol V ⋅ Itotal

(12)

where Y denotes the actuator’s Young’s modulus, ε is the generated strain, f is the actuation frequency, and Vol is the volume. The mechanical energy conversion, expressed in Eq. 12, is dependent on the actuator’s viscoelastic behavior. The loss tangent (tanδ) is a well-known ratio of the material’s loss modulus and storage modulus. As a function of frequency, temperature, environment, and residual stresses and strains, tanδ is a reliable measurement of the actuator’s material response to cyclic deformation. [82] For reference, the BP composite actuator exhibited a global efficiency of 0.052%, in which 𝜂electrical, 𝜂transduction, and 𝜂 mechanical measured to be 20%, 0.32%, and 83%, respectively.

ηmechanical =

χiii χii

Energy Conversion

1 1 + π ⋅ tanδ

(14)

FUTURE INSIGHT ON THE BP COMPOSITE ACTUATOR Actuator Preparation The BCA consists of five active material layers: two BP thin films, two Nafion adhesive thin films, and a commercial Nafion membrane. The structure is displayed in Fig. 16.

Actuators– Agriculture

Actuators: Buckypaper Composite 11

12

Actuators: Buckypaper Composite

Actuators– Agriculture

ion exchange, the commercial Nafion membrane is dehydrated at 110°C for 2 h in a vacuum oven. The dehydrated membrane is then soaked in an IL overnight at 110°C in a vacuum oven. Thus, Nafion’s ionic conductivity increases by almost 600% ­leading to significantly enhanced ­actuation response.

Buckypaper Nafion adhesive Nafion membrane Nafion adhesive

Fig. 16  Illustration of dry-layup BCA bimorph structure

MWCNT-BP Fabrication The MWCNT-BP Electrode section of this entry outlines the manufacturing process of MWCNT-BP. Commercial Nafion Pretreatment The Nafion polymer electrolyte membrane (NRE-212) used in this research is manufactured by DuPont. Conventionally, the full hydration of Nafion promotes high ionic conductivity. Water makes it easier for cations to diffuse through the membrane, and it essentially lubricates the clusters and micro-channels. However, water is volatile and limited to low working temperatures. The electrolysis of water also occurs at low voltages (1.2 V) giving Nafion a narrow electrochemical window. Silicon coatings have been proposed to preserve hydration; however, the added stiffness compromises the generated strain. Therefore, it is important to exercise Nafion’s ability as ion-­exchange membrane. Replacing water with an IL, such as 1-butyl3-­methylimidazolium tetrafluoroborate (BMI-BF4) and ­1-ethyl-3-methylimidazolium (EMI-Br), introduces a highly stable (TBoiling Point > 300°C) solvent with a large electrochemical window (1.0) and anomalous transport (0.5  ammonium alginate > alginic acid. Different alginate salt and their alginate grade product gives different stability. The industrial relevant alginate grade solution is more easily degraded by microbe in the air due to the presence of more algal particles and nitrogenous matter in product, which offer plenty of nutrition to the microbe. Solubility Alginates and their salts are soluble in water and form solution with high viscosity. Alginic acid forms water-­ soluble salts with monovalent cations, but gets precipitated upon acidification. Sodium alginate is gradually soluble in cold water, forming viscous and colloidal solution. It is insoluble in alcohol and hydroalcoholic solutions in which alcohol content is greater than 30% by weight.[162] It is also insoluble in other organic solvents such as chloroform and ether and in acids where the pH of the resulting solution

Alginate–Aviation

Alginate 119

120 Alginate

H O O

–

O O H

Ca2+

O

Ca2+

O

CO

O–

“Egg-box” structure

O Ca 2* H

O

H

The mechanism of gelation in alginates takes place in the presence of certain divalent or multivalent (Ca2+ , Ba2+ , Fe2+ , or Sr2+) and trivalent cations (Al3+), associated to ionic bonding with G blocks.[137] This key property of alginate has exploited its application as a gelling agent in different fields. However, monovalent cations and Mg2+ ions do not undergo gelation, whereas Sr2+ and Ba2+ ions produce stronger gels than Ca2+ ions. Divalent cations such as Cu2+ , Pb2+ , Cd2+ , Ni2+ , Co2+ , Mn2+ , and Zn2+ cross-link to alginate but limit their use due to their toxicity.[164] The interaction strength of alginate cross-linking cations follows the following order: trivalent cations > Pb2+ > Cu2+ > Cd2+ > Ba2+ > Sr2+ > Ca2+ .[165] Ca2+ ion does not show the highest interaction strength.[166] The gelling property of alginate depends mainly on the M:G ratios present in the alginates. Based on the M:G ratio, gels with different rigidity can be prepared for different applications. Jensen et al. have reported that alginate solutions with low M:G ratio (0.8) show higher gel strength compared to the solutions containing high M:G ratios (1.3 and 2.5).[167] It has been also reported that gelation of alginate (sol/gel transition) is almost independent of the temperature but is induced by the presence of divalent cations associated with the G-blocks.[168] Atomic absorption spectroscopy[169,170] and inductively coupled plasma mass spectrometry[136–138] are

Alginate

Fig. 4  Structural representation of “egg-box model” formed by binding of divalent ions to alginate chains

CO

Gelation

O

widely used analytical methods in determining the extent of cross-linking (i.e., to determine the total calcium content in the particles). However, these technique provides limitation for determining the data relating to calcium ion distribution throughout the polymer chain. Ca2+ ion distribution throughout alginate particles can be analyzed by using synchrotron-radiation induced X-ray emission and magnetic resonance microimaging.[174,175,179,180] Most commercially, high-M type content in alginate is obtained from giant kelp (M. pyrifera). Whereas, L. hyperborea offers alginate with high-G content. High-G alginates produce strong, brittle gels that are heat stable, whereas alginate with high-M content provides weaker and more elastic gels that are less heat stability but more freeze/thaw stability. Especially the G blocks in alginate are buckled, while the M blocks have a shape referred to as an extended ribbon. [176,177,181] Alginate with two G-block regions are aligned side by side, resulting in an ideal space for binding of calcium ions. This can be explained by the preferential binding within the hydrophilic pocket created within the polyguluronate chain. The addition of divalent ions forms “diamond shaped” arrangements with polyguluronate sequences, resulting in cross-link between two polyguluronate chains, finally to result in gelation.[178–181] This is commonly termed as the “egg-box model,” this phenomenon is often referred to as “cross-linking” (­Figure 4). The evidence for the “eggbox” junction zone configuration was detailed by using X-ray diffraction studies. [182] The detail findings on ‘eggbox’ junction, based on alginates gelling mechanism comprising long uninterrupted sequences of polyG, form the strongest gels in the presence of divalent cations as stated in above sentence.[183,184] Recently, molecular dynamic and Monte Carlo simulations [185,186] have revealed different Ca2+ -dependent modes of cross-linking two or more polyG strands, including variants of the ‘egg-box’ junction as well as a perpendicular junction and three Ca2+ ions, while a similar tilted ‘egg-box’ cross-link have been proposed independently[187] to reflect the initial stages of

O

Alginate–Aviation

falls below 3.0.[57] Sodium alginate rich in guluronic acid is more soluble in water than mannuronic acid-rich sodium alginate [163] whereas, calcium alginate is practically insoluble in water and organic solvents but soluble in sodium citrate. The pH of the solvent is an essential parameter determining the solubility of alginates in water as it will influence the presence of electrostatic charges on the uronic acid residues. High amounts of poly-alternating structure in an alginate sample have been shown to increase its ­solubility at low pH.[151]

Alginate 121

APPLICATIONS OF ALGINATE Current global trends indicate an enhanced application of alginates as potential carbohydrate polysaccharide in the fields of pharmaceutical, cosmeceutical, biomedical, food, textiles, and other relative areas. Figure 5 depicts various applications of alginate in different fields. Biomedical Applications Alginate has found a great importance in biomedical and biotechnological applications, [194] as a material for encapsulation and/or immobilization of a variety of cells due to its high biocompatible and nontoxic nature.[195] The alginate gelling property has led its application in cell transplantation and other approach of replacement tissues (tissue engineering).

Cell Encapsulation Alginate is non-immunogenic and has revealed great potential as a vehicle in drug delivery and cell transplantation.[196] The main aim behind cell encapsulation is to provide a suitable environment that helps to maintain cell viability and function. Cell microencapsulation within alginate hydrogels have been investigated earlier for a variety of diseases, such as cancer, diabetes, liver failure, metabolic deficiencies, and neurodegenerative and cardiovascular diseases.[197–199] Due to the biocompatibility nature of alginate hydrogels, it has been used to protect transplanted cells and increase the cell viability in cell encapsulation.[197,199,200] A number of cell types including osteoblasts, [194,201,202] chondrocytes, [127,203,204] pancreatic islets, [192,205,206] neural stem cells, [105,207] and MSCs [208] have been encapsulated, cultured, and expanded in alginate hydrogels both in vitro and in vivo. The first clinical trial was carried by Dr. Soon-Shiong at St. Vincent Medical Center (Los Angeles, CA, USA), in 1993, on human by utilizing alginate as the encapsulating material for pancreatic islet cells to treat insulin-dependent diabetes. The transplantation of encapsulated pancreatic islet cells was successfully achieved without any observed adverse reactions.[209] The results showed that insulin secretion could be achieved within 24 h after transplantation. Encapsulation of human SC-β cells in chemically modified alginate microsphere was reported by Vegas et al. for long-term glycemic correction of a diabetes in animal model. The authors reported that the human SC-β cells encapsulated in alginate derivatives were capable of mitigating foreign body responses in vivo when implanted into the intraperitoneal space of C57BL/6J mice. Triazole-­ thiomorpholine dioxide alginate-encapsulated SC-β cells provided long-term glycemic correction and glucose responsiveness without immunosuppressive therapy in immune-­competent diabetic C57BL/6J mice.[210] Wang et al. reported alginate-based encapsulation system for embryonic stem cells differentiation into insulin producing cells. Alginate encapsulated ES cells retained and showed high cell viability. The results showed that modified alginate-­based 3D differentiation strategy was efficient in encapsulation of ES cells with increase in cell density in culture and could provide proliferation and differentiation of ES cells into pancreatic insulin-producing cells. Compared to conventional 2D cell culture system, alginate encapsulation system showed increased ES cells ­differentiation efficiency in insulin productivity.[211] Gene Delivery Among various polymers, alginate as a 3D biomaterial offers promising function in gene delivery due to its unique properties. This hydrogel has broad potential applications to developed and control therapeutic gene delivery and cell fate within such constructs. Earlier reported by

Alginate–Aviation

cross-linking between two alginate strands. The minimum length of an oligoguluronate (oligoG) necessary to induce junction zone (i.e., leading to a stable gel formation) in the presence of Ca2+ has considered at least eight glucoronic residues [178] allowing to form stable Ca2+ mediate junction zone. Based on the earlier reports, polymers with polyG sequences shorter than eight residues leached from Ca-­ alginate gels.[188] The shorter the interaction time, the lesser in strengthening of cross-linking, leading to form weaker gel. In context, alginate gelation and r­ elaxation processes contribute to ­alginate gel rheology.[189,190] Kate et al. recently determined the minimum length of oligoG necessary to form a stable ‘eggbox’ junction zone, and characterized the onset of ‘egg-box’ formation using single molecule force spectroscopy measurement conducted by atomic force microscopy.[183] The authors found that, the buckled ribbon conformation of the GG blocks was optimal for Ca2+ binding, whereas the conformation of MM blocks and MG blocks (MM and MG blocks are like flat ribbon) is less favorable and hence resulting in lower strengthening of gel.[86,191] When controlled amounts of calcium ions are added to alginate solutions, gels are formed. The strength of the gel depends on the content and size of the G blocks of alginate and also on the concentration of calcium ions present. The calcium ions bind strongly to the G-blocks, but the formation of an insoluble precipitate is prevented by the presence of the M-blocks and MG-blocks, where the interaction with calcium ions is less. Besides calcium chloride, other calcium salts such as gluconate, nitrate, and propionate can be used to form alginate gels but it has been found that none of them can form gels as strong as calcium chloride due to their weaker ionizing properties. Breakdown or destabilization of Ca2+ cross-linked alginate gel can be accomplished by using chelating agents such as (a) ethylenediaminetetraacetic acid disodium salt (EDTA), (b) salts such as citrate, lactate, and phosphate, and (c) by adding high concentration of ions such as Mg2+ or Na+.[192,193]

122 Alginate

Fire retardent fibrics

Paper

Prevention of gastric reflux

Beverages

Alginate–Aviation

Food Construction industry Alginate applications

Bone tissue engineering

Gelling agent

Ice-cream Wound healing Dental impression Industrial waste treatment Cell entrapment

Tablets Cosmetics

Fig. 5  Illustrative image for applications of alginates in different fields

Zhou et al. developed of novel cationic carbon quantum dots (CCQDs) derived from alginate for gene delivery. Optically tunable photoluminescent carbon quantum dots (CQDs) were prepared by one-step hydrothermal carbonization route. The findings showed that negatively charged alginate can be converted to positively charged CQDs without any addition of cationic reagents. The significance of the study showed that CQDs played a dual role at the same time: as a non-viral gene vector and a bioimaging probe for gene therapy.[212] Wang and colleagues developed 3D porous scaffolds composed of chitosan-alginate complex for gene target delivery to tumor cells. Due to the structural similarity of alginate and chitosan to glycosaminoglycans, alginate showed a promising choice for in vitro modeling of tumor microenvironment. The results concluded that, in vitro cell culture system served as a useful 3D tumor model in nanoparticle-mediated gene delivery.[213] Gonzalez-­ Fernandez developed gene-activated alginate hydrogels supporting nanohydroxyapatite-mediated nonviral gene transfer to control the phenotype of mesenchymal stem cells (MSCs) for either cartilage or endochondral bone tissue engineering.[214] Gene-activated constructs, MSCs and nHA complexed with plasmid DNA (pDNA) encoding

for transforming growth factor-beta 3 (pTGF-β3), bone morphogenetic protein 2 (pBMP2), or a combination of both (pTGF-β3–pBMP2) were encapsulated into alginate hydrogels. Confocal microscopy demonstrated complexing of plasmid with nHA before hydrogel encapsulation, leding in transport of the plasmid into the nucleus of MSCs, which did not happen with naked pDNA. Gene delivery of TGF-β3 and BMP2 and subsequent cell-mediated expression of these therapeutic genes resulted in a significant increase in sulfated glycosaminoglycan and collagen production, particularly in the pTGF-β3–pBMP2 codelivery group in comparison to the delivery of either pTGF-β3 or pBMP2 in isolation. In addition, the authors observed that stronger staining for collagen type II deposition was seen in the pTGF-β3–pBMP2 codelivery group. The greater levels of calcium deposition was observed in the pTGF-β3- and pBMP2-only groups compared to codelivery, with a strong staining for collagen type X deposition, which suggested that these constructs were supporting MSC hypertrophy and progression along an endochondral pathway. However based on these results the authors reported that, the developed gene-activated alginate hydrogels were able to support transfection of encapsulated MSCs and direct their phenotype toward either a chondrogenic or an osteogenic

phenotype depending on whether TGF-β3 and BMP2 were delivered in combination or isolation.[214]

stimulation by paracrine factors from the seminiferous tubules.[216]

Cell Implantation

Pharmaceutical Applications

Since natural enzymatic degradation of alginate in mammals is slow, months can pass before alginate hydrogels are completely removed from the implantation sites. Lee and Mooney reported that alginate can be purified by the multiple step extraction process with high purity and they do not induce any significant foreign body reaction when implanted in animals.[105] Capone and his colleagues encapsulated HepG2/C3A cell in alginate-collagen beads covered with and without PLL (poly-L-lysine) layer and implanted subcutaneously in mice. The results showed that, implanted encapsulated HepG2/C3A cell in alginate-­ collagen beads could reduced inflammatory response. The PLL layer and collagen mixed to Ca-alginate increased the beads resistance as well as addition of collagen enhanced encapsulated hepatic cell line behavior.[72] Wu et al. filled an alginate sponge into a young rat spinal cord and used the alginate gel as a carrier for grafted neurosphere cells to prevent cell loss after implantation. They demonstrated that the grafted cells in the gel could survive, differentiate, extensively migrate, and integrate well into the host spinal cord tissue.[215] Chen et al. carried out in vivo and in vitro studies to determine whether testosterone-producing Leydig cells are able to develop from cells associated with rat seminiferous tubules, interstitium, or both. The authors isolated adult rat seminiferous tubules and interstitium and encapsulated separately in alginate, and implanted subcutaneously into castrated rats. With implanted tubules, serum testosterone increased through 2 months. Tubules were then removed from the implanted rats and incubated with LH produced testosterone, and cells on the tubule surfaces expressed steroidogenic enzymes. Implanted interstitial testosterone remained undetectable in castrated rats that received alginate only or alginate-encapsuled interstitial tissue or tubules. Whereas, coculture of interstitium plus tubules in vitro resulted in the formation of Leydig cells by both compartments. However, the alginate encapsulated seminiferous tubules and interstitium showed that the cells associated with seminiferous tubules are able to develop into testosterone-producing Leydig cells in vivo outside the testis and therefore in the absence of paracrine factors from the interstitial compartment and interstitial tissue, failed to form functional Leydig cells in vivo; but in vitro approach showed that the co-culture of interstitium with seminiferous tubules resulted in the formation of testosterone-producing cells by the interstitium. These results indicated that the seminiferous tubules containing both cellular and paracrine factors were necessary for the differentiation of Leydig cells, and the interstitial compartment containing precursor cells were capable of forming testosterone-producing Leydig cells but required

Wound Dressing Various literature has been reported till date on alginate as a biocompatible matrix material for wound dressing applications.[4,32,191,217,218] The major reason behind selecting alginate in wound management is the ability to form gels upon contact with exuding wounds. Alginate leads to trigger macrophage activity and increases cytokine levels in wounds [219] leading to faster wound healing property. It is also found that alginate dressing can absorb about 15–20 times of fluid compared to their own weight. When an alginate dressing comes in contact with wound exudates, ion exchange occurs between calcium ions present in wound dressing/film to that with sodium ions present in the wound exudate. Sufficient amount of calcium ions are replaced by sodium ions during application of wound dressings leading to gel formation and healing the wound. Various wound healing actives such as essential oils (lemongrass, eucalyptus, peppermint, lavender, cinnamon, chamomile blue, and elicriso), honey, and curcumin have been incorporated in alginate wound dressings/films for antimicrobial and antifungal activities.[220,221] Several other additives such as silver ions and nanoparticles,[222,223] natural chitosan and honey,[224] savlon antiseptic liquid,[225] ciprofloxacin HCl,[226] and povidone iodine [221] have been used in alginate-­based wound healing dressings for antimicrobial activity. The two commercial products available as wound dressing are Kaltostat® and Sorbsan® with the composition of calcium alginate.[227] Some of the FDA approved wound dressings include Algicell®, AlgiSite M™, Comfeel® Plus, Kaltostat®, Sorbsan®, and Tegagen™. Qin, reviewed production processes and applications of alginate fibres in wound management. The alginate material interacted with wound surface and create a moist local environment, which was suitable for epithelial cells to migrate from the edge of the wound to the wounded area leading to quicker healing. It was also reported that water-insoluble calcium alginate fiber undergo ion exchange with the sodium ions present in the body fluid and release the calcium ions, and thus acts as a haemostatic agent.[86] Alginate-based bilayer hydrocolloid films in wound dressing were earlier reported by Thu and colleagues. The developed bilayer film containing upper layer was impregnated with ibuprofen and the lower layer was drug free, which acted as a rate-­controlling membrane. The results showed that bilayer wound dressing had high mechanical and rheological properties compared to the single-layer films. The bilayers films with slow release of drug showed low-moisture vapor transmission rate, slower hydration rate, and lower drug flux in vitro compared to single layer, concluding that bilayer films prepared was useful in treating low exuding wounds. In vivo results also

Alginate–Aviation

Alginate 123

124 Alginate

Alginate–Aviation

confirmed that epidermis growth with faster tissue formation.[228] Rezvanian et al. reported the development of alginate-based composite film of simvastatin as a model drug for wound healing. Sodium alginate with pectin or gelatin was used in the development of composite films. The results from preformulation studies concluded that alginate/pectin composite film have better wound healing and mechanical properties. In vitro study showed that alginate/pectin film showed controlled-drug release profile with nontoxic properties, analyzed by cell viability study.[8] Alginates can be used in a variety of wound types, where exudate is present, such as pressure ulcers, venous leg ulcers, diabetic foot ulcers, postoperative wounds, traumatic wounds, pilonidal sinus wounds, and partial ­thickness burns.[229,230–234] Dental Impression Alginates are exploited to produce dental impressions using the reaction between a sparingly soluble calcium salt (usually calcium sulfate dehydrate) and a soluble alginate, resulting in irreversible hydrocolloid impression. Alginate impressions are usually taken to develop diagnostic study models, which are positive reproductions of the teeth and surrounding structures. Bleaching trays and mouth guards are fabricated using alginate. The use of alginate as impression material have several advantages such as the ease of use, the ability to obtain a relatively precise i­ mpression, and the lack of requirement for specific equipment [234] Alginates as elastomeric by nature offer the flexibility and duplicating properties to obtain an accurate impression. These obtained alginate impressions can be poured in gypsum to obtain models or casts. Study models also provide occlusal representation to identify the Angle’s classification of occlusion, which defines the relationship of the maxillary and mandibular teeth in the sagittal plane. Additionally, they provide recognition of wear patterns, missing teeth, drifting teeth, actual size reproductions of anatomical structures such as tooth size, shape, positions, gingival margins, interdental papillae, and the freni. Drug Delivery Various reports have been available on alginate in effective delivery of various therapeutic active solid and liquid forms to improve their therapeutic efficacy. Oral Drug Delivery.  The oral administration of alginate has not been shown to provoke much immunoresponses unlike the intravenously administered forms, and it is reported that alginate is nontoxic and b­ iodegradable when given orally. Sodium alginate is widely used in a variety of oral pharmaceutical formulations (microspheres, microcapsules, gel beads, hydrogel, film, and ­nanoparticles)[236] in delivery of low-molecular-weight drugs to large ­biomacromolecules such as proteins.

Various reports on alginate, as a carrier for controlled release of indomethacin, [235] nicardipine, [174] dicoumarol, [237] gentamicin, [238] vitamin C, [239] ketoconazole, [239] indomethacin, [42] valsartan, [240] Bacteriophage Felix O1, [241] insulin, [242] gliclazide, [243] riboflavin [45], have been reported. Mohy Eldin et al. developed amphoteric alginate-aminated chitosan-coated microbeads for oral delivery of protein (bovine serum albumin [BSA]). In their study, alginate was selected as a sensitive pH carrier for protein delivery under physiological conditions.[244] In order to control the physical instability of alginate in higher pH medium (i.e., leaching of material), Omer et al. coated the microbead with aminated chitosan to form smart pH-sensitive polyelectrolyte complex. The results showed increase in hydrophilic character of alginate microbeads through coating with aminated chitosan instead of chitosan resulting in high concentration of entrapped BSA (more than 70%) which could provide site-specific release of protein.[245] Mandal et al. prepared alginate matrix tablets containing diltiazem hydrochloride, using sodium alginate and calcium gluconate by conventional wet granulation method. The prepared tablets with ratio of 1:2 w/w of sodium alginate and calcium gluconate showed sustained release of drug up to 13.5 h. However, the study showed that, when tablet comes in contact with fluid, gelation takes place between sodium alginate and calcium ion leading to diffusion of drug through the gel matrix. [246] Goswami et al. developed calcium alginate nanocarriers for oral delivery of insulin. The obtained percent loading in nanocarriers was found to be increased (11.7%– 38.9%), with the increase in the release of insulin amount (18%–60%) from the nanocarriers. It was also observed that varying the amount of alginate (1.0–2.0 g) in the feed mixture led to decreased in the tendency of insulin release. Similarly on increasing the concentration of a cross-linking agent (0.5–1.1 mM), the release of insulin decreased constantly. However, the insulin remained stable in harsh acidic environment with 20% insulin release, while at higher intestinal pH, 90% release was observed. [247] Based on the results, the authors suggested that alginate-­based formulation with highly cross-linked network was stable for oral delivery of insulin. Mucoadhesive Drug Delivery.  Another approach to extend the residence time of active therapeutics is to develop a mucoadhesive drug delivery system. Polymer dispersions, which are applied topically are low viscous in nature and undergo hydration and provide effective mucoadhesive mechanism. Alginate, as anionic polymer with carboxyl end groups provides excellent mucoadhesive property.[248] Studies have shown that alginate has the highest mucoadhesive strength compared to polymers such as polystyrene, chitosan, carboxymethylcellulose, and ­poly­(lactic acid).[249] Kesavan et al. prepared sodium alginate- and Na-­ Carboxymethylcellulose-based ophthalmic mucoadhesive

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Whereas, the optimized batch showed 100% H pylori growth ­inhibition in 15 h in in-vitro culture.[255] Buccal Drug Delivery.  Buccal mucosa-based drug delivery has offered great advantage, especially for the drugs having poor gastrointestinal stability, low bioavailability, and susceptibility to first pass metabolism. Choi et al. developed buccal adhesive tablet of sodium alginate and HPMC containing omeprazole as a model drug. Omeprazole tablets were prepared by using bioadhesive polymers, which showed bioadhesive forces suitable for buccal adhesion of tablets. Two tablets composed of (omeprazole/sodium alginate HPMC/magnesium oxide (20/24/6/50, mg/tab)) and ((20/30/0/50, mg/tab)) were suitable for omeprazole buccal adhesive, which resulted in adhering in human cheeks without any collapse and was found to be stabilized in human saliva for at least 4 h. It was concluded that these two formulations were potential candidates for the subject of development of omeprazole buccal adhesive tablets. [256] Okeke and Boateng, developed buccal formulations (films and wafers) which comprised of HPMC and sodium alginate for nicotine replacement therapy. Sodium alginate in the formulation improved and modified the functional properties of HPMC at optimum ratio of HPMC: sodium alginate (1.25: 0.75). However, from the results it was found that, compared to HPMC-­sodium alginate composite films, HPMC-sodium alginate composite wafers showed higher swelling index with higher mucoadhesion and drug loading capacity which required for nicotine replacement therapy. [257] Ocular Drug Delivery.  Alginate offers as a promising polysaccharide in ocular drug delivery due to its gelling nature. Poor bioavailability of ophthalmic solutions results in dilution and drainage from eyes and which can be overcome by using in-situ-forming ophthalmic drug delivery systems containing polymers that exhibit reversible liquid–­ gel phase transition. Cohen et al. demonstrated that an aqueous solution of sodium alginate which can form a gel in the eye, without addition of external calcium ions or other bivalent/polyvalent cations.[258] The authors reported that, the extent of alginate gelation and consequently the release of pilocarpine depended on the percent guluronic acid (G) residues in the alginate backbone. Alginates with G contents of more than 65%, such as Manugel DMB, instantaneously formed gels upon their addition to simulated lacrimal fluid, while those having low G contents, such as Kelton LV, formed weak gels at a relatively slow rate. In vitro studies showed that pilocarpine released slowly from alginate gels, over a period of 24 h, and the release of drug was observed mostly through diffusion from the gels. Based on the overall results of the study, the authors concluded that in situ-gelling alginate system, based on high G content in alginate can acts as an excellent drug carrier for prolonged delivery of pilocarpine. Shinde et al. developed

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system for gatifloxacin. The prepared formulation was evaluated for its in vitro antibacterial activity for the microorganisms such as Staphylococcus aureus and Escherichia coli. The results for in vitro showed that the mucoadhesive systems provided sustained release of drug over a 12 h period. Noteworthy, it was also found that the reduction in the total bacterial count was observed between control and treatment groups with both the test organisms. Based on the results, the developed formulation was viable alternative to conventional eye drop due to its ability to enhance bioavailability through its longer precorneal residence time and ability to sustain the release of drug.[250] Szekalska et  al. reported the preparation of alginate-based mucoadhesive carriers of ranitidine by spray drying technique. The prepared microspheres showed ranitidine loading up to 70.9% with sustained release of drug up to 4 h. Different formulations prepared showed mucoadhesive properties expressed as maximum detachment force (Fmax) in the range of 0.16 ± 0.02 kg m s−2 to 0.91 ± 0.06 kg m s−2 and work of mucoadhesion (Wad) between 340.6 ± 1.4 and 989.2 ± 3.2 µJ. These results of their study suggested that the designed microspheres were effectively used as mucoadhesive carriers of ranitidine.[251] Kassem et al. developed mucoadhesive microbeads using thiolated sodium alginate for resveratrol intrapocket delivery. Alginate and alginate/thiolated alginate microbeads were prepared in different ratios (1:1, 2:1, 3:1, and 4:1), using calcium chloride by ionotropic gelation method. Based on the result, significant lower swelling index for thiolated alginate formulation (A/TA 1:1) with strong mucoadhesion and %EE of 83.72 was observed. The authors concluded that, the alginate and alginate/thiolated alginate microbeads in the ratio of 1:1 can be useful as prolonged drug release system for local treatment of periodontal pockets.[252] Similar study was carried by Kassem et al. for thiolated alginate-based multiple-layer mucoadhesive films for intrapocket local delivery of metformin. The results showed that 6% of carboxymethylcellulose sodium as drug loading (0.6%) middle layer and thiolated alginate (4%) as drug free outer layers exhibited strong mucoadhesion and controlled-drug release over 12 h.[253] Adebisi et al. reported formulation of floating mucoadhesive alginate beads of clarithromycin for eradication of Helicobacter pylori infections. Calcium alginate beads were prepared by ionotropic gelation method. The modified beads (calcium alginate beads with oil and further coating the beads with chitosan) provided buoyancy upto at least 24 h, with 75% beads adhering to pig gastric mucosa and 8 h release profile, ensuring that the drug release beyond 8 h in gastric pH.[254] Floating beads of amoxicillin trihydrate was reported by Dey et al. using sodium alginate and hydroxypropyl methylcellulose (HPMC) as matrix polymers and chitosan as a coating polymer to localize amoxicillin trihydrate at the stomach site against H. pylori. The results confirmed that the prepared beads from all the batches floated for more than 24 h with maximum lag time of 46.3 ± 3.2 s and showed good mucoadhesion of 75.7 ± 3.0% to 85.0 ± 5.5%.

126 Alginate

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chitosan–alginate mucoadhesive microspheres loaded with azelastine for ocular delivery. The microspheres were prepared by using modified ionotropic gelation method. The results showed that, the drug entrapment in chitosan–alginate microspheres was 73.05% with 65% of mucin-binding efficiency. The drug release was found to be controlled release over the period of 8 h. Also through in vivo study on ocular rat model, it was observed that prolonged drug release occurred in the cul-de-sac region.[259] Several other studies have been reported in literature using alginate and alginate blend for ocular drug delivery.[260,261] Novel in situ gel of sparfloxacin for sustained by ocular drug delivery was carried by Ali and colleagues. The developed systems consisted of sodium alginate as gelling agent and methylcellulose as viscosity enhancing agent. The formulated in situ gel was in sol form at pH 4.7. As the pH was raised to 7.4, the sol went on rapid gelation. The drug release in corneal region was 24 h compared to marketed eye drop. Based on the permeation study carried in situ gel of sparfloxacin carried in ex vivo goat corneal showed increase in permeation of drug.[262] Topical Drug Delivery.  Alginate as hydrocolloid films are ideal for topical drug delivery because unlike creams and gels, the matrix contains a fixed dose loaded onto a defined area and has a long residence time. In different topical formulations, sodium alginate is widely used as a thickening and suspending agent in a variety of pastes, creams, and gels, and also used as a stabilizing agent for oil-in-water emulsions. Walling et al. prepared trolamine/sodium alginate containing topical emulsion for facial ulcerations. Patient experienced tingling and pain on prompt rubbing and excoriation. After a week of twice daily application of medication (gabapentin and amitriptyline) without occlusion, patient reported healing of ulcerations and decreased dysesthesia. Six weeks later, ulcerations were resolved. Over the next 6 months, erosions occurred less frequently, emulsion application promoted rapid healing with minimized lesion duration, and prevented the development of persistent ulceration. [263] ­Bijukumar et al. developed an inflammation-­sensitive system for topical drug administration in treatment of arthritis. The prepared formulation involved multi-­ macromolecular alginate-­hyaluronic acid-chitosan (A-HC) polyelectrolyte complex nanoparticles, loaded with indomethacin employing pre-gel and post-gel techniques in the presence of dodecyl-L-­pyroglutamate (DLP). The results showed that, nanoparticle size was significantly influenced by dodecyl-L-pyroglutamate. The drug-content analysis revealed higher encapsulation efficiency (77.3%) in the presence of DLP, irrespective of the techniques used. In vitro drug release showed that indomethacin release from the nanoparticles was significantly improved (98%) in Fenton’s reagent. Drug permeation study across a cellulose membrane using a Franz diffusion cell system showed an initial surge flux (0.125 mg/cm−2/h),

followed by sustained release of drug for the post-gel nanoparticles revealing its effective skin permeation efficiency.[264] Nguyen et al. reported the nanocarriers of curcumin loaded in the oily core of calcium alginate system. The quantity of curcuminoids extracted from the skin treated with calcium alginate nanoparticles was 22% ± 1%. The size of nanocarrier (hydrodynamic diameter ∼200 nm) and surface charge (zeta potential ∼−30 mV) were both compatible for application on skin. The encapsulated curcumin showed high efficiency (∼95%) to retain ­antioxidant activity asserting that the curcumin loaded in alginate nanocarriers are effective in topical drug ­delivery.[265] Colon Drug Delivery.  Colon-targeted drug delivery has gained quite interest, especially in the treatment of IBD.[266] As a mucoadhesive agent, alginate prolong the adhesion of drug in the intestinal mucosa and increase the contact residence time in colon. Different formulations aimed to target colon include pH-dependent delivery, prodrug delivery, time-dependent delivery, and microbial degradation approach.[267] Vemula et al. developed sodium alginate compressed tablets of ketorolac tromethamine for colon delivery. Based on the in vitro results of drug release, it was found that optimized formulation showed decrease in the release (6.75 ± 0.49%) of drug at initial lag time of 5 h, followed by increase in the release (97.47 ± 0.93%) within 24 h. X-ray imaging in human proved that the tablet did not disintegrated in the upper gastrointestinal region. Pharmacokinetic study showed that Cmax was found to be 3486.70 ng/ml at Tmax of 10 h. However, in case of immediate release tablets, Cmax was found to be higher (4506.31 ng/ml) with early Tmax (2 h), which proved the ability of compression-­ coated tablets to target the colon.[267] Samak et al. developed alginate microparticles for delivery of corticosteroids to the colon. The microparticles were developed by using CaCl2 as a cross-linking solution (0.5 M and 1 M), using aerosolization and homogenization technique. The results showed that, smaller particles (45–50 mm) were obtained by the homogenization method compared to aerosolization (65–90 mm) technique. The drug loading was found higher (40% wt/wt) in microparticles prepared by aerosolization technique. However, drug release in simulated gastric fluid (SGF) and simulated intestinal fluid prior to colonic fluid was found to be suppressed in aerosolized microparticles compared to homogenized microparticles. Thus, from the results it was concluded that aerosolization and homogenization techniques showed potential in ­developing microparticles for colonic drug delivery.[268] Till date, various other reports have been published on alginate and its complex with other polymers (natural or synthetic) for effective colon drug delivery.[266,269–273] A brief study report has been reported by Aguero et al. on overview of alginate microparticles alone and with other polymers with innovative strategies for oral colon delivery.[78]

Pulmonary Drug Delivery.  Due to gelation property and high biocompatibility, alginate has been widely used as a matrix material for encapsulation of bioactive peptides/proteins and cells. However, it is difficult to obtain microparticles in a size range that is suitable for pulmonary administration. Preliminary studies have showed that atomization of alginate–drug solutions into solutions of divalent cations (Ca 2+) results in very low encapsulation efficiencies of model protein BSA and low yields of spherical particles. Alternatively, alginate– drug solutions can be spray-dried and the microparticles obtained can be cross-linked in salt solutions. [274] Biodegradable paclitaxel loaded alginate microparticles for pulmonary delivery was developed by Alipour and his colleagues. The alginate microparticles were fabricated using emulsification technique. Selection of appropriate parameters enabled preparation of microparticles with a mean volume diameter of 3 ± 0.7 µm, mass median aerodynamic diameter of 5.9 ± 0.33 µm, fine particle fraction of 13.9 ± 0.57%, and encapsulation efficiency of 61 ± 4%. The in vitro release profile showed a slower release rate for microparticles compared to pure paclitaxel. The in vitro cytotoxicity activity of paclitaxel loaded microparticles was assessed using human non-small cell lung cancer cell lines (A549 and Calu-6). Final results showed that the exposure of cells to pure paclitaxel and paclitaxel loaded microparticles effectively inhibited the growth of A549 and Calu-6 cells, similarly in a concentration and time dependent manner. [275] Parenteral Drug Delivery.  Particularly for parenteral products for administration, ultrapure grades of alginates have been widely used. The ultrapure alginates (endotoxins ≤ 100 EU/gram) are marketed under the trade name PRONOVATM. Products with different mannuronate to guluronate monomer ratios and viscosity (molecular weight) ranges are available in market. Ultrapure grades of alginates have a controllable level of pyrogenicity and have be used in implants in combination with drugs.[23] Injectable hydrogel of alginate with huge potential as an artificial 3-D cellular matrix have attained great interest in injectable cell delivery.[38] Injectable cell-based systems can be prepared with different properties, based on its type of application. Della et al. reported two ideal injectable alginate-based gels that have reached clinical trials.[276] Alginate-based solutions that gel in-situ upon administration via intracoronary injection are currently under clinical investigation in patients with acute MI.[277] Zhou and Xu developed fast release injectable microbeads, encapsulating human umbilical cord mesenchymal stem cells (hUCMSC) for bone tissue engineering. Different microbeads (alginate microbeads, oxidized alginate microbeads, and oxidized alginate–fibrin microbeads) for release of stem cells were analyzed. The authors reported that, hUCMSC encapsulated alginate–fibrin microbeads were fast degradable injectable microbeads that could

degrade and release the stem cells at 4 day. The release of hUCMSCs would enhance cell proliferation and help in bone mineral synthesis.[278] Yu and his colleagues, have reported encapsulation of human MSCs in RGD-­modified alginate injectable microspheres (400°C and will still exhibit service properties, even close to their decomposition temperatures.[69] The bond energy associated with the chemical bond in the polymer chain significantly influences its thermal stability. In addition, the polymer will degrade when the material is heated to a point where the bond breaks due to vibration energy. It is worthy to note that polymeric materials with cyclic repeating units can possess improved thermal stability and that rupture in one bond within a ring does not necessarily reduce the molecular weight of the polymer. In general, it is rare for two bonds to rupture simultaneously in a ring. Hence, the thermal stability of ladder or semiladder polymers tend to be higher than those of the open-chain ­polymers.[69] When a polymer is stable in air,

MATERIAL SELECTION The principle of material selection in the design of automobiles consists of a skillful balance between two important parameters, viz. cost and recyclability.[70] Cost is one of the most critical consumer-driven criteria in the automotive industry. Since the option of polymer composites may not be cost-effective, decisions to select these materials must be justified on the basis of improved specific required functionality.[71] On the other hand, the recycling of automotive plastics is not currently considered as a very critical factor in the selection process by the automakers. However, it is an important factor desirable by many consumers due to environmental impact considerations.[70] TECHNOLOGIES OF POLYMERS FOR AUTOMOBILES There are various feasible production techniques employed to manufacture composite polymers for automobile parts. They include injection molding, compression molding, filament winding, hand layup, prepreg molding, raising transfer molding, and pultrusion.[72] In general, manufacturing techniques for automotive polymer composites can be grouped into two major categories, as shown in Fig. 4, open molding and closed molding.[1] Open Molding Techniques In open molding process, the laminate and gel coat are exposed to the atmosphere during production owing to the application of a one-sided mold and the parts are produced Hand lay-up Open mould

Spray-up Filament winding Compression moulding Pultrusion

Closed mould

Reinforced reaction injection moulding Resin transfer moulding

Fig. 4  Overview of polymer manufacturing techniques

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On the other hand, it enhances the impact resistance and elongation. In order to reduce absorption of moisture, it is recommended that finished products should be designed such that excessive absorption is prevented or plastics exhibiting low absorption rate should be selected.[66]

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starting from the exterior to the interior of the ­components, whereas in closed molding technique, the polymer composite is produced either within a vacuum bag or in a two-sided mold set.[1] The most common and widely used techniques to produce FRP are the open molding and hand layup.[1] Alginate–Aviation

Hand Layup Hand layup is the most basic process to manufacture FRP. Typically, the technique consists of manually laying piles of dry fabric or layers prepreg, onto an open mold to form a complex shape with desired thickness of laminate stack.[73] Resin is introduced into the dry plies upon the completion of layup (i.e., through resin infusion). In wet layup, a variant of hand layup, resin is used to coat each ply and compacted or reduced after it is laid.[74] Spray-up Spray-up process is one of the manufacturing techniques for FRP.[75] In this process, air-driven chopper unit incorporated on the resin spray gone chops the continuous strands of reinforcement into short lengths and spitting out into the resin. The mixture of resin and chopped glass fibers is then sprayed to produce the component shape. The resin stream conveys the chopped fibers to the mold such that the resin and the fibers are dispensed and simultaneously positioned.[76] This technique is economical in producing certain fiber glass components due to its high rate of ­material application.[75] Filament Winding Filament winding consists of winding continuous fibers (reinforcements) onto a rotating mandrel in a predetermined pattern by using specially designed mechanisms in order to produce a closed-form hollow component.[77] The components can be cured either at room or high temperatures, and the final geometric shape is obtained after the extraction of the mandrel.[78]

Automotive Applications: Polymers in

or powder form is placed in the mold under pressure and heated by either electric heating coils or by steam.[80] The material may be reinforced by adding a binding agent.[80] Compression molding technique has largely been replaced by injection molding process for some polymers, due to the inherent benefits of automation and materials handling.[81] On the other hand, compression molding process is advantageous in processing reinforced polymers.[82] In addition, damage to the reinforced fibers is prevented due to the reduced stress and subsequent deformation level involved in the process. Thus, facilitating the incorporation of longer and high-concentration fiber into the compression-molded polymers.[82] Compression molding technique exhibits some similarities with sheet metal stamping.[82] Pultrusion Pultrusion is a continuous manufacturing technique for uniform cross-sectional composite materials.[83] It consists of pulling slit fabrics and fiber bundles through a wet resin bath before forming into a rough part shape.[84] It has found a wide application in thermoplastic matrices, which include PET and polybutylene terephthalate by wrapping it with the sheet of thermoplastic matrix or impregnating the glass fiber with the powder.[85] Reinforced Reaction Injection Molding Reaction injection molding (RIM) is a class of injection molding where only thermosetting polymers are employed; consequently, curing reaction of the compound is required within the mold. Examples of automobile components produced using this process include fenders, bumpers, and air spoilers.[85] The process is termed as reinforced reaction injection molding (RRIM) upon the addition of reinforcing agents such as mica and glass fibers.[85] A typical application of the RRIM is in the production of rigid foam automobile panels.[85] In addition, a variation of the RIM process is known as the structural reaction injection molding, where the reinforcing agents are fiber meshes, which are arranged in the mold prior to the injection of polymer mixture over the reinforcing mesh.[85]

Closed Molding Techniques Resin Transfer Molding The advantages of closed molding technique include the following: automation of parts with fewer molds, higher throughputs, and consistency in the production of superior parts, lower scrap rate, reduction of labor costs and offers new strategies for emission standards compliant.[1] Compression Molding The oldest manufacturing process of polymeric materials is the compression molding technique and is mainly employed for thermosets.[79,80] In this technique, the compound (thermosetting plastic) that may either be in a preformed tablet

Resin transfer molding (RTM) is an alternative technique to open molding. The process consists of placing preformed fibers in a closed mold before the addition of resin into the mold. The fiber reinforcement is loaded in a pair of matched tools, prior to the introduction of catalyzed polymers into the tool in order to impregnate the resin system with the reinforcement.[86] After curing of the polymer, the component is removed upon opening the tool. Finishing of the component can then be performed after molding. The technique is widely used to produce automobile doors, drive shafts, body panel (exterior), commercial

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Mechanical process

Crushing & grinding

Pyrolysis Chemical process

Fluidized bed pyrolysis

SHELF LIFE The life span of an automobile varies from country to country. However, on the average, it is estimated that an automobile life span is about 12 years.[17] In general, products with a useful or shelf life of 3 years or more are referred to as durable materials, while those with shelf life of less than 3 years are generally regarded as nondurable products.[88] The shelf life of a polymeric material is highly influenced by the type of polymer, the actual compound formulation, [89,90] and the physical properties expected from that compound after a specified number of years.[91] For example, the PVC used for packaging may have a short useful life measured in weeks or even in days, and those used for floor tiles may have a moderate useful life span of between 9 and 10 years, [92] while some buried PVC pipes exhumed after 60 years of active service were found to be fit for purpose with the possibility of another life expectancy of 50 years.[93] On the other hand, thermoplastic ­prepreg has almost an unlimited shelf life.[94] In general, the shelf life could vary dramatically depending on the environmental storage conditions.[90] Optimum environmental conditions for polymer storage include shielding from ozone, light, and humidity at a temperature lower than 32°C.[90] Other environmental factors that may also affect the shelf life during storage include deformation, and contact with foreign materials such as semisolid, liquids, and metallic and nonmetallic materials.[91] It is significant to note that the shelf life only concerns the functional application of the compound over a given period of time when properly stored. However, shelf life is not related to how the product may or may not perform in any given application.[91] RECYCLING ISSUES OF POLYMERS Modern-day automobiles consist of about 39 different types of polymers and plastics of which PVC makes up 16%, PU (17%), and PP (32%), translating to a total of 66%. The wide variety of plastics used in automobiles presents a great challenge to recycle.[95] Environmental concerns in terms of limited natural resources and waste disposal challenges are driving the increasing ­pressure to recycle m ­ aterials at the expiration of their useful

Micro-waves assisted pyrolysis

Fig. 5  General classifications of recycling processes

life.[96] However, there are some drawbacks in the recycling of polymer composites; this is due to challenges in separating different constituent of the polymers.[86] Various recycling techniques have been investigated within the last two decades, which include mechanical processes (mainly grinding), [97–99] thermal processes, [97–99] pyrolysis, [97–99] and solvolysis.[97–99] In general, recycling processes can be ­classified into two, as shown in Fig. 5. Mechanical Recycling Processes The mechanical process of recycling involves shredding or crushing of grinding of the polymeric materials followed by grinding into very fine smaller pieces.[100] It involves size reduction of the polymeric scrap materials using a slow speed cutting or crushing mill to about 100 mm in size.[96] This process is commonly applied to glass fiber-­reinforced composites (GFRCs), [101,102] particularly in bulk molding compound and sheet molding compound (SMC).[96] Ground composite materials can be used either as (i) reinforcement or (ii) filler. However, it is not commercially viable to use them as fillers because virgin fillers such as silica or calcium carbonate are relatively cheaper.[96] Polyether-ether-ketone (PEEK) resin reinforced with carbon fibers was investigated by Schinner et al., [103] observed the length distribution of fibers was more homogeneous and longer when cutting mills were used compared to when hammer mills were employed. They reported a successfully incorporation of the ground materials into a virgin PEEK resin which was molded by injection or press up to 50 wt%. However, they reported higher wear rate of the cutting blades wore faster. The authors also performed a direct reforming process without grinding although not on end-of-life materials. They reported that it was more difficult to reuse the fibrous fractions of the ground thermoset materials and that the resulting mechanical properties were poor which they attributed to the lack of good bonding between the new resin and the recyclates. On the other hand, Palmer et al.[102] observed that the mixing time can

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van roof, etc. The benefits of RTM include tight tolerances, low ­volatile emissions, versatility with different reinforcements, etc.[86] Harrison et al. of Ford Motors Company (UK) produced elevated roof using RTM for the Ford Transit van. The process used involves cutting of sandwich glass fiber mat reinforcements prior to placing them as preforms in a closed mold, and using a meticulously gated injection system, the sandwich mat reinforcement was impregnated using a ­pigmented polymer resin.[87]

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provide a positive influence on the mechanical ­properties of the resulting materials when incorporating fibrous fractions (from glass fiber SMC) identical to virgin fiber bundles at about 10 wt% in Dough molding compound. They reported enhanced mechanical properties when a longer mixing time of the paste with the recyclate was used compared to a standard mixing time. In addition, they argued that it was possible to achieve good mechanical properties comparable to the standard material due to an improved interface between the new resin and the recyclate. Thermal Processes: Pyrolysis, Fluidized Bed Pyrolysis, and Microwave-Assisted Pyrolysis Thermal processes consist of pyrolysis, fluidizedbed pyrolysis, and pyrolysis assisted with microwaves, [104–107] and operate between the temperature range of 450°C–700°C and even higher depending on the type of resin. Polyester resins are processed at lower temperatures while thermoplastics or epoxides such as PEEK require higher temperatures.[108] These processes afford the possibility of fibers recovery, including the fillers and inserts. However, the recovery of reusable monomers, which are valuable resin products, is not always possible.[108] Pyrolysis Pyrolysis, also known as thermolysis, is the thermal process of chemical decomposition of organic materials, such as biomass, in the absence of oxygen [109] and recently with the addition of steam.[109,110] The end products of the matrix degradation of the organic materials include gases, oil, and solid materials, which include fibers and consequently fillers and char.[108] The recovered fillers are heat treated in a furnace at 450°C in order to burn off the contaminant, i.e., the char from the GFRC, which consequently results in an extensive degradation of the fibers. This process has found industrial applications and has been enhanced for the ­recycling of carbon fiber-reinforced matrices.[108] The Fluidized Bed Process The fluidized bed process consists of polymer matrix thermal decomposition, which is followed by the release and collection of filler particles and glass fibers. Char formation is reduced by incorporating oxygen.[111] The advantage of this process is that it can be used to treat contaminated and mixed materials, with foam cores or painted surfaces in composites of sandwich construction or metal inserts.[96] The fluidized bed process is suitable for end-of-life wastes, and it is also a promising technique for future in the reclamation and for end-of-life wastes, especially carbon fibers.[96] However, a drawback with this process is that it hampers products recovery from the resin apart from gases. On the other hand, pyrolysis affords the recovery of oil which may contain valuable products.[96] In addition, the fluidized bed process appears

Automotive Applications: Polymers in

to have a more ­detrimental effect on the carbon fibers compared to pyrolysis. Also there are possibilities of damage to the fibers due to the attrition by the fluidized sand.[96] Microwave-Assisted Pyrolysis Microwave-assisted pyrolysis is a heating system that degrades composite matrix into oil and gases in an inert atmosphere.[108] Kritzer[112] was the pioneer workers to apply this process to recycle the composite. Industrially, this process has found a wide range of materials application which includes plastics and waste tires.[113] A major advantage of microwaveassisted pyrolysis is rapid heating and high heating efficiency resulting from the internal generation of heat within the material rather than from an external source as obtained in conventional methods.[114,115] Solvolysis Solvolysis involves degradation of resin by using a chemical treatment. The technique has found a major application in the processing of UP and SMCs as UP is one of the most widely used thermoset resins and in particular in SMCs.[108] Solvolysis provides a numerous possibilities due to a wide range of operating temperature, pressure, solvents, and catalysts. It is more advantageous over pyrolysis as degradation of polymers is possible at lower temperatures, especially with UP and epoxides. However, due to high pressures and temperatures at supercritical conditions of water, suitable reactors can become very expensive. In addition, corrosion is another drawback that can be encountered due to modifications in solvent properties.[112] CONCLUSION Plastics and polymer composite materials have been used in automobiles by the automakers for several decades due of their lightweight, reduced lead times, and lower investment costs when compared to conventional steel materials. With the advantages that conform directly to the automakers needs, polymer composites and plastics can be a major part of the solution for the automobile industry. The growth of plastics and polymer composites has been driven by the aforementioned factors and the consolidation of parts offered by the materials, as well as improved mechanical properties, good corrosion resistance, design flexibility, and material anisotropy. However, despite these numerous advantages, extensive application of polymer composite in automobiles has been hindered by high material costs (up to 10 times higher when using carbon fibers), slow lead time, and to a lesser extent, recycling challenges. Thus, the major targets for future development must include the prudent applications of hybrid composites, i.e., using ­low-cost fibers where possible and applications of carbon fibers and aramid only, were critical, for crashworthiness and stiffness

purposes) and the evaluation of high-­throughput techniques and automated processes, including the application of artificial intelligence. To fully optimize this opportunity, several challenges militating against their full potentials need to be addressed, i.e., high cost of carbon fiber-reinforced composites discourages their extensive applications in automobiles, and recycling certain polymer composites and plastic can be very challenging. In addition, original equipment manufacturers should intensify research on modeling part designs and material selection in order to ensure that a material has the necessary ­performance capabilities.

ACKNOWLEDGMENTS The financial assistances of the National Research Foundation (NRF) and Tshwane University of Technology, South Africa, are hereby acknowledged. REFERENCES 1. SusChem. Polymer composites for automotive sustainability, 2015. Available at www.suschem.org/cust/­ documentrequest.aspx?DocID=998, (accessed in August 2016). 2. Pai, A.R.; Jagtap, R.N. Surface morphology & mechanical properties of some unique natural fiber reinforced polymer compositesA review J. Mater. Environ. Sci. 2015, 6, 902–917. 3. Klein, R. Material Properties of Plastics in Laser Welding of Plastics, 1st Ed.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2011. Available at https://application. wiley-vch.de/books/sample/3527409726_c01.pdf. 4. Batzer, C. Polymere Werkstoffe Bd. 1, Chemie Und Physik; G. Thieme Verlag, Stuttgart, 1985. 5. Richeton, J. Modeling and validation of the finite strain response of amorphous polymers for a wide range of temperature and strain rate. PhD. Dissertation, Université Louis Pasteur, Stuttgart, 2005. 6. Halary, J.L.; Laupretre, F.; Monnerie, L. Polymer Materials Macroscopic Properties and Molecular Interpretations; John Wiley & Sons: New York, 2011. 7. Frank, A.; Biederbick, K. Kunststoff-Kompendium; Vogel-Buchverlag: Wurzburg, 1984. 8. Klein, R. Material Properties of Plastics Laser Welding of Plastics: Materials, Processes and Industrial Applications; Wiley-VCH: New York, 2011, 3–69. 9. Pilato, L.; Michno, M.J. Advanced Composite Materials; Springer-Verlag: New York, 1994. 10. Chawla, K.K.; Xu, Z.R.; Ha, J.S.; Schmücker, M.; Schneide, H. Effect of BN coating on the strength of a mullite type fiber. Appl. Compos. Mater. 1997, 4, 263–272. 11. PlasticEurope. Automotive—The world moves with plastics, 2013. Available at www.plasticseurope.org, (accessed in August 2016). 12. George, J.; Sreekala, M.S.; Thomas, S. A review on i­ nterface modification and characterization of natural fibre reinforced plastic composites. Poly. Eng. Sci. 2001, 41, 1471–1485.

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Automotive Applications: Reinforced Material Components Bheemappa Suresha Department of Mechanical Engineering, The National Institute of Engineering, Mysuru, India

Rajashekaraiah Hemanth Alginate–Aviation

Department of Mechanical Engineering, NIE Institute of Technology, Mysuru, India

Kundachira Subramani Nithin Department of Chemistry, The National Institute of Engineering, Mysuru, India

Abstract The shift to more balanced structures in automotive industry is not only an ingenuity toward a more feasible environment and cost efficiency but also a mandate of policies. They also play an imperative role to forcibly use sustainable materials, i.e., composites. The environmental studies have revealed that the composite in automotive industry has recently attracted attention in terms of volatile organic compound emissions, carbon footprint, and recyclability. As a result, it has become desirable to develop end-of-life strategies that avoid landfill. This has set the automotive industries to design the products, which requires 95% of the mass of each product manufactured to be recycled/recovered. Polymer matrix composites (PMCs), especially thermoplastic-based polymer composites, offer these possibilities. Alternative approach to balance sustainability and cost is with the use of PMCs in automobile panels, as introduced by a number of automobile manufacturers, which use renewable materials as reinforcements in various polymers. Composites made of renewable materials have been rampantly used in inner and peripheral body parts. Similar components are used as trim parts in dashboards, door panels, parcel shelves, seat cushions, backrests, and cabin linings. In recent years, there has been increasing interest in the replacement of fiberglass in PMCs by natural plant fibers such as jute, flax, hemp, sisal, ramie, and so on. This entry tries to impart knowledge on the structural, and tribological performances, and applications of various polymers and polymer composites reinforced with fibers, fillers, and the combination of both in the area of automobile engineering. Keywords: Polymers and their composites; Processing techniques; Structural applications; Tribological applications.

INTRODUCTION TO AUTOMOBILE Mobility has always played a crucial role in the course of human development. In almost every era, man has attempted to find the means to allow him to transport people over long distances at the highest possible speed. It took the development of reliable internal combustion engines that were operated on liquid fuels to turn the vision of a self-propelling “automobile” into reality (combination of Greek: autos = self and Latin: mobilis = mobile). [1] The alternate name “Car” is believed to originate from the Latin word Carrus or Carrum (wheeled vehicle), or the middle English Carre (cart). An automobile is a self-propelled vehicle, driven by an internal combustion engine/hybrid engine, and it is used for the transportation of passengers and goods on the ground. Automobile engineering is the one of the streams of mechanical engineering, which deals with everything

Encyclopedia of Polymer Applications, First Edition DOI: 10.1201/9781351019422-140000314 Copyright © 2018 by Taylor & Francis. All rights reserved.

about automobiles and practices (designing, manufacturing, and servicing) to propel them economically and eco-friendly. Further, automotive engineering draws on almost all areas of engineering: thermodynamics and combustion, fluid mechanics and heat transfer, mechanics, stress ­analysis, materials science, electronics and controls, dynamics, vibrations, machine design, linkages, and so forth. However, automobiles are also subject to commercial ­considerations, such as economics, marketing, [2] ­automobile insurance, sales and services, etc. MAJOR CLASSIFICATION OF AUTOMOBILES To understand the concepts of automobile clearly, they are classified into several types based on the various criteria. A brief classification of automobiles is listed in Table 1. [3]

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Table 1  Major classification of automobiles [3] Type

Description

Examples

1. Based on purpose

Alginate–Aviation

Passenger vehicles

These automobiles carry passengers

Buses, passenger trains, cars, etc.

Goods vehicles

These vehicles are used for transportation of goods from one place to another

Goods lorry, goods carrier, etc.

Special purpose vehicle

Built for a purpose other than carrying passengers and goods

Mobile cranes, concrete pump, drill rig, and fire truck

Heavy motor vehicle

Large and bulky motor vehicles

Large trucks, buses, etc.

Light motor vehicle

Small motor vehicles

Cars, jeeps, etc.

Medium vehicle

Relatively medium-sized vehicles

Small trucks, mini buses, etc.

2. Based on capacity

3. Based on fuel source Petrol engine vehicles

Powered by spark ignition petrol engine

Scooters, cars, mopeds, motorcycles, etc.

Diesel engine vehicles

Powered by compression ignition (CI) diesel engine

Trucks, buses, and cars

Gas engine vehicles

LPG (liquefied petroleum gas) and CNG (compressed natural gas) vehicles

Buses, cars, bikes, etc.

Electric vehicles

Using electricity as a power source without IC engine Electric cars, electric buses, and bikes

Hybrid vehicles

Using two or more distinct power sources, e.g., petrol Hybrid buses and hybrid cars with LPG

Hybrid electric vehicle

Using both IC engine and electric power source

Hybrid buses, cars, bikes

Solar vehicles

Using solar energy as a power source

Solar powered cars, bikes, etc.

Hydrogen vehicles

Using hydrogen as a power source

Honda FCX clarity

Steam engine vehicles

Powered by steam engine

Steamboat, steam locomotive, and steam wagon

Conventional transmission vehicles

Automotive whose gear ratios have to be changed manually

Transmission with sliding mesh, constant mesh, synchromesh, and gear boxes

Semiautomatic transmission vehicles

Vehicles that facilitate manual gear changing with clutch pedal

Clutch less manual transmission

Automatic transmission vehicles

Automobiles, capable of changing gear ratios automatically as they move

Automatic transmission cars using epicycle gear trains

Two wheeler

Having two wheels

Scooters, mopeds, and motorcycles

Three wheeler

Having three wheels

Tricycles, auto-rickshaws, tempos

Four wheeler

Having four wheels

Car and jeep

Six wheeler

Having six wheels

Large trucks and large buses

Sedan with two doors

Cars that are almost always fully closed

Cars

Sedan with four doors

Cars that are almost always fully closed

Cars

Station wagon

Roof extended rearward

Estate car and estate wagon

Convertible

Cars/jeeps with soft, foldable/open top and rollup windows

Cars, jeep, etc.

Van

Used for transporting goods or people with maximum space considerations

Ambulance, school van, milk van, etc.

Front engine

Vehicles with front mounted engine

Cars, auto-rickshaws, and buses

Rear engine

Vehicles with rear mounted engine

Cars, auto rickshaws, and buses (Continued)

4. Based on type of transmission

5. Based on number of wheels

6. Based on basis of body

7. Based on position of engine

Automotive Applications: Reinforced Material Components 181

Table 1  Major classification of automobiles [3] (Continued) Type Description

Examples

8. Based on steering side Vehicle in which steering wheel is fitted on the lefthand side

Automobiles found in the US and Russia

Right-hand drive

Vehicle in which steering wheel is fitted on the right- Automobiles found in India and hand side Australia

9. Based on type of suspension Conventional

Leaf springs and shock absorbers

Trucks and buses

Independent

Coil spring, torsion bar, and pneumatic

Cars and jeeps

GENERAL STRUCTURE OF AUTOMOBILES The primary purpose of the structure is to preserve the shape and to aid the numerous loads applied to an automobile. It is essential that the best structure is chosen to ensure acceptable performance within other design constraints such as production volume, cost, and method of p­ roduction. Few of the automobile structures are discussed here. The Underfloor Chassis Frame The underfloor chassis frame, which was regarded as the structure of the car, consisted of a more or less flat “ladder frame.” The two side frames running the full length of the vehicle connected together by cross-members running laterally and riveted to the side frames at 90° joints. These frames were also called as “grillages” (structure subjected to loads normal to its plane). It was perhaps fortunate that car bodies were “coach-built” by carpenters, out of timber resulting in very low stiffness, as the earlier chassis frame demonstrated poor torsion performance. In the 1920s, the cars had open bodies which are inherently flexible and assumed that the body carried only selfweight, passengers and payload, not for road loads. Metal clad bodies with a roof led to problems of “rattling” and “squeaking” between the chassis and the body resulting in cracking at various points within the body. The root of these problems is body-on-chassis arrangement, i.e., two structures acting as torsion springs in parallel. In the mid-1920s, bodies were made by flexible metal joints, and the use of flexible materials (fabric) for the outer cover of the body came into subsistence. The subfloor chassis frame with open section members riveted together was regarded as the structure of the vehicle in the 1930s. Further, the requirement of high volume of production led to the extensive use of pressed steel sheets welded or riveted together resulting in greater stiffness body. However, the configuration still remained as the “body-on-separate chassis” resembling springs in parallel, leading to problems of fighting between the body and the chassis frame. This has led to the use of cruciform bracing, made of open-channel section members and the ends are well connected to the chassis members. By the mid-1930s, the chassis frame and

the body made of steel were integrated by a large number of fasteners, which eventually led to the evolution of “integral body.” The stiffness of the chassis assembly improved with the use of “twin tube” or “multi tube” frame, basically a ladder-type grillage frame, with side members connected by lateral cross-members. They possess better stiffness compared to open section members. Further the members were welded together resulting in higher stiffness than the riveted connections of the earlier frames. This kind of multi tube frame was used in specialist racing vehicles between the late 1930s and 1950s such as Jaguar and Ferrari. However, in recent times, the closed tube structure has been used to effect by using larger section, but thinner walled members. The “Backbone” chassis structure based on the concept of “large section tube,” which still amounts to a “separate chassis frame” is used on sports cars, often their bodies made of glass fiber-reinforced plastic (GFRP). The combined stiffness of the chassis and the attached body together is greater than the sum of the stiffness of the individual elements. This reflects the connection between the chassis and the body is not just at the ends, but is made at many points, giving a combined structure which is highly statically indeterminate. Triangulated Tube Structure It is not limited to backbone structures. Stiffening of the edges, particularly at corners, enhances the torsional performance. This method is suited to low-volume production and not for mass production due to low tooling cost and complexity in manufacturing, respectively. Incorporation of Roll Cage into Structure The work of Roots et al.[4] revealed the incorporation of roll cage into the structure has enhanced the torsional stiffness by over 500% as compared with the basic chassis frame. The contribution of the roll cage depends on the following: a. The degree of triangulation in the roll cage and b. On how well the roll cage was connected to the rest of the structure.

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Left-hand drive

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Pure Monocoque

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“Monocoque” a French word meaning “single shell.” The outer skin performs the dual role of the body surface and structure, resulting in the reduction of weight. Pure monocoque structures are rarely used and restricted to racing cars. This is due to the provision provided for openings to passenger entry, visibility, etc., resulting in lowering of torsion stiffness. Hence, these structures require reinforcement to prevent buckling and to carry loads. Formula 1 ­racing car monocoques are made of carbon fiber composites. Punt or Platform Structure This is one of the modern structures, usually made of sheet metal with good joints between the structural members. They possess high torsion and bending properties. It is often used for low-production volume vehicles, for which different body styles or rapid model changes are required. This approach is used to create convertible version of mass produced integral sedan car structures. Perimeter Space Frame or “Birdcage” Frame Another contemporary structure is the “birdcage,” in which relatively small section members are built into stiff jointed “ring beam” bays welded together at joints. The edge members of each ring frame, specially the corners, must be stiff locally in bending. This construction method is dictated by production requirements. The beam structures must be assembled into the structural concept using welded joints. Integral or Unitary Body Structure This is the most widely used modern structure, and it is the “integral” (or “unitary”), spot welded, pressed steel sheet metal body, suited to mass production. The body is

self-supporting so that the separate “chassis” is omitted with a saving in weight. The integral body is a blend of the monocoque and the birdcage. Panels and body components of integral structure are stamped from steel sheet and secured together by spot welding, laser welding, or other methods used for particular locations. UltraLight Steel Auto Body It is a modernized version of the structures for near-future developments. Ultra-Light Steel Auto Body is made by hydroforming (manufacturing of tubular cross sections by internal hydraulic pressure into molds). Laser-welded blanks are also used widely. Laser welding, which is stiffer than spot welding, is used extensively to join the panels together. The result is a structure that was recorded to be lighter and stiffer than the “traditional” integral steel bodies. In recent developments, the competing materials such as aluminum and polymer composites have been used in the integral body, resulting in the ­reduction of weight and appraise in overall performance. The structure of an automobile comprises body and chassis. The automobile chassis houses various subsystems. The various chassis and subsystems of automobiles are discussed in the following section. Chassis A vehicle without body is called as chassis. It carries all other systems, holding all components together while driving. The main functions of chassis are to carry load, to facilitate all other systems on the structure, and to withstand road shocks and absorb engine and driveline torque. Basic layout of automotive chassis is shown in Fig. 1, containing the power plant (Engine), frame, driveline, braking, and the steering ­suspension systems along with electrical and electronic systems. The structural members of chassis are made of box-, tubular-, and channel-shaped longitudinal and

Steering

Engine

Suspension

Wheel and type

Axe

Shaft

Clutch

Gear box Final drive

Fig. 1  Typical layout of chassis of an automobile

cross-members that are welded or riveted together. Primarily, the structure supports the body. Heavy vehicles are made of conventional type of chassis, and cars are made of semi-integral and integral type of chassis. Automotive Body Another essential element of an automotive is the “body.” It unites all the different elements; it houses the various mechanisms, which help in protecting and transporting of passengers and freights. The body should be rigid enough to support weight and stress, and to secure various other components. Furthermore, it must repel and mitigate the impact of a crash to safely protect the passengers. In addition, it needs to be as light as possible to optimize fuel consumption and p­ erformance. Over the years, various body designs have been used. Principal Subsystems of an Automotive The principal subsystems of an automotive are presented in Table 2. These subsystems will be discussed further in the next section.

are integrated as auxiliary systems for an engine. Air–fuel mixture is inducted inside the combustion chamber through inlet manifold and is fired in each cylinder in accordance with the firing order. The resulting expanding gases push on pistons and connecting rods which are on crank, making crankshaft to rotate, and the burnt gases are expelled through exhaust manifold. The pulses of power from each piston are smoothed out by a heavy flywheel. Power leaves the engine through the flywheel, which is ­fitted on the end of the crankshaft, and passes to the clutch.[5] Driving System The power developed by the engine is transferred to the wheels by transmission system followed by driveline, differential, and rear axle. Transmission system makes use of clutch mechanism to engage and disengage drive during shifting of gears and starting from rest. Further, various gear ratios are obtained by the arrangement of gears in the gear box to vary speed and torque depending on road, load, and speed conditions of the vehicle. Finally, the differential allows the drive shafts to rotate at different speeds and hence the wheels to rotate at different speeds when the vehicle is cornering.

Engine Control and Suspension Systems The engine is power plant of the vehicle, and the power required for the propulsion is produced by the burning of fuel inside the internal combustion chamber with petrol or diesel as fuel. An engine may be either a two-stroke engine or a four-stroke engine. An engine consists of a major components such as cylinder, piston, valves, valve operating mechanism along with accessories like carburetor (or Multi Point Fuel Injection (MPFI) in modern cars)/fuel injection pump, fan, fuel feed pump, and oil pump. Major systems such as cranking, ignition cooling, and exhaust systems

Control system consists of the braking and steering systems which are integrated with suspension system as well as the wheels and tires. Fluid power is used to activate the brakes to slow down or stop the vehicle. Rotating discs are  gripped between pads of friction lining. The hand brake uses a mechanical linkage to operate parking brakes. Both front wheels are linked mechanically and must turn together to provide steering control. The most common method is to use a rack and pinion. The steering wheel

Table 2  The principal subsystems of an automotive

Power system (engine)

Driving system

Control system

Subsystems Frames and suspension system

Electrical and electronics system

Instrumentation and accessories

Examples Mechanical linkages and components

Clutch

Steering system

Frames with structural members

Charging system

Visual display on dashboard, sensors, GPS

Fuel system

Transmission (gearbox)

Braking system

Springs, shock absorbers

Cranking system

Signaling system

Lubricating system

Drive line

Stabilizer bar

Wheels and tires

Ignition system

Wind screen wipers and mirrors

Cooling system

Differential

Lighting system

Central locking system

Exhaust system

Rear axle

Audio/video system

Air conditioning and ventilation

Equipment for engine management

Seat heating system

Horn system

Safety devices

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is linked to the pinion and as this is turned it moves the rack to and fro, which in turn moves the wheels. Tires also absorb some road shock and play a very important part in road holding. Most of the remaining shocks and vibrations are absorbed by springs in the drivers and passenger seats. The springs can be coil type and are used in conjunction with a damper to stop them oscillating.[5] Electrical and Electronics Systems with Instrumentation and Accessories The electrical system covers many aspects such as lighting, wipers, and instrumentation. A key component is the alternator, which is driven by the engine, produces electricity to run the electrical systems and charge the battery. A starter motor takes energy from the battery to crank over and start the engine. Electrical components are controlled by a range of switches. Electronic systems use sensors to sense conditions and actuators to control a variety of things—in fact, on modern vehicles, almost everything.[5] Today, automobiles play an unimaginable role in the social, economic, and industrial growth of any country. Today’s economies are dramatically changing, triggered by the development in emerging markets, the accelerated rise of new technologies, sustainability policies, and changing consumer preferences around ownership. Digitization and new business models have revolutionized other industries, and automotive will be no exception. The future of the automotive industry presents many challenges but also many new opportunities.[6] To cope with the new challenges and to explore opportunities for welfare of society, automobile concepts are needed to be studied thoroughly from the basics to redefine it in terms of materials, methods, and technology with respect to time. In this context, an effort is made to refer some of the major emerging polymers with a variety of reinforcing materials for automobile components. INTRODUCTION TO POLYMER SCIENCE AND TECHNOLOGY PERTAINING TO AUTOMOBILE Polymers are produced by the chemical reaction of single molecules (monomer) such as ethane, propylene, and fluorine which are basically made of carbon, hydrogen, and chlorine. Naturally occurring polymers such as wood, rubber, cotton, wool, and silk are being used since from thousands of years. Artificial polymers such as polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), and urea formaldehyde (UF) are, indeed, relatively recent and mostly date from after World War II. In many instances, the artificial polymers are both better and cheaper than the natural alternatives. Polymers are also termed as plastics. In general, these polymers are light in weight, resistance to corrosion, with varied mechanical, electrical, thermal, and tribological properties. No single polymer alone can demonstrate better performance in the said

Automotive Applications: Reinforced Material Components

area of engineering to put them to real-time applications. Hence, they are modified with suitable polymer–polymer ­addition, reinforcement materials, and various types of fillers to tailor the material to suit the desired applications. Such modified polymers are termed as polymer blend or polymer alloy or polymer-based composites or polymer matrix composites (PMCs). Basically, composites are materials consisting of two or more chemically distinct constituents, on a macroscale, having a distinct interface separating them. “One or more discontinuous phases are, therefore, embedded in a continuous phase to form a composite” was the definition given by Agarwal and Broutman.[7] The intermittent phase is usually harder and stronger and is called the reinforcement, whereas the interminable phase is termed as the matrix. The matrix ­material can be metallic, polymeric, or can even be ceramic. When the matrix is a polymer, the composite is called polymer matrix ­composite (PMC). Gramman and Krapp [8] in their work revealed that the composite industries have recently attracted attention in terms of volatile organic compounds emissions, carbon footprint, and recyclability. As a result, it has become desirable to develop end-of-life strategies that avoid landfill. This has set the industries to design the products, which require 95% of the mass of each product manufactured to be recycled/ recovered. It was noted by Gibson and his coworkers[9–11] that the thermoplastic composites offer these possibilities. Further, rises in oil prices due to increasing demand, prominence of global warming, environmental concern and technological advancements to improve the automotive efficiency are becoming increasingly important to competitiveness in the global automotive market. One way for improving the automotive efficiency is the reduction of vehicle mass. This not only enhances the fuel efficiency but also lowers automotive emissions and improves driving performance. The two main approaches for reducing weight in the automotive are to make architectural alternates and material substitution. Among architectural alternates, the unibody is most mass efficient and is already prevalent. Hence, the mechanism available for reducing the weight of the automotive is the use of alternative sustainable materials. The applications of polymers in automotive manufacturing began in 1950, when thermoplastics made their debut, starting with acrylonitrile–butadiene–styrene (ABS) and other polymers, together with the introduction of their alloys/blends and their composites. General Motors (GM) introduced the Chevrolet Corvette in 1953. It was designed by chief stylist Harley Earl, who was fascinated with the use of GFRP as a body material. A total of 300 Corvettes were produced in the first year of production, each containing 41 glass fiber-­reinforced unsaturated polyester body parts. The 1953 Corvette was available only with a white body and red interior, and sold for $3,498.00. While the Corvette has changed dramatically over its 60-year history, one thing that has not changed is the use of the GFRP body.

Working independently, both Hermann Schnell of Bayer A.G. in Germany and Daniel Fox of the General Electric Company in the US discovered polycarbonate (PC) in 1953. This optically transparent engineering thermoplastic offers a great balance of stiffness and toughness, heat resistance, and electrical insulating properties. It is widely used for durable products such as automotive headlights, helmets, tool housings, and computer enclosures. Classification of Polymers and Polymer Composites Polymers can be classified on the basis of various aspects. Basically they are categorized into thermoplastics and thermosets depending upon their behavior in the vicinity of heat. Thermoplastics can be reformed/recycled upon application of heat and pressure, whereas the thermosets are cured in the vicinity of heat and pressure, but cannot be reformed/ recycled as in the case of thermoplastics. Examples for thermoplastics are PE, PP, PVC, polyamide (PA), polyurethane (PUR), ABS, PC, polystyrene (PS), urethane elastomers, etc.; however, epoxy resin, phenol f­ ormaldehyde (PF), UF, etc., are the examples for thermosets. Another significant classification is based on polymer performance and their service application. They are classified into commodity polymers, engineering polymers, and specialty/high-performance polymers. Commodity polymers are low-performance, high-volume plastics, characterized by heat deflection temperature (HDT) < 100°C and tensile strength (σ) < 50 MPa, e.g., PE, PP, PS, PVC, low-density polyethylene (LDPE), linear low-density ­polyethylene (LLDPE), and high-density polyethylene (HDPE). Engineering polymers have a great diversity of performance, and these are characterized by HDT > 100°C and σ > 40 MPa. They can be formed to precise and stable dimensions, e.g., PC, polyoxymethylene (POM), PA-6, PA-66, polyethylene terephthalate (PET), and polybutylene terephthalate (PBT). Specialty/high-performance polymers are low-volume, high-performance, high-temperature, and high-cost polymers. It is characterized by high modulus, tensile and impact strength. Their continuous use temperature is >150°C, e.g., poly ether ether ketone (PEEK), poly phenylene sulfide (PPS), and poly ether sulfone (PES). Performances of Polymers and Polymer Composites The exact date of the usage of naturally available polymers has not been recorded; however, report reveals that in 1839 Charles Goodyear discovered the vulcanization process for natural rubber. He discovers that adding sulfur to natural rubber greatly enhances its elasticity and toughness. This is the stepping stone for the development of polymers. This development revolutionized the automotive ­manufacturing industries. The ongoing development of advanced, high-performance polymers and their composites has

spectacularly increased their usage. The polymers and their composites were specified for applications because they offered good mechanical properties combined with aesthetics, including the possibility of self-coloring (tailor ability to suit the desired performance). The application of polymeric and polymeric composite components in the automotive industry has been increasing over the decades. They are used mainly to make automotive more energy efficient by reducing weight, together with providing durability, corrosion resistance, toughness, design flexibility, resilience, and high performance at low cost. The automotive manufacturing industries use polymer composites and plastics in a wide range of products. They are the second most common class of automotive materials after ferrous metals, their alloys (cast iron, steel, nickel) and nonferrous metals (copper, aluminum, magnesium, zinc, titanium), and their alloys contribute to 65% by weight. The presence of polymers and their composites in a commercial vehicles is about 50% of all interior components, including safety subsystems, doors, and seat assemblies. Due to the rapid growth in the area of science and technology, the growth in use of polymers and their composites in automotive industries has the following advantages. The component mounting costs are being met by the ability of the polymers and their composites to be molded into complex geometries, often replacing several parts with other materials, and offering integral fitments that all add up to easier assembly resulting in reduced lead time. Further, the leading experts of automotive industries have estimated that every 10% reduction in vehicle weight results in 5%–7% fuel saving. Thus for every kilogram of vehicle weight reduction, there is the potential to reduce carbon dioxide emissions by 20 kg. The incorporation of the lightweight materials in automobile is a necessity. Selection of Polymer and Their Composites for Automobiles Many types of polymers and their composites are used in the manufacturing of thousands of parts with varied shapes and sizes of an automotive. These are subjected to various parameters such as type of application, working environment, and safety issues during in and off service. Further, the properties of the materials also play a vital role in choosing the material for specific applications. Hence, it is the responsibility of the product design engineer to select the proper materials for manufacturing the components to sustain for the desired period with low cost. An instant gaze inside any automobile shows that polymers and their composites are now used for manufacturing the components such as bumpers, doors, safety and windows, headlight, side view mirror housings, hoods, grilles, and wheel covers. Few of the important polymers and their composites have been discussed, which are as follows: PP—It is made from propylene gas with least specific gravity of 0.903. They are extremely chemical resistant and

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almost completely resistant to water. Black has the best UV resistance. Applications are as follows: automotive bumpers, mud flaps, dashboards, steering wheel covers, ­integral hinges, cable insulation, battery boxes, petrol cans. PUR—This is widely used in high resilience, flexible foam seating, suspension bushings, microcellular foam seals and gaskets, rigid insulation panels, elastomeric wheels and tires, and cushions. PVC—This is prepared from vinyl chloride monomer with a specific gravity ranging from 1.18 to 1.7. It is a good resistance to chemical and solvent attack. Applications are as follows: automobile instruments panels, sheathing of electrical cables, seat belts, arm rests, spark plug covers, seat covers, etc. ABS—It is made by the combination of acrylonitrile, butadiene, and styrene with specific gravity ranging from 1.03 to 1.06, and was introduced in the market in the late 1940s. It is a rigid polymer with rubber-like characteristics, which gives good impact resistance. Applications are as follows: car dashboard and covers. PA—It is also known as nylon and is available in variants such as nylon 6, nylon 66, nylon 610, nylon 612, nylon 11, and nylon 12. Good abrasion resistance, low friction characteristics, good chemical resistance, and high water absorption. Applications are as follows: gears, bushes, cams, bearings, carburetor floats, seat slides, wind shield wiper motor parts, mirror housings, window lift system, steering wheels, etc. PS—This is produced by ethylene and benzene, crystal clear, rigid and easily processed. Applications are as follows: automotive reflectors, doom lights, automotive panel covers, display signs, etc. PE—It has the simplest chemical structure with the ethylene as repeating unit. It is the largest produced polymer having different types such as LDPE, HDPE, LLDPE, high molecular weight high-density polyethylene (HMWHDPE), and ultra-high molecular weight high-density polyethylene (UHMWHDPE). Applications are as follows: bushes in dashboards in cars, glass fiber-reinforced car bodies, wiper strips, door handles, luggage stands, radiator closure seals, car door interiors, decorative covers on car wheels, pressure pipes, blow molded fuel tanks, scooter stand bushes, porous filters, pinion gears, battery separators, etc. PC—It has good weather and UV resistance, with transparency levels almost good as acrylic. Applications are as follows: security screens, bumpers, headlamp lenses, etc. POM—This is also known as polyacetal, better stiffness, rigidity and excellent yield, good chemical and fuel resistance. Applications are as follows: interior/exterior trims, small gears, etc. Poly(methyl methacrylate) (PMMA)—This is also known as acrylics, is more transparent than glass, and has reasonable tensile strength (shatter proof grades are available), good UV and weather resistance, high optical quality, and surface finish with a huge color range. Applications are as follows: windows, displays, screens, etc.

Automotive Applications: Reinforced Material Components

PBT—It has good chemical resistance and electrical properties, hard and tough material with water absorption, very good resistance to dynamic stress, thermal and dimension stability and easy to manufacture due to fast ­crystallization and fast cooling. Applications are as follows: fog lamp housings and bezels, sunroof front parts, locking system housings, door handles, bumpers, ­carburetor ­components, etc. PET—It has similar properties as PBT such as good thermal stability, good electrical properties, very low water absorption, and excellent surface properties. Applications are as follows: wiper arm and their gear housings, headlamp retainer, engine cover, connector housings, etc. Acrylonitrile styrene acrylate (ASA)—This has better toughness and rigidity, good chemical resistance, thermal stability, outstanding resistance to weather, aging and high gloss. Applications are as follows: housings, interior parts and outdoor applications. Phenol formaldehyde (PF) —They are thermosetting resins which are hard, stiff and brittle, good dimension ability, good retention of properties at elevated temperatures and easy mold ability with specific gravity of 1.40. Applications are as follows: disk brake pistons, brake liners, propellers, clutch disks, lamp sockets, ignition parts, solenoid covers, engine blocks, intake manifolds, etc. Polyester resins—They are first synthesized in 1936 and commercialized in 1941. They can be rigid, resilient, flexible, corrosion resistant, and weather resistant. Applications are as follows: automobile structural parts, auto body putty, automobile springs, truck cabs, automobile exterior parts, etc. Epoxy resins—They are first developed in 1930 in Germany and was commercialized in 1946. The term epoxy is a three-membered ring containing one oxygen and two carbon atoms. Their characteristics include good mechanical strength, dimensional stability, wear resistance, corrosion resistance, flame retardant, susceptible to UV radiation. Applications are as follows: automotive doors, seats, l­uggage carriers, partition walls, etc. Processing Technologies for Making Automobile Components The polymer- and polymer composites-based automotive components are manufactured by several processing techniques, which are discussed in brief in the following section. Generally, the polymer or the polymer composites are required to be preheated at the prescribed temperature and duration, according to the suppliers’ recommendation to eliminate the moisture content in the material, to obtain the optimum part performance. Injection Molding It is the most important and widely used technique involving the cyclic procedure of processing polymers and their

Automotive Applications: Reinforced Material Components 187

Blow Molding It is a very important processing procedure, meant for production of hollow articles or one-side open hollow bodies. In the first phase, the production of hollow bodies by blow molding a preform is produced by extrusion or injection molding. In the second phase, the workpiece is shaped. The blow molding process is classified according to the method of producing the preform—extrusion blow ­molding and injection blow molding. Extrusion blow molding is most often used for the production of components of many thermoplastics such as polyolefins (PP, PE, LDPE, HDPE), ABS, PVC, and PA. The products can be axis-symmetrical, but also of Movable platen

1 Cavities

3

Mold closing

4

Blowing

Fig. 3  The stages of extrusion blow molding process

irregular shape (e.g., fuel tanks in automobiles). Figure 3 demonstrates the extrusion blow molding process. In the first stage of the extrusion blow molding process, the preform is obtained from the extruder in the form of a flexible pipe (hose). In the second stage, a part of the pipe is then enveloped in a mold because of efficient heat supply. Then the mold is closed and in the process, one end of the pipe, usually the bottom, is squeezed by the mold and welded. The compressed air is then blown through the nozzle of the blow molding machine, which widens the pipe and pushes it to the mold wall. The same polymeric materials are used for injection blow molding process. Figure 4 depicts the stages involved in the injection blow molding process. Gas or air blow pin Injection molded preform, heated & inserted in blow mold

Polymer granules

Nozzle

Blow mold cavity

Barrel

Band heaters Reciprocating screw Fixed platen

Fig. 2  The injection molding process

Extruder nozzle

Extruded preform

Hopper Mold

2

Blow pin

Gas/air blown to preheated preform

Fig. 4  The injection blow molding process

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composites for manufacturing automotive components. Apart from the materials and skilled manpower, the other prerequisites for this process are the injection molding machine and an injection mold. The raw material (polymer/ polymer composites) in the form of granules or pellets is fed into an injection molding machine and heated to obtain molten form and forced through the injection nozzle of the machine into the metal mold to give the desired shape and size. The molten material flows through the feed system (sprue, runner, and gate, respectively) and enters the space called impression (formed by locking of core and cavity) as shown in Fig. 2. This molten material is allowed to cool. The cooled object is ejected out of the mold and this cycle (i.e., melting, injection, cooling and ejection) continues for obtaining mass production. The injection molding procedure can be automated, and it is suitable for manufacturing molded parts of high dimensional stability and complexity, as well as of different sizes. Injection molding can be applied for low-viscous liquids or polymeric melts (e.g., thermoplastic melts). Successful development of injection molding of thermosets has significantly expanded their field of application. The injection molding of thermosets is especially competitive in the production of thick-wall molding parts, due to the much shorter cycle duration. Elastomers can also be injection molded, and elasto-thermoplastics are injection molded in c­ ompliance with the rules of injection molding of thermoplastics.

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Injection blow molding consists of two phases. In the first phase, the preform molded in injection molding process. This preform is transferred into the cavity of the blow mold for blow molding process. After inserting the preform and held firmly in the blow mold, the compressed air is blown through the nozzle of the blow molding machine to the desired pressure and the product continues to be shaped as described for extrusion blow molding. However, unlike extrusion blow molding, the products of injection blow molding feature higher quality surfaces since there are no welded edges and consequently no flash or waste material either. Thermo (Heat) Forming Large thermoplastic sheet moldings are economically made by thermoforming. Figure 5 illustrated the various thermoforming processes. In vacuum thermoforming (Fig. 5a), a thermoplastic sheet is heated to its softening point, is sucked against the contours of a mold, taking up its contour. It is then cooled and allowed to solidify against the mold. (a) Vacuum forming

(b) Drape forming Heater

Heater

Plastic sheet

Vacuum

Vacuum (c) Pressure forming

Air blow

(d) Plug assisted forming

Heater

Plastic sheet

Plug

Vents

Vacuum

Drape thermoforming (Fig. 5b) relies partly on vacuum and partly on the natural sag of the hot polymer to form the shape. Pressure thermoforming (Fig. 5c) uses a ­pressure of several atmospheres to force the hot polymer sheet into the mold. Finally, the plug-assisted thermoforming (Fig. 5d) augments the vacuum with a ­compression plug. Extrusion Extrusion is the most widely used processing procedure of polymeric materials. In this process, the polymer in powder form is processed by a rotating screw through a heating chamber, and the resulting melt is forced through a shaped die or orifice as shown in Fig. 6. The extrudates are cooled as it leaves the die. The extrudates are then drawn down to a similar cross section. The process is used to coat the wire, which can be achieved at very high speeds. Extrusion is also used as a preprocess to many of the molding processes such as injection molding. The process has the advantage of relatively low tooling costs, though capital costs are high, and the output usually requires further processing, varying from simply cutting to size to remelting and injection molding. Die design is complicated by die swell, hence tolerances are not as tight as for the pressure molding processes. The most frequently used are the single screw extruders, but the twin screw extruders provide a homogeneous mixture of the polymer blends/composites. Compression Molding A premeasure quantity of polymer—usually a thermoset in the form of granules or a preformed tablet containing resin and hardener—is placed in a heated mold. The mold is closed creating a sufficient pressure to force the polymer into the mold cavity. The polymer is allowed to cure, the mold is opened, and the component is removed. Compression molding is widely used to shape the composites. Figure 7 illustrates the compression molding technique.

Fig. 5  The various thermoforming processes Plastic pellets in Some of the commonly extruded cross-sections Hopper

Barrel

Screw

Band heaters Screen/mesh

Hydraulic drive system

Plastic extrudates out

Feed zone

Fig. 6  The extrusion process

Extrusion die

Compression zone

Metering Zone

Applied pressure

Upper die (core)

Apart from the above process, molding technique is practiced, where one material is molded into another one. These find wider applications in the field of automotive industries as the different polymers can be better utilized. Also multilayer extrusion technique is developing rapidly, which helps in the production of fuel tubing and to reduce permeability to nearly zero.

Thermoset plastic powder

POLYMERS AND THEIR COMPOSITES FOR AUTOMOTIVE STRUCTURAL APPLICATIONS

Cartridge heater Lower die

(cavity)

Ejector pin

Fig. 7  The compression molding technique

Reaction Injection Molding It is low-pressure molding process used for the in situ polymerization of parts. The process uses preheated low-viscosity chemicals (e.g., polyol + isocyanate for PUR). These are fed under pressure into the mixing head, from which they are injected into the mold where polymerization occurs. The process is generally used for large parts and may be used for complex shapes. The process is most commonly used with thermosetting PUR but other polymers also can be used (e.g., nylon 6 and epoxy resins). Figure 8 illustrates the reaction injection molding (RIM) process. The process may be used to produce structural foam products and fiber-reinforced composites (reinforced RIM or RRIM). Capital costs are high, but tooling costs are low due to the low injection pressures. Energy consumption is also lower than conventional injection molding. Cycle time is largely controlled by cure time and open mold time (part ejection may be difficult with thermosets). Isocyanate

The automotive industry is on the edge of a revolution, and the polymer industry poised to play an important role. The real polymer revolution in automotive industry began in 1950 when thermoplastics made their first appearance, starting with ABS and going on to PA, POM and PC, PP together with the introduction of alloys and blends of various polymers. The ongoing progress of advanced, high-performance polymers has dramatically increased their usage. Originally polymers were specified because they offered superior mechanical properties combined with excellent exterior, including the possibility of self-­ coloring. The application of polymer-based components in the automotive industry has been increasing over the last decades. Nowadays, the polymer and their composites are used mainly to make cars more energy efficient by reducing total weight, together with providing sturdiness, corrosion resistance, toughness, design flexibility, resilience, and high performance at low cost. The average vehicle uses about 150 kg of polymers and their composites versus 1163 kg of iron and steel—currently, it is moving around 10%–15% of total weight of the car. The automotive industry uses engineered polymers and PMCs in a wide range of applications, as the second most common class of automotive materials after ferrous metals and alloys which represent 68 wt%; other nonferrous metals used include copper, zinc, aluminum, magnesium,

Polyol

Ejector pins Supply line

Return line Mixing head Return line Supply line

Fig. 8  The RIM technique

Core Mold Cavity

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Automotive Applications: Reinforced Material Components 189

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titanium, and their alloys. The polymers and PMCs contents of commercial vehicles comprise about 50% of all interior components, including safety subsystems, door, and seat assemblies. During the enormous growth of polymer-based components in automotive, the advantages of using PMCs have changed. Increasing costs are being met by the ability of PMCs to be molded into components of complex geometries, often replacing several parts in other materials, and offering integral fitments that all add up to easier assembly. Many types of polymers are used in more than thousand different parts of all shapes and sizes. A quick look inside any model of the car shows that polymers are now used in exterior and interior components such as bumpers, doors, safety and windows, headlight and side view mirror housing, trunk lids, hoods, grilles, and wheel covers. Although up to 13 different polymers may be used in a single car model, just three types of plastics make up some 66% of the total plastics used in a car: PP (33%), PUR (17%), and PVC (16%) (Table 3).[12] The manufacture of natural fiber-reinforced composites includes the use of either a thermoset or thermoplastic polymer matrix system combined with the natural fiber (short fiber, preform, or mat). In automotive applications, the most common system used today is PP, particularly for non-load-bearing components. PP is favored due to its low density, excellent processability, mechanical properties, excellent electrical properties, and good dimensional stability and impact strength. However, several synthetic

Table 3  Plastics used in a typical car[13]

Component

Types of plastics

Weight in average, in car (kg)

Bumpers

PS, ABS, PC/PBT

10.0

Seating

PUR, PP, PVC, ABS, PA

13.0

Dashboard

PP, ABS, SMA, PPE, PC

7.00

Fuel systems

HDPE, POM, PA, PP, PBT

6.00

Body (incl. panels)

PP, PPE, UP

6.00

Under-bonnet components

PA, PP, PBT

9.00

Interior trim

PP, ABS, PET, POM, PVC

20.00

Electrical components

PP, PE, PBT, PA, PVC

7.00

Exterior trim

ABS, PA, PBT, POM, ASA, PP

4.00

Lighting

PC, PBT, ABS, PMMA, UP

5.00

Upholstery

PVC, PUR, PP, PE

8.00

Liquid reservoirs

PP, PE, PA

1.00

Total

105.00

thermoplastics are utilized including PE, PS, and PAs (PA 6 and PA 66). Common thermoplastic properties and their applications are listed in Table 4. In the mid-1980s, GM Chairman Roger Smith set out to “rethink” the way automobiles were designed and manufactured. Working with a clean slate, GM rethought everything from marketing to materials of construction to manufacturing. Plastics played a large role in this effort. The new automobile, the Saturn, was the very first passenger vehicle to make extensive use of injection-molded “thermoplastics” for exterior body panels. Most of the Saturn’s body panels are molded from a blend of PC and PC/ABS. The use of the PC/ABS gave designers much greater design freedom when compared to traditional sheet metal body panels. The PC/ABS body is also, lightweight, ­corrosion resistant, and durable. The very first Saturn was driven out of the Spring Hill, Tennessee, assembly plant by Roger Smith on July 30, 1990. More than 2.5 million Saturn have been produced since that time. POLYMERS AND THEIR COMPOSITES FOR AUTOMOBILE—TRIBOLOGICAL APPLICATIONS One of the most important demands in food processing industry, textile machinery, power plants, and other industries today stems from the increasing prominence on preserving the environment. The recent push toward introducing environmental friendly solutions has led to many efforts in utilizing solid lubricants in machinery as well as in power plants. The recent efforts in replacing liquid lubricants and introduction of solid lubricants have contributed to the reduction of the risks posed to the environment. These concerns for protecting the environment have led to the concept of using solid lubricant and attempts toward developing unlubricated bearings for food processing machineries. This, however, poses many engineering challenges which require rethinking of the different aspects of bearings, e.g., design, operating conditions, and selection of shaft and constituents of bearing materials. Therefore, the choice of the materials and their tribological behavior are very important for the proper performance and extended life span of bearings used in food processing machinery.[13] The materials of contacting surfaces play an important role in the performance of dry-sliding bearings. The choice of materials is not only determined by the mechanical and tribological properties, but also by the cost, ease of processing, and the practical limitations in the real-time application. Application of polymers and their composites as bearing materials can improve the performance of the sliding bearings in machinery. In view of this, the mechanical properties and tribological behavior of solid lubricants filled fiber-reinforced polymer composites with consideration toward application in unlubricated sliding

Automotive Applications: Reinforced Material Components 191

Automotive applications

PP

Density (0.92 g/cm3) Melting point (171°C) Shore D (55–65) Coefficient of friction (0.3) Young’s modulus (1300–1800 MPa) Thermal conductivity (0.17–0.22 W/m oK)

Automotive bumpers, chemical tanks, cable insulation, battery boxes, bottles, petrol cans, indoor and outdoor carpets, carpet fibers

ABS

Density (1.04 g/cm3) Melting point (130°C) Hardness Rockwell R (103–112) Tensile strength yield (42.5–44.8 MPa)

Car dashboards, covers

PA

Density (1.08 g/cm3) Melting point (220°C–265°C) Hardness Rockwell M (82–89) Tensile modulus (2.6–3.3 GPa) Coefficient of friction (0.2–0.3)

Gears, bushes, cams, bearings, weather proof coatings

PC

Density (1.2 g/cm3) Melting point (117°C–127°C) Hardness Rockwell M (70) Tensile strength (72 MPa) Tensile modulus (2.2 GPa)

Security screens, aircraft panels, bumpers, spectacle lenses, headlamp lenses

PVC

Density (1.15–1.45 g/cm3) Melting point (140°C–160°C) Hardness Rockwell R (115) Tensile modulus (4.11 GPa)

Automobile instruments panels, sheathing of electrical cables, pipes, doors, waterproof clothing, chemical tanks

PUR

Density (0.87 g/cm3) Tensile strength (26 MPa) Hardness Shore D (60–70)

Flexible foam seating, rigid foam insulation panels, microcellular foam seals and gaskets, durable elastomeric wheels and tires, automotive suspension bushings, electrical potting compounds, hard plastic parts (such as for electronic instruments), cushions

PS

Density (1.0 g/cm3) Tensile strength (32–44 MPa) Tensile modulus (1.9–2.9 GPa) Hardness Shore D (784–80)

Equipment housings, buttons, car fittings, display bases

PE

Density (0.88–0.96 g/cm3) Tensile strength (20 MPa) Tensile modulus (0.7 GPa) Hardness Shore D (59–64)

Glass reinforced for car bodies, electrical insulation, packaging

LDPE

Density (0.91–0.93 g/cm3) Tensile strength (14.5–38 MPa) Tensile modulus (0.4–1.5 GPa)

Electrical cables, consumer packaging, bags, bottles, and liners

HDPE

Density (0.94–0.96 g/cm3) Tensile strength (25–69 MPa) Tensile modulus (0.4–0.5 GPa)

Toys, utensils, films, bottles, pipe, and processing equipment. Wire and cable insulations

Ultra-high-molecular weight polyethylene (UHMWPE)

Density (0.93 g/cm3) Tensile strength (40 MPa) Tensile modulus (0.69 GPa)

Snowboard bottoms, material handling, packaging, food processing, and automotive parts

POM

Density (1.42 g/cm3) Tensile strength (60 MPa) Tensile modulus (2.9–3.5 GPa)

Interior and exterior trims; fuel systems; pump and filter housings; shower heads; machinery parts such as gears, bearings, rollers, and conveyor chains; airflow valve fittings and valves; steering column—gear shift assemblies; and household appliances such as food mixer parts, pump, and water sprinkler parts (Continued)

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Table 4  Polymers and their properties in automotive applications Polymers Properties

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Table 4  Polymers and their properties in automotive applications (Continued) Polymers Properties

Automotive applications

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PMMA

Density (1.18 g/cm3) Hardness Rockwell M (92) Tensile strength (70 MPa) Tensile modulus (1.8–3.1 GPa)

Automotive rear light housings (windows, displays, screens), badges, steering wheel insignia, and fascia panels

PBT

Density (1.31 g/cm3) Hardness Rockwell M (70) Tensile strength (50 MPa) Tensile modulus (2 GPa)

Automotive components—under-bonnet and power tool casings; fog lamp housings and bezels, sunroof front parts, locking system housings, door handles, bumpers, carburetor components

PET

Density (1.39 g/cm3) Hardness Rockwell M (94) Tensile strength (80 MPa) Tensile modulus (2–4 GPa)

Wiper arm and their gear housings, headlamp retainer, engine cover, connector housings

Epoxy

Density (1.1–1.4 g/cm3) Hardness (Shore D) (90) Tensile strength (35–100 MPa) Tensile modulus (3–6 GPa)

Load bearings, valve bodies, and downhole plugs, and components for the oil and gas industry, interior display systems, seating, consoles, sensors, actuators, airbag sealing, headliners, and overhead systems

Polyester

Density (1.2–1.5 g/cm3) Hardness (Shore D) (87) Tensile strength (40–90 MPa) Tensile modulus (2–4.5 GPa)

Firemen’s helmets, firefighting equipment components, composite toe caps, and bump caps

Vinyl ester

Density (1.2–1.4 g/cm3) Hardness (Shore D) (83) Tensile strength (69–83 MPa) Tensile modulus (3.1–3.8 GPa)

Gimbal rings and cowlings, out drive gimbal housing, and power boat seat shells

bearings for food processing and other machinery have been discussed. Besides the conventional thermoplastic polymers used as matrices for tribo-components (e.g., POM, PA, PTFE, PE), [14,15] now newer high-performance thermoset ­polymer-based composites (e.g., synthetic fibers and solid lubricants) have found more and more entrance into tribological applications. Very special candidates in this respect are also the epoxies which allow tailoring of mechanical properties and tribological behavior by reinforcing glass and carbon fibers and solid lubricants such as graphite, molybdenum disulfide, and nanoclay fillers. The addition of synthetic fibers to epoxy matrix is mainly intended to improve properties other than wear, such as hardness, stiffness, strength, and creep resistance. Additions of solid lubricants, in particular graphite, molybdenum disulfide, and nanoclay, for reduction of the coefficient of friction usually alter the mechanical properties but this can be offset by fiber reinforcement. A vast range of epoxy with fibers and solid lubricated filler filled composites is now available, presenting the designer with multiple choice since the properties of many of epoxy-based composites tend to be very similar.[16–18] A few examples of how graphite, alumina, and molybdenum disulfide affect the wear behavior of carbon fabric-reinforced epoxy (C-E) composites are given in Figs. 9 and 10. Plot of wear loss as a function of load for different weight % of graphite-filled C-E composites is shown in Fig. 9a–d,

respectively. From the plot, it was noticed that the wear loss increases with increase in load for all composites. The wear loss of graphite-filled C-E composites is lower than that exhibited by the unfilled CE composites under the condition of 6000 m sliding distance. The amounts of graphite in C-E composite are 5 and 10 wt%, respectively. For this reason, the filler-loaded C-E wear loss was small and wear loss had been caused mainly by matrix wear. The order of wear resistance behavior of composites is as follows: 10% > 5% > 0% by weight of graphite. This behavior can be attributed to the presence of graphite particles, which act as effective barriers to prevent large-scale fragmentation of epoxy. Several authors [14,15] have discussed the role of graphite as an effective filler material. The carbon fabric strengthens the composite, while the carbon as graphite acts lubricant together providing enhanced wear resistance. Figure 10 shows the wear volume loss increases with increase in sliding velocity for all the composites. The wear volume loss decreases with increase in filler loading is shown in Fig. 10, which highlights the beneficial effect of inclusion of micro-fillers into C-E composite. The wear volume loss of unfilled C-E composite is highest among all the composites tested. It was also noticed that inclusion of particulate micro-fillers to C-E composite reduces the wear volume loss. The wear volume decreases with increase in filler loading and 10 wt% MoS2-filled C-E (C-E-10M) composite showed the least wear volume loss under different loads. The reduction in wear volume loss

Automotive Applications: Reinforced Material Components 193

CE-1 CE-2

CE-2

0.002 25

(d) 0.025

CE-1 CE-3

50 75 Load(N)

CE-1

0.004 0

CE

25

CE

0.006

100

Weight loss(g)

(c) 0.018 0.016 0.014 0.012 0.01 0.008 0.006 0.004 0.002 0

50 75 Load(N)

Weight loss(g)

25

0.01 0.008

50

75 Load(N)

100

0.02 0.015

CE CE-1

0.01

CE-2

0.005 0

100

25

50 75 Load(N)

100

Fig. 9  Wear loss versus load of unfilled and graphite-filled C-E composites at (a) 3 m/s, (b) 4 m/s, (c) 5 m/s, and (d) 6 m/s [16]

7

Wear volume (mm3)

6

(b)

C-E C-E-5A C-E-5M C-E-10A C-E-10M

5 4 3 2 1

(c)

7

Wear volume (mm3)

6

20

40 60 Normal load (N)

5 4 3 2

0

80

(d)

C-E C-E-5A C-E-5M C-E-10A C-E-10M

7 6

5 4 3 2 1 0

C-E C-E-5A C-E-5M C-E-10A C-E-10M

1

Wear volume (mm3)

0

7 6

Wear volume (mm3)

(a)

20

40 60 Normal load (N)

80

40 60 Normal load (N)

80

C-E C-E-5A C-E-5M C-E-10A C-E-10M

5 4 3 2 1

20

40 60 Normal load (N)

80

0

20

Fig. 10  Wear volume loss of unfilled and particulate-filled C-E composites as a function of load and velocity: (a) 0.5 m/s, (b) 1 m/s, (c) 1.5 m/s, and (d) 2 m/s [16]

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CE

Weight loss(g)

(b) 0.012

Weight loss(N)

(a) 0.008 0.007 0.006 0.005 0.004 0.003 0.002 0.001 0

194

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due to the addition of particulate fillers such as Al2O3 and MoS2 can be attributed to the presence of particulate fillers which act as effective barriers to prevent large-scale fragmentation of epoxy matrix. It can be seen in Fig. 10a–d that the wear volume loss of composite with 10 wt% MoS2 (C-E-10M) increased about 68% compared to unfilled C-E under a sliding velocity of 0.5 m/s and at 80 N load. In the case of C-E with 10 wt% Al2O3 (C-E-10A), the wear volume loss of composite was approximately reduced by about 36% than that of unfilled C-E. It should be noted that the reduction in wear volume loss was greater in MoS2filled C-E composites. In addition, it can be seen that the wear volume loss of unfilled and particulate-filled C-E composites increased with increase in sliding velocities from 0.5 to 2 m/s.

REFERENCES 1. Reif, K. (Ed.) Fundamentals of Automotive and Engine Technology. Standard Drives, Hybrid Drives, Brakes, Safety Systems. Bosch Professional Automotive Information; Springer Vieweg: Berlin, 2014. doi:10.1007/978-3-658-03972-1. 2. Richard, S.; Ball, J.K. Automotive Engineering Fundamentals; SAE—International: Warrendale, PA, 2002. 3. h t t p s : / / m e - m e c h a n i c a l e n g i n e e r i n g . c o m / classification-of-automobiles/. 4. Roots, M.; Brown, J.C.; Anderson, N.; Wanke, T.; Gadola, M. The contribution of passenger safety measures to structural performance in sports racing. In MSC World Users Conference, Newport Beach, CA, 1995. 5. Tom, D. Automobile Mechanical and Electrical Systems Automotive Technology: Vehicle Maintenance and Repair; Butterworth-Heinemann is an imprint of Elsevier: Oxford, 2011. 6. McKinsey & Company. Automotive Revolution—­Perspective Towards 2030. Advanced Industries, January 2016.

Automotive Applications: Reinforced Material Components

7. Agarwal, B.D.; Broutman, L.J. Analysis and Performance of Fiber Composites, 2nd Ed.; John Wiley & Sons, Inc.: Hoboken, NJ, 1990, 2–16. 8. Gramman, H.; Krapp, R.V.B. Disposal and recycling of HSC materials. In 6th International Conference on High-­ Performance Marine Vehicles, Naples, 2008, 271–280. 9. Gibson, A.G.; Manson, J.A.E. Impregnation technology for thermoplastic matrix composites. Compos. Manuf. 1992, 3 (4), 223–233. 10. Gibson, A.G. Continuous molding of thermoplastic composites. In Comprehensive Composite Materials; Kelly, A.; Zweben, C.; Eds.; Pergamon: Oxford, 2000, 979–998. 11. Ijaz, M.; Robinson, M.; Gibson, A.G. Cooling and crystallization behaviour during vacuum-consolidation of commingled thermoplastic composites. Compos. Part A: Appl. Sci. Manuf. 2007, 38 (3), 828–842. 12. http://www.plasticsconverters.eu/organisation/division/ automotive (cit. on 30.4.2010). 13. Friedrich, K.B.; Lu, Z.B.; Hager, A.M. Recent advances in polymer composites’ tribology. Wear 1995, 190 (2), 139–144. 14. Byett, J.H.; Allen, C. Dry sliding wear behaviour of polyamide 66 and polycarbonate composites. Tribol. Int. 1992, 25 (4), 237–246. 15. Bohm, H.; Betz, S.; Ball, A. The wear resistance of polymers. Tribol. Int. 1990, 23 (6), 399–406. 16. Suresha, B.; Siddaramaiah; Kishore; Seetharamu, S.; Kumaran, P.S. Investigations on the influence of graphite filler on dry sliding wear and abrasive wear behaviour of carbon fabric reinforced epoxy composites. Wear 2009, 267, 1405–1414. 17. Li, X.; Gao, Y.; Xing, J.; Wang, Y.; Fang, L. Wear reduction mechanism of graphite and MoS2 in epoxy composites. Wear 2004, 257 (3–4), 279–283. 18. Kishore, Sampathkumaran, P.; Seetharamu, S.; Thomas, P.; Janardhana, M. A study on the effect of the type and content of filler in epoxy-glass composite system on the friction and slide wear characteristics. Wear 2005, 259, 634–664.

Aviation: Polymer Composites, Processes, and Properties I.A. Aderibigbe, P. Nombula, and A.P.I. Popoola

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Department of Chemical and Metallurgical Engineering, Tshwane University of Technology, Pretoria, South Africa

Abstract Polymers and polymer composites are increasingly becoming the preferred construction materials for aircraft and spacecraft due to their unique combinations of properties such as durability, good stiffness, high strength, corrosion resistance, and light weight. Thus, providing them with the capacity to reduce the overall weight of the aircraft improves the fuel efficiency while simultaneously reducing carbon footprint on the environment. This entry gives an overview of the application of polymers in aviation industry. Other important topics discussed include polymer composites, reinforcing fibers, properties of polymers, selection criteria and manufacturing techniques, shelf life, and recycling processes for polymers. Keywords: Aerospace; Aircraft; Aviation; Composites; Lightweight; Plastics; Polymers.

INTRODUCTION Every extra kilogram an aircraft weighs requires more energy to power it and thus costs more money. The application of modern polymeric materials (polymers and polymer composites) in aircraft affords the possibility of achieving the objective of lightweight constructions and hence fuel economy. In addition, aviation structures are quite different from other engineering structures due to the requirements of high-standard performance placed on the parts. As a result, the industry is constantly under pressure to design, develop, and modify aircraft components that will minimize carbon emissions by reducing the overall weight of the aircraft without compromising performance and safety. One of the ways the aircraft makers have been attempting to resolve the carbon emission issue is by increasing the use of polymers in the aircraft structure. A large number of modern aviation structures typically necessitate the applications of lightweight, environmental friendly, thin-walled advanced polymer composites, and multifunctional materials in order to achieve fuel economy and subsequently lower carbon emission. This has led to the increased market demand of polymer composite from the aviation industry. Advantages of polymer composites include weight reduction, superior stiffness, and strength. Polymer composites are used in key areas of the aircraft such as structural elements, navigational devices as well as interior components such as overhead stowage bins, bulkheads/class dividers, galleys, window surrounds, lavatory modules, cabin sidewalls, and floor and ceiling panels. Growth in aviation and aerospace industries is increasing the applications of advanced polymer composites in areas such as the leading edge of the wing and the Encyclopedia of Polymer Applications, First Edition DOI: 10.1201/9781351019422-140000094 Copyright © 2018 by Taylor & Francis. All rights reserved.

ailerons; it is further expected that the use of advanced polymeric composites will be high in the nearest future even in more critical areas such as the aircraft fuselage. The terms polymers, plastics, and resins are usually regarded as synonymous, though technically with slight distinctions. A precise description of pure polymer (though rarely used on its own) is a family of materials, including rubbers, which exhibit long-chained molecules and are formed by reactions between monomers (simple molecules) and can be chemically engineered to provide the desired toughness or strength requirement for a component while simultaneously retaining uniform composition. On the other hand, plastics are soft and can be molded, and tend to reach a liquid condition during processing. However, they are solid in the finished condition. Polymer matrix composites consist of an organic polymer matrix which binds a variety of short or continuous fibers together within the matrix. The polymeric matrix functions include the bonding of the fibers together and transferring of load between them, while the reinforcing fibers provide the required strength and stiffness to support the mechanical loads encountered in service. Polymer matrix composites are classified into two groups based on the inherent mechanical properties (usually stiffness and strength): (i) reinforced plastics are relatively cheaper and consist of reinforced polyester resins with low-stiffness glass fibers, and (ii) advanced polymer matrix composites also known as advanced composite materials exhibit superior strength and stiffness; they are relatively more expensive compared to reinforced plastics and have found major applications in the aviation industry. Advantages of polymeric materials which make them suitable for use in the aircraft structures include lightweight, high strength, enhanced stiffness, excellent corrosion

195

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resistance, aesthetics, and high degree of design flexibility. However, there are some drawbacks to the use of polymer composite materials in aviation industry, namely, polymeric materials are not suitable for aircraft engine parts due to the characteristic high temperatures, limited supply base worldwide of polymeric composite materials, poor fire performance, and high initial costs by certain reinforced polymers such as carbon fiber-reinforced plastic (CFRP).

the reinforcing material, and decrease as the shape changes in the order of fiber, whisker, and particle. This accounts for the reason why long, continuous fibers are used for structural applications in aircraft.[1] The fibers (Fig. 1) are stacked in piles or layers, usually in different directions in order to support multidirectional loads.[1] Selected fibers used in aviation industry are shown in Table 1. Fiberglass

POLYMER COMPOSITES (REINFORCED PLASTICS) In aviation industry, besides aluminum alloys, the composites used are largely polymer matrix composites, and in most cases, the polymer is usually a thermoset resin. The polymer composites are in the form of sandwich materials or laminates, made up of polymer matrix and continuous fibers. They are applied in areas where structural parts are heavily loaded and are usually supported by frames, ribs, and spars. Depending on the designed payload, the laminate thickness can vary from a few millimeters to about 25 mm.[1] Carbon fiber-epoxy is the most common laminate or composite used in aircraft. On the other hand, carbon fiber or carbon fiber-bismaleimide may also be used.[1] Polymer composites can be divided based on the length and shape of the reinforcement used in the composites. There are three major classes of composites: fiber-reinforced, whisker-reinforced, and particle-reinforced composites (Fig. 1a–c). However, in structural applications, particle- or whisker-reinforced composites are rarely used.[1]

Fiberglass is a type of fiber-reinforced polymer where the reinforcing element is in the form of fine glass. The arrangement of the glass fiber may be random, interwoven into a fabric or flattened into a sheet referred to as chopped strand mat. The typical composition of glass fiber is based on alumina-lime-borosilicate. A wide range of commercial composition can be produced by varying the individual content. For example, the E-glass (electrical grade) composition is calcium aluminoborosilica with less than 2% alkali content, C-glass (chemical resistant grade) composition is soda-lime-borosilicate, while high-strength S-2 glass composition is a low-alkali magnesium-alumina-silicate.[4] Fiberglass has found a wide range of applications in aviation industry. Their applications often include secondary structures on aircraft, such as wind tips, radomes, and fairings. They are also used for rotor blades of helicopters.[2] Glass fibers suffer from certain disadvantages such as lower modulus; thus, additional design requirements are needed in applications where stiffness is critical.[5] In addition, they pose health risks during processing, as toxic fumes are emitted when heated or incinerated, which can cause ­irritation to the respiratory system and dermal tissue.[6]

Reinforcing Fibers Aramid Fiber The fibers are the major reinforcements or load-bearing elements of the composite material. Thus, they provide the composite structure with strength, stiffness, creep performance, and fatigue resistance.[1] In some applications, the reinforcement may be modified to alter non-mechanical properties such as thermal or electrical conductivity of the composite structure.[1] The strength and stiffness depend on the shape of

(a)

Aramid (aromatic polyamide [PA]) fiber is the first organic fiber that exhibits sufficient tensile strength and modulus to be used in advanced composites as reinforcements and exhibits between 5% and 10% improved mechanical properties compared to synthetic fibers.[7] In 1970, DuPont introduced an organic fiber, Kevlar™ aramid, which

(b)

(c)

Fig. 1  Illustration of fibers

Aviation: Polymer Composites, Processes, and Properties 197

Application

References

Fiberglass

White

Lower cost, chemical/galvanic corrosion resistance

Lower strength compared to other fibers, i.e., Kevlar, boron, and carbon

Fairings, domes, and wing tips

[2]

Aramid (Kelva®)

Yellow

Lower cost, galvanic/chemical resistance

Lower modulus (lower stiffness)

Military ballistic, body armor rotor burst protection in jet engines

[2,3]

Graphite/ carbon

Black/ gray

3–10 times stronger than fiberglass. Good corrosion resistance

Expensive lower conductivity than aluminum

Aircraft floor beams, stabilizers, flight controls, primary fuselage, and wing structure

[2]

Boron

Black

Very stiff, high tensile, and compressive strength

Expensive, health risk less flexible, difficult to cut

To repair cracked aluminum aircraft skin. Good corrosion resistance

[2]

Ceramic

White

Suitable for high-temperature applications up to 1,200°C

Less flexible, expensive, difficult to fabricate complex shape

Gas turbine engine blades

[2]

exhibits high-specific tensile modulus and strength.[4] This material has been credited to be displacing other organic fibers and metal wires from high-performance applications used in automotive, marine, bulletproof vests, and aircraft applications.[7] Important physical and mechanical properties of aramid fibers include low density, high stiffness, improved tensile strength, low nonlinear compressive properties, and unique toughness characteristics.[4] The density of aramid is 1.44 gm/cm3, about 40% lower than that of glass and about 20% lower compared to carbon. It is significant to note that aramids do not melt, but they decompose at a temperature of about 500°C. The tensile strength of aramid yarn can range between 3.4 and 4.1 GPa when measured in twisted configuration. In the axial direction, the coefficient of thermal expansion of aramid fibers is 5 × 10−6 m/m/C°, with remarkable thermal stability; aromatic PA polymers and exhibits improved chemical and dielectric properties.[4] At room temperature, epoxy-reinforced composites with aramid exhibit a nominal tensile strength and modulus of 1.4 and 76 GPa, respectively. The composites also exhibit good ductility under flexure and compression and show reduced values compared to those of carbon or glass-reinforced composites. They also show high resistance to stress and fatigue rupture. It is interesting to note that unidirectional aramid-reinforced composite samples (Vf ~ 60%) under tension/tension fatigue were not affected at 50% of their ultimate stress when operating at 3,000,000 cycles.[4] Carbon Fibers Carbon fibers possess high strength and stiffness, about three to ten times stiffer compared to glass fibers.[2] It has found structural applications in aircraft, which include wing structure, floor beams, primary fuselage, flight controls, and stabilizers.[2] There are three chemical routes

of producing carbon fibers: petroleum pitch, rayon, and polyacrylonitrile (PAN).[5] Pitch fibers are characterized by high tensile modulus and good thermal expansion coefficients, whereas PAN-based fibers exhibit more improved mechanical properties for structural applications and ­constitute about 90% of the total carbon fibers produced.[5] PAN-based fibers constitute about 90% of the total carbon fibers produced and exhibit excellent mechanical properties for structural applications, whereas good thermal expansion coefficients and higher modulus values are associated with pitch fibers.[5] Carbon fibers are produced in different forms, such as chopped fibers, mats, or continuous filament tows. Advantages of carbon fibers include high stiffness, improved strength, and corrosion resistance. However, they are very brittle and costly.[5] Other limitations include a necessity to provide a protective mesh or coating for aircraft components which are susceptible to lightning due to their lower conductivity compared to aluminum. In addition, they have a high tendency to cause galvanic corrosion when used in conjunction with metallic structures and fasteners.[2] They are available in the form of prepregs or dry fabrics and can be found in black or gray color.[2] The properties of carbon fiber are influenced by the microstructure, which is in turn highly influenced by processing, such that there can be a dramatically difference in the properties of fibers with the same precursor but different processing methods. On the other hand, the properties of carbon fiber can also be influenced by the precursor itself. In addition, the processing may be optimized to yield high strength or modulus, or traded off with economics.[4] Boron Fibers Boron fibers exhibit high stiffness, tensile, and compressive strength, [2,6] and has been credited to be unmatched

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Table 1  Reinforcing fibers commonly used in aviation industry Fiber Color Advantages Disadvantages

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due to its unique combination of stiffness, strength, and density. [4] The tensile strength and modulus of boron fiber are 3,600 MPa and 40 GPa, respectively. However, they exhibit low thermal conductivity and thermal expansion. The coefficient of thermal expansion is 4.5–5.4 × 10 −6 /C°. [4] The fibers are characterized by relatively large diameter and exhibit low flexibility; [2] thus, the material is exclusively available in filament form or epoxy matrix prepreg. Boron fibers have found applications in aerospace, in areas requiring high strength and/ or stiffness, [4] and are primarily used in military aviation applications. [2] The major hindrances to the wide use of born fibers are the exorbitant cost of boron fiber, which is approximately an order of magnitude higher compared to standard carbon fiber, [4] and the increased health risks for personnel. [2] Ceramic Fibers Ceramic fibers consist of a wide variety of crystalline or amorphous synthetic mineral fibers distinguished by their refractory properties. [8,9] They are employed in high-temperature applications up to temperatures of about 1,200°C. [4] Ceramic fibers are basically composed of silica, alumina, and other metallic oxides or, less commonly, of non-oxide-producing materials such as silicon carbide. Most ceramic fibers consist of silica and alumina in a 50/50 mixture ratio. [10] They are used as insulating materials due to their ability to withstand high-­ temperature applications. [10] Ceramic fibers have found applications in aircraft and space vehicles. Teflon coated with ceramic fibers are used as sewing threads to produce high-temperature insulating materials for aircraft and space vehicles. They are also used for heat shields in the aerospace industry and for space shuttle tiles. [8] A drawback in the use of the ceramic fibers is the health risks that generally characterize the manufacturing of mineral-fiber products due to the exposure to the released airborne respirable fibers. [8]

CLASSIFICATION OF POLYMER MATRIX The matrix of an advanced polymer composite consists of resin which is a generic term designated for the polymer to which a wide range of short or continuous fibers are bonded together. The polymer matrix functions include transfer of load transfer between the reinforcing fibers. The polymer composite matrix can be classified as ­thermosetting and thermoplastic resins (Fig. 2). Thermosetting Resin Thermosetting resin are commonly used for aircraft structures.[11] The physical properties and chemical composition of the matrix fundamentally affect the fabrication, processing, and ultimate properties of a composite material. The resins are easily cast into any shape and cure readily either by the application of catalyst or heat into an insoluble solid. The presence of contaminants or impurities in a resin coupled with changes in the composition, morphology, or physical state of the resin may affect the processability and handleability, lamina/laminate properties, long-term durability and overall performance of the composite material.[4] An important characteristic of thermosetting resins is that curing is required once they crosslinked, which is an irreversible process. Thus, they cannot be melted and reshaped upon curing. As a result, they exhibit high thermal stability, hardness, creep resistance, and good rigidity.[12] Thermosets are the most widely used matrices due to their processing condition that prevents breakage.[13] Thermosetting resins that are commonly used in aviation include epoxy, phenolic, polyesters, and PAs.[14] Table 2 shows the properties and applications of selected thermosets used in aviation industries. Thermoplastic Resins Thermoplastics, unlike thermosetting polymers, do not absorb much moisture and exhibit less reduction in

Polymer matrix

Thermoplastic resin

Amorphous thermoplastic

Fig. 2  General classifications of polymer matrix

Crystalline thermoplastic

Thermosetting resin

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Amorphous Thermoplastic Polymer chains in amorphous thermoplastics are arranged randomly in a coil status void of any near order.[11,20] Overall, these types of resins mostly exhibit brittleness and optically transparent. Typical examples of amorphous thermoplastic resins are polyvinylchloride (PVC) or polystyrene (PS), polymethylmethacrylate (PMMA), and polycarbonate (PC).[20] The properties and applications of some commonly used amorphous thermoplastic resins in aviation are shown in Table 3. It is significant

Table 2  Properties and applications of selected thermoset resin used in aviation Resin types Key properties Advantages Disadvantages

Application

References

Epoxies

The most commonly used resin in aerospace applications

Greater strength and stiffness toughness, excellent adhesion, chemical and corrosion resistance

Expensive

Coatings, electronic/ electrical insulation, adhesives

Phenolic

High-temperature tolerance accounts for 80%–90% of passenger aircraft interior furnishing

Flame resistant and dimensional stability

Produces toxic fumes upon burning. Discoloration when exposed to light

Insulation, laminates, coatings, and vanishes. To impregnate Nomex honeycomb floors and interior cabin liners

Polyesters

Premixed with glass fiber Ease of processing, low to form BMC cost, good performance

Variable anisotropy

Autobody parts, bathtubs, and boats

[15,16]

Polyimides

High elongation and toughness exhibit imide structure and high resistance to combustion

Excellent oxidative and thermal properties

Expensive, poor resistance to alkalies, hydrolysis, and concentrated acids

Aerospace and electronics jet engine parts

[16,19]

Table 3  Properties and applications of selected amorphous thermoplastic resins Resin types Key properties Advantages Disadvantages

Application

Exhaust gas ducts, Sensitive to UV tubes, hoses, pipes/ and oxidative degradation. Thermal fittings decomposition evolves HCl

[15,16]

[16–18]

References [21–23]

PVC

Chemically stable Resistant to acid, excellent fire retardant alkali, and almost resistant to oxidation all inorganic chemicals. Versatility. Lightweight

PC

High toughness, transparent, flame, and heat resistant. High notched impact strength at low temperatures

Shatter resistance, durability, and heat resistance

Subject to stress cracking, moderate chemical resistance, and aromatic sensitivity

Aircraft-grade mirror

PS

Transparent, high stiffness, and processability

Very clear

Poor hightemperature properties. Low light UV stability. Lower chemical resistance

Model aircraft

[16,27]

PMMA

High surface hardness, good light transmission properties

Excellent clarity. High Poor solvent resistance. Poor light transmission fatigue resistance economical. 100% recyclable

Lenses, light covers, meter covers, signs

[28,29]

[16,24,25,26]

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mechanical properties at high temperature. Thermoplastic polymers possess better toughness compared to thermosets. Thus, they exhibit improved interlaminar strength and impact resistance. They require short processing time since they do not undergo any chemical reaction. However, increased pressures are required, higher than those for the thermosets, with the associated increase in costs. In addition, thermoplastic matrix flaws can be remedied by welding the parts. [11] Thermoplastic polymers can be grouped into two major classes crystalline and amorphous (Fig. 2).

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to note that amorphous thermoplastics can be dissolved in common industrial solvents. Thus, the reinforcement impregnation can be done with low viscosity solution to prevent high-melt viscosity problem. However, it should be noted that the resulting composite will exhibit low solvent resistance.[11] Consequently, these are severe drawbacks to the applications of these composites in aviation industry where aviation fuels, hydraulic fluids, and paint strippers are widely used, consequently placing soluble composites at a serious disadvantage. In addition, amorphous thermoplastics tend to be more prone to fatigue damage and creep deformation compared to semicrystalline polymers.[11] As a result of their poor solvent resistance, the applications of amorphous thermoplastics are limited in aviation to nonstructural areas where their unique properties such as good smoke, fire, and low-toxicity properties could still be beneficial particularly where some level of exposure to solvent can be accommodated and high-temperature characteristic is required.[11] The temperature window at which amorphous thermoplastic resins can be applied is below the glass temperature Tg, referred to as the glass condition. In this region, the molecular structures of thermoplastics are frozen in a precise shape resulting in the mechanical properties to be hardly flexible and thus brittle.[20] On the other hand, there will be an increased molecular mobility when the glass transition temperature is exceeded; the resin becomes soft elastic consequently reducing the mechanical strength of the resin.

The resin becomes frozen on approaching the flow temperature Tf. The molecular structure decomposition will commence within the molten phase on approaching the ­decomposition ­temperature Td.[20] Crystalline Thermoplastic Majority of thermoplastic matrices suitable for high-­ performance composites contain certain degree of crystalline structure, which exhibit enhanced resistance to chemical attack by hydraulic oil, fuel, and paint stripper.[11] The degree of crystallization of semicrystalline thermoplastic is influenced by the molecular weight, the regularity of the chain structure, and the molecular chain mobility, which can be restricted by loop formation.[30] The crystallization process can be influenced by the processing condition, i.e., fast cooling of the melt will not favor crystallization, whereas slow cooling or tempering at the crystallization temperature will result in increased crystallization. The semicrystalline thermoplastics characterized by low crystallization and smaller crystallite phases are more optically transparent compared to those with high crystallization and bigger crystallite phases.[18] Table 4 indicates the properties and applications of selected semicrystalline thermoplastics. It is significant to note that it is not feasible to attain 100% crystallinity due to the interference at large interfacial regions where there is presence of some level of disorder .[11] The amorphous phase of the semicrystalline

Table 4  Properties and applications of selected semicrystalline thermoplastics Resin types Key properties Advantages Disadvantages Brittleness, warpage, high expensive

Application

References

Electrical insulation pumphousing, connectors, and sockets. Filter material for fluids

[2,31]

PPS

Unique hightemperature performance, flame retardant, chemical resistance

High modulus when reinforced

PEEK

Outstanding mechanical properties including toughness

High resistance against Not suitable above 120°C, it Aircraft composites fire, aircraft oil, and is hydrophobic grease

PAs or nylons

They are semicrystalline plastics. Originally produced as highstrength textile fibers

High resistance against Attacked by strong acids oil and grease and bases, high moisture pickup, requires UV stabilization

Gears, housings, connectors

HDPE

High tensile strength/ stiffness and abrasion resistance

Low cost, chemical resistance, and moisture resistance

High thermal expansion prone to stress cracking, difficult to bond, flammable

Pipes, wire, and cable insulations

[33,34]

LDPE

High tensile/stiffness resistant to stains

Low cost, chemical resistance, and moisture resistance

High thermal expansion, flammable difficult to bond, poor weather resistance

Pipes, wire, and cable insulations

[33,35]

PP

Tough translucent, Good chemical, heat, semirigid, and resistant and fatigue resistance to stress cracking

High thermal expansion and Battery cases, high flexural strength carpeting, and upholstery

[11]

[2,15,16,32]

[36,37]

thermoplastics remains frozen below the glass temperature Tg and exhibits brittleness. However, upon exceeding the glass temperature (Tg), which corresponds to the application state, [38] the amorphous phase macromolecules become increasingly mobile leading to melting of the amorphous phase. It is noteworthy that the crystalline phase still exists and the mechanical behavior of the material ranges between tough elastic to hard. Upon exceeding the crystal melt temperature (Tg), melting of the crystalline phase will commence and the material becomes malleable. The semicrystalline thermoplastics melt temperature (Tm) is controlled by certain factors which include the crystallites size and the ratio between the crystalline and the amorphous phases. Increased number of crystallites and their sizes will lead to increased melt temperature. Similar to the behavior of the amorphous thermoplastics, semicrystalline thermoplastics will also start to degrade beyond the decomposition temperature Td in the molten phase.[39] TYPES OF POLYMER COMPOSITES USED IN AVIATION Epoxy Resin Epoxy is a general term used to describe a family of polymerizable thermosetting polymers which contain one or more  epoxide groups which are curable by reacting them with amines, amides, phenols, mercaptans, acids, alcohols, or acid anhydrides. Epoxies can be obtained in a wide range of viscosities from liquid to solid.[4] Epoxies are commonly used for high-performance composites in the aviation industry,[11] due to their high strength and modulus, excellent adhesion, ease of processing low shrinkage, low levels of volatiles, good chemical resistance, and ease of processing.[4] ­However, they suffer from drawbacks such as brittleness and ­degradation of properties when exposed to moisture. In addition, the processing or curing time for epoxies is longer compared to polyester resins.[4] Epoxies are often cured in a temperature range between 120°C and 180°C, and curing in higher temperature generally yield improved temperature resistance. The curing pressure ranges from vacuum to about 700 kPa, which is generally regarded as low-pressure molding ­process. The production techniques include filament ­winding, ­autoclave molding, resin transfer molding, vacuum bag molding, ­pultrusion, and press molding.[4] Polyester and Vinyl Ester Resins Polyesters are synthetic polymers that are very versatile, [40] due to their wide range of applications, such as fibers, composites, plastics, and coatings.[41] Polyester resins are relatively cheaper with fast processing time compared to epoxy resins, and they are generally used for low cost applications. Polyester resins with low smoke-producing grades are commonly used for aircraft interiors.[2] Common processing

techniques for fiber-reinforced polyesters include injection molding, wet layup, press (vacuum bag) molding, pultrusion, filament winding, autoclaving, and matched metal molding.[2] Polyesters can be processed from the ambient temperature up to 180°C, depending on the formulation method, and are more easily processed compared to epoxies. Careful application of proper temperature will lead to prompt curing. However, absence of sufficient heat will result in plasticized resin/catalyst system. They are much tougher compared to epoxies.[4] On the other hand, vinyl ester composites exhibit enhanced mechanical properties and corrosion resistance compared to polyesters composites. However, it is significant to note that the appearance, curing, and handling characteristics of conventional ­polyester are similar to those of vinyl ester resins.[2] Phenolic Resins Phenolics are formaldehyde-based thermosetting resins.[42] In general, they are cured by condensation process with the off-gassing of water to yield a matrix by characterized by good hardness, thermal and chemical resistance with low smoke and toxic degradation properties. Phenolic resoles or novolacs polymers are produced by condensation either by reacting a base catalyst with excess formaldehyde and phenol (resole), or by reacting formaldehyde (novolac) and acidic catalyst with excess phenol (resole) (see Fig. 3). The resins are characterized by improved viscosities and molecular weights compared to the individual parent material. Phenolic resins can be processed by using either press or autoclave cure or subjected to relatively high temperature while freely standing after curing.[4] Polyimide Resins The polyimide resins family consists of a wide range of polymers with an aromatic heterocyclic ring structure. They may occur as either thermoplastic or thermoset resins. Polyimide matrix composites exhibit high-temperature resistance and good oxidative stability, solvent resistance, and low coefficient of thermal expansion. They are primarily used for circuit boards, and aerospace structural and hot engine parts. Majority of polyimide resin monomers are in powdered form. Thus, impregnation of unidirectional fiber and woven fabrics are possible by adding solvents. In general, for fabrics, a 50:50 ratio by weight mixture is used, while a 90:10 ratio by weight high solids mixture is used to produce a film for unidirectional fibre and a low areal weight fabric prepregs.[4] Bismaleimides Resins Bismaleimides thermosetting resins recently developed and are available in fabrics, rovings, prepreg tapes, and sheet molding compound (SMC). They are the maleimide produced from the reaction of a maleic anhydride

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HO

OH

OH

O

OH OH

OH

OH

OH

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OH

OH OH

OH

O OH

OH

OH + Water

+

OH

Water

OH

OH

+

Water

Resol Resin A

Fig. 3  General of classifications of polymer composites

and diamine. They have earned a good reputation among thermosetting polyimides due to a combination of excellent features such as unique physical property retention in wet environments and at high temperatures, nonflammability properties with fairly constant electrical characteristics over a wide range of temperatures, [43] with improved toughness compared to epoxy resins and excellent performance at ambient and elevated temperatures.[2] Their processing techniques are identical to that for epoxy resins, which include autoclave processing, SMC, injection molding and resin transfer molding. Applications of bismaleimides thermosetting resins include high-temperature parts and aero engines.[44] Polyketones Polyketones belong to a group of high-performance thermoplastic resins. The polymers are crystalline with an excellent high-temperature resistance characteristic, [11] attributed to the strong attraction between the molecular chains, [45] which increases the melting

point of the material. [46] Polyketones exhibit high solvent resistance and excellent mechanical properties. [46] There are various types of aromatic polyketones which include polyetherketone (PEK), polyetheretherketone (PEEK), etc. The most common of the polyketones is the PEEK. They exhibit improved mechanical properties, high-­temperature and good solvent resistance. PEEK exhibits poor resistance to certain concentrated acids such as nitric acid, hydrochloric acid, etc. [11] The processing history plays a key role in the degree of crystallinity of PEEK materials. Fast cooling can lead to amorphous polymers which can subsequently produce the desired level of crystallinity upon annealing. Overall, the highest content of crystallinity achievable with PEEK resins ranges between 25% and 40%. [11] A drawback to the use of PEEK is high cost, but is justifiable in high-performance composite applications in the defense and aerospace industries. PEEK materials are available in the form of films, fibers, and continuous fiber-­reinforced materials. [11] Carbon and glass can also be used to reinforce PEEK. [2] PEEK polymers can be

Aviation: Polymer Composites, Processes, and Properties 203

produced using step-growth polymerization by the dialkylation of bisphenolate salts. A typical production route is the synthesis of 4,4′-difluorobenzophenone in addition to disodium salt of hydroquinone (Fig. 4). [47]

can be obtained in the form of long fiber reinforcements [45] with carbon and other form of reinforcing elements.[48] It is relatively expensive.[45] Polyamide-Imides

Polyphenylene sulfide (PPS) is a semicrystalline, high-­ performance thermoplastic which possesses a combination of unique properties such as broad chemical resistance, inherent flame resistance, and outstanding high-­temperature resistance (Fig. 5);[45] in fact, the resin is unaffected by organic solvents below 200°C.[48] PPS polymers and compounds possess diverse combinations of high strength, impact property, and electrical insulation, coupled with low arc tracking and high arc resistance.[48] They can be obtained with tensile strength ranging between 69 and 172 MPa and possess good stiffness, with a wide range of flexural moduli between 12,000 and 17,000 MPa. PPS O

ONa

Polyamide-imide (PAI) resins are engineering thermoplastic materials that exhibit high strength at elevated temperature, good impact property, and exceptional dimensional stability. A common type of PAI is known as Torlon, which is available in various grades such as injection-molded and general-purpose grades.[45] PAI molded parts are inherently nonflammable; they possess excellent electrical characteristics and exhibit elevated temperature structural stability in service from cryogenic up to 300°C.[49] PAI polymers can be produced by reacting an aromatic diamine, i.e., methylenediphenylamine (MDA) and trimellitic acid chloride [49] as shown in Fig. 6. However, this route is not cost effective; a more economical route is shown in Fig. 7, which consists of methylenediphenyl disocyanate (MDI) and trimellitic anhydride (TMA) to produce PAI resin.[50] PROPERTIES OF POLYMER/POLYMER COMPOSITES

X

X

Thermal Properties of Polymer Composites ONa

X-Halide O

O

O

Influence of temperature on polymers is of high importance because majority of polymers are processed at moderately high temperature and are applied in a wide range of temperature.[51] Polymeric materials thermal conductivity is significantly dependent on the chain segment orientation.[52,53] The thermal conductivity of pure polymer is generally low, stretching between 0.1 and 0.6 W/(m K).[51] Furthermore, polymer foams characterized by lower density have more pockets of air and thus exhibit lower thermal conductivity.[51] Thermal properties of polymer composites are influenced by O

Peek

X

X

O

Y O

Fig. 4  Typical production route of PEEK

O

X

N

O

X-NH Y-CI

Polyphenylene sulfide

Fig. 5  Chemical structure of PPS

PAI RESIN scheme 1

Fig. 6  Typical production route of PAI by reacting an aromatic diamine, i.e., MDA and trimellitic acid chloride

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Polyphenylene Sulfide

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NCO O X O

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O

OCN NH

O X N

O X-COOH PAI RESIN scheme 2

Fig. 7  A more economical route of production of PAI by reacting MDI and TMA. General classifications of recycling processes

both macromolecular matrix and the additives.[51] Polymers with aromatic structures are characterized by high-temperature resistance due to the tendency of organic materials to form aromatic compounds when heated to elevated temperatures. Thus, polymers with aromatic structures exhibit high-temperature resistance.[54] Overall, most polymers usually tend to flow when heated to a temperature below 200°C. On the other hand, thermally stable polymers possess high transition temperature; they tend to flow at a temperature in excess of 400°C and still exhibit service properties even close to the decomposition temperature.[55] The bond energy associated with the chemical bond in the polymer chain significantly influences the thermal stability. In addition, the polymer will degrade when the material is heated to a point where the bond breaks due vibration energy. It is worthy to note that polymeric materials with cyclic repeating units can exhibit increased thermal stability and that breaking in one bond within a ring does not reduce the molecular weight of the material. Overall, it is uncommon to have the two bonds broken in a ring. Hence, the thermal stability of the ladder or semiladder polymers appears to be higher than that of the open-chain polymers.[54] Polyurethane (PU) is commonly used in aircraft. PU composites reinforced with short polyester fibers with and without different bonding agents showed that degradation of PU resin occurred in two steps, whereas that of PU composites occurred in three steps.[51] Correa et al. investigated the thermal behavior of short-fiber-reinforced PU composites

using thermogravimetry and differential scanning calorimetry techniques. They observed that the thermal resistance of aramid fiber-reinforced composites was higher compared to that of carbon fiber-reinforced composites or the pure matrix polymer.[55] In general, thermal conductivity of amorphous polymers improves gradually in the glassy region but remains constant or slowly decreases in the rubber region. On the other hand, there is a steady decrease in the thermal conductivity of crystalline polymers as the temperature increases below the melting point (Tm).[56] Mechanical Properties of Polymers Polymeric materials often experience a wide range of loading conditions in terms of strain rate and temperature in aviation and automotive industry.[57] They are classified as viscoelastic materials due to their intermediate position between viscous liquids and elastic solids. The mechanical properties of polymers are influenced by the strain rate, temperature, and environmental factors. A polymer may exhibit glass-like property at low temperatures with a Young modulus ranging between 1 and 10 GPa and will flow or rupture at strains in excess of 5%. At elevated temperatures, the same polymer may exhibit high viscosity property.[58] It is significant to note that the material can withstand substantial extensions (~100%) without experiencing permanent deformation. In addition, the polymer undergoes permanent deformation at higher temperatures when subjected

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Chemical Properties of Polymers Polymers are generally characterized by their molecular and supramolecular structures. Polymer materials properties are significantly controlled on the molecular level by their chemical compositions and their constituent monomers, while their properties are further controlled by their chains three-dimensional arrangements.[69] The properties of polymeric materials can also be affected by their chemical environment, either by chemical action or by dissolving the polymer which may result in environmental stress cracking. Overall, all polymers are susceptible to stress cracking by some chemical environments; it is significant to note that various polymers are susceptible to different reagents and specific chemicals that may initiate stress cracking in one polymer may not have any detrimental effects on another polymer. Thus, it is significant to identify chemical environments with potential harmful effects on a particular polymer prior to application.[16] However, the dissolution of polymers in solvents is very crucial in engineering and polymer science applications in industry and in the recycling process of polymers. Compared to nonpolymeric materials, polymers undergo delayed dissolution which is influenced by either the diffusion of the chains through a boundary layer close to the polymer–­solvent interface or the polymer chain disentanglement.[70] MATERIAL SELECTION Material selection for the manufacturing of aircraft components is a very crucial process. The criteria for selection of appropriate materials in aviation industry are premised on the desire to reduce environmental impact, market demands, and the crucial need to reduce cost.[71] The decision making on the selection of materials is usually carried out during the design stage of the components which involves taking into consideration a number of factors such as safety, comfort, reliability, and efficiency,[72] in order to meet specific requirements which affect the complexity of the construction materials and the aircraft structure.[73] In the design of an aircraft, a wide range of construction materials may be used in order to take the advantages of their properties such as corrosion resistance, strength, specific weight, and elasticity.[73] Selection of different materials can also be done for the design of specific aircraft components in order to satisfy the requirements for the preferred directions of the applied loads and strength-to-weight ratio.[73] It is significant to mention that the selection process needs to include certain indicators such as materials sustainability, social dimensions, and economic sustainability indicators with emphasis on how they affect the different phases of the product life cycle.[72] Developing a single eco-indicator for individual evaluated material to study all environmental impacts combined together may not be effective due to complex requirements at local, regional,

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to higher load, subsequently exhibiting very high viscous behavior.[58] The mechanical properties of polymers can be analyzed in terms of: (i) molecular description using physical structure and (ii) microscopic description of the behavior of the particular polymer facet.[59] At the microscopic level, the shape, orientation, and volume fraction of reinforcements strongly influence the effective behavior of a composite material.[60] The molecular structures of polymers are long, coiled, and flexible, and exhibit random configuration under Brownian thermal motion. However, they become straightened out when deformed under an applied load and spring back to assume random shapes as quickly as their thermal motion permits.[59] Walley et al. investigated the rate sensitivity behavior of a number of polymers over the range of strain rates between 10 −4 and 103 s−1. They observed that at high strain rate, all the polymers examined exhibited improved yield stress.[61] During the deformation of a polymer, there is corresponding increase in stress as the strain increases. The high elasticity property of rubber is due to its inherent molecular structure.[59] Chou et al. examined the compressive behavior of various plastic materials and reported that the yield strength increased with the strain rate.[62] This is corroborated by the results obtained by Briscoe and Nosker, [63] who also stated that the flow stress increases with the strain rate. The influence of temperature at high strain rate was examined by Rietsch and Bouette. They studied the PC polymers compression yield stress over a wide range of strain rates and temperatures, and reported the significance of the secondary transition in order to account for low stress increase.[64] Chen et al. showed that there is a difference in the dynamic stress–strain behavior of polymers subjected to tensile stress compared to those subjected to compression.[65] The molecular structures of polymers are long, coiled, and flexible, and exhibit random configuration under Brownian thermal motion. However, they become straightened out when deformed under an applied load and spring back to assume random shapes as quickly as their thermal motion permits. [59] Nielsen [66] conducted the investigation on the atactic PS and observed that before reaching the glass transition temperature (Tg), there was an increase in the relative effect of molecular weight on mechanical properties as the experimental regimen advanced from elastic to viscoelastic, then to large strain and eventually to fracture. Matsuoka [67] observed that the molecular weight influences the polymer impact strength and toughness in the glassy state, and that the impact strength increases with the molecular weight. Walsh and Termonia investigated the fracture toughness dependence on the temperature and the molecular weight for PMMA. They observed a strong dependence on the fracture toughness on the temperature and the molecular weight distribution. [68]

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and global levels. As a result, material selection strategies which prioritize how the product life cycle is impacted only in certain phases of the product life cycle, i.e., by using Ashby Eco-Audits analytical methodology to determine the carbon footprint and the impacts of the energy consumption.[74] On the other hand, it may not be effective to use Ashby Eco-Audits strategies to assess the selected materials for components that are produced with nonrecyclable materials and which may evolve toxic fumes such as those in the interiors of the aircraft.[72] With regard to environmental concerns, it is essential to include indicators which measure the impact of toxic substances on the passengers and the environment, including the pollution resulting from the products disposal at the end of their life cycles.[72] In addition, the environmental concerns and complex passenger safety are expected to satisfy the restrictions on the chemical products as well as product disposal regulations.[72]

Hand lay-up Open mould

Spray-up Filament winding

Compression moulding Pultrusion Closed mould

Reinforced reaction injection moulding Resin transfer moulding

Fig. 8  Manufacturing techniques of polymer composites

Injection Molding MANUFACTURING TECHNIQUES OF AVIATION POLYMERS There are various feasible production techniques employed to manufacture composite polymers used in aviation (Fig.  8). They include injection molding, compression molding, filament winding, hand layup, prepreg molding, raising transfer molding, and pultrusion.[75] Characteristics, applications, advantages, and disadvantages of selected polymer composite manufacturing techniques are shown in Table 5.

Injection molding is one of the techniques used for composite materials to produce automotive components. [82] It is a process in which liquidized plastic or reinforced plastic material is injected into a mold and allowed to solidify, resulting in the production of the desired component. The mold refers to the hollow shape or structure of the component to be made, which is the main component of the injection molding technique. [83] The benefits of using this method include the possibility of mass production of products with little or no variation in the weight or related properties. The process can be done

Table 5  Selected manufacturing techniques for polymer composites Manufacturing techniques Characteristics Advantages

Disadvantages

Applications

References

RTM

Low operating Rapid, suitable for pressures complex geometry, fiber orientation can be controlled

Handling of high-toxic Automotive substances structural parts

[76,77]

Injection molding

Rapid, large volume production

Low labor cost and waste design flexibility

High initial tooling cost, part design restriction, difficult accurate costing

Fan blades, gears

[76,78]

Prepreg tape layup

Fibers, thermoplastic materials. Slow process, expensive, tasking. Speed can be automated for enhancement

Possibility of automation

Slow, labor intensive, high-cost materials

Aerospace structures

[76,79]

Filament winding

Suitable for complex parts, moderate speed, hollow component

Low cost, high speed

Needed mandrel can be expensive or complicated

Drive shafts, pipes, aircraft fuselage

[76,80]

Pultrusion

Continuous and consistent part cross section

Thin-walled parts can High-energy be produced. High requirement volume production

Columns and I-beams

[76,81]

Aviation: Polymer Composites, Processes, and Properties 207

Compression Molding Compression molding is one of the first molding techniques of plastics. The molding for this process is such that it is divided into two parts: the top and bottom parts. The material is placed onto the bottom part of the molding, and it is then pressed by the top part of the molding into the desired shape of the mold, which is then cooled in the mold and released as soon as the product is formed.[85] Compression molding is usually adapted for thermoset polymers, but it can also be used for thermoplastics. It can be used for mass production like the injection method, and it can be automated but still requires labor. The materials used in compression molding can also be used in the form of granules, soft masses, or performs.[86] Since the manufacturing does not involve the injection or the transfer cycle, it means that the tooling costs are much lower compared to other molding methods such as the injection molding. Compression molding is viable for low production since it is relatively costs less and can be employed for various sizes of components. It is mainly used to make large components. Unlike injection molding, the product made from the compression molding is not precise. This results in residual waste of thermosets or thermoplastics which can barely be recycled. Compression molding is not a fully automated process, hence requires a high labor cost. The products formed from this process are prone to contamination. This is because the plastic is exposed to dirt in the air which is also compressed in as the product.[87]

Filament Winding Filament winding is a manufacturing technique in which the thermoplastic material that is semihardened is wound up in strands onto a rotating mandrel until a layer is formed. Filaments are continuously wound onto the mandrel until the desired product is formed.[88] It had been originally used to make pipes and rocket parts.[88,89] Products of this manufacturing process are precise and well defined in design and orientation since the winding process is continuous and is of symmetry, and the products that can be formed are hollow structures. Over the years, more complex structures have been made by this process.[89] The process of the filament winding consists of the material rack, tension controller, preheating, heating zone, and the mandrel. The winding angel is the angle between the winding direction and the rotation axis, and the winding patterns can be adjusted in three ways—the pattern can be helical, be polar, or have a hoop pattern. The winding pattern depends on the angle formed between the winding path and the rotational axis of the mandrel. [90] The machines for filament winding are designed in such a way that they have individual motors in order to be able to adjust the angles for winding. Complex structures are formed by making various combinations to form geometries which are cylindrical, spherical, ellipsoid, cavity, disk, cylinder with domed ends, and cylinder without domed ends. [89] The fascinating thing about the filament winding process is that a two-dimensional material is transformed into a three-dimensional reinforcement structure. This opens an opportunity to make structures that are fabricated using the combination of braiding and winding.[90] Hand Layup Process Hand layup (Fig. 9) is a manufacturing technique which uses a one-sided mold which can be either a female or a male mold. The female mold is characterized by the mold dimensions that lie on the outer part of the mold, whereas the male mold is characterized by the dimensions of the mold being in the inner part of the mold. [91] The mold is layered with a mold release agent which facilitates easy removal of the component which is produced from this technique. A gelcoat resin is either sprayed or painted onto the mold so as to ensure an aesthetically smooth surfaced component without display of ridges of the mold. The component is made by lamination, on top of the gelcoat a certain amount of resin is poured onto the surface and is layered with a dry reinforcement. A roller is then used to evenly distribute the resin all over the mold. It also gets rid of voids and compacts the laminate. When this is complete, the process begins again by applying resin onto the dry reinforcement which is also layered with another dry reinforcement and rolled out. This process continues until the desired amount of reinforcement

Alginate–Aviation

with little machine supervision with no requirements for further production processes after the product is solidified and the products made are not porous. [82,83] When using injection process, the material is first presented in the form of pellets or powder. The material is then melted by rotating in a screen and a heated barrel. The melted material is then transported into a reservoir from where it is plunged into a steel mold. The mold is cooler than the material; hence, as soon as they come in contact, the molten material starts to solidify from the outer surface and boarders. Once the cooling is complete, the composite is ejected. [83,84] Injection molding can be used for the thermosets and thermoplastics. It is often used to mold fiber-reinforced thermoplastics. Since the length of the fiber can be relatively short (0.2–0.4 mm), fiber mats are embedded into the matrix of the thermoplastics to overcome such hindrances.[84] There are variations of injection molding. This is because the main components made in this process are predominantly for decorative purposes. There are three types of injection molding which are used to design structural and more complex components: reaction injection molding, reinforced reaction injection molding, and structural reaction injection molding which produces structural member strength.[82]

208

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Roller

Resin

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Gel coat Contact mould

Fig. 9  Hand layup technique

is accomplished. A topcoat like the gelcoat is then applied onto the surface. [92] The hand lay-up manufacturing technique is feasible due to its low tooling and capital costs. This technique can be used to manufacture components of varying sizes.  One can design the components of creative decorative finishes; it is used for prototyping, scaling up, and on-site fabrication, and it can accommodate ­special designs and strength requirements of the designed components. The hand layup process has limitations in that the component is designed perfectly only on one side. It is a process that is more labor intensive, and the quality of the product is only as good as the operator; hence, one runs the risk of inconsistencies in the design and the quality of the product. The shapes also depend on the skill of reinforcing the materials to take the shape and design of the mold and dangerous chemicals that are emitted from the resin system.[93] Prepreg Molding Prepreg is a manufacturing process in which fiber or fabric is reinforced and pre-impregnated with resin. [94] Layers of fibers are unidirectionally orientated and then they are pre-impregnated with matrix resin. The prepreg can then be laid up onto a mold to form a final product. [94] This can be done in two ways: using the vacuum bag molding and the autoclave molding. Both these processes are used in aerospace industry. The autoclave is used for material which requires the volume of the fiber to be high because unlike the vacuum, autoclave can achieve high pressures. [95] Prepreg does not require skilled workers, the amount of resin can easily be adjusted without affecting the ­geometry. The prepregs can make composites of any shape and does not require a lot of labor. Actually, this process can be totally automated and mass production with consistent qualities can be produced. There are barely poisonous chemicals involved in this process.[96] The disadvantages of this method are that it is expensive, the material is high in cost, and even though it can be automated, it still requires labor content and many layers are required to ­prepreg mold.

Resin Transfer Molding It is a manufacturing technique in which matching molds, the male and female molds, are connected to make a single mold and the sides are sealed. The preform which is impregnated is first compacted into the mold.[97] The injection phase is where the resin is put into the mold through a resin injection port which is on the female mold together with the vent port which removes air and prevents the loss of resin.[97] The process is complete once the product is cured. For aerospace structures, to ensure that the matrix is polymerized and that the internal stress is released, an additional curing process is applied.[97] Resin transfer molding (RTM) is beneficial because it can provide enhanced close dimensional tolerance, excellent surface finish, and is relatively the cheapest closed-mold process in terms of tooling, [98] and is the preferred manufacturing process for complex strong structures which require joint problems to be eliminated and have  high-quality strength and versatility.[97] The disadvantage of the RTM is that since complex designs are made, the mold and the tooling are quiet expensive. There is a relatively high waste product which results from the process and it is not reusable. The mold requires high maintenance and tool cleaning which is time consuming and dry ice blasters.[99] Pultrusion Pultrusion is a manufacturing technique which produces polymer composites that are reinforced with fiber. It is a continuous process in which the fiber is continually reinforced with thermosetting matrix.[100,101] The fibers are in strands on a creel which is then directed to be continually impregnated with liquid matrix resin which are then pulled through a heated die in order to make the final product.[100,102] This process cures the composite and the final product needs no other processes.[102] The final product of the composites is limited to the following symmetries, it is either rectangular, cylindrical, a hollow square, or any other shapes which are elongated.[101] The advantages of using the pultrusion are that it requires low labor since it is an automated process, it produces little waste and consistent quality products. The

Aviation: Polymer Composites, Processes, and Properties 209

AU

Urethane

CR

Neoprene

CSM

Hypalon®

®

Chemical name

Recommended shelf life

Polymer urethane

5 years

Chloroprene

15 years

Chlorosulfonated

Unlimited

EPDM

EPDM

Ethylene propylene

Unlimited

EU

Urethane

Polyether propylene

5 years

FEPM

Aflas

Tetrafluoroethylene propylene

Unlimited

FFKM

Kalrez®

Perfluorocarbon

Unlimited

FKM

Viton , Fluorel

Fluorocarbon

Unlimited

FVMQ

Fluorosilicone

Fluoro methyl vinyl silicone

Unlimited

IIR

Butyl

Isobutylene isoprene

15 years

NBR

Buna N, Nitrile

Butadiene acrylonitrile

15 years

®

®

®

PVMQ

Silicone

Phenyl methyl vinyl silicone

Unlimited

VMQ

Silicone

Methyl vinyl silicone

Unlimited

Source: Reprinted with permission by SAE © 2017 SAE International. Further distribution of this material is not permitted without prior permission from SAE.

limitations are the complex shapes; the only shapes that can be made are those with the same cross section along the length of the composite. The pultrusion cannot make composites with high accuracy in terms of the inner and the outer dimensions.[100,102] SHELF LIFE The shelf life of a polymeric material is the maximum timescale, beginning from the production time, that the material may be stored under certain conditions, beyond which it is considered as unserviceable for the specific original intended purposes. Optimum ambient conditions for safe storage of polymers include shielding from ozone, light, and humidity at a temperature lower than 32°C.[103] In addition, other factors that may also affect the shelf life during storage include deformation and contact with foreign materials, such as semisolid, liquids, metallic, and nonmetallic materials.[104] It is significant to note that the shelf life only concerns the functional application of the compound over a given period of time when properly stored. However, the shelf life is not related to how the product may or may not perform in any given application.[104] The shelf life of commonly used polymeric materials in aviation industry is indicated in Table 6.[105]

of polymer materials.[106] There are basically two types of recycling processes (Fig. 10): mechanical and chemical recycling (also called feedstock recycling).[107] Mechanical Recycling Process Mechanical recycling deals with the processing of polymer wastes via mechanical operations (grinding, washing, separating, drying, regranulating, and compounding), thus yielding recyclates that are convertible to new polymer products, often replacing virgin plastics. [108] This process is suitable particularly for only thermoplastic materials, i.e., plastic materials that can be remelted and reprocessed into new devices using processes such as extrusion or injection molding. They are commonly used for the following thermoplastic materials: polyethylene terephthalate (PET), polypropylene (PP), PS (solid-PS, expandable-EPS), PVC, and polyethylene (including

Mechanical process

Crushing and grinding

Pyrolysis

RECYCLING OF POLYMER MATERIALS Environmental concerns include driving the limiting use of finite resources, and preventing the continuous accumulation of polymer wastes in unregulated areas and landfills. In addition, tax incentives and legislation have been introduced by the government to encourage the recycling

Chemical process

Fluidized bed pyrolysis Micro-waves assisted pyrolysis

Fig. 10  General classifications of recycling processes

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Table 6  Shelf life of selected polymers ASTM designation Common or trade name

210

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low-density polyethylene (LDPE), linear LDPE, and high-density polyethylene (HDPE)).[107] The basic operations of mechanical recycling consist of cutting of the large plastic parts using shears or saw for further processing. The plastics are shredded by chopping them into small flakes to allow for the separation of materials (e.g., paper, glass, metals, etc.) and plastic types. This is followed by additional sorting upon shredding the material. Contaminants which may include ferrous metals and papers  are separated from the plastics in the cyclone separators while wet phase can be used to separate glues and liquids. Various types of plastics can be separated in a floating tank according to the individual density. This is followed by extrusion, where the pellets, flakes, and agglomerates are then fed into an extruder and heated. The melt is then forced through, converting it into a strand of continuous plastic product, followed by filtering using a 100–300 µm metal mesh. The strands produced are then cooled by water and chopped into pellets, which may be used for manufacturing new plastic products. [107] Chemical Recycling Process Chemical recycling (feedstock chemical) is an alternative route of recycling plastic materials other than mechanical recycling and petrochemical feedstock recycling. It deals with the processing of plastics by means of heat (thermolysis) and/or chemical means (chemolysis) by depolymerization or breaking down of polymer structures into their original building blocks (monomers) or other hydrocarbon products, which may be used to produce new polymers, fuels, or refined chemicals. It is best suited for clean single plastic fractions due to the chemical specificity of various resins. [109] An advantage of the chemical recycling process is that it is possible to recover the polymer petrochemical constituents, which can be subsequently used to reproduce plastic materials or to produce other synthetic chemicals. [110] Thermal Recycling Processes Thermal recycling processes are a group of processes which include pyrolysis, fluidized bed pyrolysis, and microwave-assisted pyrolysis. [111–115] Thermal recycling process is regarded as technically feasible, [110] and it basically involves the reduction of polymers down to their monomers or to produce derivatives of petro-based chemicals. [115] In general, a major drawback associated with this method is that it is very expensive and the initial cost of plant. [116]

Aviation: Polymer Composites, Processes, and Properties

of heat and pressure, [115] with limited or in the absence of oxygen.[117] When the pyrolysis process is carried out under an oxygen-free atmosphere, liquid and gaseous products are produced by the organic parts of the degraded material, which can be used as chemical sources and/or as fuels. Important parameters that can affect the process are time and temperature. There are various temperature spectra suggested by different authors for the process, which range between 350°C and 900°C and timelines of between 15 min and 2 h.[118–122] Fluidized Bed Process Fluidized bed process is an effective method of combusting polymer matrix and subsequently releasing and collection of filler materials and glass fibers. Oxygen gas is used to minimize the formation of char.[123] Polymer matrix can be volatized with organic contaminants from the fluidized bed, i.e., mineral oil, whereas metal inserts and other inorganic solid materials, i.e., metal inserts are submerged. The metal inserts can be recovered by degradation of the fluidized bed.[123] Fluidized bed process is beneficial for the treatment of mixed or contaminated materials with painted surfaces, metal inserts, or foam cores, while its major disadvantage is that it hinders the recovery of solid products from the resin besides the gases.[124] Microwave-Assisted Pyrolysis Microwave-assisted pyrolysis is a promising heating system that reduces composite matrix into oil and gases in an inert atmosphere through internal rapid heating by irradiation from a microwave. [125] An important benefit of microwave-assisted pyrolysis is associated with the fast-heating and high-heating efficiency arising from the heat generated within the material as compared to the conventional methods with external heat source. [126,127] In general, microwave-assisted pyrolysis exhibits high-­ energy recovery between 60% and 80% of the system input energy, i.e., electrical energy input and the added waste material calorific value. [128–131] Russell et al. [132] investigated the effects of microwave-assisted pyrolysis of HDPE with the aid of catalytic activated carbon reactor bed. The activated carbon served two major purposes: a catalyst in the cracking process and acting as both envelope and transferring energy for processing the microwave-transparent material. The authors reported an increased cracking across all processing temperatures and production of a lighter liquid, which exhibit a narrower chain length compared to when conventional coke bed is used.

Pyrolysis Solvolysis Pyrolysis is the chemical degradation of organic substances by heating to elevated temperatures in order to break polymer chains into smaller units under the application

Solvolysis is a collective term for a class of depolymerization processes which include hydrolysis, alcoholysis,

aminolysis, acidolysis, and various reactions that result in production of monomers or oligomers. Solvolytic recycling processes are primarily applicable to thermoplastics and thermosets made by step growth polymerization such as PU, PC nylon, and PET.[133] Advantages of solvolysis recycle process include a wide spectrum of operating temperature, pressure, catalysts, and solvents. In addition, it exhibits better degradation of polymers even at lower temperature, making it to be more beneficial compared to pyrolysis, particularly for processing unsaturated polyesters and epoxides. The drawbacks associated with this process include corrosion challenges of the process equipment and very expensive reactors to operate at the required high pressures and temperatures at supercritical conditions of water.[134] CONCLUSION: OPPORTUNITIES AND CHALLENGES From the time that phenolic resin was developed in 1909, the use of polymers and polymer composites took off in aircraft and continued to develop rapidly in the 1940s by attracting great interest from the aviation industry by exploiting their unique properties such as durability, improved corrosion resistance and specific strength, and stiffness to enhance lightweight structural design. Polymers and polymer composites have since gone a long way. In addition, a high performance laminated composite material structure enables designers to adjust the optimum mechanical properties by aligning the fiber direction along the primary load paths. Furthermore, certain polymer composites exhibit relative ease to produce complex shapes and possess excellent fatigue properties which enable them to be increasingly desirable in the renewable energy sector. However, polymers and polymer composites are still far from reaching their full potentials. The major challenges in the application of polymers in aviation industry include corrosion challenges encountered over time when CFRPs come in direct contact with the aluminum shell structure. Thus, contact between aluminum parts and carbon at lug attachments must be prevented. Low wear resistance, changes to polymer composite properties due to water pickup, and recycling problems also pose challenges. Problems can also be encountered due to the mismatch stresses which may be created at attachment points due to the variation in thermal expansion and thermal conductivities of different polymer composites used within and outside of the aircraft. Weight savings can also be compromised due to poor electrical conductivity of polymer composites, which often necessitate the inclusion copper mesh in the aircraft laminates to prevent lightning strike hazards. In addition, there are a wide range of composite failure modes in polymer composites which include resin failure, delamination, fiber failure, fiber/matrix debonding, etc. It is significant to note that it can be very challenging

to accurately predict the failure load since the mode of failures are often related. The failure of polymer composites is usually catastrophic and sudden with no prior signs or warning of loading conditions. However, it is significant to state that some of these problems have been solved individually by researchers. Current and future research activities in the aviation industry and higher education in the field of advanced composites are believed to find solutions to the challenges in the nearest future.

ACKNOWLEDGMENTS The financial support of the National Research Foundation (NRF) of South Africa toward this work is acknowledged. The views and opinions expressed in this entry are ­exclusively those of the authors and not of NRF.

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behaviour of poly-methyl-methacrylate: Experimental and modelling analyses. Mater. Des. 2012, 37, 500–509. 58. Ward, I.M.; Sweeney, J. Mechanical Properties of Solid Polymers, 3rd Ed.; John Wiley & Sons, Ltd: London, 2013, 19–29. 59. Bhowmick, A.K. Mechanical properties of polymers. In  Materials Science and Engineering; Eolss Publishers: UK, Vol. 1, 2009, 157–172. 60. Microstructures and Properties of Polymers. Available at http://pimm.paris.ensam.fr/en/node1443, (accessed in September 15, 2016). 61. Walley, S.M.; Field, J.E.; Pope, P.H.; Safford, N.A. A study of the rapid deformation behaviour of a range of polymers. Philos. Trans. R. Soc. London, Ser. A 1989, 328 (1597), 1–33. 62. Chou, S.C.; Robertson, K.D.; Rainey, J.H. The effect of strain rate and heat developed during deformation on the stress–strain curve of plastics. Exp. Mech. 1973, 13 (3), 422–432 63. Briscoe, B.J.; Nosker, R.W. The flow stress of high density polyethylene at high rates of strain. Polym. Commun. 1985, 26 (10), 307–308. 64. Rietsch, F.; Bouette, B. The compression yield behaviour of polycarbonate over a wide range of strain rates and temperatures. Eur. Polym. J. 1990, 26 (10), 1071–1075. 65. Chen, W.; Lu, F.; Cheng, M. Tension and compression tests of two polymers under quasi-static and dynamic loading. Polym. Test. 2002, 21 (2), 113–121. 66. Nielsen, L.E. Mechanical Properties of Polymers and Composites, Vol. 2; Marcel Dekker Inc: New York, 1974, 379–452. 67. Matsuoka, S. Relaxation Phenomena in Polymers; Hanser: Munich, Germany, 1992, 8–34. 68. Walsh, D.J.; Termonia, Y. How molecular structure affects mechanical properties of an advanced polymer. Polym. Commun. 2000, 1–14. 69. Sawyer, L.S.; Grubb, D.T.; Meyers, G.F. Polymer Microscopy, 3rd Ed.; Springer: New York, 2008, 2–4 70. Miller-Chou, B.A.; Koenig, J.L. A review of polymer dissolution. Prog. Polym. Sci. 2003, 28 (8), 1223–1270. 71. Farag, M. Materials and Process Selection for Engineering Design, 2nd Ed.; CRC Press: New York, 1997, 304. 72. Santos, C.V.D.; Leiva, D.R.; Costa, F.R.; Gregolin, J.A.R. Materials selection for sustainable executive aircraft interiors. Mater. Res. 2016, 19 (2), 339–352. 73. Spampinato, A. The Materials Used in the Design of Aircraft Wings. Available at http://www.azom.com/­a rticle. aspx?ArticleID =12117, (accessed in December 22, 2016). 74. Ashby, M.F. Materials and the Environment—Eco-­ Informed Material Choice; Elsevier: London, 2009, 227–274. 75. Mohammad, F. Specialty Polymers: Materials and ­Applications; I.K International Publishing House Pvt. Ltd: New Delhi, 2007, 246. 76. U.S. Congress, Office of Technology Assessment, Advanced Materials by Design, OTAE-351 (Washington, DC: U.S. Government Printing Office, June 1988). 77. Vlachopoulos, J.; Strutt, D. Polymer processing. Mater. Sci. Technol. 2003, 19 (9), 1161–1169. 78. Advantages and Disadvantages of Injection  Moulding. Available at http://www.avplastics.co.uk/advantages-and-​

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115. Johnson, M; Derrick, S. Pyrolysis: A method for Mixed Polymer Recycling. Green Manufacturing Initiative, ­September 07, 2010. 1–21. Available at https://wmich.edu/ mfe/mrc/greenmanufacturing/pdf/GMI%20Pyrolysis%20 of%20Mixed%20Polymers%20Review.pdf, (accessed in December 21, 2016). 116. Patel, M.; von Thienen, N.; Jochem, E.; Worrell, E. Recycling of plastics in Germany. Resour. Conserv. Recycl. 2000, 29 (1), 65–90. 117. Hartulistiyosoa, E.; Sigiroa, F.A.P.A.G.; Yuliantoa, M. Temperature distribution of the plastics Pyrolysis process to produce fuel at 450°C. Procedia Environ. Sci. 2015, 28, 234–241. 118. López, A.; De Marco, I.; Caballero, B.M.; Laresgoiti, M.F.; Adrados, A. Influence of time and temperature on pyrolysis of plastic wastes in a semi-batch reactor. Chem. Eng. J. 2011, 173 (1), 62–71. 119. Miskolczi, N.; Angyal, A.; Bartha, L.; Valkai, I. Fuels by pyrolysis of waste plastics from agricultural and packaging sectors in a pilot scale reactor. Fuel Process. Technol. 2009, 90 (7), 1032–1040. 120. Park, S.S.; Seo, D.K.; Lee, S.H.; Yu, T.U.; Hwang, J. Study on pyrolysis characteristics of refuse plastic fuel using labscale tube furnace and thermogravimetric analysis reactor. J. Anal. Appl. Pyrolysis 2012, 97, 29–38. 121. Sugano, M.; Komatsu, A.; Yamamoto, M.; Kumagai, M.; Shimizu, T.; Hirano, K.; Mashimo, K. Liquefaction ­process for a hydrothermally treated waste mixture containing plastics. J. Mater. Cycles Waste Manage. 2009, 11 (1), 27–31. 122. Mastral, F.J.; Esperanza, E.; Garcia, P.; Juste, M. Pyrolysis of high-density polyethylene in a fluidised bed reactor. Influence of the temperature and residence time. J. Anal. Appl. Pyrolysis 2002, 63 (1), 1–15. 123. Francis, R. (Ed.) Recycling of Polymers: Methods, Characterization and Applications; John Wiley & Sons, Ltd: London, 2016, 115–140. 124. Pickering, S.J. Recycling technologies for thermoset composite materials—Current status. Compos. Part A 2006, 37 (8), 1206–1215. 125. Schinner, G.; Brandt, J.; Richter, H. Recycling carbon-­ fiber-reinforced thermoplastic composites. J. Thermoplast. Compos. Mater. 1996, 9 (3), 239–245. 126. Neves, D.; Thunman, H.; Matos, A.; Tarelho, L.; GómezBarea, A. Characterization and prediction of biomass pyrolysis products. Prog. Energ. Combust. Sci. 2011, 37 (5), 611–630 127. Lam, S.S.; Chase, H.A. A review on waste to energy processes using microwave pyrolysis. Energies 2012, 5 (10), 4209–4232. 128. Lam, S.S.; Russell, A.D.; Lee, C.L.; Chase, H.A. ­Microwave-heated pyrolysis of waste automotive engine oil: Influence of operation parameters on the yield, composition, and fuel properties of pyrolysis oil. Fuel 2012, 92 (1), 327–339. 129. Huang, Y.F.; Kuan, W.H.; Lo, S.L.; Lin, C.F. Hydrogen-rich fuel gas from rice straw via microwave-induced pyrolysis. Bioresour. Technol. 2010, 101 (6), 1968–1973. 130. Zhao, X.; Zhang, J.; Song, Z.; Liu, H.; Li, L.; Ma, C. Microwave pyrolysis of straw bale and energy balance analysis. J. Anal. Appl. Pyrolysis 2011, 92 (1), 43–49.

131. Tian, Y.; Zuo, W.; Ren, Z.; Chen, D. Estimation of a novel method to produce bio-oil from sewage sludge by ­m icrowave pyrolysis with the consideration of efficiency  and safety. Bioresour. Technol. 2011, 102 (2), 2053–2061. 132. Lam, S.S.; Russell, A.D.; Lee, C.L.; Lam, S.K.; Chase, H.A. Production of hydrogen and light hydrocarbons as a potential gaseous fuel from microwave-heated pyrolysis of

waste automotive engine oil. Int. J. Hydrogen Energy 2012, 37 (6), 5011–5021. 133. Azapagic, A.; Emsley, A.; Hamerton, I. Polymers: The Environment and Sustainable Development; John Wiley & Sons, Ltd: London, 2003, 189–190. 1 34. Kritzer, P. Corrosion in high-temperature and supercritical water and aqueous solutions: A review. J. Supercrit. Fluids. 2004, 29 (1), 1–29.

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Aviation: Thermoplastic and Thermoset Polymers in M. Özgür Seydibeyoğlu Department of Materials Science and Engineering, Izmir Katip Celebi University, Izmir, Turkey

Fatma Erdoğan Alginate–Aviation

Department of Materials Science and Engineering, Ege University, Izmir, Turkey

Elif Alyamaç Department of Petroleum and Natural Gas Engineering, Izmir Katip Celebi University, Izmir, Turkey

Abstract This chapter presents a review of polymers and composites used in the aviation industry with new developments and standard protocols. This chapter will enable the readers to understand the aviation industry from a polymer and materials science perspective. It helps to understand the rapid growth of the aviation industry with high demand of lightweight structures. The chapter deals with all aspects of polymers including thermoplastics, thermosets, and composites. Qualification tests and some tests are given to get a better understanding of polymeric materials used in the aviation industry. Finally, some recent new developments are presented with a futuristic outlook. Keywords: Aerospace; Aviation; Composites; Polymers; Qualification tests; Thermoplastic; Thermoset.

INTRODUCTION

Aerospace and Defense

The aerospace industry is one of the fastest growing sectors today. Along with the developing technology, the materials used in this area have started to change. In particular, demand for polymer materials that have a significant share of lightness and fuel saving has begun to increase. Aircrafts, helicopters, space, and defense industries are some of the industries in which the use of polymers is widespread. Therefore, this chapter contains some information about the p­ olymer ­materials and derivatives used in the aviation industry.

The aerospace industry is one of the largest manufacturing industries in the world in terms of both the number of employees and the value of output. As a result, a country possessing an aerospace industry also has economic and military strength. In this regard, the United States has the largest market size in the global aerospace industry, and it is followed by Europe dominantly and grown by Asian participants, respectively (Fig. 1).[4,5] The global airline industry remains a large and ­growing industry. Based on the International Air Transport

AEROSPACE INDUSTRY OVERVIEW Aerospace is a special industry that includes research, development, and manufacture of vehicles moving through air and space.[1] The aerospace industry has many different areas such as unpowered gliders, sailplanes, balloons, aircrafts, missiles, and spacecrafts.[2] Classification of these products can be defined in many ways. Airplane can be classified into various types based on the size of the aircraft such as airliners, commuter transports, business jets, and general aviation airplanes. On the other hand, it can be divided into two main categories, i.e., civil products and military products or according to technological development of the products such as avionics, structural components, hydraulic systems, propulsion systems, and flying vehicles.[3]

216

Space General aviation

Rotarty aircrafts Aerospace

Commercial aircrafts

Military aircrafts Regional jets

Fig. 1  The aerospace market segments [4] Encyclopedia of Polymer Applications, First Edition DOI: 10.1201/9781351019422-140000331 Copyright © 2018 by Taylor & Francis. All rights reserved.

Association (IATA) data, the total revenue of airline ­industry has doubled over the past decade.[6] During this time, travel demand has also increased at an annual growth rate of 4.7%.[7] Besides, with international business growth creating investment opportunities, supply and production have led to a rise in business travel. The travel demand has been driven by global demographics and welfare increase in the Middle East and Asian countries results in a ­considerable increase for new aircrafts [7] According to Fig. 3, aircraft production from 2009 as annual estimated production through to 2035 shows the growth experienced by the commercial aerospace subsector (Fig. 2). Similarly, the amount of military products in aerospace sector is also predicted to increase in the coming years. Especially, with growing global tensions, international requisition for defense industry is rising in the Middle East, Eastern Europe, North Korea, and the East and the South China Seas. This situation results in the increased military expenditure globally, particularly in Saudi Arabia, India, South Korea, and Japan, many of which have increased the procurement of new generation military equipment.[7]

Technological improvements are the basis for c­ ompetitiveness of companies and advancements of the aerospace industry. Especially high quality and performance expectation from aerospace products increase the need for advanced technology in the production of these parts.[9] Nowadays, a there is dense research conducted on aerospace structures, aerodynamics, propulsion, materials, and manufacturing. There is a new trend to make aerospace parts with 3-D printing. Nowadays, sustainable and environment-­friendly production techniques also influence the aerospace industry. Nanotechnology looks promising, but still not many commercial products are in the market. In this chapter, besides all these developments, the recent developments in the materials used in aerospace will be described. As the materials develop, many standards change and novel applications arise. Especially, lighter materials are very critical for aerospace and the whole transportation industry as there is a significant reduction in oil consumption and CO2 emission. A special emphasis will be given to polymers and composites for the Encyclopedia of Polymer Applications.

Aircraft production (2009 to 2035F)

2500

Aircraft units

2000 1500 1000

0

2009 2010 2011 2012 2013 2014 2015 2016E 2017E 2018E 2019E 2020E 2021E 2022E 2023E 2024E 2025E 2026E 2027E 2028E 2029E 2030E 2031E 2032E 2033E 2034E 2035E

500

Fig. 2  Aircraft production since 2009, annual estimated production through to 2035 [7]

Global A&D sector revenues

680

658.7

660 Revenues (US $ billions)

674.4

634.6

640 622.8

620 603.5 600 580 560

2012

2013

2014

Fig. 3  Five-year history of global aerospace and defense sector revenue [8]

2015

2016

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INCREASED ROLE OF POLYMERS AND COMPOSITES FOR AVIATION INDUSTRY IN THE NEW CENTURY

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With the development of science and technology, much progress has been made in accordance with the requirements of the aviation industry. Aerospace structures requirements and their effect on the design of the structure are shown in Table 1.[10] Based on Table 1, all space programs need to decrease weight and have high reliability requirements due to fuel saving and providing easy access for maintenance of equipment. Passenger transportation requirements contain safety standards such as fire retardancy and crashworthiness on the materials and design of vehicle. For spacecrafts, fatigue and corrosion resistance are very important, besides for the space environment vacuum, thermal cycling and radiation are crucial parameters being considered. Scientific and technological field has two main elements that guide the development of the aerospace industry. The first of these are the improvements in the computational power and the second is the advanced composite material technology using fiber-reinforced polymeric materials. Table 1  Features of aircraft structure Requirement •

Light weight

Almost 40 years ago, aluminum was used predominantly in the aviation industry. Other raw materials such as alloys and composites were also used, including titanium, graphite, and fiberglass in a small amount of 3%–7%.[11] However, since the 1970s the use of plastics in the aviation industry has quadrupled [12] For example, typical jet contains almost 20% pure aluminum today. Carbon fiber-­ reinforced composites and honeycomb structures are the most commonly used nonmetal materials in the aviation industry. Particularly, some features such as lightweight, ease of handling, low-cost, corrosion resistance, and higher temperature resistance are also the reasons for plastics and composites to increase demand in the aviation sector. Polymer Classification in Aerospace Polymers in the aerospace industry have been successfully used both in composite and unreinforced form for several years. Some advantages of polymers are as follows: • • •

Easy fabrication of products Lighter than metals Good toughness and damage resistance

Applicability All aerospace programs

Effect •

•

• •

High reliability

All aerospace programs

• • •

•

Passenger safety

Passenger vehicles

• •

•

Durability and fatigue and corrosion degradation: vacuum radiation thermal

Aircraft Spacecraft

•

• • • • •

Aerodynamic performance

Aircraft Reusable spacecraft

• •

•

•

Multi-role or functionability

All aerospace programs

• •

•

Fly-by-wire

Aircrafts, mostly for fighters but also some in passenger

•

• • •

Stealth

Specific military aerospace applications

•

All-weather operation

Aircraft

•

•

Semi-monocoque construction Thin-walled-box or stiffened structures Use of low-density materials: Wood, Al alloys, composites High strength/weight, high stiffness/weight Strict quality control Extensive testing for reliable data Certification: proof or design Use of fire-retardant materials Extensive testing: crashworthiness Extensive fatigue analysis testing Al-alloys do not have a fatigue limit Corrosion prevention schemes Issues of damage and safe life, life extension Extensive testing for required environment Thin materials with high integrity Highly complex loading Thin flexible wings and control surfaces Deformed shape-aero elasticity, dynamics Complex-contoured shapes Manufacturability: machining: molding Efficient design Use: composites with functional properties Structure–control interactions Aeroservoelasticity Extensive use of computers and electronics EMI shielding Specific surface and shape of aircraft Stealth coatings Lightning protection, erosion resistance

• • • •

Corrosion resistance Flame retardancy High thermal and mechanical stability Good electrical insulation [13,14]

•

Polymers can be divided into two main parts: thermoplastics and thermosets. The composite applications of polymers will also be described in this chapter (Fig. 4). STRUCTURAL AND NONSTRUCTURAL MATERIALS FOR AVIATION

•

a. Thermoplastics b. Thermosetting resins c. Composites Thermoplastics Thermoplastics are high-density polymers which have linear or branched structures. This property gives the ability to be repeatedly melted and formed into product unlike conventional thermoset resins.[16,17] Therefore, thermoplastics are very convenient materials to use in many sectors as well as aerospace industry. Most commonly used thermoplastics in aerospace industry are as follows: polyetheretherketone (PEEK), polyphenylenesulfide (PPS), ­polytetrafluoroethylene (PTFE), and polyetherimide (PEI).[18,19] •

PEEK: It is a semicrystalline organic polymer. It is commonly used in commercial area. It is the basis of aerospace thermoplastics and being used in the industry for more than 20 years.[20,21] PEEK has excellent mechanical performance and thermal resistance, flame retardancy, and chemical resistance. In addition, it has perfect hydrolysis resistance, so it can be exposed to high-­pressure water and steam for a long time. Because of these properties, PEEK is preferred in aerospace industry and generally in conditions such as low temperatures or particulate atmospheric ­environments. Its usage areas are commonly valve seats; pump

Propulsion systems

•

gears; and primary, secondary, and internal aircraft s­ tructures.[18,19] PPS: It is also a semicrystalline polymer like PEEK but has a lower Tg and Tm. It is thermally stable and shows flame retardant property. It is also chemically resistant even at high temperatures and has good mechanical properties. PPS is mainly used in the external sides of airplanes in a composite form. Some examples are fixed wing leading edges of Airbus A340 and A380, keel beams, and so on.[20,21] PTFE: It is also known as teflon, and it is a semi­ crystalline fluorocarbon polymer. It has a great ­thermal stability and chemical resistance. In addition, it is flame retardant, has electrical and high tear resistance. Despite all superior properties, PTFE has relatively lower mechanical strength than other polymers; so, if it is used with some reinforcing materials as composites, it will give better results for the required more mechanical stability. PTFE is usually used for insulating the myriad wires and cables in aircrafts. PEI: It is an amorphous polymer, which has good mechanical strength and rigidity. It has also good dimensional stability, electrical insulation, very high heat resistance, and radiation resistance. PEI is suitable for many aircraft structures, for example, Airbus floor panels, pressure bulkheads and Gulfstream rudder ribs etc. It is also used in aerospace components for antenna structures and aircraft interior equipment.

Thermosetting Resins Among polymer types, thermosetting resins and curing have been the starting points of the polymers since the 1750s with the discovery of natural rubber. In 1839, it was discovered that rubber can be cross-linked with sulfur and this has been the starting point for the thermosetting resins. In 1847, unsaturated polyester resins were developed and later further resins were developed. In the context of the polymers in aviation, the structural polymers and composites play an important role, and therefore only ­structural resins will be mentioned in this chapter.

Cabin areas

Thermal control materials

Polymers in aerospace

Satellite

Fig. 4  Applications of polymers in aerospace [15]

Airframe

Components

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Unsaturated polyester resin was the first structural resin used in composites and still for most of the composite applications; unsaturated polyester is commonly used but for aircraft and aerospace structures, it is not preferred much due to low-mechanical properties. Vinyl ester, which is another form of unsaturated polyester, is also under dense research to optimize price and performance having properties between unsaturated polyester and epoxy. Phenolic resins have been used for long time but still due to its brittleness and low shrinkage during processing, it is less preferred. For the aviation industry, epoxy is the commonly used resin because of its high-strength value. Aerospace industry has always made great and successful use of epoxy resins not just as composite matrices, but also for adhesive bonding. In some applications, especially for high temperature applications, polyimides are the suitable resins because of their high heat resistance, high temperature stability, good impact and tensile performance, and an inherent resistance to combustion. There is also a growing interest in the range of modified polyimide resins for aerospace industry. These include bismaleimide resins, due to their higher thermal stability and comparable mechanical properties, processability, and cost with high-performance epoxies.[22] Cyanate ester resins are high-temperature thermosetting resins, and their use is limited to aerospace a­ pplications due to their high cost. Composites Composites have played an important role throughout human history, and they have been used since ancient times in the form of natural fiber and clays. In the last century, with the development of polymers, polymer matrix composites (PMCs) have gained considerable attention due to their lightweight structure, no corrosion problem, and better fatigue performance. Moreover, the processing conditions of PMC materials are much cheaper compared to metal manufacturing. Fiber-reinforced PMC composites are very crucial for the transportation industry including automotive, railways, marine, and aviation due to low fuel consumption and reduced CO2 emissions. Among many composites, glass fiber and carbon fiber-reinforced composites have been commonly used.[23] Besides fiber-reinforced composites, sandwich structures have been also very critical for many aerospace structures including honeycomb material being used for many decades.[24] The use of composites in the aerospace industry has grown with a very fast pace with 100% growth rate and with compound annual growth rate of 11.1%.[25] The composite materials with a unique design that can combine two different properties are very attractive for the aviation industry as composites are very light and much stronger than steel alloys and especially aluminum alloys.[26] The class of polymer matrix composites was mainly ­developed after the discovery of the glass fiber in

Aviation: Thermoplastic and Thermoset Polymers in

1937 and the 1950s, there was interest in the hand layup technique, where the glass fibers were woven and combined with polyester resins and phenolic resins. In the 1960s, there were studies to replace metallic alloys with PMC. In 1969, with the introduction of Nomex from DuPont, honeycomb sandwich composites were used in Boeing airplanes. Sandwich structures and fiber composites developed in the same period but fiber composites have been more predominant, especially in the last two decades, due to various processes that have been developed, and more important new types of fibers, including carbon fiber and aramid fiber, were developed changing the whole structure of the composites.[27] With the introduction of various novel-manufacturing techniques, such as resin transfer molding, quickstep manufacturing techniques, prepreg techniques, and other techniques, the composites are transformed into many different forms and this resulted in material replacement with composites. Recent trend is to replace thermoset resins with thermoplastics and the use of advanced nanotechnology in composite structures.[27] QUALIFICATION PROCEDURE FOR AVIATION INDUSTRY Materials that are used for medical, defense, and aerospace have to be formally qualified to avoid any risk. Although the qualifications tests are various for different applications and industries, the main goal for these tests is to demonstrate that the process and the material used will perform as expected with sufficient data. Aviation industry is highly critical for civil and military applications since a failure in an airplane results in a catastrophic manner, and it is developed both for civil people and, during the war, for military purposes. Extensive tests including thousands of individual tests and data would generally take minimum five years, and these tests can be extended to 15 years. Moreover, a small change in the material or ­process requires re-qualification tests. Qualification tests take long time to fully understand  the real behavior of the materials under various conditions including fatigue tests, corrosion tests, and conventional mechanical tests. There are mainly three paths to ­qualification tests:[28] a. Statistical based qualification based on extensive empirical testing. b. Equivalence-based qualification via moderate testing to measure properties of new materials and processes. c. Model-based qualification test where the ­performance of new process and/or materials is ­demonstrated with a computer model. For testing aerospace parts, there are various tests including nondestructive testing and destructive testing.

Among these tests for polymers and composites, the ­following tests are highly critical: a. b. c.

Mechanical testing i. Tensile testing ii. Compressive testing iii. Flexural testing iv. Interlaminar (interfacial) shear strength tests Fatigue test Creep test

There are many other tests but they are out of the scope of this chapter.

is very catastrophic due to number of death in airplane accidents. Recently, self-healing materials have been developed in many areas including concrete and ceramics. For this chapter, self-healing polymers and composites are critical and there are three main groups for the self-healing resins. The first one is capsule based, the second one is vascular, and the third one is intrinsic.[31] These three different composites have been studied. Capsule based composite is already commercialized using various resins in capsules for automotive industry. Vascular composite which is a kind of biomimetic material is under heavy research and some significant outcome is about to be commercialized. Intrinsic composites are the new generation ­self-healing composites and many new findings are appearing for ­aerospace structures.

SPECIAL TOPICS IN POLYMERS FOR AVIATION As the polymer science and materials science develop, many new forms of materials are formed. Smart materials including many different aspects like biomimetic materials, self-healing materials, shape memory materials, and nanomaterials are some of the new generation materials. These new materials influence the design and performance of many structural components in different applications including construction applications, architectural materials, textile materials, automotive materials, and aviation materials. Biomimetic Materials As the name refers, the biomimetic materials are important class of materials that mimic the nature and try to form new materials using natural hierarchical orders. Biomimetic is very critical for aviation as the first biomimetic studies have been observed in flying objects. Leonardo da Vinci studied anatomy in depth and formed numerous sketches for flying objects.[29] The airplane was first invented by the Wright Brothers in 1903 which was heavier than air and hence could fly.[30] Another interesting example for aviation is the design of helicopters, which was inspired by a helicopter insect, also known as dragon fly. There are numerous studies on polymers and composites that are inspired by the nature and also affect the aerospace materials. A very recent material was developed by Airbus mimicking the shark skin and the material has been studied for 30 years. A shark minimizes energy when it is in motion due to expansion, and this structure is adapted to airplanes to minimize fuel consumption.[30] Self-Healing Polymers Cracks are one of the key problems in today’s structural materials. Many materials experts work on failure a­ nalysis of materials including metals, ceramics, and polymers. Many researchers have tried to avoid cracks using ­various materials manufacturing techniques. Cracks are very detrimental for the airplane industry as the failure of a plane

Nanotechnology Nanomaterials and nanotechnology have been present throughout the world and human history, and currently, it is possible to detect nanostructures in the materials world via advanced microscopy techniques. Nevertheless, materials scientists are constantly searching for new structures at the nanoscale and they try to find new applications especially in the polymer and composite area.[32] In the area of polymers, nanoparticle-enhanced composites have been critical for structural applications replacing fiber-­reinforced composites due to improved toughness and transparent properties. There are also various studies that use nanoparticles and nanotechnology to improve the properties of fiber-­reinforced composites.[32] These kinds of studies show that mechanical properties and/or toughness of the fiber-­reinforced composites can be further improved with nanoparticles. Moreover, new studies show that some functional properties can be incorporated into the composites such as lightning preventing composites, special sensing capabilities, and radiation shielding properties.[32] FUTURE OUTLOOK FOR AVIATION As the need for green technologies and less fuel consumption increases, the need for lightweight aerospace structures will be more demanding. There is enormous research to manufacture lighter airplanes using polymers. The driving force to replace the metallic and ceramic materials by high-performance polymers for aerospace applications is to reduce the weight of the spacecraft. There has been a significant transformation from metals to polymers and composites in the last 50 years of aviation. During this transformation, many resins and new fibers have been developed. Carbon fiber is changing all the rules of composites with advanced mechanical properties enabling aerospace engineers to design new structures with s­ ignificant weight reduction.

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With industry 4.0 and increased usage of automation, new manufacturing techniques like automatic tape ­placement/automatic fiber placement methods are becoming more and more important for composites in aerospace structures. Automation helps to minimize production faults and help the companies economically reduce the number of workers. As the need for eco-friendly materials increases, the shift to composites is transforming to use environment-friendly composites. In this respect, thermosetting resins are now being replaced with thermoplastic resins. There is quite much interest on polyetherimide, polyphenylene sulfide, polyether ether ketone, and polysulfone polymers to use in the form of carbon fiber/glass fiber-reinforced composites for structural applications. For nonstructural applications, polyamide and its composites are commonly used. For some elastic applications, thermoplastic polyurethane and other elastomers are also commonly used. In the area of polymers, the need for green polymers and bio-compostable polymers is not the priority in aviation but still need for greener materials is under extensive research with many qualifications tests. The natural fiber composites are not comparable to carbon fiber and other structural composites but still some nonstructural ­applications are to be commercialized soon. For aerospace and aviation, the use for smart materials decorated with nanotechnology and other electronic structures is the current ultimate goal for this industry. Smart piezo electric materials, conductive polymer composites, and other electrochemical system embedded systems are on the way to be commercialized. This encyclopedia of polymer applications is very important for many polymer researchers both in the industry and in the academia, and it is also very critical for graduate and undergraduate students. In this chapter, all the aspects of polymers and composites in the aviation sector have been given with a slight in-depth knowledge. Further references have been stated in the references and we hope that it will be the key for many advanced applications of polymers as aviation and defense industries are the starting point for many commercial-scale polymers and composites. ACKNOWLEDGMENT Izmir Katip Celebi University Scientific Project, 2016-ÖNP-MÜM-0002 is highly acknowledged for the composite research. Support from “PARE” project within Horizon 2020 is greatly appreciated. REFERENCES 1. Ong, A. Study on the Aerospace Industry Part One: Introduction to Aerospace; 2011.

Aviation: Thermoplastic and Thermoset Polymers in

2. Weiss, I.S.; Amir, R.A. Aerospace Industry. Available at https://www.britannica.com/topic/aerospace-industry, (accessed in June 2017). 3. Spreen, W.E. Marketing in the International Aerospace Industry; Routledge: New York, 2016. 4. Aerospace Consulting/Aerospace and Defense ­Market Research. Available at http://www.lucintel.com/­aerospace_ consulting.aspx, (accessed in July 2017). 5. Capgemini Consulting Technology Outsourcing, The Changing Face of the Aerospace & Defense Industry. Available at www.capgemini.com/aerospace-defense, (accessed in June 2017). 6. 2015 Aviation Trends. Available at https://www.­strategyand. pwc.com/trends/2015-aviation-trends, (accessed in May 2017). 7. 2017 Global Aerospace and Defense Sector Outlook, Deloitte; 2017. 8. 2017 Global Aerospace and Defense Sector Financial ­Performance Study, Deloitte; 2017. 9. Tam, W.F. Improvement Opportunities for Aerospace Design Process; American Institute of Aeronautics and Astronautics Report, Aerojet: Sacramento, CA, 2004, 1–8. 10. Nayak, N.V. Composite materials in aerospace applications. Int. J. Sci. Res. Publ. 2014, 4 (9). ISSN: 2250-3153. 11. Standridge, M. Aerospace Materials-Past, Present and Future. Aerospace Manufacturing and Design Magazine, posted August 13, 2014. Available at http:// www.­a erospacemanufacturinganddesign.com/article/ amd0814-materials-aerospace-manufacturing/, (accessed in June 2017). 12. Craftech Industries. Why the Aerospace Industry Loves the Plastic Materials, 2015. Available at http://www.craftechind.com/why-the-aerospace-industry-loves-plastic-­ materials/, (accessed in June 2017). 13. Technology and Applications of Advanced Thermoplastic Composites. Tencate Advanced Composites Magazine, published December 17, 2012. Available at http://www.azom. com/article.aspx?ArticleID=7983, (accessed in July 2017). 14. High-Performance Plastics for Aviation and Aerospace. San Diego Plastics Technical Report, Ensinger. 15. Meador, M.A. Polymeric Materials for Aerospace Power and Propulsion-Nasa Glen Overview. Polymers Branch, Structures and Materials Division NASA Glenn Research Center Report, 2008. 16. Taranenko, I.M. Non-metals and composites. In Materials Engineering and Aviation Structural Materials, Part 2; Taranenko, I.M.; Ed.; Kharkiv Aviation Institute: Ukraine, 2014. 17. Vodicka, R. Thermoplastics for Airframe Applications: A Review of the Properties and Repair Methods for Thermoplastic Composites; Airframes and Engines Division Aeronautical and Maritime Research Laboratory: Fishermans Bend, VIC, 1996. 18. Malnati, P. Thermoplastic Composites Take Off in Aircraft Interiors; Plastics Engineering, digital issue published May 2016. Available at http://read.nxtbook.com/wiley/­ plasticsengineering/may2016/thermoplasticcompositestakeoff.html, (accessed in June 2017). 19. Red, C. Composites in Commercial Aircraft Engines 2014–2023. Composites World Magazine, published June

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2015. Available at http://www.compositesworld.com/­ articles/the-outlook-for-thermoplastics-in-aerospace-­ composites-2014-2023, (accessed in January 2017). Favaloro, M. A Comparison of the Environmental ­Attributes of Thermoplastic vs. Thermoset Composites, Ticona Engineering Polymers Report, 2012. High Performance Plastics, Ensinger, 2017. Available at https:// www.ensingerplastics.com/en/shapes/high-­performanceplastics/pps, (accessed in May 2017). Seydibeyoğlu, M.Ö.; Mohanty, A.K.; Misra, M. Introduction. In Reinforcing Fibers for Fiber Reinforced Composites; Seydibeyoğlu, M.Ö.; Mohanty, A.K.; Misra, M.; Eds.; Woodhead Publishing, Elsevier: Cambridge, 2017. Abbadi, A.; Koutsawa, Y.; Carmasol, A.; Belouettar, S.; Azari, Z. Experimental and numerical characterization of honeycomb sandwich composite panels. Simul. Model. Pract. Theory 2009, 17, 1533–1547. 2014–2023 Global Composite Aerostructures Market Outlook. Composites Forecasts and Consulting, published July 2014. Ridge, W. Aerospace Composites: A Design and Manufacturing Guide; Gardner Publications, Inc.: Wheat Ridge, 2008.

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Seydibeyoğlu, M.Ö.; Mohanty, A.K.; Misra, M. Fiber Technology for Fiber Reinforced Composites; Woodhead Publishing: Cambridge, 2017. Qualification for Additive Manufacturing ­Materials, Processes, and Parts, 2014. Available at https://www.nist.gov/ programs-projects/qualification-additive-­m anufacturin g-materials-processes-and-parts, (accessed in July 2017). Romei, F.; Leonardo Da Vinci; The Oliver Press: Minneapolis, MN, 2008, 56. ISBN: 978-1-934545-00-3. Howard, F. Wilbur and Orville: A Biography of the Wright Brothers; Dober Publications: Mineola, 1998, 33. ISBN: 978-0-486-40297-0. Airbus Aircraft Design Inspired by Nature, 2016. Available at http://www.biomimeticsummit.com/airbus-aircraft-­designinspired-by-nature/, (accessed in July 2017). Blaiszik, B.J.; Kramer, S.L.B.; Olugebefola, S.C.; Moore, J.S.; Sottos, N.R.; White, S.R. Self healing polymers and composites. Annu. Rev. Mater. Res. 2010, 40, 179–211. Demiroğlu, S.; Mohanty, A.K.; Misra, M.; Seydibeyoğlu, M.Ö. The use of nanotechnology for fiber reinforced composites. In Reinforcing Fibers for Fiber Reinforced Composites; Seydibeyoğlu, M.Ö.; Mohanty, A.K.; Misra, M.; Eds.; Woodhead Publishing, Elsevier: Cambridge, 2017.

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Biomaterials: Soft Tissue Injuries Anurag Singh Department of Mechanical Engineering (DEMec), Faculty of Engineering, University of Porto (FEUP), Porto, Portugal

Rui Miranda Guedes Department of Mechanical Engineering (DEMec), Faculty of Engineering, University of Porto (FEUP), Porto, Portugal; Laboratory of Optics and Experimental Mechanics, Institute of Science and Innovation in Mechanical and Industrial Engineering (INEGI), Porto, Portugal; LABIOMEP, Porto Biomechanics Laboratory, University of Porto, Porto, Portugal

Biomaterials– Biomedicine

Abstract In human body, the group of tissues which bind, support, and protect the body and connect other tissues are called soft connective tissues. Some of the soft tissues are ligaments, tendons, blood vessels, skins, and articular cartilage. Tissue engineering (TE) is an interdisciplinary subject to help solve the critical medical problems like tissue loss and organ failure by using the knowledge from chemistry, physics, engineering, life science, and clinical science. Regenerative implantable medical devices are expected to prosper in the near future, providing viable solutions to many healthcare problems. In many cases, these devices consist of scaffolds that temporarily replace the biomechanical functions of a biologic tissue, while these progressively regenerate and gradually support the body imposed loads. Hence, the biodegradable materials employed to fabricate these scaffolds should fulfil the necessary mechanical and biological properties. This chapter is focused in the aliphatic polyesters since they constitute the major class of biomaterials. In this context, it revisited the importance of scaffolds in the tissue engineering, the methods of fabricating the scaffold, the mechanism of polymer degradation, the important mechanical and material characterization techniques, and the application of these biomaterials in tissue engineering. At last, the possible future trends are briefly analyzed. Keywords: Aliphatic polyesters; Biocompatibility; Hydrolytic degradation; Mechanical properties; Scaffold; Tissue engineering.

INTRODUCTION Research in the field of biodegradable materials has increased rapidly in the past decade. The reasons for this development are the advantages that biodegradable materials offer over the traditional methods. As they are gradually absorbed leaving no traces in the human body, they do not impose foreign body reactions. Traditional methods like allograft and autograft possess limitations such as limited availability, requirement of additional surgery for tissue harvest, [1,2] restrictions in the movement accompanied with pain, donor site scarcity, rejection by an immune-induced response, elevated harvesting costs, and diseases transfer.[3] Therefore, they do not provide long-term surgical alternative to this problem. Research in the field of biodegradable polymers has helped in overcoming the various drawbacks of traditional methods used in the field of TE. Rate of degradation of a biomaterial is the most important factor in deciding the application for which it  is intended for, as different medical devices have different

224

requirements in terms of rate of degradation, mechanical properties, biocompatibility, bioactivity, etc. The biodegradable material offers to be used as a temporary scaffold for load bearing application by gradually replacing the biomechanical functions, and at the same time, progressively regenerating and supporting the loads imposed by the body, [4] which eliminates the need for second surgery. The acceptance of biomaterial in orthopedic implant is limited to mechanical properties and rate of degradation. During the recovery period, the strength of the device should reduce progressively, and biological tissues must restore to its full capacity. Through this time, the cultured cell over the scaffold should be able to form new tissues.[5] Apart from that, the rate of degradation of device must match the rate of growth of new tissues. The device must supply the necessary mechanical support to handle the forces generated due to mobilization, [6] in case of load bearing applications. The essential requirements of the biodegradable ­material can be summarized as that it must be biocompatible; Encyclopedia of Polymer Applications, First Edition DOI: 10.1201/9781351019422-140000374 Copyright © 2018 by Taylor & Francis. All rights reserved.

Biomaterials: Soft Tissue Injuries 225

it should have enough endurance strength, and should be creep resistant and on swelling to saturation limit; it must display similar mechanical behavior as that of natural tissues. In addition, the degradation rate must not harm the new and surrounding tissues while not causing the stress shielding effect. This chapter discusses the aliphatic polyesters, which is the major class of biomaterials, properties of various biodegradable materials, necessary requirements of selecting a biomaterial for application, importance of scaffold in the field of tissue engineering, methods of fabricating the scaffold, mechanism of polymer degradation, important mechanical and material characterization techniques, and application of these biomaterials in the field of tissue engineering. At last, conclusion is ­presented with the future trend.

ester, amide, and urethane, or polymers with carbon backbones, in which additives like antioxidants are added. Synthesis, properties, and biodegradability of the main class of synthetic polymers will be discussed in this section. Biodegradable polymers obtained from renewable resources have attracted much attention in recent years. This new interest results from global environmental respect awareness and the fossil depletion problem. Biopolymers research and development as well as their production have been the ­fastest for several years.

Polylactide (PLA) is derived from the renewable sources like corn and rice (carbohydrates) as shown in Fig. 1. By bacterial fermentation of these carbohydrates, the lactic acid is obtained after that it is dehydrated to obtain the lactides.[7,8] It is followed by the ring-opening polymerization to obtain the polylactic acid. Depending upon its molecular weight, crystallinity, and shape, it degenerates completely into lactic acid in the period of 10 months  to 4 years.[9] It degrades further into carbon dioxide and water in the presence of aerobic bacteria; these products are easily assimilated within the body.[8,10] These attributes make it a suitable candidate for medical devices and drug ­delivery system.

BIODEGRADABLE POLYMERS Apart from metals, ceramics, and glass, polymers represent the largest class of biomaterials. Polymers can be derived from either natural processes or synthetic processes. Aliphatic polyesters are the most studied class of biodegradable polymers, and in this section, the emphasis has been given only on the aliphatic polyesters. These are synthetic polymers with hydrolysable functions, such as

CO2 + H2O Aerobic bacteria

O H3C

Photosynthesis

OH

OH (Lactic acid)

Corn and rice (carbohydrates)

Hydraulytic degradation

Fermentation

O H 3C

PLA Ring opening polymerization

Dehydration

O CH3

O O

CH3 O

(Lactide)

Fig. 1  The cycle of PLA in nature

OH (Lactic acid)

OH

Biomaterials– Biomedicine

Polylactide

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PLA has extensively been used as a biomaterial for human body because of its absorbability and nontoxicity after degradation. PLA degenerates completely into lactic acid in the period of 10 months to 4 years.[9] It degrades further into carbon dioxide and water in the presence of aerobic bacteria; these products are easily assimilated within the body. These attributes make it a suitable candidate for medical devices and drug delivery system. It also has some disadvantages such as the rate of degradation is very slow, it is very brittle, and it is highly hydrophobic which can cause inflammatory response in the tissues. PLA has two stereoisomers: poly l- lactide (PLLA) and poly d- lactide (PDLA) or as a racemic mixture, designated as PDLLA. Biomaterials– Biomedicine

Polyglycolide

Polycaprolactone Polycaprolactone (PCL) is another degradable aliphatic polyester; at room temperature, it remains in the rubbery state. Structure of PCL is a semicrystalline, and hence, it is hydrophobic in nature with a low rate of degradation. Because of the presence of five hydrophobic CH2 moieties in its repeating units, it degrades more slowly.[17] Therefore, due to its slow degradation rate it becomes less favorable to biomedical applications but, on the other hand, is a good candidate for long-term implants and other controlled drug delivery system.[6] The glass transition temperature of PCL is very low around −62°C [18] along with low melting point around 59°C–64°C.[19] It is produced by the ring-opening polymerization of a monomer caprolactone unit. Polybutylene succinate

Polyglycolide (PGA) is hydrophilic in nature, as hydrophilic materials have more tendencies to swell by presenting a higher degree of saturation which leads to high rate of degradation and because of which it quickly loses its mechanical properties in 2–4 weeks.[11,12] PGA is commonly synthesized by ring-opening polymerization of glycolic acid. It has a high degree of crystallinity. PGA degrades into glycolic acid in in vivo condition, glycolic acid is nontoxic, and it further degrades into carbon dioxide and water, which can be easily defecated by the body.[10,13]

Polybutylene succinate (PBS) is a thermoplastic polymer, which is white in color and has a semicrystalline structure, and it has a low melting point of about 110°C–118°C.[20,21] Condensation polymerization of 1,4-butanediol and succinic acid is used to synthesize the PBS. Its degradation behavior is similar to that of PLA and PGA; it degrades into carbon dioxide and water with the help of enzymes and microorganisms.

PHA

Polycarbonate—Poly(Trimethylene Carbonate)

Polyhydroxyalkanoates (PHA) is class of polyesters derived from the microorganisms via fermentation in unbalanced growth condition.[14] Their chemical structure depends on the specie of bacteria and the carbon source composition on which the microorganisms are grown.[6,12] Figure 2 shows the general molecular structure of PHA; here, m varies as 1, 2, and 3. The value of n can range from hundreds to thousands. When R is Ch3, it is known as poly-3-­hydroxybutyrate (PHB), when R is CH 2CH3; it is called poly-3-hydroxyvalerate (PHBV). Apart from the microorganisms, PHB can also be derived by ring-opening polymerization of β- butyrolactone.[15] Due to the semicrystalline nature of PHB, it possesses relatively low degradation rate in in vivo condition, along with hydrophobic nature and brittle character of PHB, and it becomes less popular for many medical applications. However, by changing the composition of PHA, it is possible to achieve the favorable properties according to the application. Some other representatives of this class are poly-4-­hydroxybutyrate (P4HB), 3-­hydroxyhexanoate (PHBHHx), and poly-3-hydroxyoctanoate.[16]

Poly(trimethylene carbonate) (PTMC) is obtained by ring-opening polymerization of trimethylene carbonate, catalyzed with diethylzinc. A high molecular weight flexible polymer was prepared, but it displays poor mechanical performance. Consequently, its applications are limited, and copolymers are more often used. Poly(propylene carbonate) is synthesized via copolymerization of propylene oxide and carbon dioxide. It has good properties such as compatibility, impact resistance. Its thermal stability and biodegradation need to be improved. A classical way is to blend it with other polymers. TMC is one of the constituents of the biodegradable sutures Biosyn, Caprosyn, and Maxon.

R C H

O H2 C

m

C

O

n

Fig. 2  General molecular structure of polyhydroxyalkanoates

Blends of Biodegradable Polymers Polymer blends generally have two segments in their structure namely hard segment and soft segment. Hard segment has the glass transition temperature above that of the room temperature and is glassy and semicrystalline in nature, whereas the glass transition temperature of soft segments is well below the room temperature, and the soft segments are rubbery at room temperature. Rate of degradation is controlled by c­ ontrolling the concentration of hard and soft segments. Let us consider the case of PGLA, which is a blend of PLA and PGA. PLA is known for its slow degradation rate, while on the other hand PGA has the high rate of ­degradation. So, to achieve intermediate degradation rate

Biomaterials: Soft Tissue Injuries 227

for orthopedic applications. Table 2 shows the various mechanical ­properties of the biodegradable polymers. SELECTION OF THE BIODEGRADABLE MATERIAL FOR SCAFFOLD The selection of the biomaterial is very important in the design and development of the device in tissue engineering. The primary concern is the sustainability of the strength of material over the period for the orthopedic applications. Biomaterial in study for the replacement must be able to interact with the tissues to be repaired rather than act as a static replacement. The material must possess some ­necessary requirements which are as follows: 1. The capability of the material to be incorporated as a final product. 2. Properties of the biodegradable material must closely match to that of the application for which it is intended for. 3. Degradable products should not be toxic and should not cause local tissue response. 4. It should be able to maintain the mechanical integrity during the healing of the said tissue. 5. There should be ease in manufacturing of the bulk component. 6. Flexibility in the design of the component. 7. Material should be lightweight. 8. Possible miniaturization of components. 9. Thermal insulation and conductivity. 10. Tailorability of the properties by adding fillers and additives. POLYMER SCAFFOLD FOR RECONSTRUCTION

PROPERTIES OF BIODEGRADABLE POLYMERS The key role of the biomaterial for the device is to provide adequate support to carry loads, and it should have enough compliance to diminish the damage done by the device to the adjacent tissues while keeping the mechanical integrity of the same. For avoiding the short- and long-term health impairment, the properties of the material of the device should match that of the ligament. Physical Properties of Various Biodegradable Polymers The physical properties of the material are intrinsic to the success of the scaffold. Table 1 shows the various physical properties of the biodegradable polymers.

In our body, tissues are organized in three-dimensional (3D) spaces; in TE for the regeneration of new tissues, a 3D support is required, and this support is provided by the scaffold which acts as the substrate for cellular growth, proliferation, cell adhesion, differentiation, and formation of ECM, and this provides support to the new tissues formed.[20] Scaffold is like a warehouse which supplies essential nutrients, water, and cytokines, and regulates the growth and proliferation of new cells. They also help in vascularization of new tissues.[6,46] Cultured cells are grown on a bioactive scaffold that guides differentiation and assembly of cells inside the 3D structure. A scaffold selected must match biological, morphological, and biomechanical requirements, [47] and the ultimate aim should be to restore the functioning during the rehabilitation and to p­ rovide a suitable microenvironment for the growth of cells.

Mechanical Properties of Biodegradable Polymers

Scaffold Parameters

Mechanical properties of the biodegradable polymers are of main concern for selecting the material

There are various parameters which affect the property of a scaffold; these can be classified according to geometry,

Biomaterials– Biomedicine

various mixing ratios (50:50, 65:35, 75:25: 85:15, 90:10) of lactic acid and glycolic acid have been employed to synthesize PLGA to maintain its mechanical integrity for several months.[22,23] PLA reduces the local pH, thereby inducing the inflammatory response. So, they are blended with PCL which results in a balanced pH, thus decreasing the inflammation.[24] PGA being hydrophilic degrades very fast in aqueous solution and loses its mechanical integrity in between 2 and 4 weeks. Chen et al. concluded that by the addition of PDLLA or PCL, the properties of PLLA change, which were originally hard and brittle and also that PLLA/ PDLLA blend has the lower elongation and higher mechanical properties when compared to the PLLA/PCL.[9] So, the selection of material along with blending counterpart is important for the biomedical devices and it should be carefully chosen keeping in view of the application. Di-block polymers are quite common in practice, and few common examples are PLGA, PLA-PCL, PGLA, Maxon (one hard segment of glycolide at 67%–75% and one soft segment of trimethylene carbonate at 32.5%) etc. In certain cases, more than one type of soft building block is combined with the hard building blocks; but in all cases, the hard segments have always been the predominant component (>50%). Biosyn is a triblock polymer having three copolymers: one hard segment (glycolide, 60%) and two soft segments (p-­­dioxanone and trimethylene carbonate). Caprosyn is a quad-block polymer of one of its kind having two hard segments (glycolide and l-lactide) and two soft segments (ε-­caprolactone and trimethylene carbonate), the rate of degradation of Caprosyn is very high. It can completely degrade in 56 days, and it is primarily used for the surgical sutures, but it has the potential to be applied in other biomedical applications.

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Table 1  Physical properties of the biodegradable polymers Average Density ρ molecular weight Material (g/cm3) (Mn/g mol-1)

Melting point Tm (°C)

Glass transition temperature (°C)

Decomposition temperature (°C)

Crystallinity (%)

Biomaterials– Biomedicine

PLA

1.21–1.25 [25] 1.23 [26]

3.34 × 105 ± 0.03[27] 150–162 [25]

58.61 ± 0.58 [27] 45–60 [25] 53 [28]

8 [29]

PLLA

1.24–1.30 [25] 1.25–1.29 [30]

1.1 × 105[31]

170–200 [25] 173–178 [32,33] 175[34] 175.1[9] ~180 [20,35,36] 170–190 [30] 184.2 [31]

55–65[25] ~200 [36] [9] 57.4 50–60 [30] 60 [37] 60–65 [20,32,33,34,38] 55–60 [36] 65[31]

70 [35]

~180 [36]

50–60 [36]

PDLA

~200 [36]

PDLLA

1.25–1.27[25] 1.27[30]

3.25 × 105[39]

Amorphous [25,32,35] 51.6 [9] 55–60 [20,34] 50–60 [25,30,35]

PCL

1.11[34] 1.06–1.13 [30]

530–630 000 [34,40] 2.7 × 105[30] 2.7 × 104[9] 2.66 × 105[39]

58–63 [33] 55–60 [34] 65[40,41] 60 [30] 58–65 [25] 59–64 [20] 53.1[9] 62 [28]

−60 to −65[25] −65 to −61[40,41] −60 [20,30,33,34]

67[34,40,41]

PGA

1.5–1.707[25] 1.5–1.69 [30]

>200 [34] 225–230 [30,32,33] 220–233 [25]

35–40 [32–34] 35–45[25]

45–55[34]

PDO

1.5 × 105[42]

PGA-PCL

1.5 × 10

185–200 [36]

32 [42]

5[42]

PLA/PCL/2.5

152 [29]

30.4 [29]

PLA/PCL/5

152.7

[29]

32.5[29]

PLA/PCL/7.5

152.5[29]

33.5[29]

PLA/PCL/10 PHB

1.18–1.262 [25]

168–182 [25]

5–15[25]

Polydioxanone (PDS) PLGA (50/50)

(−10 to 0)[32,34] 1.30–1.40 [25]

Amorphous [25,33]

40–50 [25] 50–55[33]

PLGA (85/15)

Amorphous [33]

50–55[33]

PLGA (90/10)

Amorphous

50–55[33]

Poly(propylene fumarate) (PPF)

30–50 [33]

PBS

110–118 [20]

PLLA/PDLLA (80/20)

174 [9]

[33]

–60 [33]

58.1[9]

PLLA/PDLLA (60/40)

171.8

[9]

56.6 [9]

PLLA/PDLLA (50/50)

171.7[9]

57.6 [9]

PLLA/PDLLA (40/60)

172.5

[9]

53.3 [9]

PLLA/PDLLA (20/80)

173.8 [9]

52.2 [9]

Biomaterials: Soft Tissue Injuries 229

Table 2  Mechanical properties of biodegradable polymers Material

Modulus (MPa)

Elongation at break (%)

Tensile stress at break (MPa)

PLA

350–3,500 [25] 3,300 ± 80 [26] 3,200 [29]

2.5–6 [25] 3.8 ± 0.07[26] 2.2 [29]

21–60 [25] 57.8 ± 0.86 [26] 51.3 [29]

PLLA

2,700–4,140 [25] 4,800 [32,34] 1,500–2,700 GPa [33] 19,800 ± 3,000 [9] 2,397[31]

3–10 [25] 5.4 [31] 20–30 [36] 56.3 ± 1.9 [9]

15.5–150 [25] 50.8 [31]

20–30 [36]

PDLLA

1.9 [32] 2,800 [9] 1,000–3,450 [25]

11.4 [9] 2–10 [25]

26 [9] 27.6–50 [25]

PCL

190 [34,40,41] 400–600 [33] 210–440 [25]

>500 [40,41] >700 [34] 300–1,000 [25]

14 [34,40,41] 23 [34] 20.7–42 [25]

PGA

12,500 [34] 12,800 [32] 5,000–7,000 [33] 6,000–7,000 [25]

1.5–20 [25]

60–99.7[25]

PDO

62 [42]

139 [42]

PGA-PCL

55

192.1[42]

[42]

PLLA/PCL (80/20)

20.7 ± 1.4 [9]

129.5 ± 32.9 [9]

41.2 ± 1.5[9]

PLLA/PCL (60/40)

10.7 ± 2.2 [9]

152.1 ± 11.8 [9]

19.3 ± 1.9 [9]

PLLA/PCL (50/50)

8.1 ± 2.8 [9]

139.6 ± 17.4 [9]

16.9 ± 1.3 [9]

PLA/PCL/2.5

1.9 [29]

43.7[29]

31.8 [29]

PLA/PCL/5

1.9

[29]

38.1

29.5[29]

PLA/PCL/7.5

1.4 [29]

74.4 [29]

25.0 [29]

PLA/PCL/10

1.3

32.6

24.8 [29]

PHB

3.5–4 [25]

PDS

1,500

PLGA (50/50)

1,000–4,340 [25] 1,400–2,800 [33]

PLGA (85/15)

1,400–2,800 [33]

[29]

[29]

[29]

5–8 [25]

40 [25]

2–10 [25]

41.4–55.2 [25]

[32,34]

PLGA (90/10) PPF

45[43] 50, 70 [44]

Biomaterials– Biomedicine

PDLA

34.1 ± 2.5[9]

Accelerated test (Temp.) (°C)

2,000–3,000 [33]

PBS PLLA/PDLLA (80/20)

11.9 ± 1.7[9]

59.8 ± 9.3 [9]

35.2 ± 2.4 [9]

PLLA/PDLLA (60/40)

20.5 ± 3.9 [9]

38.1 ± 4.6 [9]

41.1 ± 8.1[9]

PLLA/PDLLA (50/50)

10.1 ± 0.1

56.2 ± 3.4

[9]

36.2 ± 1.5[9]

PLLA/PDLLA (40/60)

22.0 ± 3.4 [9]

60.8 ± 6.6 [9]

39.2 ± 5.4 [9]

PLLA/PDLLA (20/80)

17.6 ± 2.1

62.5 ± 9.0

32.6 ± 2.1[9]

[9]

[9]

[9]

25, 37, 44, 54, 65, 80 [45]

230

Biomaterials: Soft Tissue Injuries

Architecture

Fiber diameter D

Fig. 4  Multilayered braided scaffold architecture [50]

Twisting angle

Methods of Fabricating 3D Scaffold

Maximum tensile load Young's modulus Material selection Fibers diameter

Fabrication parameters

L layers

Brainding angle

Porosity Surface area

Mechanical properties

Braiding angle α

way for navigation of nutrients and waste product through the scaffold structure.[6,49] Porosity affects the mechanical integrity of the scaffold as highly porous structure may lead to catastrophic failure as it is more fragile when compared to the scaffold less porous. Therefore, equilibrium must be maintained between connectivity and mechanical integrity. The various methods to measure the pore size and porosity are gravimetric method, porosimetry, liquid displacement method, scanning electron microscopy, microcomputed tomography, permeability-based methods, and capillary flow porometry.[54] Degradation must be designed in such a way that at the time the complete proliferation of cells occurs, the scaffold must be completely degraded till that time. For the cell to adhere, proliferate, grow, and deposition of ECM, the fibers of the biodegradable scaffold can be coated with nanofibers and collagens.[55] Other than the properties of the raw material, fabrication technique plays a major role in determining the final characteristics and geometry of the scaffold. This architecture has a well-connected pores which will help in the formation of new tissues and the removal of wastes (generated from the degradation) from the scaffold, by altering the parameters like number of layers, diameter of fibers, and braiding angle, and a wide tailorability can be obtained to use in various other fields.[50] Figure 5 shows the spherical pores within a scaffold obtained from X-ray tomography. Different colors correspond to the pore radius.

Pore diameter

Scaffold parameters

Biomaterials– Biomedicine

mechanical properties, and fabrication parameters [48] as shown in Fig. 3. For example, when the braiding angle is increased, there is an increase in the length of toe region, [49] but stiffness and capacity of yield load are reduced.[50] Also, the pore diameter and porosity are reduced with an increase in the pore surface area.[51] Architecture of the scaffold must provide the necessary conditions for the seeded cells to proliferate and generate the tissue; for that, porosity and surface area must be optimized.[52] For soft tissues, the size of pores must be in between 200 and 250 µm for the growth of cells.[51] However, porosity is not a significant parameter as far as cell growth is considered, but a good pore size is vital for the transport of oxygen and other important nutrients required for the growth. They also help in carrying the metabolic waste out of the system. Porosity affects the mechanical integrity of the scaffold as highly porous structure may lead to catastrophic failure as it is more fragile when compared to the scaffold less porous. Therefore, equilibrium must be maintained between connectivity and mechanical integrity. The various methods to measure the pore size and porosity are gravimetric method, porosimetry, liquid displacement method, scanning electron microscopy, microcomputed tomography, permeability-­ based methods, and capillary flow porometry. Figure 4 shows the multilayered braided scaffold architecture. Apart from being biocompatible, biodegradable, and having mechanical properties similar to that of tissue it is regenerating, it should not cause stress shielding while promoting the tissue growth as discussed in previous section. Other than these mentioned things, high porosity, adequate pore size, and interconnectivity are must-have requirements for the cell seeding and diffusion.[53] Interconnecting the pores results in an increase in the surface area for the cells to adhere on the surface while giving the

Yarn density

Fig. 3  Various parameters that affect scaffold properties

Commonly used methods for fabricating the 3D scaffolds are particulate leaching, gas foaming, freeze-drying, rapid prototyping (RPT), electrospinning (ES), thermally

Biomaterials: Soft Tissue Injuries 231

leached inside the polymer while evaporating the solvent.[53] Now, the mold is submerged inside the water bath for the sufficient amount of time to remove the porogen from the polymer matrix,[57] which leaves the porous structure. Pore size can be tailored accordingly by varying the size, type, and amount of porogen.[53,58,60] Average pore size diameter of 500 µm and porosity of 93% can be achieved by this method.[61,62]

Fig. 5  Generation of spherical pores within a scaffold issued from X-ray tomography. Different colors correspond to the pore radius [56]

induced phase separation (TIPS), etc.[53] as shown in Figs. 6 and 7. The different methods of fabrication and the way it is done affect numerous aspects of the scaffold characteristics like porosity, rate of degradation, mechanical strength, and the ability to incorporate various bioactive molecules. For an instance, the polymers with higher crystallinity will have a high tensile strength, but it will come at an expense of very slow rate of degradation. In the next subsections, these methods are discussed briefly. Particulate Leaching This technique is used for the fabrication of biocomposite scaffold. The polymer solution is mixed with the solvent, and then, it is poured into the mold of desired shape which also contains a water-soluble salt, known as porogen particle. Solvent is removed by heating, and the mold leaves the salt

Polymer is dissolved in an organic solvent which is then combined with the appropriate volume of water and is thoroughly mixed until the homogeneity is achieved. Now, it is poured into the metal mold of required shape and is freezedried with the help of a liquid nitrogen. After this, the water is directly sublimed into the gas phase as the applied pressure is lower than the equilibrium vapor pressure of the frozen solvent. These sublimed ice crystals leave behind the porous structure.[63] The porosity of 90% with the pore size varying from 20 to 200 µm can be achieved by this method.[53] Pore size diameter can be controlled by various parameters by either varying the freezing temperature, rate of freezing, pH or ionic concentration.[64] Thermally Induce Phase Separation Solvent with the low melting point is used in this method for the dissolution of polymer, to ease the sublimation. Now, it is placed in a mold, which is cooled to the point when solvent freezes and the liquid phase gets separated. After this, it is quenched with the help of liquid nitrogen, resulting in two-phase solid. Solvent is removed by sublimation, leaving behind the porous structure.[6,59] By varying the polymer concentration, composition of solvent, quenching temperature and rate of cooling, the diameter of the pores, and the configuration can be controlled.[65] Pore size diameter g­ enerated by this method can be in the range of 10–100 µm.[53] Particulate leaching

By using porogen

Gas foaming Freeze drying Phase separation

Thermally induced Solid liquid Liquid liquid

Selective laser sintering

Methods of fabrication Rapid prototyping

3D printing Stereolithography Fused deposition modeling

Fibers

Fig. 6  Various methods for fabrication of 3D scaffold

Electro spinning

Biomaterials– Biomedicine

Freeze-drying

232

Biomaterials: Soft Tissue Injuries

Polymer solution

Water Porous structure

Porogen Solvent evaporation

Pgas >> P˚gas

Pgas = P˚gas

Gas bubbles

Porogen leaching (A)

Ice Tfreeze cristals

(B) Pores P < P˚solution

Phase separation

Homogenous solution Termodyn. instability

(C)

(D)

Biomaterials– Biomedicine

Fig. 7  Various methods of fabricating scaffold: (a) particulate leaching, (b) gas foaming technique, (c) freeze-drying process, (d) phase separation method [70]

Gas Foaming Process

Electrospinning

The use of organic solvent may leave behind traces during their removal, which may lead to the inflammatory response after implanting in the body. Gas foaming process does not utilize any solvent for the fabrication of scaffold, [66] thus eliminating that risk. High-pressure carbon dioxide is used as a porogen, [67] which is kept over a polymer disk in an isolated chamber for several days. This makes the polymer saturated with the carbon dioxide, and the pressure in the chamber is again brought back to the atmospheric pressure.[68] This drop in pressure makes the carbon dioxide leaving the polymer, which leads to nucleation and pore formation in the polymer matrix, leaving behind the sponge-like structure.[69] Figure 7b shows the fabrication process of gas foaming technique. PLGA scaffold is fabricated by this process with the pore size in between 100 and 200 µm in diameter with the interconnected pores.[48,66]

Electrospinning is used to produce nonwoven micro- or nanofibrous mats. It uses polymer solution to produce ultra-thin fibers, and the diameter of these fibers varies from nanometer to the micrometer.[73] Fibers produced by ES will have a large surface area being small in diameter along with better mechanical properties. Figure 8 shows the ES setup; it consists of a syringe with a blunt needle, and polymer solution along with a suitable solvent. Solvent is fed into the barrel, and electric field is applied with a high voltage power supply of 5–30 kV, electrode being in contact with the high voltage power supply.[74,75] When surface of the solution in the syringe overcomes the surface tension, a single fiber is emitted in the form of jet, and a constant rate is maintained with the help of the syringe pump. This jet of fiber is collected on the opposite charged electrode, as these fibers are collected over the plate, solvent evaporates to form a nonwoven fiber mat (porous scaffold).[76] ES produces fibers of diameter in between 100 and 400 nm, and these small diameter fibers provide a large surface area-to-volume ratio which will result in increased cell adhesion. By varying the process parameters such as concentration of solute, solvent, capillary diameter, material of the collecting plate, rate of ejection, and applied electric strength, the scaffold diameter, average pore diameter, and thickness of fibers can be controlled.[73] Fibers made from collagen, PLA, PLGA, PCL, and their copolymers, elastin, and spider silk have been prepared by using ES technique.[76,77]

Rapid Prototyping In RPT technique, the design of scaffold is created with the computer-aided design (CAD) software along with the suitable algorithm to move the nozzle for shaping the scaffold layer by layer. The RPT system can be of three types: liquid-based, solid-based, and the powder-based. 3D Printing  In this technique, sequences of 2D geometries are formed layer by layer, ultimately to form a 3D structure of interest. A binder solution is added onto the print bed by an inkjet printing head which is followed by the layer of powder.[71,72] It can be used to form some complex shapes with high precision and resolution. As fabrication involves the use of high temperature, not all the polymers can be fabricated by 3D printing. Recent advancement involves printing of layers with different biodegradable material, which can also be coated by using a bioactive material to obtain a better structure as a scaffold.

POLYMER DEGRADATION MECHANISM Degradation is an important phenomenon in the field of tissue engineering, as the rate of degradation determines the success of a material to be used in the biomedical appli­ cation. Rate of degradation helps in checking the cellular response of the material, functioning of biomaterial in in

Biomaterials: Soft Tissue Injuries 233

Fibers are collected Syringe pump

Polymer solution with solvent Syringe

Solvent volatization

Blunt needle Jet innitiation

Collector

Taylor cone at the needle tip

Biomaterials– Biomedicine

Polymer jet

High voltage power supply

Fig. 8  Electrospinning setup

Structure • Shape and dimension • Functionalized group

Processing parameters • Pore size • Interconnectivity • Morphology

Polymer properties • Molecular weight • Crystallinity

Surface properties • Wettability • Porosity

Table 3  Degradation properties of various biodegradable polymers Half-life in 37°C (normal Loss of saline) strength Loss of mass (months) (months) (months) Material PLLA

4–6 [36]

6 [32,34,85]

12–18, [33] 24–66, [34] >24 [24]

PDLA

4–6 [36]

—

12–16 [24]

PDLLA

2–3 [36]

1–2 [32,34,85]

12–16 [32,34,85]

PCL

—

—

>24 [24,33]

PGA

—

1–2 [32]

6–12, [24,32] 3–4 [33]

PDO

—

1–2 [32]

6–12 [32]

PLGA (50/50)

1.5[24]

—

3–6, [33] 1–2 [20]

PLGA (85/15) —

—

3–6, [33] 5–6 [20]

PLGA (90/10) —

—

PLA/PEG > PLA/ PEG/C30B. It was deduced that PLA/PEG and PLA/PEG/ C30B with optimal concentration of PEG and organo-clays have ability to be used for packaging and food grade applications.[129] It is found that the process-ability of PLA/ PEG can be improved using low molecular weight PEG (MW = 200). Results exhibited that PLA/PEG with lower molecular weight PEG will become better candidates for producing easily biomedical products.[130] Radical-induced reactive blending method was used to partially graft a low molecular weight PEG monoacrylate (PEGA) to the PLA chain. The optimization of Tg and elongation at break of blends was achieved via controlling the grafted PEGA content in PLA.[131] In order to improve compatibility, stability, and ductility of PEG/PLA blends, hydroxylated PEG, and citrate plasticizers in the presence of MA moieties grafted onto PLA chains using reactive extrusion.[132] Essa et al. synthesized a series of PEG-modified PLA graft (PL-g-PEG) and multiblock copolymers (PLA-PEG-PLA). Their findings showed that when PEG was grafted onto the PLA backbone, the polymer exhibited different physicochemical properties than when PEG was copolymerized to PLA. Such increase has been reported to be more pronounced for pure PLA particles without any PEG attached on the surface compared to particles exhibiting PEG on their surface.[133] PLA fibers were reinforced by graphene oxide with PEG as a scaffold for tissue engineering. Fibers exhibit the suitable biocompatibility and cells attach and proliferate on fibers in an appropriate manner.[134]

Polyethers and Copolymer Annually Renewable Biodegradable Materials PEG/PLA Blends Starch/PLA Blends PEG is an outstanding plasticizer in terms of reducing glass transition temperature and stiffness for PLA, which can significantly boost the elongation at break of PLA. However, the impact resistance of PLA/PEG blend is very limited. Moreover, compared to pure PLA, plasticized PLA has lower viscosity and more notable elastic properties. Thus, researchers have accomplished some effort to overcome these problems. It was reported that the addition of low molecular weight PEG result in an increase of elongation at break (>7,000%) and crystallinity; on the other hand, a decrease in tensile strength and tensile modulus was observed.[127,128] In addition, it was observed that when PEG content exceeded 10 wt%, phase separation take place in l-PLA/PEG blends.[128] Cloisite 93A (C93A) and Cloisite

Many efforts have been accomplished to develop PLA/ starch blends to reduce total raw materials cost and tune their biodegradability.[135] However, the main problem of this kind of blend systems is the poor interfacial ­interaction between hydrophilic starch granules and hydrophobic PLA. As a result of poor compatibility between PLA and starch, mechanical properties of this blends is not appropriate.[14,136] Overcoming drawbacks various biocompatible compatiblizers were used such as methylene diphenyl diisocyanate, MA, [137] glycerol, sorbitol, [138] GMA, [139] adipate, [140] tung oil anhydride, [141] epoxidized soybean oil, [142] which improved interfacial adhesion between starch and PLA resulted in improvement of thermal and mechanical

Composites– Conducting

Composites: Polylactic Acid-Based Blends 581

582

Composites: Polylactic Acid-Based Blends

(a)

Chitosan/PLA Blends Chitosan/PLA blends have been developed by many researchers and it was found that in vitro assessment of PLA/chitosan nanoparticles/nanofibers demonstrated low cytotoxicity, which makes it a tremendous option for biomedical application.[145,146] MMT presence in chitosan/PLA blend affected the control release which was influenced by pH variation.[147] Chitosan nanoparticles were used to coat PLLA scaffolds using ultrasonication and ionic gelation techniques. It was observed that nanoparticles enhanced hydrophilicity, biodegradation, and cytocompatibility compared with pure PLLA.[148] Corporation of Ag nanoparticles in hybrid chitosan/PLA nanofibers reported by Au et al. They found out that the antibacterial activity of containing against gram-negative and gram-positive bacteria (E. coli and Staphylococcus aureus) is higher compared to that of chitosan/PLA mats and AgNPs/ PLA mats. Also, It was observed that Ag nanoparticles fabricated during electrospinning are uniformly distributed in chitosan/PLA fibers.[149] Increment of chitosan content resulted in decrement of the average diameter of chitosan-blended PLA nanofibers.[146] Also, carboxymethyl nanopowder chitosan/PLA mats were prepared by electrospinning exhibited appropriate blood compatibility.[150] Properties of various blend of PLA are mentioned in Table 3.[150–162] PLA APPLICATIONS PLA because of its intrinsic nature exhibits the appropriate biocompatibility and degradability, which can be used in various fields like tissue engineering, textile, packaging, and printing (Fig. 6). In regenerative medicine and (b) 9.0

10.0

8.5

9.5

8.0

9.0 Log (E’’) (Pa)

8.5 Log (E’) (Pa)

Composites–­ Conducting

properties of the blend. Hwang et al. used a similar technique to compatibilize PLA/starch blends by use of the DCP as an initiator for the formation of PLA-g-MA, which enhanced interfacial adhesion and has compatibilization effect between PLLA and starch due to the compatibilization effect of MA grafted onto PLLA and starch. However, reactive compatibilization remarkably diminished molecular weight of PLA/starch blends. [137] It was found that the sorbitol-plasticized TPS exhibited higher tensile strength, modulus, finer blend morphologies, and lower crystallization rate compared to glycerol-plasticized TPS blends.[138] Moreover, The PLA/starch blends compatibilized by 15% the PLA-g-GMA copolymer enhanced the mechanical properties and interfacial adhesion and exhibited stronger medium resistance in comparison with PLA/starch blends without the PLA-g-GMA copolymer.[139] Some investigators, used maleinized linseed oil and others synthesized new compound, MA-grafted PEG-grafted starch (mPEGg-St) as compatibilizers in PLA/thermoplastic starch blends.[143] Moreover, reactive plant oils such as tung oil anhydride acted as a plasticizers, while epoxidized soybean oil appeared as plasticizer and compatibilizer simultaneously, at the same time in the PLA/starch/oil ternary mixtures and formed a flexible interphase between PLA and starch, which result in improvement in thermal and mechanical properties of PLA/starch blends. [141,143] The ternary and binary blend of thermoplastic starch (TPS), PLA, and GMA-grafted poly (ethylene octane) (GPOE) was prepared. GPOE presence in ternary blend resulted in elongation at break, compatibility, and impact strength increment. The storage modulus of ternary blend was higher than pure PLA. Storage modulus of PLA blends is higher than pristine PLA in high temperature, which is depicted in Fig. 5.[144]

8.0 7.5 7.0 6.5 6.0 5.5 –100

PLA/ 10wt% TPS PLA/10wt% TPS/ 10wt% GPOE PLA/ 20wt% TPS PLA/20wt% TPS/ 10wt% GPOE PLA/ 40wt% TPS PLA/ 40wt% TPS/ 10wt% GPOE Pure PLA –50

50 0 Temperature (°C)

7.5 7.0 6.5 6.0 5.5

100

5.0 –100

Fig. 5  (a) Storage modulus and (b) loss modulus of PLA/TPS/GPOE [144]

PLA/ 10wt% TPS PLA/10wt% TPS/ 10wt% GPOE PLA/ 20wt% TPS PLA/20wt% TPS/ 10wt% GPOE PLA/ 40wt% TPS PLA/ 40wt% TPS/ 10wt% GPOE Pure PLA –50

50 0 Temperature (°C)

100

Composites: Polylactic Acid-Based Blends 583

Table 3  PLA blends properties Preparation method

Properties

Compatiblizer

PE/PLA

Melt blending

Immiscible

PE/PLA

Melt blending

Strong interfacial tension

block copolymer GMA

PP/PLA

Melt blending in Twin screw extruder

Increased tensile strength, impact strength

PP-g-MA

[33]

PP/PLA/clay

Melt blending in Twin screw extruder

Increased tensile modulus

n-butyl acrylate GMA ethylene terpolymers (PTW)

[34]

PLA/PS/ MWCNT

Melt mixing

Conductive(10 −5 S cm−1)

—

[40]

PLA/PS

Spin-casting of polymer solutions

Nanotopographic film for cell stimulation

—

[150]

graphite/layeredsilicate clay/PLA

Melt mixing

Appropriate flame retardancy

—

[151]

PBAT/PLA

Melt mixing

Solid like behavior due to branching and network structure

PLA/GO/PEG

Freeze drying

Excellent dispersion of nanofiller

—

[152]

TiO2 /PLA

thermally induced phase separation

High porosity scaffold with well dispersion of filler

—

[153]

PLA/Chitosan

Electrospinning

Nanofiber as a drug carrier, scaffold for vascular gasket

—

[154,155]

Starch/PLA

Melt mixing

Excellent biodegradibility

—

[141]

PLA/Polyaniline

Electrospinning

Sensor

—

[156,157]

PLA/NR- PLA/ ENR

Melt blending

High toughness

—

[51]

PLA/Gelatin

Electrospinning

Neural scaffold, neural differentiation

—

[158]

PLA/PEG

Gelation

Hydrogel, biocompatible, thermosensetive

Fig. 6  Various applications of PLA

—

References

DCP

[148] [148,149]

[123]

[159,160]

tissue engineering, using materials should exhibit appropriate compatibility with organs to regenerate the impaired one.[163,164] Moreover, various materials can be added to PLA blends to endow the various features like antibacterial properties.[165] PLA have been used in medicine for various purpose like surgical suture, scaffold, drug career, and fixture.[27] PLA due to inappropriate m ­ echanical properties like brittleness cannot be utilized merely in ­packaging; hence, PLA blended with various materials to achieve desire features. Binary blends of PLA exhibited the appropriate mechanical characteristics. By adding the third component and preparing ternary blend other ­features like antibacterial is endowed to PLA. These blends can be utilized as a plasticulture, food packaging, and disposable one as a green packaging.[99,166] Textile industry is another destination for PLA productions. PLA due to capability of organic compound absorbent and wicking properties cab be used as a fiber and disposable products. PLA fibers have been used in automotive industry and nowadays by blending with other polymers like PET enhance the PLA properties. PLA

Composites– Conducting

Blend

584

because of appropriate resiliency can tolerate the laundry service; hence, it can be used as in garment industry. Moreover, PLA has been used as a bottle for water and juice in which its usage is just limited to non-carbonated beverage because of inadequate creep behavior and high permeability toward CO2.[167,168] The 3D printers have been developed based on PLA to fabricate the 3D structures.[169]

Composites: Polylactic Acid-Based Blends

5.

6.

CONCLUSION AND FUTURE PERSPECTIVE

Composites–­ Conducting

PLA proffers an appropriate option to the regular nonbiodegradable polymers especially when their recycling is demanding or not economical. Despite the fact that there are many restrictions due to PLA mechanical properties, a number of these challenges are expected to overcome through compounding of PLA with other polymers, by making microstructure and nanostructure, coating with high barrier materials, and also with polymer modification in order to make this material more engaging to many applications as compression to petrochemical plastics. In this regard, the range of improvement in thermal and mechanical properties is more pronounced when plasticizer or nanofillers was utilized as modifiers. According to processing condition and material formulation (plasticizers, blend, composites, etc.), the mechanical properties of PLA can vary astonishingly, ranging from soft and elastic materials to stiff and high strength materials. This opens interesting possibilities for tailoring new materials within a perspective of eco-design or sustainable development with physical, thermal, and mechanical performances of PLA blends. This enhancement in mechanical properties may have influence on biomedical and ­packaging application. REFERENCES 1. Yuwono, S.D.; Kokugan, T. Study of the effects of temperature and pH on lactic acid production from fresh cassava roots in tofu liquid waste by Streptococcus bovis. Biochem. Eng. J. 2008, 40 (1), 175–183. 2. Iacondini, A.; Mencherini, U.; Passarini, F.; Vassura, I.; Fanelli, A.; Cibotti, P. Feasibility of industrial symbiosis in Italy as an opportunity for economic development: Critical success factor analysis, impact and constrains of the specific italian regulations. Waste Biomass Valorization 2015, 6 (5), 865–874. 3. Rahmati, M.; Milan, P.B.; Samadikuchaksaraei, A.; Goodarzi, V.; Saeb, M.R.; Kargozar, S.; Kaplan, D.L.; Mozafari, M. Ionically crosslinked thermoresponsive chitosan hydrogels formed in situ: A conceptual basis for deeper understanding. Macromol. Mater. Eng. 2017, 302 (11), 1700227. 4. Ghasemi Hamidabadi, H.; Rezvani, Z.; Nazm Bojnordi, M.; Shirinzadeh, H.; Seifalian, A.M.; Joghataei, M.T.; Razaghpour, M.; Alibakhshi, A.; Yazdanpanah, A.; Salimi, M.; Mozafari, M. Chitosan-intercalated montmorillonite/poly (vinyl alcohol) nanofibers as a platform to guide neuronlike

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Conducting Polymers: Applications Anupreet Kaur Basic and Applied Sciences, Punjabi University-Patiala, Punjab, India

Abstract Intrinsically conducting polymers have been studied extensively due to their intriguing electronic and redox properties and numerous potential applications in many fields since their discovery in the 1970s.Recently, the conducting polymers have attracted much interest in the development of sensors. The electrically conducting polymers are known to possess numerous features, which allow them to act as sensors. In this entry, an attempt has been made to describe the salient features of conducting polymers and their wide applications in different fields.

ABBREVIATIONS PANI: Poylaniline PPY: Polypyrrole CP: Conducting polymer ICPS: Intrinsically conducting polymers HOMO: Highest Occupied Molecular Orbital LUMO: Lowest Unoccupied Molecular Orbital VRH: Variable range hopping INTRODUCTION Nanotechnology is rapidly evolving to open new materials useful in solving challenging problems. During the past decades, nanotechnology has become an active field of research because of its tremendous potential for a variety of applications. Conducting polymers (CPs) are used mainly in chemical, optical, pH biological sensors, and also in other sensors. CPs are classified as insulators, semiconductors, conductors, and superconductors based on their electrical properties.[1–5] A material with conductivity less than 10 −7 S/cm is regarded as an insulator. Metals have conductivity larger than 103 S/cm, whereas the conductivity of a semiconductor varies from 10 −4 to 10 S/cm depending upon the degree of doping. It was generally believed that plastics (polymers) and electronic conductivity were mutually exclusive and the inability of polymers to carry electricity distinguished them from metals and semiconductors. As such, polymers were traditionally used as inert, insulating, and structural materials in packaging, electrical insulations, and textiles where their mechanical and electrically insulating properties were paramount.[6–8] In fact, any electrical conduction in polymers was generally regarded as an undesirable phenomenon. Research on Encyclopedia of Polymer Applications, First Edition DOI: 10.1201/9781351019422-140000004 Copyright © 2018 by Taylor & Francis. All rights reserved.

CPs intensified soon after the discovery of poly (sulfur nitride) [(SN) x] in 1975 which becomes superconducting at low temperatures. Although CP complexes in the form of tetracyano-platinates, tetraoxalato-platinates, and the Krogman salts charge transfer complexes had been known earlier, significance lies in the rediscovery of polyacteylene by several orders. The breakthrough happened in the year 1977 when, somewhat accidently, Alan J. Heeger, Alan G. MacDiarmid, and Hideki Shirakawa discovered that plastics that are generally referred to as insulators can, under certain circumstances, be made to behave like metals. This path-breaking discovery of high conductivity in polyacetylene in 1977 resulted in a paradigm shift in thinking and opened up new vistas in chemistry and physics. Their work was finally rewarded with Nobel Prize in chemistry in 2000 for the discovery and development of electronically conductive polymers. In recent years, distinct development has been made in understanding of structure–property relationships for many of the CPs. This fascinating progress rate has been stimulated by the field’s fundamental synthetic novelty, importance to interdisciplinary research and to the emerging technological applications of these materials in different areas such as molecular electronics, electrodes for redox supercapacitors, electrochromic displays, chemical sensors, actuators, electromagnetic ­shielding, and nonlinear optics.[9,10] Intrinsically CPs, also known as “synthetic metals,” are polymers with a highly π-conjugated polymeric chain. Intrinsic CPs are completely different from other CPs in which a conducting material such as a metal or carbon powder is dispersed in a nonconductive polymer. These polymers often referred to as conjugated polymers belong to a totally different class of polymeric materials with alternate single and triple bonds in their main chain and are capable of conducting electricity when doped. ICPs,

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Keywords: Conducting polymers; Intrinsically conducting polymers; Pollutants; Sensors.

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similar to other organic polymers, usually are described by σ bonds and π bonds. While the σ electrons are fixed and immobile due to the formation of covalent bonds between carbon atoms, the remaining π-electrons can be easily delocalized upon doping. DIFFERENTIATION: CONJUGATED POLYMERS AND CONVENTIONAL POLYMERS •

• •

•

Band gap (e.g., electronic band gap) is small (~1–3.5 eV) with corresponding low excitations and s­ emiconducting behavior. Can be oxidized or reduced through charge transfer reactions with atomic or molecular dopant species. Net charge carrier mobilities in the conducting state are large enough and because of this high electrical ­conductivity is observed. Quasi particle, which under certain conditions, may move relatively freely through the material.

CONDUCTION MECHANISM

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The doping process in conjugated polymers is, however, essentially a charge transfer reaction, resulting in the partial oxidation of the polymer. Unlike inorganic semiconductors, doping in conjugated polymer is reversible in a way that upon de-doping the original polymer can retained with almost no degradation of the polymer back bone.[11–16] Another very important difference between the doping in conjugated polymers than in inorganic semiconductors is that doping in conjugated polymers is interstitial, whereas in inorganic semiconductors, the doping is substitutional (Fig. 1).[17–21] CPs can be p or n doped chemically and electrochemically to obtain a metallic state. Doping of conjugated

polymers can also be carried out by methods that introduce no dopant ions such as field induced charging. In doped state, the backbone of a CP consists of highly delocalized π electrons. Doping of conjugated polymers either by oxidation or by reduction in which the number of electrons in the polymeric backbone gets changed is generally referred to as redox doping. The charge neutrality of the CP is maintained by the incorporation of counter ions. Redox doping can be further subdivided into three main classes: ­p-doping, n-doping, and doping involving no dopant ions. Both chemical and electrochemical redox doping techniques can be employed to dope conjugated polymers either by the removal of electrons from the polymer backbone chain or by the addition of electrons to the chain. In chemical doping, the polymer is exposed to an oxidizing agent such as iodine vapors or reducing agents, whereas in electrochemical doping process, a polymer-coated working electrode is suspended in an electrolyte solution in which the polymer is insoluble along with separate counter and reference electrodes. On the application of a potential difference between the ­electrodes, charge cross into the polymer in the form of electron addition (n-doping) or (p-doping) and the appropriate counter ion from the electrolyte enters into the p­ olymer film in order to maintain charge neutrality. Photo-doping is a process where CPs can be doped without the insertion of cations or anions simply by irradiating the polymer with photons of energy higher than bandgap of the CP. This leads to the promotion of electrons to higher than band gap of the CP. This leads to the promotion of electrons to higher energy levels in the band gap. However, due to the rapid recombination of electrons and holes, photo-doping does not sustain after the irradiation of the polymer is stopped. Charge injection doping is another type of redox doping that can also be used to dope an undoped conducting.

Doping in conducting polymers

Redox doping

p-doping

n-doping

Non redox doping

No doping ions

Charge injection doping

Fig. 1  Different methods for doping conducting polymers

Doping ionimplantation

Doping by heating

Photo-doping

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The non-redox doping of CP is a process of doping CPs in which the number of electrons associated with the polymer chain is kept constant. In fact it is the energy level in the CP that gets rearranged in the non-redox doping process. The best example of non-redox doping is the conversion of emeraldine base form of polyaniline (PANI) to protonated emeraldine base (polysemiquinone radical cation) when treated with protic acids. It has been observed that the conductivity of PANI is increased by approximately 10 orders of magnitude by non-redox doping. Ion implantation and heat treatment methods have also been used to dope CPs. Ion bombardment of PANI by 100 KeV Ar+ ion and 24 KeV I+ at an affluence of 106 ions/cm2 has been reported, and it has been observed that upon I+ ion implantation, the films become environmentally stable showing enhanced ­conductivity by 12 orders of magnitude. Heat treatment induced doping has been observed for ladder type of CPs. It has been observed that for ladder type of polymers conductivity increases from 10 −8 to 10 −4 S/cm upon heat treatment, which has been attributed to the improved ordering of the polymer structure and thermally excited charge carriers. However, this technique has rarely been used for doping CPs. METAL–INSULATOR TRANSITION IN DOPED CONDUCTING POLYMERS Metal–insulator (M–I) transition is one of the most interesting physical aspects of CPs. When the mean free path becomes less than the interatomic spacing due to an increase in the disorder in a metallic system, coherent metallic transport is not possible. When the disorder is sufficiently large, the metal exhibits a transition from the metallic to insulating behavior. As a result of this transition, which is also known as the Anderson transition, all the states in a conductor become localized and it converts into a “Fermi glass” with a continuous density of localized states occupied according to the Fermi statistics. Although there is no energy gap in a Fermi glass but due to the spatially localized energy states, a Fermi glass behaves as an insulator. It has been found that the electrical conductivity of a material near the critical regime of Anderson transition obeys the power-law temperature dependence. This type of M–I transition has been observed for different CPs, such as polyacetyle, PANI, polypyrrole (PPy), and poly(p-phenylenevinylene), and is particularly interesting because the critical behavior has been observed over a relatively wide temperature range. In CPs, the critical regime is easily tunable by varying the extent of disorder by means of doping or by applying external pressure and/ or magnetic fields. In the metallic regime, the zero temperature conductivity remains finite, and σ(T) remains constant as T approaches zero. In the critical region, the

conductivity follows a power law, whereas in the insulator regime, transport occurs through variable range hopping among localized states. Although disorder is generally recognized to play an important role in the physics of “metallic” polymers, the effective length scale of the disorder and the nature of the M–I transition are yet central unresolved issues. In particular, it has been a matter of in-depth discussion that whether the disorder is present over a wide range of length scales or whether the properties are ­dominated by more macroscopic in homogeneities. In the former case, the metallic state and the M–I transition can be described by conventional localization physics (e.g., the Anderson transition), whereas in the latter case, the M–I transition would be better described in terms of percolation between metallic islands. BAND STRUCTURE AND CHARGE CARRIERS IN CONDUCTING POLYMERS A continuous system of strongly interacting atomic orbitals leads to the formation of band-like electronic states. The atomic orbitals of each atom in an inorganic semiconductor or in a metal overlap with each other in the solid state giving rise to a number of continuous energy bands. The electrons provided by each orbital are delocalized ­throughout the entire array of atoms. The extent of delocalization and the bandwidth are determined by the strength of interaction between the overlapping orbitals. In the case of conjugated polymers, the band structure originates from the interaction of the p orbitals of the repeating units throughout the chain. A set of bonding and antibonding molecular orbitals is formed by the combination of two or more adjacent ­p-orbitals, in which the electron pairs are shared by more than two atoms resulting in a delocalized pi-band. The bonding pi-orbital is referred to as the highest occupied molecular and LUMO in case of trans-polyacetylene. Unless doped, most of the conjugated polymer systems behave as insulators. Although this property of CPs is very similar to that of semiconductors, the underlying physics is quite different. Three-dimensionally bonded materials have rigid structures owing to their four-fold (or six-fold, etc.) coordination of each atom to its neighbors through covalent bonds.[22–26] Due to the rigidity of the lattice, charge carriers added to the system are accommodated in the conduction and the valence bands without negligible rearrangement of the bonding. In such systems, therefore, the conventional concept of electrons and holes as the dominant excitations has been followed. Bonding in conjugated polymers, on the other hand, has reduced dimensionality since the intra-chain interactions are much stronger than the interchain interactions between the adjacent chains. These polymers, therefore, have two-fold coordination and are hence susceptible to structural distortion.[27,28] As a result, the dominant electronic excitations are inherently

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THE NON-REDOX DOPING

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Conducting Polymers: Applications

Composites–­ Conducting

coupled to chain distortions, and the equilibrium geometry is determined by the occupancy of the electronic levels through electron–phonon coupling.[29] When a conjugated polymer is doped, the accommodation of an added charge becomes much easier if the charge is localized over a smaller section of the chain. The charge can be localized in the conjugated polymer if the reduction in the ionization energy of the chains due to addition of the charged species can offset the elastic energy gained by the system due to the local rearrangement of the bonding configuration in the vicinity of the charge. The nature of charged defects formed on the polymer backbone during doping depends on the structure of the polymer chain. Different types of charged defects have been observed for CPs with degenerate ground state structures such as polyacetylene and for those with nondegenerate ground state structures such as PANI or poly(p-phenylene).[30,31] Figure 2 shows a schematic depiction of the formation of different types of charge carriers in CPs and the corresponding modifications in their band structure. When an electron is removed from the π-system of a nondegenerate polymer such as poly(p-phenylene) or PANI by chemical oxidation, an unpaired electron with spin 1/2 (a free radical) and a ­spinless positive charge (­cation) are created as is evident from Fig. 2. The radical and cation are coupled to each other by a local bond rearrangement, creating a polaron that appears in the band structure as localized electronic states symmetrically located within the gap with the lower energy states being occupied by a single unpaired electron.[32,33] Unless doped, most of the conjugated polymer systems behave as insulators. Although this property of CPs is very

Polyene (a)

+e–

–

+e–

Polaron (radical anion) (b)

– Bipolaron (dianion) (c)

–

– –

Soliton pair (d)

Fig. 2  Mechanism of conduction in the conducting polymers

similar to that of semiconductors, the underlying physics is quite different. Three-dimensionally bonded materials have rigid structures owing to their four-fold (or six-fold, etc.) coordination of each atom to its neighbors through covalent bonds. Due to the rigidity of the lattice, charge carriers added to the system are accommodated in the conduction and the valence bands without negligible rearrangement of the bonding. In such systems, therefore, the conventional concept of electrons and holes as the dominant excitations has been followed. Bonding in conjugated polymers, on the other hand, has reduced dimensionality since the intra-chain interactions are much stronger than the inter-chain interactions between adjacent chains. These polymers therefore have two-fold coordination and are hence susceptible to structural distortion. As a result, the dominant electronic excitations are inherently coupled to chain distortions, and the equilibrium geometry is determined by the occupancy of the electronic levels through electron-phonon coupling. When a conjugated polymer is doped, the accommodation of an added charge becomes much easier if the charge is localized over a smaller section of the chain. The charge can be localized in the conjugated polymer if the reduction in the ionization energy of the chains due to addition of the charged species can offset the elastic energy gained by the system due to the local rearrangement of the bonding configuration in the vicinity of the charge. The nature of charged defects formed on the polymer backbone during doping depends on the structure of the polymer chain. Different types of charged defects have been observed for CPs with degenerate ground state structures such as polyacetylene and for those with non-degenerate ground state structures such as PANI or poly(p-phenylene).[34–40] ENERGY STORAGE Electrochemical studies on CP nanostructures have demonstrated that they usually have higher specific capacitance values and can be beneficial in the development of the next-generation energy storage devices. Zhang et al. reported that PANI nanofibers showed higher capacitance values and more symmetrical charge/discharge cycles due to their increased available surface area. It was also reported that MnO2 /PEDOT poly(3,4-­ ethylenedioxythiophene) coaxial nanowires prepared by a one-step co-electrodeposition in a porous alumina template show very high specific capacitances even at high current densities. The excellent electrochemical and mechanical properties indicate that the coaxial nanowires may lead to new types of nanomaterials in electrochemical energy storage devices. Fuel and photovoltaic cells are also concerned by polymeric nanofibers. Kim et al. reported template synthesized PPy nanowire-based enzymatic biofuel cells. It was found that the nanowire-type biofuel cell exhibited a

Conducting Polymers: Applications 595

DRUG RELEASE AND PROTEIN PURIFICATION Drug delivery devices have flourished during the last few decades and are extensively used in various kinds of treatments. In this context, CP-based devices have been investigated to examine how they can serve as electrically controlled drug delivery devices inside the body. One major challenge is to develop a drug delivery system that allows strict control of the ON/OFF state. In addition, such a device must be able to deliver the drug of interest at doses that are required to obtain the therapeutic effect. Abidian et al.[41] reported an on-demand drug realizing system based on PEDOT nanotubes. In addition, Shi et al.[42] reported chemically modified poly(1-(2-carboxyethyl) pyrrole) microtubes could be used as an affinity matrix for protein purification. Elution of the protein showed that the desorption ratio was up to 99.5%. Furthermore, it was found that the adsorption–desorption cycle was repeated ten times using the same microtubes without significant loss in the hemoglobin adsorption capacity. Sensors are very important devices in industry for quantity control and online control of different processes. In order to measure parameters such as temperature, pressure, vacuum, and flow, physical sensors were used. However, in some special cases such as the detection of evolution of hazardous gases during industrial processes, which are very harmful for the environment, chemical sensors are required. Chemical sensors based on metal oxides have, therefore, been synthesized for the detection of various toxic gases produced during industrial processes that destruct the environment. However, the metal-oxide sensors suffer from a major drawback in spite of being selective. Sensors based on metal oxides generally operate at very high temperature, which is not desired for detecting hazardous chemicals evolving from industrial processes. In order to overcome this problem, the active layers of ­sensors have been replaced by CPs such as polypyrrole (PPy), PANI, polythiophene, and their derivatives since the early 1980s. Due to variable electrical conductivity, CPs are used as gas or chemical sensors. In its simplest form, a sensor consists of a planar interdigital electrode coated with a CP thin film. If a particular vapor is absorbed by the film and it affects the conductivity, its presence may be detected as a conductivity change.[43–45] Redox active materials, when doped, exhibit changes in their color, volume, mass, conductivity, ion permeability, and m ­ echanical strength.[46–49] Detecting the variations in any one of these physical properties indirectly allows the detection of the

analyte responsible for provoking the physical change in the CP. Compared to the commercially available m ­ etal-oxide sensors, CP-based sensors have many improved characteristics such as high sensitivities and short response times at room temperature.[50–57] Another advantage of sensors based on CPs is that they can be easily synthesized by chemical or electrochemical processes, and their molecular chain structure can be modified conveniently by ­copolymerization or structural derivations.[58–60] CPs also have good mechanical properties, which permit simplistic manufacture of sensors.[61–63] SOLID-PHASE EXTRACTION In recent years, intrinsic CPs with conjugated double bonds have been attracted much attention as advanced materials. There has been a growing interest in CPs due to their multifunctional properties and potential applications, including ion exchangers, energy storage materials, corrosion-resistant coating, catalysts, chemical sensors, and materials for separation.[64–66]These are versatile materials in which molecular/analyte recognition can be achieved in different ways, including (i) the incorporation of counter ions, (ii) utilizing the inherent and unusual multifunctionality (hydrophobic, acid–base, and π–π interaction; polar functional groups; ion exchanger; hydrogen bonding; and electroactivity) of the polymer, and (iii) the introduction of functional groups to the monomers. Also, these materials have additional advantages because they can be easily synthesized in both aqueous and nonaqueous medium, chemically and electrochemically. There are varieties in type of dopant and additives during synthesis. All these conditions and varieties affect the chemical, mechanical, morphological, and electronic properties of the polymers. Among those, CPs, PPy is especially promising for ­commercial applications because of its good environmental stability, facile synthesis, and higher conductivity than other conductive polymers. Recently, aniline-based polymers were used as sorbents for solid-phase extraction of chlorophenols from water samples.[67,68] Also, Pawliszyn and coworkers have applied PPy and poly-N-phenyl pyrrole as a coating material for solid-phase microextraction of some inorganic anions and organic compounds.[69–73] APPLICATIONS AS SENSORS CP shows almost no conductivity in the neutral (uncharged) state. Their intrinsic conductivity results from the formation of charge carriers upon oxidizing (p-doping) or reducing (n-doping) their conjugated backbone. Oxidation of the neutral polymer and the following relaxation processes causes the generation of localized electronic states and a so-called polaron is formed. If now an additional electron is removed, it is energetically more favorable to remove the

Composites– Conducting

higher power density than the film-type biofuel cell by two orders of magnitude. Particularly, single PPy-CdS nanowire photovoltaic cells were reported recently. We note that conjugated polymer-based photovoltaic elements (plastic solar cells) have been extensively reported, for a good review.[37–40]

596

Composites–­ Conducting

second electron from the polaron than from another part of the polymer chain. This leads to the formation of one bipolaron rather than two polarons. However, it is important to note that before bipolaron formation, the entire CP chain would first become saturated with polarons.[73] Sensors are usually composed of two parts: receptor and transducer. The receptor has high specificity, which can also greatly enhance the detection sensitivity. The transducer is usually a separate chemical or physical sense component, which can also greatly enhance the detection principles. The transducer is usually a separate chemical or physical sense component, which works with electrochemical, optical, thermal, piezoelectric, and other detection principles. The high application potential of CPs in chemical and biological sensors is one of the main reasons for the intensive investigation and development of these materials. Sensors are very important devices in industry for quantity control and online control of different processes. In order to measure parameters such as temperature, pressure, vacuum, and flow, chemical sensors based on metal oxides have, therefore, been synthesized for the detection of various toxic gases produced during industrial processes. However, the metal-oxide sensors suffer from a major drawback in spite of being selective. In order to overcome this problem, the active layers of sensors have been replaced by the CPs. CPs such as PANI, PPY, and PI, which get protonated and deprotonated show sensitivity for pH change. Deprotonation of these CPs results in a decrease of charge carriers along the polymer chains and is accompanied by changes in the electrical optical properties.NH3, NO, and pH sensors based on individual PANI, PEDOT, and PPy nanofibers/wires have also been reported. For instance, single PANI nanowire chemical sensors showed a rapid and reversible resistance change upon exposure to NH3 gas at concentrations as low as 0.5 ppm. Conductivity of CP can be caused by ions and/or electrons. The affinity of amino groups to protons may be the reason for high proton permeability of these polymers in aqueous solutions: films of PANI and PPY on glassy carbon and platinum electrodes show almost Nernstian ­behavior relative to pH. The potentiometric response of PPY is linear between pH 2 and 11. PH-SENSITIVITY OF CONDUCTING POLYMERS Many CP possess acidic or/and basic groups which can be protonated or deprotonated. To this family of CP belong PANI, PPY, PI, polycarbazole, and polyazines. Deprotonation of these CP results in a decrease of charge carriers along the polymer chains and is accompanied by changes in the electrical and optical properties. These changes lead to the modification of redox properties of CP; this effect is very strong for PANI and PI but almost not recognizable in PPY.

Conducting Polymers: Applications

PANI has a variety of oxidation states that are both pH and potential dependent. The protonation enhances the conductivity of PPY, whereas deprotonation decreases the conductivity. It is generally agreed that PANI exists in three different base forms: leucoemeraldine (fully reduced), emeraldine (half-oxidized, and pernigraniline (fully oxidized).[74] The only electrically conducting form is, however, the emeraldine salt form (ES), which is the protonated form of EB. In contrast to other CPs, protons must thus be added to the PANI backbone in order to make it electrically conductive. The EB form of PANI can be protonated with sufficiently strong acids to ES due to the presence of basic sites (amine and imine groups) in the polymer structure. The pKa values found for the transfer between these three pH dependent forms. It is well known that the chemistry of PANI is more complex than that of other CPs. CONDUCTING POLYMERS WITH AFFINITY TO INORGANIC IONS Unmodified CP films can display some intrinsic affinity to metal ions.[75,76] Films of PI and polycarbazol provide a selective potentiometric response to Cu(II) ions.[77] The complexation of Cu(II)-ions to polycarbazol enhances the conductivity of this polymer.[78] It was suggested that the complexation of Cu(II)-ions changes the conformation of the polymer from a compact coil to a higher conducting expanded coil. Films of poly-3-octylthiophene show aselective Nernstian response toward Ag+ ions. Introduction of ligands leads to a modification of this ionic sensitivity. Such modification can be realized by an introduction of corresponding monomer into the polymer backbone, [79–87] by using counter ions with such ligands, [88–92] or by inclusion of ionophores into the polymer matrix.[93] ­2,3-disubstituted 5-nitroquinoxalines bearing 2-pyrrolyl and 2-thienyl substituents have been electropolymerized. The potentiometric response of the resulting films toward various cations was tested and compared to the response of the monomers immobilized in a PVC membrane. It was concluded that the binding mechanism for the monomer and the polymer is the same.[85] CONDUCTING POLYMERS WITH AFFINITY TO ORGANIC MOLECULES A number of new chemosensitive CP based on derivatives of PANI, PPY, and other heterocyclic compounds was synthesized and studied in; potentiometric responses to dicarboxylates and amino acids were observed. Poly(3-aminophenylboronic acid) upon binding with sugars, this leads to changes in pKa of p­ oly(3-aminophenylboronic acid) corresponding to changes in H+ activity, which is detected as potentiometric response. Simultaneously, changes

Conducting Polymers: Applications 597

CONDUCTING POLYMERS WITH AFFINITY TO GASES An application of CP for detection of gaseous analytes belongs to the well-developed field of chemosensor design. Gases interacting with CP can be divided into two main classes: gases that chemically react with CP and gases that physically adsorb with CP. Chemical reactions lead to change in the doping level of CP and hence alter their physical properties such as resistance or optical absorption. Electron acceptors such as NO2, I2, O3, and O2 are able to oxidize partially reduced CP and therefore increase their doping level. To oxidize CP, the gas should have a higher electron affinity than the CP. NO2 was found to increase the number of charge carriers in PANI and (P3HTH) poly-(3-hexyl-thiophene) through oxidative doping with NO2–ions and therefore decreases the resistance. In the detection of nitroaromatics, an electron transfer from CP to analyte can be also used for optical detection. The electron-deficient nitroaromatics, which bind through a π–π to the polymer, act as electron acceptors for the ­photoexcited electrons of the polymer[]. The protonation of PANI or PPY by HCl vapor leads to an increase of the polymer conductivity. This was used to design a multilayer conductometric sensors based on the subsequent polymerization of aniline and PEDOT on p-aminothiophenol-modified gold electrodes. The PEDOT layer provides electrical contact between the two sensing parts and operates as filter and protective layer covering the sensors surface. The conductivity responses of PANI on exposure to ammonia and hydrochloric acid. Deprotonation of PPY or PANI by ammonia leads to an increase of the polymer resistance, for PPY this process is reversible at low ammonia concentrations but irreversible at higher concentrations, especially under high humidity. Composites of CP with metal- [96] or metal-oxide nanoparticles, carbon nanotubes, organic and metal-­ organic compounds, and insulating polymers [97–99] provide new analytical possibilities. Metal-oxide particles are assumed to form n−p hetero junctions with the CP containing a depletion region. Adsorbed gases change the depletion region. Adsorbed gases change the depletion region

and thus modulate the conductivity of the junction.[100,101] ZnO nanowires in composites with P3HT assumed to reduce gases such as chlorine gas and therefore enhance its sensitivity to oxidizing gases and reduce the P3HT and therefore enhance its sensitivity to oxidizing gases and reduce its sensitivity to reducing gases. Ferrocene was immobilized in PPY to enhance the sensitivity of a PPYbased CO sensor. The CO is assumed to interact with the iron ions of ferrocene, which then transfer an electron to PPY chain. The interaction of short-chain alcohols with various PANI derivatives increases the order in the p­ olymer films which is accompanied by the expansion of polymer chains and an increase in the conductivity. The adsorption of short-chain aliphatic alcohols to PANI/PSS blends is assumed to enhance the charge transfer between adjacent PANI particles by reducing the potential barrier for ­hopping/tunneling processes, or by increasing of ­inter-chain and inter-particle charge mobility.[102] MOLECULARLY IMPRINTED CONDUCTING POLYMER The formation of artificial receptors by means of molecularly imprinted polymerization includes the following essential steps: 1. Preparation of non-covalent complex or covalent conjugate between analyte (or its analog) and ­polymerizable functional monomers 2. Polymerization of these functional monomers 3. Removal of the analyte Electrical and optical properties of conjugated polymers based on acrylic compounds with different functional groups provide higher flexibility in the selection of ­polymerizable monomers and cross-linkers. MODIFICATION OF CONDUCTING POLYMER BY RECEPTORS The immobilization procedures are based on non-­covalent interactions (physical adsorption, electrostatic assembly, hydrophobic interactions) or covalent binding of r­ eceptors to a matrix. Different CP were used for the preparation of  biosensors, [103] but the most attention was  paid to PPY.[104,105] Examples of immobilization of CP are ­presented in Table 1. CONCLUSIONS Nanocomposites are very important inclusions in the list of novel materials and composites although their history is

Composites– Conducting

of resonance frequently of quartz microbalance were measured.[94,95] ATP binding to the PTH Polythiophene derivative poly(3-(3,-N, N,N-trimethylamino-1-­propyloxy)4-methyl-2,5-thiophene hydrochloride) in solution leads to changes in the absorption spectra and to fluorescence quenching. Suggestively, an electrostatic interaction of negatively charged triphosphate group of ATP with positively charged ammonium group promotes planarization of (PTH) polythiophene backbone resulting in efficient π–π stacking between the (PTH) polythiophene backbones.

598

Conducting Polymers: Applications

Table 1  Conducting nanocomposite polymers as sensors Analyte Polymer/polymerization method

Biological receptors

Uric acid

Uricase

PANI

Type of transducing Photometric(UV–visible)

References [106]

H 2 O2

Chemical deposition to pretreated PET plates HRP

Photometric(UV–visible)

[107]

Oligosaccharides

PANI

Glucoamilase

Photometric(UV–visible)

[108]

Glucose

Electrochemical polymerization

GOx modified by PEG

Amperometric

[109]

Phenol

PEDOT electrochemical polymerization

Tyrosinase

Amperometric

[110]

Phenolic compounds Copolymer; PPY/thiophene derivative

Polyphenol oxidase

Photometric(UV–visible)

[111]

H 2 O2

Electrochemical polymerization PANI

HRP

Amperometric, colorimetric

[112]

Glucose

Electrochemical polymerization PANI

GOx modified by AMPS

Amperometric

[113]

not older than 10 years. CP-based nanostructured materials offer the advantages of low dimensionality and enhanced surface-to-volume ratio that make them promising materials in various fields. Polymers are highly radiation-sensitive materials, and irradiation can cause useful modification in the polymer that cannot be achieved by other means. In this entry, an attempt has been made to describe the salient ­features of CPs and their wide ­applications in different fields. REFERENCES

Composites–­ Conducting

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80. Dabke, R.B.; Singh, G.D.; Dhanabalan, A.; Lal, R.; Contractor, A.Q. An ion-activated molecular electronic device. Anal. Chem. 1997, 69 (4), 724–729. 81. Ion, A.C.; Moutet, J.C.; Pailleret, A.; Popescu, A.; SaintAman, E.; Siebert, E.; Ungureanu, E.M. Boronate-­ functionalized polypyrrole as a new fluoride sensing material. J. Electroanal. Chem. 1999, 464 (3), 24–30. 82. Buda, M.; Moutet, J.C.; Pailleret, A.; Saint-Aman, E.; Ziessel, R. Electrosynthesis and coordination chemistry of poly(ferrocene-bipyridyl) films. J. Electroanal. Chem. 2000, 484 (2), 164–171. 83. Liu, B.; Yu, W.L.; Pei, J.; Liu, S.Y.; Lai, Y.H.; Huang, W. Design and synthesis of bipyridyl-containing conjugated polymers: Effects of polymer rigidity on metal ion sensing. Macromolecules 2001, 33 (24), 7932–7952. 84. Heitzmann, M.; Bucher, C.; Moutet, J.C.; Pereira, E.; Rivas, B.L.; Royal, G.; Saint-Aman, E. Complexation of poly (pyrrole-EDTA like) film modified electrodes: ­Application to metal cations electroanalysis. Electrochim. Acta 2007, 52 (9), 3082–3087. 85. Breznov, H.; Volf, R.; Sessler, J.L.; Try, A.C.; Shishkanova, T.V. Potentiometric responses and mechanism of anionic recognition of anionic recognition of heterocalixarene-based ion selective electrodes. Anal. Bioanal. Chem. 2007, 587 (2), 247–253. 86. Tarabek, J.; Jahne, E.; Rapta, P.; Ferse, D.; Adler, H.; Dunsch, L. New acetophenone-functionalized thiophene monomer for conducting films on electrodes in chemical ion-sensorics: The synthesis and spectroelectrochemical study. Russ. J. Electrochem. 2006, 42 (3), 1169–1174. 87. Seol, H.; Shin, S.C.; Shim, Y.B. Electrochemical synthesis and characterization of poly[3-(4-formyl3-­hydroxyphenyl)-­5,2 :5,2-terthiophene] film. Electroanalysis 2004, 16 (24), 2051–2056. 88. Migdalski, J.; Blaz, T.; Paczosa, B.; Lewenstam, A. Magnesium and calcium-dependent membrane potential of poly(pyrrole) films doped with adenosine triphosphate. Microchim. Acta. 2003, 143 (4), 177–185. 89. Mousavi, Z.; Alaviuhkola, T.; Bobacka, J.; Latonen, R.M.; Pursiainen, J.; Ivaska, A. Electrochemical characterization of poly(3,4-ethylenedioxythiophene) (PEDOT) doped with sulfonated thiophenes. Electrochim. Acta 2008, 53 (11), 3755–3762. 90. Mousavi, Z.; Bobacka, J.; Lewenstam, A.; Ivaska, A. Response mechanism of potentiometric Ag+ sensor based on poly(3,4-ethylenedioxythiophene) doped with silver hexabromocarborane. J. Electroanal. Chem. 2006, 593 (1–2), 219–226. 91. Zanganeh, A.R.; Amini, M.K. A potentiometric and voltammetric sensor based on polypyrrole film with electrochemically induced recognition sites for detection of silver ion. Electrochim. Acta 2007, 52 (11), 3822–3830. 92. Vazquez, M.; Bobacka, J.; Luostarinen, M.; Rissanen, K.; Lewenstam, A.; Ivaska, A. Potentiometric sensors based on poly(3,4-ethylenedioxythiophene) (PEDOT) doped with sulfonated calix[4]arene and calix[4]resorcarenes. J. Solid State Electrochem. 2005, 9 (5), 312–319. 93. Cortina-Puig, M.; Munoz-Berbel, X.; Del Valle, M.; Munoz, F.J.; Alonso-Lomillo, M.A. Characterization of an ion-selective polypyrrole coating and application to the joint

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104. Njagi, J.; Andreescu, S. Stable enzyme biosensors based on chemically synthesized Au-polypyrrole nanocomposites. Biosens. Bioelectron. 2007, 23 (1), 168–175. 105. Ramanavicius, A.; Ramanaviciene, A.; Malinauskas, A. Electrochemical sensors based on conducting polymer— polypyrrole. Electrochim. Acta 2006, 51 (27), 6025–6037. 106. Caramori, S.S.; Fernandes, K.F. Covalent immobilisation of  horseradish peroxidase onto poly (ethylene terephthalate)–­poly (aniline) composite. Process Biochem. 2004, 39 (3), 883–888. 107. Silva, R.N.; Asquieri, E.R.; Fernandes, K.F. Immobilization of Aspergillus niger glucoamylase onto a polyaniline polymer. Process Biochem. 2005, 40 (3–4), 1155–1159. 108. Piro, B.; Dang, L.A.; Pham, M.C.; Fabiano, S.; TranMinh, C. A glucose biosensor based on modified-­ enzyme incorporated within electropolymerised poly (3, ­4-ethylenedioxythiophene)(PEDT) films. J. Electroanal. Chem. 2001, 512 (1), 101–105. 109. Vedrine, C.; Fabiano, S.; Tran-Minh, C. Amperometric tyrosinase based biosensor using an electrogenerated polythiophene film as an entrapment support. Talanta 2003, 59 (3), 535–544. 110. Kiralp, S.; Toppare, L.; Yagci, Y. Immobilization of polyphenol oxidase in conducting copolymers and determination of phenolic compounds in wines with enzyme electrodes. Int. J. Biol. Macromol. 2003, 33 (1), 37–41. 111. Morrin, A.A.; Guzman, A.; Killard, A.J.; Pingarron, J.M.; Smyth, M.R. Characterisation of horseradish peroxidase immobilisation on an electrochemical biosensor by colorimetric and amperometric techniques. Biosens. Bioelectron. 2003, 18 (2), 715. 112. Sung, W.J.; Bae, Y.H. A glucose oxidase electrode based on polypyrrole with polyanion/PEG/enzyme conjugate dopant. Biosens. Bioelectron. 2003, 18 (3), 1231–1239. 113. Asberg, P.; Inganas, O. Hydrogels of a conducting ­conjugated polymer as 3-D enzyme electrode. Biosens. Bioelectron. 2003, 19 (1), 199–207.

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Conducting Polymers: Electrospun Materials Thomas Kerr-Phillips and Jadranka Travas-Sejdic Polymer Electronics Research Centre, School of Chemical Sciences, The University of Auckland, Auckland, New Zealand; The MacDiarmid Institute for Advanced Materials and Nanotechnology, Wellington, New Zealand

Abstract Electrospinning is a fabrication technique that is easy to setup, simple, and inexpensive. It is widely used to produce continuous microfibers/nanofibers, and thus is a popular approach to fabricate microscale/nanoscale polymeric materials. Conducting polymers (CPs) are versatile electroactive materials, which are gaining increased interest for the development of sensors, optoelectronics, flexible electronics, responsive biomaterials, and many other applications. The high surface area of electrospun CPs is highly desirable; for example, in sensor applications, in 3D platform for advanced tissue engineering, and in producing flexible, porous electronic devices. Therefore, the combination of electrospinning and CPs affords versatile, smart microfiber/nanofiber materials useful for a wide range of applications. In this entry, we first review the methodologies of electrospinning of CPs, followed by discussion on the applications of electrospun CPs and their composites. Keywords: Conducting polymers; Electrospinning; Nanofibers; Sensors introduction; Tissue engineering.

INTRODUCTION Electrospinning Composites–­ Conducting

Electrospinning is a nanofabrication technique that has been around for the last eight decades.[1–4] With a new drive for nanotechnology, however, electrospinning has recently regained significant interest. The process involves slowly pumping out a polymer solution typically through the tip of a metallic needle. A potential difference is applied between the needle and a fiber collector, usually a metal plate, generating an electrostatic force.[1,3] This electrostatic force acts directly against the surface tension of the solution at the needle tip.[3] As the electric field increases in strength, the hemispherical droplet at the needle tip elongates and forms what is known as a Taylor cone.[1,3] The polymer jet is initiated when the electrostatic forces overcome the surface tension of the droplet. Ideally, the solvent is vaporized before the solution hits the collector, leaving behind continuous polymer microfiber/nanofiber mats. It is worthy to note that polymer melts can also be electrospun,[5–7] but as this would require high temperatures, it is not as often used. A general schematic for electrospinning is given in Fig. 1. The electrospinning process is affected by a number of parameters that determine the quality of the nanofiber mats produced. These are excellently discussed in detail in recent articles and reviews on electrospinning.[1,3,8–11] In short, these parameters include: the applied potential

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difference between the tip and the collector (i.e., the strength of the electric field), the distance between the needle tip and the collector, the viscosity and viscoelasticity of the polymer solution, the solution ionic strength and ionic conductivity, the polymer properties (glass transition temperature, solubility, molecular weight, etc.), the ­humidity, and the temperature. Wet electrospinning of polymer solutions is the most commonly used method of electrospinning as it is the simplest. Other variants include: melt, [5–7] bath electrospinning (into a liquid bath as the collector), [12,13] electrospinning of aligned fibers on a modified collector of various designs, [13–16] and bubble electrospinning (from a bubble formed at the needle tip).[17] Electrospun polymers have had a wide range of applications. These include filters, [18–20] desalination membranes, [21] tissue engineering, [2,22–25] drug-release, [25,26] sensors, [27–32] polarized organic LEDs, [33,34] and many others.[35–38] Electrospun fiber mats applications in tissue engineering are particularly widely explored in the recent literature [2,22–25,39] due to their excellent porosity, desired in cell scaffolds mimicking cell’s extracellular matrix.[2,22–25,39] Conducting Polymers Since the first report[40] of high conductivity of iodinedoped polyacetylene in 1977, the research field of electrically conducting polymers (CPs) has been rapidly expanding. CPs, in principle, combine the processability of Encyclopedia of Polymer Applications, First Edition DOI: 10.1201/9781351019422-140000033 Copyright © 2018 by Taylor & Francis. All rights reserved.

Conducting Polymers: Electrospun Materials 603

Taylor cone

Fig. 1  Schematic of an electrospinning setup

conventional polymers, with the electrical conductivities of metals or semiconductors. Due to their plastic mechanical properties, organic origin, and electrical conductivity, CPs have been utilized in numerous applications, including flexible and printable electronics, [41] plastic solar cells [42] and light-emitting diodes (LEDs), [43] sensors, [44] artificial muscles [15,45] and other actuators, [46,47] anticorrosive coatings, [48] and redox mediators.[49] Figure 2 shows the most common classes of CPs. In conventional polymers, the bandgap between conductive and valence band is large, making these materials insulators. In CPs, the bandgap is such (due to the conjugation in the backbone) that it can be modified upon a so-called “doping” process, [50,51] practically achieved by means of removing (or sometimes adding) an electron from (to) a neutral polymer chain.[50] This process modifies the bandgap of the polymer through distortion of the lattice and through charge delocalization over several monomeric units.[50] The formed radical ion (created upon oxidation or reduction of the polymer backbone) is commonly balanced by a counter ion.[50,52] The as created states with energies between the valence and conduction band, are called polarons (radical cations and anions), and those created upon further doping, bipolarons (radical dications or dianions). Bipolarons are energetically more favorable than

Polyacetylene

O

While CPs are more processable than other (semi)conductors, they are still far less processable than conventional polymers.[73,74] The majority of CPs are insoluble, intractable, and brittle materials. Only a few CPs are partly soluble in organic solvents, and their solubility is often achieved through chemical functionalization with side chains.[16,55,73,75,76] Therefore, CPs are difficult to process by electrospinning and hence pristine CPs are rarely electrospun on their own. More commonly, CPs are either co-­electrospun with a support polymer or incorporated on/ into the fibers post-electrospinning.

N n H Polypyrrole

n

S n Polythiophene

O S

n

S

Poly(3,4-ethylenedioxythiophene)

n Poly(phenylene vinylene)

Fig. 2  Common types of CPs

FABRICATION METHODS OF ELECTROSPUN CPS

HN

n

Poly(3-hexyl thiophene)

n Poly(phenylene ethylene)

n

Polyaniline

n Poly(para-phenylene)

Composites– Conducting

Syringe and syringe pump

two polarons over the same polymer segment.[50] These states are considered responsible for the observed charge transport and conductivity in CPs. The doping process can be typically induced either chemically[52] or electrochemically.[52] Conductivity is determined by the number and mobility of these charge carriers, often limited by the mobility of the dopant counter ion. CPs are versatile and “smart” materials, amenable to nano/ micro fabrication,[46,53–55] including electrospinning.[55–57] Electrospun nanofibers that contain CPs possess dynamic properties imparted by the CP and have been explored in many applications, including advanced tissue engineering,[23,38,58,59] chemical and biological sensors with higher sensitivities and lower detection limits,[30,57,60–63] breathable hydrophobic fabrics,[38] drug delivery,[64] flexible electronics,[65–69] and many others.[67,70–72] The following sections first summarize the methods used to produce electrospun CPs, followed by outlining prominent examples of applications of these ­materials. Lastly, we provide a brief d­ iscussion and an outlook.

Metal collector

Nanofibers

604

Electrospinning of Pristine CPs

Composites–­ Conducting

As mentioned above, there are not many examples where pristine CPs are electrospun. Recently, some clever macromolecule engineering and processing methodologies have been developed to make this possible, as outlined below. One of the most common methods to electrospin pristine CP is the combination of adding solubilizing side groups on the CP and with carefully selecting a binary solvent system. The solvent system must be designed such that the best solubility can be obtained from the CP, the solution has good conductivity, and the solvents encourage interchain interactions. The interchain interactions contribute to the physical chain entanglement improving the viscoelasticity of the solution and suppressing bead formation. These techniques have been used to electrospin pristine poly(2-methoxy-5-(20-­ ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) [16] and poly(3-hexylthiophene) (P3HT). [66] In the example of MEH-PPV, a binary solvent system of chloroform, as a good solvent and either methanol or isopropanol as a bad solvent was utilized. Here, the bad solvent is chosen to increase the solution conductivity and increases interchain interactions. Chloroform is used as a good solvent to insure complete dissolution of the polymer. Electrospinning of that polymer/solvent system afforded a fluorescent MEH-PPV aligned fiber mat that displayed polarized light emission. González, Pinto et al.[55] electrospun poly(3-­ hexylthiophene) (P3HT) from a 7%wt/v P3HT in chloroform as unary solvent. A single polymer nanofiber was isolated during the process that was used in a nanoscaled field effect transistor. In the above cases, the CP had to be dissolved either at elevated temperatures or by sonication to overcome their limited solubility. Using a core shell morphology can help add stability to the electrospun fibers, allowing pristine CP cores to be electrospun. Recently, this is demonstrated by Chang, Liu et al.[66] In this study, electrospun core-shell nanofibers are made from a poly(methy methacrylate (PMMA) shell and P3HT core. Gold nanoparticles were included into the P3HT solution made from a binary solvent mixture and were embedded into the resulting fibers. The composite fiber was used as a flexible nonvolatile transistor memory device. The binary solvent system (chloroform and chlorobenzene) was used to increase the interchain interactions and solution conductivity, as before. In the examples above, the CPs had side substituents that improve solubility such that the polymer concentrations in the electrospinning solutions were maximized. This helps improve the solutions’ viscosity and viscoelasticity, which helps the solutions to be electrospun. In a different approach by Huang, Lo et al.[57] magnesium nitrate was used to electrostatically crosslink poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)

Conducting Polymers: Electrospun Materials

(PEDOT:PSS), thereby increasing the viscosity of the electrospinning solution. This unique strategy produced nanofibers with diameters ranging from 70 to 100 nm. The authors demonstrated the use of the obtained fibrous material for gas sensing of dimethyl formamide (DMF), dimethyl sulfoxide (DMSO), and propylene carbonate (PC) vapors. CP Blends The most common method for electrospinning polymers lacking desirable mechanical properties, including CPs, is to utilize a support polymer.[74] There are several common polymers used for this purpose depending on the application. For tissue engineering and other biomedical applications, polylactic acid (PLA), poly(lactic-co-glycolic acid) (PLGA), or polycaprolactone (PCL) are commonly used, as these polymers are biocompatible and biodegradable. A common limitation with electrospun CP/conventional polymer blends, however, is the moderately low CP content compared to the support polymer. As a result, electrospun polymer blends have moderated CP properties (conductivity, electroactivity, and electroluminescence). Still, a number of applications, such as wound dressing or tissue engineering, do not require substrates of high conductivity.[77] Furthermore, low concentrations of CPs have been shown to be still able to provide antioxidant and antimicrobial properties.[23,58] Thus, a common focus for electrospun CPs for tissue engineering studies focuses on balancing the CP properties with fiber morphology and mechanical properties.[14,23,58,59,76,78] Due to the superior hydrophilicity of polyaniline (PANI) and polypyrrole (PPy), these classes of CP dominate such biological and biomedical studies.[14,58,74,79,80] As an example, Hsiao, Bai et al.[14] electrospun aligned fibers from a blend of polyaniline emeraldine base (PANI-EB) and PLGA for cardiac tissue engineering applications. A 0.5%wt/v solution of PANI-EB in hexafluoropropanol was prepared by stirring overnight before adding PLGA at 6 to 12%wt/v. It was found that higher ratios of PLGA to PANI-EB had better fiber morphology but worse conductance. The analysis of cell growth indicated that 8%wt/v of PLGA was the preferred ratio. Gizdavic-Nikolaidis, Ray et al.[23,58] synthesized a carboxylic acid functionalized PANI and blended it with PLA. The fibers, with ratios of PANI: PLA from 5:95 to 45:55, respectively, were electrospun from a 4%wt/v polymer solution in DMSO/tetrahydrofuran (THF). They demonstrated successful growth of fibroblasts, and it was argued the functionalized PANI added antimicrobial and antioxidant functionality. A similar study, conducted by Abdul Rahman, Feisst et al.[76,78] with the same polymer system used mixed DMF:THF solvents. The resulting fiber mats were used to demonstrate a tissue engineering application by successfully growing human adipose-derived stem cells on the substrates.

CP fibers blended with biocompatible polymers for tissue engineering have further been developed to include drug delivery capabilities. [81] For example, PANI/PLA fibers made for tissue engineering applications have been loaded with silica nanoparticles containing a drug to help cell vascularization. In this study, solutions of PLA in chloroform: DMF (4:1) and a PANI (doped with dinonylnaphthalene sulfonic acid) emulsion were mixed. Before mixing, the PANI emulsion (purchased commercially) was further diluted with chloroform. Mesoporous silica nanospheres, loaded with a model drug compound (DiO), were then entrapped in the porous electrospun fiber structure by drop casting the solution containing the nanoparticles onto the fiber mat followed by removal of the solvent. As the cells (myoblasts) grew on the substrates, the entrapped silica nanoparticles were internalized by the cells, following by the particles’ slow release of the drug. While biological based applications use biocompatible and biodegradable support polymers, the field of sensing is much more diverse. Thus, there is a much wider range of support polymers commonly used. When choosing a support polymer for CP fiber sensors, there are two main considerations: the mechanical properties and how the support polymer effects the sensing. For example, one may choose polystyrene for its superior mechanical properties. Such an example is provided by Aussawasathien, Dong et al.[63] In this study, electrospun PANI/polystyrene fibers for hydrogen peroxide detection were demonstrated. Polystyrene (7.5%wt/v) as a support polymer was utilized, in a solution with PANI (2%wt/v) and a dopant (camphorsulfonic acid, HCSA, 4%wt/v) dissolved in chloroform. The fibers showed a good electrochemical response to hydrogen peroxide, with increased currents observed for higher H2O2 concentrations. Liu, Fang et al.[60] developed an electrospun strain sensor and demonstrated the importance of the properties of the support polymer on the sensor characteristics. Here, a blend of PEDOT:PSS with poly(vinyl alcohol) (PVA)

Motor

Crankshaft connecting rod

was electrospun. The electrospinning solution contained a commercially available PEDOT:PSS aqueous solution into which PVA was added. This was then electrospun onto nickel coated Kapton tape. The resulting fibers were then embedded into an elastomer, poly(dimethylsiloxane) PDMS, and the solvent DMSO was used to further tune the conductivity. PVA was chosen as the support polymer as PVA has moderate elastic properties, [82] thus it facilitates the strain required for the sensing. Similarly, Sun, Long et al.[83] developed a strain sensor made of PEDOT:PSS, poly(vinyl pyrrolidone) (PVP), also with DMSO as an additive to improve the conductivity. The blend of PEDOT:PSS/PVP was electrospun onto a PDMS covered aluminum foil. Like above, both PVP and PDMS afforded the elastic properties of the sensor. In this study, the strain of the resulting material was further improved by the use of a mechanical arm that oscillated the electrospinning syringe during electrospinning in such a way that coiled fibers formed (Fig. 3). The coiled fiber morphology improved the maximum strain, which enhanced the sensors capabilities. Occurrence of beading in electrospun fibers is generally avoided. However, Lin, Li et al.[62] reported beading to have a positive impact on their PANI/poly(vinyl butyral) (PVB)/poly(ethylene oxide) (PEO) nanofibers humidity sensor. The fibers were prepared from a solution containing various ratios of PANI (doped with poly(styrene sulfonic acid), PSSA), PVB, and PEO in DMF. The optimal weight ratios of PANI-PSSA/PEO/PVB were deemed to be 20:3:14 with a maximum total polymer content of 3.7 %wt/v. This ratio generated beading of the fibers that show an optimal sensing response and good adhesion to glassy carbon electrodes. Carbonization is an innovative way to overcome the limitations of low CP content in the CP blends and to increase the resulting fibers conductivity. For example, Zhang, Liu et al.[84] electrospun PPy nanoparticles blended with polyacrylonitrile (PAN), which was subsequently carbonized at 900°C. Here, PAN in a DMF solution with

Slender rod Syringe DC power supply

Speed controller

Needle

Base

Collector

Fig. 3  Schematic diagram of electrospinning apparatus to form coiled PEDOT:PSS fibers [83]

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606

Conducting Polymers: Electrospun Materials

Composites–­ Conducting

PPy nanoparticles added at varying quantities (6–18 %wt, Fig.  4) was electrospun. The different content of PPy nanoparticles resulted in varying pyrolic nitrogen levels that affected the electroactivity of the final fibers. After carbonization, the electroactivity was enhanced and the fibers were used as a hydrogen peroxide sensor. In a characteristic study to produce light emitting smart textiles, blends of poly(9,9-dioctylfluoreny-2,7-diyl) (PFO), MEH-PPV, and PMMA were electrospun. This study illustrated that multiple optically active CPs can be blended in electrospun fibers to tune the resulting fibers fluorescence and color. The resulting materials in this study could emit the full spectrum of visible light by controlling the PFO/ MEH-PPV ratios.[85] The authors also used a common method to improve the electrospinning solution’s ability to be electrospun, by adding a small amount of salt.[86] In this case, benzyl triethylammonium chloride was added to the solution. This increased the solution’s ionic conductivity and thus allowed for uniform beadless fibers to form during electrospinning. Researches have shown that one may take an advantage of CPs’ poor solubility to obtain pristine CP fibers. A simple method to achieve this is to employ a carrier polymer that can be later removed, usually by dissolving it. Sujith, Asha et al.[71] reported a study on electrospinning of a carbon black/PANI composite fibers with poly(vinyl alcohol) as a support polymer that was subsequently dissolved. These fibers demonstrated excellent conductivity and after a heat treatment showed excellent porosity (71%).

While conventional electrospinning uses homogeneous solutions, emulsions and colloidal polymer dispersions can also be electrospun. Emulsion electrospinning provides a method to electrospin polymers that are relatively insoluble such as CPs. In emulsion (and colloidal dispersion) electrospinning of CPs, a CP is typically prepared by polymerizing it in the presence of a surfactant that usually also acts as a dopant, resulting in a CP colloid/emulsion. In most cases, the resulting colloid/emulsion is mixed with a support polymer solution forming another emulsion. The previous mentioned study by Shokry, Vanamo et al.[81] utilized this method with a commercially obtained PANI based emulsion. Similarly, Ju, Park et al.[87] prepared PPy electrospun fibers using a chemically modified electrospun rubber, sulfonated poly(styrene-ethylene-butylene-styrene (sulfonated SEBS). The authors’ first polymerized pyrrole using ferric chloride in the presence of dodecylbenzenesulfonic acid as a surfactant. A slurry is then formed with a solution of the modified SEBS in chloroform, the PPy slury, and DMF. Due to the sulfonated SEBS also acting as a dopant for PPy, the resulting fibers had excellent electroactivity and high electrochemical capacitance. In a similar manner to the PPy nanoparticle dispersed fibers prepared by Zang, Lie et al.[84] Zhou, Cheng et al.[79] produced PPy nanoparticles dispersed in PLA fibers. This was achieved by first preparing PPy nanoparticles. To that end, pyrrole (0.7%wt/v) was added to a Pluronic P123 (an oligomeric surfactant) aqueous solution (1%wt/v) and

(a)

(b)

(c)

(d)

Fig. 4  Transmission electron microscopy (TEM) images of NCNFs (A), NCNPFs-6 %wt (B), NCNPFs-12 %wt (C), and NCNPFs-18 %wt (D)[84]

polymerized by the dropwise addition of an aqueous FeCl3 solution (1 g/ml) under continuous stirring for 6 h at 18°C. This was then dried and ground producing a PPy nanoparticle powder. These particles were then dispersed in DMF (5.6%wt/v) and the resulting solution was added to a DCM solution containing PLA (18.75%wt/v), with the final solution containing a 2:1 DCM:DMF ratio. The resulting colloidal suspension was then electrospun on a rotating drum, producing aligned conducting fibers. In this study, the anisotropy of the mechanical properties is discussed, showing a significantly greater maximum elongation at break in the radial direction. Interestingly, the conductivity does not show any anisotropy and this is attributed to the nanoparticle content exceeding a percolation threshold (Fig. 5). These fibers were then used to cause stem cell differentiation (into neural cells) through a synergy between the fiber orientation and electrical stimulation. One of the most common CP colloidal dispersions used in electrospinning is PEDOT:PSS.[57,65,68,69,88] In a study by Kara, Frey et al.[88] PVA and 0.5%wt of a nonionic surfactant (Trition X) were added to a commercially available PEDOT:PSS solution. The mixture was then electrospun as is, or with the addition of various solvents including ethylene glycol, DMSO, DMF, and THF. The fibers produced without the solvent additives were heavily beaded, whereas the fibers with ethylene glycol had the most desirable ­morphology with thin and uniform fibers with few beads. In a work by Zhao, Nugay et al.[69] flexible, stretchable, transparent films were prepared by electrospinning a solution containing PEDOT:PSS and PEO onto drying

(a)

polymer films (PMMA, polyimide (PI), or poly(urethane) (PU)). The PMMA, PI, or PU films were casted onto a conducting belt that dried during the electrospinning process such that the electrospun fibers were partially embedded into the cast films. The authors emphasized that in order to maximize the amount of the PEDOT:PSS, a high molecular weight support polymer is needed. Thus, 0.7%wt of high molecular weight PEO was added to a commercial PEDOT:PSS emulsion. The resulting transparent polymer films were proposed to be used in electro-optical applications. Novel core-shell fibers were prepared from a cross-linked thermoresponsive polymer (poly(N-­ isopropylacrylamide-co-N-methylolacrylamide) (PNN)) and PEDOT:PSS as the shell and poly(butyl acrylate-b-­ styrene) as the core.[68] The core-shell fibers were obtained by using coaxial needles. The PEDOT solution was prepared by dissolving PNN into a PEDOT:PSS solution, at various ratios, while keeping the solid content at 6%wt/v. Both electrospun solutions had sodium 1,4-bis(2-ethylhexyl)sulfosuccinate (AOT) as a surfactant added to prevent beading. The PEDOT:PSS:PNN core was then thermally cross-linked at 110  °C post-electrospinning. Like in other studies, [60,83,88] DMSO was added as an additive to improve the conductivity; in this case, with DMSO swelling the crosslinked PNN polymer network. which further emphasized the effect. A number of the above-mentioned studies demonstrate the advancements in electrospinning technique in recent years. Various modifications of the technique have been

(b)

0.25

(e)

(315.2 ± 3.7) nm

Intensity/%

0.20 0.15 0.10 0.05 0.00 100

(c)

(d)

200

300 400 Diameter/nm

500

600

(f) Fiber PPy Current path

Fig. 5  (a) Scanning electron microscopy (SEM) image of random PPy nanoparticle containing nanofibers; (b) SEM image of aligned PPy nanoparticle containing nanofibers; (c) TEM image of electrospun nanofibers embedded with PPy nanoparticles; (d) magnification of the square part in (c) showed the distribution of PPy particles in the fibers; (e) histogram of diameter distribution of the nanofibers; and (f) illustration of current path formed by PPy in and between PLA fibers [79]

Composites– Conducting

Conducting Polymers: Electrospun Materials 607

608

developed to produce complex fiber morphologies. These include: mechanical oscillation of the electrospinning needle, [83] modified needles for fabricating core-shell fibers, [66,68] implementing specialized collectors, [14–16,77,89] and mechanical manipulation of the electrospun fibers.[65] Such modifications allow for unique fiber morphologies that can add extra functionality or improve a devices performance.[83,92] For example, mechanical manipulation of the electrospun fibers by means of twisting an electrospun bundle of PEDOT:PSS–PVP, a micrometer-sized PEDOT “microrope” was prepared.[65] The electrospinning solution here was comprised of a commercial PEDOT:PSS, which was further diluted with ethanol before adding PVP and ionic liquid dopant, 1-ethyl-3-methylimidazolium acetate. The conductivity of the microropes was shown to exhibit linear responses to strain (up to 90%), and thus, the authors suggested applications of their “microropes” in sensors and as stretchable semiconductors. Table 1 summarizes the examples of CP/conventional polymer blends, revealing common electrospinning parameters and applications. Incorporation of CPs Post-Electrospinning

Composites–­ Conducting

An alternative approach to fabricate electrospun fibers containing CPs involves incorporating CPs onto the electrospun fibers post-electrospinning. Such approaches are utilized often when the applications require highly conductive and electroactive substrates, which are not easily achieved with electrospun CP blends. The most common method to incorporate CPs post-­ electrospinning is by chemical polymerization of the CP onto the exterior of the electrospun fibers. For example, Merlini, Barra et al. [90] electrospun poly(vinylidene fluoride) (PVDF) fibers and coated them with PANI by immersing the fibers into an acidic solution of aniline, followed by addition of iron chloride solution as the oxidant. They studied the change in conductivity as a function of the aniline concentration. In a similar approach, Cho, Nam et al. [38] electrospun PU and then coated the resulting fiber mats with PANI, using the same method as above with ammonium persulfate as the oxidant. The fibers where then further coated with poly(tetrafluoroethylene) (PTFE), affording a super-hydrophobic porous material which showed gas breathability. In another example of chemically polymerized CP coatings, PANI coated aligned PU fibers were prepared by Gu, Ismail et al.[15] An interesting feature of this study is the method used to produce aligned fibers, which was achieved with an innovative design of the fiber collector that generated aligned fiber bundles (Fig. 6). Aligned fibers are typically produced using a rotating drum collector or a grid collector with parallel lines.[16,77] Like in the work done by Cho et al.[38] the fibers were coated with PANI post-­electrospinning using an aqueous solution

Conducting Polymers: Electrospun Materials

containing aniline and ammonium persulfate. The fiber bundles showed linear actuation upon PANI oxidation of up to 1.65% and desirable mechanical properties for actuator applications, such as flexibility, stretchability, and toughness. Coating CPs onto electrospun fibers opens up the possibilities for producing unique morphologies of CP fibers, not possible with blends and pristine electrospun CP fibers. For example, in a study by Xie, MacEwan et al.[91] electrospun PLA and PCL fiber mats were prepared and immersed in a 0.04 M pyrrole solution, followed by the addition of an oxidant (FeCl3, 0.084 M). The authors also investigated ammonium persulfate as an oxidant and p-toluenesulfonic acid as the dopant. PPy hollow fibers were then formed by dissolving out the original PLA and PCL fibers with DCM (Fig. 7), allowing them to determine the thickness of the PPy shell. The fiber mats with PLA or PCL core intact were used as tissue scaffolds for neural tissue engineering. As mentioned above, the collector in electrospinning may be used to induce unique fiber morphologies. While coiled fibers can be obtained from mechanical manipulation of the electrospinning needle, similar morphology is also obtained with a modified electrospinning collector.[89] Yu, Yan et al.[89] fabricated a strain sensor from PVDF fibers with PANI coatings (using sulfosalicylic acid as the dopant). In this work, PANI was polymerized onto PVDF fibers electrospun onto a patterned grid. The grid caused the fibers to buckle allowing the fibers to stretch even after  they were coated with PANI. However, stretching caused the PANI coating to fracture, reducing conductivity. While the above examples demonstrate coating of CPs onto fibers Zou, Qin et al. studied the mechanism responsible for adhesion of a CP onto electrospun fibers.[77] In this work, aligned PLA fibers were produced by using a rotating drum collector. The resulting fibers were coated with PPy by sonicating the fibers in a solution containing pyrrole (14 mM), dodecylbenzenesulfonate (DBS, 7 mM), and poly(glutamic acid) (PGlu), 7 mM), before the oxidant (an iron chloride solution) was added. PGlu and DBS were included in the polymerization solution, and it was shown that the DBS contributed to the binding of PPy to the PLA fibers. FTIR measurements indicated chemisorption between the ester groups of the PLA and sulfo-acid groups in DBS, which were coupled to pyrrole segments. These fiber mats were then used as a scaffold for neural cell growth, with electrical stimulation improving neurite formation. Similarly, Thunberg, Kalogeropoulos et al. [80] electrospun cellulose acetate fibers and coated them with PPy to produce a tissue scaffold for the growth of neural cells. Cellulose acetate fibers were first immersed in a pyrrole solution (0.05–0.45 M PPy, 0.1 M HCl), followed by immersion in iron chloride solution (0.12–1.08 M). It was shown that PPy grew as nanoparticles clustering together to uniformly coat the electrospun fibers. This resulted in an increase in the average surface roughness

Conducting Polymers: Electrospun Materials 609

Table 1  Electrospun conducting polymer/conventional polymer blends Strength of Total polymer Conducting polymer Conducting the electric concentration concentration polymer Solvent(s) field (kV) %wt/v %wt/v PPy

Hexafluoroisopropanol (HFP)

12

PPy[87]

Chloroform/DMF

20

PPy[84]

DMF

PPy nanoparticles [79]

DMF/DCM

[59]

PANI DMSO/THF (functionalized)

7

1.05 and 2.1

80% vs overall polymer weight

Support polymer

Application(s)

PCL: gelatin (50:50)

Cardiac tissue engineering

Modified SEBS

Batteries

Not specified

16–28

6–18

PAN

Carbonized fibers used as a H2O2 sensor

15

14.37

5.6

PLA

Directed stem cell differentiation (into neural cells)

10–15

4

0.2–1.8

PLA

Tissue engineering

12

10.13–10.58

1.3–5.8

PLA

Tissue engineering

12.3–13.6

6.5–12.5

0.5

PLGA

Cardiac tissue engineering

30

9.5

2

Polystyrene H2O2 and glucose sensor

[10,27]

PANI DMF/THF (functionalized) PANI [14]

HFP

PANI [63]

Chloroform

PANI (with carbon black)[71]

Chloroform/Water

15 kV

8

4

PVA

Porous conductor

PANI [81]

Chloroform/DMF with a PANI emulsion with xylene, ethyl benzene, and 2-butoxy-ethanol

10–25

2–4.6

0.7–1.5

PLA

Tissue engineering with drug delivery

PANI:PSSA[62]

DMF

6

3.2–5

2

PVB and PEO

Humidity sensor

PANI (doped with CSA)[74]

HFP

14

12.5

2.5

Gelatin

Removal of chlorinated toxins from water

PANI (doped with peptide, FLQDV)[74]

HFP

14

12.5

2.5

Gelatin

Removal of chlorinated toxins from water

MEH-PPV[85]

Chloroform

15

10

10–0

PMMA and PFO

Full color light emitting material

MEH-PPV[34]

Chloroform

15

2.05 to 2.4

0.05%–0.4%

PEO

Polarized light emission

P3HT[34]

Chloroform

15

2.05–2.4

2.05–2.4

PEO

Polarized light emission

PEDOT:PSS [69]

Water

5

1.8

1.1

PEO

Electro-optical material

PEDOT:PSS [88] PEDOT:PSS [60] PEDOT:PSS [83] PEDOT:PSS [68]

Water Water Water Water

15 20 8 16

5.3 7–7.4 53 6

1.3 1–1.4 2.8 0.024–1.2

PVA PVA PVP PNN

Characteristic study Strain sensor Strain sensor Characteristic study of thermo-responsive flexible fibers

PEDOT:PSS [65]

Water/EtOH

15

15.084

0.084

PVP

Characteristic study of twisted micro ropes

Composites– Conducting

[76,78]

610

Conducting Polymers: Electrospun Materials

F2 F1 F2

Fig. 6  The fibers collector setup to form aligned fiber bundles, as designed by Gu, Ismail et al. [15] Source: Reprinted with permission from Gu et al.[15]

(a)

(b)

250 nm

250 nm (c)

(d)

Composites–­ Conducting

1 µm

50 nm

Fig. 7  (a, b) SEM images of PLA nanofibers and PPy nanotubes, respectively; (c, d) TEM images of the PPy nanotubes. The PPy ­nanotubes were produced by soaking the PLA-PPy core–sheath nanofibers in DCM to selectively remove the PLA cores Source: Reprinted with permission from Xie et al.[91]

of the fibers, leading to an improved adhesion of neural cells to the fibers. The above-discussed two studies illustrate a need for an affinity for the underlying electrospun fibers and the chosen CP in order to obtain a uniform CP coating. Electrochemical polymerization is also used to fabricate CP coatings onto electrospun fibers. This requires the electrospinning collector to also act as the working electrode for the electropolymerization. As an example, Yang, Kampstra et al.[31] electrochemically polymerized 3,4-ethylenedioxythiophene (EDOT) onto electrospun PLA. The PLA fibers were electrospun onto platinum microelectrodes, which were then used to electropolymerize EDOT. The resulting PLA/PEDOT matrix, while still attached to the electrodes, was then used as an enzymatic electrochemical glucose sensor as glucose oxidase was embedded in the fiber matrix. In another example, a neural probe with drug-delivery functionality was produced.[92] Abidian, Kim et al.[92] electrospun PLA and PLGA fibers with dexamethasone in the

electrospinning solution, onto micro-fabricated gold, neural probe electrode arrays. These probes were then used as the working electrode for electropolymerization of PEDOT. After PEDOT polymerization, the PLA was removed, forming PEDOT nanotubes and the fibers controlled drug release through electrical stimulation and were suggested for neural probes with capability of ­dexamethasone ­delivery upon PEDOT oxidation. A simple method to obtain CP coated electrospun fibers was demonstrated by Chang, Sun et al.[93] who coated PEDOT:PSS onto PLA/poly(3-hydroxybutyrate-co-3-­ hydroxyvalerate) (PHBV) fibers. The PEDOT:PSS coating was obtained by soaking the fiber mats in isopropanol/ PEDOT:PSS solutions (10%v/v and 30%v/v), followed by vacuum drying, with no further steps or modifications required. Extensive characterization was performed on these fibers, including a cell assay (human fibroblasts) showing more favorable results with PEDOT:PSS coatings than without.

In research by Wang, Kim et al.[29] an anionically functionalized polythiophene (PTh)—hydrolyzed poly[2-(3thienyl)ethanol butoxy carbonyl-methyl urethane]—was electrostatically coated onto electrospun cellulose acetate fibers. This simple method of electrostatically binding the CP to the electrospun fibers worked well with the resulting materials used as optical biosensor with up to two orders of magnitude greater sensitivity compared to the film form of the same material. Electrostatic binding can also be used to attach ­molecules onto electrospun CP fiber’s surface. This was demonstrated with an example of electrospun CPs for ­biosensing.[30] In this example, PCL fibers were electrospun onto an indium tin oxide coated polyethylene ­terephthalate slide, followed by immersing the substrate into a pyrrole solution and adding an oxidant (FeCl3) solution. A single stranded DNA probe was electrostatically bound to the fibers surface through the charged PPy layer. Upon target DNA hybridization, there was an increase in the charge transfer resistance of the substrate measured by electrochemical impedance, providing a quantitative measure of the target DNA sequence. A different, yet interesting, approach to obtain electrospun CP fibers was demonstrated by Han, Shi et al.[56] Here, the authors synthesized an electrospinnable oxidant (ferric 1,4-bis(2-ethylhexyl)sulfosuccinate) which was then electrospun. The resulting electrospun fibers were then exposed to pyrrole vapors, resulting in PPy forming within the electrospun fibers. The non-polymeric oxidant acts as a dopant, resulting in the fibers to be considered as pristine doped PPy fibers. In some cases, the oxidant-based fibers were also produced with multi-walled carbon nanotubes (MWCNTs) present in the electrospinning solution. The produced material (both with and without MWCNTs) had high surface area and good electrical properties, as probed

by cyclic voltammetry, conductivity (10–15 S/cm-1), and i-V characteristics; therefore, sensing ­applications were suggested. Vapor phase polymerization of CP monomers has shown to give the resulting polymer higher conductivities than other polymerization methods.[94–96] This method is employed by Laforgue, Robitaille et al.[95] to produce highly conductive PEDOT coated PVP fibers. PVP fibers were electrospun from a solution containing iron(III) p-toluenesulfonate (FeTos). These fiber mats were then placed in a vacuum chamber containing EDOT. Under vacuum (here, 15 min active vacuum followed by 3–4 h passive vacuum), the EDOT evaporates and absorbs onto the PVP fibers where polymerization occurs due to the presence of the oxidant, FeTos. The resulting fibers were reported to have a conductance of 60 S/cm. The authors suggested the use of the resulting material in sensors or energy storage systems. We have recently demonstrated the fabrication of conducting, semi-interpenetrated network (sIPN) fiber mats.[67] These were formed by electrospinning a nitrile butadiene rubber/poly(ethylene glycol) dimethacrylate composite, embedded with PEDOT post-electrospinning. That was achieved by swelling of the rubber fibers with EDOT monomer vapors using the chemical vapor deposition method. The EDOT swollen fibers were then exposed to an oxidant solution (aqueous ferric chloride), resulting in PEDOT embedded into the fibers. A key distinction here is that the PEDOT penetrates into the rubber, forming a sIPN with the rubber, that maintained, to a good extent, the flexibility of the rubbery component of the composite. Table 2 summarizes the methods and base polymers used to incorporate CPs post-electrospinning onto underlying base polymers and reveals common applications of such fibers.

Table 2  Electrospun fibers produced by incorporating CPs post-electrospinning Conducting Base CP incorporation CP polymer polymer methodology distribution PANI

CP content

Applications

[38]

PU

Fibers soaked in polymerization solution

PANI Coating on the PU fibers

Not measured

Super-hydrophobic breathable membrane, with applications in underwater oxygen sensors

PANI [15]

PU

Fibers soaked in polymerization solution

PANI Coating on the PU fibers

Not measured

Artificial muscle applications suggested

PANI [90]

PVDF

Fibers soaked in polymerization solution

PANI Coating on the PVDF fibers

12–40 %wt

PANI [89]

PVDF

Fibers soaked in polymerization solution

PANI Coating on the PVDF fibers

Not measured

Strain sensor

(Continued)

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612

Conducting Polymers: Electrospun Materials

Table 2  Electrospun fibers produced by incorporating CPs post-electrospinning (Continued) Conducting Base CP incorporation CP polymer polymer methodology distribution CP content PPy

Applications

Composites–­ Conducting

Novel compound (FeAOt)

Vapor phase polymerization

Doped PPy

100%

Sensor-based applications suggested

PPy[30]

PCL

Fibers soaked in polymerization solution

PPy coating on the PCL fibers

36-nm thick shell around 161 nm (diameter) PCL fibers

DNA sensor

PPy[77]

PLLA

Fibers soaked in polymerization solution

PPy Coating on the PLLA fibers

250-nm thick shell around 300 nm (diameter) PLA fibers

Aligned fibers used as tissue scaffold to direct and orientate neurite growth

PPy[80]

Cellulose Acetate

Fibers soaked in polymerization solution

PPy coating on the cellulose fibers

Not measured

Aligned, conducting fibers for neural tissue engineering

PPy[91]

PLA or PCL

Fibers soaked in polymerization solution

PPy nanotubes

50-nm thick PPy coating on 270 and 250 nm (diameter) PCL and PLA nanofibers

Neural cell growth applications

PPy[97]

PLGA

Fibers soaked in polymerization solution

PPy coating on the PLGA fibers

85-nm thick PPy coating on 520 and 430 nm (diameter) PLGA nanofibers

Neural cell growth applications.

Functionalized PTh [29]

Cellulose Acetate

Electrostatic binding using charge site on functionalized PTh

PTh coating on the surface of cellulose acetate fibers

Qualitative measurements by UV–Vis absorption

Optical biosensing

PEDOT[31]

PLA

Fibers used as working electrode

PEDOT coated on the surface of PLA fibers

15-nm thick shell around 80 nm (diameter) PLA fibers

Glucose sensor

PEDOT[95]

PVP

Oxidant containing fibers exposed to EDOT vapors under reduced pressure

PEDOT after PVP dissolution

100%

Sensing or energy storage applications suggested

PEDOT[67]

NBR/ PEGDM

Fibers swollen with monomer followed by chemical oxidation

Interpenetrated into the fiber

10 %wt

Reversible pore size change upon oxidation/ reduction demonstrated; tunable filtration applications suggested

PEDOT[93]

PLA/ PHBV

Fibers immersed in PEDOT:PSS solution, with subsequent drying

PEDOT Coating on the PLA/ PHBV fibers

PEDOT:PSS presence and relative amount confirmed with EDX looking at the sulfur content

Tissue engineering

PEDOT[92]

PLA or PLGA

Fibers used as working electrode

PEDOT nanotubes

100% PEDOT

Neural interfacing with drug delivery capability

[56]

APPLICATIONS OF ELECTROSPUN CP FIBERS The combination of the unique properties of both electrospun fibers and CPs are beneficial in numerous applications. Properties of electrospun fibers such as high surface area to volume ratios, 3D polymer network, and high porosity are combined with properties such as tunable

conductivity, ability to actuate at low working voltages, and ease of chemical functionalization of the CPs. As a result, the electrospun CPs are often utilized as stimuli responsive fiber mats. Another advantage of electrospun CPs, over “bulk” CPs, is that the resulting material forms flexible freestanding films: a property quite difficult to achieve with solely CPs as they are generally brittle and

hard materials. Most commonly reported applications of electrospun fibers with CPs incorporated are in tissue engineering (with [14,58,59,77] and without drug delivery[81,92]) and sensing, [31,83,84,89] with some other noteworthy applications such as in super-­hydrophobic membranes, [38] membranes for energy storage systems [95] or tunable filters.[67] Optically active conjugated polymers such as PPV or PTh often feature in applications such as white light displays, [85] polarized light sources, [16,34] or optical biosensors.[29] Tissue Engineering and Drug Delivery One of the most typical use of electrospun polymers is in tissue engineering applications, [2,22,26,98] which utilizes biodegradable and biocompatible conventional polymers. Since CPs are generally biocompatible, [99–101] electrospun CPs are no different; however, CPs add functionality to the substrates that cannot be achieved with conventional polymers including the ability to apply electrical stimulation to cells [23,58,59,76,77,80,93] and in some cases along with mechanical stimulation through actuation, [14,15,79] or add antioxidant properties, [58] and impart antimicrobial properties.[58] In spite of the relatively low conductivities of electrospun CP blends with biocompatible support polymers, such fibers are still effective in tissue engineering applications, as cells stimulation usually requires relatively low voltages, commonly in the range of 0.1–10 V.[77,102,103] It was also shown that certain cells proliferate better if the scaffold has low to moderate conductivity rather than being insulating.[59,81,104,105] This is attributed to muscle cells communicating through the extracellular matrix electrochemically[14,106] and thus scaffolds that are used to mimic the extracellular matrix perform better if they are conductive. A good example of electrospun fibers incorporated with a CP, to add further functionality, to improve a tissue engineering scaffold is demonstrated by Gizdavic-Nikolaidis, Ray et al.[23,58] In these studies, a blend of PLA and a PANI functionalized with both ester and carboxylic acids was electrospun. It was suggested that such functionalization of PANI affords antibacterial and radical savaging properties. Furthermore, a unique honeycomb structure is obtained during electrospinning under particular electrospinning (a)

parameters (Fig. 8). It was reported that the cells proliferation was enhanced due to the high degree of interconnectivity in the honeycomb structure as compared to the conventional random fiber orientation. There are several reports on neural[77,80,91,92,97] and cardiac tissue engineering[14,59] applications of electrospun CP fibers, both relying on the electrospun CP fibers to mimic the extra cellular matrix and provide electrical stimulus to the cells to improve cell growth and vitality. Of these, Hsiao, Bai et al.’s [14] work is particularly noteworthy as it utilizes all the interesting features of both electrospun polymers and CPs. This in-depth study involves cardiac tissue engineering where aligned fibers were used to produce aligned cardiac muscle cells. The conductivity, actuation, and anisotropy of the cell scaffold were used to induce synchronized cell beating, as illustrates in Fig. 9. In more recent research by Zhou, Cheng et al.[79] aligned PLA fibers with PPy nanoparticles embedded have been used to direct stem cell differentiation into neural cells. This work attributed the success of stem cell differentiation to a synergy between the anisotropic affect caused by the fiber alignment and by an applied electrical stimulation. Another interesting aspect of this work is the study on the effect of the alignment of the fibers on the mechanical properties of the mat. It was revealed that fibers with and without alignment show different mechanical properties, such as Young’s modulus, maximum tensile strength, and maximum elongation at break. The aligned fibers demonstrated better properties in the radial direction than in the axial. Interestingly, this anisotropy was not observed with the conductance and this was attributed to the high concentration of PPy nanoparticles exceeding a percolation threshold (Fig. 5). As CPs actuate by changing their volume upon oxidation/­reduction, [45,107] this property of CPs is utilized in drug delivery.[108,109] This is achieved if the drug can act as a dopant for the CP or is highly soluble in the solvent that the CP absorbs during actuation. In the case of electrospun CPs for drug delivery, the drug delivery feature is usually combined with tissue engineering applications. Examples of this are reports by Abidian, Kim et al.[92] and Shokry, Vanamo et al.[81] In the work by Abidian, Kim et al.[92] CP (b)

Fig. 8  Honeycomb structure obtained in the electrospinning system containing PLA and functionalized PANI Source: Reproduced with permission from Gizdavic-Nikolaidis et al.[23]

Composites– Conducting

Conducting Polymers: Electrospun Materials 613

614

Conducting Polymers: Electrospun Materials

Chemical structures H N N H

HO

N N

Polyaniline (PANI) O

Electrospinning

CH3

n

x

O

O y

Poly(lactic-co-glycolic acid) (PLGA)

PANI/PLGA nanofibrous mesh Glass well Cell

HCl

H

O

Cell seeding and electrical stimulation

PANI/PLGA in HFIP

Doping Before doping

Seeding After doping

Rotating mandrel

Glass slide Wire

Isolated Cardiomyocytes

Synchronous cell beating mechanisms

Intra-cluster beating: mediated by gap junctions (Cx 43)

Cluster beating frequency

Electrical stimulation

Cluster beating frequency Inter-cluster beating: mediated by electrical stimulation Electrical stimulation

Unsynchronized beating

Synchronized beating

Electrical potential frequency Isolated cell Gap junction cluster (Cx 43)

Fig. 9  Illustration showing the synthesis, development, and working principle behind aligned PANI/PLGA electrospun conducting fibers for use in cardiac tissue engineering[14]

Composites–­ Conducting

nanotubes were fabricated to enhance the amount of drug loaded into the fibers (Fig. 10). The 3D network also provided a scaffold to improve the interactions with the neural cells. Sensors Versatile chemical[60,62,110–116] and biochemical sensors [44,53,54,117–121] have been demonstrated using CPs. This is largely due to their ease of chemical functionalization and the fact that their conductance/resistance can be used directly as a method for signal transduction and readout. The latter point makes them particularly attractive in labelfree sensor and biosensor formats.[54,117–119,122] Increasing the surface area to volume ratio of a sensor can increase its sensitivity and detection limit; thus, CP microfibers/­ nanofibers are ideally suited to fabricate such sensors. Electrospinning is a popular fabrication route to such materials due to its simplicity in creating high surface area 3D structures that form excellent platforms for sensors. As a way of examples, a glucose sensor based on an electrospun CP with excellent performance was developed by Yang, Kampstra et al.[31] Here, PLA was electrospun onto platinum microelectrodes, after which PEDOT was electropolymerized galvanostatically from a solution containing glucose oxidase (Fig. 11). This resulted in glucose oxidase being entrapped within the nanofibers covered with PEDOT. A bare PEDOT control was also synthesized, and it was found that the nanofiber sensor performed

significantly better. This was attributed to the high surface area of the electrospun sensor, causing better entrapment of glucose oxidase and lower impedance. The improvement of these two properties resulted in higher sensitivity with a lower detection limit. Similar work was conducted by Aussawasathien, Dong et al.[63] where PANI doped with HCSA was blended with polystyrene and electrospun to form a H2O2 sensors and a glucose sensor. For the glucose sensor, glucose oxidase was immobilized via electrodeposition. In both cases, the sensing results were compared to bulk films and it was found that the electrospun, high surface area sensors had a superior performance and higher sensitivity. Optically active CPs also make great materials for optical sensors. In the work by Wang, Kim et al.[29] optical sensors for cytochrome c and methyl viologen were fabricated from electrospun cellulose acetate with an anionically functionalized PTh electrostatically bound to the cellulose fibers. Sensing was achieved by fluorescence quenching of the PTh by cytochrome c and methyl viologen. The sensitivity was reported to be 1 to 2 magnitudes of order larger than that of the PTh films (prepared using an electrostatic layer-by-layer complexation technique). The improved sensing response was directly attributed to the increase in the surface area. Strain sensing applications of electrospun CP fiber mats are also attractive as such materials present CPs in a suitable format for effective strain sensing. In most of the examples, the conductivity of the fiber mats decreases

Conducting Polymers: Electrospun Materials 615

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(c)

(d)

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Fig. 10  Scanning electron micrographs of PLGA nanoscale fibers and PEDOT nanotubes. (a) PLGA fibers. (b) Electropolymerized PEDOT nanotubes on the electrode site of an acute neural probe tip after removing the PLGA core fibers. (c) A section of (b). (d) Higher-magnification image of (c). (e) A single PEDOT nanotube. (f) Higher-magnification image of a single PEDOT nanotube Source: Reproduced with permission from Abidian et al.[92]

as the material is stretched due to the breaking of the CP coating, [38,89] Fig. 12. In the work by Yu, Yan et al.[89] a patterned grid (Fig. 13) was used to form coiled fibers that showed a superior strain sensing capability. In this work, a strain sensor is developed, which can repeatedly sense strains up to 22.4%, with a maximum strain of 110%. The large maximum strain before break was attributed to the  coiled morphology of the fibers, allowing a larger ­sensing window. For sensing applications, higher proportions of CP in the composite fibers are generally desirable, [57] driving efforts in producing fibers with a high proportion of CPs. As an example, pristine electrospun PEDOT:PSS fibers were used as a gas sensor.[57] In that work, PEDOT:PSS electrospun fibers were used to sense gases such as DMF, DMSO, and PC. The mechanism behind the fibers sensing

capabilities relies on the fiber swelling with the solvent, defined by the Hildebrand solubility parameter, with a greater similarity in the solubility parameters resulting in greater sensitivity. As expected, nanofiber mats performed better than films, the increased porosity improves the sensing performance due to a quicker uptake of the gas and higher surface area. Also, during electrospinning, PEDOT migrates to the surface of the fibers. Since PEDOT is the active component of the sensor, the greater the proportion of PEDOT exposed, the better the performance of the ­sensor is observed. Specialized Fabrics, Membranes, or Coatings An advantage of highly porous materials such as electrospun fibers is they are comparatively lightweight compared

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PEDOT GOx

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(h)

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Fig. 11  Electrospun PLA fibers on a Pt microelectrode, covered with PEDOT and used for a glucose sensor Source: Reproduced with permission from Yang et al.[31]

to the bulk form. This is useful for application such as batteries, coatings, and membranes as it reduces the amount of material required to produce an effective device. One of the more distinctive applications of electrospun CPs is demonstrated by Cho, Nam et al. [38] where they produced a super-hydrophobic membrane that enabled an oxygen sensor to operate underwater. In this case, PANI coated PU fibers were further coated with PTFE to form a super-hydrophobic, stretchable, breathable fabric. The role of the PANI was to induce a nanostructured surface on the fibers, referred to as “nanohairs.” This provided extra porosity and thus enhanced the overall hydrophobicity of the PTFE, as illustrated in Fig. 14. The waterproofing was hence achieved by allowing oxygen bubbles to diffuse across the membrane while being water impermeable.

Materials with high porosity, high capacitance, and at least moderate conductance make excellent materials for cathodes in batteries or electrochemical cells. [123,124] An example is reported by Ju, Park et al. [87] In this work, three lithium ion electrochemical cells were prepared from three CP membranes (utilized as the respective cathodes); a membrane prepared though a casting method with PPy, an electrospun membrane from SEBS with PPy embedded and, an electrospun membrane from a sulfonated SEBS with PPy embedded. Both electrospun membranes showed better performance than the casted membranes and this was attributed to the higher surface area resulting in lower interfacial resistance. The sulfonated SEBS membrane showed significantly better properties to that of the non-sulfonated membrane, attributed to the higher degree of doping (from the

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Fig. 12  (a) SEM image of randomly oriented nanostructured PU fibers (as-prepared). Scale bar: 10 μm. (b) PU fibers shown in (a), horizontally stretched by 200% (in the direction of the yellow arrows). Scale bar: 10 μm. (c) Magnified image of (b). The PANI structures on the stretched fibers were slightly cracked. Scale bar: 1 μm. (d) 0% stretched fibers oriented in one direction, produced by a parallel collector. Scale bar: 1 μm. (e) 100% stretched image of (d). Scale bar: 1 μm. (f,g) Interval spacing and crack width, I ∗ and W ∗, ­respectively, as a function of strain Source: Reproduced with permission from Cho et al.[38]

sulfonic acid groups), a better intercalation of the Li ion, and improved charge transfer as a consequence of higher conductivity. CPs such as PPV and poly(phenylene ethynylene) are photoluminescent and electroluminescent.[125–127] These type of materials have been shown to be suitable in organic LEDs [128,129] and solar cells.[42] When these types of CPs are electrospun, an interesting phenomena may occur: the ­electrospinning can induce molecular alignment of the polymer chains that results in polarization of the light emitted by the polymers.[16] It has been further observed that the fluorescence of electrospun pristine MEH-PPV with such morphology is more intense when excited by polarized light. In a similar work by Yin Zhang et al.[34] polarized light was observed from electrospun ­MEH-PPV/PEO and P3HT/PEO blends. Such materials have been suggested for optoelectronic applications such

as one dimensional organic lasers, polarized light sources, and nanoscale sensors. Due to the nano-sized morphological features induced by electrospinning, a unique phase separation may occur. In the study reported by Chen, Wang et al., [85] fluorescence efficiency was improved when electrospun fibers made from blends of PPV/PFO/PMMA were compared to the casted films. This was attributed to the smaller domain sizes of the fluorescent polymers inside the electrospun fibers. These fibers blends were used to fabricate light emitting membranes, whose color could be adjusted, including white light emission, by altering the PPV:PFO ratio. Nanowires While electrospinning is typically used to form nonwoven, high surface area, freestranding mats, it is possible to form

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(f ) 600

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Fig. 13  Patterned collector (a), optical microscope image of electrospun fibers removed from the collector (b), electrospun PVDF fibers (c), and PANI/PVDF electrospun fibers (d)[89]

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single nanowires. Two examples of this are provided by Chang, Liu et al.[66] and González, Pinto et al.[55] In both of these cases, P3HT was electrospun and single nanowires were used for electronic applications. In the work by González, Pinto et al., [55] pristine P3HT nanofibers were prepared by intercepting the electrospinning jet during its flight. These fibers were used as nano-sized field effect transistor. However, the authors suggested that further optimization is possible if prepared in an inert atmosphere and reducing the fibers dimensions. In the work by Chang, Liu at al.[66] gold nanoparticle/P3HT composites fibers were produced. Single nanofibers were isolated manually, and used for flexible nonvolatile flash transistor memory devices. The functionalized gold nanoparticles maintained a charge during the transistor gate pulses, which resulted in a memory affect in the transistors. The devices were reported to be stable with negligible differences between device batches. SUMMARY AND OUTLOOK Applications of electrospun CP fibers utilize the best properties of electrospun fibers and CPs. This combination affords high surface area, low density, responsive, and versatile conductors. Such materials find applications typically in advanced tissue engineering, high sensitivity sensors, or energy storage devices. However, due to

CPs versatile, easily functionalized nature the potential ­applications are much broader. This review summarizes the current fabrication techniques and applications of electrospun CPs. Based on the  reports discussed herein, PEDOT and PANI are the most commonly employed CPs to electrospin, with tissue engineering and sensing as the most common applications. However, as new functionalization chemistry of CPs is developed,[75,130–133] CPs are becoming more processable, achieving higher conductivities, and obtaining improved mechanical properties. Overcoming such limitations of traditional CPs enables more widespread use of CPs in electrospinning. With more possibilities, such as removing the need for a support or carrier polymer, will in turn, improve the electrochemical capabilities of the fiber mats. With such barriers removed, it is expected that many more applications will come to light. A main attraction of electrospinning is its ease of setup and cost effectiveness. Thus, future directions will most likely involve large-scale production of pristine electrospun CPs, enabling these materials to surpass the research stage and enter commercial applications. ACKNOWLEDGMENTS Thomas Kerr-Phillips would like to thank the MacDiarmid Institute for Advanced Materials and Nanotechnology for his PhD scholarship.

Conducting Polymers: Electrospun Materials 619

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Fig. 14  (a) SEM image of a PANI-nanostructured PU fibrous membrane. Scale bar: 1 μm. (b) Schematic diagram of the membrane consisting of PU fibers, PANI nanostructures (≈40 nm diameter), and a PTFE layer. (c) Photograph showing the resistance to a water jet. (Inset: after jetting). (d) Optical images of the super-hydrophobic membrane under various strain. Scale bar: 1 cm. (e and f) Gas-breathability test. (g) The membrane can conform to a cylindrical geometry (white dotted line) while maintaining its ­anti-­wettability and gas breathability Source: Reproduced with permission from Cho et al.[38]

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Conducting Polymers: Electrospun Materials 623

Construction: Rigid Bio-based Polyurethane Foams for Sandwich Panels Rafael de Avila Delucis and Sandro Campos Amicos Post-Graduation Program in Mining, Metallurgical and Materials Engineering, Federal University of Rio Grande do Sul, Porto Alegre, Brazil

Washington Luiz Esteves Magalhães Embrapa Florestas, Colombo, Brazil

Cesar Liberato Petzhold Institute of Chemistry, Federal University of Rio Grande do Sul, Porto Alegre, Brazil

Abstract Polyurethane (PU) foams are dominant materials in several markets due to their characteristics such as low cost, wide range of densities, low fragility, easy adhesion to coatings, thermal and acoustic insulation, and high versatility. Due to environmental concerns, bio-based polyester polyols have been used to partly replace traditional hydroxylated polyether polyols from petroleum. Biobased PU properties may be modified by chemically treating the polyol, or with the aid of chemical additives or synthetic and vegetable-based fillers, e.g., wood flour and its derivatives, which affect their rheological, morphological, thermal, and mechanical characteristics, even though this effect still lacks comprehensive studies, especially about filled foams. This review also encompasses the use of neat and filled PU foams as cores in structural insulation panels, including aspects such as ­processing, properties, advantages, and challenges. Indeed, although metallic profiles and c­ omposite plates are most used as face sheets of sandwich panels due to their high performance, solid wood and wood-based panels can be interesting alternatives to widen the range of applications of bio-based PU cores, retaining their sustainable character. Other applications are also discussed in this entry.

Keywords: Bio-based foams; Insulation material; Lightweight panels; Polyether polyols; Sandwich structures; Wood-based composites.

INTRODUCTION Construction– Cosmetics

Polyurethane (PU) is a synthetic polymer invented by a German chemist called Otto Bayer in 1937. When used as a foam, PU synthesis occurs when polyisocyanate (–N=C=O) reacts with polyalcohol (–OH), which forms urethane linkages (Fig. 1a), with a heat involved of about 100–110 kJ mol−1. Meanwhile, the NCO groups react with the available water (blowing agent), which forms amine and carbon dioxide (Fig. 1b) and liberates a heat of about 196 kJ mol−1. Then, the synthetized amine can react with the isocyanate, generating urea groups (Fig. 1c). Simultaneously, the carbon dioxide is encapsulated and then acts as a blowing gas, leading to the formation of cells.[1,2] There are several theories about the development of PU foams; therefore, its reaction formation is usually described as follows: spherical gas bubbles are formed in the liquid polymer, and when they touch each other, the bubbles deform, gradually composing a denser ­system. Thereby, thin walls (edges) are formed between the b­ ubbles,

624

e­ ncapsulating each bubble in their contact regions, forming a cellular material. Then, edges and faces collapse, leading to an open cell formation, with dimensions governed by the cross-linking degree.[1,3] Currently, PU foams represent about a third of the whole PU market and are among the main insulation materials used in coatings for refrigerators and freezers, hot water tanks, pipelines, heating pipes, and refrigerated warehouses in both industrial and domestic sectors. Automobile industry is another important application, wherein they constitute automobile seats or thermal coatings in refrigerated roads and rail transports.[4] PU foams are dominant in these markets mainly due to their low cost and low thermal and acoustic conductivities. Other advantages include wide range of densities, low fragility, and high versatility in the production of complex shapes. Moreover, expanded PU may be easily combined with other materials due to their easy adhesion to several surfaces, extending its applicability. According to  their cross-linking degree, PU foams can be classified into Encyclopedia of Polymer Applications, First Edition DOI: 10.1201/9781351019422-140000307 Copyright © 2018 by Taylor & Francis. All rights reserved.

Construction: Rigid Bio-based Polyurethane Foams for Sandwich Panels 625

(a)

(b)

(c)

R΄

R N C O

OH

Alcohol

N

R΄ N C O

Water

Isocyanate

NH2

R

Amine

C(=O)

O

R΄

Urethane

Isocyanate

H2O

R΄

R

R΄

CO2

NH2

Carbon

Amine

N C O

R

NH

dioxide C(=O)

Isocyanate

NH

R΄

Urea

flexible, semirigid, and rigid, each of them more suitable for certain applications, as shown in Table 1. Currently, rigid PU foams (RPUFs) are popular ­insulation materials due to both their lower cost and lower thermal conductivity than other cellular materials, including expanded polystyrene (EPS) and extruded polystyrene, porous concrete, cork tile, and mineral wool. RPUFs with densities of at least 40 kg m−3 present suitable properties (see Table 2) to be used as the core for structural insulation panels (SIPs), and they will be later discussed in details. In SIPs, core materials comprise most of the panel ­volume and have to complement the properties of face sheets. The core is normally responsible for ­withstanding compressive, shear, and impact loads. Besides, low thermal and acoustic conductivities, low flammability, and ­hydrophobic character are appreciated features for these parts. Because of that, core materials are known as insulating materials. Nevertheless, it is hard for a single material to meet all these requirements.

Table 1  Typical applications of PU foams [5] Types of foam Markets Applications Flexible

Transportation

Seating, pads, liners, dampening, carpet backing, filters, flooring, armrests, trim

Furniture

Bedding, padding, flooring

Recreation

Sport mats, toys, helmet liner, chest protection

Packaging Semirigid Rigid

Electronic, computer, china, equipment

Automotive

Dash panel, liner

Footwear

Soles

Construction

Insulation, flooring, siding

Appliance

Refrigerator frame, dishwasher door

Regarding the current scientific scenario of the PU production, polyester polyols have been developed ­following environmental concerns related to the use of petroleum-based chemicals and materials, i.e., l­egislative requirements for environmental protection guide and ­economical problems due to unstable oil prices. Bio-based oils used as raw materials for biopolyol production have low cost, high availability, and sustainable nature, related to essentially zero carbon dioxide emissions. Even though there are controversies related to the use of food crops to produce raw materials for polymers. Moreover, products from the biodiesel industry are also strong rivals for ­particular applications. Seeds rich in triglycerides are the main plant sources for bio-polyol production. Among these vegetable ­matrices, castor oil stands out, since it is a naturally hydroxylated feedstock with about 160 mg of KOH g−1.[6,7] Castor oil (­Ricinus communis) is a light liquid pigmented in a pale-yellow shade and has a molecular weight of about 932 g mol−1.[8] Among their potential applications, polyols are designed to produce flexible Pus, such as foams and elastomers. In fact, bio-based PU foams with interesting ­mechanical and thermal performances have been reported in the literature, especially those filled with natural fibers or ­particles.[9,10] Based on the current studies, the main ­challenge is to make them resemble the mechanical and thermal behavior of oil-based foams. Moreover, it is preferable to achieve

Table 2  Typical properties of rigid PU foams [2] Property

Value

Density (kg m )

40–50

Young’s modulus (MPa)

124–152

−3

Compressive strength (kPa)

180–220

Thermal conductivity (W m  K )

0.0233

Closed-cell content (%)

>90

Water uptake (% volume)

0.2

Dimensional stability to −20°C (%)

20%) of water within their structure, without dissolving in water.[52–54] Special hydrogels as superabsorbent polymers (SAPs) are widely found in hygienic products, particularly disposable diapers and female napkins, where they can capture secreted fluids, e.g., urine, blood, etc. Agricultural grade of such hydrogels are used as granules for holding soil moisture in arid areas. Hydrogels can be classified into two groups based on the origin of polymers: natural and synthetic. Examples of natural polymers are proteins such as collagen and polysaccharides such as chitosan, dextran, and alginate. Hydrogels of natural origin support cellular activities and are biocompatible and biodegradable. On the other hand, they may contain biological pathogens or evoke an immune response. Two other disadvantages are low mechanical strength and batch variation. Synthetic polymers are made from monomers such as vinyl acetate (VAc), acrylamide (AM), ethylene glycol, and lactic acid. Not all monomers for synthetic polymers are derived from petroleum, e.g., lactic acid is made from plants such as corn and sugarcane. Synthesis of polymers can be precisely controlled and tailored to give a wide range of properties. They have a low risk of biological pathogens and evoke immune response. Disadvantages are low biodegradability and absence of inherent bioactive properties. Also, toxic substances may be present.[55] SAPs may be categorized into four groups based on the presence of ionic charges:[50,55]

3. Cationic, e.g., chitosan 4. Ampholytic, e.g., collagen

1. Neutral, e.g., dextran 2. Anionic, e.g., carrageenan

Table 1 shows the works performed by different authors for the synthesis of SAP by solution polymerization.

Mechanism Figure 2 shows the working mechanism of SAP Important Starting Materials A variety of monomers, mostly acrylics, are employed to prepare SAPs. Acrylic acid (AA) and its sodium or potassium salts and AM are most often used in the industrial production of SAPs. However, on a laboratory scale, a number of monomers such as methacrylic acid, methacrylamide, acrylonitrile, 2-hydroxy ethyl methacrylate, 2-acrylamido-­ 2-methyl propane sulfonic acid, ­N-vinylpyrrolidone, vinyl sulfonic acid, and VAc are also used. The bifunctional compound N,N′-methylene bisacrylamide (MBA) is most often used as a water-soluble cross-linking agent. Ethylene glycol dimethacrylate, 1,1,1-trimethylolpropane triacrylate, and tetraalyloxy ethane are known examples of two-, three- and four-­functional cross-linkers, respectively. Potassium persulfate (KPS) and ammonium persulfate (APS) are water-soluble thermal initiators used frequently in both solution and inverse suspension polymerization. SYNTHESIS OF POLYMERIC DESICCANTS Solution Polymerization

Construction– Cosmetics Dry SAP particle

Chains collapsed in the particle

H2O H2O H2O H2O

Aqueous medium

Fig. 2  Working mechanism of SAP

H2O H 2O

H2O

H2O

H 2O

H 2O

H2O H2O

H2O

Chains collapsed in dry state

H2O H2O

H2O

H2O

H 2O H2O

H 2O

H2O H 2O

Chains expanded in water

Corrosion Inhibitor: Polymeric Desiccant 645

Table 1  Works done by different authors for SAP synthesis by solution polymerization technique S.no. Works done

References

1

Free radical-initiated polymerization of AA and its salts (and AM) with a cross-linker is frequently used for SAP preparation. The carboxylic acid groups of the product are partially neutralized before or after the polymerization step. Initiation is most often carried out chemically with free radical azo or peroxide thermal dissociative species or by reaction of a reducing agent with an oxidizing agent (redox system). In addition, radiation is sometimes used for initiating the polymerization

[50,54,56]

2

The solution polymerization of AA and its salts with a water-soluble cross-linker, e.g., MBA in an aqueous solution is a straightforward process. The reactants are dissolved in water at desired concentrations, usually about 10%–70%. A fast exothermic reaction yields a gel-like elastic product, which is dried and the macro-porous mass is pulverized and sieved to obtain the required particle size. This preparative method usually suffers from the necessity to handle a rubbery/solid reaction product, lack of a sufficient reaction control, non-exact particle size distribution, and increasing the sol content mainly due to undesired effects of hydrolytic and thermal cleavage. However, for a general production of a SAP with acceptable swelling properties, the less expensive and faster technique, i.e., solution method may often be preferred by the manufacturers

[57–60]

3

Hydrogels were prepared via graft copolymerization of mixtures of AA and AM onto gelatin backbones by a free radical polymerization technique using APS as an initiator and MBA as a cross-linker

[61]

4

SAPs were prepared via graft polymerization of AM onto chitosan backbone in the presence of gelatin, CTS-g-PAAm/Ge, using potassium persulfate and MBA as an initiator and a cross-linker, respectively. These hydrogels were also partially hydrolyzed to achieve SAPs with ampholytic properties and uppermost swelling capacity

[51]

5

The preparation and characteristics of SAPs from AA/acrylate, N,N′-methylene-bis-acrylamide as a crosslinker and KPS as an initiator via free radical polymerization technique was studied. Swelling studies of the candidate polymer in tap water and different pH were also done

[62]

6

SAP was prepared using copolymers comprising AA and AM using solution polymerization. The copolymer formed which absorbed about 900 g water/g dry copolymer was used to study the influence of sodium chloride on the absorption capacity at 24°C

[63]

Table 2 presents the work performed by different authors for the synthesis of SAP by inverse suspension polymerization. Gamma Radiation Technique This technique seems to be an excellent method for the preparation of hydrogels because a polymer in an aqueous solution, with a monomer dissolved in it, undergoes cross-linking and graft copolymerization on irradiation to yield a hydrogel. A simple procedure control, no initiators, cross-linkers, no waste, and relatively low operating cost make an irradiation technique a suitable choice for the synthesis of hydrogels.[75] Some of the advantages of ­radiation-initiated polymerization are as follows: i. Absence of foreign matter, such as initiator and catalyst ii. Polymerization at low temperature or in solid state iii. Rate of the initiation step can easily be controlled by varying the dose rate iv. The initiating radicals can be produced uniformly by gamma irradiation [76] Table 3 presents the works performed by different authors for the synthesis of SAP by gamma radiation technique.

SYSTEMS WHERE VCI AND DESICCANTS ARE USED TOGETHER IN INDUSTRIES The scientists at the Institute of Physical Chemistry in the USSR Academy of Sciences used primary, secondary, and tertiary amines as VCIs along with a solid porous carrier, i.e., zeolites or silica gel.[84,85] The function of the zeolite or silica gel was to absorb a maximum amount of moisture present in the surrounding, which further helped in decreasing the corrosion rate of the material. In Ref. [86], a patent was submitted in the United States, in which aliphatic ester of an amino acid was used along with silica gel for corrosion protection. Cortec Corporation submitted several patents, [87–89] in which the VCI was applied on a desiccant material such as silica gel which had a large water absorption property. A mixture of VCIs (­Dicyclohexyl amine nitrite cyclohexylamine, diisopropylammonium nitrite, BTA, tolyltriazole) with a water-absorbing polymer, e.g., polyacrylate, was claimed as a formulation for metal protection.[90] A VCI-related component that is new in patent literature was introduced in formulations by Metpro Technical Services Ltd. It is an ester of phosphoric acid. In combination with amines, organic acids (caprylic or nonylic), and silica gel, it provides an efficient protection of metals in the gas phase. In Ref. [91], a combination of desiccants and VCIs was used. The desiccants used

Construction– Cosmetics

Inverse-Suspension Polymerization

646

Corrosion Inhibitor: Polymeric Desiccant

Table 2  Works done by different authors for SAP synthesis by inverse suspension polymerization technique S. no. Works done

References

1

Dispersion polymerization is an advantageous method since the products are obtained as powder or microspheres (beads) and thus grinding is not required. Since water-in-oil process is chosen instead of the more common oil-in-water the polymerization is referred to as “inverse-suspension.” In this technique, the monomers and initiator are dispersed in the hydrocarbon phase as a homogenous mixture. The viscosity of the monomer solution, agitation speed, rotor design, and dispersant type mainly govern the resin particle size and shape 2–6. Some detailed discussions on heterophase polymerizations have already been published

[64,65]

2

The dispersion is thermodynamically unstable and requires both continuous agitation and addition of a low hydrophilic-lipophilic-balance suspending agent. The inverse-suspension is a highly flexible and versatile technique to produce SAPs with high swelling ability and fast absorption kinetics

[66]

3

A water-soluble initiator shows a better efficiency than the oil-soluble type. When the initiator dissolves in the dispersed (aqueous) phase, each particle contains all the reactive species and therefore behaves like an isolated micro-batch polymerization reactor. The resulting microspherical particles are easily removed by filtration or centrifugation from the continuous organic phase. Upon drying, these particles or beads will directly provide a free flowing powder. In addition to the unique flowing properties of these beads, the inverse-suspension process displays additional advantages compared to the solution method

[67]

4

Hydrogels have been prepared by inverse suspension polymerization, using SPAN 80 as a dispersant, heptanes as the organic phase, MBA as a cross-linker, and KPS as an initiator

[68]

5

Hydrogels was prepared by inverse suspension polymerization, using SPAN 80 as a dispersant, heptanes as the organic phase, MBA as a cross-linker, and KPS as an initiator

[69,71]

6

A novel copolymer was prepared using AM and AA as monomer by inverse suspension polymerization using APS as an initiator, MBA as a cross-linking agent and OP-10, and SDS as a complex surfactant

[70,72]

7

AA/Kaolin powder superabsorbent composite was synthesized by inverse suspension method, and the influence of dispersant agent on configuration of the product was investigated

[71,73]

Table 3  Works done by different authors for SAP synthesis by gamma radiation technique S. no. Works done

References

Construction– Cosmetics

1

The synthesis and swelling behavior of an environmental-sensitive superabsorbent hydrogel (SAH) via graft copolymerization of poly-acrylonitrile onto a homogeneous solution of starch and kappa-carrageenan using gamma rays is reported. The use of gamma rays as a clean energy source, an initiator, and a cross-linker, instead of toxic reagents such as APS and MBA, and shorter routes for the synthesis optimization are the advantages of the presented work

[74]

2

Polymeric hydrogels composed of chitosan and AA by using gamma irradiation as a source of initiation and cross-linking and investigate the swelling properties at different conditions. In addition, the suitability of chitosan/acrylic hydrogel as a carrier material for the controlled release of drug chlortetracycline HCl was investigated

[59]

3

SAP was synthesized by radiation-induced grafting of AM onto carboxymethyl cellulose (CMC) in the presence of a cross-linking agent, MBA

[75]

4

The authors synthesized poly(acrylamide-co-acrylic acid)–NaAlg SAHs via cross-linking gamma radiation, and the effect of irradiation dose (20–40 kGy) and NaAlg concentration (0.1%–0.7%) on the swelling of hydrogels was studied

[76]

5

A SAH was synthesized from partially neutralized AA by using different degrees of neutralization using gamma radiation at a dose rate of 5 kGy/h at room temperature

[77]

6

A SAH based on natural polymer chitosan and AA using a 60 Co gamma radiation as a source of initiation and cross-linking was synthesized

[59]

7

Synthesized a copolymer of poly(acrylamide-co-acrylic acid), (poly(AAm-co-AAc))-based SAHs was prepared by gamma radiation at a dose rate of 15 kGy, and the amount of monomer was varied in between 30% and 70%

[78,81]

8

A hydrogel was prepared using a co-monomer such as AA and collagen using an irradiation dose of 25 kGy in aqueous solutions of 2-acrylamido-2-methylpropanesulfonic acid

[79,82]

9

The superabsorbent, self-cross-linking sodium polyacrylate was synthesized by means of reversed-phase suspension polymerization under gamma-ray irradiation, and the water-absorbing mechanism of the selfcross-linking hydrogel was discussed

[80,83]

Corrosion Inhibitor: Polymeric Desiccant 647

CONCLUSION Corrosion plays a significant role in our life. A huge amount of money is spent to control and prevent corrosion. VCIs are one of the most effective compounds utilized to prevent corrosion and avert the consequences such as plant shutdown, machine failure, loss of efficiency, and time consumption due to failure in keeping corrosion in check. The VCI molecules render the metal surface hydrophobic, and thus prevent the reaction of metal with the environment. Referring to the literature available a large number of VCIs are currently available, which can prevent the corrosion of ferrous and nonferrous materials. Desiccant materials when used alone can help in reducing corrosion to some extent by absorbing the moisture in the surrounding, which in turn reduces corrosion. But if desiccants and VCIs are used together, corrosion rate can be reduced drastically. It is one of the most effective and cost-effective method that can be used in industries especially in packaging industries. To protect complicated machinery during shipment, storage, and use, VCIs still offer an excellent solution ­without impairing the functional properties of the machinery used. ACKNOWLEDGMENTS The completion of this study could not have been possible without the expertise of Dr. P.A. Mahanwar, my respected guide for the research project. I thank my seniors Mr. Nikesh Samarth and Mr. Vinayak Kamble for their guidance and words of advice. I also thank my other seniors and fellow classmates for their constant encouragement and motivation.

Last but not least, I thank my parents, Mr. Subodh Kulkarni and Mrs. Susmita Kulkarni, without whom none of this would be possible.

REFERENCES 1. Pereira, E.A.; Tavares, M.F.M. Determination of volatile corrosion inhibitors by capillary electrophoresis. J. ­Chromatogr. A 2004, 1051 (1–2), 303–308. doi:10.1016/j. chroma.2004.08.044. 2. Shreir, L.L. 1.05- Basic concepts of corrosion. Shreir’s Corros. 2010, 1, 89–100. doi:10.1016/B978-044452787-5.00006-8. 3. Carneiro, J.O.; Teixeira, V.; Azevedo, S.; Maltez-da costa, M. Smart self-cleaning coatings for corrosion protection. In Handbook of Smart Coatings for Materials Protection, 2014, 489–509. doi:10.1533/9780857096883. 3.489. 4. Franke, G.; Heinzelmann, U. Internal cathodic protection of water tanks and boilers. In Handbook of Cathodic Protection, 1997, 441–463, doi:10.1016/B978-088415056-5/50027-7. 5. Fitzgerald III, J.H.F. Engineering of cathodic protection systems. In Uhlig’ Corrosion Handbook; Winston Revie, R.;Ed.; John Wiley & Sons, Inc.: Hoboken, NJ, 2011, 1001–1011. 6. Norsok: Design Principles. Materials Selection, Norsok, December 1994. 7. Dariva, C.G.; Galio, A.F. Corrosion inhibitors—Principles, mechanisms and applications. In Developments in Corrosion Protection, 2014, 366–379. doi:10.5772/57255. 8. Boerwinkle, F.P.; Kubik, D.J. Corrosion Inhibitors, 10.1 and 10.2, n.d. 9. Skinner, W. A new method for quantitative evaluation of volatile corrosion inhibitors. Corros. Sci. 1993, 35 (5–8), 1491–1494. doi:10.1016/0010–938X(93)90376–R. 10. Wachter, A.; Nathan Stillman, B.C. Vapor-Phase Corrosion Inhibitor. 2,629,649 United States, Issued 1953, 2–3. 11. Zhang, D.; An, Z.; Pan, Q.; Gao, L.; Zhou, G. Volatile corrosion inhibitor film formation on carbon steel surface and its inhibition effect on the atmospheric corrosion of carbon steel. Appl. Surf. Sci. 2006, 253, 1343–1348. doi:10.1016/j. apsusc.2006.02.005. 12. Sastri, V.S. Adsorption in corrosion inhibition. In Green Corrosion Inhibitors: Theory and Practice, 2011, 103–138. doi:10.1002/9781118015438. 13. Andreev, N.N.; Kuznetsov, Y.I. Volatile inhibitors of atmospheric corrosion. III. Principles and methods of efficiency estimation. Int. J. Corros. Scale Inhib. 2013, 2 (1), 39–52. doi:10.17675/2305-6894-2013-2–1-039-052. 14. Nontete, N.S. TGA-FTIR Study of the Vapours Released by Volatile Corrosion Inhibitor Model Systems, 2013. 15. Balezin, I.N.P.A.V.P.B. Journal of Electrochemical Society, October 1961, 2014. 16. Wachter, A.; Stillman, N.; Skei, T. Dicyclohexylammonium nitrite, a volatile inhibitor for corrosion preventive packaging. Corros. Nat. Assoc. Corros. Eng. 1951, 7 (9), 284–294. 17. Ngobeni, P.; Vander, G.H.; Vuorinen, E.; Klashorst, G.H.; Skinner, W.; Ernst, W.S. Derivatives of cyclohexylamine and morpholine as volatile corrosion inhibitors. Br. Corros. J. 1994, 29 (2), 120–121.

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were mostly alkali metal poly acrylate and an alkali metal partial salt of cross-linked poly propenoic acid. The VCIs used in this case were benzotriazole and tolyltriazole. In Ref.  [92], the authors tried several methods to prevent ­corrosion, of which in one of the cases they used a VCI and a desiccant material. In Ref. [93], an enclosed chamber comprising desiccants in the form of a capsule and a VCI was used. The desiccants used were silica gel, calcium oxide, sodium bicarbonate, and zinc borate. In Ref. [94], three samples were prepared and exposed to VCI consisted in a barrier film laminate, with desiccant and oxygen absorber. ­Maximum corrosion protection was obtained by a system containing desiccants followed by a barrier film laminate, and it was found that least corrosion protection was achieved by the oxygen absorber. In Ref. [95], a mixture of primary, secondary, tertiary amines, and their derivatives were used as VCIs along with zeolite and ­silica gel as desiccants, and thus, an effective reduction in ­corrosion was obtained.

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18. Henriksen, J.F. The distribution of a volatile corrosion inhibitor (CHC) on corroded Fe. Corros. Sci. 1970, 12, 433–436. 19. Cano, E.; Bastidas, D.M.; Simancas, J.; Bastidas, J.M. Dicyclohexylamine nitrite as volatile corrosion inhibitor for steel in polluted environments. Corrosion 2005, 61 (5), 473–479. 20. Focke, W.W.; Nhlapo, N.S.; Vuorinen, E. Thermal analysis and FTIR studies of volatile corrosion inhibitor model systems. Corros. Sci. 2013, 77, 88–96. doi:10.1016/j. corsci.2013.07.030. 21. Wachter, R.M. A Process for Protecting Metals Against Corrosion Anti Corrosive Packing Material and the ­Manufacture Thereof. 627801 United Kingdom, Issued 1949. 22. Milošev, I.; Finšgar, M. Inhibition of copper corrosion by 1, 2, 3-benzotriazole : A review. Corros. Sci. 2010, 52, 2737–2749. doi:10.1016/j.corsci.2010.05.002. 23. Chen, Z.; Huang, L.; Zhang, G.; Qiu, Y.; Guo, X. Benzotriazole as a volatile corrosion inhibitor during the early stage of copper corrosion under adsorbed thin electrolyte layers. Corros. Sci. 2012, 65, 214–222. doi:10.1016/j. corsci.2012.08.019. 24. Abraham, K.; Cromwell Paper Co. Vapor Phase Corrosion Inhibitor and Wrapping Material Containing Same. 2829945 United States, Issued 1958, 8–9. 25. Haruhito Sato, K.O. Vapor Phase Corrosion Inhibitor ­Compositions and Method of Inhibiting Corrosion Using said Compositions. 4308168 United States, Issued 1981. 26. Miksic, B.A.; Foley, J.M. Vapor Phase Corrosion Inhibitor Desiccant Material United States, Issued 1994. 27. Miksic, B.A.; Foley, J.M. Vapor Phase Corrosion Inhibitor Desiccant Material. 5320778 Unites States, Issued 1994. 28. Foley, J.M. Vapor Phase Corrosion Inhibitor Desiccant Material. 0662527A1 United States, Issued 1994. 29. Thomas, E.; Brown, C.A.; Anthony, R.; Russel Wendt, R.-H.W. Magnetic Recording Device with Improved ­Reliability United States, Issued 2003. 30. Goldade, V.A.; Neverov, A.S.; Semenovich, L.; Alexander, A.L. Anticorrosive Material. GB 2187466A United ­K ingdom, Issued 1986. 31. Goldade, V.A.; Neverov, A.S.; Semenovich, L.; ­Valentina, S.; Alexander, A.L. Anti Corrosive Material United ­K ingdom, Patent no. GB 2187466 A, Issued 23 May 1985. 32. Martin, P.; Nelson Malwitz, B.H. Vapor Phase Corrosion Inhibitor Product and Method Containing a Desiccant. 4973448 Unites States, Issued 1990. 33. Miksic, B.A. Vapor Phase Corrosion Inhibitor Material. 0639657A1 United States, Issued 1992. 34. Trufanova, A.I.; Ivanova, S.A. Volatile Inhibitor of ­Corrosion. RU2023753C1 Russia, Issued 1994. 35. Hahn, G.L.S.L.U. VCI their Use and Method of Producing the Same. Ep000976851B1 Russia, Issued 1999, 1–13. 36. Boris Miksic, M.K. Water Soluble Containers for Water Cooling Towers and Boilers. 6085905 United States, Issued 2000. 37. Nascimento, R.S.V. Modifcations in the volatilization rate of volatile corrosion inhibitors by means of host ± guest systems. Corros. Sci. 2001, 43, 1133–1153.

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38. Boerwinkle, F.P.; Kubik, D.A. Volatile Corrosion Inhibiting Article, 1981, 5–7. 39. Gao, G.; Liang, C.H. Volatile corrosion inhibitor for brass. Corros. Sci. 2007, 49, 3479–3493. doi:10.1016/j. corsci.2007.03.030. 40. Hashiudo, K.; Imanishi, K. Corrosion Inhibiting Plastic Film, Patent no. US 4124549A, 1978. Available at https:// patents.google.com/patent/US4124549. 41. Chio, H.H. VCI Powder with Improved Props to Produce Good Quality VCI Product Korea, Issued 2003. 42. Bester, P.; Okkers, D.J.A. Desiccant Composition. WO 2005/058481 A1WO, Issued June 2005. 43. Quraishi, M.A.; Jamal, D. Development and testing of all organic volatile corrosion inhibitors. Corrosion 2002, 58 (5), 387–391. 44. Sudheer, Q.M.A.; Ebenso, E.E.; Natesan, M. Inhibition of atmospheric corrosion of mild steel by new green inhibitors under vapour phase condition. Int. J. Electrochem. Sci. 2012, 7 (8), 7463–7475. 45. Vuorinen, E; Kalman, E.W.F. Introduction to vapor phase corrosion inhibitors in metal packaging. Surf. Eng. 2004, 20 (4), 281–285. doi:10.1179/026708404225016481. 46. Chandler, C.; Miksic, B.A.; Bradley, S.J. Biodegradable VCI, Patent no. US6028160, 2000, 3–8. 47. Chandler, C. VCI Package Utilizing Plastic Packaging Envelopes, Issued 1998. 48. Subramanian, A. An overview: Vapor phase corrosion inhibitors. Corrosion 2000, 56 (2), 144–155. doi:10.5006/1.3280530. 49. Poongothai, N.; Natesan, M.; Palanisamy, N.; Murugavel, S.C.; Ramachandran, T. Azole, amine, benzoate and nitrate compound mixture as VPI for metals in NaCl and SO2 environments. Ind. J. Chem. Technol. 2007, 14 (5), 481–487. 50. Zohuriaan-Mehr, M.J.; Kabiri, K. Superabsorbent polymer materials: A review. Iran. Polym. J. 2008, 17 (6), 451–477. Available at http://journal.ippi.ac.ir. 51. Aiouaz, N.; Dairi, N.H.F.H. Synthesis and properties of chitosan-graft polyacrylamide/gelatin superabsorbent composites for wastewater purification. Int. J. Chem. Mol. Nucl. Mater. Metall. Eng. 2015, 9 (7), 849–856. 52. Brannon-Peppas, L.; Mark, H.F. Structural representation of polymers. Encyclopedia Polym. Sci. Technol. 2002, 8, 106–121. 53. Gross, J.R. The Evolution of Absorbent Materials, 2nd Ed.; Elsevier Inc., 1990. Available at http://dx.doi.org/10.1016/ B978-0-444-88654-5.50006-6. 54. Ratner, B.; Hoffman, A. Synthetic hydrogels for biomedical applications. In Hydrogels for Medical and Related Applications; University of Washington: Seattle, WA, 1976, 1–36. 55. Niu, H.; Wang, F.; Weiss, R.A. Hydrophobic/hydrophilic triblock copolymers: Synthesis and properties of physically cross-linked hydrogels, 2012. doi:10.1021/ma502133f. 56. Pó, R. Water-absorbent polymers : A patent survey. J.M.S.Rev. Macromol. Sci. Part C 1994, 34 (4), 37–41. 57. Omidian, H.; Hashemi, S.A.; Askari, F.; Nafisi, S. Modifying acrylic based superabsorbents. II. Modification of process nature. J. Appl. Polym. Sci. 1994, 54 (2), 251–256. doi:10.1002/app.1994.070540211.

58. Omidian, H.; Hashemi, S.A.; Askari, F.; Nafisi, S. Modifying acrylic based superabsorbents. I. Modification of crosslinker and comonomer nature. J. Appl. Polym. Sci. 1994, 54 (2), 241–249. doi:10.1002/app.1994.070540210. 59. Ismail, S.; Hegazy, E.-S.; Shaker, N.; Badr, E.; Deghiedy, N. Radiation synthesis of superabsorbent hydrogels based on chitosan and acrylic acid for controlled drug release. J. Macromol. Sci. Part A 2009, 46 (10), 967–974. doi:10.1080/10601320903158560. 60. Kabiri, K.; Mirzadeh, H.; Mehr, M.J.Z. Undesirable effects of heating on hydrogel. J. Appl. Polym. Sci. 2008, 110, 3420–3430. doi:10.1002/app. 61. Hosseinzadeh, M.S.H. Synthesis and properties of ­biopolymer based on Gelatin-G-Poly (Sodium acrylateco -acrylamide) for cephalexin controlled release. Turk. J. Biochem. 2011, 36, 334–341. 62. Kabiri, K.; Zohuriaan-Mehr, M.J. Superabsorbent hydrogel composites. Polym. Adv. Technol. 2003, 14 (6), 438–444. doi:10.1002/pat.356. 63. Ahmed, E.M.; Aggor, F.S.; Nada, S.S.; Hawash, S.I. Synthesis and characterization of super absorbent polymers for agricultural purposes. Int. J. Sci. Eng. Res. 2015, 6 (3), 282–287. 64. Li, A.; Zhang, J.; Wang, A. Utilization of starch and clay for the preparation of superabsorbent composite. Bioresour. Technol. 2007, 98 (2), 327–332. doi:10.1016/j. biortech.2005.12.026. 65. Liu, Z.S.; Rempel, G.L. Preparation of superabsorbent polymers by crosslinking acrylic acid and acrylamide copolymers. J. Appl. Polym. Sci. 1996, 64, 1345–1353. 66. Hunkeler, D. Synthesis and characterization of high molecular weight water soluble polymers. Polym. Int. 1992, 27 (1), 23–33. doi:10.1002/pi.4990270105. 67. Watanabe, N.; Hosoya, Y.; Tamura, A.; Kosuge, H. Characteristics of water absorbent polymer emulsions. Polym. Int. 1993, 30 (4), 525–531. doi:10.1002/pi.4990300417. 68. Trijasson, P.; Pith, T.; Lambla, M. Hydrophilic polyelectrolyte gels by inverse suspension. Makromolekulare Chemie. Macromol. Symp. 1990, 35–36 (1), 141–169. doi:10.1002/ masy.19900350111. 69. Lee, W.W.; Hsu, C.C. Superabsorbent polymeric material. V. Synthesis and swelling behavior of sodium acrylate and sodium 2‐acrylamido‐2‐methylpropanesulfonate copolymeric gels. J. Appl. Polym. Sci. 1998, 69 (2), 229–237. 70. Askari, F.; Nafisi, S.; Omidian, H.; Hashemi, S.A. Synthesis and characterization of acrylic based superabsorbents. J. Appl. Polym. Sci. 1993, 50 (10), 1851–1855. doi:10.1002/ app.1993.070501022. 71. Bajpai, S.K.; Bajpai, M.; Sharma, L. Inverse suspension polymerization of poly (methacrylic acid-co-partially neutralized acrylic acid) superabsorbent hydrogels: Synthesis and water uptake behavior. Des. Mono. Polym. 2007, 5551 (February), 181–192. 72. Chen, X.; Shan, G.; Huang, J.; Huang, Z.; Weng, Z. Synthesis and properties of acrylic-based superabsorbent. J. Appl. Polym. Sci. 2004, 92, 619–624. 73. Wan, T.; Wang, L.; Yao, J.; Ma, X.; Yin, Q.; Zang, T. Saline solution absorbency and structure study of poly (AA-AM) water superabsorbent by inverse microemulsion

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Corrosion Inhibitor: Polymeric Desiccant 649

650

89. Iosif Lvouich Rozenfeld, V.P.J.I.N.M. Method of Protecting Metals Against Atmospheric Corrosion. 1414025 United Kingdom, Issued 1973, 251–255. 90. Desiccant and Oxygen Scavenger for Packaging Material, n.d. 91. Mayeaux, D.; La, P. Combination Desiccant and Vapor Corrosion Inhibitor. 5324448 United States, Issued 1993, 1–4. 92. Brighenti, A. Ronnie Singh Combined Methods with VCI for Extended Corrosion Protection. Technologia de Equipmentos, 2002.

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93. Lyublinski, E.Y.; Kubic, D. Systems and Methods for Preventing and/or Reducing Corrosion in Various Articles. 6551552B1 United states, Issued 2003. doi:10.1074/ JBC.274.42.30033.(51). 94. Rollo, P. A Protective Packaging Evaluation Involving a High Barrier Film Lamiation, Desiccants and Oxygen Absorbers, 1996. 95. Iosif, R.; Persiantseva, V.P.; Igorevich, J.; Petrov, N. Method for Inhibiting the Corrosion of Metals with Vapor Phase Inhibitors Disposed in a Zeolite Carrier. 3967926 United States, Issued 1976. doi:10.1074/JBC.274.42.30033.(51).

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Corrosion Protection: Natural Polymer in I. B. Obot and Saviour A. Umoren Centre of Research Excellence in Corrosion, Research Institute, King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia

Ubong M. Eduok Department of Chemistry, King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia

Abstract Natural polymers represent the vast class of polymers derived mainly from plants and animals. They are macromolecules whose molecular units are linked in a continuous chain of complex structures in living systems. Depending on their applications, food or nonfood, most of them are also produced in various amounts all over the world. Polysaccharide biopolymers, for instance, make up between 60% and 85% of two billion tonnes of the world’s renewable resources produced annually, and more than half come from natural sources. They have also been widely deployed as corrosion inhibitors for industrial metals in various media. The presence of special atoms on the sugar moiety provides metal surface adsorption sites, while CH2COOH grouped increases the possibility of charge transfer thereby aiding adsorption. The large molecular sizes of the polysaccharide biopolymers further promote surface adsorption by “blanketing” the metal surfaces from corrosive ions and molecules. Among some of the corrosion control methods deployed in the industrial marine and oilfield companies, the use of natural polymers in corrosion inhibitor formulations and anticorrosive-paint composites for metal surfaces have been effective. In this entry, we discuss in detail the application of natural polymers in corrosion protection of industrially important metals and alloys.

INTRODUCTION Natural polymers simply represent the vast class of polymers (and their derivatives) widely distributed in nature, both flora and fauna sources alike. They are macromolecules whose molecular units are linked in a continuous chain of complex structures in living systems. Depending on their applications, food or nonfood, most of them are also produced in various amounts all over the world. Polysaccharide biopolymers, for instance, make up between 60% and 85% of two billion tonnes of the world’s renewable resources produced annually, and more than half come from natural sources.[1] Most of them are readily modified (physically and/or chemically) to attain practical requirements for an end use with interesting and unique properties. The production of natural polymer feeds both light and heavy industries (see examples in Fig. 1) and also promotes the use of these biomaterials in human endeavor. The gelling properties of alginates and pectates of most natural polymers have been utilized in medicine and pharmaceutical drug composition. Highly oxidized pectates are enriched with reactive chemical groups that aid drug ­controlled delivery.[2]

Encyclopedia of Polymer Applications, First Edition DOI: 10.1201/9781351019422-120054050 Copyright © 2018 by Taylor & Francis. All rights reserved.

Modern consumers’ tastes for food are not without health and nutrition consciousness, especially in terms of total fat content since in most cases, only most low-fat comminuted meat products (contain about 20–30 g/100 g fat) are recommended for consumption.[3] Hydrocolloids and biopolymers are used in such products as fat substitutes and also deployed to stabilize food product textures and rheology. Most of them are thickening, gelling, and stabilizing agents and some with taste-release properties (e.g., gellan). Most food products have more than one biopolymer blended with the needed vitamins, minerals, fat, and water in a food product. Shredded chicken meats are made into “meat balls” with pectin and gelatin while the addition of carrageenan instills a unique sensory acceptance to the food.[4] Gelatin extracts from fish as well as carrageenan and chitosan are used as edible coatings due to their barrier properties for seafood products against oxygen, drought, and sunlight. They are also excellent gelling agents in food when blended together.[5,6] Most natural polymers with emulsifying abilities are amphiphilic, and this surface-active property makes them unique candidates for surface protection. As hydrocolloids, some biopolymers

651

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Keywords: Aluminum; Copper; Corrosion; Inhibitors; Natural polymers; Steel.

652

Corrosion Protection: Natural Polymer in

Petrochemicals, energy/biofuel processing Construction and building materials

Core laboratory research (e.g., chromatographic separation) and industrial (adhesive, fibers, resistant films/ paints, etc.) production

Industries Pharmaceutical and biomedical (drug delivery, cell transplantion, dental impression materials, wound dressing, etc.)

Cosmetics, papermaking, textile and clothing/ fabric

Food and agriculture

Fig. 1  Some industries utilizing natural polymers in product manufacturing

possess hydrophobic groups that aid molecular adsorption and surface adhesion.[7] Except for some polysaccharides (including gum arabic, cellulose, pectin), most of them show predominant hydrophilic surface activity due to the ­presence of the nonpolar functional groups on their molecular chain.[7–9] This contribution will dwell on the advancement in the use of natural polymers with such ­surface-active properties in anticorrosion applications. OCCURRENCE OF NATURAL POLYMERS

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Natural polymers abound in nature and most of them are found in biomasses of varieties of living systems, both plant and animals. Most of them are products of physiological (e.g., metabolic product) processes in plants, unicellular microorganisms as well as higher animals. To meet their insatiable demand for various industrial applications, an appreciable amount of most of them are now also synthesized in the laboratory. For instance, pectin, a polysaccharide biopolymer known to be extracted from fruits (e.g., citrus, apples, cherries),[10] has also been synthesized and modified via controlled chemical processes. Sharma and Ahuja[11] have reported the thiolation of pectin from t­hioglycolic acid esterification. Recently, Suchaoin et al.[12] have reported the synthesis and characterization of a ­thiolated carrageenan from κ- and ι-­carrageenans. The procedure involved the bromine displacement of the hydroxyl groups of the carrageenan, followed by thiolation reaction using thiourea. The synthesized polymer exhibited about 176 and 109 µmol thiol groups per matrix for thiolated κ- and ι-carrageenans, respectively. These chemically modified forms of carrageenan demonstrated enhanced cytotoxicity against Caco-2 (heterogeneous human epithelial ­colorectal adenocarcinoma) cell line in tumor below 0.1% (w/v). The synthesis of ­κ-carrageenan-g-N-(hydroxymethyl) acrylamide as metal adsorbent and antiflocculant has also

been reported.[13] Chemical grafting of N-(hydroxymethyl) acrylamide to carrageenan was possible through the hydroxyl group from spectroscopic evidence. The new grafted polymer matrix developed superior adsorption of bivalent ions, with the order of metal-ion sorption selectivity: Ni2+ > Zn2+ > Pb2+. Qui et al.[14] have synthesized nanosized carboxymethyl cellulose (CMC) derivative (CMC-lithium) for use as an anode material in Li-ion battery. Improved battery performance was realized after electrospinning CMC-Li on the cell (Fig. 2); the synthesis route for CMC-Li is displayed in Eqs. 1 and 2. NaOH Cell – (OH)3 ClCH  → Cell – ( OH )3– x (ONa)x – n COOH 2

(OCH2COO– Na+ )n → (Cell– CH2COONa)

(1)

Cell–CH2COONa(CMC–Na) HCl → Cell–CH2 COOH (2) LiOH (CMC–H)  → Cell–CH2COOLi(CMC–Li) A procedure for grafting docetaxel (DTX, an anticancer drug) on carboxymethyl chitosan (CMCh) has also been reported.[15] The authors in this novel polymer–drug conjugate design discovered a 12% DTX release on incubation in plasma for two consecutive days. This conjugate inhibited tumor growth after a month (using a melanoma mouse model) with increased cytotoxicity against HepG2 and B16 cells. The synthesis route of CMCh–DTX conjugate is displayed in Fig. 3; DTX is grafted to CMCh via a succinate linker for ease of release to exhibit the needed therapeutic effect. Four triazoles have also been grafted on starch molecules and the resultant materials were tested for their in vitro antibacterial activity against Escherichia coli and Staphylococcus aureus.[16] Most of the starch-triazole matrices (at 1 mg/mL) exhibited 60% and 40% inhibitory effect against the growth of both bacteria after 8- and 16-h culture periods, respectively. The

Corrosion Protection: Natural Polymer in 653

(a)

: M(anode precursor) : CMCLi

High voltage source

(b)

: CMCLi

: M(anode precursor)

: AQ nanoparticles

As-spun nanofibers

Heat up to 280°C at 0.5°Cmin–1 Dwell at 280°C for 2 h AQ-NR

AQ-NF

bacterial inhibitory activity increased in the following order: unmodified starch < 6-hydroxymethyltriazole-6-­ deoxy starch (HMTST) < 6-bromomethyltriazole-6-deoxy starch (BMTST) < chloromethyltriazole-6-deoxy starch (CMTST) < 6-carboxyltriazole-6-deoxy starch (CBTST), with CBTST attaining more than 97%. The antibacterial properties of these matrices were linked with the electron-­ withdrawing ability of each substituted triazole group grafted to the starch molecule. The synthesis route for these starch-triazole materials is displayed in Fig. 4. CLASSIFICATION OF NATURALLY OCCURRING POLYMERS Natural polymers could be classified based on the chemistry of the molecular structures, prevalence chemical properties, their applications, and sources. In this contribution, they will be classified based on the natural sources to which they could be extracted from. Most of these polymers are polysaccharides and proteins and are sourced from plants and animals. Figure 5 displays a summary of the source-based ­classification of natural polymers as used in this entry. Natural Polymers Sourced from Plants Most forms of lignin, such as kraft lignin and lignosulfonates, are made available via chemical pulp production, and they are present in stems of higher vascular plants.[17,18]

Polysaccharides are stored in various plant parts including roots and seeds; the presence of glucans (normal, xylo, and galactoxy types) and galactomannans is prevalent in terrestrial plants.[19] In combination with some proteins, these polysaccharides play key roles in the physiology of living systems.[20] Chitin and its derivatives are the second most abundant natural polymers in nature; chitins are mostly present in cuticles of adult insects, shells of invertebrates, and fungi.[21] The carboxymethyl (CM) derivatives possess unique antimicrobial abilities against bacteria, though improved antibacterial potentials have been reported with the addition of quaternary ammonium groups to the N-substituted CM group.[22] Agar is another class of polysaccharide biopolymer. They are made up of nonionic linear agarose, abundant in red (Rhodophyta) and brown (Phaeophyceae) seaweeds as branched agaropectin. They are widely used in culture media and agar gels to induce electroosmotic flux.[23] Alginates and carrageenans are also present in seaweeds. Highly abundant in Chondrus crispus, carrageenans are linearly structured polyelectrolyte heteropolysaccharides (mostly sulfated) that readily form highly crossed-linked gels with cations.[24] Alginates are film-forming compounds found in marine kelp.[25] The discovery of alginic acid, an alginate derivative, dates far back to the 1800s, but it was not until 1923 that Thornley started a small-scale alginate business enterprise using alginates as anthracite/coal dust binders in the United Kingdom. It was later mass-produced on a larger scale between 1934 and 1939.[26] Large quantities of starchy flours are mostly

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Fig. 2  (a) Electrospinning procedure of the anthraquinone (AQ)/CMC-Li nanofiber before 2032 Li-ion battery assembling and (b) mechanism of formation of carbon nanofibers/AQ/Li electrode Source: Reprinted with permission from Elsevier, © 2015.[14]

654

Corrosion Protection: Natural Polymer in

O

O O

NH

OH

O

O

O OH

O OH

+

O

O

NH

DMAP

O

O OH

O

O

HO OO O

OH

O

O O

HO

O

O

O

OO

O O

OH + CH2OCH2COOH CH2OCH2COOH OH

O

O

OH

O

O

CH2OCH2COOH CH2OCH2COOH CH2OH

CH2OH OH

O

EDC, NHS

y

x NHOCCH3

z

NH2

NH

OH

O

O

NH2

OH

x

O

O y

NHOCCH3

OH

O NH2

z

O O

OH

O O

O HO

O O

Self-assemble

O O

OH

O

CMCS-DTX nanoconjugates

NH O O

Fig. 3  Synthesis route of CMCh–DTX conjugate assemble Source: Reprinted with permission from Elsevier, © 2013.[15]

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derived from herbaceous, perennial root crops (e.g., yam, potato, cassava), grains, and legumes from regions with tropical climates. The increasing demand for starch in the food and pharmaceutical industries alone has necessitated the search for alternative sources.[27] Starch consists of amylose and amylopectin in varying composition, and depending on the plant source, one of them could be lacking. Starch extracted from glutinous rice in China has been reported not to have amylose, but it is still used in the preparation of most traditional delicacies (e.g., rice-made cakes and biscuits). The problem of indigestibility associated with food products of glutinous rice has been ­heavily linked with the absence of amylose.[28] Starch is one of the major constituents on man’s diet and also a principal additive in food and nonfood industries; it is deployed as glue, gelling, emulsifying, and thickening agents. Another polysaccharide biopolymer extracted from plant sources is a heteropolysaccharide gum-type substance, pectin. Pectins are found in all plant primary cell walls.[29] This hydrophilic material is composed chemically of rhamnose, arabinan, galactan, and arabinogalactan units in one long polysaccharide molecular chain.[30] Cellulose is the most

abundant polysaccharide biopolymer on earth; it makes up the plant cell wall and forms between one-half and twothirds of the tissues in green plants.[31] Approximately 1011 tonnes are biosynthesized worldwide per year. Most derivatives of cellulose have huge industrial applications due to their biocompatibility, biodegradability, hydrophilicity, and unique mechanical properties.[32] Secretions from Higher Plants and Unicellular Microorganisms Natural gums are polysaccharide-enriched secretions, or simply exudates. Depending on the source, some ­contain a mixture of highly branched and complex polymers with varying adhesive and cohesive properties. [31] This surface property allows for the use of gums in ­pharmaceutical formulations as stabilizing, thickening, gelling, and ­suspending agents. Their uses in ­industrial applications still have few drawbacks if used in the unmodified forms [32] (Fig.  6). Most plant exudations are mixtures of solid/­semisolid gums and volatile oil, mostly gums/ resins. Gum arabic is a typical native exudate gum

Corrosion Protection: Natural Polymer in 655

Br

HO O

H

H

OH

*

Starch N

O OH

H

OH

* n

*

O * n

OH

6-Bromo-6-deoxy-starch NaN3

N

R

N

N3

H *

O

H

NBS/Ph3P

O OH

H O

OH

O

H

R

H

OH

TEA/CuI or * * n without TEA/CuI

O OH

* n

6-Azido-6deoxy-starch HMTST: R =

OH

;

BMTST: R =

Br ; O

CMTST: R =

CI ;

CBTST: R =

OH .

Fig. 4  Synthesis route for the four starch-triazole materials (HMTST, BMTST, CMTST, and CBTST) Source: Reprinted with permission from Elsevier, © 2014.[16]

Natural polymers

Plant (Mostly polysaccharides) Terrestial plant parts: (e.g., leaves, stems and roots of tropical plants)

Animal (Mostly proteins)

Marine plants: (e.g., sea weeds are enriched with agar, alginate, carrageenan)

Extracts (e.g., cellulose in plant cell walls; tropical root crops are enriched with starch; while pectins are present in leaves of citrus and peach)

They can also be synthesized (e.g., cellulose and cellulose drivatives).

Products of biophysiological processes: (e.g., gelatins are biodenaturation products of animal-based collagen)

Animal parts: (e.g., serums and eggs are rich in albumin; shells of most crustaceans are enriched with chitins/chitosans)

Exudates (e.g., natural gums like oleo gum and Gum Arabic)

Fig. 5  Classification of natural biopolymers by natural sources

collected for Acacia senegal tree; it basically consists of highly branched arabinogalactan heteropolymers. These polysaccharide biopolymers are covalently bound to ­surface-active proteinaceous materials inherent in the gum material.[33] Not just from plant sources, other gums

are also interestingly sourced for unicellular microorganisms. A gram-­negative Xanthomonas campestris is known to secrete a high-­molecular-weight heteropolysaccharide, xanthan gum, [34] whereas guar gum is collected from Cyamopsis ­tetragonolobus.[35]

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Bacteria (e.g., Cyamopsis tetragonolobus secretes Guar gum, Xanthomonas campestris and Sphingomonas elodea secrete Xanthan and Gellan gums, respectively)

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Corrosion Protection: Natural Polymer in

Obscurity (opaque upon storage) Uncontrollable rheological properties Mold growth and microbial contamination Deformed  and non-uniform microstructure Reduced viscosity upon storage Unpredictable rates of hydration Unpredictable rates of thickening 

Fig. 6  Some demerits and drawbacks to use of natural (native and unmodified) gums for industrial applications

Natural Polymers Sourced from Animals Most natural polymers found in animal sources are proteins; gelatin and albumin are typical examples. Gelatins are heterogeneous mixtures of peptides and proteins. They  are denaturation products of collagen.[36] Collagen is the main structural protein of connective tissues in higher animals.[37] Like the polysaccharide biopolymers, its ­biodegradable and biocompatible properties make it a candidate for most biomedical (e.g., tissue engineering) and pharmaceutical applications. Gelatins are mainly produced from the organs of higher animals such as the skin and bone of African buffalos and pigs.[36] Albumin is protein found in body fluids including the blood serum and cerebrospinal fluid; it regulates blood volume and other biological functions. It is also found in eggs as well as amniotic fl ­ uids.[38] In humans, the serum albumin possesses multiple ligand-binding sites for many endogenous and exogenous compounds.[39]

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APPLICATION OF NATURAL POLYMERS FOR CORROSION PROTECTION Metals are a part of human life, and they have been deployed for various domestic and industrial applications including the construction and fabrication of engineering structures with steel,[40] building materials with copper,[41] bone implants with titanium and magnesium alloys,[42] etc. For most material applications, the lifetime of these metals depends on their chemistries and the environment to which they are being deployed. Several researches worldwide have focused on the study of the dynamics of metal degradation in order to reduce the rate of this spontaneous electrochemical process. These have been achieved via new designs and novelties in alloying technology, reliable corrosion inhibition strategies, and better prevention approaches. These corrosion mitigation methods are necessary since the cost of metal corrosion is enormous. Companies incur cost due to new constructions and replacements of corroded machine parts, general maintenance and repairs, corrosion-related downtime, total material failures, and cost of periodic corrosion inhibitor purchase. In the marine sector of most

advanced economies, localized corrosion has been a major cause of concern. Ships and other marine vessels are susceptible to corrosion due to the constant exposure to water and other corrosive molecules/ions in the sea. These physical defects on ship structures cost the U.S. marine shipping industry approximately US$2.7 billion in 1998, and a record 275.7 billion more was expended that year in other sectors of the economy in corrosion-related issues.[43] The cost of corrosion keeps rising, and the figures from other developed and developing countries are outrageous. Among some of the corrosion control techniques deployed in the marine and oilfield applications, the use of protective organic coatings, corrosion inhibitor formulations, and anticorrosive paints composites on metals surfaces has been effective.[44] Over the years, the use of inorganic/organic compounds (surfactants, emulsifiers, promoters, oil-based media, etc.) in corrosion inhibitor formulations has been the most applied mitigation strategy. Figure 7 shows a proposed mechanism of protection with chemical inhibitors. The first step in this mechanism at metal surfaces normally involves molecular adsorption—either physisorption or chemisorption, and in some cases, a combination of both. These adsorption processes could be spontaneous, and if successful, layers of protective oxide films are formed, and there is also a possibility of reaction between adsorbed species and potential corrosive components.[45] Anodizing and chromating processes are some of the most pretreating procedures for industrial metals. However, pretreatment electrolytes with hexavalent chromium have been prohibited due to e­ nvironment toxicity and the health effects on humans.[46] The use of borates,  phosphates, arsenates, molybdate, tungstate, chromates, dichromates, and nitrites as corrosion inhibitor compounds has also been banned as their usage is against modern industrial safety legislations.[47] The influence of anions of chromates and molybdates on embryonic development has been studied.[48,49] Other inorganic compounds evaluated for their potential corrosion inhibition properties are polyphosphates, vanadates, phosphonates; their ­efficiencies and robustness in corrosion applications make them economically profitable.[50] Corrosion inhibition efficiencies of some inorganic ­compounds have been established in the following order:[51] CrO42 − > WO42 − > MoO42 − > Cr2 O72 − > NO2− ≈ NO3− . Recent industrial researches in this field are geared toward replacing the obnoxious inorganic corrosion inhibitors with greener and more efficient alternatives.[52] The use of natural polymers as corrosion inhibitors is gaining much grounds in material science; most of these compounds are available in their pure forms, biomass extracts, or as essential oils. They are known to form stable protective inhibitor films on metal surfaces, thereby reducing the passage of corrosive ions and molecules. The a­ dsorption of these compounds of metal surfaces is influenced by their concentration in the media, their molecular sizes, and the chemistry of the metal/electrolyte interface.[53] Most of these

Corrosion Protection: Natural Polymer in 657

Molecular adsorption on the metal surface intiates the formation of protective thin film

More layers of protective oxide films could be formed on the base metal

Adsorbed inhibitor species could further react with the electrolyte/corrodent to form more complex corrosion product(s)

biopolymers are endowed with larger molecule sizes for wider coverage on metal surfaces. The molecular chains of some of these compounds bear heteroatoms (e.g., N, S, O) as well as electron-rich polar chemical groups and conjugated double bonds. Most of these electrons are donated to empty orbitals of the metal atoms for bond formation. Natural polymers are readily available as their sources are abundant; they are low cost, renewable, and robust compared to their synthetic counterparts. They are also eco-friendly and benign, and possess optimal biocompatibility and biodegradability. They could serve as an efficient anticorrosive additive for field-based inhibitor formulations.[47] Tables 1–5 present typical examples of metal substrates and their related alloys reported in the literature for the anticorrosion applications involving various classes of natural polymers. Lignin Probably well known for structurally supporting the tissues of vascular plant, thereby rendering firmness to the woody stems (including seaweeds) and barks of trees, lignin (Lignum; Latin name for “wood”) is that class of complex organic polymer with highly cross-linked phenolic polymers.[54,55] Lignin is covalently bonded to various ­polysaccharides in plants and confers the needed mechanical strength and agility to the cell walls depending on the plant.[56] This complex polymer is soluble in alkali but not in alcohol and water, and can also be precipitated from solution at acidic pH. Its biosynthesis and complex molecular structure have been widely reported.[57,58] Lignin is readily available in many forms; as monolignols, it is widely found as guaiacyl and syringyl residues, and they are nontoxic.[59] Many forms of lignin have been employed as corrosion inhibitors for some metals in various media. Akbarzadeh et al.[60] have reported the effect of monomers of lignin (p-coumaric acid, ferulic, and hydroxybenzaldehyde) as corrosion inhibitors for mild steel in neutral medium. These compounds demonstrated remarkable corrosion protection with more than 70% efficiency at very low concentration. The authors attributed the reduction in corrosion to

molecular adsorption at the metal surface [confirmed by scanning electron microscope (SEM)], and they related the trend of experimental corrosion parameter with theoretical analyses. The effects of increasing concentration of lignin on carbon steel corrosion have been studied in 1 M HCl in 30°C–70°C using weight loss technique by Yahya et al.[61] Corrosion inhibition was observed to increase with concentration of lignin but not with temperature, and authors attributed this to molecular (physical) adsorption at carbon steel surface. Adsorption was explained by means of adsorption isotherm, while the inhibition mechanism was elucidated by means of thermodynamics and kinetics parameters. The effect of dimethyl diallyl ammonium chloride and acrylamide grafted on lignin has been investigated by Ren.[62] These lignin-derived terpolymer composites demonstrated improved corrosion inhibition in 10% HCl medium at 25°C–80°C up to 95% efficiency from both weight loss and potentiodynamic polarization techniques. This was attributed to the enhanced adsorption of the secondary compounds (dimethyl diallyl ammonium chloride and acrylamide) to the metal surface; a synergistic effect with lignin. Adsorption was approximated by Temkin isotherm model. Paddy-based lignin residue has been tested as corrosion inhibitor in 1 M HCl for steel compared to commercial alkali lignin using both chemical and electrochemical techniques. Corrosion inhibition was attributed to the adsorption of syringyl compound in the lignin [confirmed by Fourier-transform infrared spectroscopy (FTIR)].[63] The anticorrosion potential of alkali extracts of lignin for ferrous and Al alloys has been studied using weight loss method, SEM, and microbeam X-ray fluorescence. It was found from the three techniques employed in the investigation that corrosion inhibition of alkali lignin increased with its concentration in 1 M HCl with 54% and 21% magnitudes of corrosion inhibition efficiency obtained for 1500 ppm lignin extract.[64] The corrosion protection properties of rubber/benzene–sulfonate and lignosulfonate-doped polyaniline coating have been studied for mild steel in 3.5 wt% NaCl solution using accelerated salt spray and immersion tests. The lignosulfonate-doped polyaniline (PANI) coating

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Fig. 7  A proposed mechanism of protection with chemical inhibitors

Construction– Cosmetics 658

Table 1  Typical examples of ferrous alloy substrates reported in the literature for the anticorrosion applications involving various classes of natural polymers in various corrosive media Trend of corrosion inhibition Type of metal with: substrate/ corrosion Inhibitor Inhibitor Corrosion monitoring Reason(s) for corrosion medium/ Type of inhibitor/ classification/ S. concentration Temperature technique deployed in the study inhibition Reference corrodent inhibitor system type No. Uninvestigated

Increased

Uninvestigated Electrochemical and chemical methods

Molecular adsorption at the metal surface (this was confirmed with SEM analysis); more than 70% inhibition efficiency was recorded against mild steel corrosion.

[60]

Carbon steel/1 M Lignin HCl

Uninvestigated

Increased

Decreased

Weight loss technique

Molecular adsorption at the metal surface; 1000 ppm lignin recorded the highest magnitude of inhibition efficiency (η% = 75%) for the metal substrate in 1 M HCl

[61]

Carbon steel/10% HCl

Mixed- type

Increased

Increased

Weight loss and potentiodynamic Inhibition of carbon steel polarization techniques corrosion was attributed to the adsorption of the secondary compounds (dimethyl diallyl ammonium chloride and acrylamide) on the metal surface prior to lignin; a synergistic effect with lignin. Metal surface adsorption was confirmed with FTIR spectroscopy; this was also explained by means of Temkin isotherm. The highest magnitude of η% in this study was observed for polymer-grafted lignin composite (above 95% in 10% HCl acid at 25°C)

1.

Mild steel/near neutral medium

2.

3.

Monomers of lignin (p-coumaric acid, ferulic, and hydroxybenzaldehyde)

Dimethyl diallyl ammonium chloride and acrylamide grafted on lignin terpolymer

[62]

Corrosion Protection: Natural Polymer in

(Continued)

[63]

4.

Mixed- type Carbon steel/1 M Paddy-based lignin HCl residue (compared with commercial alkali lignin, Sigma Aldrich)

Increased

Uninvestigated Weight loss and potentiodynamic Corrosion inhibition was polarization techniques attributed to the adsorption of syringyl compound inherent in the lignin structure on steel (this was confirmed by FTIR)

5.

Ferrous and Al alloys/1 M HCl

Alkali extract of lignin

Uninvestigated

Increased

Uninvestigated Weight loss technique, SEM, and Formation of protective microbeam X-ray fluorescence inhibitor film at the metal surface (this was confirmed by SEM)

[64]

6.

Mild steel/3.5 wt% NaCl

Rubber/benzene– sulfonate and lignosulfonate-doped polyaniline coating

Not applicable

Decreased with concentration of polyaniline

Formation of protective Uninvestigated EIS, potentiodynamic inhibitor film on the metal polarization techniques, and by surface visual examination of exposed surfaced in the solution of the corrosive electrolyte; accelerated salt spray and immersion tests

[65]

7.

Mild steel/1 M HCl

N-(2-hydroxy-3-trimethyl Mixed-type (but Increased ammonium)propyl predominantly chitosan chloride cathodic)

Decreased

Formation of protective film on the surface of mild steel; this was confirmed by spectroscopic characterization

[70]

8.

API X65 pipeline Modified CMCh steel/1 M HCl

Inhibitor molecular adsorption at the surface of steel; this was marked by increased surface hydrophobicity. The highest recorded magnitude of η% in this study was 93.23% for 250 ppm at 25°C

[147]

Mixed-type

Increased

Weight loss, EIS, and potentiodynamic polarization techniques

Uninvestigated Potentiodynamic polarization technique

(Continued)

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Corrosion Protection: Natural Polymer in 659

Table 1  Typical examples of ferrous alloy substrates reported in the literature for the anticorrosion applications involving various classes of natural polymers in various corrosive media (Continued) Trend of corrosion inhibition Type of metal with: substrate/ corrosion Inhibitor Inhibitor Corrosion monitoring Reason(s) for corrosion medium/ Type of inhibitor/ classification/ S. concentration Temperature technique deployed in the study inhibition Reference corrodent inhibitor system type No.

Construction– Cosmetics 660

Table 1  Typical examples of ferrous alloy substrates reported in the literature for the anticorrosion applications involving various classes of natural polymers in various corrosive media (Continued) Trend of corrosion inhibition Type of metal with: substrate/ corrosion Inhibitor Inhibitor Corrosion monitoring Reason(s) for corrosion medium/ Type of inhibitor/ classification/ S. concentration Temperature technique deployed in the study inhibition Reference corrodent inhibitor system type No. 9.

304 steel/2% CH3COOH

10.

11.

Mixed-type (predominantly anodic)

Increased

Uninvestigated Potentiodynamic polarization for the corrosion test/differential scanning calorimetry was used to study the thermal properties of the modified chitosan material; FTIR.

Formation of protective inhibitor film on steel; corrosion inhibition increased in the presence of the carbazide and hydrazide compounds; a 92% inhibitor efficiency was recorded for 60 mg/L carbazide/chitosan composites

Galvanized Chitosan-heptanoate steel/3 wt% NaCl modified beidellite

Uninvestigated

Increased with chitosan modification

Uninvestigated Characterization of the chitosan composite was successful with diffuse reflectance FTIR, thermogravimetry analysis (TGA) coupled to mass spectrometry, and X-ray diffraction analysis (XRD); EIS was deployed for corrosion studies

A magnitude of film resistance of 16.6 and 10.0 MΩ were recorded for unmodified and chitosanheptanoate modified beidellite inhibitor composites at room temperature

Carbon steel/1 M Chitosan grafted HCl polyethylene glycol (PEG) assembled on silver nanoparticles

Mixed-type (but Increased predominantly cathodic)

Uninvestigated Characterization of the modified chitosan material with gel permeation chromatography, FTIR, proton nuclear magnetic resonance (NMR), and XRD techniques; high-resolution transmission electron microscopy and energy dispersive analysis of X-rays; potentiodynamic polarization and EIS techniques were deployed for the corrosion studies

Improved surface properties of adsorptive film in the presence of silver nanoparticles and PEG. The highest recorded magnitude of η% was 92.75 and 76.64% for 1 × 10 −3 M inhibitor composite in the presence and absence of silver nanoparticles

Thiosemicarbazide and thiocarbohydrazide functionalized chitosan

[148]

[149]

[150] Corrosion Protection: Natural Polymer in

(Continued)

12. Mild steel/0.1 N HCl

13.

Nanostructured chitosan/ Uninvestigated ZnO nanoparticle films

Mild steel/ N-Acetyl thiourea naturally aerated chitosan 0.5 M H2SO4

Mixed-type

Increased with the number of layers of chitosan/ZnO films

Uninvestigated Characterization of the protective films with UV–visible absorption spectroscopy (UV– visible), FTIR, XRD, SEM, EDAX. Linear polarization studies, potentiodynamic polarization, and EIS techniques were deployed for the corrosion studies

Molecular adsorption/ formation of protective film on the surface of the mild steel; adsorbed film with ZnO nanoparticles was probed using FTIR, XRD, and SEM techniques. Improved corrosion inhibition was observed for the ZnO nanoparticlemodified chitosan composite with the magnitude of corrosion inhibition efficiency of 73.80% compared to the unmodified chitosan films (32.47%)

Increased

Decreased

Formation of protective chitosan film on steel was initiated by molecular adsorption; this physical phenomenon was approximated by Langmuir isotherm adsorption model. The highest recorded magnitude of inhibition efficiency was 94.5% for 0.76 mM chitosan concentration

Potentiodynamic polarization, EIS measurements, and surface examination via SEM technique

[151]

[152]

(Continued)

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Corrosion Protection: Natural Polymer in 661

Table 1  Typical examples of ferrous alloy substrates reported in the literature for the anticorrosion applications involving various classes of natural polymers in various corrosive media (Continued) Trend of corrosion inhibition Type of metal with: substrate/ Inhibitor corrosion Inhibitor Corrosion monitoring Reason(s) for corrosion medium/ Type of inhibitor/ classification/ S. concentration Temperature technique deployed in the study inhibition Reference corrodent inhibitor system type No.

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Table 1  Typical examples of ferrous alloy substrates reported in the literature for the anticorrosion applications involving various classes of natural polymers in various corrosive media (Continued) Trend of corrosion inhibition Type of metal with: substrate/ corrosion Inhibitor Inhibitor Corrosion monitoring Reason(s) for corrosion medium/ Type of inhibitor/ classification/ S. concentration Temperature technique deployed in the study inhibition Reference corrodent inhibitor system type No. Mild steel/1 M HCl

O-Fumaryl-chitosan

Cathodic

Increased

Uninvestigated Potentiodynamic polarization and EIS techniques

Corrosion inhibition was attributed to the adsorption of inhibitor molecules as well as the formation of protective inhibitor film on surface of mild steel; this was confirmed by SEM, AFM, FTIR, and XRD techniques. Adsorption was approximated by Langmuir adsorption isotherm model; the highest recorded magnitude of η% was 94.1% for 500 ppm O-fumaryl chitosan

[69]

15.

Q235 carbon steel/5% HCl

Polyamine grafted chitosan copolymer

Cathodic

Increased with concentration of polyamine

Uninvestigated Gravimetric measurements, metallographic microscopy, potentiodynamic polarization, and EIS techniques

Adhesion of stable inhibitor film on carbon steel surface at room temperature; the highest value of η% was recorded for 0.1% polyamine in the chitosan coating (90.34%) at room temperature

[153]

16.

Mild steel/0.5 M Electrophoretic deposited Uninvestigated H2SO4 of chitosan film

Uninvestigated Potentiodynamic polarization Mechanical and EIS techniques; FTIR and and barrier SEM analyses properties of chitosan was enhanced using a cross-linker (glutaraldehyde).

The magnitude of η% up to 98.1% was observed for the coating with the optimal barrier properties. Electrophoretic deposition technique enabled the formation of more compact coatings on mild steel

[154]

(Continued)

Corrosion Protection: Natural Polymer in

14.

[155]

17.

Q235 steel/1 M HCl

Bi-modified amphoteric oligochitosan derivatives

Mixed-type (but Increased for predominantly each derivative anodic)

Uninvestigated Oligochitosan derivatives were characterized using FTIR and proton NMR. Potentiodynamic polarization and EIS techniques followed by SEM analysis of metal surface for evidence of protective film formation

Molecular adsorption on the metal surface by chitosan and subsequent charge-transfer reduction due to the presence of the inhibitor film. 100 mg/L of oligochitosan recorded 87.36% value of η%; this was the highest in this study

18.

Mild steel/1 M HCl

Agar

Mixed-type

Increased

Increased

Weight loss and galvanostatic polarization techniques

Adsorption on molecules of agar on mild steel; adsorption was approximated by Langmuir adsorption isotherm model; the magnitude of η% = 98% was observed for 1000 ppm agar at room temperature

[156]

19.

Steel/0.1 M NaOH

Gelatin

Mixed-type

Increased

Uninvestigated Potentiodynamic polarization technique

Adsorption of gelatin on the surface of steel

[157]

20. Mild steel/H3PO4 Gelatin

Uninvestigated

Gelatin, addition to electropolishing solution, enhanced corrosion inhibition

Uninvestigated Potentiodynamic polarization technique

Formation of protective film on mild steel in H3PO4 (this was confirmed by SEM)

[158]

21.

Mixed-type

Increased

Increased then gradually dropped

Molecular adsorption on steel; the adhesion of protective gelatin film was probed by SEM analysis. 100 ppm gelatin recorded 77% and 92% inhibition efficiency at 25°C and 60°C, respectively

[120]

Mild steel/1 M HCl

Gelatin

Potentiodynamic polarization, EIS, and weight loss techniques

(Continued)

Construction– Cosmetics

Corrosion Protection: Natural Polymer in 663

Table 1  Typical examples of ferrous alloy substrates reported in the literature for the anticorrosion applications involving various classes of natural polymers in various corrosive media (Continued) Trend of corrosion inhibition Type of metal with: substrate/ Inhibitor corrosion Inhibitor Corrosion monitoring Reason(s) for corrosion medium/ Type of inhibitor/ classification/ S. concentration Temperature technique deployed in the study inhibition Reference corrodent inhibitor system type No.

Construction– Cosmetics 664

Table 1  Typical examples of ferrous alloy substrates reported in the literature for the anticorrosion applications involving various classes of natural polymers in various corrosive media (Continued) Trend of corrosion inhibition Type of metal with: substrate/ corrosion Inhibitor Inhibitor Corrosion monitoring Reason(s) for corrosion medium/ Type of inhibitor/ classification/ S. concentration Temperature technique deployed in the study inhibition Reference corrodent inhibitor system type No. Octadecylamine/ hydroxypropylcyclodextrin supramolecular complex

Mixed type (but Corrosion predominantly inhibition anodic) increased due to the presence of hydrophobic octadecylamine inherent in complex

Uninvestigated Weight loss, potentiodynamic polarization, and EIS techniques for the corrosion test; characterization of the complex was conducted with FTIR, XRD, and proton NMR

Increased hydrophobicity of the octadecylamine adsorbed film on the surface of steel; more than 95% inhibition efficiency was recorded for steel in the saline corrodent

[159]

23. X70 steel/0.5 M H2SO4

Polyacrylamide grafted cyclodextrin

Mixed-type

Increased

Decreased

Adsorption of dextrin on the metal surface

[160]

24. X70 steel/0.5 M H2SO4

β-Cyclodextrin grafted polyacrylamide

Mixed-type

Increased

Uninvestigated Potentiodynamic polarization, EIS, and SEM/EDS techniques

Metal surface dextrin adsorption was confirmed by SEM/EDX analyses; molecular adsorption was approximated by Langmuir adsorption isotherm; 150 mg/L modified cyclodextrin recorded 84.9% inhibition efficiency for steel in 0.5 M H2SO4 at 303 K.

[161]

25. Q235 carbon steel/0.5 M HCl

β-Cyclodextrin/PEG compound

Mixed-type

Increased

Uninvestigated Weight loss, EIS, and potentiodynamic polarization techniques

Modified dextrin adsorption on the metal surface

[162]

26. Q235 carbon steel/0.5 M HCl

β-Cyclodextrin grafted chitosan

Mixed-type (but Increased predominantly cathodic)

Weight loss, potentiodynamic polarization, EIS, and SEM technique

Uninvestigated Weight loss, potentiodynamic Formation of protective polarization, and EIS; SEM/EDS cyclodextrin film on techniques carbon (confirmed by SEM); adsorption was approximated by Langmuir adsorption isotherm; the highest value of η% in this study was 96.02%

[163]

(Continued)

Corrosion Protection: Natural Polymer in

22. Q235A steel/ CO2 saturated deionized water (at 5.6 pH)

27.

Ferritic stainless steel, Fe–17Cr/0.05 M H2SO4

BSA

Cathodic

Increased; also increased with immersion period

Uninvestigated EIS and polarization modulation-infrared reflectionadsorption spectroscopy, XPS, and FTIR

Adsorption of albumin on steel (this was confirmed by Polarization modulationinfrared reflectionadsorption spectroscopy PMIRRAS and XPS analyses)

[164]

28. Cast iron/1 M HCl

Gellan (with glucose and hydroxypropyl cellulose)

Mixed-type

Increased

Decreased

Weight loss, potentiodynamic polarization, and EIS techniques

Molecular gellan adsorption on the surface of cast iron; adsorption was approximated by Langmuir adsorption isotherm model. Improved corrosion inhibition was achieved in the presence of KI (an inhibition synergy between gellan and KI molecules)

[83]

29.

Acryl amide grafted cassava starch

Mixed-type at 20°C (predominantly cathodic at 50°C)

Increased

Decreased

Weight loss, potentiodynamic Adsorption of starch on polarization, and EIS techniques; steel; corrosion inhibition SEM was enhanced after grafting acryl amide to starch molecule. Metal surface adsorption was probed by SEM. Absorption was further approximated by Langmuir adsorption isotherm model

[165]

Cold rolled steel/1 M H2SO4

(Continued)

Construction– Cosmetics

Corrosion Protection: Natural Polymer in 665

Table 1  Typical examples of ferrous alloy substrates reported in the literature for the anticorrosion applications involving various classes of natural polymers in various corrosive media (Continued) Trend of corrosion inhibition Type of metal with: substrate/ corrosion Inhibitor Inhibitor Corrosion monitoring Reason(s) for corrosion medium/ Type of inhibitor/ classification/ S. concentration Temperature technique deployed in the study inhibition Reference corrodent inhibitor system type No.

Construction– Cosmetics 666

Table 1  Typical examples of ferrous alloy substrates reported in the literature for the anticorrosion applications involving various classes of natural polymers in various corrosive media (Continued) Trend of corrosion inhibition Type of metal with: substrate/ corrosion Inhibitor Inhibitor Corrosion monitoring Reason(s) for corrosion medium/ Type of inhibitor/ classification/ S. concentration Temperature technique deployed in the study inhibition Reference corrodent inhibitor system type No. 30. Carbon Activated and steel/2 wt% NaCl carboxymethylatedmodified cassava starch matrices

Uninvestigated

31.

Mixed type (but Increased predominantly anodic)

Mild steel/0.1 M H2SO4

Formation of protective starch film at the surface of steel; AFM was used in studying the surface morphology. Reduced corrosion inhibitive effect after chemical modification (compared to the activated starch) is linked with the substitution of the active hydroxyl groups. Metal surface adsorption was further approximated by Langmuir adsorption isotherm

[88]

Weight loss and potentiodynamic The presence of molecules polarization techniques of the surfactants in the protective film ascribed a slight hydrophobic character to the inhibiting layer of starch on steel; this led to a maximum inhibition efficiency (η%) of 66.21% for 200 ppm modified starch at 30°C

[166]

Uninvestigated EIS technique

Decreased

(Continued)

Corrosion Protection: Natural Polymer in

Starch modified with two surfactants (sodium dodecyl sulfate and cetyl trimethyl ammonium bromide)

Increased

32. Mild steel/I N HCl

Starch (chemically modified with piperidin-4-one)

Mixed-type

Increased

Decreased

Weight loss, potentiodynamic polarization, and EIS techniques

Improved corrosion protection for steel was attributed to an adsorption synergy between starch and piperidin-4-one. FTIR was deployed to further probe the adsorption of these compounds on the metal surface

[167]

33.

Alginate surfactant (N-(2-hydroxyethyl)N,N-dimethyldodecan1-aminium bromide cationic surfactant) derivative

Mixed-type (but Increased predominantly cathodic)

Decreased

Material characterization using FTIR, proton NMR, and UV– visible spectroscopy technique. Corrosion evaluation was conducted using weight loss, potentiodynamic polarization, and EIS techniques; SEM/EDX

Molecular adsorption of alginate on the metal surface; formation of more stable passive alginate film in the presence of the surfactant (this was confirmed by SEM/EDX). Adsorption was further approximated by Langmuir adsorption isotherm model. The highest magnitude of η% in this study was 96.27% recorded for 5 × 10 −3 M alginate/surfactant concentration

[168]

Hydroxyl propyl alginate

Mixed-type

Uninvestigated Weight loss, potentiodynamic polarization, and EIS techniques

Molecular adsorption of alginate at the surface of steel in 1 M HCl (this was confirmed by SEM SEM/EDX, AFM, and FTIR spectroscopic analyses). Adsorption was approximated by Frumkin adsorption isotherm

[169]

Low carbon steel/1 M HCl

34. Mild steel/1 M HCl

Increased

(Continued)

Construction– Cosmetics

Corrosion Protection: Natural Polymer in 667

Table 1  Typical examples of ferrous alloy substrates reported in the literature for the anticorrosion applications involving various classes of natural polymers in various corrosive media (Continued) Trend of corrosion inhibition Type of metal with: substrate/ corrosion Inhibitor Inhibitor Corrosion monitoring Reason(s) for corrosion medium/ Type of inhibitor/ classification/ S. concentration Temperature technique deployed in the study inhibition Reference corrodent inhibitor system type No.

Construction– Cosmetics 668

Table 1  Typical examples of ferrous alloy substrates reported in the literature for the anticorrosion applications involving various classes of natural polymers in various corrosive media (Continued) Trend of corrosion inhibition Type of metal with: substrate/ Inhibitor corrosion Inhibitor Corrosion monitoring Reason(s) for corrosion medium/ Type of inhibitor/ classification/ S. concentration Temperature technique deployed in the study inhibition Reference corrodent inhibitor system type No. Uninvestigated

Increased as the imidazoline corrosion inhibitor molecules are released at the metal/solution interface.

Uninvestigated Material characterization using UV–visible spectrophotometry and FTIR, SEM, TGA; corrosion evaluation was conducted using SEM and EIS techniques

The autorelease of imidazoline molecules at the metal/alginate film interface promoted corrosion retardation

[92]

36. Carbon steel/0.5 M HCl

Sodium aliginate

Mixed-type

Increased

Decreased

Weight loss, potentiodynamic polarization, EIS, and electrochemical frequency modulation techniques

Adsorption of sodium aliginate on carbon steel in 0.5 M HCl

[170]

37.

ι-, κ-, and µ-carrageenans Anodic

Increased

Decreased

Weight loss and galvanostatic polarization techniques

Formation of inhibiting carrageenans films on the metal surface

[96]

Mixed-type Increased (but dominantly cathodic)

Increased

Weight loss, potentiodynamic polarization, and EIS techniques

Formation of protective pectin film on the surface of steel; this was further studied by SEM and FTIR analyses as well as the evaluation of the aqueous contact angle of the adsorbed surfaces

[119]

Mixed-type

Increased

Weight loss, potentiodynamic polarization, and EIS techniques; spectroscopic (UV–visible) and SEM analyses for surface analyses

Formation of complex-type species on mild steel; the highest value of η% in this study was 90.3% and 94.3% at 14°C and 44°C for 2 g/L pectin. SEM was deployed to further study the metal surface in the presence of the inhibitor

[118]

P110 steel/ CO2- saturated 3.5 wt% NaCl

Low carbon steel/1 M HCl

38. X60 pipeline Commercial pectin steel grade/0.5 M extracted from apple HCl

39.

Mild steel/1 M HCl

Pectin extracted from fresh lemon peel

Increased

(Continued)

Corrosion Protection: Natural Polymer in

Calcium alginate gel capsules (doped with imidazoline quaternary ammonium salts)

35.

40. Mild steel/1 M HCl

41.

Pectin extracted from cactus

Carbon Pectin [propyl steel/60 ppm phosphonic acid and chloride solution Zn2+]

42. Mild steel/3.5 wt% NaCl

Pectin-g-polyacrylamide and Pectin-g-polyacrylic acid grafted polymers

Uninvestigated Weight loss, potentiodynamic polarization, and EIS techniques

Pectin adsorption and formation of protective inhibitor film on steel; adsorption was further approximated by Langmuir adsorption isotherm. A 96% magnitude of η% was recorded for 1 g/L at 308 K

[171]

Mixed-type (but Increased in predominantly the presence of cathodic) phosphonic acid and Zn2+

Uninvestigated Weight loss, potentiodynamic polarization, and EIS techniques; FTIR, XPS, AFM, and SEM surface analyses

Corrosion inhibition synergy between pectin and propyl phosphonic acid/Zn2+ ions; this was attributed to simultaneous molecular adsorption. A 94% magnitude of η% was recorded for 50 ppm phosphonic acid, 20 ppm Zn2+, and 250 ppm pectin at room temperature

[172]

Mixed-type Increased (but dominantly cathodic)

Decreased

Formation of protective polyacrylamide/polyacrylicgrafted pectin film on steel; a 95% magnitude of η% was recorded for 300 ppm pectin/ polyacrylamide at 30°C

[173]

Mixed-type (predominantly cathodic)

Increased

Characterization of the pectin inhibitor using FTIR, TGA, and SEM. Corrosion studies was conducted using potentiodynamic polarization and EIS techniques

(Continued)

Construction– Cosmetics

Corrosion Protection: Natural Polymer in 669

Table 1  Typical examples of ferrous alloy substrates reported in the literature for the anticorrosion applications involving various classes of natural polymers in various corrosive media (Continued) Trend of corrosion inhibition Type of metal with: substrate/ corrosion Inhibitor Inhibitor Corrosion monitoring Reason(s) for corrosion medium/ Type of inhibitor/ classification/ S. concentration Temperature technique deployed in the study inhibition Reference corrodent inhibitor system type No.

Construction– Cosmetics 43.

Mild steel/2 M H2SO4

Uninvestigated

Increased

Decreased

Weight loss, thermometric and hydrogen evolution techniques

Adsorption/electrostatic interaction of caboxymethyl cellulose with the charged mild steel surface via protonated COOH group. Adsorption was approximated by Langmuir and Dubinin–Radushkevich adsorption isotherm models; while the highest recorded inhibition efficiency in this study was 56.3% for 0.5 g/L CMC in 2 M H2SO4 at 30°C

[112]

44. Mild steel/1 M HCl

Na CMC

Mixed-type

Increased

Decreased

Weight loss techniques, potentiodynamic polarization, linear polarization resistance, and EIS

Adsorption on the surface of mild steel via inherent chemical groups; this was further approximated by Langmuir adsorption isotherm model

[113]

45.

CMC in combination with potassium halides (KCl, KBr, KI)

Uninvestigated

Increased

Decreased

Weight loss and hydrogen evolution techniques

Improved protection in the presence of KI was attributed to the adsorption of iodide ions prior to CMC at the mild steel surface (synergistic corrosion inhibition effect). Metal surface molecular adsorption was further explained by Langmuir adsorption isotherm model while the obtained magnitudes of η% for 0.5 g/L CMC without and with 5 mM KI were 56% and 85% at 30°C

[174]

Mild steel/2 M H2SO4

(Continued)

Corrosion Protection: Natural Polymer in

CMC

670

Table 1  Typical examples of ferrous alloy substrates reported in the literature for the anticorrosion applications involving various classes of natural polymers in various corrosive media (Continued) Trend of corrosion inhibition Type of metal with: substrate/ Inhibitor corrosion Inhibitor Corrosion monitoring Reason(s) for corrosion medium/ Type of inhibitor/ classification/ S. concentration Temperature technique deployed in the study inhibition Reference corrodent inhibitor system type No.

46. Mild steel/ neutral media with 60 ppm Cl−

CMC in combination with Mixed-type 1-hydroxyethanole-1,1diphosphonic acid–Zn2+ system

Increased

Decreased

Weight loss, potentiodynamic polarization, XRD, and FTIR techniques

47.

Hydroxypropyl cellulose [authors also studied glucose and gellan gum] in combination with KI

Mixed-type

Increased

Increased

Weight loss, potentiodynamic Improved protection in polarization, and EIS techniques; the presence of KI was XRD, FTIR, and SEM attributed to the adsorption of iodide ions prior to hydroxypropyl cellulose at the cast iron in 1 M HCl; an 89% magnitude of η% was recorded for 500 ppm at 30°C

[83]

Hydroxyethyl cellulose

Mixed-type

Increased

Decreased

EIS, potentiodynamic polarization, and electrochemical frequency modulation techniques; SEM/EDX

Metal surface molecular adsorption; this was approximated by Langmuir adsorption isotherm. The highest recorded magnitude of η% was 95% for 0.5 mM at 30°C

[176]

Mixed-type

Uninvestigated

Uninvestigated Weight loss and potentiodynamic Metal surface molecular polarization and EIS techniques adsorption; and the presence of KI aided the bridging of hydroxyethyl cellulose and the charged metal surface

Mixed-type

Uninvestigated

Uninvestigated Weight loss, potentiodynamic polarization, and EIS techniques

Cast iron/1 M HCl

48. Carbon steel (1018 grade)/3.5 wt% NaCl

49.

Mild steel/0.5 M Hydroxyethyl cellulose H2SO4 (in combination with KI)

50. Mild steel/1 and Hydroxyethyl cellulose 1.5 M HCl; 0.5 M H2SO4

Formation of protective inhibitor complex films on mild steel in the chloride solution. A combination of 10 ppm Zn2+ and 300 ppm hydroxyethanol-1,1diphosphonic acid recorded the highest magnitude of η% of about 40% and 80% on addition of 50 ppm CMC

Metal surface molecular adsorption of hydroxyethyl cellulose in the acid electrolytes

[175]

[115]

[177,178]

(Continued)

Construction– Cosmetics

Corrosion Protection: Natural Polymer in 671

Table 1  Typical examples of ferrous alloy substrates reported in the literature for the anticorrosion applications involving various classes of natural polymers in various corrosive media (Continued) Trend of corrosion inhibition Type of metal with: substrate/ corrosion Inhibitor Inhibitor Corrosion monitoring Reason(s) for corrosion medium/ Type of inhibitor/ classification/ S. concentration Temperature technique deployed in the study inhibition Reference corrodent inhibitor system type No.

Construction– Cosmetics Uninvestigated EIS technique; SEM Barrier properties increased with pH and the scratched surface healed due to surface release of cellulose nanofiber composites

The pH-controlled selfhealing properties of the protective polymer coatings was due to the surfacerelease presence of the cellulose nanofiber

[108,109]

52. Mild steel/0.5, 1 and 2 M H2SO4 and HCl

Gum arabic

Mixed-type

Increased

Decreased

Weight loss, hydrogen evolution, and potentiodynamic polarization techniques.

Formation of protective inhibitor film; this was further probed by SEM, FTIR, and XPS techniques. For 0.6 mg/L Gum arabic at 30°C, the recorded corrosion inhibition efficiency for mild steel corrosion was 17% and 76%, respectively, in 2 M H2SO4 and HCl.

[130]

53.

Gum arabic

Mixed-type

Increased

Decreased

Potentiodynamic polarization and EIS techniques.

The physical adsorption of gum arabic on steel surface; this was approximated by Langmuir adsorption isotherm. The maximum value of η% in this study was 92% for 2 g/L gum arabic

[179]

Gum arabic in combination with KCl, KBr, and KI

Uninvestigated

Increased

Increased

Weight loss, hydrogen evolution, and thermometric methods

Synergistic inhibition effect between gum arabic and KI molecules. Adsorption of gum arabic obeyed Temkin adsorption isotherm within the range of temperature under study

[180]

Cold-rolled steel/0.05 wt% NaCl

API 5L X42 pipeline steel/ HCl

54. Mild steel/0.1 M H2SO4

(Continued)

Corrosion Protection: Natural Polymer in

Polymer-type (doped Uninvestigated with cellulose nanofibers)

51.

672

Table 1  Typical examples of ferrous alloy substrates reported in the literature for the anticorrosion applications involving various classes of natural polymers in various corrosive media (Continued) Trend of corrosion inhibition Type of metal with: substrate/ corrosion Inhibitor Inhibitor Corrosion monitoring Reason(s) for corrosion medium/ Type of inhibitor/ classification/ S. concentration Temperature technique deployed in the study inhibition Reference corrodent inhibitor system type No.

55.

Carbon steel/1 M Guar gum H2SO4

Mixed-type

Increased

Uninvestigated Weight loss and potentiodynamic Adsorption of guar gum on polarization techniques steel; the presence of guar gum shifted pitting potential of steel to more positive values. This was further explained by means of adsorption isotherm model. A 93.9% corrosion inhibition efficiency was obtained for 1500 ppm guar gum at 30°C

[134]

56. Mild steel/1 M HCl

Polyacrylamide grafted guar gum with various grafting percentages

Mixed-type

Increased

Uninvestigated Potentiodynamic polarization and EIS techniques; FTIR spectroscopy

Formation of inhibiting polyacrylamide/guar gum polymer layer on mild steel; this was also found to follow Langmuir adsorption isotherm

[135]

57.

Polyacrylamide grafted xanthan gum

Mixed-type

Increased

Decreased

SEM analysis revealed the presence of protection inhibitor film on mild steel in the presence of xanthan gum. Adsorption of xanthan gum obeyed Langmuir adsorption isotherm. 0.4 g/L xanthan gum recorded a corrosion inhibition efficiency of 92.8% and 98.1% after being grafted with polyacrylamide

[140]

Mild steel/15% HCl

Weight loss, potentiodynamic polarization, and EIS techniques

(Continued)

Construction– Cosmetics

Corrosion Protection: Natural Polymer in 673

Table 1  Typical examples of ferrous alloy substrates reported in the literature for the anticorrosion applications involving various classes of natural polymers in various corrosive media (Continued) Trend of corrosion inhibition Type of metal with: substrate/ corrosion Inhibitor Inhibitor Corrosion monitoring Reason(s) for corrosion medium/ Type of inhibitor/ classification/ S. concentration Temperature technique deployed in the study inhibition Reference corrodent inhibitor system type No.

Construction– Cosmetics Xanthan gum (in combination with synergistic surfactant additives)

Mixed-type

Increased

Decreased

Weight loss, potentiodynamic polarization, and EIS techniques; UV–visible spectroscopy for surface analysis

Formation of protective layer on mild steel in the presence of xanthan gum and xanthan gum-surfactants composite. Molecular adsorption obeyed Langmuir adsorption isotherm. The highest magnitude of η% (74.24%) was obtained for 1000 ppm xanthan in combination with Triton X-100 at 30°C

[139]

59.

Exudate oleo-gum extracts from F. assa-foetida and D. ammoniacum

Mixed-type

Increased

Decreased

Weight loss, potentiodynamic polarization, and EIS techniques

Gum adsorption on steel in 2 M HCl corrodent; adsorption was found to obey Langmuir adsorption isotherm and SEM was deployed to study the film formation at room temperature. Maximum values of inhibition efficiency up to 91% and 96% were recorded for 0.8 g/L gum from D. ammoniacum and F. assafoetida, respectively

[145]

Oleo-gum extracted F. gummosa

Mixed-type

Increased

Unchanged

Weight loss, potentiodynamic polarization, and EIS techniques

Adsorption of the phytoconstituents of the gum extract on stainless steel; adsorption was found to obey Temkin adsorption isotherm; 1000 ppm gum extract inhibited corrosion of stainless steel up to 90.7% at room temperature

[146]

Mild steel/2 M HCl

60. 304 stainless steel/2 M HCl

(Continued)

Corrosion Protection: Natural Polymer in

58. Mild steel/1 M HCl

674

Table 1  Typical examples of ferrous alloy substrates reported in the literature for the anticorrosion applications involving various classes of natural polymers in various corrosive media (Continued) Trend of corrosion inhibition Type of metal with: substrate/ corrosion Inhibitor Inhibitor Corrosion monitoring Reason(s) for corrosion medium/ Type of inhibitor/ classification/ S. concentration Temperature technique deployed in the study inhibition Reference corrodent inhibitor system type No.

61.

Mild steel/1 M H2SO4

62. Mild steel/1 M H2SO4

63.

Albizia zygia gum

Uninvestigated

Increased

Decreased

Weight loss, hydrogen evolution, and thermometric techniques

Same as Behpour et al. (2009); adsorption was further probed by SEM analysis; 87% inhibition efficiency obtained for mild steel corrosion by 0.5 g/L gum extract at 30°C.

[181]

Ficus glumosa gum

Uninvestigated

Increased

Increased

Weight loss, hydrogen evolution, and thermometric technique; SEM

Same as Behpour et al. (2009); adsorption followed Langmuir adsorption model; 71.11% inhibition efficiency obtained for mild steel corrosion by 0.5 g/L gum extract at 60°C

[182]

Uninvestigated

Increased

Decreased

Weight loss technique.

Same as Behpour et al. (2009); adsorption was further probed by FTIR spectroscopic analysis; 78% inhibition efficiency obtained for zinc corrosion by 0.5 g/L gum extract at 30°C. Adsorption of gum obeyed Langmuir adsorption model

[183]

Mild Steel/0.1 M Ficus platyphylla gum HCl

Construction– Cosmetics

Corrosion Protection: Natural Polymer in 675

Table 1  Typical examples of ferrous alloy substrates reported in the literature for the anticorrosion applications involving various classes of natural polymers in various corrosive media (Continued) Trend of corrosion inhibition Type of metal with: substrate/ corrosion Inhibitor Inhibitor Corrosion monitoring Reason(s) for corrosion medium/ Type of inhibitor/ classification/ S. concentration Temperature technique deployed in the study inhibition Reference corrodent inhibitor system type No.

Construction– Cosmetics 676

Table 2  Typical examples of aluminum and aluminum alloy substrates reported in the literature for the anticorrosion applications involving various classes of natural polymers in various corrosive media Trend of corrosion inhibition Corrosion Type of metal with: monitoring substrate/ Inhibitor Inhibitor technique deployed corrosion Type of inhibitor/ classification/ S. concentration Temperature in the study Reason(s) for corrosion inhibition Reference inhibitor system type No. medium/corrodent Formation of protective film on the metal surface; this was probed by SEM analyses

[184]

2-Mercaptobenzothiazole, Uninvestigated Uninvestigated poly(ethylene-alt-maleic anhydride) and poly(maleic anhydride-alt-1-octadecene) modified chitosan coating

Uninvestigated Water contact angle Surface hydrophobicity after bulk measurement, FTIR coating modification (up to contact angle of 140°) and EIS; SEM analysis

[77]

Aluminum alloy 2024/0.05 M NaCl

Cerium nitrate doped Uninvestigated Uninvestigated chitosan-based self-healing protective coating

Uninvestigated Proton NMR and FTIR characterization of the coating; EIS was deployed for the corrosion studies

4.

Aluminum alloy 2024/0.05 M NaCl

Cerium doped chitosan protective coating

Uninvestigated Uninvestigated

Uninvestigated EIS, SEM/EDAX, Self-healing properties of cerium optical micrograph, ions in the chitosan coating and scanning vibrating electrode technique mapping; FTIR spectroscopy

5.

Aluminum/0.1 M HCl

Chitosan

Mixed-type

Uninvestigated Potentiodynamic polarization and EIS techniques; IR and UV–visible spectroscopies.

1.

Aluminum alloy 2024/1 M HCl

Nonionic amphiphilic chitosan nanoparticles

2.

AA2024-T3/ 50 mM NaCl

3.

Mixed-type

Increased

The improved barrier and selfhealing properties of the coating was attributed to the presence of cerium nitrate. Authors did not probed the surface morphology by SEM but presented photograph images of the immersed surfaces of the coating. The recorded coating resistance ranged between 103 and 105 MΩ between the initial hours of immersion and the 10th day

Formation of protective inhibitor film on aluminum in 0.1 M HCl; the highest recorded value of η% was 90% for 0.028 g/L chitosan at room temperature

[185]

[186]

[187]

(Continued)

Corrosion Protection: Natural Polymer in

Increased

Uninvestigated Proton NMR, FTIR, SEM, EIS, and potentiodynamic polarization techniques

Decreased Efficiency decreased with the concentration of alkali but increased with concentration of agar-agar

Weight loss technique

Sorption of the inhibitor on the aluminum surface; η% = 64% was recorded for 1.5% agar-agar

[101]

Decreased

Weight loss technique

Molecular adsorption on metal surface; values of η% up to 43% was observed for 1 g/L at room temperature

[100]

Decreased

Weight loss and polarization techniques

Adsorption of agar on the alloy; 1% agar and gelatin exhibited 64.3% and 56% inhibition efficiencies, respectively, for aluminum corrosion in 0.1 M NaOH

[102]

Uninvestigated Efficiency decreased with the concentration of alkali but with concentration of gelatin

Decreased

Weight loss technique

Absorption of molecular gelatin on the aluminum surface; at room temperature, 1.5% gelatin recorded values of η% up to 39%

[101]

Gelatin

Uninvestigated Increased

Decreased

Weight loss technique

Gelatin adsorption on the metal in 0.3 M NaOH; values of η% up to 41% was recorded for 1 g/L gelatin

[100]

Aqueous extracts of Moringa oleifera, Terminalia arjuna, and Mangifera indica

Mixed-type

Increased

Weight loss, potentiodynamic polarization, and EIS techniques; SEM and AFM

Adsorption of phytoconstituents [principally 4-(alpha-lrhamnopyranosyloxy)benzylglucosinolate]/formation of protective film on the metal surface; the highest inhibition efficiency (85.3%) for aluminum was recorded in the presence of 0.6 g/L at M. oleifera at 300 K. Metal surface adsorption was approximated by Langmuir adsorption isotherm model

[188]

6.

B26S aluminum/1 M NaOH

Agar-agar

Mixed-type

7.

Aluminum (M578)/0.3 M NaOH

Agar-agar

Uninvestigated Increased

8.

Aluminum copper alloy/0.1 M NaOH

Agar-agar and dextrin

Mixed-type

9.

B26S aluminum/ 0.1 M NaOH

Gelatin

10.

Aluminum (28, 38 and M57 8)/0.3 M NaOH

11.

Aluminum alloy/ 1 M NaOH

Increased

Increased

(Continued)

Construction– Cosmetics

Corrosion Protection: Natural Polymer in 677

Table 2  Typical examples of aluminum and aluminum alloy substrates reported in the literature for the anticorrosion applications involving various classes of natural polymers in various corrosive media (Continued) Trend of corrosion inhibition Corrosion Type of metal with: monitoring substrate/ Inhibitor Inhibitor technique deployed corrosion Type of inhibitor/ classification/ S. concentration Temperature in the study Reason(s) for corrosion inhibition Reference inhibitor system type No. medium/corrodent

Construction– Cosmetics [189]

Uninvestigated Potentiodynamic polarization and weight loss techniques

Formation of protective gelatin film which was confirmed by SEM analysis of aluminum surface; the highest recorded magnitude of η% was 61.74% for 0.6% gelatin in 0.1 N HCl

[190]

Uninvestigated Inhibition efficiency decreased with concentration of alkali but not with gelatin concentration

Decreased

Weight loss technique

Adsorption of molecules of dextrin on aluminum surface; the highest recorded magnitude of η% was 47% for 1.5% dextrin in 0.1 M NaOH

[101]

Uninvestigated Increased

Decreased

Weight loss technique

Adsorption of molecules of dextrin on the aluminum surface; 1 g/L dextrin recorded 8.7% magnitude of η% for aluminum in 0.3 M NaOH

[100]

Decreased

Weight loss, potentiodynamic polarization and EIS techniques

Adsorption of phytoconstituents (mostly sugars and phytoproteins) on the surface of the alloy; 500 ppm extract recorded 50.54% magnitude of η% for aluminum in 1 M phosphoric acid. Metal surface adsorption was approximated by Langmuir adsorption isotherm model and further analysis was conducted with SEM and FTIR spectroscopy

[191]

Mixed-type

Increased

Decreased

13.

Aluminum/0.1N HCl

Gelatin

Mixed-type

Increased

14.

B26S aluminum/0.1 M NaOH

Dextrin (47%)

15.

Aluminum (28, 38 and M57 8)/0.3 M NaOH

Dextrin

16.

Aluminum alloy/1 M phosphoric acid

Aqueous extract of seeds of Mixed-type Coriandrum sativum L.

Increased

Potentiostat polarization, EIS, cyclic voltammetry, and potentiodynamic anodic polarization techniques

(Continued)

Corrosion Protection: Natural Polymer in

Adsorption of gelatin was approximated by Freundlich isotherm; the obtained magnitude of η% for Al/Si and Si were 65.22% and 60.00% for 2000 ppm at 30°C. Gelatin adsorption exhibited lower affinity for silicon compared to aluminum

12. Aluminum/silicon Gelatin alloys/0.1 M NaOH

678

Table 2  Typical examples of aluminum and aluminum alloy substrates reported in the literature for the anticorrosion applications involving various classes of natural polymers in various corrosive media (Continued) Trend of corrosion inhibition Corrosion Type of metal with: monitoring substrate/ Inhibitor Inhibitor technique deployed corrosion Type of inhibitor/ classification/ S. concentration Temperature in the study Reason(s) for corrosion inhibition Reference inhibitor system type No. medium/corrodent

Enhanced corrosion resistance of the coatings after the adsorption of albumin/alginate

[123]

Uninvestigated EIS technique. Surface analyses followed electrochemical corrosion evaluation using XRD, XPS, TGA, DSC, and IR spectrophotometry

Molecular adsorption of starch on the aluminum in the alkaline corrodent was enhanced after grafting the polysaccharide chain with polyorganosiloxane

[192]

Uninvestigated Uninvestigated

Uninvestigated EIS, FTIR, and XPS techniques; salt-spray test.

Adsorption of starch molecules/ formation of cerium-bridged carboxylate complexes on aluminum surface

[193]

Increased Mixedtype (but predominantly anodic)

Uninvestigated Weight loss, potentiodynamic polarization, linear polarization resistance, and EIS techniques

Corrosion inhibition was attributed to the precipitation of tapioca starch on the surface of aluminum. Adsorption was approximated by Langmuir adsorption isotherm. The highest recorded inhibition efficiency in this study was 91.4% for 1000 ppm starch in artificial water

[89]

Aluminum coating/E. coli induced microbiologically influenced corrosion (MIC) in artificial seawater

Albumin (in combination with alginate)

18.

Aluminum/an alkaline solution consisting of 0.4 wt% NaOH, 2.8 wt% tetrasodium pyrophosphate, 2.8 wt% sodium bicarbonate, and 94.0 wt% water.

Polyorganosiloxane-grafted Uninvestigated Uninvestigated starch (sourced from Ipomoea batatas) coatings

19.

6061–T6 aluminum/neutral solution

Potato starch doped with cerium (IV) ammonium nitrate Tapioca starch

20. AA6061 alloy/ artificial seawater

Mixed-type

Uninvestigated Potentiodynamic Albumin polarization adsorption technique; increased corrosion FTIR/AFM/ inhibition Field Emission Scanning Electron Microscopy (FESEM) techniques.

17.

(Continued)

Construction– Cosmetics

Corrosion Protection: Natural Polymer in 679

Table 2  Typical examples of aluminum and aluminum alloy substrates reported in the literature for the anticorrosion applications involving various classes of natural polymers in various corrosive media (Continued) Trend of corrosion inhibition Corrosion Type of metal with: monitoring substrate/ Inhibitor Inhibitor technique deployed corrosion Type of inhibitor/ classification/ S. concentration Temperature in the study Reason(s) for corrosion inhibition Reference inhibitor system type No. medium/corrodent

Construction– Cosmetics [93]

Same as Zaafarany[93]

[94]

Uninvestigated Potentiodynamic Uninvestigated Alginate polarization adsorption technique increased corrosion inhibition

Molecular adsorption of protective alginate and albumin films on aluminum. Metal surface adsorption was confirmed with FTIR, AFM, and FESEM analyses

[123]

ι-carrageenan

Uninvestigated Increased

Increased; then gradually dropped

Weight loss technique; SEM

A combined molecular adsorption between carrageenan and pefloxacin; metal surface adsorption was further approximated by Langmuir isotherm model

[97]

25. Aluminum/4 M NaOH

Pectate (and alginate) water-soluble natural polymer anionic polyelectrolytes

Uninvestigated Increased

Decreased

Weight loss and hydrogen evolution techniques

Molecular adsorption of protective pectate and alginate films on aluminum in 4 M NaOH

[93]

26. Aluminum/4 M NaOH

Sodium pectate (in combination with sodium alginate)

Uninvestigated Uninvestigated

Uninvestigated Weight loss and hydrogen evolution techniques

Same as Zaafarany[93]

[94]

27.

Water-soluble natural Polymeric Pectate

Uninvestigated Increased

decreased

Formation of protective pectate film on aluminum in 1 M HCl; molecular adsorption was further approximated by Freundlich isotherm model. The magnitude of η% up to 81.67% was recorded for 1.2 wt% pectate at room temperature

[194]

Uninvestigated Increased

Decreased

22. Aluminum/4 M NaOH

Sodium alginate (in combination with sodium pectate)

Uninvestigated Uninvestigated

Uninvestigated Weight loss and hydrogen evolution techniques

23. Aluminum coating /E. coli induced MIC in artificial seawater

Alginate (in combination with albumin)

24. Aluminum/2 M HCl

Aluminum/4 M NaOH

Aluminum/1 M HCl

Weight loss and hydrogen evolution techniques

Weight loss and hydrogen evolution techniques

(Continued)

Corrosion Protection: Natural Polymer in

Metal surface adsorption via hydroxyl groups of the alginate/ pectate macromolecules; the polyelectrolytes acted as bridgeformers between alginate and the aluminum surface

Alginate (and pectate) water-soluble natural polymer anionic polyelectrolytes

21.

680

Table 2  Typical examples of aluminum and aluminum alloy substrates reported in the literature for the anticorrosion applications involving various classes of natural polymers in various corrosive media (Continued) Trend of corrosion inhibition Corrosion Type of metal with: monitoring Inhibitor substrate/ Inhibitor technique deployed corrosion Type of inhibitor/ classification/ S. concentration Temperature in the study Reason(s) for corrosion inhibition Reference inhibitor system type No. medium/corrodent

28. Aluminum/2 M HCl

29.

Pectin extracted from citrus peel (average molecular weight 30–100 kg/mol and degree of esterification = 60%)

Aluminum and Methyl cellulose aluminum silicon alloys/0.1 M NaOH

Uninvestigated Increased

Decreased

Weight loss and SEM techniques

Adsorption of pectin macromolecules on aluminum surface; this was approximated by Langmuir isotherm model. 8.0 g/L pectin recorded values of corrosion inhibition efficiency up to 91% and 31% at 10°C and 40°C, respectively

[117]

Anodic

Decreased

Potentiostatic polarization, EIS cyclic voltammetry, and potentiodynamic anodic polarization techniques

Metal surface molecular adsorption of methyl cellulose on the alloys; this was approximated by Langmuir adsorption isotherm. The highest recorded magnitude of η% was 61% for 2000 ppm methyl cellulose at 30°C for aluminum corrosion alone

[195]

Formation of protective and stable adsorbed layer on the metal substrates. The obtained values of corrosion inhibition efficiency for aluminum and mild steel were 70% and 43%, respectively for 0.5 g/L at 30°C. Metal surface adsorption was further approximated with Temkin adsorption isotherm/El-Awady et al. thermodynamic–kinetic adsorption isotherm models

[129]

Increased

30. Aluminum and mild steel/0.1 M H2SO4

Gum arabic

Uninvestigated Increased for both metal substrates

Decreased for aluminum but not for mild steel

Weight loss and thermometric techniques

31.

Gum arabic

Uninvestigated Increased

Decreased

Hydrogen evolution Molecular adsorption of and thermometric polysaccharides and glucoprotein techniques constituents of the gum on aluminum; adsorption was further approximated by Langmuir and Freundlich adsorption isotherms

Aluminum/1 M NaOH

[196]

(Continued)

Construction– Cosmetics

Corrosion Protection: Natural Polymer in 681

Table 2  Typical examples of aluminum and aluminum alloy substrates reported in the literature for the anticorrosion applications involving various classes of natural polymers in various corrosive media (Continued) Trend of corrosion inhibition Corrosion Type of metal with: monitoring substrate/ Inhibitor Inhibitor technique deployed corrosion Type of inhibitor/ classification/ S. concentration Temperature in the study Reason(s) for corrosion inhibition Reference inhibitor system type No. medium/corrodent

Construction– Cosmetics 682

Table 2  Typical examples of aluminum and aluminum alloy substrates reported in the literature for the anticorrosion applications involving various classes of natural polymers in various corrosive media (Continued) Trend of corrosion inhibition Corrosion Type of metal with: monitoring substrate/ Inhibitor Inhibitor technique deployed corrosion Type of inhibitor/ classification/ S. concentration Temperature in the study Reason(s) for corrosion inhibition Reference inhibitor system type No. medium/corrodent 32. Aluminum/0.1 M HCl

Ficus sycomorus gum

Uninvestigated Increased

Decreased

Weight loss technique

Same as Behpour et al. (2009); adsorption followed Langmuir adsorption isotherm with 80% inhibition efficiency obtained for aluminum corrosion by 0.5 g/L gum extract at 30°C

[197]

33.

Ficus benjamina gum

Uninvestigated Increased

Decreased

Weight loss technique

Same as Behpour et al. (2009); adsorption followed Frumkin and Dubinin–Radushkevich adsorption models with 87% inhibition efficiency obtained for aluminum corrosion by 0.5 g/L gum extract at 30°C

[198]

Uninvestigated Increased

Decreased

Hydrogen evolution Same as Behpour et al. (2009); and thermometric adsorption of gum obeyed Langmuir techniques and Frumkin adsorption model; 73% inhibition efficiency obtained for aluminum corrosion by 0.5 g/L gum extract at 30°C

Aluminum/0.1 M HCl

34. AA 3001 aluminum Commiphora pedunculata gum alloy/0.1 M HCl

[199] Corrosion Protection: Natural Polymer in

Uninvestigated Weight loss, EIS potentiodynamic polarization, and electrochemical frequency modulation; Quantum chemical calculations

Molecular adsorption/the presence of protective film on copper; this was probed by SEM and FTIR spectroscopy. Metal surface adsorption was approximated by Langmuir adsorption isotherm model and a 93.0% magnitude of η% was recorded for 8 ppm at room temperature

[200]

Corrosion protection increased with thiazole modification

Uninvestigated Potentiodynamic polarization and EIS techniques; Field emission SEM/EDX

The improved barrier properties of the coating in the presence of thiazole compound was attributed to the self-healing of thiazole compound

[201]

Uninvestigated

Increased

Decreased

Molecular adsorption of gelatin on the surfaces of copper and brass

[202]

Gelatin

Uninvestigated

Uninvestigated Uninvestigated Voltammetric studies and electrochemical quartz crystal microbalance) and SEM

Formation of protective film confirmed by XPS, polarization modulation-infrared reflectionadsorption spectroscopy and cyclic voltammetry

[203]

BSA

Uninvestigated

Uninvestigated Cyclic voltammetry Corrosion and EIS techniques inhibition increased in the presence of 1 g/L BSA

Albumin protein interfacial resistance was also dependent on the applied potential and the characteristics of the inhibitive oxide film on copper substrate. An ultrasound irradiation procedure was subsequently deployed to remove the copper oxide/albumin film from the surface of the substrate

[204]

1.

Copper/0.5 M HCl

2.

Copper/3.5 wt% 2-Mercaptobenzothiazole Mixed-type behavior (only in the presence of NaCl encapsulated with chitosan/11-alkanethiolate Mercaptobenzothiazole) acid coating

3.

Copper and Gelatin brass/5% H2SO4

4.

Copper and copper–tin/ H2SO4

5.

Copper/0.1 M phosphate buffer solution pH 7

Chitosan

Mixed-type inhibitor Increased (appears more cathodic)

Weight loss technique

(Continued)

Construction– Cosmetics

Corrosion Protection: Natural Polymer in 683

Table 3  Typical examples of copper, tin, and cadmium (and their alloys) substrates reported in the literature for the anticorrosion applications involving various classes of natural polymers in various corrosive media Trend of corrosion inhibition Type of metal with: Corrosion substrate/ monitoring corrosion Inhibitor technique deployed Reason(s) for corrosion medium/ Type of inhibitor/ Inhibitor classification/ S. concentration Temperature in the study inhibition Reference corrodent inhibitor system type No.

Construction– Cosmetics 684

Table 3  Typical examples of copper, tin, and cadmium (and their alloys) substrates reported in the literature for the anticorrosion applications involving various classes of natural polymers in various corrosive media (Continued) Trend of corrosion inhibition Type of metal with: Corrosion substrate/ monitoring corrosion Inhibitor technique deployed Reason(s) for corrosion medium/ Type of inhibitor/ Inhibitor classification/ S. concentration Temperature in the study inhibition Reference corrodent inhibitor system type No. Pectin extracted from tomato waste

6.

Tin/2% NaCl, 1% acetic acid, and 0.5% citric acid

7.

Cadmium disc CMC electrode/0.5 M HCl

Increased

Uninvestigated Characterization of the pectin inhibitor using NMR, FTIR spectroscopic techniques; corrosion studies was conducted using EIS and potentiodynamic polarization techniques

Pectin adsorption on the metal surface; the highest recorded value of η% in this study was 73% for 4 g/L at 25°C

[205]

Mixed-type (but predominantly cathodic at higher concentrations of CMC)

Increased

Uninvestigated Potentiodynamic Metal surface molecular adsorption polarization and EIS on cadmium electrode in 0.5 M HCl; this was further explained techniques by Temkin adsorption isotherm. The highest magnitude of η% in this study was recorded for 0.2 g/L CMC at 30°C (62.5%)

[114]

Corrosion Protection: Natural Polymer in

Mixed-type (but dominantly cathodic)

1.

Chitosan AZ31E and AZ91E magnesium alloys/ synthetic sweat medium

Mixed-type

Increased

Uninvestigated Potentiodynamic polarization and EIS techniques; SEM.

Corrosion inhibition was attributed to the formation of protective film on the magnesium substrate

[206]

2.

Mg–Ca alloy/0.9 wt% NaCl

BSA

Cathodic

Increased

Uninvestigated EIS, hydrogen evolution, and potentiodynamic polarization techniques

Synergistic effect of the negatively charged adsorbed albumin molecules; albumin adsorption at the surface of the alloy was confirmed by FTIR spectroscopy

[207]

3.

Plasma electrolytic oxidation coated magnesium/0.9 wt% NaCl

Albumin

Cathodic

Addition of albumin improved corrosion inhibition

Uninvestigated Potentiodynamic polarization and EIS techniques

Molecular albumin adsorption/formation of protective albumin film at the metal surface; this was confirmed by SEM/EDX and XPS analyses. Corrosion inhibition was attributed to the synergistic effect of protein adsorption and precipitation of insoluble salts (from the NaCl corrodent)

[208]

4.

AZ31 alloy grade/3.5 wt% NaCl

Sodium alginate

Mixed-type (but Increased predominantly anodic)

Decreased

Formation of compact protective alginate film on the surface of the alloy (this was probed by SEM/EDX and FTIR spectroscopy; the maximum values of corrosion inhibition efficiency of 90% was recorded for 500 ppm alginate at room temperature

[209]

5.

Pure magnesium/ 0.2–0.8 wt% NaCl solution.

Albumin

Anodic

Increased

Uninvestigated EIS and Adsorption of albumin on magnesium in potentiodynamic NaCl solution polarization techniques

[210]

6.

Zinc/0.5 and 1 N NaOH Gelatin

Anodic

Increased

Uninvestigated Galvanostatic technique Deposition of ZnO/formation of protective film and this was confirmed by SEM analysis; adsorption was approximated by Langmuir isotherm

[211]

Weight-loss, potentiodynamic polarization and EIS techniques; SEM/EDS and FTIR spectroscopy

(Continued)

Construction– Cosmetics

Corrosion Protection: Natural Polymer in 685

Table 4  Typical examples of magnesium and zinc (and their alloys) substrates reported in the literature for the anticorrosion applications involving various classes of natural polymers in various corrosive media Trend of corrosion inhibition Type of with: Corrosion monitoring inhibitor/ Inhibitor Type of metal Inhibitor technique deployed in inhibitor classification/ S. substrate/corrosion concentration Temperature the study Reason(s) for corrosion inhibition Referemce system type No. medium/corrodent

Construction– Cosmetics 686

Table 4  Typical examples of magnesium and zinc (and their alloys) substrates reported in the literature for the anticorrosion applications involving various classes of natural polymers in various corrosive media (Continued) Trend of corrosion inhibition Type of with: Corrosion monitoring inhibitor/ Inhibitor Type of metal Inhibitor technique deployed in inhibitor classification/ S. substrate/corrosion concentration Temperature the study Reason(s) for corrosion inhibition Referemce system type No. medium/corrodent Zinc electroplated on mild Steel/3 M HCl

Uninvestigated Dextrin (in combination with Thiourea additives)

Increased

Uninvestigated SEM analysis and surface adhesion tests

Synergistic corrosion inhibition between dextrin and the thiourea additive; SEM/ EDS was deployed to probe metal surface adsorption after immersion in the inhibitor/ HCl solution

[212]

8.

Cylindrical zinc substrate (zinc-carbon battery/26% NH4Cl

Hydroxyethyl cellulose

Mixed-type

Increased

Uninvestigated Potentiodynamic polarization and EIS techniques; FTIR and SEM

Inhibitor molecular adsorption on zinc surface; this was further approximated by Langmuir adsorption isotherm. The highest recorded magnitude of η% was 92.0% for 300 ppm hydroxyethyl cellulose at 25°C

[213]

9.

Zinc (A72357 grade)/0.1 M H2SO4

Acacia sieberiana gum

Uninvestigated

Increased

Decreased

Same as Behpour et al. (2009); adsorption was further probed by SEM analysis; 80% inhibition efficiency is obtained for zinc corrosion by 0.5 g/L gum extract at room temperature. Adsorption of gum obeyed Frumkin adsorption model

[214]

Weight loss, hydrogen evolution, and thermometric technique; SEM

Corrosion Protection: Natural Polymer in

7.

1.

CoCrMo alloy/0.14 M NaCl

BSA

2.

CoCrMo BSA biomedical alloy/ PBS

3.

AISI 316L, Co–28Cr–6Mo, and Ti–6Al–4V alloys/PBS solutions

4.

Albumin adsorption on the surface of the alloy after immersion in 0.14 M NaCl; adsorption was approximated by Langmuir adsorption isotherm model

[215]

Mixed-type

Increased

Decreased

EIS technique was used to investigate the interfacial behavior of BSA at open circuit potential (OCP); potentiodynamic polarization technique

Inconclusive

Increased with immersion period

Uninvestigated

Albumin adsorption on the metal EIS technique was used to investigate the interfacial behavior surface of BSA; potentiodynamic polarization technique

BSA

Anodic

Increased

Uninvestigated

OCP, EIS, potentiodynamic polarization, linear polarization resistance, X-ray photoelectron spectroscopy techniques

The presence of albumin initiated the formation of stable oxide layer growth/ protein adsorbed films on the surfaces of the alloys

[217]

Niobium/PBS solution

BSA

Cathodic

Increased

Uninvestigated

OCP, EIS, polarization resistance techniques

Adsorption of albumin on the metal surface (this was confirmed by XPS and SEM techniques)

[122]

5.

Titanium/ phosphate buffer saline solution (at pH 7.4)

BSA (in combination with fibrinogen)

Anodic

Albumin adsorption increased corrosion inhibition

Uninvestigated

Potentiodynamic polarization and EIS techniques

The presence of BSA gradually shifted the OCP in negative values; this is attributed to the chelating effect by the albumin protein on the dissolution process thereby inhibiting corrosion protection

[218]

6.

Ti-6Al-4V alloy/1 M NaCl

BSA and egg albumin.

Anodic

Increased

Uninvestigated

Potentiodynamic polarization and potentiodynamic cyclic anodic polarization techniques

Formation of protective albumin films on alloy surface (this was confirmed by SEM analysis)

[219]

7.

CoCrMo alloy surfaces/PBS, with 0.8g NaCl, 0.2 g KCl, 1.44 g Na2HPO4, and 0.24 g KH2PO4 per liter.

BSA

Anodic at lower pH

Uninvestigated

Uninvestigated

Cyclic potentiodynamic polarization technique

Adsorption of albumin and subsequent formation of monolayer albumin film on the CoCrMo alloy surface; metal adsorption was confirmed by AFM and surface kelvin potential force microscopy analyses

[220]

Construction– Cosmetics

[216]

Corrosion Protection: Natural Polymer in 687

Table 5  Typical examples of other forms of alloys reported in the literature for the anticorrosion applications involving various classes of natural polymers in various corrosive media Trend of corrosion inhibition Type of metal with: Type of substrate/ Inhibitor inhibitor/ corrosion Inhibitor Corrosion monitoring technique inhibitor classification/ medium/ S. concentration Temperature deployed in the study Reason(s) for corrosion inhibition Reference system type corrodent No.

688

Corrosion Protection: Natural Polymer in

(at 1.5%–3% PANI) demonstrated significant protection as revealed in the salt spray/immersion tests, with improved self-healing ability compared to PANI/benzene–sulfonate coating. The corrosion test was conducted by electrochemical impedance spectroscopy (EIS), potentiodynamic polarization, and visual examination of exposed surfaced in the solution of the c­ orrosive electrolyte.[65] Polysaccharide Biopolymers Sugar-type polymers with monosaccharide units ranging from oligosaccharide molecular sizes to long chains of glycosidic-bonded compounds are classified as polysaccharides. These macromolecules could be reclassified based on their chemical properties normally controlled by the inherent monosaccharide building blocks of their molecules, similar or varying repeat units for homoglycans and heteroglycans, respectively. Some are found in nature though they can also be synthesized. Polysaccharide biopolymers are found in microorganisms and in cell walls of both plants and animals. Because of their enormous applications, polysaccharides are very much in demand in food, biomedical, pharmaceutical, and other industrial purposes.[66] The annual production in 2009 alone in the United States and Europe reached about $1 billion for only seaweed-derived polysaccharides (not limited to alginates and carrageenans), which are some of the most used polysaccharides polymers [66] with almost 40% sales increase compared to the past decade.[67] In corrosion protection, the use of polysaccharide biopolymers is slowly gaining popularity in the formulation of greener and benign ­inhibitor composites for metal surfaces, ­linings, and as coatings. Chitosans (Poly-(d)glucosamine) Construction– Cosmetics

Chitosan may be found in most shelled sea animals (e.g., shellfish and shrimps) compared to other seabed-­dueling marine animals. Chitosan is a polysaccharide whose molecule is composed of acetylated and deacetylated ­d-glucosamine groups within its molecular chain (Fig. 8). The versatility of chitosan for most industrial applications is linked with its unique film-forming and superior adhesion ability and other physicochemical properties (e.g., biocompatibility, antibacterial activity). This polymer is also easily functionalized chemically to industrial products with robust properties; hence it serves as a precursor for most fine chemicals.[68] Like any other polymer with OH HO HO

O NH2

OH O HO

O NH2

OH O HO n

O OH NH2

Fig. 8  Molecular structure of chitosan (composed of acetylated and deacetylated d-glucosamine groups)

anticorrosion property, the ability for chitosan to inhibit metal corrosion is drawn from this electronic and molecular structure; the lone pairs of electrons of the amino and hydroxyl functional groups (on the d-glucosamine moiety) on chitosan readily form coordinate bonds with iron (Fe) from industrial steel. Chitosan has been deployed as a corrosion inhibitor and protective coating for both alloyed and nonalloyed metal substrates in various media ranging from acid, alkali, to saline media. Recently, Sangeetha et al.[69] have reported the anticorrosion properties of O-fumaryl-chitosan in 1 M HCl  for mild steel using chemical (weight loss) and electrochemical (EIS and polarization methods) techniques. ­O-Fumaryl-chitosan was synthesized using 1 g chitosan as the precursor; it was suspended in distilled water at room temperature with 5 g fumaric acid before 2 M H2SO4 was added at 80°C. The functionalized chitosan s­ uspension was allowed to cool at room temperature before acetone-­ assisted precipitation and subsequently, the H2SO4 residue was washed off using ethanol and dried. The modified chitosan prepared was readily soluble, and used for the corrosion studies. It was found to reduced mild steel corrosion by 93% at 500 ppm, and the corrosion inhibition efficiency (%η) was found to increase with the concentration of the fumaryl derivative chitosan inhibitor. The presence of this compound affected both anodic and cathodic reactions as revealed by results of the polarization studies. Corrosion inhibition of steel was attributed to the formation and adsorption of fumaryl chitosan film on the surface of the mild steel substrate; and this phenomenon was approximated by Langmuir adsorption isotherm. In another work using the same steel grade in the acid electrolyte, [70] these authors have also studied the effectiveness of another modified chitosan (N-(2-hydroxy-3-trimethyl ammonium) propyl chitosan chloride). The magnitude of %η was also found to be concentration dependent, while 500 ppm of this compound recorded more than 98% corrosion reduction efficiency in the solution of the acid at room ­temperature. The proposed mechanism of molecular adsorption at the mild steel electrode surface is displayed in Fig. 9; adsorption of chitosan molecules is via nitrogen atom (lone electron pairs used for coordinate bond formation with Fe) of the glucosamine moiety and positive charge on RNR′3 group attracted by the negatively charged electrode surface. Recent works on the use of various forms of ­chitosan (including pure forms) as corrosion inhibitors and ­protective coatings focus on magnesium, [71,72] low carbon/­ stainless steel, [73–76] and ­aluminum, [77–79] respectively. Agarose (Agar) This polysaccharide biopolymer is made up of galactopyranose and d-galactose units covalently joined by α(1→3) and β(1→4) glycosidic bonds, and it is found predominantly in red and brown seaweeds though can be obtained from agar by simply extracting the agaropectin component

Corrosion Protection: Natural Polymer in 689

Gellan O

O

O HO

NH H3C H3C

Fe

Fe

OH

+

N Cl–

CH3

Mild steel

Fe

Fe

Fig. 9  Mechanism of N-(2-hydroxy-3-trimethyl ammonium) propyl chitosan chloride adsorption/corrosion inhibition on mild steel Source: Reprinted with permission from Elsevier, © 2016.[70]

(https://en.wikipedia.org/wiki/Agarose). The molecular structure of agarose is presented in Fig. 10. It is widely employed for chemical derivatization due to its hydroxyl group density and the molecule is nonionic. Agar is widely used in tissue engineering in the biomedical field as well as in electrophoresis (gel) and chromatographic (gel filtration, ion exchange, and affinity chromatography) separations. It is also a bioinert and thermoreversible gel.[66] Roy et al.[80] have reported the application of agar for mild steel corrosion reduction in 0.5 M H2SO4. In this work, authors combined the synergistic protective ability of agar in combination with thiourea, and the study was conducted using the Tafel polarization and EIS techniques. Corrosion inhibition was enhanced in the presence of the agar–thiourea system (%η = 96) compared to each of them studied separately (%ηagar = 96%) at 1000 ppm agar. Agar–thiourea system was revealed as a mixed-type inhibitor system from the results of the polarization studies, and this composite initiated corrosion reduction for mild steel by physisorption (cooperative adsorption between both agar and thiourea molecules). The authors opined that this must have been enhanced by ion–dipolar interaction between the protonated amine group on thiourea and the hydroxyl group on agar.

OH OH

OH

Starch This is the energy store for every green plant, and it is also present in almost every human diets. It is also richly endowed in a great variety of tropical root crops with annual industrial production reaching 150 million tonnes in China alone.[84] Starch is a large carbohydrate molecule with relatively large glucose unit compared to other carbohydrates (Fig. 11). It is unstable in solutions of acid pH and possesses low solubility in water.[85] The demand for starch has been enormous in the past century; in fact, those from native agricultural sources have not been able to meet the entire industrial demand. Starch has also been synthesized, physically and chemically modified for use in various applications in food and nonfood industries.[86,87] Bello et al.[88] have studied the anticorrosion properties of modified cassava (Family Euphorbiaceae) starch as corrosion inhibitor for carbon steel in 200 mg/L NaCl using EIS. The chemical structures of both forms of starch (activated and carboxymethylated type) deployed in this study

O

O H O

Secreted by Sphingomonas elodea, gellan is a linear anionic-type heteropolysaccharide that physically exists as a highly strengthened “gelly gum” as well as in solutions of reduced temperature depending on their pH and ionic strength.[81,82] Gellan is oligo(tetra)saccharide with two d-glucose units (residues of l-rhamnose and ­d-glucuronic acid) joined together by α(1→3) glycosidic bonds. It is commonly used in food and pharmaceutical industries as thickeners and stabilizers (https://en.wikipedia.org/ wiki/Gellan_gum). Like any other polysaccharide, lone pairs of electrons on the hydroxyl groups attached to the glucose units are used for bond formation during metal surface bonding. Gellan has been reported as an effective corrosion inhibitor for cast iron in 1 M HCl using chemical (weight loss) and electrochemical (polarization and EIS) techniques.[83] Corrosion inhibition of cast iron in the presence of gellan increased with the concentration of the inhibitor molecule in the solution of the acid; a magnitude of %η equal to 81% was recorded at 500 ppm. Gellan demonstrated a mixed-type inhibitor behavior from polarization results in the solution of the electrolyte at all temperatures studied, while chemisorption was proposed as the mechanism of adsorption of the polysaccharide molecule on iron. Adsorption of gellan on iron followed the Langmuir adsorption isotherm.

O

OH HO

CH2OH

O n

Fig. 10  Molecular structure of an agarose polymer (made up of galactopyranose and d-galactose units covalently joined by α(1→3) and β(1→4) glycosidic bonds)

OH

O

O

O OH

Fig. 11  Molecular structure for linear amylose (starch)

Construction– Cosmetics

OH

690

were elucidated by nuclear magnetic resonance spectroscopy. Improved corrosion protection was realized for the physically modified starch and authors attributed this to the presence of strong ionic interaction between the starch molecule and the Fe2+ ions at the metal/solution interface; this was not observed for the chemically modified starch due to the CM group substitution and lower hydrophilicity. Corrosion inhibition was further explained by electrostatic potential mapping of the repetitive monomeric starch units in order to explore the nature of active binding sites for metal surface adsorption. Corrosion inhibition was attributed to the resistance to charge-transfer processes as explained by the EIS result. Prolonged immersion of the steel substrate in starch solution further improved corrosion inhibition for the metal substrate due to the formation of dense inhibitor film. This was confirmed by atomic force microscopy (AFM). Tapioca starch has also been reported to inhibit the corrosion of AA6061 alloy in seawater.[89] The authors attributed the corrosion inhibition process to the precipitation of tapioca starch on the metal surface, thereby reducing the passage of corrosive ions and molecules to the metal surface. Alginates

Construction– Cosmetics

This polysaccharide is present in seaweeds and can be collected after alkaline (usually Na2CO3) extraction at elevated temperature. Present in a thick slurry as sodium ­alginate, it is diluted with water and filtered with diatomaceous earth before precipitation.[26,90] Depending on the solvent used in this step, alginate can take any form since it readily undergo room temperature gelation and cross-­linking (and subsequent formation of microparticles) in the presence of divalent cations (e.g., Ca2+ and Zn2+).[26] These processes aid the strengthening of the overall gel strength.[91] These biodegradable and biocompatible compounds have been used as raw materials for various industrial processes as thickening, stabilizing, and gelling agents.[26] Recent corrosion-related applications of alginates are those reported as inhibitor composites and coatings with superior protective properties. Wang et al.[92] have studied the autorelease mechanism of imidazoline quaternary ammonium salts loaded in Ca alginate gel capsules prepared by piercing-­ solidifying method. The protective properties of this hybrid coating applied on a P110 grade steel substrate was investigated using EIS in a CO2-saturated saline (3.5 wt% NaCl) solution. The protective coating developed a self-healing ability with improved barrier strength due to the release of the imidazoline compound at the surface of the steel substrate. Inhibitor release was further enhanced by increasing the temperature of the media in order to aid gel swelling for only denser alginate gel composites encapsulated with BaSO4 particles. The corrosion inhibition effect of a water-soluble alginate inhibitor has been investigated for aluminum in 4 M NaOH using hydrogen evolution and weight loss techniques.[93,94] Corrosion inhibition was

Corrosion Protection: Natural Polymer in

found  to increase with the concentration of alginate and not with temperature; this was attributed to the physical adsorption of molecules of alginate to the metal surface. Adsorption of alginate on aluminum was approximated by Langmuir and Freundlich isotherms and the mechanism was further explained by means of kinetic parameters. Carrageenan Highly abundant in red seaweeds (e.g., C. crispus), carrageenans are linearly structured polyelectrolyte heteropolysaccharides (mostly sulfated) that readily form highly crossed-linked gels with cations. Their linear structure is made up of β-d-galactose and 3,6 anhydrogalactose repeating units covalently joined together by α(1→3) and β (1→4) glycosidic bonds with an unstable helical symmetry. Carrageenans are water soluble and this physical property makes them suitable candidates for “sustained-release” materials for drug delivery in modern pharmaceutics as well as in food industries as gelling and stabilizing agents.[95] Carrageenans have also been reported to possess anticorrosion potentials for various aluminum alloys in HCl medium. They are known to form protective geltype films on these metals thereby preventing further corrosion attack. Normally, film formation by polysaccharides is initiated by molecular adsorption at the metal/solution interface; in the case of carrageenans, this is partly due to the presence of the sulfonic acid group on the compound. Zaafarany[96] has studied the protective properties of κ-, ι-, and λ-carrageenans for aluminum corrosion in 1 M HCl using gravimetric (weight loss) and galvanostatic polarization techniques. Corrosion inhibition was found to increase with the concentrations of these compounds and decrease with temperature. This was attributed to the adsorption and subsequent blocking of the aluminum surface by gel-forming carrageenan molecules; more at reduced temperature. Results from the galvanostatic polarization experiment revealed that these compounds were anodic-type inhibitors for aluminum in 1 M HCl. In the presence of a mediator (pefloxacin mesylate), the effect of ι-carrageenan addition to 2 M HCl containing aluminum (99% pure) sheets has been also been investigated using gravimetric technique.[97] Improved corrosion inhibition was attributed to the initial adsorption of a primary inhibitor layer in the presence of pefloxacin before the formation of the gel-like carrageenan film. The magnitudes of %η in the absence and presence of pefloxacin were 66% and 91%, respectively, and this trend reveals synergistic corrosion inhibition effect in the presence of both compounds. Molecular adsorption was confirmed by SEM analysis of the metal surface exposed to the acid electrolyte. Dextrin Dextrin is one of the relatively low-molecular-weight c­ arbohydrates obtained from the hydrolysis of glycogen

or starch.[98,99] Unlike gellan, the glucose units in dextrin are joined by α(1→4) or α(1→6) glycosidic bond. Dextrin is widely deployed in biomedical and pharmaceutical applications partly due to its biodegradability and biocompatibility as it is hydrophilic with considerable amounts of hydroxyl groups that can be easily modified.[99] Pioneer works in this area must have been those conducted for simple dextrin in combination with other simple biopolymers (e.g., gelatin, agar, acacia extracts) for aluminum alloys (published 1975)[100] and aluminum–copper alloys (published 1976/1981).[101,102] Cyclodextrin dominates this application due to its unique molecular structure; bearing a hydrophobic interior (with 1o hydroxyl groups) and hydrophilic exterior (with 2o hydroxyl groups).[103] Most recently, Zou et al.[103] have reported the anticorrosion properties of β-cyclodextrin-modified acrylamide for steel (X70 grade) in 0.5 M H2SO4 solution using polarization and EIS techniques. The authors found out that corrosion inhibition was concentration dependent recording the highest magnitude of 84.9% for 150 mg/L at 30°C. Corrosion inhibition was attributed to the physical adsorption at the surface of the metal substrate and confirmed with SEM coupled with energy-dispersive X-ray spectroscopy (EDS). Physical adsorption phenomenon was approximated by Langmuir adsorption isotherm. Khramov et al.[104] have reported a procedure for caging two organic inhibitor compounds (­mercaptobenzothiazole and mercaptobenzimidazole) within β-cyclodextrin. The substituted azole/ dextrin ­composites were further encapsulated within a protective sol–gel-type coating synthesized from tetramethoxysilane and 3-glycidoxypropyltrimethoxysilane. This ­functionalized coating was applied on aluminum substrate (AA2024-T3 grade) and analyzed for its barrier properties in dilute Harrison’s solution using polarization and scanning vibrating electrode techniques. The improved ­corrosion protection for this coating was attributed to the release of the two organic corrosion inhibitors from the β-cyclodextrin/inhibitor inclusion complex cage. The authors also concluded that this autorelease mechanism within the coating ascribed a self-healing ability to the coating. Cellulose and Cellulose Derivatives Cellulose was probably first extracted from plant matter by Anselme Payen (a French chemist) in the mid-1800s [105] and later chemically synthesized by Kobayashi et al.[106] in 1992 via nonbiosynthetic path catalyzed by cellulase. Cellulose is a principal component of biomasses of green plants and the floral community as a whole. Apart from cellulose, the major components of lignocellulosic biomass include lignin and extractables. Isolation of cellulose from cell walls of plants poses enormous challenge to biorefinery industries since chemical treatment of plant biomass normally breaks the cellulosic chain.[107] The percentage cellulose extract from lignocellulosic biomasses normally

depends on the type and concentration of the extractive solvent (e.g., NaOH) and temperature. Cellulose is a linear polysaccharide with d-glucose units joined with regular β(1→4) glycosidic bonds (Fig. 12).[107] Derivatives of cellulose have been widely deployed as raw materials for various industrial applications, such as papermaking, biofuel processing, thin-layer chromatography, and building materials. For metal protection, these compounds have been reported as a potential safe, relatively cheap, and renewable component of inhibitor formulations and protective coatings.[108] Yabuki et al.[108,109] have recently investigated the ability of cellulose nanofibers to retain and autorelease calcium nitrite inhibitors within a polymer-type coating. In one of the works, [108] the authors have elaborately explained the step-wise procedures for encapsulating the inorganic corrosion inhibitor within the cellulose nanofiber. The barrier property of the functionalized coating was studied using polarization and EIS techniques with the protective coating applied on cold-rolled steel plates and completely immersed in 0.5 wt% NaCl for more than 2 h. The polymer coating was found to prevent the corrosion of steel to a reasonable extent, but the improved protection realized after the incorporation of the calcium nitrite/cellulose nanofiber composite was attributed to a self-healing potential in the presence of the composite. The concentrations of the c­ ellulose nanofibers and corrosion inhibitor needed to generate an optimum self-healing coating ability were studied. Results obtained from corrosion electrochemistry were confirmed with SEM analysis. The presence of nanosize holes of the polymer coating was evidence for the autorelease of the encapsulated inorganic corrosion inhibitor. The use of charged cellulose nanocrystals as anodic ­corrosion inhibitor has also been reported for some ­metallic substrates.[110] Derivatives of cellulose are basically obtained by simply substituting the hydroxyl groups (OH–) on cellulose with the functional groups of the desired reagents. A typical example is the formation of methyl and ethylcellulose from chloromethane and chloroethane, respectively, as well as CMC from chloroacetic acid. CMC is one of the most reported cellulose derivatives with anticorrosion potential. This anionic polymer is water soluble due to the presence of the CH2COOH group on its cellulosic chain.[111] CMC is a biocompatible and biodegradable polymer, making it a suitable additive for corrosion inhibitor formulation and for protective coating as well, due to its transparency and OH HO

O OH

HO O

OH O OH

O n

Fig. 12  Molecular structure of cellulose (a linear polysaccharide with d-glucose units joined with regular β(1→4) glycosidic bonds

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692

Corrosion Protection: Natural Polymer in

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surface hydrophilicity. Other molecular properties of CMC are due to the presence of hydroxyl and carboxylic group attached to the glucopyranosic group on the cellulosic chain (Fig. 13). CMC is present as sodium salt. Solomon et al.[112] have reported the inhibition of corrosion of mild steel in 2 M H2SO4 using CMC. Corrosion studies were conducted using gravimetric, gasometric, and thermometric techniques between 30°C and 60°C. The authors found that corrosion inhibition increased with the concentration of CMC but decreased with increase in temperature. This was attributed to the physical adsorption of CMC molecules onto the metal surface, and this was approximated by Dubinin–Radushkevich and Langmuir isotherm models. Using electrochemical methods only, Bayol et al.[113] have also studied CMC adsorption of the same metal substrate in 1 M HCl; evidence of metal surface adsorption was affirmed using SEM. Khairou and El-Sayed [114] have also investigated the corrosion inhibition of this compound (in combination with other water-soluble polymers) in 0.5 M HCl using EIS and polarization techniques. Results from the polarization experiments revealed that CMC adsorption affected only cathodic process at higher inhibitor concentration; CMC adsorption obeyed Temkin adsorption isotherm. The anticorrosion potentials of other cellulose derivatives have also been investigated; widely studied are those of alkyl substituted hydroxycellulose. The corrosion inhibition of hydroxyethyl cellulose (Fig. 13) has been studied for mild steel in 0.5 M H2SO4 medium using gravimetric and electrochemical (EIS and potentiodynamic polarization) techniques.[115] Hydroxyethyl cellulose adsorption was found to affect both cathodic and anodic corrosion processes, and the compound protected mild steel by chemical adsorption mechanism on the metal surface gradually reducing the magnitude of double layer capacitance. Corrosion inhibition increased with increase in both temperature and concentration of this cellulose derivative, with the highest inhibition efficiency (76%) recorded for 2000 mg/L and 92% in combination with KI. The effect of immersion time and temperature on the corrosion inhibition of hydroxypropyl cellulose (Fig. 13) has also been reported in 1 M HCl for cast iron using Tafel polarization and EIS.[83] Corrosion inhibition was found to increase with immersion period and not with increase in

OR

RO O

temperature; the highest magnitude of corrosion inhibition was recorded as 90% for 500 ppm for both experimental techniques. The degree of protection further increased with the addition of KI, and this was attributed to the synergistic effect between the principal polysaccharide inhibitor and the KI molecules. Traces of antagonism were also obtained within some range of defined concentration of hydroxypropyl cellulose though inhibition was linked with metal surface molecular adsorption which was also found to obey Langmuir adsorption isotherm model. Arukalam et al. [116] have studied the inhibition effect of hydroxypropyl methylcellulose (Fig. 13) for aluminum corrosion in 0.5 M H2SO4 using chemical (weight loss) and electrochemical techniques. Corrosion inhibition was found to be concentration dependent from results of both techniques. This was attributed to the formation of stable passive films in the presence of the inhibitor at higher concentrations. Adsorption in the presence of hydroxypropyl methylcellulose was approximated by Freundlich adsorption isotherm model. A weak physisorption was proposed as the adsorption mechanism and preferential interaction between the cellulose d­ erivative and the aluminum surface. Pectin This dirty-white carbohydrate biopolymeric powder is mostly found in cell walls of vascular plants [117] and could be extracted too from terrestrial citrus, peaches, and most varieties of berries. Pectin is a polysaccharide with relatively large linear chains of poly(d-galacturonic acid) bonded via α-1,4-glycosidic linkages (Fig. 14). According to Fiori-Bimbi et al., [118] the carboxyl groups on the ­d -galacturonic acid moiety could be simultaneously esterified and deprotonated to anions. The hydroxyl groups could aid metal surface adsorption and strong interaction between the adsorbed pectate film and the metal surface. Just like any other protective polysaccharide layer, ­pectin replaces absorbed water molecules on metal surfaces, thereby reducing metallic dissolution/ corrosive effects. [117] Recently, Fiori-Bimbi et al. [118] have reported a step-bystep procedure for extracting pectin from citrus peel. The purified pectin extract was further tested for its anticorrosion ability in 1 M HCl for mild steel using gravimetric (weight loss) and electrochemical (EIS and Tafel polarization) techniques. Pectin from this source was found to inhibit corrosion of mild steel to a great extent with

O OR

COOH

n

Fig. 13  Molecular structure of substituted cellulose polymer: If the chemical group R is CH2COOH, then the polymer is hydroxyethyl cellulose or CMC. If R is CH3 or CH2CH(OH) CH3, then the compound is hydroxypropyl methylcellulose, and hydroxypropyl cellulose if R is CH2CH(OH)CH3). For the four derivatives, R could also be H atom

O

O

COOCH3 O HO

Fig. 14  Molecular structure for pectin

O OH

O

n

Corrosion Protection: Natural Polymer in 693

Protein Unlike polysaccharide biopolymers, the use of proteins and protein mixtures from natural sources as additives in industrial-based inhibitor formulations (or as simple components) are rare, though the application of gelatin and albumin has been reported in simple-component systems. Gelatin Gelatin is normally extracted from connecting tissues, skin, and bones of farm animals as well as scales of fishes.[120] Though gelatin is commonly used in food and nonfood industries as gelling agents, it is also being deployed as additives for inhibitor formulations. Stankiewicz et al. [121] have reported the preparation and codeposition of Ni-P/ gelatin hybrid coating on an inert solid substrate; the presence of gelatin in the protective coating offered a self-­healing property. Authors also found that more stable gelatin microsize gels were formed around 80°C far below critical micelle concentration, and this could improve the protective properties of the coating. The preparation of this hybrid coating matrix offered a reliable method of codepositing gelatin inhibitor-loaded coating on the substrate for surface corrosion mitigation. Pal et al. [120] have also studied the inhibition of mild steel corrosion in 1 M HCl between 20°C and 50°C with gelatin. The corrosion inhibition of mild steel increased with the concentration of gelatin and this was attributed to the reduction of charge transfer rate across the steel/HCl interface due to molecular gelatin adsorption. Gelatin adsorption followed Langmuir adsorption isotherm, while results for the polarization experiments revealed this molecular adsorption affected both anodic and cathodic reactions. The authors

opined that corrosion inhibition was possible due to gelatin film formation at the metal surface; this must have been possible via coordinate bonding between the amide linkages (of the polypeptide chain) on gelatin and the empty 3d orbital of Fe on the steel surface. Albumin Albumin extracts obtained from various sources (albuminoids) have also been deployed for metal protection, and this has been successful due to its solubility and a wide range of corrosive media just like any other protein. Recently, the influence of bovine serum albumin (BSA) on the corrosion of implant grade niobium (Nb) alloy sheets has been studied in phosphate-buffered saline (PBS) electrolytes. ­Corrosion inhibition of Nb in the presence of BSA was attributed to the adsorption/formation of stable passive films on the metal substrate. [122] Molecular adsorption affected the trend in open circuit potential (E oc) and polarization resistance (Rp) as the immersion period for Nb sheets increased in the test solution containing varying concentrations of BSA. He et al. [123] have also reported the mitigation of biocorrosion associated with the bacterial (E. coli) biofilm formation by adsorbing albumin (in combination with alginate) on  sprayed aluminum coatings. The adsorption of albumin was found to inhibit bacterial adhesion/biofilm formation and also improve the barrier performance of the coating to a great extent. Corrosion inhibition was linked with the hydrophobicity of the ­coating after albumin/ alginate adsorption. Natural Gums Gums are water soluble (and not in organic solvents) plant or animal (even bacterial) polysaccharide secretions that readily form hydrocolloids in sufficient amount of water. These semisolid matters are complex mixtures of simple sugars, oligosaccharides, and even polysaccharides. [124] Exudate gums have been used as thickeners and stabilizers about 5000 years ago. Gum arabic was the principal additive in ancient Egyptian adhesives and pigment binders used in making the hieroglyphs and other artistic inspirations and also in the embalmment of mummies. [125] In the modern time, gums have also been widely used in the food and nonfood (e.g., pharmaceuticals and cosmetics) industries as emulsifier, fat replacers, and cross-­linking agents.[126] Gums have also been deployed as corrosion inhibitors for industrial metals in various media. The presence of special atoms on the sugar moiety provides metal surface adsorption sites, while CH2COOH grouped increases the possibility of charge transfer thereby aiding adsorption. The large molecular sizes of the polysaccharides in gum further promote surface adsorption by “blanketing” the metal surfaces from corrosive ions and molecules.[124]

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the highest corrosion inhibition efficiency (%η = 94%) recorded for 2 g/L at 45°C. Magnitude of %η was found to increase with pectin concentration and with temperature for the three corrosion evaluation techniques. The results from the polarization technique revealed that pectin was a mixed-type corrosion inhibitor (denoting that corrosion inhibition was via “geometric blocking effect” of adsorbed molecular species of pectin). The adsorption of a protective pectin/Fe2+ film was revealed by spectroscopic evidence and SEM analysis of mild steel immersed in 2 g/L pectin for 3 h at room temperature. Umoren et al. [119] have also investigated the corrosion inhibiting properties of pectin (extract from apple) in 0.5 M HCl for X60 pipeline steel using chemical and electrochemical techniques. Results from both techniques revealed increased corrosion inhibition with pectin concentration and with temperature. Authors opined that pectin must have reacted with freshly generated Fe2+ ions, forming a protective film on the steel. This mode of adsorption was approximated by Langmuir adsorption isotherm model, while SEM and water contact angle measurements confirmed the adsorption.

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Gum Arabic

Guar gum

This natural gum must have derived its name from its place of origin; it was widely transported for supply from the ports of Arabia to Europe and other regions of the world. [125] A policy report on the distribution and marketing of Sudanese gum arabic has accounted for up to 80% production between the 1950s and early 1990s. [127] According to FAO/WHO Joint Expert Committee for Food A ­ dditives, gum arabic is a dried exudate from acacia (Acacia senegal and Acacia seyal (Family Leguminosae). [128] It is a mixture of plant proteins and sugars not limited to glucoproteins, arabinogalactan, oligosaccharides, and polysaccharides. [124] Like most polysaccharides, its food and nonfood applications include being used as gelling and emulsifying agents, stabilizer, binder, and thickener. It has also been deployed as ­corrosion inhibitor in various media for some industrial metals, and the success of this application is linked with the polysaccharide constituents of the gum as well as the polar functionalities on each molecule (oxygen, nitrogen, sulfur, and phosphorous). These atoms serve as metal surface adsorption sites; they facilitate the adherence of the formed inhibiting film on substrates. [129] Umoren [129] has investigated the efficiency of gum arabic for mild steel and aluminum corrosion in 0.1 M H 2SO 4 using chemical evaluation techniques (weight loss and thermometric techniques). Corrosion studies were conducted between 30°C and 60°C within 0.1 and 0.5 g/L gum arabic concentration in the aqueous acid corrodent. The corrosion rates of both metals were found to reduce with the concentration of the inhibitor with higher magnitudes of corrosion inhibition efficiency (%η) recorded for mild at 60°C (37.8%) and 30°C (79.7%) for aluminum at 0.5 g/L. Corrosion reduction in the presence of gum arabic is attributed to the molecular adsorption on the metal surfaces; physical and chemical adsorption ­mechanisms were proposed for aluminum and mild steel, respectively, from the trend of values of %η with temperature. Adsorption of gum arabic on aluminum and mild steel followed El-Awady et al. and Temkin adsorption isotherms. Author concluded that gum arabic was a better corrosion inhibitor for aluminum than for mild steel. Another research group has reported a more comprehensive anticorrosion studies with this inhibitor using electrochemical and surface analytical techniques. [130] Corrosion studies was conducted using chemical (weight loss and hydrogen evolution) and electrochemical (polarization) techniques for mild steel sheets in 1 M HCl and 1 M H 2SO 4, and results revealed that corrosion inhibition was concentration dependent. Evidence of protective film formation by gum arabic on mild steel surface was revealed by FTIR, SEM, and X-ray photoelectron spectroscopy (XPS). Corrosion reduction of mild steel with gum arabic was also found to increase in the presence of an external magnetic field.

Guar gum is chemically composed of galactose and mannose (Fig. 15). The ratio of galactose to mannose is between 1.2 and 1.8 depending on the temperature.[131] Guar gum is widely being involved in most biomedical and pharmaceutical applications, especially colon-­specific and oral-controlled drug delivery, due to its surface hydrophilicity, biodegradability and bioavailability, biodegradability, and zero-level toxicity.[132,133] Guar gum’s ability to build hydrogen bridges in water makes it a reliable candidate for gelling and thickening agents for food and cosmetics applications.[131–133] It has also been reported as having the potentials for corrosion inhibition for some metal substrates. Abdallah [134] has investigated the inhibition of carbon steel corrosion using guar gum in 1 M H2SO4 medium using weight loss and polarization techniques. Results from the later technique revealed that guar gum was a mixed-type inhibitor, while corrosion reduction was realized at the increased guar gum concentration. The presence of guar gum shifted pitting potential of the working electrode to more positive values, and this was linked with resistance to surface pitting. Corrosion inhibition was attributed to the formation of protective inhibitor film by molecular adsorption which was found to follow Langmuir adsorption isotherm. To improve the corrosion inhibition properties of guar gum, it was further grafted to polyacrylamide and deployed for similar application for mild steel in 1 M HCl.[135] Investigated with potentiodynamic polarization and electrochemical impedance techniques, corrosion inhibition of steel was found to be dependent on the degrees of polymer grafting and exposure period to the corrodent. A magnitude of corrosion inhibition efficiency (%η) more than 90% was obtained for 86% grafting (500 ppm) after 50 h of immersion in 1 M HCl. The experimental results from the polarization technique revealed that that the polyacrylamide/guar gum composite was a mixed-type inhibitor system. The authors also attributed corrosion inhibition to synergistically molecular adsorption between polyacrylamide and guar gum at the metal surface, evidence of which was confirmed by FTIR analysis of the adsorbed film. They proposed metal surface inhibitor adsorption via the hydroxyl groups on the sugar moiety of guar gum and OH OH HO

O OH O

H

O HO

OH O

HO O OH

O OH

OH n

Fig. 15  A linear molecular chain of 1,4-linked β-d-­ mannopyranose residues in which α-d-galactopyranose residues are branched at the 1,6 positions at every second mannose

Corrosion Protection: Natural Polymer in 695

Xanthan gum It is chemically composed of pentasaccharide units in a 2:2:1 ratio of mannose, glucose, and glucuronic acid, respectively (Fig. 16).[136] This highly charged polysaccharide molecule possesses a relatively rigid polymer chain. Beyond critical concentration, xanthan undergoes ­nonequilibrium jamming transition and reduces cosolute mobility/diffusion of other polysaccharides (e.g., agarose) in the same solution with it.[137] Xanthan research has attracted much attention compared to other polysaccharide biopolymer due to the wide applicability of the final products. Xanthan gums have been used as thickening and gelling agents, emulsifiers, and stabilizers in nonfood industries (e.g., pharmaceutical and biomedical). Xanthan gum is also widely utilized as a medium in most drug delivery systems.[138] Recently, Mobin and Rizvi[139] have reported the anticorrosion potential of xanthan gum for mild steel in 1 M HCl between 30°C and 60°C using gravimetric analysis, potentiodynamic polarization, and EIS. For both techniques, corrosion inhibition was found to increase with xanthan concentration but not with temperature. The highest magnitude of %η in this study was 70% at 30°C for 1000 ppm xanthan gum. Reduction of mild steel corrosion was enhanced by introducing three surfactants (surfactants sodium dodecyl sulfate, cetyl pyridinium chloride, and Triton X-100), with sodium dodecyl sulfate showing better protection compared to other. Improved corrosion protection in the presence of the surfactants was attributed to the synergistic effect between surfactants and xanthan gum. Results from the polarization technique

CH2OH OH

O COOH H3C

H3C

R6

R4

O

CH2OH

O

O

O

OH

OH O

O

n

O OH OH

R 6O

COOH O OH OH

R 4O

O

OH

O

O

OH

Fig. 16  Xanthan gum is chemically composed of pentasaccharide units in a 2:2:1 ratio of mannose, glucose, and glucuronic acid, respectively

revealed that the adsorption of a xanthan gum-type protective film at the metal surface influenced both anodic and cathodic processes. Metal surface adsorption was probed by surface analytical techniques, and UV–visible spectrophotometry revealed the presence of xanthan– Fe2+ corrosion inhibiting complex and was further confirmed by SEM analysis. Adsorption followed Langmuir adsorption isotherm. Biswas et al.[140] have investigated the corrosion inhibition of poly(acrylamide) grafted xanthan gum for mild steel in a more corrosive acid medium (15% HCl). The corrosion electrochemistry was studied using chemical and electrochemical techniques. Results from polarization technique reveal that xanthan gum and the ­poly(acrylamide) grafted xanthan gum demonstrated a mixed-type inhibitor behavior toward steel dissolution. Improved corrosion protection was revealed for xanthan after chemical grafting with poly(acrylamide), with 75% and 92% being the recorded magnitudes of corrosion inhibition efficiency for the pure polysaccharide and its grafted form, respectively. Corrosion reduction was attributed to the adsorption of protective inhibition film on the metal surface, and this phenomenon followed Langmuir ­adsorption isotherm. Oleo gum Oleo gums are widely known as solid exudations from fibrous herbs of the genus Ferula, constituting more than 100 species in the Mediterranean region to northern Africa and central Asia.[141] In Ferula gummosa alone, oleo gum is abundant throughout the vegetative period of the plant, with each plant being capable of producing more than 10 g gum resin mostly from the roots. Normally, gum harvesting is done by dripping of the viscous liquid substance as it slowly hardens to a thick solid or semisolid and reactively dark matter. The oleo-gum-resin from the plant species indigenous to Iran is best harvested between June and September.[142] Besides the food applications of this gum resin, it also possesses some unique pharmacological activities as enlisted in Fig. 17.[143] The ­general arabinan and galactan constituents as well as the effect of collection time of the gum on the chemical ­composition of oleo gums have been widely studied.[142,144] Oleo gum has been deployed as a corrosion inhibitor for some metallic substrates in acidic media. Behpour et al.[145] have investigated the anticorrosion abilities of oleo gum extracted from Ferula assa-foetida and Dorema ammoniacum for mild steel in 2 M HCl. Corrosion inhibition was investigated using chemical (weight loss) and electrochemical (EIS and potentiodynamic polarization) techniques. The variation of current-potential values on Tafel curves revealed that gum resins from both sources were mixed-type inhibitors for mild steel. Corrosion inhibition was found to increase with the concentration of the resins and decrease with the rise in temperature. The authors attributed the corrosion reduction of mild steel to the formation of protective films from adsorbed inhibitor species, confirmed by SEM analysis. This

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a chelation-type binding between the endocyclic/exocyclic oxygen atoms (also on the sugar moiety) and Fe2+ ions from the mild steel.

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Antinociceptive/ carminative activities Anticonvulsant and anticatarrh activities

Digestive and expectorant activities Pharmacological activities of oleogum resin

Antiseptic and analgesic activities

Antihysteric activity Laxative and aphrodisiac activities

Fig. 17  Some oleo-gum resin

pharmacological

properties/activities

of

adsorption ­phenomenon was approximated by Langmuir isotherm for both resins. Oleo-gum extract from F. assa-foetida (η = 96%) showed a better corrosion inhibiting effect for mild steel compared to that of D. ammoniacum (η = 91%) at equal concentration. Experimental results were supported by theoretical-based quantum chemical calculations in order to reasonably explain the relationship between corrosion inhibition and molecular structures of the major constituents of both gums. The authors have also reported a similar study for 304 stainless steel in 2 M HCl.[146] Construction– Cosmetics

and bioproteins most especially denature at higher temperature. Some chemical reactions such as carboxymethylation, oxidation, and thiolation can introduce new functional groups into the carbohydrate chain to promote solubility. Graft-­copolymerization reactions are quite easy in appropriate reagents and mild reaction conditions. Since there are a lot of derivable chemical groups present in this class of compounds, various polysaccharide derivatives could be ­synthesized in a single reaction step. More natural polymers should be deployed for the corrosion inhibition of other metals, including alloys and non-alloyed substrates. There are already so many studies on steel and other ferrous alloys. Attention should also be focused on the development of natural polymers with scale inhibition potential. The earlier mentioned chemical modification methods will introduce functional groups with strong binding strength (other than the hydroxyl group) within the polymer chain. This will improved the antiscalant potential of these c­ ompounds, especially against CaSO4 scaling. Since most gum extracts are mixtures of varying compositions of bioproteins, glucans, oligosaccharides and lowand high-molecular-weight polysaccharides, [124] it is almost impossible to ascribe the causative reason for corrosion inhibition to only one for the phytoconstituent. To avoid this, research of gum extracts should focus on the extraction and isolation of the gum constituent with the most efficient corrosion inhibiting potential. ­Theoretical-based approach to corrosion inhibition is also recommended in this field of study. The use of appropriate computational tools is necessary to correlate corrosion inhibition efficiency with the molecular structures of the inhibitor compounds. The principles of density functional theory (DFT) should be deployed to analyze the ground-level molecular properties of the inhibiting polymers as well as compute their local reactivities at any chosen molecular state.

FUTURE AREAS OF APPLICATION Natural gums are efficient class of corrosion inhibitors for a wide variety of industrial metals but they also lack the needed efficacy for prolonged protection in the most corrosive media. This drawback is due to their poor bulk stability and solubility in these aqueous media. Gums from some floral sources seem to also “flake out” of metal surfaces due the formation of protective inhibitor films with poor adhesion. Most of these problems could be solved by modification of the native gum matrices. By some physical or/and chemical modification procedures, with appropriate reagents, desired chemical groups can be introduced to aid gum dissolution in aqueous media thereby promoting its stability in solution. Exudate gums from most terrestrial plants can be modified with alcohol to enhance bulk solubility. Most highmolecular-­weight polysaccharide ­biopolymers also have poor solubility at room temperatures, though they are water soluble at elevated temperatures. Some starch polysaccharides

CONCLUDING REMARKS Natural polymers and their derivatives are found in nature, plants, and animals sources. They also found in biomasses of varieties of living systems; most of them are products of physiological processes in plants, animals, as well as unicellular microorganisms. These classes of polymers are produced in various amounts all over the world for many food and nonfood industrial applications. To meet their insatiable demand for these applications, natural polymers are not only extracted from natural sources, most of them are also synthesized in the laboratory. They have also been widely deployed as corrosion inhibitors for industrial metals in various media. The presence of special atoms on the sugar moiety provides metal surface adsorption sites, while CH2COOH grouped increases the possibility of charge transfer, thereby aiding adsorption. The large molecular sizes of the polysaccharides biopolymers further promote surface adsorption by

“blanketing” the metal surfaces from corrosive ions and molecules. Among some of the corrosion control methods deployed in the industrial marine and oilfield companies, the use of natural polymers in corrosion inhibitor formulations and in anticorrosive paints composites for metal surfaces have been effective. Their efficient metal surface protection and prolonged shelf life is gradually replacing the use of the obnoxious inorganic compounds (e.g., chromates, borates, arsenates) that have been banned by modern industrial safety legislation due to their toxic effect. Natural polymers are available, low cost, renewable and robust, green alternatives to the synthetic inhibitor compound. Their biocompatibility and biodegradability make them efficient anticorrosive ­additives for field-based inhibitor formulations. ACKNOWLEDGMENTS The authors greatfully acknowledge the support of the ­Center of Research Excellence in Corrosion (CORE-C), King Fahd University of Petroleum and Minerals (KFUPM), Saudi Arabia. UME is also grateful to KFUPM for her support while he was there; this work was initiated and completed while he was still at KFUPM. REFERENCES 1. Yang, X.; Qiao, C.; Li, Y.; Li, T. Dissolution and resourcfulization of biopolymers in ionic liquids. React. Funct. Polym. 2016, 100, 181–190. 2. Riyajan, S. Robust and biodegradable polymer of cassava starch and modified natural rubber. Carbohyd. Polym. 2015, 134, 267–277. 3. Candogan, K.; Kolsarici, N. Storage stability of low-fat beef frankfurters formulated with carrageenan or carrageenan with pectin. Meat. Sci. 2003, 64, 207–214. 4. Yasin, H.; Babji, A.S.; Ismail, H. Optimization and rheological properties of chicken ball as affected by ­k-carrageenan, fish gelatin and chicken meat. LWT Food Sci. Technol. 2016, 66, 79–85. 5. Feng, X.; Bansal, N.; Yang, H. Fish gelatin combined with chitosan coating inhibits myofibril degradation of golden pomfret (Trachinotus blochii) fillet during cold storage. Food Chem. 2016, 200, 283–292. 6. López, C.M.E.; Gómez, G.M.C.; Pérez, M.M.; Montero, P.  A chitosan– gelatin blend as a coating for fish patties. Food Hydrocoll. 2005, 19, 303–311. 7. Vilela, J.A.P.; Cunha, R.L. High acyl gellan as an emulsion stabilizer. Carbohydr. Polym. 2016, 139, 115–124. 8. Dickinson, E. Hydrocolloids at interfaces and the influence on the properties of dispersed systems. Food Hydrocoll. 2003, 17, 25–39. 9. Dickinson, E. Hydrocolloids as emulsifiers and emulsion stabilizers. Food Hydrocoll. 2009, 23, 1473–1482. 10. Grassino, A.N.; Brncic, M.; Vikic-Topic, D.; Roca, S.; Dent, M.; Brncic, S.R. Ultrasound assisted extraction and characterization of pectin from tomato waste. Food Chem. 2016, 198, 93–100.

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chitosan based pre-layer reservoir of corrosion inhibitor. J. Mater. Chem. 2011, 21, 4805–4812. 187. Abd-El-Nabey, B.A.; Goher, Y.M.; Fetouha, H.A.; Karam, M.S. Anticorrosive Properties of chitosan for the acid corrosion of aluminium. Portugaliae Electrochimica Acta 2015, 33 (4), 231–239. 188. Chaubey, N.; Savita; Singh, V.K.; Quraish, M.A. Corrosion inhibition performance of different bark extracts on aluminium in alkaline solution. J. Assoc. Arab Univ. Basic Appl. Sci. 2016, doi:10.1016/j.jaubas.2015.12.003. 189. Abdallah, M.; Kamar, E.M.; ElEtre, A.Y.; Eid, S. Gelatin as corrosion inhibitor for aluminum and aluminum silicon alloys in sodium hydroxide solutions. Prot. Met. Phys. Chem. Surf. 2016, 52 (1), 140–148. 190. Meena, S.L.; Verma, P.S. Effect of inhibitors on corrosion of aluminium in acidic medium. Ind. J. Chem. Technol. 2014, 21, 220–223. 191. Prabhu, D.; Rao, P. Coriandrum sativum L.—A novel green inhibitor for the corrosion inhibition of aluminium in 1.0 M phosphoric acid solution. J. Environ. Chem. Eng. 2013, 1, 676–683. 192. Sugama, T.; DuVall, J.E. Polyorganosiloxane-grafted potato starch coatings for protecting aluminum from corrosion. Thin Solid Films 1996, 289, 39–48. 193. Sugama, T. Oxidized potato-starch films as primer coatings of aluminium. J. Mater. Sci. 1997, 32, 3995–4003. 194. Hassan, R.M.; Zaafarany, I.A. Kinetics of corrosion inhibition of aluminum in acidic media by water-soluble natural polymeric pectates as anionic polyelectrolyte inhibitors. Materials. 2013, 6, 2436–2451. 195. Eid, S.; Abdallah, M.; Kamar, E.M.; El-Etre, A.Y. Corrosion inhibition of aluminum and aluminum silicon alloys in sodium hydroxide solutions by methyl cellulose. J. Mater. Environ. Sci. 2015, 6, 892–901. 196. Umoren, S.A.; Obot, I.B.; Ebenso, E.E.; Okafor, P.C.; Ogbobe, O.; Oguzie, E.E. Gum Arabic as a potential corrosion inhibitor for aluminium in alkaline medium and its adsorption characteristics. Anti-Corros. Methods Mater. 2006, 53 (5), 277–282. 197. Ja’o, A.M.; Eddy, N.O.; Alhassan, S.I.; Habib, I.Y. Adsorption and inhibitive properties of Ficus sycomorus gum on the corrosion of aluminium in HCl. Int. J. Sci. Res. Publ. 2015, 5 (3), 1–6. 198. Eddy, N.O.; Ameh, P.O.; Ibrahim, A. Physicochemical characterization and corrosion inhibition potential of Ficus benjamina (FB) gum for aluminum in 0.1 M HCl Walailak. J. Sci. Tech. 2015, 12 (12), 1121–1136. 199. Ameh, P.O.; Eddy, N.O. Commiphora pedunculata gum as a green inhibitor for the corrosion of aluminium alloy in 0.1 M HCl. Res. Chem. Intermed. 2014, 40, 2641–2649. 200. El-Haddad, M.N. Chitosan as a green inhibitor for copper corrosion in acidic medium. Int. J. Biol. Macromol. 2013, 55, 142–149. 201. Bao, Q.; Zhang, D.; Wan, Y. 2-Mercaptobenzothiazole doped chitosan/11-alkanethiolate acid composite coating: Dual function for copper protection. Appl. Surf. Sci. 2011, 257, 10529–10534. 202. Todorović, D.; Dražić-Janković, Z.; Marković, D. Determination of the degree of adsorption on copper and brass tins by changing the temperature on the surface before

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Cosmetics: Active Polymers Mayuree Kanlayavattanakul and Nattaya Lourith School of Cosmetic Science, Mae Fah Lung University, Chiang Rai, Thailand

Abstract Polymers are widely served in several industries including personal care and cosmetic products. They are applied as the cosmetic raw materials and packages. Those for raw materials are ­categorized as functional and active polymers, which could be defined on the basis of the sources, i.e., ­synthetic and natural. Natural-derived polymers, are also known as biopolymers, are biodegradable, whereas synthetic polymers can be either biodegradable or nonbiodegradable. In addition to these general technical terms of natural and synthetic polymers, the emerging trends of biopolymers, bioplastics, and biodegradable polymers are becoming the keywords and gaining a top level of the highlighted spot among the consumers with different meaning according to the productions. Of particular interest, active polymers have been commercializing at a higher price than the functional polymer. This group is therefore exclusively focused in this context and classified on the basis of their sources. Keywords: Active polymer; Biopolymer; Cosmetics; Dermal matrix; Natural polymer.

Applications of polymers in cosmetics and personal care are generally classified into natural, semisynthetic, and synthetic polymers. Among these, natural polymers derived from plant, animal, and microorganism are of particular interests in this industry. In the meantime, modifications of the natural-based polymers tailoring the polymeric properties fitting with variations of the dosage forms are widely undertaken and regarded as the ­semisynthetic ones. Differently, synthetic-based polymers are those for the natural-mimetic or the fabricated polymer with diverse properties. In addition to classification on the basis of the polymeric sources, their actions in the products that are functional and active polymers are classified in cosmetic applications.[1] In this context, these polymers are summarized with their trade and INCI (International ­Nomenclature of Cosmetic Ingredients) names. Functional Polymers Polymers are incorporated into cosmetic products to function as a gelling agent, viscosity adjuster, thickener, and emulsifier as well, according to its polymerized network holding water by means of its swelling capability. Therefore, functional polymers are traditionally classified into anionic, cationic, nonionic, and amphoteric polymers on the basis of their electrochemical charges in the structure. On the basis of sources, those of functional polymers are

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majorly obtained from semisynthetic or modification of the native polymers and synthetic ones. Active Polymers Applications of polymers acting as active ingredients are widely used in cosmetics. Active polymers have been commercializing at a higher price than the functional polymers. This group is exclusively focused in this context and classified on the basis of their sources. PLANT-BASED POLYMERS Cellulose Cellulose is the most abundant renewable polymer composed of repeating units of monosaccharides that also known as cellobiose. This anionic water-insoluble polymer is hygroscopic material that can be implied in ­skin-conditioning products particularly skin-hydrating cosmetics.[1] Gum or Hemicellulose The commonly used gums in cosmetics and pharmaceutics are gum Arabic, karaya, tragacanth, and guar and locust bean gums.[2,3] Although they are generally known as the functional excipients, they also provide a protective coating

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INTRODUCTION

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Cosmetics: Active Polymers

and a smooth feel with a resistant to bacterial degradation. Gum tragacanth was reported for their emollient property contributing to skin-hydrating effect.[1] Although gum can be isolated and named by its sources, gum for cosmetic applications is simply classified into four groups based on backbone chain that are (i) xyloglucans, (ii) glucomannans, (iii) arabinoxylans, (iv) ß-D-glucans, and (v) arabinogalactans. Among these, ß-D-glucans are the majorly used functional ingredient in cosmetics [1] with some of the commercializing ones as shown in Table 1. Starch Starch is a primary polysaccharide consisting of carbohydrates mainly amylose and amylopectin.[4] Native form of starch from different source is widely used as functional ingredients (Table 1). Novel application forms of starch are microcapsules, nanoparticles, and composites and are recently used in the cosmetic industry. Pectin Pectin is non-starch water-soluble linear polysaccharide with an ability to form gel depends on the molecular size and degree of esterification (DE). The pectin classes

based on the DE are high methoxyl (HM) and the low methoxyl (LM) pectins. The values for commercial HM pectins typically range from 60% to 75% and those for LM pectin range from 20% to 40%. Similar to cellulose, gum, and starch, pectin is widely used as functional polymer in cosmetics.[1] Those of commercializing pectins are ­exemplified in Table 1. Mucilage Mucilage is traditionally applied in topical products as a functional ingredient. This water-soluble polysaccharide, similar to gum, is widely used in cosmetics regarding to its high-water absorption capacity. Those of commercializing mucilages are exemplified in Table 1. Recently, applications of mucilage as the active polymer are of challenged in cosmetics, for instance skin-hydrating okra mucilage.[5] Vegetable Proteins Flours from cereal grains (wheat, oat, rice, corn) and defatted oilseeds (soy, peanut, sunflower, almond, sesame) are cheap and widely available sources of vegetable proteins. They are used as functional polymeric ingredients (Table 1).

Table 1  Example of some commercializing plant-based polymers for cosmetics Trade name INCI name or source

Supplier

Glucans Oat COM

Avena sativa (oat) kernel flour

Oat cosmetics

ABS aloe beta-glucan

Aloe barbadensis leaf extract & yeast extract

Active concepts

Bio-beta-glucan

BetaGlucan

Bioland

SymGlucan

Oat seed

Symrise

Oat®

Avena sativa meal extract

Oat cosmetics

Purity®

Corn starch

AkzoNobel

Zea mays starch

Agrana

Genu® pectin

HM pectin from citrus peel and sugar beet pulp

CP Kelco

Apple pectin extract

Pyrus malus (apple) fruit

Carruba

Arabinose L-(+)

Botanical pectin sugar

Kaden biochemicals

Actiphyte® mustard seed

Brassica alba seed extract

Active organics

Amidroxy sugar cane

Sugar cane mucilages

Alban Muller

Amidroxy tamarind

Tamarind mucilages

International

Blue Malva LG

Malva sylvestris flower extract

Naturex

Ritaloe

Aloe barbadensis leaf juice

R.I.T.A

Hydrolyzed vegetable protein

Induchem

®

Construction– Cosmetics

Starch

AGENAJEL

®

Pectin

Mucilage

®

Vegetable proteins Unirepair T-43 Phytoprotein

Sinerga

Phytoprolin complex

Lonza

Cosmetics: Active Polymers 707

Chitin and Chitosan Chitin is formed in the exoskeleton of arthropods or in the cell walls of fungi and yeast including other lower plants and animals as summarized in Table 2 for reinforce and strength proposes. The possible sources of chitin with ­estimate amounts were exhibited in Table 3.[6–8] Chitin itself has low toxicity and biodegradable polymer with antibacterial properties, hydrophilicity, gel-­ forming properties, and affinity for proteins. Therefore, various applications according to its biological activities and wound healing effect were demonstrated. Despite the valuable biological activities of this biomaterial chitin, it is insoluble in the usual solvents that complicated the applications. Modification of chitin into the soluble form, chitosan, is adopted with continuous development for application improvement prospects. Deacetylated chitin up to 51% becomes soluble in aqueous acidic solvent which is chitosan that enables many applications due to its solubility and largely used as solutions, gels, or films and fibers. The antimicrobial activities, wound healing, and antioxidant properties including asthma and atopic dermatitis curing effects enable high quality of cosmetic applications as its ability to maintain skin moisture, acne treatment, skin toning; improve suppleness of hair; and reduce electrostatic of hair as well as efficient surfactant and Table 2  Chitin source Sea animals

Insects

Microorganisms

Annelida

Scorpions

Green algae

Mollusca

Spiders

Yeast (β-type)

Coelenterate

Lobster

Brachiopods

Fungi

Crab

Ants

Mycelia penicillium

Shrimp

Cockroaches

Brown algae

Prawn

Beetles

Spores

Krill

humectant functions. High-molecular weight chitosan is especially utilized for skin care cosmetics, and a molecular weight of 104 –106 Da is used as a broad-spectrum ingredient in hair care cosmetics. Significant transepidermal water loss (TEWL) reduction of skin treated with high-molecular weight chitosan was found. This consequently brings softer and more flexible skins accordingly in addition to irritation reduction efficacy. Improvement of sun protection product’s water resistance was attributed by the emulsion-containing chitosan as well as adhering affinity of perfume-containing chitosan onto skin that prolongs wearing duration by reducing the evaporation rate. Moreover, its applications for deodorant and antiperspirant products were also demonstrated concerning to humectancy of chitosan. The hydrophilic character of chitosan sufficiently conserves moisture in hair under low humidity affecting hair electrostatic and sustains hair style accordingly. The hydrophilicity of chitosan efficiently removes sebum and oils from hair, scalp, and skin. Those of available chitosan for cosmetic preparations are exemplified in Table 4. Polymeric Materials of Skin The extracellular matrix (ECM) is a gel-like material filling the extracellular space in animal tissues that holds cells together and provides a porous pathway for the diffusion of nutrients and oxygen to individual cells. This structure is composed of fibrous proteins (collagen, elastin, fibronectin, and laminin) and heteropolysaccharides namely ­glycosaminoglycans (GAGs).[9–11] Collagen In mammals, collagen is the most abundant component of ECM. Around 28 types of collagen have been identified and, among these, type I collagen is the most prevalent type found in ECM. Those of active collagen for cosmetics derived from animal are exemplified in Table 5.

Ascomydes

Table 4  Example of commercializing chitosan for cosmetics INCI name or Trade name source Supplier

Blastocladiaceae

Chitosan

Chytridiaceae

Chitosan

Anubhav Biotech Jeen International

Table 3  Chitin content in various sources Source

Chitin (%)

Crab cuticle

15–30

Shrimp cuticle

30–40

Krill cuticle

20–30

Squid pen

20–40

Clam/oyster shell

3–6

Insect cuticle

5–25

Fungi cell wall

10–25

Hydagen®

BASF

Chitosan™

Sino Lion

Zenvivo™

Clariant

Ritachitosan lactate

Chitosan lactate

R.I.T.A

Categel™

Chitosan adipate

Collaborative Laboratories

Chitanide™ 222

Chitosan succinamide

M.M.P

Chitoglycan

Carboxymethyl chitosan

Sinerga

Construction– Cosmetics

ANIMAL-BASED POLYMERS

708

Cosmetics: Active Polymers

Elastin Elastin is a highly cross-linked, insoluble scleroprotein rich in nonpolar amino acids. After collagen, elastin is the most abundant constituent of the connective tissues. Elastin forms highly reticulated fibers by intermolecular condensation of two K residues (lysinonorleucine) or three (isodesmosine) or four (desmosine). Mature elastin contains many of these cross-linkages, which make it ­insoluble and nonextractable in the native form. Soluble elastin, suitable for cosmetic use, is obtainable only as partial hydrolysate and is normally extracted from bovine nuchal ligaments. Examples of some commercializing hydrolyzed elastin are summarized in Table 5. Glycosaminoglycans The heteropolysaccharides are unbranched polysaccharide chains, composed of repeating disaccharide units. They are called GAGs because one of the two sugar residues in the repeating disaccharide is always an amino sugar (N-acetylglucosamine or N-acetylgalactosamine), which in most cases is sulfated. The second sugar is usually an uronic acid (glucuronic or iduronic), except for dermatan sulfate (DS) that contains galactose. GAGs are negatively Table 5  Example of collagen and elastin for cosmetics INCI name Trade name or source Supplier Collagen

charged, due to the presence of sulfate or carboxyl groups on most of their sugar residues. Based on the disaccharide composition, linkage type, and presence of sulfate groups, GAGs have been divided into four main groups: (i) ­hyaluronic acid (HA), (ii) chondroitin sulfate (CS) and DS, (iii) heparin sulfate (HS) and heparin, and (iv) keratan sulfate.

Construction– Cosmetics

Hyaluronic Acid  HA, also named hyaluronan, is a high molecular weight (105–107 Da), naturally occurring biodegradable polymer. HA is a polyanion that can self-­associate and can also bind to water molecules (when not bound to other molecules), giving it a stiff, viscous quality similar to gelatin. It is one of the major elements in ECM of vertebrate tissues and available in almost all body fluids and tissues. It is also involved in several important biological functions, such as regulation of cell adhesion and cell motility, manipulation of cell differentiation and proliferation, and providing mechanical properties to tissues. HA is responsible for providing the viscoelasticity of some biological fluids. HA’s characteristics including its consistency, biocompatibility, and hydrophilicity have made it an excellent moisturizer in cosmetic dermatology and skin care products. In cosmetic applications, HA is simply divided into high-molecular weight HA (HMWHA; 105–107 Da) and low-molecular weight HA (LMWHA; 2 × 104 –4.5 × 105 Da). HMWHA with antiangiogenic activity also exhibits fibrinogen-binding enhancement assisting in clot formation in wound healing and therefore inhibits scar formation in addition to its anti-inflammatory activity. LMWHA is a good collagen type I and VIII synthesis enhancer and also expression activities towards the biosynthesis enzymes, matrix metalloproteinases (MMPs): MMP-9, MMP-13 and MMP-3, and hyaluronan synthase (HAS): HAS-2.[12,13] Among these, biological functions of HA depend on chain length, molecular mass, and synthetic circumstances. In addition, those of commercializing HA types that are ­supplied for cosmetic formulation are exemplified in Table 6.

Collagen

Soluble collagen

DSM

Collagen hydrolysate

Hydrolyzed collagen

Kelisema

Collagen CLR

Soluble collagen

CLR Berlin

Bio-collagen V-50

Hydrolyzed collagen

Cobiosa

Crotein™ A

Hydrolyzed collagen

Croda

Collaplex 1.0

Soluble collagen

GfN-Selco

Hydrolyzed elastin

Gattefosse

LMW hyaluronic acid

Hydrolyzed elastin powder

Spec-Chem Industry

SMW hyaluronic acid

Elastin CLR

Chemisches Laboratorium Dr. Kurt Richter

AC elastin

Active Concepts

Sodium hyaluronate

Biotein®

BioOrganic Concepts

Hyaluronic acid

DSM

Crolastin™ 30

Croda DSM

Hyaluronic acid (sodium salt)

Spec-Chem Industry

Hydrolastan

Elastin Elastin marine

Table 6  Example of commercial HA for cosmetics INCI name or Trade name source Supplier Hyaluronic acid

Contipro Biotech

Sodium hyaluronate

Nikkol

HYSILK HyActive

Cosmetics: Active Polymers 709

Keratin In addition to collagen and elastin, keratin is one of the important fibrous proteins of dermal matrix. [11] In spite of its animal origin, the use of keratin derivatives in cosmetics is generally well accepted by the supporters of animal rights as the protein can be obtained without cruelty from wool and hairs. Mammalian keratins consist of filaments about 7 mm in diameter, embedded in a nonfilamentous matrix. The filaments are composed of helical structures of relatively low sulfur content (α-keratins); the matrix consists of two groups of nonhelical proteins: one cystine rich (18%–45%, β-keratins) and the second rich in glycine and tyrosine (γ-keratins). The high content of cystine ­residues is considered to be the special characteristic of keratins. While the individual peptide chains have molecular weights of 50,000 Da, cross-linking of

Table 7  Current commercial CS Name Source Condrosulf®

Shark fins

peptide chains by disulfide bridges results in huge aggregates having molecular weights of many millions, highly insoluble, and resistant to the action of chemicals and enzymes. Major sources for keratin extraction include poultry feathers, animal hoofs and horns, wool, and human hair. Those are used as raw material in the production of keratin for ­cosmetics (Table 8). Melanin Melanin is a biological polymer which is responsible for the pigmentation of many animals and plants. Throughout the body, melanin is a homogenous biological polymer containing a population of intrinsic, semiquinone-like ­radicals. Additional extrinsic free radicals are reversibly photo-generated by UV and visible light. Melanin is the only known biopolymer containing stable free radicals. Several melanin monomers exist, and the predominant monomer in a melanin polymer depends on its location within an organism. In the skin, melanin differs in eumelanin or pheomelanin, whereas in the eye it is exclusively eumelanin. The existence of melanin in skin is strongly correlated with the prevention of free radicals generated by UV radiation. Especially in the skin, melanin (mainly eumelanin) ensures the only natural UV ­protection by eliminating the generated free radicals.[16]

Table 8  Keratin and melanin as active ingredients in cosmetics INCI name Trade name or source Supplier Keratin Nutrilan® keratin Hydrolyzed keratin powder

BASF Spec-Chem Industry

Keramois®

IKEDA

HydroSal™ SalSilk

Salvona Technologies

Keratec PEP

Croda

Therapeutics

Crotein™

Anti-inflammatory

ProSina™

Osteoarthritis

Biotein®

—

Squid cartilage

Antiviral

Keramino®

—

Whale and squid cartilages

Antimalarial

Antiaging melanin

—

Shark cartilage

Anticancer

Lipo® melanin 10% solution

—

Human

Biomarker ovarian epithelial cancer

Liposhield® HEV melanin

Biomarker prostate cancer

Sunscreen melanin

—

Hydrolyzed keratin

Creanatural® sepia melanin

—

Bovine trachea, shark cartilage

Central nervous system

Sigma (C4170)

—

Neuroprotective

Creanatural® vegetable melanin

C4-S, C6-S Sigma

—

Wound healing

Vegetan premium

Keratin amino acids

BioOrganic Concepts

Melanin

Lipo Chemicals (Vantage Groups)

Melanin

Cosmetics Innovations and Technologies

Lonza

Soliance

Construction– Cosmetics

Chondroitin Sulfate  CS is a ubiquitous homopolymeric GAG containing only one type of repeating disaccharide unit that in vivo, during polymerization, undergoes sulfation at various positions through the activity of a variety of sulfotransferases therefore resulting in chains decorated with O-sulfo groups at various positions. CSs extracted from animal sources (human, pig, shark, squid) are often a combination of the different types. CSs are abundant and widely distributed in humans, other mammals, and invertebrates reflecting a central role in biological processes, in particular they may function as regulators of growth factors, cytokines, chemokines, adhesion molecules, and lipoproteins through interactions with the ligands of these proteins via specific saccharidic domains. Isolation and analysis has been reported for many sources.[14,15] Commercial CS is mainly derived from bovine trachea, pig nasal septa, chicken keel, shark fins, and fish cartilage (Table 7).

710

Cosmetics: Active Polymers

Protection against UV rays is guaranteed by the tanning response in which UV radiation triggers the production of melanin. The key upstream components of the melanin cascade process are the α-melanocyte-stimulating hormone and its receptor including tyrosinase. Tyrosine is hydroxylated to dihydroxyphenylalanine (DOPA) and to DOPA quinone by tyrosinase. After the generation of DOPA quinone, two separate pathways, which include several intermediate steps, lead to the formation of eumelanin and pheomelanin.[17] Those of melanin that are commercializing prepared for cosmetic products with different claims are exemplified in Table 8. Milk Proteins Milk was probably the first proteinaceous cosmetic ingredient used (Secchi, 2008). Milk proteins are composed of about 80% casein, an acidic phosphoprotein present as colloidal dispersion, and a soluble protein fraction composed of lactalbumin and lactoglobulin, which are found in milk serum after acid precipitation of casein. These three ­protein substances are useful as functional cosmetic ingredients (Table 9) as partial hydrolysates (casein, l­actalbumin) or in native form (lactoglobulin). Silk Proteins

Construction– Cosmetics

Silk is a continuous protein filament secreted by special glands located in the head of the silkworm (larva of the Bombyx mori). The special structure of the silk fiber is made of two distinct strands of fibroin, coated and connected by a second protein (sericin) exuding from other glands together with fibroin. Silk powder, whole silk protein hydrolysates, and sericin hydrolysates are the most common silk derivatives used in cosmetics. Fibroin is a glycoprotein formed of two subunits of 370 and 25 kDa connected by disulfide bonds. It is mainly composed of three amino acids, glycine, alanine, and serine, in the ratio 3:2:1, forming repeating sequences of six residues (GSGAGA) over the polypeptide chain. As silk is composed of about 70%–80% fibroin, products obtained by hydrolysis of whole silk are mainly fragments of fibroin. The second protein of silk, sericin, is a group of glycoproteins with a small carbohydrate content of about 3% and composed of five to six principal species having molecular masses between 65 and 400 kDa.[18–20] Biomedical applications of sericin including cosmetics

are shown in Table 10. Those of protein derivatives used in a variety of skin and hair care formulations including ­cleansing products are listed in Table 11. Enzyme Matrix Metalloproteinases MMPs, matrixins, are a family of secreted and membrane-bound zinc-dependent endopeptidases that have the combined capacity to degrade all the components of the ECM. These enzymes have a common zinc-binding motif (HEXXHXXGXXH) in their active site and a conserved methionine turn following the active site. MMPs have been found in vertebrates, invertebrates, and plants. They are distinguished from other endopeptidases by their dependence on metal ions as cofactors, their ability to degrade ECM, and their specific evolutionary DNA sequence. In skin, several different types of cells are capable of producing MMPs. Those are keratinocytes, fibroblasts, macrophages, endothelial cells, mast cells, eosinophils, and neutrophils. On the basis of substrate specificity and homology, MMPs can be divided into six groups: collagenases, gelatinases, stromelysins, matrilysins, membrane-type MMPs (MT-MMPs), and other MMPs. However, those that are relevant to skin elasticity are emphasized in Table 12. Those are needed to be inhibited or suppressed the activity, p­ rotecting degradation of collagen and elastin accordingly.[21–23] Miscellaneous Enzyme Proteolytic enzymes have been used for skin peeling and smoothing in addition to others application [22] as shown Table 10  Silk proteins in cosmetics INCI name or Trade name source Silk Pro-Tein

Hydrolyzed silk protein

Promois®

Supplier TRI-K Industries R.I.T.A Lonza

Solu-Silk® Akopro® silk

Akott

HYDROSILK

I.R.A. Istituto Ricerche Applicate

Crosilk™ protein complex

Croda

®

Setakol® powder

Hydrolyzed sericin

DSM

Silk sericin

Sericin

Bioland

Silk proteins

Quest

Silk amino acids (silk essence)

Huzhou Nanxun Shengtao Botanical

Milkpro®

IKEDA

Silkall 100

Promois® milk

R.I.T.A

Silkpro®

Pentacare HP

DSM

PromaEssence®

Table 9  Milk proteins Trade name

INCI name or source

Supplier

Naturein Casein Peptide®

Hydrolyzed casein

IKEDA Uniproma Chemical

Cosmetics: Active Polymers 711

Table 11  Proteins in cosmetics Protein

Function

Concentration

Skin care products Protein hydrolysates

Moisturizer, film former, conditioner, buffering agent

0.2%–5% in creams, lotions, and aftershave

Soluble collagen, desamino collagen, serum albumin, sodium caseinate

Moisturizer, conditioner

0.01%–0.1% in creams and lotions

Gelatin

Thickener, film former, humectant, stabilizer

1%–2% in emulsion

Potassium and sodium cocoyl hydrolyzed proteins

Coemulsifier

0.5%–2% in O/W emulsions

Silk power, insoluble elastin, and keratin

Oil absorber, cohesive agent

1%–5% in powder makeup

Protein hydrolysates

Conditioner, buffering agent

0.2%–2% in shampoo, conditioner, rinse, lotion, waving, and styling products

Hydrolyzed wheat proteins

Conditioner

0.5%–3%

Hair care products

AMP-isostearoyl hydrolyzed proteins

0.05%–1% in alcoholic conditioner

Alkyldimonium hydroxypropyl or ethyl hydrolyzed proteins

0.5%–2% in conditioner, rinse, conditioning perm, relaxer, lotion

Soluble proteins

Permanent conditioning

0.5%–5% in conditioning perm

Potassium undecylenoyl hydrolyzed collagen

Antidandruff

0.5%–2% in shampoo and conditioner

Potassium abietoyl hydrolyzed collagen

Scalp lipid regulator

1%–2% in shampoo and conditioner

Protein hydrolysates

Anti-irritant, conditioner, moisturizer

0.2%–5%

Soluble collagen, deamido collagen, serum albumin, sodium caseinate

Moisturizer, conditioner

0.01%–0.5%

Gelatin

Thickener, film former, humectant

1%–3%

Potassium and sodium cocoyl hydrolyzed

Detergent, foaming enhancer, anti-irritant

1%–10%

Alkyldimonium hydroxypropyl hydrolyzed

Anti-irritant, conditioner

1%–5%

Table 12 Metalloproteinases Type MMP Collagenase

Enzyme

Starting material

MMP-1

Collagenase-1

Collagen (I–III, VII, VIII, and X), gelatin

MMP-8

Collagenase-2

Collagen (I–III, VII, VIII, and X), gelatin, fribronectin

MMP-13

Collagenase-3

Collagen (I–IV, IX, X, and XIV) gelatin, fribronectin

MMP-18

Collagenase-4

Collagen I

Gelatinase

MMP-2

Gelatinase-A

Gelatin, collagen (IV–VI and X), elastin, fibronectin

MMP-9

Gelatinase-A

Gelatin, collagen (IV, V, VI, X, and XIV) elastin, fibronectin

Elastase

MMP-12

Metalloelastase

Gelatin, collagen (IV), gelatin, fibronectin, vibronectin, laminin

in Table 13. Enzymes have the capacity to degrade stains at low washing temperatures and are less toxic than surfactants and are used as supplements in detergents that largely preferred nowadays. In addition, specific enzyme dealing with pigment synthesis, i.e., tyrosinase, is one of the most commonly used in skin and hair care preparation (Table  14) in addition to other enzymes for cosmetics as exemplified in Table 15.

MICROORGANISM-BASED POLYMERS Polymers derived from microorganisms, including bacteria, yeasts, and moulds, represent an unexploited market. The polysaccharides produced by microorganisms can be classified into three main groups: (i) capsular polysaccharides, which provide a carbon and energy source for the cell; (ii) lipopolysaccharides, which make up the cell wall;

Construction– Cosmetics

Cleansing products

712

Cosmetics: Active Polymers

Table 13  Functions of enzyme applicable for personal care products Product Enzyme Cosmetics

Detergents

Hair dyeing

Protein disulfide isomerase, glutathione sulfhydryl oxidase, transglutaminase

Hair waving

Papain, bromelain, subtilisin

Gentle peeling agent in skin care

Amyloglucosidases, glucose oxidase

Toothpaste and mouthwash

Protease

Hydrolyzing protein-based stains in fabrics into soluble amino acids

Lipase

Decomposing fatty material stain on collars and cuffs

Amylase

Removing resistant starch residues

Cellulase

Modifying cellulose structure to increase the brightness and soften the cotton

Table 14  Tyrosinase for skin and hair care products Trade name INCI or source Supplier ZYMO TAN COMPLEX®

Technical benefit

Oxidase, peroxidase

Tyrosinase

I.R.A. Istituto Ricerche Applicate

ZYMO TAN COMPLEX PF® HYDROSOLUBLE ZYMO TAN COMPLEX Brookosome® TA

time, Aspergillus niger, Humicolo lutea, and Fusarium moniliforme are reported to be the producer of chitosan. In addition, two other strains that are commercially used to prepare chitosan are also reported. However, Bacillus pumilus chitosanase is more effective than Streptomyces griseus c­ hitinase.[1] Microorganism GAGs

Lonza

and (iii) exopolysaccharides (EPSs), which are exuded into the extracellular environment in the form of capsules or biofilm [24] Bacterial Polysaccharides Construction– Cosmetics

Bacterial polysaccharides have been exploited in a number of industrial uses. The particular application of a specific polysaccharide is a reflection of its unique physical ­properties.[25–27] Therefore, microbial polysaccharides are accounted as the important active ingredients, [1] particularly those of moisturizing products (Table 16) with different safety used concentrations (Table 17). Depending on their subunit composition, structure, and molecular mass, EPSs can have commercially relevant material properties that are attractive for cosmetic ­industrial applications (Table 18). Microorganism Chitin and Chitosan Chitin can be produced in the form of aminoglucan (poly-GlcNAc) by Saccharomyces cerevisiae (β-(1,6)-­ glucan), Aspergillus fumigatus (β-(1,3/1,4)-glucan), Candida albicans, Fusarium oxysporum, Microsporum fulvum, and Epidermophyton stockdaleae. On the means

Microorganisms can be regarded as the important source of GAGs production, [28] which are HA, CS, and HS (Table 19). Among them, HA is of particularly interested in cosmetic industry as shown in Table 20. Mushroom Polysaccharides Fungal polysaccharides comprise a large group of biopolymers which are either part of the cell wall or may form intracellular inclusions and serve as energy reserve, or are excreted extracellularly providing a mechanism for cell protection or attachment to other surfaces. Many of them derive from edible mushrooms, such as the maitake or shiitake or oyster mushroom, are generally recognized as safe or GRAS [29] as summarized in Table 21. In addition, those of commercializing mushroom polysaccharides available are summarized in Table 22. Seaweed Polysaccharides Seaweeds or marine macro algae are potential renewable resources in the marine environment and known to be extremely rich sources of bioactive compounds. Therefore, algae can be a very interesting natural source of new metabolites with various biological activities that could be used as functional ingredients. Biological activities are correlated to the presence of chemical compounds, particularly secondary metabolites. The presence of these compounds may assist in predicting some traditional uses of

Cosmetics: Active Polymers 713

Table 15  Enzymes for skin and hair care products Trade name

INCI or source

Supplier

For skin and hair care products Melaclear™ 2

Keratolytic enzyme

Sederma

Preregen

Oxido reductases

DSM

Papain

Papain

Spec-Chem Industry

Cyclolipase™

Lipase

Sederma

Regu® -AGE

Oxido reductases

DSM

Coenzyme Q10 (ubidecarenone)

Ubiquinone

Spec-Chem Industry

AC Coenzyme Q10 Liposome For personal care products

Active Concepts

BioNatural® enzyme

Lysozyme, glucoxidase, amylase, papain, amylogucoside and peptizyme sp and lactoferrin

BioOrganic Concepts

Biovert® enzymes and substrate

Lactoperoxidase & glucose oxidase

Lonza

DEPIL ENZYME

Subtilisin

I.R.A. Istituto Ricerche Applicate

ZYMO WRINKLE

Alkaline phosphatase

®

Protease

ZYMO ACID® ZYMO CLEAR MD

®

Lipase, subtilisin

Table 16  Bacterial polysaccharides in moisturizing cosmetics Polysaccharide Xanthan gum

Strain

Xanthomonas campestris

Xanthan gum cross polymer Xanthan gum hydroxypropyl trimonium chloride Sclerotium gum

Sclerotium rolfsii, Sclerotium glucanicum

Biosaccharide gum-1 Biosaccharide gum-2 Biosaccharide gum-3 Biosaccharide gum-4 Pseudoalteromonas EPSs

Pseudoalteromonas

Dextran sulfate

Leuconostoc mesenteroides

Beta-glucan

Aureobasidium pullulans, Agrobacterium biobar, Agrobacterium radiobacter

Beta-glucan hydroxypropyltrimonium chloride Beta-glucan palmitate Hydrolyzed beta-glucan Oxidized beta-glucan Alcaligenes polysaccharides

Alcaligenes latus

medicinal plants.[30–32] However, novel potential areas have to be explored in order to maximize the effective ­utilization of seaweeds. Alginates are one of the most well-known marine polysaccharides prepared from brown seaweeds in ­Phaeophyceae family. Macrocystis pyrifera is a major commercialized seaweed for alginates production. In addition, Laminaria hyperborea, Laminaria digitata, and Laminaria japonica are becoming important sources of good quality alginates.[33]

On the means time, marine red algae (Rhodophyceae family) largely produce carrageenans. Carrageenan is hydrocolloid that can be classified into κ-, ι-, and ­λ-carrageenans. Red-purple algae of the Rhodophyceae additionally synthesize agar of which Gracilaria and ­Gelidium sp. are the major grown genus for agar ­production.[31] Those of commercializing seaweed polysaccharides for cosmetic applications are exemplified in Table 23. Among these, Carrageenan is mostly used in cosmetic preparation

Construction– Cosmetics

Biosaccharide gum-5

714

Cosmetics: Active Polymers

Table 17  Bacterial polysaccharides in cosmetics Bacterial polysaccharides

Leave-on

Xanthan gum

0.001–6

Xanthan gum cross polymer

0.03–5

Concentration (%) in cosmetics Dermal contact 0.001–6

Biosaccharide gum-1

0.002–6

0.002–6

1

1

Biosaccharide gum-4

0.004–5

0.00001–5

Dextran sulfate

0.01–0.1

0.01–0.1

Sclerotium gum

0.003–2

0.003–2

1

1

0.0002–0.1

0.0002–0.1

0.3

0.005–0.3

Beta-glucan Alcaligenes polysaccharides

0.2–0.6

No report

Biosaccharide gum-2

Hydrolyzed sclerotium gum

Baby products

No report

Table 18  Example of commercializing bacterial polymers in cosmetics INCI name or Trade name source Supplier

Table 20  Example of commercializing bacterial HA INCI name or Trade name source Supplier

Schizophyllan

Biopolymer BHA-10

Contipro Biotech

Contipro biotechhyaluronic acid OligoHyaferre

Contipro biotechhyaluronic acid Phytohyaluronate

Kelcogel®

Gellan gum

Keltrol®

Xanthan gum

SC-Glucan

Beta glucan

Bioland

Herbex™ yeast beta-glucan

Yeast

Biospectrum

Carboxymethyl yeast beta-glucan (CMG) C90

Lipo Chemicals (Vantage Groups) Contipro Biotech

Construction– Cosmetics

Orina-G™ Yeast polysaccharides Scleroglucan

Table 19  Microorganism GAGs Microbe

Lonza

Table 21  Health beneficial mushroom polysaccharides Scientific name Common name Agaricus subrufescens

Almond/himematsutake

Cordyceps sinensis

Caterpillar fungus

Ganoderma lucidum

Reishitake

Grifola frondosa

Maitake

Welltech Biotechnology

Hericium erinaceus

Lion’s mane

Inonotus obliquus

Chaga

Induchem

Lentinula edodes

Shiitake

Pleurotus ostreatus

Oyster

Alban Muller International

Poria cocos

Tuckahoe

Trametes versicolor

Turkey tail

Angel Yeast

Glucano™

Yeast extract

CP Kelco

Yeast Polysaccharides M60

Amigum PF000015

Hyaluronic acid

HYSILK

Carboxymethylglucan

Uniglucan

Biopolymer HA-24

GAG

Streptococcus groups A or C

HA

Escherichia coli K4

CS with fructose on C3 of GlcUA

Chlorella PBCV-1 virus

HA

Pasteurella multocida type A

HA

Pasteurella multocida type F

Chondroitin (unsulfate)

Pasteurella multocida type D

Heparan (unsulfate)

Avibacterium paragallinarum

HA, CS, or HS

with different products as shown in Table 24. Furthermore, another interesting seaweed-producing polysaccharides are also summarized with their INCI listing functions as shown in Table 25. SEMISYNTHETIC-BASED POLYMERS They are chemically modified natural polymers such as esters or ethers of cellulose such as cellulose nitrate and methyl cellulose (MC).[25,34]

Cosmetics: Active Polymers 715

Table 22  Commercializing mushroom polysaccharides for cosmetics Trade name INCI name or source Supplier Tremoist™ TP

Tremella fuciformis polysaccharide

NIPPON FINE CHEMICAL

ACB mushroom extract SM

Ganoderma lucidum (Reishi)/Lentinus edodes (Shiitake)

Active Concepts

Reishi mushroom extract

Ganoderma lucidum

Carruba Lonza

Laricyl® LS 8865

Mushroom

Laboratoires Serobiologiques

Oliglycan

Tremella fuciformis, Lentinus edodes

Cosmetic Research & Development

Gandoderma extract

Ganoderma lucidum

Bioland

Carboxymethyl Cellulose The ionic polymer is water soluble can be produced and commercialized in free or salt forms. Of particular, commercial carboxymethyl celluloses (CMCs) are those with DS of 0.4–1.5 that make them pH media unaffected between 5 and 9. CMC is widely available in a number of grades for specific applications in the industry. For instance, extra pure CMC grade is used in food products, pharmaceuticals, and toothpastes while semi purified and technical grades may be used for oil drilling muds, paper coatings, textile sizing, mining operations, detergents, etc. Methyl Cellulose

Cellulose Ethers in Cosmetics Derivatization of cellulose into ether forms changes the anionic nature of cellulose into nonionic polymers with the maximum DS of 3 (3-hydroxyl group derivatizable).

Due to strong intermolecular hydrogen bonding between the hydroxyl groups on the cellulosic chains, unmodified cellulose is insoluble in water. However, methylation of cellulose introduces hydrophobic groups on the cellulosic chains producing MC which can be easily dissolved in cold water.

Table 23  Commercializing seaweed polysaccharides for cosmetics Trade name INCI name or source Algae polysaccharide

Brown algae

Supplier Spec-Chem Industry

Alginates

Alginates

C.E. Roeper

Flavikafine™

Calcium alginate

Nisshinbo Chemical

Protanal®

Propylene glycol alginate/algin

Soliance

Hydrintense

Porphyridium cruentum extract

Kelacid®, Kelcoloid®, Kelcosol®, Kelmar®, Keltone®

Kelp alginate

Ashland Specialty Chemical

EPS SEAMAT®

Plankton extract

Codif

Agar agar extract

Gelidium amansii (red seaweed)

Carruba

Organic irish moss powder

Chondrus crispus

Natural Sourcing

EPIDERMIST

®

Lichen

Lessonia

Irish moss seaweed extract

Carruba

Chondrus crispus flakes

Presperse

Gelalg

®

Oligogeline® Sea moss Hawaiian sea extract Q

C. crispus (carrageenan)

TRI-K Industries

Genu carrageenan

Ashland Specialty Chemical

Gelcarin®

Soliance

®

Viscarin

®

Genugel® Genuvisco

CP Kelco ®

Construction– Cosmetics

NAB® mushroom extract

Cellulose ethers are GRAS similarly to the native form. Therefore, they gain highly acceptance to be incorporated as functional and active polymers in cosmetics (Tables 26–28).

716

Cosmetics: Active Polymers

Table 24  Carrageenan in cosmetics Commercial carrageenan

Cosmetic products

Table 26  Water-soluble cellulose ethers Cellulose ether

-R

Danagel® AF 9254

Air freshener gel

Carboxymethyl cellulose (CMC)

Gelcarin® GP-379NF

Gelling and viscosifying agent

Sulfoenyl cellulose (SEC)

-CH2CH2SO3Na

Methyl cellulose (MC)

-CH3

Viscarin® GP-109NF

-CH2COONa

Viscarin® TP-389-CP

Toothpaste

Ethyl cellulose (EC)

-CH2CH3

GENUGEL® (kappa)

Air freshener gel, cell entrapment

Hydroxyethyl cellulose (HEC)

-CH2CH2OH

Hydroxypropyl cellulose (HPC)

GENUGEL® RLV

Air freshener gel

-CH2CH2CH2OH

Cyanoethyl cellulose (CyEC)

GENUGEL® CG-130

Gel, face mask, shower gel, emulsion

-CH2CH2CN

Methylcarboxymethyl cellulose (MCMC)

-CH3CH2COONa

GENUGEL® C1-102

Mould, encapsulation, air freshener

Hydroxyethylcarboxymethyl cellulose (HECMC)

GENUGEL® C1-121

Gelling agent, air freshener

-CH2CH2OH, -CH2COONa

GENUGEL® C1-124

Air freshener gel

Hydroxyethylmethylcarboxy methylcellulose (HEMCMC)

-CH2CH2OH, -CH3, -CH2COONa

Sulfoenylcarboxymethyl cellulose (SECMC)

-CH2CH2SO3Na, -CH2COONa

GENUGEL® C1-125 GENUGEL® C1-153 GENUVISCO® (iota and lambda)

Personal care and cosmetics

Hydroxyethylhydroxypropyl cellulose (HEHPC)

-CH2CH2OH, -CH3CH2CH2OH

GENUVISCO® CG-131 (iota)

Face mask, shaving product, spa, and nutricosmetics

Hydroxyethylethyl cellulose (HEEC)

-CH2CH2OH, -CH2CH3

GENUVISCO® CG-129 (lambda)

Lubricant, sprayable system

Hydroxypropylmethyl cellulose (HPMC)

-CH2CH2OH, -CH2CH2CH3

GENUVISCO® TPH-1/TPC-1 (iota)

Toothpaste, gel

Hydroxyethylsulfoethyl cellulose (HESEC)

-CH2CH2OH, -CH2CH2SO3Na

Cellulose ester Table 25  Candidate seaweed producing polysaccharides in cosmetics Species INCI listing functions

Construction– Cosmetics

Laminaria japonica

Skin protecting

Ascophyllum nodosum

Skin conditioning

Fucus vesiculosus

Soothing, smoothing, emollient, skin conditioning

Undaria pinnatifida

Skin protecting

Durvillea gelatinize

Skin protecting

Macrocystis pyrifera

Viscosity controlling

Once derivatized into ether forms, they turn to be nonionic polymer acting as thickening agent in the formulation with surface activity in addition to its task as film former. At a greater molecular weight, they become greater pseudoplasticity cellulose. Substitution with a higher methyl residue results in a harder gel. In contrary, if higher hydroxypropyl substitution took place, a softer gel will be offered. Therefore, MC including HPMC are DS ­unaffected on viscosity of cellulose ethers. Hydroxyethyl Cellulose This cellulose ether is the most hydrophilic with cloud point > 100°C. HEC (hydroxyethyl cellulose) tolerates extreme

-R

Cellulose acetate

-COCH3

Cellulose xanthogenate

-CSSNa

Cellulose sulfate

-SO3Na

Cellulose phosphate

-PO(OH) 2

Cellulose phthalate

-COC6H4COONa

Table 27  Cosmetic functions of cellulose ethers Application Celluloses Function Detergent

CMC, HEMC, HPMC

Anti-redeposition, wetting ability, suspender, emulsifier

Cream, shampoo, lotion, ointment, gel

CMC, MC, HEC, HEMC, MPMC

Thickener, binder, emulsifier, stabilizer, film former

pH and salts. Therefore, it is accounted and ­extensively used as surfactant. Hydroxypropyl Cellulose This polymer with cloud point ~45°C is more lipophilic than HEC. Therefore, it is easily dissolved in cosmetic solvents and giving a higher thickening effect. In addition, its thermoplastic nature (film forming during melts) with liquid crystallinity makes it insensitive to pH fluctuations.

Cosmetics: Active Polymers 717

Table 28  Example of some commercializing semisynthetic cellulose for cosmetics Trade name INCI name or source Benecel® MP943W

Hydroxypropyl methylcellulose

Supplier Ashland Specialty Chemical

Culminal® hydroxypropyl methylcellulose Tegocel® fluid HMP

Evonik

Walocel HM ®

Polyjel HV

United-Guardian

Methocel™

Dow Chemical

Cellosize™ HEC QP

Hydroxyethylcellulose

ETHARAMNOSAN

Vevy

Tylose® H

SE Tylose

Structure® CEL

Cellulose, ethyl 2-hydroxyethyl ether

AkzoNobel

CMC

Na carboxymethyl cellulose

C.E. Roeper

Cell-U-Lash

Cellulose

Kobo Products

Table 29  Modified starch for cosmetics Modified

Starch

Property

Amylose-free waxy

Maize, sorghum, amaranth, wheat, sweet potato, potato

Easily gelatinized Clear paste Stabilizer Thickener Emulsifier Freeze-thaw stability increases

High amylose starch

Maize, cereals, potato

High gelling strength Film former Adhesive

Altered amylopectin

Potato, rice

Low gelatinization (105

>1016

1015

>3 × 1016

1016

Mott Memory  Mott memory can be regarded as a type of ReRAM with a switching mechanism that is different from conventional resistive memories. The cell of Mott memory usually comprises a correlated electron insulator (also called Mott insulator) whose phase can be switched between an insulator state and a metallic state through a “Mott transition.” [16] A two-terminal Mott memory cell is shown in Fig. 8 along with the resistivity profile of a Mott insulator under a cyclic external perturbation in the form of electrical, optical, or thermal excitation. As the perturbation is slowly increased, the material resistivity does not change much as it is the insulating phase. However, at a threshold perturbation, the resistivity decreases by several orders of magnitude signaling a metal-to-insulator transition to the metallic phase (MP). Near the threshold, both insulating and MP can coexist. When the perturbation is reduced, the resistivity switches back to the original high resistivity state at a lower threshold resulting in a hysteresis.

V or I Metal Insulator Metal

IP Resistivity

The preceding equation shows that if ln(J/E) is proportional to 1/ 2 in the J–E data, then Poole–Frenkel conE duction can be established. Trap charging and discharging behavior manifests itself by a negative differential resistance regime followed by N-shaped I–V characteristics. Fowler–Nordheim and direct tunneling occur at high electric fields for thicker and thinner layers, respectively. Hence, J has an E2 dependence in the case of the former, whereas J is independent of E in the case of direct tunneling. ReRAM with crossbar technology has the potential applications for computer memory.[55] Although the reported characteristics of ReRAM devices using transition metal oxide materials seem to be adequate to replace Flash memory, practical demonstration of the same with a number of cells in integrated circuits is surprisingly pending.[56] Considering the progress made in ReRAM, it will be soon available in the market as computer memory devices and mobile devices. However, more research has to be performed in understanding the switching mechanisms for improving the storage density and scaling down of ReRAM in the future. Table 3 shows projected parameters for future NVMs in 2022.[57] NAND Flash is compared with the four promising NVMs, namely, ReRAM, PCM, magnetic RAM, and FeRAM. The other emerging NVMs are briefly discussed in the next Sections (Mott Memory, Carbon Memory, Molecular Memory and Millipede Memory).

FeRAM

NVR

MP External excitation

Fig. 8  A Mott memory cell and the resistivity profile across the metal–insulator transition in a Mott insulator with NVR

Within the hysteresis loop, the system’s resistance could be nonvolatile resistivity.[58] Recently, Mott memory based on materials such as Ge, V, Nb, Ta, Se, and Ga exhibited nonvolatile behavior with fast erasing and writing times ranging from 10 to 50 ns.[59,60] Picket et al.[61] demonstrated a Mott device with NbO2 exhibiting a fast switching speed of 103) was developed by Dellmann et al.[63] with hydrogenated amorphous carbon-based memory with a Cu/a-C:H/TiN structure with a switching speed of < 30 ns and nondestructive readout. Recently, Tsai et al.[64] observed a high ON/OFF ratio of 5 × 105 in the nanosecond regime in CNT/AlOx / CNT crossbar device with an estimated energy of 0.11 fJ/ bit and the power at 10–100 nW. Schematic diagram of the device with top and bottom carbon nanotube (CNT) electrodes sandwiching an AlOx thin film is shown in Fig. 9. Fullerene-based materials in polymer matrix have shown a high ON/OFF ratio of >105.[65] Although carbon-based

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C AlOx

NT

CNT CN

Crossbar

T

RRAM OFF

0.1 fJ/bit

ON

Fig. 9  CNT crossbar ReRAM with single nanometer bit scaling Source: Reprinted with permission from Tsai et al.[64]

memories have high ON/OFF ratio, the exact physical mechanism causing the resistive switching in carbon materials is not clear. Such carbon–polymer composites would be reviewed later under ReRAM section. Nanotube RAM or nano-RAM (NRAM) is a proprietary emerging NVM of Nantero. [66] The second-generation NRAM is a two-terminal device based on the position of CNT deposited on a chiplike substrate in nonwoven fabric style. Each NRAM cell consists of an interlinked network of CNTs located between two metal electrodes. The crossed CNTs may be touching or slightly separated. When the CNTs are not touching, the resistance of the CNT fabric is high. When the CNTs are brought into contact, the resistance of the CNT fabric is low. NRAM acts as a resistive memory because the LRS and HRS are very stable. Since nanotubes are very small, they can store large amounts of data which is 10–100 times of current memories. NRAM is expected to challenge NAND Flash in power consumption, speed, endurance, and capacity. The main advantage of NRAM over NAND Flash is its resistance. Nantero claims that NRAM can last for 1,000 years against heat up to 85°C and 10 years up to 300°C without data loss. [66] A 22 nm technology NRAM NVM has been fabricated and tested on CMOS substrates. [67] A joint venture by Nantero and Fujitsu is expected to produce commercial NRAM chips in 2018. Molecular Memory  Molecular memory makes use of molecular species for data storage. Memories using ­polymer or long-chain molecules are generally called as macromolecular memory. The macromolecular memory cell is similar to the Mott memory cell. The storage or memory function of molecular memory is based on molecular switching involving a change in either the capacitance or the resistance of the cell.[16] A typical capacitive molecular memory consists of a redox active monolayer such as metal porphyrins (say, ZnAB3P) or ferrocene grafted on a Si substrate.[68] These methods exploit self-assembled monolayers of molecular species capable of capturing or releasing electrons. Jonathan et al.[69] fabricated a 160 kb molecular memory by placing a monolayer of rotaxane molecules between Si and Ti nanowire electrodes. The crossbar design yielded a high density of 1011 bits/cm2. Resistive molecular memory consists of organic material sandwiched between two metal electrodes.[16] Radha et al.[70] demonstrated an

ON/OFF ratio of 103 with reduced power consumption and excellent stability and retention in a molecular device with palladium hexadecylthiolate. Macromolecular memory label with graphene electrode has been fabricated on a Polyethylene terephthalate (PET) substrate by a simple and cost-effective strategy.[71] This memory label-based device exhibited an ON/OFF ratio of ~106. Scaling beyond 100 nm is a core challenge in macromolecular memory. Simplicity, flexibility, and cost-­effective macromolecular devices hold promise in p­ ractical applications in portable electronic devices.[16] Millipede Memory  Millipede is a nano-cantilever tipbased data storage concept developed by IBM in 2002 that combines ultrahigh density, terabit capacity, small form factor, and high data rate.[72] Millipede storage attempts to combine features of both a magnetic HDD and a DRAM. Like a hard drive, millipede stores data in a medium and accesses the data by moving the medium under the head as well. However, millipede uses many nanoscopic heads that can read and write bits in parallel in nano-indentations burnt on thin polymer layers. Initial areal densities of 100–200 Gb/in2 have been achieved. [73] However, Millipede memory has not yet been ­commercialized due to competition from other NVMs. RECENT MARKET TRENDS OF EMERGING NONVOLATILE MEMORIES A recent report by Yole Development Company predicts the future market of the new emerging NVMs.[74] This report includes a roadmap for future NVMs; identifies key industrial key players in NVM market; describes NVM technologies by price, endurance, and cell size; and covers the trends and challenges in main application areas and a market forecast for MRAM/STT-RAM. Considering all the emerging NVMs in Fig. 1, market survey shows PCM, ReRAM, and STT-RAM as the most promising NVMs. The 2016 commercial product performance of PCM, STTRAM, and ReRAM is briefly compared in Table 4. By 2021, total market for these three NVMs is expected to be US$4,622 million. Storage class memory (SCM) will be the biggest emerging NVM market. PCM leads the race to SCM for now thanks to 3D XPoint developed by Intel/Micron, but ReRAM is expected to catch up soon. In the stand-alone market mostly focused on SCM for the next 5 years, Micron/Intel has chosen PCM, whereas SK Hynix and Sandisk/Western Digital have selected RRAM as the competitor to PCM, and Samsung seems also to be favoring ReRAM thanks to its compatibility with the vertical 3D approach used for 3D NAND. Substitution of 3D NAND by ReRAM and DRAM by STT-RAM will commence very slowly before 2021. The big question is, which NVM will be chosen by whom? Panasonic and SMIC made the early move and selected ReRAM, and some top

Table 4  2016 commercial product performance of leading NVMs Feature of product in 2016 PCM STT-RAM ReRAM Endurance (Nb cycle)

108

1012

106

Technological node produced (nm)

20

40

130

Cell size (F2)

Unknown

Medium (6–12)

Medium (6–12)

Read latency (ns)

Fast (50–100)

Fast (10–20)

Medium (250)

Power consumption

Medium

Medium (50 pJ/bit)

Medium (6 nJ/bit)

Price (US$/Gb)

102

Electron trapping

[172]

6

ITO/PVA + GO/Al

3

3×10

Pool–Frenkel emission, ohmic

[173]

7

Al/Au-DT NPs + 8HQ + PS/Al

2.8

104

F–N tunneling, direct tunneling

[174]

8

MLG/PI:PCBM/Al

4

>10

Charge trapping

[175]

9

ITO/PS/graphene/PS/Al

3.2

107

Charge trapping

[176]

10

Al/PS:C60/Al

2

10

F-N tunneling

[177]

11

ITO/PVK-C60/Al

3

>105

Charge transfer

[178]

12

Al/PI/Au NPs/P3HT:PCBM/Al

3

10

SCLC, thermionic emission, ohmic

[179]

13

ITO/PMMA-ZnO NR/Al

2.5

104

SCLC, TCLC

[180]

14

ITO/ZnO NP:PMMA/Al

1.5

>10

F–N tunneling

[181]

15

ITO/Ag:PMMA/Al

2

10

Thermionic emission, SCLC, ohmic

[182]

16

rGO/P3HT:PCBM/Al

0.5–1.2

10

Ohmic, thermionic emission

[183]

17

Al/PVK:Au NPs/PEDOT–PSS/Al

4.3

5 × 109

Schottky emission

[184]

18

Al/Au-2NT NPs + PS/Al

2

10

Charge transfer

[185]

19

Al/Au NP + P3HT/Al

3–4

105

Poole–Frenkel emission

[186]

20

ITO/PET/pEGDMA/Cu

1.5

>10

Carbon filament formation

[187]

21

ITO/Au NPs:PVK/Al on PET sheet

0.7

4×105

SCLC, F-N tunneling

[188]

22

Al/PMMA/graphene/PMMA/ITO

103

Filament formation

[195]

29

Mg/Ag:chitosan/Mg

1.63

>102

SCLC and ohmic

[196]

30

ITO/GO + PANI/Al

1.3

>10

4

Electron trapping

[197]

31

Ag/MoS2-PVA/Ag

0.7

>102

SCLC and ohmic

[198]

32

Al/PVP + C60 /Al

2.5

>10

Electron trapping

[199]

33

ITO/nanocellulose fibers/Ag

1.5

>107

Filament formation

[200]

and gelatin. Additives used along with the polymers in the active layer include zero-dimensional (Au, Ag, ZnO, PCBM, C60, etc.), one-dimensional (ZnO nanorods, CNT, nanofibers), and 2D (graphene, rGO, MoS2) nanomaterials. Although some pure polymers (configuration (i) in Fig. 11) have showed bistable behavior under an applied field, efficient switching is obtained in polymer nanomaterial composite layer (configuration (iv) in Fig. 11). The mostly used substrate is glass with ITO coating which serves as a base as well as the bottom electrode. However, polymer ReRAM cells with flexible substrates such as PET coated with ITO have also been fabricated and tested. The wide varieties of polymers with different molecular

3

4

6

4

3

3

6

3

2

7

3

6

2

weights and nanomaterials have opened up the scope of fabricating polymer ReRAMs with a variety of properties. Table 5 provides a list of polymer ReRAM cells chosen specifically from the literature to illustrate the variety of switching characteristics and conduction mechanisms possible in this class of NVMs. The complexity in the behavior and reason for the lack of a unified and generalized understanding of the resistive behavior in polymer ReRAMs is evident from the examples shown in Table 5. While these organic ReRAMs offer plenty of advantages over the existing memory technologies, there are several challenges such as reproducibility and retentivity. These aspects can be addressed by improving the cell uniformity,

understanding of the conduction mechanisms and switching characteristics in a wholesome manner, and obtaining better polymeric dielectrics. If the intense research efforts under way provide the expected breakthroughs, it will not be long before the markets see polymer-based ReRAMs.

CONCLUSION In this entry, existing and emerging memory technologies have been reviewed. Although DRAM and NAND Flash technologies are the main players in the global market, NVMs are slowly and steadily replacing them. Although it is not easy to identify the most promising NVM technology from the ones reviewed as this is mainly driven by the preference and R&D success of the memory industry, there are several indicators. Recent advent of 3D XPoint NVM in the market with features superior to the baseline memories is considered as the first breakthrough in this sector. NRAM is also poised to enter the market soon. Such breakthroughs are also expected in STT-RAM and ReRAM soon. It will not be long before the immense potentials demonstrated by polymer-based ReRAMs are harnessed into products.

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Electronics: Polymer–Graphene Composites Seema Ansari and M.N. Muralidharan Centre for Materials for the Electronics Technology, Thrissur, India

Abstract Polymer–graphene composites have emerged as a new class of materials which combines the unique electrical, optical, and thermal properties of graphene with the structural and functional properties of polymers to yield low cost, flexible, and lightweight devices. This entry reviews various methods of preparation for polymer–graphene composites and their applications in various emerging areas including sensors, actuators, shape memory, electromagnetic interference shielding, energy storage, etc. In addition to the basic working principles of polymer–graphene composites, we have also briefly reviewed the recent developments in each area of application. The improvement in properties with the use of polymer–graphene composites as active materials in different application has been highlighted with suitable examples. The methods used to improve the properties of these composites by various researchers are discussed in detail in order to enable the readers to find out more and more new application by suitable modifications. The entry gives an introduction to the electronic applications of polymer–graphene composites. Various other related applications based on the electrical, thermal, chemical, optical, and structural properties of polymer–graphene c­ omposites are also discussed in this entry. Keywords: Actuators; Composites; Electrodes; Graphene; Polymer; Sensors; TCEs.

INTRODUCTION Miniaturization and multifunctionality are the two major factors that drive the current and future electronic industries. Development of suitable multifunctional materials which can address simultaneously several parameters such as lightweight, flexibility, conductivity, environmental impact, and production cost is the key factor to attain multifunctional and miniaturized devices. Polymer electronics that can address these factors effectively opens up a new and high potential technological field, leading to many novel applications and products. Graphene is a monolayer of graphite where the atoms are arranged in a honeycomb network. It has attracted technological interest due to high electron conductivity, high aspect ratio, remarkable mechanical properties, quantum Hall effect at ambient temperatures, controllable band gaps, excellent thermal and electrical properties, electromagnetic interference (EMI) shielding, and flexibility. These unique properties make graphene to be used in many electronic applications such as electronic circuits, actuators, sensors, electrodes for supercapacitors and batteries, transparent and flexible electrodes for displays, and solar cells. Recently, considerable interest has been attracted in polymer nanocomposites with graphene as filler. Many studies reported that graphene as filler is better than conventional nanofillers such as sodium montmorillonite, layered double hydroxide, carbon nanotubes (CNTs), and exfoliated or

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expanded graphite (EG). The improvement in the properties of the polymer by the incorporation of graphene depends on the homogeneous distribution of graphene in the polymer matrix and interfacial bonding between the graphene layers and the polymer matrix. The interface between graphene and polymer determines the final characteristics of polymer–­graphene nanocomposites. Due to the incompatibility of graphene with polymers, pristine graphene is very difficult to homogeneously distribute in the polymer matrix. Hence, surface modification of graphene is usually carried out for obtaining a molecular level dispersion of graphene in a polymer matrix. In the present review, we will be dealing with the polymer–graphene composites for electronic applications. Polymer–graphene composites have applications in energy storage as transparent electrodes for solar cells, electrodes for supercapacitors and batteries, etc. They have great potential as transducers, which include electrical, optical, and acoustic actuators. Polymer–graphene composites can be used for the development of low-cost sensors. The review details the preparation, properties and applications of plastics (both thermoplastic and thermosets), e­ lastomers, and blends filled or coated with graphene. The review will cover the following applications of polymer–graphene composites in the field: 1. Actuators 2. Energy storage Encyclopedia of Polymer Applications, First Edition DOI: 10.1201/9781351019422-120054048 Copyright © 2018 by Taylor & Francis. All rights reserved.

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EMI shielding and antistatic application Shape memory application Sensors Flexible and printed electronics

SYNTHESIS OF GRAPHENE Graphene can be produced mainly by four different methods: (1) chemical vapor deposition (CVD), (2) mechanical exfoliation, (3) thermal decomposition of silicon carbide, and (4) chemical synthesis through graphite oxide ­intermediate.[1,2] CVD is a process in which carbon is supplied in gas form (e.g., ethylene, methane, etc.) and a metal (nickel, copper, etc.) is used as both a catalyst and a substrate to grow the graphene layer. Obraztsov et al. have reported 1–2-nm-thick graphene sheets grown on Ni substrates by using a precursor gas mixture of H2 and CH4 (92:8 ratio), at a total gas pressure of 80 torr.[3] They have also reported three- to four-layer graphene formation on Ni foils of 500 μm thickness, through CVD with the use of a precursor gas mixture of CH4, H2, and Ar (0.15:1:2 ratio), at a total flow rate of 315 sccm, at 1,000°C for 20 min.[4] Growth of single- to few-layer graphene (FLG) on the Ni film by CVD was reported by many other researchers.[5–8] High-quality and uniform graphene was synthesized on a 1-cm2 area of Cu foil by CVD process.[9] To get high-purity graphene, mechanical exfoliation of highly oriented pyrolitic graphite is used.[10] One-millimeter-thick graphite sheet was subjected to oxygen plasma dry etching to create many 5-μm-deep mesas and then put on a photoresist. By using a scotch tape, layers were peeled off from the graphite sheet. Since this process is very simple, it was well accepted and adopted by scientific community.[11–13] However, the bulk production of graphene using this technique is limited. Modifications of this process were also reported.[14] Single- to few-layer graphene sheets were produced from either graphite or graphite oxide powder and using different solvents such as N-methyl-pyrrolidone, sodium dodecylbenzene sulfonate (SDBS), and dimethyl formamide (DMF).[15–20] Thermal decomposition of silicon carbide is one of the highly popular techniques for graphene growth.[21–25] In this process, silicon carbide is heated in ultrahigh vacuum to temperatures between 1,000°C and 1,500°C. The number of graphene layer grown epitaxially on the surface is being dependent on the decomposition temperature. One-atomthick graphene was produced using a similar t­ echnique by Rollings et al.[22] For the bulk production as well as for the use of graphene as a filler in polymer matrix, the most commonly used method is wet chemical production of graphene through graphite oxide intermediate. For the production of graphite oxide, the modified Hummers method is commonly employed. In this process,

graphite is treated with an anhydrous mixture of sulfuric acid, sodium nitrate, and potassium permanganate for several hours, followed by the addition of water. The resulting graphite oxide contains a combination of hydroxide, carbonyl, carboxyl, and epoxide groups covalently bonded to the graphite lattice. Graphite oxide is exfoliated to form graphene oxide (GO) by ultrasonication. The subsequent reduction of exfoliated GO sheets to graphene was carried out using several methods including chemical reduction (using reducing agent such as hydrazine hydrate, dimethylhydrazine, sodium borohydride, and sodium hydroxide), thermal reduction, photocatalytic reduction, etc.[26–36] SURFACE MODIFICATION OF GRAPHENE Pristine graphene is highly prone to agglomerate in a polymer matrix and is not suitable for intercalation by large species, such as polymer chains.[37,38] Surface modification of graphene is required to make soluble graphene in a stabilization medium. The chemical modification of graphene is found to be highly attractive in which functionalization of graphene with small molecules or polymer chains is carried out.[39–47] This can improve the processability and solubility of graphene, thereby enhancing the interactions with polymers. Much research has been carried out on the esterification, amination, isocyanate modification, and polymer wrapping for the functionalization of graphene.[43–45,47–49] Covalent modification of graphene by alkyl lithium reagents, different organic amines, isocyanates, and diisocyanate compounds was also reported.[46–48] GO sheets contain a large amount of functional groups (hydroxyl, diols, epoxide, ketones, and carboxyls), which make them strongly hydrophilic and can be used as fillers directly in water-soluble polymers. Ionic liquid (IL)-functionalized graphene (f-G) sheets were prepared from graphite by the electrochemical process. In the single-step electrochemical process, ionic liquid-functionalized graphite sheets were prepared, and the exfoliation yielded ionic-liquid f-G sheets.[50] Poly(N-isopropylacrylamide) (PNIPAAm)-modified graphene was prepared by the π–π interaction between the π orbitals of graphene and PNIPAAm. PNIPAAm-modified graphene is a water-dispersible system.[51] POLYMER–GRAPHENE NANOCOMPOSITES Different allotropic forms of carbon are used as filler in polymer matrix to prepare conducting composites. Nano-conducting fillers such as CNT or graphene can enhance the conductivity at very low filler loading compared to conventional fillers such as carbon black. The extent of improvement is highly depended on the degree of

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dispersion of nanofillers in the polymer matrix. Graphene is the new addition to the carbon family with unique electrical, optical, thermal, and mechanical properties. Polymer–graphene nanocomposites have immense potential applications in the fields of electronics, optoelectronics, aerospace, defense, automobile, green energy, etc. There have been many studies on polymer composites using graphene, GO, and reduced GO (rGO)in combination with a wide range of polymers.[52–77] Based on the application, researchers have prepared polymer–graphene composites with different polymer systems including polyethylene (PE), [52,64] polymethylmethacrylate (PMMA), [53,65] polypyrrole (PPY), [54,55] epoxy, [56–58] polyaniline (PANI), [59–63] polyacrylonitrile, [65] polyvinyl alcohol (PVC), [66–70] polystyrene, [71] polyacrylamide, [72]polyimide, [73] polycarbonate, [74] poly(allylamine), [75] polycaprolactone (PCL), [76] polyurethane (PU), [77] etc. Preparation of Polymer–Graphene Nanocomposites Graphene has a higher surface-to-volume ratio compared to other nanofillers such as CNTs, which makes them more favorable for improving the properties of polymer matrices. To get advantages of its properties, integration of individual graphene in polymer matrices is very important. There are mainly three synthesis routes for the production of graphene–polymer nanocomposites, which are as follows:

Electronics: Polymer–Graphene Composites

Solution Mixing In this process, the colloidal solution of graphene or modified graphene is mixed with polymer or prepolymer solution by sonication or shear mixing. Solvent compatibility of graphene and the polymer is critical for attaining homogeneous dispersion. It also depends on the degree of exfoliation of the graphene platelets. Graphene or surface-modified graphene can be dispersed in a suitable solvent, such as DMF, chloroform, acetone, tetrahydrofuran (THF), toluene, or water. GO that contains hydroxyl, carboxyl, and various other residual functional groups can be directly mixed with water-soluble polymers, such as PVA. The polymer chains will adsorb onto the delaminated graphene sheets, and when the solvent is removed, the sheets restack, intercalating the polymer to form the nanocomposites. The entropy gained due to the desorption of the solvent molecules from the sheets forms the driving force for the intercalation of polymer from solution thereby compensating the decrease in conformational entropy of the intercalated polymer chains. Solution mixing techniques can be used even for the preparation  of nanocomposites having polymers with low or even no polarity. This technique is used to prepare graphene nanocomposites with PE, [64] PMMA, [65,66] PVA, [67–70] PS, [71] polyacrylamide, [72] polyimides, [73] polycarbonate, [74] poly(allylamine), [75] PCL, [76] PU, [77] etc. Melt Mixing

1. In situ polymerization 2. Solution mixing 3. Melt mixing In Situ Polymerization In this process, graphene or modified graphene is dispersed in monomer or a solution of monomer, followed by polymerization in the presence of graphene. A variety of polymer nanocomposites have been produced using this method including poly(ethylene),[52] PMMA,[53] and poly (pyrrole),[54–55] epoxy,[56–58] and PANI.[59–63] Graphene–epoxy nanocomposite is prepared by mixing a graphene-based filler in the epoxy resins under high-shear forces followed by the addition of a curing agent for initiating the polymerization.[56] Graphene–PANI is prepared by the in situ oxidative process, using ammonium persulfate as an oxidative agent to facilitate the polymerization.[62] Graphene–PANI composite is prepared by the in situ anodic electropolymerization by directly depositing PANI on the working electrode made from graphene paper (G-paper) in a three-electrode cell.[63] Compared to solution mixing methods, a high level of dispersion of fillers is achieved in in situ polymerization without a prior exfoliation step. There are reports in which monomer is intercalated between the layers of graphite or GO, then polymerized to exfoliate the layers.

In this process, graphene or modified graphene is mixed under high shear conditions with the polymer in the molten state. In the case of thermoplastic polymer, after mixing, the composites are processed using conventional  methods, such as extrusion and injection molding. [78] Since no solvent is used, melt mixing is often ­considered more economical compared to solution mixing, and moreover, it is compatible with many current industrial practices. [79] A wide range of polymer nanocomposites, especially thermoplastic nanocomposites, such as PP/EG, [78,80,81] LLDPE/EG, [82] HDPE/EG, [83] PPS/EG [84] PA6/EG, [85] polylactide (PLA)-(EG), [86] and PET-rGO, [87] have been prepared using this method. The high shear forces used in melt mixing can cause breakage of the filler materials, such as CNTs and graphene nanosheets (GNSs). [88] The low bulk density of graphene powders makes handling of the dry powders difficult and cause processing difficulties. In order to circumvent this issue, a solution mixing process is used to disperse the graphene in the polymer matrix prior to compounding, [89] and another approach is used to “premix” the polymer and filler prior to mixing.[80] It is also reported that melt processing and molding operations may cause a substantial reduction of the platelets due to their thermal instability.[90]

ELECTRONIC APPLICATIONS OF POLYMER– GRAPHENE NANOCOMPOSITES Graphene–polymer nanocomposites have found applications in various electronic and photonic devices including actuators, sensors, capacitors, fuel cells, etc. The following sections will elaborate on the major applications of polymer–graphene composites and recent developments in each field. Polymer–Graphene Composites for Actuator Applications “Actuator” is a term generally used to represent materials or devices, which can undergo a mechanical deformation when excited with an appropriate external stimulus. [91] In the recent past, graphene–polymer composite actuators have attracted considerable attention of researchers because of their unique advantages such as low cost, ease of processing, large displacements obtained, and their resemblance to the properties of human muscles. These features make them the most promising candidates for many applications including artificial muscles and robotics. Graphene–­polymer composite actuators are considered as one of the most versatile actuator systems since they can be triggered by an electrical, optical, thermal, or chemical stimulus. Actuators Driven by Electrical Stimulus The electrically triggered polymer actuators can basically follow two different mechanisms: by utilizing Maxwell’s stress, which is generated as a result of the electrostatic attraction between the two electrodes, or by pure electrostrictive effect.[92] However, there can be another class of polymer actuators in which the large mechanical deformation is caused due to the movement of charge by ions at low voltages as in the case of ionic polymer–metal composites.[93] There have been several reports during the last few years to develop electromechanical actuators with excellent actuator performance by combining the unique properties of graphene with different polymer systems. Unlike the conventional polymer based electromechanical actuators, graphene–polymer composite actuators follow different mechanisms in different systems depending on the materials and the design of the actuators. This makes them the most versatile actuator materials for various ­practical applications. In a study by Chen et al., using poly(methylmethacrylate) f-G–PU (MG–PU) composite actuators, it was found that the introduction of f-G into PU matrix significantly improved the electric field induced strain behavior compared to pure PU films. There was an increase in the electric field induced strain from 17.6% for pure PU to about 32.8% for 1.5 wt% MG–PU composite film which is almost two times that of pure film.[94]

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Fig. 1  Graphene-on-organic film, which is in the form of a dragonfly wing Source: Reprinted with permission from American Chemical Society, © 2011.[95]

Graphene can undergo contraction on heating due to its negative temperature coefficient of thermal expansion. This unique property was utilized by Zhu et al. for designing a bimorph actuator using graphene–epoxy hybrid system.[95] The graphene-on-organic film actuator was developed as a cantilever in which graphene acted both as the conducting layer and heating layer. Upon applying electric power, the graphene was directly heated and the epoxy was warmed up by diffused heating. Due to the mismatch in thermal expansion of graphene and epoxy, the cantilever exhibited a deflection toward the graphene-coated side. The device exhibited high actuation behavior at very low power. For instance, the cantilever tip showed a deflection of 1 µm with an input voltage as low as 1 V within 0.02 s and returned back to its original position within 0.1 s. They have also reported that the flapping and bending motion of the actuators can be controlled by changing the frequency and duration of applied voltage. They have demonstrated the working of this graphene-on-organic film actuator in the form of a dragon fly wing as illustrated in Fig. 1. The performance of the graphene-based organic bimorph ­actuator is depicted in Fig. 2. Figure 2a gives the temperature extracted from the emitted infrared (IR) radiation as a function of the input power, and the solid line shows the best fit to the data obtained by finite element analysis (FEA). The inset shows a color contour plot of the temperature profile of the cantilever, and the dotted gray lines represent the beam boundaries. Color scale from 17°C to 40°C is shown on the right. In Fig. 2b, the tip displacement of the cantilever beam as a function of temperature is given. Each data point is the average value of five measurements, and the standard deviation is shown as error bars. The solid line shows the fit to the data obtained from FEA. The insets show the SEM images of the initial position of the cantilever beam (top left) and bend up state upon applying electrical power (right bottom). Figure 2c shows the tip displacement of the cantilever beam as a function of time extracted from eight repeated cycles of input sweep voltage from1 to 4 V

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Fig. 2  Performance characterization of a graphene-based organic bimorph microactuator Source: Reprinted with permission from American Chemical Society, © 2011.[95]

for 1 s followed by off power period for 0.1 s in each cycle. In Fig.  2d, the resistance of graphene serpentine microheater as a function of the displacement of cantilever tip is depicted. The dashed line shows a polynomial fit (order of three) to the data. A similar bimorph actuator based on a bilayer of graphene and polydiacetylene (PDAC) was reported by Liang et al.[96] The actuator generated large actuation motion under low electric current in response to both dc and ac signals. For example, for a bimorph of size 10 mm length by 2.7 mm width, at a very low dc current of 20 mA (a current density of 0.74 A/mm2), a displacement as large as 1.8 mm and a curvature of 0.37 cm−1 were obtained. Similarly, an actuation stress as high as 160 MPa/g was obtained under an applied dc of only 0.29 A/mm2. Under ac signal, this actuator displayed reversible swing behavior. Moreover, it was also demonstrated that a strong resonance can be generated when the frequency and the value of applied ac and the state of the actuators reach an appropriate value. This interesting actuation behavior was explained based on two mechanisms. The electric field-induced deformation is considered as the dominant one, and the thermal-induced expansion of PDAC was proposed as the second mechanism contributing to this high actuation performance. With an aim of meeting the stringent requirements for human related bio-engineering and biomimetic robotics, a high fidelity bioelectronic soft actuator was reported based on biofriendly 2,2,6,6-tetramethylpiperidine-1-oxyl radical-oxidized bacterial cellulose (TOBC), chemically modified graphene (G), and IL with PEDOT:PSS as the electrode.[97] The introduction of 0.1 wt% of chemically modified graphene into the TOBC–IL system enhanced the ionic conductivity by 120%. The specific capacitance and

the mechanical properties were also enhanced drastically with the addition of graphene. This altogether resulted in an increased bending deformation by 2.3 times without serious back relaxation phenomena. Moreover, the strong ionic interactions among the carboxylic groups of TOBC, chemically modified graphene, IL, and PEDOT:PSS resulted in increased IL absorption. This in turn facilitated easy and fast ion transport within the nanocomposite matrix, thereby enhancing the bending actuation performance. Recently, the electroactive performance of grapheneloaded cellulose composite actuators was reported by Sen et al.[98] The films of microcrystalline cellulose loaded with graphene nanoplatelets were prepared by solvent casting method. An IL, 1-butyl-3-methylimidazolium chloride, was used as the solvent. The composite films were converted into actuator strips by forming electrodes using gold leaf. The incorporation of graphene enhanced the conductivity and mechanical properties of the composite. The actuator performance was measured at 3–7 V. The study reported that the graphene loading reduced the actuation speed but enhanced the ability to operate at higher excitation voltages. Moreover, an increase of 267% in the maximum displacement at an excitation voltage of 3 V was achieved with the addition of graphene. This clearly indicates that the loading of graphene nanoplatelets resulted in better electroactive performance of the cellulose-based composite actuator. Electro-active actuators based on graphene-­reinforced Nafion composite electrolytes were developed, and their actuation performances were investigated by Jung et al.[99] In addition to the enhanced mechanical properties imparted by graphene loading, the proton conductivity was drastically improved on incorporation of graphene. The displacement obtained for graphene-loaded Nafion at 0.5 V

was twice that of recast Nafion. When the exciting voltage was increased to 1.5 V, the increase in displacement for the composite was three times. This displacement was higher than that for single-walled carbon nanotube reinforced ionic polymer metal composites (IPMCs) reported earlier. The bending deformations and blocking force obtained for graphene–Nafion composites is much higher than that of recast Nafion-based IPMC actuators. The efficiency analysis confirmed that the electromechanical efficiency of the graphene–Nafion composite actuators is almost twice than that of recast Nafion. In a similar Nafion-based actuator system, instead of incorporating graphene into the Nafion matrix, graphene–PANI composite was used as the electrolyte.[100] Maximum conductivity was obtained for films with aniline and 90% graphene. A tip displacement of 8.1 mm was obtained for 6.8 V. Actuators Driven by Optical Stimulus Polymer nanocomposite based optical actuator is one of the fast developing fields in the contemporary research. The unique features, such as wireless actuation and remote controllability make them one of the prime focuses of actuator research. Optically triggered actuators are potentially important in biomedical field where any stimulus other than electricity is more preferred to drive the ­actuators. Basically, an optically triggered actuator consists of an “energy transfer unit” which absorbs the light energy and a “molecular switch unit” where the mechanical deformation takes place. In a polymer composite-based optical actuator, the filler acts as the “energy transfer unit” and the polymer matrix itself acts as the “molecular switch unit.” Due to the excellent IR absorption characteristics of graphene, there have been considerable interest to utilize graphene–­polymer composite actuators, which can work under IR light as the stimulus. The optical actuation performance of graphene–polymer composite depends mainly on the quality of graphene, the type of polymer system used, and the interaction between the polymer and graphene. In a graphene–polymer composite actuator, the homogeneous dispersion of graphene is one of the important aspects to be ensured for good optical actuation behavior. The homogeneity of graphene dispersion was achieved by many researchers using functionalization techniques. IR-triggered actuation of a thermoplastic PU (TPU)/sulfonated graphene nanocomposite was demonstrated by Liang et al.[101] Under IR illumination, a pre-stretched sample of 1 wt% sulfonated grapheneloaded TPU composite actuator could contract in length and lift a weight of 21.6 g to a height of 3.1 cm with 0.21 N force. The energy density achieved in this demonstration was estimated to be more than 0.33 J/g. When stretched, the PU matrix undergoes strain-induced crystallization. The graphene platelets dispersed in the polymer matrix absorbs the IR light and convert it into thermal energy. Each graphene particle acts as a nanoheater that melts the

strain-induced crystallites in the TPU matrix resulting in contraction. Similar experiments were also performed with isocyanate-f-G and reduced graphene as fillers in PU matrix. However, actuators with sulfonated graphene composite displayed much better actuation performance than the other fillers due to its homogeneous dispersion and better IR absorption characteristics. The actuation performance was also found to increase with increasing sulfonated graphene content. However, no overall improvement in actuation performance was observed above 1 wt% filler loading. Thermally rGO (TRGO)/TPU composites were also reported to exhibit excellent photomechanical actuation properties under IR light illumination.[102] The optical actuation properties and the photomechanical response were found to increase with increasing TRGO content. Maximum stress and strain were obtained for an optimum filler loading of 2 wt%. For an actuator sample with 2 wt% TRGO and pre-stretched to 220%, a photomechanical strain of 50.2% and a stress of 1,680 kPa were obtained, which indicate that it can lift a weight of 42.85 g to a height of 25 mm. The optically triggered actuation behavior of PU composites was further improved when a hybrid filler containing sulfonated rGO (SRGO) and sulfonated carbon nanotube was used.[103] The crystallization of the soft segments of the PU was drastically improved even with a low loading of 1% filler which resulted in larger strains. Moreover, the uniformly dispersed and interconnected SRGO/sulfonated CNT hybrid fillers exhibited excellent IR absorption characteristics. The ratio of SRGO to sulfonated CNT was found to have profound effect on the actuation properties. Maximum actuation performance was displayed with a 3:1 weight ratio of SRGO to sulfonated CNT. Under IR illumination, it could lift a weight of 107.6 g up to 4.7 cm in 18 s. This demonstrated a very high-energy density of 0.63 J/g and a shape recovery force of 1.2 MPa. The improved thermal conductivity of the composite helped in improved actuation performance. Compared to graphene–PU-based systems, graphene– polydimethylsiloxane (PDMS) systems showed less photomechanical stress and strain. For example, 2 wt% graphene nanoplatelet–PDMS system exhibited a change in stress of only less than 40 kPa.[104] Even with single-layer graphene (1 wt%), the photomechanical stress of the PDMS system was 80%), low sheet resistance (30°C). The incorporation of RTILs such as N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide, PYR14TFSI, into PEO–LiX polymer electrolytes allows to reduce the operative temperature of LMPBs down to room temperature without depleting the chemical/electrochemical properties. All-solid-state, solvent-free Li/LiFePO4 polymer batteries based on PYR14TFSI RTIL are able to deliver the theoretical capacity (170mAh g−1) at 30°C and still large capacities (>100mAh g−1) at 20°Cwith excellent cycle capability and coulombic efficiency close to 100% at 100% of DOD. At 40°C, large capacities are discharged even at medium rates (e.g., 125mAh g−1 at C/3). The performance decay below 30°C is almost fully ascribed to the remarkable increase in the resistance at the Li/SPE interface, which represents more than 93% of the overall b­ attery impedance. A further development of LMPBs will strongly depend on the improvement of the lithium/polymer electrolyte interface that represents the bottleneck for room-temperature applications. Very important to notice, however, is that the incorporation of the RTILs improves the Li/SPE interfacial properties. Although the observed improvement is limited to a resistance decrease of about 30%, it certainly supports for the search of tailored ILs, which would be able to decrease the Li/SPE interface resistance further.[71] Macroporous Organic Polymers Microbial fuel cells (MFCs) are sustainable and green energy sources that can convert chemical energy in organic wastes into electricity and integrate environmental bioremediation with power production [72,73] Electrons stored in organic matters can be released by microbial metabolism and subsequently passed to solid electrodes in MFCs through different extracellular electron transfer (EET) mechanisms including direct electron transfer via the redox active proteins on the bacterial outer membrane and/or through the conductive pili (bacterial nanowires), and indirect electron transfer mediated by shuttle

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molecules.[74,75] However, the low output power density from MFCs due to low bacteria loading onto the electrode and low EET efficiency between bacteria and electrodes is the major bottleneck that obstructs practical applications of MFCs. Development of novel anodic materials that could facilitate bacterial biofilm formation and EET is vital to enhance power production of MFCs. Towards this aim, various strategies have been developed to increase the specific surface area, biocompatibility, conductivity, and electron-accepting ability of the electrodes.[76,77] For example, conducting nanostructured materials (e.g., carbon nanotubes) have been employed to coat the standard electrode in order to increase the specific surface area and promote EET. However, such flat (two-dimensional [2D]) porous anodes have small pore sizes. Consequently, bacteria only clog on the surface and are inaccessible to the interior of the anode. This seriously limits the anode efficiency. In view of this, three-dimensional (3D) structured anodes have been devised based on graphite fiber brush, [78] reticulated vitreous carbon, [79] granular activated carbon, [80] carbon fiber nonwovens, [81] or carbon nanotube textile.[82] Compared with the conventional flat anodes, the 3D anodes provide larger surface area to interface with bacteria. But the problems associated with these 3D structures include low specific surface area due to the lack of microscopic or nanoscropic structures, or too small pore sizes for bacteria penetration, or poor conductivity, or disruption of bacterial membrane by sharp nanomaterials.[83] Graphene is a single atomic layer of carbon atoms arranged in a hexagonal lattice. Owing to its extraordinary electrical, physiochemical, and structural properties, this recently discovered allotrope of carbon has already demonstrated great potentials in many fields of science and technologies.[84,85] It shall provide new opportunities to MFC as well, taking advantage of its unique properties, such as outstanding electrical conductivity, extremely high specific surface area (up to 2,600 m2g−1), mechanical robustness and flexibility, chemical inertness, and biocompatibility.[86] Recently, thinfilms of chemically derived graphene sheets [87] and graphene oxide nanoribbons [88] have been employed to improve the performance of MFC anodes. But the conductivity of those graphene materials is largely compromised by the chemical groups and defects introduced during the synthesis processes.[89] In addition, like other 2D structures, graphene thinfilms have limited bacteria loading capacity and the stacking between individual sheets largely sacrifices the high intrinsic specific area of graphene. Herein, we demonstrate a novel 3D macroporous anode, which is a free-standing, flexible, conductive, and monolithic grapheme foam [90] decorated with the conductive polymer PANi. To the best of our knowledge, this is the first demonstrated 3D monolithic carbon anode for MFC. We show that it greatly outperforms the standard planar carbon electrode owing to its abilities to interface with bacterial biofilms three-dimensionally, facilitates electron transfer, and ­provides multiplexed and highly ­conductive pathways.

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The 3D graphene was synthesized by chemical vapor deposition (CVD) with nickel foam as the substrate and using ethanol as the carbon source. The nickel substrate was then etched away using HCl (3M) solution at 80°C to produce free-standing yet light graphene foam. The 3D graphene exhibits a honeycomb structure with a pore size of 100–300 μm. At a higher magnification, it is observed that the surface of the graphene skeleton is seamlessly continuous and exhibits micrometer scale smooth (flat) topographic domains (comparable to the size of a bacterium). The morphology, surface topology, and dimensions of 3D graphene are identical to those of the nickel substrate resulting from the conformal CVD growth.[91] Therefore, these structural parameters of the 3D graphene can be varied by the choice of different growth substrates. Raman spectroscopy was also conducted to examine the obtained graphene. The absence of Raman D band indicates that the grown graphene is of high quality, while the integrated intensity ratio between 2D and G band indicates the coexistence of single-layer and few-layer domains.[92,93] The lack of defects and the absence of contacts between separated graphene sheets ensure a high conductivity of this 3D graphene monolith. Despite the fact that the graphene layers are extremely thin (one-atom thick for single layer), the monolithically and continuously networked graphene structure can stand alone and be manipulated due to the extraordinary mechanical strength of graphene. As the 3D grapheme foam gives a high specific surface area (850m2g−1), it is able to provide a large surface area for bacteria attachment. Also desirably, the porosity of the grapheme foam is much higher than that of carbon cloth (99% vs 65%). Because the pore size of graphene foam is much larger than a bacterium (1–2 μm), bacteria can easily diffuse in and colonize inside. In addition, such macroporous structure guarantees unhindered substrate transport. Graphene, however, just like other carbon materials, is highly hydrophobic which is unfavorable for bacteria adhesion. We thus decorated the surface of grapheme with hydrophilic conducting polymer PANi through in situ polymerization in order to promote bacteria adhesion and biofilm formation. The success of PANi deposition was confirmed by scanning electron microscopy (SEM) and cyclic voltammetry in which the characteristic redox peaks from PANi (originated from the redox transition between the leucoemeraldine and the polaronicemeraldine form) are evident. In summary, we demonstrate the use of a novel 3D graphene/PANi structure as the MFC anode. It impressively outperforms the commonly used carbon cloth owing to the higher bacterial biofilm loading and higher EET efficiency. The former is due to the large specific surface area of the 3D graphene/PANi electrode and its ability to integrate with bacterial biofilms three-dimensionally. The latter is because of the large surface area to accept electrons from riboflavin molecules (electron shuttles released by Shewanella oneidensis MR-1), the realization of direct

electron transfer from OmcA cytochrome proteins on cell membrane, and the multiplexing and highly conductive pathways provided by the graphene network. In addition, this 3D anode is promising for practical large-scale MFCs because its lightness ensures high specific power density and its power output can be boosted simply by increasing its thickness. Furthermore, because the 3D graphene electrode is monolithic (continuous scaffold as a whole) and with smooth surface, the bacterial membrane will not be ­disrupted due to the penetration of sharp nanoscale features, and cytotoxicity will not arise due to uptake of nanomaterials. This study adds a new dimension to MFC anode design as well as to the emerging grapheme applications.[94] Polymer–Graphene Nanocomposite for Supercapacitor Application The graphene-based materials are promising for applications in supercapacitors and other energy storage devices due to the intriguing properties, i.e., highly tunable surface area, outstanding electrical conductivity, good chemical stability, and excellent mechanical behavior. In this study, we will summarizes recent development on graphenebased materials for supercapacitor electrodes, based on their macrostructural complexity, i.e., zero dimensional (e.g., free-standing graphene dots and particles), one dimensional (e.g., fiber-type and yarn-type structures), two dimensional (e.g., graphenes and graphene-based nanocomposite films), and three dimensional (e.g., graphene foam and hydrogel-based nanocomposites). There are extensive and ongoing researches on the rationalization of their structures at varying scales and dimensions, development of effective, and low-cost synthesis techniques, design, and architecture of graphene-based materials, as well as clarification of their electrochemical performance. It is indicated that future studies should focus on the overall device performance in energy storage devices and largescale process in low costs for the promising applications in portable and wearable electronic, transport electrical, and hybrid vehicles. A novel graphene–polyaniline (G–PANi) nanocomposite material synthesized using chemical precipitation technique is reported as an electrode for supercapacitors. The G–PANi composite material has recently been used for energy applications.[95] As a result of the high quality of the sp2 carbon lattice, electrons have been found to move ballistically in the graphene layer, even at ambient temperatures. In this work, the nanocomposite formation of graphene is produced in order to understand how the graphene could be exploited for energy application.[96] The G–PANi has been synthesized using graphene platelets which have been successfully applied in conjunction with an aniline monomer to produce highly conductive nanocomposite material. G–PANi nanocomposites have been synthesized by varying the monomer-to-graphene ratio for obtaining highly conducting systems suitable to

supercapacitor applications. The morphology of the G– PANi has been characterized by using SEM, TEM, FTIR, and Raman to understand the graphene effect over the PANi network system. The presence of graphene in PANi shows the penetrating network-like structure in the G– PANI film, whereas the TEM shows how the graphene platelets are making the network structure with PANi. The high specific capacitance and good cyclic stability have been achieved using an aniline-to-grapheme ratio of 1:2 by weight of G–PANi polymer. This result has proved that the presence of graphene in network of PANi changes the composite structure and could easily be exploited for high-powered supercapacitor applications for portable devices. Based on these results from G–PANi polymer nanocomposite, new synthesis applying G–PANi and G–polythiophenes are in progress as a future work for ­supercapacitor applications.[97] CONCLUSION Polymers are increasingly finding applications in the areas of energy storage and conversion. A number of recent advances in the control of the polymer molecular structure control, which allows the polymer properties to be more finely tuned, have led to these advances and new applications. PHEMA is a polymer that is well known as hydrogel type. The good chemical stability, high biocompatibility, and physicochemical properties of PHEMA make it a widely used polymer in the field of biomedicine. The biomedical and pharmaceutical field studies showed that PHEMA is used for a variety of applications including soft contact lenses, drug delivery applications, and fibro-resistant coatings. Another main application is that PHEMA hydrogel is used in the preparation of transistor. The organic, polymer-based photovoltaic elements have introduced at least the potential of obtaining cheap and easy methods to produce energy from light. The general structure used for organic solar cells is similar to the organic LEDs. Organic semiconductors have several advantages: (a) low-cost synthesis and (b) easy manufacturing of thin-film devices by vacuum evaporation/sublimation or solution cast or printing technologies. Polymer electrolytes have provided the attractive possibility of developing a new type of lithium battery, the so-called LPB with thin layers of laminated material making thin-film applications and also a variety of conformal shapes possible. LPBs represent an excellent choice as an electrochemical power source in a number of applications characterized by high energy density, good cyclability, reliability, and safety. The further development of LMPBs will strongly depend on the improvement of the lithium/polymer electrolyte interface that represents the bottleneck for room-temperature applications. MFCs are sustainable and green energy sources that can convert chemical energy in organic wastes into electricity and integrate environmental bioremediation with power

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production. Compared with the conventional flat anodes, the 3D anodes provide a larger surface area to interface with bacteria. A novel 3D macroporous anode, decorated with the conductive polymer PANi, is able to interface with bacterial biofilm three-dimensionally, facilitates electron transfer, and provides multiplexed and highly conductive pathways. The graphene-based materials are promising for applications in supercapacitors and other energy storage devices due to the intriguing properties, i.e., highly tunable surface area, outstanding electrical conductivity, good chemical stability, and excellent mechanical behavior. ACKNOWLEDGMENT The authors are thankful to the American Chemical ­Society for copyright permission.

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Energy: Polymer Electrolytes for Lithium Ion Batteries Guang Yang and Qing Wang Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania, U.S.A.

Youngmin Lee Department of Chemical Engineering, New Mexico Tech, Socorro, New Mexico, U.S.A.

Abstract Advancements of electrolytes would enrich the options of high-capacity electrode materials for high-energy density lithium ion batteries (LIBs) that are needed for next-generation transportations such as electric vehicles. The scaling up of the battery production is limited by the safety issues such as overheat and thermal runaway caused by high temperature, mechanical vibration, or short circuit. The currently available flammable liquid electrolyte is easy to catch fire under those conditions. Polymer electrolyte is one of the best candidates to substitute the liquid organic electrolyte because it is ion conducting, flexible, thermally stable, nonflammable, compatible with electrodes, and easy processing. Extensive efforts have been taken to enhance the ionic conductivity of Li+ ions in the polymer electrolytes using a variety of approaches such as molecular design and synthesis, physical blending, and film fabrication. Herein, major classes of polymer electrolytes for LIBs are reviewed, including intrinsic dry polymer electrolytes and combination of dry polymer electrolytes with other small molecule plasticizers, fillers, and additives. The most recent polymer structures designed as electrolytes for LIBs are also included and discussed. Keywords: Ionic liquid; Lithium ion battery; Polymer composite; Polymer electrolyte; Single ion conductor; Solid polymer electrolyte.

INTRODUCTION TO LITHIUM ION BATTERY Our current energy economy is still dominated by fossil fuels consumption. Unfortunately, the burning of these nonrenewable resources urges scientists to seek new power sources. Also, the emission of carbon dioxide leads to the rising of the temperature worldwide. Among the choices of new and clean energy sources, lithium ion batteries (LIBs) are considered as a reliable option since their appearance in market (1991) because these nongaseous emission devices have already proved to be able to power portable electronic devices such as cell phones and cameras. Recently, LIBs are increasingly used in hybrid electric vehicles (HEVs) and electric vehicles (EVs) due to their reasonable energy density and cycle life.[1–3] Conventional rechargeable LIBs are composed of cathode (e.g., lithium cobalt oxide  (LiCoO2) and lithium ion phosphate (LiFePO4)), anode (e.g., graphite, lithium titanate (LTO), or lithium metal), liquid electrolyte containing dissolved lithium salt of certain concentration and organic solvent (such as ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC)), and a separator (Fig. 1). When the battery discharges, oxidation reaction happens on the anode side, electrons will flow from the anode to the cathode via an external circuit, and ions will flow inside the battery via

972

the electrolyte. As a result, chemical energy is converted into electrical energy. During the charging process, the electrons and ions move reversely. During the process of charging or discharging, the voltage of batteries can be described as V = Voc ± IRb, where Voc is the open-circuit voltage and Rb is the internal resistance. The open-circuit voltage of a battery is influenced by both the difference in the electrochemical potential of anode and cathode and the energy gap in the electrolyte.[4] In general, the electrochemical potential of cathode, µ A, and the electrochemical potential of anode, µ c, must lie within the HOMO (highest occupied molecular orbital)– LUMO (lowest unoccupied molecular orbital) gap of liquid electrolyte or the energy gap between the conduction band and the valence band of solid electrolyte. For the future applications in HEVs and EVs, scale-up is crucial. One of the major challenges for scaling up LIBs arises from the safety issue. Most of the commercial LIBs contain unsafe alkyl carbonate electrolytes composed of volatile and flammable mixtures. Exposure to harsh conditions in terms of thermal and mechanical stresses can lead to potential dangers in safety such as fire, electrolyte leakage, and explosion.[2,5] For example, in case of metal lithium metal as the anode, short circuits can occur by bridging the two electrodes originated from irregular lithium deposits. Encyclopedia of Polymer Application, First Edition DOI: 10.1201/9781351019422-140000028 Copyright © 2018 by Taylor & Francis. All rights reserved.

µA

Eg

(c)

e– –

+

Cu

Al

(a)

LUMO

eVoc µc

HOMO

Reductant

Electrolyte

Oxidant

Li+

Separator

µA

Anode (graphite)

Electrolyte

HOMO

(b)

C.B.

Eg Cathode (LiCoO2)

V.B. Reductant

Electrolyte

eVoc LUMO

µc

Oxidant

Fig. 1  Relative energies of the electrolyte window and electrochemical potentials of cathode and anode: (a) liquid electrolyte; (b) solid electrolyte; (c) schematic illustration of a common “rocking chair” LIB Source: From Goodenough [1] © 2013 American Chemical Society.

In practice, two safety incidents of Boeing 787 Dreamliner were reported in 2013 due to the smoldering of LIBs.[6] In order to address the challenges in safety, organic liquid electrolyte has been attempted to be replaced by the solvent-free safer polymer electrolyte capable of dissolving the Li salts, dissociating into ions, and transporting Li+ ions.[7–10] Also, lithium polymer batteries (LPBs) using gel polymer electrolytes (GPEs) have been widely used in mobile phones, laptop computers, and other electronic devices. We will introduce the polymer electrolyte in the following section. INTRODUCTION TO POLYMER ELECTROLYTES Polymer electrolytes can be defined as any polymers with high ionic conductivity. P.V. Wright initially discovered the capability of oxygen atoms on poly(ethylene oxide) (PEO) to form a complex with alkali metal ions, and the motion of cations is related to the dissociation and association among coordination sites (Fig. 2).[11,12] However, the PEO/ lithium salt system had a limitation that the ionic conductivity, σ, was only satisfactory above a certain temperature (larger than 10 −4 S/cm when T is higher than 60°C) due to the crystallization of PEO (σ 1 mS/cm) but also low interfacial resistance which indicated fast transport of Li+ both in the bulk of electrolyte and at the electrolyte/electrode interface. Galvanostatic cycling test of Li symmetrical cell showed that the presence of Al2O3 impeded the Li dendrite proliferation. In another study, garnet type tetragonal Li7La3Zr2O12 (LLZO) was embedded in a PEO matrix. LLZO itself is known to have high σ (within the order of 104 S/cm).[35] After it

PVDF-HFP in DMF

PVDF-HFP Nanoporous alumina PVDF-HFP

Fig. 5  Illustration of structure of PVDF-HFP/Al2O3 separator. SEM images show cross sections of different parts of the separator Source: From Tu [30] © 2013 Wiley–VCH Verlag GmbH & Co. KGaA, Weinheim.

was introduced to the PEO/LiClO4 system, the optimal σ of 4.4  × 10 −4 S/cm at 55°C was achieved at 52.5 wt% of LLZO.[32] Using ceramic nanowire fillers is another option for polymer composite electrolyte. Nanowires were reported to provide uninterrupted Li+ conduction path that significantly decreased the interface junctions prevailing in particle-reinforced systems.[36–38] For example, Liu et  al. prepared Li0.33La0.557TiO3 (LLTO) nanowires utilizing electrospinning technique and embedded them into PAN matrix. σ value of 0.24 mS/cm could be achieved with the addition of 15 wt% of LLZO nanowires into the PAN/ LiClO4 system. This was three orders of magnitude higher than that of PAN/LiClO4 electrolyte. The reason was inferred that (a) the acidic groups of the ceramic nanowires had a high affinity for ClO4− favoring the dissociation of LiClO4; (b) the surface of LLZO nanowires contained a large number of vacancies allowing Li+ hopping between the vacant sites; and (c) the nanowires constructed a three-dimensional conduction pathway promoting Li+ movement.[37] SINGLE-ION POLYMER ELECTROLYTES Conventional liquid or polymer electrolytes which prevail in prototype or commercial LIBs are conductors for binary Li-salt. Both Li+ ions and counter ions are t­ ransported toward the opposite direction in the electrolyte. Moreover, anions move much faster than cations, and less than one-third of the current of the circuit comes from the cations. The lithium transference number (tLi+), the fraction of current from Li+ ions to the total current for polymer electrolyte/binary salt system, is between 0.1 and 0.3.[5] Only Li+ ions participate in the electrochemical reaction at the electrode/electrolyte interface, whereas anions do not. Consequently, anions build up at one electrode interface and diminish at the other electrode interface, which causes concentration polarization. Concentration polarization is a negative factor for battery performance. Concentration polarization decreases the overall σ, voltage, cycling rates, and power. It causes other undesirable reactions and lithium dendrite formation.[39,40] If the free movement of the anions can be hindered or eliminated, the concentration polarization effect can be avoided. This will allow a single-ion conductor (tLi+ close to unity); as a result, cell power and stability will be improved accordingly. One of the possible approaches is the formation of covalent bonds between anions and polymer chains.[41,42] Various structures and combinations of polymers have been designed for single-ion polymer electrolytes.[13,41,43–45] For example, single-ion conductors based on the mixture of PEO and lithium poly(4-styrenesulfonyl(trifluoromethylsulfonyl)imide) (PSTFSI), [46] copolymers consisting of lithium (4-styrenesulfonyl)(trifluoromethanesulfonyl) imide (LiSTFSI) and methoxy-polyethylene glycol a­ crylate

(MPEGA) monomers, [47] and AB diblock and BAB triblock copolymers where B block was P(STFSILi) and A block was PEO.[13,48] For the BAB triblock copolymer, Li was weakly associated with the TFSI anion in the B block enabling the fast cation transport. Meanwhile, the separation of the dissociable TFSI anion phase and the PEO phase indicated that the complexation conformation of Li+ was different from that of the LiTFSI/PEO blend. The σ could reach 1.3 × 10 −5 S/cm at 60°C, which was higher than the LiTFSI/PEO mixture. For the Li/BAB triblock copolymer/ LiFePO4 (LFP) battery, at C/2 rate, over 85% of the capacity could be retained, and this was superior to other reported dry polymer batteries. This study provided hints for enhancing the σ of polymer electrolyte without sacrificing the mechanical properties. Sun et al. synthesized comb-branched single-ion polymer conductors based on copolymer of polyethylene glycol monomethyl ether acrylate and sulfonated polyethylene glycol acrylate with Li+ cation. The σ of dry polymer exhibited between 10 −7 and 10 −8 S/cm at room temperature. After imbibing 50 wt% of liquid carbonate electrolyte PC/EMC, the σ value increased on the order of 10 −4 S/cm. In the galvanostatic cycling experiment, no concentration polarization effect occurred with a current density of 0.1 mA/cm2 at 85°C.[41] Nafion membrane, which was originally designed as proton exchange membrane for fuel cells, was also modified to produce lithiated single-ion conductors due to its excellent chemical stability, mechanical strength, nanoporous structure, and insolubility in battery electrolyte solvents. Lu et al. studied the effect of lithiated Nafion film (Li-PEM) on the stability of lithium deposition on the electrode. They found that the Li-PEM film showed high tLi+, facilitated uniform lithium stripping/plating, and impeded dendrite formation.[49] It is known that the commonly used LiPF6 salt is not thermally stable (decompose above 60°C) and can degrade in the carbonate liquid electrolyte.[5,50] The by-­ products after decomposition such as HF corrode the cathode materials. To replace LiPF6, lithium chelatoborates such as lithium bis(oxalate)borate (LiBOB) and lithium bis[1,2-benzenediolato(2-)-O,O′]borate (LBBB) became attractive because of their excellent thermal and electrochemical stability, low cost, and capability of promoting solid electrolyte interface formation. These lithium borates have been either added to polymer electrolytes such as PVDF-HFP or chemically incorporated to polymer structures to make single-ion conductors. For example, low-­ molecular-weight polyborates such as polymeric l­ ithium pentaerythrite borate (PLPB) and polymeric lithium tartaric acid borate (PLTB) were added, as novel lithium salts, to polymer matrices such as PVDF-HFP.[51] Lithium polyvinyl alcohol oxalate borate (LiPVAOB) was synthesized via modification of the hydroxyl groups on polyvinyl alcohol (PVA). Lithium oxalate polyacrylic acid borate (LiPAAOB) was obtained by the modification of carboxyl groups of ­polyacrylic acid (Scheme 1).[42,52]

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OH

n

+ H3BO3

O

B

O

n

OH

Oxalic acid

O

Li2CO3

O

B–

O O

n Li+

O

O

LiPVAOB n n n + COOH

H3BO3

O

O

B O OH

O

Oxalic acid LiOH

O

O

O –O + B Li O O O

O

LiOPAAB

O O O O

O O + B– Li O O

LiBOB

Scheme 1  Synthetic route of LiPVAOB and LiPAAOB Source: From Zhu [52] © 2012 Elsevier Ltd. All rights reserved and Zhu [42] © 2012 Elsevier B.V. All rights reserved.

Most of the single-ion polymer electrolytes mentioned above have relatively low glass transition temperature (Tg) due to PEO component with high chain flexibility. Although low Tg polymers are common selections for enhancing the room temperature σ (usually, the σ is still less than 10 −5 S/cm for the dry polymer itself), their mechanical properties and thermal stabilities are partially sacrificed. Rigid aromatic structures were introduced to the main chains of polymers to synthesis high Tg single-ion conductors. As an example, Oh et al. reported a single-ion poly(arylene ether)s with lithium perfluoroethyl sulfonate side chains (PAE-LiPFS) (Fig. 6).[44] This polymer was fabricated into microporous membrane (45% porosity estimated from surface images by scanning electron microscopy) which facilitated the permeation of liquid electrolyte and charge transport. The dry PAE-LiPFS showed low σ on the order of 10 −7 S/cm at room temperature. However, after soaking with DEC/ EC/PC (1:1:1 by volume), σ increased dramatically to 3.1 × 10 −3 S/cm. This was even superior to some liquid electrolyte-containing polyolefin films. Negligible concentration polarization effect was confirmed by the galvanostatic cycling test and the steady-state current method reported by Vincent and Bruce. The tLi+ was close to unity (0.98) measured by the steady-state current method.[53]

Fig. 6  First charge/discharge profile of LiFePO4/PAE-LiPFS electrolyte film/Li cell at room temperature. The inset graphs show the polymer structure and surface morphology Source: From Oh [44] © 2016 American Chemical Society.

Another type of rigid chain polymers is polyamide (PA).[54,55] Pan and coworkers demonstrated a single-ion PA via polycondensation of aromatic dicarboxylic acid and aromatic diamine. Thin films of the ionomer could serve as not only separators but also binders of the cathode LFP for the purpose of enhancing the electrode/electrolyte interaction and ion transport within the cathode layer. This was expected to fully utilize the capacity of the electrode materials because conventional PVDF binders could not promote the Li+ movement inside the electrode. The tLi+ was 0.92, a typical value for single-ion conductors. Meanwhile, the interfacial resistance between the ionomer and cathode obtained from impedance measurement decreased almost 50% which indicated improved compatibility at the electrode/electrolyte interface.[54] SOLID POLYMER ELECTROLYTES The ultimate goal for LPB is all solid-state battery whose electrolytes are free of volatile liquid components. Physically and chemically stable SPE with high ionic conductivity (σ), good electrochemical stability, and compatibility with electrodes are perfect alternatives to organic liquid electrolyte.[56] Nonflammable SPEs will not suffer from evaporation problems and can operate at high temperatures. The ion transport of Li+ in the polymer matrix mainly results from either ion hoping on polymer chains or polymer segmental movement.[21] Currently, although some interesting works have been done to produce SPEs with decent σ and good mechanical strength, most of the reported SPEs still suffer from relatively low σ due to low ion mobility caused by crystallization of polymers.[5,21,57,58] High operation temperature is usually required to achieve good σ and sufficient interfacial contact. PEO-Based SPEs The merits of PEO as SPEs have been well recognized.[59,60] Currently, researchers are putting great efforts to improve

10–3 Comb PEO (viscous electrolyte)

σ (S/cm)

the low σ at room temperature and poor mechanical properties of PEO, such as blends with amorphous polymers and additives, grafting oligomeric PEO to the polymer backbones, BCPs with rigid and soft blocks to integrate high mechanical strength and free conduction pathway at the same time, cross-linking chemistry to prepare polymer networks, and addition of inorganic fillers to enhance Li+ transport, mechanical properties, transference number as well as anodic stability.[57,60–63] BCPs could be an attractive option for SPEs because BCPs provide a flexible design in chemical structures and enable the formation of conductive channels for ions by self-assembly. Usually, one block is PEO to solvate the Li+ dissociated from salt, and the other block is polymer containing stiff backbones with high Tg such as PS. The PEO block can be both linear or comb shaped. The σ is influenced by the block composition, the molecular weight of each block of BCPs, and the concentration of the salt.[64] Balsara group investigated the effect of PEO chain lengths on the σ of diblock copolymers. They found that an increase in PEO chain length led to a decrease in σ at the beginning, and gradually the σ stabilized (the largest σ at 60°C could reach 2 × 10 −4 S/cm when the ratio of ethylene glycol unit to Li+ was 11).[65] This phenomenon was different from that of PEO homopolymer whose σ decreased when molecular weight rose. The authors explained that the proportion of dead zone (noneffective zone for conduction) at the PEO/PS interface increased when the molecular weight of PEO decreased.[66] The σ as a function of volume fraction of PEO (Φc) phase is shown in Fig. 7. When Φc was high, log[σ] has a linear relationship with Φc. When the salt concentration was high, the σ dropped rapidly because the formation of ion aggregates slowed down the mobility of ions. Also, the increase of Φc raised the melting point (Tm) and the degree of crystallinity (Xc) of the linear BCPs. On the other hand, by increasing the fraction of PS, both linear and comb-shaped BCPs exhibited enhanced mechanical properties (tensile strength and Young’s modulus). When applied to lithium metal batteries using porous LFP cathode, linear BCPs seemed to be better than comb-shaped BCPs in terms of the cycling stability at low rates, but comb-shaped BCPs are more ­promising for room-­temperature application.[64] Besides the PEO-based BCPs SPEs, strategies such as making polymer networks [3] and adding plasticizers, ceramic fillers, [14,59,63,67] and other inorganic moieties are also adopted and tested.[68,69] It is known that lithium dendrite formation on lithium anodes of lithium metal battery will do harm to the battery life because these uneven deposited lithium dendrites could pierce through the electrolyte and bridge cathode and anode causing short circuit.[38,70] The Chazalviel model proved that low tLi+ in the electrolyte could accelerate the growth of dendrites.[71] Monroe proposed that improving the shear modulus above 7 GPa could significantly suppress dendrites.[72] Khurana et  al. reported a method to synthesize cross-linked PE using

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Comb PEO 10–4

SEO35S SEO20S SEO10S SEO9S

10–5 0.4

0.5

0.6

0.7 φc

0.8

0.9

1

Fig. 7  σ change of comb-shaped and linear BCPs as a function of volume fraction of conducting phase at 60°C (S is styrene, EO is ethylene glycol, salt is LiTFSI). Empty symbols are ­comb-shaped BCPs. □, comb-shaped PEO homopolymer (cEO23); ◇, B−A diblock; △, B−A−B triblock; ○, A−B−A ­triblock. Filled symbols are linear BCPs. ■, PEO35 and SEOxS with (left-­pointing triangle) x = 9; ▼, x = 10, (right-pointing ­triangle) x = 20; ▲, x = 35 Source: From Devaux[64] © 2015 American Chemical Society.

PEO-based cross-linker (PEOX). Although these PE-PEO cross-linked polymers could not show increased σ compared to other amorphous PEO-based polymers, it gave better mechanical strength and electrochemical stability. In order to meet the minimum requirement for room-­ temperature application of batteries, σ has to be more than 1 × 10 −4 S/cm. When poly(ethylene glycol) (PEG) dimethyl ether (Mw = 275 g/mol) incorporated to the PE-PEO as a plasticizer, σ further increased to 2 × 10 −4 S/cm. Galvanostatic strip/plate experiments were carried out to study how the plasticized PE-PEO performs to resist dendrite induced short-circuit. It was found that the PE-PEO/PEG polymer exhibited more than one order of magnitude higher of the charge passed before cell short circuit (Cd, 1790 C/cm2 for (70PEOX0.34)(34PE0.35)(5PEG0.31) under a current density of 0.26 mA/cm2 at 90°C) than that of PEO (Mn = 900 kDa) and PS-b-PEO BCPs. Nanohybrid polymer electrolytes based on PEO and lithium [(4-methylphenyl)sulfonyl] [(trifluoromethyl)sulfonyl)amide anion co-grafted Al2O3 and SiO2 nanoparticles were reported by Lago et al.[63] σ of 1.9 × 10 −4 S/cm was found at 70°C. The LFP/Li coin cells using abovementioned electrolyte could still deliver 120 mA h/g after 130 cycles at C/2 rate, 70°C. Polyhedral oligomeric silsesquioxane (POSS) is composed of a thermally stable rigid inorganic SiO1.5 ­silicon-oxygen framework and eight attached organic groups which can be adopted as a filler to improve the

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support. The Li+ ions of −Si−O−BF3Li groups were solvated by the conduction phase POSS-PEG8. Consequently, the σ at 30°C was 2.5 × 10 −4 S/cm when the O/Li ratio was 16/1. In addition, since the POSS-benzyl7(BF3)3−3 anion was bulky, larger portion of the current could be carried by the Li+ ions. The tLi+ was 0.5 ± 0.01, indicating the slowing down of anion movement.

mechanical properties of polymers.[16,73,74] POSS as a filler can form a dendritic-like structure providing additional free volume, which allows to decrease the glass transition temperature (Tg) of polymers and thus enhances the ion conduction.[74] Wunder group investigated eight PEO chains grafted POSS-type Li+ ion conductors.[16,75–78] They found that the σ at room temperature of Octa PEO (four ethylene oxide repeating units/chain) functionalized POSS could reach 10 −4 S/cm level (Mw = 3000–5000, LiClO4 as salt), which was comparable to low-molecular-weight PEOs (Mw = 300–500).[75] In another work, hybrid SPE composed of Octa PEO functionalized POSS (POSS-PEG8) and phenyl groups/−Si−O−BF3Li ionic groups co-functionalized POSS (POSS-benzyl7(BF3Li)3) was prepared (Fig. 8). Due to the electron withdrawing POSS molecules and BF3 anions, Li+ dissociation was improved.[69] The phenyl functional groups could crystallize which provided structural

H 3C

O

Non-PEO-Based SPEs Other than PEO, different polymers and additives (e.g., poly(propylene carbonate), PVA, succinonitrile (SN), and perfluoropolyether) have been tried in SPE.[79–83] Nitrile compounds such as SN were proved to increase σ for SPE because the high polarity of –C≡N group in SN enabled the solvation of salts.[79,84,85] When mixed with Li salts, SN is viscous liquid rather than solid at room temperature.

CH3

O

O

O

n

n H3C

O

Me2SiO

O n

H 3C

O

Me2SiO

O

Me2SiO

n O

H3C O

O Si

Me2SiO

O

O

Si

O

Si

O O

OSiMe2

O

Si

O

O Si

Si

O

Si

OSiMe2 O

Si O

O

O

O

O

CH3 n

CH3 n

OSiMe2

OSiMe2

n

O

O

CH3 n

POSS-PEG8

OBF3Li

OLi Si O

O Si

Si

O

O

O Si

OLi

OLi

O

Si

O

O

O

O

POSS-benzyl7Li3

Fig. 8  Structure of POSS-PEG8 and synthetic route of POSS-benzyl7 (BF3Li)3 Source: From Chinnam [69] © 2011 American Chemical Society.

OBF3Li

O

Si

OBF3Li

Si

Si O

BF3O(Et)2

O Si

Si

Si

O

O

Si

O

Si

Si O

O

POSS-benzyl7(BF3Li)3.

For this reason, SN was mixed with mechanically stronger polymers. Cyanoethyl PVA was synthesized in the SN/ LiTFSI solution which was prefilled in the PAN fiber mat with 17 µm thickness (named SEN).[85] The mechanical strength of SEN was 15.4 MPa which was higher than that of a PAN fiber mat. CV test showed that SEN was electrochemically stable up to 5 V. The σ at room temperature of SEN was 4.49 × 10 −4 S/cm. The log σ–T relationship is presented in Fig. 9. The curve could be well fitted by empirical Vogel−Tammann−Fulcher equation shown in Eq. 1. σ (T) =

 A −B  exp   T  R T − T0 

(

(1)

)

A is the pre-exponential factor representing the number of charge carriers, B is the pseudoactivation energy for ion motion, T0 is the ideal glass transition temperature at which the free volume disappears, and R is the gas constant. The B value obtained by fitting was 4.25 × 10 −2 eV, which was close to the activation energy of liquid electrolyte. The LFP/SEN/Li battery could deliver 97.7 mA h/g at 1 C rate. Wong et al. reported a new perfluoropolyether-based electrolyte (PFPE-DMC) using commercial Celgard membrane as physical separator.[81] They found that PFPE-DMC was able to solvate LiTFSI and promote the Li+ conduction with σ of 2.5 × 10 −5 at 30°C. The tLi+ was close to unity (0.91). Poly(propylene carbonate)-based SPE in combination with cellulose nonwovens supports (CPPC-SPE)

–1

100

80

60

(a)

40

t/°C 20

0

–20

*Solid lines represent VTF fiting results.

–2

log σ/S/cm

–3 –4 –5 –6 –7 2.6

SN based solid electrolyte PVA-CN/SN solid electrolyte Liquid electrolyte PVA-CN/SN solid electrolyte (With commercial separator) SEN Liquid electrolyte (With commercial separator)

2.8

3.0

3.4 3.2 1000 T–1/k–1

3.6

3.8

4.0

Fig. 9  Ionic conductivity–temperature relationship of SN-based solid electrolyte, PVA-CN/SN solid electrolyte, 1 M LiPF6-EC/ EMC/DMC liquid electrolyte, PVA-CN/SN solid electrolyte with commercial separator, SEN, and liquid electrolyte with commercial separator. Solid lines are VTF-fitted curves Source: From Zhou [85] © 2015 Wiley–VCH Verlag GmbH & Co. KGaA, Weinheim.

was also reported.[82] At 20°C, the σ reached 3 × 10 −4 S/ cm which was two orders of magnitude higher than that of PEO-SPE counterparts. The discharge capacity of LFP/ CPPC-SPE/Li cell at 20°C and 0.1 C was 142 mA h/g. Moreover, the cell demonstrated an excellent cycling stability. Only 5% of capacity loss was found after 1,000 cycles at 20°C and 0.5 C. When used in higher voltage LIBs with LiFe0.2 Mn0.8PO4 as the cathode, flat charge and discharge curves were observed around 4.1 V, and 96% of capacity was retained after 100 cycles. These non-PEObased SPEs greatly enriched the exploration of safe LIBs electrolytes. Table 1 lists the properties and performance in LIB of representative SPEs. IL-BASED POLYMER ELECTROLYTES Ionic Liquids ILs are molten organic at room temperature. They have been considered important in electrochemical devices especially as potential electrolyte candidates for LIBs because of their unique properties: ionic conductive (σ  =  10 −4 to 10 −2 S/cm), nonvolatile, nonflammable, and electrochemically stable (generally > 4 V).[86–88] The cations of ILs include 1,3-dialkyl imidazolium, N-alkyl pyridinium, tetraalkyl ammonium, tetraalkyl phosphonium, and N-alkyl pyrrolidinium. The anions of ILs can be bis((trifluoromethyl) sulfonyl)imide (TFSI), tetrafluoroborate (BF4−), hexafluorophosphate (PF6−), trifluoromethane sulfonate(Tf−), chloride (Cl−), bromide (Br−), nitrite (NO3−), iodide (I−), acetate (CH3CO2−), etc. (Fig. 10). Polymer/IL Electrolytes The introduction of ILs into the polymer electrolyte system is one of the effective ways for enhancing the σ because the plasticizing ILs can provide more “free volume” in the polymers for ion diffusion.[9,89–91] ILs also play a role in adjusting the tLi+ in the system due to their ionic features. Blends of polymer electrolytes and ILs have been widely studied. Basically, the hybrid polymer/ILs electrolytes can be prepared via (a) mixing polymers with ILs with or without addition of solvents, (b) soaking polymer membranes in ILs, and (c) polymerizing in IL solvents.[34,86,92–94] PEOand PVDF-HFP- containing polymers are the most frequently used polymers to dope ILs. Due to their excellent electrochemical stability, alkyl pyrrolidinium and alkyl imidazolium cations-based ILs are commonly studied.[93,95] The free-standing polymer/IL membranes could be fabricated by the solution-­casting or hot-press method. Passerini group and Scrosati group have done some solid works on studying the electrochemical properties of dimensionally stable polymer/IL ­membranes.[9,21,89,92,93,96,97] N-Methyl-N-propyl-­pyrrolidinium bis(trifluoromethanesulfonyl)imide (PyR13TFSI), ­N-n-butyl-N-ethylpyrrolidinium

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Table 1  List of properties and performance in LIB of representative SPEs σ value Cathode/ SPE components Li salt (mS/cm) anode

Interfacial impedance (Ω)

Specific capacity delivered (mA h/g)

Reference

P(STFSILi)-b-PEO-bP(STFSILi)

No

0.013 at 60°C

LFP/Li

100 at 90°C

144 at 80°C, 0.5 C

[13]

PEO and anion co-grafted Al2O3

No

0.04 at R.T.

LFP/Li

N/A

125 at 70°C, 0.5 C

[63]

PEPTA and SN in PET nonwoven

LiTFSI

0.57 at 30°C

LiCoO2 / LTO

N/A

128 at R.T., 0.2 C

[79]

Comb-like PEO-based copolymers

LiTFSI

0.068 at 25°C

LFP/Li

N/A

146 at 80°C, C/24

[57]

Hybrid POSS-PEO networks

LiTFSI

0.095 at 30°C

LFP/Li

N/A

144 at 90°C, 0.5 C

[70]

Perfluoropolyether

LiTFSI

0.025 at 30°C

LiNMCa / Li

N/A

120 at 30°C, 0.1 C

[81]

Polypropylene carbonate

LiTFSI

0.3 at 20°C

LFP/Li

83 at 20°C

142 at 20°C, 0.1 C

[83]

PVA-CN and SN in PAN membrane

LiTFSI

0.449 at 25°C

LFP/Li

211 at 25°C

150 at 25°C, 0.1 C

[115]

PEG-borate ester

LiTFSI

N/A

LFP/Li

174 at 60°C

125 at 60°C, 0.5 C

[67]

PEO-poly(cyano acrylate)

LiBOB

0.3 at 60°C

LFP/Li

80 at 80°C

153 at 80°C, 0.1 C

[116]

PEO, POSS containing graft polymers with core/shell fillers

LiClO4

0.16 at 30°C

V2O5/Li

300 at 60°C

240 at 60°C, 0.1 C

[14]

Cations

R1

N + N R 2 + N

Anions 1,3-dialkylimidazolium

N-alkylpyridinium

O – O F3C S N S CF3 O

Bis((trifluoromethyl) sulfonyl)imide

R R1 + R2 N R4 R3

Tetraalkylammonium

R1 + R2 P R4 R3

Tetraalkylphosphonium

R H 1+ N

O

N-alkylpyrrolidinium

TFSI– F – F B F F Tetrafluoroborate BF4– CI– Chloride Br–

Bromide

I– Iodide

F F –F P F F F Hexafluoro phosphate PF6– O – F3C O S O

O Trifluoromethane sulfonate TF– NO3– Nitrate CH3CO2– Acetate

Fig. 10  Common anions and cations used in IL Source: From Ye [86] © 2012 Royal Society of Chemistry.

­ ,N-bis(trifluoromethane)sulfonamide N (PyR 24TFSI), and N-butyl-N-methylpyrrolidinium bis(fluorosulfonyl) imide (PyR14FSI) are mostly used ILs because of their wide cathodic stability limit, good thermal stability, and compatibility with lithium.[93,96,97] For example, Kim et al. developed a series of PEO-LiTFSI electrolytes

incorporated with PyR1ATFSI ILs (the subscript A stands for different alkyl side chain lengths and alkyl types. For example, A = n3 means n-propyl group). It was found that ILs in the polymer matrix did not give any negative influence on the electrochemical stability of the polymers. The PEO-PyR1n3TFSI-LiTFSI hybrid electrolyte exhibited σ of

1.38 × 10 −4 S/cm at 20°C when the PyR1n3/Li+ molar ratio was 0.96. This was significantly larger than the IL-free PEO-LiTFSI sample (1.33 × 10 −6 S/cm). The increase of σ was due to the suppression of PEO crystallization and the coordination of Li+ to TFSI− (from both LiTFSI and PyR1ATFSI) in addition to PEO chains.[94] The PEO matrix was further strengthened by UV cross-linking that enabled higher ILs content in the polymer film. The σ of crosslinked PEO-PyR14TFSI-LiTFSI was slightly higher than that of linear PEO-PyR14TFSI-LiTFSI over the temperature ranging from −40°C to 100°C.[89] At temperature above 30°C, this Li/SPE/LFP battery showed more than 150 mA h/g of delivered capacity at 0.1 C and good term stability. In the case of IL-trapped PVDF-HFP polymeric films, it was discovered that by adding a small amount of EC-PC mixture to the PVDF-HFP/IL membrane, ion transport was improved, and passivation layer on the ­lithium electrode was formed.[9,97] Imidazolium-based ILs are another choice for the polymer/IL systems. The tertiary nitrogen of the imidazole ring can be readily quaternized producing positively charged ring which was thermally stable (up to 275°C). Kumar et al. investigated PEO/lithium trifluoromethanesulfonate (LiCF3SO3 or LiTf) doped by ILs 1-ethyl 3-methyl imidazolium trifluoromethanesulfonate (EMITf). High σ of 3  ×  10 −4 S/cm was obtained when the composition was PEOLiTf(EO/Li = 25) + 40 wt.% EMITf.[98] Nair et al. reported a facile way to fabricate free-standing cross-linked PEO/LiTFSI films with trapped EMI-TFSI. Basically, the ternary mixture of PEO/LiTFSI/EMI-TFSI was hotpressed into 90 µm-thick film followed by exposure to UV light for cross-linking. This SPE after UV irradiation was much tougher than the uncross-linked polymer. It showed σ of 2.5 × 10 −4 S/cm at 20°C and above 10 −3 S/cm at 50°C. To ensure excellent contact of the SPE with ­electrode materials, the precursor film of PEO/LiTFSI/EMI-TFSI mixture was first obtained by hot pressing onto the LFP electrode, and the UV light was then turned on to initiate the ­photo-cross-linking.[99] Poly(IL)s Polymeric ILs (or poly(IL)s) have been attracting much attention because of the excellent affinity and compatibility with ILs minimizing the chance to leak.[100–107] Poly(IL)s are polymerized form of IL monomers so that the advantages of ILs are partially integrated into the polymers. Poly(IL)s as a new type of polyelectrolyte can be classified as polycations, [102,108] polyanions, [106,109] and ­polyzwitterions (Fig. 11).[110] Different types of anions in the poly(IL)s strongly influence the physical properties of polymers such as solubility, σ, Tg, and thermal stability. For example, the imidazolium-based poly(IL)s with halide anions are water soluble. When the anions are replaced with other fluorine-containing counter ions, the polymers

Polycations

n

Counter ions

n N + N R

n + N

SCN– n

n

+ N R

N N

CF3SO3– PF6–

+

+

N

N

BF4–

(CF3CF2SO2)2N–

(CF3SO2)2N–

Polyanions

n HN

n

n 2– PO3

–

O m

SO3–

R

–

N SO2 F3C

SO3

R n COO–

N + N R

N + R

R

+ N R’’ R R’

+ N R

Fig. 11  Typical chemical structures of poly(ILs) Source: From Mecerreyes [115] © 2011 Elsevier Ltd. All rights reserved.

tend to be hydrophobic. It has also been found that different anions change Tg of poly(IL)s as much as following the order of Br > PF6 > BF4 > TFSI. Their σ values, however, follow the opposite trend as TFSI > BF4 > PF6. This is not surprising because lower Tg allows higher chain flexibility favoring ion transport. In addition, the cation ring structure and spacer structure are other two main factors influencing the σ of poly(IL)s. For imidazolium-based poly(IL)s, if the C2 position of the imizadole ring is substituted with alkyl groups, the cathodic electrochemical stability can be improved, whereas the σ will be dropped obviously. Meanwhile, using ethylene oxide derivatives to substitute the quaternized ammonium leads to an improvement of σ. If the IL structure was located far from the polymer main chain, the σ could also be enhanced.[9,111] Most of the poly(IL)s reported for battery application are polycations due to the convenience of synthesis, they were combined with Li-salt and plasticizers (usually ILs) to form three component system.[102,104,107,112,113] For example, Yin and coworkers designed a new synthetic route to prepare imidazolium-based poly(IL)s: poly(1-ethyl-­3vinylimidazolium ­bis(trifluoromethanesulfonylimide)) that had higher molecular weight than poly(IL)s ­polymerized directly from IL monomers.[114] The poly(IL), (1,2-dimethyl-­3-ethoxyethyl imidazolium bis(trifluoromethanesulfonyl)imide (IM(2o2)11TFSI)) as a plasticizer IL, and LiTFSI salt were dissolved in acetonitrile. The poly(IL) films were prepared by the solution-casting method. The Li/LFP coin cells using the poly(IL)s could deliver higher specific capacity than conventional poly(IL) s (more than 40 mA h/g at 0.1 C rate, 60°C). Zhang et al.

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developed a poly(IL) network electrolyte containing a high density of ion pairs from the IL structure.[101] It was expected that the densely arranged charged units provided many weak electrostatic interaction sites for Li+ movement, and the space among the high charge density poly(IL) domains provided channels for fast Li+ transport. The poly(IL)/LiTFSI cross-linked membrane showed σ at room temperature of 1.17 × 10 −4 S/cm. After mixing with 1-ethyl-3-methyl-­imidazolium bis(trifluoromethane)sulfonimide (EMIM-TFSI), the σ could be further increased to 5.32 × 10 −3 S/cm, and the anodic electrochemical stability could be above 5 V. The Li/poly(IL)@LiTFSI-EMIMTFSI/LFP batteries showed the initial discharge capacity above 143 mA h/g at room temperature. Poly(IL)s could also be grafted onto inorganic nanoparticles through living/controlled radical polymerization to produce new SPE with continuous ion conductive ­network. Sato et al. successfully polymerized IL monomer, N,N-­ diethyl-N-(2-methacryloylethyl) -N-methylammonium bis(trifluoromethylsulfonyl)imide (DEMM-TFSI), on silica nanoparticles using atom transfer radical ­polymerization (ATRP). By careful mixing of these hybrid silica particles with ILs under the optimal composition in ­volatile solvent followed by solution casting, interesting colloidal crystal films were prepared (PSiP/IL; Fig. 12).[104] The σ of the film was 1.7 × 10 −4 S/cm at 30°C. The potential value as a solid electrolyte in LTO/LiMn2O4 battery was confirmed as well.

O

EtO EtO Si EtO

O Br

O

O

BHE

SiO2

O O Si O

LIBs are widely used in electronics, automobiles, and other industrial machineries. The LIBs’ market is still expanding year by year. People are constantly seeking high-capacity stable electrodes and high safety electrolytes for batteries. Although carbonate liquid electrolytes with dissolved Li salt have strong electrochemical stability, they bring about leakage, flammability, and thermal instability problems. The logic of using polymer electrolytes lies in their ability to address the safety issues in batteries (nonflammability, flexibility, thermal stability, and electrochemical stability). We have discussed the major types of polymer-based electrolytes herein. Starting from the PEO, which is a prototype of polymer electrolyte, various PEO-based and non-PEO-based polymers (including random copolymers, BCPs, grafted copolymers, single-ion polymers, PILs) and polymer composites with or without liquid plasticizers (carbonate solvent or ILs) have been covered. The good news is GPE-based LIBs have already been widely used in our daily life such as our cell phones and mobile DVDs. The most challenging issues toward the complete replacement of liquid electrolytes are still unsatisfactory ambient-temperature σ and inferior electrode/electrolyte interfacial properties. To address these ­problems, four key factors should be paid attention to (a) polymer physics (e.g., contribution of the functional groups to ion transport, ion interactions, crystallization) to acquire

N + –

N(CF3SO2)2

PSiP

O

N + –

N(CF3SO2)2

DEMM-TFSI

Ionic liquid

LRP

Cast

O O

Br

(a)

(b)

fcc structure

tp-plane

cp-plane

Fig. 12  Synthesis of PSiP/IL solid electrolyte and self-assembly of PSiPs providing channels for ions’ movement. (a) Photograph of the solid film; (b) organization of PSiP in solid Source: From Sato [104] © 2011 Wiley–VCH Verlag GmbH & Co. KGaA, Weinheim.

deeper understanding and guidance for the polymer chemist to achieve more efficient molecular design, (b) the electrochemical properties of each single component of the polymer electrolyte (e.g., electrochemical stability limit of ILs), (c) electrode/electrolyte interfacial properties as well as the ion transport properties inside the electrode, and (d) closer collaboration between synthetic chemist, ­electrochemist, and battery engineers.

ACKNOWLEDGMENT

14.

15.

16.

We thank the support from the U.S. National Science Foundation. 17.

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95. Sutto, T.E. Hydrophobic and hydrophilic interactions of ionic liquids and polymers in solid polymer gel electrolytes. J. Electrochem. Soc. 2007, 154 (10), P101. doi:10.1149/1.2767414. 96. Shin, J.H.; Henderson, W.A.; Scaccia, S.; Prosini, P.P.; Passerini, S. Solid-state Li/LiFePO4 polymer electrolyte batteries incorporating an ionic liquid cycled at 40°C. J.  Power Sources 2006, 156 (2), 560–566. doi:10.1016/j. jpowsour.2005.06.026. 97. Navarra, M.A.; Manzi, J.; Lombardo, L.; Panero, S.; Scrosati, B. Ionic liquid-based membranes as electrolytes for advanced lithium polymer batteries. ChemSusChem 2011, 4 (1), 125–130. doi:10.1002/cssc.201000254. 98. Kumar, Y.; Hashmi, S.A.; Pandey, G.P. Lithium ion transport and ion–polymer interaction in PEO based polymer electrolyte plasticized with ionic liquid. Solid State Ion 2011, 201 (1), 73–80. doi:10.1016/j.ssi.2011.08.010. 99. Nair, J.R.; Porcarelli, L.; Bella, F.; Gerbaldi, C. Newly elaborated multipurpose polymer electrolyte encompassing RTILs for smart energy-efficient devices. ACS Appl. Mater. Interfaces 2015, 7 (23), 12961–12971. doi:10.1021/ acsami.5b02729. 100. Yuan, J.; Mecerreyes, D.; Antonietti, M. Poly(ionic liquid) s: An update. Prog. Polym. Sci. 2013, 38 (7), 1009–1036. doi:10.1016/j.progpolymsci.2013.04.002. 101. Zhang, P.; Li, M.; Yang, B.; Fang, Y.; Jiang, X.; Veith, G.M.; Sun, X.; Dai, S. Polymerized ionic networks with high charge density: Quasi-solid electrolytes in lithium-metal batteries. Adv. Mater. 2015, 27 (48), 8088–8094. doi:10.1002/adma.201502855. 102. Li, M.; Wang, L.; Yang, B.; Du, T.; Zhang, Y. Facile preparation of polymer electrolytes based on the polymerized ionic liquid poly((4-vinylbenzyl)trimethylammonium bis(trifluoromethanesulfonylimide)) for lithium secondary batteries. Electrochim. Acta 2014, 123, 296–302. doi:10.1016/j.electacta.2013.12.179. 103. Meek, K.M.; Sharick, S.; Ye, Y.; Winey, K.I.; Elabd, Y.A. Bromide and hydroxide conductivity–morphology relationships in polymerized ionic liquid block copolymers. Macromolecules 2015, 48 (14), 4850–4862. doi:10.1021/acs. macromol.5b00926. 104. Sato, T.; Morinaga, T.; Marukane, S.; Narutomi, T.; Igarashi, T.; Kawano, Y.; Ohno, K.; Fukuda, T.; Tsujii, Y. Novel solid-state polymer electrolyte of colloidal crystal decorated with ionic-liquid polymer brush. Adv. Mater. 2011, 23 (42), 4868–4872. doi:10.1002/ adma.201101983. 105. Li, X.; Zhang, Z.; Li, S.; Yang, L.; Hirano, S. Polymeric ionic liquid-plastic crystal composite electrolytes for lithium ion batteries. J. Power Sources 2016, 307, 678–683. doi:10.1016/j.jpowsour.2016.01.032. 106. Porcarelli, L.; Shaplov, A.S.; Salsamendi, M.; Nair, J.R.; Vygodskii, Y.S.; Mecerreyes, D.; Gerbaldi, C. Single-Ion Block Copoly(ionic liquid)s as Electrolytes for All-Solid State Lithium Batteries. ACS Appl. Mater. Interfaces 2016, 8 (16), 10350–10359. doi:10.1021/acsami.6b01973. 107. Yin, K.; Zhang, Z.; Li, X.; Yang, L.; Tachibana, K.; Hirano, S. Polymer electrolytes based on dicationic polymeric ionic liquids: Application in lithium metal batteries. J. Mater. Chem. A 2015, 3 (1), 170–178. doi:10.1039/ C4TA05106H.

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Li4Ti5O12 lithium ion battery anode. J. Power Sources 2014, 247, 112–116. doi:10.1016/j.jpowsour.2013.08.080. 114. Yin, K.; Zhang, Z.; Yang, L.; Hirano, S.I. An imidazolium-based polymerized ionic liquid via novel synthetic strategy as polymer electrolytes for lithium ion batteries. J. Power Sources 2014, 258, 150–154. doi:10.1016/j. jpowsour.2014.02.057. 1 15. Mecerreyes D. Polymeric ionic liquids: Broadening the properties and applications of polyelectrolytes. Prog. Polym. Sci. 2011, 36 (12), 1629–1648. doi:10.1016/j. progpolymsci.2011.05.007. 116. Zhang, J.; Yue, L.; Hu, P.; Liu, Z.; Qin, B.; Zhang, B.; Wang, Q.; Ding, G.; Zhang, C.; Zhou, X.; Yao, J.; Cui, G.; Chen, L. Taichi-inspired rigid-flexible coupling cellulose-supported solid polymer electrolyte for high-performance lithium batteries. Sci. Rep. 2014, 4, 6272. doi:10.1038/srep06272.

BIBLIOGRAPHY Sequeira C.; Santos D.(Eds.) Polymer Electrolytes: Fundamentals and Applications; Elsevier: The Netherlands, 2010.

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Energy: Polymer Supercapacitors Anish Benny Department of Electrical and Electronics Engineering, Amal Jyothi College of Engineering, Kottayam, India

Soney C. George Centre for Nanoscience and Nanotechnology, Amal Jyothi College of Engineering, Kottayam, India

Abstract Demand for power sources is getting increased due to tremendous usage of portable electronic devices. Batteries are the most popular power sources. For energy storage, capacitors can be considered as an alternate device compared to batteries. Supercapacitor, also called an ultra-capacitor, is an electrical component capable of holding electrical charge hundred times more than a standard dielectric capacitor but is much lower than those of secondary batteries. Supercapacitors are electrochemical capacitors, and their structure is entirely different from conventional dielectric capacitors. Electrochemical capacitors can be classified into three types: the first type is electric double layer capacitor that depends on the charge storage of ion adsorption, the second type is pseudocapacitor that is based on charge storage involving fast surface redox reactions, and the last type is hybrid capacitor that depends on the charge storage of ion adsorption and fast surface redox reactions. In most of the cases, a supercapacitor can take the place of a rechargeable low-voltage electrochemical battery. Supercapacitors can be used in backup power applications because of their infinite life span. Due to the above reasons, study on supercapacitors is of high interest among the researchers. This entry reviews the latest progress in supercapacitors in charge storage mechanisms, electrode materials, electrolyte materials, characterization methods, and applications. The aim of this review is to know the benefits and challenges involved in the use of several materials in supercapacitors and to recognize areas for further development. Keywords: Asymmetrical supercapacitor; Electric double layer capacitor; Electronically conducting polymers; Polymer electrolytes; Pseudocapacitor; Symmetrical supercapacitor.

INTRODUCTION TO SUPERCAPACITORS Natural resources are depleting day by day, and there is an increasing need for high efficient rechargeable batteries, fuel cells, and supercapacitor technology to satisfy the future development of a low-carbon and sustainable economy. Rechargeable batteries are promising candidates for electric vehicles (EVs) and capable energy storage devices for sustainable energy such as wind or solar energy. However, the charge storage mechanism of current rechargeable batteries mainly depends on the intercalation or de-intercalation of cations within the crystalline structure of electrode materials. This is under the control of diffusion of cations within the crystalline structure, which significantly limits the charge/discharge rate of batteries. Some of the renewable energy sources are difficult to produce because of their irregular nature. Therefore, reliable and safe energy storage devices are needed to meet the heavy energy demand of the world. Other than batteries, the remaining promising candidates for energy storage

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purpose are fuel cells, supercapacitors, and photoelectrochemical cells. But supercapacitor has been found to be the most exciting device due to quick bursts of power, extended cycle life, low cost, and ease of fabrication processes than other energy storage devices. Supercapacitors can provide higher power density than their counterparts such as batteries because their charge storage mechanism is based on the surface reactions of electrode materials, without ion diffusion within the bulk of materials. When high power delivery and/or fast energy harvesting are required, supercapacitors are promising alternatives to rechargeable ­batteries. But supercapacitor lacks capacity much lower than that of batteries because of the restriction of charge storage to the surface. Many researches have been conducted to improve their energy density. It should be noted that the energy density (E) of a supercapacitor depends on both the specific capacitance (F) of the electrode materials and the overall cell voltage (V). To increase the capacity, one efficient technique is to develop porous and nanosized electrode materials. Another method is to build hybrid or Encyclopedia of Polymer Applications, First Edition DOI: 10.1201/9781351019422-140000165 Copyright © 2018 by Taylor & Francis. All rights reserved.

asymmetric supercapacitors, which can efficiently utilize the potential gap between the two types of electrodes to increase the overall cell voltage (V). During the past years, many efforts have been made to develop nanostructured electrode materials to shorten the diffusion length and increase the outer surface area in order to improve the performance of rechargeable batteries or supercapacitors. In short, we can say that supercapacitors currently fill the gap between batteries and electrostatic capacitors. Conducting polymers (CPs), metal oxides (MOx), and carbon materials are three major material categories used in supercapacitor electrodes. Hybrids or nanocomposites of CPs and carbon materials find a suitable alternative for supercapacitors due to cost-effectiveness, the ability to meet the energy demand for future applications, and ease of synthesis in the fabrication of electrodes. The active research based on hybrid nanostructures of CPs and carbon materials toward the supercapacitor development, its properties, and ­characterization has been discussed.

Current collector Electrode

Electrolyte Separator

Fig. 1  Basic arrangement of a supercapacitor cell

A separator is placed in between the electrodes and filled with electrolytes which enable the movements of ions. The current collectors are connected with both electrodes which conduct the electrical current from working electrodes. The specific energy is calculated according to the relationship given as follows:[2]

CLASSIFICATION OF SUPERCAPACITORS There are three types of supercapacitors found in the literature: electric double-layer capacitor (EDLC), surface redox reaction-based pseudocapacitor, and the hybrid type  formed by combining the EDLC and pseudocapacitive charge s­ torage mechanisms.[1,2] The working principle of the EDLC mechanism was first described in a US patent in the 1950s.[2] The carbon electrode having high surface area was immersed in sulfuric acid electrolytes to collect the polarized charge. Later, NEC (Japan) introduced the usage of aqueous-electrolyte capacitors in the 1970s.[2] Its charging and discharging mechanisms are different from the chemical reactions found in batteries during charging and discharging processes. The capacitance of EDLC mainly comes from electrochemical adsorption/desorption of cations and anions at the electrode/electrolyte interface. When the electrochemically stable electrode of EDLC is immersed in an electrolyte, under external electric field, there will be a sudden arrangement of charges at the interface of the electrode and in the electrolyte, approaching the electrode to form electrochemical double layer.[3] This process of charge collection in an EDLC is schematically shown in Fig. 1. The major difference between conventional capacitor and supercapacitor is that the solid dielectric layer in a dielectric capacitor is replaced by electrolyte in a supercapacitor. In EDLC, no Faradaic reaction takes place and the electrical energy in an EDLC is accumulated at the surface of electrode material. In order to obtain the best EDLC performance, electrochemically stable electrode materials with high specific surface area and conductivity should be used. Porous carbon is a good choice. However, recent researches prove that graphene is a very good candidate for making EDLC because of its high specific area. The basic EDLC supercapacitor contains two working electrodes, anode and cathode, as shown in Fig. 1.

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Ecell =

1 CV 2 2

(1)

where C is the total capacitance and V is the voltage of the cell. The power, P, of the supercapacitor can be expressed by[2] P=

V2 4R

(2)

where R indicates the equivalent resistance of the capacitor and V is the voltage of the cell. Another important parameter, the specific capacitance, depends on the electrode materials. Selection of electrolytes is another vital matter, and it is chosen based on the operating ­voltage. The other factors determining the supercapacitor performance are electrolyte/electrode interface, the stability of electrodes in the electrolyte, voltage window, porosity, temperature, self-discharge, and the internal resistance. In short, we can say that proper selection of electrode and electrolyte plays the most important role in the fabrication of supercapacitors. Capacitance again depends on the ion size. But thermal stability of the capacitor depends on the boiling point, freezing point, and salt solubility of electrolyte. The power density is influenced by ionic conductivity and ion–electrode interaction. The capacitance retention ability depends on the electrochemical stability and ion–electrode interaction. [4] Nonaqueous acetonitrile-based electrolytes can be used up to an operating voltage of 2.7 V, and for ionic electrolytes, the operating voltage range is 3.5–4 V. [2,4] The working principle of pseudocapacitors involve Faradaic oxidation–reduction reactions at the electrode/electrolyte interface. Transition MOx or conductive polymers (CPs) exhibit typical pseudocapacitance and display

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Ion

De intercalation Electrode Electrolyte Ion

107 106 Specific power / Wh kg–1

Electronics–Energy

Intercalation Electrode Electrolyte

Capacitors

104 103

much higher specific capacitance than carbon materials for EDLCs. [5,6] Because of Faradaic reactions at electrode surfaces, pseudocapacitors have short life cycle than EDLCs. The advantage of fast reversible Faradaic reaction is that a higher energy density can be obtained for pseudocapacitors than for EDLCs. The process of charge collection in a pseudocapacitor is shown in Fig. 2. Recent researches are focus more on the development of pseudocapacitors. In electrochemical batteries, the bulk of the electrode participates in the electrochemical reaction leading to crystallographic and structural changes which affects the stability during cycling. But in pseudocapacitors, the surface redox reactions take place only in the top few nanometers in the electrode’s surface, and it will not affect the crystallographic and structural nature of the material leading to very good stability during cycling. Most modern nanoengineering skills improve the properties of pseudocapacitors. Widely employed electrode materials include MOx such as RuO2, MnO2, NiO, Co3O4, Fe3O4, and V2O5; metal hydroxides such as Ni(OH)2 and Co(OH)2; and CPs such as polyaniline (PANI) and polypyrrole(PPy). Based on the redox reactions for different electrolyte ions, it alters various properties of supercapacitors such as specific capacitance, power characteristics, and working voltage window. Latest researches show that CP is very good for making pseudocapacitive electrodes because it has high electrical conductivity and is low cost. However, CP pseudoelectrodes degrade severely during cycling because of doping and de-doping processes involving the intercalation and de-intercalation of ions. The third type of supercapacitor is hybrid-type supercapacitor. It incorporates the working principles of both EDLCs and pseudocapacitors. It exhibits the highest capacitance than the other two capacitors. However, fabrication complexity and construction cost may be a disadvantage for these types of capacitors. The energy storage density and power delivery capability of various storage and conversion systems

Super capacitors

100

Batteries

10 1 0.01

Fig. 2  Charge storage mechanism of a pseudocapacitor

Combustion engine, gas turbine

105

0.05 0.1

0.5 1

5 10

50 100

Specific energy / Wh kg–1

Fuel cells

500 1000

Fig. 3  Ragone plot of various energy storage and conversion devices Source: Reprinted with permission from American Chemical Society, © 2004.[5]

are concluded in a special chart called the Ragone plot as shown in Fig. 3. From the chart, it is clear that supercapacitors can store up to 10 Wh kg–1 of energy and provide power up to 106 W kg−1. In the Ragone plot, we find that researchers are trying to match performance data of other systems to those of conversion systems such as combustion engines and fuel cells. It is clear that none of the electrochemical systems matches the performance of combustion engines. Moreover, in the Ragone plot, the performance of supercapacitors overlaps at both ends with that of capacitors (at high-power, low-energy end) and that of batteries (at high-energy, low-power end). COMPARISON BETWEEN SUPERCAPACITOR AND BATTERY Here, the working principle of supercapacitors is compared with the charge storage mechanism of batteries. It is very important to distinguish between a capacitive behavior and a battery behavior. The energy storage mechanism of rechargeable batteries mainly works on the principle of intercalation/de-intercalation of cations (H+ or Li+) within the crystalline structure of electrode materials, combined with the redox reactions of metal ions within the crystalline structure. The charging/discharging process of aqueous Ni–MH batteries depends on H+ intercalation/de-intercalation. Similarly, the charging/discharging process of conventional Li-ion batteries depends on Li+ intercalation/de-intercalation.[7,8] The capacitance of supercapacitors is due to the surface reactions of electrode materials such as electrochemical adsorption/desorption of cations and anions at the electrode/electrolyte interface and surface Faradic redox reactions as discussed in the previous section. It should be noted that in all rechargeable batteries,

INTRODUCTION TO POLYMER SCIENCE BEHIND SUPERCAPACITORS The charge storage action of a supercapacitor is confined by the electrodes in most cases. Hence, we can say that the development of electrode materials relates to the development of supercapacitors. When two electrodes of a supercapacitor are fabricated from the identical material, a symmetrical supercapacitor is formed. Otherwise, an asymmetrical supercapacitor can be fabricated. In symmetrical supercapacitors, an electrode may be connected as either a positive or a negative electrode. But in an asymmetrical supercapacitor, the positive electrode operates at the more positive potential and the negative electrode is operated at the more negative potential. The merit of symmetrical supercapacitors is its easy fabrication because both electrodes are the same. Asymmetrical supercapacitors can be specially designed to achieve a higher operating voltage and thus achieve higher energy storage density.[14–17] Carbon is the common material used for making symmetrical supercapacitors.[18–20] For asymmetrical supercapacitors, carbon is a good choice for the negative electrode, and for the positive electrode, a wide range of materials including MOx[18,21,22] and electronically conducting polymers (ECPs) or CPs can be used.[23–27] Asymmetrical supercapacitors made from two different MOx or two different ECPs are also common.[25,28–31] The schematic diagram of symmetrical and asymmetrical supercapacitors is shown in Fig. 4. Carbon and its different forms are the most extensively used marketable material for supercapacitors. Carbon materials are widely used because of their low cost, high conductivity, good corrosion resistance, high temperature stability, availability, and established electrode production

Positive/ negative

Positive/ negative

(P type CP) ECP/MOx electrode

Electrolyte

Electrolyte

(N type CP) Carbon electrode

Separator

Symmetrical supercapacitor

Negative

Positive

Asymmetrical supercapacitor

Fig. 4  Schematic diagram of symmetrical and asymmetrical supercapacitors

2100

EDLC pseudocapacitance

EDLC/ pseudocapacitance

1800

CNT/CP

1500

GO/CP MOx/CP

1200 900 600 300 0

CNT GO

Carbon

Polyaniline, PANI Polypyrrole, PPy PEDOT Conducting polymers

Composite materials

Fig. 5  Comparison of various materials according to their ­specific capacitance for supercapacitor applications

technologies. But carbon material-based supercapacitors show only lower capacitance compared to the MOx and the ECP-based supercapacitors. Based on the literature survey, Fig. 5 summarizes the specific capacitance of the EDLCs and pseudocapacitor systems based on various active materials. From Fig. 5, it is clear that the specific capacitance of MOx and ECPs is very large compared to carbon. Because of the nonideal behavior, each of the material discussed above has several pros and cons, which are summarized in Table 1.

Electronics–Energy

Carbon electrode

Electrolyte

Electrolyte

Carbon electrode

Specific capacitance (Fg–1)

a phase-transformation and/or alloying reactions occur, in addition to the intercalation reaction mentioned above. Hence, the charge storage principle of a supercapacitor is different from that of a rechargeable battery. The charge storage of battery is limited by cation diffusion within the crystalline structure of active material, while the charge storage of supercapacitor is not under the control of the diffusion process.[9–11] Since the capacitive behavior is not diffusion controlled, the current should vary linearly. Most of the battery electrode potential is determined by the Gibbs energies of pure, well-defined phases and also the composition and concentration of the electrolyte.[12,13] A pseudocapacitive electrode shows a linear dependence of the charge stored with the charging potential. The charge storage initiates from electron-transfer mechanisms, rather than the accumulation of ions in the EDLC. But the charge storage of a battery electrode comes from the special potential. Hence, the charge/discharge curves of battery electrodes are not linear as in the case of supercapacitors.

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Table 1  Comparison of physicochemical properties of the various supercapacitor materials Physicochemical Carbon properties materials MOx CPs Non-Faradaic capacitance

Very high

Medium

Medium

Faradaic capacitance

Very low

Very high

Very high

Conductivity

Very high

Low

Very high

Energy density

Low

High

Medium

Power density

High

Low

Medium

Cost

Medium

High

Medium

Chemical stability

Very high

Low

High

Cycle life

Very high

Medium

Medium

Easy fabrication process

Medium

Low

High

Flexibility

Medium

Very low

High

and polyethyl enedioxy thiophene (PE DOT). Because of their high ­conductivity, ease of preparation, cost-­ effectiveness, and lightweight, higher specific capacitance can be obtained.[37–39] Although n-type ECPs also exist, e.g., ­certain polythiophene (PTh) derivatives, they are not commonly used in supercapacitor applications due to their relative instability.[40,41] For all ECPs, the theoretical mass specific capacitance can be approximately computed using the following equation: Ctmsp =

nF ∆VMW

(3)

where n indicates the average number of electrons transferred per unit monomer of the ECP electrode material during the redox process, F denotes the Faraday constant (96,485 C mol−1), ΔV is the potential range across which the charging occurs, and M W indicates the relative ­molecular weight per monomer unit.

ROLE OF CONDUCTING POLYMERS

Types of Polymers Suitable for Electrodes

ECPs are CPs which have a π-conjugated backbone and contain regularly alternating single (C–C) and conjugated (e.g., C=C) bonds. The π-conjugated backbone helps in the formation and propagation of charge carriers, making the polymers intrinsically conducting. Thus, all ECPs share the same π-conjugated structure.[32] In the 1970s, Alan J. Heeger, Alan G. MacDiarmid et al. first developed the basic underlying principle of the electronic conductivity in ECPs. It was based on polyacetylene (PA), and it is the simplest form among all ECPs.[33,34] The PA film exhibited a tremendous increase in conductivity when it was doped with bromine.[34] In that case, the doping with bromine corresponds to the oxidation of the ECP also called p-doping, and de-doping indicates the reduction in ECPs. Hence, we can say that in the oxidized state, p-type ECPs are conductors, and in the neutral state, they are good insulators. Because of the above property, ECPs are also called semiconducting polymers and it defines the nature of the material’s conductivity. The semiconducting nature of ECPs is the reason behind the charge storage in these materials.[35,36] Another method to oxidize and reduce the ECP is by applying a potential. If these pseudocapacitive materials are grown on electrodes, they can be oxidized or reduced by electrochemical action. Normally, the redox process due to electrochemical action is accompanied by either absorption or expulsion of counter ions to maintain ­electroneutrality. ECPs are the promising candidates to fabricate supercapacitor electrodes because at a faster rate, the redox processes in ECPs can be operated. However, a suitable electrolyte is needed to achieve this operation. Because of ECPs, higher capacitance, higher power, and rapid charging and discharging ability in the supercapacitor can be improved.[37] The most popular p-type ECPs for supercapacitor applications are PPy, PANI,

Polypyrrole After PA, PPy is one of the simplest CPs. The π-conjugated backbone of a neutral PPy resembles that of cis-polyacetylene, except PPy that has an additional nitrogen heteroatom for every four carbon atoms, linking between the first and fourth carbon atoms as shown in Fig. 6. It is a p-type CP for the Faradaic pseudocapacitor application. The important features of PPy are high conductivity, fast charge–discharge mechanism, good thermal stability, low cost, and high energy density. The cross-linked PPy exhibits very high capacitance because of high ion diffusivity and porosity of the active material. The nitrogen heteroatom and the ring structure of the repeating unit help to stabilize the molecular structure of PPy. PPy is very useful for supercapacitor fabrication. The pyrrole monomer is soluble in water, and the polymer has good conductivity and environmental stability. The specific capacitance values of PPy materials lie in the range of 200–500 F g−1. An et al. achieved a specific capacitance of 433 F g−1 for their supercapacitor using the PPy/ carbon aerogel composite synthesized by chemical oxidation polymerization technique as shown in Fig. 7. Because of high internal resistance in the occurrence of binders, it is very difficult to make PPy electrodes for supercapacitor

N

N N

N

Fig. 6  Chemical structure of PPy ECP in their neutral state. Hydrogen atoms are omitted for clarity

N

Pyrrole

SDS

N

N

Fig. 9  Chemical structure of PANI ECP in their neutral state. Hydrogen atoms are omitted for clarity CA

FeCl3

SDS Pryyole

Polymerization PPy

Fig. 7  Schematic of the synthesis of PPy/CA composite Source: Reprinted with permission from Elsevier, © 2010.[42]

21%-PPy/CA 35%-PPy/CA 63%-PPy/CA 10%-PPy/CA CA PPy

Specific capacitance (F g–1)

480 400 320 240 160 80

0

2

4 6 Scan rate (mV s–1)

8

10

Fig. 8  Plot of specific capacitances of PPy/CA composite, CA, and PPy electrodes Source: Reprinted with permission from Elsevier, © 2010.[42]

application using chemical oxidation–polymerization techniques in the presence of excess surfactants. Figure 8 shows the specific capacitance variations of different PPy contents in PPy/carbon aerogel (CA) composites with different scan rates. From the figure, we can see that the specific capacitance of 35% PPy/CA composite reaches to 433 F g−1 at 1 mVs−1 and for pure PPy, the specific capacitance reaches 320 F g−1 at 1 mVs−1. Moreover, the results reveal that all PPy/CA electrodes exhibit higher capacitance than the pure CA. The electropolymerization technique can be employed for the growth of PPy directly on the current collector for supercapacitor application. A capacitance of 0.42 F cm−2 with a high energy density of 1 mWh cm−3 at a power density of 0.27 W cm−3 was obtained when the electrode was made of PPy/paper composite. The PPy nanostructure on a suitable flexible substrate can be used for making the electrodes of future supercapacitor application. Researches show that the PPy-coated paper retained its conductance almost constant after 100 cycles of bending.

Polyaniline PANI is another CP discovered in 1934 as aniline black. The chemical structure is shown in Fig. 9. PANI is a special ECP because the transition from insulator to metallic conductivity depends on the oxidation state and induced protonation.[43] Protic solvents are required for PANI to become conducting, and to enable proper charging and discharging. The main advantage of using PANI is its higher capacitance (higher than 600 F g−1) than other ECPs. This is because a PANI can remove up to one electron for every two monomer units during the reversible redox reaction. For PPy, a maximum of one electron is removed every three monomer units during the reversible redox reaction. Chemical and electrochemical oxidation methods can be used to synthesize PANI. Researches are showing that PANI-based supercapacitors achieved a specific capacitance up to 3,000 F g−1. The specific capacitance depends on different factors such as the structural morphology, dopant concentration, polymerization process, and the ionic diffusion length of the electroactive material. In another research, using the potentiodynamic method, the porous carbon has been deposited with PANI achieved a very high specific capacitance of 1,600 F g−1 at a current density of 2.2 A g−1. Figure 10 shows the scanning electron microscope (SEM) images of pure PANI and PANI/SA, and the transmission electron microscope (TEM) image of PANI/SA. From Fig. 10a, we can see that the pure PANI appears as random aggregation, and their surface is not smooth. The reason is that a large number of nanoparticles are adsorbed on them because of the secondary growth. But Fig. 10b and c indicates that the PANI/sodium alginate (SA) nanofiber has uniform and well-extended random network and exhibits a mat-like nanostructure. Such composites with a porous network help in supercapacitor applications. The TEM image in Fig. 10d further illustrates the uniform network of PANI/SA nanofibers. Generally, the PANI-based electrode exhibits very good specific capacitance value. However, its cycle stability is limited. The stability problem can be overcome by using suitable composite materials along with PANI while ­making supercapacitor electrodes. Polythiophene Derivatives Poly(3,4-ethylenedioxythiophene) or PEDOT is another ECP for the fabrication of supercapacitors. PEDOT was developed in the 1980s at the Bayer AG research lab in Germany. The structure of PEDOT is shown in Fig. 11.

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(a)

(c)

(b)

(d)

Fig. 10  SEM images of (a) pure PANI, (b and c) PANI/SA, and (d) TEM image of PANI/SA Source: Reprinted with permission from American Chemical Society, © 2011.[44]

During polymerization of PEDOT, the unwanted couplings do not take place as both of the β positions are occupied by the ethylenedioxy functionalities. Hence, higher conjugation length is obtained, giving rise to a higher intrinsic electronic conductivity. Higher stability of the polymer is also obtained in the oxidized state because of the ethylenedioxy functionalities. PEDOT is insoluble, but this can be overcome by using poly(styrene-sulfonate) or PSS as the dopant during polymerization. The combination of PEDOT:PSS becomes soluble. [45] PEDOT:PSS is commercially available as BAYTRON P. The main merit is its wider potential range and higher stability. The mass specific capacitance lies in the range of 100–200 F g−1. PTh and its derivatives can exist as both p- and n-type ECPs. Normally, the PTh shows poor conductivity. However, the p-doped polymers are highly stable in air and humidified environment. The specific capacitance of the PTh is usually lower than with PANI or PPy. The main advantage is that it can work in a higher potential window (~1.2 V).[44,45] This helps to construct asymmetric supercapacitor device based on ECPs. Other than PEDOT, the various thiophene-based

O

O

O

S

S S

O

O

O

S O

O

Fig. 11  Chemical structure of PEDOT ECP in their neutral state. Hydrogen atoms are omitted for clarity

polymers are poly(3-(4-flurophenyl)thiphene), poly(3methyl thiophene), and poly(ditheno(3,4-b:3 0, 4 0 d) thiophene). These are popularly used for the supercapacitor application. The reported specific capacitance values of these polymers are in the range of 70–200 F g−1.[44–46] A new class of dendritic poly(tris(thiophenylphenyl)amine) (pTTPA) CP is developed. This dendritic pTTPA is having a high specific capacitance (~950 F g−1) in an organic electrolyte.[47] The PTh-based supercapacitor researches open

a door for the future development of the CP-based flexible energy storage device.

Table 2  The main electrolytes currently used for supercapacitors Electrolyte Voltage window (V)

Types of Polymers Suitable for Electrolytes

TEAB F4/PC

3.5

H2SO4

1.0

KOH

1.0

Proper choice of electrolytes is a vital factor to improve the supercapacitor performance. The most effective way to increase both the energy and power density of the supercapacitors is to enhance the working potential window. In any electrochemical process, the safe working potential window depends on the electrochemical stability of the electrolytes and the generation of gaseous products at cathode and anodes. Various types of electrolytes have been developed by researchers. Other important properties of an electrolyte are ionic conductivity and charge compensation on both electrodes of the cell. Hence, the electrolyte, including solvent and salt, is one of the most important constituents of electrochemical supercapacitors. Electrolytes can be categorized as organic electrolytes, aqueous electrolytes, ionic liquids, and solid-state polymer electrolytes. According to the advanced supercapacitor electrolyte research, organic (acetonitrile, propylene carbonate (PC), linear sulfones, etc.) and ionic liquid (imidazolium, pyrrolidium, etc.)-based electrolytes could improve the working potential window almost 1.5–2 times than that of acid-based aqueous electrolytes.[48] Conducting salts dissolved in organic solvents can provide a voltage window of 3.5 V, which is higher than that of aqueous electrolytes. The main disadvantages associated with organic electrolytes are higher cost, lower specific capacitance, inferior conductivity, flammability, volatility, and toxicity. The typical organic electrolytes for commercial EDLC consist of conductive salts (e.g., tetraethyl-ammonium tetrafluoroborate [TEAB F4]) dissolved in acetonitrile or PC solvent. Acetonitrile is harmful to the environment. But PC-based electrolytes are environment friendly and can provide a wide voltage window, a wide range of working temperatures, and good electrical conductivity. Because of these reasons, TEAB F4/PC is commonly used in EDLC studies.[49] While choosing an electrolyte, the ion size of the selected electrolyte should be matched with the pore diameter of electrode materials to achieve the maximum specific capacitance. The other is that organic electrolytes must contain very little water, i.e., below 3–5 ppm. Water content will reduce the capacitor’s working voltage. Aqueous electrolytes can provide a higher ionic concentration, a lower resistance, and a much higher ionic conductivity than organic electrolytes. But the main disadvantage is the lower potential window (about 1.2 V) than organic ­electrolytes due to water decomposition. Research on ionic liquid electrolyte has also received significant interest, and it can be used as alternative electrolytes for supercapacitors. The main reasons are negligible volatility, high thermal, chemical, and electrochemical stability, low flammability, and wide electrochemical stability

Na2SO4

1.8

Li2 SO4

2.2

Pyrrolidinium dicyanamide

2.6

PVA/H3PO4 hydrogel

0.8

window of 4.5 V and good conductivity.[50] It is found that ionic conductivity of the ionic liquid electrolyte could be further improved by adding a small amount of single-walled CNTs. It enhances capacitance, energy density, and cycling stability.[51] Solid polymer electrolyte-based supercapacitors have attracted the researchers very much due to the quick demand of power for various types of electronics. The solid polymer electrolyte is a good candidate that can act as an ionic conducting media and as an electrode separator. There are three types of polymer-based solid electrolyte for supercapacitors: dry polymer electrolyte, gel polymer electrolyte, polyelectrolyte. Among these, the gel polymer electrolyte has recently been the most extensively examined electrolyte due to its high ionic conductivity. The gel polymer electrolyte is also known as a hydrogel polymer electrolyte when water is used as the plasticizer. Hydrogel polymer electrolyte generally owns three-dimensional (3D) polymeric networks. Poly(vinyl alcohol) (PVA) is one of the most selected polymer matrices for electrolyte preparation for supercapacitor. This is because of its easy preparation, good hydrophilicity, outstanding film-­ forming properties, nontoxic features, and low cost. PVA is commonly mixed with other aqueous solutions.[52] Chen et al., synthesized six different types of PVA-based hydrogel electrolytes by using different aqueous solutions, such as H3PO4, H2SO4, KOH, NaOH, KCl, and NaCl, among which the PVA/H3PO4 electrolyte provided the best capacitive performance.[53] Table 2 shows the common electrolytes used for supercapacitor application and their working voltage window. The perfluorosulfonic acid polymer known as Nafion, a well-known proton CP, has also been applied as the electrolyte in Solid-state supercapacitors recently because of its high ionic conductivity.[54,55] Analysis of Polymers The most CPs used for supercapacitor fabrication are PA, PPy, PANI, and PEDOT. They are considered as pseudocapacitive electrode materials for supercapacitors. Their conducting nature is rendered conductive through a conjugated bond system along the polymer backbone. The redox

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processes are highly reversible because no phase transformation is occurring during the redox reactions. The ideal capacitive behavior can be obtained only when keeping the CPs in the conducting state. PANI and PPy are the most suitable members because of low cost, environmental stability, and simplistic synthesis. Both PANI and PPy can only be p-doped in order to get a good reduction potential. Hence, they are treated as positive electrode materials. If the electrode is PANI, then the electrolyte should be an acidic solution, a protic solvent, or a protic ionic liquid because a proton is required during charging and discharging of PANI. Unfortunately, during the process of intercalation and de-intercalation, CPs may swell and shrink which results in a low cycling stability if it is used in supercapacitors. But the above problem can be removed by compositing ECP with other species which will improve their cycling stability. Zhao et al. developed a PANI-coated hierarchical porous carbon composite via a vapor deposition polymerization approach. This structure gives a high capacitance of 531 F g−1 at a current load of 0.5 A g−1 and an outstanding capacitance retention of 96.1% after 10,000 cycles.[56] Liu et  al. constructed a type of three-dimensional highly ordered graphene–PANI hybrid material that shows a high specific capacitance (1,225 F g−1), an outstanding rate capability, and an excellent cycling stability.[57] Wang et al. made a nanocellulose-coupled PPy/graphene oxide (GO) paper through in situ polymerization technique for use in supercapacitors, providing stable cycling over 16,000 cycles at 5 A g−1 and a large specific volumetric capacitance of 198 F cm−3.[58] By suitably selecting potential window, the cycling stability of CPs can be improved.[59] Figure  12 shows the comparison of theoretical mass-specific capacitances, average number of electrons transferred per unit monomer of the ECP electrode material during the redox process (n), the potential range across which the charging occurs (ΔV), and the relative molecular weight per ­monomer unit (MW) of various electronic CPs.[60]

∆V = 0.8 n = 0.33 Mw = 84

600

400

∆V = 0.8 n = 0.33 Mw = 67

∆V = 0.7 n = 0.50 Mw = 93

∆V = 1.2 n = 0.33 Mw = 142

200

0

PEDOT

PTh

PPy

Fig. 12  Comparison of various parameters of CPs

PANI

POLYMER DEGRADATION, STABILITY, AND FAILURE ASPECTS All CPs discussed above share a common drawback which limits their application alone in supercapacitors. They degrade during repeated charge/discharge cycles or redox cycles. This is due to the swelling and shrinkage of the ECP when the associated ion and solvent transfer across the ECP/electrolyte interface.[60,61] This weakness is very deep especially for PPy and PANI. Among the ECPs, PPy has the worst redox cycling stability performance.[60] The poor redox cycling stability is due to the structure of the PPy polymer which is nonporous structures and contains large micrometer sized particles. These structures are not good for electrolyte access during the redox cycling as the electrolyte must infiltrate thick layers of solid PPy to achieve the redox processes. Even though PANI can achieve porous structure using different fabrication methods, it is mechanically much weaker than PPy and PEDOT.[60,62,63] The instability of PANI toward repeated redox cycling indicates that just a porous structure is inadequate to use it in supercapacitor. Mechanical strength is needed to improve the performance of ECP materials. A range of ECP nanocomposites, containing nanoparticulate materials, has been developed to impart mechanical strength, and porosity to ECPs.[60,64,65] The ideal capacitive behavior can be obtained only when keeping the CPs in the conducting state. Porosity of CPs, electrode/electrolyte interaction, boiling point, and freezing point will determine the conductivity of CPs. When the ECP composites with suitable materials are used, we can improve the conductivity. In short, overall performance of CPs is improved by using ECP composites with suitable materials. Without this, the operation and stability of a supercapacitor can fail during its working. POLYMER COMPOSITES FOR FLEXIBLE SUPERCAPACITORS We have already seen that mechanical strength is desirable to improve the performance of ECP materials. A range of ECP nanocomposites, containing nanoparticulate materials, has been developed to impart mechanical strength, as well as porosity to ECPs.[60,64,65] Figure 13 shows the ­schematic of the formation of ECP and ECP nanocomposites. A number of composite materials based on CPs combined with carbon nanotubes (CNTs), [66–68] GOs, [69–71] carbon cloth (CC), [72,73] MOx, [74,75] and cellulose [76] have been confirmed, showing highly stable supercapacitive performance. The main attributes are conductivity and/or redox behavior of the CPs composites by CNTs or GO or MOx or cellulose integration. Composite electrodes are not only to improve the cycle life and conductivity but also to open the possibility of having additional physical properties depending on the individual components and their mutual interactions.

Oxidization

Collision

Polymerization EC monomers

ECP aggregates

ECP chains

In presence of CNT or CNXL Supporting structures with ECP aggregates

CNT or CNXL

Collision

Fig. 13  The schematic of the formation of ECP and ECP nanocomposites

CNT-Based Composites CNTs are famous for their high conductivity and mechanical strength. Both single-walled and multiwalled CNTs have been used in the research of EDLCs. The presence of the CNTs can improve the specific capacitance due to the better charge accumulation.[77] Literature reviews show that electrodes made from CPs showed the advantage of possessing very high pseudocapacitance. Figure 14 shows the SEM image of PPy-MWCNT composite films. The image reveals the porous structure of PPy-MWCNT composite films. The weakness of the CP-based electrode is its low mechanical stability due to the considerable volume change, repetitive swelling–shrinking, and ion insertion and release during the repeated charge–discharge cyclic process. Studies show that the composites of CPs with CNTs demonstrated a remarkable improvement of mechanical stability and performance of the supercapacitor electrode.[78,79] π–π interaction helps the conjugated CPs to effectively attach on the surface of the CNTs. Significant progress has been made in preparing CNTs/ECP composites in n­ anostructure to use it for supercapacitor applications.

(a)

(b)

Graphene/Graphene Oxide-Based Composites Graphene has wide potential applications in energy systems. Graphene is widely accepted because it has high electric and thermal conductivity, great mechanical strength, inherent flexibility, and huge specific surface area. Researchers are trying to mix CPs with graphene materials to make composites to fabricate supercapacitor electrodes. Figure 15 indicates the SEM image showing the structure of pure graphene and graphene/PPy after electrodeposition technique. The white particles are PPy.

Fig. 14  SEM image showing the porous structure of PPyMWCNT composite films: (a) at the film surface and (b) the fractured film cross section Source: Reprinted with permission from American Chemical Society, © 2002.[77]

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Electronics–Energy Fig. 15  SEM image showing the structure of (a) pure graphene and graphene/PPy after a (b) 60-, (c) 120-, and (d) 360-s ­electrodeposition. PPy can be seen as white particles Source: Reprinted with permission from American Chemical Society, © 2011.[80]

The porous structures of graphene increase the electrolyte ions travel within the electrodes to improve the specific capacitance. In 2010, graphene/PANI composite was fabricated, in which PANI nanostructures have vertically grown on the graphene surface by electrodeposition method, achieving a high specific capacitance (550 F g−1).[81] Various researches are being made in preparing graphene/ECP composites in nanostructure to use it for supercapacitor applications. CC-Based Flexible Composites CC is a low-cost material suitable for current collection in supercapacitors due to their unique 3D structure, high surface area, remarkable chemical stability, electrical conductivity, and flexibility. Hence, by combining CC with CPs, we can make supercapacitors soft portable electronic equipments, such as rollup display, electric paper, and wearable systems for personal multimedia. Horng et al. have demonstrated direct growth of PANI nanowires on CC by electrochemical polymerization, and the specific capacitance obtained was (1,079 F g−1).[82] Figure 16 shows

the SEM images of original CC, surface of PANI-NWs/CC electrode, and higher magnification SEM image of PANINWs/CC electrode. The high-magnification SEM image illustrates free-standing wirelike structures of PANINWs, with 60–80 nm in diameter and several microns in length. This will provide a significant enhancement in the active-surface area for high-performance electrochemical capacitors. Because of binder-free interface between the PANI-NWs and CC, a rapid electron transfer will occur resulting in improved charge storage and delivery. In their work, the flexible symmetrical supercapacitor was made of PANI nanowires/CC electrodes with cellulose film as the separator and 1 mol L−1 H2SO4 as the electrolyte. It has been observed that the major considering factors to improve supercapacitor performance are the shape and size distribution of the CPs on the CC substrate as well as the characteristic of the electrolyte used in the system. MOx-Based Composites MOx are one of the most promising materials for the next-generation supercapacitors. It provides higher

(a)

(b)

(c)

Specific capacitance (Fg–1)

Fig. 16  SEM images of (a) CC, (b) surface of PANI-NWs/CC electrode, and (c) higher magnification SEM image of PANI-NWs/CC electrode Source: Reprinted with permission from Elsevier, © 2010.[82]

2100

Pseudocapacitance

EDLC/ pseudocapacitance

1800 1500 1200 900 600 300 0

RuO2 MnO2 NiO CO3O4 V2O5 Fe2O3 SnO2 Metal oxides

MnO2/CP RuO2/CP CO3O4/CP MnO2/CNT

Composite materials

Fig. 17  Comparison of various MOx and their c­ omposites a­ ccording to their specific capacitance for supercapacitor applications

pseudocapacitance through bulk redox reactions. It was found that large volume variation-induced structural change limits the charge/discharge cycle stability of these electrode materials. Another problem is that MOx suffers from low capacitive behaviors due to the poor electrical conductivity. However, CPs provides a good electrical conductivity. By combining MOx and CPs at the molecular scale, a novel flexible supercapacitors with improved capacitive properties can be expected.[74,75] In MOx/CP composite, CPs provide polymeric flexibility and high electrical conductivity for flexible supercapacitors application. Based on the literature survey, Fig. 17 summarizes the specific capacitance of the EDLCs and p­ seudocapacitors systems based on various MOx and their composites. The MOx provide high specific capacity depending on their redox properties and structural morphologies. The resultant MOx/CPs composite is a new supercapacitor material with new functionalities and properties. Several CPs/MOx composites materials are proposed based on various MOx such as RuO2, MnO2, and TiO2. with PANI, PPy, and PEDOT CPs.[83,84]

ECP Cellulose Nanocomposites The use of nanocellulose for the fabrication of ECP nanocomposites is a new research development. This is mainly because CP/cellulose composites are more environment-friendly materials. PPy-cellulose nanocrystal (PPyCNXL) nanocomposites showed that the conductivity of the CP filler is not necessary. The PPy-CNXL showed high performance compared to PPy-CNT nanocomposites in the same work.[85] The PPy-CNXL nanocomposites have a porous structure and the negative charge on the CNX L surfaces. Recently, good performance had also been reported for PPy-CNXL nanocomposites made by the chemical deposition method.[86] CNXLs are also very strong materials, and it may be helpful to make strong thick ECP-CNXL films. Recent researches show that CNXLs can bring similar reinforcements on PANI and PEDOT as they did for PPy. It was shown that PANI-CNXL nanocomposites were promising materials for supercapacitors similar to PPy-CNXL, and a thick PANI-CNXL film which had high electrode capacitance exceeding 2 F cm−2 was obtained using electrodeposition. Moreover, cellulose nanofibrils derived from a bacterial source have been used for making PANI composites.[87,88]

TESTING AND CHARACTERIZATION OF SUPERCAPACITORS Material Characterization Supercapacitors give capacitance through the surface reactions of electrode materials. Hence, characterization of surface area, pore structure, and surface structure (containing functional groups) is very important. Brunauer–Emmett– Teller (BET) method can be used to determine the surface area and the pore-size distribution of the electrode material. Also, surface analysis, using FTIR and X-ray photoelectron spectroscopy (XPS), is most important for the pseudocapacitance electrode materials.

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The capacitance of supercapacitors strongly depends on the surface area of the selected electrode materials. However, the electrolyte ions could not access the whole specific surface area. Specific surface area characterization including the pore-size distribution, mesopores, and micropores can be determined and performed by nitrogen (77.4 K) or argon (87.3 K) sorption experiments using the BET method. Additionally, CO2 adsorption/desorption is performed to assess the specific surface area of the ultramicropores with pore diameters less than 1 nm. By using the BET method, Li et al.[44] found that the specific surface area of PANI/SA (66 m2 g−1) is larger than that of pure PANI (28 m2 g−1), which is favorable for ion transportation and, hence, provides larger capacitance.

Nuclear magnetic resonance (NMR) spectroscopy is a technique which can analyze local structure and dynamics. It can observe individual ionic species independently due to the advantage of element selectivity, and thus, it is employed to study the charge storage mechanism during the charge/discharge process for a supercapacitor. Ex situ NMR is employed for holding the supercapacitor at a specific voltage level, disassembling it, and then obtaining the NMR spectrum of the electrolyte species that remain inside the electrode film. However, in situ NMR permits changes in the local environments of the ions in the electric double layer to be observed for working devices. This technique gives a qualitative insight into the charging mechanism for a range of supercapacitor systems.[92,93] Griffin et al.[94] used an in situ NMR method to examine variations at the electrode/electrolyte interface of supercapacitors during charging and discharging process.

X-ray Photoelectron Spectroscopy Pseudocapacitance mainly relates to the surface Faradaic reactions of active electrode materials. Hence, valence state and structure of surface atoms should be studied. XPS helps to detect the valence state of surface atoms and also the existence of surface functional groups or heteroatoms. With the help of XPS, Toupin et al.[89] observed the change in manganese oxidation state, which varied from III to IV for the reduced and oxidized forms of thin film electrodes, respectively, during the charge/discharge process. Wu et al.[90] used XPS to detect the existence and relative concentrations of N and B heteroatoms in graphene sheets to confirm the homogeneous heteroatom doping in the graphene. Sullivan et al.[91] employed XPS to find the formation and reduction in oxygen-containing groups of glassy carbon electrodes after oxidation and reduction. XPS is very sensitive which can be used to detect the functional surface species. But FTIR is employed to verify the formation of a surface layer by checking differences between the reference spectrum and the ­spectra of the oxidized, or oxidized and reduced sample. (a)

4 I/mA

3 2

Electrochemical profile evaluation is employed to find the electrochemical performance of an electrode material or a supercapacitor. There are several types of electrochemical measurements for the evaluation of supercapacitors. Cyclic voltammetry (CV) can be directly used to evaluate the average capacitance for an EDLC behavior or a typical pseudocapacitive behavior that displays a rectangular CV curve. If CV curve is not rectangular containing redox peaks, then the corresponding average capacitance cannot be directly calculated. Another test is galvanostatic charge/discharge test. This is an efficient measurement for capacitance evaluation using working potential window. Figure 18 shows the cyclic voltammograms of a PANI/SA electrode at various scan rates in 1M H2SO4 and displays the galvanostatic charge/ discharge curves of pure PANI and PANI/SA nanofiber electrodes in 1 M H2SO4. For plotting CV curve, the scan rate

1 0 100 200 300 400 500 Scan rate/mV s–1

0

0.8

0.6

0.4

PANI

PANI/SA

0.2

–2

–0.2

(b)

E/V vs.CSE

2

Electrochemical Profile Evaluation

500 mv 300 mv 200 mv 150 mv 100 mv 50 mv 20 mv

5

4

I / mA

Electronics–Energy

In situ Nuclear Magnetic Resonance Spectroscopy

BET Method

0.0

0.2 0.4 E / V vs.SCE

0.6

0.8

0.0

0

500

1000

1500 Time / s

2000

2500

3000

Fig. 18  (a) Cyclic voltammograms of a PANI/SA electrode at various scan rates and (inset) evolution of the current density at different potential scan rates. (b) Galvanostatic charge/discharge curves of pure PANI and PANI/SA nanofiber electrodes Source: Reprinted with permission from American Chemical Society, © 2011.[44]

used was 100 mV s−1, and for galvanostatic charge/­discharge curves, the current rate used was 1 A g−1. The high power property of an electrode material in supercapacitors can be identified from their voltammetric response at various scan rates, and the electrochemical capacitance is proportional to their CV curve area. An additional test called electrochemical impedance spectroscopy measurements is popularly performed by collecting the supercapacitor impedance data at a specific potential, with a small voltage amplitude of 5 or 10 mV over a wide range of frequencies, 0.01 Hz–100 kHz. It can display various resistances such as interface resistance, charge transfer resistance, and the capacitive behavior. The Ragone plot (energy density vs. power density) has been widely used to find the overall performance of a supercapacitor. Figure 3 is a Ragone plot and can only be used to characterize the electrochemical profile of a supercapacitor, rather than a single electrode. CHALLENGES INVOLVED IN BUILDING SUPERCAPACITORS From the above section, it can be made out that different composites with CPs can drive the future of CP-based high-performance flexible supercapacitors. Research studies show that the composite development can change the overall electrochemical Faradaic charge transfer process and the final performance of supercapacitors and stability of the electrolyte materials. It is found that CP-based composites were the best performing flexible supercapacitors when directly fabricated on the suitable current collector. Most of the research works have mainly concentrated on the fabrication of high-performance supercapacitor materials. Even though the CP-based composites show very high specific capacitance, their device performances such as power density, energy density, and cycle stability are not up to the mark for practical applications. The supercapacitor parameters such as optimization of the supercapacitor cell fabrication and their effect on device performance, such as power density, energy density, and stability, were less discussed. More advanced study is carefully needed in this field to fit the supercapacitor for commercial energy storage device applications. Lastly, it is essential to define the industrial standards for the commercialization of supercapacitors. Carbon-based supercapacitors are commercially available in the market, but there is lack of industrial standards for both EDLCs and pseudocapacitors at this moment. Hence, it is compulsory to form some general industrial standards, such as electrode structural and dimensional parameters, performance, and stability, depending on the type of supercapacitors and their applications. APPLICATION AND FUTURE ASPECTS The various applications for supercapacitors are reviewed by Refs. [95–97]. The vital application for supercapacitors

Table 3  Summary of the budding applications of supercapacitors Sl No. Target Applications 1

Lead–acid battery replacement

ECs

Frequent stop and go EDLCs vehicles • • • • •

Electric bikes Electric carts Energy backups Anti-voltage drop Electrical networks

2

Regeneration Large cranes applications Elevators

EDLCs

3

Renewable energies

Storage for PV Wind/solar generators

EDLCs/ pseudocapacitors

4

Vehicles

Electric bus Electric cars Electric tramcar

EDLCs/ pseudocapacitors

has been prolonged to the EV application, including hybrid power with fuel cells or LIBs, as well as pure supercapacitor power. Two examples of supercapacitor applications are electric bus (e-bus) and electric tramcar. They are introduced to illuminate the future application trend of supercapacitors. Because of supercapacitors, the fast charging is possible for an e-bus, and it is one of the important advantages for public city traffic systems. ECs are the most promising candidate for such applications, as they can be charged within seconds. Aowei Technology Co., Ltd. (Shanghai, China) has successfully applied Ni(OH)2/hybrid ECs (or hybrid supercapacitors) in a trolley bus. The above trolley bus can be fully charged within 90 s and drive 7.9 km at a time with an average speed of 22 kmph and a maximum speed of 44.8 kmph. It takes only 16.5 s to increase the throttle from 0 to 40 kmph. Recently, EDLCs have been applied in an electric tramcar by the Chinese CSR Co., Ltd. (Beijing, China). The electric tramcar can be charged in a very short time of 30 s and drive 3–5 km at a time. Table 3 summarizes the budding applications of supercapacitors. The main advantages of a high-capacity supercapacitor are as follows: There is no explosion or fire risk because no chemical reactions are taking place, the cycle number or cycle stability can reach one million, and its lifetime can be up to 10 years. This type of electric tramcar and e-bus can lessen city traffic pressure and reduce environmental pollution. Table 4 gives energy and power performances of some selected supercapacitors fabricated using ECP nanocomposites. Both symmetric and asymmetric supercapacitors are taken for performance comparison. In the table, V is the working voltage, Wsp is the specific energy, and Psp is the specific power. Table 5 shows the comparison of some selected supercapacitor performances in terms of specific capacitances, specific energy, and power and stability.

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Energy: Polymer Supercapacitors

Electronics–Energy

Table 4  Energy and power performances of some selected supercapacitors fabricated using ECP nanocomposites Electrode and electrolyte Supercapacitor characteristics Positive

Negative

Electrolyte

V (V)

Wsp (Wh kg−1)

Reference

Psp (kW kg−1)

PPy-GO

Activated carbon

Aq. 1 M Na2SO4

1.6

21.4

2.1

[15]

MnO2-RGO

PPy-RGO

LiCl-PVA gel electrolyte

1.8

16

7.4

[31]

PPy-CNT

PPy-CNT

1 M LiClO4/PC

2

21.22

—

[98]

PANI-CNT

PANI-CNT

1 M LiClO4 in 1:1:1 mixture of EC/DEC and DMC

2

131

62.5

[99]

Table 5  Summary of the selected high-performance CP-based flexible supercapacitor Energy density and Electrode materials Specific capacitance power density

Stability

Reference

PANI NW/CC

1,079 F g−1 at 1.73 A g−1 in 1 mol L−1 H2SO4

100.9 Wh kg−1 and 12.1 kW kg−1

14% loss after 2,100 cycles

[82]

MnO2 /PEDOT:PSS-SS-PET

1,670 mF cm−2 at 0.5 mA cm−2 in 0.5 mol L−1 Na2SO4

1.80 mWh cm−3

0.5% loss after 4,000 cycles

[100]

PEDOT-NWs/CC

256 F g−1 at 0.8 A g−1 in 1 mol L−1 Na2SO4

182.1 Wh kg−1 and 13.1 kW kg−1

30% loss after 1,000 cycles

[101]

PANI/eCFC

1,035 F g−1 at 1 A g−1 in 1 mol L−1 H2SO4

22.9 Wh kg−1 and 36.5 kW kg−1

18% loss after 1,000 cycles

[102]

CNT/PPy/MnO2 sponge

305.9 F g−1 at 2 mV s−1 in 2 mol L−1 KCl

8.6 W h kg−1 and 16.5 kW kg−1

9.8% loss after 1,000 cycles

[103]

CONCLUSION The research and developments of the flexible supercapacitor based on CPs have been progressed rapidly in the last several years. As a result of several researches, CP-based supercapacitor emanated as a new platform for flexible energy storage device. The current discussion began with some history of the supercapacitor development and how ECP was used in the supercapacitor development to reach higher capacitance. This review has discussed the latest development, illustrating some of the successful research works achieving high specific capacitance based on CP-based flexible supercapacitors. However, the weaknesses of ECP would limit their use in supercapacitor applications and these weaknesses are mainly mechanical. Nanocomposites of ECP with strong materials such as CNTs, graphene, and nanocellulose have been developed to improve the mechanical strength and other functionalities of the composite materials. We hope that this review will help you to think in the mission of lucid designs for more efficient CP-based pseudocapacitors. However, there are several issues that remain to be overcome to achieve the supercapacitor performances, such as power density, energy density and cycle stability, higher working potential window in the eco-friendly ionic liquid electrolyte system, and apposite for the practical industrial

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69. Wang, H.; Hao, Q.; Yang, X.; Lu, L.; Wang, X. Graphene oxide doped polyaniline for supercapacitors. Electrochem. Commun. 2009, 11 (6), 1158–1161. 70. Biswas, S.; Drzal, L.T. Multilayered nanoarchitecture of graphene nanosheets and polypyrrole nanowires for high performance supercapacitor electrodes. Chem. Mater. 2010, 22 (20), 5667–5671. 71. Basnayaka, P.A.; Ram, M.K.; Stefanakos, E.K.; Kumar, A. Supercapacitors based on graphene– polyaniline derivative nanocomposite electrode materials. Electrochim. Acta 2013, 92, 376–382. 72. Cheng, Q.; Tang, J.; Ma, J.; Zhang, H.; Shinya, N.; Qin, L.C. Polyaniline-coated electro-etched carbon fiber cloth electrodes for supercapacitors. J. Phys. Chem. C 2011, 115 (47), 23584–23590. 73. Hsu, Y.K.; Chen, Y.C.; Lin, Y.G.; Chen, L.C.; Chen, K.H. Direct-growth of poly (3,4-ethylenedioxythiophene) nanowires/carbon cloth as hierarchical supercapacitor electrode in neutral aqueous solution. J. Power Sources 2013, 242, 718–724. 74. Yu, M.; Zeng, Y.; Zhang, C.; Lu, X.; Zeng, C.; Yao, C.; Yang, Y.; Tong, Y. Titanium dioxide @ polypyrrole coreshell nanowires for all solid-state flexible supercapacitors. Nanoscale 2013, 5 (22), 10806–10810. 75. Zhou, C.; Zhang, Y.; Li, Y.; Liu, J. Construction of high-­ capacitance 3D CoO @ polypyrrole nanowire array electrode for aqueous asymmetric supercapacitor. Nano Lett. 2013, 13 (5), 2078–2085. 76. Zhu, C.Z.; Zhai, J.F.; Wen, D.; Dong, S.J. Graphene oxide/ polypyrrole nanocomposites: One-step electrochemical doping, coating and synergistic effect for energy storage. J. Mater. Chem. 2012, 22 (13), 6300–6306. 77. Hughes, M.; Chen, G.Z.; Shaffer, M.S.; Fray, D.J.; Windle, A.H. Electrochemical capacitance of a nanoporous composite of carbon nanotubes and polypyrrole. Chem. Mater. 2002, 14 (4), 1610–1613. 78. Wu, T.M.; Lin, Y.W.; Liao, C.S. Preparation and characterization of polyaniline/multi-walled carbon nanotube composites. Carbon 2005, 43 (4), 734–740. 79. Gupta, V.; Miura, N. Influence of the microstructure on the supercapacitive behavior of polyaniline/single-wall carbon nanotube composites. J. Power Sources 2006, 157 (1), 616–620. 80. Davies, A.; Audette, P.; Farrow, B.; Hassan, F.; Chen, Z.; Choi, J.Y.; Yu, A. Graphene-based flexible supercapacitors: Pulse-electropolymerization of polypyrrole on free-­ standing graphene films. J. Phys. Chem. C 2011, 115 (35), 17612–17620. 81. Xu, J.; Wang, K.; Zu, S.Z.; Han, B.H.; Wei, Z. Hierarchical nanocomposites of polyaniline nanowire arrays on graphene oxide sheets with synergistic effect for energy storage. ACS Nano 2010, 4 (9), 5019–5026. 82. Horng, Y.Y.; Lu, Y.C.; Hsu, Y.K.; Chen, C.C.; Chen, L.C.; Chen, K.H. Flexible supercapacitor based on polyaniline nanowires/carbon cloth with both high gravimetric and area-normalized capacitance. J. Power Sources 2010, 195 (13), 4418–4422. 83. Zang, J.; Bao, S.J.; Li, C.M.; Bian, H.; Cui, X.; Bao, Q.L.; Sun, C.Q.; Guo, J.; Lian, K. Well-aligned cone-shaped nanostructure of polypyrrole/RuO2 and its electrochemical supercapacitor. J. Phys. Chem. C 2008, 112, 14843–14847.

84. Liu, R.; Lee, S.B. MnO2/poly (3,4-ethylenedioxythiophene) coaxial nanowires by one-step coelectrodeposition for electrochemical energy storage. J. Am. Chem. Soc. 2008, 130 (10), 2942–2943. 85. Liew, S.Y.; Thielemans, W.; Walsh, D.A. Electrochemical capacitance of nanocomposite polypyrrole/cellulose films. J. Phys. Chem. C 2010, 114 (41), 17926–17933. 86. Wu, X.; Chabot, V.L.; Kim, B.K.; Yu, A.; Berry, R.M.; Tam, K.C. Cost-effective and scalable chemical synthesis of conductive cellulose nanocrystals for high-performance supercapacitors. Electrochim. Acta 2014, 138, 139–147. 87. Wang, H.; Zhu, E.; Yang, J.; Zhou, P.; Sun, D.; Tang, W. Bacterial cellulose nanofiber-supported polyaniline nanocomposites with flake-shaped morphology as supercapacitor electrodes. J. Phys. Chem. C 2012, 116 (24), 13013–13019. 88. Wang, H.; Bian, L.; Zhou, P.; Tang, J.; Tang, W. Core– sheath structured bacterial cellulose/polypyrrole nanocomposites with excellent conductivity as supercapacitors. J. Mater. Chem. A 2013, 1 (3), 578–584. 89. Toupin, M.; Brousse, T.; Belanger, D. Charge storage mechanism of MnO2 electrode used in aqueous electrochemical capacitor. Chem. Mater. 2004, 16 (16), 3184–3190. 90. Wu, Z.S.; Winter, A.; Chen, L.; Sun, Y.; Turchanin, A.; Feng, X.L.; Mullen, K. Three-dimensional nitrogen and boron co-doped graphene for high-performance allsolid-state supercapacitors. Adv. Mater. 2012, 24 (37), 5130–5135. 91. Sullivan, M.G.; Schnyder, B.; Bartsch, M.; Alliata, D.; Barbero, C.; Imhofand, R.; Kotz, R. Electrochemically modified glassy carbon for capacitor electrodes characterization of thick anodic layers by cyclic voltammetry, differential electrochemical mass spectrometry, spectroscopic ellipsometry, X-ray photoelectron spectroscopy, FTIR, and AFM. J. Electrochem. Soc. 2000, 147 (7), 2636–2643. 92. Blanc, F.; Leskes, M.; Grey, C.P. In situ solid-state NMR spectroscopy of electrochemical cells: Batteries, supercapacitors, and fuel cells. Acc. Chem. Res. 2013, 46 (9), 1952–1963. 93. Borchardt, L.; Oschatz, M.; Paasch, S.; Kaskel, S.; Brunner, E. Interaction of electrolyte molecules with carbon materials of well-defined porosity: Characterization by solid-state NMR spectroscopy. Phys. Chem. Chem. Phys. 2013, 15 (36), 15177–15184. 94. Griffin, M.; Forse, A.C.; Tsai, W.Y.; Taberna, P.L.; Simon, P.; Grey, C.P. In situ NMR and electrochemical quartz crystal microbalance techniques reveal the structure of the electrical double layer in supercapacitors. Nat. Mater. 2015, 14 (8), 812–819. 95. Miller, J.R.; Simon, P. Electrochemical capacitors for energy management. Science 2008, 321 (5889), 651–652. 96. Kotz, R.; Carlen, M. Principles and applications of electrochemical capacitors. Electrochim. Acta 2000, 45 (15–16), 2483–2498. 97. Miller, J.R.; Burke, A.F. Electrochemical capacitors: ­Challenges and opportunities for real-world applications. Electrochem. Soc. Interface 2008, 17 (1), 53–57. 98. Lee, H.; Kim, H.; Cho, M.S.; Choi, J.; Lee, Y. Fabrication of polypyrrole (PPy)/carbon nanotube (CNT) composite electrode on ceramic fabric for supercapacitor applications. Electrochim. Acta 2011, 56 (22), 7460–7466.

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nanowires/carbon cloth as hierarchical supercapacitor electrode in neutral aqueous solution. J. Power Sources 2013, 242, 718–724. 102. Yu, P.; Li, Y.; Yu, X.; Zhao, X.; Wu, L.; Zhang, Q. Polyaniline nanowire arrays aligned on nitrogen-doped carbon fabric for high-performance flexible supercapacitors. Langmuir 2013, 29 (38), 12051–12058. 1 03. Li, P.; Yang, Y.; Shi, E.; Shen, Q.; Shang, Y.; Wu, S.; Wei, J.; Wang, K.; Zhu, H.; Yuan, Q.; Cao, A. Core-doubleshell, carbon nanotube@ polypyrrole@ MnO2 sponge as freestanding, compressible supercapacitor electrode. ACS Appl. Mater. Interfaces 2014, 6 (7), 5228–5234.

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Energy: Polymer-Functionalized Graphene Arun K. Nandi, Pousali Chal, and Arnab Shit Polymer Science Unit, Indian Association for the Cultivation of Science, Kolkata, India

Abstract This entry emphasizes the modern trends of the synthesis of polymer-functionalized graphene by means of both covalent and noncovalent approaches to utilize them for energy generation and energy storage devices. Mostly, the functionalization of graphene oxide, reduced graphene oxide, and graphene quantum dots with polyaniline, polypyrrole, and polythiophene is discussed. The applications of these hybrids in the energy generation by means of photovoltaic and fuel cell devices  are embodied. The energy storage behavior of these conducting polymer/graphene composites in the field of battery and supercapacitor applications is thoroughly reviewed. Apart from the binary blends of conducting polymer and graphene, the use of ternary composites containing inorganic nanoparticles for energy harvesting applications is also embodied. Keywords: Battery; Covalent functionalization; Fuel cell; Nanocomposite; Noncovalent functionalization; Polyaniline; Polypyrrole; Polythiophene; Solar cell; Supercapacitor.

INTRODUCTION Graphene, a two-dimensional and one-atom-thick ­carbon sheet with a honeycomb-like structure, is an excellent ­stimulating material due to its high surface area and exceptional electrical and thermal conductivities.[1–4] It has the highest mechanical strength with a Young’s modulus of ~1.0 TPa.[5] It also has high electron mobility with an exceptional value of 200,000 cm2 /v/s and high o­ ptical transparency.[6] Because of these unique properties, graphene has attracted a huge attention for various applications in a broad field of physics, chemistry, biology, and materials science.[7] Interestingly, in the past two decades, graphene is the most studied material among other carbon allotropes as a most promising component for many ­potential applications. The continuous increase in energy consumption makes the scientists to develop effectual energy conversion and storage systems such as solar cells, Li ion b­ atteries (LIBs), and supercapacitors. The efficiency of energy conversion and storage systems strongly depends on various factors, especially, structure and properties of the respective materials. In this regard, carbon nanomaterial, particularly graphene, has attracted a great attention due to cost-­effective and excellent electronic properties. The bottleneck of using graphene is its nonsolubility/less dispersibility that forbids its exciting surface features from different applications. Therefore, functionalization of graphene is of utmost importance to improve the processability of graphene, thus enhancing its energy harvesting applications. Several new methodologies for the synthesis of effective graphene materials have been developed, Encyclopedia of Polymer Applications, First Edition DOI: 10.1201/9781351019422-140000192 Copyright © 2018 by Taylor & Francis. All rights reserved.

including chemical vapor deposition (CVD), [8] epitaxial growth on a single-crystal SiC to produce ultrathin graphite sheet, [9] chemical coupling reaction, [10] chemical production of graphene, [11] exfoliation of graphite powder through solution oxidation, [11] micromechanical exfoliation of graphite known as Scotch tape method, [1] electrochemical reduction, [12] thermal and photocatalytic reduction of graphene oxide (GO), [13] sonication/intercalation, [14] and ball milling.[15] These approaches of graphene preparation develop different extraordinary properties required for potential applications in the targeted research fields, including batteries, fuel cells, capacitors, solar cells, and sensors. In spite of the above potential applications of graphene, its zero bandgap value and its chemical inertness make it a great challenge for its straightforward application in the field of energy applications such as inorganic semiconductors. For that reason, several research groups are devoted to produce new strategies for the functionalization of graphene, doping, and intercalation methods to generate a bandgap of graphene materials for suitable applications on electronic and optoelectronic devices. The discovery of graphene–polymer nanocomposites has opened a new select in the field of nanotechnology and materials science.[16] On the other side, inorganic nanomaterials have also been introduced as polymer/ inorganic ­composites particularly for energy harvesting applications.[17–20] Various limitations such as poor electrical and thermal conductivities create difficulty for the potential applications of these polymer/inorganic ­composites. These shortcomings led to the incorporation of carbon-based materials such as carbon black, fullerenes, and carbon nanotube (CNT) for the preparation

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of polymer/carbon nanocomposites. Although the conductivity of the composite increases, due to being an expensive material, it is not cost-effective to use fullerenes and CNTs for mass production of these composites. So, implementation of graphene as composite material resolves the above problem due to its low cost. Unambiguously, the graphene materials have a high surface area of 2,600 m2 /g and an excellent tolerance for chemical activity and a wide-­ ranging electrochemical gap that is highly advantageous for energy application of these materials. By introducing conducting polymer material into the graphene sheet as a spacer will not only minimize the aggregation propensity but also help to increase the surface area and conductivity of the active material. In an attempt to explore graphene–polymer composite in the field of solar cell, our laboratory reconnoitered the effectiveness of different graphene–polymer nanocomposites as an alternative to TiO2 nanostructures of dye-­ sensitized solar cell (DSSC).[21–25] The designed polymer and graphene sheet nanocomposites effectively assisted the electron diffusion in which the excited electrons from the dye molecules become injected to conducting polymers en route to graphene sheet, leading to a reduction in ­unfavorable recombination reactions of the device. In this study, aminofunctionalized reduced GO (a-rGO) and polyaniline (PANI) were used as active materials, and in another study, poly[3-(2-hydroxyethyl)-2,5-­thienylene] poly(HET) grafted reduced GO (PHET-g-rGO) was used as a key material for DSSCs, respectively. Other two ­studies focus on the use of graphene quantum dots (GQDs)-­polythiophene-g-poly[(diethylene glycol methyl ether methacrylate)-co-poly(N,N-­dimethylaminoethyl methacrylate)] [PT-g-P-(MeO2MA-co-DMAEMA), P] composite and GQDs-PANI composite as alternative active materials for DSSCs. In a different strategy of using a three-­dimensional network, DSSCs were fabricated with the xerogel of 5,5-(1,3,5,7-tetraoxopyrrolo[3,4-f]isoindole-­ 2,6-diyl)diisophthalic acid (P), GO, and poly(3,4-­ethyle nedioxythiophene):polystyrene sulfonate (PEDOT:PSS) ­trihybrid hydrogels as an active material. In this entry, we discuss both covalent and noncovalent functionalized polymer–graphene nanocomposites suitable for energy applications. The different methods of preparation of required polymer–graphene nanocomposites are addressed here. We have reviewed the application of these materials in the field of energy conversion and energy storage devices. Finally, a summary and conclusion are embodied discussing its future scope. FUNCTIONALIZATION OF GRAPHENE To attain stable dispersions of graphene noncovalent or covalent functionalization of graphene with polymers is necessary. The noncovalent functionalization mainly depends on the van der Waals forces, π–π stacking, or

Energy: Polymer-Functionalized Graphene

electrostatic interaction, [26–28] and it is easier to achieve without varying the chemical structure and nature of the graphene sheets.[29] The covalent functionalization of graphene derivatives is mainly based on the reaction between the oxygenated groups present in GO or reduced GO (r-GO) surfaces, such as epoxides and hydroxyls on their basal planes, and with carboxylic acid groups around the periphery of the sheets [11] with the functional groups of molecules.[30,31] Compared to noncovalent functionalization, the covalent functionalization of graphene holds wide possibilities due to the closer proximity of the donor (conducting polymer) and acceptor (graphene) molecules reach to exciton diffusion length, facilitating easier flow of excitons in the nanocomposite. It can also be noted that the noncovalent functionalization of graphene to the polymer chains can improve several properties, but the efficiency of load as well as charge transfer is expected to be low as the interaction between the wrapping molecules and the graphene surface is weak. However, a drawback of covalent functionalization on graphene surface is the deterioration of the transport properties along the graphene sheets due to the conversion of conjugated sp2 carbon into nonconjugated sp3 carbon. Nonetheless, both the methods of functionalization of graphene are presently used for the energy harvesting and energy storage applications. Precursor of Functionalized Graphene GO is the main precursor for the functionalization of graphene with polymers because of its multiple oxygen-­ containing functional groups, such as hydroxyl, epoxy, and carboxyl groups.[11] GO is usually produced using different variations of the Staudenmaier[32] or Hummers [33] method, in which graphite is oxidized using strong oxidants such as KMnO4, KClO3, and NaNO2 in the presence of nitric acid or its mixture with sulfuric acid.[34,35] The presence of these oxygen-containing groups disrupts the aromatic graphene network and renders GO electrically insulating, which limits its use for the synthesis of composites. However, the reduction of GO removes most (but not all) of the oxygen-containing functionalities such as epoxy, hydroxyl, and carboxylic acid groups. Therefore, some functionalization reactions are based on reduced GO. Generally, GO can be exfoliated using a variety of methods, usually by solvent-assisted exfoliation and reduction in appropriate media or by thermal exfoliation and reduction.[11] In the former route, the hydrophilic nature and increased interlayer spacing of GO facilitate direct exfoliation into solvents (water, alcohol, and other protic solvents) assisted by mechanical exfoliation, such as ultrasonication and/or stirring, forming colloidal suspensions of “GO.” The chemical reduction of GO is usually fulfilled by using hydrazine, [36–38] dimethylhydrazine, [16] hydroquinone, [39] NaBH4, [40,41] pyrrole, [42] chitosan, [43] amines, [44] hydroxylamines, [45] vitamin C, [46] and urea [47] as reducing agents. However, the hazardous nature and cost of the chemicals

used in reduction may limit its application. The most promising methods for large-scale production of graphene is the thermal exfoliation and reduction of GO. Thermally reduced GO can be produced by rapid heating of dry GO under inert gas and high temperature, [48,49] the epoxy and hydroxyl sites of GO decompose to produce gases such as H2O and CO2, yielding pressures that exceed van der Waals forces holding the graphene sheets together, causing the occurrence of exfoliation. The polymer functionalization of graphene relevant to energy applications are ­presented in Scheme 1. Covalent Functionalization of Graphene It is anticipated that stronger bonds are typically formed between the graphene and polymers by covalent bond formation of graphene with small molecules/polymers. The GO contains sufficient amount of oxygenated functionalities; therefore, it is hydrophilic in nature, making it incompatible with most hydrophobic polymers. The oxygenated functionalities of GO provide an opportunity to chemically functionalize with polymers, but it is usually difficult for rGO because ideal rGO lacks functional groups that can be conjugated with. Hence, chemical attachment with small molecules containing functional groups brings sufficient functionality, facilitating both GO and rGO to give remarkable opportunities for further modification with polymer either by “grafting to” or by “grafting from” technique.

“Grafting to” Method In this method, the polymer chains are at first synthesized, and finally, these presynthesized polymers are attached with the functional groups of GO or rGO or directly with their aromatic surface. This technique involves the direct covalent linkage of the functional polymers on the GO surface using esterification, amidation, click chemistry, nitrene chemistry, radical addition, etc. Yu et al. synthesized regioregular poly(3-hexylthiophene) (P3HT) chains that have been covalently grafted onto graphene sheets via esterification between the carboxylic groups in GO and CH2OH-terminated P3HT (Fig. 1). The resultant P3HTgrafted GO sheets possess good solubility in common organic solvents (e.g., THF), facilitating the structure/ property characterization and device fabrication by solution processing. Detailed spectroscopic and electrochemical measurements indicated that chemical grafting of P3HT onto graphene induced a strong electronic interaction, leading to an enhanced electron delocalization and a slightly reduced bandgap for the graphene-bound P3HT, with respect to pure P3HT.[50] Furthermore, Dai et al.[27] conjugated graphene with poly(ethyleneimine) (PEI), obtaining a water-soluble product via the formation of amide bonds (N–C=O) and subsequently assembled with acid-treated CNTs to form hybrid carbon films with interconnected carbon structures possessing well-­ defined nanoscale pores. These hierarchical hybrids with

Graphene-polymer conjugates

Covalent ATRP

Redox polymerization

Noncovalent Solution mixing

Insitu polymerization

Scheme 1  Schematic presentation of the different ways for making graphene–polymer composites for energy harvesting application

Fig. 1  Synthesis procedure for chemical grafting of CH 2OH-terminated P3HT chains onto graphene, which involves the SOCl 2 t­ reatment of GO (step 1) and the esterification reaction between acyl chloride functionalized GO and MeOH-terminated P3HT (step 2) [50]

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controlled architectures and thickness displayed a higher average specific capacitance of 120 F/g that make them promising candidates as supercapacitor electrodes.[27] Conjugated polymer-modified graphene materials are also prepared by amidation reactions. In these cases, graphene is linked to the ends of the polymers, leading to products that are soluble in common solvents, enabling device preparation by solution processing. Thus, GO modified with triphenylamine-based polyazomethine acts as a hole-transport agent due to its excellent hole injection and high mobilities and low ionization potentials. It can be incorporated into specific devices by simple spin coating to obtain materials that exhibit a nonvolatile rewritable memory effect.[51] Jianhua et al.[52] proposed a novel route for obtaining a graphene–PANI (GP) hybrid by grafting PANI onto graphene with an amide group. The oxygen-­ containing groups of graphene were reduced with the carbonyl group preserved using hydrazine. The obtained chemically reduced graphene sheets were activated using thionyl dichloride (SOCl2) to obtain a graphene derivative containing acyl chloride groups. These acyl chloride groups reacted with the amine groups of PANI, forming amide groups that anchored PANI nanofibers onto the graphene sheets. The amide linkage acts as an electron bridge joining the π-conjugated PANI chains with graphene. As a result, a larger scale π–π conjugated system is formed, and this π–π conjugated structure facilitates faradic charges to be transported effectively through the highly ­conductive network of graphene. The facile charge transfer and decreased resistance between graphene and PANI improve the electrochemical stability of the GP hybrid. Hence, a rapid charge–discharge characteristic at high sweeping rate was achieved.[52] “Grafting from” Method Generally, this technique is associated with immobilizing initiators onto graphene sheets to initiate polymerization. These initiators are covalently attached directly from the hydroxyl or carboxylic acid groups of GO or grafting the small molecules at first to bring the desired functionality followed by attachment of the initiator. Atom Transfer Radical Polymerization  Atom transfer radical polymerization (ATRP) is the most widely studied type of the “grafting from” polymerization method, but the use of this process in the field energy harvesting application is somewhat less explored. Fang et al.[53] have reported a new method for attaching polymer brushes to GO sheet. The hydroxyl groups present on the surface of GO were first functionalized with a well-known ATRP initiator 2-bromoisobutyl bromide by Schötten–­Baumann reaction. ATRP reaction of 2,5-dioxopyrrolidin-1-yl-4-­vinylbenzoate was carried out in dimethyl sulfoxide (DMSO) at 80°C with CuBr/1,1,4,7,7-­pentamethyldiethylenetriamine as the catalyst/ligand system and RGO-methyl bromoisobutyrate

Energy: Polymer-Functionalized Graphene

as the initiator. Additionally, they have functionalized the PS polymer brushes with pendant [Ru(4-CH2NH24′-CH3-bpy)(bpy)2] 2+ complex units (4-CH2NH2-4′-CH3bpy = 4-aminomethyl-4′-methyl-2,2′-bipyridine and bpy = 2,2-bipyridine). This graphene grafted with p­ olypyridyl Ru(II)-modified PS brushes were used for solar cell ­application.[53] Poly(2,2,6,6-tetramethylpiperidin-1-oxyl-4-yl methacrylate) (PTMA) is a typical polymer with potential application in cathodic materials. It is grafted from GO surface using the ATRP technique (Fig. 2). PTMA shows a theoretical capacitance of 111 mAh/g, but the composite cathode material prepared by directly mixing PTMA with RGO shows a much higher specific capacitance (222 mAh/g). However, graphene-graft-PTMA (G-g-PTMA) along with rGO conductive additive was adopted for the preparation of composite materials, and the cathode showed a higher specific capacity up to 466 mAh/g with good cycling performance and excellent rate capability, highlighting the importance of covalent functionalization of graphene sheets via surface-initiated ATRP.[54] Other Methods Other methods include oxidative ­polymerization of graphene-functionalized monomer in  the presence of oxidant. In a pioneering work, Baek et al.[55] established an effective route to prepare conducting PANI-grafted reduced GO (PANI-g-rGO) composite (Fig. 3). In this method, GO was acylated in the presence of excess SOCl2 and then reacted with amine-protected 4-­aminophenol. Upon subsequent deprotection by hydrolysis with trifluoro acetic acid, PANI-g-rGO was prepared by in situ oxidative polymerization of aniline in the presence of an oxidant and amine-terminated rGO (rGO-NH2) as an initiator. This composite showed an electrical conductivity as high as 8.66 S/cm and a capacitance of 250 F/g with good cycling stability than that of pristine graphene because of the minimal phase separation and excellent synergism between the components.[55] Chatterjee et al.[21] also took advantage of the amino functionalized reduced graphene oxide, (a-RGO by functionalized GO via the diazotization of ­p -phenylenediamine to covalently graft PANI chain with the a-RGO. After polymerization in the presence of a-RGO, the morphology has changed from nanotube of pure PANI to a new flat rectangular nanopipe (FRNP) morphology that can provide a direct conduction path increasing the electronic transport. When this PANI-a-RGO FRNP system was used as active material for the dye-sensitized application, it showed a power conversion efficiency (PCE) of 2.01%. [21] In another work, Chatterjee et al. have established a covalent grafting of HET with reduced GO (rGO) followed by oxidative polymerization to get PHET-g-rGO. The grafting of the polymer chain onto the rGO surface has improved the electrical conductivity significantly from 2.6 × 10 −8 S/cm to 7.9 × 10 −4 S/cm due to the increase in

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G-g-PTMPM

Fig. 2  Preparation of PTMA-functionalized graphene sheets [54]

carrier mobility. PHET-g-rGO when used as an active material for DSSC shows an overall PCE of 3.06%.[22] Wang et al.[56] proposed an effective route to prepare vertical PANI nanowire array-modified rGO nanocomposites based on covalent connection. First, they functionalized rGO by spontaneously grafting nitrophenyl groups on rGO based on the C–C bond. Then, the nitrophenyl groups were reduced to aminophenyl groups, and subsequently, polymerization was carried out in the presence of aniline monomer to yield vertical PANI arrays on rGO. A large-scale π–π conjugated system is formed between PANI and rGO, which has improved the charge transfer significantly.[56] Noncovalent Functionalization Noncovalent functionalization strategies include mixing of the components and also in situ polymerization of monomers in the presence of graphene. Solution mixing is the simplest approach to prepare graphene/polymer composites, and it requires both graphene material and polymer to be stably dispersed in a common solvent. Liu et al.[57] have mixed an organic solution-processable functionalized graphene material (SPF graphene) with poly(3-octyl thiophene) (P3OT) in dichlorobenzene and have fabricated P3OT/SPF graphene hybrid solar cell showing a

maximum efficiency of 1.4%.[57] Li et al.[58] have fabricated a GQD-based bulk heterojunction (BHJ) polymer solar cell by blending 10% GQD with P3HT(w/w) using the device architecture indium tin oxide (ITO)/PEDOT:PSS/ P3HT:GQDs/Al, yielding a maximum efficiency of 1.28%, whereas pristine P3HT-based device exhibits only 0.008% efficiency. In the P3HT/GQD device, the GQDs provide a large surface area for the formation of p–n junction interfaces. GQDs incorporated into the polymer matrix effectively afford exciton separation and carrier transporting pathways, which led to a dramatic increase in short circuit current Jsc. The enhanced Voc for P3HT:GQD device inevitably improves the electron-collecting efficiency, and hence the photocurrent. On the other hand, GQDs have high electron mobility, which accordingly are conducive to electron transport in the active film. Therefore, charge separation could effectively take place in the P3HT:GQD device, but the transport of charge carriers could be enhanced significantly; as a result, PCE was enormously improved.[58] Gupta et al.[59] have demonstrated that composites of aniline-functionalized graphene quantum dot (GQD with P3HT in dichlorobenzene exhibit an excited-state charge transfer and shows a maximum solar cell efficiency of 1.14%.[59] Routh et al.[23] have ­prepared ­polythiophene-g-poly[(diethylene glycol methyl ether methacrylate)-co-poly(N,N-dimethylaminoethyl

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HO O

HO Hummers and Offeman’s method Cone. H2SO4, NaNO3/KMnO4

OH O

HO

Sonication

O Graphite

C O OH

C OH

Graphite oxide (GO)

SOCl2 Cl O

Reflux

Cl C

O C Cl

O

PANi-g-rGO

O CO

H N

O CO

H N H N

O CO

THF/Pyridine Reflux

n H N n

H N

H N

H2N

O

O O

APS

O O H2N

O

NH2

O

O

H N O

Boc Boc

O

O O

TFA/CH2Cl2 Hydrolysis

O NH2

O

O O

HN

Cl

Boc NH

HO

HN Boc

O O O

n NH2

NH2

O

NH

O

Boc

Boc N H

Fig. 3  Schematic governing the preparation of PANI-g-rGO with a digital picture of the sample in the middle [55]

methacrylate)], a water-soluble polythiophene derivative via ATRP technique, and mixed it with GQD in the water medium that showed the interaction between −NMe2 group of the polymer and –COOH group of GQD and also the π–π interaction between the thiophene units of polymer and the sp2 π clouds of GQD, resulting in a solar cell ­efficiency of 1.76%.[23] In situ polymerization method involves the polymerization of monomer in a system containing the graphene material. Many graphene/polymer composites, such as graphene/PANI, [24,60–65] graphene/polypyrrole (PPY) (Table 1)[23,61,66–69] and graphene/polyindole, [70,71] can be prepared using this approach (Table 1). In a typical example, aniline and graphene are well mixed using ultrasonication to generate a fine dispersion; an amount of ammonium persulfate (APS) is dissolved in distilled water as an oxidizing agent. Then, the two solutions are mixed by stirring for a specific time to produce PANI/graphene nanocomposites. This method is able to provide various structures such as nanofibers, nanoparticles, nanowires, nanorods, and nanotubes, [60,72–74] depending on the specific ­experimental conditions. Among the graphene/polymer composites, graphene/ conducting polymer composites can be produced not

only by in situ chemical polymerization but also by in situ electrochemical polymerization. This technique provides some advantages over the in situ chemical oxidative polymerization, such as simple operation, short reaction time, and the capability to produce binder-free, thin-film electrodes, and highly flexible electrodes. Additionally, electrochemical polymerization can be accurately controlled by the applied potential, polymerization time, and current density. Wang et al. [75] synthesized graphene/ PANI paper by in situ electrochemical polymerization of the ­a niline monomers adsorbed on the surfaces of graphene to form PANI between neighboring graphene sheets. After polymerization, the graphene paper still maintained its layered structure. The mechanical and electrochemical properties of the composite can be controlled by simply regulating the time of polymerization since that changes its PANI content. [75] ENERGY HARVESTING APPLICATIONS In the energy harvesting application, the graphene-­ conducting polymer hybrids are mainly used for both ­photovoltaic and fuel cell applications (Table 2).

Table 1  Comparison of typical synthetic methods for graphene–polymer conjugates related to energy applications Form of Method of Type of functionalization/ Polymer graphene functionalization polymerization Application

References

P3HT

GO

Grafting to

Esterification

Solar cell

[50]

PEI

rGO

Grafting to

Amidation

Supercapacitor

[27]

PANI

GO

Grafting to

Acylation

Supercapacitor

[52]

PS

rGO

Grafting from

ATRP

Solar cell

[53]

PTMA

rGO

Grafting from

ATRP

Supercapacitor

[54]

PMAA

GO

Grafting from

ATRP

Photovoltaic

PANI

rGO

Grafting from

Redox polymerization

Supercapacitor

PANI

a-rGO

Grafting from

Redox polymerization

Solar cell

[21]

PHET

rGO

Grafting from

Redox polymerization

Solar cell

[22]

PANI

rGO-Azo

Grafting from

Redox polymerization

Supercapacitor

[58]

N-substituted carboxyl PANI (NPAN)

Aniline modified GO

Grafting from

Redox polymerization

Supercapacitor

[59]

PANI

GO–NH2

Grafting from

Redox polymerization

Supercapacitor

[60]

PPY

4-Aminophenyl modified graphene

Grafting from

Redox polymerization

Supercapacitor

[61]

Photovoltaic Cells An excellent electrical conductivity, good carrier mobility, and moderately high optical transmittance make polymer-functionalized graphene as one of the most promising materials for solar cell applications. Mullen et al.[104] first reported a transparent and ultrathin graphene film electrode for solid-state DSSC as a complementary of metal oxide-based electrode.[104] Polymer-functionalized graphene has been used as a competitive electrode material than graphene electrode for highly efficient dye sensitized solar cells (DSSCs) and organic solar cells (OSCs). Transparent conducting films are prepared from ­chemically modified graphene (CMG) and chemical vapor deposition (CVD)-developed graphene materials that are used for solar cell application. Shi et al. reported a graphene and ­PEDOT-PSS (graphene/PEDOT-PSS) composite deposited on ITO as counter electrodes for DSSCs with a PCE of 4.5%.[105] These composite films were produced without any heating at room temperature. Pyrene-1-sulfonic acid sodium salt (PyS)-graphene nanocomposites were used as an anode electrode at P3HT– phenyl-C61-butyric acid methyl ester (PCBM) heterojunction solar cells. The composite showed a higher PCE of 1.12% compared to only graphene electrode with a PCE of 0.78%.[106] Valentini and his group [107] used graphene/PEDOT–PSS ­composite coated ITO as an anode material for polymer bulk heterojunction (BHJ) solar cells with a PCE of 0.75%. Zhou and coworkers developed a chemical vapor deposited ­transparent conductive graphene electrode for organic solar cells with a PCE of 1.27%.[108] Figure 4 represents different properties of an organic solar cell. Solar cells from these CVD graphene showed an extra bending stability of flexible polyethylene terephthalate (PET) substrates up

[57] [55,56]

to 138° than ITO-based devices that cracks at bending of 60° only. The persistent nature of CVD/graphene system provides a minimum surface roughness of approximately 0.9 nm and a sheet resistance down to 230 Ω/sq. Figure 4 denotes different representations of CVD graphene solar cells. In another study, Kamat and his group [109] developed a CdSe quantum dot-sensitized solar cell with rGO-Cu2S nanocomposite as a counter electrode. The device showed a PCE of 4.4% that is comparable with a platinum counter electrode-based device. This group also reported a CdSe colloidal quantum dots-graphene solar cell.[110] Introduction of graphene material into colloidal CdSe quantum dots increases both the charge separation and electron conduction through the film. These cells showed a p­ hotocurrent response of 150% than without any graphene material. In another study, CdS/CdSe quantum dot on graphene–­ polymer template was used as a potential active material for photovoltaics.[111] The anionic functional polymer (polymethacrylate cadmium) was synthesized by ATRP using polymerized pyrene units as a macroinitiator. The polymer anchored on the graphene surface through pyrene units and polymer brushes were used as a nonreactor for the synthesis of CdS/CdSe quantum dot. The CdS–graphene photoelectrode exhibited an open-circuit voltage (Voc) of 0.87 V and a short circuit c­ urrent (Jsc) of 2.15 mA/cm2, whereas CdSe–graphene photoelectrode produced Voc and Jsc of 0.77 V and 0.71 mA/cm2, respectively. a-rGO is used to covalently functionalize PANI by polymerization of aniline in the presence of a-rGO using the initiator ammonium persulphate (APS). The PANI morphology has changed from a nanotube to an flat rectangular nanopipe (FRNP) (Fig. 5).[21] An efficient (~500 times) improvement in photocurrent is observed in FRNP over PANI nanotubes on irradiation with white

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Table 2  Synthesis and applications of graphene/polymer composites produced by noncovalent approach Polymer Form of graphene Strategy Application

References

P3OT

Solution-processable functionalized graphene material (SPF graphene)

Mixing in solution

Photovoltaic

[57]

P3HT

GQD

Mixing in solution

Photovoltaic

[58,59]

Polythiophene-g-poly[(diethylene glycol methyl ether methacrylate)-co-poly(N,Ndimethylaminoethyl methacrylate)]

GQD

Mixing in solution

Photovoltaic

[23]

PANI

GQD

In situ polymerization

Photovoltaic

[24]

PANI

GO

In situ polymerization

Photovoltaic

[76]

PANI

GO/rGO

In situ polymerization

Supercapacitor

[60,77]

PANI

GO

In situ polymerization

Supercapacitor

[73,78–80]

PANI

3D-RGO

In situ polymerization

Supercapacitor

[81]

PANI

PANI-functionalized reduced graphene oxide (PORGO)

In situ polymerization

Supercapacitor

[82]

PANI

SG

In situ polymerization

Supercapacitor

[64]

PPY

GQD

In situ polymerization

Photovoltaic

[23]

PPY

GO

In situ polymerization

Photovoltaic

[83]

PPY

Na-PSS-modified graphene nanosheets

In situ polymerization

Supercapacitor

[84]

PPY

RGO

In situ polymerization

Supercapacitor

[85]

PPY

Claisen graphene

In situ

Supercapacitor

[86]

PPY

GO

In situ

Supercapacitor

[68,69]

Poly[(thiophene-2,5-diyl)-co(benzylidene)]

GO

In situ polymerization

Supercapacitor

[87]

PEDOT

GO

In situ electropolymerization

Supercapacitor

[88,89]

PEDOT/PSS

rGO

Mixing in solution

Supercapacitor

[90]

Polythiophene

rGO

In situ polymerization

Supercapacitor

[91]

P3HT

rGO

Mixing in solution/ in situ

Supercapacitor

[92]

Polyindole

rGO

In situ polymerization

Supercapacitor

[71]

Poly(anthraquinonyl sulfide)

GO

In situ polymerization

LIBs

[93]

Poly(4-vinyloxy-2,2,6,6-tetramethylpiperidine-N-oxyl) (PTVE)

Graphene

Codeposition

LIBs

[94]

Poly(2,2,6,6-tetramethyl-1piperidinyloxy-4-yl methacrylate) (PTMA)

Graphene

Mixing in solution

LIBs

[95]

PPY

GO

In situ polymerization

Zinc/polymer battery

[96]

PPY

rGO

In situ polymerization

Lithium batteries

Polyimide (PI)

GO

In situ polymerization

Lithium and sodium batteries

[100]

PEO

GO

Mixing in solution

Fuel cell

[101]

PPY

GO

In situ electropolymerization

Fuel cell

[102]

PVA/chitosan

Sulfonated grapheme

Mixing in solution

Fuel cell

[103]

[97–99]

(a)

(b)

2.2eV 3.3eV PEDOT CuPc 4.5eV

Graphene

5.2eV 5.2eV

3.0eV 4.0eV

C60

Ni

BCP

PMMA/Graphene

Si/SiO2

4.3eV

Transfer

Al PMMA coating

6.2eV 6.4eV

C60

Graphene film

Al

Substrate BCP

CuPc

Ni etching

PEDOT Substrate

Graphene (d)

(e)

100

Transmittance (%)

(c)

80 60

CVD Graphene ITO SWNT

40 20 0 400

Graphene roughness: 0.9 nm

ITO roughness: 0.7 nm 50 nm

50 nm

0.2

0.8 1.0 um 0.4 0.6

80 8.30 kΩ/sq 3.40 kΩ/sq 0.70 kΩ/sq 0.23 kΩ/sq

40 20 600

800

Wavelength (nm)

1000

SWNT roughness: 8.4 nm

0.8 1.0 um 0.4 0.6

0.2

0.8 1.0 um 0.4 0.6

CVD Graphene Eda et al. Blake et al. Li et al. Wu et al.

105

60

0 400

0.2 (h) 107

100 Transmittance (%)

(g)

600 800 1000 Wavelength (nm)

50 nm

Rsheet(kΩ/sq)

(f )

Acetone

Graphene

103 101

10–1

70

75 80 85 90 95 100 Transmittance (%)

Fig. 4  (a) Schematic representation of the energy level of CVD graphene/PEDOT/CuPc/C60/BCP/Al. (b) Schematic representation of the CVD graphene transfer process onto transparent substrates. (c, d) Photographic image of highly transparent graphene films transferred onto glass and PET. (e) Transmission spectra for CVD graphene, ITO, and single-walled carbon nanotube (SWNT) films on glass. (f) Atomic force microscopy (AFM) images of CVD graphene surface, ITO, and SWNT films on glass. (g) Transmission spectra of CVD graphene sheet with a variation of sheet resistance (Rsheet). (h) Comparison of Rsheet vs. light transmittance at 550 nm for CVD graphene sheet and reported reduced GO films [113]

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Electronics–Energy

1. 98% H2SO4

HO

2. NaNO3

COOH

HO HOOC

3. KMnO4

HO

NaBH4

NH2 OH

COOH

HO

NH2 NaNO2

OH

HCl 0 - 5°C NH2

HOOC OH 1. N2H4, H2O 2. H2O 3. 24h. 100°C

NH2 OH HO

NH2

(a)

NH OH

COOH

APS, 20°C, CH3COOH medium

HOOC NH2

n

OH NH2

COOH

HO

HN

HOOC

H2N

NH2

COOH

HO HOOC HN

NH n

NH

NH n

(b)

Fig. 5  Scheme of the preparation of a-RGO and formation of PANI-a-RGO hybrid SEM images of PANI produced (a) without and (b) with a-RGO [21]

light, and it is quite reproducible even after several cycles. A donor–acceptor-based “electron–hole” pair mechanism is proposed for the photocurrent behavior of FRNP, where PANI chains act as a donor and rGO acts as an acceptor. A DSSC using the PANI-a-RGO hybrid with Rose Bengal dye is constructed yielding a PCE of 2.012%. In the traditional DSSC, the TiO2 layer is the active material, through which the photoelectrons generated from the dye molecules are transported to the anode. We have used a newly synthesized semiconducting poly[3-(2hydroxyethyl)-2,5-thienylene] grafted reduced graphene oxide (PHET-g-rGO to replace TiO2 layer.[22] The GO is refluxed with SOCl2 to produce acyl chloride, which is reacted with 3-hydroxy thiophene to produce HET-grGO, and on oxidative polymerization of HET, from it PHET-g-rGO is produced (Fig. 6). The fibrous network morphology of PHET remains appended on the graphene surface, making it a good electron transport promoter. The absorption band of PHET at 374 nm shows a red shift to 383 nm, indicating a decrease of bandgap from 1.86 eV in PHET to 1.38 eV in PHET-g-rGO due to the covalent bonding of PHET with rGO. Here, the fluorescence intensity of pure PHET gets quenched, and the emission peak shows a red shift by 13 nm, suggesting an efficient ­ electron–hole pair separation. With irradiation of white

Fig. 6  Formation of a PHET-g-rGO composite [22]

light, a stable photocurrent is produced in the PHET-grGO. The Brunauer–Emmett–Teller (BET ) surface area for the PHET-g-rGO is 227.5 m2 /g, and the respective pore volume is 0.19 cc/g, which is very much comparable with the bare TiO2 surface area (typically ~290 m2 /g). We have replaced the TiO2 used in traditional DSSC for the  first time with PHET-g-rGO, and its open-circuit voltage (0.61V), short-circuit current (7.5 mA/cm2), and fill factor (0.668) yields a PCE of 3.06%.The 2D graphene having a large surface area provides sites for the adsorption of large number of dye molecules, which harvest more light, leading to the injection of more photogenerated electrons into the conduction band of the composite. High photoelectron injection, a reduced (e− /h+) recombination, and high electron transport through graphene makes it an efficient DSSC. The energy level of graphene is in between the conduction band of PHET and that of ITO, and the operational principle of the DSSC is shown in Fig. 7.[22] On ­illumination with white light, the conduction band of PHET receives the electrons from the  photoexcited dye. As PHET is anchored with graphene, the excited electrons of PHET are captured by it without any obstruction, and these electrons can transport from PHET to the conductive substrate quickly through graphene to the ITO (Fig.  7), thus suppressing the recombination and back reaction.

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Energy: Polymer-Functionalized Graphene

Electronics–Energy Fig. 7  Operation mechanism of PHET-g-rGO-based DSSCs [22]

DSSC fabricated with bulk TiO2 and graphite counter electrode shows a PCE of 2.66%, whereas DSSC made with a physical mixture of PHET and rGO shows a PCE of only 0.82%, which is much lower than the DSSC made by the grafted composite (PHET-g-rGO). This result indicates that chemical ­grafting has a commanding role in increasing the cell efficiency. We have produced GQDs from the m ­ icrometer-sized GO sheets using the sono-Fenton reaction. The water-soluble GQDs are used to form composites with a water-soluble polythiophene-graft-­copolymer [polythiophene-g-poly[(diethylene glycol methyl ethermethacrylate)-co-poly(N,N-dimethyl aminoethyl ­methacrylate)] [PT-g-P(MeO2MA-co-DMAEMA), P], using the interaction between –OH and –COOH groups of GQDs with the dimethyl amino group of the polymer P.[23] The components possess fluorescence properties, but the emission peak of the composite at 537 nm quenches and shift towards the lower wavelength (~430 nm) with aging time due to energy transfer from P to GQDs followed by a delayed up-converted emission of GQDs. The composite on photoillumination produces stable photocurrent and DSSC fabricated with an ITO/P-GQD/graphite device using an N719 dye exhibit a PCE of 1.76%. The GQDs are further used to fabricate DSSC with polypyrrole (PPY), where PCE increases with an increase in GQD concentration, showing a maximum of 2.09%. PANI-GQD hybrids (PAGD) are produced by in situ polymerization of aniline without using an external dopant. The carboxylic acid groups of GQDs dope PANI well, showing a gradual red shift of π to polaron band transition of PAGD hybrids, with an increase in the intensity on increasing the

amount of GQDs.[24] (Fig.  8) It is attributed to the gradual uncoiling of PANI chain and the increase of polarons in the doped PANI chains, respectively. In the hybrids, fluorescence intensity of GQDs drastically quenches for an effective charge ­transfer between GQDs and PANI chains. The polaron formation in the PANI chains causes a three-order increase in the dc-conductivity in the PAGD composites from that of GQDs. DSSCs fabricated with PANI-GQD hybrids and N719 dye indicate a highest PCE of 3.12%. The Nyquist plot obtained from the impedance spectra of DSSCs indicates the presence of three semicircles comprising three R–C circuits, analysis of the data yields the lifetime values of photoinjected electrons, and the highest lifetime (4.46 mS) supports the highest PCE of a PAGD hybrid. GQD-P3HT-based device were fabricated by Qu and his group. GQDs act as an electron acceptor material for this polymer solar cell, exhibiting a PCE of 1.28%. The GQDs have an average size of 3–5 nm showing green luminescence and are stable for several months without any color change.[58] The cathode interlayer (CIL) has an important role in the PCE of solar cells. Hence, GQDs are used as a CIL for polymer solar cells. Liu et al. designed a GQD functionalized with tetramethyl ammonium (TMA) as a CIL for PCDTBT:PC71-BM and PTB7Th:PC71-BM solar cells.[112] GQD-TMA helps to reduce the work function of the metallic cathode by forming an interfacial dipole at the interface. The polymer solar cells have efficiencies of 7.01% and 8.80%, respectively, which are much higher than that of Ca or Li interlayer. F ­ igure 9 represents ­different layers of the cell. Zhang et al. [113] have tested a reduced polydopamine-­ functionalized graphene (RPFGO)/silver nanowire

Electronics–Energy

Energy: Polymer-Functionalized Graphene 1021

Fig. 8  In aqueous dispersions of GQDs, aniline is in situ polymerized to produce PANI-GQDs hybrids, and DSSC fabricated with the hybrids indicates the highest PCE of 3.12%[24]

N+ N+ –O

–

N+

Cathode interlayer

O

+ C O– N

O

C

ITO

C

O C

O–+ N

O O–+ N

GQD-TMA S N N

Active layer

PEDOT:PSS

O

C

O

Al

O– O C

N

C8H17 C8H17

O O S

S

CH3

n

PCDTBT PC71BM

Fig. 9  A schematic representation of different layers of PCDTBT:PC71-BM solar cell with a GQD-TMA interlayer[116]

composite to act as a transparent electrode. [113] RPFGO has improved the adhesion force between silver nanoparticle and the substrate material by lowering the sheet resistance. Figure 10 illustrates the preparation of polydopamine-­functionalized graphene/silver nanowire composite materials. The composite works well due to a good interaction between graphene and polymer. Graphene is also demonstrated to act as an alternative of PCBM electron acceptor. In a study, Chen et al. reported a graphene-P3OT-based organic solar cell having an ­efficiency of 1.4%. [57] In a recent study, Zheng et al.[114] have reported an rGO-zinc oxide (ZnO) as a CIL for PTB7:PC71BM and PTB7-Th:PC71BM solar cells. This nanocomposite with higher electrical conductivity displays efficiencies of 8.04% and 9.49%, respectively. Researchers have not only

improved graphene–polymer as active electrode materials but also demonstrated as an electrolyte material for DSSCs. Liquid electrolyte has low stability and limited robustness, which limits the solar cell efficiency, and it is very challenging to replace this liquid electrolyte with other materials. To overcome this drawback, Marchezi and his group [115] have presented an rGO-polymer gel electrolyte as an alternative of liquid electrolyte. rGO is added to poly(ethylene oxide) (PEO), γ-butyrolactone, LiI, and I2, and used them as a gel–polymer electrolyte. The DSSC constructed from 0.5 wt% of rGO shows an efficiency of 5.07 ± 0.97%. This nanocomposite electrolyte helps to decrease the recombination reaction at photoanode through the interaction of polyiodide with rGO and increase both Voc and Jsc. This result demonstrates that rGO-polymer gel electrolyte is a new addition for electrolyte with low cost.

1022

Energy: Polymer-Functionalized Graphene

Electronics–Energy Fig. 10  Schematic diagram of the preparation process of RPFGO–silver nanowire nanocomposite [117]

20

Gel/0.0% RGO Gel/0.1% RGO Gel/0.3% RGO Gel/0.5% RGO Liquid electrolyte

18

Current/mA cm–2

16 14 12 10 8 6 4 2 0 0.0

0.1

0.2

0.3

0.4 0.5 Potential/V

0.6

0.7

0.8

0.9

Fig. 11  J–V curve of DSSCs constructed from different variations of RGO and DSSC using the standard liquid electrolyte under AM 1.5 conditions with an intensity of 100 mW/cm2[119]

Figure 11 shows how Voc and Jsc vary with variation of rGO in the gel electrolyte. Xerogels of trihybrid hydrogels containing GO and a conducting polymer along with a gelator molecule are found to exhibit good PCE. Dihybrid (GP) and trihybrid (GPPS) hydrogels are produced by using 5,5′-(1,3,5,7-tetraoxopyrrolo[3,4-f]isoindole-2,6-diyl) diisophthalic acid (P), GO and P, GO, and PEDOT:PSS, respectively. The dc-conductivity of GPPS xerogels are four to five orders higher compared to the GP and P xerogels. The GP and GPPS xerogels exhibit a photocurrent on white light irradiation, and the on–off cycles display a stable photocurrent for the later system. The GPPS xerogels are used as active materials to fabricate DSSCs using the N719 dye, and the PCE increases with an increase in the PEDOT:PSS

concentration showing a maximum PCE of 4.5%. The incident photon to current conversion efficiency (IPCE) curve of the DSSC shows a wide absorption range (360–700 nm) with a maximum absorbance of ∼57%. The Nyquist plot of DSSC obtained from impedance spectroscopy consists of three semicircles, and the equivalent resistance–­ capacitance circuit yields the highest lifetime values of photoinjected electrons to be 3.2 ms supporting the highest PCE value of 4.5%. After discovery of perovskite solar cell, it acquired a drastic attention because of high PCE.[116–118] These organic–inorganic halide perovskites solar cell are considered to be the most promising material in the field of photovoltaics because of their ease of fabrication and inexpensive material.[119] But still there are some factors that

have to be overcome to get a high efficiency and commercialization of perovskite solar cell. Due to low electron mobility of TiO2 and low stability of ZnO, graphene–­ polymer composite is introduced as an alternative for electron transporting layer (ETL) in perovskite solar cells. Loh and groups [119] demonstrated a mesoporous graphene/ PANI composite as an ETL for perovskite cells. The PCE of the cell is observed at 13.8% under air mass (AM) 1.5G solar illumination, with a low-temperature processing ability and good stability. Graphene with a highly conducting network structure and PANI with a well-defined granular structure can assist as a good thermally stable ETL to form a highly crystalline and uniform perovskite layer. Batmunkh et al.[120] have used graphene as a transparent conductive graphene film instead of conventional transparent conducting oxide. Incorporation of graphene into both compact TiO2 and mesoporous TiO2 layers increases the PCE to 0.81%. So this result opens a new hypothesis for perovskite solar cells. Dong et al. designed a graphene– silver nanowire–polymer electrode material for perovskite solar cells.[121] The graphene–polymer composite helps to circumvent metal mesh from oxidation and corrosion from air. The perovskite solar cells assembled from these hybrid electrode materials exhibit a PCE of 10.42%. These lightweight hybrid electrodes show good stability and high mechanical strength. Fuel Cell Application Noncovalent functionalized polymer–graphene nanocomposite has been demonstrated as an efficient catalyst for oxygen reduction reaction (ORR) through intermolecular charge transfer for fuel cell application.[122,123] The electrochemical and basic properties of graphene remain intact after interaction with polymer. Wang et al.[124,125] described how poly(diallyldimethylammonium chloride) (PDDA) is used for exploitation of graphene sheet to reduce GO with sodium borohydride (NaBH4). The following PDDA– graphene composite successively enhances the electrocatalytic activity of ORR of fuel cell via intermolecular charge transfer. The intermolecular charge transfer occurs between graphene and positively charged PDDA, which opens up a new model for ORR process with a cost-­effective metal free polymer–graphene material. Khilari and group [126] reported a proton-exchange polymer membrane consisting of GO, poly(vinyl alcohol) (PVA), and silicotungstic acid  (STA) for microbial fuel cells (MFCs) where biodegradable materials are used to convert electricity.[126] With addition of GO to the membrane, the electrochemical behavior is also improved. The PVA–STA–GO composites showed a maximum power density of 1.9 W/m3 and could be used as a low-cost effective material for microbial fuel cells. Sulfonated GO (SGO) is widely used as a membrane material in fuel cells. In a consecutive study, Gahlot and group [127] demonstrated an SGO-sulfonated poly(ether sulfone) composite material as a proton-exchange membranes for

methanol fuel cells. Incorporation of SGO into sulfonated poly(ether sulfone) rises the electrochemical properties, such as ion-exchange capacity, water retention, and proton conductivity, and simultaneously decreases ­methanol permeability. Figure 12 represents the variation of different membrane properties with the incorporation of SGO. In a further report, poly(vinyl alcohol) (PVA) and aryl SGO composite was used as a crosslinked nanocomposite membrane for proton-exchange membrane fuel cell. [128] Introduction of SGO into the PVA substrate enhances the thermal stability (melting temperature, Tm = 223°C), mechanical stability (tensile strength, TS = 67.8 MPa), and proton conductivity (σ =  0.050 S/cm) of the membrane. A maximum power density of 16.15 mW/cm2 was observed for this fuel cell. So, these studies suggested that PVASGO composite could be used as a future material for proton exchange membrane fuel cells. In an exciting study, Beydaghi and Javanbakht introduced iron oxide (Fe3O4) nanoparticles into PVA-SGO composite methanol fuel cell.[129] The aligned PVA/SGO/Fe3O4 membranes have excellent thermal stability and good proton conductivity. The highest power density of 25.57 mW/cm was observed at 30°C for this three-component membrane fuel cell. Interestingly, SGO and Nafion composite were also testified as a membrane material for polymer electrolyte fuel cell.[130] Incorporation of SGO into Nafion increases the proton mobility. A peak power density of 300 mW/cm2 was observed for these composite membranes at a load current density of 760 mA/cm2. ENERGY STORAGE APPLICATION The energy storage devices are gaining significant attention with energy conversion devices, because of their increasing demand in the field of nanotechnology. Supercapacitors

Water permeability

Proton conductivity

Ion exchange capacity

SPES SGO-1

Methanol permeability

SGO-2

SGO-5

Fig. 12  Different membrane properties by incorporation of ­sulfonated graphene oxide [131]

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and batteries are two essential electrochemical energy storage devices. In particular, several research efforts have been devoted to improve the charge storage capacity, high power transfer capability, and good reproducibility of cycles of batteries.[131–133] Ideally, batteries have high energy storage ability with a poor energy transfer capability; however, supercapacitors have high energy transfer capability with low energy storage ability.[134–137] (Fig. 13). So, supercapacitors and batteries are complementary to each other. Currently, polymer-functionalized graphene materials are employed for high-performance supercapacitors and batteries due to their high conductivity and high surface area.[138] Batteries Li ion batteries (LIBs) have high energy density with a theoretical value of 400 Wh/kg approximately.[134,139,140] Graphite and LiCoO2 are used as an electrode material in LIB due to reversible charge/discharge capability and a specific storage capacity of these materials under a static potential. In a traditional LIB, three important components are Li+, intercalation anode and cathode, and the corresponding electrolyte. During charging, Li+ ion moves to the anode from the cathode through the electrolyte ­solution, and during discharging, it follows the opposite route. Honma and coworkers developed a graphene-CNT composite-based LIB intercalation anode material.[141] In this study, CNTs and fullerenes (C60) have been incorporated

e–

(a)

into graphene sheet by tuning interspacing between the adjacent layer morphology and the thickness of graphene sheet. They suggested that CNTs and C60 may cause expansion of d-spacing of the graphene interlayer because of their electron affinity and lower the stacking of graphene layers. Therefore, CNTs or C60 and graphene composites are promising anode materials for imminent high-capacity LIBs. In another study, Zhang and coworkers [142] reported that based on the three components, TiO2 nanoparticles incorporated PANI and graphene nanosheet composite (PTG) for anode electrode in LIB. This three-component anode material is highly active compared to pure TiO2, with a fast charging and discharging rate and good cycling performance. So, PTG-based nanocomposite undoubtedly is a new addition for anode material in LIBs. Polymer–graphene composite can also behave as a cathode for LIBs. Song et al.[93] reported a new polymer– graphene nanocomposite as a highly efficient cathode for LIB, which can deliver greater than 100 mAh/g in a few seconds. In this study, poly(anthraquinonyl sulfide) and polyimide–graphene composites are used as active cathode materials. The electrical conductivity and the electrochemical activity of cathode increase drastically due to high dispersion of the graphene sheet in the polymer material and enhanced noncovalent interaction between the graphene surface and the polymers. In a subsequent study, Guo and his group [95] established a PTMA/graphene-based nanocomposite for a cathode material with a long life of more than 20,000 cycles at 100°C and a high capacity value of

(b)

e–

Charge

Charge

Discharge

Discharge

e– Electrode

Separator

e– Electrode

Anode

Separator

Charge

Discharge Electrolyte

Fig. 13  Graphical representation of working principle of (a) supercapacitor and (b) battery[142]

Cathode

222 mA h/g at 1°C. The PTMA/graphene composite displayed a two-stage charge/discharge process reversibly with a two-electron reaction process. Supercapacitor Supercapacitors are energy storage devices with a great power capability and comparatively large energy density.[135] Supercapacitors can store energy in two ­different ways: such as, one is the electrostatic charge accumulation at the interface of electrode and electrolyte in an electrical double layer and another one is the pseudocapacitor where transfer of charge to the redox materials occurs via redox reaction at the surface of the electrode. (Fig. 14) These two different processes strongly depend on the nature of the electrode material and the surface area of the electrode. Graphene sheets possess a huge surface area; therefore, they are the  perfect electrode material for supercapacitor applications. Recently, GO is chosen by tuning its reduction and hence the band gap for its supercapacitor performance. For this purpose, we have reduced GO using KI in water (KG)

Electrolyte

Separator

Current collector

Current collector

Active porous electrode material

+ve ions –ve ions

ψ

ν

C1

C2

Fig. 14  Schematic representation of an EDLC based on porous electrode materials [139]

and also in the acidic medium of different strengths.[143] It is observed from spectroscopic results that KI in water acts as a mild reducing agent; however, it is lower than that in the acidic medium but better than thermally reduced GO. KG exhibits a dc conductivity of 0.18 mS/cm and a high specific capacitance of 414 F/g at a current density of 0.5 A/g. It exhibits long cyclic stability (~95% after 10,000 cycles) and excellent rate capability (56.5% retention at a current density of 20 A/g), exhibiting an excellent energy density of 10.76 W h/kg at a power density of 125 W/kg. Both electrical double-layer capacitance and pseudocapacitance are responsible for the supercapacitor performance of KG, the latter arising from the reversible redox processes of the oxygenated functional groups of reduced GO. It is found that an optimum reduction of GO using KI in water is suitable for the best supercapacitor performance. In the presence of acid, higher reduction takes place and decreases the redox active hydroxyl groups, causing a decrease in the interlayer distance of rGO sheets. This causes a decrease of electrical double layer capacitance (EDLC) due to stacking of the graphene sheets, causing difficulty to use whole of its available surface area for charge storage. Also, the removal of redox-active oxygenated functional group decreases the pseudocapacitance. For this reason, functionalization of graphene is required to separate the stacking of layers. Yu and Dai[27] reported the use of PEI-modified graphene sheets and acid-oxidized multiwalled CNT films as an active electrode for supercapacitors. These composite films electrode exhibited a rectangular cyclic voltametric (CV) curve at a high scan rate of 1 V/s and showed the average specific capacitance value of 120 F/g though the stability of the materials during charging/discharging process is very challenging. Conducting PANI is gaining interest because of its low cost in the field of supercapacitors. To overcome this factor, there are numerous reports on the PANI–graphene-based composite as an electrode material for supercapacitor application. Zhang et al.[60] observed that chemically modified graphene (CMG) with PANI displayed a very good cycling stability during charging/ discharging process. The nanocomposite also has a very good specific capacitance value of 480 F/g at a current density of 0.1 A/g. Not only earlier research work but also other studies are reported to overcome the poor stability and low cycling life of supercapacitor based on graphene– PANI composite. Wang et al.[144] incorporated PANI on a single-layer graphene sheet via in situ polymerization to synthesize graphene–PANI nanocomposite with different mass ratios of 12,500 and 500 mesh. It has been found that the specific capacitance varied with a variation of mass ratio. The enhanced specific capacitance and cycling lifetime during charging–discharging confirmed the presence of synergistic effect between PANI and the graphene sheet. This synergistic effect between them arises probably from electrostatic interaction, H-bonding, and π–π interaction. The earlier two studies implies that graphene–PANI

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Electronics–Energy Fig. 15  Schematic presentation of the preparation of graphene–PANI composites [65]

nanocomposite acts as a potential electrode material for supercapacitor and energy storage devices (Fig. 15). Most of the polymers have low conductivity and poor mechanical strength, and to improve them, graphene has been introduced in the composites. To address this concern, Wang et al.[75] reported a graphene/PANI-based composite material for application in flexible supercapacitor with a high tensile strength of 12.6 MPa and an excellent electrochemical capacitance. The composite exhibited g­ ravimetric and volumetric capacitances of 233 F/g and 135 F/cm3, respectively. The morphologies of PANI are also greatly affected by the addition of graphene, which might increase the electrochemical properties of the composite.[78] Also, ­nanowire arrays are effectively used in ­supercapacitors by Xu et al.[73], who presented the growth of PANI n­ anowire arrays on GO. The ionic mobility increases due to the nanowire morphology of PANI, and the ionic diffusion decreases due to the network ­structure of PANI. Scanning electron microscope (SEM) images of graphene–PANI nanowires arrays are shown in Fig. 16. Well-­organized nanowires were formed at low ­concentration, but ­random nanowires were ­developed at high ­concentration. A ­ developed electrochemical ­capacitance and good ­stability of the composite was observed due to an excellent synergism effect between PANI and graphene. To lower the stacking between graphene sheets, different s­ trategies were introduced by chemists. Interestingly, PANI plays a crucial role in overcoming these shortcomings of graphene. PANI nanowafer was developed on an rGO nanosheet to prevent the adjacent sheet coming closer to the template sheet.[145]

Fig. 16  SEM images of graphene–PANI nanowires in the ­concentration of (a, b) 0.05 M and (c, d) 0.06 M [78]

A specific capacitance of 329.5 F/g was found for this composite and also has an excellent life cycle. This study reveals that PANI acts as a spacer material for graphene sheet, and facile synthesis of this composite make it more promising material for energy storage devices. In a similar study, PANI nanoparticles are used as a spacer material to keep neighboring layers of graphene sheet far away.[146] The resulting nanocomposite electrode have a high ­specific surface area of 891 m2 /g compared to graphene sheet and exhibit a specific capacitance of 257 F/g at a current density of 0.1 A/g. The mesoporosity of the composite helps

to diffuse the electrolyte ions. Due to high surface area, these graphene/PANI nanocomposites possess a good specific capacitance and are capable to produce a wonderful supercapacitor. Flexible all-solid-state supercapacitors are a most conversed subject in the field of energy storages. Duan and his group have demonstrated the use of three-dimensional porous graphene/polymer composites (3DGPCs) as an active material for all-solid-state supercapacitors.[147] Three-dimensional porous graphene hydrogel–PANI nanocomposites are explored here as a flexible electrode material. Ramphal and Hagerman [148] incorporated Laponite clay into the graphene/PANI ­nanocomposite to promote the supercapacitive nature of the material. Incorporation of Laponite clay improves the water dispersion of the composite and makes it easy to prepare homogenous films during fabrication. Sekar et al.[149] used 3D porous PANI anchored with a graphene sheet to utilize as an efficient electrode for supercapacitor. The composites show a high conductivity of 3.74 S/cm with a high surface area of 330 m2 /g. The supercapacitor constructed from these composite have a specific capacitance of 547 F/g with a mass loading of 3 mg/cm2. Recently, Malik et al.[150] prepared GQD-doped PANI composites by the oxidative polymerization of aniline in the presence of GQDs. The novel fibrous composites show an excellent specific capacitance value of 1,044 F/g at a current density of 1 A/g with a moderate cyclic stability.[150] PPY is well known for its good environmental stability and good conductivity. So, graphene–PPY composites are the most promising nanomaterial for supercapacitor application because of the synergism existing between them. Electropolymerization is the most common technique for the synthesis of PPY on graphene backbone. Till date, several studies have been performed to explore graphene–PPY composite in the field of supercapacitors. Davies et al.[151] reported a flexible supercapacitor based on the graphene–PPY composite that is synthesized via pulsed electropolymerization. The supercapacitor has a specific capacitance of 237 F/g and the highest energy of ∼32.9 W h/kg. de Oliveira et al. [86] demonstrated a CMG– PPY composite for a high-performance supercapacitor. They have introduced a –CH2CO2H group to the graphene surface by Johnson–Claisen functionalization scheme. As a result, a strong electrostatic interaction occurs between graphene surface and polymer material, and the interfacial polymerization raises the surface area and specific capacitance of the composite. The capacitance of devices increases drastically due to a good synergistic interaction between polymer and graphene composite. Bora et al. [152] explored PPY and sulfonated graphene composite as an electrode material for supercapacitor. These materials have exceptional electrical conductivity and good electrochemical reversibility showing a capacitance of 360 F/g at a current density of 1 A/g. Another interesting study on the graphene–PPY composite electrode material was carried out by Yang and ­coworkers. [153] Electropolymerization

of pyrrole on the graphene sheet leads to the formation of macroporous graphene/PPY (MGPPy) composites. The MGPPy composite electrode shows an excellent specific capacitance of 196 mF/cm 2 at a current density of 1 mA/cm 2 due to the presence of well-conducting 3D macroporous graphene and pseudocapacitance nature of PPY. Figure 17 illustrates the different studies of supercapacitors of these ­composites. These studies suggest that PPY–graphene composites can be used as an adaptable electrode material for s­ upercapacitors due to their various ­advantages. In a different study, Jiang et al.[154] reported a three-­ component supercapacitor (titania–graphene–PPY) (TiO2–G–PPy) with high electrochemical stability and improved current density of electrodes. Here, titania develops the electrochemical stability and pseudocapacitance of the graphene electrode, whereas PPY improves the conductivity. PPy lowers the electrochemical stability of the components due to a good synergistic effect between TiO2 and PPy, augmenting the capacitance and electrochemical stability to a remarkable value. In a recent study, Asen and Shahrokhian [155] have reported a ternary active electrode material consisting of rGO/PPY/Cu2O–Cu(OH)2 components for supercapacitor application. These nanocomposites show a good gravimetric specific capacitance of 997 F/g at 10 A/g due to double-layer capacitance of graphene and pseudocapacitance of PPY and Cu2O–Cu(OH)2. These electrode materials also have a decent reproducibility in nature. It is noteworthy that excluding PANI and PPY, other polymer–graphene nanocomposites are also reported for supercapacitor application. A ternary nanocomposite (graphene, single-walled CNT, and poly(3-­ methylthiophene)) was used as an electrode material for a high-­performance supercapacitor.[156] The nanocomposite shows an electrical conductivity of 4.68 S/cm and reveals a maximum specific capacitance of 561 F/g at a 5 mV/s scan rate. In a further study, Lehtimäki et al.[157] reported a PEDOT- and rGO-based composite material that was electrodeposited on a flexible substrate to get a highly efficient supercapacitor. The capacitive properties of PEDOT/rGO composites are much better than those of PEDOT, and the specific capacitance persists at 90% of original value after 2,000 cycles of charging and discharging. FUTURE SCOPE The functionalization of graphene with well-defined conducting polymers or small donor molecules can introduce different exciting architectures onto the graphene surface, facilitating to import new optoelectronic properties. It would be useful to produce tailor-made materials for fulfilling the widespread demands of harvesting energy, while the covalent functionalization ensures a closer contact between the donor small-molecule or conducting

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Fig. 17  (a) CV spectra of MG, PPy, and MGPPy composite at a potential from −0.8 to 0.2 V in 1 M KCl. (b) CV spectra of MGPPy composite at different scan rates (10–500 mV/s). (c) Charge–discharge curves of MG, PPy, and 120MG120PPy at a current density of 0.5 mA/cm2. (d) Charge–discharge curves of 120MG120PPy at various current densities. (e) A real capacitance of MGPPy composite with different mass loadings of PPy with respect to discharge current (120MG120PPy, 120MG240PPy, 120MG360PPy, 120MG480PPy, 120MG600PPy, 120MG900PPy, 120MG1200PPy, and 120MG1500PPy). (f) Cycling presentation of the 120MG600PPy for charging and discharging at a current density of 1 mA/cm [153]

polymer and the acceptor graphene; the weaker noncovalent approach avoids the introduction of sp3 defects onto the graphene surface, retaining its good charge carrier property. The functionalization of graphene for energy

harvesting applications has grown to some extent and there is an immense scope for the development of new and improved strategies. The block copolymers grafted from graphene surface using surface-initiated ATRP/RAFT

polymerization have a wide scope for the preparation of energy harvesting materials with tunable properties. The layer-by-layer structures may be developed by self-­ assembly between the two oppositely charged graphene polymer composites, and it may find great applications particularly in the field of supercapacitors. Photovoltaic devices with superior PCE can be fabricated by attachment of graphene with conducting block copolymers, as the block copolymer due to their self-organizing capability can tune the bandgap of the material. Further, the use of graphene/conducting polymer hybrid in the perovskite solar cell is at the initial state and yet to be vividly explored. Also, the field of the graphene/conducting polymer hybrids in fuel cell and battery application is in its very infant stage. From the viewpoint of its production cost and application, graphene  shows brighter promise over CNTs. Hence, the future research of graphene would be oriented towards fabrication of photovoltaic devices, supercapacitors, batteries, fuel cells, semiconducting chips, etc. Graphene functionalization with appropriate donor moiety in an appropriate manner would certainly help to fetch better energy harvesting properties in these important materials. The incorporation of inorganic nanoparticles into the graphene-conducting polymer hybrid would stimulate to enhance both energy generation and energy storage ­property of the hybrids. ACKNOWLEDGMENT

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127. Gahlot, S.; Sharma, P.P.; Kulshrestha, V.; Jha, P.K. SGO/ SPES-based highly conducting polymer electrolyte membranes for fuel cell application. ACS Appl. Mater. Interfaces 2014, 6 (8), 5595–5601. 128. Beydaghi, H.; Javanbakht, M.; Kowsari, E. Synthesis and characterization of poly(vinyl alcohol)/sulfonated graphene oxide nanocomposite membranes for use in proton exchange membrane fuel cells (PEMFCs). Ind. Eng. Chem. Res. 2014, 53 (43), 16621–16632. 129. Beydaghi, H.; Javanbakht, M. Aligned nanocomposite membranes containing sulfonated graphene oxide with superior ionic conductivity for direct methanol fuel cell application. Ind. Eng. Chem. Res. 2015, 54 (28), 7028–7037. 130. Sahu, A.K.; Ketpang, K.; Shanmugam, S.; Kwon, O.; Lee, S.; Kim, H. Sulfonated graphene–nafion composite membranes for polymer electrolyte fuel cells operating under reduced relative humidity. J. Phys. Chem. C 2016, 120 (29), 15855–15866. 131. Doh, C.-H.; Kim, H.-S.; Moon, S.-I. A study on the irreversible capacity of initial doping/undoping of lithium into carbon. J. Power Sources 2001, 101 (1), 96–102. 132. Xue, Y.; Chen, H.; Yu, D.; Wang, S.; Yardeni, M.; Dai, Q.; Guo, M.; Liu, Y.; Lu, F.; Qu, J.; Dai, L. Oxidizing metal ions with graphene oxide: The in situ formation of magnetic nanoparticles on self-reduced graphene sheets for multifunctional applications. Chem. Commun. 2011, 47 (42), 11689–11691. 133. Yang, Z.; Zhang, J.; Kintner-Meyer, M.C.W.; Lu, X.; Choi, D.; Lemmon, J.P.; Liu, J. Electrochemical energy storage for green grid. Chem. Rev. 2011, 111 (5), 3577–3613. 134. Tarascon, J.M.; Armand, M. Issues and challenges facing rechargeable lithium batteries. Nature 2001, 414 (6861), 359–367. 135. Zhang, L.L.; Zhao, X.S. Carbon-based materials as supercapacitor electrodes. Chem. Soc. Rev. 2009, 38 (9), 2520–2531. 136. Park, M.; Zhang, X.; Chung, M.; Less, G.B.; Sastry, A.M. A review of conduction phenomena in Li-ion batteries. J. Power Sources 2010, 195 (24), 7904–7929. 137. Brodd, R.J.; Bullock, K.R.; Leising, R.A.; Middaugh, R.L.; Miller, J.R.; Takeuchi, E. Batteries, 1977 to 2002. J. Electrochem. Soc. 2004, 151 (3), K1–K11. 138. Dai, L. Functionalization of graphene for efficient energy conversion and storage. Acc. Chem. Res. 2013, 46 (1), 31–42. 139. Armand, M.; Tarascon, J.M. Building better batteries. Nature 2008, 451 (7179), 652–657. 140. Dunn, B.; Kamath, H.; Tarascon, J.-M. Electrical energy storage for the grid: A battery of choices. Science 2011, 334 (6058), 928–935. 141. Yoo, E.; Kim, J.; Hosono, E.; Zhou, H.-S.; Kudo, T.; Honma, I. Large reversible Li storage of graphene nanosheet families for use in rechargeable lithium ion batteries. Nano Lett. 2008, 8 (8), 2277–2282. 142. Zhang, F.; Cao, H.; Yue, D.; Zhang, J.; Qu, M. Enhanced anode performances of polyaniline–TiO2–Reduced graphene oxide nanocomposites for lithium ion batteries. Inorg. Chem. 2012, 51 (17), 9544–9551. 143. Mandal, D.; Routh, P.; Nandi, A.K. Supercapacitor and photocurrent performance of tunable reduced graphene oxide. Chem. Select 2017, 2 (10), 3163–3171.

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153.

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155.

156.

157.

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Electronics–Energy

Energy: Polymers in Harvesting, Conversion, and Storage Shahram Mehdipour-Ataei and Zahra Tabatabaei-Yazdi Iran Polymer and Petrochemical Institute, Tehran, Iran

Abstract In the past few decades, polymer science and technology have been extensively contributed in energy production, energy conversion, and energy storage. There are numerous research studies and strategies in which polymeric materials play the key role in providing energy requirements from pilot scale toward industrial scales. The most important problem facing the world today is global warming, and environmental catastrophes caused mainly by the excessive use of fossil fuels which produces large amounts of greenhouse gases. The most reliable and logistic approach to reduce the emission of greenhouse gases and other pollutants is the use of clean energy instead of ­petroleum-based fuels. On the other hand, the world is running out of oil and other ­hydrocarbon-based reservoirs, and therefore, finding sustainable and/or renewable energy resources and developing their large-scale usage is of crucial importance. Sunlight, hydrogen and oxygen gases, wind and ocean energy, and internal earth heat are examples of sustainable or renewable energy resources under investigation for generation of clean and efficient power. Polymers have been significantly contributed in energy production and energy saving owing to their unique combination of desirable properties and excellent versatility they offer. The main energy areas in which polymeric materials and polymer-based composites play irreplaceable role involve energy harvesting, energy conversion, and energy storage. Each category includes several important polymer-based applications that will be introduced and briefly described. Keywords: Energy; Polymer batteries; Polymer electrolyte electrolysis cells; Polymer electrolyte membrane fuel cells; Polymer solar cells; Porous polymers.

ENERGY HARVESTING Solar Energy Harvesting Solar energy, the most abundant renewable energy on the earth, is radiant light and heat from the sun. Nowadays, sunlight has become as a valuable source for a wide range of ever-evolving technologies such as photovoltaics, solar cells, concentrated solar power, and other solar plants. These technologies are labeled as either passive or active, depending on the way they capture and distribute solar energy or convert it into other energy forms. The large number of available solar energy technologies makes it very interesting source of energy to be converted to electricity. A report released by the United Nations in 2000 announced that the annual potential of solar energy is in the range of 1,575–49,837 exajoules (1 EJ = 1018 J). This amount of energy is several times larger than the most recent estimate of total world energy consumption, which was 567 EJ in 2013. Such a rough comparison readily highlights the importance and potential of development and employment of solar energy technologies to supply a great part of world energy needs. Development of solar energy technologies will reduce pollution, enhance sustainability, Encyclopedia of Polymer Applications, First Edition DOI: 10.1201/9781351019422-140000256 Copyright © 2018 by Taylor & Francis. All rights reserved.

and lower the costs of mitigating global warming, which are all considered as distinct global advantages. Moreover, solar devices require very little space to create power compared with many other energy sources, and this makes solar technologies more appealing. Most commercial solar cells are now made from refined, highly purified silicon crystals similar to the materials used in the manufacture of integrated circuits and computer chips. The high fabrication cost of silicon solar cells and their complicated production process drew the attention of research groups and individuals toward the ­alternative technologies, e.g., polymer solar cell technology. Organic Polymer Solar Cells/Photovoltaic Cells Polymer solar cells have several advantages over the conventional silicone solar cells. For instance, they are ­flexible, lightweight, relatively inexpensive to manufacture, potentially disposable, and more environmentally friendly. Polymer solar cells also have the potential to be transparent which is highly required in many applications, e.g., in windows, walls, and flexible electronics. Nevertheless, polymer solar cells exhibit some disadvantages: lower efficiency compared to hard materials and the substantial

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photochemical degradation are the most serious ones.[1] It is well known that organic materials are inherently more receptive to degradation agents such as oxygen and water than inorganic materials. Hence, organic devices must be improved from the viewpoint of chemical stability to become technologically superior competitors in the solar market. Low efficiency and instability of polymer solar cells are the most challenging problems and, therefore, are the main research focus in many solar cell research groups and R&D centers of companies.[2] Conjugated polymers such as poly(alkyl thiophene)s and poly(p-phenylene vinylene)s (PPVs), and their various derivatives are excellent candidates to be used in electronic devices and photovoltaics, owing to their high mobility of charge carriers and the high absorption coefficient in the visible wavelength region as well as their excellent ­thermal, mechanical, and physical properties.[3] In polymer solar cells, like all other types of solar cells, a flux of photons is converted to a flux of charged particles within the semiconductor layer. Incident photons generate an electron/hole pair called exciton. Photo-generated excitons are tightly bound together so that only those migrate to the donor–acceptor interface can separate into free ­electron and hole species. Since the migration distance is typically about 10 nm, the energy conversion efficiency of the polymer solar cells has remained only less than 1% for long times. An excellent solution for obtaining higher yields of charge separation is to enlarge the interface area between donor and acceptor layers of materials by mixing them into a single-active layer called bulk heterojunction. The efficiency of polymer solar cells having a ­heterojunction interface has been reported to reach up to 5%.[4] The main properties and synthetic methods for the production of two conjugated polymers that are widely used in organic polymer solar cells, i.e., polythiophenes (PTs) and PPVs, are reviewed in the following section. Polythiophenes  PTs are a class of sulfur heterocyclic polymers that are synthesized by the polymerization of thiophene monomers. Thiophene is polymerized either electrochemically via applying a potential to a ­solution of monomer or chemically using an oxidant or a ­cross-coupling catalyst. Poly(3-hexylthiophene) (P3HT) is an important example of PTs and the most popular semiconductor polymer that is extensively used in bulk heterojunction solar cells. Scheme 1 shows the chemical structure of the repeating unit of P3HT. The most characteristic feature of PTs is their conductivity, originating from the delocalization of electrons along the macromolecular chains. Electron delocalization also leads to the optical properties of PTs, making them responsive to environmental stimuli such as temperature changes and applied potential variation. The conductivity and optical properties of PTs and other conjugated polymers make them suitable choices for the production

Energy: Polymers in Harvesting, Conversion, and Storage

(CH2)5-CH3

S n

Scheme 1  Chemical structure of the repeating unit of P3HT

of polymer solar cells and optical/electronic-responsive ­sensors as well. Poly(p-phenylene vinylene)s  PPVs are an important class of conducting polymers having a rigid-rod conjugated backbone. They can be synthesized by a variety of methods, dictating the purity and molecular weight of the polymer. The most utilized method is a base-induced elimination from α,αʹ-disubstituted para-xylenes. Other synthetic routes for the production of PPVs include stepgrowth ­coupling reaction, Heck coupling reaction, and ring-opening polymerization of a bicyclooctadiene compound. Scheme 2 shows the chemical structure of the repeating unit of PPV. PPVs can be processed into a highly ordered crystalline thin film. They are good candidates for polymer solar cells owing to their small optical bandgap and bright yellow fluorescence. They can be doped to form electrically conductive materials and provide the photovoltaic cells with enhanced physical and electronic properties. Indeed, PPVs are used as an electron-donating material in organic solar cells. Photovoltaic devices made by PPVs and their derivatives are usually subjected to poor absorption and photodegradation problems; however, they have found many applications in research photovoltaic cells.[5] Deposition of polymeric materials using various techniques such as screen printing, doctor blading, inkjet printing, and spray deposition is possible because they are mainly soluble in common solvents. Such a wide range of deposition techniques along with lightweight and flexibility of polymeric materials are the most important qualities needed to feasible and cost-effective manufacturing of solar panels. Additionally, these deposition techniques are performed at low temperature allowing the manufacturers to fabricate thin film solar layers on plastic substrates for flexible devices. Flexible photovoltaics that are mainly based on plastics, such as portable power generation and aesthetic photovoltaics in building design, have gained a specific market share.

CH

CH n

Scheme 2  Chemical structure of the repeating unit of PPV

Polymer–Inorganic Hybrid Solar Cells Polymer–inorganic semiconductor hybrid solar cells have received much attention since the last decade for their potential in the production of cost-effective solar energy on a large/industrial scale. The efficiency of power conversion of organic–inorganic solar cells has reached above 3%; however, it is still lower than some other types of solar cells such as polymer–fullerene solar cells. In polymer– fullerene solar cells, conjugated polymers mainly function as electron-donor materials. Fullerene derivatives are mainly used as electron-acceptor materials owing to their high electron mobility and their high solubility.[6] Recently, n-type inorganic semiconductor materials have also been extensively investigated because of their high electron mobility comparable to or higher than that of the fullerene-based organic semiconductors. Various combinations have also been reported such as ­poly(2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene/TiO2 and P3HT/TiO2. Nano-sized metal oxides such as TiO2, ZnO, and SnO2 are fascinating inorganic semiconductors for their ease of fabrication, proper control of film morphology, and desirable interfacial properties.[7,8] Polymer–inorganic hybrid solar cells may offer advantages of both materials, i.e., advanced solution processability and high photosensitivity of conjugated polymers in addition to the high electron mobility of inorganic materials. Nevertheless, the energy conversion efficiency of metal oxide-based polymer–inorganic hybrid solar cells is still lower than their organic counterparts (about 1%) originating from a weak interfacial contact between inorganic and organic materials.[9] Electronic junction between organic and inorganic phases is a crucial factor in the performance of the cells. A useful approach to improve the electronic junction at the interface of these phases is the chemical modification of inorganic materials with organic materials via different available techniques.

poorly sealed, and therefore, replacement of liquid electrolytes by polymeric electrolytes solves such problem. Also, they offer an additional advantage of playing the role of binder for electrodes. More recently, intrinsically conducting polymers have been used as hole conducting materials in DSSCs, exhibiting promising results. Generally, when properly modified plastic materials substitute for inorganic or simple organic material, excellent processing advantages will be obtained. Indeed, by this substitution, the specific mechanical properties of polymers are combined with the desired electrical, optical, electronic, and magnetic properties of metals and semiconductors to improve the final properties. Besides, the use of flexible polymeric materials instead of rigid and brittle substrates opens up the ­possibility of producing DSSCs through a continuous roll-to-roll, high-speed coating process. Moreover, design patterns in different shapes being suitable for application at different surfaces such as glass windows will be ­facilitated.[10–12] Among various polymers used in DSSCs, PET foils coated with thin conductive layer are widely used as the polymer substrate. The main features, synthetic routes, and applications of PET are explained in the next section. Poly(ethylene terephthalate)  PET is the most common thermoplastic polyester having a chemical structure as shown in Scheme 3. It is a semicrystalline polymer that processes a combination of desired physical properties. For example, it is lightweight, colorless, strong and impact resistant, durable in various conditions, and an excellent barrier to moisture and chemicals. Therefore, it has found numerous applications in the fiber and liquid packaging industry. Moreover, PET and its engineering composites found high-tech applications, for instance, in thin-film solar cells. PET is synthesized from the polycondensation reaction between ethylene glycol and dimethyl terephthalate (via transesterification reaction) or terephthalic acid (via esterification reaction).[13]

Dye-Sensitized Polymer Solar Cells

Wind Energy Harvesting

Dye-sensitized solar cells (DSSCs) are composed of a combination of several materials including optically transparent electrodes, nano-particulated semiconductors, coordination compounds, inorganic salts, solvents, and metallic catalysts. Each material plays a specific role in harvesting solar light and transforming it into electricity. Some of these materials are recently substituted by polymers to improve the efficiency of energy conversion and also to increase the technological perspectives of DSSCs. For instance, films of poly(ethylene terephthalate) (PET) coated with indium-doped tin oxide (ITO) can be used as substitution for glass electrodes, leading to improved flexibility and impact resistance and lower weight and cost of a solar cell. On the other hand, liquid electrolytes are usually volatile compounds and may leak if the cell is

Wind energy is absolutely the least expensive renewable energy source for generating electricity. In regions geographically suited for harnessing wind power, this clean energy can be one of the primary sustainable sources to produce electricity. Generation of electrical power from wind energy is actually without the emission of any greenhouse gases, and hence is environmental friendly. ­Moreover, recent improvements in the turbine technology

O

O

O

C

C

O

CH2

CH2 n

Scheme 3  Chemical structure of PET

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have significantly reduced the cost of wind energy, making the electricity generated by wind farms very cost competitive with the electricity produced by fossil fuels such as coal or natural gas. Wind Turbines Wind turbines convert the kinetic energy in the wind into mechanical power that is transferred to a generator which produces electrical output from the mechanical input. In fact, the wind energy turns the blades, which spin a shaft that is attached to a generator. The generator then converts the received mechanical energy into electricity. Wind turbines are one of the fast-paced growing applications of the polymer composite industry. Many wind turbine parts are made from polymer composite materials. Indeed, the production of wind turbine blades accounts for a large proportion of all thermoset composites manufactured worldwide. Ultralight, strong and stiff, thermally stable, and durable composites are ideal for producing turbine blades having excellent performance capabilities. Generally, a wind turbine should work for about 20–25 years without repair, which means that minor blade deformation should exist. In other words, the fatigue effect caused by gravitation forces at rotation and cyclic wind loads must be negligible. The only materials that meet the requirements of such condition are engineering polymer composites. Composite materials dominate the wind turbine market owing to their superior fatigue resistance and their highly desirable stiffness-to-weight ratio. Composites can withstand very harsh atmospheric condition and meet strict mechanical requirements. For instance, they must have an excellent resistance to torsion and fatigue, and endure a service life for about 20 years under high static and dynamic load. Besides, their ability to fabricate various and complex geometries, and high potential for ­optimizing blade designs are other significant advantages they offer.[14–16] A wind turbine is constituted by several composite parts. Turbine nacelle that provides weather protection for the components is made of composites, usually polyester-­ based composites, for it must have low weight, excellent strength, and corrosion resistance. Other turbine parts such as blades and hub are also made of polyesters. Blades are the most crucial part of a wind turbine, and their mechanical features determine the performance and lifetime of the wind turbine. Turbine blades are mainly made of ­fiber-reinforced epoxy or unsaturated polyesters.[17] They must be very strong and lightweight to perform their function well. The blades are made up of particular composite materials. Matrix of the composite determines fracture toughness, strength, and stiffness of the composite, and hence plays a critical role in the overall performance of the turbine blade. Thermoset polymers are typically used as the matrix material. The main advantages of thermosets are the possibility of curing at room or low temperature,

Energy: Polymers in Harvesting, Conversion, and Storage

and their relatively low viscosity, which facilitate their impregnation and adhesion. Among the numerous thermoset polymers, epoxy, polyester, and vinyl ester (to a lesser extent) were the main polymers applied in the wind blade production market in the early years of their commercialization. Nevertheless, epoxy composites received more attention as the most suitable material for manufacturing of turbine blades as they grew longer. Although ­polyesters are easier to process and less expensive, epoxy-based composites exhibit better mechanical performance—­particularly, tensile and flexural strength—for blade  products. Unlike epoxy, polyester needs no post-curing process, but the blades are almost always heavier. Moreover, the production of epoxy-based composites is more environmentally friendly, and it can be another reason for industry to switch away from polyester to epoxy.[18,19] Thermoplastics are fascinating alternatives for thermosets to be used as the matrix material. The main advantages of thermoplastics are their recyclability and higher fracture strain. For instance, BASF recently developed a new acrylonitrile styrene acrylate copolymer for wind turbine use.[20] Nevertheless, thermoplastics generally suffer from several disadvantages: high processing temperature, high melt viscosity, lower melting temperature, and hence low ­resistance to deformation upon heating. E-glass is by far the most widely used reinforcing agent in polymer composites for turbine blades. However, carbone fiber is the best reinforcing agent to provide the necessity of stiffness and lightweight. S-glass, aromatic polyamide (aramide) fibers, and basalt fibers are the other commonly used reinforcing agents. The use of carbon nanotubes as reinforcing agent in polymeric matrices is still under investigation. It can be concluded that the blades in service are affected by a combination of different external loads including bending, compression, tension, gravitation, and inertia. Therefore, the material used for producing blades must have outstanding mechanical properties ­including excellent fatigue resistance, strength, stiffness, and ­environmental stability. All these requirements are fulfilled by polymer-based composite materials as mentioned earlier.[21] In the following section, a brief introduction of the two most widely used polymers for turbine blades is given. Unsaturated Polyester  Unsaturated polyesters are condensation polymers synthesized by the reaction between polyols with saturated or unsaturated dibasic acids. Typical polyols used are glycols, e.g., ethylene glycol, and diacid monomers used are phthalic acid or maleic acid. Once the polymerization reaction is completed, the p­ repared liquid polymer is converted to a solid thermoset polymer in the cross-linking process. Unsaturated polyesters are mainly used in the form of composite materials containing fibrous-reinforcing fillers. For preparation of unsaturated polyester composites, fillers are added into the

CH3 O

C

O

CH2

H C

O

CH3

CH3

C

C H

C H

C

O

O

O

H C

H2C

CH3 n

Scheme 4  A typical structure of unsaturated polyester resin

initial resin prior to the curing process. These composites exhibit excellent mechanical strength and heat resistance, as well as corrosion and chemical stability.[22] Scheme 4 shows a typical structure of unsaturated polyester before cross-linking. Epoxy Resins  Epoxy resins, also called polyepoxides, are a class of reactive prepolymers and polymers containing epoxide functional groups (Scheme 5). Epoxy resin is synthesized from polycondensation reaction of a diepoxide or equivalent with a reactive diol or polyol. Epichlorohydrin is the most widely used diepoxide equivalent for its commercial availability and low price. Bisphenol-A is the most widely used diol owing to its aromatic nature that leads to enhanced hydroxyl reactivity and improved mechanical strength of the resultant epoxy. Epoxy resins are then cross-linked either by themselves through catalytic homopolymerization, or by various cross-linking agents such as polyfunctional hardeners such as amines, acids, and phenols. The prepared cross-linked epoxy compounds are thermosetting polymers having high m ­ echanical properties, thermal stability, and chemical resistance. Indeed, the chemistry of epoxies and the diversity of available ­modification methods allow for the production of cured polymers having a very broad range of properties. Therefore, cured (cross-linked) epoxy is a suitable choice to be used in a wide range of applications, particularly as fiber-reinforced plastic materials, where high thermomechanical performance even in the harsh environmental condition is required.[23,24] Scheme 5 shows the chemical structure of an epoxy prepolymer. Vibration Energy Harvesting Another form of energy that can be harvested and converted into other useful forms of energy like electricity is vibration energy. The vibration energy that is harvested is most of the time the waste energy being useless otherwise. For instance, engine vibration can stimulate piezoelectric materials generating electricity therefrom.

Piezoelectric materials have attracted considerable attention during the past few decades for their ability to convert mechanical kinetic energy into electrical potential energy, and vice versa. For instance, when automobiles move on the road, the piezoelectric materials under the road are vibrated in response to the force applied from the tires leading to the formation of noticeable amount of electricity. The ability of certain crystalline materials to generate electric charge in response to applied mechanical stress (when the substance is stretched or compressed) is called the “direct” piezoelectric effect or generator effect. On the contrary, the substance undergoes geometric deformation (the substance shrinks or expands) once an electric field is applied to a piezoelectric material. This is known as the “inverse” piezoelectric effect or the actuator or motor effect.[25] The nature of piezoelectric effect originates from the occurrence of electric dipole moments in solids. Once a mechanical stress is applied, piezoelectric effect is produced by the change in polarization of material atoms or reorientation of molecular dipole moments upon external stress and hence formation of an electric field.[26] Piezoelectric Composite Devices Piezoelectric composite is a piezoelectric material constituted by a combination of piezoelectric ceramic and/or a polymer and a non-piezoelectric polymer. Early versions of piezoelectric composites include barium titanate and lead zirconate titanate (PZT) ceramic powders dispersed in polymer matrices. Recent developments in this field are preparation of piezoelectric composites based on the following: (1) flexible composites made by dispersing PbTiO3 or PZT in a synthesized rubber, (2) polyvinylidene fluoride (PVDF)-based piezoelectric composites, (3) woven PZT ceramic/polymer composites, and (4) modified lead ­titanate rods in a polymer matrix.[27] There are various types of natural and synthetic crystals, ceramics, polymers, and composites showing piezoelectric

CH3 O

O

C

O

CH3

Scheme 5  Chemical structure of an epoxy prepolymer

CH2

CH OH

H2 C

CH3 O

C CH3

O

O

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effect. Among different classes of synthetic ­crystalline polymers, PVDF is a ferroelectric polymer exhibiting piezoelectric and pyroelectric properties. These characteristics make it useful in sensor and battery applications. A brief introduction of PVDF and its synthetic route and properties are discussed in the next section.

highly complex equipment such as accelerometers used in aerospace industry rely on piezoelectric effect occurred in piezoelectric materials.

Polyvinylidene Fluoride  PVDF is a thermoplastic fluoropolymer that is highly nonreactive and chemically stable. It is synthesized by free radical polymerization of gaseous vinylidene difluoride monomers. PVDF is a ­semicrystalline polymer and has a very low glass transition temperature of about −35°C. In order to induce the piezoelectric properties to PVDF, its molecular chains must be oriented by mechanically stretching or annealing followed by poling under tension. Actually, this process changes PVDF chain conformation and converts non-piezoelectric alpha phase to the piezoelectric beta phase. Scheme 6 shows the ­chemical structure of the repeating unit of PVDF. PVDF copolymers such as P(VDF-trifluoroethylene) and P(VDF-tetrafluoroethylene) have an enhanced crystalline structure and thus exhibit piezoelectric properties. These copolymers are used in applications where enhanced piezoelectric response is required. Polymer-based piezoelectric materials are mainly classified into three categories. The first category is the bulk polymer which is a solid polymer film showing the piezoelectric effect through its molecular structure and molecular arrangement. The second category is the polymer-based piezoelectric composite in which the polymer structure is filled with piezoelectric ceramic particles that generate the piezoelectric effect. These composites take the advantages of both phases, namely, the mechanical strength and flexibility of polymers and high electromechanical coupling of the piezoelectric ceramic particles. The third type of piezoelectric polymers is the voided charged polymer that is basically a polymer film in which a number of gas voids are introduced in the film and surfaces are charged in a way that internal dipoles are formed. Once a mechanical stress is applied on the polymer film, the polarization of these dipoles changes and the piezoelectric response reveals.[25,28] The main markets for piezoelectric devices are industrial and manufacturing units. There is high demand for piezoelectric devices from automotive, medical, and telecommunication industries. A large number of the modern technologies ranging from sensors, transducers, and actuators used in simple devices such as gas igniters to

Various forms of energy can be transformed to other form using various devices and systems developed for this purpose. For instance, chemical and electrical energies can be converted into each another by electrochemical devices. Fuel cell and electrolysis cell are electrochemical energy conversion devices that generate electricity using hydrogen fuel and generate hydrogen using electricity, ­respectively. Fuel cell converts chemical energy from hydrogen into electricity through a chemical reaction between H+ ions and oxygen and produces water. On the contrary, electrolysis cell dissociates water into hydrogen and oxygen gases through a redox reaction when an electric current is applied. Fuel cell is a promising technology for the production of clean power to meet a large portion of global energy demands. In the transportation sector, e.g., fuel cells are competitive alternatives to conventional pollutant combustion engines. In fact, fuel cell technology possesses the major advantage of nonpollutant process over the conventional combustion engines. In addition to transportation, fuel cells are potentially very attractive in other application sectors such as stationary power plants and portable devices.[29] Hydrogen is the fastest growing industrial gas throughout the globe. Developing novel and more efficient ­methods for the production of hydrogen is therefore the concern of many research groups. Electrolysis of water is a ­process that has been widely studied for hydrogen production; however, it is still considered as an expensive method since the energy required for water dissociation is higher than the energy that could be generated from the produced hydrogen. Batteries are devices capable of converting stored chemical energy into electricity. They use almost the same mechanism as fuel cell and electrolysis cell use. There are a wide variety of commercially available batteries ­developed for special application. In the next section, the main principles and applications of these energy-converting devices that are based on ­polymeric materials are explained.

F CH2

C F

n

Scheme 6  Chemical structure of the repeating unit of PVDF

ENERGY CONVERSION

Polymer Electrolyte Membrane Fuel Cells In polymer electrolyte membrane fuel cells (PEMFCs)— also called proton exchange membrane fuel cells—a solid polymer membrane in the form of a thin film is used as the electrolyte. Proton-conductive groups, typically sulfonic acid, are predominantly introduced in the main chain or side chains of the polymer support. Fluorinated polymers used as PEMFC membranes are one of the most famous

membranes due to their thermal and chemical stability under the operating condition. The role of membrane in PEMFC is allowing only the hydrogen ions generated at the anode to pass through and get to the cathode. Hence, the electrons have to transfer via an external electric circuit where they produce an electric current. This electric current can be used on site, e.g., to power a vehicle-like automobile or bus. PEMFCs generate high power density and offer several advantages including compact size and lightweight, clean by-products, and silent operation compared with other types of fuel cells. They only need hydrogen, oxygen, and low amounts of water (humidity) to operate. Figure 1 shows a schematic image of a PEMFC operation. PEMFCs operate at relatively low temperature, typically around 60°C–80°C, leading to a quick start-up that results in less damage and corrosion on the system components. Nevertheless, this type of fuel cell requires a noble metal catalyst, typically platinum, to separate electrons and protons from the hydrogen atoms, which imposes costs on the system. Membranes in PEMFC have multiple roles including charge carriers for proton, separator of reactant gases, and electronic insulator for electrons through the membrane. Indeed, polymeric membrane is the core component of a PEMFC. Nowadays, the only used commercial membrane for PEMFCs is Nafion, a perfluorosulfonic acid (PFSA) polymer discovered in the late 1960s by Walther Grot at DuPont. Although extensive research efforts have been exerted to develop high-performance alternatives to Nafion membranes, no successful commercial products

Electric

e

with reasonable price have launched in the market yet. However, Nafion has some deficiencies. In order to overcome the drawbacks of Nafion membrane, various modification methods have been developed, and a large number of polymers having desirable properties such as high proton conductivity and thermal stability have been reported in the literature. Sulfonated aromatic polymers such as poly(ether ether ketone)s (PEEKs), polysulfones, polybenzimidazoles (PBIs), polyimides (PIs), and their copolymers and composites are some examples of alternatives to Nafion membrane.[30] A brief introduction, synthetic methods and properties of some most-studied polymers for PEMFC membrane are reviewed in the following section. Poly(ether ether ketone) PEEK is a multipurpose thermoplastic polymer and a member of poly(arylene ether ketone) family that founds a large number of engineering applications owing to its excellent combination of desired properties. It has a glass transition temperature of around 143°C and a wide operating temperature range up to 250°C. Most important characteristic features of PEEK include excellent mechanical strength, chemical stability, and high resistance to thermal degradation. Aromatic PEEK polymers are usually synthesized via the step-growth polymerization reaction between aromatic difluoroketone or dichloroketone and bisphenolate. Scheme 7 shows the chemical structure of the repeating unit of PEEK. Sulfonated PEEKs that are needed for obtaining high proton transfer can be prepared either through the use of

current

–

e

–

Excess fuel outlet

H2O – e

H H

+ +

H2 H

H2

H2O H2O

Water & heat outlet

H2O

+

H

O2 +

O2

– e

Fuel inlet

Fig. 1  Schematic view of a PEMFC operation

Anode

Electrolyte

Cathode

Air inlet

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O

O

C O

n

Scheme 7  Chemical structure of the repeating unit of PEEK

sulfonated monomers and following the routine polymerization process or post-sulfonation of prepared polymer using concentrated acid solutions. Sulfonated PEEKs have the advantage of better solubility in organic solvents that allows easier casting from solution leading to the more convenient fabrication process compared with PFSA membranes. Sulfonated PEEK membranes with improved proton conductivity have been received remarkable attention in PEM cells.[31] Poly(ether sulfone) Poly(ether sulfone) (PES) is a high-temperature engineering thermoplastic that maintains its desirable properties, dimensional stability, and extremely high resistance to hydrolysis, e.g., at high temperatures. It is primarily synthesized by polycondensation reaction between aromatic difluorosulfone or dichlorosulfone and bisphenolate. Scheme 8 shows the chemical structure of the repeating unit of PES. PESs are widely used as a membrane material in dialysis; micro-, nano-, and ultrafiltration; and reverse osmosis. Sulfonated PESs have been used as a PEMFC membrane material in many research efforts, exhibiting appropriate  performance. PES usually consists of diphenylene ­sulfone, diphenylene ether, and an aromatic isopropylidene unit. Aromatic polyether part, which is also presented in PEEK, induces chain flexibility in the polymer ­backbone, whereas diphenylene sulfone moiety induces t­ hermo-oxidative stability to the polymer ­structure.[32,33] O O

Polybenzimidazoles PBIs are a class of high-performance and extremely heat-­ resistant heterocyclic thermoplastics. They exhibit an excellent chemical resistance to a wide range of chemicals and solvents, and possess a very high glass transition temperature (about 425°C) and a high decomposition temperature (above 500°C). Therefore, PBIs are known as one of the best high-performance engineering plastics on the market for many applications such as semiconductor, aerospace, and membrane separation industries. Functionalized PBIs have been extensively used as high-temperature PEMFC membrane. Although PBI has very low proton conductivity, once it is doped by some acids, its proton conductivity dramatically increases even in the anhydrous state. For instance, phosphoric acid-doped PBI-based membranes are the most promising candidate for high-temperature PEMFCs.[34,35] PBIs are generally synthesized from polycondensation reaction between aromatic tetra-amine and aromatic dicarboxylic acid (or its derivatives) monomers. Scheme 9 demonstrates the chemical structure of the repeating unit of PBI. Sulfonated PBIs can be synthesized via three common approaches: (1) polymerization of sulfonated aromatic diacid with an aromatic tetra-amine, (2) chemical grafting of functionalized monomers onto the PBI backbone, and (3) direct sulfonation of the polymer backbone by immersing in sulfuric acid or phosphoric acid solutions. It is noteworthy that the first method has several advantages over the two others including the limitation of side reactions, the ability to control the degree of sulfonation, and the ­distribution of sulfonated units.[36] Polyimides

S O

n

Scheme 8  Chemical structure of the repeating unit of PES

PIs are a well-known class of high-temperature polymers that exhibit the outstanding mechanical performance, dimensional stability, and chemical resistance even at

N

N

N H

N H

Scheme 9  Chemical structure of the repeating unit of PBI

n

an elevated temperature. These unique combinations of thermal, physical, and mechanical properties come from the strong intra- and intermolecular interactions in the PI backbone.[22] Scheme 10 shows the chemical structure of the repeating unit of PI. PIs are synthetized from condensation polymerization of stoichiometric amounts of diamine and dianhydride monomers via two main methods: (1) classical two-step method via polyamic acid prepolymer and thermal imidization process, and (2) one-step high-temperature imidization or direct conversion of diamine and dianhydride monomers using a dehydrating agent and an imidization catalyst. Sulfonated PIs have been widely studied to be used as high-temperature PEMFC membranes because they fulfill almost all the requirements for this application. Generally, sulfonated PIs show relatively high proton conductivity, low gas and methanol permeability, suitable mechanical properties, and unique water uptake behavior in fuel cell condition.[37] Radiation-Grafted Copolymers for PEMFC Ion exchange membranes prepared by radiation-induced graft copolymerization have been widely studied and used in different separation technologies such as water desalination and dialysis. Their application in PEM fuel cells is also investigated. Radiation-grafted PEMFC membranes are based on stable fluorinated polytetrafluoroethylene (PTFE) or partially fluorinated ethylenetetrafluoroethylene (ETFE) polymer backbones that are functionalized to introduce ion exchange sites and thus proton conductivity. Base polymer films and grafting monomers can be selected from a wide variety of commercially available products, and this opens up the possibility of using the cost-effective and more available starting materials. Besides, this method offers several other advantages. For example, the graft polymerization can be carried out at low temperatures unlike common radical polymerizations. Also, the degree of grafting can be easily controlled by monitoring the radiation dose and rate and by adjusting the reaction time.[38,39] Polymer Electrolyte Electrolysis Cells Electrolysis is an important fundamental technology for production of the most efficient clean energy carrier, i.e., hydrogen. By definition, electrolysis of water is the O

O

N

N O

O

n

Scheme 10  Chemical structure of the repeating unit of PI

decomposition of water molecules into oxygen (O2) and hydrogen (H2) gases once an electric current is passed through the water. Water electrolysis is a promising and efficient approach for the production of high-purity hydrogen gas because of the large operational scales and high efficiency of the process. Water electrolysis primarily takes place in alkaline or polymer electrolyte membrane (PEM) electrolysis cells. Alkaline electrolyzers use aqueous solution of 30% potassium hydroxide (KOH) as the electrolyte, whereas PEM electrolyzers utilize solid polymer membranes. PEM electrolysis was developed to overcome the challenges associated with alkaline electrolysis such as low current density and low pressure operation. PEM electrolysis offers several advantages over the alkaline technology in that it has stack operation at four- to fivefold higher current density (this counteracts higher cost material), ecological cleanness (lack of corrosive electrolytes), differential pressure operation (elimination of need for strict pressure controls), higher stack efficiencies, higher purity of the product, fast response time to current signal, smaller size and mass, higher safety level, and the potential to be integrated with solar and wind power.[40] PEM electrolysis process is just the reverse of a PEM fuel cell process; however, the materials used are generally different from those used in PEMFC. The core component in a PEM electrolyzer is the proton exchange membrane (or solid polymer) electrolyte. Figure 2 shows the ­schematic representation of PEM electrolysis cell. A PEM electrolysis cell is a cell equipped with a solid polymer electrolyte (SPE) in which the electrolysis of water takes place. The SPE plays several roles simultaneously comprising conduction of protons, separation of product gases, and electrical insulation of the electrodes. Polymeric electrolysis membranes with high efficiency have been developed during the last several years. For example, supported membranes such as PFSA ionomer incorporated in an engineering plastic support exhibited high mechanical strength and very low dimensional changes in wet/dry or freeze–thaw cycling. Alternative organic electrolysis membranes are hydrocarbon-based membranes such as biphenyl sulfone membrane (BPSH) and hydrocarbon/phosphonate membrane. Scheme 11 shows the chemical structure of a typical PFSA. The requirements of proton exchange membrane for efficient service operation include high proton conductivity for ion transportation, high oxidative stability, high mechanical and thermal stability, low permeability or high barrier properties to gases to prevent mixing of the ­resultant gases, electric insulator to prevent short circuits within the cell, and high thermomechanical stability to t­olerate the vigorous conditions in a PEM electrolysis cell.[41] A recent development in proton exchange membrane water electrolysis is the design and development of microblock aromatic ionomers in which sulfonated and nonsulfonated segments are repeated along the polymer chain.

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Electric

e

current

–

e

H

– e

H

2 H2O

O2 +

+ +

H

4H++ 4e–

–

+

H

4H++ 4e–

+

2 H2

– e Anode

Electrolyte

Cathode

Fig. 2  Schematic representation of PEM electrolysis cell

C F2

F2 C X

F C O

F2 C y CF2

F C

F2 C

O C z F 2

SO3H

CF3

Scheme 11  Chemical structures of a typical PFSA

Such ionomers exhibit a much higher degree of microphase separation between hydrophilic and hydrophobic regions compared to conventional random copolymers that results in lower swelling ratios and improved mechanical stability in hydrogen fuel cell and water electrolysis applications.[42] Lithium Polymer Batteries An electric battery is a device constituted by one or more electrochemical cells connected to an external circuit delivering electrical energy to devices that need it to perform required work. Indeed, chemical reactions are occurred at the electrodes, and when ions are passed through the electrolyte, generated electrons are released into the external circuit. A lithium-ion battery is a rechargeable battery where charge and discharge processes are happened via transfer of ions from negative electrode to positive electrode and vice versa, respectively. A lithium polymer battery or Li-polymer battery, like other types of batteries, is a device in which

chemical energy converts into the electricity by transfer of lithium ions through a polymer electrolyte and release of generated electrons into the external circuit. A solid or gel-like polymer electrolyte provides a conductive medium for ion transfer between two electrodes. Indeed, ­Li-polymer batteries follow the same chemical mechanism as lithium-­ ion batteries since the materials used for cathode and anode are almost the same. Thus, L ­ i-polymer batteries are not ­classified into a distinct class from ­lithium-ion batteries. Lithium polymer batteries provide several distinct advantages for manufacturers because they can be easily produced in almost any desired shape and size to fit in an array of device, and this allows them to be suitable for many diverse portable applications. For instance, the weight and space needs of mobile phones, laptops, and tablet computers are completely met once these batteries are applied. Another advantage of lithium polymer cells over conventional nickel–cadmium and nickel–metal hydride cells is the much lower rate of self-discharge. A typical lithium polymer cell has four main components including positive and negative electrodes, a separator, and an electrolyte. Polymer electrolytes are based on different polymers such as polyethylene oxide, poly(acrylonitrile) (PAN), poly(methyl methacrylate) (PMMA), or PVDF. The SPE can generally be categorized into three types: dry SPE, gelled SPE, and porous SPE. For example, an SPE can be a compound of lithium bis(fluorosulfonyl) imide and a high-molecular-weight poly(ethylene oxide), or a high-molecular-weight poly(trimethylene carbonate).[43] Apart from the electrolyte, the separator in many cell types is made up of a polymer, e.g., a microporous film of polyethylene or polypropylene. Separator is usually

covered in an electrolytic gel that could also serve as a catalyst and reduce the energy barrier of the chemical reactions between the cathode and the anode. Moreover, positive and negative electrodes contain a polymer binder to uniformly bind electrode components. PVDF, e.g., provides an efficient cohesion of metal oxide and additives in the cathode, and hence mechanical integrity of the electrode structure. Thus, even in the case of liquid electrolyte cells, there are still some polymer components playing critical roles in the cells. In the following section, a brief introduction, synthetic routes, and properties of the two most widely used polymers for the electrolyte of Li-polymer batteries are reviewed. Poly(acrylonitrile) PAN is a semicrystalline thermoplastic polymer synthetized by free radical polymerization of acrylonitrile monomer. It has a glass transition temperature of about 95°C and a melting temperature above 300°C. It has a good balance of desirable properties including good processability, high mechanical strength, good thermal stability, and oxidative resistance as well as high electrochemical stability. Owing to these great properties, PAN is used in a wide range of applications from membrane to textile to reinforced engineering composites.[44] Scheme 12 shows the chemical structure of the repeating unit of PAN. Poly(methyl methacrylate) PMMA is a transparent thermoplastic polymer synthesized by polymerization of methyl methacrylate. It is also known as acrylic glass owing to its excellent transparency and glass-like appearance. It has a combination of advantageous properties such as excellent tensile and flexural strength, good UV tolerance, ease of processability, and low weight and cost. Therefore, PMMA is a versatile polymer that can be used in several applications such as building windows, LCD screens, lens systems, transparent coatings, medical instruments, and vehicle lighting parts. Depending on the synthesis method, PMMA products could be atactic, syndiotactic, or isotactic. Commercial grades of PMMA are atactic and completely amorphous. Scheme 13 shows the chemical structure of the repeating unit of PMMA. For application as a polymer electrolyte for Li-polymer batteries, PMMA blends with PAN, PCV, and PVDF have been studied and showed better performance.[45]

H CH2

C N

C

n

Scheme 12  Chemical structure of the repeating unit of PAN

CH3 H2 C

C O

C

OCH3

n

Scheme 13  Chemical structure of the repeating unit of PMMA

ENERGY STORAGE For many decades, energy storage has been a main concern of nations, especially in developed and underdeveloping countries, throughout the globe. Nowadays, more than ever before, the depletion of fossil fuels and environmental damages caused by an excessive use of these natural resources highlights the importance of energy storage. Until now, various energy storage technologies have been developed in order to save and reduce the consumption of various types of energy, particularly thermal energy. For example, polymer-based foams and nanofoams have been widely studied for thermal insulation of buildings and industrial plants. Phase change materials (PCMs) are another class of energy storage materials developed recently in order to reduce the energy used for heating and cooling of buildings. PCMs undergo a phase change upon a defined temperature variation and release or adsorb ­specific values of heat that conditions the room temperature. Storage of energy carrier gases and liquids has also been the focus of many attempts since the past few decades, and different techniques were developed for this purpose. In the following section, two primary technologies developed so far for energy storage are explained. Gas Storage in Porous Polymers The tremendously growing demand for clean energy throughout the world has made significant emphasis on exploring new methods for clean energy harvesting and storage. For energy storage, there are two ways: (1) storage of energy molecules in the pores of adsorbents, such as storage of hydrogen or methane gas (direct storage), and (2): transformation of energy into chemical bonds within the porous polymers, i.e., a battery-type storage mode (indirect storage). Also, energy can be stored in secondary valences or structural rearrangements within the pores of the porous polymers, e.g., compressed double layer, which is the base of super-capacitors. In terms of energy applications, direct storage of energy carriers such as hydrogen or methane via adsorbents is of prime interest. Adsorbents have found several applications in the fields related to energy such as gas storage and gas separation. For example, metal–organic frameworks (MOFs) have a Langmuir surface area of 10,400 m2 g−1 and have attracted large attention for their outstanding large surface area. However, MOFs have low thermal and chemical

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stability that limits their applications in harsh conditions. Porous polymers are a class of adsorbents having several advantages over other conventional adsorbents such as zeolites, MOFs, or covalent organic frameworks (COFs). High thermal and chemical stability, easy processing, and low cost of porous polymers make them excellent candidates for gas storage applications. In the case of porous polymers, a possibly maximal surface area is required for applications such as gas storage, whereas the pore size and pore size distribution are rather more important factors for applications where molecular selectivity is needed as in the case of gas separation membranes.[46] Hypercross-linked polymers are a classic type of permanent porous polymeric material initially developed for their potential applications in energy and environmental fields such as gas storage, carbon capture, molecular ­separation, and catalysis. Polymers having intrinsic microporosity are an important type of porous polymers synthesized via introduction of bulky groups in the monomer structure and hence prevention of strict polymer chain packing. Remarkable p­ rogress has been occurred during the past few years in design and synthesis of porous polymers and is still going on.[47] Conjugated microporous polymers (CMPs), developed by Cooper’s group, are a type of polymeric f­ rameworks having high microporosity and chemical resistance. These materials have been extensively studied in the field of gas storage owing to their high surface areas and porosity. CMPs based on poly(aryleneethynylene), ­poly(p-dichloroxylene), and 1,3,5-triethynylbenzene were studied by different research groups for their gas storage capacities. Pd-incorporated and Li-doped polymer networks were also investigated to enhance gas storage ­capacities of CMPs. More recently, a porous aromatic framework was developed by Ben et al. via homocoupling of tetrahedral monomers. The porous aromatic framework has an exceptionally high surface area, i.e., a Langmuir surface area of 7,100 m2 g−1 leading to the excellent hydrogen and carbon dioxide storage capacities.[48,49] Thermal Energy Storage (Polymer Insulator) Nowadays, as environmental concerns about global warming and fossil fuel depletion are growing, much attention is attracted to developing technologies with the focus on enhanced energy efficiency of air-conditioning infrastructures, and thus, sustainable energy saving. Prevention of energy lost through walls, roofs, and windows in residential buildings (the largest waste of energy in most buildings) or industrial plants is an extremely important way to save large amounts of energy and hence operating cost. Among various technologies developed for this aim, novel methods for efficient thermal insulation of buildings have a significant impact on reduction of energy consumption, and as a consequence, reduction of greenhouse gas emissions.

Energy: Polymers in Harvesting, Conversion, and Storage

Many types of thermal insulation materials have been considered over the past decades. Among all studied and applied materials, vacuum insulation panels, polymer foams, and aerogel-based materials are the most promising ones. These materials show very low thermal conductivity values as compared with other conventional insulating materials owing to their reduced vacuum and gas phase conductivity. Different types of cellular materials are widely used in thermal insulation to benefit from the desirable insulation capacity of some gases. In other words, thermal conductivity of cellular materials having low density is, in many cases, in the range of that of gases. Polystyrene and polyurethane (PU) foams, e.g., show very low thermal ­conductivity values in the range of those of gaseous phases. Aerogels have outstanding high insulation efficiency; however, their low processability and high price are the limiting factors for their widespread and large-scale applications.[50] On the contrary, polymeric foams have gained much research and industrial attention originating from their excellent processability, lightweight, affordable price, and many other physical advantages. Polymeric nonocellular foams or nanofoams are the most attractive polymeric insulation materials. Such nanoporous foams show very low thermal conductivity, in many cases even lower than that of the relevant gas, originating from the Knudsen effect that limits the heat conduction through a cramped gaseous phase.[51] Thermal conductivity of insulating polymeric foam depends on three parameters: (1) the conductivity of cell gas or gas mixture, (2) the conductivity of polymer frame, and (3) the heat radiation between cells. The conduction of heat in the polymer frame can be reduced by decreasing the density of foam, and the radiation of heat can be reduced by decreasing the size of cells. Nevertheless, the conduction of heat within the cell gas or gas mixture accounts for the main portion of the thermal conductivity of a foam structure. Highly thermal efficient walls and roofs allow for sensible reduction of the size of heating/cooling systems by approximately as much as 35%, and as a consequence, the maximum usage of floor space. Among numerous types of polymers that can be used as polymeric insulating foam, PU, polyisocyanurate, polystyrene, polyvinyl chloride, and polyolefines are the most used ones. PU and polyisocyanurate foams have some of the highest insulation efficiency values among all commercially available insulation products. PU insulation products are probably the most suitable candidates for energy efficient thin wall designs and low profile roofs as well. In the following paragraphs, a brief introduction of PU and its main features are presented. Polyurethane PUs are one of the most versatile condensation polymers available in both forms of thermoplastic and thermoset

O C

HN

HN

O C O

CH2

CH2 O n

Scheme 14  Chemical structure of a typical PU repeating unit

products. They are synthesized mainly from polycondensation reaction between polyol and diisocyanate in the presence of a suitable catalyst or by activation with ultraviolet light. Since a wide variety of diisocyanate and polyols are available, various PUs can be synthesized to meet the requirements of specific applications. Scheme 14 shows the chemical structure of a typical PU repeating unit. In addition to the excellent characteristic features of PUs such as high flexibility and ease of processability, favorable mechanical performance, abrasion and impact resistance, and high load bearing capacity, PU foams have particularly good thermal insulating properties. Such a combination of advantageous properties and diverse manufacturing processes such as spraying and molding techniques makes it possible to design and develop PU thermal insulating products for a variety of applications. Spray products, e.g., can be applied to many substrates, and other insulation ­products can be molded to obtain desired shapes and sizes.[52] CONCLUSION As explained throughout the entry, polymers are versatile materials with exceptional desired properties that can be used in different energy fields, namely, energy harvesting, energy conversion, and energy storage. In fact, many other types of materials such as metals, glass, and ceramics can be substituted by polymers in various energy-related applications. It is clear that to fulfill the ever-increasing demand of energy, novel technologies and materials with improved properties are continuously required. Polymers are undoubtedly the best materials of choice to be used in many research and industrial energy-related applications, where high mechanical and physical performance, excellent processability, and lightweight are simultaneously required. The application window of polymers and polymer composites in energy will be broaden remarkably in future, and more progresses will take place in polymer synthesis and processing ­techniques for specific markets. REFERENCES 1. Helgesen, M.; Søndergaard, R.; Krebs, F.C. Advanced materials and processes for polymer solar cell devices. J. Mater. Chem. 2010, 20 (1), 36–60.

2. Jørgensen, M.; Norrman, K.; Krebs, F.C. Stability/­ degradation of polymer solar cells. Sol. Energy Mater. Sol. Cells 2008, 92 (7), 686–714. 3. Kudo, N.; Shimazaki, Y.; Ohkita, H.; Ohoka, M.; Ito, S. Organic–inorganic hybrid solar cells based on conducting polymer and SnO2 nanoparticles chemically modified with a fullerene derivative. Sol. Energy Mater. Sol. Cells 2007, 91 (13), 1243–1247. 4. Hadziioannou, G.; Van Hutten, P. Semiconducting ­Polymers; Wiley VCH: Weinheim, New York, 2000. 5. Stübinger, T.; Brütting, W. Exciton diffusion and optical interference in organic donor–acceptor photovoltaic cells. J. Appl. Phys. 2001, 90 (7), 3632–3641. 6. Xu, T.; Qiao, Q. Conjugated polymer–inorganic semiconductor hybrid solar cells. Energy Environ. Sci. 2001, 4 (8), 2700–2720. 7. Song, M.Y.; Kim, J.K.; Kim, K.J.; Kim, D.Y. Photovoltaic characteristics of TiO2 /conjugated polymer junctions. Synth. Met. 2003, 137 (1–3), 1387–1388. 8. Anderson, N.A.; Hao, E.; Ai, X.; Hastings, G.; Lian, T. Ultrafast and long-lived photoinduced charge separation in MEH-PPV/nanoporous semiconductor thin film composites. Chem. Phys. Lett. 2001, 347 (4), 304–310. 9. Beek, W.J.; Wienk, M.M.; Janssen, R.A. Hybrid polymer solar cells based on zinc oxide. J. Mater. Chem. 2005, 15 (29), 2985–2988. 10. Longo, C.; De Paoli, M.A. Dye-sensitized solar cells: A successful combination of materials. J. Braz. Chem. Soc. 2003, 14 (6), 898–901. 11. Nogueira, A.F.; Longo, C.; De Paoli, M.A. Polymers in dye sensitized solar cells: Overview and perspectives. Coord. Chem. Rev. 2004, 248 (13), 1455–1468. 12. Pichot, F.; Ferrere, S.; Pitts, R.J.; Gregg, B.A. Flexible solid-state photoelectrochromic windows. J. Electrochem. Soc. 1999, 146 (11), 4324–4326. 13. Shah, A.V.; Schade, H.; Vanecek, M.; Meier, J.; Vallat‐­ Sauvain, E.; Wyrsch, N.; Kroll, U.; Droz, C.; Bailat, J. Thin-film silicon solar cell technology. Prog. Photovoltaics Res. Appl. 2004, 12 (2–3), 113–142. 14. Mishnaevsky, L.; Brøndsted, P.; Nijssen, R.; Lekou, D.J.; Philippidis, T.P. Materials of large wind turbine blades: Recent results in testing and modeling. Wind Energy 2012, 15 (1), 83–97. 15. Manwell, J.F.; McGowan, J.G.; Rogers, A.L. Wind Energy Explained: Theory, Design and Application; John Wiley & Sons: Chichester, 2010. 16. Burton, T.; Jenkins, N.; Sharpe, D.; Bossanyi, E. Wind Energy Handbook; John Wiley & Sons: Chichester, 2011. 17. Langemeier, P.; Scheuer, C. Big challenges: The role of resin in wind turbine rotor blade development. Reinf. Plast. 2010, 54 (1), 36–39. 18. Hayman, B.; Wedel-Heinen, J.; Brøndsted, P. Materials challenges in present and future wind energy. MRS Bull. 2008, 33 (4), 343–353. 19. Mishnaevsky, L., Jr.; Favorsky, O. Composite Materials in Wind Energy Technology; Riso Nantional Laboratory for Sustainable Energy, Technical University of Denmark: Roskilde, 2011. 20. Stewart, R. Thermoplastic composites—Recyclable and fast to process. Reinf. Plast. 2011, 55 (3), 22–28.

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21. Brøndsted, P.; Lilholt, H.; Lystrup, A. Composite materials for wind power turbine blades. Annu. Rev. Mater. Res. 2005, 35, 505–538. 22. Mehdipour-Ataei, S.; Tabatabaei-Yazdi, Z. Heat resistant polymers. In Encyclopedia of Polymer Science and Technology; Mark, H.F.; Ed.; Wiley: New York, 2015, 1–31. 23. May, C. Epoxy Resins: Chemistry and Technology; CRC press, Marcel Dekker, Inc.: New York, 1987. 24. Jureczko, M.E.Z.Y.K.; Pawlak, M.; Mężyk, A. Optimisation of wind turbine blades. J. Mater. Process. Technol. 2005, 167 (2), 463–471. 25. Gupta, M. N.; Suman; Yadav, S.K. Electricity generation due to vibration of moving vehicles using piezoelectric effect. Adv. Electron. Electr. Eng. 2014, 4 (3), 313–318. 26. Dineva, P.S.; Gross, D.; Müller, R.; Rangelov, T. Dynamic Fracture of Piezoelectric Materials: Solution of Time-­ Harmonic Problems via BIEM; Springer International Publishing: Switzerland, 2014. 27. Sakamoto, W.K.; Souza, E.D.; Das-Gupta, D.K. Electroactive properties of flexible piezoelectric composites. Mater. Res. 2011, 4 (3), 201–204. 28. Ramadan, K.S.; Sameoto, D.; Evoy, S. A review of piezoelectric polymers as functional materials for electromechanical transducers. Smart Mater. Struct. 2014, 23 (3), 1–26. 29. Larminie, J.; Dicks, A.; McDonald, M.S. Fuel Cell Systems Explained, Vol. 2; John Wiley & Sons: Chichester, 2003. 30. Chen, J.; Asano, M.; Yamaki, T.; Yoshida, M. Preparation of sulfonated crosslinked PTFE-graft-poly(alkyl vinyl ether) membranes for polymer electrolyte membrane fuel cells by radiation processing. J. Membr. Sci. 2005, 256 (1), 38–45. 31. Xing, P.; Robertson, G.P.; Guiver, M.D.; Mikhailenko, S.D.; Wang, K.; Kaliaguine, S. Synthesis and characterization of sulfonated poly(ether ether ketone) for proton exchange membranes. J. Membr. Sci. 2004, 229 (1), 95–106. 32. Genova-Dimitrova, P.; Baradie, B.; Foscallo, D.; Poinsignon, C.; Sanchez, J.Y. Ionomeric membranes for proton exchange membrane fuel cell (PEMFC): Sulfonated polysulfone associated with phosphatoantimonic acid. J. Membr. Sci. 2001, 185 (1), 59–71. 33. Oroujzadeh, M.; Mehdipour-Ataei, S.; Esfandeh, M. Microphase separated sepiolite-based nanocomposite blends of fully sulfonated poly(ether ketone)/non-­sulfonated poly(ether sulfone) as proton exchange membranes from dual electrospun mats. RSC Adv. 2015, 5 (88), 72075–72083. 34. Zhang, J.; Tang, Y.; Song, C.; Zhang, J. Polybenzimidazole-membrane-based PEM fuel cell in the temperature range of 120–200°C. J. Power Sources 2007, 172 (1), 163–171. 35. Yu, S.; Benicewicz, B.C. Synthesis and properties of functionalized polybenzimidazoles for high-temperature ­PEMFCs. Macromolecules 2009, 42 (22), 8640–8648. 36. Mader, J.A.; Benicewicz, B.C. Sulfonated polybenzimidazoles for high temperature PEM fuel cells. Macromolecules 2010, 43 (16), 6706–6715. 37. Akbarian-Feizi, L.; Mehdipour-Ataei, S.; Yeganeh, H. Survey of sulfonated polyimide membrane as a good candidate for Nafion substitution in fuel cell. Int. J. Hydrogen Energy 2010, 35 (17), 9385–9397.

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38. Gode, P.; Ihonen, J.; Strandroth, A.; Ericson, H.; Lindbergh, G.; Paronen, M.; Sundholm, F.; Sundholm, G.; Walsby, N. Membrane durability in a PEM fuel cell studied using PVDF based radiation grafted membranes. Fuel Cells 2003, 3 (1–2), 21–27. 39. Gubler, L.; Slaski, M.; Wallasch, F.; Wokaun, A.; Scherer, G.G. Radiation grafted fuel cell membranes based on co-grafting of α-methylstyrene and methacrylonitrile into a fluoropolymer base film. J. Membr. Sci. 2009, 339 (1), 68–77. 40. Grigoriev, S.A.; Porembsky, V.I.; Fateev, V.N. Pure hydrogen production by PEM electrolysis for hydrogen energy. Int. J. Hydrogen Energy 2006, 31 (2), 171–175. 41. Sapountzi, F.M.; Gracia, J.M.; Fredriksson, H.O.; Niemantsverdriet, J.H. Electrocatalysts for the generation of hydrogen, oxygen and synthesis gas. Prog. Energy ­Combust. Sci. 2017, 58 (1), 1–35. 42. Smith, D.W.; Oladoyinbo, F.O.; Mortimore, W.A.; Colquhoun, H.M.; Thomassen, M.S.; Ødegård, A.; Guillet, N.; Mayousse, E.; Klicpera, T.; Hayes, W. A microblock ionomer in proton exchange membrane electrolysis for the production of high purity hydrogen. Macromolecules 2013, 46 (4), 1504–1511. 43. Stephan, A.M. Review on gel polymer electrolytes for lithium batteries. Eur. Polym. J. 2006, 42 (1), 21–42. 44. Raghavan, P.; Manuel, J.; Zhao, X.; Kim, D.S.; Ahn, J.H.; Nah, C. Preparation and electrochemical characterization of gel polymer electrolyte based on electrospun polyacrylonitrile nonwoven membranes for lithium batteries. J. Power Sources 2011, 196 (16), 6742–6749. 45. Rao, M.; Geng, X.; Liao, Y.; Hu, S.; Li, W. Preparation and performance of gel polymer electrolyte based on electrospun polymer membrane and ionic liquid for lithium ion battery. J. Membr. Sci. 2012, 399 (5), 37–42. 46. Ma, S.; Zhou, H.C. Gas storage in porous metal–organic frameworks for clean energy applications. Chem. Commun. 2010, 46 (1), 44–53. 47. Lu, W.; Yuan, D.; Zhao, D.; Schilling, C.I.; Plietzsch, O.; Muller, T.; Bräse, S.; Guenther, J.; Blümel, J.; Krishna, R.; Li, Z. Porous polymer networks: Synthesis, porosity, and applications in gas storage/separation. Chem. Mater. 2010, 22 (21), 5964–5972. 48. Cooper, A.I. Conjugated microporous polymers. Adv. Mater. 2009, 21 (12), 1291–1295. 49. Ben, T.; Ren, H.; Ma, S.; Cao, D.; Lan, J.; Jing, X.; Wang, W.; Xu, J.; Deng, F.; Simmons, J.M.; Qiu, S. Targeted synthesis of a porous aromatic framework with high stability and exceptionally high surface area. Angew. Chem. 2009, 121 (50), 9621–9624. 50. Baetens, R.; Jelle, B.P.; Gustavsen, A. Aerogel insulation for building applications: A state-of-the-art review. Energy Build. 2011, 43 (4), 761–769. 51. Forest, C.; Chaumont, P.; Cassagnau, P.; Swoboda, B.; Sonntag, P. Polymer nano-foams for insulating applications prepared from CO2 foaming. Prog. Polym. Sci. 2015, 41 (5), 122–145. 52. Papadopoulos, A.M. State of the art in thermal insulation materials and aims for future developments. Energy Build. 2005, 37 (1), 77–86.

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Energy: Polymers in the Active Layer of Solar Cells Bianca P. Santos and Maria de Fatima V. Marques Institute of Macromolecules Professor Eloisa Mano, Universidade Federal do Rio de Janeiro (UFRJ), Rio de Janeiro, Brazil

Abstract Polymeric solar cells have become a promising technology for converting photons energy from sunlight into electrical energy due to mechanical flexibility, low production cost, lightweight, and environmentally friendly. One of the most developed areas is the active layer composed of a conjugated polymer as the electron donor, and usually the fullerene derivatives as the electron acceptor, forming the bulk heterojunction (BHJ). The development of polymers with donor and acceptor building blocks has become one of the progresses in the area of organic solar cells. This strategy allows to adjust the highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels, shift the absorption bands to length of larger waves where they promote greater collection of photons, and, consequently, increase the energy conversion efficiency of solar cells. The optimization of the active layer using various strategies such as different types of solvents, use of additives, insertion of side groups in the polymer chains, and configuration of the device is to improve the photovoltaic properties. In addition, an inverted configuration of the photovoltaic cell has been presented as a prosperous path to promote greater durability of the device due to the use of materials in the layers that are more important or the chemical inertia to moisture and oxygen. Thus, a review of the main current studies, as well as the study of the active layer highlighting the applied conjugated polymers, is carried out in the field of BHJ-type solar cells. Keywords: Active layer; D-A copolymer; Device configuration; Organic solar cells; Photovoltaic polymer; Stability; Terpolymers.

INTRODUCTION In the past two decades, many efforts have been employed to improve the efficiency and durability of polymer solar cells (PSCs). The organic photovoltaic cell (OPV) is a promising alternative for manufacturing devices with high flexibility, low weight, resistant, and environmentally friendly.[1] The evolution of polymer photoactive layer is shown in Fig. 1. Most of the OPVs are based on the bulk heterojunction (BHJ) proposed by Yu et al.[3] in 1995. The term “bulk heterojunction” corresponds to a large contact area between the donor and the acceptor. In this type of solar cell, the electron donor (photovoltaic polymer) is mixed with an acceptor (fullerene) soluble in an organic solvent and then placed as a thin film on the indium tin oxide (ITO) previously deposited on glass or on a flexible surface of ­polyethylene terephthalate (PET).[4]

Encyclopedia of Polymer Applications, First Edition DOI: 10.1201/9781351019422-140000280 Copyright © 2018 by Taylor & Francis. All rights reserved.

MECHANISM OF INTERACTION BETWEEN PHOTON AND ELECTRON IN OSC The mechanism of converting solar light into electrical energy in an organic solar cell (OSC) can be divided into four steps:[5] 1. Absorption of the photons leading to the generation of exciton 2. Exciton diffusion to the donor/acceptor heterojunction 3. Dissociation of the exciton in heterojunction (free charge carriers) 4. Transport of charge carriers to the respective ­electrodes and collection of charges Figure 2 shows the mechanism of interaction between photons and electrons in BHJ solar cells. By absorbing light, an electron undergoes photoinduced excitation

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Monolayer PCE = 0.1%

Bilayer PCE = 1%

Cathode

Cathode

ETL Polymer or

ETL Donor Acceptor HTL Anode Substrate

small molecule HTL Anode Substrate

Bulk heterojunction PCE > 10%

Cathode Donor

ETL

Acceptor HTL Anode Substrate

Fig. 1  Evolution of the active organic solar cell layer (energy conversion performance = PCE) Source: The authors.

hv

LUMO

Energy

LUMO

Donor Acceptor

ETL

Cathode

LD Anode HTL

HOMO HOMO

Fig. 2  Mechanism of interaction between the photon and the electron in OSC [6]

from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) of the macromolecule (conjugated polymer), forming the exciton (coulombically bound electron–hole).[6]

After the exciton generation, the pair (electron–hole) needs to be separated so as to produce the free charge carriers. In order to achieve the dissociation of the exciton, an acceptor is required to ensure the formation of an internal

Cathode

(a)

Acceptor

2LD

Donor

(b)

Anode

Fig. 3  Schematic morphology of a BHJ solar cell [9]

electric field, so these two materials with aligned bands are placed adjacent to each other in the so-called bulk ­heterojunction.[7,8] The diffusion length (LD), around 10 nm, corresponds to the distance that the excitons can diffuse to the donor– acceptor interface in the organic materials. If the excitons are generated at a greater distance from the heterojunction, an electron–hole combination can occur and result in low dissociation efficiency. Thus, the active layer must have small thickness to ensure phase separation between the acceptor and the donor within the dissociation length of the exciton.[9] After the dissociation of the excitons, the electrons and holes are transported through the electron transport layer (ETL) and the hole transport layer (HTL), which corresponds to the collection step, and are then forwarded to the respective electrodes.[7,8] ACTIVE LAYER The photoactive layer of the PSC is composed by a blend of a donor-conjugated polymer and an electron acceptor (fullerene derivative or small molecules). The ideal arrangement of an active layer is shown in Fig. 3a, which would perform well since the donor–acceptor arrangement could ensure good charge transport. However, this morphology is difficult to be achieved because the manufacture of devices often depends on phase separation of the donor and acceptor materials during the formation of the active layer. A real morphology of the active BHJ cell layer is shown in Fig. 3b. The active layer is a bicontinuous interpenetrating network composed of a donor and an acceptor with a maximum interfacial area to ensure good dissociation of the exciton and a mean domain size compatible with the diffusion of the exciton (~10 nm).[9] The morphology of the active layer depends on the interaction between intrinsic and extrinsic properties. Intrinsic properties are the crystallinity of two materials, relative miscibility, molar mass, and chemical structure of

components. Extrinsic factors include all external influences associated with the fabrication of device, such as solvent choice, total concentration of the blend components, preparation process (spin coating, screen printing, etc.), evaporation rate of the solvent, as well as the thermal and/ or solvent treatment. Depending on the type of solvent, the solubility of each donor/acceptor material, as well as the good interaction and miscibility between them may be different. In addition, the boiling temperature of each solvent plays an important role in the formation of film and ­morphological organization of OPV–BHJ.[10] Among the most commonly used solvents are chlorinated solvents such as chlorobenzene (CB), dichlorobenzene (DCB), trichlorobenzene (TCB), and chloroform (CF). These solvents are considered to be good solvents for a wide variety of organic semiconductors; in addition, other aromatic solvents such as xylene and toluene may be employed. Many studies have been developed to evaluate the effect of solvent on donor and acceptor solubility properties as well as the morphology of the resulting active layer. The improvement in the performance of poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene], MDMO-PPV:PC60BM, was reported by replacing toluene with CB during the deposition of the active layer, and the energy conversion performance obtained was 0.9% and 2.5%, respectively. Through transmission electron microscopy (TEM) images, the influence of solvent on the size and phase separation of the active layer was observed (Fig. 4).[11] Ruderer and collaborators [12] studied the influence of solvents on the morphology and performance of BHJ cell based on the photoactive layer P3HT:PCBM: a mixture of poly(3-hexylthiophene), conductive polymer, and PCBM, a fullerene derivative C60 ([6,6]-phenyl-C61-butyric methyl ester). The solvents employed were CF, toluene, CB, and xylene using the conventional configuration ITO/PEDOT:PSS/ P3HT:PCBM/Al. The films made using CF solvent presented a layer with disadvantageous material distribution, generating accumulation of PCBM and phase separation.

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(a)

(b)

Fig. 4  TEM image (1.8 μm × 1.8 μm) of MDMO-PPV:PCBM: (a) toluene and (b) CB on the PET substrate [11]

Therefore, these films had the worst photovoltaic performance (PCE = 0.43%). Similar PCEs were obtained from CB, xylene, and toluene solutions (~2.6%). The choice of solvent can be a determining factor in PCE of a solar cell. Kadem et al.[13] reported the improvement of PCE in device composed of ITO/PEDOT:PSS/P3HT:PCBM/Al employing different types of solvents. This configuration is widely employed, where ITO is the transparent anode, the film of the ionomer blend poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (polymeric structure shown below) is used as the HTL, and aluminum is the cathode. O

O

O S

S O

O S

O

O S

O

O

x

y

S O

PEDOT:PSS

O S O O

O S O OH

The influence of the solvent in energy conversion efficiency was verified through scanning electron microscopy (SEM) images (Fig. 5). Using CF and CB, no phase separation or aggregation was observed in the study. In these films, the lighter regions correspond to the PCBM; this difference between the regions of the film is due to different boiling points between the solvents, which cause different drying processes. PCBM molecules can be carried by the solvent in a vertical direction leading to a rough surface film and large aggregates. The composite film with the CF solvent presented a more uniform morphology when compared with CB film, which indicates that the boiling point plays an important role in the morphology of the formed film. On the other hand, the film composed of the blends with DCB showed the separation of phases.

Some aggregates are formed in this film as a result of the low evaporation rate of the DCB, leading to competition between the phase (amorphous and crystalline regions) in film.[13] The effect of solvent is also observed in the HTL. Wang et al.[14] studied surface modification of the PEDOT:PSS with solvent dimethylsulfoxide (Fig. 6), where an increase in PCE from 5.95% to 6.52% was obtained. Zhao et al.[15] studied the use of the solvent 1,2,4-trimethylbenzene (TMB) with 1-phenylnaphthalene (PN) as an additive to replace the halogenated solvents (CB and DCB, among others), which are toxic and costly. The configuration of BHJ solar cell was ITO/ZnO/PffBT4T-C9C13 (poly[(5,6-difluoro-2,1,3-benzothiadiazol-­4,7-diyl)-alt(3,3‴-di(2-nonyltridecyl)-2,2′;5′,2″;5″,2‴-quaterthiophen5,5‴-diyl)]):PC71BM/V2O5/Al. The highest energy conversion efficiency (11.7%) was obtained using TMB– PN. The device without the PN additive presented a ­significant reduction in PCE to 6.4%. The use of additive provides improved device performance due to simultaneous reduction in the domain and higher molecular orientation of the polymer:fullerene interface. Better molecular orientation and high domain purity are the key factors for efficient charge generation and to minimize recombination. In this work, the influence of the alkyl chain length of the polymer on the morphology and performance of the BHJ solar cell was also reported. Poly(benzothiadiazole) (PBT)-4T-C8C12, PBT-4T-C9C13, or PBT-4T-C10C14 as electron donor polymers (structures shown below) were employed using TBM–PN or CB–DIO (diiodooctane). Through the obtained results, the C9C13 side chain presented the best result, having obtained good orientation, small domain size, and an increase in the homogeneity of the BHJ film.

0.44%

0.96%

CB

DCB

0.36%

1.42%

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CF

CF:CB

PCE

Fig. 5  SEM image of P3HT:PCBM using different solvents: CB, CF, DCB, and CF:CB, and their efficiency of conversion energy (PCE)[13]

Fig. 6  Solar cell configuration and chemical structure [14]

PHOTOVOLTAIC POLYMERS The performance of BHJ solar cell is affected by several factors highlighted the structure of electron polymer donor. The photovoltaic polymer should have low HOMO to provide high open-circuit voltage (Voc), wide absorption in the solar spectrum, low bandgap, high mobility of charge carriers, and a LUMO that promotes sufficient compensation during charge separation.[16] Structural scheme of conjugated polymer backbone typically used as an electron donor is shown in Fig. 7. Generally, a conjugated polymer may be divided into three components: main chain, side chain, and substituents. The conjugated main chain is the most important

component because it dictates the physical properties of conjugated polymer as energy level, bandgap, and inter- and intramolecular interactions. The side chains increase some properties such as molar mass, solubility, processability, and interaction with the acceptor. The substituents are used to adjust the physical properties of the conjugated polymers as energy levels, bandgap, and mobility. [17] The main characteristic that allows the conjugated polymer or photovoltaic polymer to be used in photovoltaic cells is the alternation of single (σ) and double (σ and π) chemical bonds in the backbone, where the π bond is responsible for properties that make these materials ­candidates for optoelectronic applications.

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Homopolymers Poly(phenylenevinylene) (PPV), polythiophene (PT), polycarbazole (PCz), polyfluorene (PF), PBT, and poly(benzodithiophene) (PDT) are the most reported polymers in literature (Fig. 8). PPV-based polymers were the first electron donor materials applied in PSC; but because of the low mobility of holes, limited range of light absorption, and insolubility in organic solvents, it has been suggested to employ Side chain Main chain

Substituent groups

Fig. 7  Structural scheme of a conjugated polymer typically used in PSC [17]

Fig. 8  Conjugated polymers used in PSC [18]

modified PPVs such as poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH–PPV) and MDMO– PPV. The addition of branching improved the solubility of polymer, reducing the difference between HOMO and LUMO and shifting the emission color to larger wavelengths.[18] The MDMO-PPV:PCBM as the photoactive layer achieved the PCE of 2.5%. Thiophene is an important heterocyclic component to form donor conjugate with high efficiency. As mentioned, in order to improve the solubility of the PT, different R groups are employed (alkyl, alkyloxy, acid, ester, and phenyl). The thiophene ring has five atoms, and the monomer is polymerized through positions 2 and 5, where position 2 corresponds to head and 5 corresponds to tail (Fig. 9). P3HT is the most representative conjugated polymer applied in OSC. This is an excellent electron donor and mostly used in OSCs due to good solubility, high hole mobility, and good crystallinity, which is attributed to its well-defined molecular structure. P3HT is able to absorb photons at longer wavelength compared to PPV derivatives.[20] The best energy conversion efficiency was 5% using the P3HT:PCBM system.[21] According He et al.[22], P3HT could reach over 8% of PCE if new fullerene derivative acceptors were developed.

In this way, some advances have been achieved in the development of new photovoltaic materials to optimize the structure of the conjugated polymer. However, the limitation of this electron donor is related to the high energy level of HOMO, which interferes with the open-circuit voltage in the absorption spectrum, and large amount of energy is lost when a photoexcited electron is transferred between LUMO levels of P3HT and PCBM. The main routes used to produce this polymer are those of Rieke, McCullough, and Grignard due to the simplicity, high yield, and easy adjustment of the molar mass through changes in the reaction conditions. Other routes

Fig. 9  PT and its derivatives HH, TT, and HT couplings [19]

Fig. 10  Absorption spectrum of PTB7-DT and PTB7[24]

may be employed to synthesize conjugated polymers, e.g., the Kumada catalyst-transfer polycondensation, developed by the McCullough and Yokozawa groups, also known as quasi-living polymerization.[23] Liu et al.[24] studied a new low bandgap conjugated polymer, the poly[[4,8-bis[(5-ethylhexyl)thienyl]benzo­ [1,2-b;3,3-b]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl) carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7-DT). This synthesized polymer is based on poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl] [3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7), where the main difference between the two polymers is the presence of 5-alkylthiophene-2-yl side chains in the polymer PTB7-DT, which provide better absorption of photons and benefit the mobility of charges. The UV–Vis spectroscopy technique (Fig. 10) was able to observe absorption band shift, where the maximum peak at 704 nm was obtained for the functionalized polymer, which is attributed to the better planarity achieved due to the presence of side chains. In addition, the bandgap was reduced from 1.68 (PTB7) to 1.59 eV (PTB7-DT). This result indicates greater hole mobility and, consequently, improves the photovoltaic properties of device. The energy conversion efficiency employing the polymers PTB7 and PTB7-DT as the photoactive layer was evaluated using the same configuration. The device with the active layer composed of the polymer PTB7-DT presented a PCE of 10.12%, while using the polymer PTB7 the efficiency obtained was

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7.4%, indicating the influence of ­optimization of polymers applied in OPV. Polymers based on carbazole (Cz) units are very important in OSC because they have high thermal stability, good hole transport, and high photoconductivity. It is a material with relatively low cost and contains nitrogen atom, which can be easily functionalized with a variety of substituents such as alkyl, alkyloxy, and alkylated benzene, thus increasing the solubility and stability of the material. Cz can be prepared by the substitution at 3.6, 2.7, and 1.8 position (structures shown below). The Cz unit is electron rich and binds the electron-deficient fullerene unit, creating a donor–acceptor arrangement, and introduces the intramolecular charge transfer (ICT) effect, which enhances absorption in the UV spectrum.[4]

N H 3.6 carbazole

N H 2.7 carbazole

N H 1.8 carbazole

PF, on the other hand, presents high hole mobility, good chemical and thermal stability, and high absorption coefficient, and can act as electron donors in OPVs. Moreover, PF, in general, has high bandgap exhibiting good optical absorption in the blue region in the visible spectrum. In addition, it exhibits high stability due to the rigid and planar biphenylic structure of the fluorene unit. The introduction of substituent at the carbon 9 position renders the PF derivatives soluble and therefore easy to process in organic solvents. The main limitation is the absence of absorption bands in the low-energy region in which there is abundance of solar irradiation (infrared region in the spectrum). However, many studies have been dedicated to associate its high charge mobility and increase light absorption by shifting the absorption band to the infrared region.[17] Both homopolymers with Cz and fluorene units show a narrow absorption, thus limiting the OPV performance.

D-A Copolymers The major development strategy in designing low bandgap conjugated polymers is to alternate a conjugated electron-rich donor (D) unit and a conjugated electron-­ deficient one (A), called D-A copolymer. Monomers such as Cz and fluorene when copolymerized with an electron acceptor unit reduce the bandgap and improve absorption (Fig. 11).[18] The most convenient way to improve efficiency is by means of construction of D-A copolymers using the coupling strategies. Cross-coupling reactions comprise a transformation group for the formation of C–C bonds based on the transmetalation of metallic nucleophilic components with organic electrophiles in the presence of transition metal compounds as catalyst (nickel or palladium). The most efficient and widely used methods for preparing alternating copolymers are those of Stille and Suzuki. Suzuki coupling reactions are the most widely utilized to produce PF-based copolymers. The main reagents used in this reaction are the catalyst Pd(PPh3) 4 used in the presence of K2CO3 and a phase-transfer catalyst, Aliquat 336, in a solvent mixture (toluene/degassed water). During the polymerization process, the fluorenylboronic ester and the aromatic dibromide react with each other to give well-­defined alternating PF copolymers as shown in the ­following figure:[25] O O

B

B R

R

O O

LUMO LUMO

LUMO Eg HOMO

HOMO Donor

Br Ar Br

Aliquat 336 Toluene/H2O

Heeger[26] reported a PCE of 6.1% using (poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′benzothiadiazole) (PCDTBT):PC71BM as the photoactive layer, and Kuznetsov et al.[27] reported that the efficiency of the F10TBT:[70]PCBM system was 4.5%. According to theoretical models, fluorene-based copolymers should give an ultimate efficiency of 8.0%–8.5% in OSC.

E

HOMO

+

Pd(PPh3)4 K2CO3

D-A

Acceptor

Fig. 11  Molecular orbital interactions leading to a smaller bandgap in a D–A conjugated polymer[18]

R

R

Benzothiadiazole (BT) is widely used in the second-generation D-A copolymer as a strong electron-­ withdrawing unit associated mainly with fluorene, Cz, and thiophene units. The benzodithiophene (BDT)-based polymer is a low bandgap material extensively studied due to excellent optical and electronic properties, as well as good stability. A structure analog to the BT is benzoxadiazole (BO), which is used as the electron acceptor in conjugated materials. This polymer shows lower HOMO and LUMO energy levels and higher air stability; nevertheless, the main problem is low solubility that led to the formation of low-molecular-weight polymers, consequently reducing PCE. Cyclopentadithiophene (CPT) shows stronger electron-rich properties and has narrow bandgap; the best performance of a device with this unit achieved a PCE of 6.04% in PCPDTBT/PC71BM using 1% additive, DIO (diioctane).[28] Terpolymers The development of conjugated polymers containing several electron-rich (D) and electron-deficient (A) bonds, the alternating D-A copolymer, plays a crucial role, but the maximum efficiency reached was 9%. A method to increase the optical, electrical, and structural synergism is the addition of a third component in the D-A structure, producing terpolymers. The advantages of employing terpolymers are tuning the absorption range and HOMO/LUMO

energy levels and optimizing the effects between polymer solubility and molecular packing. In fact, the great advance in polymer synthesis was the development of low bandgap D-A copolymers, which allows efficient collection of photons, although some have the ICT between D and A very close to the infrared region, which reduces the absorption near the visible region and Jsc. The incorporation of an additional third unit can extend the light absorption range through the appearance of new π–π* or ICT peaks. Also, optimizing and balancing both polymer solubility and molecular package is possible using terpolymers.[29] The difference between D–A copolymers and terpolymers was reported by Akkuratov et al.[30] The addition of 10% Cz (terpolymer P3) shifts the wavelength to 624 nm compared to the copolymer with fluorene units P2 (609 nm). The introduction of 5% fluorene shifts the absorption band to 643 nm compared to polymer with Cz units P1 (605 nm). This indicates that the Cz and fluorene units in the chain induce some synergistic effects, enhancing the optical properties of the polymer. The best PCE of 7.0% was obtained in terpolymer P3, as shown in Fig. 12. ELECTRON ACCEPTORS The fullerene and its derivatives (Fig. 13) are most widely used as electron acceptors in OSCs, due to high electron affinity, fast electron transfer, and low-energy reorganization.[8]

Fig. 12  Chemical structures of P1 and P2 are copolymers and P3 terpolymer in relation to efficiency[30]

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O O O O C60

PC60BM

PC70BM

ICBA

Fig. 13  Chemical structure of electron acceptors based on fullerene [8]

Table 1  Photovoltaic performance of Cz–fluorene–TTBTBTT-based devices with PC60BM and PC70BM [33] Donor(X:Y ratio):acceptor

Voc (mV)

Jsc (mA/cm²)

FF (%)

PCE (%)

X:Y = 10:90/PC70BM

796

13.1

57

5.9

X:Y = 90:10/PC70BM

729

11.8

52

4.5

X:Y = 10:90/PC60BM

813

10.3

61

5.1

X:Y = 90:10/PC60BM

725

11.5

51

4.3

X = carbazole and Y = fluorene.

Fullerene (C60) presents good electronic mobility and high degree of symmetry, but its use has produced solar cells with low conversion efficiency for electrical energy. In order to overcome this limitation, increase solubility and avoid phase separation, fullerene derivative, [6,6]-phenyl-C71-butyric acid methyl ester (trade name) (PC60BM), has been applied in OPVs. According to He and Li, [31] PC60BM offers advantages such as good solubility in organic solvents (CF, CB, and DCB), high mobility, and electronic affinity. However, its limitation is related to poor absorption in the region of visible spectrum, due to the high degree of symmetry of the C60 and the low energy level of LUMO, resulting in low Voc. In this way, it is very important to develop and synthesize new derivatives of fullerene that strongly absorb light in the visible spectrum and that present a higher LUMO level than PCBM. C84 derivatives have been synthesized as acceptors; however, the PCE obtained was relatively low. The absorption property, especially in the visible region, is very important in photovoltaic materials. The PC70BM derivatives have also been widely used as electron acceptors in PSCs associated with low bandgap donor polymers, aiming to increase the wavelength range of solar collecting and PCE. Many PSCs using PC70BM achieved a 10% increase in PCE compared to PC60BM; however, due to the process of PC70BM purification, its cost becomes relatively high, limiting its large-scale application.[32]

Another promising fullerene acceptor is indene-C60 (IC60BA), which presents good solubility in organic solvents such as CF, higher LUMO level, and high absorption in the visible spectrum compared to PCBM. Kuznetsov et al.[33] synthesized Cz–fluorene–TTBTBTTbased statistical terpolymers (where T = thiophene and B = benzothiadiazole) as electron donors (as shown below) and associated them with two types of electron acceptors, PC60BM and PC70BM. Optical and electronic properties were higher using PC70BM as the electron acceptor in all active layers. In addition, the combination of Cz and fluorene in the polymer backbone produced synergistic effects and improved the energy conversion efficiency of the devices, as can be seen in Table 1. Mi et al.[34] synthesized a new C60 fullerene derivative containing a carbazole group (CBZ-C60) as the electron acceptor and P3HT as the electron donor. The configuration of photovoltaic device was ITO/PEDOT:PSS/ P3HT:CBZ-C60 /LiF/Al. An increase was achieved both in the LUMO energy level of electron acceptor and Voc, compared to devices manufactured using PCBM as the acceptor. Graphene has been considered a promising electron acceptor in OPVs because of its excellent optical, electronic, thermal, and mechanical properties, and can also be applied as a transparent electrode and electron acceptor material. ITO is the most widely used transparent electrode,

but it has great limitations, such as high cost with preparation methods (spraying, evaporation, light deposition, and electroplating). In addition, indium is a limited source in the earth and presents some toxic properties causing risks to health and environment; therefore, the use of graphene would be a good alternative.[35,36] The most widely used electron acceptor is fullerene derivative; however, it has some limitations such as poor absorption of light in the visible spectrum, low energy level LUMO, and being difficult to tune high Voc in BHJ devices. In view of excellent electronic properties such as high mobility and good solubility in organic solution, it is hoped that graphene and its derivatives may be the alternative acceptor material in OPVs. One of the main factors that justifies the use of graphene as an electron acceptor is the size of its structure in relation to donor macromolecule. This factor is very important to form a good donor–­ acceptor interface and nanoscale interpenetrating network, thus ensuring efficient dissociation of excitons.[37] PREPARATION OF ORGANIC PHOTOVOLTAIC DEVICE Organic BHJ photovoltaic cells basically consist of an active layer (donor:acceptor) arranged between two different electrodes. The transparent electrode (anode) allows the diffusion of visible light in the devices; the cathode is responsible for collecting electrons, and the substrate is usually a layer of glass or flexible PET.[2] The BHJ–OPV can present two types of configurations: conventional and inverted (Fig. 14). Generally, in BHJ devices, two layers are inserted: the HTL in which metals with high work function are employed and ETL, which is composed of metals with low

work function, aiming to improve the efficiency of collecting charges carriers and reduce the barrier potential at the interface. The polymer mixture of PEDOT:PSS is usually employed in the HTL (conventional configuration). PEDOT:PSS is a mixture of ionomers, formed by sulfonated polystyrene, partially deprotonated (carries negative charge), and a conjugated polymer-based PT (carries positive charge), and this layer promotes an ohmic contact between the active layer and the anode, in addition to being able to reduce the roughness of surface of ITO electrode, benefiting the transport of holes.[37] The main advantages of the PEDOT:PSS layer are high electrical conductivity, high optical transparency in the visible region, good film-forming capacity, and high resistance to organic solvents, which are used to solubilize the components of active layer. However, when applying the PEDOT:PSS layer, there may be a reduction in the stability of a photovoltaic device, caused mainly by PEDOT:PSS acidity at the anode (ITO), negatively impacting the lifetime of a solar cell. In addition, due to lack of the electrical and structural homogeneity, a loss in the capacity of charge collection may occur. However, some thermally evaporated metallic oxides (MoO3 V2O5, NiO, and WO3) with high transparency and stability are applied as HTL in order to promote an ohmic contact between the active layer and ITO, improving PCE.[24] In the conventional configuration, low-working metals are employed in ETL. In general, lithium, magnesium, barium, or calcium fluoride, which are inserted in the aluminum/photoactive layer interface, are used to increase the efficiency, but they are sensitive to air and humidity, besides the high cost with the thermal deposition method.[39] Conventional device has inherent stability problems; so in the inverted configuration, charge collection is opposite.

Cathode

Anode

ETL

HTL

Active layer

Active layer

HTL

ETL

Anode

Cathode

Substrate

Substrate

Conventional configuration

Inverted configuration

Fig. 14  Structure and configuration of the OPV device: (a) conventional and (b) inverted [38]

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(a)

Conventional

(b)

(c)

Inverted

CH 6 13

O O S S

S S

O

NC

C6H13 O

CN

S

O

O

S

n O

HO

DC-IDT2F

OH N

N N

y

x N

CN

C6H13

C6H13

PBDTTT-CT

NC

N H OH

z

OH

PEIE

Fig. 15  TEM micrographs of PBDTTT-CT:DC-IDT2F (2:1) conventional configuration (a), inverted configuration (b), and chemical structures (c)[41]

Fig. 16  Contact angle of (a) PBDTTT-CT (102.7°) and (b) DC-IDT2F (24.2°)[41]

By inverting the solar cell, a gain in stability is due to applying stable materials (Au and Ag) at the top of the anode exposed to air. Silver is considered the most stable material when compared with other metals. In addition, it is possible to employ techniques in low-cost solution such as spin coating and screen printing, thus reducing the cost of production. In inverted configuration, ZnO and TiOx are more used as ETL due to high optical transparency, high transport mobility, and processability in solution.[40] One of the limitations to use the conventional configuration is related to the oxidation of electrode with low working function (e.g., Al), which may lead to destabilization of the device; however, encapsulation techniques are necessary, making the process very costly. In order to overcome these limitations, the use of the inverted configuration is presented as a viable alternative (SU; LAN; WEI, 2012).[18] Wang et al.[14] fabricated two devices, conventional and inverted, using poly{[4,8-bis-(2-ethyl-hexyl-thiophene-5-yl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl]-alt-[2-(20-ethylhexanoyl)-thieno[3,4-b]thiophen-4,6-diyl]} (PBDTTT-CT)

as a donor polymer and 4,4,9,9-tetrakis(4-hexylphenyl)-indaceno[1,2-b:5,6-b]dithiophene as a central building block, furan as π-bridges, and 1,1-dicyanomethylene-3-indanone as end acceptor groups (DC-IDT2F), obtaining 2.26% and 3.08% of PCEs in the conventional and inverted structures, respectively. The configuration of the conventional and integrated devices was composed of ITO/PEDOT:PSS/ PBDTTT-CT:DC-IDT2F/Ca/Al and ITO/PEIE/PBDTTTCT:DC-IDT2F/MoO3/Ag, respectively. The active layer in the inverted structure had a smoother structure, smaller interconnected domains, and less phase separation (Fig. 15), which explains the improvement in the photovoltaic ­parameters such as JSC, fill factor (FF), and PCE. In addition, the surface energy of PBDTTT-CT was found to be lower than that of DC-IDT2F (Fig. 16), through contact angle analysis and X-ray photoelectron spectroscopy. Then, the PBDTTT-CT polymer tends to aggregate on the top active layer surface, which is close to the anode in an inverted solar cell. As a result, the best morphology was observed in the inverted configuration since it is more favorable for transporting charges.

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Energy: Polymers in the Active Layer of Solar Cells 1061

Fig. 17  Stability ranking of units in the polymer chain [44]

STABILITY AND DURABILITY The most studied parameter in photovoltaic devices is energy conversion efficiency (PCE). The three aspects that need to be associated are efficiency, durability, and cost of OPV. The best lifetime of this kind of device was 7 years, employing carbazole-based polymer (PCDTBT). Earlier, for the device using P3HT in the active layer, the best l­ ifetime was 4 years.[42] However, other parameter such as the stability of a solar cell is also extremely important. The active layer is susceptible to surfer degradation due to the presence of oxygen and moisture. A photochemical stability of different polymers is shown in Fig. 17. The number of publications in this area is still limited, and the mechanism of degradation of the device is not fully understood.[43] PPV derivatives are instable under photooxidative conditions due to the presence of the substituents and the vinylene bonds. The degradation of P3HT occurs by the oxidation of side chain with the formation of hydroperoxides in the benzylic position; nevertheless, this polymer is more stable compared to the PPV derivative. Other polymers with fluorene and CPT units show instability due to the presence of quaternary carbon atoms. The improvement in the stability was observed, replacing quaternary carbon atom by a silicon atom (Si-CPT). The acceptor

group used to form a D–A copolymer was BT, which shows high ­stability because this moiety is free of side chains.[44] The most common polymer applied in the HTL in OPV is PEDOT:PSS. These polymers are susceptible to degradation, as reported by Kawano et al.[45] The devices were composed of ITO/PEDOT:PSS/MDMO-PPV:PCBM/Al. Rapid degradation was observed in cells illuminated under humid conditions either in air or in nitrogen. The devices without a PEDOT:PSS layer indicate that the degradation is associated with water absorption into the PEDOT:PSS layer. METHODS OF PRODUCTION OF DEVICES Most of the reported efficiency of PSC is fabricated on the rigid substrate, which therefore does not fully use the processing advantages of organic semiconducting materials and, as a result, limits their potential applications. In order to fully exploit the potential of PSCs, it is desirable to develop devices on a flexible plastic substrate which are lightweight, mechanically flexible, and compatible with roll-to-roll manufacturing, which can reduce the cost of installation and production of solar cells. Burgués-Ceballos et al.[46] studied different polymers composed of fluorene, Cz, thiophene, and BT units and

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Spin coating

Doctor blade coating

Inkjet printing

Slot die coating

Fig. 18  Methods of layer deposition [47]

employed different deposition methods to produce the ­photovoltaic devices (Fig. 18). The polymers P1, P2, and P3 showed molar mass of 190, 290, and 299 kg/mol, respectively. The highest efficiency using the spin coating technique in the inert (5.88%) and ambient atmosphere (5.44%) was obtained using P3. Also, it was observed that spin coating technique under the inert condition promotes a slight increase in Jsc and a decrease in Voc. This observation was attributed to hydration of PEDOT: PSS, resulting in an increase of its working function. On the other hand, the drop of Jsc could be explained by the reduction of charge carrier mobility, allowing the generation of electronic traps, energy levels located in the bandgap, under light and air exposure. In addition, the use of doctor blade coating was observed to increase PCE and FF to all the polymers compared with slot die coating. According to the literature, applying high temperature in the substrate during doctor blade coating (95°C) and slot die coating (70°C), as well as subjecting samples to post-annealing, reduces morphological differences. Inkjet printing technique showed limitations such as the nozzle clogging due to the high molar mass of polymer, as observed for P3 and P1 polymers. These results indicated that inkjet printing is a more restrictive technique for depositing layers of polymers with high molar mass. It was possible to obtain a satisfactory result using only P2 since this polymer is highly soluble and has longer gelatinization time. The best result of PCE among all polymers and techniques evaluated was 6.31% in P3 employing the doctor blade technique. SOLAR CELL EFFICIENCY In order to obtain good energy conversion efficiency (PCE), the design and synthesis of polymeric components must have some properties such as sufficient solubility to

ensure miscibility between the donor and the acceptor, and high hole mobility for an efficient transport of charges and narrow bandgap, to improve absorption of photon in solar spectrum, aiming at a good absorption of light and an increase in the short-circuit current density (Jsc).[8] The performance of the photovoltaic devices is characterized by the energy conversion efficiency (PCE), calculated using Eq. 1. This is defined as the ratio between the maximum electric power (Pmax) and the incident light power (Pin).[6] PCE =

P máx Pin

(1)

where Jsc, Voc, FF, and Pin are, respectively, the short-circuit current density, the open-circuit voltage, the fill factor, and the incident light power. The short-circuit current density (Jsc) can be defined as the product of extent of spectral absorption with the absorption intensity of the active layer. The control of some optical–electric parameters allows an increase in Jsc, such as reducing bandgap and phase separation and increasing the absorption coefficient and charges transport.[5] Another important parameter is the open-circuit voltage (Voc), which can be estimated through the difference between the HOMO of the donor and the LUMO of the acceptor. Ideally the donor polymer should have the lowest HOMO level to ensure high Voc. However, this reduction of HOMO needs to be controlled since the maximum energy difference between LUMO levels of the donor polymer and the acceptor polymer is ~0.3 eV. In addition, the continuous reduction in the HOMO level of the donor polymer could increase the bandgap, decreasing the ability to absorb light, and also decrease Jsc. The Voc is affected by many factors beyond the HOMO level of the polymer, such as side chains, intercalation distance, and morphology of active layer.[17]

Power

Current

0

Jsc

Jmp *Vmp

F.F.=

PCE=

Jsc *Voc

Pmax Pin

=

Voc

F.F. *Jsc *Vov Pin

Voltage (V)

Fig. 19  Current–voltage (I–V) characteristics and the corresponding power–voltage curve for a BHJ solar cell under illumination [25]

The FF is defined according to Eq. 2 and can be attributed to the active layer morphology and charge transport through the BHJ cell for a given area unit. This parameter represents the quality of photovoltaic solar cell. The morphology of the active layer can be optimized to promote the separation and transport of charge, aiming to maximize FF and Jsc.[48] FF =

Jmáx × Vmáx Jsc × Voc

(2)

where Jmax and Vmax are the current density and voltage at the maximum power output, respectively. These parameters can be obtained through the J–V curve (Fig. 19). The FF describes the characteristic shape of the J–V curve of a solar cell. The ideal device has a rectangular J–V curve with an FF ~1. The intersections of the J–V curve with x and y axes give the open-circuit voltage (Voc) and the short-circuit current density (Jsc), respectively. The point where the product J * V is the highest represents the maximum power point (Pmax). CONCLUSIONS The efficiency of an OSC is closely related to the active layer where the conversion of light into energy occurs. It is influenced by several factors such as solvent, the presence of additives, humidity, and especially to both the electron donor (conjugated polymer) and the electron acceptor (generally, fullerene derivative). Different configurations of the device (conventional and inverted) are developed to absorb more light, improve the dissociation of excitons, reduce impediments in the transport of charges, and increase the durability of the devices. Analysis of the morphology

of the active layer using techniques such as SEM, TEM, atomic force microscopy, and photovoltaic properties is extremely important to obtain a good correlation between energy conversion efficiency (PCE) and morphology. To optimize the device, it is important to understand how the device degrades during operation. The reduction of the durability of the solar cell is the result of several processes that occur simultaneously to the degradation of the material and, as a consequence, interfere in its physical, electrical, and mechanical properties. Inverted solar cells have been studied in recent years, mainly due to the increase in PCE of the device and the higher stability when compared to conventional solar cells. It is believed that the greatest current challenges to be circumvented in the field of PSCs are the energy conversion efficiency and the stability of the device. Aiming to make these devices more competitive in front of inorganic solar cells, it is necessary to develop research related to the optimization of the active layer through the study of electron donor-conjugated polymers with derivatives of fullerene that exhibit durability and efficiency of at least 10% and 10 years, respectively.

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26. Heeger, A.J. Semiconducting polymers: The third generation. Chem. Soc. Rev. 2010, 39 (7), 2354. 27. Kuznetsov, I.E.; Akkuratow, A.V.; Susarova, D.K.; Anokhin, D.V.; Moskvin, Y.L.; Kluyev, M.V.; Peregudov, A.S.; Troshin, P.A. Statistical carbazole–fluorene–TTBTBTT terpolymers as promising electron donor materials for organic solar cells. Chem. Commun. 2015, 51 (35), 7562–7564. 28. Kim, H.Y.; Choi, M.H.; Han, W.Y.; Moon, D.H.K.; Haw, J. R. Deep HOMO polymers comprising anthracene unit for bulk heterojunction solar cells. J. Ind. Eng. Chem. 2016, 33, 209–220. 29. Kang, T.E.; Kim, K.-H.; Kim, B.J. Design of terpolymers as electron donors for highly efficient polymer solar cells. J. Mater. Chem. A 2014, 2 (37), 15252. 30. Akkuratov, A.V.; Mühlbach, S.; Susarova, D.K.; Sebler, M.; Zimmermann, B.; Razumov, V.F.; Würfel, U.; Troshin, P.A. Positive side of disorder: Statistical fluorene-­carbazoleTTBTBTT terpolymers show improved optoelectronic and photovoltaic properties compared to the regioregular structures. Sol. Energy Mater. Sol. Cells 2016, 160, 346–354, 31. He, Y.; Li, Y. Fullerene derivative acceptors for high performance polymer solar cells. Phys. Chem. Chem. Phys. 2011, 13, 1970–1983. 32. Huang, F.; Yip, H.L.; Cao, Y. Polymer Photovoltaics: Materials, Physics and Device Engineering; Royal Society of Chemistry: London, 2016. 33. Kuznetsov, I.E.; Susarova, D.K.; Inasaridze, L.N.; Klyuev, M.V.; Troshin, P.A. Synthesis of statistical carbazole–­ fluorene–thiophene–benzothiadiazole copolymers and their investigation in organic solar cells. Mendeleev ­Commun. 2015, 25 (4), 277–279. 34. Mi, D.; Kim, J.; Yoon, S.C.; Lee, C.; Lee, J.; Hwang, H. Synthesis and characterization of a novel fullerene derivative containing carbazole group for use in organic solar cells. Synth. Met. 2011, 161, 1330–1335. 35. Bégué, D.; Guille, E.; Metz, S.; Arnaud, M.A.; Silva, H.S.; Seck, M.; Fayon, P.; Dagron-Lartigau, C.; Iratcabal, P.; Hiorns, R C. Graphene-based acceptor molecules for organic photovoltaic cells: A predictive study identifying high modularity and morphological stability. RSC Adv. 2016, 6 (17),13653–13656. 36. Pechlivani, E.M.; Papas, D.; Zachariadis, A.; Laskarakis, A.; Logothetidis, S. Organic photovoltaic cells based on graphene interfacial anode electrodes. Mater. Today Proc. 2016, 3 (3), 788–795. 37. Mao, H.Y.; Lu, Y.H.; Lin, J.D.; Zhong, S.; Wee, A.T.S.; Chen, W. Manipulating the electronic and chemical properties of graphene via molecular functionalization. Prog. Surf. Sci. 2013, 88 (2), 132–159. 38. Yi, C.; Hu, X.; Gong, X.; Elzatahry, A. Interfacial engineering for high performance organic photovoltaics. Mater. Today 2016, 19 (3) 169–177. 39. Meyer, F. Fluorinated conjugated polymers in organic bulk heterojunction photovoltaic solar cells. Prog. Polym. Sci. 2015, 47, 70–91. 40. Oseni, S.O.; Mola, G.T. Properties of functional layers in inverted thin film organic solar cells. Sol. Energy Mater. Sol. Cells 2017, 160, 241–256. 41. Wang, Y.; Bai, H.; Zhan, X. Comparison of conventional and inverted structures in fullerene-free organic solar cells. J. Energy Chem. 2015, 24 (6), 744–749.

42. Peters, C.H.; Sachs-Quintana, I.T.; Kastrop, J.P.; Beaupré, S.; Leclerc, M.; Mcgehee, M. D. High efficiency polymer solar cells with long operating lifetimes. Adv. Energy Mater. 2011, 1 (4), 491–494. 43. Jorgensen, M.; Norrman, K.; Gevorgyan, S.A.; Tromholt, T.; Andreasen, B.; Krebs, F.C. Stability of polymer solar cells. Adv. Mater. 2017, 24 (5), 580–612. 44. Manceau, M.; Bundgaard, E.; Carlé, J.E.; Hagemann, O.; Helgesen, M.; Sondergaard, R.; Jorgensen, M.; Krebs, F.C. Photochemical stability of ?-conjugated polymers for polymer solar cells: A rule of thumb. J. Mater. Chem. 2011, 21 (12), 4132. 44. Kawano, K.; Ito, N.; Nishimori, T.; Sakai, J. Open circuit voltage of stacked bulk heterojunction organic solar cells. Appl. Phys. Lett. 2006, 88, 073514-1–073514-3.

46. Burgués-Ceballos, I.; Hermerschmidt, F.; Akkuratov, A.V.; Susarova, D.K.; Troshin, P. A.; Choulis, S.A. High-performing polycarbazole derivatives for efficient solution-processing of organic solar cells in air. ChemSusChem. 2015, 8 (24), 4209–4215. 47. Krebs, F. Fabrication and processing of polymer solar cells: A review of printing and coating techniques. Sol. Energy Mater. Sol. Cells 2009, 93, 394–412. 48. Yip, H.; Jen, A.K.-Y. Recent advances in solution-­processed interfacial materials for efficient and stable polymer solar cells. Energy Environ. Sci. 2012, 5 (3), 5994.

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Environmental Applications: Biopolymer Sorbents for Heavy Metal Removal Patrícia Concórdio-Reis and Filomena Freitas Environmental–Fire Protection

UCIBIO-REQUIMTE, Department of Chemistry, Faculty of Science and Technology (FCT), New University of Lisbon, Caparica, Portugal

Abstract Heavy metals are found naturally throughout the earth’s crust, and they are not only essential for several biological processes in plants and animals but also vital for our industry and daily life. Despite being required to maintain the organisms’ metabolism, heavy metals are toxic at higher concentrations. Since industrial revolution, industrial operations and anthropologic activities have increased the presence of these metals in terrestrial and aquatic environments, raising health and environmental concerns. In recent years, there has been an urge to find new eco-friendly and economic alternatives for controlling and removing metal pollution. One of the most promising strategies is biosorption, which utilizes different natural materials from biological sources as sorbents of these pollutants. Prokaryotic and eukaryotic biopolymers are rich in adsorption sites which can interact with the metallic cations, binding them with varying degrees of specificity and affinity, thus concentrating and removing heavy metals from aqueous solutions. This entry reports the current investigation and advancements in biosorption of heavy metals using biopolymers from natural sources. The biopolymer–metal interactions are reviewed, considering the mechanisms involved and the environmental factors that affect the biosorption process. A brief overview of the sorption isotherms and models used to characterize the biosorption process is provided. The potential of biosorption technology, commercialization attempts, future work, and prospects are also discussed. Keywords: Algae; Bacteria; Biosorbent; Biosorption; Desorption; Fungi; Heavy metal removal; Metal sorption mechanisms; Toxic metals.

INTRODUCTION Heavy metals are high-density metallic substances found in trace concentrations in nature that have a potential adverse effect on the living organisms. These metals represent a major threat to the environment and human health, since they are nondegradable and extremely toxic and ­poisonous even at low concentrations.[1] Despite their negative ecological impact, the use of heavy metals has increased considerably over the past decades due to industrialization. Industrial operations such as mining, fossil fuel combustion, metallurgy, and electroplating, as well as agriculture and domestic activities, rely on the use of metals and chemicals and generate effluents loaded with heavy metals.[2] Several physical and chemical methods are available to remove metal ions from aqueous solutions, namely, ­chemical precipitation, ion exchange, electrochemical treatment, reverse osmosis, and chemical extraction. ­However, these conventional methods generate undesirable metallic by-products, are difficult to implement at large scale and are very expensive. Moreover, they are ineffective, especially for concentrations in the 1–100 ppm range.[2–6] Therefore,

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there is a growing need for novel, eco-friendly alternatives. In view of that, the use of ­biopolymers, obtained from different natural sources (plants, algae, animals, microorganisms), has been proposed as bio-based ­methods to remove heavy metals from contaminated effluents. Numerous biopolymers, including chitosan or ­alginate, have ­demonstrated to be effective in the sequestration of different metals. These biopolymers are biodegradable, selective, and nontoxic, and their p­ roduction is easy and cost-effective.[7] Thus, biopolymers represent a promising eco-friendly solution for heavy metal ­sequestration and recovery. TOXICITY AND HEALTH EFFECTS OF HEAVY METALS There are three types of heavy metals of concern: toxic metals (e.g., mercury, chromium, lead, zinc, and ­copper), precious metals (e.g., silver, platinum, and gold), and ­radionuclides (e.g., uranium, thorium, and radium).[2,5,8] However, the metals most commonly discharged into aquatic systems are arsenic, mercury, cadmium, chromium, Encyclopedia of Polymer Applications, First Edition DOI: 10.1201/9781351019422-140000381 Copyright © 2018 by Taylor & Francis. All rights reserved.

copper, lead, nickel, and zinc.[8,9] The major sources of these metals, their toxicity mechanisms, and health effects are summarized in Table 1. The toxic effect of heavy metals is due to their interference with the organisms’ metabolic processes or by promoting mutagenesis in living organisms.[10] In fact, exposure to these metals affects the nervous system and is associated with the occurrence of numerous cancers, kidney malfunction, and can even cause death.[1,10] Moreover, heavy metals accumulate in living organisms, since they are difficult to be excreted or metabolized. Thus, they persist in the organisms and accumulate throughout the food chain, which magnifies their concentration.[10,34] Heavy metals have high affinity to sulfur present in the functional groups of enzymes or proteins, thus altering their structure, leading to the inactivation of critical metabolic pathways. Furthermore, some heavy metals can compete and substitute essential elements and molecules required for cellular maintenance and function, due to structural and chemical similarity.[1,9,10] Examples include lead and mercury that can imitate calcium, being incorporated into the bones, [10,39] and arsenic that can replace inorganic phosphate in several biochemical processes.[19] Heavy metals also fasten the production of reactive oxygen species (ROS), which cause oxidative stress in living cells and cellular damages.[40] Furthermore, these metals induce cell death processes and damage the genome, increasing the occurrence of cancer and cancer-related diseases.[34] The oxidation state of the metallic cations has a high impact on their solubility, bioavailability, and, therefore, their toxicity.[1,9,10] For example, while Cr(III) is an essential micronutrient for cellular metabolism, Cr(VI) is a strong oxidizing agent, being corrosive, irritant, and toxic to living organisms. Furthermore, in contrast to Cr(III) which is poorly absorbed, Cr(VI) can pass through biological membranes and react with proteins and nucleic acids, compromising the cell integrity and functions.[1,24,26] Another example is As(III) that is up to 10 times more toxic than As(V), since it can lead to the inactivation of over 200 enzymes, due to its high affinity for ­sulfur-containing groups.[1] In plants, heavy metals affect seed germination, root expansion, and plant growth. A decrease in chlorophyll pigment content and an inhibition of photosynthesis are also associated with the toxic effect of heavy metals, such as lead, mercury, arsenic, and cadmium.[9,11,14,19,41–44] BIOSORPTION: BIOPOLYMER SORBENTS AND NATURAL SOURCES Biosorption techniques can use either living or dead microorganisms, or products of their metabolism, such as polysaccharides, proteins, nucleic acids, or lipids.[2,5,45] Microbial cells can uptake metals from solution and accumulate them bound to the cell wall, reducing their

bioavailability. Cellular metabolism can eventually lead to the conversion of those metal ions into less soluble and/or toxic forms, by precipitation with metabolites or by redox reactions.[2,46] Using microbial products such as biopolymers instead of the biomass is advantageous since it reduces the complexity and avoids metabolic interference and possible pathogenicity issues.[8,47] Natural Sources of Biopolymer Sorbents Bacteria Both Gram-negative and Gram-positive bacteria produce biopolymers capable of removing metal ions from ­aqueous  solutions. Some of these biomolecules are compounds of the cell wall, and the most common examples include lipopolysaccharides (LPSs), glycoproteins, lipoproteins and peptidoglycan in Gram-negative bacteria, and teichoic acids, teichuronic acids and peptidoglycan in Gram-­positive bacteria.[48–50] Cell wall biopolymers are associated with granting protection and structure to the cell (peptidoglycan)[51], and conferring asymmetry to the outer membrane and negative charge to the cell wall of Gram-negative bacteria (LPS).[5,52] Teichuronic and teichoic acids are believed to be associated with maintaining wall structure, provide negatively charged sites in Gram-positive bacteria cell wall, and participate in metal tripping.[5] The carboxyl and phosphate groups present in these molecules are the main binding site for metal ions.[53] Moreover, extracellular polymerics substances (EPSs), such as water-soluble and amphipathic exopolysaccharides, capsular polysaccharides, and capsular polyglutamic acid, have a major role in bacterial biosorption.[50] EPSs are localized outside the bacterial cell and comprises a variety of high-molecular-weight microbial biopolymers, along with other low-molecular-weight by-products.[47] They are mainly composed of polysaccharides, proteins, lipids, nucleic acids, uronic acids, and humic substances.[3,54] Nevertheless, extracellular polysaccharides or exopolysaccharides are the most abundant component in microbial EPSs [47] and the most important for metal adsorption.[3] Exopolysaccharides can be covalently bound to the cell membrane forming the bacterial capsule (capsule polysaccharides) or as a slime loosely bound to the cell surface.[55,56] Capsular polymers usually function as surface antigens and virulence factors [57] and have lower metal sorption capacity than the soluble, loosely bound, ­exopolysaccharides.[58] Although exopolysaccharides are majorly associated with protective mechanisms, since they can prevent cellular desiccation due to their water retention capacity and protect against predatory microorganisms, [54,55] these biopolymers also act as structural elements in the formation of biofilms and play a major role in the adherence to surfaces.[54] They are important in interactions with other

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Environmental–Fire Protection Toxic effect

Maximum concentration (mg L −1) in drinking watera

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Table 1  Toxic effects associated with exposure to heavy metals Heavy Metal sources and metal utilizations Toxicity mechanism

References

Mining, smelting, lead-based paints, fossil fuels, lead-acid battery manufacturing and recycling, pigment and metal product manufacturing (e.g., ammunitions, solder, and pipes), crystal and ceramic industry

Leads to the formation of ROS Causes DNA damage by inducing gene mutations Inhibits synthesis of hemoglobin Causes defects in mitosis Replaces essential ions (e.g., Ca2+, Mg2+, Fe2+, and Na+) in the cell and inhibits calcium transport Binds to biomolecules (sulfhydryl and amide groups)

Affects the central and peripheral nervous system, the kidneys, the gastrointestinal tract, and the reproductive system Causes anemia and increased blood pressure Causes brain damage, mental retardation in children and congenital malformations Potentially carcinogenic

0.015

[1,8–13]

Mercury

Production of caustic soda, power generation plants, rubber processing, paints and fertilizers industries, paper manufacturing and is used in antifungal agents and dental amalgams

All forms of mercury are toxic, but organomercurics have higher toxicity. They can accumulate in biological tissues due to their high lipophilicity, eventually undergoing biomagnification Leads to the formation of ROS Causes DNA damage Reacts and binds with biomolecules (sulfhydryl and selenohydryl groups), damaging their structures and inhibiting the normal cellular processes Replaces essential ions in the cell Affects calcium homeostasis, and causes mitochondrial damage and accumulation of neurotoxic molecules

Affects the central and peripheral nervous system, the kidneys, the gastrointestinal tract, the respiratory and cardiovascular system, and causes reproductive damage Causes brain damage, and neuromuscular and psychological changes Leads to spontaneous abortions and congenital malformations Carcinogenic

0.002

[1,9,10,14–16]

Arsenic

Smelting and mining processes, coal-fired power stations and natural mineralization Used in agricultural products, as insecticides, herbicides, fungicides, algicides and wood preservatives, and in veterinary medicine to treat parasitic diseases

Both inorganic and organic methylated forms are toxic Toxicity depends on the oxidation state: As(III) has a higher toxicity than As(V). Both are more toxic than organic As compounds As(V) replaces Pi in biological systems and As(III) reacts with sulfhydryl groups, disrupting enzymatic processes Leads to the formation of ROS Causes DNA damage Inhibits ATP production Binds to biomolecules

Affects the central and peripheral nervous system, the kidneys, the gastrointestinal tract, and the respiratory, hepatobiliary, dermatologic and cardiovascular systems, and causes reproductive damage Can be associated with hearing loss, diabetes, hypertension, neurological complications, hematologic problems (anemia, leukopenia, and eosinophilia) and can possibly cause reproductive damage Fetal death and congenital malformations Carcinogenic (cancers of the skin, lungs, liver, kidneys, bladder, and prostate and hematopoietic systems)

0.010

[1,10,17–19]

(Continued)

Environmental Applications: Biopolymer Sorbents for Heavy Metal Removal

Lead

Toxic effect

Maximum concentration (mg L −1) in drinking watera

References [1,10,12,20,21]

Cadmium

By-product of lead and zinc smelting, burning of fossil fuels, manufacturing of nickelcadmium batteries, production of alloys and paint pigments, impurity of phosphate fertilizers, stabilizer of PVC products, used for coating against corrosion, and in nuclear power plants.

Leads to the formation of ROS, causing oxidative stress Causes DNA damage Binds to biomolecules and blocks calcium transporters Competes with the physiological action of Zn2+, Cu2+, or Mg2+, due to chemical similarities Inhibits heme synthesis Affects mitochondrial function, inducing apoptosis Impairs vitamin D metabolism, reducing bone mineralization

Damages in respiratory, gastrointestinal, hepatic, renal, cardiovascular, nervous, and reproductive and hematopoietic systems Hypertension, diabetes, and myocardial disease Endocrine disruption and effects on thyroid function Decrease in bone mineral density, leading to bone fractures Osteomalacia and/or osteoporosis, anemia, renal dysfunction, and calcium deficiency, which could cause itai-itai disease Carcinogenic

0.005

Chromium

Anticorrosive agent and paint pigment. Electroplating, tanneries, metal processing, mining, leather finishing, wood preservation, cement, paper, chemical, textile, and photographic industries

Different oxidation states but Cr(III) and Cr(VI) are the more stable forms Cr(III) is an essential trace element to carbohydrate, protein, and fat metabolism, while Cr(VI) is extremely toxic and poisonous Cr(VI) is a strong oxidant and efficiently generates ROS Cr(III) has low membrane permeability and is poorly absorbed by the cells Cr(VI) can easily pass through cellular membranes and once inside the cells, Cr(VI) is reduced to reactive intermediates, Cr(V) and Cr(IV), and ultimately Cr(III). High intracellular concentrations of Cr(III) lead to the formation of adducts with DNA and the occurrence of mutations

Damages in the kidney and liver, as well as negative effects on the neurological, respiratory, hematological, and cardiovascular systems Skin irritation and ulcers in the gastrointestinal organs Can cause allergy, asthma, anemia, and sperm damage Carcinogenic

0.1

[1,10,22–26]

(Continued)

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Table 1  Toxic effects associated with exposure to heavy metals (Continued) Heavy Metal sources and metal utilizations Toxicity mechanism

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Table 1  Toxic effects associated with exposure to heavy metals (Continued) Heavy Metal sources and metal utilizations Toxicity mechanism

Toxic effect

Maximum concentration (mg L −1) in drinking watera

References

Electrical equipment, copper pipes, alloys, mining, metallurgical, electroplating, paper and pulp industries, wood preserving, fertilizers, fungicidal products, and additives to control algae growth

Vital for many biological processes such as enzymatic activity and electron transport. Toxic in high concentrations Copper homeostasis is regulated through complex metal transport systems and chaperone proteins that bound this metal, since free Cu is toxic and causes cellular damages Toxicity is due to the formation of ROS: Cupric ion (Cu++) is reduced to Cu+ in the presence of reducing agents. Cu+ is involved in the formation of hydroxyl radicals, which are powerful oxidizing agents that can damage biological structures Formation of DNA adducts Alters metabolic processes

Anemia, liver and kidney damage, and stomach and intestinal irritation Cu toxicity can be due to hereditary factors. Wilson’s disease is an autosomal recessive disease associated with chronic copper poisoning. Gene mutations result in the accumulation of copper in various organs, leading to brain and liver damage

1.3

[8,10,27–29]

Nickel

Nickel alloys (stainless steel), coins, jewelry, mining, combustion of coal and fuel oil, batteries, pigments, and welding and plating processes

Not required for human survival, but essential for other organisms Promotes alterations in genetic expression (mutations and chromosomal aberrations) and epigenetic events (modifications in histone acetylation, in the DNA methylome and in microRNA expression) Disrupts ion homeostasis (Ca2+, Fe2+/ Fe3+, Mg2+, Cu2+ or Zn2+). A decrease in Ca2+ availability leads to a reduction in exoskeleton, shell, and bone formation Binds with biomolecules and replaces essential ions Production of ROS, inducing oxidative stress

Dermatitis, allergy, asthma, damage to lungs, and nasal and oral epithelium Impact on bone growth Carcinogenic

0.04

[30–34]

(Continued)

Environmental Applications: Biopolymer Sorbents for Heavy Metal Removal

Copper

Zinc

a

Mining, burning coal, metallurgical processing, fungicides, algaecides, and fertilizers

Essential trace element in all organisms, zinc deficiency can cause serious health problems. Despite its low toxicity, high concentrations have harmful effects on health Causes copper deficiency, alters iron homeostasis, reduces lymphocyte function and perturbs cholesterol metabolism Induces apoptosis in different cell types, having an important role in neuronal death

Toxic effect Nausea, vomiting, diarrhea, and abdominal pain Lethargy and ataxia Lung and reproductive disorders Anemia, cholesterol, and impaired immune response

Maximum concentration (mg L −1) in drinking watera 5

According to the United States Environmental Protection Agency, National Primary Drinking Water Regulations, EPA 816-F-09-004 (2009).

Environmental–Fire Protection

References [8,35–38]

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Table 1  Toxic effects associated with exposure to heavy metals (Continued) Heavy Metal sources and metal utilizations Toxicity mechanism

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Environmental Applications: Biopolymer Sorbents for Heavy Metal Removal

Environmental–Fire Protection

microorganisms, namely, in bacterial cell–cell recognition and aggregation processes, and are also responsible for the sorption of exogenous compounds and inorganic ions, due to their anionic charge.[54,59,60] Furthermore, exopolysaccharides may be secreted by some bacteria as a defensive mechanism against toxic metal ions, since these polymers can shield the cell and sequestrate the metallic ions, preventing them to penetrate the cell surface, thus protecting bacteria from their toxicity.[3] EPSs produced by different genera of bacteria, including Azotobacter, Bacillus, Paenibacillus, and Pseudomonas, have been reported to remove heavy metals from solutions with different efficiencies, as given in Table 2. Fungi and Yeasts Fungal cell walls are a complex and rigid structure mainly composed of polysaccharides, but also proteins, lipids, pigments (e.g., melanin), polyphosphates, and inorganic ions.[5,53,97,98] Chitin is a strong but flexible nitrogen-­ containing polysaccharide (composed of N-acetyl-d-­ glucosamine units) found in the cell wall of most fungal species. Chitin, chitosan (the N-deacetylated form of chitin), and other ­chitin derivatives are effective biosorbents for heavy ­metals.[5,50,53,99] Their binding capacity is presumably due to the coordination of the metal cation with hydroxyl and amine groups.[50,53,99] Other polysaccharides, such as glucans and mannans, are usually the main constituents of a thin outer layer in the cell wall, being present in all fungi.[5,100] β-Glucans can be complexed to chitin, forming ­chitin–glucan complexes [100] that also bind metal ions by ion exchange and coordination.[97] Many fungi and yeasts also secrete exopolysaccharides, which are believed to be important in the adhesion to host surfaces in the case of pathogenic cells. Similarly to bacteria, fungi and yeast cells increase exopolysaccharides production in the presence of metal ions. Therefore, these extracellular polymers may be involved in protective mechanisms to overcome metal toxicity.[86,101,102] For example, Mikes et al.[102] documented that increasing the metal ­concentration in the medium positively influenced EPS ­synthesis by Candida utilis and Rhodotorula mucilaginosa. Most of these exopolysaccharides are homopolysaccharides (e.g., pullulan or scleroglucan), but heteropolysaccharides composed of different neutral ­sugars together  with uronic acids are also common.[100] The metal sorption capacity of some fungal exopolysaccharides has been reported. For example, a Pb2+ removal of over 90% by the fungus Aureobasidium pullulans KFCC was attributed to the presence of the EPS.[103] Moreover, both Pestalotiopsis sp. KCTC (Korean collection for type cultures) 8637P and Aspergillus fumigatus produced exopolysaccharides with binding capability towards Pb2+ and Zn2+, and Cu2+ and Cd2+, respectively.[83,85] Soluble EPSs produced by two Yarrowia spp. strains immobilized up to 59% Hg2+, while its bound-EPSs were fundamental for the

cellular biosorption of mercury. In that study, EPS sequestration, concomitant with volatilization and bioaccumulation in cellular components, led to a removal of 97% of the Hg bioavailable in the medium (Table 2).[86] Breierová et al.[101] studied eight yeast species, including A. pullulans, Hansenula anomala, and Saccharomyces cerevisiae, and determined that the mechanisms of their resistance to cadmium were highly dependent on the production of extracellular glycoproteins. Those exopolymers adsorbed ions through ionic interactions and via physical entrapment. The composition of the saccharide moiety of the biopolymer was influenced by the metal concentration in the cultivation medium and had a major impact on its sorption capacity. In fact, when the metal content in the cultivation medium increased, the polymer produced had a higher content of mannose, phosphorus, and glutamic acid, and a lower content of glucose, galactose, and arabinose. Algae and Cyanobacteria Cyanobacteria (formerly known as blue-green algae) are phototrophic bacteria which have a similar cell wall to Gram-negative bacteria. Peptidoglycan, consisting of linear chains of N-acetylglucosamine and β(1→4)-Nacetylmuramic acid with peptide chain, is the main component responsible for their biosorption capability due to the presence of carboxylic groups. However, some species also produce EPS, sheaths, and slimes, rich in polysaccharides and negatively charged groups, with sorptive ­properties.[4,53,98,104] Algae are photosynthetic eukaryotic organisms with high sorption capacity.[5,97,105] According to their characteristics, such as the nature of the pigments, cell wall chemistry, and flagellation, algae can be divided into algal sub-groups: Chlorophyta (green algae), P ­ haeophyta (brown algae), Rhodophyta (red algae), ­Euglenophyta  (euglenoids), Chrysophyta (diatoms), and Pyrrophyta (dino-­ flagellates).[5,106,107] Nonetheless, green, red, and brown algae have shown a higher metal-complexing performance, due to the chemistry of the cell wall.[106] Despite the variation in composition, most algae cell walls are composed of a fibrillar skeleton of cellulose, β(1→4)-­glucan, which can be replaced by xylan, β(1→3)- and β(1→4)-linked ­d-xylose, in Chlorophyta and Rhodophyta spp., or mannan, β(1→4)linked d-mannose, in Chlorophyta spp.[53,106,108] Pectin, a polysaccharide composed of polygalacturonic acid and small amounts of rhamnose, can also be present in the cell wall of some algae.[5] Other than cellulose (structural support), brown algae (Phaeophyta) cell wall generally contains an amorphous extracellular polysaccharide composed of alginic acid, a polymer of mannuronic and guluronic acids monomers, complexed with salts of sodium, potassium, magnesium, and calcium; and sulfated polysaccharides (fucoidan), a polymer mainly composed of l-fucose 4-sulfate building blocks.[4,106–108] Structures of algal cellulose, xylan, ­mannan,

Environmental Applications: Biopolymer Sorbents for Heavy Metal Removal 1073

Table 2  Metal-binding efficiency of biopolymer-based sorbents from bacteria, fungi, algae, and cyanobacteria Maximal biosorption Condition remarks Organism Biosorbent composition capacity (mg g−1 or %)

References

Activated sludge

EPS: mainly protein Polysaccharides, humic acids, uronic acids, and nucleic acids

Cu2+: 231.05 Pb2+: 392.85 Cd2+: 2.25

pH 7, T 22

[58]

Aerobic activated sludge

EPS: proteins and polysaccharides Humic acids

Hg2+: 452.8 Sb5+: 648.7

pH 7, T 20, Mc 30 and BSc 0.197

[61]

Aerobic granules

EPS: mainly protein Polysaccharides

Zn2+: 6.9 Co2+: 5.5

pH 5, T 20

[62]

Alteromonas macleodii subsp. fijiensis

Exopolysaccharide Man, ManP, Glc, Gal, GlcA, GalA Uronic acids, proteins

Pb2+: 316 Cd2+: 125 Zn2+: 75

pH approx. 5, BSc 0.1%

[63]

Azotobacter spp.

EPS: proteins and polysaccharides

Cd2+:15.17

pH 6, BSc 12

[64]

Azotobacter spp.

Exopolysaccharide

Cu2+: 15.5 Zn2+: 20

Mc 10

[65]

Bacillus firmus MS-102

Exopolysaccharide (Glc, Fru, Man, Gal) Pyr, uronic acids

Pb2+: 1,103 or 98.3% Cu2+: 860 or 74.9% Zn2+: 722 or 61.8%

pH 4.5 for Pb2+, pH 4 for Cu2+ and pH 6 for Zn2+ BSc 1, Mc 2000, T 25

[66]

Bacillus sp. CH15

EPS: higher content in protein Polysaccharides, HexN, Uronic acid

Cd2+: 64%

pH 7, Mc 1, BSc 10

[67]

Bacillus sp. F19

EPS: 66.4% polysaccharide (Man, Glc) and 16.4% protein Uronic acids

Cu2+: 89.62

pH 4.8, T 25, Mc 500

[68]

Chryseomonas luteola TEM05

EPS immobilized in Ca-alginate beads 33% polysaccharide and 26% protein

Cd2+: 64.10 Co2+: 55.25 Cu2+: 126.39 Ni2+: 71.84 Pb2+: 111.11

pH 6, T 25, BSc 1

Desulfovibrio desulfuricans

EPS: mainly protein Polysaccharides and nucleic acids

Cu2+: 899.1 Zn2+: 932.1

pH 5

[71]

Desulfovibrio desulfuricans

EPS: mainly protein Polysaccharides and nucleic acids

Cu2+: 899.1 Zn2+: 932.1

pH 5, room temperature

[71]

Escherichia coli K-12

Cell envelopes (peptidoglycan, LPS, proteins)

Pb2+: 31.5 Hg2+: 12.8 Zn2+: 25.5 Au3+: 11 Cu2+: 5.7 Ru3+: 9.1 Ni2+: 0.1 Co2+: 10.5 Mn2+: 7.7 Pt4+: 0.4

T 23, Mc 5 mM, BSc 2.5

[72]

Herbaspirillium sp. CH7

Exopolysaccharide Proteins, HexN, uronic acid

Hg2+: 80% Cd2+: 66%

Mc 1, BSc 0.001 for Hg2+ and 10 for Cd2+

[67]

Klebsiella sp. J1

EPS: mainly protein Polysaccharides and nucleic acids

Cu2+: 112.5 Zn2+: 88.9

pH 5, Mc 0.1–1 mM

[73]

Methylobacterium organophilum

EPS: 80.4% polysaccharide (Man, Gal, Glc) and 6.1% protein Uronic acids (ManA, GalA, GlcA)

Pb2+: 184.2 Cu2+: 200.3

pH 7, T 20, Mc superior to 400

[74]

[69,70]

(Continued)

Environmental–Fire Protection

Bacteria

1074

Environmental Applications: Biopolymer Sorbents for Heavy Metal Removal

Table 2  Metal-binding efficiency of biopolymer-based sorbents from bacteria, fungi, algae, and cyanobacteria (Continued) Maximal biosorption Condition remarks References Organism Biosorbent composition capacity (mg g−1 or %)

Environmental–Fire Protection

Paenibacillus jamilae

EPS: 62.9% polysaccharide (Glc, Gal, Rha, Fuc, Man) and 1.5% protein Uronic acids (Pyr), HexN, Ace

Pb2+: 303.03 Cd2+: 30.12 Co2+: 20.49 Ni2+: 17.66 Cu2+: 12.31 Zn2+: 7.81

pH 5.5

[75]

Paenibacillus jamilae CECT 5266

Exopolysaccharide (Fuc, Xyl, Rha, Man, Gal, Glc)

Pb2+: 2.28

T 25, Mc 20.7

[76]

Paenibacillus jamilae CECT 5266

Exopolysaccharide Fuc, Xyl, Rha, Ara, Man, Gal, Glc

Pb2+: 228 Cd2+: 55 Cu2+: 40 Zn2+: 37 Ni2+: 15 Co2+: 10

T 25, Mc 0.1 mM

[77]

Paenibacillus polymyxa P13

Exopolysaccharide

Cu2+: 1,602

pH 6, T 25, BSc 0.1

[78]

Paenibacillus sp. CH11

EPS: mainly proteins Polysaccharides, HexN, uronic acid

Cd2+: 93%

pH 3, Mc 1, BSc 10

[67]

Pseudomonas aeruginosa ATCC-10145

EPS: 89% polysaccharide and 27% protein Uronic acids and aminosugars

Cu2+: 87.39% Hg2+: 89.09% Pb2+: 79.70% Cd2+: 73.93% As2+: 72.96% Zn2+: 80.59%

pH 7, T 30, BSc 0.1 Mc 20 for Cu2+ and Hg2+, 40 for Pb2+ and Cd2+, 60 for As2+ and Zn2+

[79]

Pseudomonas putida X4

EPS

Cu2+: 169.24

pH 5, T 25, Mc 12.7–190.7

[80]

Pseudomonas sp. CH6

EPS: equal content in protein and carbohydrates HexN, uronic acid

Zn2+: 46% Cd2+: 66%

Mc 1 for Zn2+ and 10 for Cd2+, BSc 1

[67]

Pseudomonas stuteri AS22

Exopolysaccharide (Glc, Man, lactyl rhamnose)

Pb2+: 215.60 Co2+: 1.40 Cu2+: 0.55 Cd2+: 0.08

Room temperature, BSc 10.2, Mc 0.1 mM

[81]

Rhizobium etli M4

EPS

Mn2+: 67

pH 5.2–5.8, Mc 100

[82]

Rhizobium radiobacter F2 and Bacillus sphaeicus F6

Exopolysaccharide (Rha, Man, Glc, Gal)

Pb : 189.31

pH 7, T 25–35, Mc 2, BSc 0.001

[6]

Aspergillus fumigatus

Exopolysaccharide Uronic acids, lipids, proteins, and nucleic acids

Cu2+: 40 Cd2+: 85.5

pH 5, T 25, Mc 25 for Cu2+ and approx. 45 for Cd2, BSc 0.81

[83]

Aspergillus fumigatus

EPS: mainly polysaccharides Proteins and nucleic acids

Cu2+: 26.1 Pb2+: 20.1 Cd2+: 4.05

pH 7 for Cu2+ and Cd2+, pH 5 for Pb2+, T 25

[84]

Pestalotiopsis sp. KCTC 8637P

Pestan, exopolysaccharide Glc, GalN, GlcA, Gal, GalA

Pb2+: 120 Zn2+: 60

T 25, Mc aprox.400 for Pb2+ and aprox. 550 for Zn2+

[85]

Two strains of Yarrowia spp.

EPS: protein and polysaccharides

Hg2: 43.8%–58.7%

Mc between 2 and 32

[86]

EPS

Cu2+: 20

2+

Fungi and yeasts

Algae Ankistrodesmus densus

[87] (Continued)

Environmental Applications: Biopolymer Sorbents for Heavy Metal Removal 1075

Laminaria digitata

Ca-alginate beads

Cu2+: 87.38 Cd2+: 129.95 Pb2+: 372.38

pH 4.5, BSc 1, Mc 5–500

[88]

Macrosytica pyrifera

Ca-alginate beads

Zn2+: 136.2

pH 6.7, T 30, Mc 400

[89]

Sargassum vulgaris

Cell with alginate and sulfated polysaccharides (fucoidan)

Pb : 124.3 Cd2+: 39.34 Ni2+: 16.43

pH 6

[90]

Sargassum vulgaris

Cell without alginate and sulfated polysaccharides (fucoidan)

Pb2+: 93.24 Cd2+: 17.7 Ni2+: 4.11

pH 6

[90]

Anabaena spiroides Klebahn

EPS: 88% polysaccharide (Glc, Man, Rha) and 12% protein Uronic acids

Mn2+: 8.52

pH 6.5

[91]

Cyanothece sp. CCY 0110

Exopolysaccharide

Cd2+: 23.6 Pb2+: 16.6

pH 5, Mc 10

[92]

Gloeothece sp.

Sulfated exopolysaccharide Glc, Gal, Man, Fru, Ara, Xyl, Rib, Fuc, Rha, GlcA, GalA

Cu2+: 68.3 Pb2+: 185.7

pH 4.5–5.5

[93,94]

Lyngbya putealis HH-15

Exopolysaccharide

Cr6+: 157

pH 2, T 45, Mc 30–40

[95]

Nostoc linckia HA-46

Exopolysaccharide

Cr : 14.3 Co2+: 17.9

pH 2 for Cr and pH 4 for Co2+, T 25, Mc 20

[96]

2+

Cyanobacteria

6+

6+

Mc, metal concentration in mg L−1; BSc, biosorbent concentration in g L−1; T, temperature in oC; Man, mannose; ManP, pyruvated mannose; Glc, glucose; Gal, galactose; GlcA, glucuronic acid; GalA, galacturonic acid; Fru, fructose; Pyr, pyruvate; HexN, hexoamine; Ca-alginate, calcium alginate; ManA, mannuronic acid; Rha, rhamnose; Fuc, fucose; Ace, acetyls; Xyl, xylose; Ara, arabinose; GalN, galactosamine; Rib, ribose.

fucoidan, and alginate were illustrated by Davis et al.[106] Their excellent sorption capacity is specially related to the high content of alginates.[109] As shown in Table 2, when the cells of Sargassum vulgaris were treated to eliminate alginic acid and fucoidan, their binding efficiency towards Pb2+, Cd2+, and Ni2+ decreased by 25%, 55%, and 75%, respectively.[90] These results envisage the importance of those polymers in the biosorption process. Red algae (Rhodophyta) contain cellulose in the cell wall as well, but their binding ability is related to the presence of sulfated polysaccharides made of galactans (e.g., agar and carrageenan). Besides cellulose, green algae (Chlorophyta) cell wall also contains glycoproteins.[106–108] These components provide diverse binding sites, such as amino, carboxyl, hydroxyl, phosphate, and sulfate groups, responsible for their metal-complexing properties.[4,53,107,110] Similarly to cyanobacteria, diatoms (Chrysophyta) and green algae (Chlorophyta) can be used as biosorbent materials due to the interaction between EPS and heavy metals.[111] Both algae, such as Chlorella vulgaris, Synura sp., Dunaliella tertiolecta, Cylindrotheca closterium, Lingulodinium polyedrum, Gymnodinium sp. and Prorocentrum micans, and cyanobacteria, such as Synechococcus spp. and Synechocystis sp., produce EPSs as protective barriers to reduce the toxicity of the heavy metals towards the cells.[112–120] For example, in the presence of

1 mg L−1 of Cd2+, C. vulgaris increased the production of bound (50.6%) and soluble EPS (19.7%). Moreover, both EPS had higher contents of humic acid-like substances, and tryptophan-like and tyrosine-like components in the protein fraction. Such molecules are known to bind with metals.[112] Kaplan et al.[114] reported that the carboxylic groups of uronic acids present in the exopolysaccharides produced by C. stigmatophora, as well as their relatively homogenous distribution, are fundamental for the metal-­ complexing properties of the polymer. When exposed to heavy metals, the cyanobacterium Synechocystis sp. BASO671 produced exopolysaccharides with a higher content of monomers with acidic functional groups (glucuronic and galacturonic acid), suggesting the relevance of those groups in their metal removal capacity.[119] Biosorbent Modifications Biopolymer biosorption capacity can be increased by physical/ mechanical treatments and ­chemical ­modifications, such as acetylation, methylation, ­sulfonylation, ­carboxymethylation, or phosphorylation.[3] For ­example, chitosan stability in acid solutions can be increased by  cross-linking chitosan using chemical agents, through radiation or ultraviolet light (physical method). Chitosan composites and blends can be achieved by the addition of polar groups that increase the

Environmental–Fire Protection

Table 2  Metal-binding efficiency of biopolymer-based sorbents from bacteria, fungi, algae, and cyanobacteria (Continued) Maximal biosorption Condition remarks References Organism Biosorbent composition capacity (mg g−1 or %)

1076

Environmental Applications: Biopolymer Sorbents for Heavy Metal Removal

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ability of the polymer to interact with other polymers.[99] Chitosan composites towards heavy metal removal were extensively reviewed by Wan Ngah et al.[121] Through physical methods, other forms of chitosan with improved sorption capacity can be produced, such as gels, beads, membranes, and films.[122] Liu et al.[123] utilized chitosan beads as support for Fe0 nanoparticles, which led to the removal of over 89% of the metals in electroplating wastewater. Another example is the increased metal biosorption performance of cellulose produced by bacteria of the genus Acetobacter after phosphorylation. Furthermore, the selectivity of the biosorption was related to the presence of phosphoric acid groups.[124] Several studies also reported the higher efficiency in metal removal using calcium ­alginate beads.[88,89,125–128]

BIOSORPTION: BIOPOLYMER–METAL INTERACTION As previously referred, biosorption also occurs in inactive/ dead cells, which indicates that a metabolism-independent process is involved in the association of the metal with the cell. It is believed that this process occurs in a passive way because the metal cation is able to interact with active functional groups present in the cellular structures. Biopolymers are rich in those groups (e.g., carboxyl, phosphoryl, sulfhydryl, amine, and hydroxyl groups), thus playing a major role in the metal-binding process [5,45] and exhibiting different degrees of specificity and affinity.[47,50] Functional Groups Involved in Biosorption The biosorption performance of biopolymers varies remarkably depending on their composition, as the properties of the active functional groups influence the mechanism of biosorption itself.[4,129] Several studies show that previous exposure of the microorganisms to metals affects the production and composition of the synthesized biopolymers, thus affecting their biosorption performance.[101,102,112,114,119] Cation binding usually occurs through electrostatic interactions between the positively charged cation and the negatively charged functional groups present in the polymers, such as phosphate and carboxyl groups. However, cation binding by the positively charged groups (hydroxyl groups) can also take place.[3,5,47,53] The chemical groups of biopolymers involved in metal binding are summarized in Table 3. Biosorbent preferences for metallic cations can be explained bearing in mind the Pearson concept and the Irving–Williams series of stability of metal complexes, [53,90] considering that the biosorbent and the metal act as a Lewis base and a Lewis acid, respectively.[93] According to the hard and soft acid–base theory (HSAB), metal ions can be classified as hard acids (e.g., Na+, Co3+, Cr3+, Ca2+,

Mg2+), soft acids (e.g., Hg2+, Cu+, Cd2+), and intermediate ions (e.g., Zn2+, Ni2+, Cu2+, Pb2+, Co2+).[131] Hard metals preferentially bind to oxygen-containing ligands through electrostatic interactions, whereas soft metals e­ stablish covalent or coordinative interactions with sulfur- or nitrogen-­containing ligands.[5,53,90] However, this principle is not absolute since several conditions such as metal concentration, competitive effect, and availability of surface ligands affect their predictable behavior.[53] Mechanisms of Metal Biosorption When the biosorption is only due to electrostatic interactions, van der Waals interactions or the concentration imbalance between the surface of the biosorbent and the solution, the process is considered a physical sorption.[2,132] However, other mechanisms could be responsible for the biosorption of metals by biosorbents, such as microprecipitation, ion exchange, complexation, chelation, and ­coordination.[2,5] Ion Exchange In ion exchange, a change in ions occurs between the biosorbent and the solution, releasing counter ions (e.g., H+, Ca2+, or Mg2+) into the aqueous solution.[5,45] Ion exchange is dependent on the charge density of the cation and the ionization state of the functional groups. The main ionizable groups in biopolymers capable of exchanging ions are carboxyl, phosphate, and sulfate groups.[133] The presence of other cations can negatively affect this mechanism.[129] For example, the adsorption of copper, lead, and mercury, by EPS produced by the cyanobacterium Anabaena spiroides, was related to the release of previously adsorbed Mn2+ ions, therefore pointing out the involvement of an ion exchange mechanism.[91] Furthermore, the biosorption of Cr3+, Cd2+, and Cu2+ by green algae Spirulina sp. cells was mainly associated with a chemical process, since the ­contribution of physical sorption never exceeded 3.7% of the total amount of metal removed from solution. Ion exchange was the dominating mechanism of biosorption, and the functional groups capable of exchanging ­cations (carboxyl, phosphate, and hydroxyl) were identified in the cell surface.[134] A study performed by Yin et al.[83] also showed that ion exchange was the main mechanism in Cu2+ and Cd2+ biosorption by the exopolysaccharide ­produced by Aspergillus fumigatus. Surface Precipitation or Microprecipitation Microprecipitation can be either dependent on or independent of cellular mechanism. In the former case, the metal ions interact with the biopolymer, leading to local accumulation. When the solubility reaches its limit, the metal precipitates in the surface of the biosorbent.[5,45,104] Microprecipitation onto active binding sites with weak affinity

Environmental Applications: Biopolymer Sorbents for Heavy Metal Removal 1077

Hydroxyl Carbonyl (ketone) Carboxyl

–OH >C O

=

C O

Occurrence in biomolecules

Ligand atom

9.5–13 — 1.7–4.7

Hard Hard Hard

O O O

PS, UA, SPS, AA Peptide bond UA, AA

8.3–10.8 1.3

Soft Hard

S O

AA SPS

— 8–11 13 —

Soft Int. Int. Int.

S N N N

AA Cto, AA Cti, PG, peptide bond AA

11.6–12.6 6.0

Int. Soft

N N

AA AA

0.9–2.1 6.1–6.8

Hard

O

PL

1.5

Hard

O

TA, LPS

OH

Sulfhydryl (thiol) Sulfonate

–SH O S O O

Thioether Amine Secondary amine Amide

>S –NH2 >NH C O NH2

Imine Imidazole

Phosphonate

= NH C N H >CH H C N OH P O OH

Phosphodiester

>P

O

OH Source: Reproduced with permission from Volesky. [130] PS, polysaccharide; UA, uronic acid; SPS, sulfated PS; Cto, chitosan; Cti, chitin; PG, peptidoglycan; AA, amino acid; TA, teichoic acid; PL, phospholipid; LPS, lipoPS.

was responsible for over 85% of the Pb2+ sorbed by Rhizopus arrhizus biomass.[135] A combined mechanism of ion exchange and microprecipitation led to the biosorption of Ni2+ on the cell surface of Pseudomonas fluorescens 4F39.[136] Microprecipitation using biopolymers has been particularly investigated for the recovery of precious ­metals as nanoparticles.[137–145] Metal Complexation, Coordination, and Chelation When a metal complex is formed due to the association of the metal with another molecule (ligand), the mechanism involved is complexation. When the ligand interacts with

the metal through two or more sites, it is called chelation, and usually, the biosorbent forms a chelation ring attaching the metal. Coordination is a mechanism similar to complexation, but the ligand (biosorbent) binds covalently to the metal.[2] Despite the differences in concept, all these mechanisms require the presence of one or several atoms within the biopolymer moieties that have a pair of free electrons available to interact with the metallic cation. The most common are trivalent and neutral nitrogen atoms, and neutral and divalent atoms of oxygen and sulfur.[133] A study performed by Nakajima et al.[146] revealed that copper ions interact with nitrogen and oxygen atoms in cell wall polymers of Micrococcus luteus through coordination

Environmental–Fire Protection

Table 3  Main functional groups of biopolymers involved in metal biding HSAB Binding group Structural formula pKa classification

1078

Environmental Applications: Biopolymer Sorbents for Heavy Metal Removal

Environmental–Fire Protection

bonds. According to Merroun et al., [147] U(VI) was biosorbed by extracellular and cell-associated polymers from three species of Acidithiobacillus ferrooxidans, through the formation of uranyl phosphate complexes. Complexation was also reported to be the main mechanism of copper and zinc binding by an EPS secreted by Desulfovibrio desulfuricans.[71] Cadmium biosorption by Sargassum fluitans was reported to occur through bridging or bidentate complex formation with the carboxyl groups present in alginate, a polysaccharide present in the algae.[148] In literature, organometal complexes have been classified as inner-sphere or outer-sphere complexes, depending on the type of bond. In the first case, the ligand atoms replace the water molecules that surrounded the metal; thus, a strong covalent bond is formed. Soft metals tend to form this type of complexes. Outer sphere complexes form long-range electrostatic bonds. Therefore, the metal retains its coordinated water molecules when the metal–ligand complex is formed. These are weaker covalent bonds with partial electrostatic character and are usually preferred by hard metals.[106,129] Stearic and conformational effects should be taken into consideration in metal binding through complexation/ chelation mechanisms.[49,106] For example, alginic acid has regions rich in glucuronic acid residues that display higher selectivity towards divalent cations through multidentate complexation, whereas monodentate complexation was dominant in mannuronic acid-rich regions, providing weaker environments for complexation. These differences are related to the orientation of the binding moieties, therefore impacting the spatial availability of the oxygen atoms to interact with the metal.[106] Analytical Techniques Knowledge of the active sites involved in metal sequestration and the structures and composition of the polymers is essential for a better understanding of the adsorption mechanism. This information can be obtained using different analytical techniques, including potentiometric ­titration, that allows the determination of the intrinsic pKa and acid/base properties of the polymer; spectroscopic analysis, infrared absorption, or Fourier transformed infrared spectroscopy (IR or FTIR), that can provide structural and bonding information on the metal–ligand interaction and the active functional groups of the polymer; nuclear magnetic resonance (NMR), which can reveal useful information about the molecular structure and the mechanism of biosorption; X-ray diffraction analysis, to study the crystallographic structure and chemical composition of metal–polymer complex; and microscopy techniques, such as scanning electron microscopy (SEM) and transmission electron microscopy, for visual confirmation of the morphology of the cells and the extracted polymers. SEM imaging combined with energy-dispersive X-ray spectroscopy gives information about the quantity and the distribution

of the metal in the biosorbent. To study the chemistry of the surface, X-ray photoelectron spectroscopy (XPS) can be used. This technique allows the determination of the oxidation state of the metal bound to the biosorbent and the nature of the bond between the biopolymer and the metal. Electron paramagnetic resonance can be used to investigate the conformation of the polymer’s sites interacting with the metal.[2,129,149] Recently, three-­dimensional excitation–­ emission matrix spectroscopy has been employed to study the changes in conformation and composition in the ­biopolymers after metal interactions.[71,112,129,150] The information provided by these different techniques is complementary, and a comprehensive study of the biosorption process often requires the combination of several analytical tools. To understand the biosorption mechanism of cadmium, nickel, and lead to functional groups on the cell wall of Sargassum vulgaris, a combination of several techniques was used (FTIR, XPS, SEM, and extraction of the metal-binding moieties, namely, alginic acid and sulfated polysaccharides). Carboxyl, amino, sulfhydryl, and sulfonate groups from the cell wall polymers (alginic acid, sulfated polysaccharides, proteins, and peptidoglycans) were shown to be responsible for the metal-binding properties of S. vulgaris. Chelation and ion exchange were the main mechanisms behind cadmium and nickel removal, respectively, whereas lead biosorption mechanism was a combination of ion exchange, chelation, reduction ­reactions, with lead precipitation on cell wall matrix.[90] Ngwenya et al.[151] combined surface complexation ­modeling of macroscopic adsorption data with X-ray adsorption spectroscopy measurements to study the adsorption of lanthanide onto bacterial cells. The combination of both methods allowed a better understanding of the mechanism: coordination of lanthanides to phosphate groups at low pH and secondary involvement of carboxyl sites at high pH. Furthermore, the coordination of lanthanides to phosphate was monodentate, and these groups were deprotonated around neutral pH. FACTORS AFFECTING BIOSORPTION The effects of pH, temperature, and other ions in solution (competition), metal, and biopolymer concentration on the biosorption process are discussed in this section. pH Value The pH affects the functional groups responsible for the metal-binding activity, the metal solubility, and the competition with coexisting ions.[8,52,53,136] In fact, pH determines the ionization state of important functional groups involved in the metal-binding process since they can be protonated or deprotonated depending on the pH value of the medium.[6] For example, carboxyl groups are deprotonated at neutral pH (pKa 1.7–4.7, Table 3), becoming

Environmental Applications: Biopolymer Sorbents for Heavy Metal Removal 1079

Temperature It has been reported that the biosorption efficiency remains unaffected within the temperature range of 20°C–35°C.[8,52,97] Higher temperatures usually enhance biosorptive removal due to an increase in surface activity and kinetic energy of the solute.[52,129,154] Furthermore, the temperature increases the rate of diffusion of the metal across the external boundary layer into internal pores of the biosorbent, since the viscosity decreases.[98] Nevertheless, an excessive increase in temperature could lead to changes in the physical structure of the biopolymers, affecting the biosorption performance.[52,129,154] Theoretically, the biosorption mechanism is given by ­thermodynamics. Low ΔH (enthalpy) favors physical ­sorption, while chemical biosorption requires higher ­activation energy.[132] For example, at low temperatures (20°C), P. putida binds heavy metal ions through a physical adsorption process.[155] However, it should be taken into account that chemical adsorption (e.g., ion exchange) usually involves several reactions. Thus, the binding of the metal ion can be exothermic, whereas the overall reaction is endothermic, because the release of the counter ion requires energy (positive ΔH).[153] The study performed by Uslu and ­Tanyol [155] indicated that the metal biosorption process by P. putida could be either endothermic or exothermic, ­depending on the biosorption of Cu(II) or Pb(II), respectively. Presence of Other Ions An increase in ionic strength leads to a reduction in biosorption due to competition for the binding sites of the biosorbent.[149,154] Since real water environments always contain other ion species, several studies reported the biosorption performance in multimetallic systems [62,63,66,125,156–158] and in the presence of other ions (Ca2+, Mg2+, or Na+).[159,160] Metals with stronger (covalent) binding affinity, such as lead and copper, were preferentially biosorbed, whereas for cations with weaker, mostly electrostatic bonding (nickel, zinc, and chromium), competition was a problem.[129,153]

Metal and Biosorbent Concentration An increase in biosorbent dosage usually has a positive effect on the amount of metal biosorbed since it increases the number of binding sites.[52] However, high biosorbent concentrations also increase the interactions occurring between the polymers’ molecules, which decreases the binding sites available to capture the metal cations.[66] Also, the concentration of metal could be insufficient to cover all the free binding sites, thus resulting in lower metal uptake.[52] Increasing the initial metal concentration provides more cations available to interact with the biosorbent’s surface area, therefore increasing the biosorption. However, high metal concentration may lead to the saturation of the active binding groups, and a plateau value of metal uptake is obtained.[52,153] MODELING IN BIOSORPTION The adsorption process can be expressed as a batch equilibrium isotherm curve that can be modeled by empirical and mechanistic equations.[52,53] An adequate modeling study can reduce the experimental work and is useful to understand the biosorption mechanism. Furthermore, an appropriate mathematical model can help predict the ­polymer–metal interaction under different conditions, which is useful for process optimization and scale-up.[129,132,154,161] In the field of biosorption, several models have been proposed and are described in detail in several useful review papers.[2,8,52,153,161–168] The most frequently used single i­sotherm adsorption models are ­represented in Table 4. The Langmuir and Freundlich models are the most widely accepted and frequently used models. However, these models are mainly empirical and do not reflect the biosorption mechanism. Also, they do not take into account the effects of variable environmental factors and may be oversimplified to fully describe the complex b­ iosorption processes.[5,53] Mechanistic models have also been proposed, but they are generally highly simplified and case specific.[129] Therefore, much more work is required to develop new accurate mechanistic models that consider not only the operational factors (pH, ionic strength, or metal concentration) but also the mass transfer and kinetic factors. DESORPTION AND BIOSORBENT REGENERATION Desorption techniques must be implemented, so biosorption processes become economic viable alternatives for industrial implementation. The biosorbent loaded with metals should be treated with a desorbing agent that allows

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negatively charged and attracting the positively charged metal through electrostatic interactions. The optimal pH for metal binding is usually higher when amine groups are responsible for the biosorption.[97] Depending on the type of polymer–metal interaction, usually acidic conditions tend to decrease the metal uptake, due to the interaction between the negative binding groups and the hydrogen ion H+.[67] Generally, increasing the pH leads to an increase in the number of deprotonated functional groups, therefore enhancing cation biosorption.[53,136] However, under alkaline conditions, hydroxylated species tend to be formed, decreasing the metal complex solubility, and precipitation can occur, which interferes with the biosorption process.[52,136,152] Furthermore, extreme pH values can damage the structure of the biosorbent.[153]

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Environmental Applications: Biopolymer Sorbents for Heavy Metal Removal

Table 4  Isotherms for empirical modeling of single-component adsorption systems Isotherm model Equations Nomenclature and remarks

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Freundlich

qe = KF Ce1/ n

Langmuir

qe =

qmax bCe 1 + bCe

Langmuir–Freundlich

qe =

qmax bCe1/ n 1 + bCe1/ n

Temkin

qe =

RT ln aCe b

Brunauer–Emmer– Teller (BET)

qe =

BQCe Cs − Ce 1 + ( B − 1) Ce / Cs 

Redlich–Peterson

qe =

KRP Ce 1 + aRP Ceβ

Radke–Prausnitz

qe =

arCep a + rCep−1

qe, equilibrium metal sorption capacity Ce, equilibrium metal concentration in solution KF and n, Freundlich constants representative of sorption capacity and intensity, respectively Simple expression for a single-component system qmax and b, Langmuir constants related to maximum adsorption capacity and bonding energy of adsorption, respectively Monolayer sorption Homogenous surface but the adsorption is a cooperative process due to adsorbate–adsorbate interactions

( )

(

)

a and b, Temkin isotherm constant and Temkin constant related to the heat of sorption R and T, constant (8.314 kJ/mol K) and temperature

(

)

the recovery of the biosorbed metals. Furthermore, the desorbing solution should not damage the biosorbent structure or affect its efficiency, so it can be reutilized.[5,49,53,97] However, if a cheap biosorbent is used for the recovery of valuable metals, then a destructive method, such as incineration or dissolution with strong acids or alkalis, may be implemented.[154] The selection of an appropriate elutant is strongly dependent on the type of biosorbent and the mechanism of biosorption. Also, the elutant must be less expensive, ecofriendly, efficient, and preferentially metal selective.[52,153] A variety of elutants have been efficiently used as metal desorbents, such as dilute solutions of HCl or NaOH, carbonates and bicarbonates, chelating agents (EDTA), and organic acids (lactic acid, citric acid, and acetic acid).[5,49,97] In some cases, heating or microwaving can increase the efficiency of the desorption process.[154] Zhang et al.[61] tested the efficiency of EDTA, Ca(II), pH, and temperature shocks to desorb Hg(II) and Sb(V) from the EPS produced by aerobic active sludge and found that it was dependent on the EPS–metal complex stability. Nitric acid was more efficient than EDTA in the desorption of different metals bound to the algae Spirulina sp.[134]

Cs, saturation concentration B and Q, constants for the energy of interaction and the amount of metal adsorbed to form a monolayer Multilayer adsorption, assuming Langmuir isotherm applies to each layer K RP, aRP, and β are model parameters For β = 1, the model converts into Langmuir isotherm Wide metal concentration range a, r, and p are model constants

References [169]

[170]

[171]

[172]

[173]

[174]

[175]

PROSPECTS AND CONCLUSIONS Biosorption has potential to be a cost-effective biotechnological method for the treatment of high volumes of complex waste streams contaminated with heavy metals, especially at low concentrations.[5] Numerous biopolymers obtained from different microbial sources have been shown to be effective and selective in metal sequestration and recuperation, over a broad range of conditions (pH, temperature, presence of other ions, and contaminants). Biopolymer modifications, hybrid biopolymer composites, and blends can be further explored to increase not only biosorption efficiency but also the metal specificity and biopolymer stability, regenerability, and reusability.[3] However, it should be taken into consideration that these alterations increase the overall costs of the processes and should not generate toxic by-products. Moreover, differences in the methodologies implemented and in the experimental conditions (pH, temperature, metal and biosorbent concentration, presence of other contaminants) increased the ambiguity of the results reported in the literature, which makes the comparison of biosorbents’ performance more difficult.

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ACKNOWLEDGMENTS The authors acknowledge the support of the Unidade de Ciências Biomoleculares Aplicadas—UCIBIO which is financed by national funds from FCT/MEC (UID/Multi/04378/2013)

and cofinanced by the ERDF under the PT2020 Partnership Agreement (POCI-01–0145-FEDER-007728). Exploratory project IF/00589/2015 attributed within the 2015 FCT Researcher Program, and grant SFRH/BD/131947/2017”.

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Despite their potential, few studies use real metal-­ contaminated wastewaters and even fewer explore the implementation of continuous systems for biosorption. Mechanistic and structural information is required for a deeper understanding of the biosorption mechanism, being fundamental for the development of new mathematical models that describe the process. Eventually, computational modeling can be applied to increase the predictability and efficiency of the biosorption process, simplifying the ­continuous-flow sorption experiments; thus, more conclusive studies describing continuous biosorption tests arise. Considering the biopolymers’ extraction costs, a more economical alternative would be the usage of biopolymer-­ rich biomass in the biosorption process.[104] In fact, several commercialization attempts have been performed, including AlgaSORB™, Bio-Fix, AMT-Bioclaim™, B.V. Sorbex, and RAHCO Bio-Beads. These products used algae or bacteria immobilized in bed reactors.[5] However, these have yet to be proven as commercially successful. Fomina and Gadd [53] stated that those technologies lacked fundamental information about the thermodynamics, mechanism, and limitations of the process. Furthermore, competitive technologies based on physical and chemical methods have been intensely studied and are well established in the wastewater treatment field. Thus, the identification of potential partnerships is required to overcome the direct competitor: ion exchange technology.[5,130] Until now, the majority of the work performed is related to the sorption capacity of the biopolymer. However, the desorption process should also be taken into account. In fact, it is known that environmental and operational changes can lead to the sudden release of accumulated heavy metals, causing major toxicity problems.[61,129] Besides, desorption envisages the reutilization of the biosorbent, reducing the costs associated with production, and the recovery of the metals. This could be particularly useful for precious metals, especially if they were retrieved as nanoparticles. Therefore, to implement biosorption technologies, desorption kinetics and desorbing agents must be further explored. Biopolymer composition is affected by different environmental factors during production. This opens the possibility to design different polymers with distinct characteristics through engineering and manipulation of the operational conditions. The tailoring of the biopolymers could increase metal-binding efficiency and selectivity, getting these biosorbents one step closer to be used in wastewater treatment plants.

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determined by polarographic method. Environ. Monit. Assess. 2013, 185 (8), 6713–6718. Moon, S.-H.; Park, C.-S.; Kim, Y.-J.; Park, Y.-I. Biosorption isotherms of Pb (II) and Zn (II) on Pestan, an extracellular polysaccharide, of Pestalotiopsis sp. KCTC 8637P. Process Biochem. 2006, 41 (2), 312–316. Oyetibo, G.O.; Miyauchi, K.; Suzuki, H.; Endo, G. Mercury removal during growth of mercury tolerant and self-­ aggregating Yarrowia spp. AMB Express 2016, 6 (1), 99–111. Vieira, A.A.H.; Nascimento, O.R. An EPR determination of copper complexation by excreted high molecular weight compounds of Ankistrodesmus densus (Chlorophyceae). J. Plankton Res. 1988, 10 (6), 1313–1315. Papageorgiou, S.K.; Katsaros, F.K.; Kouvelos, E.P.; Nolan, J.W.; Le Deit, H.; Kanellopoulos, N.K. Heavy metal sorption by calcium alginate beads from Laminaria digitata. J. Hazard. Mater. 2006, 137 (3), 1765–1772. Lai, Y.; Annadurai, G.; Huang, F.; Lee, J. Biosorption of Zn(II) on the different Ca-alginate beads from aqueous solution. Bioresour. Technol. 2008, 99 (14), 6480–6487. Raize, O.; Argaman, Y.; Yannai, S. Mechanisms of biosorption of different heavy metals by brown marine macroalgae. Biotechnol. Bioeng. 2004, 87 (4), 451–458. Freire-Nordi, C.S.; Vieira, A.A.H.; Nascimento, O.R. The metal binding capacity of Anabaena spiroides extracellular polysaccharide: An EPR study. Process Biochem. 2005, 40 (6), 2215–2224. Mota, R.; Rossi, F.; Andrenelli, L.; Pereira, S.B.; De Philippis, R.; Tamagnini, P. Released polysaccharides (RPS) from Cyanothece sp. CCY 0110 as biosorbent for heavy metals bioremediation: Interactions between metals and RPS binding sites. Appl. Microbiol. Biotechnol. 2016, 100 (17), 7765–7775. Pereira, S.; Micheletti, E.; Zille, A.; Santos, A.; Moradas-Ferreira, P.; Tamagnini, P.; De Philippis, R. Using extracellular polymeric substances (EPS)-producing cyanobacteria for the bioremediation of heavy metals: Do cations compete for the EPS functional groups and also accumulate inside the cell? Microbiology. 2011, 157 (2), 451–458. Micheletti, E.; Pereira, S.; Mannelli, F.; Moradas-Ferreira, P.; Tamagnini, P.; De Philippis, R. Sheathless mutant of cyanobacterium Gloeothece sp. strain PCC 6909 with increased capacity to remove copper ions from aqueous solutions. Appl. Environ. Microbiol. 2008, 74 (9), 2797–2804. Kiran, B.; Kaushik, A. Chromium binding capacity of Lyngbya putealis exopolysaccharides. Biochem. Eng. J. 2008, 38 (1), 47–54. Mona, S.; Kaushik, A. Chromium and cobalt sequestration using exopolysaccharides produced by freshwater cyanobacterium Nostoc linckia. Ecol. Eng. 2015, 82, 121–125. Ahemad, M.; Kibret, M. Recent trends in microbial biosorption of heavy metals: A review. Biochem. Mol. Biol. 2013, 1 (1), 19–26. Ayangbenro, A.; Babalola, O. A new strategy for heavy metal polluted environments: A review of microbial biosorbents. Int. J. Environ. Res. Public Health 2017, 14 (1), 94. Wang, J.; Chen, C. Chitosan-based biosorbents: Modification and application for biosorption of heavy metals and radionuclides. Bioresour. Technol. 2014, 160, 129–141.

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Environmental Applications: Hydrogels G. Roshan Deen Environmental–Fire Protection

Soft Materials Laboratory, Natural Sciences and Science Education, National Institute of Education, Nanyang Technological University, Nanyang Walk, Singapore

Abstract The release of industrial effluents from chemical industries containing toxic substances, such as heavy metals, dyes, organic solvents, and pharmaceutical waste in the environment, has caused considerable attention due to their toxicity and carcinogenicity to living organisms including humans. The increasing demand for the removal of these toxic pollutants from industrial effluents has led to the development of new functional polymeric materials as effective adsorbents. Adsorption methods for removal of toxic pollutants are popular due to high level of efficiency, simplicity, and economical feasibility. In this entry, adsorbents based on natural and synthetics polymeric materials for the removal of heavy metal ions and dyes from aqueous solution are reviewed. The kinetic models and thermodynamics of adsorption, and the various external factors that influence the adsorption process are also presented in this entry. Keywords: Adsorption isotherms; Adsorption kinetics; Environment pollution; Heavy metals; Hydrogels; Polymers; Toxic dyes.

INTRODUCTION Extensive industrialization and technological advances have been achieved over the last 30 years. However, this progress has been accompanied by a negative impact on the environment in terms of pollution. The industrial and agricultural activities and the consumption of natural resources enhance the stress on the environmental system, through indiscriminate disposal of wastes. For example, industries that facilitate metal plating, mining, dyeing, and leather tanning discharge extensive effluents containing a wide variety of toxic substances such as solvents, heavy metals, and dyes. These toxic materials when found above the tolerance level have detrimental effects on living orgasms and the ecosystem.[1–5] In terms of human health, the dyes and heavy metals cause severe dysfunctions of reproductive system, kidneys, brain, and the central nervous system.[5–10] The major sources of water pollution are shown in Fig. 1. Even at low concentration, the organic dyes cause coloration of water and changes in pH, thus affecting water quality. Therefore, very strict laws are in place for the removal of dyes and heavy metals from industrial wastewater before they are discharged into water bodies. It has to be pointed out that it is impossible to remove all of the dye substances present in industrial wastewater before their discharge, and the discharge thus contains a small concentration of dye substances. Therefore, the removal of heavy metals and toxic dyes from industrial effluent is a stringent requirement to prevent the pollution of environment. This requires Encyclopedia of Polymer Applications, First Edition DOI: 10.1201/9781351019422-140000173 Copyright © 2018 by Taylor & Francis. All rights reserved.

the continuous search for new materials and development of various technologies for satisfactory removal of dyes and heavy metals from industrial wastewater before their safe discharge on land and in water bodies.[11–15] A wide range of methods, such as biological treatments, [15–17] flocculation and coagulation, [18–20] advanced oxidation, [21–23] and adsorption processes, [24–26] are available for removal of dyes and heavy metals from wastewater. Among these, adsorption processes are increasingly ­popular because they are cost-effective and efficient. ­Different types of activated carbon are generally employed as sorbents in adsorption processes; however, their use is often limited due to the high cost involved in the p­ roduction and regeneration processes.[12,27,28] Polymer-based adsorbents (natural and synthetic) have many advantages over conventional adsorbents such as activated carbon and ion exchange resins. These include simple processing methods, introduction of specific chemical functional groups for target-specific removal of pollutants, and effective regeneration of adsorbent under mild conditions.[29] In addition, this type of sorbents can be developed in various architectures such as hydrogels, microgels, beads, fibers, interpenetrating networks (IPNs), and resins with surface grafting with linear ­polymers.[30–33] In this entry, the use of natural and synthetic polymer-­ based adsorbent materials for the removal of dyes and heavy metals from aqueous solutions is presented. The ­different types of adsorption models and thermodynamics of adsorption process are also discussed.

1087

1088

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DYES METALS FLUORIDE

WATER POLLUTION

PESTICIDES

Environmental–Fire Protection

MICROPLASTICS PHARMACEUTICAL WASTE

Fig. 1  Major sources of water pollution

Classification of Dyes Organic dyes possess color because they (i) absorb light in the visible range of electromagnetic spectrum (400– 700 nm), (ii) have at least one color-bearing group called the chromophore, (iii) have a conjugate system (chemical structure with alternating double and single bonds), and (iv) exhibit resonance of electrons.[9] In addition to chromophores, most organic dyes also contain groups known as auxochromes, and these intensify the color of the chromophores. The most important chromophores are azo (−N=N−), carbonyl (−C=O), methane (−CH=), nitro (NO2), and quinoid functional groups. The important auxochromes are amine (−NH3), sulfonate (−SO3H), carboxyl (−COOH), and hydroxyl (−OH) functional groups. Dyes can be classified based on their chemical structure and solubility. Dyes containing one or more azo groups (azo dyes) comprise the largest family of organic dyes.[1,11] The important types of azo dyes are (i) acid dyes for polyamide and protein substrates such as nylon, wool, and silk, (ii) disperse dyes for hydrophobic substrates such as polyester and acetate, and (iii) reactive and direct dyes for cellulosic substrates such as cotton, rayon, paper, and linen. The effectiveness of a dyeing or printing process largely depends on the affinity between the dye and substrate. As a result of this, dyes are designed to have greater affinity for the substrate to be dyed than the medium in which they are dissolved. Some typical azo dyes used in commercial applications are given in Fig. 2. Toxicological Considerations The annual production of dyes is almost 109 kg, of which azo dyes represent about 70% by weight, and azo dyes are widely used in industries such as leather tanning, textile, and paper.[34] These dyes are known to transform to carcinogenic aromatic amines as metabolites in the environment, and therefore, their disposal is a major environmental concern.[35] For example, the dye, Direct Red 28, can lead to the formation of benzidine in the body, which is a carcinogen that affects the bladder.[3,35] The chemical reaction

of reductive cleavage of Direct Red 28 in the ­presence of an enzyme, azoreductase, is shown in Fig. 3. It is generally believed that the carcinogenic effect of the aromatic amines and azo compounds arise from the metabolic conversion of these compounds to electrophilic species that interact with electron-rich sites in DNA. This leads to the formation of DNA adducts causing mutation and subsequent adverse effects in the cell.[35] There exists a correlation between chemical structure of dyes and their related carcinogenicity.[35,36] Hydrophobic azo dyes containing amino groups in the para-position are found to be carcinogenic, while the ortho-isomers are not.[36,37] Water Pollution Caused by Heavy Metals and Dyes Wastewater from leather processing, metal plating, textile, paper, and food industries pose a threat to the environment, as they contain large amount of chemically different dyes and metal salts. The presence of dyes and heavy metals in very low concentrations in such effluents is highly visible in terms of coloration, and the degradation products of the pollutants are mostly carcinogenic.[38] Any metal or metalloid can be considered a contaminant if it occurs above the permissible concentration in water bodies and toxic to living organisms. In general, such metals are called heavy metals.[6] These include lead (Pb), cadmium (Cd), mercury (Hg), arsenic (As), chromium (Cr), copper (Cu), nickel (Ni), silver (Ag), and zinc (Zn). Other less common metallic contaminants are cesium (Cs), cobalt (Co), aluminum (Al), manganese (Mn), ­molybdenum (Mo), strontium (Sr), and uranium (U). The heavy metals are mainly found in effluents from industries involved in the manufacture of metals, alloys, batteries, and electronic devices.[39] Living organisms require varying amounts of heavy metals such as iron, cobalt, copper, manganese, molybdenum, and zinc, as they provide essential cofactors for metalloproteins and enzymes. However, at higher concentrations, these metals can be detrimental to the functions of cells and tissues. A  few heavy metals such as mercury, lead, cadmium, and arsenic are toxic to living organisms even at very low concentrations, as they directly affect many biochemical and physiological processes.[39,40] The permissible concentrations of heavy metals in drinking water and their ­hazardous effects are given in Table 1. Similar to heavy metals, organic dyes also possess a significant impact on living organism and environment. Dyes cause coloration of water bodies even at very low concentration. The coloration caused by dyes hinders light penetration in water bodies and significantly affects the photosynthesis cycle in aquatic life.[41] Many organic dyes do not biodegrade easily due to their complex molecular structure and synthetic origin, leading to accumulation and irreversible formation of toxic by-products or metabolites.

Environmental Applications: Hydrogels 1089

CH3 H3C

N

CH3 N

CH2

N

N

CH3

Cl CH3

COOH Acid Red 2 (Methyl red)

Malachite Green

H3C

CH3

SO3

N

H3C

CH3

O

NH

SO3 Na

N Brilliant Blue G H3C

NH2 N

SO3 Na N

N

Na O3S Congo Red

N H2N

CH3 O

H3C

N

SO3 Na CH3 N

OH O

Cl CH3

OH

Mordant Red (Alizarin Red S) H3C

N CH3

Fig. 2  Chemical structure of typical synthetic ado dyes

Basic Violet 3 (Crystal Violet)

Environmental–Fire Protection

N

1090

Environmental Applications: Hydrogels

SO3 Na NH2 N

N

N

N

Environmental–Fire Protection

NH2

Direct Red 28 SO3 Na azo-reductase enzyme NH2

NH2 H2N

NH2

+

2

Benzidine

SO3 Na

Fig. 3  Chemical reaction of reductive cleavage of Direct Red 28 in the presence of an enzyme Table 1  Permissible limits of heavy metals in drinking water and their hazardous effect Metal ion Symbol Permissible limit (mg L −1)

Hazardous effects

Arsenic

Ar

0.010

Skin problems, visceral cancers, vascular disease

Cadmium

Cd

0.003

Kidney damage, renal disorder

Chromium

Cr

0.050

Headache, diarrhea, nausea, carcinogenic

Copper

Cu

2.000

Liver damage, insomnia, Wilson disease

Lead

Pb

0.800

Damage to fetal brain, kidney damage, nervous disorder

Manganese

Mn

0.400

Hallucinations, nerve damage, forgetfulness

Nickel

Ni

0.070

Dermatitis, chronic asthma, carcinogenic

Uranium

U

0.015

Nephrotoxicity, genotoxicity, developmental defects

Zinc

Zn

0.800

Depression, lethargy, increased thirst

These metabolites have toxic as well as carcinogenic, mutagenic, and teratogenic effects on aquatic life and humans.[42] Therefore, it is important to remove these toxic dyes from wastewater before they are safely discharged in the environment. Common Separation Methods for Removal of Toxic Metals and Dyes Many conventional techniques are commonly used for the removal of heavy metals and dyes from wastewater, and these include chemical oxidation, reverse osmosis, biological treatment, coagulation, and chemical or physical adsorption (sorption).[15–26] The heavy metal ions of industrial wastewater can also be effectively removed to acceptable levels by precipitating the metal ions in an insoluble

form such as hydroxides, carbonates, sulfides, or sulfates. The main drawback of this method is the coprecipitation of other metals such as iron or aluminum and also the requirement of certain pretreatment processes.[43–45] Ion exchange resins are used in water treatment and ­pollution control to remove traces of ion impurities from water. The method has poor selectivity towards heavy metal ions, alkali, and alkaline earth metals. Membrane filtration (ultrafiltration, nanofiltration, and reverse osmosis) is widely used in the removal of pollutants from water.[46–50] The main disadvantages of this method are high operational cost and membrane fouling. The other forms of membrane filtration methods based on electrotreatments are electrodialysis, membrane electrolysis, and electrochemical precipitations. Due to the high operational cost involved, these methods are used less extensively.[45,51–53]

Solvent extraction is another separation technique for the removal of metal ions in the form of liquid complexes. This technique is based on differences in solubility of two immiscible liquid phases (aqueous and organic) in contact with each other. This method of separation is widely used due to simplicity in operation, but the use of large volumes of organic solvents and the loss of materials through phase disengagements are some of the limitations of this ­technique.[54–57] Despite the large number of methods available for removal of heavy metals and dyes from wastewater, their application is often limited to high operating costs, nonselectivity, formation of toxic degradation products, and the need for sludge removal after the treatment processes.[56,57] Adsorption processes using natural and synthetic materials have become increasingly popular to treat industrial effluents containing heavy metals and dyes, due to their ­simplicity, efficiency, and economical feasibility.[28,58–60] Adsorption of Heavy Metals and Dyes on Polymer Materials In recent years, adsorption materials that are cost-effective and have high adsorption capacity have gained much attention. Different types of activated carbons are generally used for removal of a wide variety of pollutants such as heavy metals, organic molecules, dyes, and dissolved gases.[12,61] The use of activated carbon materials is limited because of high capital cost involved in their manufacture and regeneration processes. Other alternative materials explored are natural materials that require minimal pretreatment and are renewable.[11–13] Adsorbents based on polymers exhibit superior properties over conventional adsorbents (activated carbons, ion-exchange resins, etc.). The processing of polymer-based adsorbents is simple, and desired chemical functional groups for effective adsorption can be easily incorporated. Further, under mild conditions, the materials can be regenerated (desorption) for subsequent adsorption cycles. The chemical reagents required for desorption process mainly depends on the properties of the adsorbed heavy metals and dyes. The typical reagents used are inorganic acids (hydrochloric acid, sulfuric acid, nitric acid), bases (sodium hydroxide, sodium carbonate), or organic solvents (methanol, acetone).[62–64]

The process of removal of heavy metal ions and dyes from water using polymer-based adsorbents is mainly driven by physical or chemical interactions between the pollutant and the adsorbent. If the adsorbate molecules (heavy metals, dyes) are attracted by weak van der Waals forces by the adsorbent, the adsorption process is known as physical adsorption or physisorption; on the other hand, when the adsorbate is bound to the surface of adsorbent by chemical bonds, the adsorption is known as chemical adsorption or chemisorption. Some general characteristics of physisorption and chemisorption are given in Table 2. Adsorbents based on functional polymers (functional polymer adsorbents) have gained much interest in recent years due to their interesting properties such as large surface area, mechanical strength, tunable surface chemistry, and regeneration under mild conditions. Such functional ­polymers can be prepared with a wide range of physicochemical properties (size distribution, porosity, hydrophobicity, etc.), and suitable functional groups can be incorporated easily for target-specific removal of ­pollutants.[33,65–67] Using a range of controlled polymerization methods such as atom-transfer radical polymerization, reversible addition-fragmentation chain transfer polymerization, and ring-opening metathesis polymerization, functional polymers in various interesting architectures (linear, blocks, brushes, grafts, crosslinked networks) can be synthesized.[68,69] Functional polymer adsorbents can be easily and completely regenerated with no significant loss in sorption capacity,[29] and with the correct choice of monomers, ­environmentally compatible adsorbents can be obtained. HYDROGELS One of the most used polymer material in the adsorption of pollutants from water is hydrogels. Hydrogels are three-­ dimensional polymer networks with the ability to absorb large amounts of water or physiological fluids, while maintaining their dimensional stability.[70] The structural integrity of hydrogels in their swollen state is maintained by either physical or chemical cross-linking. Chemically cross-linked networks have permanent cross-link

Table 2  General features of physisorption and chemisorption Physisorption

Chemisorption

Low heat of adsorption

High heat of adsorption

Monolayer or multilayer adsorption

Monolayer adsorption

Low activation energy for adsorption

High activation energy for adsorption

Adsorption is rapid, nonactivated, reversible

Adsorption is nonrapid and reversible only at high temperature

No dissociation of adsorbed species

Increase in electron density in the adsorbent-adsorbate interface

Adsorption takes place at low temperature

Adsorption takes place at high temperature

Adsorption is nonspecific

Adsorption is specific

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1092

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junctions, while physical cross-linking have nonpermanent or transient junctions. The transient junctions arise from either entanglement of polymer chains or physical interactions such as ionic interactions, hydrogen bonds, or ­hydrophobic interactions.[70–72]

electrostatic interactions, and hydrophobic interactions between the functional groups and dye molecules are involved in the adsorption process.[94] A list of few synthetic functional polymer adsorbents for the removal of targeted dyes and their adsorption capacities is given in Table 4.

Hydrogels in the Removal of Toxic Metal Ions

Adsorbents Based on Biopolymers

The heavy metal uptake mechanism of the hydrogels is related to the high water permeability and to the presence of specific functional groups. Specifically, functional hydrogels composed of acrylic acid, acrylamide, 1-­acrylamido-2-methyl-1-propane sulfonic acid, N-vinyl imidazole, 4-vinyl pyridine, and N-methyl piperazine have been used as adsorbents in the removal of heavy metals from water.[73–76] A few examples of functional monomers used for the development of functional hydrogels are given in Fig. 4. In addition, other natural polymeric materials such as chitosan, pectin, sodium alginate, guar gum, and cellulose have also used in the removal of heavy metals.[77–79] A few hydrogels for the adsorption of toxic metal ions from aqueous environment and their adsorption capacities are given in Table 3.

Natural polymers or biopolymers are an attractive alternate to synthetic polymer-based adsorbents. This is due to their ease of availability, physicochemical characteristics, chemical stability, high reactivity, and selectivity towards dyes and heavy metal ions.[11] Polysaccharides in general, are renewable and biodegradable, and have the capacity to remove a wide variety of pollutants through physical and chemical interactions. However, the main disadvantages of adsorbents based on polysaccharides are their low porosity and pH dependence. Despite the limitations, polysaccharides such as starch, alginate, cyclodextrin, cellulose, and chitosan have gained wide research attention as ­biosorbents.[11,107,108] Cellulose is one of the most abundant organic materials, and is obtained mainly from wood pulp and cotton. The molecular structure of cellulose comprises β-d-­glucopyranose repeating units that are covalently linked through acetal functions between the OH group of the C4 and C1 carbon atoms, as shown in Fig. 5. The presence of many hydroxyl groups in cellulose favors the adsorption of heavy metals and azo dyes from wastewater. Modified cellulose, [109,110] cellulose gels regenerated from aqueous alkali–urea solvent, [111] and IPN based on cellulose–­polymethacrylic acid–bentonite [112] have been used as adsorbents for the removal of dyes such as methyl orange, Reactive red 120, Congo red, and methylene blue.

Hydrogels in the Removal of Dyes Specific functional groups can be incorporated in adsorbents based on functional polymers for target-specific removal of dyes or group of dyes from wastewater. Microgels,[30] IPN,[31,92] cross-linked polymer resins with grafted polymer chains, polymer beads, polymer fibers, etc. have been investigated as adsorbents for the removal of various azo dyes.[30–33,93] Depending on the functional group present in these adsorbents, hydrogen bonding, ion exchange,

O O

O

OH

OH

HO

Acrylic acid

Itaconic acid

H2C

O

N

N H2C

Vinyl pyridine

O Acryloyl morpholine

N

O H2C

N

O

H2C

Vinyl pyrrolidine

Fig. 4  Chemical structure of some important functional monomers

N

Acryloyl ethyl piperazine

Environmental Applications: Hydrogels 1093

Cu(II)

Reference

1

Magnetic nanocomposite based on pectin

48.99

5.73

[80]

2

Polymer blend of chitosan and PVA

3

PVA-TiO2 nanofibers

U(VI)

156

5.0–6.0

[81]

U(VI) Th(IV)

196.1 238.1

6.0

[82]

4

Acrylamide-acrylic acid hydrogels

5

Hydrogels of poly(propylene glycol), 1,3-propanediamine, and 1,2-ethanedithiol

U(VI)

236.6

13

Fe (III) Co(II) Ni(II) Cu(II) Zn(II) Cd(II) Hg(II) Pb(II)

329 108 143 190 241 272 82 294

6

Hydrogels of modified guanidine with 2-acrylamido-2-methylpropane sulfonic acid

Pb(II) Cd(II)

22.73 27.78

5

[85]

7

Hydrogels of N-hydroxymethyl methacrylamide and thiourea

Pt(II) Pd(II)

477 407

~1

[86]

8

Hydrogels of acrylamide and sodium acrylate

Cu(II) Cd(II)

24.05 32.99

5.0–6.0

[87]

9

Microspheres based on modified polystyrene

Hg(II)

33.2

7.0

[88]

10

Polymer beads based on hydroxyethyl methacrylate and methacrylamide histidine

Cu(II)

122.7

5.0

[89]

11

Surface-modified magnetic Fe3O4 nanoparticles

U(VI)

12.33

4.5

[90]

12

Hydrogels based on chitosan and acrylic acid

Ni(II)

161.8

3.0–7.0

[91]

[83] [84]

Table 4  A few important polymeric adsorbents for the removal of dyes and their adsorption capacities No. Type of adsorbent Dye Uptake capacity (mg g−1)

References

1

Poly(glycidyl methacrylate)-grafted sulfonamide-based polystyrene resin with tertiary amine

Everzol black Everzol red Calcon

980 1,000 1,000

[95] [33]

2

Porous PVA hydrogels

Congo red Methyl orange

35 NA

[96] [95]

3

Poly(amidoamine-co-acrylic acid) copolymer

Direct red 31 Direct red 80

3,400 3,448

[97] [98]

4

Poly(N-vinyl-2-pyrrolidone)

Congo red Methyl orange

NA NA

[99] [97]

5

Poly(N-vinyl-2-pyrrolidone-co-acrylonitrile) treated with hydroxylamine-hydrochloride

Acid fast-yellow G Direct blue 38 Reactive red-SH

7.6 7.0 7.4

[100] [101] [101]

6

Poly(acrylamide-co-acrylic acid) hydrogels

Janus green B

44.0

7

Poly(acrylamide-co-sodium acrylate) hydrogels

Janus green B

7 × 10  mol g

[103]

8

Poly(acrylamide-co-sodium 4-styrenesulfonate)

Janus green B

67%

[31]

[102] −4

−1

9

Poly(acrylamide-co-sodium methacrylate)

Janus green B

90%

[104]

10

Poly(N-isopropylacrylamide-co-acrylic acid) microgel

Orange 2

73%

[30]

11

Poly(N-acryloyl-N’-ethyl piperazine-co-vinyl caprolactam) hydrogels

Congo red

90%

[105]

12

Poly(N-acryloyl-N’-ethyl piperazine-co-maleic acid) hydrogels

Congo red

85%

[106]

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Table 3  A few important polymeric adsorbents for the removal of heavy metals ions and their adsorption capacities pH No. Type of adsorbent Metal ion Uptake capacity (mg g−1)

1094

Environmental Applications: Hydrogels

O

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chemical modification of starch, water-resistant adsorbents based on starch for the removal of heavy metal ions and dyes have been developed.[113–115] Chemically cross-linked starch-based adsorbents are an important class of starch derivatives. Some of the cross-linked materials include starch cross-linked with epichlorohydrin,[108] starch cross-linked with quaternary ammonium groups, [116] starch cross-linked with tertiary  amine groups, [117] and starch modified with ethylenediamine.[118] These materials possess a three-­ dimensional network  structure and swell in aqueous ­solution without dissolution. Alginate is a natural polymer and is present in brown algae. It is a linear 1,4-linked copolymer of β-d-­ mannuronic acid (M) and α-l-guluronic acid (G) residues in varying proportions, order, and molecular weights. The chemical structure of alginate is shown in Fig. 7. One of the most important properties of alginates is the ability to form hydrogels and has the capacity to remove toxic pollutants from aqueous solutions.[119,120] In the presence of divalent cations, e.g., Ca2+ and Ba2+, an aqueous solution of alginate readily transforms into a hydrogel. The hydrogel is formed due to the interaction between guluronic acid residues and divalent cations, leading to the formation of a network of alginate filaments that are held together mainly by ionic interactions. Biosorbents based on alginates exhibit high sorption capacities towards dyes such as Basic red 18 and Basic blue 41.[121] Chitosan is a natural polymer produced mainly from the partial deacetylation of chitin, which occurs in the exoskeleton of crustaceans.[122] It is composed of units of β(1– 4)-d-glucosamine and β (1–4) N-acetyl-d-glucosamine in various proportions. The chemical structure of chitosan is shown in Fig. 8. Its functional properties depend on average molar mass, degree of acetylation, viscosity, and pH of the medium. Gels based on chitosan can be developed in various forms such as flakes, powders, beads, and membranes [123] for the removal of heavy metals and dyes from aqueous

HO H

H OH O H

H O H

H

O

OH

HO

HO

OH

H

H

H

H

n

Fig. 5  Chemical structure of cellulose

Next to cellulose, starch is the most abundant polysaccharide and is present in living plants. Starch is a mixture of amylopection and amylose but contains only a single type of carbohydrate, i.e., glucose. The structure of starch is shown in Fig. 6. Starch, in its unmodified form has many advantages, which makes them excellent materials for industrial use, but the hydrophilic nature of the ­material is a major drawback that limits its use as adsorbents. Through H OH O HO

H

H HO H

H

O

H O HO

H H

OH O

H

O

OH

1,6-glycosidic linkage

H O H

HO H

H

O

OH

H OH

O

1,4-glycosidic linkage

HO H

H H

O

OH O

Fig. 6  Chemical structure of starch

OH HOOC HOOC

HO

OH O

OH O

O HO

OH O

HOOC M M = β-D-Mannuronic acid

M

O

O

O

OH G G = α-L-guluronic acid

Fig. 7  Chemical structure of alginate

OH

COOH G

Environmental Applications: Hydrogels 1095

CH2OH O

H O

H

H

OH

O

H OH

O

H

Environmental–Fire Protection

CH2OH

H

H H H

H

NH2

NH2

Fig. 8  Chemical structure of chitosan

solutions. Introduction of specific functional groups onto chitosan-based adsorbents by grafting greatly improves the chelation properties for effective removal of anionic and cationic dyes from water.[124,125] The use of raw natural materials based adsorbents is limited due to various factors such as high solubility, low affinity towards heavy metals and dyes, and poor regeneration. Chemically modified natural materials or their derivatives are widely used as commercial adsorbents due to high efficiency and high rate of regeneration. KINETICS OF ADSORPTION To design an appropriate adsorption system and to quantify the uptake rate of heavy metals and dyes, the kinetics of sorption process must be understood. The adsorption process or sorption process, in general, consists of several independent processes that act in series or parallel, such as the following[126]: 1. Transport of heavy metals and dyes (sorbate) from bulk liquid to the external surface of the adsorbent. This process is called the film diffusion process. 2. Diffusion of heavy metals and dyes into the adsorbent with small of adsorption on the surface of the ­adsorbent. This process is called the particle diffusion. 3. Binding of heavy metals and dyes on the adsorption sites on the surface of adsorbent. The velocity at which these processes occur is dependent on factors such as concentration of sorbate, pH, ionic strength, temperature, and nature of adsorption material. Analysis of adsorption kinetics allows the prediction of adsorption process and the possible adsorption mechanism. The following four kinetic models are widely used to study the kinetics of adsorption:

1. 2. 3. 4.

Lagergren pseudo-first-order model Lagergren pseudo-second-order model Elovich model Intra-particle diffusion model

Lagergren Pseudo-First-Order Model Lagergren in 1898 presented a first-order rate equation to describe the kinetic process of liquid–solid phase adsorption of oxalic acid and malonic acid onto charcoal.[127] This model is the earliest model pertaining to the a­ dsorption rate based on adsorption capacity. It is given by the ­following equation: dqt = k1 qe − qt dt

(

)

(1)

where qe and qt (mg g−1) are the adsorption capacities at equilibrium and time t (min), respectively, and k1 is the pseudo-first-order rate constant. Integrating Eq. 1 with the boundary conditions of qt = 0 at t = 0, and qt = qt at t = t, gives:[128]  qe  ln   = k1t  qe − qt 

(2)

The variable qt is expressed as qt =

(c

)

− ct V

0

m



(3)

where c0 is the initial concentration of heavy metals or dyes, ct is the concentration at time t, V is the volume of initial solution (L), and m is the amount of adsorbent (g). The degree of deviation of qeexp and qe is defined as

α=

(q

ex e

− qe

qeex

) × 100

(4)

1096

Environmental Applications: Hydrogels

Environmental–Fire Protection

where qeex (mg g−1) is the experimental value of heavy metal ions or dyes adsorbed on the adsorbent. A plot of log (qe − qt) vs. t should be a straight line, the slope of which gives the rate constant k1. If the plot is not linear, it indicates the inappropriateness of the pseudo-first-order equation to describe the sorption process. This model considers the rate of occupation of adsorption sites to be proportional to the number of unoccupied sites. In recent years, this model has been widely used to describe the adsorption of pollutants from wastewater.[129–131] Lagergren Pseudo-Second-Order Model Ho described a kinetic process of adsorption of divalent metal ions onto peat through cation-exchange mechanism.[132] The interaction of heavy metal ions on the surface of adsorbent involves valence forces (through sharing or exchange of electrons) and tends to find sites that maximize their coordination number with adsorbent surfaces.[133] The pseudo-second-order model is given by the ­following equation: 2 dqt = k2 qe − qt dt

(

)

(5)

Rearranging the preceding equation and integrating with the boundary conditions qt = 0 t = 0, and qt = qt at t = t, yields 1 t t = + qt k2 qe2 qe

qt =

1 1 ln(α β ) + ln t β β

(8)

where α (mg g−1 min−1) is the constant of initial adsorption rate because the value of dqt /dt approaches a when qt approaches 0, and β (g mg−1) is the extent of surface coverage. The Elovich coefficients can be obtained from a plot of qt vs. ln t. Elovich equation has been widely used to describe the adsorption of gas onto solid systems and the adsorption process of pollutants from aqueous solutions using ­adsorbents based on natural polymers.[137,138] DIFFUSION MODELS FOR ADSORPTION PROCESS A typical liquid–solid adsorption involved film diffusion, intra-particle diffusion, and mass action. For physical adsorption processes, the mass action is a very rapid process and can be negligible for kinetic investigations. The kinetic process of sorption is always controlled by ­liquid-film diffusion or intra-particle diffusion.[139] Liquid-Film Diffusion Model Linear Driving Force Rate Law

(6)

where k2 (g mol−1 min−1) is the pseudo-second-order rate constant, and qe and qt are the adsorbed amounts at any time t (min) and equilibrium, respectively. Based on Eq. 5, a plot of t/qt vs. t is linear, and the values of qe and k2 are determined from the slope and intercept, respectively. The adsorption of heavy metal ions, dyes, herbicides, oils, and organic substances from aqueous solutions on many adsorbents follow the pseudo-second-order model. The pseudo-second-order rate constant k2, in general, is a function of experimental conditions such as temperature, and properties of sorbents, metal ions, and dyes.[134–136] Elovich Equation This equation has general application in sorption kinetic modeling, and it assumes that the actual solid surfaces are energetically heterogeneous, and at low surface coverage, neither desorption nor interactions between the adsorbed species have a substantial effect on the ­adsorption process.[137] The equation is given as: dqt = α e( − β qt ) dq

The integrated form of this equation is

(7)

In liquid–solid adsorption, the rate of solute accumulation in the solid phase is equal to that of solute transfer across the liquid film according to the mass balance law. The rate of solute accumulation in a solid particle is equal to the product of volume of particle (Vp) and average solute ­concentration in the solid. The rate of solute transfer across the liquid film is proportional to the surface area of the particle (As) and the concentration gradient (C-Ci). The equation is given as  ∂q  Vp   = k f As C − Ci  ∂t 

(

)

(9)

where Ci and C are the concentrations of solute at the particle/liquid interface and in the bulk of the liquid (far from the surface), respectively, and k f is the mass transfer ­coefficient of the film.[140] The equation can be rearranged as: As  ∂q  C − Ci   = k f ∂t Vp

(

)

(10)

This equation is called the linear driving force rate law and is usually applied to describe the mass transfer through the liquid film.

Environmental Applications: Hydrogels 1097

The equation [141] to describe the film diffusion mass ­transfer rate is given as  q ln  1 − t  = − R1t q  e

(11)

where R1 is the liquid-film diffusion constant and is given by the following equation: R1 =

3 De1 r0 ∆ro k ′

(12)

where De1 (cm2 min−1) is the effective liquid film diffusion coefficient, r0 (cm) is the radius of sorbent beads, Δr0 (cm) is the thickness of liquid film, and k′ is the adsorption ­equilibrium constant. A plot of ln(1 − qt /qe) vs. t should be linear with a slope equal to −R1, if film diffusion is the rate-limiting step. The adsorption of phenol by a polymeric sorbent at various temperature and initial concentrations has been studied using the film diffusion mass transfer rate equation.[139]

Homogeneous Solid Diffusion Model The typical intra-particle diffusion model called the homogeneous diffusion model describes the mass transfer in an amorphous and homogeneous sphere, [140] and is given by the following equation: (13)

where Ds is the intra-particle diffusion coefficient, r is the radial position, and q is the adsorption quantity of ­solute in the solid that varies with radial position at time t. The solution to the above equation was given by Crank, [142] for the “infinite-bath” case, where the sphere is initially free of solute and the concentration of the solute at the surface remains constant. Neglecting the external film resistance according to the constant surface concentration, the solution is given as q 2R = 1+ qs πr

∞

∑ n =1

(−1)n nπ r  − Ds n2π 2t  sin exp    n R R2

(14)

where R is the total particle radius. The average value of q in a spherical particle at any time is defined as q=

3 R3

∫

q 6 = 1− 2 q∞ π

∞

∑ n1 exp  2

n =1

− Ds n2π 2t   R2

(16)

where qs is the average concentration in the solid at infinite time. For early time, when q /q∞ < 0.3, Eq. 16 is simplified to give: q  D  = 6  2s  Rπ q∞

1/ 2

t1/ 2

(17)

From a plot of q /q∞ against t1/2, the value of Ds for ­early-time adsorption can be determined. This equation predicts that the rate of adsorption decreases with an increase in the particle size of adsorbent and vice versa. For a ­long-time adsorption process, Eq. 16 is given as q 6  − DLπ 2t  = 1 − 2 exp   R2  q∞ π

(18)

where DL is the diffusion constant for long-time ­adsorption. The linear form of Eq. 18 is written as:

Intra-particle Diffusion Model

∂q Ds ∂  2 ∂q  = 2 r  ∂t r ∂r  ∂r 

where q(r) is the local value of the solid-phase concentration. Using the solution for q(r) into the earlier equation, the following solution is obtained:

 q  − DLπ 2 6 t + ln 2 ln  1 −  = 2 q∞  R π 

(19)

From a plot of ln(1 − q /q∞ ) against t, the value of DL can be obtained. The assumption in this model is that the constant surface coverage is likely to be violated during the longtime diffusion process. Therefore, this model is generally valid for short-time diffusion processes.[140] Weber–Morris Model To estimate the overall rate of sorption of a porous sorbent, the sorption rate on an active site, the external mass transfer, and intra-particle diffusion must be considered.[143] A simplified intraparticle diffusion model is widely used to identify the diffusion mechanism, which refers to the ­theory proposed by Weber and Morris.[144] It is assumed that three steps occur during sorption process: (i) the first sharp linear stage is the rapid external diffusion of sorbate through the solution toward the interface space (external diffusion); (ii) the second stage is a gradual sorption stage, where intraparticle diffusion is rate limiting; and (iii) the third state is the intraparticle diffusion beginning to slow down due to low concentration in the solution phase and less availability of sorption sites. The model is expressed as

R

0

q(r )r2 dr

(15)

qt = kpt1/ 2 + c

(20)

Environmental–Fire Protection

Film Diffusion Mass Transfer Rate Equation

1098

Environmental Applications: Hydrogels

Environmental–Fire Protection

where kp is the intra-particle diffusion rate constant (mg g−1 min−1/2) and c (mg g−1) is a constant that is related to the thickness of the boundary layer. According to this model, if the plot of qt vs. t1/2 is a straight line, then intra-particle diffusion is involved during the sorption process, and if this line passes through the origin point, then the rate-controlling step is intra-particle diffusion. If the plot shows multilinear regions, then two or more steps are included in the sorption process such as external diffusion and intra-particle diffusion.[145]

A negative value of ΔG° suggests that the sorption process is spontaneous and thermodynamically favorable under the conditions studied. The term chemisorption refers to the formation of a chemical bond between the sorbate ­molecule and the solid sorbent (solid surface), and physisorption refers to the formation of physical association (van der Waals force) between the sorbate and sorbent surfaces.[94] The value of ΔG° is between −20 and 0 kJ mol−1 for physisorption, −80 and −400 kJ mol−1 for chemisorption, and −20 and −80 kJ mol−1 for physisorption together with chemisorption.[94,147]

Dumwald–Wagner Model Thermodynamic Adsorption Models The intraparticle diffusion model proposed by Dumwald and Wagner[146] is given by the following equation: F=

q 6 = 1− 2 qs π

∞

∑ n1 exp( −n kt ) 2

2

(21)

n =1

where k (min−1) is the rate constant of adsorption, and this equation is simplified as: log (1 − F2 ) = −

k t 2.303

(22)

A plot of log (1 − F2) against t is linear, and the rate ­constant k is obtained from the slope.

Thermodynamic Sorption Parameters Useful information concerning the inherent energy change of the sorption process is obtained from thermodynamic parameters such as Gibbs free energy change (ΔG°), standard enthalpy change (ΔH°), and standard entropy change (ΔS°). These quantities can be obtained using the ­following equations: ∆S ∆H  − R RT

(23)

where Kd is the sorption coefficient (ml g−1), R is the gas constant (8.314 J K−1 mol−1), and T is the absolute ­temperature (K). The sorption coefficient, Kd, is defined as Kd =

c0 − ce V ce m

(24)

where c0 and ce are the initial and equilibrium concentrations of sorbate (mg L−1), respectively; V is the volume of sorbate solution; and m is the weight of sorbent (g). The sorption spontaneity can be evaluated by calculating the Gibbs free energy change (ΔG°) using the ­following relationship: ∆G = ∆H  − T ∆S

Langmuir Model This model describes monolayer adsorption on the surface of adsorbent (solid surface), with a finite number of identical adsorption sites, and no subsequent interactions among the adsorbed molecules. The linearized expression of the model is given as [94] ce ce 1 = + qe qm qm KL

THERMODYNAMICS OF SORPTION

ln Kd =

The thermodynamics and adsorption capacity of sorbent materials can be understood by analyzing the sorption equilibrium data with different isotherm models such as Langmuir, Freundlich, and Dubinin–Radushkevich.

(25)

(26)

where ce is the concentration of sorbate in solution at equilibrium (mg L−1), qe is the amount of sorbate on the adsorbent (mg g−1), qm is the theoretical saturation capacity of the monolayer (mg g−1), and KL is the Langmuir sorption constant (L mg−1). A plot of ce /qe against ce should be linear, and the values of qm and KL can be calculated from the intercept and slope, respectively. The dimensionless equilibrium parameter, R L , is used to indicate the favorability of adsorption process and is given by the following expression [148]: RL =

1 1 + KL c0

(27)

where c0 is the highest initial concentration of sorbate solution. The following R L values indicate the favorability of adsorption process: i. R L > 1: the process is unfavorable ii. 0  1: the adsorption affinities increase as a result of increasing adsorbate concentration. iii. 1/n = 1: the amount of adsorbates adsorbed on the adsorbent is proportional to that in the solution at a fixed ratio. This indicates a linear adsorption and equal adsorption energies for all adsorption sites. Temkin Isotherm Model This model describes the interaction between the adsorbent and the adsorbing molecules. It assumes that the heat of adsorption of all the molecules in the layer decreases linearly. The linearized Temkin model is expressed as [151] qe = B ln A + B ln ce

qm RT ∆H

ln qe = ln qm − kε 2

(31)

where qe and qm are the amount of heavy metal or dye adsorbed per mass of adsorbent at equilibrium and at saturation (mg g−1), respectively; k is a constant related to the adsorption energy (mol2 J−2); and ε is the Polanyi p­ otential that is related to the equilibrium heavy metal or dye ­concentration expressed as [94]:  1 ε = RT ln  1 +  ce  

(32)

From a plot of ln qe vs. ε2, the values of k and qm are obtained from the slope and intercept, respectively. Using the calculated value of k, the mean adsorption energy E can be evaluated using the following equation [94,152,153]: E=

1 −2 k

(33)

The sorption energy, E (kJ mol−1), is the mean free energy for transferring one mole of adsorbate from infinity to the surface of adsorbent. This is widely used to evaluate the type of adsorption process, namely, physisorption (E = 1–8 kJ mol−1) and chemisorption (E > 8 kJ mol−1).[152,153] FACTORS THAT INFLUENCE ADSORPTION Influence of Nature and Properties of Adsorbent Materials

(29)

where qe is the amount of heavy metal or dye adsorbed per mass of adsorbent at equilibrium (mg g−1), A is the equilibrium binding constant (L mg−1), and B is the Temkin constant that is related to heat of adsorption (mg g−1) according to the following equation, B=

between the physical and chemical adsorption processes. The model is expressed as [152]

(30)

The characteristics of adsorbent materials such as physical properties (surface area, size, morphology, surface charge, and aggregation state) and chemical properties (type of functional groups and reactivity) play a key role in the adsorption of pollutants from aqueous solutions (heavy metals, dyes, pharmaceuticals, and pathogens).[11,94] Influence of Properties of Heavy Metals and Dyes

where R is the gas constant (8.314 J K−1 mol−1), T is the absolute temperature (K), and ΔH is the heat of adsorption or adsorption energy (J mol−1). From a plot of qe vs. ln ce, the values of A and B can be calculated from the intercept and slope, respectively.

The properties of heavy metals (ionic radius, valence, ratio of charge to ionic radius, and electronegativity) and dyes (chemical structure and type) can influence the adsorption process. These properties affect the energy of surface binding and interaction and accessibility for heavy metal ions and dye molecules.[154,155]

Dubinin–Radushkevich Isotherm Model

Influence of Solution Properties

This model does not assume a homogeneous surface or constant adsorption potential. It is used to distinguish

The pH of solution is the most important parameter controlling the adsorption of heavy metal ions and dyes at the

Environmental–Fire Protection

This model describes multilayer adsorption, assuming heterogeneous surface energies at different sites of the ­adsorbent.[149] The linearized equation is expressed as [150]

1100

Environmental–Fire Protection

solid–water interface. It can affect the charge density and colloidal behavior of polymeric adsorbents. Neutralization of surface charge may result in aggregation and decrease in  availability of surface sites for the adsorption to take place.[156] Solution pH also controls the speciation of heavy metal ions in the solution influencing the adsorption process. Ionic strength of solution also plays an important role in the adsorption process of pollutants. Electrolyte ions can shield the electrostatic charge and thus reduce the stability of colloids and promote aggregation due to electrical ­double layer forces.[157] The Effect of Coexisting Heavy Metal Ions Adsorption of heavy metal ions under competitive condition by adsorbents is a matter of concern, as many heavy metal ions coexist in industrial wastewater. Although some adsorption sites favor only certain heavy metal ions, the presence of other metal ions can lead to a decrease in adsorption capacity of the material. Significant competition for adsorption can occur at high concentration of heavy metal ions.[158] Heavy metal ions with greater charge to ionic radius ratio or electronegativity have higher affinity to the adsorption sites. Some metal ions such as Na +, Ca2+, Mg2+, Cu2+, and Ni2+ do not compete for active sites and are not known to influence the sorption of other heavy metal ions. [159] The Effect of Natural Organic Matter Natural organic matters are ubiquitous in aquatic systems, and have a variety of functional groups, such as phenolic, hydroxyl, carbonyl, amine, amide, and ester, which can bind with heavy metal ions and organic molecules through physical and chemical interactions. Adsorbents can also become coated with the natural organic matters  which can increase the sorption process of heavy metals and dyes through chemical complexation and electrostatic ­interactions.[160,161] CONCLUSIONS AND PERSPECTIVE Due to rapid industrial development, large volume of wastewater from chemical industries containing toxic substances (heavy metal ions, dyes, pharmaceutical waste, organic compounds, etc.) is discharged in the environment. In terms of environmental protection and sustainability, there is a need to introduce new processes for wastewater treatment and regeneration of materials. New functional polymeric materials and derivatives are becoming increasingly popular as adsorbent materials for the removal of pollutants from wastewater due to adsorption efficiency and low cost for regeneration, in comparison to conventional adsorbents. The properties of these adsorbent

Environmental Applications: Hydrogels

materials and the solution have a significant effect on the adsorption process and adsorption capacity. ACKNOWLEDGMENT Financial support from the National Institute of Education, Nanyang Technological University, under the research grant NIEAcRF RI 1/15 RGR is gratefully acknowledged. REFERENCES 1. Duruibe, J.O.; Ogwuegbu, M.D.C.; Egwurugwu, J.N. Heavy metal pollution and human biotoxic effects. Int. J. Phys. Sci. 2007, 2 (5), 112–118. 2. Kadirvelu, K.; Kavipriya, M.; Karthicka, C.; Radhika, M.; Vennilamani, N.; Pattabhi, S. Utilization of various agricultural wastes for activated carbon preparation and application for the removal of dyes and meal ions from aqueous solutions. Biores. Technol. 2003, 87 (1), 129–132. 3. Weisburger, E. Cancer-causing chemicals. In Cancer— The Outlaw Cell; LaFond, R.F.; Ed.; American Chemical ­Society: Washington, DC, 1978, 173–186. 4. Shen, D.; Fan, J.; Zhou, W.; Gao, B.; Yue, Q.; Kang, Q. Adsorption kinetics and isotherm of anionic dyes onto organo-bentonite from single and multi-solute systems. J. Hazard. Mater. 2009, 172 (1), 99–107. 5. Forgacs, E.; Cserhati, T.; Oros, G. Removal of synthetic dyes from wastewaters: A review. Environ. Int. 2004, 30 (7), 953–971. 6. Sivagangi Reddy, N.; Krishna Rao, K.S.V. Polymeric hydrogels: Recent advances in toxic metal ion removal and anticancer drug delivery applications. Ind. J. Adv. Chem. Sci. 2016, 4 (2), 214–234. 7. Yang, Q.; Adrus, N.; Tomicki, F.; Ulbricht, M. Composites of functional polymeric hydrogels and porous membranes. J. Mater. Chem. 2011, 21 (9), 2783–2811. 8. Meade, R.H. Contaminants in the Mississippi River. ­Geological Survey Circular 1133, Reston, Virginia; U. S. Geological Survey: Denver, CO, 1995. 9. Abrahart, E.N. Dyes and their Intermediates; Chemical Publishing: New York, 1977, 1–12. 10. Aplin, R.; Wait, T.D. Comparison of three advanced ­oxidation processes for degradation of textile dyes. Water Sci. Technol. 2000, 42 (5–6), 345–354. 11. Panic, V.V.; Seslija, S.I.; Nesic, A.R.; Velickovic, S.J. Adsorption of azo dyes on polymer materials. Hem. Ind. 2013, 67 (6), 881–900. 12. Amin, N. Removal of direct blue-106 dye from aqueous solution using new activated carbons developed from pomegranate peel: Adsorption equilibrium and kinetics. J. Hazard. Mater. 2009, 165 (1–3), 52–62. 13. Deng, S.; Peng-Ting, Y. Polyethyleneimine-modified fungal biomass as a high-capacity biosorbent for Cr(VI) anions: Sorption capacity and uptake mechanisms. Envir on. Sci. Tech. 2005, 39 (21), 8490–8496. 14. Panic, V.V.; Madzarevic, Z.P.; Volkov-Husovic, T.; Velickovic, S.J. Poly(methacylic acid) based hydrogels as sorbents for removal of cationic dye basic yellow 28:

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143. Figaro, S.; Avril, J.R.; Brouers, F.; Ouensanga, A.; Gaspard, S. Adsorption studies of molasses wastewaters on activated carbon: Modeling with a new fractal kinetic equation and evaluation of kinetic models. J. Hazard. Mater. 2009, 161 (2–3), 649–656. 144. Weber, W.J., Jr.; Asce, A.M.; Morris, J.C. Kinetics of adsorption on carbon from solution. J. Sanit. Eng. Div. Am. Soc. Civ. Eng. 1963, 89 (S-A2), 31–59. 145. Tofighy, M.A.; Mohammadi, T. Adsorption of divalent heavy metal ions from water using carbon nanotube sheets. J. Hazard. Mater. 2011, 185 (1), 140–147. 146. Wang, H.L.; Chen, J.L.; Zhai, Z.C. Study of thermodynamics and kinetics of adsorption of p-toluidine from aqueous solution by hyper crosslinked polymeric adsorbents. Environ. Chem. 2004, 23 (2), 188–192. 147. Liu, C.C.; Kuang-Wang, M.; Li, Y.S. Removal of nickel from aqueous solution using wine processing waste sludge. Ind. Eng. Chem. Res. 2005, 44 (5), 1438–1445. 148. Weber, T.N.; Chakravarti, R.K. Pore and solid diffusion models for fixed bed adsorbers. J. Am. Inst. Chem. Eng. 1974, 20 (2), 228–238. 149. Chen, C.L.; Wang, X.K.; Nagatsu, M. Europium adsorption on multi wall carbon nanotube/iron oxide magnetic composite in the presence of polyacrylic acid. Environ. Sci. Technol. 2009, 43 (7), 2362–2367. 150. Wang, X.L.; Lu, J.L.; Xing, B.S. Sorption of organic contaminants by carbon nanotube influence of adsorbed organic matter. Environ. Sci. Technol. 2008, 42 (6), 3207–3212. 151. Tempkin, M.I.; Pyzhev, V. Kinetics of ammonia synthesis on promoted iron catalyst. Acta Phys. Chim. USSR. 1940, 12 (1), 327–356. 152. Naiya, T.; Bhattacharya, A.K.; Das, S.K. Adsorption of Cd(II) and Pb(II) from aqueous solutions on activated ­alumina. J. Colloid Interf. Sci. 2009, 33 (1), 14–26.

153. Ghasemi, Z.; Seif, A.; Ahmadi, T.S.; Zargar, B.; Rashidi, F.; Rouzbahani, G.M. Thermodynamic and kinetic studies for the adsorption of Hg(II) by nano-TiO2 from aqueous solution. Adv. Powder Technol. 2012, 23 (2), 148–156. 154. Gao, Z.M.; Bandosz, T.J.; Zhao, Z.B.; Han, M.; Qiu, J.S. Investigation of factors affecting adsorption of transition metals on oxidized carbon nanotubes. J. Hazard. Mater. 2009, 167 (1), 357–365. 155. Nightingale, E.R. Phenomenological theory of ion solvation: Effective radii of hydrated ions. J. Phys. Chem. C. 1959, 63 (9), 1381–1387. 156. Illes, E.; Tombacz, E. The effect of humic acid adsorption on pH-dependent surface charging and aggregation of magnetite nanoparticles. J. Colloid Interf. Sci. 2006, 295 (1), 115–123. 157. Chen, C.L.; Wang, X.K. Adsorption of Ni(II) from aqueous solution using oxidized multiwall carbon nanotubes. Ind. Eng. Chem. Res. 2006, 45 (26), 9144–9149. 158. Peerez-Aguilar, N.V.; Munoz-Sandoval, E.; Diaz-Flores, P.E.; Rangel-Mendez, J.R. Adsorption of cadmium and lead onto oxidized nitrogen-doped multiwall carbon nanotubes in aqueous solution: Equilibrium and Kinetics. J. Nanopart. Res. 2010, 12 (3), 467–480. 159. Hu, J.; Chen, G.H.; Lo, I. Removal and recovery of Cr(IV) from wastewater using maghemite nanoparticles. Water Res. 2005, 39 (18), 4528–4536. 160. Bigalke, M.; Weyer, S.; Wilcke, W. Copper isotope fractionation during complexation with insolubilized humic acid. Environ. Sci. Technol. 2010, 44 (14), 5496–5502. 161. Liang, L.; Lv, J.T.; Luo, L.; Zhang, J.; Zhang, S.Z. Influences of surface-coated fulvic and humic acids on the adsorption of metal cations to SiO2 nanoparticles. Colloids Surf. A. 2011, 389 (1–3), 27–32.

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Environmental Applications: Polymers in Chow Wen Shyang School of Materials and Mineral Resources Engineering, Engineering Campus, University of Science, Malaysia, Nibong Tebal, Malaysia Environmental–Fire Protection

Abstract This entry focused on the environmental applications of polymers, including polymers for sustainable waste management (e.g., oxo-biodegradable polyolefins and biodegradable plastics in a­ griculture and packaging applications), polymers for soil protection (e.g., superabsorbent ­polymers), polymers for pollutant sensing and detection, polymers for water treatment, and polymers for gas capture and separation. The functionality and importance of polymers in the environmental applications are discussed. This entry ends with a summary of selection of polymers for environmental applications, as well as the future trend and prospective of the related polymers. Keywords: Gas capture; Pollutant sensors; Soil protection; Sustainable polymer; Water treatment.

INTRODUCTION Polymers from renewable resources may become a sustainable solution, due to the increasing of oil prices and the waste accumulation problem. The society is moving towards sustainable development. This indicates a major focus on the sustainable utilization of renewable resources.[1,2] Nevertheless, it is hard to replace all the large volume polymers for agriculture and packaging (­ fossil fuel-based, e.g., polyolefins) with polymers made from renewable resources. Parallel developments and innovations based on renewable resources and synthetic polymer could potentially deliver solutions for the waste management. The society would benefit from parallel innovations that focus on renewable resources in order to reduce the pressure on the environment.[3] The routes of design, production, and disposal of bio-based polymers have to be systematically strategized. The balance between the longterm properties and the end of life of bio-based polymers is the key to sustainability. Superabsorbent polymers (SAPs) have gained in importance in the soil stabilization and protection. SAPs could retain large quantities of water and nutrients; thus, they have been widely used for agricultural water saving and ecological recovery.[4] Developing a simple, effective, and reusable polymer-based sensor with high sensitivity and selectivity will be of great interest from a global perspective. Demand for continuous reliable sensors for online and real-time monitoring of heavy metals and gaseous species in environmental and industrial processes is increasing. There is a wide range of membrane-­based sensing devices, including electrochemical sensors, optical sensors, and biosensors for

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applications in monitoring heavy metals and gas analytes in the environment.[5] The nanomaterial-based membrane with different structures and combinations has exhibited a high potential application in water treatment and purification.[6] The development of hybrid and nanocomposite ultrafiltration (UF) and reverse osmosis (RO) membranes is progressing well in water and wastewater treatment, desalination of brackish and seawater, and removal of organic and micropollutant from water.[7] The introduction of polymer-based nanomaterials for the CO2 and CH4 capture is a feasible technique attributed to their unique properties.[8] In moving towards the sustainable development, it is important to develop polymer materials that are eco-friendly.[9] This can be achieved by using earth-­ abundant materials and renewable resources, combined with the energy-saving advanced technology. In this entry, the environmental applications of polymers for ­sustainable waste management, soil protection, pollutant sensing and detection, and water treatment, as well as gas capture and separation, will be discussed. POLYMERS FOR SUSTAINABLE WASTE MANAGEMENT Sustainability development is a development that fulfills the needs of the present without compromising the ability of future generations to meet their own needs. Three important pillars of sustainability should be concerned when designing polymers for the future: social (people), economy (profit), and environment (planet). There should be a good balance between the performance and

Encyclopedia of Polymer Applications, First Edition DOI: 10.1201/9781351019422-140000342 Copyright © 2018 by Taylor & Francis. All rights reserved.

the functionality of the polymers during their service life (long-term properties) and strategy for the waste management after their end of life.[1] Approaches commonly used for plastic waste management are 3R (reduce, reuse, recycle), eco-friendly polymers, oxo-biodegradable polymers, and biodegradable polymers. The use of plastics in agriculture results in increased yields, earlier harvests, better protection of food products through the mitigation of extreme weather changes, and efficient water conservation. Agricultural films, mainly made from polyethylene (PE), are used as coverings for greenhouses or tunnels over crop rows, silage covers, balewrap films, and mulching films to cover soil.[10] Since the most utilized plastic is PE, thus any reduction in the accumulation of PE waste alone would have a major impact on the overall reduction of the plastic waste in the environment. The rate of biodegradation of PE, even during many years of exposure to microbial consortia of soil, has been found to be very low, thus accounting for less than 1% carbon mineralization. Since PE is considered to be practically inert, efforts were made to isolate unique microorganisms capable of utilizing synthetic polymers. It showed that biodegradation of plastic waste with selected microbial strains became a viable solution. Degradable polymers are classified into two types: (1) polymers that are intrinsically biodegradable, whose chemical structure enables direct enzymatic degradation, and (2) polymers that undergo photo-oxidation or thermo-oxidation upon exposure to ultraviolet (UV) radiation or heat, respectively. Oxo-biodegradable polymers often contain prooxidants. For example, photodegradation of low-density polyethylene (LDPE) and polypropylene films can be activated using metal oxides as catalysts. Those materials require oxidative degradation to induce chain scission in order to reduce the molecular weight and to produce oxygenated groups (e.g., carbonyl) through Norrish type I and type II mechanisms, which are then more easily metabolized by microorganism during biotic process.[11] Primary applications of these ­oxo-degradable polymers are mainly in the mulching and packaging film applications as well as other products of limited lifetimes, e.g., carry bags. These oxo-biodegradable materials are intended to be mineralized in soil in the case of agricultural films or during ­composting in the case of packaging applications. Husarova et al.[12] have investigated the aerobic biodegradation of calcium carbonate-filled LDPE film containing Mn ion-based prooxidant additives. The levels of carbon mineralization reached 13% and 16% for the 40and 80-day pre-oxidized PE-containing fillers, respectively, after approximately 16 months in a soil environment at 25°C, and both types of sample were mineralized to about 19% in the compost environment at 58°C during the same period. Chatterjee et al.[13] have investigated enzyme-­mediated biodegradation of heat-treated commercial LDPE by staphylococcal species. The degradation of LDPE has been shown by the growth of bacterial strain

in the nutrient medium separated from the inoculums through LDPE film. Sivan [11] found that PE samples mineralized about 12% of the original carbon in compost at 58°C for 3 months after being exposed for 1 year to natural weathering. There are a few microbial strains capable of degrading standard nonoxidized PE. These include the actinomycete Rhodococcus ruber (strain C208), the thermophilic bacterium Brevibacillus borstelensis (strain 707), and the fungus Penicillium simplicissimum. Owing to its high durability, PE is often photo-oxidized and/ or thermo-oxidized as a pretreatment before the incubation with the degrading culture. Abrusci et al.[10] have reported the biodegradation of photodegraded mulching films based on PE and prooxidant additives (i.e., calcium stearates and iron stearates). Three bacterial species common in soil were found attached to the PE and identified as Bacillus cereus, B. megaterium, and B. subtilis. Corti et al.[14] have examined the synergistic effects of sunlight exposure, thermal aging, and fungal biodegradation on the oxidation and biodegradation of LLDPE films containing prooxidants. The results suggested that the degradation of ­oxo-biodegradable LLDPE was enhanced by the synergistic action of both abiotic and biological factors after its initial oxidation by exposure to direct sunlight. Nowak et  al.[15] have evaluated the degree of biodegradation of LDPE films modified with aliphatic polyester in different soils under laboratory conditions. Films were incubated in soils from waste coal, a forest, and an extinct volcano crater. Bacteria belonging to the genus, Bacillus, and the fungi, Gliocladium viride, Aspergillus awamori, and Mortierella subtilissima, could colonize PE both with and without aliphatic polyester. The effect of transition metal stearates (i.e., iron, cobalt, and manganese stearates) is examined on the post-bacterial photochemical and thermal degradation of LDPE. The biodegradation of degraded LDPE films by Brevibacillus borstelensis and the mixture of Bacillus (B. cereus, B. megaterium, and B. subtilis) was highly effective following the order LDPE-Co > LDPE-Mn > LDPE-Fe and in the range of 9.0%–59.2% of mineralization after only 90 days of bacterial bioassay at 45°C.[16] Although some of the oxo-degradable additives increased the abiotic photodegradation, the molecular weight reduction in compost was not sufficient to reach the maximum biotic degradation level established by international standards for biodegradable materials.[17] Musioł et al.[18] have evaluated the aging process of commercial oxo-degradable PE bag under real industrial ­composting conditions and in distilled water at 70°C. The results indicated that such an oxo-degradable product offered in markets degrades slowly under industrial ­composting conditions. The slow degradation and fragmentation are most likely due to partial cross-linking after long time of degradation, which results in the limitation of low molecular weight residues for assimilation. Moreno et al.[19] have reported a deterioration pattern of six biodegradable, potentially low environmental impact mulches (e.g., LLDPE, corn thermoplastic

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starch-filled copolyester, potato thermoplastic starch-filled biodegradable recycled polymers bioplastic, polylactic acid (PLA) plastic film, and paper mulches) in field conditions. Considering the state of the mulch residues in the soil at 200 days, the potato thermoplastic starch-filled biodegradable polymer is suitable in the edaphoclimatic conditions of the trial. It could give a potential alternative to PE mulch in order to reduce e­ nvironmental pollution in the ­agricultural fields. Biodegradable polymers can be classified into four ­categories depending on their type of synthesis and source: (1) biopolymers generated from biomass such as agro-­polymers from agro-resources including polysaccharides, proteins, and lipids; (2) polymers obtained via microbial production (e.g., polyhydroxyalkanoates); (3) ­polymers  chemically synthesized using monomers obtained from agro-resources (e.g., polyglycolic acid and PLA); and (4) polymers chemically synthesized from fossil fuel resources (e.g., poly(butylene adipateco-­terephthalate),  poly(butylene succinate adipate), polycaprolactones PCL, and polyesteramides).[20] The development of high-­performance bio-based and renewable materials is one of the important factors for sustainable growth of the bio-based industry. Biopolymers present  a large spectrum of application such as collection bags for compost, agricultural foils, horticultures, n­ ursery products, toys, fibers, textiles, and food packaging.[21] There is an increasing demand for the biopolymers to replace ­traditional fossil-based polymers, attributed to the lower environmental impact and carbon footprint of the biopolymers. The bioplastic market is growing by 20%–30% each year. Biodegradation can be a key feature of synthetic polymers within the frame of sustainable development. In polymer research, biodegradation is a useful property to obtain plastics for certain ­applications where biodegradation enhances the value of the application (e.g., mulching films, food-­packaging materials, or polymers used in oilfield and gas-field chemicals). The value of the application increases because the products may have lesser or no ­environmental impact when biodegradable.[22] Biopolymers made from renewable resources are important and innovative materials because they are expected to minimize our dependence on fossil fuel-based polymers. PLA has been extensively studied and used for packaging and biomedical applications. It is a thermoplastic aliphatic polyester produced from nonfossil renewable natural resources by the fermentation of polysaccharides or sugar, e.g., extracted from corn, potato, cane molasses, and sugar beet. PLA brings a new combination of a­ ttributes to packaging, including high stiffness and strength, high clarity, dead fold and twist retention, low-temperature heat sealability, biocompatibility, and biodegradability, as well as an interesting combination of barrier properties including flavor, and aroma barrier characteristics. The ­properties of PLA can be tailored by using polymer blending, rubber toughening, and nanotechnology.[23–31] PLA is

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ideally suited for many applications in the environment where recovery of the product is not practical, such as agricultural mulch films and bags. Example of PLA usage also includes packaging container and paper coatings, sustained release systems for pesticides and fertilizers, compost bags, service ware, textiles, fibers, and environmental remediation films. Efforts have been made to utilize biobased sources such as fermentable sugars from nonfood cellulosic biomass, agricultural wastes, and nonfood crops (e.g., switchgrass) as an alternative to agricultural foods to synthesize PLA.[32,33] The concept of using nonfood cellulosic biomass and nonfood crops as raw materials for PLA synthesis and manufacturing well fitted to the philosophy of sustainable development. Analysis of biochemical processes shows that PLA biodegradation is specifically related to the presence of the microorganisms and environmental conditions. Biochemical processes of PLA degradation mainly include chemical hydrolysis and biodegradation in the natural soil microcosm.[34] Accordingly, understanding the fundamental of biochemical processes of PLA biodegradation is a key factor for exploring the highly efficient methods of PLA biodegradation and assessing the degradability of the PLA under real composting conditions. It was found that the type of soil, d-enantiomer content of PLA, and the geometry and shape of the material had a significant effect on the rate of biodegradation of PLA following soil burial.[35] ­Biodegradable polymer is not a panacea for the waste problem, but at least it can be part of the solution for the waste management and sustainable development. As a ­compostable polymer, PLA is considered a promising alternative to reduce the municipal solid waste disposal problem by offering additional end-of-life scenarios. Chemical recycling of polyurethanes is not feasible at the industrial scale for the degradation of wastes from discarded customer products, but biodegradation using microbial catalysts is a valuable alternative. Cregut et al.[36] have documented a review on polyurethane biodegradation and realistic prospects for the development of a sustainable waste recycling process. Discovering polyurethane (PU) degrading fungus should help to reduce the worldwide concern caused by polyurethane wastes. Khan et al.[37] have reported biodegradation of polyester polyurethane by Aspergillus tubingensis, which was isolated from soil. The A. tubingensis can grow on the PU film surface and leads to cracking and erosion. Widespread studies on the biodegradation of rubbers have been carried out in order to overcome the environmental problems associated with rubber waste. Shah et al.[38] have documented a review on the biodegradation of natural and synthetic rubbers, and emphasized the importance of biodegradation in environmental biotechnology for waste rubber disposal. It is known that both natural and synthetic rubbers can be degraded by microorganisms. It has confirmed that the latex-clearing protein (Lcp) from the Gram-positive Streptomyces sp. K30 and a rubber-cleaving dioxygenase RoxA

from the Gram-negative Xanthomonas sp. 35Y are responsible for the degradation of natural and synthetic rubbers. Understanding the knowledge on the enzymatic activity and biochemical pathways of rubber biodegradation could be a solution for the rubber waste management. In addition to that, Sienkiewicz et al.[39] have reported a review on ecofriendly polymer–rubber composites obtained from waste tires. One of the sustainable management of the used tires is grinding and utilizing them as fillers for elastomer or thermoplastic composites. The rubber composites prepared by this waste-free technology exhibited good mechanical and functional properties. POLYMERS FOR SOIL PROTECTION This subtopic focuses on the polymers used for soil protection, in the context of clay slope surface, agriculture soil, and contaminated soil. The capability of polymers used as soil stabilizer, superabsorbent, and soil amendment agent will be discussed. Clay slope surface suffers from soil erosion during rainfall, attributed to the water stability, and the strength of the topsoil is weak. Chemical stabilization of the soil with polymer soil stabilizer has been developed to control the soil erosion. According to Liu et al., [40] acetic-ethylene-­ester polymer-based soil stabilizer can significantly increase the unconfined compression strength, shear strength, water stability, and erosion resistance of clayey soil, while ­reducing the soil loss and protecting the v­ egetation growth. SAPs are highly cross-linked polymers of high molecular weight mostly synthesized using acrylamide (AM), acrylate, or acrylic acid. They have been widely utilized in many applications such as medicine, horticulture, sanitary products, and agriculture, due to their excellent water retention abilities. The application of SAPs as soil amendments can be a feasible approach to enhance the soil water holding capacity and stabilize the soil structure.[41] SAPs are usually applied as soil conditioner in agriculture to hold soil moisture, improve soil stability or aeration, and prevent soil erosion. SAPs have been employed in combination with pesticides to control their release rates in order to promote the efficient use of both pesticides and water.[42] SAP amendments can reduce soil penetration resistance, increase soil aggregation, and aid the protection of soil organic matter. Achtenhagen and Kreuzig[41] have investigated the effect of cross-linked potassium-polyacrylate SAPs on the transformation and sorption of organic xenobiotics in soil. In their study, (R,S)-1-[2-(2,4-dichlorophenyl)-2-­ (2-propenyloxy)ethyl]-1H-imidazole was selected as the 14 C-labeled model substance. This imidazole derivative is used as a systemic pesticide in seed protection of cereals, postharvest protection of fruits, vegetables, and protection of ornamental plants. It was found that the increased microbial activity, usually promoting transformation processes of xenobiotics, was compensated by the enhanced

sorption in the amended soils (containing 0.4% SAPs (w/w)) evidenced by the increase in soil/water distribution coefficients (Kd) of 26–42 L kg−1 for the silty sand and 6–25 L kg−1 for the sand, respectively. Yang et al.[42] have prepared self-synthesized SAPs (starch-graft-polyacrylamide (St-g-PAM) superabsorbent cross-linked by N,N-methyl bisacrylamide) combined with 14 C-labeled carbendazim to evaluate the transformation of carbendazim (a broad-spectrum benzimidazole fungicide) in soils. The results showed that compared to the SAP-free control, a 11.4% relative reduction in 14C-carbendazim extractable residue was detected in red clayey soil with SAP amendments after 100 days of incubation. After 100 days of SAP treatment, the mineralization of 14C-carbendazim was significantly reduced by 37.6% and 41.2% in loamy soil and saline soil, respectively, relative to the SAP-free treatment. These findings suggest that SAP amendments significantly affected the fate of carbendazim. Cao et al.[4] have reported the effect of polyacrylamide-acrylic SAPs on soil and water conservation on the terraces of the Loess Plateau (China). The SAPs are capable to relieve the pressure of water shortages and change the process of rainfall runoff. Moreover, the SAPs could significantly increase root density as well as root biomass and subsequently increase soil erosion resistance. Thus, the using of SAPs can ensure ecological sustainability in dryland systems as well. Several soil moisture conservation techniques can be used to achieve sustainable farming of the agricultural lands by saving the water resources, such as mulching, terracing, and reduced tillage, being practiced in agricultural production. Mulching is a technique widely used in agriculture consisting of modifying the condition of agricultural soils by covering the soil surface. The application of mulching practices conserves soil moisture, controls soil structure, and temperature, enhances the efficiency of fertilizer and water levels, improves total soil porosity, and increases crop yield and quality.[19,43] Kader et al.[43] have documented a review on mulching materials and methods for modifying the soil environment. Mulching protect the soil from physical, chemical, and biological degradation, and reduce irrigation requirement by conserving water. More research is needed to be carried out to understand and evaluate the effects of biodegradable mulching materials on the microclimate modifications, soil biota, soil ­fertility, crop growth, and yields. The main problem of the world’s land is the degradation of agricultural soils. In order to prevent the erosion-mediated transfer of contaminated soils, Zezin et al.[44] have designed and developed original binders for dispersed systems, for soils, and fine grounds, which mainly focus on a broad class of amphiphilic binders based on interpolyelectrolyte complexes (IPCs). IPCs relate to amphiphilic block copolymers composed of hydrophilic and hydrophobic fragments. Examples of IPCs containing commercially available polyelectrolytes are cationic ­poly-N,N-diallyl-N,N-dimethylammonium

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chloride (PDADMAC) and anionic copolymer of acrylic acid and acrylamide obtained via partial alkaline hydrolysis of polyacrylonitrile (HPAN). For soil remediation, ­hetero-chain polyelectrolytes (modified cellulose, lignin, and chitin) are preferable, which biodegrade with the ­formation of nontoxic products. Environmental–Fire Protection

POLYMERS FOR POLLUTANT SENSING AND DETECTION The need for miniature sensors with high sensitivity, good selectivity, and high efficiency is growing rapidly due to the increasing global industrialization. The designing of polymer sensor is a challenging task due to the effect of toxic pollutants on human health and environment. This subtopic discusses the function and importance of ­polymers used in pollutant sensing and detection. Polymer-­ based sensors could offer many advantages, such as simple operation, low cost, high effectiveness and efficiency, high reliability and accuracy, high sensitivity and selectivity, and quick detection and fast response, as well as low operating temperature and low power consumption. All these benefits will certainly adding value for the polymer-based sensors. Molecular imprinting is a simple and well-established methodology to prepare materials with recognition sites for specific molecules. Molecular imprinting polymers (MIPs) possess high selectivity and affinity for the target molecule and have been widely used as a molecular recognition tool for sensor development. They are typically obtained by the copolymerization of an appropriate monomer with a cross-linker in the presence of a template molecule. Magnetic molecularly imprinted polymers (MMIPs) have been shown to provide an effective way for immobilization and MIP renewal from the solid support. They bear selectivity due to its biomimetic receptor, and facilitate ­pre-concentration and separation as well as manipulation of the analyte attributed to their outstanding magnetism. Combination of electrochemical sensors and MMIPs enhances the sensor stability and signal amplification.[45–47] Heavy metal ions (e.g., lead (Pb), mercury (Hg), arsenic (As), chromium (Cr), and cadmium (Cd)) are considered to be highly toxic and hazardous to human health even at trace levels. Bansod et al.[48] have documented a review on various electrochemical techniques for heavy metal ions detection with different sensing platforms. This leads to the requirement of fast, accurate, and reliable techniques for the detection of heavy metal ions in various environments, e.g., biological samples (blood, serum, saliva), food, natural and wastewater, air, and soil. Highly sensitive spectroscopic techniques, e.g., atomic absorption spectroscopy, inductively coupled plasma mass spectroscopy, X-ray fluorescence spectrometry, neutron activation analysis, inductively coupled plasma-optical emission spectrometry, atomic fluorescence spectrometry,

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UV–Vis spectrophotometer, high-performance liquid chromatography (HPLC), and gas chromatography-mass spectrometry, are employed for the detection of heavy metals and pollutants in complex matrices. Electrochemical techniques, on the contrary, are more economic, user-friendly, reliable, and suitable for in-field applications. This technique is also capable for in situ and online monitoring of contaminated water sample. Mercury ions are potentially toxic environmental pollutants. When released into the water bodies, mercury is converted by bacteria to its more toxic form methyl mercury, which can accumulate in aquatic organisms. Dithizone (diphenylthiocarbazone) is one of the sensitive, selective, and effective chelating agents with nitrogen and sulfur donor atoms to prepare specific sorbents for detection and removal of heavy metal ions. Therefore, it is an efficient reagent which applies for the detection of heavy metal ions. Sedghi et al.[49] have prepared a colorimetric sensor based on dithizone-anchored poly(vinyl pyridine-N,N-methylenebisacrylamide-acrylic acid) nanocomposite for the detection of mercury and lead ions trace levels in aqueous media. The developed method is suitable for the determination of ultratrace levels of Hg and Pb ions in real samples. The detection of Hg and Pb ions in aqueous media using the colorimetric sensor can be achieved at low concentration (about 10 ppb) in a very short time. Sahin et al.[50] have decorated the polyacrylamide-­epoxy ­cryogel with “pyrene” by using click chemistry and obtained a fluorescence active cryogel (PAAm-Pyr). The PAAmPyr can sense very low concentration of Hg2+ (2  ppb) in aqueous solutions and demonstrates a high selectivity for Hg2+ in the presence of various competing metal ions in pure water. Palanisamy et al.[51] have reported a sensitive and disposable electrochemical sensor for the detection of Hg(II) in various water samples using a polypyrrole (PPy)-decorated graphene/β-cyclodextrin (GR-CD) composite modified screen-printed carbon electrode (SPCE). The GR-CD/PPy composite modified SPCE can detect the Hg(II) up to 51.56 μM L−1 with a limit of detection (LOD) of 0.47 nM L−1. Cobalt salts are some of the main groups of salts that are widely used in industrial materials, paint products, fertilizers, feeds, and disinfectants. The direct determination of trace cobalt in complex samples is usually limited owing to the interferences caused by matrix elements. Thus, ion imprinted polymer (IIP) has attracted much attention as a highly selective sorbent for metal ions overcomes the above problem. Torkashvand et al.[52] have constructed a ­Co2+ -selective sensor based on IIP nanobeads and applied it for differential pulse cathodic stripping voltammetric determination of cobalt ions. The IIP nanobeads were fabricated by the polymerization of acryl amide as a functional monomer, N,N-methylenebisacryl amide as a crosslinker, and ammonium persulfate as an initiator, around the ­complex of Co2+ 8-hydroxyquniline linked to the casted magnetic nanoparticles (MNPs) at the surface of glassy

carbon electrode. The sensor showed 2 linear ranges of 0.5–20 and 20–500 nM with a LOD of 0.1 nM (S/N = 3). Moreover, the sensor was successfully applied for low-level determination of cobalt in natural water. The presence of silver (Ag(I)) in the natural water environment has been a great concern because of its toxicity to aquatic plants and animals, especially when it is in the free ion form (Ag+). The major sources of silver in the environment are wastes from the photographic and imaging industry, and wastes from the manufacturing of electronics, jewelry, and silverware.[53] Lindfors et al.[54] used polyaniline (PANI) nanoparticles–silicon rubber solid contact electrode for Ag+ detection in the range of 10 −7 to 10 −4 M. Copper is widely used in industry and is accumulating in their waste streams. Copper is harmful if it is discharged without treatment. The adsorption of copper has shortand long-term effects on human health.[55,56] Conducting polymers, carbon nanotubes, and metal oxides have been explored to improve the detection of heavy metal ions. Among all conducting polymers, PANI is one of the most prominent materials due to its excellent electrical and mechanical properties, easy synthesis, and significant environmental stability, which have been demonstrated in many potential electrical devices and sensor applications. Deshmukh et al.[57] have developed a hybrid electrochemical/electrochromic Cu(II) ion sensor prototype based on the PANI/indium tin oxide (ITO) electrode. The PANI/ ITO electrode shows both electrochromic and electrochemical responses towards Cu(II) ions in aqueous solutions. This indicated that PANI/ITO electrodes can be used for both chronoamperometric and electrochromic detection of Cu(II) ions. Nitrophenols (NPs) are classified as hazardous pollutants, and they are widely present in pesticides, pharmaceuticals, and dyes. Zhang et al.[58] have fabricated silver nanowire-polyaniline (AgNW-PANI) composite modified glassy carbon electrode (GCE) for the sensitive electrochemical determination of 4-nitrophenol (4-NP). The modified electrode demonstrated an excellent electro-­ catalytic activity towards the redox of 4-NP, and a relatively low LOD 52 nM was observed for the fabricated sensor with better anti-interference ability. Bogale et al.[59] have developed a luminescent terbium(III)-based coordination polymer for highly selective and sensitive detection of 4-NP through fluorescence quenching mechanism. The luminescent terbium(III)-based coordination polymer shows green emission characteristics and remarkable capabilities for identification and quantification of 4-NP, even in the presence of other competing nitroaromatics such as 2,4,6-trinitrophenol, 2,6-dinitrotoluene, nitrobenzene, 4-nitrotoluene, 4-bromophenol, phenol, bromobenzene, and 1,2-dimethylbenzene. Conjugated microporous polymers (CMPs) are emerging as fluorescence sensing materials for nitroaromatic derivatives, because the CMPs inherently combine ­π-conjugated skeletons with stiff permanent nanopores

and have advantages such as low mass density, high chemical stability, numerous selectable monomers, and synthetic methods. Geng et al.[60] have synthesized two thiophene-based and carbazole-based CMPs with triphenylamine core structural monomers via straight forward anhydrous ferric chloride oxidative coupling polymerization. Both CMPs exhibited strong fluorescent emission properties and function as fluorescent chemosensors towards electron-deficient nitroaromatic analytes, attributed to their propeller-like structure, high porosity, extended π-conjugation, and electron-rich characteristics. Nitrites are of both environmental and biological importance, which are the intermediate species in the nitrogen cycle. It is commonly used as a preservative in food industry and a corrosion inhibitor in industrial water. However, toxic nitrite is hazardous to human health and the e­ nvironment. Liu et al.[61] have reported the highly sensitive nitrite sensor based on poly(diallyldimethylammonium chloride) (PDADMAC)-coated Fe1.833 (OH) 0.5O2.5-­decorated N-doped graphene ternary hierarchical nanocomposite. The modified electrode exhibits an excellent sensitivity (0.0183 μA μM−1), with the wide linear ranges (0.1–347 and 347–1,275 μM) and a low LOD (0.027 μM) for nitrite sensing. Phthalic acid esters are often used in the manufacture of plastics and other industrial or consumer products. They are usually not chemically bonded to the substrate and may leak into the environment. Thus, they can be found in water, soil, air, food, and human organisms. Zhou et al. [62] have developed fluorescence molecularly imprinted polymers sensor via in situ precipitation polymerization for the determination of dibutyl phthalate (DBP). Based on Mn-doped ZnS quantum dots (QDs) and by anchoring a MIP layer on the surface of SiO2 nanoparticles, the fluorescence MIPs (SiO2@QDs@MIPs) were synthesized by precipitation polymerization, with AM as the functional monomer and ethylene glycol dimethacrylate (EGDMA) as the cross-linker. The fluorescence intensity decreases linearly with increasing concentration of DBP in the range of 5–50 μmol L−1 under optimized experiment conditions. 17β-Estradiol has been identified as one of the major contributors to the endocrine-disrupting activity detected in environment water samples. Ming et al.[63] have synthesized MMIPs, a core–shell structured composite of Fe3O4 nanoparticles and MIPs. The methacrylic acid (MAA) was used as the functional monomer and the ethylene glycol dimethacrylate acted as the cross-linking agent for the surface imprinting polymerization. The fluorescence increase linearly coincided with the concentration of 17β-estradiol within the range of 0.10–70 μM with a LOD of 0.03 μM. Lahcen et al.[64] have reported the synthesis and c­ haracterization of a MMIP (Fe3O4-PANI-MIP) for the determination of 17β-estradiol in real environment samples from river water. This MIP-based sensor exhibits a wide linear range of 0.05–10 μM and a high selectivity

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and sensitivity towards 17β-estradiol. In addition, the LOD of this sensor reaches 20 nM. Among all persistent toxic substances, polychlorobiphenyls (PCBs) and heavy metal ions attract special attention because of their toxicity, bioaccumulation, and notorious difficulty in degradation. Various nanomaterials including porous anodic aluminum oxide (PAAO) template, PPy nanotubes imbedded in PAAO membrane, PPy nanowires, Fe3O4 NPs, and peanut shells are used as the building blocks to fabricate fluorescence probes for trace detection of both PCBs (2,20,4,5,5′-pentachlorinated biphenyl (PCB101) and 2,3,3′-trichlorobiphenyl (PCB20)) and heavy metal ions (including Cu2+, Cr3+, Zn2+, Cd2+, and Hg2+).[65] 2,4-Dichlorophenol (2,4-DCP) is widely employed in synthesizing insecticides, herbicides, preservatives, antifungal agents, and dyes. Liang et al.[66] have developed a sensitive and selective electrochemical sensor based on molecularly imprinted polymer/graphene oxide (MIP/ GO) modified GCE for the determination of 2,4-DCP in lake water samples. MIP was synthesized via precipitation polymerization, using 2,4-DCP as the template, MAA monomer, ethylene glycol dimethacrylate (EGDMA) cross-linking agent, and azodiisobutyronitrile initiator. The oxidation peak current and the concentration of 2,4-DCP had a good linear relationship in the range of 0.004–10.0 μM with a LOD of 0.5 nM. Chloroacetamide herbicides are primarily used in agriculture to control the growth of weeds. However, an extensive use of chloroacetamide herbicides can cause pollution to the environment. Ji et al.[67] have prepared an MMIP by using Fe3O4 microspheres as the magnetic core, 4-vinyl pyridine (4-VP), and alkenyl glycosides glucose as functional comonomers. The developed MMISPE-HPLC method exhibited good linearity (0.1–200 μg L−1) and low LOD (0.03–0.06 μg L−1) of chloroacetamide herbicides under the optimized conditions. Formaldehyde is considered as one of the most important (indoor) pollutants due to its high toxicity and potential carcinogenicity. Menart et al.[68] have developed an e­ lectrochemical sensor for convenient detection of gaseous formaldehyde. The sensor is composed of a three-­electrode screen printed system modified with hydrazinium polyacrylate. The sensors exhibited satisfactory linear behavior in the concentration range of 4–16 ppm associated with 120  min accumulation/exposure. Subppm gaseous concentrations can be readily measured; e.g., 500 ppb of formaldehyde can be detected after 64 h exposure with a signal approximately equivalent to 4 ppm at 2 h exposure. González-Vila et al.[69] have ­proposed to coat a TFBG-SPR (tilted fiber Bragg gratings–surface plasmon ­resonance) sensor with a conductive MIP. A PPybased MIP was chosen due to the ease of production of a ­single-molecule-sensitive material. The sensor can detect tiny formaldehyde concentrations in gaseous state, with a sensitivity of 2.10 pm ppm−1.

Environmental Applications: Polymers in

Nitrogen oxides are one of the major exhausted pollutants from the society needs like a vehicle and domestic and industrial exhaust. Polythiophene (PT) has received much attention due to its unique properties, e.g., high flexibility, good environmental stability, and high electrical conductivity. Kamble et al.[70] have prepared interconnected nanofibrous PT thin film which was deposited on the glass substrate through the chemical bath deposition method. The film deposited at a 0.5 M monomer concentration showed the highest NO2 gas response of 47.58% at room temperature. The interconnected nanofibrous PT thin-film sensor demonstrated its ability to detect very low NO2 ­concentration of 1 ppm. Most volatile organic compounds (VOCs) such as aldehydes, aromatic compounds, polycyclic aromatic hydrocarbons (PAHs), alcohols, and ketones are highly toxic and carcinogenic.[71] Triethylamine (TEA), as a VOC, is used widely as an organic solvent, a catalyst in polymerization, a preservative, and a synthetic dye. PPy is a promising polymer used in sensor application attributed to its charge characteristic that can be modified by doping technique. Sun et al.[72] have fabricated the PPy/WO3 (tungsten trioxide) hybrids with p–n heterojunction by in situ chemical oxidation polymerization and loaded on the substrate of a flexible polyethylene terephthalate thin film to structure a smart TEA sensor. The sensor exhibits the highest response of 680–100 ppm TEA at room temperature. In an effort to detect ethanol, Bahoumina et al.[73] have reported the feasibility of microwave flexible gas sensor based on poly(3,4-ethylenedioxythiophene) polystyrene sulfonate–multiwall carbon nanotubes as a sensitive material, deposited by inkjet printing technology. The sensor sensitivity to ethanol vapor exposition has been estimated to be −642.9 Hz ppm−1 and −7 μdB ppm−1 for resonant frequency and insertion losses variations in differential mode, respectively, according to the values at 4 min of exposure to 500, 1,000, and 2,000 ppm. Sulfur dioxide (SO2) constitutes one of the major environmental pollutant gases and is responsible for the formation of the corrosive acid rain and acid smog after reacting with the water vapor in the atmosphere. It  is known that PANI has good acid/base properties, rendering the better molecular and/or physical interactions between the target gas and the sensing material. It has been found that at low concentrations of sulfur dioxide, the poly ­(2-methoxyaniline)-based quartz crystal microbalance gas sensor can serve as an analytical sensor for its determination. The methoxy-substituted PANI was tested to show the unique gas sensing properties for the sulfur dioxide at the low concentrations (50–250 ppm) and function as the adsorbing material at the high concentrations (500–1,250 ppm).[74] Ammonia (NH3) is a kind of colorless, stimulating, toxic, and flammable gas, which mainly comes from industrial processes, biological metabolism, and automobile exhaust. NH3 sensors operated at room temperature

for environment protection and health monitoring, in which the superior sensing materials are very important for improving the performance of sensors. Conducting polymers (e.g., PANI, PPy, and PT) are widely investigated for NH3 sensors due to their excellent electronic properties, low operating temperature, and intrinsic redox reaction, in which nanostructure PANI with the high surface area is an attractive sensing material on account of high sensitivity and fast response. Liu et al.[75] have prepared a polyaniline-­ titanium dioxide-gold (PANI-TiO2-Au) ternary nanocomposite thin film on gold interdigital electrodes by the in situ self-assembly method at room temperature (25°C) for NH3 detection. TiO2 nanoparticles and Au nanorods were wrapped by PANI, and the typical core–shell structure was formed successfully. The PANI-TiO2-Au ternary thin-film sensor exhibited excellent repeatability, remarkable selectivity, acceptable stability (68%), and low detectable concentration (1 ppm) to NH3. Andre et al.[76] have reported on sensing units made with nano-structured layer-by-layer (LbL) films containing the semiconducting PANI, GO, and zinc oxide (ZnO). The films with three tetralayers were found to be the most adequate for detecting NH3 in the range of 25–500 ppm with a response time of 30 s. POLYMERS FOR WATER TREATMENT The environmental importance of water is considered as basic necessary everywhere in the world. The serious environmental burden is rising due to the water contamination and water insufficiency, and its limited availability is increasing nowadays due to the destruction of natural water supports. The disposal of highly polluted wastewater is rising during the past decades because of certain actions such as urbanization, industrialization, and agricultural practices. Generally, wastewater is classified as domestic wastewater and industrial wastewater. [77] ­Wastewater from different sources, such as petroleum industry (e.g., oily wastewater), coal industry, printing and dyeing industry, tannery, electroplating, metal-­ containing effluent, pulp, and paper industry, has caused serious treatment issues all around the world. Generally, wastewater contains complicated organic and inorganic compounds, such as high concentrations of phenolic compounds, pesticides, dyes, PAHs, alkylphenol, salts, oils, heavy metals (e.g., arsenic), ammonia, cyanide, and ­thiocyanate. There are various types of water treatment technologies, such as membrane technology, ion exchange technology, electrochemical techniques, electrodialysis, flotation, oxidation, biological treatment, ­magnetic nanophotocatalysis, biologically activated ­carbon, chemical precipitation, flocculation and coagulation, adsorption, RO, membrane bioreactor, nanotechnology, crystallization (e.g., evaporation ­crystallization, cooling crystallization, reaction crystallization,

drowning-out crystallization, and membrane distillation ­crystallization), and ­polymer-assisted UF. [78–87] Nanomaterial and nanotechnology have emerged as one of the possible methods to develop high-performance separation membrane and help to solve the global water crisis. These nanomaterial-based membranes, including ­nanoparticles, nanofibers, two-dimensional (2D) layer materials, and nanocomposites, exhibit outstanding permeation properties which is important for water p­ urification. The addition of nanoparticle (e.g., TiO2, silica, alumina, and ZnO) into polymeric UF membranes could increase membrane surface hydrophilicity, water ­permeability, and fouling resistance.[6] Natural organic matter (NOM) is considered as a complex matrix of heterogeneous mixture of organic compounds such as humic substances, polysaccharides, amino sugars, proteins, peptides, lipids, and small hydrophilic acids. NOM gives rise to problems in the production of drinking water. For example, NOM adds taste, odor, and color to raw drinking water. The removal of NOM and its constituents from water is a challenging issue worldwide. Modified nano-adsorbents and composite materials (e.g., PPy-coated glass beads, aminated polyacrylonitrile fibers, PANI/attapulgite composite, cross-linked chitosan-­ epichlorohydrin beads, poly(amidoamine) dendrimers (PAMAM), and PANI/silica-coated Fe3O4 nanoparticles) have shown promising results for NOM removal from water.[88] Emerging contaminants (ECs) include compounds such as pharmaceutical and personal care products, pesticides, and hormones that have adverse effects on human and wildlife endocrine systems. Sustainable reuse of water contaminated with ECs is a challenging task. Membrane processes are another type of phase-changing processes with a variety of applications in the removal of ECs. Membrane filtration can be classified as UF, nanofiltration (NF), microfiltration, RO, and forward osmosis (FO).[89] The demands of NF membranes are gradually increasing for water and wastewater treatment due to their high selectivity and energy efficiency. Primarily aromatic and semi-aromatic polyamide (PA) derived from the aromatic amines such as 1,3-phenylenediamine and the aliphatic diamines such as piperazine (PIP) found suitable for RO and NF membranes. Poly(piperazine amide) (PIPA) offers high hydrophilicity and water permeability, and low surface roughness. PIPA and polyethyleneimine (PEI)-based PA membranes show promising applications for water softening (hardness removal) in the form of flat sheet and hollow fiber configuration. Interfacial polymerization is the fundamental technique for in situ formation of PA layer of nanoscale thickness on porous polymeric support, and this composite structure has successfully used NF and RO membranes for various applications.[7] Heavy metals in wastewater are a major problem in the environment, because the high risk is associated with

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ecosystems and human health even at very low concentration. There are several types of polymeric material used in NF membrane for the removal of heavy metals from the wastewater, e.g., polybenzimidazole (PBI), PAMAM, PA, polysuphone, polyvinylidene fluoride (PVDF), and PEI.[77] A combination of cellulose-based materials and nanostructured photocatalyst has been developed for water treatment. The development of cheap, renewable, and ecofriendly photocatalysts based on cellulose hybrid on a large scale for practical applications is the goal towards worldwide sustainability.[90] Grafted cellulose-based copolymers are reported as potential candidates for the effective metal ion sorption applications.[91] In addition, cyclodextrin polymers and cyclodextrin-based nanosponges proved to have the ability to remove dissolved pollutants in ­contaminated water.[92] In addition, nanosponge cyclodextrin ­polyurethanes are good adsorbents for the removal of organic ­contaminants including dyes, fertilizers, and ­pesticide.[93] It has been well known that seawater constitutes 97.5% of total water resource on the earth. Desalination technique  can acquire freshwater from seawater through membrane separation via RO and thermal distillation (multistage flash and multi-effect distillation). PA thin-film composite (PA-TFC) membranes are finding great popularity in desalination via both RO and FO processes, which can effectively reduce worldwide freshwater ­crisis through turning seawater into potable water. Typical PA-TFC RO membranes exhibit a three-layer structure in terms of nonwoven fibrous mechanical support, p­ olysulfone or ­polyethersulfone sub-layer, and surface PA layer.[94] Large amounts of clay minerals present in wastewater are discharged from industrial activities such as mineral processing, sludge dewatering, and papermaking. Separation of clay minerals from industrial wastewaters is of industry importance. Coagulation–flocculation remains the most commonly utilized technique for colloidal wastewaters in industries due to its simplicity, effectiveness, and versatility. Flocculating agents such as p­ olyelectrolytes have the potential to separate the clay minerals from industrial wastewater effluents. Several examples of the synthetic polyelectrolytes used in flocculation of clay ­minerals are polyacrylamides (nonionic, cationic, anionic, and ­amphoteric), chitosan-graft-polyacrylamide, polymethacrylic acid, comb-like polycarboxylate copolymers, amylopectin grafted with poly(acrylic acid), cationic polyamine, and poly(diallyldimethylammonium chloride) (­PDADMAC).[95] Functionalized MNPs have been used as an option of effective adsorbent of heavy metal ions. The magnetic nanomaterials (e.g., magnetite (Fe3O4) and maghemite (γ-Fe2O3)) combining magnetic separation and nanotechnology could give good performance in the removal of heavy metal ions.[96] The magnetic property of nano-­ adsorbent leads to easy separation of adsorbent from the sample media by using external magnetic field without

Environmental Applications: Polymers in

centrifuging/filtration step.[97] PPy has attracted attention attributed to its high electrical conductivity, ease of preparation, and nontoxicity.[98,99] PPy as conducting polymer is proven one of the promising materials that can be used in the modification of MNPs. PPy can effectively remove heavy metal ions through ionic exchange when it is positively doped. Using magnetic nanocomposites (MNCs) is an attractive approach in heavy metal adsorption. Using PPy/γ-Fe2O3 provides the adsorption affinity for heavy metal ions by the PPy and ease of recovery from MNPs by the application of magnetic field.[100] Halloysite nanotube (HNT) is a potential adsorbent for a variety of pollutants for both positively and negatively charged molecules due to its unique bivalent morphology with spatially separated negative and positive surfaces. It was incorporated into MNPs by using the concept of coprecipitation of magnetite on the HNT surface. PPy/Fe3O4/HNT was synthesized through in situ chemical oxidative polymerization by microemulsion. Figure 1 shows the TEM images of PPy/ Fe3O4/HNT MNC. It is shown that some middle hollow parts of the HNT nanotube were filled by PPy. This indicates that the PPY could polymerize both inside and outside of the HNT. Figure 2 shows that the removal percentage and adsorption capacity, qt, are increasing with interaction time until 30 min. Both removal percentage of Cu(II) ions and adsorption capacity reach maximum after adsorption for 30 min which shows that an adsorption equilibrium is achieved. The maximum removal percentage of Cu(II) is 11%, and the qt at 30 min which is equal to the equilibrium adsorption capacity, qe, is 63.70 mg g−1.

PPY

HNT

Fe3O4

200 nm

Fig. 1  TEM image of PPy/Fe3O4/HNT nanocomposites at 40,000× magnification

14

80 70

12

60 50

8

40

6

30 20

4 Removal

2 0

qt (mg/g)

Removal (%)

10

qe

10 0

0

10

20 30 40 Interaction time (min)

50

–10 60

Fig. 2  Effect of interaction time on the removal percentage of Cu(II) ions and adsorption capacity at time (pH = 5)

POLYMERS FOR GAS ADSORPTION, CAPTURE, AND SEPARATION Global warming is the result of the increased concentration of greenhouse gases (GHGs), e.g., carbon dioxide (CO2), methane (CH4), nitrous oxide, hydrofluorocarbons, perfluorocarbons, and sulfur hexafluorocarbon. CO2 is produced in many industrial processes, e.g., fossil fuel power plants. Three options are available to reduce the CO2 emission: reducing energy intensity, reducing c­ arbon footprint, and improving carbon capture and sequestration (CCS). One of the approaches to minimize CO2 emissions is to convert CO2 to value-added products and fuels using clean energy. Nitrogen-containing polymers can be used as a platform for CO2 electroreduction. Based on the mechanism of their conductivity, the nitrogen-­ containing polymers tested in CO2 electroreduction can be divided into two main groups: (1) electroactive polymers which include (a) π-conjugated polymers (e.g., PANI and PPy) and (b)  polymers containing readily-reducible ligands (e.g., bipyridine-based polymers, polyporphyrins, and polyphthalocyanines), and (2)  ion-­exchanging polymers such as poly(4-vinyl pyridine) (P4VP), PEI, PDADMAC, and ­poly(2-trimethylammonium)ethyl methacrylate, where nitrogen atoms can participate in the Lewis base–Lewis acid interactions.[9] The CO2 capture is preferred to be applied directly on-site, since the capture materials and technologies have demonstrated better performances at high CO2 concentrations rather than at atmospheric levels.[8] Various technical options in pre- and post-combustion modes are available: adsorption, absorption, membrane separation, chemical looping combustion, and cryogenic separations. Among all these, absorption technology which could be used as a post-combustion option to be integrated with power plant has been commercialized with amines as solvents.[101] The development of viable CO2 storage systems has required

the needs for highly selective CO2 capture materials suitable to selectively enrich CO2 in preference to other gases in the flue gas stream.[102] In the CCS technology, a variety of techniques may be used to capture CO2, including gas-phase separation, absorption into a liquid, and adsorption onto a solid, as well as hybrid processes (e.g., an adsorption/­membrane system).[103] Many solid sorbents have been synthesized for CO2 capture such as activated carbon, zeolites, metal organic frameworks (MOFs), covalent organic frameworks, porous polymers, alkali metal carbonates, modified porous silica, amine functionalized sorbents, and biochar.[104,105] Nanomaterial adsorbents are one of the approaches to capture carbon dioxide and methane due to the fact that they have relatively high CO2 capture and high CH4 storage capacities, reusable throughout multiple sorption/desorption cycles, and relatively low energy requirements. In addition, it is expected that nanoporous carbonaceous materials for CH4 storage will be extensively developed and used in the future. Qi et al.[106] have developed a high efficiency nanocomposite sorbent for CO2 capture based on oligomeric amine(PEI and tetraethylenepentamine (TEPA)) functionalized mesoporous silica capsules. The capsules offer increased amount of amine incorporation and reactive sites for CO2 capture leading to exceptional capturing performance of up to 7.93 mmol g−1 in simulated flue gas. Karunakaran et al.[107] have produced CO2-selective GO nanocomposite membranes by embedding GO into the poly(ethylene oxide)/poly(butylene terephthalate) (PEO/PBT) copolymer. The PEO/PBT/GO membranes showed a high CO2 permeability (143 Barrer) and a CO2 /N2 selectivity suitable for practical separation applications. The addition of GO to the PEO/PBT membrane increased the CO2 /N2 selectivity from 52 to 73, maintaining the same high permeability of a pristine PEO/PBT membrane. Zerze et al.[108] have prepared poly(allylamine) (PAA) via free-radical polymerization and physically impregnated on fumed silica at various amine loadings for CO2 capture from trace concentration streams under continuous flow of ambient air. The PAA-­silica composites were found to have a significant potential as trace CO2 adsorbents under ambient conditions. A maximum adsorption capacity for trace CO2 of 188 mg g−1 ­adsorbent was achieved using the PAA adsorbent. I­ midazole/benzimidazole and carboxylic acid-based coordination polymers have gained attention due to their tunable architectures such as rigidity, interpenetrating ­properties, and design flexibility for CO2 sorption/­desorption. As an important family of multidentate ­N-donor ligands, ­imidazole/benzimidazole-based derivatives are the ­excellent building blocks as bridging ligands with excellent coordination ability, and versatile ­conformations, as well as outstanding stability for constructing ­one-dimensional, 2D, and three-dimensional frameworks.[109] Membrane materials used for CO2 separation could be broadly classified into two categories: porous and

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Environmental Applications: Polymers in 1115

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nonporous types. Nonporous membranes, also known as dense film membranes, are usually polymeric membranes, e.g., poly(arylene-ether), poly[1-(trimethylsilyl)1-­propyne], poly(dimethyl siloxane), poly(urethane-urea), ­polyether amine, polyimide, PBI, polyvinyl amine, polyvinyl alcohol, PANI, and polyacrylates.[110] Polyimide membranes show great potential for gas separation and reveal good selectivity for CO2 /N2 and CO2 /CH4 gas pairs.[111] Mixed-matrix membranes (MMMs) are the combination of inorganic and organic membrane materials, which are expected to have the advantages of both the materials, i.e., low cost of organic membranes together with high permeability, selectivity, and stability of inorganic counterparts. Atomically thin, 2D nanomaterials such as graphene or GO nanosheets are highly regarded as promising nanofillers to improve MMM properties.[112] The MMMs show the potential to exceed the Robenson upper bound for gas ­separations. Moreover, it is easy to prepare large-scale MMMs with low cost.[113] Permeability and selectivity of the gas could be enhanced by the fabrication of MMMs in which the inorganic particles are well dispersed in the polymer matrix. ZIF-90 (zeolitic imidazolate framework-90) is an attractive MOF for application in CO2-selective MMMS. It has a sodalite cage-like structure with 0.35 nm pore windows, through which size exclusion of CH4 from CO2 /CH4 mixtures is possible. Furthermore, the imidazole linker in ZIF-90 contains a carbonyl group, which has a favorable chemical noncovalent interaction with CO2. MMMs were fabricated using polyimide as polymer matrices. The ZIF90/polyimide membranes showed high CO2 permeability (>700 Barrer) combined with a good CO2 /CH4 mixed-gas selectivity of 37.[114]

POLYMER SELECTION FOR ENVIRONMENTAL APPLICATIONS Table 1 summarizes the selection of polymers for environmental applications. The polymer selection is based on the molecular structure, chemical and physical nature, polymer properties (e.g., mechanical, thermal, optical, and electrical properties), and processability. The suitability of the environmental applications also attributed to the tunable properties of the related polymers with other ­materials, e.g., nanofillers and bio-resources.

RECOMMENDED READING 1. Susheel Kalia, Luc Averous (2016). Biodegradable and Bio-Based Polymers for Environmental and Biomedical Applications. Wiley-Scrivener Publishing, Hoboken, NJ. 2. Peng Wang (2016). Smart Materials for Advanced Environmental Applications. Royal Society of Chemistry, Cambridge.

Environmental Applications: Polymers in

3. Vijay Kumar Thakur, Manju Kumari Thakur (2015). Eco-Friendly Polymer Nanocomposites: Chemistry and ­Applications. Springer, New Delhi, India.

FUTURE TRENDS AND PROSPECTIVE Biopolymers fulfill some of the environmental concerns; however, they still have some limitations in terms of thermal, barrier, and mechanical properties, as well as their cost-effectiveness. Understanding the fundamental of the efficient degradation methods based on biopolymer-­ degrading microorganism is important for controlling the biodegradation of the biopolymers. The effects of degraded plastics on the ecology and marine life need more attention to maintain their sustainability. Various types of polymer waste management technologies and strategies (e.g., 3R, biopolymers) can be applied simultaneously and appropriately to solve the solid waste problem. The performance of biopolymers can be improved by using alternative feedstocks (e.g., waste or ­by-products) and nanotechnology. In terms of soil protection, SAPs demonstrated their ability in improving the erosion resistance of slope topsoil, reducing the soil loss, protecting the vegetation growth, and conserving water and soil, as well as maintaining land fertility. Nevertheless, SAP might affect the biological and abiotic degradation behavior of pesticides in soils. Thus, the biological effects and toxicity of pesticides in SAP-modified soil should be always taken into consideration. Based on the continuous progress in the development of new nanomaterials (e.g., graphene-based polymer nanocomposite) and the excellent prospects in sensitivity, selectivity, and convenience offered by MMIP-based electrochemical s­ ensors, the use and application of new electrochemical sensors based on novel multifunctional MMIPs are expected to be widely developed and used in the future. The introduction of nanotechnology into wastewater treatment proved to be a feasible approach for environmental remediation. More researches are needed to prove the long-term efficacy of the nanomaterials as sorbents in the real scale applications. It is also necessary and important to study the toxicity of the nanomaterials used for water treatment and purification for the safety of the human health and environmental wellness. Nanotechnology-based MMM is a feasible technology for gas separation. Further research on molecular modeling should be performed in order to provide useful information of the membrane properties for carbon dioxide capture in the real conditions. In conclusion, polymers can demonstrate their functionality for the environmental applications in sustainable waste management, soil protection, pollutant sensors, water treatment, and gas capture. Sustainable development of polymer materials can be achieved if management of the resources is well ­strategized and properly controlled.

Environmental Applications: Polymers in 1117

Sustainable waste management Soil protection

Pollutant sensing and detection

Water treatment

Gas capture and separation

Agriculture mulching film, packaging Organic polymer soil stabilizer, mulching film

Hg2+ detection. Co2+ detection. c Ag+ detection. d Cu2+ detection. e 4-nitrophenol detection. f nitroaromatic detection. g nitrite detection. h NO2 gas detection. i TEA detection. j ethanol detection. k SO2 detection. l NH3 detection. m ultrafiltration membrane. n metal ion adsorbent. o flocculating agent. a

b

Polymers

Oxo-biodegradable PE Biodegradable polymer (e.g., PLA) Acetic-ethylene-ester polymer-based soil stabilizer SAPs (e.g., cross-linked potassium–polyacrylate, polyacrylamide-acrylic superabsorbent resin, starch-graft-polyacrylamide superabsorbent cross-linked by N,N-methyl bisacrylamide) Interpolyelectrolyte complexes (e.g., cationic poly-N,N-diallyl-N,N-dimethylammonium chloride and anionic copolymer of acrylic acid and AM) Heavy metal ions and Dithizone-anchored poly(vinyl pyridine–N,N-methylenebisacrylamide-acrylic acid)) pollutant sensors nanocompositea Polyacrylamide-epoxy cryogel with “pyrene”a PPy-decorated graphene/β-cyclodextrin composite modified screen-printed carbon electrodea IIP nanobeads fabricated from acryl amideb PANI nanoparticles–silicon rubber solid contact electrodec Hybrid electrochemical/electrochromic Cu(II) ion sensor prototype based on PANI/ITO electroded Silver nanowire-PANI composite modified glassy carbon electrodee Thiophene-based and carbazole-based CMPs with triphenylamine core structural monomersf Poly(diallyldimethylammonium chloride)-coated Fe1.833(OH) 0.5O2.5-decorated N-doped graphene ternary hierarchical nanocompositeg Interconnected nanofibrous PT thin film h PPy/tungsten trioxide hybridsi Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate–multiwall carbon nanotubesj Methoxy-substituted PANIk PANI-titanium dioxide-gold ternary nanocomposite thin film on gold interdigital electrodesl Nano-structured LBL films containing the semiconducting PANI, GO and ZnOl PIPAm High-performance membrane PBIm filtration, metal Poly(amidoamine) dendrimer m ions adsorbents, PAm flocculating agents of Polysuphonem clay mineral in water PVDFm Polyethyleneiminem PPyn Polyacrylamideso Cationic polyamineo Polydiallyldimethylammonium chlorideo CO2 capture PEI and TEPA functionalized mesoporous silica capsules PEO/PBT/GO membrane PANI Poly(arylene-ether) Poly(urethane-urea) Poly(2-N,N-dimethyl aminoethyl methacrylate-co-acrylic acid sodium) Polyimide/zeolitic imidazolate framework-90 (ZIF-90) PAA-coated fumed silica solid materials

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Table 1  Polymer selection for environmental applications Category Applications

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73. Bahoumina, P.; Hallil, H.; Lachaud, J.L.; Abdelghani, A.; Frigui, K.; Bila, S.; Baillargeat, D.; Ravichandran, A.; Coquet, P.; Paragua, C.; Pichonat, E.; Happy, H.; Rebière, D.; Dejous, C. Microwave flexible gas sensor based on polymer multi wall carbon nanotubes sensitive layer. Sens. Actuators B 2017, 249, 708–714. 74. Tian, Y.H.; Qu, K.; Zeng, X.Q. Investigation into the ring-substituted polyanilines and their application for the detection and adsorption of sulfur dioxide. Sens. Actuators B 2017, 249, 423–430. 75. Liu, C.H.; Tai, H.L.; Zhang, P.; Ye, Z.B.; Su, Y.J.; Jiang, Y.D. Enhanced ammonia-sensing properties of PANITiO2-Au ternaryself-assembly nanocomposite thin film at room temperature. Sens. Actuators B 2017, 246, 85–95. 76. Andre, R.S.; Shimizu, F.M.; Miyazaki, C.M.; Riul, A., Jr.; Manzani, D.; Ribeiro, S.J.L.; Oliveira, O.N., Jr.; Mattoso, L.H.C.; Correa, D.S. Hybrid layer-by-layer (LbL) films of polyaniline, graphene oxide and zinc oxide to detect ammonia. Sens. Actuators B 2017, 238, 795–801. 77. Carolin, C.F.; Kumar, P.S.; Saravanan, A.; Joshiba, G.J.; Naushad, M. Efficient techniques for the removal of toxic heavy metals from aquatic environment: A review. J. Environ. Chem. Eng. 2017, 5, 2782–2799. 78. Imyim, A.; Sirithaweesit, T.; Ruangpornvisuti, V. Arsenite and arsenate removal from wastewater using cationic polymer-modified waste tyre rubber. J. Environ. Manag. 2016, 166, 574–578. 79. Gómez-Pastora, J.; Dominguez, S.; Bringas, E.; Rivero, M.J.; Ortiz, I.; Dionysiou, D.D. Review and perspectives on the use of magnetic nanophotocatalysts (MNPCs) in water treatment. Chem. Eng. J. 2017, 310, 407–427. 80. Besha, A.T.; Gebreyohannes, A.Y.; Tufa, R.A.; Bekele, D.N.; Curcio, E.; Giorno, L. Removal of emerging micropollutants by activated sludge process and membrane bioreactors and the effects of micropollutants on membrane fouling: A review. J. Environ. Chem. Eng. 2017, 5, 2395–2414. 81. Korotta-Gamage, S.M.; Sathasivan, A. A review: Potential and challenges of biologically activated carbon to remove natural organic matter in drinking water purification ­process. Chemosphere 2017, 167, 120–138. 82. Crini, G.; Morin-Crini, N.; Fatin-Rouge, N.; Déon, S.; ­Fievet, P. Metal removal from aqueous media by polymer-assisted ultrafiltration with chitosan. Arab. J. Chem. 2017, 10, S3826–S3839. 83. Yu, L.; Han, M.; He, F. A review of treating oily wastewater. Arab. J. Chem. 2017, 10, S1913–S1922. 84. Toczyłowska-Mamińska, R. Limits and perspectives of pulp and paper industry wastewater treatment—A review. Renew. Sustain. Energy Rev. 2017, 78, 764–772. 85. Dubey, S.; Banerjee, S.; Upadhyay, S.N.; Sharma, Y.C. Application of common nano-materials for removal of selected metallic species from water and wastewaters: A critical review. J. Mol. Liq. 2017, 240, 656–677. 86. Lu, H.J.; Wang, J.K.; Wang, T.; Wang, N.; Bao, Y.; Hao, H.X. Crystallization techniques in wastewater treatment: An overview of applications. Chemosphere 2017, 173, 474–484. 87. Aslam, M.; Charfi, A.; Lesage, G.; Heran, M.; Kim, J.H. Membrane bioreactors for wastewater treatment: A review

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Environmental Applications: Polymers in 1121

Fire Protection: Flame-Retardant Additives and Fillers for Polymers Hüsnügül Yilmaz Atay Environmental–Fire Protection

Department of Material Science and Engineering, İzmir Katip Çelebi University, Çiğli İzmir, Turkey

Abstract Due to their lightweight and ease of processing, the use of polymers arises by their remarkable combination of properties in our daily life. However, although polymers have benefited our society in a number of ways, they are also known for their relatively high flammability. Moreover, most of them are accompanied by corrosive or toxic gases and smoke, which are produced during combustion. Therefore, obtaining fire-resisting properties of polymers is becoming an important issue to extend their usage for many applications. Keywords: Cone calorimeter; Flame retardant; Halogenated flame retardants; Inorganic minerals; Loss of ignition; Polymer composites.

FIRE

ceiling height), location of fire, and ambient conditions (­temperature, wind, etc.).[4]

Fire Phenomena Traditional Fire Development Fire can be described as the heat and light from a rapid combination of oxygen and other materials. Fires start when a flammable and/or combustible material, in combination with a sufficient quantity of an oxidizer such as oxygen gas or another oxygen-rich compound (though non-oxygen oxidizers exist that can replace oxygen), is exposed to a source of heat or ambient temperature above the flash point of the fuel/oxidizer mix, and is able to sustain a rate of rapid oxidation that produces a chain reaction. This is commonly called the fire tetrahedron (Fig. 1). Fire cannot exist without all of these elements in place and in right proportions. For example, a flammable liquid will start burning only if the fuel and oxygen are in right proportions. Some fuel–oxygen mixes may require a catalyst, a substance that is not directly involved in any chemical reaction during combustion, but enables the reactants to combust more readily.[1,2] Once ignited, a chain reaction must take place, whereby fires can sustain their own heat by further release of heat energy in the process of combustion and may propagate, provided there is a continuous supply of an oxidizer and fuel. For fire to exist, a combustible substance must be present, the temperature must be high enough to cause combustion, and enough oxygen must be present to sustain rapid combustion.[3] Fire development is a function of many factors including fuel properties, fuel quantity, ventilation (natural or mechanical), compartment geometry (volume and

1122

The traditional fire development curve shows the time history of a fuel-limited fire. In other words, the fire growth is not limited to the availability of oxygen. As more fuel added into the fire, the energy level continues to increase until all of the fuel available is burnt (fully developed). Then as the fuel is burned away, the energy level begins to decay. The key is that oxygen is available to mix with the heated gases (fuel) to enable the completion of the fire triangle and the generation of energy (Fig. 2). [4] Fire Behavior in a Structure The fire behavior in a structure curve demonstrates the time history of a ventilation-limited fire. In this case, the fire starts in a closed structure. In the early stage of fire growth, there is adequate oxygen to mix with the heated gases, resulting in flaming combustion. As the oxygen level within the structure decreases, the fire decays, the heat release from the fire decreases, and as a result, the temperature decreases. When a vent is opened, such as when the fire department enters a door, oxygen is introduced. The oxygen mixes with the heated gases in the structure, and the energy level begins to increase. This change in ventilation can result in a rapid increase in fire growth, potentially leading to a flashover (fully developed compartment fire) condition (Fig. 3).[4] Encyclopedia of Polymer Applications, First Edition DOI: 10.1201/9781351019422-140000164 Copyright © 2018 by Taylor & Francis. All rights reserved.

HEAT

OXYGEN

FLAME FUEL

Fig. 1  Fire tetrahedron

Temperature

Fully developed Growth

Decay

Ignition Time

Fig. 2  Traditional fire development [4]

composed of glowing particles of burning material and luminous gases. It is caused by a highly exothermic reaction taking place in a thin zone.[5] Very hot flames are hot enough to have ionized gaseous components of sufficient density to be considered plasma. Color and temperature of a flame are dependent on the type of fuel involved in the combustion, as, e.g., when a lighter is held to a candle. The applied heat causes the fuel molecules in the candle wax to vaporize. In this state, they can then readily react with oxygen in the air, which gives off enough heat in the subsequent exothermic reaction to vaporize yet more fuel, thus sustaining a consistent flame. Other oxidizers besides oxygen can be used to produce  a flame. Hydrogen burning in chlorine produces a flame and, in the process emits, gaseous hydrogen chloride (HCl) as the combustion product. Another of the many possible chemical combinations is hydrazine and nitrogen tetroxide which is hypergolic and commonly used in rocket engines. Fluoropolymers can be used to supply fluorine as an oxidizer of metallic fuels, e.g., in the magnesium/teflon/ viton composition.[5] The chemical kinetics occurring in the flame is very complex and involves typically a large number of ­chemical reactions and intermediate species, most of them radicals. For instance, a well-known chemical kinetics scheme, GRI-Mech [6] uses 53 species and 325 elementary reactions to describe the combustion of biogas.

Temperature

Typical Temperatures of Flames

Fire under ventilated

Ignition

Fire dept. vents Time

Fig. 3  Fire behavior in the structure [4]

Flashover is the transition phase in the development of a contained fire in which surfaces exposed to the thermal radiation, from fire gases in excess of 600°C, reach ignition temperature more or less simultaneously and fire spreads rapidly through the space. This is the most dangerous stage of fire development.[4] Flame and Flame Temperatures Flame The flame, comes from the Latin word of “flamma,” can be described as the visible and gaseous part of a fire. It is

When looking at a flame’s temperature, there are many factors that can change or apply. An important one is that a flame’s color does not necessarily determine a temperature comparison because black-body radiation is not the only thing that produces or determines the color seen; therefore, it is only an estimation of temperature. Adiabatic flame, atmospheric pressure, percentage oxygen content of the atmosphere, and the fuel being burned are some of the ­factors that determine its temperature.[7] In fires (particularly house fires), the cooler flames are often red and produce the most smoke. Here the red color compared to the typical yellow color of the flames suggests that the temperature is lower. This is because there is a lack of oxygen in the room, and therefore, there is incomplete combustion and the flame temperature is low, often just 600°C–850°C. This means that a lot of carbon monoxide is formed (which is a flammable gas), which is when in fire and arson investigation, there is the greatest risk of backdraft. When this occurs, combustible gases, already at or above the flash point of spontaneous combustion, are exposed to oxygen, carbon monoxide, and superheated hydrocarbons, and temporary temperatures of up to 2,000°C occur.[7] Dicyanoacetylene, a compound of carbon and nitrogen with chemical formula C4N2 burns in oxygen with a bright blue-white flame at a temperature of 4,990°C and at up to

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Fire Protection: Flame-Retardant Additives and Fillers for Polymers

Table 1  Flame temperatures for various common substances (in 20°C air at 1 atm. pressure)[9] Material burned

Charcoal fire

Methane (natural gas)

Bunsen burner flame

Candle flame

Magnesium

Oxyacetylene

Flame temperature

750–1,200°C

900–1,500°C

900–1,600°C

~1,100°C

1,900–2,300°C

Up to 3,300°C

Environmental–Fire Protection

5,730°C in ozone.[8] This high flame temperature occurs partially due to the absence of hydrogen in the fuel (dicyanoacetylene is not a hydrocarbon); thus, there is no water among the combustion products. Table 1 lists the flame temperatures of some substances.[9] Role of Oxygen Combustion requires the fuel to be oxidized. Oxygen is the most commonly available oxidizer, which means that the molecule accepts electrons – electronegativity. It is supporting life in many ways, for instance, when we eat, oxygen is oxidizing the food (fuel) we ingested to generate the energy we need to live. So there is “combustion” going on inside the body. Complex molecules get reduced to simpler ones. For example, wood on combustion will give carbon dioxide and water as its main products. When this happens, the molecules of the wood get rapidly oxidized, forming carbon dioxide and water as its final components. Besides, this can be technically provided with fluorine too. As compared with oxygen, fluorine is more electronegative. Fluorine can be described as a superb oxidizer that blows fluorine gas at nearly any substance and bursts into flames.[7] Role of Heating Rate The term “heat rate” simply refers to the energy conversion efficiency, in terms of “how much energy must be expended in order to obtain a unit of useful work.” In a combustion power plant, the fuel is the energy source, and the useful work is the electrical power supplied to the grid, the steam heat supplied to an industrial customer or used for heating, or both. Because “useful work” is typically defined as the electricity and steam that is delivered to the final ­customers, engineers tend to work with the net plant heat rate. In the United States, heat rate is typically expressed using the mixed English and SI units of Btu/kWh. Though confusing at first, this merely indicates how many Btu/h of energy are required to produce 1 kW of useful work. Other countries commonly use kJ/kWh, kcal/kWh, or other measures.[10] Heat rate has a decided effect on fire behavior. Very low rate of heat can result in a low-intensity, creeping fire. On the other hand, high rate of heat could result in a blow-up fire that is difficult to control. The more fuel burning, the more heat produced. Generally, the greater the heat rate, the more intense the fire will be.

Role of Pressure Fire naturally moves from areas of higher, or positive, ­pressure to areas of comparatively lower, or negative, ­pressure. The largest area of negative pressure during a fire is outside the structure. The positive pressure area inside and the negative pressure area outside are separated by the walls and roof of the structure. There are two causes that create the substantial increase in pressure when a structure is on fire: The first is the fire itself. As molecules heat and expand, they substantially increase the interior pressure within a building or within a fire area. This increase has been measured at ­approximately 7% overall. The second substantial increase in interior pressure is caused by water application. This increase, due to steam production, is less dependent on stream selection than on extremely hot interior temperature, the hot surfaces our water strikes, and the volume of the fire space relative to the negative area where the pressure could exhaust.[11] Role of Fuel Most fires involve combustible solids, although in many sectors of industry, liquid and gaseous fuels are to be found. The range of fuels is very wide, from the simplest gaseous hydrocarbons to solids of high molecular weight and great chemical complexity, some of which occur naturally, such as cellulose and others that are man-made, e.g., polyethylene and polyurethane (Table 2).[12–14] All will burn under appropriate conditions, reacting with oxygen from the air, generating combustion products, and releasing heat. Thus, a stream or jet of a gaseous hydrocarbon can be ignited in air during the oxidation process. Flame is a gas-phase phenomenon and, clearly, flaming combustion of liquid and solid fuels must involve their conversion to gaseous surface, but for almost solids, chemical decomposition or pyrolysis is necessary to yield products of sufficiently low molecular weight that can volatilize from the surface and enter the flame. As this requires much more energy than simple evaporation, the surface temperature of a burning solid tends to be high (typically 400°C). Exceptions to this rule are those solids that sublime on heating, i.e., pass directly from the solid to the vapor phase without chemical decomposition. The composition of the volatiles released from the ­surface of a burning solid tends to be extremely complex. This can

Fire Protection: Flame-Retardant Additives and Fillers for Polymers 1125

Density (kg/m³)

970

Polypropyleneisotactic 940

Polystyrene

Polyoxymethylene

1,100

Polyvinyl chloride

1,430

1,400

Heat capacity (kJ/kg K)

2.3

1.9

1.2

1.4

1.05

Thermal capacity (W/m K)

0.44

0.24

0.11

0.29

0.16

46.5

46.0

41.6

15.5

19.9

130–135

186

240

181

—

Heat of combustion (kJ/g) Melting point (°C)

be understood when the chemical nature of the solid is considered. All those of significance are ­polymeric materials of high molecular weight, whose individual m ­ olecules consist of long “chains” of repeated units, which in turn are derived from simple molecules known as ­monomers.[15] Fire Extinguish Fire can be extinguished by removing any one of the elements of the fire tetrahedron. Consider a natural gas flame, such as from a stovetop burner. The fire can be e­ xtinguished by any of the following: • •

•

•

turning off the gas supply, which removes the fuel source covering the flame completely, which smothers the flame as the combustion both uses the available oxidizer (the oxygen in the air) and displaces it from the area around the flame with CO2 application of water, which removes heat from the fire faster than the fire can produce it (similarly, blowing hard on a flame will displace the heat of the currently burning gas from its fuel source, to the same end) application of a retardant chemical such as halon to the flame, which retards the chemical reaction itself until the rate of combustion is too slow to maintain the chain reaction

In contrast, fire is intensified by increasing the overall rate of combustion. Methods to do this include balancing the input of fuel and oxidizer to stoichiometric proportions,  increasing fuel and oxidizer input in this balanced mix, increasing the ambient temperature so the fire’s own heat is better able to sustain combustion, or providing a catalyst; a nonreactant medium in which the fuel and ­oxidizer can more readily react.[3] FIRE AND POLYMERS Polymer Combustion Due to their lightweight and ease of processing, the use of polymers arises by their remarkable combination of

properties in our daily life. A polymer can be described as a macromolecule composed of repeating structural units typically connected by covalent chemical bonds.[16] Although polymers have benefited our society in a number of ways, they are also known for their relatively high flammability. Moreover, most of them are accompanied by the corrosive or toxic gases and smoke, which are produced during combustion.[17] Therefore, obtaining fire-resisting properties of polymers is becoming an important issue to extend their usage for many applications.[18,19] Before passing to the fire resistivity of polymer, it is better to look to the polymer combustion mechanism. The polymer is first heated to a temperature at which it starts to decompose and gives out gaseous products that are usually combustible. These products then diffuse into the flame zone above the burning polymer. If there is an ignition source, they will undergo combustion in the gas phase and liberate more heat. Under steady-state burning conditions, some of the heat is transferred back to the polymer surface, producing more volatile polymer fragments to sustain the combustion cycle (Fig. 4).[20] Condensed Phase Flame-retardant materials can act chemically and/or physically in the condensed phase or gas phase. In reality, combustion is a complex process occurring through simultaneous multiple paths that involve competing chemical reactions. Heat produces flammable gases from pyrolysis of the polymer, and when the required ratio between these gases and oxygen is achieved, ignition and combustion of the polymer will take place.[21] Three types of processes can be seen in the condensed phase: 1. The polymer can be broken down, and this can be accelerated by flame retardants and leads to its ­pronounced flow by decreasing the impact of the flame. 2. Flame-retardant material can leave a layer of carbon (charring) on the polymer’s surface. This occurs through the dehydrating action of the flame retardant, generating double bonds in the polymer. As a result of cyclization and crosslinking, these processes form a carbonaceous layer (see Fig. 1 for details). 3. Heat absorption through materials.

Environmental–Fire Protection

Table 2  Properties of some solid fuels [13,14] Polyethylene– Materials high density

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Fire Protection: Flame-Retardant Additives and Fillers for Polymers

Thermal feedback from the flame Exothermic (-∆)

Endothermic (+∆) Solid charred residue Polymer

Liquid products and tar

Environmental–Fire Protection

Combustible and Non-combustible gases Pyrolysis

+O2

FLAME Combustion

Fig. 4  Thermal feedback from the flame [20]

Flame-retarding polymers by intumescence are essentially a special case of a condensed-phase activity without apparent involvement of radical trap mechanisms in the gaseous phase. Intumescence involves an increase in volume of the burning substrate as a result of network or char formation. For ingressing of oxygen to the fuel, this char serves as a barrier and also as a medium in which heat can be ­dissipated as seen in Fig. 5.[21,22] The produced fuel amount is greatly diminished in intumescence and char, rather than combustible gases being formed. The char constitutes a two-way barrier, both for hindering the passage of combustible gases and molten polymer to the flame as well as for shielding the polymer from the heat of the flame.[4] Gas Phase In the gas phase, the process of combustion is slowed by reactive species that interfere chemically with the propagation process of the fire. The flame retardants themselves or species derived from them interfere with the free radical mechanism of the combustion process. This slows or stops the exothermic processes that occur in the gas phase and results in a cooling of the system and a reduction in the supply of flammable gases.[21] Hydrogen halides, HX (X = Br or Cl), are produced by the reaction of halogenated organic compounds, R-X, with a polymer, P-H, and can react with the excited-state HO· and H· radicals X· to produce the less-reactive, halogen-free radicals X, leading to an overall decrease in the kinetics of combustion:[21,22]

Heat source

POLYMER

CHAR

Fig. 5  Char and intumescence formation [21]

INTUMESCENT CHAR

Figure 6 shows the components of polymer combustion. In the presence of halogenated fire retardants, the active radical species such as OH·, O·, and H· can be quenched in the gas phase to form species such as H2O, H2, and HX, which are relatively less reactive in the combustion cycle. The mechanism of action of flame retardants is given in Fig. 6 as a schematic summary.[21,22] Inhibition of Polymer Combustion The most efficient way to prevent polymer combustion is to design inherently fire-resistant polymers that have high thermal stability, resistance to the spread of flame, and low burning rate even under high heat flux. However, these materials are generally not easy to process and are very expensive. Another strategy is to use flame-retardant additives to inhibit the combustion of polymers, especially for the commodity polymers.[20] FIRE-SAFE POLYMERS Purpose and Methods of Fire-Retardant Systems In the point of saving life, flame-retardant materials play a significant role. Flame retardants save lives and protect the environment by helping to prevent fires from starting. Those materials act for delaying the spread of flame even if ignition occurs, so that they provide extra time in the early stages when the fire can be extinguished or an escape can be made. Hence, over the past 20 years, flame retardants have played a major role in increasing safety in all areas of our lives—at home, at work, at leisure, in transport, in the healthcare industry. In the room containing flame-retarded products, there was 15 times more available escape time, only 25% of the heat is released, 50% less material is consumed by the fire, one-third of the toxic gases (expressed at CO equivalents) is released, and there is little difference in the production of smoke. Without the use of flame retardants in all these areas, lives would be at far greater risk and some of the rapid developments that have occurred during this period would not have taken place because

Fire Protection: Flame-Retardant Additives and Fillers for Polymers 1127

Ever increasing concentrations of the excited-state reactive OH, O and H free radical species. CO2, OH, O2, O, CO, OH, H

The excited-state free radicals have been rendered unavailable to the combustion cycle. CO2, OH, HOH, HX, H, HH Polymer with RX flame retardant

Polymer without RX flame retardant

Fig. 6  Components of polymer combustion [21]

our safety and the safety of our families would have been ­jeopardized.[23–25] The UK Department of Trade and Industry published a report from the University of Surrey on Risks and Benefits in the Use of Flame Retardants in Consumer Products. The University of Surrey report emphasizes the clear benefits, explaining that flame retardants bring in terms of saving lives. European Flame Retardant Association (EFRA) views this report as an important contribution to the scientific understanding of flame retardants. The report comes at a time when there are isolated proposals to phase out the use of certain flame retardants. In this context, the report represents a clear warning against any rush to move out of individual flame retardants in view of the demonstrated benefits these products provide in terms of saving lives.[23] Another example can be given for TV fires. In 1974, there were over 2,300 recorded TV set fires in the United Kingdom. In 1989, despite the fact that the number of TV sets in use increased many times over, there were only 470 fires (Fig. 7). In Europe since the early 1990s, this

Fires / Million TVs

20 15

History

10

Fig. 7  TV set fires occur in the United Kingdom [24]

1995

1994

1993

1992

1991

1990

1989

1988

1987

1986

1985

5 0

trend has tended to level off, or even to increase again in countries such as the United Kingdom and Sweden. The range of TV set fires in Europe is 12–100 fires per million TV sets per year, an order of magnitude higher than in the United States, where the fire safety ratings for TV set enclosure materials has been historically high. TV set fires can have a dramatic impact on life and property. To avoid a possible future general increase in TV set fires, fire safety requirements in standards should be increased, and public awareness of the importance of fire safety in home appliances needs to be heightened.[24] Moreover, a life-cycle assessment study recently ­completed in Sweden has found that over the lifetime of TV set, there are less emissions to the environment from a TV set containing flame retardants in the outer casing than from a TV set without such flame-retardant protection. Since flame retardants have been removed from TV set casings, TV set fires occur about 10 times more often. As fires themselves are a major source of polyaromatic hydrocarbons (a class of carcinogenic substances found in combustion smoke) and dibenzodioxin and—furan emissions, the use of flame retardants significantly decreased such environmental emissions (~60 and 10 times lower, respectively). Therefore, the use of flame retardants to prevent fires is beneficial from a human health and an ­environmental ­protection standpoint.[24,25]

Although used in the public interest in our lives, fires stood out as a major problem ever since the invention of fire. In fact, in 360 BC, the early history of fire-retardant systems begins with wooden fortifications painted with vinegar. Fire-retardant canvas, producing a combination of clay and plaster, was used in the 1600s. The first patent was received for fire retardancy of textiles using alum, borax, and vitriol in England in 1735.[26–28]

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Gas-phases components of polymer compostion

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Fire Protection: Flame-Retardant Additives and Fillers for Polymers

Table 3  Early historical developments [29] 200–450 BC 1638

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Alum is used to reduce flammability of wood by Egyptians The Romans use a mixture of alum and vinegar on wood

Mixture of clay and gypsum used to reduce flammability of theater curtains

1735–1783

1821

1912

Mixture of alum, ferrous sulfate and borax used on wood and textiles by Wyld in Britain. Alum used to reduce flammability of balloons

Gay-Lussac reported a mixture of (NH4) PO4, NH4Cl, and borax to be effective on linen and hemp

Perkin described a flameretardant treatment for cotton using a mixture of sodium stannate and ammonium sulfate

Actually even if earlier civilizations did not understand the physics and chemistry taking place, they figured out ways to create fire retardants.[29] Some early historical developments in flame retarding are shown in Table 3. The earliest of polymer-compatible flame retardants was a treatment consisting of chlorinated paraffins and an insoluble metal oxide with a binder resin. The insoluble metal was mainly composed of antimony oxide. Investigation into taming fire goes back to centuries to demonstrate that Chinese was seen as the first innovated solution. They covered wood with vinegar and alum before encasing it with clay to prevent the spread of fire, [30] a tactic copied by the Romans to protect the boats of the Empire thousands of years later.[31] This tried and tested method was still being copied in the United Kingdom by theater owners as recently as the 16th century, where they would apply the alum, ammonium, and clay mix to fabric stage curtains to reduce the risk of them catching alight. In fact, alum is still used today in fire extinguishers to smother chemical and oil fires. Evolution was not forthcoming because alum worked and had been reasonably effective in halting fires, although the Great Fire of London in 1666 starkly proved that fire still could be devastatingly destructive. The first scientific attempt at making fire-retardant materials did not occur until the 19th century when our understanding of ­chemistry had developed.[32] In 1821, Frenchman Joseph Louis Gay-Lussac used his knowledge of chemistry to make a serious step forward in our understanding and production of flame-retardant materials. He formulated the law stating that if the mass and volume of a gas are held constant, then gas pressure increases linearly as the temperature rises—Gay-Lussac’s Law. He also discovered boron and iodine, and could be found ­investigating the earth’s atmosphere in a hot air balloon.[32] Other chemists took up his research, such as William Perkins who, in 1912, added stannic oxide to the mix that allowed up to 2 years of regular washing. But the real ­evolution in fire-resistant studies came in the late 1900s. Over the past 40 years, the increased public demand for safety has spurred the development of more rigorous legislation regarding life safety in both public and private settings. In the 1970s, the most common flame-retardant polymers were thermosets such as unsaturated polyesters and epoxy resins. These thermosets included reactive ­halogen compounds and aluminum hydrate as additives

to give them flame-retardant properties. Phosphate esters were also used as flame-retardant additives in plasticized polyvinyl chloride, cellulose acetate film, unsaturated polyesters, and modified polyphenylene oxide.[29] The use of halogen-containing flame-retardant additives was considerable less than the use of the aforementioned additives in the 1970s. Throughout the 1970s, many new brominated additives, as well as a number of chlorinated products, entered the market. As the use of thermosets and thermoplastics became commonplace in large-scale applications such as buildings, transportations, electrical engineering, and electronics, and more rigorous safety legislation was introduced, new flame-retardant systems were developed. These systems consist of inorganic or organic compounds based on bromine, chlorine, phosphorus, nitrogen, boron, and metallic oxides and hydroxides. Polyethylenes were flame retarded with chloroparaffins and antimony trioxide. However, a number of chlorinated chemicals were shown to cause environmental problems, including polychlorinated biphenyls, which were banned in 1970 due to high toxicity.[29] Intrinsically Fire-Resistant Polymers Most polymers with high thermal stability are intrinsically fire resistant. Due to their high decomposition temperature, the initial breakdown will be effectively prevented and the combustion process will not be initiated. This high-temperature property of polymers can be improved by increasing the interactions between polymer chains or by chain stiffening.[33] Chain interactions can be enhanced by several means, such as increasing crystallinity, introduction of polar groups, and hydrogen bonding. Chain stiffening can be accomplished by the use of aromatic or heterocyclic structures in the polymer backbone, such as in poly(phenylene), aromatic polyamides, and polyesters. In addition, polymers containing considerable numbers of aromatic groups in the structural units have great tendencies to condense into chars on heating. Therefore, they produce less-flammable gaseous products in a flame. In all, polymers that have high thermal stability and generate less-flammable volatiles on decomposition are the most desired fire-­resistant polymers. There are three general types of structures for the intrinsically fire-resistant polymers: linear single-strand polymers consisting of a sequence of cyclic aromatic or

Fire Protection: Flame-Retardant Additives and Fillers for Polymers 1129

Flame-Retardant Additives and Fillers From the manufacturing point of view, the introduction of flame-retardant additives undoubtedly constitutes the easiest way of making a polymer less flammable. There are two types of additives: reactive and additive flame retardants.[38] The reactive flame retardants are the compounds that contain heteroatoms known to confer some degree of flame retardance and are built chemically into the polymer molecule. Alternatively, the additive flame retardant can be physically mixed with existing polymers. In this case, the compounds do not react chemically with the polymers. The flame retardants most abundantly used at the present time are based largely on six elements: boron, aluminum, phosphorus, antimony, chlorine, and bromine. In addition, nitrogen and silicon can also confer some degree of flame retardance. Other elements and their compounds

Inorganic Cl Br P based Melamine

Fig. 8  Major families of flame retardants and their prevalence in the FR market in 1997[29]

have proved to be less effective. Combinations of flame retardants often have synergistic or antagonistic effects. Sometimes, a heteroatom already present in the polymer backbone may interact with a flame retardant and, thus, exhibit synergism or antagonism.[29] Currently, between 150 and 200 flame retardants have been designated to fit the requirements of the various markets they serve. The main chemical components used in these products are halogens (bromine and chlorine), phosphorus, inorganics, and melamine compounds. Figure 8 shows the percentages of each chemical component in the overall market of flame retardants. From this figure, it is evident that bromine-based flame retardants dominate the market. Although additive flame retardants are widely used in polymers, there are some limitations such as poor compatibility, high volatility, deleterious effects on the properties of polymers, and increase of the production of carbon monoxide (CO) and smoke.[29] Flame-retardant additives can act by a variety of ­mechanisms in either the condensed phase or the gas phase.[38] They can terminate the free-radical reactions in the ­condensed phase, act as heat sinks due to their heat capacity, form a nonflammable protective coating or char to insulate the flammable polymer from the source of the heat and o­ xidant, and interrupt the flame combustion in the gas phase. It is difficult, however, to unequivocally attribute a single mode of action to a particular additive or class of additives. Many flame retardants appear to be ­capable of functioning simultaneously by several different mechanisms, often depending on the nature of organic polymers.[29] Halogenated Flame Retardants (Containing Chlorine or Bromine Atoms) Halogenated flame retardants act by effectively removing the H+ and OH radicals in the gas flame phase. This considerably slows or prevents the burning process, thus reducing heat generation and so the production of further gaseous flammable material. In fact, the mechanism is as follows. When exposed to high temperatures, the flame-­ retardant molecule releases bromine (Br) or chlorine (Cl) as free radicals (Br or Cl), which react with hydrocarbon molecules (flammable gases) to give HBr or HCl. These

Environmental–Fire Protection

heterocyclic structures, ladder polymers, and inorganic or semiorganic polymers.[33] So far, most carbon-based, fire-resistant polymers are prepared by incorporating highly stable, rigid, aromatic, or heterocyclic ring systems directly into the polymer chain, [34] such as polyimide, polybenzoxazole, polybenzimidazole, and polybenzthiazoles (see Figs. 1 and 2). The synthetic routes to such polyaromatic heterocyclic polymers involve a two-step process in which soluble high-molecular-weight prepolymers are first synthesized, and then rigid stable rings are formed by thermally or chemically induced condensation of reactive groups on the polymer chains. Ladder polymers are a very special type of rigid-chain polymers.[35] These polymers are double-stranded structures consisting of two polymer chains periodically bound together by chemical bonds such as cyclized polyacrylonitrile. In principle, these materials should show superior thermal stability because the polymer chains cannot be severed by breaking a single bond.[29] The synthesis of inorganic and semiorganic polymers has been aimed at the production of stable, polymeric materials having linear chains consisting of such typical repeating units as silicon-nitrogen, boron-nitrogen, and phosphorus-nitrogen.[36] The nonburning characteristics of inorganic elements and the formation of nonflammable protective coatings are the two main reasons for the fire resistance of these polymers.[29] Substantial research effort has been made in the manufacture of intrinsically fire-retardant textile fibers. All the major types with the exception of polyolefins have fire-­retardant versions. The chemical nature of the modifications is not generally revealed. It is however reasonable to assume that the fire-retardant properties are obtained by addition of one or more fire-retardant chemicals to the polymer mass before spinning the fiber. Viscose F.R., Fidion F.R., Trevira C.S. Panox, and Acrylic copolymers (Sironil F.R.) are some randomly chosen commercial examples of these types of flame-retardant materials.[37]

Fire Protection: Flame-Retardant Additives and Fillers for Polymers

Smoke density E-622 Test (flaming mode)

Environmental–Fire Protection

then react with the high-energy H+ and OH radicals to give water and the much lower energy Br or Cl radicals, which are then available to begin a new cycle of H+ and OH radical removal. The effectiveness of halogenated flame retardants thus depends on the quantity of the halogen atoms they contain (e.g., 10 bromine atoms in one molecule of deca-DBP) and also, very strongly, on the control of the halogen release. Inasmuch as chlorine is released over a wider range of temperatures than bromine, it is then present in the flame zone at lower concentrations, and so is less effective. Bromine is released over a narrow temperature range, thus resulting in optimal concentrations in the flame zone. Many different bromine-containing flame retardants have been developed, with bromine atoms bound into different organic molecules. These offer different properties, in terms of how the bromine is bound into the flame-­retardant molecule (aliphatically, aromatically), and of how the flame-retardant molecule interacts with different ­plastics. Different specific brominated compounds can thus be added to or chemically bound into different plastics without deteriorating their properties (flexibility, durability, color, etc.). The many varying brominated products ­available thus offer high flame retardancy effectiveness solutions for all plastics currently on the market and for most of their varied applications. Similarly, ­several chlorinated flame retardants are also available and are e­ ffective flame retardants in standard and technical ­plastics, ­thermosets, textiles, and rubber materials.[24] Halogen-containing flame retardants act in the gas phase and produce incomplete burned substances like black smoke (soot particles) and toxic CO. Thus, halogenated systems, especially brominated systems, play an important role in North America, an explanation for the increasing percentage of people killed by smoke inhalation. Figure 9 shows the smoke formation of polypropylene compounds containing different flame retardants.[39] A lot of toxic gases are found in real fires. The most serious is carbon monoxide, CO, a highly toxic, nonirritating gas. The presence of CO disturbs the respiration process immediately as it blocks the oxygen transport of

700 600 500 400 300 200 100 0

no FR

Halogen/SbQ3 Phosphate

Mg(OH)2

Fig. 9  Smoke density of PP compounds with different flame retardants [39]

Brominated FR Chlorinated FR No FR Mg(OH)2

5 4 Amount CO (G)

1130

3 2 1 0 0

100

200

300

400

500

Time (seconds)

Fig. 10  CO formation of PP compounds with different flame retardants [39]

blood. Again, traditional solutions based on halogens have serious drawbacks compared to mineral flame retardants, namely magnesium hydroxide. Figure 10 illustrates the CO amounts of different flame retardants.[39] Antimony Trioxide (Sb2O3) Antimony trioxide does not have flame-retarding properties on its own but is an effective synergist for halogenated flame retardants. It acts as a catalyst, facilitating the breakdown of halogenated flame retardants to active molecules. It also reacts with the halogens to produce ­volatile ­antimony halogen compounds, which are themselves directly effective in removing the high-energy H+ and OH− radicals that feed the flame phase of the fire, thus reinforcing the flame-suppressing effect of the halogenated flame retardants. When added to polyvinyl chloride (PVC), ­antimony trioxide acts to suppress flames by activating the chlorine present in the plastic materials.[24] Phosphorus Flame Retardants Phosphorus-containing flame retardants usually act in the solid phase of burning materials. When heated, the phosphorus reacts to give a polymeric form of phosphoric acid (PO3). This acid causes the material to char, inhibiting the “pyrolysis” process (breakdown and release of flammable gases) which is necessary to feed flames. Different phosphorus-containing flame retardants can be either simply mixed into plastics (and then held in the material when the plastic sets) or be reactive and ­chemically bind into the plastic molecules at polymerization. This will depend on the properties required of the plastic in terms of finished product performance, facility of processing (melting, extrusion, and molding), and flame ­retardancy (temperature of onset of the charring process).

Fire Protection: Flame-Retardant Additives and Fillers for Polymers 1131

Nitrogen Flame Retardants The mechanisms of nitrogen-containing flame retardants are not fully understood, but it is thought that they have several effects: formation of crosslinked molecular structures in the treated material. These are relatively stable at high temperatures, thus physically inhibiting the decomposition of materials to flammable gases (needed to feed flames), release of nitrogen gas that dilutes the flammable gases, and thus reduces flame synergy with phosphorus-containing flame retardants by reinforcing their function. To be effective, nitrogen-based flame retardants are used at high concentrations or in conjunction with other flame retardants. They can be either simply added to plastics or reacted into the plastic molecules. Melamine-based products are the most widely used type of nitrogen flame retardant, used in foams and nylons, but other products are also available or being developed.[24]

Inorganic Flame Retardants A number of inorganic compounds are used as flame retardants, interfering by various physical processes with the burning process: release of water or nonflammable gases, which dilute the gases feeding flames; absorption of heat energy (in these gas-release reactions), thus cooling the fire; and production of a nonflammable and resistant layer on the surface of material. These mechanisms of inorganic compounds are, however, of a relatively low efficiency and the products having to be used often in relatively large concentrations, or more usually, in combination with other types of flame retardants. Specific application forms of these products (for instance, within organic coatings) can enable such high concentrations to be added to plastics without modifying their performance properties.[24] Those inorganic flame retardants include aluminum trihydrate (ATO), magnesium hydroxide, boron compounds, zinc borate, and other zinc and tin compounds. •

Intumescent Coatings Intumescent coatings are fire protection systems that are not only used to protect materials such as wood or plastic from fire (prevent burning) but also to protect steel and other materials from the high temperatures of fires (thus preventing or retarding structural damage during fires). The coatings are made of a combination of products, applied to the surface like paint, which are designed to expand to form an insulating and fire-­resistant covering when subject to heat. The products involved contain a number of essential interdependent ingredients: s­ pumific compounds, which (when heated) release large quantities of nonflammable gas (such as nitrogen, ammonia, CO2), a binder, which (when heated) melts to give a thick liquid, thus trapping the released gas in bubbles and producing a thick layer of froth: an acid source and a carbon ­compound. On heating, the acid source releases p­ hosphoric, boric, or sulfuric acid that chars the carbon compound, causing the layer of bubbles to harden and ­giving it a fire-­resistant coating. Often the binder can also serve as a carbon compound. In a fire, the coating expands to a thick nonflammable layer of bubbles, offering good insulation protection to the material coated. As well as being used to protect flammable materials and structural elements, intumescent systems are now being incorporated into certain plastics, thus providing an inherent fire protection capacity materials. [24]

•

•

Aluminum Trihydrate (ATO): This simple inorganic compound acts with all three of the mechanisms indicated earlier. At around 200°C, it is decomposed to aluminum oxide (which forms a protective, nonflammable layer on the material surface) and water. The water (as steam) forms a layer of nonflammable gas near the material’s surface, inhibiting flames. The reaction is endothermic (absorbs heat energy), thus cooling the material and slowing burning.[40,41] Magnesium Hydroxide: This acts with the same three mechanisms as ATO, but is only decomposed at somewhat higher temperatures (around 300°C), meaning that it can be used in plastics that are molded or ­processed at relatively higher temperatures.[40,41] Huntite/Hydromagnesite:  Huntite/hydromagnesite mineral is used in the polymer application for its flame retardancy properties. This mineral was introduced to the market in the late 1980s as one of the magnesium-­ containing sources for mineral flame retardants.[23,42] At present, the commercially used deposits are located in Greece and Turkey. The deposit normally consists of physical blends of two minerals, huntite and hydromagnesite, with varying ratios in between 40%–30% huntite and 60%–70% hydromagnesite. The level of impurity is very low; the most important ones are other white carbonate minerals such as aragonite, calcite, and dolomite. Physical densities of huntite (Mg3Ca(CO3)4) and hydromagnesite (Mg4(OH)2(CO3)3·3H2O) minerals are 2.70 g/cm3 and 2.24 g/cm3, respectively.[42] The advantages of this mineral can be arranged in order such as being noncorrosive to processing equipments, low smoke generation, no acid gas emission, halogen-free, environmentally safe, recyclable, no combustion gas corrosion, no limitation in coloring and low combustion. In addition, huntite/hydromagnesite offers good cost/performance relationship in flame-retardant applications.[39]

Environmental–Fire Protection

Phosphorus-based flame retardants vary from elemental red phosphorus (P), which is oxidized to phosphoric acid with heat, to complex P-containing organic molecules offering specific performance properties. Certain products contain both phosphorus and chlorine or nitrogen, thus combining the different flame-retarding mechanisms of these elements.[24]

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Fire Protection: Flame-Retardant Additives and Fillers for Polymers

The decomposition reactions of mentioned inorganic materials mostly used in the flame-retardant market are ­demonstrated in the following[43]:

Environmental–Fire Protection

Huntite:

Mg3Ca(CO3) 4 → 3MgO + CaO + 4CO2

Hydromagnesite:

Mg4 (CO3)3(OH)2·3H2O → 4MgO + 3CO2 + 4H2O

Magnesium hydroxide:

Mg (OH)2 → MgO + H2O

Aluminum hydroxide:

2Al(OH)3 → Al2O3 + 3H2O

•

•

•

Boron compounds: These also act by releasing water, in a heat absorbing reaction, and forming a protective glassy layer on the material’s surface. They can release boric acid, which also acts by causing ­charring of the material, reducing the release of flammable gases.[40,41] Zinc borate: This is a multifunctional flame retardant that can function as a flame retardant (synergist of halogen), smoke suppressant (promote char formation), and after-glow suppressant. In some halogen-containing systems, it can display synergy with antimony oxide. In certain halogen-free systems, it can also promote ceramic char formation.[40,41] Other zinc and tin compounds: These materials act to reduce smoke emission from PVC, to promote ­charring or as synergists to increase the effectiveness of halogenated or nitrogen (melamine) flame-retardant materials.[24]

Table 4  Principle candidate flame-retardant fillers [14] Candidate material Approximate onset (common names and of decomposition formula) (°C) Nesquehonite, MgCO3 ·3H2O

70–100

Calcium sulfate dihydrate, Gypsum CaSO4 2H2O

60–130

Magnesium phosphate octahydrate, Mg3(PO4)2·8H2O

140–150

Alumina trihydrate, Aluminum hydroxide, Al(OH)3

180–200

Basic magnesium carbonate, Hydromagnesite, 4MgCO3 ·Mg(OH)2·4H2O

220–240

Dawsonite (sodium form), NaAl(OH) 2CO3

240–260

Magnesium hydroxide, Mg(OH) 2

300–320

Magnesium carbonate subhydrate (MCS), MgO·CO2 (0.96)H2O(0.30)

340–350

Boehmite, AlO(OH)

340–350

Calcium hydroxide, Ca(OH) 2

430–450

The flame-retardant materials in Table 4 are seen to cover a wide range of decomposition temperatures and to include release of carbon dioxide as well as water.[41] Those materials remove a good deal of heat evolved in degradation and thus can prevent further degradation. Nonetheless, to be effective, they must be used at very high loadings, which lead, in some instance, to the loss of mechanical properties of interest. Nanocomposites are formed when a small amount of organically modified aluminosilicate clay is added to a polymer. The presence of only a small amount of clay can give a significant reduction in the peak heat release rate (HRR). The difference between the microcomposite and the nanocomposite is the dispersion of the material in the polymer. In a nanocomposite, the clay or the nanofiller/ additive is well dispersed throughout the polymer.[23,28] Nanocomposites Nanocomposites are a new class of inorganic materials that only somewhat recently have begun to be used to achieve fire retardancy. The initial discovery is that a polyamide-6 clay nanocomposite, containing 5% clay, shows an increase of 40% in tensile strength, 68% in tensile modulus, 60% in flexural strength, and 126% in flexural modulus, while the heat distortion temperature increases from 65°C to 152°C and the impact strength is lowered by only 10%.[28,44] The initial work, which was not yet recognized as nanocomposites, actually took place sometime earlier when Blumstein synthesized poly(methyl methacrylate) in the presence of a clay and found that the clay had a templating effect on the formation of the polymer. The significance of these observations was not realized for several years, and this work has taken on more importance since the advent of the nanoera.[45] Nanocomposites may be produced using several different materials for the nanodimensional material, including clays, graphites, carbon nanotubes, and polyhedral oligosilsesquioxanes. Most work to date has been with clays, particularly with montmorillonite clay, an alumina-silicate material. A wide variety of other clays naturally occur, but, for some reason, montmorillonite has been by far the chosen material, probably because interesting results were obtained with this clay.[28] Surprisingly, graphite has not been more widely used; one concern may be that the d-spacing in most organically-modified montmorillonites is in the range of 2 or 3 nm, while graphite has a d-spacing of about 2 or 3 Å. To form a nanocomposite, the polymer must be able to enter into the gallery space of the nanomaterial, and this may require that this space be large enough to permit the polymer to begin to enter. Graphite does form a number of intercalation compounds in which the d-spacing is large. For instance, potassium graphite, KC8, has a d-spacing of 5.5 Å and that of graphite sulfuric acid is even larger. Possibly, if one begins with an already expanded graphite, a d-spacing in the range of 2–3 nm at least, that

graphite may become more useful as a nanodimensional material for nanocomposite formation. Carbon nanotubes are, of course, a newer discovery, and they are still quite expensive. There is still some activity in this area; the major difficulty with the single wall nanotubes appears to be the need to organically modify the nanotubes to make them more organophilic, this is probably also a limitation with the graphite system. The multiwall nanotubes do not require organic modification for nanocomposite ­formation. There has been little work on the fire retardancy of n­ anocomposites using carbon nanotube.[46,47] Testing Parameters in Flame-Retardant Testings Tests play a vital and somewhat controversial role in determining the effect of fillers on polymer flammability, and some discussion is essential before examining the fire-­ retardant performance of fillers. Every fire starts with the ignition of something that can be ignited under given circumstances. There are a lot of completely different ignition sources regarding the heat content and the time of ignition. A decision has to be made on how safe an individual part or the whole installation should be.[41] Large-scale tests are used for assessing the real fire resistance of articles containing polymers. The construction of the article and even its method of mounting can be important, and these tests are usually carried out on complete items. They are also usually designed to replicate the fire hazard situation the article is likely to experience. Thus, the appropriate test for a cable jacketing will be ­different to that for chair upholstery or a wall covering.[41] Various small-scale tests are used for product development and quality control purposes. These are usually carried out on flat specimens and are more related to material properties. The main characteristics one tries to assess in small-scale tests are demonstrated in Table 5. Unfortunately, all of these parameters are dependent on the test conditions, especially the amount of radiant heat from an external source. Indeed, the rating of materials can be reversed in some tests merely by changing the ­radiant heat conditions. Cone Calorimeter The cone calorimeter is a fire-testing device based on the principle of oxygen consumption during combustion. This device is used by most leading fire research groups as a data source for properties of materials and as a source for input data to models when predicting fire behavior. The cone calorimeter is considered the most significant benchscale instrument in fire testing. A fuel sample surface can be radiated with different heat fluxes by this device. The fuel sample ignites and burns in excess air. Among the

results are time to ignition, mass loss, smoke amounts, gas analyses, HRR, and other parameters related to burning properties of a fuel. HRR is defined as the mass loss rate of the material times its heat of combustion.[48] Heat release is the key measurement required to assess the fire development of materials and products. Traditionally, it has been very difficult to measure and more recently full-scale testing of items (e.g., furniture) has been possible by burning these articles and measuring the evolved heat using a technique called oxygen depletion calorimetry. In the early 1980s, workers at NIST (formerly NBS), in the United States, decided to produce an improved benchscale heat release test that would overcome the deficiencies of existing small-scale heat release tests that relied on the measurement of the outflow enthalpy of enclosed systems. Oxygen depletion calorimetry was identified as the best measurement method. This is based on the empirical observation that heat released by burning materials is directly proportional to the quantity of oxygen used in the combustion process. The instrument was called a Cone Calorimeter. This name was derived from the shape of the truncated conical heater that is used to irradiate the test specimen with fluxes up to 100 kW/m2 in the test.[48] This apparatus has been adopted by the International Organization for Standardization (ISO 5660–1) for measuring HRR of a sample. It has been shown that most fuels generate approximately 13.1 MJ of energy per kg of oxygen consumed. Therefore, HRR is based on the fact that the oxygen consumed during combustion is proportional to the heat released. This device analyses the combustion gases and measures the produced smoke from a test specimen that is being exposed to a certain heat flux. At least Table 5  The main characteristics in the flame-retardant tests [41] Ignitability

How readily a material will catch fire under certain condition.

Propagation

How rapidly will fire travel once ignition is achieved.

Heat release

How much fire will raise the temperature of the surrounding, thus causing the problem to spread.

After glow and smoldering

This is an important, but often ignored factor, as it can cause reignition after a fire has apparently been extinguished.

Dripping

Dripping can reduce the apparent flammability by removing heat from the specimen. On the other hand, it can also lead to propagation, if the drops themselves are burning.

Smoke and gases

Smoke and toxic and corrosive gases.

Char integrity

This is becoming of some importance for building products.

Environmental–Fire Protection

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the oxygen concentration must be analyzed to calculate the released heat, but to improve the accuracy, carbon monoxide and carbon dioxide concentrations can also be analyzed. The data collected from this bench-scale real fire test can be used for fire modeling, prediction of real-scale fire behavior, pass/fail tests, etc.[49] Environmental–Fire Protection

Limiting Oxygen Index The definition of flame retardancy needed for a specific application is often misleading. The material’s behavior, the “flammability” of the compound, is a first hint that a finished product which uses this material may survive a real fire attack. Such material behavior is generally and easily checked by loss on ignition (limiting oxygen index (LOI)). LOI tests determine the oxygen concentration, which is needed to keep a sample of material burning. The main problem with testing conditions is that an oxygen concentration is always used, which is higher than the usual oxygen concentration in the air. The LOI classification specifications are shown in Table 6.[41] Keeping in mind that oxygen is needed and used during a fire, the oxygen content of the air surrounding a burning plastic part will in any case be lower than the standard amount. Therefore, it becomes clear that LOI testing

Table 6  LOI classification specifications [41] LOI Classifications—Grouping 34

Flammable

Limited flame resistance

Flame resistance

Extra flame resistance

Fire propagation

Fire resistance

Fire gases

Integrity of service

(a)

might be used as a quick test for development purposes in the lab. Here it can differentiate between samples created under similar conditions by slightly changing the type and amount of ingredients. Finished products can be better checked using full-scale tests. There are many different full-scale tests. In general, these tests intend to simulate potential practical fire attacks. In most cases, there is one test to check the material behavior against a match, a cigarette, or a lighter. A second much more severe test simulates an arson attempt with 20 L of gasoline. For cables, there are single cable tests to check the burning behavior of just one cable. Cable bundle tests simulate today’s standard type of applications with more than one cable in a cable duct.[23] A real system test would not only include the fire-­ retarded product, such as the cable, but also all other ­system parts like adhesives, paints, joints, or repair kits. For cables, the afterinstallation of one cable after the other should also be kept in mind when judging the efficiency of a complete installation.[23] UL94 UL94 is a technique used in plastic industry according to UL94 standardization. The material is conducted to the flame in a specific angle and distance. Extinguishing time of the flame and dripping ability are measured.[5] The UL94 test standard is generally used as an indicator of the acceptability of plastic for general use with regard to its flammability.[50] There are three different tests: surface burn tests, vertical burn test, horizontal burn tests, as indicated schematically in Fig. 11 and Table 7. The samples are subjected to flame for 3 min after the calibration process is complete (flame height: 12 mm, it is 23.5 s:1 s to reach from T1:100°C to T2:700°C). The used gas is propane with 99.5% of purity.

(b)

(c)

45°C

Surface Burn

Vertical Burn

Horizontal Burn

Doesn’t ignite under

Self-extinguishing

Slow burn rating

hotter flame

UL 94 V-0 (best)

Takes more than to 3 min.

UL94 5VA

UL 94 V-1 (good)

To burn 4 inches.

UL 94 5VB

UL 94 V-2 (drips)

Fig. 11  Schematic of UL94 test requirements: (a) surface, (b) vertical, and (c) horizontal burns [51]

Fire Protection: Flame-Retardant Additives and Fillers for Polymers 1135

5VA

Burning must stop within 60 s after five applications of 5 s each of a flame to a test bar, and there must be no burnthrough hole. The flame is larger than that used in Vertical Burn testing. This is the highest (most flame retardant) UL94 rating.

5VB Vertical burn tests

As for 94 5VA, but a burnthrough hole is allowed.

V-0

Specimens must extinguish within 10 s after each flame application and a total combustion of less than 10 s after 10 flame applications. No samples are to drip flaming particles or have glowing combustion lasting beyond 30 s after the second flame test.

V-1

Specimens must extinguish within 30 s after each flame application and a total combustion of less than 250 s after 10 flame applications. No samples are to drip flaming particles or have glowing combustion lasting beyond 60 s after the second flame test.

V-2

Specimens must extinguish within 30 s after each flame application and a total combustion of less than 250 s after 10 flame applications. Samples may drip flame particles, burning briefly; and no specimen will have glowing combustion beyond 60 s after the second flame test.

VTM

Thin material version of the vertical burning test applied to thin or flexible materials which may distort, shrink, or flex during the 94 V test. A specimen 200 × 50 mm is rolled longitudinally around a 12.7 mm diameter mandrel and taped on one end. When the mandrel is removed, the specimen forms a cone shape, which gives it longitudinal rigidity.

VTM

Have the same three classifications as 94 V. Differences are that a flame is applied twice for only 3 s, and no specimens may have flaming or glowing combustion beyond a point 125 mm from the bottom of the specimen.

Horizontal burn test HB

Slow horizontal burning on a 3 mm thick specimen with a burning rate is less than 75 mm/min or stops burning before a mark 125 mm away from the point of flame application. HB-rated materials are considered “self-extinguishing.” This is the lowest (least flame retardant) UL94 rating.

Recycling Recycling activities on polymeric materials are increasing and becoming more important in recent years. For ­polymers containing no flame retardants, suitable recycling strategies already exist. To investigate the recyclability of flame-­ retarded polymers that contain flame-retardant ­additives, various researches have been done by ­researchers.[52] For certain polymers from electrotechnical ­applications, suitable recycling strategies have been used. Due to the variety of flame-retarded polymers, new rapid analytical methods such as pyrolysis gas chromatography with mass spectrometric detection (py-GC/MS) together with infrared spectroscopy (FT-IR) have been developed.[52,53] In this manner as a first step in recycling, the analytical methods employed for the characterization of materials are listed in Table 8. Riess et al.[52] studied a purification method developed for the quantification of the additive content in polymer extracts containing high concentrations of polybrominated diphenylethers. Used sample purification procedure adopted is shown in Fig. 12. Analysis of the flame retardants was carried out following the H2SO4/silica column ­procedure. For the quantification of additive, two further Al2O3 purification steps were included. The analysis of these compounds was carried out in the single ion mode

using internal and external standards, respectively. Following chemical characterization, the polymer samples were recycled. For this purpose, the material was grinded to a size of 4 mm. A Leistritz 30.34 twin screw extruder was used. For the high-impact polystyrene materials, the p­ rocessing temperature was 190°C and 210°C for the ­acrylonitrile butadiene styrene (ABS) samples, respectively. Recycling of plastics containing flame retardants in electronic waste is also an important challenge for a sustainable solution. In 2011, the EFRA initiated a full-scale recycling project on flame-retardant plastics from postconsumer flat TV sets. Directive could impose much higher recycling rates on waste of electrical and electronic equipment. Therefore, waste of electric and electronic equipment plastics increasingly have to go through mechanical recycling as opposed to energy recovery. These developments are likely to trigger significant growth in the mechanical recycling of plastics, which also can contain different types of flame retardants.[54] The steps in the recycling are as follows: •

Identification and sorting of black plastics, the effect of contaminants of one plastic type on the other, and understanding the physical properties of virgin plastics when mixed with and compared to recycled plastics.

Environmental–Fire Protection

Table 7  Characterizations of materials according to UL94 test [51] Surface burn tests

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Table 8  Analytical methods for characterization of polymers [58] Analytical method

Purpose

Environmental–Fire Protection

Infrared spectroscopy (FT-IR) in combination with thermogravimetric methods

Polymers

Pyrolysis gas chromatography (py-GC/MS) with mass spectrometric detection

Polymer and flame retardancy class

Energy dispersive X-ray fluorescence (ED-XRF)

Halogen-containing samples

High resolution gas chromatography with mass spectrometric detection (HRGC/MS)

Quantification of polybrominated dioxins and furans (PBDD/F)

High-pressure liquid chromatography with ultraviolet detection (HPLC)

Chemical reactivity of flame retardants, determination of other additives in polymers

Atom spectroscopic methods (AAS and ICP-AES)

Reference and trace analysis

Flame retardant polymer

Addition of 5 13C12-labeled PBDD/F

Soxhlet-extraction with toluene 24h

Column chromatographic purification

HRGC/MS

Fig. 12  Isolation and purification of PBDD/F [52] •

•

After characterizing and defining the main physical properties of plastics, the first step is setting up a scheme for further mechanical size reduction and ­separation of plastics. These starting points can be used, and they provide guidance on how to achieve the desired plastic quality, and whether these requirements will take into account the miscibility limitations of different plastics. Recycling of plastics containing flame retardants in electronic waste is a technical and environmental challenge for a sustainable solution.

Plastics containing brominated flame retardants have proven to be fully compatible with all methods of waste management, especially recycling and recovery. Certain plastics/brominated flame-retardants combinations are actually already being specified by leading manufacturers of photocopiers, in part because of their excellent stability in the recycling process. Recycling is already taking place with 30% of some new copiers containing recycled plastic with brominated flame retardants. A recent study concluded that ABS plastic containing brominated flame retardants was superior to other plastics in terms of recyclability and could be recycled five times in full compliance with the strictest environmental and fire safety requirements. In

short, the presence of plastics containing brominated flame retardants in the waste stream provides producers of many products with a wide variety of environmentally sound and ­economically feasible options for waste recovery and ­recycling.[55] Phosphorus, inorganic, and nitrogen flame retardants (PIN) flame retardants are compatible with polymer recycling regarding following processes: Mechanical recycling provides reusing the plastics as such after separation of different kinds, grinding, melting, etc., and turning into new products. Feedstock recycling helps to breakdown the plastics into chemical constituents, which can then be used to produce new plastics or other materials. Energy recovery is possible by capturing the energy content of plastics, usually by combustion with proper energy generation (electricity, heat, steam). Those types of flame retardants can be produced using recycled (secondary) raw materials. Full-scale pilot industry tests have demonstrated that phosphorus can be recovered from sewage sludge or other ashes, and used as a raw material for industry, including those flame retardants. Besides, it is possible to recover some of the basic chemicals used in the PIN flame retardants in processes where waste polymers are treated (e.g., metal smelters used to recover copper and rare earths from electronic wastes, incinerators producing energy from waste plastics).[56] Research into those flame-retarded products is developing as these are increasingly used in consumer and industrial products to replace halogenated substances.[57] On the other hand, Fraunhofer LBF in Germany is leading a project to boost mechanical recycling of plastics containing halogen-free flame retardants. The works continuously improve the environmental and health profile of their flame-retardant products, working groups and research projects in the areas of fire safety, environment and ecolabels, recycling, and communications and outreach. The halogen-free retardant plastics research being carried out by Fraunhofer LBF will be of particular use to polymer, flame-retardant and additive manufacturers, compounders, masterbatch producers, producers of p­ lastic parts, recycling companies and consulting firms, the ­institute says.[58,59]

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Fire Protection: Flame-Retardant Epoxy Resins in Seema Agrawal and Anudeep Kumar Narula Environmental–Fire Protection

University School of Basic and Applied Sciences, Guru Gobind Singh Indraprastha University, New Delhi, India

Abstract Epoxy resins have been widely used for coatings, adhesives, electronic materials, and matrices for fiber-reinforced composites because of their outstanding mechanical properties, good heat resistance, high adhesion strength, and high electrical resistance. The properties of cured epoxy resins are affected by the structure of epoxy resin, curing agents, additives, and curing process. An overview of the recent literature on new developments in epoxy monomers, curing agents, and fire-­ retardant additives is described. The literature sources are mostly taken from publications from 2006 and later. This entry discusses the latest advancements of phosphorus-containing fire retardants for electrical and electronic applications and compares them with commercially available ones. Silicon, boron, metals, or nitrogen-containing products and inorganic additives remain of great interest as supplementary materials to phosphorus fire retardants. The mechanism of thermal degradation and fire retardancy of phosphorus fire retardants in epoxy resins is also discussed. Keywords: Curing agents; Epoxy resins; Flame retardancy; Smoke suppression; Thermal stability.

INTRODUCTION Epoxy resins (EPs) are widely used as matrix materials for the fabrication of advanced composites in the electrical and electronics industry due to their good impregnation and adhesion to fiber reinforcement, high tensile strength and modulus, good chemical and corrosive resistance, low shrinkage upon cure, and excellent dimensional stability. Their use is in particular justified in application areas where their technical advantages balance their relatively higher costs compared to other thermosetting polymers, e.g., in aviation and aircraft industry. The manufacture of large parts for both commercial aircrafts and general aviation has become an important force driving the development of ever-improving carbon fiber-reinforced epoxy systems. Composites are to make up a large share of the structural weight of new aircraft: more than 50% of the new Boeing 787 and over 20% of the new Airbus A 380. However, the flammability of EPs still represents a limitation in structural applications, as an incidental fire event involves not only health risks but also loss of mechanical properties. To meet application requirements, their flame-retardant (FR) properties have to be improved, while maintenance of other important char characteristics such as thermal and mechanical properties and consideration of environmental issues is also required. Supporting that aircraft manufacturer will be forced to demonstrate that the polymer composite structures provide safety equivalent to the current material systems, there is a demand for Encyclopedia of Polymer Applications, First Edition DOI: 10.1201/9781351019422-140000200 Copyright © 2018 by Taylor & Francis. All rights reserved.

research and development of fire-retardant fiber-reinforced EP composites. The challenge involves achieving multifunctional composites with effective flame retardancy and ­high-performance mechanical properties. As reported by several researchers, the fire retardancy of EP can be improved by incorporation of bromine-­ containing additives or by copolymerization with brominated epoxy compounds. Halogenated FRs have shown consistent growth over the past 20 years, especially with exponential increases in the sales of computers and other electronic equipment. However, the generation of corrosive and toxic substances such as HBr or halogenated dibenzodioxines during combustion led to concerns about the exposure to halogen-containing contaminants, which in turn stimulated the development of alternative methods of flame retardancy. Therefore, phosphorus-based FRs have been investigated in an attempt to find halogen-free FR solutions. The increasing focus on the issues has been drawn to the attention to halogen-free additives, especially phosphorus-containing FRs in the field of advanced composites. FR PROPERTIES Generally speaking, the flammability of polymers can be decreased either by alternating the products of thermal degradation in such a way that the amount of nonflammable combustion products is increased at the expense of

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flammable volatiles or by inhibiting oxidation reactions in the gas-phase through the trapping of free radical species or by a combination of these mechanisms. Nevertheless, other modes of effectiveness of FRs play a role: (1) endothermic reactions of degradation products from FRs with species present in the flame or substrates; (2) generation of noncombustible gases, which dilute the oxygen supply at the surface of burning polymer; (3) endothermic decomposition of FRs; and (4) formation of nonvolatile char of glassy film barrier, which minimizes diffusion of oxygen to the polymer substrate and also reduces heat transfer from flame to polymer substrate. The mechanism of flame retardation discussed earlier does not contradict each other, since several principles can simultaneously contribute to the action or effectiveness of an FR system. To consider combinations of FRs, it is important to point out the widely discussed but poorly understood phenomena of synergism. Typically, a combination of phosphorus and halogen is widely used, and it is postulated that an FR effectiveness of systems containing these elements includes a solid-­ phase effectiveness component (­ phosphorus) as well as a ­vapor-phase activity (halogen). Fire retardants and methods for making flammable materials resistant to fires have been around since the beginning of recorded history. Vinegar and potash alum or ammonium aluminum sulfate were used as paints or coatings for wood to help impart flame resistance. Over time, different types of gypsum, clays, borax, and asbestos have been used to make canvas and clothing impervious to flames. Inorganic materials and salts are stable FRs that are still used today. World War II was another dominant milestone in the creation of FRs. In the 1950s, the polymer industry was starting to grow into a big market. During this period, a majority of commodity plastics (e.g., polyurethane, polystyrene, and polyethylene) were developed on a widespread and economically favorable scale and was preferred over other materials such as metal alloys and wood. The government also established guidelines for firesafe materials in fabrics and transportation materials, particularly aeroplanes. To satisfy the laws, the development of FRs became a multibillion dollar industry. FR polymers can be divided into two distinct classes: additives and inherently FR polymers. The additives, as the name implies, are various inorganic or organic compounds that are added to commercial polymers in ­conjunction with a synergist to make the polymers fire retardant. Additives make up the majority of the fire-retardant market due to lower cost for processing these commodity materials into FR polymers. Additives work well for fire-retardant applications, but they have few drawbacks. FR additives leach out of the plastics over time, polluting the environment and making the polymers less flame resistant. Another drawback to the approach of using FR additives is their tendency to degrade the physical properties such as impact resistance of tensile strength of the native polymer. Inherently,

Fire Protection: Flame-Retardant Epoxy Resins in

FR plastics are more complicated, since these polymers have FR moieties incorporated either in the backbone of the polymer or as pendent groups. These polymers are usually found in specialty applications where cost is not an important factor. In principle, to be seriously considered, an FR added to a polymer should (1) reduce flammability ­compared to the unmodified polymer to a level specified for the product in terms of product performance in a specific flammability test; (2) reduce smoke generation under specified conditions of testing; (3) be retained in the ­product through normal use (exposure, cleaning, aging, etc.); (4) not increase the toxicity of combustion products from the modified polymers compared to the unmodified polymers; and (5) have an acceptable minimal effect on other performance properties of the product in use. In addition, consideration must be given to the effects that the added FRs may have on fabrication, processing conditions, and cost in the manufacture of modified product; to health hazards that may result from the presence of the compound in the workplace, in the environment (physiological effects, industrial hygiene, ecological consideration), and in the product; and of course, to the availability and ­economics of the fire retardant itself. Most of the products that fit the criteria of being an FR polymer are usually marketed as heat-resistant materials, which means that these polymers do not burn but rather degrade at elevated temperatures. There are five main principles that have to be considered while developing flame— and heat-resistant polymers: (1) There should be no easy pathway for the intermolecular rearrangement, (2) compounds with strong bonds should be used, (3) there should be resonance stabilization in aromatic rings to minimize the bonding energy, (4) there should be no bond strain, and (5) multiple bonding to several centers should be utilized. There are different steps where a fire scenario can be stopped, as shown in the flame cycle (Fig. 1). An FR can act in the gas-phase by inhibition of the exothermic oxidation reaction in the flame by radical scavenging, thus decreasing the energy feedback to the polymer surface. A  fire retardant can also promote the formation of a ­protecting char layer at the surface of the condensed-phase, which blocks the release of gaseous fuel and prevents the heat back transfer to the burning polymer.[1] CHEMICAL MODIFICATION WITH FRs FRs can be introduced in several ways during the processing sequence of monomers to polymers and subsequently to plastics, fibers, or other end products. Each approach has its own advantages and limitations, which depend on many factors including processing requirements, level of FR needed, properties of fire retardants, and critical product properties. The approaches that are considered are briefly explained later.

Fire Protection: Flame-Retardant Epoxy Resins in 1141

Flame Smoke and fume

Heat

Air Combustible gaseous fuel

Thermal degradation

Char residue

Polymeric material

Heat

Condensed phase

Fig. 1  Combustion cycle of polymer fire Source: Vandersall [1] © 1971 Sage.

Use of FR Monomers The advantage of this approach is that the FR becomes an integral part of the polymer molecule, is resistant to removal or leaching, and thus loses the effectiveness during use. The disadvantage resides in the effect of resulting changes in the polymer properties such as mechanical behavior (recovery, tensile strength), morphology ­(crystallinity, orientation, intermolecular forces), as well as physical properties such as glass transition temperature and melting point. Comonomers have been used commercially as FRs  in polyester fibers, acrylic fibers, and ­polyurethane foams. Use of FR Additives FR additives are introduced in the polymer before spinning of fabrication. These additives are stable under the conditions of spinning and are uniformly dispersed and retained in the polymer fluid during processing in the amount needed to impart the desired level of flame retardancy. This approach is most widely used for plastics and foams.

The most important tool for the laboratory evaluation of flammability of polymers is the limiting oxygen index (LOI) test developed at the General Electric Company for polymer sticks and since then adapted to a wide variety of polymeric materials and compositions. The oxygen index of a material is the minimum percentage of oxygen in the oxygen–nitrogen atmosphere required to sustain ­combustion of the material after ignition. This test is generally carried out with the sample burning downward in a candle-like manner, producing a ­gaseous diffusion flame above the polymer surface. PROBLEMS IN POLYMER MODIFICATION WITH FRs Incorporation of Effective Amounts of FRs The incorporation of effective amounts of FR compounds in polymers without impairing polymer properties is an extremely difficult problem since the effect of added FRs on polymer properties is considerable and difficult to overcome. The effect of FRs on polymer properties may cause the polymer to fail one or more critical requirements, either in use or product manufacture. The minimum amounts of FRs on polymer flammability can be defined only in relative terms with reference to carefully specified test procedures. Cost Chemical modification of a given polymer with FRs include many components such as chemical cost of FRs, processing cost of modification, cost of adjustments in manufacturing process for modified polymer, and cost of quality control (flame retardance). The costs were initially high, but decreased with improved technology and increased use of FRs. Retention in Use This entails the evaluation of the effect of environmental conditions, which the product is likely to encounter in use (e.g., temperature, light, humidity, etc.) and aging.

Graft Copolymerization

Environmental

Graft copolymerization has been extensively investigated and is reportedly approaching commercial development for regenerated cellulose fibers in Europe. While commercialization has not been reported, the flexibility of the approach and the multitude of disclosures in the technical literature suggest that this route may become more ­important in the future.

The FR compounds must be safely handled in industry (occupational health) and must not accumulate in the environment. Effective FRs that can be retained adequately in polymeric substrates in use are generally organic compounds of halogens and phosphorus. These may possess physiological activity and may pose problems of several kinds. The status of the controversy on environmental

Environmental–Fire Protection

EVALUATION OF FLAME RETARDANCY

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Fire Protection: Flame-Retardant Epoxy Resins in

pollution by FR polymers is currently under review. These FR polymeric materials should not yield abnormally toxic degradation products on burning and must not pose a health hazard in use.

Environmental–Fire Protection

grades, where flame retardancy is mandatory, is reactive ­tetrabromobisphenol A (TBBPA) (Fig. 2). It should be considered that the primary cause of death in household fires is smoke. The Underwriters Laboratories examined the different combinations of household materials and found that synthetic materials cause hotter fires and increased toxic smoke compared to natural furnishings. It is the fact that consumer products and household objects, in general, make home fires distinctively more ­dangerous. A fire-retardant system always has limitations which, when exceeded, may lead to a sustained fire. The nature of FRs added has a nonnegligible impact on the toxicity of the fumes released.[2] Despite their b­ enefits of slowing down the flame spread by reducing the fire growth, FRs can increase the yield of toxic gases or even decompose into toxic gases during a fire scenario.[3] Environmental persistence and the ability to bioaccumulate add more concerns. EPs frequently have to meet a flame retardancy grade that could be accomplished by incorporating brominated reactive compounds such as TBBPA cured by a number of hardeners.[4] A few brominated EPs (BERs) were prepared by curing a mixture of diglycidyl ether of bisphenol A (DGEBA)/diglycidyl ethers of TBBPA (Fig. 3) and different hardeners: dicyandiamide (DICY), ­4,4′-diaminodiphenyl sulfone (DDS), and polyethylene polyamine (POEPA). The use of different hardeners strongly affected the ­thermal and degradation behavior of BER. ER containing bromine compound was melt-blended with poly(ethylene terephthalate) (PET) to obtain an FR polymer.[5] DSC data revealed that the EP was not located in the crystalline region but appeared in the amorphous region of the PET matrix. Good miscibility of EP resulted in the decrease of crystallization temperature and glass transition temperature of PET. 4,4,5,5-Tetra(3,5-dibromophenyl)-2,2-diphenyl-1,3-­ dioxa-2-silole (Fig. 4) was prepared from the brominated precursor diol and diphenylchlorosilane.[6] This compound was used to serve as an initiator for vinyl polymerization to incorporate a flame-retarding moiety into the polymer main chain. Bromine carbon alkyd resin was synthesized with soya oil acid, pentaerythritol (PER), phthalic anhydride, and BER as materials, [7] and then bromine carbon alkyd resin-retardant coatings for steel structure were prepared

Smoke Evolution and Toxicity of Combustion Products Evolution of smoke and toxic degradation products from fire-retardant polymeric materials exposed to fire is a complex function of composition and of the conditions of burning. The presence of FRs in polymeric materials may increase smoke and toxicity of degradation products, but generalizations are not possible on the basis of knowledge available at this time. Modification of polymers with FRs is designed to decrease the probability of ignition and of sustained combustion on exposure to flame. This is an important aspect of fire hazard. Other aspects include formation of degradation products and smoke evolution, either particulate or gaseous, which may be toxic or lethal. FR COMPOUNDS Halogen FRs Halogen-containing FRs are effective in the gas-phase by releasing halogen radicals, which react with the high-­ energetic H and OH radicals responsible for the chain reaction of burning organic gases. Generally, halogen-­ containing FRs for EPs are brominated phenols, which react with epoxy components of polymer systems, and so bromine is integrated into the polymer matrix. Until recently, the majority of FRs were halogen based, and the most widely used FR in EPs, especially in electronic Br

Br CH3

HO

OH CH3

Br

Br

Fig. 2  Structure of TBBPA

Br H2C

H H2 C C O O

Br

CH3

H2 H H2 O C C C

C Br

Br

CH3

Br

OH

O

CH3

H2 H O C C CH2 O

C Br

CH3

Diglycidyl Ether of Tetrabromobisphenol A (DGEBBA).

Fig. 3  Structure of diglycidyl ethers of TBBPA Source: Luda et al.[4] © 2007 Elsevier.

Br

Br

n

Fire Protection: Flame-Retardant Epoxy Resins in 1143

Inorganic FRs Boron-Containing Compounds as FR Additives

Br Si O

A flame retardation system for epoxy molding compounds (EMCs) used in semiconductors packaging was investigated using calcium borate as a fire retardant.[9] By adding a small amount of calcium borate as an FR, the flammability of the EMCs decreased. But the flammability did not decrease proportionally with the volume of calcium borate added. The FR mechanism of this epoxy system was considered to be the cooling effect by water released from calcium borate and the sealing effect by the glass matters formed from B2O3 at high temperature. The possibility of char formation by Ca and CaO was considered. To further improve flammability, an excess of phenolic resin (Fig. 5a,b) was added to the EMC. With the effect of calcium oxide and excess phenolic resin, an EMC that has an FR level sufficient to satisfy the V0 classification of the UL94 (Underwriters Laboratory) rating was developed. It was found that CaO component accelerates the curing reaction of the epoxy compounds due to water absorption from the phenolic resin hardener. Ca(OH)2 formed from CaO and water also contributes to enhance the flame ­retardancy, releasing water at high temperature. Smoke is considered to be the main hazard of fires involving EPs, but its production depends on many variables, principally the chemical character and the burning rate of the polymer as well as the availability of oxygen. The work reported by Formicola et al.[10] aimed to study the smoke-suppressant effect and flammability performance of zinc-based compounds (FR system) in epoxy matrix composites used in the aerospace and aeronautical industry. The FR performance of neat and FR-loaded systems

O Br

Br Br

Br

Br

Br

Fig. 4  Structure of 4,4,5,5-tetra(3,5-dibromophenyl)-2,2-­ diphenyl-1,3-dioxa-2-silole Source: Howell and Cho.[6] © 2011 Elsevier.

using bromine carbon alkyd resin compounded (ground, stirred, and dispersed homogeneously) with FR additives as a binder. New modified reactive FR alkyd resins (short, medium, and long oil alkyd) were produced by means of a condensation polymerization reaction between a linseed oil fatty acid and glycerol, to produce monoglyceride as the ingredient source of the polyol used.[8] This was then reacted with phthalic anhydride, which was partially replaced with tetrabromophthalic anhydride as the ingredient source of dibasic acid. The fire-retardant capacity of the modified reactive FR alkyd resins was assessed using the LOI test. The results of the LOI test indicated that the modified reactive FR alkyd resins exhibited an improved flame ­retardancy effect. H3C

H C

H2C

H2 C

CH3

O

O

O

H2 C

H C

CH2 O

H3C OH

CH3

(a)

OH

OH

n

(b)

Fig. 5  Structure of (a) EP and (b) phenolic resin Source: Ishii et al.[9] © 2006 Society of Plastics Engineering.

Environmental–Fire Protection

Br

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Environmental–Fire Protection

was screened using microcombustion calorimetry, while smoke generation, in terms of carbon dioxide (CO2) and carbon monoxide (CO) production. The results indicated that the dispersion of zinc borate (ZB) and zinc hydroxyl stannate (ZHS) into epoxy matrices led to a significant variation in fire-retardant properties, reducing both total heat release (THR) by about 25% and 30%, respectively. The epoxy system containing ZHS showed an enhancement in all  smoke-suppressant properties; both tin compounds (zinc stannate (ZS) and ZHS) gave a reduction of CO2 /CO ratio from 41 to 25 for ZS and from 41 to 36 for ZHS compared to neat epoxy. A series of fire-retardant EPs containing boron and silicon (Scheme 1) were obtained through a cross-linking reaction by using tris(2-hydroxypropyl) borate (THPB) together with curing agent octaaminophenyl polyhedral oligomeric silsesquioxanes (OapPOSS) (Fig. 6).[11] The LOI reached 30.5% when the contents of boron and silicon in cured EP were 1.5% and 0.5%, respectively, indicating that THPB and OapPOSS exhibited a good fire-retardant effect on EPs. Moreover, the results showed that epoxy hybrids possessed lower initial decomposition temperature but higher char residues than neat epoxy, mainly owing to the degradation of THPB in advance and releasing boric acid (BA) that could promote the formation of char layer. The incorporation of THPB and OapPOSS catalyzed the degradation of epoxy to form a heat-resistant char layer, reduced the release of flammable gas products, and thus retarded the combustion. The effect of ZB, BA, and boric oxide was reported on the fire retardancy and thermal properties of EPs

containing red phosphorus (RP).[12] The results showed that the incorporation of 6.45–7.05 wt% was enough to achieve the highest UL94 rating (V0) with a LOI value of 32.5. RP showed a fire-retardant effect through the formation of a thermally stable char layer in the condensed-phase and the flame inhibition effect in the gas-phase. According to LOI and UL94 tests, the maximum beneficial effect was seen at a ratio of 9:1 with the addition of BA and ZB. The highest UL94 rating (V0) was achieved with the inclusion of BA at 7:3. The addition of boron complexes showed an adjuvant effect by favoring the char residue and the formation of boron phosphate in the condensed-phase. The boron compounds used favor the formation of char via the  formation of boron phosphate in the condensed-phase. The inclusion of BA and ZB also increased the char formation by water release, which favors the oxidation of RP. The FR properties of boron compounds with respect to aluminum trihydroxide in an epoxy system based on epichlorohydrin bisphenol A-based EP and cycloaliphatic polyamine-based hardener were investigated.[13] Six different boron compounds, including colemanite (C), boric oxide (BO), ulexite (U), BA, melamine (MEL) borate (MB), and guanidinium nonaborate (GB), were used as fire-­retardant additives. The results of boron compounds, except for C and U, showed better performance than aluminum trihydroxide. LOI results showed that 40% BA-­containing ­samples had the highest LOI value of 28.5, while 35% GB, 30% MB, and 40% BA-containing samples had the highest UL94 V rating (V0). The formation of an insulating protective layer predominantly improved the fire p­ erformances of boron compounds in the condensed-phase. OH

OH

OH

HO

B

+

3

OH

HO

O

HO

H2C O

CH3 C CH3

O

O

+

H H2 C C O

B

OH H2 H2 O C C C O H n Heating

OapPOSS

Boron/silicon-containing epoxy thermosets

Scheme 1  Synthesis of boron/silicon-containing thermosets Source: Yang et al.[11] © 2012 Elsevier.

CH3 C CH3

OH

H2 H O C C CH2 O

Fire Protection: Flame-Retardant Epoxy Resins in 1145

H2 N NH2 O

H2N

Si

Si O

Si

O

O O

NH2

O O

H2N

Si

Si

O

O Si

Environmental–Fire Protection

Si

O

O

Si

NH2

O

H 2N

H2N

Fig. 6  Structure of OapPOSS Source: Yang et al.[11] © 2012 Elsevier.

Smoke suppression and synergistic flame retardancy properties of ZB and diantimony trioxide (Sb2O3) in epoxy-based intumescent fire-retardant (IFR) coating were investigated by Zhang et al.[14] With the addition of ZB and Sb2O3, the epoxy-based IFR coating could effectively suppress smoke and retard heat during the whole combustion process. The ZB and Sb2O3 could improve the shielding layer to be more stable and compact, so that it could restrain the smoke generation and heat release. Moreover, a lot of heat was absorbed during the melting process of ZB and Sb2O3. Hence, the smoke-suppression and synergistic flame retardancy properties of ZB and Sb2O3 in ­epoxy-based IFR coating were quite excellent. To develop organic/inorganic fire retardants, the aromatic boronic acid derivative 2,4,6-tris(4-boronic2-­thiophene)-1,3,5-triazine (3TT-3BA) (Fig. 7) and magnesium hydroxide (MH) were selected.[15] These two compounds were added to EP to improve the fire-retardant properties. The results showed that mixing EP with both 3TT-3BA and MH results in better thermal stability and fire-retardant properties than mixing with only one of the compounds, indicating the synergistic effect of the two components. A halogen-free fire retardant (DTB) (Fig. 8) containing phosphorus, nitrogen, and boron was successfully synthesized and then blended with DGEBA to prepare FREPs.[16] The results indicated that the flame retardancy and smoke inhibition performance of EP/DTB thermosets were significantly improved with the incorporation of DTB, and the char yields of epoxy thermosets were increased by 45.1%–72.8% compared with that of the neat EP thermoset. The LOI value of the neat EP thermoset was just

HO B S

HO B HO

OH

N S N

N

S

B

OH

HO

Fig. 7  Structure of 3TT-3BA Source: Zhang et al.[15] © 2016 Elsevier.

22.5%. With the incorporation of DTB, the LOI values of EP/DTB thermosets with different amounts of DTB reached 31.5%–35.6%. In addition, EP/DTB thermosets had no dripping phenomenon during UL94 vertical burning test. DTB contributed to form intumescent and glassy char layers with barrier effect and decomposed to generate free radicals with a quenching effect. DTB was an effective radical trapper as well as an efficient charring agent for EP and exerted FR effect in the gas and condensed-phases simultaneously. The boron-containing phenol–formaldehyde resin (BPFR) was used to cure TBBPA EP (TBBPAER).[17] The results of thermogravimetric analysis (TGA) showed

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Fire Protection: Flame-Retardant Epoxy Resins in

OH O

OH

N

Environmental–Fire Protection

O

HO

N

N

O

B

O

n

O OH

O

P

O

Fig. 8  Structure of halogen-free fire retardant (DTB) Source: Yang et al.[16] © 2016 Elsevier.

that with the increasing BPFR content, the heat-resistant ­property was better, which might be due to –OH of – C6H4 –CH2OH and –C6H4OH that reacts with the epoxy group. When the quantity of TBBPAER was 10 wt%, the temperature of 5% weight loss was 386.03°C, which was 38.78°C higher than that of 40 wt%. The LOI and the fire resistance increased with the addition of BPFR, so therefore the LOI could reach 68.5. As the content of BPFR increased and the content of TBBPAER decreased, the fire resistance was better. This was mainly because BPFR played a leading role in the fire resistance, which could form a nonpenetrable coating in these materials upon their thermal degradation, and the coatings excluded oxygen and prevented further propagation combustion, so the LOI was higher. Metallic Compounds as FRs Metal hydroxides, such as MH and aluminum hydroxide, have several positive effects when applied as FRs. They are easy-to-obtain, nontoxic, very cheap, and environmentally friendly FR compounds. Nonetheless, very high loadings are required to obtain fire retardancy, and such high loadings have detrimental effects on the properties of the end product. Metal hydroxides have a strong tendency to react via a condensed-phase mechanism. They decompose into metal oxide and water, which is highly an endothermic reaction (Eq. 1: endothermic reaction of aluminum hydroxide which leads to release of water). 2Al ( OH )3 + 1075 kJ/kg Al2 O3 + H2 O

(1)

The released water evaporates, thus cooling the surface of the polymer and diluting the burnable gases at the same time. The remaining metal oxide forms a protective layer on the surface of polymer to shield the polymer against further degradation and reduce the amount of toxic gases released. Aluminum-oxide-hydroxide (AlOOH or boehmite) has much higher thermal stability and can be applied in epoxy systems that undergo lead-free soldering. Leadfree soldering required higher processing temperatures that demand high thermal stability from the additives used. However, the recently used system for printed wiring board consisting of a novolac EP and DICY as a hardener must endure 288°C without delaminating. Metal hydroxides are often used as synergists with phosphorus-based FRs (e.g., metal phosphinates). ZS, including zinc hydroxystannate (ZHS), was used within synergistic FR systems, usually in conjunction with halogenated species in a number of polymers.[18] Their properties were found to be similar to antimony (III) oxide (ATO) in that they enhanced the effectiveness of the halogenated and principally brominated retardant (Br-FR), present. Unlike ATO, they were nontoxic but were specific in their synergistic activity and were effective as smoke suppressants. It was reported that ZHS precipitated as a crystalline on the surface of other solid particulate fire retardants such as aluminum and MHs to form a coated material of enhanced efficacy. ZHS was used in polymeric formulations that were heated up to 180°C, while ZS may be used at all processing temperatures up to 400°C. FREPs were prepared by a simple mixed method using ammonium aluminum carbonate hydroxy hydrate

(AACHH) as a halogen-free FR.[19] The effects of AACHH content on LOI of ER/AACHH composite and FR mechanism were investigated and discussed. Results showed that AACHH exhibited excellent FR properties in EP. When the content of AACHH was 47.4%, the LOI of EP reached 32.2%. AACHH could not only reduce the decomposition heat of EP but also improve the thermal stability and the antimelting capacity of EP. It had multifunctions of diluting, cooling, decomposition resisting, and obstructing. So it was a very good and promising FR. An FR mechanism of magnesium oxychloride (MOC) in epoxy was investigated.[20] The investigation showed that MOC was a good inorganic FR to ERs and reached a relatively good FR effect. The FR properties showed that there was no droplet dropped during the combustion of EP/ MOC composite. When the MOC content was 28.6 wt%, the LOI of the composite rose from 19.8 (the LOI of pure EP) to 23.4 wt% and reached a self-extinguishing standard (22.6 wt%). When the MOC content reached 44.4 wt%, the LOI reached 27.2 wt%, which exceeded the FR standard (26.8 wt%).[21] When the content of MOC was 50%, the LOI of EP reached 29.6% and the mass of residual char reached 9.6%. MOC enhanced the thermal decomposition stability and promoted the mass of residual char and had a good FR effect. MOC, possessing the functions of diluting, cooling, catalyzing char forming, and obstructing, was a very promising FR. Martins and Pereira [22] reported the effect of nano-MH (nano-Mg(OH)2) on the FR properties of EPs and compared with the traditional FR used in just small amounts for EPs, ammonium polyphosphate (APP). It was observed that the flammability of the unfilled resin was significantly changed with nano-Mg(OH)2 addition, and reductions of 33%, 22%, and 23% in the epoxy composite peak heat release rate (PHRR) by adding 10%, 5%, and 1% of nanoMg(OH)2, respectively, were achieved. From these results, it was suggested that nano-magnesium hydroxide (NMH) was acting as an FR by energy absorption and in formation of a continuous network-structured protective layer that behaved as a thermal shield and gas barrier to the oxygen diffusion and to the escape of degradation compounds. Organo-modified layered double hydroxide (LDH) MgAl intercalated with glycinate was prepared and used H2N

(CH2)2

H N

(CH2)2

to formulate epoxy composites with a distinct content of LDH.[23] The positive aspect of produced composites was their much lower burning rate compared to the pristine epoxy, revealing the potential of MgAl LDH to be used as an FR filler in epoxy matrices, bearing in mind that this characteristic was dependent on the dispersion degree. A cheap macromolecular IFR (Zn-MIFR) with a ­structure of a caged bicyclic PER diphosphonate was synthesized [24] and used as an FRto modified EPs. The EPs obtained showed a UL94 V0 rating at low Zn contents of 3.0% to get a LOI of 27.5% and a char yield of 20.5%. Dilatation, smoke density rating (SDR), and maximum smoke density (MSD) of EP/Zn-MIFR were found to be decreased. In the thermal degradation of EP/Zn-MIFR, phosphorus groups decomposed at a low temperature, and then catalyzed the dehydration and carbonization of EP to form a heat-resistant char, retarding the weight loss rate of EP at high temperatures. It was concluded that Zn and MIFR had a synergistic effect on the flame retardancy of EP. Hexabromocyclododecane and antimony trioxide were introduced into the bisphenol A (BPA) EP to improve its flame retardancy.[25] With the addition of hexabromocyclododecane, it did not show any flash and explosion during the 20 times of mechanical impact, whereas a slightly empyreumatic scent was detected. An explosion was observed for the other specimens, and the resin particles on the surface of the specimen after the mechanical impact were more than that before the mechanical impact, which was attributed presumably to the mechanical impact at the low temperature, resulting in the crushing of resin materials. It also indicated that BPA EP cured by 593 (Fig. 9a) with antimony trioxide at the low temperature had low flexibility. The specimens cured by T-31 (Fig. 9b), with the addition of hexabromocyclododecane, before and after the mechanical impact were statistically approximate to 0.223 and 0.238, respectively, which revealed that the specimen was compatible with liquid oxygen. A macromolecular IFR (Al-MIFR) with a structure of a caged bicyclic PER diphosphonate was synthesized.[26] EPs were modified with Al-MIFR to get the fire-retardant EP. The results showed that a UL94 V0 rating at a low Al content of 4.0% got a LOI of 26.8% and a char yield of 19.5%. H N

H2 H C C

CH2

OCH2(CH2)2CH3

OH (a) H2N

R

H N

H2 C

OH

H2 H C N R

(b)

Fig. 9  Structure of (a) 593 and (b) T-31 Source: Li et al.[25] © 2013 Springer.

H H2 N C

OH

H2 H C N R

NH2

Environmental–Fire Protection

Fire Protection: Flame-Retardant Epoxy Resins in 1147

1148

Environmental–Fire Protection

In the thermal degradation of EP/Al-MIFR, phosphorus groups decomposed at a relatively low temperature, and then catalyzed the dehydration and carbonization of EP to form a heat-resistant char, retarding the weight loss rate of EP at high temperatures. It was observed that Al and MIFR had a synergistic effect on the fire retardancy of EP. A macromolecular IFR (Mg-MIFR) with a structure of a caged bicyclic PER diphosphonate was synthesized.[27] EPs were modified with Mg-MIFR to get the fire-retardant EP. The results showed that a UL94 V0 rating at low Mg contents of 4.0% got a LOI of 27.0% and a char yield of 19.8%. In the thermal degradation of EP/Mg-MIFR, phosphorus groups decomposed at a relatively low temperature, and then catalyzed the dehydration and carbonization of EP to form a heat-resistant char, retarding the weight loss rate of EP at high temperatures. Mg and MIFR had a ­synergistic effect on the fire retardancy of EP. A macromolecular IFR (Cu-MIFR) caged bicyclic PER diphosphonate structure was synthesized.[28] By using Cu-MIFR, EPs were modified to get the ­fire-retardant EP. The results showed that a UL94 V0 rating at low Cu ­contents of 3.0% got a LOI of 27.0% and a char yield of 19.4%. The thermal properties showed that phosphorus groups in EP/Cu-MIFR decomposed at a relatively low temperature, and then catalyzed the dehydration and ­carbonization of EP to form a heat-resistant char. A macromolecular IFR (Mn-MIFR) with a structure of a caged bicyclic PER diphosphonate was synthesized.[29] EPs were modified with Mn-MIFR to get an FREP. The results showed that a UL94 V0 rating at low Mn contents of 4.0% got a LOI of 26.5% and a char yield of 18.2%. Dilatation, SDR, and MSD of EP/Mn-MIFR were found to be decreased. During the thermal degradation of EP/ Mn-MIFR, phosphorus groups decomposed at a relatively low temperature, and then catalyzed the dehydration and carbonization of EP to form a heat-resistant char, retarding the weight loss rate of EP at high temperatures. The smoke suppression properties and synergistic FR effects of iron oxide green on IFREPs using APP and PER as IFRs were investigated.[30] Cone calorimeter test data revealed that a moderate amount of iron oxide green could apparently reduce the heat release rate (HRR), THR, total smoke release (TSR), etc. On the other hand, the smoke density test (SDT) results showed that iron oxide green could catalyze the carbonization of IFREP at low temperatures. Here, iron oxide green was considered to be an effective smoke suppression agent and a good synergist with IFR in fire-retardant EPs, which could greatly improve the structure of char residue. Chen et al.[31] reported the influence of organic-­ modified montmorillonite (Fe-OMMT) on smoke suppression properties and combustion behavior of FREP. A series of IFREPs were prepared with different contents of Fe-OMMT based on EP resin as a matrix resin, and APP and pentaerythrite as IFRs. The results showed that the HRR and smoke production rate (SPR) of FR samples

Fire Protection: Flame-Retardant Epoxy Resins in

decreased greatly with the increasing Fe-OMMT content. Fe-OMMT enhanced the char residues of FR samples and decreased the SPR. IFREP coatings including APP, PER, MEL, MH, EP, and polyamide resin (PA) were prepared.[32] Thermal analysis of IFR-EP coatings showed that 15 parts per hundred parts of resin MH was more appropriate to be selected in the formulation of IFREP coatings. Smoke suppression properties and synergistic FR effects of iron oxide brown ((Fe2O3 + FeO)·nH2O) on IFREP were reported using PER and APP as IFRs.[33] The volatilized products formed on thermal degradation of IFREP compositions showed that the volatilized products were CO, CO2, H2O, carboxylic acid, and aliphatic hydrocarbons. Initially, iron oxide brown helped to change the structure of char residue layer that restrained the heat release and smoke generation. Second, iron oxide brown represented excellent smoke suppression properties in fire-retardant epoxy composites based on IFR. Here, iron oxide brown was considered to be an effective smoke-suppressive agent and a good synergist with IFR in fire-retardant EPs, which greatly enhanced the char residue. Smoke suppression properties and synergistic FR effects of ferrocene (Fe(C5H5) 2) on IFREPs using APP and PER as IFRs were reported by Liu et al.[34] Data revealed that the addition of ferrocene into IFREP compositions significantly reduced HRR, THR, SPR, and TSR. On the other hand, ferrocene greatly improved the structure of char residue. Moreover, the volatilized products formed on thermal degradation of IFREP compositions demonstrated that the volatilized products were H2O, CO2, CO, carboxylic acid, and aliphatic hydrocarbons. Here, ferrocene is considered to be an effective smoke-suppression agent and a good synergist with IFR in FREPs, which enhanced the char residue. Addition of ferrocene remarkably improved the FR properties of IFREP. The scheme of the combustion mechanism for IFREP system with ferrocene is shown in Scheme 2. From the results of IFREP, the following conclusions can be drawn: ferrocene can help to change the structure of char residue layer that restrain the heat release and smoke generation, ferrocene represents dramatically excellent smoke suppression properties in FREP composites based on IFR, and the synergistic FR effect and smoke suppression between ferrocene and IFR are very apparent. The synergistic multicomponent system containing MEL poly(metal phosphate)s was proposed as a fire retardant.[35] This work was focused on the decomposition pathway and morphology of the fire residues of EP retarded with MEL poly(zinc phosphate) (MPZnP) to explain the modes of action and synergistic effects with selected synergists (MEL polyphosphate (MPP) and AlO(OH), respectively). The total load of FRs was 20 wt% EP + (MPZnP + MPP) and formed a highly voluminous residue that showed structural features of both EP + MPZnP and EP + MPP, resulting in a highly effective protective layer.

Fire Protection: Flame-Retardant Epoxy Resins in 1149

Fe

∆T O

ONH4

ONH4 O

P O

P

O

∆T

O

Fe

n+

+

NH3

O

O

PER

O P

HO

O

HO

OH

O

O

O

O

P

+

H2O

Fe n+ Thermal pyrolysis

Char

Crosslinking CH3 C

H2 C O

Environmental–Fire Protection

O

P

NH3

Intumescent char

H O C CH2CH2CH2CH2 OH

CH3

Scheme 2  Possible reaction mechanisms of char formation during the combustion of IFREP system with ferrocene Source: Liu et al.[34] © 2015 Springer.

EP + (MPZnP + AlO(OH) preserved the entire quantity of phosphorus content during combustion due to formation of Zn2P2O7 and AlPO4. P-OH and NH2 functionalities of MPP and MPZnP were proposed to be able to scavenge O

O

P

O

P

O

O

H2O

O

amine and carbonyl moieties (Scheme 3), which resulted in increased residues compared to EP. The crystals of diethylenetriamine copper(II) sulfate (DETA-CuSO4) (Scheme 4) were prepared by direct

O

2

OR

OR

P OR

R1 [P] OH

[P] OH

+

+

H

N

R1 –H2O

R2

[P]

O

O

R2

Mel

NH2

[P]

[P]-OH

OH

–H2O

R2 R1

O R2

–H2O

Mel

N

N R2

R1

R1

+

[P]-OH

OH

R1 R2

R1 [P]

O

O

[P]

R2

Polyaromatic cross-linked structures

Suggested reactions between melamine and polyphosphate derivatives provided by MPZnP and MPP and carbonyl and amine moieties from the decomposition of EP

Scheme 3  Suggested reactions between MEL and polyphosphate derivatives provided by MPZnP and MPP and carbonyl and amine moieties from the decomposition of EP Source: Muller et al.[35] © 2016 Elsevier.

1150

Fire Protection: Flame-Retardant Epoxy Resins in

O

O S

O

H2O

S

O H2O

Environmental–Fire Protection

2+

− 3H2O

OH2

O

H 2O

O

H N

+ H2NC2H4NHC2H4NH2 Cu

O

O

Cu

2+

OH2

N

H2O OH2

NH

CuSO4.5H2O

DETA-CuSO4

Scheme 4  Synthesis of DETA-CuSO4 Source: Lavrenyuk et al.[36] © 2016 Elsevier.

interaction of copper(II) sulfate with diethylenetriamine (hardener of EPs) and used as an FR in epoxy amine polymers (DGEBA/DETA).[36] The addition of DETA-CuSO4 to DGEBA appreciably improved the thermal stability and antiflammability of DGEBA/DETA-CuSO4. Here, the effectiveness of combustibility suppressing of epoxyamine polymers depends on CuSO4 interlinking strength with amine hardener (DGEBA-CuSO4 chelation) that was accompanied by formation of Cu-N (Scheme 5) bonds within the framework of polymeric composite. Cu-doped graphene (graphenit-Cu) was successfully prepared to increase FR properties of EPs. By using slightly oxidized graphene nanoplatelets (graphenit-ox) and graphenit-Cu, graphenit-ox/epoxy and graphenit-Cu/ epoxy composites were prepared.[37] The LOI value of epoxy composite with 3% graphenit-ox was improved to 25.8% from 23.8% for pure epoxy, whereas the LOI value of epoxy composite with graphenit-Cu was increased to 26.4%. The cone calorimetry results showed that the addition of graphenit-Cu reduced the HRR, total smoke production (TSP), and SPR of epoxy composites; however, the addition of graphenit-ox had a little FR effect on EP.

N

H

+

O

Dodecyl sulfate (DDS) intercalated magnesium aluminum-LDH (MgAl DS LDH) and the MEL salt of PER diphosphate (MPP) (Fig. 10) were synthesized and used as additives for the preparation of epoxy composites.[38] The MgAl DS LDH addition to MPP (Scheme 6) in EP protected the polymer from thermal oxidation. Due to the presence of MgAl DS LDH, the epoxy nanocomposites developed a rigid and dense upper layer with stable charring, which prevented the escape of the decomposed flammable volatiles and protected the lower layer of the nanocomposites from further decomposition. However, MgAl DS LDH showed a synergistic effect in the thermal and flammable properties on epoxy nanocomposites, in the presence of MPP. A series of IFREPs were prepared based on BPA EP, APP, PER as an IFR, and ferrite yellow (goethite) as a smoke suppressant.[39] Then, the synergistic FR and smoke suppression properties of α-FeOOH on IFR epoxy c­ omposites were investigated. The results showed that goethite reduced THR, HRR, SPR, and TSR. It was found that there were obvious synergistic FR and smoke suppression effects between goethite and IFRs in epoxy composites.

CH2

N

HC

CH2 CH HO

N

N

H +

Cu2+ OH2

Ο

H2 C

CH2 HC

Scheme 5  Mechanism of curing of epoxy using DETA-CuSO4 Source: Lavrenyuk et al.[36] © 2016 Elsevier.

− H2O

Cu2+ O H

H C

Fire Protection: Flame-Retardant Epoxy Resins in 1151

O

O

–O

O

O O–

P

P

NH3+

O

+

O

H3N N

H2N

NH2

N

N

Environmental–Fire Protection

N

N

N H2N

NH2

Melamine salt of pentaerythritol diphosphate

Fig. 10  Structure of MEL salt of PER diphosphate (MPP) Source: Kaul et al.[38] © 2017 Elsevier.

Mg

OH

Mg

OH

+H N 3

O O

Al

−

+

O

O

O

P

O

− O

P O

OH

N

O

NH2

2 N N

O

O

H 2N Mg

Mg O

OH

OH

+H N 3 O

OH − O

Mg

O P

O Al

O P

O

OH

O

N − O

N

O NH3+

O

H2N

N

Mg OH

N

N H2 N

NH2 Schematic representation of MPP interaction on MgAl DS LDH

Scheme 6  Schematic representation of MPP interaction on MgAl DS LDH Source: Kaul et al.[38] © 2017 Elsevier.

NH2

N

1152

Environmental–Fire Protection

An intrinsic fire-retardant composite was prepared from EP by curing with diethylenetriamine and a functional magnesium organic composite salt (FMOCS).[40] It was found that the flame retardancy and mechanical properties of cured composite were significantly enhanced compared with DETA/EP. The LOI of FMOCS/EP reached 33%, which was much higher than the DETA/EP (19%) and its IFR composite (31%) in the addition of APP (18.69 wt%), PER (6.21 wt%), and FMOCS (3.50 wt%), and could pass the V0 rating. The ZHS /reduction graphene oxide (ZHS/RGO) hybrid with a multilayer sandwich structure was successfully prepared through the hydrothermal route.[41] ZHS/RGO was applied to reduce fire hazards of epoxy as a fire retardant and smoke suppressant. The incorporation of 3.0 mass% ZHS/RGO hybrid into EP led to the PHRR and THR values to be significantly reduced by 50%–39%, respectively, compared to those of pure EP. The results indicated that ZHS/RGO could suppress the formation of smoke and toxic carbon monoxide as well as improved the fire retardancy of EP composites. The notable reduction of the fire hazard was mainly attributed to the synergistic action between the physical barrier effect of graphene and the catalytic effect of ZHS. The HRR, THR, SPR, and TSP of EP-ZHS/RGO were decreased compared with the pure EP. An environmentally friendly method was used to synthesize nano-boehmite (AlOOH) that was added in BPA EP to enhance the fire retardancy of the EP.[42] The results of TGA of neat EP and AlOOH/epoxy nanocomposites with different contents of AlOOH (2.3%, 4.7%, 5.6%, and 10.2%, respectively) showed that the decomposition temperature of AlOOH epoxy nanocomposites increased compared with the neat EP, and a significant improvement in the mass loss rate (MLR) was observed when the content of AlOOH reached 10.2%. By incorporating the AlOOH nanoparticles, the fire retardancy and Tg of EP were significantly improved. As the content of AlOOH further increased to 10.2%, the oxygen index values of AlOOH/ epoxy nanocomposites also increased by 10.7% (from 33.8 to 37.4), which indicated that AlOOH improved the fire-­ retardant properties of EP. If the content of AlOOH further increased to 10.2%, the oxygen index values of AlOOH/ epoxy nanocomposites increased by 10.7% (from 33.8 to 37.4). The epoxy composites were prepared using epoxy-­ diamine resin ED-20, POEPA as a hardener, BA fine powder, and aluminum nanopowder as an FR filler.[43] It was found that an incorporation of all fillers improved the mechanical properties and thermal stability of the epoxy composites. The highest flexural properties showed that the epoxy composite is based on the combination of BA and aluminum nanopowder. However, the combination of BA and aluminum nanopowders resulted in the highest flexural properties. A polyether amine (M2070) was covalently grafted into the surface of graphene nanosheets (GNS) decorated

Fire Protection: Flame-Retardant Epoxy Resins in

by ZHS to obtain an organic hybrid material (GNSZHS-M2070).[44] It was then incorporated into EP to improve the fire-retardant properties. It was observed that the GNS-ZHS-M2070/EP composites possessed superior fire-retardant performance such as the lowest PHRR and fire growth rate index values, which was attributed to the synergism between the catalysis effect of ZHS and the adsorption effect of GNS. It had been demonstrated that the superior FR performance of GNS-ZHS-M2070/EP should be ascribed to the excellent processability and good compatibility of GNS-ZHS-M2070, derived from the soft organic shell, and the unique flowability of such surface modification of graphene could help to form a continuous and compact char layer. A strategy was developed to prepare a 3D nanostructure to improve the FR properties and toxic effluent elimination of EPs. For this purpose, cross-linked organic–­inorganic polyphosphazene nanoshells (PZM) with amine-rich groups were synthesized via condensation polymerization of hexachlorotriphosphozene (HCCP) and 4,4′-diaminodiphenylether on silica nanospheres as templates.[45] Then, cuprous oxide nanoparticles (Cu2O NPs) (Scheme 7) were synthesized on the surface of PZM. The obtained SiO2@ PZM@Cu spheres were incorporated into the EP to get FREPs. Incorporation of 2 wt% of SiO2@PZM@Cu into EP increased the char yield with a decrease in PHRR, THR, TSR, and peak smoke production rate (PSPR) values. The results showed that the amount of toxic CO and other volatile products from the EP decomposition significantly suppressed after incorporation of SiO2@PZM@Cu hybrids, implying a reduced toxicity. An FR mechanism (Scheme 8) was suggested based on the analysis of char residue and pyrolysis fragments. From all results, it was reasonable to believe that enhanced flame retardancy and toxic effluent elimination for nanoparticles were attributed to a synergistic action between the catalytic effect of SiO2 and Cu2O NPs and the intumescent effect of PZM. A nano ZB/epoxide resin composite was prepared using ZB and epoxide resin as raw materials by coordination homogeneous precipitation method.[46] The epoxy thermosets were found to contain an excellent inflaming retarding effect for epoxide resin. It showed that the products might be an inorganic flame-retarding material with vast ­foreground in applications. Epoxy was infused with 15 wt% intumescent alumina trihydrate (ATH) FR formulations into an 18 wt% oil palm natural fiber (NF) mat.[47] The effects of ATH and its blend with APP and ZB on flammability, mechanical, and thermal properties of the composites were investigated. Compared to neat NF-filled epoxy composites, specimens loaded with intumescent blend of FR formulations showed improved thermal properties and greater mass residual that attributed to the formation of cross-linked network among the NF and FRs. Incorporation of fibers improved the mass residue and lowered the HRR compared to the pure epoxy. Incorporation of FR formulations drastically reduced the

Fire Protection: Flame-Retardant Epoxy Resins in 1153

P Cl

H 2N

N

P

Cl Adhesion

Cl

O

Polymerization

NH2

SiO2

SiO2@PZM R

P

R

P

N

P

R

R Cl P N N P

R

N

P

N

Cl

P

R R P N N R

R

P

R Cl P N N

R

R

R

R = H2N

P

N

P

N

P

R R

Reduction

R

R R P N N

R R

Cu(CH3COO)2·H2O

R R P N N

NH2

O

SiO2@PZM@Cu

Scheme 7  Synthesis route of SiO2@PZM@Cu Source: Qiu et al.[45] © 2017 Elsevier.

Redox O2 Cu2O

SiO2

Toxic effluents elimination

Cu1+

CO

O2

CO

Cu2+

Cu0

CuO

Solid acid catalyze PZM

R

EP

R

N P R

Cat

aly z

P N

e

R N P R

P-rich intumescent char

R

c ati om rks r A two ne

Residue Flame retardancy

Scheme 8  Schematic illustration of mechanism for flame retardancy and toxic effluent elimination of SiO2@PZM@Cu in flaming EP composites Source: Qiu et al.[45] © 2017 Elsevier.

combustion heat release, total mass loss, and zero drip flame in the NF composites.FR formulation with 5 wt% ATH and 10 wt% APP exhibited a self-­extinguishing property, achieved the lowest mass loss and no drip flame under Bunsen burner tests, signifying the synergistic effects

between ATH and APP within the NF epoxy composites. APP reacts with the carbonaceous network of NF throughout the ignition period, resulting in the formation of a thick char layer that acts as a gas and thermal barrier against the fire mechanism.

Environmental–Fire Protection

Cl

Cl Cl N P N

1154

Fire Protection: Flame-Retardant Epoxy Resins in

Inherently FR ERs

Environmental–Fire Protection

Becker et al.[48] investigated the use of glycinate intercalated LDHs for the preparation of two-component (LDH) and three-component (LDH/glass fiber) epoxy composites to achieve flame retardancy. Concerning fire-retardant properties of the two and three components of epoxy composites, it was concluded that all composites with LDH showed burning rate much lower than that of pristine epoxy. The inclusion of LDH in the fibrous composite systems showed a very remarkable effect on their response to fire, preventing thorough matrix consumption and leaving an LDH residue on the fiber surface. Thus, the combination of LDH and glass fibers yielded a synergistic effect on mechanical and fire-retardant properties. The biobased amine functional benzoxazine (Bnz) resin (Fig. 11) was synthesized from cardanol, N,N′-bis(2-­ aminoethyl)ethane-1,2-diamine and paraformaldehyde.[49] The additional amine functionality in Bnz resin was introduced by using multifunctional amine (N,N′-bis(2-aminoethyl)ethane-1,2-diamine) to react with monofunctional cardanol. The synthesized Bnz resin was copolymerized with conventional EPs to prepare poly(benzoxazine-co-­ epoxy) (Scheme 9) coatings for anticorrosive application. The poly(benzoxazine-co-epoxy) coatings had shown good chemical, mechanical, and solvent resistance properties compared to the cross-linked polybenzoxazine coating. Poly(benzoxazine-co-epoxy) coatings showed a lower char yield at 800°C compared to polybenzoxazine coating. Poly(benzoxazine-co-epoxy) coatings display high solvent and chemical resistance properties compared to the polybenzoxazine coating, and this could be due to the dual cross-linked network. The mechanical properties of polybenzoxazine coating were improved by copolymerization with more flexible and functional EPs. Phosphorus-Containing FRs The range of phosphorus-containing FR compounds is extremely wide, and the materials are versatile, since the element exists in various oxidation states. Phosphorus,

phosphonium compounds, phosphonates, phosphine oxide, elemental RP, phosphates, and phosphites are used as FRs. Their importance is permanently growing as they perform an adequate effect also in low percentage and since they do not influence the properties of the polymer matrix considerably. For example, the metal hydroxides, on the other hand, considering the impact on health and environment are more advantageous than halogen FRs. Organophosphorus compounds used as monomers or as additives form an important group of FRs. Organophosphorus compounds provide good physical properties and requires less loading compared to regular fillers (e.g., ATH). However, a broad application is only gradually taking place, since these are still more expensive than conventionally used FRs. Phosphorus-containing compounds mostly perform their FR function in the condensed-phase by increasing the amount of carbonaceous residue. There are two char-­forming mechanisms: (1) redirection of the chemical reactions involved in degradation in favor of reactions yielding carbon rather than CO or CO2 and (2) formation of a ­surface layer of protective char. Phosphorus-Containing Additives Braun et al.[50] investigated aluminum diethyl phosphinates as an FR for polyesters (without glass fiber). The results showed that diethyl phosphinic acid was released during degradation of polymer in the gas-phase. A UL94 V0 rating was achieved with a combined FR loading of 20 wt% since aromatic phosphate-like triphenyl phosphate was known to increase the flame retardancy of polymer. Perret et al.[51] reported halogen-free phosphorus-based oligomeric star-shaped FRs containing DOPO (DOPP and DOPI) for RTM6 and RTM6-CF (carbon fiber RTM6 composite) (Figs. 12 and 13). Both FRs worked in the condensed and gas-phases of RTM6. In the gas-phase, they worked via flame inhibition, which indicated phosphorus release detected in evolved gases. Condensed-phase analysis showed that the phosphorus-containing part was incorporated into the char network by the reactions of the phosphorus with partially decomposed RTM6. Therefore, O

R

R

O H N

R=

N N H

N

H2C

CH2 Amine functional Bnz resin

Fig. 11  Structure of bio-based amine functional Bnz resin Source: Patil et al.[49] © 2017 Elsevier.

Fire Protection: Flame-Retardant Epoxy Resins in 1155

O

R

O

+

H N

O

O R

N N H

N

3h

200°C

Environmental–Fire Protection

R

R R

HO

HO

HO

R

OH

R

N

N N

N OH

R HO OH R

Curing reaction of amine functional Bnz and epoxy resins

Scheme 9  Curing reaction of functional Bnz and EP Source: Patil et al.[49] © 2017 Elsevier.

P

O O

O O P

O O

O

O

O O

O P

O

O O O P

O

(DOPP)

Fig. 12  Structure of oligomeric star-shaped FR, DOPP Source: Perret et al.[51] © 2011 Elsevier.

these FRs acted only via flame inhibition in the composites, but no effect was found of the structures of FRs on thermal decomposition. Qian et al.[52] synthesized an FR additive hexa-­ (phosphaphenanthrene-hydroxyl-methyl-phenoxyl) cyclotriphosphazene (HAP-DOPO) (Fig. 14) with phosphazene and phosphaphenenthrene double functional groups from hexachloro-cyclotriphosphazene, 4-hydroxy-benzaldehyde, and DOPO. Phosphaphenenthrene/phosphazene synergistic FR system brought higher FR efficiency to epoxy thermosets. By forming a slightly expanding, stronger, and phosphorus-rich char layer, the epoxy thermosets containing HAP-DOPO reduced the heat ­transmission and the rate of heat release during combustion. Hexa(4-maleimido-phenoxyl)-cyclotriphosphazene (HMCP) (Fig. 15) was synthesized by nucleophilic substitution reaction between N-(4-hydroxyphenyl) maleimide (HPM) and HCCP by Yang et al.[53] The prepared HMCP was then blended with DGEBA to prepare FREPs. The fire properties indicated that these EPs contain good FR properties and thermal stability. An FR additive with phosphaphenanthrene and phosphazene groups

1156

Fire Protection: Flame-Retardant Epoxy Resins in

P

O O

Environmental–Fire Protection

O O

O

O

O

P

N N

O

O O

N

O

O

O

O P

O

(DOPI)

Fig. 13  Structure of oligomeric star-shaped FR, DOPI Source: Perret et al.[51] © 2011 Elsevier.

O O P O

HO

OH

O

HO P

CH

O

HC

O

O

P

C H

P N O

N

P

O P N

O

OH HC

O

Fig. 14  Structure of FR additive, HAP-DOPO Source: Qian et al.[52] © 2011 Elsevier.

O P

CH P

O O

O

C H

P O

O

O

HO

OH

Fire Protection: Flame-Retardant Epoxy Resins in 1157

O

O

N N O

O

O O N

P N

N

P N

P

O

O

O

N

O

O

O O O

N O

N O

O

Fig. 15  Structure of HMCP Source: Yang et al.[53] © 2016 Elsevier.

hexa-[4-(p-hydroxyanilino-phosphaphenanthrene-­methyl)phenoxyl]-cyclotriphosphazene (CTP-DOPO) (Fig. 16) was synthesized from hexachlorocyclotriphosphazene, ­p-hydroxybenzaldehyde, 4-aminophenol, and DOPO.[54] The results indicated that prepared CTP-DOPO possessed good flame retardancy for DGEBA thermosets. Lim et al.[55] have reported the effects of halogen-free FRs such as intumescent APP and MEL cyanurate (MC) on the mechanical properties and flammability of epoxy/ glass fiber composite systems. Overall, intumescent APP showed better flammability results compared with MC. The composites with 15 vol% APP performed sufficiently well in the UL94 test and LOI, whereas 20 vol% MC was required to achieve similar results. The addition of 1 vol% MC into 4 vol% APP had shown some improvement on the composite flame resistance. The composite attained the maximum flexural strength at 15 vol%, while the dynamic mechanical analysis showed that the addition of fire retardants increased the storage modulus but did not change the Tg. The FRs BA-(DOP)2, BA-(DOP)2-O, BA-(DOP)2-S, DDM-(DOP)2, and DDM-(DOP)2-S (Fig. 17) derivatives of DOPO were synthesized and incorporated in EPs, consisting of epoxy novolac (DEN 438), DICY, and fenuron.[56] UL94 tests showed that nitrogen-containing ­substituents at the phosphorus atom increased the fire-retardant ­efficiency of DOPO-based additives. Two nitrogen-­substituted DOPO derivatives, 4,4′-Diaminodiphenylmethane (DDM)(DOP)2 and DDM-(DOP)2-S, were found to be very powerful fire retardants in DICY-cured DEN 438 EPs. The V-0 rating was achieved at approximately 1% phosphorus

in the resin, and the glass transition temperature (Tg) was maintained at a high level of the pure DEN 438/DICY epoxy material. Di(acryloyloxyethyl) benzenephosphonate (DABP) and acryloyloxyethyl phenyl benzenephosphonate (APBP) (Fig. 18) were synthesized and blended in the ratios of 10–50 wt% with a commercial epoxy acrylate (EA) oligomer (EB600) to obtain a series of FR ultraviolet (UV)-­ curable formulations.[57] The results revealed that the blended EAs with DABP or APBP possess improved thermal stability at elevated temperature and have higher char yields together with higher LOI values. The cross-link density increased along with the content of DABP or APBP in the blend, whereas the glass transition temperatures of the blended resins decreased compared to pure cured EB600. The curing of mixtures of bis(m-aminophenyl)methylphosphine oxide-based benzoxazine (Bz-BAMPO) (Fig. 19) and glycidylether or Bnz of BPA was studied.[58] In all samples, the molar ratio of Bnz monomers or the Bnz– epoxy system was varied to achieve different phosphorus contents. All the obtained materials showed high Tg values and higher cross-linking densities were observed for Bnz– epoxy ­system. However, the LOI value was ­significantly higher for Bnz–epoxy. EP modifier, DOPO-1,2-diethylidene-1,1,2,2-­ tetramethyldisiloxane (DOPO–TMDS) and DOPO1,2-­d iethylidene-1,2-dimethyl-1,2-diphenyldisiloxane (DOPO–DMDP), (Fig. 20) were synthesized.[59] Halogen-­ free FREPs were obtained through modification of o-cresol novolac EP cured by phenol novolac resin using

Environmental–Fire Protection

O

1158

Fire Protection: Flame-Retardant Epoxy Resins in

OH

HO HN

Environmental–Fire Protection

O NH

P

O

O

P O O

O

P

H N

O O

P N O

N

P

O P N

H N

O

O

O

P

OH

O

HO O

O

P

NH

P

O

O

HN OH

HO

Fig. 16  Structure of CTP-DOPO Source: Xu et al.[54] © 2016 Elsevier.

O

P

O

O

P

O

O

P

BA-(DOP)2

O

P O

O

O BA-(DOP)2-O

P

O

O

O

S

BA-(DOP)2-S

P

O

O

O

N H

N H DDM-(DOP)2

P S

N H

DDM-(DOP)2-S

N H

P

O

S

Fig. 17  Structures of FRs BA-(DOP)2, BA-(DOP)2-O, BA-(DOP)2-S, DDM-(DOP) 2, and DDM-(DOP) 2-S Source: Ciesielski et al.[56] © 2008 Wiley.

P

O

S

P

O

Fire Protection: Flame-Retardant Epoxy Resins in 1159

O

H2C=HCCO(H2C)2O

O

P

O

O(CH2)2OCCH=CH2

O

P

O O(CH2)2OCCH=CH2

(APBP)

(DABP)

Fig. 18  Structures of DABP and APBP Source: Wang and Shi.[57] © 2006 Elsevier.

O

O

N

N O P CH3 (Bz-BAMPO)

Fig. 19  Structure of Bz-BAMPO Source: Sponton et al.[58] © 2008 Elsevier.

R1 O P O

R1

O

Si O Si R2

DOPO-TMDS: R1 =

CH3

R2 =

CH3

P O

R2

DOPO-DMDP: R1 =

CH3

R2 =

Fig. 20  Structures of DOPO–DMDP Source: Ding et al.[59] © 2009 Springer.

DOPO–DMDP and DOPO–TMDS. The cured EPs exhibited better mechanical properties and greatly improved fire-retardant properties due to the presence of phosphorus-­ containing siloxanes. The cured EPs with phosphorus loading of 2.0 wt% showed LOI values of 32–33 and achieved UL94 V0 ratings. A series of FREP with different contents of poly(DOPO substituted dihydroxyl phenyl PER diphosphonate) (PFR) (Fig. 21) were prepared.[60] The results showed that the incorporation of PFR into EP improved the thermal stability. The mechanical results represented that PFR enhanced failure strain slightly accompanied by a decrease in tensile strength. EP/PFR hybrids possessed lower initial

decomposition temperature and higher char residue than pure EP. The degradation products of PFR, including PER, phosphorus acid, phosphoric acid, or polyphosphoric acid (PPA), associated with amine curing agent constitute an intumescent FR system. Advanced FREPs with different contents of poly(DOPO substituted phenyl dimethanol PER diphosphonate) (PFR) (Fig. 22) were prepared.[61] LOI values increased from 21.5 for pure EP to 36.0 for phosphorus-containing resins, and UL94 V0 materials were obtained with the 15 wt% PFR. The TGA results indicated that incorporation of PFR significantly enhanced the char yield and the thermal stability of char layer at higher temperature. It was suggested that the addition of PFR can reduce the release of combustible gas, trap the H and OH radicals by releasing the PO radical, and induce the formation of char layer, thus r­ etarding the polymer degradation and combustion process. PFR was synthesized, and by using it, the thermal degradation behaviors and FR properties of the EP (ER)/ PFR systems were investigated.[62] When the PFR content reached 10 wt%, the EP system met the UL94 V0 classification and the LOI value of 30.2. The TGA results showed that the EP with 10 wt% PFR exhibited high char yields. The high char yields and the high LOI values were found to certify the excellent flame retardancy of this ­phosphorus-containing EP. The FREP was prepared by mixing the FR additive hexa(phosphaphenanthrene-hydroxyl-methyl-phenoxyl)-­ cyclotriphosphazene (HAP-DOPO) (Fig. 23) into DGEBA.[63] After curing by DDS, the FR properties of thermosets were characterized. The results showed that lower the PHRR, higher the flammability rating, than that of FR EP by DOPO, hexa-phenoxyl-cyclotriphosphazene (HPCP), and their mixture cloning the ratio of group component of HAP-DOPO. During combustion, HAP-DOPO continued to release the PO radicals and o­ -phenylphenoxyl radical during two degradation stages from 200°C for its unstable trisubstituted methyl structure of HAP-DOPO, inhibited the chain reaction of decomposition, and exerted the FR effect in the gas-phase. The phosphazene group linked with the residual phosphate from degraded phosphaphenanthrene, which increased the cross-link

Environmental–Fire Protection

O

1160

Fire Protection: Flame-Retardant Epoxy Resins in

O

P

O

Environmental–Fire Protection

O CH2

O O

O

P

H2 C

P

C O

CH2

(PFR)

O

O

C H2

n

O

Fig. 21  Structure of poly(DOPO substituted dihydroxyl phenyl PER diphosphonate) (PFR) Source: Wanga et al.[60] © 2010 Elsevier.

O

O P

H C HC

O

C HC H

O

C

O

O P

O O

P

O

H C

C H O (PFR)

P

O

n

O

Fig. 22  Structure of poly(DOPO substituted phenyl dimethanol PER diphosphonate) (PFR) Source: Wang et al.[61] © 2011 Elsevier. CH3

H2 C O

O

R

O

C CH3

R=

H2 C

O or

H2 C

OH C H

H2 C

O P O

(DOPO-DGEBA)

Fig. 23  Structure of FR, HAP-DOPO Source: Qian et al.[63] © 2011 Elsevier.

density of residue and effectively promoted the formation of h­ igh-strength, high-yield, and phosphorus-rich char. A phosphorus-containing oligomeric FR, poly(DOPO substituted hydroxyphenyl methanol PER diphosphonate) (PDPDP) was synthesized and applied to FREPs.[64] The identification of pyrolysis fragmentations provided insight into the FR mechanism (Scheme 10). The results showed that MLR of the EP/PDPDP composites was clearly lower than pure EP when the temperature was higher than 300°C in air or nitrogen atmosphere. The results also suggested

that the main decomposition fragmentations of the EP/ PDPDP composite were H2O, CO2, CO, benzene, and phenol. The incorporation of PDPDP reduced the release of combustible gas and induced the formation of char layer; hence, the fire potential hazard was reduced. An intumescent FR piperazine-N,N′-bis(acryloxyethylaryl-phosphoramidate) (NPBAAP) (Fig. 24) containing phosphorus and nitrogen used for UV-curable coating was synthesized.[65] Thermogravimetric (TG) results revealed that N-PBAAP enhanced the char residues of EA films at a high temperature region. It was demonstrated that the thermal degradation mechanism (Scheme 11) of the film takes place initially along the side chain due to the hexagon hydrogen bond effect, and the formation of various P–O–P and P–O–P complex structures and the polyaromatic carbonous char contribute to the flame retardancy of the N-PBAAP film. The results showed that the ­incorporation of N-PBAAP in the resins can significantly decrease the THR, heat release capacity (HRC), and PHRR of EA/N-PBAAP composites. The UV-curing FR film consisting of EA used as an oligomer, triglycidyl isocyanurate (TGIC) acrylate (TGICA) and tri(acryloyloxyethyl)phosphate (TAEP) (Fig. 25) used as an FR, was synthesized.[66] The flame retardancy and thermal properties of films were reinforced by using alpha-­ zirconium phosphate (α-Zr(HPO4)2H2O,α-ZrP). The

Fire Protection: Flame-Retardant Epoxy Resins in 1161

O

H2 H2 O C C C O H

C CH3

+

HO

OH

CH3 C CH3

CH3 OH2

H

CH3

+

HO

O

O O

C H 2C H2

O

H2 C H2C

P

O HO

O

P

O

P

O

C H

O

P

OH

+

O

C H2C H2

O

O

O

C O

ether

bisphenol-A

O

O

C

P

H N

CH3

4-isopropylphenol

H2 C H2C

H2 C

OH2 +

C

CH3 phenol

H2 C C H

O

O

(SDA)

P

O

H (DOPO)

Scheme 10  Schematic outline of the degradation process of EP/PDPDP composite Source: Wang et al.[64] © 2011 Elsevier.

O O O

O

P O

O N

N

P O

O

O O

(N-PBAAP)

Fig. 24  Structure of FR, NPBAAP Source: Chena et al.[65] © 2011 Elsevier.

results showed that the organophilic α-ZrP (OZrP) layers were dispersed well in EA resin. It was found that the incorporation of TAEP and TGICA could reduce the flammability of EA. The results showed that OZrP promoted to form

a stable and smooth char layer in the condensed-phase, which could prevent the underlying materials from further degradation. Pyrolysis and fire behavior of phosphorus polyester (PET-P-DOPO) were investigated. The glycol ether of the hydroquinone derivative of DOPO was used as a reactive halogen-free FR in PET-P-DOPO (Fig. 26).[67] PET-PDOPO was proposed as an alternative to poly(butylene terephthalate) (PBT) with established halogen-free additives. It exhibited a high LOI (39.3%) and achieved V0 classification in the UL94 test. Three different mechanisms (charring, flame inhibition, and a protection effect by the intumescent char) (Scheme 12) contributed to the flame retardancy in PET-PDOPO and were quantified with respect to different fire risks. The fire load was reduced by 66% of the PBT characteristic. The reduction was the superposition of the

Environmental–Fire Protection

OH

CH3

1162

Fire Protection: Flame-Retardant Epoxy Resins in

O

P O O

O

OH

CH2 CH

H

O

O

O

+

Environmental–Fire Protection

O H O O

O

+

CH CH2 P

O ∆T

–H2O

O

∆T

∆T

+

CH3CHO ∆T

O O P O N

+

+ H 2O

O

O P

N2 +

O

P

∆T

aldehyde(RCHO) + P-N rich compounds + alkene + alkene(CH4)

O

∆T

O

HO

N

N O P OH O

volatilized compounds

O

–CO2 –CO ∆T –alkane ∆T O2 char residue + CO + CO2

Scheme 11  Mechanism of thermal degradation of N-PBAAP film Source: Chena et al.[65] © 2011 Elsevier. O O O O

O

P

O

O O

O O

O

O

O

O

(TAEP)

N

HO O

O

N N

OH O

HO O (TGICA)

O

Fig. 25  Structure of TAEP and TGICA Source: Xing et al.[66] © 2011 Elsevier.

relative reduction due to flame inhibition (a factor of 0.625) and charring (a factor of 0.545). The PHRR was reduced by 83% due to flame inhibition, charring, and protection properties of the char (a factor of 0.486).

A hyperbranched polyphosphate ester (HPE) (Fig. 27) was synthesized via the polycondensation of BPA and phosphoryl trichloride.[68] Then, a series FR EPs from BPA epoxy cured with HPE and BPA were prepared. The LOI

Fire Protection: Flame-Retardant Epoxy Resins in 1163

O

O

O

O P

Al O n

(PBT)

3 (AlPi-Et)

O

O

O

O

O

O

(PET-P-DOPO)

P

n

O

O

Fig. 26  Structure of phosphorus polyester, PET-P-DOPO Source: Brehme et al.[67] © 2011 Elsevier.

value increased from 23 to 32 when HPE, instead of BPA, was used as a curing agent. The data revealed that the cured BPA epoxy with HPE as a curing agent possessed improved flame retardancy. FR nanocomposites (Scheme 13) were prepared from diglycidylphenylphosphate (DGPP) and modified montmorillonite (MMT) clay blended with DGEBA in different ratios.[69] Tg of all formulations increased with increasing clay content in the respective series while decreasing with increasing DGPP content. The results indicated that all nanocomposite materials were thermally stable with good fire retardancy, resulting from synergetic effect of phosphorus and inorganic clay. Compared to the neat epoxy system, a maximum increase of 59.3%, 45.5%, and 93% of tensile, flexural, and impact strengths was observed for the prepared nanocomposites. NF composites containing DGPP resin were prepared from DGPP, DGEBA, and Borassus fruit fiber.[70] The tensile strength, flexural strength, and tensile modulus increased up to 10% by the addition of DGPP and decreased with high percentage of DGPP. The flexural strength of composites was increased up to 15% by the addition of DGPP due to good dispersion and toughening of DGPP in DGEBA. As observed by the results, the matrix–fiber adhesion was poor in the case of 20% DGPP-containing composites and failure occurred through fiber pullout, whereas for composites with 5% and 10% of DGPP, the interaction of fiber and matrix was strong and failure occurred through fiber breakage rather than fiber pullout. Addition of up to 15% DGPP improved the desired thermal and mechanical properties of these composites. The incorporation of phosphorus-containing DGPP into DGEBA thermosets promoted excellent fire retardancy compared with the flammable DGEBA curing system.

A phosphorus- and nitrogen-containing compound (POPHA) (Fig. 28) was synthesized, and then a series of UV-curable FR resins were obtained by blending POPHA with EA in different ratios.[71] The high nitrogen and phosphorus content of POPHA contributed excellent flame retardancy to EA. The results revealed that the resins of EA with POPHA showed lower PHRR and THR compared to those of pure EA, indicating an improvement in the flame retardancy of EA. The thermogravimetric-Infrared spectroscopy (TG-IR) result indicated the possible outline for the char-forming mechanism (Scheme 14) of FR resins. On the basis of the analysis, the main gases released were piperazine and its derivative, which influenced char ­formation during combustion. The intercalated hydrotalcite (HT)/epoxy (EP) nanocomposites based on RP (HT-RP/EP) were prepared.[72] The results showed that HT that was dispersed in the epoxy matrix evenly showed a significant flame retardancy. When the content of both HT and RP was 5%, the flame retardancy of nanocomposites was achieved to be V0 level. The effect of IFR-containing microencapsulated RP on the flame retardance of E-44 EPEP was studied.[73] The results indicated that a good flame retardancy was observed when EP was treated with 30% IFR. Thermal analysis showed that the charring amount at a high temperature of EP increased substantially when IFR was incorporated. The thermal degradation dynamics showed that the IFRs improved the flame retardance of the EP with a decrease in the degradation speed. The hexaphenylamine cyclotriphosphazene (HPACTPZ) was synthesized and used as an FR in the EMC for packaging of large-scale integrated circuits with halogen-free flame retardants.[74] The fire-retardant properties results showed that the flame retardance of EMC flame-­retardanced by

Environmental–Fire Protection

O

1164

Fire Protection: Flame-Retardant Epoxy Resins in

O

O

O

O

O

O

O

Environmental–Fire Protection

O

P

O

n

-–CO2

O

O

O

O

O O

HO

O

O

P

O

O O

O

O

O

O

O

P

O

OH P

P

O

O

O HO

O

HO

O

HO

OH

HO

O

P

O O HO

OH

+

O

O

P

O P

+

Proposed decomposition scheme of PET-P-DOPO. Products in solid boxes were unambiguously identifed, for products in dashed boxes the functional group was detected.

Scheme 12  Proposed decomposition scheme of PET-DOPO. Products in solid boxes were unambiguously identified, and for products in dashed boxes, the functional group was detected Source: Brehme et al.[67] © 2011 Elsevier.

O O

O

P

O

O

O (HPE)

O

Fig. 27  Structure of HPE Source: Chen et al.[68] © 2011 Springer.

DGEBA

+

H2N

H2 H2 H C C N

H2 H2 H C C N

H2 H2 C C NH2

TETA

+ O O

O

P O

O

O

DGPP

MMT clay Clay-Nano composite

Scheme 13  Synthesis of clay nanocomposites Source: Sudhakara et al.[69] © 2011 Springer.

HPACTPZ was up to UL94 V0 rating, and the oxygen index of EMC was up to 35.8%, which indicated that HPACTPZ had better flame retardance for EMC than t­ raditional ­halogen FRs. Tri(o-phenylenediamine) cyclotriphosphazene (TPCTP) was synthesized using TPCTP as an FR, and the EMC for packaging of large-scale integrated circuits with halogen-free flame retardance was prepared.[75] The results indicated that the flame retardance of EMC flame-retardanced by TPCTP was up to UL94 V0 rating, and the oxygen index of EMC was up to 34.5%, which showed that TPCTP had much better flame retardance for EMC than traditional ­halogen FRs. A natural material-based FR chitosan phosphate acrylate (GPCS) containing a phosphorus and acrylate structure was prepared (Scheme 15), and its effect on thermal properties and combustion behaviors of EA was investigated.[76] The results showed that GPCS reduced the PHRR and THR of samples greatly, which meant that GPCS was efficient in reducing the flammability of EA. The results of TGA exhibited that GPCS improved the thermal stability of materials at high temperature. It was found that GPCS O

promoted the formation of char and reduced the release of combustible gas. Copolymer (SPDA) of p-phenylenediamine (p-PDA) and bispirocyclic PER diphosphate was prepared [77] and was used to synthesize EP/SPDA composites. Thermal studies showed that the addition of SPDA improved the char residue of EP. Scanning Electron Microscope (SEM) results showed that the residual chars had a honeycomb-like structure, indicating an intumescent flame-­ retarding effect of SPDA in composites. All of the earlier results confirmed that accelerated carbonization plays a key role in i­ mproving the flame retardancy of EP. An FR BPA bis(diphenyl phosphate) oligomer (BDP) was synthesized, and its 30% weight was doped into EPs to get 26.2% of LOI and UL94 V0.[78] In the thermal degradation of EP-containing BDP, the initial decomposition temperature, smoke production, and heat release were decreased, while char yields and inherent thermal stability were improved. A reactive phosphorus-containing monomer [1-oxo-­ 2,6,7-trioxa-1-phosphabicyclo-[2.2.2]octanemethyl diallyl phosphate (PDAP) (Fig. 29) was synthesized, and various amounts of PDAP were combined with unsaturated polyester by radical bulk polymerization.[79] Due to the relatively high phosphorus content of PDAP (18.2 wt%), incorporation of this monomer into unsaturated polyester resin (UPR) led to a marked decrease in the THR and HRC, and an increase in the LOI and the char yield upon combustion. An FR tri-(phosphaphenanthrene-(hydroxyl-methylene)-­ phenoxyl)-1,3,5-triazine (Trif-DOPO) was synthesized and was incorporated in different amounts into DGEBA and 4,4′-diamino-diphenyl sulfone (DDS) to prepare FR thermosets, respectively.[80] The results indicated that the TrifDOPO/DGEBA/DDS thermoset with 1.2 wt% phosphorus possessed the LOI value of 36% and UL94 V0 flammability rating, and Trif-DOPO decreased the PHRR and reduced the THR of thermosets. The results Py-GC/MS represented that the groups in Trif-DOPO decomposed and produced PO2 fragments, phosphaphenanthrene, and phenoxy fragments, which jointly quenched the free radical chain reaction during

O O

O

P

N

N

O

P

O

O

n

O

(POPHA)

O O

Fig. 28  Structure of POPHA Source: Qian et al.[71] © 2011 American Chemical Society.

O

O

O O

Environmental–Fire Protection

Fire Protection: Flame-Retardant Epoxy Resins in 1165

1166

Fire Protection: Flame-Retardant Epoxy Resins in

O N

N

P

N

N

P

O n

Environmental–Fire Protection

O

O

O O

O

O

O

O +

O

O

O

H2 H C C CH

O

C H2

O

CH CH2

O H2 C

O

C H

CH

Poly(phosphoric acid) catalyze

C H2

Phosphoris acid crosslink Char

Scheme 14  Schematic outline for the char-forming mechanism of FR EP-containing POPHA Source: Qian et al.[71] © 2011 American Chemical Society. O OH O HO

C6

P2O5

O O

O C4

CH3SO3H

NH2

O C3

O

CS

P

HO

O C8

C2

O

OH

NHR

C1

C9

O

PCS

O

C13

O

C7

OH

C5

OH

Chitosan O

P

OH

P

O

C11 C10

O

O

C12 O

GPCS R = -SO3H, -PO(OH)2

Scheme 15  Synthesis of GPCS Source: Hu et al.[76] © 2012 Elsevier.

combustion. Therefore, the excellent flame retardancy of Trif-DOPO was attributed to its FR group. A phosphorus-containing hyperbranched macromonomer (PHM) (Fig. 30) was successfully synthesized and blended with EA to prepare UV-curable FR coatings.[81]

The residues of epoxy composite (which contains 45 wt% PHM) at various temperatures were analyzed, and the results displayed that the formation of phosphorus–carbon structure could well protect the carbonaceous char from thermal-oxidative degradation at 800°C. It was found that

results showed that when 5 wt% of the FR was added, the LOI value of the EP/NPM system was improved than that of the EP⁄Al(OH)3 system. When the NPM was added in the amount of 5 wt%, the tensile shear strength of the ­composite was increased to 18.27% than that of the pure composite. Hexaphenoxycyclotriphosphazene (HPCTP) was synthesized and incorporated into DGEBA to prepare EP composites using DDM as a curing agent.[83] Thermal analysis demonstrated that the resulting composites achieved high thermal stability with high char yields. The UL94 V0 rating was achieved with the addition of HPCTP, and the heat release intensity decreased simultaneously. It was found that HPCTP enhanced the thermal stability and flame retardancy of EP. A maleimido-substituted aromatic s-triazine (TMT) (Fig. 31) was synthesized successfully.[84] The flame-­ retarded EPs were obtained via thermal curing reactions among DOPO-modified epoxy prepolymer (DOPOER), DDM, and TMT. The results indicated that TMT promoted the curing reaction of EPs and decreased its apparent activation energy (Ea). Introduction of TMT could greatly improve the FR, thermal, and mechanical properties of the cured EPs. Compared with the DOPOER/DDM system

O O

P

O

O CH2

O O P O (PDAP)

Fig. 29  Structure of PDAP Source: Dai et al.[79] © 2013 Elsevier.

the addition of PHM increased the LOI value and reduced the THR and PHRR. An FR epoxy electronic packaging material was ­synthesized through the reaction of matrix resin (E-51), FR (neopentyl glycol phosphate MEL sale, NPM), curing agent DDS, toughening agent (nitrile rubber, CTBN), and curing catalyst (dosage of benzoyl peroxide).[82] The

O

O

O O

O

O P O

O O

O

N

O

O

N

O O

O P

O

O

N

O

O

O O P

O

O

O

N

O O P O

O O

O O P O O O

O

N

O

O

O

O

(PHM)

O O

O

O

P

O

O O O

N

O O

Fig. 30  Structure of PHM Source: Wang et al.[81] © 2013 Springer.

O O

O

O

O

O

O

O

O

O O

Environmental–Fire Protection

Fire Protection: Flame-Retardant Epoxy Resins in 1167

1168

Fire Protection: Flame-Retardant Epoxy Resins in

O

N

O O

Environmental–Fire Protection

N N

O

O

N

O

N O

N

O (TMT)

O

Fig. 31  Structure of TMT Source: Wang et al.[84] © 2014 Elsevier.

without TMT, the LOI value of the cured EP increased from 36.4% to 51.8%, and all samples passed the UL94 V0 rate when the TMT content ranged from 1.98 to 7.44 wt%. A graphene-based hybrid FR (GFR) (Fig. 32) containing phosphorus, nitrogen, and silicon was synthesized.[85] The EP/GFR epoxy composites were prepared, and their thermal stability and flammability properties were ­investigated. The residual char obtained in EP/GFR was higher than EP and EP/GO, and increased by 10.4% with only 1 wt% addition. The TG results demonstrated that the char residual

can be considerably promoted due to rich FR ­elements in GFR. A series of FREPs were prepared with 1-oxo-4-­ hydroxymethyl-2,6,7-trioxa-l-phosphabicyclo[2.2.2] octane (PEPA), APP, and DOPO, respectively, with or without octaphenyl polyhedral oligomeric silsesquioxane (OPS).[86] It was found that PEPA showed a more condensed-phase flame retardancy, and DOPO showed a more gas-phase flame retardancy action. It was considered that the kind of phosphorus-containing FRs decide

O

R

=

O

O

O

H H N P N

Si

O O

Si

O O

O

Si O

OH

R

Si O Si

O O

OH O

O R

O Si

O

O Si

Si

Si

R

R

Si

O

R Si

O

R

O Si

O

Si

O O

R

OH

O

O

Si

Si

O

O

O

Si

O O Si

HO

Si O O

O

OH

R O GFR

Fig. 32  Structure of graphene-based hybrid FR Source: Wang et al.[85] © 2014 Elsevier.

O

Fire Protection: Flame-Retardant Epoxy Resins in 1169

O P

O

O

O

O

O P

n

O

(PCPBO)

O

Fig. 33  Structure of PCPBO Source: Tian et al.[87] © 2014 RSC Advances.

P+ 1/2 SO42

–

NH N

N

P

+

NH

Environmental–Fire Protection

the structure of char layer of EPs, which could seriously affect their fire-retardant performance. The results indicated that the organophosphorus PEPA and DOPO showed better flame retardancy effective in the EPs than that of ­inorganophosphorus FR APP did. A organophosphorus, poly(4,4-dihydroxy-1-methyl-­ ethyl diphenol-o-bicyclic PER phosphatephosphate) (PCPBO) (Fig. 33), was synthesized, and the flame retardancy of EP with different PCPBO loadings was investigated.[87] The results showed that the incorporation of PCPBO into EP significantly improved its flame retardancy and thermal stability. The reduced THR and PHRR, and the increased char yield of epoxy composites at high temperature further confirmed the improvement of flame retardancy. It was revealed that the addition of PCPBO could induce the formation of an intumescent char layer, which retarded the degradation and combustion process of EP. A nitrogen-containing cyclic phosphate (NDP) was synthesized and used as an additive FR to improve flame retardancy of DGEBA cured by DDS with different phosphorus contents. [88] The results showed that NDP-­ modified DGEBA/DDS thermosets exhibited good flame retardancy, moderate changes in thermal stability, and Tg. As the phosphorus content reached only 1.5 wt%, the NDP-modified DGEBA/DDS thermoset resulted to satisfy flame retardancy (UL94, V0). It was observed that NDP had a good ability for char formation, and there existed a distinct ­synergistic effect between phosphorus and nitrogen. A cross-linked triazine phosphine polymeric FR-­ additive poly MEL tetramethylene phosphonium sulfate (PMTMPS) (Fig. 34) was synthesized.[89] The s­ ynthesized PMTMPS and curing agent m-phenylenediamine were blended into EPs to prepare fire-retardant epoxy thermosets. The results showed that the cured EP/11 wt% PMTMPS composites successfully passed UL94 V0 flammability rating, and the LOI value was as high as 32.5%. The TGA results indicated that the incorporation of PMTMPS stimulated EPs matrix decomposed and char forming ahead of time, which led to a higher char yield and thermal stability for EP thermosets at high temperature.

N

NH 2–

SO4

P+

PMTMPS

Fig. 34  Structure of PMTMPS Source: Xu et al.[89] © 2015 Elsevier. CH3

O P O

O

C CH3

O

n

(PBDP)

Fig. 35  Structure of PBDP Source: Zhao et al.[90] © 2015 RSC Advances.

The PBDP was synthesized (Fig. 35) and incorporated into EP to improve its FR property.[90] The incorporation of PBDP into EP led to an excellent FR performance, such as reduced HRR and higher char yield during combustion compared to pure EP. The UL94 V0 rating was achieved at 20 wt% of PBDP in the composite, and Tg was maintained at the high level of pure EP. A cross-linked organophosphorus–nitrogen polymetric FR additive poly(urea tetramethylene phosphonium sulfate) (PUTMPS) was synthesized. [91] The synthesized PUTMPS with the curing agent m-phenylenediamine was blended into EPs to prepare fire-retardant epoxy thermosets. The results showed that the EP/12 wt% PUTMPS thermosets passed UL94 V0 flammability rating and the LOI value reached 31.3%. It was found that the incorporation of PUTMPS promoted EP matrix decomposed and char forming ahead of time, which led to a higher char yield and thermal stability for EP thermosets at high temperature. HPCTP and DOPO were used as an FR for N,N,N ′,N ′-tetraglycidyl-4,4′-diaminodiphenylmethane EPs.[92] The reduction in THR and PHRR showed that the incorporation of FRs improved the flame retardancy of EP. The higher smoke release and CO yield of the FR epoxy showed that both DOPO and HPCTP represented a gas-phase flame retardancy action. TGA of the FREPs

1170

Fire Protection: Flame-Retardant Epoxy Resins in

Environmental–Fire Protection

indicated that the addition of the FRs reduced the initial decomposition temperature of EP. A series of FR thermosets were prepared by melt blending of a phosphonate-triazine-based compound 2[4-(2,4,6-tris{4-[(5,5-dimethyl-2-oxo-2λ 5-[1,3,2]dioxaphosphinan-2-yl)hydroxymethyl]phenoxy}- (1,3,5) -­ triazine (TNTP), a triazine-based compound TN, and a phosphonate-based compound TP, respectively (Fig. 36).[93] The curing systems consisted of DGEBA and DDS. TGA results showed that the char formation of FR thermosets could be significantly improved due to the presence of phosphonate moiety rather than the triazine unit. The LOI value of TNTP/DGEBA/DDS with 1.5 wt% phosphorus content could achieve 32.4% and reach UL94 V0 rating, while that of TN/DGEBA/DDS with 0 wt% phosphorus content was 29.0% and failed in UL-94 test, and TP/DGEBA/DDS with 1.5 wt% phosphorus content with a LOI value of 31.8% just reached UL94 V1 rating. Phosphorus-containing tri(phosphaphenanthrene-­ maleimide-phenoxyl)-triazine (DOPO-TMT) (Fig. 37) was successfully synthesized as an FR additive for EPs.[94] The results indicated that DOPO-TMT exerted a biphase FR effect. In the gas-phase, DOPO-TMT released phosphorus- and nitrogen-containing free radicals with a quenching effect under thermal decomposition. The morphologies of the char residues exhibited intumescent and honeycombed structure with a small number of holes on the surfaces. The honeycombed char structure served as an

excellent protective layer. The functional groups of DOPOTMT synergistically interacted to endow EP with excellent flame retardancy. A phosphorus–nitrogen containing polymer wrapped carbon nanotubes (CNT-PD-x, x denoted the feed ratio) was prepared via strong p–p stacking interactions between the poly(phenylphosphonic-4,4′-diaminodiphenyl-­methane) (PD) and the walls of CNTs, and then incorporated into EP (Scheme 16) for improving the flame retardancy.[95] The LOI value reached 33.6% when the mass fraction of CNT-PD-x (x = 20) was 4 wt%. Compared with CNTs, the same addition of CNT-PD-x (x = 10) reduced the PHRR and THR of EP more effectively. Phosphorus-based compounds, DOPO and HPCP, and expandable graphite (EG) were adopted as fire retardants for EP.[96] The results indicated that EG affected the thermal decomposition process of EP composites and led to the earlier arisen PHRR and later lower HRR due to its high thermal conductivity and strong barrier effect. The further enhanced flame retardancy of EP composites containing both EG and phosphorus-containing compounds indicated the synergy between different FR components. The morphology study showed that the residual char of EP/EG exhibited an intumescent but fluffy and wormlike structure with low adhesion. A bridged-cyclotriphosphazene FR, bisphenol-S bridged penta(anilino) cyclotriphosphazene (BPS-BPP) (Fig. 38), was synthesized and used to flame-retard EP

O TP

TNTP

O

P OH

O

O

TN N

O O

O

O

N N

O

O

P

O P

OH

Fig. 36  Structures of TNTP, TN, and TP Source: You et al.[93] © 2015 American Chemical Society.

OH

O

Fire Protection: Flame-Retardant Epoxy Resins in 1171

O O N

O

O

O

N

N

N

O

O

P

O

N

O

O

(DOPO-TMT)

O

N O

P

O

O

Fig. 37  Structure of DOPO-TMT Source: Yang et al.[94] © 2015 American Chemical Society.

(EP, DGEBA type).[97] The results showed that the incorporation of BPS-BPP improved the thermal stability of EP at high temperature. The LOI value of EP sample increased to 29.7% with a limited amount of 9 wt% BPS-BPP. The peak of THR, PHRR, and TSP values of the same sample were declined conspicuously, indicating good FR and smoke suppression efficiency of BPS-BPP. FR semi-interpenetrating polymer networks (IPNs) based on a BPA epoxy network and an oligophosphonate PFR (Fig. 39) having high phosphorus content were ­prepared by thermally cross-linking in the presence of DICY, 4-aminophenol as a curing agent, and 1,1-dimethyl-­ 3-phenylurea as an accelerator.[98] Differential scanning calorimetry analysis revealed that cured EPs containing PFR possessed slightly higher Tg’s than phosphorus-free cured EP. The results indicated that the incorporation of PFR into EP substantially enhanced the thermal stability and fire retardancy of the char layer at high temperature. A flame-resistant UV-curable EP composite was prepared using the organophosphorous FR dimethyl methylphosphonate (DMMP) that was loaded in the halloysite nanotubes (HNTs) (Scheme 17a,b).[99] It was found that the

EP/HNTs-D systems improved flame retardancy with an obvious decrease in the HRR, THR, TSR, MLR, and SPR. Thermal stability and fire-retardant behavior of EP ­composites filled with GNS and DOPO were investigated.[100] Addition of DOPO and GNS changed the decomposition pathway of ER (Scheme 18). During combustion, DOPO played a key flame retardancy role in the gas-phase and char enhancement in the condensed-phase, while GNS played an effect in the condensed-phase. Addition of 5 wt% GNS and DOPO separated; the PHRR of epoxy was reduced. With the combined addition of GNS and DOPO, the flame retardancy of ER composites was significantly improved. The combined addition of GNS and DOPO extended the diffusion path for heat and combustible gas, while DOPO captured the free radicals that further retarded EP degradation. The polyphosphate oligomer polyphosphate poly(6oxido-6H-dibenzo[-c,e][1,2]oxa-phosphinin-6-yl phenyl phenylphosphate) (POBPP) (Fig. 40) was synthesized, and by blending the FR resin with DGEBA, an FR antidripping EP was obtained.[101] The results revealed that the FR improved the thermal stability of EP and helped the

Environmental–Fire Protection

O

P

1172

Fire Protection: Flame-Retardant Epoxy Resins in

O Cl P Cl

+

H 2N

PPD

CH2

NH2

CNT

DDM

Environmental–Fire Protection

HN

O

+

Polycondensation

TEA/THF

HN P

Schematic connection of CNT and PD

CNT-PD-x (x = 5, 10, 20) O P NH

CNT

NH

CH2

n

Scheme 16  Schematic illustration of the synthesis procedure of CNT-PD-x Source: Wang et al.[95] © 2016 Elsevier.

HN NH N P P O N N H N P NH NH

HN HN N O P P

O S

N

O

HN

P

N N H

NH

BPS-BPP

Fig. 38  Structure of BPS-BPP Source: Liang et al.[97] © 2017 Elsevier.

matrix to leave more residues by forming a net structure in char. The cured EP passed the UL94 V0 grade at a lower ­addition amount of 5 wt%. A branched poly(phosphonamidate-phosphonate) (BPPAPO) oligomer (Fig. 41) was successfully prepared and applied into DGEBA/DDM EP systems.[102] BPPAPO exhibited good FR efficiency in EPs. With only 5 wt% loading, the EP composite reached a UL94 V0 rating with a LOI value of 35.5%. BPPAPO catalyzed the early degradation of EP and promoted the formation of more char residue. When 7.5 wt% BPPAPO was incorporated, the PHRR and THR were decreased by 66.2% and 37.3%, respectively, with a delayed ignition and formation of a

highly intumescent char residue. Combination of gasphase and condensed-phase FR mechanisms was verified (Scheme 19a,b). Three DOPO-phosphonamidates were synthesized, and their influence on fire performance of EP was comparatively investigated.[103] UL94 tests results of EP composites indicated that DOPO-phosphonamidates exhibited good flame retardance performance. The EP formulation ­containing extremely low loading of PiP-DOPO (0.5 wt%  P) passed the UL94 V0 rating, while EDA-DOPO and DDM-DOPO made the EP pass the V1 rating. The results of thermal degradation indicated a higher thermal stability with the initial degradation temperature above 340°C under nitrogen.

Fire Protection: Flame-Retardant Epoxy Resins in 1173

H N

H C O

P

O

P

O

H N

C H

O

P

O

n

O (PFR)

Fig. 39  Structure of oligophosphonate, PFR Source: Hamciuc et al.[98] © 2016 RSC Advances.

Va c

cu

m

Su

ct

io n

(a)

DMMP Removing air from halloysite

Air

Vaccum broken and DMMP enters

Si Al O OH (b)

Loaded HNTs UV

Flame retardant releasing

able Cur xy Epo in Res

UV

ignition

Fire retardant

Scheme 17  (a) The process of halloysite loading with DMMP using vacuum suction and (b) schematic illustration of the mechanism for the enhance flame resistance of EP/HNTs-D composite Source: Zheng and Ni[99] © 2016 RSC Advances.

Environmental–Fire Protection

O

1174

Fire Protection: Flame-Retardant Epoxy Resins in

Environmental–Fire Protection Scheme 18  Schematic illustration of the flame retardancy effect of GNS and DOPO in ER combustion Source: Liu et al.[100] © 2016 RSC Advances.

O P

O

O n

O O

P

(POBPP) Fig. 40  Structure of POBPP Source: Min et al.[101] © 2016 Springer.

O

DDM-DOPO with high content of aromatic structures showed the highest thermal stability and left more char residues. Large amount of gas products during the combustion of PiP-DOPO were released in a short time, promoting flame inhibition, which explained its superior FR efficacy among all the DOPO-phosphonamidates. APP–MMT nanocompound was used to enhance the fire retardancy of EP compared to the mixture of APP and MMT.[104] The APP–MMT nanocompound showed much better flame retardancy compared to APP + MMT mixture. The APP and MMT intimately mixed or prereacted gave a reaction that quickly helped to form char during burning. EPs were modified by using 10-(2,5-dihydroxyl phenyl)9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide

Fire Protection: Flame-Retardant Epoxy Resins in 1175

HO

O

D

O P g O P

O

O

O

O

P

O

O

d

P k

P

O

O

P

j

P

O

O

O O

O O

O

P

O

P

c

O

O P HN l

O

P

P

O O P

NH

O

O O

P

O O

P

O e L

HO

O i P

O O

O P

O

BPPAPO

Fig. 41  Structure of BPPAPO oligomer Source: Ma et al.[102] © 2016 American Chemical Society.

and 1,10-(methylenedi-4,1-phenylene) bismaleimide (BMI), and the resulting blends were then cured with DDS.[105] The results indicated that the cured resins exhibited excellent fire-retardant properties and high Tg. BMI promoted the carbonization property of phosphorus-­containing EP and facilitated the formation of cross-linked char layer with a compact structure and thus enhanced the ­fire-retardant properties. A phosphaphenanthrene and triazinetrione group containing FR (TAD) (Fig. 42) was OMMT in EP thermosets to improve the FR properties.[106] When only 1 wt% OMMT/4 wt% TAD was introduced into EP, the LOI increased from 26% to 36.9% and a V-0 rating was achieved in a UL94 test. An FR DOPO-PEPA (Fig. 43) was used as an additive-type FR in EP.[107] EP/DOPO-PEPA showed good mechanical properties and a relatively high degree of cross-linking. Furthermore, the synergy as DOPO-PEPA was more efficient than DOPO or PEPA alone to FREP. When the FR additives were 9.1%, the EP/DOPO-PEPA reached UL94 V0 rating with a LOI value of 35%. Phosphorus-Containing Reactive FRs Many phosphorus-containing additives have low molecular weight and are volatile, leading to the possibility of their loss from the polymer by migration. There is a need to increase the performance of the additive within the polymer; therefore, the incorporation of organophosphorus FR functionality within the polymeric chain is a logical

progression of their field; new developments in their field of organophosphorus FR compounds are the reactive FRs, which can be used both as monomers for polycondensation polymers and as FRs. Phosphorus-Containing Epoxy Monomers  It was found that such phosphinates and phosphonates are more efficient at the same level of phosphorus content than phosphates. Indeed, the reactive P–H bond of hydrogen ­phosphonates and phosphinates enables to covalently bind the FR to the polymeric backbone by reaction with the epoxy ring. Seibold et al.[108] reported the synthesis and success incorporation of 2,8-dimethyl phenoxaphosphin-10-­oxide (DPPO) in novolac EP. Similar to DOPO, the Tg of the cured EP decreased with increasing phosphorus content; however, lower phosphorus content was required to impart a UL94 V0 rating (0.6%) when cured with DDM. Xia et al.[109] synthesized a phosphorus-containing ­DGEBA-based monomer using 2-(5,5-dimethyl-4-phenyl2-oxy-1,3,2-dioxa-phosphorin-6-yl)-1,4-benzenediol as a chain elongator. This synthesized monomer was cured with cresol formaldehyde novolac resin and DDS. The results showed that the cured resin reached a UL94 V0 rating and produced more char than the unmodified resin. The 2,6-dimethylphenol-dicyclopentadiene EP (MDE) was obtained by epoxidation of the intermediate 2,6-dimethylphenol-dicyclopentadiene novolac, which was synthesized from the reaction between dicyclopentadiene and 2,6-dimethylphenol.[110] Subsequently, the

Environmental–Fire Protection

HO

1176

Fire Protection: Flame-Retardant Epoxy Resins in

O N P H HN

(a)

N H HN

N O P

O

O P

N H

NH2

NH2

Environmental–Fire Protection

O P

HO

OH OH

O O P O a

P

O

OH

c, d, e

bOc O P O

O P e O

P d

O OH P g OH O P f O O

O P

O

O

NH2 f

HO a, b

HO

g

P NH O i j

O

OH

h

Oh

c O P

O

HN

N H

j

NH

i O

NH2 N

O –H2O

H2N

NH2

NH

O P

HN

O P

HO

O

O O

O

O

–H2O

O P

N

O P

O O

O

(b) N

CH2

HN

CH2

N

N

O HO d

OH a

O

O

b

CH2

HN

O

c

h g

e

OH N

CH2

HN

N

NH

j

i

O

HO

O

OH

N

CH2

f

N

k O

O

m HO

O

OH

–CH3 OH

HO n

–H

+H

HO OH

o

OH

OH OH

Scheme 19  (a) Proposed pyrolysis mechanism of BPPAPO and (b) proposed pyrolysis mechanism of EP Source: Ma et al.[102] © 2016 American Chemical Society.

2,6-dimethylphenol-dicyclopentadiene advanced EP were synthesized by reactions of 2-(6-oxido-6-H-dibenzo[c,e] [1,2]oxa-phosphorin-6-yl)-1,4-benzenediol (ODOPB) with MDE (Scheme 20). The char yields and thermal stabilities of cured ODOPB-modified epoxy networks uniquely increased with ODOPB contents. The char yields at 800°C were 11%–24%. High LOI value (found 33.8) could be achieved with a low phosphorus content around 1.0%, which implied that the flame retardancy was improved by the incorporation of ODOPB. Two phosphorus-based heterocyclic FRs, DPPO and DOPO, and their derivatives were synthesized and incorporated in the epoxy novolac (DEN 438) to obtain FR EPs.[111] FR EPs having a phosphorus content of up to 2% based on heterocyclic DOPO and DPPO were cured with DDM; therefore, the high-performance polymers with Tg’s around 190°C and a UL94 rating of V0 were obtained. These polymers were compared with ­EP-containing diphenyl phosphite and diphenyl phosphate, which are nonheterocyclic and did not pass the UL94 test up to 2% phosphorus. A reactive phosphorus-containing compound, bis-­ phenoxy (3-hydroxy) phenyl phosphine oxide (BPHPPO), was first successfully synthesized to produce the phosphorus-­containing FR EP (BPHPPO-EP) using DDS as a curing agent (Scheme 21).[112] The high char yields and high LOI values were found to certify the great flame retardancy of this phosphorus-containing EP. Two FRs (DOPO-TA and DPPO-TA) based on terephthaldialdehyde (TDA), DOPO, and DPPO as FRs were

O P O O

O P

N

O

O

N N

O O O

P

Fig. 42  Structure of TAD Source: Tang et al.[106] © 2017 RSC Advances.

P O O

O

O

(DOPO-PEPA)

O

O P

O

Fig. 43  Structure of FR DOPO-PEPA Source: Zhang et al.[107] © 2017 American Chemical Society.

O

H3C

O

CH3

C O H2

O

H3C

C H2

+

P

O

HO

CH3

(MDE)

O

OH (ODOPB)

Catalyst/fusion Advancement

O C O H2

O

OCH2CHCH2 O R1 OCH2CHCH2 O R1

R1

OH

OH

H3C

CH3

H3C

CH3

R1 =

The advancement of epoxy from MDE with ODOPB

Scheme 20  The advancement of epoxy from MDE and ODOPB Source: Ho et al.[110] © 2006 Elsevier.

O n

C H2

Environmental–Fire Protection

Fire Protection: Flame-Retardant Epoxy Resins in 1177

1178

Fire Protection: Flame-Retardant Epoxy Resins in

O O

HO

P

O

OH

O +

Environmental–Fire Protection

(BPHPPO)

O H2C

C H

H2C

C H

H2 C Cl

NaOH

O

H2 C O

O

P

O

O

H2 C C H

O CH2

(BPHPPO-EP) Synthesis of BPHPPO-EP

Scheme 21  Synthesis of BPHPPO-EP Source: Ren et al.[112] © 2007 Elsevier.

synthesized.[113] These FRs were incorporated into epoxy novolac resin to form prepolymers. The resultant FR EPs were cured with DDM and 4,4′-diamino-dicyclohexylmethane. Depending on the quantity and nature of hardener used, the polymers obtained qualified for the UL94 V0 rating at low phosphorus contents of 1.0%–1.7% phosphorus with a LOI of up to 33. The char-forming abilities of the phosphacyclic-modified epoxies were compared with those of samples based on the inefficient diethylphosphite (DEPP). It was concluded that DOPO2-TDA and DPPO2TDA performed similar as FRs, while the DEPP2-TDA at this content had no effect on the flame retardancy of the EP materials. 2- (6 -Oxido-6H-dibenz (c,e)(1,2) ­o xaphosphorin6-yl)-1,4-naphthalenediol (DOPONQ) was prepared by the addition reaction of DOPO with 1,4-naphthoquinone.[114] The phosphorus-containing diol (DOPONQ) was used as a reactive FR for DGEBA resin at various stoichiometric ratios (Scheme 22). DOPONQ-­containing advanced epoxy was separately cured with various dicyanate esters (Fig. 44) to form FR epoxy/cyanate ester (CE) systems. The effect of the phosphorus content and dicyanate ester structure on the curing characteristic, Tg, dimensional stability, thermal stability, flame retardancy, and dielectric property was studied and compared with that of the control advanced BPA epoxy system. The DOPONQ-­ containing epoxy/CE systems exhibited higher glass transition temperatures as well as better thermal dimensional and thermal degradation stabilities. The flame retardancy of the phosphorus-containing epoxy/dicyanate ester system increased with the phosphorus content, and a UL94 V0 rating could be achieved with phosphorus ­content as low as 2.1%.

An FREP was synthesized based on a reactive p­ hosphorus-containing monomer, 4-[(5,5-dimethyl-2-­ oxide-1,3,2-dioxaphosphorinan-4-yl)oxy]-­phenol (DODPP) (Scheme 23).[115] The DODPP–EP/low-­molecular-weight PA(LWPA), which contains 2.5% phosphorus, could reach a UL94 V0 rating and a LOI value of 30.2%. It was investigated that the Tg’s of cured EPs decreased with increasing phosphorus content. TGA showed that the onset decomposition temperatures and the maximum rate decomposition temperatures decreased, while char yields increased, with an increase in the ­phosphorus content. A halogen-free phosphorus- and nitrogen-containing heterocyclic FR 5,10-dihydro-phenophosphazine-10-­ oxide (DPPA) was synthesized.[116] DPPA and DOPO were successfully incorporated in the backbone of DGEBA (Scheme 24). The resultant prepolymers were cured with DDM and diethyldiamino toluene (DETDA 80). Due to the bridging incorporation of DPPA in DGEBA by fusion process, high Tg’s for these samples were achieved. DDM and DETDA 80 cured samples reached the V0 rating in the UL94 vertical test at a phosphorus content of 3.2%. The highest observed LOI values of these samples amounted to 28.5 (DDM cured) and 29.3 (DETDA cured). The curing behaviors, flame resistance, and thermal properties of a halogen-free epoxy hybrid thermoset, prepared by the curing reaction of hexakis (ethoxymethyl) melamine (HEMM), a phosphorous-containing EP with DOPO group (EPND) and phenol formaldehyde novolac (n-PF) (Fig. 45), were reported by Huang et al. [117] The synthesized thermosets showed high Tg’s (123°C–147°C), excellent thermal stability with high 5 wt% decomposition temperatures, and high char yields (39.4 wt%). All the cured epoxy EPND/HEMM/n-PF hybrid resins

Fire Protection: Flame-Retardant Epoxy Resins in 1179

CH3 HO

OH CH3 OR

+ OH

Environmental–Fire Protection

DGEBA

O P O

OH O

O C O Ar H2

OCH2CHCH2 O Ar

OCH2CHCH2 O Ar

OH

O x

OH

C H2

OR

O

O C O Ar H2

P

OCH2CHCH2 O Q

OCH2CHCH2 O Ar

OH

P O

O y

OH

O Q=

O

O C H2

CH3 Ar = CH3

Scheme 22  The advancement of DGEBA epoxy with DOPONQ or BPA Source: Hoa et al.[114] © 2008 Elsevier.

achieved the UL94 V0 grade with high LOI (LOI > 45.7). It was found that nitrogen and phosphorous elements in the cured EPND/HEMM/n-PF hybrid resins showed a synergistic effect and hence an improvement in the flame retardancy. Hexaglycidyl cyclotriphosphazene (HGCP) (Fig. 46) was synthesized and used as a reactive FR to blend with commercial EP DGEBA.[118] The results showed that HGCP presents a good dispersion in DGEBA, and the blend thermoset with 4,4′-methylene-dianiline (MDA) curing agent leads to a significant improvement of the thermal stability at elevated temperature with higher char yields compared with pure DGEBA thermoset with the same curing agent. ­Improvement was observed in the fire behavior of blend sample.

Hexakis(4-hydroxyphenoxy)-cyclotriphosphazene (PN– OH) was synthesized as a reactive FR for EPs.[119] A new phosphazene-based EP (PN-EP) (Scheme 25) was synthesized through the reaction between DGEBA and PN–OH. Four PN-EP thermosets were obtained by curing with DDM, DICY, novolac, and pyromellitic dianhydride (PMDA). An investigation on their thermal properties showed that the PN-EP thermosets achieved higher Tg and decomposition temperatures in comparison with the corresponding DGEBA ones, while their char yields increased significantly. The PN-EP thermosets also exhibited excellent flame retardancy. The thermosets with novolac, DICY, and PMDA achieved LOI values above 30 and flammability rating of UL94 V0, whereas the one with DDM reached the V1 rating.

1180

Fire Protection: Flame-Retardant Epoxy Resins in

NCO

NCO

OCN

oxydiphenol dicyanate (ODDCy)

bisphenol dicyanate (BIDCy)

H3C

Environmental–Fire Protection

CH3 NCO

OCN

O

C

OCN

CH3 H2 C

NCO

CH3

OCN

H3C

bisphenol-A dicyanate (BADCy)

CH3

tetramethyl bisphenol-F dicyanate (TBFDCy)

Fig. 44  Structures of dicyanate ester curing agents Source: Hoa et al.[114] © 2008 Elsevier. O O DGEBA

+

HO

O

P O

(DODPP) Ph3P OH H2 H2 OC C C O H n

CH3 C CH3

H H2 H2C C C O O

CH3 C CH3

OH H2 H2 OC C C O H

O O O

P O

Scheme 23  Synthesis of phosphorus-containing EP Source: Gao et al.[115] © 2008 Elsevier.

O H

P

NH

+

2 DGEBA

DPPA

CH3 C CH3

H H2 H2C C C O O

OH H2 H2 N C C CO H

O H2 OH H2 P OC C C H

O DOPO

+

DGEBA

P O

Scheme 24  Synthesis of DPPA- and DOPO-containing FR Eps Source: Schafer et al.[116] © 2008 Elsevier.

OH H2 H2 C C CO H

CH3 C CH3

CH3 C CH3

H2 H O C C CH2 O

H2 H O C C CH2 O

Fire Protection: Flame-Retardant Epoxy Resins in 1181

OH

OH

OH

H2 C

H2 C

OX

OH H2 C

H2 C

n

n

(n-PF)

OH H2 H2 C C C H

O C H

X

CH2

CH2

H3CH2COH2C

N

O

CH2OCH2CH3

N N

H3CH2COH2C

P O DOPO group (EPN-D)

N N

CH2OCH2CH3

N

CH2OCH2CH3

CH2OCH2CH3

(HEMM)

Fig. 45  Structures of HEMM, EPND, and n-PF Source: Huang et al.[117] © 2009 SCImago. O O

O

H2 C H2 C

O O

P N

N

O

H2C

O P

O

H2C

N

P

O

O CH2

CH2

O

O

(HGCP)

Fig. 46  Structure of HGCP Source: Gouri et al.[118] © 2009 Elsevier.

Thermally reworkable, phosphorus-containing, di- and trifunctional liquid cycloaliphatic EPs bis(3,4-epoxycyclohexylmethyl)phenyl phosphate (Fig. 47a) and tris(3,4-­ epoxycyclohexylmethyl) phosphate (Fig. 47b) were designed and synthesized.[120] After curing, the products were transparent and stable up to 220°C, while exhibiting a quick thermal decomposition at the temperature range

of 255°C–280°C. This unique degradation behavior was attributed to the synergistic effect of two factors: the thermally labile phosphate groups evenly distributed within the three-dimensional network and the catalysis of phosphoric acid generated from the cleavage of phosphate bond on the pyrolysis of adjacent other phosphate and ester groups. In addition, compared to the commercial cycloaliphatic EP ERL-4221, the newly synthesized EPs had increased LOI by 31%. CNTs were functionalized by vinyltriethoxysilane (VTES) to incorporate the –O–C2H5 functional group to synthesize VTES-CNT. The VTES-CNTs were added to the modified DGEBA EP (Scheme 26) containing silicon to induce the sol–gel reaction.[121] The Tg was increased from 118°C to 160°C, and the char yield of composites that contained 9 wt% CNT at 750°C was increased by 46.94%. The integral procedural decomposition temperature was increased from 890°C to 1,571°C, whereas the LOI of composites was increased from 22 to 27 and the UL94 changed from V1 to V0 when the contents were increased to 9 wt%. The nanocomposites had a higher char yield and were highly fire retardant. A new phosphinate-substituted BPA, 1,1-bis(4-­ hydroxyphenyl)-1-(6-oxido-6H-dibenz oxaphosphorin-6-yl)ethane (a), was prepared.[122] Based on (a), a diglycidyl ether derivative (b) (Scheme 27) was prepared and cured by commercially available curing agents.

Environmental–Fire Protection

OH

1182

Fire Protection: Flame-Retardant Epoxy Resins in

HOH2C CH2OH O O

N

P

Environmental–Fire Protection

N O

HOH2C

HOH2C

P N

P

O O

H C

H C

R

O

O

CH2 O

175°C

P(C6H5)3

CH2OH

CH2OH

(PN-OH)

H 2C

H C

H2C

R OH2C

O

H C

R

CH2O

CH2 O

O O H C

H2C

N

P N O

R OH2C

P N

P

O O

O

CH2O

H C

R

CH2 O

O H C

H2C

R OH2C

(PN-EP)

O

CH2O R

O

C

O

CH3

CH2 O

CH3 Where R =

H C

OH H2 H2 C C C n H

n = 1,2,3, .....

Scheme 25  Synthesis of PN-EP Source: Liu and Wang[119] © 2009 Elsevier. O

O

O

O O O

P

O

O

(a)

O

O

O

O

P

(b)

O

Fig. 47  Structures of cycloaliphatic EPs: (a) bis(3,4-epoxycyclohexylmethyl)phenyl phosphate; (b) tris(3,4-epoxycyclohexylmethyl) phosphate Source: Liu et al.[120] © 2010 Elsevier.

The resulting thermosets showed low thermal expansion, good thermostability, and excellent flame retardancy. Epoxy group-modified phosphazene-containing nanotubes (EPPZTs) were successfully synthesized through the reaction between epichlorohydrin and phosphazene-­ containing nanotubes with active hydroxyl groups

(Fig.  48).[123] EPPZTs/EP composites (Scheme 28) were prepared by introducing EPPZTs into an EP matrix (EP618). The best effect in reinforcing the matrix was observed when the content of EPPZTs was 0.1%. It was found that the addition of EPPZTs effectively increased the residue and decreased the weight loss rate.

Fire Protection: Flame-Retardant Epoxy Resins in 1183 OEt +

NCO

H2 C

CH2

H2 C

Si

CH3

H2C

C

H2 H O C C CH

C

n

O

CH3

O

CH3

H2 H H2 O C C C O

C

Δ

OEt

IPTS H H2 C C O

THF

OEt

2

CH3

O

O

HN CH2

CH2 CH2 EtO

Si

OEt

OEt

Silane-Epoxy Step I

Modified of MWCNT

1

CH

H2C

CH2

Si (OEt)3 VTES BPO

2

OEt

OEt

CH2

Δ

MWCNT

Si

EtO

3

Si OEt

EtO

OEt

Step II Silane-MWCNT

T1

R OH

Si

HO

O HO Step I + Step II

R

Sol-Gel, DDM Δ T2

R = Epoxy Chain

Si

–

O HO

Si

HO R

Si

Si

O

CH2O R

Si HO

Scheme 26  Reaction scheme of epoxy/CNT composites Source: Kuan et al.[121] © 2010 Elsevier.

O

O O

Si

OH

Si

O

OH R

Si

OH

HO OH

Step III T3

Environmental–Fire Protection

DGEBA

1184

Fire Protection: Flame-Retardant Epoxy Resins in

O

O

P

Cl

O

P

O

C

NaOH (aq)

C

Environmental–Fire Protection

HO

O

OH

O

O

(a)

O

O

(b)

Scheme 27  Synthesis of diglycidyl ether derivative (a) of 1,1-bis(4-hydroxyphenyl)-1-(6-oxido-6H-dibenz ­oxaphosphorin-6-yl)ethane (b) Source: Lina et al.[122] © 2010 Elsevier. OH OH

OH

(PZTs)

R N R

P R

P N

R

R

N

N

P

R R

N R

P R

P

OH

R

R

P N

R P

OH HO

N

N

N

N R

P R N

P

R R

P R

R

P N

P N P

R

P

N

N

R

P

R=

R

O S O

O

O

R

R

Fig. 48  Structure of phosphazene-containing nanotubes with active hydroxyl groups Source: Gu et al.[123] © 2011 Elsevier.

O HC OH

OH

H2C

OH H2C

OH HO

OH

H C

(PZTs)

HC

O

CH2

CH2 CH

CH2 O

O

CH2 O

CH2 O

Cl

O

CH2

O

(EP)

H2C O

CH C H2

O CH2

HC O CH 2

O CH2 CH O H2C

Scheme 28  Schematic synthesis procedure of EPPZTs Source: Gu et al.[123] © 2010 Elsevier.

A cyclolinear phosphazene-based EP (Scheme 29) was synthesized.[124] The curing behaviors of this EP with methyl tetrahydrophthalic anhydride, DDM, and novolac as hardeners were investigated. These thermosets achieved high glass transition temperatures Tg’s above 150°C and also gained good thermal stabilities with high char yields. The high LOI values and UL94 V0 classification of these epoxy thermosets indicated that the incorporation of

phosphazene rings into the molecular backbone imparts nonflammability to the EP as a result of the unique combination of phosphorus and nitrogen followed by a s­ ynergistic effect on flame retardancy. FREPs (Scheme 30) were synthesized from phosphoric acid and BPA EP (BAEP).[125] The results showed that HRRs and SPRs decreased greatly, and the char residue increased with increasing FREP. It indicated that good

Fire Protection: Flame-Retardant Epoxy Resins in 1185

OH O O P N O

O H2 H2C C C H

N

O P N

P

H H2 H2C C C Cl O

+

O

O

OH H2 H2 O C C C H

O O O N P P O N

O

P

N

H2 O O C C CH2 H

O n

O O N P O P

O

N

O

O

P

N

O

O

Scheme 29  General synthetic route for cyclolinear phosphazene-based EP Source: Liu et al.[124] © 2012 RSC Advances.

O

O

CH3

+

O

HO

CH3

P

OH

OH

O

CH3 O

O CH3

OH

O

P

O

O

OH

CH3

O

CH3 CH3

Tri-phosphate OH

O

O CH3

+

O

O

CH3

O

CH3

O

OH

P

CH3 O

OH

OH

O

O CH3

Di-phosphate

+ O

CH3 O

O CH3 Mono-phosphate

Scheme 30  Synthesis of FREP Source: Jiao et al.[125] © 2013 Springer.

O

OH

O

P OH

OH

Environmental–Fire Protection

HO

1186

Fire Protection: Flame-Retardant Epoxy Resins in

O H 2C

C H H C

H2C O

Environmental–Fire Protection

the synthesized EP. Curing was done with DDM and bis(3-aminophenyl) phenylphosphine oxide (BAPPO) curing agents. The results showed that BAPPO cured tetra epoxies, irrespective of nanoclay and nanoreinforcement showed better char yield and LOI values in spite of their degradation at lower temperature. The incorporation of nanoclay and nanoreinforcement to the epoxy improved the initial decomposition temperature (IDT), char yield, and LOI values. A study was focused on developing advanced EPs with the characteristic of low water absorption for halogen-free copper clad laminates.[127] To achieve this goal, a bisphenol (1) with two phosphinate pendants was synthesized. A series of advanced EPs (2) (Scheme 31) were prepared based on the advancement reaction of (1) and DGEBA. Experimental data showed that the prepared epoxy thermosets displayed moderate-to-high Tg, good flame retardancy, high modulus, and very low water absorption. To design and synthesize reworkable EP for electronic packaging, which are required to be sufficiently stable before 200°C and could rapidly decompose in the temperature range of 200°C–300°C, a new trifunctional cycloaliphatic epoxide (Epo-A) (Fig. 50a) with a phosphite center linked by three  epoxycyclohexyl groups was prepared [128] Compared to the phosphate-type analog (Epo-B) (Fig. 50b) and commercial epoxide ERL-4221 (Fig. 50c), Epo-A

O CH2

C H

N

N C H2

CH2

H C

O

CH2 O

P

(Phosphorus containing tetraglycidyl epoxy resin)

Fig. 49 Structure of triphenyl phosphine ­phosphorus tetraglycidyl epoxy Source: Meenakshi et al.[126] © 2012 Springer.

oxide-based

flame-retardant properties were related to the formation of a protective phosphorus-rich char layer. TG results represented that FREP decreased the initial decomposition temperatures of cured epoxy samples and enhanced the residue amount at high temperature. Triphenyl phosphine oxide-based phosphorus tetraglycidyl epoxy (Fig. 49) nanocomposites were prepared for use in high-performance applications.[126] Nanoclay and POSS-amine nanoreinforcements were incorporated into

HO

O P O N

N

O P O

+

140°C, 3 h

DGEBA (excess)

self-catalyzed

OH

(1)

O

H2 C O

CH3 CH3

H2 OHH2 O C C C O H

O P O N

N

O P O

O

H2 OHH2 C C C O H

(2) (2-X)

Scheme 31  Synthesis of advanced EPs (2-x). x means the phosphorus content in the resin Source: Chang et al.[127] © 2013 Elsevier.

CH3 CH3

H2 O O C m

X = 1.0, EEW = 241 g/eq X = 1.5, EEW = 286 g/eq X = 2.0, EEW = 292 g/eq X = 2.5, EEW = 362 g/eq

Fire Protection: Flame-Retardant Epoxy Resins in 1187

O O

O

O O

P

O

O P

O

O

O

O

O

O

O

O

(Epo-A)

O

(Epo-B)

O

Fig. 50  Structures of Epo-A, Epo-B, and ERL-4221 Source: Chen et al.[128] © 2013 Elsevier.

exhibited apparently high curing reactivity. The shearing strength of the cured Epo-A at room temperature was 5.67 MPa, much superior to that of ERL-4221 (3.29 MPa). More importantly, the cured Epo-A could maintain a high shearing strength up to 210°C. Upon further increasing the temperature, the network rapidly decomposed, and the strength was almost completely lost at around 255°C. In addition, the incorporation of phosphorus in the network resulted in a significantly increased LOI from 18.2 (ERL-4221) to 23.2 (Epo-A). A cyclotriphosphazene-based epoxy compound (PN-EPC) (Scheme 32) as a halogen-free reactive-type FR was synthesized.[129] These epoxy thermosets achieved a significant improvement in glass transition temperature and also gained good thermal stability with a high char yield. The incorporation of PN-EPC could impart an excellent nonflammability to the epoxy thermosets due to a synergistic flame-retarding effect as a result of the unique combination of phosphorus and nitrogen from the phosphazene rings, and these epoxy thermosets achieved high LOIs and a UL94 V0 rating when 20 wt% of PN-EPC was added. A halogen-free FREP was prepared consisting of DOPO-based glycidyl ether of cresol formaldehyde novolac and DGEBA cured by micronized DICY with Cl Cl

P N Cl

N P

Cl P N

Cl

CH3

O

P

O CH3

Cl

R P N R

N P

R P N R

Scheme 32  Synthetic route for PN-EPC Source: Liu et al.[129] © 2013 Elsevier.

N Cl

TEA/ACE

R

accelerator U-52 (Fig. 51).[130] The optimized EP formula contained 1.5 wt% of phosphorus and achieved UL94 V0 grade and a LOI of 32%, with high tensile strength. FREP was synthesized based on p­ hosphorus-containing 2-(diphenylphosphinyl)-1,4-benzenediol (DPO-HQ) (Scheme 33).[131] EP with a phosphorus mass fraction of 2.0% reached a UL94 V0 level and had a LOI value of 32.4%. It was found that the DPO-HQ-modified EPs formed lacunars and compact charred layers, which ­inhibited the transmission of heat during combustion. A series of thermosetting systems consisting of DGEBA and linear polyphosphazene-based EP (LPN–EP) (Scheme  34) were prepared, and their thermal stabilities and FR properties were investigated.[132] The resulting thermosets exhibited good flame resistance with the UL94 V0 rating but also achieved a significant improvement in impact toughness as a result of the incorporation of ­r ubbery LPN–EP. A phosphorous/nitrogen-containing reactive phenolic derivative (DOPO-HPM) was synthesized.[133] The studied FREP (Scheme 35) systems were prepared by copolymerizing DGEBA with DOPO-HPM, DDS, and TGIC. The results indicated that the modified EP systems exhibited excellent flame retardancy. The sample with 1 wt% phosphorus content and with 1.25 wt% phosphorus content N P

Cl P N

H2 H C C

HO

Cl

CH2 O

Cl

23°C/12 h

CH3 O

R O

CH3

P N R

N P

R P N R

R

R = O

H2 H C C

CH2 O

Environmental–Fire Protection

(ERL-4221)

1188

Fire Protection: Flame-Retardant Epoxy Resins in

OX H3C

OX

H2 C

H2 C

CH3 n

CH3 OX

Environmental–Fire Protection

X = A or B

n=5

A/B = 5/2

O A=

H2 C C H

OH

O CH2

H2 C C H

B=

P

H2 C O

O OH

(Structure formula of multifunctional P-containing epoxy resin) H3C

O N

C

H2 C

H N

O

H N

H3C

C

CH3 N CH3

(Accelerator U-52)

Fig. 51  Structures of multifunctional P-containing EP and accelerator U-52 Source: Hu et al.[130] © 2014 Elsevier.

O

CH3 C CH3

H2 H2C C C O H

O H2 O C C CH2 H

OH

HO P

O (DPO-HQ)

H H2 H2C C C O O

CH3 C CH3

H2 OHH2 OC C C O H

OHH H2 2 OC C C O H P

CH3 C CH3

O H2 O C C CH2 H

O

Scheme 33  Synthetic scheme of phosphorus-containing EP Source: Xiujuan et al.[131] © 2014 Springer.

acquired LOI values of 37% and 38.5%, respectively, and achieved a UL94 V0 rating. In addition, the TSP of the modified EP systems decreased with the increasing content of fire retardants, indicating the smoke-suppression effect of FR systems. The effect of dodeca phenyl POSS (DPHPOSS), glycidyl POSS (GPOSS), epoxycyclohexyl POSS (ECPOSS), and triglycidylcyclohexyl POSS (TCPOSS) (Fig. 52a–d) to act as FRs of the resin was evaluated.[134] The incorporation of 5 wt% of GPOSS into the epoxy matrix resulted

in a LOI value of 33 with respect to 27 of the pure epoxy mixture. LOI and PHRR values were compared with those obtained for the same resin, replacing the DDS with the bis(3-­aminophenyl) methyl phosphine oxide (BAMPO) and BAPPO. BAMPO and BAPPO proved to be more effective than POSS compounds to increase LOI values. CNTs, embedded inside the EP to increase electrical conductivity, were found to affect the fire properties of epoxy systems significantly by preventing the epoxy systems from forming intumescent charring.

Fire Protection: Flame-Retardant Epoxy Resins in 1189

CH2OH

P

N

P

n

O

N

H2C

m

O

H2 C

H C

CI

Hexadecyl trimethyl ammonium bromide,

NaOH

70°C–85°C, 6 h

O

Environmental–Fire Protection

O

O

OH H2 C

CH2O

H2 C

H2 C

H C

CH2

z

O

O

O P

C H

N

O

P

n

N

m

O

m, n, z = 0, 1, 2, 3,......... Target product (LPN-EP)

Scheme 34  Synthesis of linear polyphosphazene-based EP (LPN–EP) Source: Liu et al.[132] © 2015 Elsevier.

O

O

O

O

P O

N

OH

N

O

O (DOPO-HPM)

N N

(TGIC)

O

O

O H2O H N O C C CH2

N

OH

O

O

OH H2 H2 OC C C O H (DGEBA)

CH3 C CH3

n

N

N

O

O P O

TPP

CH3 C CH3

H H2 H2 C C C O

(DOPO-HPM)

O

O

O

O

O H2 O C C CH2 H

TPP O P O

O N O

OH H2 H2 O C CC O H

CH3 C CH3

OH H2 H2 OC C C O H

n

CH3 C CH3

O H2 O C C CH2 H

Scheme 35  Reactions between DOPO-HPM, TGIC, and DGEBA Source: Yang et al.[133] © 2015 Elsevier.

Phosphorus-containing EPs were synthesized by o-cresol-novolac EP (CNE) and DOPO (Scheme 36).[135] The epoxy thermoset with 1.5% phosphorus content achieved a UL94 V1 rating and a LOI of 27%, and the epoxy thermoset

with 2.4% phosphorus content achieved a UL94 V0 rating and a LOI of 31%. It was found that the char of DOPOCNE(2% P)/PN and DOPO-CNE(3% P)/PN ­present a compact and continuous surface.

1190

Fire Protection: Flame-Retardant Epoxy Resins in

R

R

R Si

O R

Environmental–Fire Protection

O R

Si

O Si

O O

Si O

O

Si

Si

Si

O

Si O

R

R

O

R

O

Si

O

Si

O

O

Si

O R

O

R

O

R

Si

O

Si

O

R

O

Si R

O

R

O

Si

R

R

Si

R

O Si

R Si

O

O

Si

O

O

Si

O

Si OR

O

R=

R O Me O

R

Si

O

R

Si O

R

Si OR

O

R

R R=

(c)

R

O

(b)

R

O Si Si

O

O

O

Si

R = Phenyl

(a)

O

O

O

R

R

O

Si

O

R

O Si Si

R O

O

Si Me

Me O R

O

O

Si

O

Me

Si O R O Si O R

O

O

O

Si

Si

Si

O

(d)

Me

O

Si Me

R = Cyclohexyl

Fig. 52  Structures of (a) DPHPOSS, (b) ECPOSS, (c) GPOSS, and (d) TCPOSS Source: Raimondo et al.[134] © 2015 RSC Advances.

O

O

O

O

H3C

H3C

H3C

O

O H

H2 C

H2 C

P O

n

(DOPO)

(CNE) 130°C

OX H 3C

H3C

OX

O

H3C

Catalyst

OX

H2 C

H2 C n (DOPO-CNE)

Scheme 36  Synthesis of phosphorus-containing EP from CNE and DOPO Source: Zhang et al.[135] © 2016 Elsevier.

X=

OH H2 H2 C C C H

P O

O

O

Phosphorus-Containing Curing Agents  Sun and Yao [136] synthesized one symmetric diamine (Fig. 53) and two symmetric phenols (Figs. 54 and 55) as p­ hosphorus-containing FRs via condensation of p-PDA with benzaldehyde, 4-hydroxybenzaldehyde, and 2-hydroxybenzaldehyde, respectively, followed by the addition of DOPO to the imine linkage. These compounds served as cocuring agents of DDM for EPs. All epoxy thermosets exhibited excellent flame retardancy, moderate changes in Tg and thermal stabilities. When the phosphorus content reached 10 wt%, the EP system met the UL94 V0 rating and the LOI value reached more than 35.6, probably because of the nitrogen– phosphorus synergistic effect. The curing behavior of cycloaliphatic EP 3,4-­epoxycyclohexylmethyl 3,4-epoxycyclohexane carboxylate was investigated by using phosphorus -containing poly(amide-imide)s (Fig. 56) having free amine groups, DDM and p-PDA.[137] When phosphorus was incorporated in EP, the EP system met the UL94 V0 classification and the LOI reached 37.8, because of nitrogen–phosphorus synergistic effect. DGEBA was cured with different phosphorus containing diamide-diimide-tetraamines (Fig. 57) derived from 1 mole of various aromatic dianhydrides and 2 moles of l-tryptophan (T) in a mixture of acetic acid and pyridine (3:2 v/v) followed by activation with thionyl chloride and then condensation with excess of phosphorus-­containing triamines tris(3-aminophenyl) phosphine and tris(3-­ aminophenyl) phosphine oxide.[138] These cured epoxy

polymers were observed to have good char yield on pyrolysis, and UL94 test exhibited that all epoxy t­ hermosets had a V0 flammability rating. An FR bisdiglycol spirocyclic PER bisphosphorate (BDSPBP) was synthesized as an FR for EP DGEBA.[139] The various results showed that the EP-containing BDSPBP had a high yield of residual char at high temperatures, indicating that BDSPBP was an effective charring agent. The FR BDSPBP improved the flame retardancy of cured EPs effectively. The cured EP system having 18 wt% BDSPBP reached a LOI value of 29.4 and V0 rating of UL94. It was observed that the char yield of the cured resin with 18 wt% BDSPBP was higher than that of the cured resin with no BDSPBP. The SEM results of the residues of cured EP-­ containing 18 wt% BDSPBP confirmed the formation of the compact charred layers, which inhibited the ­transmission of heat and heat diffusion during contact of fire. A systematic and comparative evaluation of the pyrolysis of halogen-free flame-retarded EPs containing phosphine oxide, phosphinate, phosphonate, and phosphate (phosphorus contents around 2.6  wt%) (Fig. 58) and the fire behavior of their carbon fiber composites were reported.[140] All organophosphorus-modified hardeners containing phenoxy groups led to a reduced decomposition temperature and mass loss step for the main decomposition of the cured EP. With increasing oxidation state of the phosphorus, additional charring was observed, whereas the flame inhibition, which played the more important role for the performance of the composites, decreased.

O O

P

O

O

N H

N H

P

O

O

O

N H

N H

O

P

O

Fig. 55  Structure of phosphorus-based symmetric phenol (b) Source: Sun and Yao.[136] © 2011 Elsevier.

O

O

N H

N H

Fig. 54  Structure of phosphorus-based symmetric phenol (a) Source: Sun and Yao.[136] © 2011 Elsevier.

P

HO

OH

Fig. 53  Structure of phosphorus-based symmetric diamine Source: Sun and Yao.[136] © 2011 Elsevier.

HO

P

P

O OH

Environmental–Fire Protection

Fire Protection: Flame-Retardant Epoxy Resins in 1191

1192

Fire Protection: Flame-Retardant Epoxy Resins in

N H

O

O O H H N C C N CH2 O

Ar

O H N C C CH2 O

HN

NH

Environmental–Fire Protection

Poly(amide-imide) CH3

Where Ar is NH2

NH2

P

P

O

NH2

P

Fig. 56  Structures of poly(amide-imide)s Source: Agrawal and Narula.[137] © 2014 Springer.

O H2N

Ar΄

C

NH

O H C

N

CH2

NH2

O Ar

O

N O

NH

Where Ar is

H C

O C

Ar΄

NH

CH2

NH2

NH2

HN

Diamide-diimide-tetramine O O

Ar΄ is

H2N

P

NH2

H2N

P

O

NH2

Fig. 57  Structures of phosphorus-containing diamide-diimide-tetraamines Source: Agrawal and Narula.[138] © 2014 Springer.

Two different commercial EPs, tetraglycidyl methylene dianiline (TGDDM) and DGEBA, were cured separately with DETDA and bis(4-aminophenoxy)phenyl phosphonate (BAPP) (Fig. 59).[141] The lower decomposition temperatures and higher char yields of the TGDDM were therefore provided superior fire performance compared to the bifunctional (DGEBA) epoxy. An IFREP system was prepared from DGEBA, LWPA (cure agent), and APP.[142] The cured EP served as a

carbonization agent as well as a blowing agent itself in the fire-­retardant formulation. The results of LOI and UL94 showed that APP improved the flame retardancy of LWPAcured EPs. It was found that only 5 wt% of APP increased the LOI value of EPs from 19.6 to 27.1 and improved the UL94 ratings, reaching V0 rating from no rating when the mass ratio of EP to LWPA was 100/40. It was observed that LOI values of FR cured EPs (FR-CEP) increased with decreasing LWPA. The thermal analysis results showed

Fire Protection: Flame-Retardant Epoxy Resins in 1193

O

O

H2N

NH2

Environmental–Fire Protection

NH2

(Phosphate)

(Phosphinate)

O

O P

O

O

O H 2N

O

P

O

P

P

O

H2N

H2N

NH2

NH2

(Phosphonate)

(Phosphine oxide)

Fig. 58  Structures of phosphine oxide, phosphinate, phosphonate, and phosphate Source: Braun et al.[140] © 2006 Elsevier.

O H2N

O

P

O

NH2

O

O H2 C

N

O

N

O

(TGDDM) (BAPP) CH3

CH3 H2N

NH2

H3CH2C

CH2CH3

H3CH2C

NH2 CH2CH3

NH2 (3,5-Dimethyltoluene-2,4-diamine)

(3,5-Dimethyltoluene-2,6-diamine) (DETDA)

Fig. 59  Structures of TGDDM, DETDA, and BAPP Source: Liu et al.[141] © 2007 Elsevier.

that the process of thermal degradation of FR-CEP consists of two main stages: the first stage was that a phosphorus-rich char was formed on the surface of the material below 500°C, and then a compact char yields above 500°C; the second stage was that the char residue layer gave more effective protection for the materials than the char formed at the first stage. A study was made on the development of DOPO-based phosphorus tetraglycidyl epoxy nanocomposites and to find its suitability for use in aerospace and high-performance

applications.[143] Phosphorus-containing diamine (DOPO– NH2) was synthesized from 4,4-diaminobenzophenone and DOPO. Nanoclay and POSS-amine nanoreinforcements were introduced into the synthesized EP and were cured with BAPPO and DDM curing agents. It was observed that the flame retardancy of EP was enhanced on incorporation of both POSS and clay.[144,145] This might be due to the char-forming nature of clay and low surface energy of Si-O-Si present in POSS, which migrates to the surface and protects the underlying matrix. The tetraglycidyl

1194

Environmental–Fire Protection

phosphorus nanocomposites gave better flame retardance performance due to the high thermal stability of Si–O-Si linkage of nanoclay and POSS nanoreinforcement. Two halogen-free FREPs were prepared by DGEBA epoxy.[146] The aryl phosphinate dianhydride and 1,4-­bis(phthalic anhydride-4-carbonyl)-2-(6-oxido-6Hdibenz[c,e][1,2]-oxaphosphorin6-yl)-phenylene ester (BPAODOPE) were used as a hardener and FR when coupled with curing agents, such as methylhexahydrophthalic anhydride and maleic anhydride. The results showed that phosphorus-containing EP composites had a higher char yield and UL-94 grade. Furthermore, the 1.75% phosphorus content was enough to achieve a UL94 V1 grade and the best combination properties for the two composites with different hardeners. Bis(2,6,7-trioxa-l-phosphabicyclo[2.2.2]octane-4-­ methanol) melaminium salt (Melabis) and microcapsules of Melabis with MEL resin shell as FRs were synthesized. [147] Twenty weight percent of microcapsules was doped into EPs to get FREP with 28.5% of LOI and UL94 V0. Heat release and smoke production of EP containing the Melabis microcapsules were much decreased as ­compared to EP. Hexakis-(4-aminophenoxy)-cyclotriphosphazene (PNNH2) was synthesized and used as curing agent for DGEBA EP and was compared with conventional curing agents DDS and DDM.[148] Thermal studies showed that the thermal properties of the PN-NH2-containing cured EP were higher than those of others. The phosphorus–nitrogen containing curing agent resulted in an excellent ­improvement of the flame retardance for their thermosetted EPs. To improve the fire behavior of UPRs, a phosphorus-­ containing monomer, dimethyl-vinylbenzyl phosphonate (S1), was used to replace a part of the reactive diluent (styrene).[149] TGA pointed out a decrease in thermal stability with increasing S1 content but also an increase in char formation. Cone calorimetry results showed that PHRR decreased, and from 50% of styrene substitution, a barrier effect was evidenced. At high S1 content, interactions between phosphonate groups enable a very efficient condensed-phase action by creation of a charred protective barrier. A DOPO-containing MEL Schiff base (P-MSB) was synthesized and used as a reactive FR in CNE for electronic application.[150] The FR and thermal properties of the EPs cured by phenol formaldehyde novolac (PN) and P-MSB were investigated. The results demonstrated that the cured EPs possessed high Tg (165°C) and good thermal stability (T5%, 321°C). Moreover, the P-MSB/CNE systems exhibited higher LOI (35) and maintained more char than the PN/CNE system, and the effective synergism of phosphorus–nitrogen indicated their excellent flame retardancy. An aryl phosphinate dianhydride BPAODOPE was synthesized and used as an FR and hardener for preparing halogen-free flame-retarded EPs when coupled with another

Fire Protection: Flame-Retardant Epoxy Resins in

curing agent.[151] The results showed that the BPAODOPE had better flame-resistant properties. The flame resistance and char yield of flame-retarded EPs improved with an increase in the phosphorus content, and the tensile strength and impact strength of samples gradually decreased with the addition of BPAODOPE. The flame-retarded epoxy sample with phosphorus contents of 1.75% showed the best combination properties, the LOI value was found to be 29.3, and the vertical burning test reached UL94 V0 level. Two FR-curing agents DOPO-containing 4-[(phenylamino) methyl]phenol (P-Ph) and DOPO-containing ­Mannich-type bases (P-DDS-Ph) were synthesized for EPs.[152] The thermal properties and fire retardancy of CNE cured with different contents of the phosphorus-containing compounds were analyzed. The results showed that more char was formed while containing lower contents of phosphorus in the P-Ph/CNE and P-DDS-Ph/CNE, indicating their excellent flame retardancy. Moreover, the P-DDS-Ph/ CNE exhibited higher Tg (224°C) and better thermal ­stability (T10%, 330°C) compared to that of P-Ph/CNE. DOPO was grafted onto the surface of graphene oxide (GO) by reacting epoxy ring groups together with the reduced graphene structure (DOPO-rGO) (Fig. 60).[153] Furthermore, the flame retardancy and thermal stability of DOPO-rGO/epoxy nanocomposites containing various weight fractions of DOPO-rGO were investigated. Significant increases in the char yield and LOI were achieved with the addition of 10 wt% DOPO-rGO in epoxy, giving improvements of 81% and 30%, respectively. DOPO-rGO/ epoxy nanocomposites with phosphorus and graphene layer structures were found to contribute to good flame  retardancy compared to that of neat epoxy. Therefore, the synergistic effect of DOPO-rGO is quite useful, and this material can be utilized as a potential FR. A neopentyl glycol phosphate MEL salt (NPM) was synthesized as an FR compound for EP, [154] and then the FR properties of NPM/EP system were studied. The result showed that when the quantity of NPM was 27%, the LOI of EP was 32.4 and the char yield was 18.7% at 600°C. NPM played a significant role in the improvement of the FR properties of the epoxy. Two organophosphorus-based diamines, phenyl phosphonic ethylene diamine diamide (PPEDD) (Fig. 61a) and phenyl phosphonic p-phenylene diamine diamide (PPPDD) (Fig. 61b), containing aromatic moieties were synthesized and used as a curing and flame retarding agent for Epon 828 EP.[155] Kinetic studies of the curing reaction of the two phosphorodiamidates were carried out in comparison with the corresponding non-phosphorus-containing reference cross-linking agents, p-PDA and ethylenediamine (EDA). The results showed that EP cured with phosphorodiamidate possesses higher thermostability than that of the nonphosphorus containing counterpart. This was evident by a significantly higher amount of char formed upon burning. More importantly, the LOIs of 27 and 31 were observed in the PPEDD-cured EP and PPPDD-cured EP compared

Fire Protection: Flame-Retardant Epoxy Resins in 1195

HO O

O O

O

P

O O OH

O

HOOC

COOH OH

OH

O

Environmental–Fire Protection

P

O

O

P

OH

O

OH

O O

P

O

(DOPO-rGO)

Fig. 60  Structure of DOPO-rGO Source: Liao et al.[153] © 2012 American Chemical Society. O

O H2N

P

N H

NH2 H2N

N H

(a)

H N

P

H N

NH2

(b)

Fig. 61  Structures of (a) PPEDD and (b) PPPDD Source: Jirasutsakul et al.[155] © 2013 Elsevier.

CH3 O

O

P

O

O CH3

n (P1)

OH

Fig. 62  Structure of phosphinated polyether (P1) Source: Lin et al.[156] © 2013 Elsevier.

with those prepared from nonphosphorus curing agents (20 for EDA and 21 for PDA). Lin et al.[156] reported the synthesis of a phosphinated polyether (P1) (Fig. 62) with a phenol pendant group in the repeating unit. The moderate-to-high molecular weight of P1 provided phenol linkages as reacting sites for EPs. Subsequently, flexible and transparent films of epoxy

thermosets were prepared from the curing of P1 with three EPs. The thermoset based on P1/cresol novolac epoxy shows a high Tg value (250°C), and flame retardancy ­vertical burn test (VTM-0). A simple and efficient approach for the preparation of organic–inorganic hybrids FRs (FRs-rGO), aiming at improving the FR efficiency, was presented.[157] The reduced graphite oxide (rGO) was incorporated into the FRs matrix, resulting in the formation of organic–­ inorganic hybrids FRs containing exfoliated rGO. Subsequently, the FR (FRs-rGO) was incorporated into EPs (Scheme 37). With the incorporation of 5 wt% of FRs-rGO into EP, the satisfied FR grade (V0) and the LOI as high as 29.5 were obtained. The char residues of the FRs-rGO/ EP nanocomposites were significantly increased in air as well as nitrogen atmosphere. Moreover, the PHRR value of FRs-rGO/EP was significantly reduced by 35%, and the Tg of FRs-rGO/EP nanocomposites shifted to higher ­temperature, compared to those of neat EP. Hexakis[p- (hydroxymethyl)phenoxy]cyclotriphosphazene (HHPCP) (Fig. 63) was prepared and then the flame retardancy and thermal decomposition behavior of

1196

Fire Protection: Flame-Retardant Epoxy Resins in

a

O O Si O

HO OH

OH

NCO

NH

O P O

O C O

O O Si O

THF

O NH

OH

Environmental–Fire Protection

Graphene

Sol-Gel process

C O

Si O O

O

Graphene

O Si O O

O Si O SiH O

O P O

O O Si O

N

O P O

O C O

FRs containing DOPO and silicon

O O NH C O P O

Epoxy resins b O O O SiO SiH P O O N OSi O OC O

H2C HC CH2 O

+

C H3C

O O Si O

O P O

DDM

O CH O O HC = CH2 O

O Si OO Si P O O

H 3C

FRs-rGO

Epoxy resins

OH Si O O

Si

CH3

O O NH C O P O

O

O

FRs-rGO: 10wt%Graphene contents

O

O P O

OH Si

C

CH3 O CH2 CH CH2 O

FRs-rGO/EP: 5wt% FRs-rGO contents

Scheme 37  Preparation of FRs-rGO hybrids and FRs-rGO/EP nanocomposites: (a) preparation routes of FRs-rGO hybrids; (b) ­preparation routes of FRs-rGO/EP nanocomposites Source: Qian et al.[157] © 2013 RSC Advances. CH2OH

HOH2C

O

N

O P N

HOH2C

O

HOH2C

O P

O

CH2OH

N P O CH2OH

(HHPCP)

Fig. 63  Structure of HHPCP Source: Xu et al.[158] © 2013 Springer.

EP-­containing HHPCP were carried out.[158] The result represented that the LOI value of EP increased from 20.5% to 26.5%, when 7.5 mass% HHPCP was added into EP. The addition of 1 mass% nano-MMT (nMMT)  into EP-7.5  mass% HHPCP sample as the synergist could increase the LOI value of EP-7.5 mass% HHPCP-1 mass% nMMT sample from 26.5% to 27.5%.

An effective FR and smoke-suppression oligomer, poly(MEL-ethoxyphosphinyl-diisocyanate) (PMPC) (Fig. 64), was synthesized and used alone as the FR for EPs. [159] The results showed that PMPC endowed EP with good flame retardancy. The LOI value of an EP/PMPC system containing 20 wt% PMPC increased to 28.0% and achieved a UL94 V0 rating. The cone calorimeter data showed that the THR and HRR were considerably reduced with the addition of PMPC, and the SPR, TSP, and carbon monoxide production (COP) were also reduced. A acrylate monomer containing phosphorus and nitrogen N,N-bis(2-hydroxyethyl acrylate) aminomethyl phosphonic acid diethylester (BHAAPE) (Fig. 65) was synthesized and then incorporated into EA (EA20) resins through a UV-curing process.[160] The functionalized EA resins exhibited enhanced flame retardancy, and the introduction of BHAAPE promoted the degradation of the EA matrix and catalyzed its char formation. An FR (DOPO-HEA) (Fig. 66) containing DOPO and unsaturated bonds was synthesized and incorporated into EAs in different ratios.[161] The FR properties of the resins showed that the incorporation of DOPO-HEA into EA improved the LOI values and greatly reduced the PHRR of EA. The thermal properties of the resins indicated that the incorporation of DOPO-HEA into EA could distinctly

Fire Protection: Flame-Retardant Epoxy Resins in 1197

NH

H N

N

O H N

C

O H N

P

C

OCH2CH3 N

O n

(DP-X)

R2 =

N

O

Fig. 65  Structure of BHAAPE Source: Jiang et al.[160] © 2013 American Chemical Society.

improve the char residues. Due to the catalyzing charring effect and the gas FR effect of phosphorus, the resins exhibited significant improvements in FR properties. Two halogen-free FRs, DP-DDE and DP-DDS (Fig. 67), were synthesized.[162] The fire-resistant properties of DP-DDE or DP-DDS modified EPs (2,2-bis(4-glycidyloxyphenyl)propane) with 4,4′-methylenedianiline as a hardener were investigated. Their high flame-retarding performance was found: the epoxy thermosets with a relatively low addition amount of DP-DDE or DP-DDS (on the account of phosphorus content of 0.75 or 0.73 wt%) reached a UL94 V0 flammability rating. These thermosets demonstrated excellent thermal stability, high glass ­transition temperature (Tg > 135°C), and high char yields. The phosphorus-containing star-shaped FR (TRIPOD-­ DOPO) (Fig. 68) was synthesized, while the FR -UPRs composites with various amounts of TRIPODDOPO were

O

O O

DOPO-HEA

Fig. 66  Structure of DOPO-HEA Source: Qian et al.[161] © 2013 American Chemical Society.

O

S

O

X = DDS

prepared.[163] The results showed that the incorporation of TRIPOD-DOPO improved the thermal stability and flame retardancy of UPR. Under the air condition, TRIPOD-DOPO showed a more obviously ­condensed-phase interaction increasing charring. An amine-terminated and organophosphorus-­ containing compound m-aminophenylene phenyl phosphine oxide oligomer (APPPOO) was synthesized and used as a curing agent and FR for EPs.[164] The EPO/APPPOO thermosets passed V1 rating and the LOI value reached 34.8%. The cone calorimeter test showed that the parameters of EPO/APPPOO thermosets, including THR and HRR, decreased compared with EPO/PDA thermosets. It was observed that the incorporation of APPPOO into EPs accelerated the formation of the compact and stronger char layer to enhance FR properties of the cured EPs during combustion. A halogen-free, phosphorus–nitrogen containing FR TNTP was synthesized.[165] A series of modified DGEBA EP with different contents of TNTP were synthesized and cured by DDS. All modified epoxy thermosets by using TNTP exhibited higher Tg compared to pure DGEBA/ DDS. The loading of TNTP at only 5.0 wt% resulted in satisfied flame retardancy (UL94, V0) 59 together with

(BHAAPE)

O O O P O

X = DDE

O

Fig. 67  Structures of DP-DDE and DP-DDS Source: Gu et al.[162] © 2014 Elsevier.

O

O

O P O O O P O O

OH

R1 =

O

O

O

OH

Fig. 64  Structure of PMPC Source: Lv et al.[159] © 2013 American Chemical Society.

O

O

O

NH2

O

P

R

(PMPC)

P

O

H N

O

N

O

H N

P

O

O O

O OH

O Epoxy acrylates

O OH

Environmental–Fire Protection

O

1198

Fire Protection: Flame-Retardant Epoxy Resins in

HO CH

Environmental–Fire Protection

R HO

R

O

C H

N

O

N R= N

O

O

O

(TRIPOD-DOPO)

CH

R

P

OH

Fig. 68  Structure of TRIPOD-DOPO Source: Bai et al.[163] © 2014 Elsevier.

high char residue (27.3%) at 700°C. The addition of TNTP improved the flame retardancy of DGEBA EPs. Phosphate ester compounds display good flame retardancy effect in EP systems. Several phosphate esters (Fig. 69) were synthesized and used as curing agents for EPs.[166] Then, a series of FR epoxy composites were prepared by curing the EPs (E-44) with the phosphate esters. The results indicated that phosphate esters significantly decreased the THR, HRR, and SPR. The sample cured by butyl phosphate ester from phosphorus pentoxide, phosphoric acid, and butanol showed the best FR performance among all samples and could enhance char residues of FR

O

P

O OH

Butyl phosphate ester (BPE)

P

HO

P

O OH

P

O O

O

O

P

OH

O

Ethylphosphonate ester (EPE) Butanediol and octanol mixed phosphate ester (BOPE)

O HO

O

OH

OH O

epoxy composites. It was concluded that good flame retardancy of FR epoxy composites was related to the f­ ormation of a protective phosphorus-rich char layer. A highly effective DOPO-based FR (D-bp) (Fig. 70) was successfully synthesized and was used as a cocuring agent to improve the flame retardancy of DDM/DGEBA system.[167] The results showed that the epoxy thermosets exhibited excellent flame retardancy and passed the V0 rating of UL94 test with a LOI of 39.7% when the phosphorus content was only 0.5 wt%. Hence, D-bp was expected to be used as a highly effective halogen-free FR for the ­application of EPs as electronic materials.

O O

O

O

P

O OH

O

Butanediol and butanol mixed phosphate ester (BBPE)

Fig. 69  Structures of phosphate esters Source: Jiao et al.[166] © 2014 Springer.

HO

P O

O O

O

P

OH

O

Hexanediol and butanol mixed phosphate ester (HBPE)

Fire Protection: Flame-Retardant Epoxy Resins in 1199

P

H2 C

H N

C H

HO

H N

O

P

O

O

Environmental–Fire Protection

O

OH

C H

(D-bp)

Fig. 70  Structure of D-bp Source: Xu et al.[167] © 2015 Elsevier.

Triazine derivative with active maleimide group (TMT) (Fig. 71) was synthesized. The FREPs were then prepared by copolymerizing DGEBA with DOPO, TMT, and DDM.[168] The Tg’s of EP/TMT/DOPO thermosets (154°C–160°C) were much higher than that of the traditional EP/DOPO thermoset (122°C). The results indicated that the flame retardancy of EP/TMT/DOPO thermosets was enhanced with low loading of phosphorus content. EP/ TMT/DOPO-1.0 sample with phosphorus content of only

1.0 wt% achieved a LOI value of 40.3% with a UL94 V0 rating. The morphologies of the char residues showed honeycombed and intumescent structures with a small number of holes on the surfaces. The two additive or reactive phosphorus- and sulfur-containing FRs (P-FRs) were synthesized and then incorporated into a DGEBA-isophorone diamine (IPDA) matrix (Fig. 72) to improve the FR properties.[169] The results showed the presence of phosphorus-containing O N

X = X

O

O N X

O

or

N

O

N O

O P

N

X

O

O Triazine derivative with active maleimide group (TMT)

Fig. 71  Structure of TMT Source: Yang et al.[168] © 2015 Elsevier. OEt EtO P O

OEt EtO P O

S

S OH O

O

P2EP1SP O

O

O

O

Fig. 72  Structures of P2EP1SP and P3SP Source: Menard et al.[169] © 2015 Elsevier.

O

OH

EtO

P

S OEt

OH

O

P3SP O

OH

S

OEt P OEt O

1200

Fire Protection: Flame-Retardant Epoxy Resins in

Environmental–Fire Protection

gases released during the thermal degradation, but these P-containing gases had no radical scavenging action in the gas-phase. Pyrolysis-Combustion Flow Calorimeter (PCFC) analysis proved the similar FR properties of the two P-FRs by reducing significantly the THR, PHRR, and EHC. The cone calorimeter tests showed the physical action of the char effect, leading to a huge intumescent effect and to an insulating residue, which protected the underlying thermoset from the heat and oxygen flow during thermal decomposition. DPPA (Fig. 73) was used as a cocuring agent of DDM for the curing reaction of DGEBA EP.[170] DPPA endowed EP with high FR efficiency due to the unique combination of phosphorus and nitrogen in the phenophosphazine ring. The cured EP could pass a V0 rating of UL94 test with a LOI of 33.6% at only 2.5 wt% DPPA. The reduced PHRR

and THR, and increased char yield further verified the excellent flame retardancy for EP. A phosphorus-containing compound diphenyl-(1, 2-dicarboxylethyl)-phosphine oxide (DPDCEPO) was synthesized as a reactive FR for EPs.[171] The DPDCEPO was mixed with phthalic anhydride (PHA) curing agent in various ratios into EPs to prepare FR epoxy thermosets. The various results showed that the EP/20 wt% DPDCEPO/80 wt% PHA thermosets passed the UL94 V0 flammability rating and the LOI value was as high as 33.2%. The cone test results revealed that the addition of DPDCEPO effectively reduced the combustion parameters of the EP thermosets, such as HRR and THR. The TGA results indicated that the incorporation of DPDCEPO ­promoted the decomposition of EP matrix ahead of time. Phosphorous-containing EP was first prepared by modifying o-cresol formaldehyde EP with DOPO.[172] Self-­emulsified nitrogen-containing epoxy curing agent (Scheme 38) was prepared by the reaction of BPA EP E-44, triethylene tetramine, and acrylonitrile. Phosphorous- and nitrogen-containing epoxy emulsion was obtained by emulsifying the phosphorous-containing EP with the self-­ emulsified epoxy curing agent (Scheme 39). The result showed that the prepared phosphorous- and nitrogen-­ containing epoxy emulsion presented excellent stability and the epoxy thermoset had excellent flame retardancy. An intumescent FR curing agent, poly(meta-­ xylylenediamine spirocyclic PER bisphosphonate)

H N

P O

H

(DPPA)

Fig. 73  Structure of DPPA Source: Luo et al.[170] © 2015 RSC Advances.

EP

H 2N D 2

H2 2 H2N D2 C

+

H2 H2 H H2 C C N C

H2 H2 NC C C H2N D2

OH C H

OH C H

EP

H2 H2 H H2 C C N C

H2 H2 NC C C H2N

D2

H2 C

H2 C

H H2 N C

OH

OH C H

C H

EP

H2 H2 H H2 C C N C

CH3 EP =

O

C CH3

O

Product I

NH2

H2 C

D2 NH3 + 2 H2C

H2 C

H H2 N C

OH

H2 C

OH

C H

EP

H2 H C C OH

H2 C

C H

C H

Product II

CN

H H2 H2 D2 N C C CN

+

CH3COOH

− CH3COO H H2 C

H H2 N C

O n

Scheme 38  Schematic of preparing amine-functional waterborne epoxy curing agent Source: Zhang et al.[172] © 2016 Springer.

H2 C

H2 H2 D2 N C C CN H

D=

H C2H4 N

Fire Protection: Flame-Retardant Epoxy Resins in 1201

mix

Agitation Phase inversion Epoxy emulsion

Curing agent

Scheme 39  Formation process of epoxy emulsion by the phase inversion method Source: Zhang et al.[172] © 2016 Springer.

(PMXSPB), was synthesized and used as a curing agent and FR for preparing halogen-free flame-retarded EP r­ esins.[173] The results showed that the incorporation of PMXSPB improved the flame resistance of EP composites, and the residual char ratio at 600°C was significantly increased. The flame-­retarded composites containing 3.01% phosphorus content (EP 100 g and PMXSPB 35 g) exhibited the best combination of properties including a higher Tg (147°C), good thermal stability, and a LOI of 31.2. A α,ω-dicarboxyl aromatic polyphosphonate (HP-1001COOH) (Fig. 74) was synthesized and used as a reactive FR for the DGEBA/methyl tetrahydrophthalic anhydride (MeTHPA) cured system.[174] The glass transition temperature of the cured EPs decreased with an increase in the HP-1001-COOH content and the char yields increased with an increase in the phosphorus content. When the mass fraction of HP-1001-COOH was 30 wt%, the epoxy thermosets reached a LOI value of 32.4% and successfully passed the UL94 V0 rating. The HRR, THR, PHRR, average of O HOOC

CH2CH2

C

CH3

O

CH3

CH CH2

O

the effective heat of combustion, and TSP of the FR EPs decreased compared with neat EP. A phosphorus/nitrogen-containing compound (DMT) (Fig. 75) constructed by maleimide, phosphaphenanthrene, and triazine-trione was successfully synthesized and then blended with DGEBA to prepare a series of FR EPs.[175] The results indicated that DMT enhanced the flame retardancy of EP. When the phosphorus content was only 1.0%, the EP/DMT-1.0 sample had a LOI value of 35.8% and achieved UL94 V0 rating. Compared with the neat EP sample, the PHRR, average of HRR (av-HRR), and THR of EP/DMT-1.25 sample were decreased by 59.4%, 28.2%, and 27.4%, respectively. A curing agent containing phosphorus and sulfur, tris(2-mercaptoethyl) phosphate (TMEP) (Fig. 76), was synthesized for Light Emitting Diode (LED) packaging EP with high refractive index and good flame retardancy.[176] The cured samples containing 21.6 wt% TMEP had a UL94 V0 rating and a LOI value of 29.2%. It was found that TEMP

O

C 3

O

P

C

O 4

CH3

CH3

CH3

CH3

CH3 O

Fig. 74  Structure of HP-1001-COOH Source: Wang and Nie [174] © 2016 Elsevier.

O N O

N

O

O H2 H C C O

N H 2C

N

O

P

O

OH (DMT)

=

H2 C

O

H C

H2 C O

O

N

OH O

Fig. 75  Structure of DMT Source: Huo et al.[175] © 2016 Elsevier.

O

P

O

CH2 CH

O 3

O C CH2CH2COOH

Environmental–Fire Protection

Epoxy resin

Water

1202

Fire Protection: Flame-Retardant Epoxy Resins in

Thermal cross-linkable EPs with two different kinds of containing azobenzene or/and phenylacetylene cross-­ linkable (Scheme 40) groups were synthesized successfully.[179] Flame-retardancy test results suggested that, with the incorporation of cross-linkable groups, the flame retardancy of these EPs was improved significantly. LOI value was increased from 21% of DGEBA-PA to 27.5% of AzoEP-HRC, 4-phenylethynylphthalic anhydride (PEPHA), PHRR, and THR of cross-linkable EPs from MCC test were all immensely decreased; PHRR of cross-linkable EPs from cone calorimetric test was also greatly decreased, except for that of DGEBA-PEPHA because of the competition between the cross-linking of phenylacetylene group and the decomposition of EP matrix. Various amounts of a phosphonium ionic liquid (IL169) (Fig. 79) were used as curing agents as well as FRs of epoxy prepolymer to prepare epoxy networks with improved fire-retardant properties.[180] Phosphonium ionic liquid significantly reduced flame retardancy without further addition of FRs due to the high amount of phosphorus (up to 3.69 wt%). The PHRR decreased when incorporating 30 phr of IL169. Phosphorus-modified pyrolysis ­pathway promoted charring and may act as a flame inhibitor. The char layer protected the underlying polymer, leading to a high unburnt polymeric fraction. A DPPA-based curing agent, 10-[(4-hydroxyphenyl)(4-hydroxyphenylimino) methyl]-DPPA (H-DPPA) (Fig.  80), was successfully synthesized to serve as a cocuring agent of DDM for DGEBA. [181] With the incorporation of 3.0 wt% of H-DPPA, the cured EP, in which

O

O

HSH2CH2C

P

O

CH2CH2SH

O

Environmental–Fire Protection

(TMEP)

HSH2CH2C

Fig. 76  Structure of TMEP Source: Luo et al.[176] © 2016 Elsevier.

could promote the formation of a compact and c­ ontinuous char foam layer during the combustion of cured EP. A phenophosphazine-containing compound, HD-DPPA (Fig. 77), was synthesized and used as a co-curing agent of DDM and an FR for DGEBA EP.[177] The cured EP passed the UL94 V0 rating with a LOI of 31.3% at only 2.5 wt% HD-DPPA, where phosphorus content was as low as 0.19 wt%. The formation of intumescent char layer and blowing-out effect during combustion were responsible for the high flame retardancy of EP. DOPO-based oligomer (PDAP) (Fig. 78) was synthesized.[178] The PDAP serving as a co-curing agent of DDM was employed to develop EPs with highly improved flame retardancy. The results represented that with the incorporation of 7 wt% PDAP, the modified epoxy thermoset achieved a LOI value of 35.3% and a V0 rating in UL94 test. Furthermore, the results revealed that the incorporation of PDAP decreased the Tg slightly and meanwhile improved the tensile strength of epoxy thermoset.

H C

HO

P

H2 C

H N

H N

O

N H

H C

O

OH

P

N H

(HD-DPPA)

Fig. 77  Structure of HD-DPPA Source: Luo et al.[177] © 2016 Elsevier.

H C

OHC O

P

O

H N

H N

P

O

H C

C H O

(PDAP)

Fig. 78  Structure of PDAP Source: Wang et al.[178] © 2016 Elsevier.

O

P

O

n

H N

NH2

Environmental–Fire Protection

Fire Protection: Flame-Retardant Epoxy Resins in 1203

Scheme 40  Curing processes of DGEBA-PA (DGEBA+o-PA), DGEBA-PEPHA (DGEBA+PEPHA) Source: Liu et al.[179] © 2015 Elsevier.

O

O

O

O OH n

O

DGEBA DER 332 H 2N

O

x~2.5 Jeffamine D230

Fig. 79  Structures of IL169 Source: Liu et al.[179] © 2015 Elsevier.

NH2

P+ IL 169

O P O O O

O

1204

Fire Protection: Flame-Retardant Epoxy Resins in

HO

H C P

H N

OH

O

Environmental–Fire Protection

N H (H-DPPA)

Fig. 80  Structure of H-DPPA Source: Luo et al.[181] © 2016 American Chemical Society.

the phosphorus content was as low as 0.22%, passed a V0 rating of UL94 test with a LOI of 31.8%. The high flame retardancy of EP modified by H-DPPA originated mainly from the formation of intumescent char layer during combustion. Zhang et al. synthesized a phosphorus-containing compound diphenyl-(2,5-dihydroxyphenyl)-phosphine oxide (DPDHPPO) to use as an FR and curing agent for EPEP. [182] The results showed that the EP/40 wt% DPDHPPO/60 wt% PDA thermosets successfully passed the UL94 V0 flammability rating and the LOI value was 31.9%. The cone calorimetry results showed that the incorporation of DPDHPPO efficiently reduced the combustion parameters of EPs thermosets, such as THR and HRR. The TGA results showed that the doping of DPDHPPO promoted EPs matrix decomposed ahead of time compared with that of pure EP and led to a higher char yield and thermal stability at high temperature. The effect of two types of phosphorus-containing FRs (P-FRs) (Fig. 81) with different chemical surroundings (phenylphosphonate-based (PO-Ph) and phenylphosphoric-based (PO-OPh)) was investigated on the FR efficiency for diglycidyl ester of BPA-type EP resin.[183] The most significant difference on flame retardancy between them was that FPx (x = 1, 2, and 3) endowed EP with V0 rating in UL94 test at 5 wt% loading, while FPOx (x = 1, 2, and 3) showed no rating at such loading. It was found that there was almost 10 times difference in the FR efficiency for EP between FPx and FPOx, though they had similar ­chemically molecular structures. A phosphaphenanthrene/triazine-trione bigroup FR, TOD (Fig. 82) containing two different chemical bridge bonds between phosphaphenanthrene and triazine-trione groups, was synthesized and then applied to prepare the FR epoxy thermoset in the DGEBA cured with DDM.[184] The results showed that introduction of 4 wt% TOD endowed the EP thermoset with a LOI value of 35.9%, UL94 V0 rating, 42.4% decreased peak of HRR, 46.5% decreased THR, and slightly elevated char yields. Two phosphorus-containing phenolic amines (DAP and its analog AP) were used to enhance the flame retardancy of EPs.[185] With the incorporation of 10 wt% DAP, the epoxy thermoset showed a LOI value of 36.1% and a

V0 rating in UL94 test, whereas the thermoset modified with 10 wt% AP achieved a LOI value of 25.7% and no rating in UL94 test. In the case of thermosets modified with DAP, they all represented blowing-out effects during UL94 test, which were attributed to the release of pyrolytic gases having phosphorus-based free radicals and nonflammable gases. A series of DOPO-based curing agents (Fig. 83) with weak electron-donating methylene groups, strong electron-withdrawing sulfonyl groups, and strong electron-­ donating ether groups were prepared and then incorporated into EPs.[186] The results showed that the curing agents with proper structure could impart EPs with high thermal stabilities and low flammability. The curing agents containing ether groups exhibited better FR efficiency compared with other curing agents. The TG results indicated that DOPO-DDE/EP resins had the highest char residues, and the high char residues could reduce the PHRR of the resins and impart EPs with high FR efficiency. Moreover, the DOPO-DDS/EP resins also exhibited excellent FR efficiency, which were due to both condensed and gaseous FR mechanisms (Fig. 84). Environmentally friendly phosphorus- and nitrogen-­ based tris-diethanolamine spirocyclic PER bisphosphorate reactive diluent (TDSPBRD) (Fig. 85) was synthesized and incorporated into a UV-curable formulation along with EA oligomer in different weight fractions.[187] The result showed that, with increasing concentration of TDSPBRD in the coating, its FR behavior was improved to a good extent. A halogen-free compound (6,6′-­(((methylenebis(4,1-ph enylene))bis(azanediyl))bis((4-hydroxy-­3-methoxyphenyl) methylene))bis(6H-dibenzo[c,e][1,2]oxaphosphinine 6-oxide))) (DP-DDM) was synthesized as an FR for EP.[188] The thermal properties and curing behaviors of EPs with different contents of DP-DDM were investigated. These DP-DDM-modified epoxy thermosets showed good thermal stability and high Tg. Moreover, the epoxy thermosets with a relatively low addition content of DP-DDM (on account of the phosphorus content of 0.75 wt%) reached the UL94 V0 rating with a LOI value of 34.5. A halogen-free FR of DOPO-containing ­H-benzimidazole (DHBI) was synthesized and used as a co-curing agent of DDM for DGEBA.[189] The resulting cured epoxy thermoset (EP-10) with 7.45 wt% of DHBI achieved a UL94 V0 rating with a LOI of 35.6% and ­without dropping phenomenon. Silicon-Containing FRs More and more often, silicone compounds are used as epoxy modifiers, including POSS of general formula (RSiO3/2)n, [190] where R stands for reactive or inert organic moiety. Applications of these compounds as fire retardants for EPs cured with amines were reported by Franchini et al.[191] The silicon dioxide protective layer formed on

Fire Protection: Flame-Retardant Epoxy Resins in 1205

Cl

P

+

Cl

2H2N

O

R

0°C–5°C, 2 h RT, 5 h

O P

+

NH

+

Cl

O

2H2N

Salts

O

R

HN

TEA 0°C–5°C, 2 h RT, 5 h

R

P

NH

O

+

FPx (x = 1, 2 and 3)

FPOx (x = 1, 2 and 3)

O N P N H H

N P N H O H

FP1

FPO1

O

O N P N H H O

FP2

FPO2

O

FP3

Salts

O

N P N H H

N P N H H

HCl N

R

PO-OPh FR (FPOx)

PDClP

HCl N

R

R

PO-Ph FR (FPx)

PPDCl

Cl

P

HN

TEA

O N P N H O H FPO3

Fig. 81  Structures of P-FRs Source: Zhao et al.[183] © 2016 RSC Advances.

the polymer surface during combustion prevents volatile products of polymer degradation from diffusing to the fire zone. Silica is widely used in molded EPs for encapsulating electronic devices. Because of high loading, it provides an FR effect mostly due to the “heat sink” effect. A typical composition consisting of a low-molecular-weight commercial EP based on BPA and aliphatic polyamine hardener; triethylenetetramine was modified by replacing a part of the composition with upto 15 wt% of low-molecular-weight bis(glycidylpropyl)- or bis(aminopropyl)-tetramethylsiloxane (Figs. 86, 87).[192] Both diepoxy and diaminosiloxane modifiers reduced the Tg of the resulting polymers. A small reduction of flammability of the modified epoxy polymers was observed at the highest

amount of siloxane modifiers. Two amines, phosphoric acid tris-(5-amino-naphthalene-1-yl) ester (Fig. 88) and bis-(5-amino-naphthalene-1-yl) dimethyl silane (Fig.  89), were used as reactive FRs as well as curing agents to blend with commercial EP DGEBA.[193] The results indicate that all epoxy thermosets exhibit excellent flame retardancy with LOI values in the range of 32.6–39.9. EPs with different silicon contents were prepared from silicon-containing epoxides or silicon-containing prepolymers (Fig. 90) by curing with 4,4′-diaminodiphenylmethane.[194] The reactivity of the silicon-based compounds toward amine curing agents was higher than that of the conventional EPs. The Tg of the resulting thermosets was moderate and decreased when the silicon content increased.

Environmental–Fire Protection

O

1206

Fire Protection: Flame-Retardant Epoxy Resins in

O P O

O P O CH2 HO CH

CH2

Environmental–Fire Protection

HC OH O P O CH2 O N C O C N CH2 CH CH2 O N C OH O H2C

O CH2

CH2 O OH C N C O CH CH2 N C N CH2 O

HO CH

HC OH CH2

CH2 O P O

O P O TGIC-ODOPB-DOPO (TOD)

Fig. 82  Structure of TOD Source: Qiu et al.[184] © 2016 RSC Advances.

H N

H C

HO

H N

X

H C O

O P

X=

OH

O

O

O (DOPO-DDE)

O

S

O

(DOPO-DDS)

P

CH2 (DOPO-DDM)

Fig. 83  Structures of DOPO-based curing agents Source: Qian et al.[186] © 2016 Springer.

The onset decomposition temperatures decreased and the char yields increased when the silicon content increased. EPs had a high LOI value, according to the efficiency of silicon in improving flame retardance. Silicon-containing EPs were prepared from diglycidyloxymethyl-phenyl silane (DGMPS) and DGEBA by using DDM as a curing agent.[195] Several DGMPS/ DGEBA molar ratios were used to obtain epoxy thermosets with different silicon contents. The weight loss rate of the ­silicon-containing resins was found to be lower than that of the silicon-free resin. Char yields under nitrogen and air atmospheres increased with increase in the silicon content. The LOI values also increased from 24 for a ­standard commercial resin to 36 for silicon-containing resins, ­demonstrating improved flame retardancy. Octa(aminoethyl)silsesquioxane (POSS-NH2) (Fig. 91), containing eight amine groups on the vertexes, was used as a curing agent for DGEBA to improve the various properties of EPs.[196] The disappearance of epoxy groups at the end of the curing process indicated that epoxy groups reacted with POSS to form a three-dimensional

cross-­linking network. Properties of the cured DGEBA/ POSS nanocomposite containing 30 wt% of POSS-NH2 were studied, and the results showed that the inorganic– organic materials possessed excellent toughness with enhanced thermal properties. EG was grafted using a coupling agent, 3-isocyanatopropyltriethoxysilane (IPTS).[197] EG had a –OC2H5 functional group that reacts with a polymer matrix through the sol–gel reaction (Scheme 41). EG was functionalized by the coupling agent to form a covalent bonding between organic and inorganic phases, increasing the compatibility between the fillers and the polymer, thereby enhancing the thermal stability of the composites. The results demonstrate that functionalized EG can improve the thermal ­stability of composites and increase the flame retardancy. A functional POSS (NPOSS) with two epoxy ring groups was synthesized via the reaction between TGIC and trisilanolisobutyl-POSS, and then a halogen-free epoxy composite containing silicon/nitrogen was prepared (Scheme  42).[198] NPOSS could retard the movement and scission of polymeric chains of EP and form a

Fire Protection: Flame-Retardant Epoxy Resins in 1207

O P H

P H

O S O

H H C N O P O

O

H H N C O O P

Char layers

O

DOPO-DDS H H C N O P O

O

H H N C O O P

O

Polymer matrix

O

DOPO-DDE DOPO structures or SO2

Fig. 84  Condensed and gases FR mechanisms Source: Qian et al.[186] © 2016 Springer. O

O O

P N

O

O

O P

O

O

O

O N O

O

(TDSPBRD)

O

O

Fig. 85  Structure of TDSPBRD Source: Chambhare et al.[187] © 2016 Springer.

H C

H2C

H2 C

CH3 O

(CH2)3

O

Si CH3

CH3 O

Si CH3

(CH2)3

O

H2 C

H C

CH2 O

Fig. 86  Structure of bis(glycidylpropyl) tetramethylsiloxane Source: Murias et al.[192] © 2012 Elsevier. CH3 H2N

(CH2)3

Si CH3

CH3 O

Si

(CH2)3 NH2

CH3

Fig. 87  Structure of bis(aminopropyl)-tetramethylsiloxane Source: Murias et al.[192] © 2012 Elsevier.

stable charred layer in the condensed-phase to prevent the ­underlying materials from further combustion. A silicon-containing trifunctional cycloaliphatic epoxide resin tri(3,4-epoxycyclohexylmethyloxy) phenyl silane (TEMPS) (Fig. 92) was synthesized.[199] A series of FR formulations by blending TEMPS with a commercial

Environmental–Fire Protection

O

1208

Fire Protection: Flame-Retardant Epoxy Resins in

H2N

NH2

O O

P

O

O

Environmental–Fire Protection

NH2

Fig. 88  Structure of phosphoric acid tris-(5-amino-­naphthalene1-yl) ester Source: Agrawal and Narula [193] © 2014 Springer. NH2

O H3C

Si

CH3

O

NH2

Fig. 89  Structure of bis-(5-amino-naphthalene-1-yl) dimethyl silane Source: Agrawal and Narula [193] © 2014 Springer.

epoxide resin DGEBA (EP828) in different ratios were prepared and exposed to a medium pressure lamp to form the cured films in the presence of diaryliodonium hexafluorophosphate salt as a cationic photoinitiator. The LOI value increased from 22 for EP828 to 30 68 for TEMPS80, demonstrating the improved flame retardancy. The thermal analysis showed that TEMPS had good miscibility with EP828. The Ts and Tg both decreased from 93°C and 138°C to 78°C and 118°C, respectively. The char yields under nitrogen and air atmospheres increased with ­increasing cross-linking density along with the TEMPS content. The silicon-containing multifunctional acrylates, tri(acryloyloxyethyloxy) phenyl silane (TAEPS) (Fig. 93a) and di(acryloyloxyethyloxy) methyl phenyl silane (DAEMPS) (Fig. 93b), were prepared using the transetherification of phenyltrimethoxyl silane and dimethoxymethylphenyl silane with 2-hydroxylacrylate, respectively.[200] The obtained TAEPS and DAEMPS as tri- and difunctional monomers were mixed with a commercial oligomer EA (EB600) in different ratios to formulate a series of silicon-­containing UV-curable resins. The char yields

measured under air atmosphere increased extraordinarily with an increase in the silicon content. The LOI values also increased from 21 for EB600 to over 30 at 70 wt% monomer loading, demonstrating that the improved flame retardancy was obtained. The thermal studies showed that TAEPS and DAEMPS have good miscibility with EB600. The cross-link density of UV-cured EB600/TAEPS film showed a greater increase along with the monomer ­content, compared with that of EB600/DAEMPS. Meenakshi et al. reported the development and characterization of high-functionality siloxane-based tetraglycidyl EP (TG-siloxane) (Fig. 94a).[201] Nanoclay and POSS-amine (Fig. 94b) nanoreinforcements were incorporated into the synthesized EP. Curing was done with DDM and BAPPO curing agents. This epoxy was cured with DDM with and without the incorporation of nanoclay and nanoreinforcement (POSS amine) to get matrix materials to be utilized for high-performance applications. From the results, it was observed that the tensile, flexural, and impact strength was found to be high for systems containing nanoclay and POSS amine due to increased cross-linking. The thermal studies indicated that nanoclay and nanoreinforcement showed better char yield and LOI values in spite of their degradation at a lower temperature. The incorporation of nanoclay and nanoreinforcement to the epoxy (TG-siloxane) improved the IDT, char yield, and LOI values. The test results clearly indicated that the modified tetra epoxy nanocomposites showed improved flame retardancy over unmodified tetra EP. The water absorption tendency decreased on addition of both POSS and clay, which could be due to the hydrophobic and partial ionic nature of the Si–O–Si linkage. A study was made in the present investigation on bis(p-aminophenoxy) dimethylsiloxane-based tetraglycidyl epoxy (Fig. 95) nanocomposites to determine its suitability for use in high-performance applications.[202] Nanoclay and POSS-amine nanoreinforcements were added into the synthesized EP. Curing was done with DDM and BAPPO, respectively. The results obtained from various studies showed that incorporation of nanoclay and nanoreinforcement to the epoxy showed better char yield and LOI values in spite of their degradation at lower temperature. Macromolecular silicon-containing IFR (Si-IFR) was synthesized and was used as an FR for EPs.[203] Twenty weight percent of Si-IFR was incorporated into EP to get 27.5% of LOI and UL94 V0. The experimental results exhibited that, on heating EP/Si-IFR, the phosphorus-­ containing groups first decomposed to hydrate the char source containing groups to form a continuous and protective carbonaceous char, which changed into a heat-­ resistant swollen char by gaseous products from the nitrogen-­containing groups. Meanwhile, SiO2 reacted with phosphate to yield silicophosphate, which stabilized the swollen char. The thermal stability and barrier properties of the swollen char were most effective in resisting the

Fire Protection: Flame-Retardant Epoxy Resins in 1209

O

O

H2 C O

O

H2 Si O C

Triglycidyloxyphenyl silane (TGPS)

O

H2 C

O

O

H2 C O

H2 Si O C

O

Environmental–Fire Protection

H2 O C

Diglycidyloxydiphenyl silane (DGDPS)

CH3

CH3

Si

Si

CH3

CH3

O

O

H2 C

1,4-bis(glycidyloxydimethyl silyl)-benzene (BGDMSB)

OH H2 H2 OC C C O H

CH3

H H2 H2C C C O

C

CH3

O

OH H2 H2 Si O C C C O H

CH3 C

CH3

H2 H O C C CH2

n

O

DGEBA-DPSD epoxy prepolymers

Fig. 90  Structures of silicon-containing epoxides or silicon-containing prepolymers Source: Mercado et al.[194] © 2006 Elsevier.

H2NH2CH2C

H2NH2CH2C H2NH2CH2C

H2NH2CH2C

Si

O Si O Si

O

O Si

O O

Si

O O Si

CH2CH2NH2

O Si

O

O

CH2CH2NH2

O

CH2CH2NH2

Si CH2CH2NH2

Fig. 91  Structure of POSS-NH2 Source: Zhang et al.[196] © 2007 Sage.

transport of heat and mass to improve the flame retardancy and thermal stability of EP. An ethylene glycol diglycidyl ether-modified acrylpimaric acid (AP-EGDE) (Fig. 96) and poly(methylphenylsiloxane) modified AP-EGDE (AESE) (Scheme 43) were synthesized.[204] TGA results showed that the thermal stability of AESE serials was better than that of AP-EGDE due to the formation of a protective residue. The char residue of AESE increased at 700°C when silicon content

increases. The LOI results suggested that incorporation of silicon enhanced the flame retardancy, but the LOI value did not increase with the char yield content. Based on the earlier results, it was concluded that AESE could form a protective residue at high temperature or burning, which acted as thermal insulation and prevented gas evolution, and achieve an ultimate improvement on the thermal ­stability and FR. Cage-type OPS and ladder-type polyphenyl silsesquioxane (PPSQ) (Fig. 97) were used as FRs in EPs in the presence and absence of DOPO.[205] In the UL94 test, the flame-retarded EP with OPS showed a weak blowing-out effect, but the flame-retarded EP with PPSQ did not; further, the flame-retarded EP with DOPO/OPS showed a significant blowing-out effect, but the flame-retarded EP with DOPO/PPSQ did not. Observation of the chars suggested that OPS could assist the EP, especially the EP with DOPO, to form stronger and denser chars than PPSQ, although PPSQ with a ladder structure had higher thermal stability than that of cage-type OPS. It was also observed that the Si concentration in the interior chars from the EPs with PPSQ was higher than that in those from the EPs with OPS. It was supposed that, in the composites of EP or EP/DOPO,

1210

Fire Protection: Flame-Retardant Epoxy Resins in

H H2 C C

H2C

CH3

H2 H H2 O C C C O

O

n

O

O

C Modified epoxy

H2 H O C C CH2

C CH3

O

O

NH

O

Environmental–Fire Protection

Si OEt

OEt

EG

Si OEt

EtO

OEt NH

C

O

OEt

O

C

O

OEt NH

C

Si OEt

OEt

O

IPTS-EG

THF/H+

H2O

Si O

Si

Si

O

Si O

O

O

Si

O

Si

Si

O

O

O O

R

O

Si Si

Si

O

O

O Si

R

O

O Si O

SiH

Si

O Si

O

O

R

O OH

OH

O O

Si

HN

C

Si

O

O

O

O

C

O

HO

Scheme 41  Composite of IPTS-epoxy and IPTS-EG Source: Chiang and Hsu [197] © 2010 Springer.

slow charring of PPSQ could not match the intumescent and charring process of the EPs during ­combustion, but OPS can. EPs containing POSS N-phenylaminopropyl (Fig. 98) nanocomposites were prepared.[206] The results showed that the dispersion of the POSS in the resin was effective when concentrations greater than 5 wt% were added. The addition of POSS promoted an increase in the Tg of the cured EP. The addition of POSS led to a reduction in the reversible heat capacity values and reduced the enthalpy relaxation (nonreversible heat capacity) associated with the interconnected microstructure formed between the i­ mperfections of the EP and the POSS.

UV-curable EA modified with octamercaptopropyl POSS (OMP-POSS) (Scheme 44) was prepared via thiol-­ ene photopolymerization.[207] The result of thermal degradation of the cured films showed that OMP-POSS had a double effect on the thermal stability of EA due to its structure containing both the relatively weak mercapto moieties and the POSS core resistant to heat shock, and the derivation from the former (thioether linkages) leads to the early decomposition of the nanocomposites, whereas the latter provides well-protective effects on some moieties of EA like aromatic groups and on the carbonaceous char. The analysis for the residues of EA2 with 20 wt% OMP-POSS exhibited that POSS could well protect the carbonaceous

Fire Protection: Flame-Retardant Epoxy Resins in 1211

H H2 C C

H2C

O

O

O

H2 C

H H2 C C OH

O

CH3 C

n

O

H2 H C C

CH2

CH3

O

DGEBA

O

O

R R

O Si

OH Si O R

O O O Si R

Si

N OH

O

Si

O O

Si R

heating

O

N

O H2 H C C

R O

Environmental–Fire Protection

O

O

N CH2

OH

R

NPOSS

NH2

polymeric chain of EP

NH2

R R

O Si

OH Si

OH

O R

R

O O O Si R

Si

O O

CH3 R=

Si

O

C

Si

O

O R

R

CH3

CH3

Scheme 42  Reaction process and structure of EP/NPOSS composite Source: Wu et al.[198] © 2009 Elsevier.

O

O O

O

Si O O

TEMPS

Fig. 92  Structure of TEMPS Source: Chiang and Hsu [199] © 2010 Springer.

char from thermal-oxidative degradation at 600°C, but do not work at 800°C. An FR additive, polyhedral oligomeric octadiphenylsulfonyl silsesquioxane (ODPSS) (Fig. 99) was used to retard combustion of an EP of DGEBA with curing agent DDS.[208] A series of flame-retarded EPs were prepared with ODPSS and DOPO loaded. The EP loading with 2.5 wt% ODPSS/2.5 wt% DOPO showed a longer TTI, lower value of PHRR, and higher flammability rating than the EP loading with 5 wt% DOPO. The results indicated that the mixture of ODPSS and DOPO had a remarkable synergistic effect on retarding flame of the EP composites. Dimethylpolysiloxane (Fig. 100a) liquid was blended with DGEBA EP, including anhydride curing agent,

1212

Fire Protection: Flame-Retardant Epoxy Resins in

O O CH2CH2O C H

H2 C

C

O

OH2CH2C

C

Si O CH2CH2O

Environmental–Fire Protection

O

CH2

C H C

C H

CH2

C H

CH2

O

(a)

H2C

C

C H

Si O CH2CH2O

O

OH2CH2C

C O

O (b)

Fig. 93  Structures of (a) TAEPS and (b) DAEMPS Source: Cheng and Shi[200] © 2010 Elsevier. O

O

H2C

C H

H2C

H C

CH2

CH3 Si

N(H2C)3

CH3

CH2

H2C

CH3 O

Si

(CH2)3 N

CH3

H2C

C H

CH2

H C

CH2

(CH2)3

NH2

O

O (a)

(b)

Fig. 94  Structures of (a) TG-siloxane and (b) (POSS)-amine Source: Meenakshi et al.[201] © 2011 Springer. O

O

H2C

C H

H2C

H C

CH2 N

H2C

CH3 O

CH2

O

Si CH3

N H2C

C H

CH2

H C

CH2 O

O

Fig. 95  Structure of bis(p-aminophenoxy) dimethylsiloxane-based tetraglycidyl epoxy Source: Meenakshi et al.[202] © 2012 Elsevier.

to improve the tensile strength of the EP at 77 K without any increase in its coefficient of thermal expansion (CTE).[209] A bifunctional polymer, silicone-modified EP (SME) (Fig.  100b), was also added to the mixture as a compatibilizer. The results showed that the incorporation of SME could stabilize effectively spherical domains of the siloxane liquid, which was immiscible with the epoxy matrix (Scheme 45). It was found that even small amount of addition of the siloxane liquid (0.05 phr) coupled with SME (20 phr) enhanced the tensile strength at 77 K by 77.6% compared to that of the neat EP. Two series of fluorinated siloxane starlike copolymers, with different molecular weights of siloxane segments and

large fluorinated segments at the tail, were synthesized from dicarboxyl terminated poly(2,2,3,4,4,4-hexafluorobutyl acrylate) (CTHFA) and dihydroxypropyl-terminated poly(dimethyl siloxane) (PDMS).[210] Then, the copolymers reacted with 44′-diphenylmethane diisocyanate and were used as a surface modifier of cured DGEBA at different concentrations (0.1–0.5 wt% with respect to DGEBA) (Scheme 46). It was found that the fluorine and silicon atoms selectively migrated toward the outermost surface of modified DGEBA resin. Moreover, the surface of modified EPs showed good chemical stability by immersing in acidic and salt solutions. Many ridges and rough crack structures of the fracture surface of modified DGEBA

Fire Protection: Flame-Retardant Epoxy Resins in 1213

O COOCH2CHCH2OCH2CH2OCH2

Environmental–Fire Protection

OH

O COOCH2CHCH2OCH2CH2OCH2 OH

(AP-EGDE)

Fig. 96  Structure of AP-EGDE Source: Deng et al.[204] © 2012 American Chemical Society. CH3 H3C

O

Si

CH3 Si

O ml

Ph

CH3 O

Si n

CH3

ORO

O

Si Ph

O ml

R

ORO

CH3

Si

Si

O

ORO

n

Ph

CH3

PMPS

m2

Ph

Tetraisopropyl titanate 0.5 wt% as catalyst OH 120oC

CH3

O

OH

O

ORO m2 AESE copolymer

O COOCH2CHCH2OCH2CH2OCH2

R O COOCH2CHCH2OCH2CH2OCH2

Scheme 43  Preparation of siloxane-modified AP-EGDE EPs Source: Deng et al.[204] © 2012 American Chemical Society.

resin, indicating that the CTHFA-PDMS toughened the DGEBA network. A polymeric IFR, poly(4,4′-diamino diphenyl sulfone 2,6,7-trioxa-1-phosphabicyclo[2.2.2]octane-4-methanol

(PEPA)-substituted phosphoramide) (PSA) (Fig. 101), was synthesized through solution polycondensation.[211] The performance of PSA and its mixtures with APP in enhancing the flame retardancy and thermal stability of EPs was

1214

Fire Protection: Flame-Retardant Epoxy Resins in

O

Environmental–Fire Protection

Si

O

Si

Si

O

O

O

Si

O Si

Si

Si

Si

n

(PPSQ)

Fig. 97  Structure of PPSQ Source: Zhang et al.[205] © 2013 Elsevier. R Si O R

O

R

O

Si

R

O

Si

Si O

Si

O

R

Si

O O

O

O

Si R

R

R=

N H

O O

Si R

Fig. 98  Structure of POSS Source: Pistor et al.[206] © 2013 Elsevier.

evaluated. The results indicated that the incorporation of PSA and APP slightly increased the Tg of EP. The maximum LOI value of EP/PSA composites reached 32.0% and passed the UL94 V0 rating. Moreover, remarkable decreases in the THR, PHRR, and TSR were observed when PSA was incorporated into EP. The addition of PSA and APP greatly enhanced the residual char and apparently reduced the amount of pyrolysis products during combustion. The flame retardancy of PSA and APP was mainly exerted in the condensed-phase and could be attributed to the formation of residual char with aromatic structures bridged by P–O–C and P–O–P bonds. Nanocomposites (DGEBA/DG-PDMS±POSS) (Scheme 47) based on DGEBA combined with diglycidylether-terminated PDMS (DG-PDMS), reinforced with 10 wt% (mono/octa) epoxy POSS nanocages (MEP or OEP-POSS), were synthesized.[212] DG-PDMS and POSS compounds were covalently incorporated into DGEBA

resin via copolymerization of epoxy groups. MEP-POSSbased nanocomposite with heterogeneous dispersion of POSS aggregates exhibited lower Tg value and thermal stability in comparison with OEP-POSS nanocomposite, which exhibited a nanoscale dispersion of the POSS cages. The obtained Tg of OEP-POSS-based nanocomposite increases with 31°C in comparison with the unreinforced matrix. Among POSS-based nanocomposites, enhanced thermomechanical properties were obtained for those ­reinforced with OEP-POSS. An FR containing silicon and nitrogen (PSiN) (Fig. 102) was synthesized, and it was used together with DOPO to prepare an FR system for EPs.[213] The results showed that synergistic effects on the flame retardancy of EP composites existed between DOPO and PSiN. When 3% PSiN and 7% DOPO were incorporated, the LOI value of EP was found to be 34%, and the UL94 test was found to be class V0. It was indicated that the surface of the char

Fire Protection: Flame-Retardant Epoxy Resins in 1215

R'

O

O

R'

O

Si

Si

R'

O

Si O Si

O O

O

R' Si O

SH

Si

O

EB600

O Si

Photo-click polymerization

R'

O

Environmental–Fire Protection

R'

Si R'

OMP-POSS

Hybrid curing network O O

O

S

O

OH R=

O

CH3 O OH

CH3

nS n = 0, 1, 2, 3.......

SH = POSS

Scheme 44  Synthesis of hybrid curing network Source: Wang et al.[207] © 2013 Elsevier.

O

R Si

O R

Si R

R Si

O

O

Si

O

O

O O Si R

Si

O

O

O

R R=

Si

Si R

S

R

O

O

O

ODPSS

Fig. 99  Structure of ODPSS Source: Li and Yang[208] © 2014 Elsevier.

CH3

CH3 H3C

Si CH3

O

Si

CH3 Si

O n

CH3

CH3

CH3

(a) H H2 H2C C C O O

CH3 C

CH3

OH

H2 O C C H

H2 C

O

H2 C

(b)

Fig. 100  Structures of (a) dimethylpolysiloxane liquid and (b) silicone-modified EP Source: Yi et al.[209] © 2014 Elsevier.

OH C H

CH3 H2 C

O

Si CH3

n

OH

1216

Fire Protection: Flame-Retardant Epoxy Resins in

Continuous phase (Epoxy resin)

for EP/DOPO/PSiN system holded a more cohesive and denser char structure compared with the pure EP and EP/ DOPO systems. A series of flame-retarded EPs loaded with methyl MQ silicone resin and a hyperbranched polysiloxane (HPSi) (Fig. 103) acting as a compatibilizer were prepared.[214] MQ resin was composed of a polycondensation chain unit of four functional siloxanes (Q) and single functional siloxane chain (M). The results showed that HPSi significantly improved the compatibility of EP/MQ and the incorporation of MQ into EP improved the thermal stability. It was observed that the LOI values of EPs increased obviously (from 21% to 31%) with MQ loading, which passed the V0 rating of UL94. Specifically, its combustion residue at 700°C was 14.5 wt%, which exceeded the value of neat EP

Bifunctional polymer

Environmental–Fire Protection

dispersed phase

Epoxy group Siloxane Scheme 45  Schematic diagram for the stabilization Source: Yi et al.[209] © 2014 Elsevier.

O

C

O

H N

N

C

O

PDMS

O

O C CH

DGEBA O

C

H N

N

C

O

PDMS

O

C

H2 H C C

O C O

O

n

H

O H2C CF2

CTHFA-PDMA-NCO FHC

CF3 H H2 C C

H2C

O

H2 H H 2 O C C C O O

O O

C

CH3 C CH3

n

H2 H O C C

H N

O

O

H N

C

CH2

O

PDMS

O

O C CH

O

C

H N

H N

H H2 C C

O

H2 H2 O C C C O H

O

CH3 C CH3

PDMS

n

CH3 HO (CH2)3

Si CH3

O

Si CH3

Scheme 46  Synthesis route of CTHFA-PDMS graft-modified DGEBA Source: Yan et al.[210] © 2014 Elsevier.

Si n

CH3

O C O

O

FHC CF3

CH3 O

C

CF2

CH2 O

CH3

O

n

H2C

H2 H O C C

CTHFA-PDMS graft DGEBA

PDMS =

O

O

O H 2C

C

H2H C C

(CH2)3

OH

H

Fire Protection: Flame-Retardant Epoxy Resins in 1217

H2 C

O H N

P

OH

O

O

O

n

O

CH2

(PEPA)

H N

S

O

(PSA)

Environmental–Fire Protection

P

O

O

O O

O

P O

Fig. 101  Structure of PSA Source: Zhao et al.[211] © 2015 Elsevier.

CH3 CH2

CH

CH2

CH3

C

O

O

CH2

O

CH

CH2

n

OH

CH3

O

O

C

CH2

DGEBA

CH3

CH

CH2 O

CH3 CH2

CH

CH2

(CH2)3

O

Si

O

+9

S

DG-PD BA/ M GE

t%

D

8w

R R

O Si

O

Si

O Si

O R

O R

O Si

O

Si O

Si

CH2

O

CH O

CH3

+2

R

O

O Si

R=*

CH2

CH2

CH

CH3

O

R

O Si

R

CH2

O

DG-PDMS

CH2

CH O

O

Si

O

CH3 R=*

0w

O

DGEBA

Si

(CH2)3

CH3

R

O

CH2

CH2

CH O

R

OEP-POSS

/D G-

/ MS PD

OEP-POSS

DGEBA / D GPD

t%

t%

0w

(5 wt%)

O

Si

+1

+1

1Mel

n

(CH2)3

CH3

R

O

R O

Si

Si

P-POSS ME / S M

CH3

O

Si

R

MEP-POSS

Si

O

O

O

CH3

R

Si

O

%

R

O Si

CH3

Si

wt

CH2

O R O

Si

(CH2)3

CH3 O

Scheme 47  The synthesis route of DGEBA/DG-PDMS±POSS nanocomposites Source: Florea et al.[212] © 2015 Elsevier.

resin (5.08 wt%). Additionally, the morphology of the residue char showed a compact, smooth, and tight structure of EP composite systems. Zhang et al.[215] synthesized a polysiloxane-containing nitrogen (PSiN), and it was added to EP as an FR. The results showed that the flame retardancy and thermal stability of EP were improved with the addition of PSiN. When 4.5 mass% PSiN was incorporated, the

LOI value of EP was found to be 29%, which is higher than 21% of pure EP. As part of a program to develop fire-resistant exterior composite structures for future subsonic commercial and general aviation aircraft, fire-retardant EPs are under investigation. MPP (Fig. 104) was used in combination with other FRs such as metal hydroxides, metal

1218

Fire Protection: Flame-Retardant Epoxy Resins in

CH3 Si

O

Si

O

x

y CH3

CH3

Environmental–Fire Protection

cycle involving the reduction of Cu2+ -Cu+ -Cu0 by CO and the oxidation of Cu0-Cu+ -Cu2+ by O2. Nanosized aluminum diethylphosphinate (AlPi) was well dispersed into EP by the long-time ­ultrasonication technique to form FR EP-AlPi nanocomposites.[218] Their flame retardancy-enhanced properties indicated  that EP-AlPi composites with a relatively low addition amount  of AlPi (on the account of 8.7 wt% or phosphorus content of 2 wt%) can reach the UL94 V0 flammability rating as well as the LOI value over 37.2. The glass transition temperatures (Tg) were about 50°C higher than that of neat EP; Tg increased with increase in the content of AlPi. The EP nanocomposites obtained in this study were green functional polymers and would become FR potential candidates in electronic fields such as printed wiring boards with high performance. The halogen-free FR amine-terminated cyclophosphazene (ATCP) and functionalized rice husk ash (FRHA)-­ reinforced CE (2,2-bis(4-cyanatophenyl)propane)-based EP composite material were developed through the ring formation of cyanurate and imino carbonyl adduct formation.[219] These results showed that the ATCP/FRHA/ CE-EP composites could be used as electrical-resistant, hydrophobic, and improved fire-retardant materials. Hence, the ATCP/FRHA/CE-EP FR system paved the new possibility of high-performance nonhalogen FR ­materials for microelectronic and electronic applications. Water absorption analysis proved the improved hydrophobic nature of ATCP/FRHA/CE-EP composite due to the presence of inorganic segments. It was found that the presence of ATCP and FRHA in the CE-EP composite could improve their thermal stability, char yield, and Tg. The UL94 vertical burning test, LOI test, and cone calorimeter test confirmed the improved FR properties of ATCP/ FRHA/CE-EP composites. The presence of phosphorous,

CH2CH2CH2NHCH2CH2NH2

(PSiN)

Fig. 102  Structure of PSiN Source: Zhang et al.[213] © 2016 Springer.

phosphinates, and phosphates. The results showed that it had good ­thermal stability and a low impact on Tg. Under thermal stress, MEL decomposes and releases inert nitrogen gases that dilute oxygen and flammable gases. Phosphoric acid was also formed as a decomposition product and promotes the char formation on polymer surface.[216] FREP composites were prepared with different c­ ontents of Cu2O and microencapsulated APP (MAPP).[217] The LOI and UL94 vertical burning test results showed that the best mass ratio of MAPP to Cu2O was determined as 9:1, because EP/18% MAPP/2% Cu2O reached the highest LOI value of 35% and a UL94 rating of V0. Moreover, Cu2O had a better synergistic effect with MAPP than CuO, ZnO, SnO, Fe2O3, Ni2O3, and Co2O3 on improving the flame retardancy of EP. The addition of Cu2O could further decrease THR, HRR, TSP, COP, and increase the char formation amount and intumescent degree. The reasons for the improvement of flame retardancy and smoke suppression by MAPP/Cu2O were concluded to be as follows: (1) MAPP/Cu2O promoted the formation of intumescent compact char layer, which hindered the decomposition of EP and diffusion of gaseous products. (2) Cu2O played a role in oxidation of CO to CO2. The mechanism of conversion of CO to CO2 on a Cu2O surface was probably a redox CH3 HO

O

Si

H 3C

Si

R1

CH3

R1 O

Si

Si

H 3C

Si

R1

HO

HO

R1

Si

O

R1

O

O

Si

H3CO

O Si

Si O

OHH3C

Si

(HPSi)

Fig. 103  Structure of HPSi Source: Jia et al.[214] © 2016 Elsevier.

O O Si

Si

O Si

CH3 R1

OCH3

CH3

Si

OH

R1

HO

O

R1 H3CO

OH

Si

CH3

O

OCH3

CH3

O

O

R1

CH3

OCH3

OH

Si

O

OH

OH

O R1 =

H2 C O

H2 H2 H2 C C C

and silicon in these POSSs. DPP-POSS, DPOP-POSS, and DOPO-POSS have combined the gas- and condensed-phase FR activities. The gas-phase activity was caused by the release of phosphorus volatiles. The main FR activities of DPP-POSS, DPOP-POSS, and DOPOPOSS were the especially strong condensed-phase activity through charring and intumescence due to the synergistic between phosphorus and silicon in these three POSSs. Phosphorus-containing POSS with perfect T8 caged structures showed a high fire-retardant efficiency for EP in very low phosphorus element content and might work for a great variety of polymeric systems as a new class of FR. The proposed decomposition mechanism (Scheme 48) started with the dehydration of the hydroxyl groups, leaving allylic ether bonds behind. The release of acetone leads to the formation of phenol derivatives and biphenol A. Further decomposition of BPA yields phenol and methane. It should be noted that no change of pyrolytic gases species is observed with DPOP-POSS, DPP-POSS, or DOPO-POSS loading in the EP system. A series of silicon-containing epoxy/PEPA phosphate FRs (EPPSi) (Scheme 49) were synthesized by polyphosphoric acid (PPA), caged bicyclic phosphate PEPA, and different ratios of silicon-containing epoxy 1,1,3,3-tetramethyl-1,3-bis(3-(oxiran-2-ylmethoxy)propyl) disiloxane (TMSEP) to 1,4-butanediol diglycidyl ether (BDE).[221] Afterward, the transparent intumescent fire-­ resistant coatings were prepared by mixing EPPSi and MEL formaldehyde resin. It was found that fire-resistant coatings obtained the best fire protection when the ratio of TMESP/BDE was 20/100, while excessive TMSEP made the fire protection of coatings decrease sharply. The result indicated that a synergistic effect existed between

O HO

O

P

H

O− + NH

3

N

N N

H 2N

NH2

n

MPP

Fig. 104  Structure of MEL polyphosphate Source: Braun et al.[216] © 2007 Elsevier.

nitrogen, and silica in the EP composites could influence the FR properties according to their percentage concentrations. The results obtained from different studies indicated that ATCP- and FRHA-incorporated CE-blended EP (ATCP/FRHA/CE-EP) composites could be used as better flame-resistant materials in place of the conventional EP matrix for enhanced performance and improved longevity. Phosphorus-containing POSSs (P-POSSs) of perfect T8 caged structures were designed as a new class of FRs. Phosphorus-containing POSSs combine several advantages from phosphorus-based FR and nanostructure inorganic–organic hybrid materials.[220] The impacts of DPOP-POSS, DPP-POSS, and DOPO-POSS (Fig. 105) on the pyrolysis and fire behavior of an EP (DGEBA/DDS) were discussed. The main FR activities of DPOP-POSS, DPP-POSS, and DOPO-POSS were in the especially strong condensed-phase activity through charring and intumescence due to the synergistic between phosphorus

R R

Si O Si

R

Si

O

O

O

SiH

O R

R

O

O

Si O

O O

O

R=

Si O Si

R

P

P

CH2

CH2

CH2

CH2 DPOP-POSS

DPP-POSS

O Si R

O

P

O

CH2 CH2 DOPO-POSS

Fig. 105  Structures of DPP-POSS, DPOP-POSS, and DOPO-POSS Source: Qi et al.[220] © 2016 Elsevier.

Environmental–Fire Protection

Fire Protection: Flame-Retardant Epoxy Resins in 1219

1220

Fire Protection: Flame-Retardant Epoxy Resins in

Environmental–Fire Protection

HN O

O OH

O

HN

HO S

O

CH3 HO

OH

C

H2 O C

CH3

O

OH C CH2 H

NH2 O

CH3

OH H2 O C C CH2 H

C CH3

OH O CH4

CO2

ROH

H2O O

Scheme 48  Decomposition model of EP/POSS composites Source: Qi et al.[220] © 2016 Elsevier.

phosphorus and silicon, which improved the foam structure and compressive strength of the char layer significantly. The high concentration silicon on the surface played an important protecting role for the inner char residue and improved the fire protection of coatings. It was found that silicon enhanced the thermo-oxidation resistance of coatings efficiently. The intumescence ratios of coatings decreased gradually with the increase in silicon content. Because connected with the phosphate directly, glycerol groups could be decomposed to molten alkene materials, while n-butyl groups and propyl disiloxane groups would be only decomposed to solid char directly through combustion decomposition. The increase of solid char could suppress the expansion of char layer. Therefore, the increase of TMSEP structure in EPPSi resulted in the increase of solid char and the decrease of intumescence ratio. In addition, the interaction between phosphorus and silicon could

produce new linkages O=P–O–Si, which acted as bridges to form the three-dimensional net of Si–O–Si framework. A phosphorus-containing polyether (PAPEG) was ­synthesized by PEPA, phosphorus oxychloride and water soluble oligomer polyethylene glycol 200 (PEG200). TG results indicated that graded decomposition of the two different kinds of phosphates in PAPEG were helpful in improving the charring efficiency of phosphorus.[222] Shi et  al. Synthesized PEPA-containing polyether (PAPEG) flame retardants with high water solubility by using pentaerythritol phosphate, POCl3 and polyethylene glycol (PEG) with different molecular weight. The fire protection test found that the coatings with low molecular weight PEG presented good flame retardancy and supplying effective fire protection for the plywood boards. The SEM results showed that the fire protection of coatings and char layers were significantly improved when the molecular weight

Fire Protection: Flame-Retardant Epoxy Resins in 1221

O HO P OH O

O

O O H2 H2 H2 H2 H2 H 2 H 2 H2 H2C C C O C C C Si O Si C C C O C C CH2 H H TMSEP

CH2 50oC, 24 h

O

O

P

PPA

O PEPA

O

O P

O O H2 H 2 H 2 H 2 H2 H2 H2C C C O C C C C O C C CH2 H H BDE

O

O PEPA dihydrogen phosphate (PDHP)

OH H2 H2 O P O C C C O H O O

H

Environmental–Fire Protection

CH2OH

O

H2 C

OH

H2 C C H

OH n

denote:

CH2 H2 H2 H2 H 2 H2 H2 C C C Si O Si C C C O

O P O

O

H2 H2 H2 H 2 C C C C

EPPSi

Scheme 49  The synthetic route of EPPSi Source: Shi and Wang[221] © 2016 Elsevier.

of PEG was low. TGA and real-time FTIR results proved that blowing agent and carbon source degraded in the same temperature range, which resulted in the formation of i­ ntumescent char layer.[223] The organic/inorganic FRs containing phosphorus, nitrogen, and silicon (organic/inorganic FRs) (Figs.  106, 107) were prepared.[224] The organic/inorganic FRs were then incorporated into EPs (Fig. 108) at different phosphorus/nitrogen ratios. The results indicated that synergistic effects on the flame retardancy of EP composites existed between the DOPO-VTS and TGIC-KH. The char residues for EP/FRs composites increased, and the highest char residues were obtained in air atmosphere (3.8 wt%) when the DOPO-VTS/TGIC-KH is 4/1. The THR of EPs was also decreased when the DOPO-VTS/TGIC-KH is 4/1, which was in accordance with the highest LOI and UL94 results. The SEM, Fourier-Transform Infrared Spectroscopy (FTIR), X-ray Photoelectron Spectroscopy (XPS), and TG-FTIR results of pyrolysis products in both the condensed and gas-phases indicated that the strategy of organic/inorganic FRs combined the condensed and gas-phase FR strategies, such as the phosphorus–­ nitrogen synergism systems, the silicon-reinforced effects in the condensed-phase, and the DOPO FR systems in the gas-phase, resulting in significant improvements in the flame retardancy of EPs. It was found that the strategy of organic/inorganic FRs combines the condensed and gas-phase FR strategies such as the IFR technology,

O O

O

Si

H N HO O

N

C N

HO

C

NH O

C N

O

OH N H Si

O O

O

Si O

O O

TGIC-KH

Fig. 106  Structure of TGIC-KH Source: Qian et al.[224] © 2014 Elsevier.

the phosphorus–nitrogen synergism systems, the silicon-­ reinforced effects in the condensed-phase, and the DOPO FR systems in the gas-phase, resulting in significant improvements in the flame retardancy of EPs. The organic/

1222

Fire Protection: Flame-Retardant Epoxy Resins in

O

O

Environmental–Fire Protection

suppression and synergistic FR properties of FeP on IFREP composites were evaluated. The results showed that FeP effectively decreased the HRR, THR, SPR, TSP, and smoke factor of FR samples, and could greatly improve the structure of char residue. The material with FeP undergoes degradation (Scheme 50) in three characteristic temperature stages, which could be attributed to the catalyzing deamination by FeP, the reaction between PPA and FeP, and the rupture of the PPA chain, respectively. The volatilized products formed on thermal degradation of IFREP indicate that the volatilized products are CO2, H2O, CO, carboxylic acid, and aliphatic hydrocarbons according to the temperature of onset formation. Here, FeP is considered to be an effective smoke suppression agent and a good synergist in IFREP composites, which can greatly improve the structure of char residue. The pyrolysis and fire behavior of EP composites based on a polyhedral oligomeric silsesquioxane containing DOPO (DOPO-POSS) (Scheme 51) and DGEBA had been investigated.[226] The EP composites with DOPOPOSS (Scheme 52) were prepared through a curing agent,

Si

P

O O

O DOPO-VTS

Fig. 107  Structure of DOPO-VTS Source: Qian et al.[224] © 2014 Elsevier.

inorganic FRs may be probably used as halogen-free FRs in many polymer composites due to their light color, high char formation and excellent water resistance. Liu et al. reported the smoke suppression properties and synergistic FR effect of ferric phosphate (FeP) on FREP using APP and PER as IFRs.[225] Then, the smoke

Si H N HO O

O

N C

C N

N C

HO

OH N H

NH O

OH

Si

O O

H2Si O O

Fig. 108  Structure of modified EP Source: Qian et al.[224] © 2014 Elsevier.

O

Si

O

O Si O

P

O

P

O

Fire Protection: Flame-Retardant Epoxy Resins in 1223

Fe3+

P

O

ΔT

O

O

P

ONH4 P O

O

ONH4 P

O

ΔT

O

O

NH3

Fe3+

O

P

PER

Environmental–Fire Protection

O

O

O

O O HO

O

P

HO

OH

O

O H2O Fe3+ Thermal pyrolysis Char

cross-linking

H2 C O

CH3 O CH3

H C

H2 C

H2 C

H2 C

NH3

Intumescent char

H2 C

OH

Scheme 50  Possible reaction mechanisms of char formation during the combustion of IFREP system with FeP Source: Liu et al.[225] © 2015 Springer.

m-phenylenediamine (m-PDA). The morphologies of the EP composites observed indicated that DOPO-POSS disperses with nanoscale particles in the EP networks, which implied good compatibility between them. The thermal analysis indicated that the DOPO-POSS changes the decomposition pathways of the EP and increases its residue at high temperature; moreover, the release of phosphorus products in the gas-phase and the existence of Si–O and P–O structures in the residue. The incorporation of DOPO-POSS into EP improved the flame retardancy of EP composites. The results indicated that DOPO-POSS had significant interaction with EP networks in the condensed-phase. The increased char yield and the release of compounds containing phosphorus crucially improved the flame retardancy of EP composites. The fire behavior of EP composites indicated that DOPO-POSS could reduce the HRR of EP composite significantly and accelerate the formation of char layer. The silicone-, phosphorous-, and sulfur-­containing nanocoatings using DGEBA as a base material, ­tris(p-isocyanatophenyl) thio phosphate as a modifier and POSS-NH2 (polyhedraloligomeric silsesquioxane) (Scheme 53, 54) as a nanoreinforcement were prepared.[227]

The nanocoatings were cured by Aradur 140 (polyamidoimidazoline) and XY 54 (polyamidoamine) (Fig. 109) curatives. It was observed that the molecular structures of curing agents as well as the nanoreinforcing effect of POSS-NH2 significantly influenced their corrosion and fouling protection behavior. The results showed that the modified nanohybrid epoxy coatings might be used as ecofriendly coatings to increase the corrosion and fouling resistance required for adverse marine environmentally. A phosphorus-containing hyperbranched polysiloxane (PeHSi) (Fig. 110) with a great amount of phosphaphenanthrene and silanol groups was synthesized by hydrolysis of self-made phosphorus-containing triethoxysilane.[228] Based on this, PeHSi was used to develop a new high-­ performance FR CE resin (Scheme 55) with simultaneously improved integrated properties. A small addition of PeHSi (5 wt%) to CE increased the flame retardancy of CE resin, where the content of P element is only as low as about 1.8 wt%. More attractively, the incorporation of PeHSi to CE resin significantly improved the thermal stability and mechanical properties, completely overcoming the disadvantages of phosphorus FRs. In addition, the PeHSi/CE resins had obviously decreased the curing temperature and

1224

Fire Protection: Flame-Retardant Epoxy Resins in

R O R

Si

O

Si

Si

Environmental–Fire Protection

O

R Si

O

O Si

O

O

Si

Si

Si

R

O

OH

O

O

P

O

R O

Si

H2C

Si

R

DOPO-POSS

R

O

O

O

O

Si

R =

O

R

R

Si

O

Si

O

+

O

Si

O

Si

R

R

R

Si

O

O

O

O

R

R

R

Si

O

R

O

R

+

H2C

OH H2 H2 O C C C H

CH3

H H2 C C O

C CH3

O

R O R

O

Si

Si

O

+

Si

O

O O

O O

Si

R

Si

Si

O

O R

O

Si

R

R O

Si

O

R

O

O Si

CH3

CH2

Si

O

Si

O

R

Si

R

n

R

Si

O

O

O

O

R

R Si

H2 H O C C

C

O

R

R

Si

O

heating

140°C

O

CH3

O

O

Si

O R

O

R CH3 C CH3

O

R Si

H2 H C C

CH2

OH

Scheme 51  Reaction process between DOPO-POSS and DGEBA Source: Zhang et al.[226] © 2011 Elsevier.

improved dielectric properties. These outstanding integrated properties of PeHSi/CE resins showed that PeHSi is an effective and multifunctional FR for developing high-performance resins. Phosphorusor silicon-containing Bnz–EPs (Scheme  56) with different phosphorus or silicon ­contents were obtained from (2,5-dihydroxyphenyl) diphenyl phosphine oxide (Gly-P) or diglycidyloxy methylphenylsilane (Gly-Si) and Bnz of BPA (Bz-BA) (Fig. 111).[229] A retardation of the cross-linking reaction

with increasing amount of EP was observed, being the most significant for the silicon-­containing epoxy. Tg values of thermosets increased with the epoxy content for the phosphorus-­containing Bnz—epoxy, while an opposite trend was observed for the silicon containing Bnz–epoxy. ­Phosphorus-containing material with outstanding LOI values were obtained, even when the phosphorus content was low, with the corresponding good FR properties. An  FR effect could not be observed for the ­silicon-containing resins.

+

DOPO-POSS/DGEBA

A POSS-containing DOPO (DOPO-POSS) (Fig. 112) was used to flame retard DGEBA epoxy cured by two amides, namely, the aliphatic oligomeric PA650 and the aromatic DDS (Fig. 113).[230] The epoxy composites with DOPO-POSS showed different FR properties depending on the amide used. The results of UL94 tests showed that the DEGBA/DDS with DOPO-POSS exhibited a blowing-out effect through vigorous emission of pyrolytic gases, but the DEGBA/PA650 did not. Moreover, only 2.5 wt% DOPO-POSS imparted to the EP DEGBA/DDS a LOI value of 27.1% and a UL94 V1 rating. In contrast, 10 wt% DOPO-POSS in the DEGBA/PA650 resulted in a LOI value of 25.9% and a UL94 V1 rating. The DEGBA/ DDS with even as little as 2.5 wt% DOPO-POSS easily formed a compact char. However, the DEGBA/PA650 with DOPO-POSS did not char until 10 wt% DOPO-POSS had been added. DOPO-POSS performed better in accelerating charring in the DDS curing system compared with the PA650 curing system. It was postulated that, for the

DGEBA

H2N heating NH2

Scheme 52  Synthesis of DOPO-POSS/DGEBA Source: Zhang et al.[226] © 2011 Elsevier. S OCN

O

O

P

NCO

O

+ NCO H H2 C C

H2C

OH H2 H2 O C C C H

O

O

CH3 O

H2 H O C C

C

n

CH3

THF

O

60°C

O

O O C

O

C

O

S

H N

O

P

H N

O

n

C

O n

O

O

O

NH C

O

O C

n

O

Scheme 53  Synthesis of phosphorus-containing polyurethane EP Source: Kumar and Sasikumar[227] © 2010 Elsevier.

O

CH2

Environmental–Fire Protection

Fire Protection: Flame-Retardant Epoxy Resins in 1225

1226

Fire Protection: Flame-Retardant Epoxy Resins in

OH OH OH

EtO

OEt R EtO Si +

Aminopropyltriethoxysilane

Environmental–Fire Protection

Toluene 90°C/ 8 h CH2CH2CH2NH2

R EtO

R

R

EtO

Si

O

Si

O

HO

O

HO O Si R EtO

Si

HO

POSS

O Si

Si

O

R Si

R R

Si O

R

OEt

EtO

Si

Si R

O

OH O

Si

O

C N

O

H Polyamidoimidazoline (Aradur 140)

Fig. 109  Structures of Aradur 140 and XY 54 Source: Kumar and Sasikumar[227] © 2010 Elsevier.

OEt Si

R OEt

OH

represent high char yield during the process of combustion. Thus, the flame retardancy of nanocomposites was improved, especially for the HRR and TSP. Furthermore, the combination of CNT-PR with MEL dramatically ­promoted the LOI value and the level of UL94 rating. A branched phosphonate acrylate monomer (BPA) (Fig. 115) containing phosphorus and POSSs was successfully synthesized and then the well-characterized comonomer was covalently incorporated into EA.[232] The presence of phosphorus and silicon elements improved the flame retardancy and thermal properties of EA. The FR EA composites exhibited significantly improved LOI values, as high as 29%, while the PHRR and THR values of the ­composites were reduced by 50.8% and 29.8%, respectively. The thermal properties of MEA indicated that the presence of BPA improved the degradation of the EA matrix and produced additional char residues. The char structure of MEA revealed that the addition of BPA increased the ratios of graphitized carbon in residual chars, thus improving the thermal stability of MEA. It was found that the

H N

C

R

OH R OEt

H N

OH

Fig. 110  Structure of PeHSi Source: Ye et al.[228] © 2013 Elsevier.

DGEBA/DDS/DOPO-POSS composites, the cross-­linking and dense char layer were created easily, which was helpful for the accumulation of gaseous products during the combustion process. The aliphatic chain of the PA650 was easy to break down and produce combustible gases, so did not ­easily form a cross-linked structure in the condensed-phase until enough DOPO-POSS had been added. Therefore, the structures of amide curing agents were as important as the  presence of DOPO-POSS and could influence the formation of the cross-linking char layer and the rate of release of pyrolytic gases, which subsequently ­influences whether the blowing-out effect happens. Molybdenum-phenolic resin (Mo-PR) was grafted onto the surface of multiwalled CNTs (MWCNTs) to obtain modified MWCNTs (CNT-PR) (Fig. 114).[231] Compared to EP, EP/CNT-PR nanocomposites showed improvements in flame retardancy and mechanical properties. Results indicated that the grafted Mo-PR improved the dispersion of MWCNTs in EP and enhanced the interfacial interaction between CNT-PR and EP. The grafted Mo-PR could

N

Si

R

O

O

O Si

O Si O R

EtO

O

R

OH

O Si

OEt

HO R Si OEt O

Si R

R

O

R

O R R OH HO Si EtO Si O R R O Si Si O O HO Si O R Si R O OEt

(C2H5O)3SiCH2CH2CH2NH2

Scheme 54 Synthesis of amine-functionalized (POSS-NH2) Source: Kumar and Sasikumar[227] © 2010 Elsevier.

Si

O H

N

N

N N H

C O

n

H

C

N H

n

Polyamidiamine (XY 54)

Fire Protection: Flame-Retardant Epoxy Resins in 1227

N

O

C

R'

O

C

N

N

Si

HO

C

O

R'

O

C

O

Si

R'

N

C

O

R'

O

C

O

2N

Si

C

O

R'

O

C

N

O

N O

R'

N

+

N

O

P

O

P

O

N

NH O

C

O

R'

OH

R'

P

O CH3CH2O

O

Si

Transesterification reaction

N

C

O

R'

N

C O R'

O Si

OH O HO

P

O

Si O Condensation reaction

N

C O R'

O

Si

CH3 R'=

CH3

Scheme 55  Chemical reactions between P-HSi and CE Source: Ye et al.[228] © 2013 Elsevier.

incorporation of FR accelerated the degradation of the EA matrix, reduced the flammable and toxic gases release, and promoted char formation. Moreover, the formation of silicon dioxide originating from the degradation of BPA on the surface of the residual chars reinforced the char layer, thus

enhancing thermal oxidative resistance. The improved flame retardancy of EA was responsible for a condensed-phase FR mechanism, including the catalyzing charring effect of phosphorus FRs and the char strengthening effect of silicon FRs, thereby forming a more effective protective barrier.

Environmental–Fire Protection

NH

1228

Fire Protection: Flame-Retardant Epoxy Resins in

O

O

N

N

Environmental–Fire Protection

O

OH N

O

R

R

O

R

O

OH

N

O

O

O

R

O

OH

N

OH

O

O

O

O

N

O O

O

OH

+

O O

N

N N

N O

OH Si

O P R=

Scheme 56  Synthesis of phosphorus- or silicon-containing Bnz–Eps Source: Sponton et al.[229] © 2009 Elsevier.

O

O

O

P N

N O

O O

O Bz-BA Bz BA Bz BzA

Gly-P P Gly-P

Si O O Si O

O

O O

O

O DG BA DGE BA DGEBA

Fig. 111  Structures of Bz-BA, Gly-P, DGEBA, and Gly-Si Source: Sponton et al.[229] © 2009 Elsevier.

Gly-S -Si Sii S Gly-Si

O

Fire Protection: Flame-Retardant Epoxy Resins in 1229

O R Si

O

Si

O

Si

O R

O

O R

R

R

Si

O R Si

O O Si

O

Si R O

Si

O

O

Si

R O

O

O R Si

O

Si

O R

R

Si

Si

O

Si

OH

Si

R

R

O P

R O O

Si

O

R=

O

O

Environmental–Fire Protection

Si

O

R

O

R

H2C

R

Fig. 112  Structure of DOPO-POSS Source: Zhang et al.[230] © 2012 Elsevier.

O H2N

R

NH2

S O

O

H

H

C

N

(CHCH2NH)2 CH2CH2N

H

R = dimer of eleostearic acid

DDS

PA650

Fig. 113  Structures of DDS and 650 (PA650) Source: Zhang et al.[230] © 2012 Elsevier. OH

OH

O O

H2 C

O

Mo

H2 C

O

n

Mo-PR O C

O

OH

O H2 C

O

Mo

O

O CNT-PR

H2 C m

n

reprents multiwalled carbon nanotubes (MWCNTs)

Fig. 114  Structure of MWCNTs Source: Yu et al.[231] © 2011 Elsevier.

Borate ester containing the phosphaphenanthrene group with N→B coordination structure (PBN) was synthesized and used as a curing agent and FR for EPs.[233] The cured epoxy thermosets with a 100:20 mass ratio of E51 to PBN passed the UL94 V0 rating with a LOI of 34.3%. DSC

results showed that PBN presented no curing activity on EPs at 140°C, whereas the reactivity was released to some degree at 170°C. Cured EP with 1.43% phosphorus achieved a V0 rating in the UL94 test and a LOI value of 34.3%. A higher char yield and more consolidated char

1230

Fire Protection: Flame-Retardant Epoxy Resins in

O O O

O P

Environmental–Fire Protection

O R'

R'

O

O

R'

O

Si

Si

R' O O

O

O Si

Si O

R' Si O

O

O Si

O

O O

BPA R'

Si R'

O

O

S Si

O

O R=

S

O

O O

O

O

P

O

O O O

Fig. 115  Structure of phosphonate acrylate monomer (BPA) Source: Yu et al.[232] © 2015 RSC Advances.

layer were responsible for the excellent flame retardance of cured EPs. The doping of PBN effectively enhanced the storage modulus, flexural strength, tensile strength, and impact strength of the cured EPs. The effect of layered silicate nanoclays, nanosilica, and double-walled CNTs (DWNTs) on the thermal stability and flame reaction properties of two aerospace-grade EPs was investigated. For this purpose, tetrafunctional EP, tetraglycidyl-4,4′-diaminodiphenylmethane (MY721) with DDS used as a curing agent and a bifunctional EP, 1,4-butanediol diglycidylether (LY5052) with a modified cycloaliphatic amine-based hardener (HY5052)) were used.[234] The addition of nanoclays (5 wt%) to both resins had a thermal destabilization effect in the low-temperature regime (1 and positive charge at amino/­sulfate ratio 10), moderate (0–1), good (1–0.1), and excellent (1,000), moderate (1,000–100), good (100–10), and excellent (50% and 90%). And this makes SPI a comparatively better film former. SPI films are smooth and flexible compared to the films prepared from other plant proteins. The gas barrier properties of SPI films are way less than other synthetic and natural polymeric films. However, due to their high hydrophilicity, the WVTR values are very less and the presence of humidity leads to decrease in the mechanical strength of the films. Blending is considered to be the most common method to improve the properties of SPI films. The SPI films have been blended with other biopolymers such as propylene glycol alginate, chitosan, PVA, PLA, polyurethane, polycaprolactone, and gelatin. Plasticizers work to decrease the hydrogen bonding and increase the interchain spacing in the protein structure leading to enhanced flexibility. The plasticizers should be compatible with the structure of the base polymer. Glycerol is hydrophilic and is compatible with SPI and hence considered to be the best plasticizer used with it. Apart from plasticizers, cross-linkers such as calcuim, formaldehyde, glutaraldehyde, and glyoxal, and surfactants such as sodium dodecyl sulfate, and several fatty acids have also been used for incorporation in SPI films to improve its properties.[22] Whey Protein Whey is the by-product of the dairy food industry and is considered as an environmental pollutant due to the lack of appropriate treatment and disposal methodologies. It is generally composed of around 93% water, and the rest 7% is made up of majorly lactose and small amounts of ­soluble proteins and lipids. The whey protein is obtained in three forms from the dairy industry: whey protein concentrate (WPC), whey protein isolate (WPI), and whey protein hydrolysate (WPH). Like SPI, the isolate form of whey also consists of very high amount of proteins (>90%) and is the generally used form that finds its application in food packaging. The WPI possesses functional properties such as water solubility, digestibility, gellability, and emulsification properties. However, it is hydrophilic and shows high moisture sorption and WVRT values. To overcome this, the WPI films have been incorporated with several hydrophobic agents such as lipids and essential plant oils. The plant essential oils, in addition to providing water resistance and water barrier properties, also imparted antimicrobial and antioxidant properties to the films which helped in ­enhancing the shelf life of the stored food ­product.[28,78] Aliphatic Polyesters Polylactic Acid Lactic acid, which is commonly known as 2-hydroxypropionic acid, is the most widely occurring hydroxycarboxylic acid. This naturally occurring organic acid is

most commonly obtained by fermentation but can also be produced by synthetic routes. The chemical route for its synthesis generally involves the hydrolysis of lactonitrile by strong acids which results in racemic mixture of the l- and d-forms of the lactic acid monomers. However, the biotechnological production provides with several advantages such as low-cost substrates and low energy consumption. This route involves lactic acid bacteria or lactobacillus and some filamentous fungi which convert carbon sources such as pure sugars (lactose, sucrose, ­glucose, etc.) or sugar-containing materials (molasses, whey, sugarcane bagasse, cassava bagasse, potato and tapioca starch, etc.) into lactic acid monomer.[24] PLA can be produced synthetically by the polymerization of lactic acid monomers by two methods: (a) direct polycondensation of lactic acid resulting in low-molecular-weight PLA and (b) ring-opening polymerization of lactic acid resulting in high-molecular-weight PLA.[79] In food packaging industry, PLA has made its mark in the past few years due to its excellent properties along with ­biodegradability. It possesses astounding tensile properties, compared to other biodegradable counterparts, which depends on its degree of crystallinity.[80] The hardness of PLA is comparable to that of acrylic plastics, and it is insoluble in water. It also shows good oil ­resistance, flavor and odor barrier, and sealing properties.[81,82] Polyhydroxybutyrate PHAs have attracted attention in the field of food packaging for the last few years as biodegradable thermoplastics. PHB is one of the most common and most widely studied PHAs having properties comparable to that of synthetically produced polyesters such as PLA. PHB is a naturally occurring β-hydroxyacid or linear polyester.[83] A wide variety of bacteria produce PHB as an intracellular material, in the form of granules, which serve as energy storage material. Under aerobic conditions, they can be completely degraded to water and carbon dioxide, whereas under anaerobic circumstances, they get converted to methane. The main constrain for the scale-up and commercialization of PHB is its high production cost compared to the conventional petroleum-based plastics. The most commonly studied and used bacteria for PHB production is Ralstonia eutropha.[84] It is capable of accumulating a large amount of PHB by using simple carbon sources. However, other bacteria such as Haloferax mediterranei, Halomonas boliviensis, and Bacillus megaterium have also been studied for this purpose.[85–87] Although the mechanical and physical properties of PHB are similar to those of synthetic thermoplastics, it has a narrow processing window and becomes brittle in unmodified form. It has a melting temperature very near to its degradation temperature which leads to its instability in the melt.[88] However, it has been used in food packaging industry for disposables and small packaging applications.[89]

Food Packaging– Composites

Food Packaging: Natural and Synthetic Biopolymers 1337

1338

COMMERCIALLY USED BIOPOLYMERS IN FOOD PACKAGING Although a lot of biopolymers have been described in the previous sections and many more have been put into research everyday to apply them in the food packaging industry, a few of them have been commercialized so far. The most important ones include the following:[56] •

Food Packaging– Composites

•

•

•

•

Chitin and chitosan a. Primex (Siglufjordur, Iceland) commercializes ChitoClear®—chitosan with potential food ­packaging application. b. Norwegian Chitosan (Kløfta, Norway) manufactures NorLife and Kitoflokk™—chitin and ­chitosan, respectively, for use in food and beverages. c. G.T.C. Bio Corporation (Qingdao, China) produces chitin and chitosan of different grades for several applications. Starch a. Novamont (Novara, Italy) commercializes Mater-Bi®, a biodegradable and compostable and commonly processable bioplastic in granular form. b. Eco-Go (Bangkok, Thailand) produces packaging products such as trays and containers made from cassava and corn starches. c. Plantic Technologies Limited (Altona, Australia) manufactures PLANTIC™, a corn starch-PE-PP multilayer food packaging sheet with high barrier properties. Cellulose a. Innovia Films (Wington, UK) manufactures ­CellophaneTM and NatureFlexTM for food packaging applications, in the form of bags, tapes, and box overwrap. b. Weifang Henglian Films Co. Ltd. (Weifang, China) produces food grade cellulose films for specific products. Alginate FMC (Philadelphia, PA, USA), Cargill (Minneapolis, MN, USA), and DuPont (Danisco) (Copenhagen, Denmark) are some of the leading producers of alginate in different forms for food packaging applications. The price of alginate inflated due to high demand in 2009–2013 but stabilized in 2014 at 11Є/kg. Pullulan a. Hayashibara Biochemical Labs (Okayama, Japan) is the first commercial manufacturer of pullulan since 1976. It granted a license to Pfizer Inc. to produce pullulan worldwide which has introduced the film form oral care product by the brand name Listerine.[90] b. Shandong Jinmei Biotechnology Co. Ltd. (Zhucheng, China) manufactures “Jinmei Pullulan” in the form of powder, capsules, or ­edible oral dissolving films.

Food Packaging: Natural and Synthetic Biopolymers

•

•

Gellan gum a. CP Kelco (Atlanta, GA, USA) commercializes low acyl and high acyl gellan gums by trade names GelriteTM and KelcogelTM, respectively. b. Dancheng Caixin Sugar Industry co. Ltd. (Zhoukou, China) is also a producer and worldwide seller of high and low acyl gellan. Xanthan gum The major producers include CP Kelco (Atlanta, GA, USA), Danisco (Copenhagen, Denmark), Merck (Kenilworth, NJ, USA), Sanofi-Elf (Gentilly, France), and Jungbunzlauer (Basel, Switzerland) that commercialize xanthan with different purity grades and trade names.

FUTURE ASPECTS The food packaging scenario has witnessed a tremendous development in the past few decades. With the increase in the market of packed food products, it has always been a necessity to develop a packaging material which increase the shelf life of the products and decrease the overall cost to consumer while being environment-friendly and biodegradable. To make this possible, a lot of research has been focused on biopolymers and exploiting their potential in this area. Although many natural polymers have already been applied in commercial food packaging, they possess several drawbacks: whether it is their not-so-good properties unlike petrochemical-based plastic, or their high cost of production. This gives an opportunity for the researchers and makes it necessary for them to focus on decreasing the overall cost of the biodegradable packaging material while ameliorating their properties. Recent technological innovation in the area of packaging materials (synthetic and natural) is also thriving to enhance packaging functionality via two methods: active packaging and intelligent packaging. Active packaging is defined as “packaging in which subsidiary constituents have been deliberately included in or on either the packaging material or the package headspace to enhance the performance of the package system,” while intelligent packaging can communicate the packaged content’s status or other messaging. It involves a combination of ­specialized materials, science, and technology.[91–94] SUMMARY To summarize, from the prehistoric times, the need to store food material for future use existed and the practice of packaging used to depend upon the type of material available. It varied from tree leaves and barks to seashells. With time and development, materials such as earthen pots, glass, paper, and metals were developed and used in different forms and shapes. With the advancements in

plastic technologies, it became very convenient to enhance the shelf life and portability of the food while maintaining its quality. However, this led to increased problem of municipal waste disposal which opened up an opportunity for the use of biopolymers for this purpose. Although a lot of research has been performed on biopolymers for food packaging, they still lag in some aspect or the other and could not compete with the petrochemical-based synthetic plastics. The research focused on improving the properties of the biopolymers by various methods such as chemical modifications, addition of fillers, cross-linkers, plasticizers, and natural oils to the base polymer, blending two biopolymers or a biopolymer with a synthetic one, and many more. Many of the biopolymers have been commercialized in this process, but still this research area is promising for the researchers and there is very much left unexplored.

ACKNOWLEDGMENTS The authors acknowledge the financial funding provided by the Ministry of Human Resource and Development (MHRD), Government of India.

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28.

29.

30.

31.

Food Packaging– Composites

32.

33.

34.

35.

36.

37.

38.

39.

40. 41.

42.

43.

Food Packaging: Natural and Synthetic Biopolymers

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Food Packaging: Polyhydroxyalcanoates (PHA) Containing Antimicrobial Additives C. R. Rech Faculty of Exact Sciences and Technology, Federal University of Grande Dourados, Dourados, Brazil

K. C. S. Brabes and S. M. Martelli Faculty of Engineering, Federal University of Grande Dourados, Dourados, Brazil

Food Packaging– Composites

Abstract Recent research has focused on the development of biodegradable products. Polyhydroxyalkanoates (PHAs), in particular, have received special attention. PHAs are biopolymers synthesized by some bacteria as a carbon source when they are exposed to a stress environment. PHAs are thermoplastic, biodegradable, and biocompatible, synthesized from renewable resources, and can substitute conventional plastics in some applications. In addition, the increased consumer demand for ­environmental friendly packaging has also gained prominence, and several studies have focused on the incorporation of antimicrobial agents into biodegradable products. This entry intends to present the technological advances regarding the development of biodegradable films of the PHA family, incorporated with antimicrobial additives and their application in foods. In addition, it addresses the main changes in the polymer properties as a function of the interaction with these agents. Keywords: Active packaging; Antimicrobial additives; Biodegradable polymers; Films; Polyhydroxyalkanoates.

INTRODUCTION The main purpose of packaging is to maintain food quality and safety during storage and transport, and to extend shelf life, [1] since exposure to pathogenic microorganisms during slaughter, processing, packaging, and transportation can lead to deterioration of food [2,3] and possible infections to consumers.[4] In addition, packaging must be inert, recyclable, exhibit good barrier properties, and serve the primary purposes such as containment, convenience, m ­ arketing, and communication.[5] Contamination of food can occur through physical, chemical, or biological factors. Physical contamination can occur during food handling and is characterized by the presence of foreign objects. Chemical contamination is due to the presence of chemical compounds, such as heavy metals, or toxins produced by microorganisms. Biological contamination is due to the presence of pathogenic microorganisms such as bacteria and fungi, and is the main ­factor of deterioration in food causing changes in the color, taste, texture, and appearance of food. The main sources of contamination by microorganisms are soil and water, plants, food-handling utensils, human and animal intestinal tract, food handlers, animal feed, animal skin, air, and powder.[6] Conventional polymers such as polypropylene (PP), polyethylene (PE), polyamide (PA), and polyethylene Encyclopedia of Polymer Applications, First Edition DOI: 10.1201/9781351019422-140000267 Copyright © 2018 by Taylor & Francis. All rights reserved.

terephthalate (PET) are widely used for food packaging because they have good mechanical and barrier ­properties[7] against external agents. However, packaging made from polymers from nonrenewable sources is ­considered an environmental problem as a function of the quantity discarded and degradation time.[8,9] To minimize these problems, alternatives have been studied, such as substitution by biodegradable plastics, which degrade in a short time under microbial action[10,11] or enzymes. However, the rate of biodegradation may vary depending on the molecular architecture of the polymer. Lignin, e.g., can take years to degrade, whereas proteins and polysaccharides degrade in hours or days.[12] These biopolymers can be classified according to their renewability and their level of biodegradability. In addition, they are classified into two main groups: (1) agropolymers, such as polysaccharides and proteins, are obtained by processes of biomass fragmentation; (2) polyhydroxyalkanoates (PHAs) are obtained by the synthesis of synthetic monomers such as poly-ε-caprolactone (PCL), and aromatic and aliphatic copolyesters may be obtained by the synthesis of monomers such as polylactic acid (PLA), poly(butylene adipate-co-terephthalate) (PBAT), and poly(butylene succinate adipate) (PBSA).[13] There is an increasing research on the development of biodegradable polymer materials from renewable sources since the availability of biopolymers is abundant in nature

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and some have a relatively low cost.[14] Biodegradable packaging consists of polymers that degrade in carbon dioxide, water, methane, and biomass.[15–17] Among the most studied biodegradable polymers is the family of PHAs, a class of biodegradable renewable polymers [18] synthesized by many microorganisms.[10,11] For this reason, a great deal of resources have been invested in the development of packaging that can overcome these problems, such as active packaging, consisting of a polymer matrix and an antimicrobial agent.[19] In this way, the combination of biodegradable packaging with antimicrobial additives is characterized by a promising line of research.

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Food Packaging– Composites

MAIN MICROORGANISMS OF FOOD CONTAMINATION The main microorganisms of interest in food, described by Jay, [6] are fungi (molds and yeasts), bacteria, and viruses. However, only those that cause some kind of deterioration in the food will be addressed.

•

Yeasts Yeasts of interest in food are as follows:[6] •

Fungi Molds •

Molds are formed by hyphae which together form the mycelium. They are mostly aerobic and thus grow on the surface of food in contact with the air. The main types of molds of interest in food are as follows:

• •

• •

• • •

• • • •

•

Alternaria: This can cause deterioration in tomatoes, peppers, apples, and citrus fruits, as well as red meats. Aspergillus: A. glaucus and A. repens species are the agents of food spoilage, and A. flavus and A. ­parasiticus are mycotoxin producers. Aureobasidium: It is the producer of black spots on shrimp and meat. Botrytis: B. cinerea species is responsible for gray rot in apples, pears, strawberries, and citrus fruits. Byssochlamys: B. fulva and B. nivea species can cause deterioration in canned juices, and canned fruit. B. fulva can produce mycotoxins. Cladosporium: C. herbarum and C. cladosporioides can cause changes in meat, butter, and margarine. Claviceps: C. purpurea produces toxic alkaloids in cereal grains. Colletotrichum: C. gloeosporioides species is responsible for anthracnose in fruits. Fusarium: Some species of this genus cause deterioration in citrus fruits, pineapples, and figs, in addition to presenting species producing mycotoxins. Geotrichum: G. candidum is important in dairy and canning equipment for tomatoes, and G. albidum is important in fruits.

Monilia: This genus causes deterioration in fruits. Mucor: This genus can be found in soil, dung, caves, vegetables, grains, etc. Neurospora: N. sitophila produces pink pigmentation and is common in breads. Penicillium: Several species of Penicillium are responsible for the degradation of fruits. P. cyclopium and P. viridicatum are cereal grain deteriorators, and other species are microtoxin producers. Rhizopus: This deteriorates the food of plant origin. R. stolonifer is a common mold in bread. Scopulariopsis: This genus can cause deterioration in dairy and meat. Sporotrichum: Some species of this genus multiply at low temperatures and can develop in meat stored in cold rooms. Thamnidium: T. elegans grow on refrigerated meats.

• • • • •

•

• •

•

Candida: Species of this genus are found in spoilage of fresh fruits, vegetables, dairy products, alcoholic beverages, and soft drinks, some of which are pathogenic to humans. Cryptococcus: Species of this genus are found in soil, plants and strawberries, sea fish, shrimp, raw beef, soft drinks, wines, and cereal grains. Debaryomyces: It forms films on the surface of salty foods or kept in brine. Hanseniaspora: Species of this genus can be found in figs, tomatoes, strawberries, citrus fruits, and wines. Issatchenkia: Species of this genus can be found in fruits, soft drinks, wines, and fish. Kluyveromyces: It causes deterioration in dairy, meat, and fruits. Pichia: Species of this genus are deteriorating agents of beers, wines, dairy products, and fruits. Rhodotorula: Species of this genus cause changes in the color of meat, dairy products, and fermented products. Saccharomyces: Despite having beneficial species used in food production processes, it presents undesirable species that cause changes in fruits, dairy products, mayonnaises, honey, vinegar, and fermented products. Schizosaccharomyces: It causes deterioration in fruits and wine; some species can grow in honey, bullets, and cane juice. Torulospora: T. delbrueckii deteriorates fruits, soft drinks, beers, breads, and cheeses. Trichosporon: Species of this genus can be found in fresh shrimp, ground beef, poultry meat, fruit juices, grains, and wines. Zygosaccharomyces: It causes deterioration in mayonnaise, salad dressings, fruit and fruit juices, and soft drinks.

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Bacteria

iv.

v. vi.

vii.

Virus Viruses of interest in food are responsible for viral diseases caused by water and food consumption, especially hepatitis A virus, poliomyelitis, rotavirus gastroenteritis, and Norwalk virus.[6] BIODEGRADABLE PLASTICS Biodegradable plastics are, by definition, polymers that are degraded by microbial action of bacteria, fungi, and algae, generating only CO2 and water at the end of the process.[15–17,20,21] Generally, they are divided into natural and synthetic, according to their origin. Natural polymers are formed during the growth cycle of living organisms, with an emphasis on polysaccharides, alginic acids, natural polypeptides, and bacterial polyesters.[17] Synthetic polymers offer advantages over natural polymers due to a wider range of applications and ability to adapt to mechanical properties and to change the rate of degradation according to need. On the other hand, natural polymers appear to be attractive due to their excellent biocompatibility.[22]

Food Packaging– Composites

Bacteria of interest in food are grouped into seven categories:[6] i. Gram-negative, aerobic, and microaerobic In this group, the genus Campylobacter stands out, and the most important species are C. jejuni, C. coli, and C. lari, pathogens that cause foodborne gastroenteritis. ii. Strict aerobic gram-negative bacteria —— Pseudomonas: P. aeruginosa produces toxic substances and are opportunistic human pathogens. —— Xanthomonas: Many species are pathogenic to plants and responsible for citrus canker. —— Halobacteriaceae: They are found in salty foods. They also exhibit limosity and produce in food, a red-colored superficial slime with a strong odor. —— Acetobacter: It is responsible for the deterioration in fruit juices and alcoholic beverages. —— Gluconobacter: G. oxydans causes deterioration in vegetables, fruits, yeasts, beer, wine, cider, and vinegar. —— Acinetobacter: These species are deteriorating agents of raw and processed foods, and meat and poultry carcasses. —— Alcaligenes: It causes deterioration in protein foods such as raw milk, meats, eggs, and dairy products. —— Alteromonas: It causes deterioration in fish. —— Brucella: B. abortus is a pathogenic bacteria for cattle, and B. suis is a pathogenic bacteria for swine. Its species can cause brucellosis in humans. —— Flavobacterium: It causes deterioration in fresh and frozen vegetables, fish, poultry, meat, and derivatives. —— Moraxella: It causes deterioration in fish and seafood. —— Psychrobacter: It has a single species, P. ­immobilis, common in meat, poultry, and fish. —— Shewanella: It causes deterioration in fish and seafood. iii. Facultative anaerobic gram-negative bacteria —— Citrobacter and Enterobacter: They belong to the group of coliforms and can cause food spoilage. —— Erwinia: It is responsible for the cause of plant diseases. —— Escherichia: E. coli is the main species that belongs to the group of fecal coliforms and may cause undesirable reactions in food. —— Hafnia and Serratia: They cause deterioration in chilled meats and vegetable.

Klebsiella: It belongs to the group of coliforms, causing undesirable reactions in food. —— Proteus: It causes deterioration in various foods. —— Salmonella, Shigella, and Yersinia: These genera are responsible for causing serious diseases. —— Aeromonas: It can cause deterioration in milk and derivatives, and darkening of egg yolk. —— Plesiomonas: It is isolated in water, fish, crabs, and raw oysters. —— Vibrio: V. costicola is isolated from cured meat and brine. V. cholarae, V. vulnificus, and V. ­parahaemolyticus species are pathogens. Gram-positive cocci In this group, Micrococcus and Leuconostoc are the important deteriorating agents in food. Staphylococcus produce enterotoxins in foods, and ­Enterococcus are indicative of fecal contamination. Gram-positive spore-producing bacilli Bacillus and Clostridium are responsible for food spoilage. Gram-positive non-sporulated bacilli —— Brochothrix: It causes deterioration in raw and ­processed meats (packaged and refrigerated). —— Carnobacterium: It is found in vacuum-packed meats and by-products, fish, and poultry. —— Lactobacillus: Species of this genus are not ­pathogenic but can cause food spoilage. —— Listeria: L. monocytogenes is the most important species in food due to its pathogenicity. Other: such Arthrobacter, Brevibacterium, Corynebacterium, Coxiella, Mycobacterium e Propionibacterium. ——

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Food Packaging: Polyhydroxyalcanoates (PHA) Containing Antimicrobial Additives

The biodegradable polymers can be further classified, according to Fig. 1, into four categories: (1) agro-polymers, obtained from agro-products (e.g., starch or cellulose); (2) polymers produced by microorganisms, such as PHAs; (3) polymers conventionally and chemically synthesized from monomers derived from biotechnological processes, such as PLA; and (4) polymers obtained from fossil resources. Only the first three categories (1–3) are obtained from renewable resources.[23,24] Polyhydroxyalkanoates: Classification, Chemical Structures, and Properties Food Packaging– Composites

Polyhydroxyalkanoates (PHAs) (Fig. 2)[25] are a class of polyesters, synthesized by many microorganisms as an intracellular carbon pool and energy, [10,16,26] from a range of substrates such as sugars and fatty acids.[27] Microorganisms capable of producing PHAs are generally bacteria.[28] In addition, they are divided into two groups: Those that require limitation of some nutrients, especially Ralstonia eutropha and Pseudomonas oleovorans, and those that do not require nutrient limitation and accumulate the polymer in the growth phase, such as recombinant E. coli and Alcaligenes latus.[29,30]

Since its discovery in 1926 by Lemoigne, [31] about 300 microorganisms have been identified as producers of PHAs, [32] including A. latus, Bacillus megaterium, Cupriavidus necator, and P. oleovorans.[33] The species R. eutropha (currently C. necator) (Fig. 3)[34] has been used industrially for presenting high rates of polymer production and, thus, obtaining higher yield.[10] The first report of PHAs was extracted from the bacterium B. megaterium and characterized the polyhydroxybutyrate (P(3HB)).[35] The production of this polyester can occur in a closed production cycle in which the carbon source metabolized by the microorganism produces a polymer biodegradable to carbon dioxide and water. Then, these elements will be absorbed by plants that will supply the carbon source required to produce PHAs, closing the production cycle. [36] In addition to these properties, this polymer is biodegradable and stands out as an alternative to chemically synthesized plastics, since it can be produced by microbial fermentation in the presence of a carbon source.[26] Several sources of carbon may be used for PHA synthesis, such as household wastewater, [37] food industry residues, [38] rice starch supplemented with soybean oil, [36] fatty residues, [39] and sunflower oil saponified.[40]

Starches Polysaccharides

Agro-polymers

Others

Biomass products

Proteins

Biodegradable polymers

Animals proteins Plants proteins

From microorganisms

Polyhydroxyalcanoates (PHA)

PHB, P4HB, PHBV, etc.

From biotechnology

Polylactides

Polylactic acid (PLA)

Biopolyesters

Polycaprolactones (PCL)

From petrochemical products

Other homopolyesters Aliphatic copolyesters Aromatic copolyesters

Fig. 1  Classification of biodegradable polymers

Ligno-cellulose products

Food Packaging: Polyhydroxyalcanoates (PHA) Containing Antimicrobial Additives 1347

O

R1

O

CH

C (CH2)x

O

R2

C

CH O

(CH2)x

O

n

Fig. 3  Transmission electron microscopy images of R. ­eutropha cell-containing PHB granules [34]

PHAs are classified into two groups, according to the number of carbon atoms that constitute their monomers: short-chain length (SCL) and medium-chain length (MCL) PHAs. The former comprises the majority of PHAs and is characterized by monomers having 3–5 atoms, and the latter contains monomers having 6–14 carbon atoms (Fig. 4).[25,41,42] In the figure, x ranges from 600 to 35,000; when n = 1, R = hydrogen: poly(3-­ hydroxypropionate), R = methyl: poly(3-hydroxybutyrate), R =  ethyl: poly(3-hydroxyvalerate), R  = propyl: poly(3-­ hydroxyhexanoate), R = pentyl: poly(3-hydroxyocatoate), and R = nonyl: poly(3-hydroxydodecanoate); when n = 2, R = hydrogen: poly(4-hydroxybutyrate) (P4HB); and when n = 3, R = hydrogen: poly(5-hydroxyvalerate).[43] The difference is in the substrate specificity in which the PHA synthase from A. eutrophus can polymerize 3HAs consisting of 3–5 carbon atoms, whereas the PHA synthase present in P. oleovorans can only accept 3HAs of 6–14 carbon atoms.[30] The copolymers scl-PHA present a better potential for application in food packaging due to its high plasticity and accessibility for melt extrusion, injection molding, or ­thermoforming.[44] The physical and chemical characteristics of the PHAs vary considerably, depending on the composition of the monomer.[45]

Among PHAs, the most widespread and currently studied is P(3HB) which is a renewable, biocompatible, and linear thermoplastic. It presents low permeability to oxygen and water and attractive barrier properties in relation to other polyesters.[46,47] However, P(3HB) is highly crystalline and brittle, [48,49] and its high cost of production prevents it from competing with conventional polymers.[49] In addition, it has low mechanical properties, such as Young’s modulus and tensile strength, and good thermoplastic properties.[50] The degree of crystallinity of P(3HB) lies in the range of 55%–80%, with a glass transition temperature (Tg) of ~5°C, a melting temperature (Tm) of 175°C, and a decrease in molar mass at a temperature above 170°C.[51] P4HB is a homopolymer of 4HB, which has its main application in the biomedical area in sutures, heart valves, or supports of cardiovascular tissue, and has a tensile strength comparable to PE.[52,53] It is produced by recombinant E. coli K12 fermentation and is characterized as a flexible, semicrystalline thermoplastic with a melting point of 60°C.[54] In addition, the degradation products of P4HB are less inflammatory, making it suitable for application in the medical field.[55] However, its application in food ­packaging has not yet been reported.[44] Poly(3-hyd roxybutyrate-co -3-hyd roxyvalerate) (PHBV) is also one of the most studied and produced copolymers, and presents the mechanical properties superior to P(3HB).[10] The formation of copolymers improves the properties of P(3HB) such as crystallinity, melting point, stiffness, and hardness. PHBV exhibits lower crystallinity, melt temperature and stiffness, and increased elongation at break, and is more flexible and resistant than P(3HB).[55] ACTIVE PACKAGING The packaging is used to protect the food from changes or contamination by physical, chemical, and biological agents, i.e., maintaining the quality of the food after processing and keeping it healthy for consumption even after transportation.[56] Active packaging is an innovation in relation to this concept because it is based on the incorporation of additives in the packages that release or absorb substances from the food or the external environment, in order to prolong the useful life and maintain quality, safety, and characteristics and sensory aspects of food.[57] In addition, they can be used to increase shelf life, minimize

Food Packaging– Composites

Fig. 2  General structure of PHA[25]

1348

Food Packaging: Polyhydroxyalcanoates (PHA) Containing Antimicrobial Additives

R O

O

CH

CH2

C

x

3-hydroxypropionate 3-hydroxybutyrate 3-hydroxyvalerate 3-hydroxycaproate 3-hydroxyheptanoate 3-hydroxyoctanoate 3-hydroxynonanoate 3-hydroxydecanoate 3-hydroxyundecanoate 3-hydroxydodecanoate

R = hydrogen R = methyl R = ethyl R = propyl R = butyl R = pentyl R = hexyl R = heptyl R = octyl R = nonyl

(3HP) (3HB) (3HV) (3HC) (3HH) (3HO) (3HN) (3HD) (3HUD) (3HDD)

Food Packaging– Composites

O O n=3 n=4

CH2

C n

x

4-hydroxybutyrate 5-hydroxyvalerate

(4HB) (5HV)

Fig. 4  Chemical structure of PHAs produced by bacteria (see text for details)[41]

microbiological deterioration, and inhibit the growth of pathogenic microorganisms, increasing food quality and safety.[5,58] This type of packaging plays a dynamic role in food preservation and allows packages to “interact” with food and the environment as they allow for the regulation of various factors that may influence food’s shelf life, such as physiological (e.g., respiration [fresh fruits and vegetables], chemical [lipid oxidation], physical [dehydration], and microbiological factors).[5] However, these packages must meet certain conditions for use by the industry: (1) The materials must be suitable and effective for the intended use; (2) materials and articles, including the active and intelligent, must be manufactured in accordance with good manufacturing practices; (3) the materials must display information on the use or permitted uses and other relevant information, such as the name and quantity of substances released by the active component; and (4) compulsory labeling with the words “not to eat” should be provided.[56] The types of active packaging described are as follows: oxygen scavengers, carbon dioxide-generating system, ethylene scavengers, flavor and odor absorber/releaser, ­antimicrobials, and antioxidants.[56,59] The presence of oxygen in packages allows the growth of several aerobic microorganisms and, consequently, the deterioration of the food. In addition, it impairs food as a function of enzyme-catalyzed reactions, lipid oxidation, and other oxidative reactions.[60] In relation to oxygenated packaging, it is a common practice to develop materials containing a dispersion of active substances to reduce the concentration of oxygen in the package. This

happens because these compounds react by entrapment or c­ onversion of oxygen to less reactive products.[61] Likewise, carbon dioxide (CO2) formed in some foods should be removed to prevent packaging collapse or food spoilage.[62] However, in certain foods, high levels of CO2 are required to inhibit microbial growth on surfaces and, in turn, prolong the shelf life of packaged foods.[56,63] The presence of ethylene accelerates the respiration of fruits and vegetables and, consequently, the senescence and shortening the useful life of these foods. Ethylene absorption systems include the use of ethylene-oxidizing materials, such as potassium permanganate (KMnO4), [64] commonly used in the form of sachets placed inside the package.[65] Another method is the addition of essences and odors, which can increase the demand for food, improve the aroma, or increase the taste of food when the package is opened. Release may be slow over the life of the product or controlled to occur when the package is opened.[66] The interaction of packaging with flavorings has long been recognized; however, commercially, few active packaging techniques have been used to selectively remove undesirable flavors and stains despite their potential.[67] The flavors may be added to compensate for degradation in storage, or a desirable flavor may be generated by an outer layer of a package rather than being released from an inner layer to compensate for scalping or processing losses.[68] A wide variety of agents can be added to act on food such as antioxidants and antimicrobials. Antimicrobial packaging consists of the use of active compounds to extend the shelf life of foods [69] by inhibiting microbial growth. Prasad and Kochhar[56] cite some examples of these compounds: ethanol, carbon dioxide, silver ions, chlorine dioxide, antibiotics, organic acids, essential oils, spices, etc. In addition, they can be in various forms: (1) addition of sachets/pads with volatile antimicrobial agents in packages, (2) incorporation of volatile and nonvolatile antimicrobial agents directly into polymers, (3) antimicrobial coating or adsorption on polymer surfaces, (4) immobilization of antimicrobial agents in polymers by ionic or covalent bonds, and (5) use of polymers that are antimicrobial.[70] Like microbial growth, lipid oxidation is also responsible for the deterioration in many foods. Therefore, an alternative is the addition of antioxidants to the package, the main advantage of which is to provide a prolonged release of antioxidants during food storage.[71] ANTIMICROBIAL ADDITIVES Antimicrobial packaging can be divided into two categories. The first is the direct incorporation of the additive, both in the polymer matrix and by the coating, and its effectiveness depends on the superficial contact with the food. In this case, it is important to contain the additive

Food Packaging: Polyhydroxyalcanoates (PHA) Containing Antimicrobial Additives 1349

where most deterioration occurs. When the antimicrobial is released from the package over time, the kinetics of microbial multiplication and the antimicrobial activity on the surface of the product can be balanced. Thus, the antimicrobial activity of the packaging can be extended, ­guaranteeing the safety during the food distribution.[81] ANTIMICROBIAL PHA FILMS FOR FOOD PACKAGING APPLICATIONS PHAs have been widely used as a raw material in various applications due to their biocompatibility, [18] with a special emphasis on cleaning, hygiene, cosmetics and food packaging, [10] molded products, paper coating, adhesives, films, etc. Biodegradable polymers are used in various types of food contact items, including disposable tableware, cups, salad bowls, plates, packaging and laminating films, lids, straws, and food containers distributed in delicatessens and fast-food restaurants.[82] In addition, new packaging systems using biodegradable polymers with the antimicrobial activity aroused their interest in studies using PHAs, as presented in Table 1. Vinhas et al.[83] evaluated the properties and biodegradability of P(3HB)/starch and PHBV/starch blends. Although the addition of starch decreases the mechanical properties of the films, their incorporation into the polymers allowed a significant increase of the biodegradability, possibly due to the ease of this carbohydrate being consumed as an alternative source of carbon by the fungi. They concluded that the blends of P(3HB)/starch and PHBV/starch are a good alternative for application in packages since the presence of starch offers an affordable cost to the market. Tannic acid is a natural additive and can be extracted from plants and vegetables. The use of tannic acid in P(3HB) increased the thermal stability of the polymer, making its applicability more attractive.[84] Plasmas of PLA, PCL, and P(3HB) with quaternary ammonium were developed and evaluated for its antimicrobial activity against E. coli, Salmonella typhimurium, and L. monocytogenes. It has been demonstrated that the effectiveness of the antibacterial activity of these polymers is due to the quaternary ammonium group; in all cases, no release of the active agents was observed, and this offers numerous advantages for antimicrobial applications where the release of active agents is prohibited.[85] PHBV materials coated with silver nanoparticles (AgNP) have been developed for use on food contact surfaces and biomaterials. The addition of very low AgNP loads provided virucidal activity against human norovirus, the main cause of gastroenteritis, and did not significantly modify the optical properties of the films.[86] In another study, the same authors demonstrated that the packaging of PHBV with AgNP presented the antimicrobial activity against L. monocytogenes and Salmonella enterica

Food Packaging– Composites

but, at the same time, allow it to disperse on the food surface. The second category does not rely on direct contact, e.g., oxygen sachets within the package that interact with food to reduce spoilage.[72] Various antimicrobial agents are incorporated into packaging, including chemical and natural antimicrobials, antioxidants, biotechnology products, and antimicrobial polymers and gas. Among the chemical antimicrobials, those commonly used in the industry are organic acids, fungicides, alcohols, and antibiotics.[73] Among the organic acids, acetic, benzoic, lactic, citric, malic, tartaric, propionic, fumaric, and sorbic acids inhibit the proliferation of bacteria and fungi.[74] Ethanol is mainly used in bakery products due to its antifungal[59] and antibacterial activity, but it does not present activity against yeasts.[73] The common gases in antimicrobial packaging are carbon dioxide and sulfur dioxide.[75] However, the increase by consumers in the demand for natural products causes the use of ­alternatives to chemical additives to be explored.[3] Natural antimicrobials can be derived from animal origin (edible, medicinal and herbaceous plants, and essential oils) (pleurocidin, defensins, lactoferrin, antimicrobial peptides, chitosan, and lipids) and microbial origin (­reuterin, pediocin, and nisin).[76] Lysozyme is a natural enzyme, produced by humans and many animals, which has activity against the cell wall of bacteria.[74] Essential oils are oily liquids extracted from different parts of plants, such as leaves, barks, flowers, and seeds. They have antimicrobial and antioxidant activities when incorporated into packaging materials, besides improving the water vapor barrier due to the hydrophobic nature of the oils. However, their use may be limited by the odor of essential oil in the packages.[77] Antioxidants can be added to preserve the lipid components of food from deterioration in the quality. The most commonly used foods include butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), propyl gallate (PG), and tert-butylhydroquinone (TBHQ). Natural antioxidants are also used, such as oils, spices, herbs, cereals, grains, protein hydrolysers, and teas.[78] Nisin is a natural preservative for many food products, and its use dates back to 1960.[74] It is mainly used in dairy and meat products, and inhibits the growth of pathogenic bacteria transmitted by foods, as well as deteriorating microorganisms such as gram-positive.[79] However, the addition of compounds directly to foods generally results in instant inhibition of unwanted microorganisms, but survivors will continue to grow, especially when the antimicrobials added by the formulation are depleted.[80] Thus, the use of packages containing antimicrobial agents has the advantage of diffusing these compounds to the surface of the food in a controlled manner. Thus, they are present in smaller quantities, taking into account a current demand of the consumer—i.e., the search for food free of preservatives—and only where its presence is required, that is, especially on the surface of the product,

1350

Food Packaging: Polyhydroxyalcanoates (PHA) Containing Antimicrobial Additives

Table 1  Antimicrobial activity of PHA film-containing additives Polyester/polymer Food additives

Microbial inhibition

Reference

PLA PCL P(3HB)

Quaternary ammonium

E. coli, S. typhimurium, and L. monocytogenes

[86]

PHBV

AgNPs

Human norovirus

[87]

PHBV

AgNPs

L. monocytogenes and S. enterica

[88]

PHBV

ZnO

E. coli and S. aureus

[89]

Food Packaging– Composites

P(3HB)

ZnOnano

E. coli

[104]

PHBV

Essential oils of oregano, carvacrol, clove essential oil, and eugenol

E. coli and L. innocua

[90]

Nanocomposites of PHBV

Nanocrystals of CNC–Ag

E. coli and S. aureus

[91]

PHBV

META, MESA, and HEMA

E. coli and S. aureus

[92]

P(3HB)/PCL/ organo-argilas

Nisin

L. plantarum

[46]

Layer of electrospun based on zein in P(3HB) in multilayer form

Cinnamaldehyde

Murine norovirus, feline calicivirus, and hepatitis A virus

[108]

P(3HB)

Vanillin

E. coli, S. typhimurium, S. flexneri, S. aureus, A. flavus, A. fumigatus, A. niger, A. parasiticus, A. ochraceus, P. viridicatum, and P. clavigerum

[94]

PHB–PHBV

Chitosan or chitooligosaccharide

E. coli, P. aeruginosa, methicillinresistant S. aureus, and S. aureus

[96]

in very low quantity (0.4 g/kg AgNP), with potential for ­application in films and coatings, as packaging of active foods.[87] Films of PHBV with zinc oxide (ZnO) appeared antimicrobial activity against E. coli and Staphylococcus aureus, in addition to levels of migration of nanocomposites below the allowed limit, thus demonstrating the promising ­application in food and beverage packaging.[88] Requena et al.[89] evaluated the antimicrobial activity of PHBV films with four active components: essential oil of oregano, carvacrol, clove essential oil, and eugenol against E. coli and Listeria innocua, with greater efficacy of oil of oregano and carvacrol. Nanocomposites of PHBV and nanocrystals of cellulose with silver (CNC–Ag) in various concentrations were ­evaluated microbiologically, against E. coli and S. aureus using the disk diffusion method, presenting strong ­antibacterial activity. In addition, they presented high mechanical strength with potential for application in food packaging.[90] It was performed for the modification of PHBV with three monomers with antibacterial potential: 2-[(methacryloyloxy)-ethyl]trimethylammonium chloride (META), 2- [(methacr yloylethyl)] -dimethyl- (3-sulfopropyl)

ammonium (MESA), and 2-hydroxyethyl methacrylate (HEMA). Both showed complete inhibition of S. aureus adhesion and 99% of E. coli adhesion. [91] P(3HB)/PCL and organoclay blends (Cloisite® 30B and 10A) were studied for mechanical and thermal ­properties, water vapor barrier, and nisin activation. In addition, adsorption of nisin on the P(3HB)/PCL matrix with and without clay was studied and compared to find the possible synergistic antimicrobial effects. Nisin-­ activated P(3HB)/PCL films were effective against Lactobacillus plantarum CRL691 (used as a model of processed meat decay bacteria) inoculated in cooked ham, extending the shelf life of this food. In this way, the mixture of P(3HB)/PCL and its nanocomposites activated with nisin showed potential for application in ­processed meat packages. [46] The multilayer system is also worth mentioning in the use of PHAs and the incorporation of antimicrobial additives. Cinnamaldehyde, the active compound of cinnamon with virucidal activity, was used in conjunction with a layer of eletrospun based on zein in P(3HB) in multilayer form. At 25°C, the results demonstrated a better antiviral activity, as well as the potential of this multilayer system for applications in contact with food, as well as for active

packaging technologies, in order to maintain or increase the quality and safety of food.[92] Ramachandran et al.[93] evaluated the antimicrobial activity of ethanolic and methanolic extract of pure Clitoria ternatea and incorporated into PHB. The disks embedded in the ethanol extract presented a halo of inhibition against the ten microorganisms tested, whereas the methanolic extract was not efficient against three of them. The extracts incorporated in PHB matrix were not effective, but there was no growth under the films as happened with the control (pure PHB). P(3HB) films were prepared with the incorporation of vanillin (4-hydroxy-3-methoxybenzaldehyde) at concentrations of 10–200 µg/g PHB and evaluated for the antimicrobial activity against E. coli, S. typhimurium, Shigella flexneri, S. aureus, A. flavus, A. fumigatus, A. niger, A. parasiticus, A. ochraceus, Penicillium viridicatum, and P. clavigerum. The minimum vanillin concentration required to exhibit the antimicrobial activity was ≥80 μg/g PHB for bacteria and ≥50 μg/g PHB for fungi (Fig. 5). [94] Acrylic acid was grafted to ozone-treated PHB and PHBV membranes. The resulting membranes were further grafted with chitosan or chitooligosaccharide via esterification. These chitosan- or chitooligosaccharide-grafted membranes showed the antibacterial activity against E. coli, P. aeruginosa, methicillin-resistant S. aureus, and S. aureus. The antibacterial activity of E. coli was the highest, whereas the antibacterial activity of methicillin-­ resistant S.  aureus was the lowest among these four ­bacteria tested.[95] EFFECT OF ANTIMICROBIAL ADDITIVES IN THE PROPERTIES OF PHA FILMS Packaging is a very important component of the product and serves not only to arouse consumer interest but to protect the food and preserve its quality until the final consumption. However, for this to occur, packaging material need to meet certain requirements: a. Wrap the food: this requires strength (such as resistance to puncture and impact), sealing, and dead fold. b. P  rotect the food from the external environment against: Humidity: The moisture content must be controlled, so the packaging is a barrier to the transmission of water vapor into or out of the package. Oxygen: Most of the food is susceptible to exposure to oxygen; in addition, the presence of oxygen facilitates microbial growth and, consequently, food degradation. Light: Visible light and ultraviolet (UV) light are the initiators of oxidative reactions. In some foods,

c. d. e. f.

g.

the light barrier is necessary to maintain its ­nutritional quality. Other flavors, odors, and chemicals: The packaging should serve as a barrier to other flavors, odors, and chemicals that may alter the characteristics of the food. Sealing quality: The way the package is closed must correspond to the barrier properties of the material. Maintain the sensorial qualities of food. Maintain the modified atmosphere in the product’s headspace. Exhibit stability under extreme storage conditions (e.g., low or high temperatures). Environmental conditions leading to biodegradation must be circumvented during storage of the product, whereas conditions optimized for biodegradation should arise after disposal. Food grade quality of the packaging material is required regarding the purity.[44,96]

Dixon [96] complements with the following requirements: a. Promotion: Appearance of packaging to attract the consumer and present the image of quality. b. Machinability: Adequate definition of design characteristics and packaging materials for the best ­functioning of food packaging machines. c. C  onvenience: Design features that facilitate consumer manipulation, easy opening, and closing. d. C  ost: It takes into account not only the monetary cost but the environmental impact that the packaging can cause. e. Food safety: Ensuring consumer safety as regards quality food. The main polymer materials used by the industry and their properties are shown in Table 2. PHAs have the potential for application as food packaging because of their characteristics. They can be processed by thermoforming and result in great packaging films. For this, they may be alone or in combination with other synthetic or biocompatible polymers, creating composite materials or mixtures, respectively.[97] In addition, they exhibit a degree of crystallinity and adaptive elasticity, and can be processed into pieces of different shapes such as flexible sheets or rigid and robust components such as ­storage boxes and containers.[98,99] PHAs are hydrophobic and thus exhibit a water vapor barrier, in the same way as a barrier against CO2 [100,101] and O2, which makes them interesting as materials basic to produce bottles for liquid food and also for CO2 containing liquid. Likewise, they act as a UV barrier, an important factor to protect against the formation of radicals of the unsaturated lipid components, which accelerate the ­deterioration of the food.[98]

Food Packaging– Composites

Food Packaging: Polyhydroxyalcanoates (PHA) Containing Antimicrobial Additives 1351

1352

Food Packaging: Polyhydroxyalcanoates (PHA) Containing Antimicrobial Additives

(a)

(b)

(e)

(c)

(f )

(d)

(g)

Food Packaging– Composites (h)

(i)

(j)

(k)

Fig. 5  Antimicrobial activity of PHB/vanillin films: (a) A. flavus; (b) A. niger; (c) A. parasiticus; (d) A. ochraceus; (e) A. fumigatus; (f) P. viridicatum; (g) P. clavigerum; (h) E. coli; (i) S. aureus; (j) S. typhimurium; (k) S. flexneri [94]

As presented, classical PHA production is based on residues from the food industry, which demonstrates an environmental benefit for the replacement of petrochemical polymers. Moreover, packaging made from PHA contaminated with food residues can be discarded through composting.[102] Various additives are added to the resins during manufacturing process of the plastic packaging, in order to improve the physical and chemical properties of the materials. [103] The additives may be antioxidants, plasticizers, glidants, stabilizers, lubricants, UV absorbers, antistatic, and anti-blockers. However, the use of the additives in plastic food packaging should not interfere with the characteristics of the food, nor be toxic and harmful to human health. [104] For these reasons, it is important to understand whether the incorporation of additives will or will not alter the properties of the PHAs. Based on the studies mentioned in “Antimicrobial Additives”, the authors also sought to evaluate the changes caused in the polymeric properties with the addition of additives. The incorporation of starch into the P(3HB) and PHBV promoted a reduction in the tensile strength values

at break. In P(3HB), a reduction of 30% was observed, whereas in PHBV, the incorporation of the starch further reduced its resistance, transforming it into very fragile films, which made it impossible to detect the tensile strength at rupture and the elongation of this material. It was also observed that the elongation at breakage of the films of P(3HB) increased with the incorporation of the starch. Incorporation of 25% starch into the blend reduced the tensile strength of PHBV in 50% and 40% in P(3HB). With this, there was a decrease in the mechanical properties of the polymers with the incorporation of the starch. Despite the decrease in the mechanical properties, the incorporation of starch into the polyesters allowed a significant increase of the biodegradability, possibly due to the ease of this carbohydrate being consumed as an alternative carbon source by the fungi.[83] The addition of tannic acid in P(3HB) inhibited the crystallization of the polymer after cooling. While the peak melt was essentially unchanged by the presence of tannic acid, the exothermic crystallization peak was absent. In addition, the higher the additive content, the higher the temperature at which the crystallization peak appears.

Polymeric material

Tg (°C)

PE

−78

Polyvinyl chloride

99 [109]

Polystyrene

100

Polytetrafluoroethylene

−73 [109]

PP

−15

141

[109]

272 [109], 285[111] 243

[109]

187

50

, 60–80

Polycarbonate

13785–20285[116] −35

PET

69 [116]

Polyoxymethylene Polybutylene terephthalate Polymethyl methacrylate

[117]

−50

[109]

−25

, −83

[116]

, 43

105[116]

[109]

[116]

[124]

, 183

255–265

1.6

[121]

[112]

—

[117]

189–209

[112]

200 [112], >50 [119] [115]

Tensile strength (MPa) Depends on the density[110] 40–60 [113] 40–65

400 [112] 80

[111]

265[118] 165–189,

[120]

−71[123]

Polyethylene adipate

30 [112]

[109]

332 [109], 1885–39885[115]

[109]

Polyvinylidene chloride

Depends on the density[110]

[109]

Nylon 6,6

[116]

Elongation at break (%)

Tm (°C)

—

[113]

50

, 34

, 26

94,5

Packaging for eggs, fruits and chocolates, plates, cups, disposable tableware, toys, etc. Teflon: nonstick pots and pans

[112]

Packing for sweets and breads [114] Manufacture of textile fibers, swimsuits, bikinis, fishing line, velcros, watch bracelets, etc.

[118]

62–724, [118] 65[119] 40–60

Manufacture of sacks and bags, films for packaging meat, fresh fruits, and vegetables Packaging of dairy and meats [114]

[112]

25[112] [50]

Applications

Manufacture of airplane windows, bottles, lenses, etc. Packing of meats and cheeses [114]

[122]

26485–283,85[115]

300 [112]

55[112]

4685–6485[115]

—

—

—

175[109]

—

—

—

220

—

[124]

15985–19985[115]

0 [119]

52

Yarns for weaving, magnetic tapes, films for X-rays, bottles, food jars, cosmetics, etc.

Automotive, electronics, and household appliances

[118]

70–76, [113] 70 [119]

Transparent tiles, car lanterns, eyeglasses and contact lenses, dentures, etc.

Notes: Tg is the glass transition temperature; Tm is the melting temperature.

Food Packaging– Composites

Food Packaging: Polyhydroxyalcanoates (PHA) Containing Antimicrobial Additives 1353

Table 2  Properties of polymeric materials used by industry

1354

Food Packaging: Polyhydroxyalcanoates (PHA) Containing Antimicrobial Additives

Food Packaging– Composites

The  processability of P(3HB) has also been altered as a result of the addition of the tannic acid, since a suitable amount widens the processing window of the polymer to lower temperatures. The authors concluded that the addition of tannic acid results in higher viscoelastic properties but, above all, clearly delays the thermal degradation process over longer times, and is very effective in improving the thermal stability and processability of P(3HB).[84] While the biopolymer materials composed of a PHBV matrix, a very low load AgNP-based coating provided the virucidal activity and did not significantly modify the optical properties of the films. In another study, the same authors observed that AgNPs may act as both a nucleating agent that promotes the crystallization of PHBV3/PHBV18 (poly(3-hydroxybutyrate-co-3 mol%-3-hydroxyvalerate)/ poly-hydroxybutyrate-co-18 mol%-3-hydroxyvalerate) and a tortuosity-enhancing element, such as the effects that lead to higher oxygen barrier and water vapor barrier materials.[87,88] Biodegradable nanocomposites based on PHBV reinforced with ZnO nanoparticles were developed. The nanoparticles acted as nucleating agents, increasing the crystallization temperature, and the crystallinity level of PHBV. In addition, they have strongly reinforced their stability, attributed to the barrier effect which delayed the transport of decomposition products from most of the compound to the gaseous phase. A remarkable increase in both the modulus of storage and the glass transition temperature of the matrix was observed after the addition of ZnO, with the hardening effect being more pronounced at temperatures above the glass transition. The traction and barrier properties of the biopolymer were also noticeably improved, and the migration levels in nonpolar and polar simulators decreased with increasing nanoparticle content.[88] However, the addition of ZnOnano in P(3HB) matrix did not significantly alter the thermal stability of the polymer. The nanostructure functioned as nucleating agent since its addition resulted in complete crystallization of P(3HB) during cooling. In addition, increasing the ZnOnano percentage increased the crystallinity of the polymer, thus ­increasing its industrial applications in the packaging area.[89] In another study of PHBV, but this time reinforced with cellulose–silver (CNC–Ag) crystals, the additive showed an efficient heterogeneous nucleation to facilitate the crystallization of PHBV. Besides, the addition of CNC–Ag induced significant improvements in thermal stability and mechanical property of the biopolymer matrix. In comparison with pure PHBV, a 140% improvement in tensile strength, a 200% increase in Young’s modulus in 1% CNC–Ag nanocomposites, an increase in CNC–Ag concentration, and a reduction in water absorption and water vapor permeability values were observed.[90] The incorporation of essential oils of oregano, carvacrol, eugenol, and clove into P(3HB) films produced antimicrobial films with adequate properties of tensile strength,

optic, and water vapor and with good thermal stability, even though this affected significantly the properties of the films.[105] This occurs because the incorporation of essential oil affects the continuity of the polymer matrix, leading to physical changes depending on the specific polymer–oil interactions. In addition, the film structure is weakened by the addition of oil, while the water barrier properties are improved and the transparency is reduced.[106] Although these essential oils did not improve the tensile properties, more transparent materials with better water vapor barrier capacity were obtained. Although carvacrol and eugenol gave rise to a slight decrease in the thermal stability of the polymer matrix, oregano and clove oil led to more temperature-resistant materials.[105] Otherwise, incorporation of vanillin into PHB films reduced mechanical strength and melting point, but PHB film-containing vanillin may be used as secondary films on the primary packaging polymer layers.[93] The mechanical properties of a polymer are affected by its composition, in addition to the influence of crystallinity and molecular weight. The presence of another component, as in polymer blends, or the use of additives is responsible for different behaviors and applicability.[107] CONCLUSIONS Food packaging is primarily designed to protect the product, thus acting as a barrier to external agents. Among the functions of a package, the most important is to preserve the product quality to the maximum, creating conditions that minimize chemical, biochemical, and microbiological changes. The incorporation of antimicrobial agents into packaging is an alternative to pathogenicity or degradability of food as a function of storage and commercialization time; besides, protecting the product from the external environment inhibits or slows down the growth of microorganisms in foods, especially fungi, bacteria, and viruses. Packages with these characteristics are known as active packages, as they have some other desirable function. The active packaging seeks to correct deficiencies present in the conventional packaging and has stood out in the ­packaging market. In addition, recent research focuses on the incorporation of antimicrobial additives into the matrix of biodegradable polymers. With a focus on the PHA family, these polyesters have been the focus of much research because of their biodegradability and biocompatibility, as sustainable development policies tend to expand with decreasing fossil fuel reserves and growing concern about the environment. In addition, these polymers make a significant contribution to sustainable development since they can be synthesized from renewable resources and are compatible for application in several areas, mainly biomedical and food.

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48.

49.

50.

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55.

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57.

58.

59.

60. 61.

62.

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81. Ferreira Soares, N.F.; Azevêdo da Silva, W.; dos Santos Pires, A.C.; Peruch Camilloto, G.; Santiago Silva, P. Novos desenvolvimentos e aplicações em embalagens de alimentos. Revista Ceres 2009, 56 (4), 370–378. 82. Bugnicourt, E.; Cinelli, P.; Lazzeri, A.; Alvarez, V. Polyhydroxyalkanoate (PHA): Review of synthesis, characteristics, processing and potential applications in packaging. Express Polym. Lett. 2014, 8 (11), 791–808. 83. Vinhas, G.M.; Almeida, Y.M.B.; Andrade Lima, M.A.G. Estudo das propriedades e biodegradabilidade de blendas de poliéster/amido submetidas ao ataque microbiano. Quím. Nova 2007, 30 (7), 1584. 84. Auriemma, M.; Piscitelli, A.; Pasquino, R.; Cerruti, P.; Malinconico, M.; Grizzuti, N. Blending poly (3-hydroxybutyrate) with tannic acid: Influence of a polyphenolic natural additive on the rheological and thermal behavior. Eur. Polym. J. 2015, 63, 123–131. 85. Belkhir, K.; Lacroix, M.; Jamshidian, M.; Salmieri, S.; Jegat, C.; Taha, M. Evaluation of antibacterial activity of branched quaternary ammonium grafted green polymers. Food Packag. Shelf Life 2017, 12, 28–41. 86. Castro-Mayorga, J.L.; Randazzo, W.; Fabra, M.J.; Lagarón, J.M.; Aznar, R.; Sánchez, G. Antiviral properties of silver nanoparticles against norovirus surrogates and their efficacy in coated polyhydroxyalkanoates systems. Food Sci. Technol. 2017, 79, 503–510. 87. Castro-Mayorga, J.; Fabra, M.; Lagaron, J. Stabilized nanosilver based antimicrobial poly (3-hydroxybutyrate-co-3-hydroxyvalerate) nanocomposites of interest in active food packaging. Innov. Food Sci. Emerg. Technol. 2016, 33, 524–533. 88. Díez-Pascual, A.M.; Díez-Vicente, A.L. ZnO-reinforced poly (3-hydroxybutyrate-co-3-hydroxyvalerate) bionanocomposites with antimicrobial function for food packaging. ACS Appl. Mater. Interfaces 2014, 6 (12), 9822–9834. 89. Requena, R.; Jiménez, A.; Vargas, M.; Chiralt, A. Poly [(3‐ hydroxybutyrate)‐co‐(3‐hydroxyvalerate)] active bilayer films obtained by compression moulding and applying essential oils at the interface. Polym. Int. 2016, 65 (8), 883–891. 90. Yu, H.; Sun, B.; Zhang, D.; Chen, G.; Yang, X.; Yao, J. Reinforcement of biodegradable poly (3-hydroxybutyrate-co-3-hydroxyvalerate) with cellulose nanocrystal/silver nanohybrids as bifunctional nanofillers. J. Mater. Chem. 2014, 2 (48), 8479–8489. 91. Manecka, G.; Labrash, J.; Rouxel, O.; Dubot, P.; Lalevée, J.; Andaloussi, S.A.; Renard, E.; Langlois, V.; Versace, D.L. Green photoinduced modification of natural poly (3-hydroxybutyrate-co-3-hydroxyvalerate) surface for antibacterial applications. ACS Sustain. Chem. Eng. 2014, 2 (4), 996–1006. 92. Fabra, M.J.; Castro-Mayorga, J.L.; Randazzo, W.; Lagarón, J.M.; López-Rubio, A.; Aznar, R.; Sánchez, G. Efficacy of cinnamaldehyde against enteric viruses and its activity after incorporation into biodegradable multilayer systems of interest in food packaging. Food Environ. Virol. 2016, 8 (2), 125–132. 93. Ramachandran, D.L.D.; Rasheed, R.; Gowri, G.R. Incorporation of Clitoria ternatea seed extract into bioplastic sheets having induced plasticity and their antimicrobial activity against multidrug resistant clinical pathogens. Int. J. Pharm. Pharm. Sci. 2013, 5 (3), 543–550.

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94. Xavier, J.R.; Babusha, S.T.; George, J.; Ramana, K.V. Material properties and antimicrobial activity of polyhydroxybutyrate (PHB) films incorporated with vanillin. Appl. Biochem. Biotechnol. 2015, 176 (5), 1498–1510. 95. Hu, S.G.; Jou, C.H.; Yang, M.C. Antibacterial and biodegradable properties of polyhydroxyalkanoates grafted with chitosan and chitooligosaccharides via ozone treatment. J. Appl. Polym. Sci. 2003, 88 (12), 2797–2803. 96. Dixon, J. Packaging Materials: 9. Multilayer Packaging for Food and Beverages; ILSI Europe: Brussel, 2011, 48. 97. Siracusa, V.; Rocculi, P.; Romani, S.; Dalla Rosa, M. Biodegradable polymers for food packaging: A review. Trends Food Sci. Technol. 2008, 19 (12), 634–643. 98. Koller, M.; Salerno, A.; Muhr, A.; Reiterer, A.; Braunegg, G. Polyhydroxyalkanoates: Biodegradable polymeric materials from renewable resources. Mater. Tehnol. 2013, 47, 5–12. 99. Mensitieri, G.; Di Maio, E.; Buonocore, G.G.; Nedi, I.; Oliviero, M.; Sansone, L.; Iannace, S. Processing and shelf life issues of selected food packaging materials and structures from renewable resources. Trends Food Sci. Technol. 2011, 22 (2), 72–80. 100. Miguel, O.; Iruin, J.J. Evaluation of the transport properties of poly (3‐hydroxybutyrate) and its 3‐hydroxyvalerate copolymers for packaging applications. Macromol. Symp. 1999, 144, 427–438. 101. Dagnon, K.L.; Thellen, C.; Ratto, J.A.; D’Souza, N.A. Physical and thermal analysis of the degradation of poly (3-hydroxybutyrate-co-4-hydroxybutyrate) coated paper in a constructed soil medium. J. Polym. Environ. 2010, 18 (4), 510–522. 102. Koller, M.; Salerno, A.; Dias, M.; Reiterer, A.; Braunegg, G. Modern biotechnological polymer synthesis: A review. Food Technol. Biotechnol. 2010, 48 (3), 255–269. 103. Coltro, L.; Machado, M.P. Migração específica de antioxidante de embalagens plásticas para alimentos. Polímeros 2011, 21 (5), 390–397. 104. de Lima Júnior, R.G.; Vinhas, G.M.; Souto-Maior, R.M.; Santos, A.M.P.; Santos, E.J.P.; de Almeida, Y.M.B. Desenvolvimento e caracterização de filmes à base de Poli (3-hidroxibutirato) aditivado com ZnOnano/development and characterization of films based on Poly (3-hydroxybutyrate) with added ZnOnano. Braz. J. Food Technol. 2016, 19, 1. 105. Rabello, M.S. Aditivos de Polímeros; Editora Artliber: São Paulo, 2000. 106. Atarés, L.; Chiralt, A. Essential oils as additives in biodegradable films and coatings for active food packaging. Trends Food Sci. Technol. 2016, 48, 51–62. 107. Fabra, M.J.; López-Rubio, A.; Sentandreu, E.; Lagaron, J.M. Development of multilayer corn starch-based food packaging structures containing β-carotene by means of

the electro-hydrodynamic processing. Starch—Stärke 2016, 68 (7–8), 603–610. 108. Lobato, M.F. Estudo do envase a vácuo de produtos ­cárneos curados e cozidos, 2005. 109. Van Krevelen, D.W.; Te Nijenhuis, K. Properties of Polymers: Their Correlation with Chemical Structure; Their Numerical Estimation and Prediction from Additive Group Contributions, 4th Ed.; Elsevier: Amsterdam, 2009, 1030. 110. Cheremisinoff, N.P. Handbook of Polymer Science and Technology; CRC Press: New York, 1989. 111. Osswald, T.; Hernández-Ortiz, J.P. Polymer Processing. Modeling and Simulation; Hanser: Munich, 2006. 112. Crompton, T.R. Physical Testing of Plastics; Smithers Rapra: Shrewsbury, 2012. 113. Klein, R. Material Properties of Plastics. In: Laser Welding of Plastics: Materials, Processes and Industrial Applications; Mittal, K.L.; Lei, W.S.; Eds.; Wiley-VCH: Weinheim, 2011, 3–69. 114. Gava, A.J. Princípios de Tecnologia de Alimentos; NBL Editora: São Paulo, 1978, 242. 115. Bicerano, J. Prediction of Polymer Properties; CRC Press: Boca Raton, FL, 2002. 116. Mark, J.E. Physical Properties of Polymers Handbook; Springer: New York, 2007. 117. Lewin, M. Handbook of Fiber Chemistry, 3rd Ed.; CRC Press: Boca Raton, FL, 2006. 118. Lampman, S. Characterization and Failure Analysis of Plastics; ASM International: Materials Park, OH, 2003. 119. Milisavljevic, J.; Ćirić, I.; Mančić, M.; Eds.; Tensile ­Testing for Different Types of Polymers. In 29th Danubia–Adria Symposium; University of Belgrade: Serbia, 2012. 120. Gregorio, R.; Chaud, M.R.; Nunes Dos Santos, W.; Baldo, J.B. Miscibility and morphology of poly (vinylidene fluoride)/poly [(vinylidene fluoride)‐ran‐trifluorethylene] blends. J. Appl. Polym. Sci. 2002, 85 (7), 1362–1369. 121. Capitão, R.C. Estudo Morfológico do PVDF e de Blendas PVDF/P (VDF-TrFE); Universidade de São Paulo: São Carlos, 2002. 122. Lovinger, A.J. Poly(vinylidene fluoride). In Developments in Crystalline Polymers-1; Bassett, D.C.; Ed.; Applied ­Science Publisher: London, 1982, 195. 123. Hayes R.A. The relationship between glass temperature, molar cohesion, and polymer structure. J. Appl. Polym. Sci. 1961, 5 (15), 318–321. 124. Devaux, J.; Godard, P.; Mercier, J. The transesterification of bisphenol‐a polycarbonate (PC) and polybutylene terephthalate (PBTP): A new route to block copolycondensates. Polym. Eng. Sci. 1982, 22 (4), 229–233.

Food Packaging: Polymer Composites Garima Agrawal Department of Polymer and Process Engineering, Indian Institute of Technology Roorkee, Saharanpur Campus, Saharanpur, India

Rahul Agrawal Department of Biochemical Engineering and Biotechnology, Indian Institute of Technology Delhi, New Delhi, India

Abstract Food Packaging– Composites

Polymers have been extensively used for packaging the food products to improve their quality, safety, and shelf life. In this entry, various methods for polymer surface modification to enhance the mechanical and barrier properties of polymers from the food packaging perspective are discussed. Further, different approaches to develop active packaging such as antimicrobial, oxygen, and ­ethylene scavenging are reviewed which significantly help to extend the shelf life of the food product from production to its delivery to the consumers. Additionally, nanotechnology developments targeting intelligent packaging capable of enhanced communication are presented, focusing mainly on oxygen, moisture, and microbes indicators. A variety of antimicrobial polymers and various methods of their commercial application are discussed. Moreover, migration of various nanoparticles and additives from the packaging to food stuff and their impact on the health of ­consumers are discussed. Keywords: Active packaging; Antimicrobial; Composites; Indicators; Intelligent packaging; Nanomaterials; Polymers.

INTRODUCTION Advancement of material science and technology along with changing consumer’s demand has led to the continuous evolution in the area of food packaging. The main requirement of food packaging is to improve the shelf life of food product while maintaining its quality and safety starting from production to its delivery to the end user. In addition to the basic mechanical, thermal, and optical properties, the packaging material is required to prevent the outbreak of spoilage microorganisms and act as a barrier against permeation of water vapor, oxygen, carbon dioxide, and other volatile compounds such as flavors by providing physical protection and creating proper physicochemical conditions.[1–4] Conventional packaging materials such as paper and paperboard, ceramics, glass, and metal are being replaced by polymers due to their functionality, lightweight, ­excellent physicochemical properties, ease of processing, and low cost.[5,6] High-density polyethylene (HDPE) for bags and milk bottles, low-density polyethylene (LDPE) for trays and general-purpose containers, polypropylene (PP) for film and microwavable containers, and polyethylene terephthalate (PET) for soft drink bottles are the most ­frequently used polymers in food packaging.[7] Application of nanotechnology in polymer-based food packaging has Encyclopedia of Polymer Applications, First Edition DOI: 10.1201/9781351019422-140000332 Copyright © 2018 by Taylor & Francis. All rights reserved.

resulted into packaging materials with additional functions such as antimicrobials and enhanced communications such as oxygen, moisture, and microbe indicators which are of significant importance from both the manufacturers’ and the consumers’ point of view.[8–10] This entry provides a comprehensive review of polymer composites applied in food packaging area with active and intelligent features. Additionally, the migration of nanomaterial from the package to the food and its impact on consumers’ health along with the requirement of regulations and investigations are discussed. POLYMERS AND THEIR SURFACE MODIFICATION FOR FOOD PACKAGING APPLICATION Polymer surface is modified by physical or chemical methods in order to improve their properties for packaging applications. Physical methods offer some advantages over chemical methods such as environmentally safe and clean processes along with no disposal of waste chemicals. Surface properties can be tuned by physical methods ranging from flame and corona treatments to advanced techniques such as ultraviolet (UV), gamma ray, electron beam irradiations, ion beam, plasma, and laser treatments. Although

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C

Benzophenone

O

OH

O UV irradiation

LDPE

O

O

C

O

LDPE

LDPE

C

of prime importance to obtain polymer films with good ­adhesion properties.[14–16] UV treatment is another method for polymer surface modification. Here, an initiator absorbs the UV energy leading to an excited singlet state having a short lifetime and rapidly transforms into a more stable triplet state. The initiator in its triplet state can abstract hydrogen from the  polymer surface and create active sides for grafting. Shin et al. developed LDPE grafted with acrylic acid and natamycin for antifungal function against mold and yeast by using benzophenone indicator under UV treatment (Fig. 1).[17] Results exhibited that the natamycin-grafted LDPE cling wrap treated with UV for more than 2 min ­displayed a ­significant antifungal activity against Penicillium chrysogenum and Saccharomyces cerevisiae on fresh cut showing 4.95–7.26 log and 0.8–3.0 log reduction, respectively. Further, Zhao et al. coated cellulose nanocrystal (CNC) films on hydrophobic PP with better adhesion achieved by UV treatment. [18] The composite films showed improved mechanical and oxygen barrier properties. Polymer surface modification can also be achieved by gamma ray treatment, electron beam treatment, ion beam treatment, laser treatments, and chemical methods, which are summarized in detail elsewhere.[19–28]

+

OH

LDPE

O

OH

Natamycin OH

O

O NH2

O

O

OH

H

OH OH

HO

HN HO

OH

O O

+

O LDPE-g-AA

Acrylic acid

80% water:20% Ethanol Stir at 40ºC, pH=5.5 HO

O

OH

O

C

O

O

O O

O

OH

HO

LDPE

C

UV irradiation

LDPE

Food Packaging– Composites

flame and corona discharge treatments are one of the most commonly used methods because they are cheap compared to electron beam or laser treatments, they are not very ­successful in polymers due to the rather short ­timescale of the improved properties.[11] Oxygen-containing polar groups are generated on the surfaces of polyethylene (PE), PP, PET, and polyacetals to improve their printability, wettability, and adhesion. X-ray photoelectron spectroscopy showed that various chemical species such as hydroxyls, carboxyls, and carbonyls could be introduced on flame-treated PE and PP surfaces.[12] Flame treatment is easy to use and inexpensive. However, it requires precise control of flame temperature, flame contact time, air-to-gas ratio, flow rate, and distance between the flame and the polymer surface in order to control the extent of surface modification. The contamination of the treated surface by chemical residues should be prevented in order to maintain the adhesion properties of modified polymers. In corona discharge treatment, air between the two surfaces is ionized, and the species such as hydroxyl, carboxyl, carbonyl, and amide groups are introduced on the surface of polyolefine.[13] A precise control of voltage and frequency of electromagnetic field, the gas composition, exposure time, and sample/electrode geometry are

OH O

OH

OH H

O O

LDPE-AA-Na

Fig. 1  Schematic representation of the process of UV introduced acrylic acid and natamycin grafting on the LDPE surface Source: From Shin et al.[17] © 2016 with permission from Elsevier.

POLYMER NANOCOMPOSITES FOR ACTIVE FOOD PACKAGING Biodegradable Polymers and Nanocomposites for Food Packaging Polylactic acid (PLA)-, polycaprolactone (PCL)-, poly­ (butylene succinate) (PBS)-, and polyhydroxybutyrate (PHB)-based nanocomposites using layered silicate nanoclays are some of the most common examples of packaging ­materials in food sector.[29] In addition to rendering active and/or smart properties to food packaging systems such as ­antimicrobial properties, these nanocomposites provide high barrier properties against the diffusion of oxygen, carbon dioxide, flavor compounds, and water vapor.[6] The characteristic of biodegradable polymers is their disintegration into smaller nontoxic fragments by naturally occurring microorganisms under suitable oxygen, temperature, and moisture conditions.[30] As shown in Fig. 2, biopolymers can be divided into three different categories based on their origin: (a) natural biopolymers, (b)  synthetic biopolymers, and (c) biopolymers produced by microbial fermentation. Here, it is to be noted that widespread commercial use of biodegradable polymers is still limited due to poor mechanical and barrier properties along with poor resistance to standard processing conditions.[31–33] To address these limitations, the idea of nanocomposite formation has been extensively explored. These nanocomposites exhibit improved mechanical, barrier, and heat resistance properties. Tammaro et al. developed PET-based nanocomposites using polar nanoclay by high-energy ball milling for food packaging applications.[34] Here, a significant decrease in

the oxygen permeation was observed due to the tortuous path provided by the highly dispersed polar clay platelets. Bodaghi et al. developed LDPE nanocomposite films with TiO2 and Cloisite 20A particles using an industrial melt blow extrusion.[35] The mechanical properties, water vapor, oxygen and carbon dioxide gas barrier, and antimicrobial activity of the developed films were tested. The composite films displayed an excellent antimicrobial a­ ctivity and ethylene photodegradation, showing their potential for fresh horticultural product packaging. Further, Maisanaba et al. developed PP/3-aminopropyltriethoxysilane-modified clay mineral nanocomposites with improved mechanical and barrier properties and reduced microbial growth.[36] The improved mechanical properties of polymer nanocomposites are due to high rigidity and aspect ratio of nanoclay together with a good interfacial interaction between the polymer matrix and the dispersed nanoclay, whereas the improved barrier properties are due to the tortuous path provided by fully exfoliated clay with higher aspect ratio (Fig. 3).[9,37] Arrieta et al. reported CNC-reinforced PLA nanocomposites as CNCs are stiff, lightweight, highly abundant in nature, low cost, and biodegradable.[38] Ethylene vinyl alcohol (EVOH) copolymer-based nanocomposite films were fabricated by Kim et al. by incorporation of organically modified montmorillonite nanoclays into EVOH matrix via a two-step mixing process and solvent cast method.[39] The experimental results exhibited that the addition of only 3 wt% clay improved the oxygen and water vapor barrier performances of the nanocomposite films by 59% and 90%, respectively, compared to the neat EVOH film. ­Additionally, these nanocomposites showed suppressed ­moisture-­derived deterioration in the oxygen barrier performance, implying

Biopolymers

Bioresources

• Protein SPI, WPI, cornzein, wheat gluten, gelatin etc. • Carbohydrates starch, cellulose, chitosan, agar, carrageenan etc.

Chemical synthesis

• From biomass PLA • From petrochemicals PCL PVA PGA

Microorganisms

• Polyester PHAs (PHB, PHBV) • Carbohydrates Pullulan Curdlan

• Lipids wax, fatty acids.

Fig. 2  Classification of biopolymers. PLA, poly(l-lactide); PGA, poly(glycolic acid); PCL, poly(ɛ-caprolactone); PVA, poly(vinyl alcohol); PHAs, poly(hydroxyalkanoates); PHB, poly(β-hydroxybutyrate); PHBV, poly(3-hydroxybutyrate-co-3-hydroxyvalerate) Source: From Rhim et al.[6] © 2013 with permission from Elsevier.

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Polymer

Nanoparticle

Nanoplatelets

Nanotubes

Nanofibers

Nanowires

Dispersing at least one nanostructure in polymer

Food Packaging– Composites

Nanocomposite

Straight diffusion path -Weak barrier in pristine polymer

Tortuous diffusion path -Improved barrier in nanocomposite

Example: Layered silicate nanocomposite

Exfoliated

Intercalated

Tactoids

Fig. 3  Schematic representation for the preparation of nanocomposites and improvement of the barrier properties Source: From Mihindukulasuriya et al.[9] © 2014 with permission from Elsevier.

the feasibility of applying the ­nanocomposite films to ­single-layer food packaging films. Meira et al. designed PP-based montmorillonite and nisin-containing nanocomposite films as a food packaging material. Nisin was incorporated at 1%, 2.5%, and 5% (w/w), and the composite films inhibited the gram-positive bacteria Listeria monocytogenes, Staphylococcus aureus, and Clostridium perfringens when tested on skimmed milk agar plates.[40] Silver nanoparticles (NPs) containing poly(3-hydroxybutyrate-co-3-hydroxyvalerate) nanocomposites were developed by Lagaron et al. for active food packaging. It was observed that nanocomposites had a prolonged biocide effect even at very low Ag NP content (0.04 wt%).[41] Edible Nanocomposite-Based Material Edible films and coatings are in direct contact with the food product and are integral part of it without changing its taste and texture. Edible coatings are formed on the food stuff by direct coating, spraying, or dipping of a liquid film-forming solution or molten compounds.[30] On the other hand, edible films are self-standing, performed by traditional methods and then applied onto the food. Various water-soluble polysaccharides such as cellulose derivatives, alginates, pectins, starches, and chitosan have been used to develop edible coatings and films.[42,43] Edible films

of various lipids such as waxes, acylglycerols, and fatty acids with excellent moisture barrier properties have been reported.[43] Waxes are commonly used for coating fruits and vegetables to retard respiration and lessen moisture loss.[44] Although edible films have been extensively studied, very less research has been performed on the e­ dible ­nanocomposite films and coatings. Mangiacapra et al. developed pectin/montmorillonite composites with improved oxygen barrier properties.[45] Similarly, Yao et al. reported gelatin/montmorillonite nanocomposites with improved physical properties.[46] Mattoso et al. fabricated chitosan/tripolyphosphate NPs loaded onto hydroxypropyl methylcellulose (HPMC) films.[47] Experimental results showed that HPMC films become more compact and dense by the addition of chitosan NPs. Further, the presence of NPs in the HPMC films improved the tensile and thermostability of these edible films. Further, Azeredo et  al. developed celloulose nanofiber (CNF)-reinforced nanocomposite edible films from mango puree. Addition of CNF significantly improved the tensile strength, Young’s modulus, and water vapor barrier properties.[48] Oxygen and Ethylene Scavenger The presence of oxygen in food packet can cause rancidity, increased growth of aerobic microbes, and depletion

of vitamins and other essential compounds leading to the reduction of product shelf life. On the other hand, formation of ethylene in some fruits can also reduce the shelf life by speeding up the postharvest ripening. Hence, various active packaging approaches are being developed in order to scavenge oxygen and ethylene. Polyolefine is one of the most commonly used packaging materials having excellent water barrier properties. However, its oxygen barrier properties are limited and thus limiting its use for the packaging of oxygen-sensitive products. Busolo et al. developed synthetic iron-containing kaolinite (10 wt%)-loaded HDPE films by extrusion for oxygen scavenging applications in food packaging.[49] It was observed that the iron kaolinite exhibited faster kinetics of oxygen uptake at 100% relative humidity (RH), while absorption capacity was reduced with decreasing RH and temperature for a given storage time. Durrant et al. designed nanocrystalline TiO2-deposited glass and acetate films resulting in the deoxygenation of a closed environment.[50] When TiO2 is irradiated by UV radiation, it leads to the promotion of its electrons from valence to the conduction band resulting into the accumulation of electrons on the surface of TiO2. Here, the transfer of electrons to oxygen in the air is the rate-determining step, and the developed holes can react with water and other compounds owing to their strong oxidation potential. Similarly, TiO2 has also been used for ethylene scavenging to delay the postharvest ripening of climacteric fruits. TiO2-based ethylene scavengers have an unlimited scavenging capacity as TiO2 is not consumed in the catalytic reaction compared to conventional ethylene scavengers whose capacity is dependent on their loading amount in the composite material. Maneerat et al. developed PP films coated with TiO2 suspensions to investigate their catalytic properties toward the photodegradation of ­ethylene.[51] A comparative study was performed to analyze the efficiency of ∼5 μm and ∼7 nm TiO2 particle-coated PP films, and the results showed that the ethylene decomposition rate increased with decreasing TiO2 particle size. The ­application of 10% TiO2 NP-coated film reduced ethylene by 88% ± 6% using black light lamps, whereas 76% ± 10% using fluorescent light lamps, thus making them a potential candidate for ethylene scavenger packaging for horticultural products. Luo et al. investigated the photoelectrocatalytic degradation of ethylene at 3°C and 90% RH on an activated carbon felt (ACF)-supported photocatalyst titanium dioxide photoelectrode [TiO2 /ACF].[52] It was observed that the rate of ethylene degradation was enhanced significantly when a bias voltage was applied to the electrode membrane assembly compared to ethylene degradation when irradiated with UV radiation. Enzymatically Active Packages The immobilization of enzymes in packaging m ­ aterial is interesting as it helps to decrease the amount of a

non-desired food constituent and/or produce a food substance beneficial to the health of the consumer. Hotchkiss et al. developed naringin-containing plastic packaging, which reduced the bitterness of grape juice.[53] Further, β-galactosidase and cholesterol reductase were used by Budny et al. in packaging for the hydrolysis of lactose and cholesterol, respectively.[54] Andersson et al. developed glucose oxidase-containing oxygen scavenger laminate. The enzyme solution with various active ingredients was deposited on a paper carrier, which in turn was placed between two PE films. These laminates showed significant oxygen-scavenging efficiency without its loss under heat treatment.[55] To improve the compatibility between apolar polyolefins and polar enzymes, Harris et al. used polyethylene glycol.[56] Other polymers such as polyesters, polyamides, or polyvinyl alcohol (PVA) can be easily covalently linked to peptides and thus making them a potential candidate for enzymatically active packaging.[57] Indicators for Intelligent Packaging Intelligent packaging improves the communication aspects of the package in contrast to “best before” approach mentioned on it. This technique provides the real-time status of the food product inside the packet using time–­temperature indicators, oxygen and carbon dioxide sensors, freshness indicators, etc.[58] Figure 4 exhibits the basic principles of intelligent packaging systems. Nanotechnology-based indicators or sensors interact with various food components, the species present in the headspace, and external environmental factors resulting into a visual or electrical signal representing the real-time status of the food.[9] This information is useful not only for the consumers to know about the quality of the food product but also for the manufacturers to make a right decision based on the real-time information. As discussed earlier, oxygen content in packaging is reduced to an optimum level so that the spoiling of food can be prevented. In order to keep a check on oxygen level, TiO2 NP-based indicators having redox dye and a sacrificial electron donor are encapsulated in a polymer matrix, which in turn is used for making packaging films. Here, indicator is irradiated with UV radiation and the electrons in valence band are promoted to the conduction band resulting into electron–hole pairs on the surface of the TiO2 semiconductor. Glycerol, the sacrificial electron donor, donates electrons and thus preventing the recombination of the excited electrons with the holes. The excited electrons in turn reduce the methylene blue dye leading to a colorless state, and the oxygen indicator is activated. Upon subsequent exposure to oxygen, reduced dye is oxidized leading to its original blue color whose intensity depends on the amount of oxygen.[59] Mihindukulasuriya et al. encapsulated TiO2 NPs, glycerol, and methylene blue within electrospun poly(ethylene oxide) fibers, which were used to develop membranes.[60]

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Intelligent components

Indicator/sensor

Nanotechnology • Nanoencapsulation • Nanocomposites • Nanofabrication • Self-assembly • etc.

Communication Product information

Food Packaging– Composites

• Consumer’s product acceptance/rejection • Information for producer’s decision support system • Retail inventory tracking • Regulatory agency inspection • etc.

Environment factors (physical, ambient, user)

Headspace

Food

(O2, CO2, H2O microbial metabolites, organic volatiles, etc.) Package

Fig. 4  Schematic representation of the intelligent packaging concept based on nanotechnology Source: From Mihindukulasuriya et al.[9] © 2014 with permission from Elsevier.

Dwarakanth et al. developed CdSe/ZnS quantum dots conjugated with various antibodies. These conjugated quantum dots exhibited blue shifts upon binding to the surface of the target bacteria.[61] Ag nanoplate-based colorimetric indicators for time–temperature history, nanocrystalline cellulose films with rigid rod crystallites for moisture indicator, and photonic crystal hydrogel based on latex spheres of poly(styrene–methyl methacrylate–acrylic acid) with photo-polymerized acrylamide for moisture indicator are some other examples in this area.[62–64] POLYMER-BASED ANTIMICROBIAL FOOD PACKAGING The increasing demand for minimally processed and ­ready-to-eat food products which can be easily cooked requires the packaging that can improve the shelf life of food product while maintaining the food safety, freshness, and quality.[65] Undesirable growth of microorganisms in food is the major challenge that is encountered starting from the production to the distribution of food products.[66,67] Traditional methods such as thermal processing, drying, freezing, irradiation, and adding antimicrobial agents or salts to control the microbial growth cannot be applied to all food products.[68] Direct application of antimicrobial substances onto food surface is limited as the active components are either neutralized or they diffuse into the food product along with the possibility of partial inactivation. Hence, the development of antimicrobial food packaging is of prime requirement to control the outbreak of foodborne

microorganisms and extend the shelf life of food.[69,70] Antimicrobial packaging is a type of active packaging that interacts with the food or the headspace between the food and the packaging to reduce, inhibit, or retard the growth of microorganisms.[71] Here, the slow migration of the active components from the packages to the surface of the food product over an extended time period helps in maintaining high concentrations of active components and thus increasing the safety during transport and storage phase of food distribution.[72] This section will give the ­readers a brief overview about various methods of designing ­antimicrobial packaging along with case studies. Addition of Antimicrobial Agent-Containing Sachets/Pads Using antimicrobial agent-containing sachets or pads is one of the most successful approaches in active packaging, especially at the commercial level to prolong the shelf life of food product.[73] The first sachet Ageless® was developed by Japan’s Mitsubishi Gas Chemical Company as an oxygen scavenger.[74] Later on, various oxygen- and moisture-­absorbing sachets have been developed.[75,76] Although oxygen- and moisture-absorbing sachets are not intended to be antimicrobial, they can indirectly affect the growth of the microbes. The most common antimicrobial agents used in sachets and absorbent pads are ethanol, chlorine dioxide (ClO2), and a variety of plant essential oils (EOs) and their main active constituents.[77,78] The first type of antimicrobial sachets are those that generate antimicrobial ­compounds in situ and release it. The most

Food Packaging: Polymer Composites 1365

Perforations (this may be in both outer layers or at least in one of them) Upper layer

Internal layer (absorbent material)

Lower layer

Flow of exuded liquid

Plastic tray or container Absorbent pad

Fig. 5  Schematic representation of the absorbent pad design Source: From Otoni et al.[86] © 2016 with permission from Elsevier.

common example of this approach is the in situ generation of ClO2 as it cannot be compressed and stored due to the risk of explosion. This is achieved by mixing sodium chloride and an acid inside the gas-permeable sachet.[78] On the other hand, the second type of antimicrobial sachets carry the active components and release them. This approach involves the adsorption of antimicrobial components onto carrier material such as porous polymer, and then these active components are released into the closed packaging system. For example, porous HDPE has been used as a carrier material for the adsorption and release of allyl isothiocyanate, [79–81] oregano EO, [82–84] and lemongrass EO.[82,83] In general, these sachets/pads consist of LDPE, linear low-density polyethylene (LLDPE), PE, paper/PP laminate, etc.[82,84,85] In general, the absorbent pads consist of the upper, lower, and intermediate layers of absorbent material (Fig. 5).[86] The upper and lower layers are sometimes made up of impermeable material in order to avoid the contact of absorbent material with the food product. In order to ensure the desired antimicrobial activity, the upper and lower layers are perforated. Here, the excess compression load caused by the food product and tearing of the upper and lower ­layers due to very close perforations should be avoided. Further, Otoni et al. reported the use of allyl isothiocyanate sachets for peanut storage (Fig. 6).[80,86] The experimental results exhibited the tenfold reduction of Aspergillus flavus growth after 7 days of storage at 25°C, thus ensuring microbiological safety of peanuts. Han et al. stored shredded mozzarella cheese in a nylon/EVOH/PE bag containing a sachet of microcellular foam starch with embedded rosemary oil and thyme oil.[87] The volatile effectively inhibited the growth of L. monocytogenes along with lactic acid bacteria and total aerobic bacteria.

Fig. 6  Allyl isothiocyanate-containing sachet to ensure microbiological safety of peanuts Source: From Otoni et al.[86] © 2016 with permission from Elsevier.

Chang et al. used oregano essential oil microencapsulated with polyvinyl alcohol (oregano EO/PVA ratio: 3/5 (w/w)) in a sachet for the reduction of Dickeya chrysanthemi, molds and yeasts, and total mesophilic aerobic bacteria on the surface of iceberg lettuce.[88] Antimicrobial Coating on the Polymer Surface Antimicrobials that cannot tolerate high processing ­temperatures used for polymers are generally applied in the form of coating.[89] Direct addition of nisin (a ­bacteriocin) into food product leads to activity loss, heterogeneous distribution in the food matrix, inactivation by proteolytic enzymes, and emergence of resistance in normally ­sensitive bacterial strains.[90] Grower et al. developed nisinbased solutions to coat the surface of LDPE films at room temperature.[91] It was observed that these coated films

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Food product

Plastic film for covering the food product

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Fig. 7  The inhibition of P. expansum on the surface of the cheese B. zlato in a contact with the developed antimicrobial packaging film after the storage of 4 days (left) and 18 days (right) at 23°C Source: From Hanušová et al.[95] © 2010 with permission from Elsevier.

could successfully release nisin and inhibit the growth of L. monocytogenes. Franklin et al. fabricated methylcellulose/HPMC-based coating solution containing 10,000 and 7,500 IU mL−1 nisin to coat the packaging film used for hot dog storage.[92] Experimental results indicated that this antimicrobial packaging causes a significant decrease in the number of L. monocytogenes cells by more than 2 log colony-forming unit per package after 60 storage days. Sheldon et al. manufactured nisin-coated polyvinyl chloride (PVC), LLDPE, and nylon to inhibit the growth of Salmonella typhimurium on broiler drumstick skin stored at 4°C.[93] Similarly, Kim et al. reported nisin-coated LDPE films for storing fresh oysters and ground beef at 3°C and 10°C, respectively.[94] Marek et al. designed coextruded polyamide/PE films coated with the polyvinyldichloride lacquer containing both nisin and natamycin to inhibit the growth of various microorganisms such as Penicillium expansum, Fusarium culmorum, Lactobacillus helveticus, and Listeria ivanovií on the surface of the packaged soft cheese Blaťácké zlato.[95] Figure 7 shows the prevention of the P. expansum growth on the surface of cheese B. zlato packaged with the developed films. Viedma et al. developed polythene films coated with the enterococcal bacteriocin enterocin EJ97 alone or in combination with ethylenediaminetetraacetic acid (EDTA) to inhibit the growth of Bacillus coagulans CECT 12 in canned corn and canned peas.[96] Lantano et al. developed natamycin-based antimicrobial coatings on PLA films via sol–gel method to inhibit the outbreak of microorganisms in packaged cheese.[97] Incorporation of Antimicrobial Agents into Polymer Extensive research has been performed on the incorporation of antimicrobial agents into polymer for extending the shelf life of food products. Silver-substituted zeolites, one of the most widely used polymer additives for food applications, are antimicrobials against a wide range of bacteria and molds. Here, sodium ions present in zeolites are substituted

by silver ions, which in turn d­ isrupt the ­microorganism cells’ enzymatic activity.[98] ­Silver-­substituted zeolites can withstand very high temperatures and therefore have been incorporated directly into polymers. These modified ­zeolites are included into polymers such as PE, PP, nylon, and butadiene–styrene at levels of 1%–3%.[99] The most common commercial ­examples of silver-substituted zeolites are Zeomic®, Apacider®, AgIon, Bactekiller, and Novaron. Further, propionic acid, benzoic acid, sorbic acid, and acetic acid have been added to ethylene vinyl a­ cetate (EVA) and LLDPE for antimicrobial food packaging applications.[53] Lacticin in LDPE, hexamethylenetetramine in LDPE, ethylparaben in LDPE, and lysozyme in polystyrene are some of the examples in this area.[53,100–103] Narkis et al. designed LLDPE or LLDPE/EVA films loaded with potassium sorbate (PS) for inhibiting the growth of yeast strain S. cerevisiae S288C.[104] Baldino et al. fabricated PS-loaded cellulose acetate (CA) membranes via the supercritical phase inversion process.[105] Gavara et al. developed EVOH-based polymer films with antioxidant and antimicrobial properties using green tea extract (GTE) and oregano EO.[106] The GTE and EO were added to the film-forming solution (5% w/w), and the films were prepared by solvent evaporation with hot air. The antimicrobial efficiency of the developed films was tested in vitro against different microbes, namely, L. ­monocytogenes, Escherichia coli, and P. expansum. The experimental results of 12 days of storage at 30°C ­indicated that EVOH films containing EO presented a strong effect against fungal growth, while significant growth was observed on the films containing GTE (Fig. 8).[106] As described earlier, antimicrobial agents are also immobilized on the polymer surface via ionic or covalent linkages, taking advantage of various functional groups either already present in polymer chain or achieved by the surface modification of polymers.[107–110] Cationic polymers such as chitosan and poly-l-lysine are inherently antimicrobial and have been used to prepare films for food packaging applications.[53] The amine

Food Packaging: Polymer Composites 1367

(a)

(b)

(c)

functionalities present in these polymers interact with the negatively charged cell membrane of the microbes, resulting in their destruction. Chitosan has also been used as a coating material for improving the shelf life of fresh ­vegetables and fruits.[111,112] Calcium alginate films and polymers containing biguanide substituents have also been used for antimicrobial food packaging applications.[113] CONTAMINATION IN FOOD FROM PACKAGING MATERIAL The consumer decision to select a food product significantly depends upon the odor, taste, quality, and safety of the delivered food product. When a natural or synthetic volatile antimicrobial agent is used in sachets for food storage, it can diffuse in the food along with the packaging headspace. Hence, monitoring the migration profile of the applied antimicrobial agent is crucial for successful commercial application. Pires et al. reported the reduction of allyl isothiocyanate content in the headspace of sliced mozzarella cheese during storage, which could be either due to interaction between the active component and food or the permeation of active component through packaging material.[81] Similar results were reported by Otoni et al. for the peanuts stored with allyl isothiocyante sachets.[80] ­Further, Kapetanakou et al. observed the increasing ­alcohol content in minced beef product stored at 4°C and 8°C with ­absorbent pads containing in total 50 mL of Tsipouro.[114] Migration of NPs Nanocomposites exhibit high potential in food packaging area by improving the shelf life and quality of the product. However, the migration of NPs from the packaging materials to food and the effect of the ingestion of these NPs inside the body from the mouth to the final gastrointestinal tract are the primary concerns for their successful wide application.[115–117] Hence, it is crucial to investigate the

extent of NPs migration in order to determine the extent of end exposure to these small particles.[118] Franz et al. described the use of advanced analytical techniques such as asymmetric flow field-flow fractionation coupled to a multi-angle laser light-scattering detector for ­separating, characterizing, and quantifying the potential release of titanium nitride NPs from LDPE films and inductively coupled plasma–mass spectrometry for determining the total content of NPs in a migration solution.[119] Hannon et al. assessed the migration of silver NPs from LDPE films to food simulant under conventional oven heating for10 days at 60°C, 2 h at 70°C, 2 h at 60°C, or 10 days at 70°C. Migration was also analyzed under microwave heating.[120] The experimental results from inductively coupled plasma–atomic emission spectroscopy showed that microwave heating resulted in significantly higher migration compared to oven heating for similar temperatures (100°C) and identical exposure times (2 min). The presence of Ag NPs in food simulant was confirmed by scanning electron microscopy.[120] As limited scientific research has been performed on migration of NPs from packaging materials to food, it is mandatory to investigate the extent of NPs ­m igration and to develop a method to prevent such migration before applying the nanocomposites for food packaging applications. Migration of Additives Used in Developing Polymer Packages Various additives such as plasticizers, antioxidants, lubricants, antistatic agents, and slip additives are used to develop different types of polymeric packaging materials.[121,122] Butyl stearate (BS), acetyl tributyl citrate (ATBC), alkyl sebacates (AS), adipates, and phthalates are some of the common plasticizers used for packaging application. BS, ATBC, AS, and adipates have been reported with low toxicity, whereas various regulations have been formed for the use of phthalate plasticizers due to its potential c­ arcinogenic and estrogenic effect as reported in the literature.[123]

Food Packaging– Composites

Fig. 8  Antifungal effect against P. expansum for the EVOH29-GTE film (a), EVOH29-OEO film (b), and EVOH29-GTE+OEO film (c) Source: From Muriel-Galet et al.[106] © 2015 with permission from Elsevier.

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Castle et al. conducted a survey to assess plasticizers level in a variety of food products packaged in vinylidene chloride copolymers (PVDCs), nitrocellulose‐coated regenerated cellulose film (RCF), and CA.[124] The food products were analyzed by a stable isotope dilution gas chromatography/mass spectrometry (GC/MS) technique for dibutyl phthalate (DBP), dicyclohexyl phthalate (DCHP), and diethyl phthlate (DEP); GC/MS (selected ion monitoring) for butylbenzyl phthalate (BBP) and diphenyl 2‐ethylhexyl phosphate (DPOP); and GC with flame ionization detection for dibutyl sebacate (DBS) and acetyl tributyl citrate (ATBC). The experimental results showed the presence of ATC in cheese (2–8 mg/kg); DBS in processed cheese and cooked meats (76–137 mg/kg); DBP, DCHP, BBP, and DPOP (0.5–53 mg/kg in various food products); and DEP in quiches (2–4 mg/kg).[124] Migration of triacetin plasticizer from starch-based films to milk under microwave heating was investigated by Huang et al.[125] It was reported that milk migrates into the amorphous region of the film during microwave treatment, and the plasticizer continues to leach out. The ­control experiment by simple immersion of the film in milk at 30°C without microwave showed that triacetin migration from the starch ester film to the milk system was accelerated during microwave heating.[125] Coltro et al. evaluated the migration of various plasticizers from PVC films to food simulants (Fig. 9).[126] It was reported that the difference in migration was due to different molecular structures of plasticizers. Detailed investigation showed that a contact of film with acidic food simulant for a prolonged time up to 40°C did not

Plasticizers (mg/dm2) Overall migration (mg/dm2) Tensile strength (MPa)

70 60 50 40 30 20 10 0

1

2

3

Samples

4

5

6

Fig. 9  Ratio between total plasticizers concentration, overall migration, and tensile strength for the PVC films. Sample 1: di(2-­ ethylhexyl)adipate (DEHA) + epoxidized soybean oil (ESBO); sample 2: DEHA + ESBO + ATBC; sample 3: DEHA + ESBO + ­mixture of glycerin acetates (MGA); sample 4: DEHA + ESBO + di (2-ethylhexyl)-1,4-benzenedicarboxylate (DEHT) ; sample 5: DEHA + ESBO + a­ cetylated glycerol monoester (AGM); sample 6: DEHA + ESBO + polyadipate Source: From Coltro et al.[126] © 2014 with permission from Elsevier.

promote the ­migration of plasticizers, but with fatty foods, 75%–90% loss of plasticizers was observed. Epoxidized seed and vegetable oils such as soybean oil are frequently used as thermal stabilizers in packaging materials. The toxicity of these materials is attributed to residual ethylene oxide, and it decreases with increasing molecular weight. These thermal stabilizers are used in the range of 0.1%–27% in various polymers such as PVC, PVDC, and polystyrene.[123] Further, triphenyl phosphite is an antioxidant which is toxic and can be used in packaging materials only when it is not in direct contact with fatty food stuffs. Impact on Health A potential threat to consumer health may be envisaged to emerge from the consumption of food and drinks containing nanosized ingredients and additives. NPs can enter the body by inhalation, skin penetration, and ingestion-­ posing risk to human health depending upon the extent of exposure.[127] NPs can also be eventually taken up by the environment and enter the food chain indirectly. Workers at the factories producing nanomaterials are prone to the exposure by inhalation and skin penetration. Hence, the use of high-quality gloves, safety glasses, and masks with high-efficiency particulate filters is recommended at these working areas.[116] On the other hand, the final consumer is at risk by exposure to NPs, mainly by ingestion of food products and drinks having nanoparticulate materials. These NPs can cross the cellular barrier and can lead to oxidative damage to the cell and inflammatory reactions.[128–131] An in vitro study conducted by Mikecz et al. showed that silica NPs smaller than 70 nm can enter the nucleus of human epithelial cells indicating the risk for impairment of DNA replication and transcription.[132] Szentkuti et al. reported that smaller particles of roughly 55 nm size with negative charge can penetrate through the porous mucus layer of gastrointestinal epithelia.[133] However, a research at the John Hopkins University in Maryland suggests that particles as large as 200 nm can pass through mucus pores when coated with polyethylene glycol.[134] Various research studies indicate that TiO2 NPs, Ag NPs, and carbon NPs/nanotubes can enter into the blood circulation via the gastrointestinal route and then reach the liver and spleen.[135–137] Research indicates that carbon nanotubes exhibit epidermal or pulmonary cytotoxicity, whereas ZnO NPs display genotoxic potential in human epidermal cells.[138] Chronic exposure can lead to the accumulation of NPs inside the major organs with consequences that are not much studied. Dedicated research needs to be conducted for the migration of NPs to the fetus and brain. On the other hand, de Abreu et al. reported that the migration of caprolactam, 5-chloro-2-(2,4-­dichlorophenoxy)phenol (triclosan), and trans,trans-1,4-­diphenyl-1,3-butadiene (DPBD) from polyamide and polyamide–­nanoclays to

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CONCLUSIONS AND FUTURE PERSPECTIVES Polymer composites with improved performance properties are an important class of packaging materials. Taking into account the increasing demands for improved shelf life along with the less usage of synthetic/petroleum-based materials, natural compounds have been widely exploited by the scientists all over the world. A significant interest has led to the extensive research on the application of bio-based and/or biodegradable materials as environmentally friendly alternatives for packaging materials. Additionally, biodegradability of the packaging materials can be tuned through a suitable choice, i.e., combination of polymer matrix and NPs. Property enhancement achieved by the addition of nanomaterials in small quantity compared to traditional materials opens a new pathway for food ­packaging applications. Bio-nanocomposite materials with added functional properties have a high potential for the development of active and intelligent packaging, high barrier packaging, nanosensors, and freshness indicator. It will help to extend the shelf life of food product along with maintaining the quality and freshness of the food product between production and delivery to the consumers. Further, the development of novel dual or multi-action sachets is encouraged in order to optimize the efficiency of sachets/absorbent pads. Increasing attempts are being made for developing sachets/pads, which have flavoring, antioxidant, nutrient-releasing, and anti-insect activities. Polymer composite-based packaging materials appear to have a very bright future for innovative active and intelligent food packaging with bio-functional properties. Participation of research institutions, industry, and government regulatory agency is of prime importance to develop new packaging technologies and their successful application.

ACKNOWLEDGMENTS GA thanks DST INSPIRE Faculty Award of Government of India (DST/INSPIRE/04/2015/003220) for the financial support. RA thanks the Department of Biotechnology, Government of India, for Senior Research Fellowship.

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Food Packaging: Polymer Composites 1373

Food Packaging: Polymers as Packaging Materials in Food Supply Chains A. B. Hemavathi and H. Siddaramaiah Department of Polymer Science and Technology, Sri Jayachamarajendra College of Engineering, Mysuru, India

Abstract

Food Packaging (cont.)–Fuel Cell

Food packaging helps in the preservation of food as well as gives mechanical support and protection in transit. Polymers have long been a vital part of food packaging due to their specific mechanical strength, ease of processing, excellent barrier properties, and low cost. This entry mainly focuses on the recent advances/trends in food packaging applications of polymers. It aims to obtain a better understanding of various food packaging polymers, key problems of current food packaging materials, food–material interactions, emerging aspects of nanotechnology for development of environmentally benign and smart food packaging materials, and regulations of food packaging. Functionalities of food packaging material play a primary role in keeping the freshness of food. Though conventional materials such as glass, metals, and paper for food packaging and storage seem to be efficient, most of them are selective in their functions and are comparatively of high cost. Over the past several years, innovations in polymer packaging materials have made them possible to store, protect, and preserve food from spoiling and damage. This entry reviews the major role of polymers in food supply chain as a packaging material. We hope that knowledgeable efforts by industry, ­government, and consumers will promote continued usage of polymer food packages in the future too. Keywords: Active packaging; Antimicrobial packaging; Barrier properties; Biopolymers; Food packaging; Intelligent packaging; Nanocomposites; Preservation; Thermoplastic.

INTRODUCTION Packaging is the technology of enclosing or protecting products for storage, distribution, sale, and use. Packaging also refers to the process of designing, evaluating, and producing packages. The mankind has experienced a wide range of food packaging materials such as leaves, bark, glass, metals, paper, paperboards, and plastics/polymers over the centuries. Among these, polymer packaging has gained immense popularity in the recent decades. The polymer packaging has evolved to an extent that it has almost completely replaced all conventional materials. This is mainly attributed to polymer properties that can be tailored to meet specific needs by varying the atomic composition of the repeating structure, by varying molecular weight, and by the incorporation of a wide range of additives to impart specific functionalities. Food products are more prone to be perishable if not properly processed and packed. Thus, food packaging plays an important role in reducing food waste. An ideal food package should exhibit many integrated characterstics such as: nontoxicity, selective barrier properties (barrier to ­flavor/aroma loss, moisture, and oxygen), product visibility, stable performance over a wide temperature range,

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good mechanical strength, easy processibility, ability to label/self-explanatory, resistance to leaching/­migration, strong marketing appeal/esthetics, environmentally benign and sustainable, and finally all the above mentioned traits, be available at affordable cost and be in line with the regulations. Polymer food packaging can exhibit most of the aforementioned characterstics if designed and handled suitably. This means that the selected polymer food packaging needs to satisfy physical, chemical, and biological role using their life cycle as packaging, and once these primary functions have been fulfilled, the packaging should be disposed without polluting the environment. Hence, a more holistic approach is necessary to balance/imbibe the multifunctionality into a food packaging material. Developing polymer food packaging should consider the future demand of society. The lifestyle pattern have been changed over the years, smaller families, less leisure time, increase in average lifespan, improvements in the quality of life/standard of living, general level of education, and market globalization resulting in longer distribution of food may also demand specific functions of packaging. Eating styles, such as ready-to-eat foods, fresh snacks, and microwaveable ready meals, demand innovation in food packaging. In case of prepacked foods, packaging material should Encyclopedia of Polymer Applications, First Edition DOI: 10.1201/9781351019422-140000032 Copyright © 2018 by Taylor & Francis. All rights reserved.

Food Packaging: Polymers as Packaging Materials in Food Supply Chains 1375

FOOD PACKAGING MATERIALS The key to successful food preservation is to select the suitable packaging material and design it to best satisfy competing needs with regard to product characteristics, marketing considerations, environmental issues, and cost. Some of these factors are interrelated, e.g., single-serving food packaging meets consumer needs, but bulk packaging is better for environmental reasons. Attempts to balance competing needs can sometimes be addressed by a combination of packaging materials such as combining different polymers through coextrusion or lamination or by laminating polymers with metal foil or paper. An exercise of balancing and negotiation in various aspects are involved in the development of a successful food packaging. In general, materials that provide optimum protection of food quality and safety are most preferred. In particular, food/package interactions also play an important role in selection of packaging materials for food applications. Each packaging material has its inherent properties. These properties dictate the selection of material for a particular food, given the characteristics of that food (e.g., moisture, oxygen, and light sensitivity). Table 1 lists the properties

Table 1  Polymer properties to be considered for food packaging applications 1 Physical properties

Density, transparency, opacity, refractive index, UV resistance/ photosensitivity

2 Chemical properties

Chemical resistance at different pH, oxidative stability, resistance towards oils, fats, and other food additives

3 Mechanical properties

Toughness, tensile strength, elongation, tear strength, flexibility, puncture resistance/impact resistance

4 Thermal properties

Melting point, softening point, heat sealability, microwave resistance, autoclave and sterilizing temperature tolerance

5 Other properties

Barrier property (against O2, CO2, water vapor, ethylene gas, aroma, and flavor), migration/leaching property, toxicity (biocompatibility), processability, printability, recyclability, and cost

of food packaging material to be considered during its selection for intended application. The right selection of packaging material and technologies ensures food quality and freshness during distribution and storage. Today’s food packages often combine several materials to exploit each material’s functional or esthetic properties. The FDA regulates food packaging material selection under section 409 of the federal Food, Drug and Cosmetic Act. It defines food contact substance (FCS) as “any substance intended for use as a component of materials used in manufacturing, packing, packaging, transporting, or holding of food if the use is not intended to have a technical effect in such food.” [1] All FCSs that may reasonably migrate to food under conditions of intended use are identified and regulated as food additives and classified as ­generally recognized as safe (GRAS) substances. Conventional Food Packaging Materials Conventional food packaging is meant for mechanical supporting of otherwise nonsolid food and for protecting food from external factors like dust, humidity, oxygen, ­microorganisms, light, etc., and by doing so, guarantees convenience in food handling and preservation for an extended time period. The key safety objective for these traditional materials in contact with food is to be as much inert as possible, i.e., there should be a minimum interaction between food and packaging material.[2] The ­conventional food packaging materials mainly includes paper, paperboards, glass, and metals (aluminum, tin, and steel). These materials however serve limited functions than advanced polymer packaging materials. Figure 1 summarizes the expected role of packaging in food supply chain management.

Food Packaging (cont.)–Fuel Cell

have the ability to stand the severity or type of process conditions, such as canning, microwaving, and retorting conditions. Irradiated foods are usually prepacked before treatment by ionizing radiation. Active packaging, modified atmosphere packaging (MAP), antimicrobial packaging (AMP), nanomaterial-based packaging, intelligent/ smart packaging, and edible food packaging are aimed to extend the shelf life of food, while maintaining their nutritional quality and safety. Food packaging also affects international trade by making shipping of food products possible and allowing seasonal products to be accessible out of season. Innovations in packaging will protect food in transport, extend the shelf life, and reduce waste along the supply chain from producer to consumer, leading to better food management. All the polymers are not permitted for food packaging applications. The Food and Drug Administration (FDA) regulates the safety of food-contact packaging, including polymers used in contact with food. All food-contact packaging materials must pass FDA’s stringent approval process before they can be put on the market. Food market globalization demands uniform and stringent regulation maintenance by all the food supply chain participants. This entry describes the role of polymer food packaging in the food supply chain, the types of materials used in food packaging, and recent innovations in polymer food packaging. Finally, it addresses regulations of food packaging. With all these challenges, creating an ideal food packaging is as much an art as science, trying to achieve the best overall result without falling below acceptable standards in any single category.

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Food Packaging (cont.)–Fuel Cell

Paper and Paperboard

Metals

Paper and paperboard are materials made from an interlaced network of cellulose fibers derived from wood, a renewable resource. FDA regulates the additives used in paper and paperboard food packaging (21 Code of Federal Regulations (CFR) Part 176). Paper and paperboards are commonly used in the form of corrugated boxes, folding cartons, wrapping paper, bags and sacks, tissue paper, paper plates, and cups. Plain paper is not used to protect foods for long period of time because it has poor barrier properties and is not heat sealable. Paperboard is thicker than paper with a higher weight per unit area and often consist of multiple layers. Typically developed paperboard and corrugated materials act as physical barriers to resist impacts, abrasions, and crushing damage, so they are widely used as shipping containers and as packaging for delicate foods such as eggs and fresh fruits, and seldom used for direct food contact. When used as primary packaging (i.e., in contact with food), paper is almost always coated, laminated, or impregnated with materials such as waxes, resins, or lacquers to improve functional and protective properties.[3] Paper can be laminated with metal film and/or selected polymers such as polyethylene (PE) to improve its barrier properties and to make it heat sealable. Recent advancement reported the potential usage of nanostructured surface coating to enhance the moisture-­ repellent properties of paper-based packaging materials. The surface of the paper was modified to create a lotus-like superhydrophobic surface by applying coating prepared with R812S silica nanoparticles (NPs) and polydimethylsiloxane silicone oil.[4] In another study, polystyrene (PS) nanocomposites containing TiO2 and silver NPs (AgNPs) applied as a coating onto a paper substrate display both  antibacterial and better barrier property.[5] The main advantages of paper and paperboards are low weight, low cost, renewable, relatively high stiffness, and excellent printability.

Metals are the most versatile of all packaging materials. It offers a combination of excellent physical protection, barrier properties, formability, impact resistance, d­ ecorative potential, recyclability, and consumer acceptance. The metals most predominantly used in food packaging are ­aluminum, tin, and steel. Aluminum is lightweight and resistant to most forms of corrosion, and has good flexibility, excellent malleability, formability, and outstanding embossing potential. It is also an ideal material for recycling because it is easy to reclaim and process into new products without much change in its original characteristics. The main disadvantage of aluminum is its high cost compared to other metals (e.g., steel) and its inability to be welded, which makes it useful only for making seamless containers.[7] Laminated food packaging involves the binding of aluminum foil to paper or plastic film to improve the barrier properties. A less expensive alternative to laminated packaging is metallized film. Metallized films are plastics containing a thin layer of aluminum metal. These films are more flexible than laminated films, have improved barrier properties, and highly reflective surface, which makes it more attractive. Metallized films are mainly used in snacks packaging.[3] Although the individual components of laminates and metallized films are technically recyclable, the difficulty in sorting and separating the material precludes economically feasible recycling. Tin is the other metal that is widely used in food packaging applications. It has excellent barrier properties to water vapor, gases, light, and odors, and tinplates can be heat treated and sealed hermetically making it suitable for sterile products. It also has good ductility and formability, and hence can be used for making different shapes of containers. Thus, tinplate is widely used to form cans for drinks, processed foods, powdered foods, aerosols, and as package closures. Its relative low weight and high mechanical strength make it easy to ship and store. Tin can be easily recycled many times without loss of quality and is lower in cost than aluminum.[7]

Glass Glass is an absolute barrier to moisture, gases, chemicals, and other environmental agents, so it maintains product freshness for a long period of time without changing the organoleptic properties of food. The ability to withstand high processing temperatures makes glass useful for heat sterilization of both low- and high-acid foods. The transparency of glass allows consumers to see the product, yet variations in glass color can protect light-sensitive contents. Glass is not recommended for frozen products or for ground or roasted coffee because of breakage costs and the difficulty of vacuum flushing. [6] However, glass packaging benefits the environment because it is reusable and recyclable. The disadvantages of glass are fragility, heavy weight, and energy-intensive ­manufacturing process.

Polymer as Food Packaging Material Polymers/plastics are made by addition or condensation polymerization of monomer units. There are several advantages of using plastics/polymers for food ­packaging such as lightweight, design flexibility, low cost, heat s­ ealable, easy to print, tunable physical, mechanical, and optical ­properties. The packaging processes can be integrated, wherein the package is formed, filled, and sealed in the same production line. There are two major categories of plastics: thermoplastics and thermosets. Thermosets are polymers that solidify or set irreversibly when heated/cured and cannot be remolded. Since they are strong and durable, they tend to be used primarily in construction and automobile applications, not in food packaging applications. On the

Food Packaging: Polymers as Packaging Materials in Food Supply Chains 1377

Manufacturer expectations Easy to process Heat sealable Printable/decorative Sustainable/renewable raw material Supplier expectations

Government expectations

Marketing appeal Self explaining Protection in transit

Meet standards and regulations Food packaging

Environmentally benign Recyclable

Shelf-stable

Pollution free

Easy to stack/handle Impact resistant

Consumer expectations Safe/non-toxic Preserve freshness Product visibility Package integrity Cooking instructions Easy to dispense Ready to cook/serve Microwavable Resealable/reusable Affordable cost

Fig. 1  Expected role of packaging in food supply chain management

other hand, thermoplastics are polymers that soften upon heating, shaped, and set in shape by cooling. Since thermoplastics can be easily shaped and molded into various products such as cups, bottles, jugs, and films, they are ideal for food packaging. Moreover, all thermoplastics are recyclable. In contrast to glass or metal packaging, packages made with polymers are permeable at different degrees to small molecules such as water vapor, oxygen, organic vapor, and other low-molecular-weight compounds such as flavor, aroma, and additives present in food. To overcome this disadvantage, polymers are often compounded, blended, laminated, and modified into composites/nanocomposites. It is reported that packaging industry consumes more than 40% of the plastics with half of it for food packaging.[8] Food packaging uses a wide range of polymer/plastic materials in both rigid and flexible forms, e.g., polyolefins, which include low density PE (LDPE), linear low density PE (LLDEP), and high density PE (HDPE), ­polypropylene (PP), and biaxially oriented PP (BOPP); copolymers of ethylene such as ethylene-vinyl alcohol (EVOH), ethylene-­ vinyl acetate (EVA), and ethylene-acrylic acid; substituted olefins such as PS, oriented PS (OPS), high impact PS (HIPS), poly(vinyl chloride) (PVC), ­poly(vinylidene chloride) (PVDC), poly(vinyl alcohol) (PVOH), and poly(tetrafluoroethylene) (PTFE) and acrylonitriles such as polyacrylonitrile (PAN) and acrylonitrile/styrene,

polyamides, and polyesters such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and ­copolymer of PET-PEN.[9] Apart from these synthetic polymers, in recent years, biopolymers such as regenerated cellulose, polylactic acid (PLA), polyhydroxyalkanoate (PHA), polyhydroxybuterate (PHB), polyhydroxybutyrateco-­hydroxyvalerate, polybutylene succinate (PBS)-adipate, poly(butylene adipate-co-terephtalate), and poly(hydroxyl ester ether) are also used for food packaging applications to ensure sustainability.[10] Some of the widely used ­polymers for food packaging applications are discussed later. Polyolefins Polyolefin is a collective term for polyethylene (PE) and polypropylene (PP), the two most widely used polymers in food packaging and other less popular olefin polymers. The simplest and inexpensive plastic made by addition polymerization of ethylene is PE. PE and PP possess a desired combination of properties such as lightweight, strength, flexibility, moisture and chemical resistance, and easy processability, and are well suited for recycling and reuse. There are three classes of polyethylene: HDPE, LDPE, and LLDPE. HDPE is stiff, strong, tough, resistant to chemicals and moisture, easy to process, and easy to form but permeable to gases. It is used to make bottles for milk,

Food Packaging (cont.)–Fuel Cell

Nutritional information

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Food Packaging: Polymers as Packaging Materials in Food Supply Chains

Food Packaging (cont.)–Fuel Cell

juice, and water; cereal box liners; caps and closures; margarine tubs; blown and cast films for pouches; and grocery, trash, and retail bags. For example, HDPE can be modified by incorporating iron-containing kaolinite to create oxygen-scavenging packaging films.[11] LDPE is flexible, strong, tough, easy to seal, and resistant to moisture and used as a water-resistant film in wrapping. Since LDPE is relatively glossy and transparent, it is predominantly used in film applications where heat sealing is necessary. Bread and frozen food bags, squeezable food bottles, and flexible lids are examples of LDPE food packages. LLDPE is more transparent than HDPE, and it is mainly used as films, sheets, stretch wrap, and multilayer films. PP is another polyolefin widely used in food packaging, which is transparent than PE, has good resistance to chemicals, and is an effective water vapor barrier. Its high melting point (170°C–190°C) makes it suitable for applications such as hot-filled, sterilizable, autoclavable, and microwavable containers. Popular uses of PP include closures, lids, yogurt containers, and margarine tubs. When used in combination with an oxygen barrier such as EVOH or PVDC, PP provides more strength and better gas barrier properties. BOPP possesses more strength, stiffness, gloss, transparency, and barrier properties than unoriented PP. But BOPP has poor heat sealability than PP. BOPP is mainly used for shrink wrapping and for fresh produce, confectionary, cereals, nuts, roasted coffee, and snack packing. It is also used in MAP of fruits.[12]

(transparent) and semicrystalline (translucent/opaque) thermoplastic materials. Amorphous PET has better clarity and ductility but less stiffness and hardness than semicrystalline PET. It is a good barrier to gases (oxygen and carbon dioxide) and moisture. It also has good resistance to heat, ultraviolet (UV) radiation, mineral oils, solvents, and acids, but not to bases. Crystalline PET can withstand both freezing and oven baking conditions and hence is used in boil-in bags, ready meals, and sterilizable food packaging. PET is a better choice particularly for beverages and mineral water.[13] The main reasons for its ­popularity are its glasslike transparency, impact ­resistance, adequate gas barrier for retention of carbonation, lightweight, shatter resistance, and ability to recycle easily. The major p­ ackaging forms of PET are containers (bottles, jars, and tubs), semirigid sheets for thermoforming (trays and blisters), and thin ­oriented films (bags and snack food wrappers). Polyethylene naphthalate (PEN) is another polyester used in food industry. It is a condensation polymer of dimethyl naphthalene dicarboxylate and ethylene glycol. The barrier properties of PEN to water vapor, carbon dioxide, and oxygen are superior to those of PET, and it provides better performance at high temperatures, allowing hot refills, rewashing, and reuse. It is used in making food trays for use in convection and microwave ovens. Because PEN provides protection against transfer of flavors and odors, it is well suited for beverages such as beer. However, PEN costs three to four times more than PET.[14]

Polystyrene

Polyamides

PS is a transparent, hard and brittle polymer with a melting point of ~190°C. It can be monoextruded, coextruded with other polymers to overcome poor barrier properties, ­injection-molded, thermoformed, or foamed to produce a range of products. Foaming produces an opaque, lightweight, rigid material with better impact protection and thermal insulation properties. PS foams are used for trays and insulated containers to keep food products cold or hot. Typical applications include protective packaging such as egg cartons, containers, disposable plastic wares, lids, hot cups, plates, bottles, and food trays. OPS is produced by stretching the extruded PS film, and it has improved strength and transparency, reduced haziness, and high stiffness. This is often used in packages where the manufacturer would like the consumer to see the enclosed product.[2] Some benefits to OPS are that it is less expensive to produce than other clear plastics such as PP, PET, and HIPS, and it is less hazy than HIPS or PP.

Polyamides (nylons) are linear thermoplastics obtained by polycondensation of dicarboxylic acids with diamines or by self-condensation of aminoacids. The major types of nylons used in food packaging are nylon 6 and nylon 66. They have excellent combination of properties such as good impact strength, flexibility, heat resistance, puncture resistance, chemical resistance, and good oxygen, carbon dioxide, and aroma barrier property, but poor moisture barrier properties due to its hydrophilic nature. The major uses of nylon as flexible films (blown/cast/biaxially oriented), coextruded or laminated structure with PE, PP, and EVOH, include packaging of fruit juices, fish, meat, ready meals, boil-in bag foods, cheese, cereals, dairy products, and other oxygen-sensitive foods. They are the vital choice for vacuum packaging of products that tends to oxidize easily. An exterior polyamide layer in multilayer films ensures high heat resistance, allowing high sealing line speed and good seal quality.[15]

Polyesters

Polyvinyl Chloride

Polyesters are produced by the reaction of a diol with a dibasic acid. PET is a widely used polyester for food packaging applications. It is produced by reacting terephthalic acid with ethylene glycol. It exists as both amorphous

PVC is an addition polymer of vinyl chloride that is rigid and transparent. It can be transformed into materials with a wide range of flexibility with the addition of plasticizers such as phthalates, adipates, citrates, and phosphates.

Phthalates are mainly used in nonfood packaging applications such as cosmetics, toys, and medical devices but not for food contact applications.[16] Since they are easily thermoformed, PVC sheets are widely used for blister packs, cling wraps such as those for meat products, and unit dose pharmaceutical packaging. PVC has excellent resistance to chemicals (acids and bases), grease, and oil. The major applications include packaging of poultry, cured meat, cheese, snack foods, tea, edible oils, liquor, coffee, and confectionary, and food wraps for fresh fruits, ­vegetables, and fresh red meat. Polycarbonate Polycarbonate is formed by melt condensation of bisphenol A with diphenyl carbonate or phosgene. It is a highly transparent, rigid, hard, durable, impact-resistant, and heat-resistant polymer, and can withstand an autoclave ­sterilization temperature. It can be readily shaped by ­injection molding, thermoforming, and extrusion blow molding. It is mainly used as a replacement for glass in items such as large returnable/refillable water bottles/­ containers and ­sterilizable baby bottles. Care must be taken when ­cleaning polycarbonate, because using harsh detergents such as sodium hypochlorite is not recommended as they catalyze the release of bisphenol A, a potential health hazard.[17] Because no known single polymer discussed earlier exhibits all the desired barrier and mechanical properties required for every conceivable food packaging application, laminates, multilayer films, composites, or polymer blends are often utilized. For example, in an application where ultrahigh oxygen barriers are required over a large humidity range, a high oxygen barrier, water-­sensitive material such as EVOH can be sandwiched between two layers composed of a relatively hydrophobic polymer such as PE.[18] Smart-blended coextrusion can be used to achieve the desired properties that cannot otherwise be attained with polymer monolayers. Polymer nanocomposites (PNCs) are other alternatives to combine many desirable properties into food packaging. They use fillers such as clay, silica, and silicate nanoparticles;[19–22] carbon nanotubes (CNTs);[23,24] graphene;[25–27] starch nanocrystals;[28,29] cellulose-based nanofibers or nanowhiskers;[30–33] chitin or chitosan NPs;[34–36] and other inorganics [37–40] along with different polymers. Though enhancing the polymer barrier properties is the most obvious application of PNCs in the food industry, PNCs are also stronger and more flame resistant, and possess better thermal properties than polymers that contain no nanoscale fillers. They can also be used to alter surface wettability and hydrophobicity. Unfortunately, PNC production involves higher material costs, requires the use of additional additives that complicate their regulation by federal agencies, and entails added difficulty when it comes to recycling. As a result, there is still a significant push in the polymer industry to

generate thick monolayer films with improved mechanical and gas barrier properties. Food is a very complex entity that needs to be safeguarded against many deteriorating parameters by a single packaging material, so many of the times, these packaging materials are used in combination to get the synergistic benefits. For example, a retort pouch consists of a laminate of flexible plastics and metal foils. It allows the sterile packing of a wide variety of food and drink handled by aseptic processing and is used as an alternative to traditional industrial canning methods.[41] A typical retort pouch consists of PET to provide gloss and rigidity, aluminum for effective gas barrier property, nylon (bi-oriented) for puncture resistance, and cast PP/PE as a sealing and bonding layer. However, the retort pouch construction varies from one application to another, as a liquid product needs different barrier properties than a dry product, and similarly, an acidic product needs a different chemical resistance than a basic product. The selection of the best packaging material is a challenging task for food scientists, because it needs to be  versatile enough to withstand handling process while maintaining physical and chemical integrity and suitable barrier properties to several gases used in MAP (e.g., O2, N2, CO2). Furthermore, the intrinsic composition of the packed food (e.g., pH, fat content, aroma) may have an influence on the sorption characteristics of the packaging materials. For some polymers, the absorption of vapor or liquid can cause an increase of polymer plasticization, resulting in loss of mechanical strength. FUNCTIONS SERVED BY FOOD PACKAGING MATERIALS Any packaging performs four main functions: product containment, protection and preservation, convenience, and communication and marketing.[42] Food packaging maintains the benefits of food processing, enabling food to travel safely for long distances from their point of origin and still be wholesome at the time of consumption. Food packaging can be described as a coordinated system of preparing foods for transport, warehousing, logistics, sale, and end use. Innovations in food packaging aim at improving, combining, or extending these four basic functions of traditional food packaging. Product Containment Containment is the most basic function of a packaging, and it refers to holding food in a form suitable for transport or handling. Food can be packed either in rigid or flexible container/package depending on the product nature. Food in the form of liquids, semiliquids, powders as well as bulk solids cannot be transported/marketed without suitable containers. It is sometimes necessary to design packaging

Food Packaging (cont.)–Fuel Cell

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that is shaped especially to contain a particular food (e.g., egg boxes) so that the product is held securely and well protected from damage. The packaging should keep food intact, which means that liquid products should not leak and that dry materials such as flour should not spill out. Protection and Preservation

Food Packaging (cont.)–Fuel Cell

The second function of packaging is to protect the food from surroundings and maintain the organoleptic quality throughout the shelf life. Protective packaging is a term applied to packaging primarily designed to protect the goods, rather than for appearance or presentation. Food packaging can retard deterioration, retain the beneficial effects of processing, extend shelf life, and maintain or increase the quality and safety of food. Food product shelf life is controlled by product characteristics, properties, storage, and distribution conditions. Physical, chemical, enzymatic, and microbiological reactions can cause deterioration in food quality. Physical protection shields food from mechanical damage and includes cushioning against the shock and vibration encountered. Chemical protection minimizes compositional changes triggered by environmental factors such as oxygen, moisture, or light (visible, infrared, or UV). In addition to light barriers, use of UV absorbers in the packaging material can decrease lipid oxidation. Biological protection provides a ­barrier to ­microorganisms (pathogens and spoiling agents), insects, rodents, thereby preventing disease and spoilage. In ­addition, biological barriers maintain conditions to control ripening and aging. The oxygen barrier property of a food container for fresh products such as fruits, salad, and ready-to-eat meals plays an important role in preservation. So, when a polymer packaging has low oxygen permeability coefficients, the oxygen pressure inside the container drops to the point where the oxidation is retarded, extending the shelf life of the product. The formation of ethylene vapor in climacteric fruits can accelerate the postharvest ripening and quickly shorten the shelf life.[43] Temperature and humidity parameters are of crucial importance for food quality preservation. Packaging is part of the transit and distribution process that delivers food to the consumer safely and facilitate handling. It is important to be aware of the distribution challenges and design of package to suit it. Packaging plays a major role in international trade, which makes safe shipping/export of food products possible across a long distance. Food/package interaction also plays a major role in the preservation of food safety and quality. This may involve the transportation of low molecular weight compounds such as water, aroma, and gases/vapors from the food through the package, from the external environment through the package, from the food into the package, and/or from the package into the food. It may also include chemical changes in the food, package, or both. These interactions result in food contamination, decrease

Food Packaging: Polymers as Packaging Materials in Food Supply Chains

in quality, and loss of package integrity. Ingress of oxygen in food package can lead to a reduction of product shelf life due to various degradation reactions (e.g., rancidity, browning, depletion of vitamins, growth of aerobic microorganisms, loss of essential flavor compounds). Carotenoid pigments can be oxidized, leading to loss of color and their beneficial effects. To avoid these undesirable effects, oxygen within the package should be eliminated or reduced to a level acceptable for a given product. For preservation of fresh food, it is important to avoid dehydration, while for bakery or delicatessen, it is important to avoid water permeation.[44] Wilful tampering with food products has resulted in special packaging features designed to reduce or eliminate the risk of tampering and adulteration. Child-­ resistant closures hinder access to potentially dangerous products. ­Tamper-evident features include banding, special ­membranes, breakaway closures, and special printing on bottle liners or cans, graphics, or text that irreversibly change upon opening. Special printing also includes holograms that cannot be easily duplicated. Tamper-evident packaging usually requires additional packaging materials, which exacerbates disposal issues, but the benefits generally outweigh its drawback. An example of a tamper-evident feature that requires no additional packaging material is a heat seal used on medical packaging that is chemically formulated to change color when opened. [45] Convenience Packaging can have features that add convenience in handling, distribution, stacking, display, and sales. Consumers are consistently looking for packages that offer convenience attributes such as ease of access, handling (lightweight materials preferred), ease of opening, reclosing/resealability, container portability, product visibility, special dispensing features, convenient preparation features such as microwavability, and recycling and ease of disposal. The convenience features greatly influences customer decision of purchase. As a consequence, packaging plays a vital role in minimizing the effort necessary to prepare and serve foods. Oven-safe trays, boil-in bags, and microwavable packaging enable consumers to cook an entire meal with virtually no preparation. Advances in food packaging facilitated the development of modern retail formats that offer consumers the convenience of one-stop shopping and the availability of food from around the world. These convenience features add value and competitive advantages to products but may also influence the amount and type of packaging waste requiring disposal. The potential of packaging use/ reuse eliminates or delays its entry to the waste stream. Food labels are intended by law to provide the information to consumers to make the necessary decisions about the purchase of food. It also provides essential product

Food Packaging: Polymers as Packaging Materials in Food Supply Chains 1381

information to facilitate the promotion and advertisement of the product. Due to consumers’ complex and busy lifestyle in the modern society, food technologists are striving to develop functional packaging systems with enhanced ­end-use convenience features.

stored, where appropriate, the date up to which it is safe, for whom it is intended, genetically modified ingredients and allergens alert, and a bar code to identify the food. Classification of food packaging based on its broad and specific functions is shown in Fig. 2.

Communication and Marketing

Product containment

Primary packaging

Secondary packaging

Bulk packaging

RECENT DEVELOPMENTS IN POLYMER FOOD PACKAGING Polymer food packaging is an evolving area with new challenges and developments to satisfy the growing needs of consumers. The food packaging demands are complex and diverse, and sometimes a unique solution is required to address the complexity. For example, in some applications, high barriers to gas migration or diffusion are ­undesirable, such as in packaging of fresh fruits and vegetables, whose shelf life is dependent on the access to a continual supply of oxygen for sustained cellular respiration. Plastics utilized for carbonated beverage containers, on the other hand, must have high carbon dioxide and oxygen barrier property to prevent decarbonation and oxidation of the beverage. As a result of these complexities, food products require sophisticated and remarkably different packaging functions, and the responsibility on the packaging industry will further increase if food has to be transported over increasingly longer distances between producers and consumers. Increasing consumer awareness about resource management, waste minimization, eco-safety, and sustainability along with food safety resulted in many of the recent developments in polymer food packaging, few of them are reviewed later.

Protection and preservation

Antimicrobial packaging

Modified atmosphere packaging OR active packaging Oxygen scavengers

Tamper-evident packaging

Package integrity indicators

Water vapor scavengers Ethylene scavengers Vacuum Packaging

Fig. 2  Classification of food packaging based on their functions

Convenience

Retail packaging

Edible packaging

Communication and marketing

Boil-in-bag packaging

Microwave safe packaging

Smart packaging

Intelligent packaging

Nanosensors Biosensors RFID tags Freshness indicators Time, temperature indicators

Food Packaging (cont.)–Fuel Cell

The showcasing of food in an attractive way is one of the marketing strategies for better sales. For a package to be effective, it must present the product in an attractive manner to the potential buyer and should do its own publicity. Packaging is the one which draws the customer attention so that it should be smart and catchy. A cleverly designed and attractive packaging can help to sell a product, which is an essential attribute of an effective marketing campaign. The packaging helps to enhance the product image and/or to differentiate the competitive products on the shelf, which is a trait especially important when marketing unique food products such as low-fat, cholesterol-free, low calorie, or nutritional products. Packaging is also designed to be visually stimulating and provide information about the product to help the customer. For children, the packaging might represent innovation or fun. A package is the face of a product and often is the only product exposure consumers experience before purchase. Consequently, distinctive or innovative packaging can boost sales in a competitive environment. Additionally, the package conveys an important information about the product such as net weight, nutritional value, ingredient declaration, cooking instructions, brand identification, manufacturer information, pricing, how it should be

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Biopolymer-Based Food Packaging

Food Packaging (cont.)–Fuel Cell

Majority of materials employed for food packaging are nonbiodegradable and do not meet the demands of sustainability and environmental safety. Biopolymers have been one of the favorable alternatives to be exploited and developed into ecologically benign, sustainable food packaging materials due to their biodegradability.[46] They are derived from replenishable agricultural feedstocks, animal sources, food processing industry wastes, or microbial sources. In addition to renewable, these biopolymers break down to produce environmentally friendly by-products such as carbon dioxide, water, and organic compounds without toxic residues. Food packaging materials are often contaminated by foodstuff, so recycling these materials is impracticable and, most of the times, economically not viable. As a consequence, biodegradability is not only a functional requirement but also an important environmental attribute. Biodegradability is directly correlated to the chemical structure of the materials rather than the origin. The use of long-lasting synthetic polymers as p­ ackaging ­materials for short application is not always justified.[47] Biopolymers can be categorized into four groups depending on the origin: natural biopolymers extracted from biomass (e.g., agroresources), synthetic biopolymers from microbial production or fermentation (e.g., PHAs), synthetic biopolymers conventionally and chemically synthesized from biomass (e.g., PLA), and synthetic biopolymers conventionally and chemically synthesized from petroleum products (e.g., PCL). The first three groups are derived from renewable resources, while the last group is derived from nonrenewable resources. Among the biopolymers, the most common type that has been studied to produce bionanocomposite materials for food packaging applications are starch and its derivates.[48–50] Starch and its derivates are generally nontoxic and edible; thus, they are safe as food packaging materials. Studies have shown that starch is completely degradable and could stimulate biodegradability of nonbiodegrable materials when mixed. However, starch has low mechanical properties that can be improved with additives such as plasticizers and nanofillers.[51] The other biopolymers for food packaging applications include cellulose, chitosan, agar, gelatin, gluten, alginate, whey protein, collagen, polyhydroxyalkanoate (PHA), polyhydroxybuterate (PHB), and a copolymer of PHB and valeric acid (PHB/V). Nowadays, the advent of technology has also led to the production of synthetic biopolymers such as polylactic acid (PLA), polycaprolactone (PCL), polyglycolic acid (PGA), polyvinyl alcohol (PVOH), and polybutylene succinate (PBS).[10] The advantages of synthetic biopolymers include the potential to create a sustainable industry as well as the enhancement in various properties such as durability, flexibility, clarity, high gloss, and tensile strength. PLA is emerging as one of the most attractive green food packaging material because of its excellent processability

Food Packaging: Polymers as Packaging Materials in Food Supply Chains

and biocompatibility. It is a versatile polymer with high transparency and water resistance. It was found that in many situations, it performs better than synthetic polymers such as OPS and PET. PLA films have better UV resistance than LDPE. It can be processed by injection molding, blow molding, thermoforming and extrusion.[52] PCL is a fully biodegradable polymer obtained from the polymerization of nonrenewable raw materials such as crude oil. It is a thermoplastic with good resistance to water, oil, and chlorine; is easy to process; and has a very short degradation time. It is not used for food application, but if mixed with starch, it is possible to obtain a good biodegradable material at a low price, used for trash bags. Another biopolymer, PHA is produced by bacterial fermentation of sugar and lipids. They can be made into thermoplastic or elastomeric materials with a melting point between 40 to 180°C depending on the monomer used in the synthesis.[46] The most common type is the polyhydroxybutyrate, obtained from the polymerization of 3-hydroxybutyrate monomer with properties similar to PP but with improved stiffness. The three most unique properties of PHB are as follows: it is 100% water resistant and 100% biodegradable, and has thermoplastic processing features. In 1993, LDPE-starch blend was commercialized under the trade name Ecostar. Other commercial biopolymers are Bioplast (from Biotec GmbH) and NOVON (from NOVON International). A commercially available aliphatic copolyester produced by Procter & Gamble Company (Cincinnati, Ohio) with the trade name Nodax can degrade in aerobic and anaerobic environmental conditions. The other one is the Eastar Bio, produced from the Eastman Chemical Company (Hartlepool, UK). Combination of PLA with montmorillonite (MMT), a layered silicate, enhanced the barrier properties and made it applicable for food packaging.[53] Sanchez-­Garcia et al.[54] reported a lower water and oxygen permeability and improved thermal and mechanical properties of poly(3-­hydroxybutyrate-hydroxyvalerate) (PHBV) and polycaprolactone (PCL) films after incorporation of carbon nanofibers and CNTs. Also, use of carboxymethylcellulose (CMC) in starch-based films with a decrease in water vapor permeability is reported.[55] At present, the biopolymers are focused mainly for short shelf-life applications such as fresh fruits and vegetables and dry products such as pasta that do not demand barrier to oxygen and/or water vapor. Storage tests and tests on the industrial packaging machines should be performed to make sure that biopolymers can be used commercially. It is however important to realize that a thorough evaluation of the functional properties of a biopolymer is essential before it can be used as an alternative for conventional polymers. Unfortunately, the use of biopolymers as food packaging materials has drawbacks such as poor mechanical, thermal, and barrier properties compared to the synthetic polymers. To overcome this, many research efforts were made to improve the properties of biopolymers.

For example, these drawbacks can be addressed by adding reinforcing nanosized fillers to form bio-nanocomposites. In bio-­nanocomposites, usually, the matrix is a biopolymer and the nanofiller may or may not be biobased. The nanosized fillers act as reinforcement to improve the mechanical and barrier properties of the matrix. For example, the biopolymer chitosan-starch with AgNPs has been found to exhibit better gas barrier and tensile properties than control films composed of the same material.[56] To improve the barrier properties of biopolymers, several other approaches are available: use of coating that improves hydrophobicity, lamination of two or more biopolymers (coextrusion), use of an edible coating with the required barrier properties, chemical and/or physical modification of biopolymers, and blending of biopolymers. Some examples of blends include PLA/Polyethylene Glycol (PEG), PLA/PHA, and PLA/ PCL. For instance, modified starch and PLA coextrusion combines the PLA’s mechanical strength and hydrophobic properties with the gas barrier properties of starch. Chemical modification of starch was employed in this case to improve the adhesion with the more hydrophobic layer of PLA.[57] Biopolymers are also used as edible films (thin layer of edible material applied to food as a coating or placed on or between food components to ensure freshness and quality). The edible films not only provide physical protection to food but also act as a barrier to water vapor, oxygen, carbon dioxide, lipids, flavors, and aromas between food and the surrounding. Edible films are derived from plant and animal sources such as zein (corn protein), whey (milk protein), collagen, and gelatin.[58] Gelatin reinforced with edible bacterial cellulose nanocrystals for food packaging applications was reported.[59] At present, food packaging grade biopolymers are more expensive than most petroleum-based polymers, so substitution would likely result in increased packaging cost. Although the complete replacement is just impossible to achieve, at least for specific ­short-term applications, the use of biopolymers can be promoted. The biopolymers in food-contact application includes disposable cutlery, drinking cups, salad cups, straws, stirrers, overwrap and lamination film, lids and cups, and plates and containers for food dispensed at delicatessen and fast-food establishments. Biopolymers can be used in contact with aqueous, acidic, and fatty foods that are dispensed or maintained at or below room temperature or dispensed at temperatures as high as 60°C. Modified Atmosphere Food Packaging Modified atmosphere packaging (MAP) is a system for holding food in an atmosphere that differs substantially from ambient air with respect to CO2 and O2 levels. Controlled atmosphere (CA) storage is a type of MAP that refers to the constant monitoring and adjustment of atmosphere within gas-tight containers by ventilation and scrubbing. Compounds that influence taste and aroma are

carbon dioxide, oxygen, water vapor, ethylene, or volatiles. There are two commonly used CA storages depending on the method of control of gases: static CA storage and flushed CA storage. In the static type, the product generates the atmosphere, and in the flushed type, the ­atmosphere is ­created from an external flowing gas stream, which purges the package continuously. The shelf life of fresh produce such as fruits and vegetables is strictly related to their respiration rate and metabolic activity. The temperature decrease with atmosphere modification leads to a reduction in respiration rate and therefore can enhance the ­storage life of fruits with climacteric respiration.[60] The main way of preserving fruits and vegetables in storage or during long-distance transport is by refrigeration. CAs are considered as a supplement to increase or enhance the effect of refrigeration. The type of atmosphere modification required is determined by three interacting processes: respiration of the product packed, gas diffusion through the product, and gas permeation through the packaging film. Each of these processes is in turn strongly influenced by several environment- and product-generated factors. The ultra low oxygen (ULO) storage uses O2 levels close to the minimum level required for maintenance of plant tissues. Using ULO storage at 1°C–2°C with preset levels of 0.5%–1% O2 and 2%–3% CO2, Elstar apples were reported to be stored for almost a whole year without ­unacceptable quality loss.[61] Vacuum packaging (VP) can be regarded as a special type of MAP, in which the initial gas composition is that of normal air, but because of the reduced partial gas pressure, the amount of O2 available at the start of storage is less than that of the normal amount. As with MAP, the lower O2 content preserves the postharvest product quality by slowing down the metabolism of the produce and the growth of spoilage microorganisms. MAP, CA, and VP all focus on changing the metabolic gases. CA storage also requires a high relative humidity, generally closer to saturation is better, as long as moisture does not condense on the product. Modified humidity packaging is designed to control not only dehydration but also condensation in products such as leafy vegetables or bell peppers, where dehydration causes the most important quality losses. High humidity increases the probability of condensation and free water accumulation on the product, especially when the package is exposed to changing temperatures. Hence, strict temperature control in the food distribution chain would be a prerequisite for optimal use of MAP in practice. Control of RH inside the packages can be done by the use of packaging materials with high water vapor permeabilities, by inclusion of sachets containing water absorbers such as CaCl2, sorbitol, or xylitol into the package or by the use of packaging materials with suitable gas permeabilities onto which such desiccants are coated. Advantages of MAP include shelf-life extension that can even be doubled; delayed ripening; a considerable decrease in respiration rate; the preservation of firmness of flesh; a

Food Packaging (cont.)–Fuel Cell

Food Packaging: Polymers as Packaging Materials in Food Supply Chains 1383

1384

high turgidity such that fruits are more juicy and crisp; less loss of acidity, sugars, and vitamin C; and higher stability of color. Some physiological changes such as chill injuries, decay, browning, water core, and scald are prevented or greatly limited. Molds can be reduced, in particular, under low O2 and high CO2 atmospheres.[60] Antimicrobial Food Packaging

Food Packaging (cont.)–Fuel Cell

It is a common fact that the spoilage of food mainly occurs by microbial contamination. Antimicrobial (AM) food packaging functions to reduce, inhibit, or retard the growth of microorganisms that may be present in the packed food or packaging material itself. Most of the food preservation methods such as refrigeration, freezing, drying, thermal processing, and modified atmosphere are effective but have their own limitations especially when applied to fresh meat that requires minimal changes in meat texture. Since microbial contamination always occurs primarily on food surface, many studies suggested the addition of AM agents onto the food surfaces to suppress the growth of microbes and prolong the shelf life of food. However, the direct application of AM agents via dipping and spraying onto food is not effective and recommended. AMP that incorporates AM agents into polymer packaging is more preferred. The AM agent could be embedded inside polymer chains by offering a slow and continuous migration to form an AM layer on the food surface. The rationale for incorporating AMs into the packaging material is to prevent surface growth of microbes in foods where a large portion of spoilage and contamination occurs. [62] The effectiveness of AMP is greater compared to direct addition of AM agents into food due to two main reasons. First, the attachment of AM agents with polymer film enables slow release of AM agents to function over an extended period of time. Second, AM agents may experience inactivation (such as neutralization, hydrolysis, and dilution) by food components when added directly into the food. Besides, direct addition of AM agents into food can reduce the food quality by changing the organoleptic and textural qualities of the food. [63] Thus, AMP plays an important role to inhibit the growth of targeted microbes on food while improving food safety and shelf life without compromising in the food quality. AM agents can be incorporated into polymers by melt- or solvent-casting method. Thermal processing of polymers such as extrusion and injection molding may be used with thermally stable AM agents. For example, silver-substituted zeolites can withstand high temperatures up to 800°C and therefore have been incorporated as a thin coextruded layer with other polymers. [64] AM agents that cannot tolerate the temperatures used in polymer processing are often coated onto the packaging films after forming or casting. Cast edible films have been used as carriers for AM agents and applied as coatings onto packaging materials and/or foods. Examples include nisin methylcellulose

Food Packaging: Polymers as Packaging Materials in Food Supply Chains

coatings for PE films and nisin zein coatings for poultry applications. [65] AM effect can also be induced by other means such as addition of sachets/pads containing volatile AM agents into packages, incorporation of volatile and nonvolatile AM agents directly into polymers, adsorbing or coating AMs onto polymer surfaces, immobilization of AMs to polymers by covalent or ion linkages, and use of polymers that are inherently AM. The most successful commercial application of AMP is to enclose sachets separately or attached to the interior of a package. Oxygen absorbers, moisture absorbers, and ethanol vapor generators are used as indirect AM agents. Oxygen and moisture absorbers are used primarily in bakery, pasta, produce, and meat packaging to prevent oxidation and water condensation. A reduction in oxygen inhibits the growth of aerobes, particularly molds. Ethanol released into the headspace within the package has been mainly used to retard molds in bakery and dried fish products. Commercial examples include Ethicap— heat-sealed packets containing microencapsulated ethanol in silicon dioxide powder and Fretek—a paper wafer in which the center layer is impregnated with ethanol in acetic acid and sandwiched between layers of polyolefin films.[66] Another commercial AM product is Microgarde, as marketed by Barrier Safe Solutions International Inc., Lake Forest, Illinois, which exists as stickers or sheets containing sodium chlorite and acid precursors. When humidity in the air is high, it enters the Microgarde sheet and initiates the solid-state dry reaction, subsequently producing chlorine dioxide that diffuses throughout the package to inhibit microbial contamination. Silver compounds are extensively used as the AM agent in food packaging films. Compared to molecular AMs, which are generally targeted to specific classes of organism, silver is a broad spectrum and toxic to numerous strains of bacteria, fungi, algae, and possibly some viruses. Being an element, silver is stable/active for a longer period. The ­largest advantage of silver-based AM agents is that silver can be easily incorporated into plastics, making it especially useful for applications where a broad spectrum of sustainable AM activity is desirable but where traditional AM agents would be impractical. Silver-containing polymers are used as refrigerator liners, cutting boards, and food storage containers. The FDA has approved over a dozen of s­ ilver-containing zeolites for use as food contact materials (There are presently 17 silver-containing substances approved by the FDA for contact with foods.). In 2009, the FDA modified the food additive regulations to permit the direct addition of silver nitrate as a ­disinfectant to bottled water at concentrations not to exceed 171 μg/kg.[67] The ZnO NP coatings have also attracted a great deal of attention due to its AM activity towards both gram-­positive and gram-negative bacteria. Tankhiwale and Bajpai[68] developed PE film coated with ZnO for food packaging applications. Of all the silver-based AM agents, silver-­substituted zeolites are the most widely used polymer

Food Packaging: Polymers as Packaging Materials in Food Supply Chains 1385

The concentrations that are required for AMP applications are much higher than the concentrations found in nature, which may raise regulatory concerns.[77] There are various chemical components of plant-origin possessing AM effects, which include flavonoids, ­thiosulfinates, ­glucosinolates, and saponins. Spices such as cumin, ­cinnamon, and cloves exhibit AM effects against many bacterial species.[78] For instance, Sudjana et al.[79] reported that olive leaves (Olea europaea) has AM effects. ­Rahman and Kang[80] have shown that ethanolic leaf extract of Lonicera japonica Thunb. has AM effects against some food-borne pathogens. Allium sativum, commonly known as garlic, is well recognized for its antibacterial functionality. AM-incorporated soy protein isolate coated onto the oriented PP/PE packaging for extended shelf life of fresh sprouts was reported.[81] However, AM packaging is still an extremely challenging technology, and there are only a few ­commercialized products found in the market. Active Food Packaging Active packaging means having active functions beyond the inert passive containment and protection of the product. They contain deliberately incorporated components intended to release or absorb substances into or from the packed food or from the environment surrounding the food to maintain quality and/or to extend the shelf life of food. In active packaging systems, the protection function of a package is enhanced by incorporating into it active compounds such as AM agents, oxygen absorbers, water vapor absorbers, preservatives, ethylene scavengers, and so on. Active food contact materials are defined as materials that are intended to extend the shelf life or to maintain or improve the condition of packaged foods. Active materials are allowed to bring about changes in the composition or the organoleptic characteristics of the food on the condition that the changes comply with the r­ egulations. The principle of active packaging depends either on the intrinsic properties of the polymer used as packaging material or on the introduction (inclusion, entrapment, etc.) of specific substances inside the polymer. An active agent can be incorporated inside the packaging material by the intentional grafting of an active group inside the polymer chain or onto its surface. In multilayer structures/ packaging sachets, labels or bottle caps can be designed to perform active functions. The active agents that can be added (organic acids, enzymes, bacteriocins, fungicides, natural extracts, ions, ethanol, etc.) as well as the nature of the materials (papers, plastics, metals, or combinations of these materials) into which they are included are very diverse. The active systems can be placed outside the primary packaging, in between different parts of the primary packaging or inside the primary packaging itself. In the last case, the systems can be in contact only with the atmosphere surrounding the food, in contact with the food ­surface, or placed inside the food itself (for liquid foods).

Food Packaging (cont.)–Fuel Cell

additive for food applications. Sodium ions present in zeolites are substituted by silver ions and are incorporated into polymers such as PE, PP, and nylon. Commercial examples of ­silver-substituted zeolites include Apacider, AgIon, ­Zeomic, Bactekiller, and Novaron. Zeomic has been widely used in chopping board, food packaging films, gloves, and lunch box. AM enzymes such as lactoperoxidase and lactoferrin;  AM peptides such as magainins, cecropins, and ­defensins; natural phenols such as hydroquinones and catechins; fatty acid esters; antioxidant phenolics; and metals such as c­ opper may be other useful AM agents.[69] Packaging systems can also be designed with volatile AMs such as chlorine dioxide, sulfur dioxide, and allylisothiocyanate (AITC). The advantage of volatile AM agents is that they can penetrate the bulk of the food and that the polymer need not necessarily directly contact the product. AM vapors or gases are appropriate for applications where contact between the required portions of the food and the packaging does not occur, as in ground beef or cut produce. Early developments in AMP include incorporation of fungicides into waxes to coat vegetables and fruits, quaternary ammonium salts coating on shrink films to wrap potatoes, sorbic acid coating for cellulose casings, and wax paper for wrapping sausages and cheeses.[70,71] A few examples of ionic and covalent immobilization of AMs onto polymers have been reported. This type of immobilization requires the presence of functional groups on both the AM and the polymer, and may also require the use of spacer molecules that link the polymer surface to the bioactive agent. Examples of AMs with functional groups are peptides, enzymes, polyamines, and organic acids. The spacers allow sufficient freedom of motion, so the active  portion of the agent can contact microorganisms on the food surface. Spacers that could potentially be used for food AMP include dextrans, PEG, ethylenediamine, and polyethyleneimine. It has been reported that cross-linking edible films such as calcium caseinate by gamma irradiation will find applications as supports for the ­immobilization of AMs and other additives.[72] Some polymers such as chitosan and poly-l-lysine are inherently AM and have been used in films and coatings. Chitosan has been used as a coating to protect fresh vegetables and fruits from fungal contamination. Although the AM effect is attributed to antifungal properties of chitosan, it is also reported that the chitosan acts as a barrier between the nutrients contained in the food and ­microorganisms.[73] Use of chitosan-based AM films has been reported for packaging of organic acids and spices.[74] Bactericidal acrylic polymers made by copolymerizing acrylic protonated amine comonomer have been reported as packaging material for fruits and vegetables.[75] Polymers containing biguanide substituents also exhibit AM activity.[76] Plant extracts and oils as AM additives for polymers are generally recognized as safe (GRAS). Thus, recent studies have focused on the use of natural volatile AM agents for both specific and broad microbial inhibition.

1386

Food Packaging (cont.)–Fuel Cell

This diversity accounts for the innovative potential in this field, but it also poses a real challenge for the safety ­assessment.[82] Bioactive packaging materials need to keep bioactive compounds, such as prebiotics, probiotics, encapsulated vitamins, or bioavailable flavonoids in favorable condition until they are released in a controllable manner into the food product. Bioactive packaging materials can help to control oxidation of food stuffs and prevent the formation of off-flavors and undesirable textures of food. Bioactive compounds that are encapsulated into packaging are a promising approach, because this would allow the release of active compounds in a controlled manner. Several approved food additives such as carrageenan, chitosan, gelatin, PLA, PGA, and alginate could be used for such nanoencapsulation.[83] It is important to emphasize that intelligent packaging is different from active packaging. Intelligent ­packaging systems are those that possess enhanced functions with respect to communication and marketing functions, such as to provide dynamic feedback to the consumer on the actual quality of the food. On the other hand, active packaging is focused on providing protection and preservation of the food through some mechanism activated by extrinsic and/or intrinsic factors. The intelligent systems monitor the condition of packed food to give information regarding the quality of the packed food during storage and transportation, whereas in active packaging of the product, the package and the environment interact in a positive way to extend the shelf life, to improve the condition of packed food, or to achieve some characteristics that cannot be obtained otherwise. But active and intelligent packagings are not mutually exclusive. Both packaging systems can work synergistically to form a smart packaging system.[84] Active packaging can be classified into two types: nonmigratory active packaging, which elicits a desirable response from food systems without the active component migrating from the packaging into the food, and migratory active packaging, which allows a controlled migration of volatile or nonvolatile agents into the atmosphere surrounding the food. The most well-known examples of nonmigratory active packaging are moisture absorbers, mostly based on the adsorption of water by a zeolite, cellulose, and their derivatives. The market trend is to introduce the moisture absorbers inside the packaging material to make the active system invisible for the consumer. Other wellknown active packaging systems are oxygen absorbers, mostly based on iron oxidation, but they can also be based on catechol or ascorbic acid oxidation, on enzymatic catalysis as well as on many other reactions. The development of oxygen scavenging was based on the design of active substances for being included in the packaging material itself, using monolayer or multilayer materials or reactive closure liners for bottles and jars.[85] Ethylene scavengers, based on various reactions, are extensively used to slow down the maturation rate of fruits, a crucial factor for import–export

Food Packaging: Polymers as Packaging Materials in Food Supply Chains

of fresh fruits and vegetables. Another example of nonmigratory packaging is the AMP based on the entrapment in a silicate network of silver ions, a widely used AM agent. The design of sufficiently effective active packaging with minimum detrimental side effects and which permits an accurate, knowledge-based assessment of potential risks is a challenge. The active agents can act either on the surface of a solid food or in the bulk of liquid foods. The ethanol-releasing active packaging will be able to slow down mold growth and thus increase the shelf life of bakery products. Water vapor-releasing sachets placed in ready-to-cook fresh vegetable packaging with a valve placed onto the plastic pouch permit to cook fresh vegetables in a microwave oven with improved organoleptic and nutritional quality. Sulfur dioxide releaser packaging is extensively used for preserving grapes from mold development. Some commercial products are based on the reaction of calcium sulfite with moisture, while others are based on metabisulfite hydrolysis. In these cases, one of the breakdown products originating from the reaction with moisture is an active agent. Another example of active material is a plastic film commercially available in Japan, containing AITC, a strong AM substance extracted from mustard. AITC, a volatile active agent, is entrapped in cyclodextrins for protecting it from thermal degradation during extrusion. When exposed to high moisture conditions, cyclodextrins have the ability to change its structure to release the AM agent into the atmosphere surrounding the food.[86] Reducing the oxidation process is an important challenge to food industry. Lipid oxidation is the main cause for deterioration of meat quality during refrigerated storage. The shelf life of fresh meat can be extended using antioxidants, AM agents, and suitable packaging materials. Some types of active packaging allow the controlled release of AM agents or antioxidants that have previously been added to the package. The active packaging film based on LDPE and EVA with a natural extract obtained from a brewery waste and functionalized nanoclay was reported to enhance the oxidative stability of beef during refrigeration. They have both antioxidant and AM properties and hence can be used to extend the shelf life of minimally processed meat products.[87] Many research studies in recent years have focused on developing active packaging containing natural antioxidants such as butylated hydroxytoluene (BHT), butylated hydroxyanisole (BHA), alpha-tocopherol, and natural extracts into packaging films.[88,89] Moore et al.[89] demonstrated that the incorporation of antioxidant substances (such as BHT, BHA, alpha-tocopherol, and rosemary extract) into packaging is more effective than the direct use of additives on the meat surface. Edible film impregnated with procyanidins, a natural antioxidant present in grapes, was found to inhibit lipid oxidation and microbial growth, thereby enhancing the quality and shelf life of pork meat.[90] Active packaging systems have also been reported using natural extracts such as rosemary, oregano, and green tea with both AM

Food Packaging: Polymers as Packaging Materials in Food Supply Chains 1387

Principle

Type

Ageless

Mitsubishi Gas Chemical Co. Ltd., Japan

Iron based

Oxygen scavenger

Freshilizer

Toppan Printing Co. Ltd., Japan

Iron based

Oxygen scavenger

Freshmax, Freshpax, Fresh Pack

Multisorb Technologies, USA

Iron based

Oxygen scavenger

Oxyguard

Toyo Seikan Kaisha Ltd., Japan

Iron based

Oxygen scavenger

Zero2

Food Science Australia, Australia

Photosensitive dye

Oxygen scavenger

Bioka

Bioka Ltd., Finland

Enzyme based

Oxygen scavenger

Dri-Loc®

Sealed Air Corporation, USA

Absorbent pad

Moisture absorber

SEALPAC, Germany

Dual compartment system

Moisture absorber

Biomaster®

Addmaster Limited, USA

Silver based

AMP

Agion

Life Materials Technology Limited, USA

Silver based

AMP

SANICO®

Laboratories STANDA

Antifungal coating

Interleavers

Neupalon

Sekisui Jushi Ltd., Japan

Activated carbon

Ethylene scavenger

Peakfresh

Peakfresh Products Ltd., Australia

Activated clay

Ethylene scavenger

Evert-Fresh

Evert-Fresh Corporation, USA

Activated zeolites

Ethylene scavenger

Tenderpac

®

®

and antioxidant properties to increase the stability of different meat products.[91–93] Table 2 lists the commercially available active food packaging systems.[94] Intelligent/Smart Food Packaging Intelligent packaging can be defined as packaging that contains an external or internal indicator to provide information about aspects of the history of the package and/ or the quality of the food. They help to extend the shelf life, monitor freshness, display information on quality, and improve safety and convenience.[85] Intelligent packaging is an extension of the communication function of traditional packaging, and it communicates information to the consumer based on its ability to sense, detect, record, and measure an attribute of the product, the inner atmosphere of the package, or its environment, and hence enhances the possibilities to monitor product quality, trace the critical points, and give information throughout the food supply chain. This information can in turn trigger active packaging functions. Programmable matter, sensors, smart materials, etc. can be used in intelligent packages. Oxygen and moisture absorbers were among the first series of active and intelligent packaging to be developed and successfully applied for improving food quality and shelf-life extension (e.g., for delicatessen, cooked meats, etc.). Numerous other concepts such as ethylene absorbers (e.g., for climacteric fruits), ethanol emitters (e.g., for bakery products), carbon dioxide emitters/absorbers, and oxygen and time/­ temperature indicators were developed later.[84] A package can be made smart through its functional attributes. Smart packaging provides a total packaging solution that, on the one hand, monitors changes in the product or the environment (intelligent) and, on the other hand, acts upon the product or the environment

changes (active). Intelligent tags such as electronic labeling designed with ink technology in a printed circuit and built-in battery radio frequency identity tags placed outside the primary packaging are being used to offer innovative communication. Diagnostic indicators are used to provide information on food storage conditions such as time, temperature, oxygen, or carbon dioxide content, and thus indirectly information on food quality, as an interesting complement to end use/best before dates. The most commonly used visual indicators are time/temperature indicators, critical temperature indicators, and leak indicators. These first-generation indicators are considered as indirect indicators of food freshness. The trend in this field changed to develop direct indicators of food quality because of their ability to provide more targeted and precise information on quality attributes.[95] Targeted quality markers can be volatile compounds (e.g., volatiles of microbial origin), such as carbon dioxide, nitrogen compounds or biogenic toxins, amines, and pathogenic bacteria. One such commercially available direct indicator is a fish freshness indicator based on the detection of volatile amines [96] or a pear maturation indicator based on the detection of a volatile aroma compound.[97] More sophisticated systems contain a plastic layer loaded with specific antibodies of pathogenic microorganisms such as Salmonella or Listeria deposited on the bar code, whose presence can be detected when the bar code is read. Despite active research in the area, the intelligent systems are rarely practiced. Reasons for this may be the high cost of an indicator label, legislative restrictions, and even acceptance of retailers and brand owners. The fear could also be that indicators would reveal possible irregularities occurring, e.g., in the management or control of the cold chain. On the other hand, a few shortcomings of these indicators also exist, e.g., their response to oscillating temperature patterns. Safety issues

Food Packaging (cont.)–Fuel Cell

Table 2  Commercially available active food packaging systems [94] Trade name Manufacturer

1388

Food Packaging (cont.)–Fuel Cell

mainly for direct indicators that are required to be placed inside the primary packaging for direct contact with the atmosphere surrounding the food or with the food itself, because of potential undesirable migration of chemical components. Three major technologies are used in intelligent packaging: sensors, indicators, and radio frequency identification (RFID) systems.[95] These technologies differ from one another not only in hardware but also in the amount and type of data that can be carried and how the data are captured and distributed. Sensors act as an alternative to the time-consuming, destructive, and expensive analytical techniques that are applied to monitor a packed food product and its environment throughout the entire food supply chain. Besides traditional sensors to measure temperature, humidity, pH level, and light exposure, chemical sensors have received increasing attention in the last few years to monitor food quality and package integrity. The sensing part of a chemical sensor (receptor)  is usually a ­chemical-selective coating capable of detecting the presence, concentration or composition, activity of specific chemical analytes or gases via surface adsorption, resulting in a change of a certain coating property. This change is typically being observed and converted into a proportional output signal by the measuring part of the sensor (transducer). Some smart packages include time–­temperature food quality labels, self-heating or self-cooling containers with electronic displays indicating use-by dates, and information regarding the nutritional qualities. Examples of smart packages are self-heating coffee container based on the CaO exothermic reaction, self-cooling beer using zeolite technology, and use of electronics and lithium battery power sources to enhance the branding citation in a crowded consumer product category.[98] However, a major challenge for the future is to combine the active and intelligent packaging concepts in one packaging material. This kind of closed-loop packaging systems offers the perspective to monitor changes in the product, packaging, and/or the environment and to respond appropriately to these changes via a feedback mechanism. Small and flexible chemical sensors to monitor volatile organic compounds and gas molecules (H2, H2S, NO2, O2, CO, CO2, NH3, CH4, etc.) related to food spoilage and package leak to evaluate the product quality and the package integrity are used in MAP. This type of chemical sensors could in time offer a valuable alternative to the cumbersome analytical instruments, such as fixed gas chromatography mass spectrometers (GCMS), which require breakage of package integrity or portable gas analyzers that are not applicable for real-time, online control, or large-scale usage. Gas indicators offer an alternative approach to traditional destructive techniques to determine the package integrity of MAP. They usually provide a semiquantitative or qualitative information about altered gas concentrations (CO2, O2, water vapor, ethanol, aroma) through visual colorimetric changes. Freshness indicators provide product quality information resulting from

Food Packaging: Polymers as Packaging Materials in Food Supply Chains

microbial growth or chemical changes within a food product. Microbial contamination may be detected visually through reactions between microbial growth metabolites and integrated indicators within the package. Freshness indicators can also be used to provide an estimate about the shelf life of perishable products.[99] Intelligent packaging is the new generation of packaging that provides the user with reliable and correct information on the conditions of the food, the environment, and/or the packaging integrity, many of which are in the development stages with sensor and nanosensor concepts. This type of advanced packaging is useful to increase the efficacy of information transfer in food supply chain and enables the detection of calamities and possible abuse through the entire food supply chain. Table 3 lists the commercially available intelligent food packaging systems.[94] Nanocomposite-Based Food Packaging Application of nanotechnology in food industry has been mainly focused on the development of innovative packaging materials. PNCs can be categorized into four types based on the purpose of application: improved food packaging PNCs, which aim at improving the packaging physical properties such as barrier, mechanical, thermal, and optical properties; active food packaging PNCs; intelligent food packaging PNCs; and biodegradable or compostable PNCs.[100] Though polymer food packaging has revolutionized the food industry, their major drawback is an inherent permeability to gases and other small molecules. Some polymers are better than others in this regard. For example, PET provides a good barrier to oxygen, while HDPE is poor. On the other hand, HDPE is significantly a better barrier against water vapor than PET. Permeability of a polymer to gases or moisture is dependent on a large number of interrelated factors such as polarity and structural features of polymeric side chains, method of synthesis, degree of crystallinity, molecular weight, polydispersity, degree of branching or cross-linking, hydrogen bonding characteristics, free volume, and specific polymer–­polymer and ­polymer–gas interactions; all of these can in turn be influenced by external parameters such as pressure and temperature. Permeability of one migrant can also be complicated by the presence of other migrants. For example, EVOH exhibits a quite excellent oxygen barrier property under dry conditions, but under humid conditions (relative humidity >75%), it fails due to swelling of the polymer and plasticization.[101] The overall rate of gas diffusion is also directly dependent on the film thickness. Higher thickness ensures better barrier property but demands more material usage that is not economically feasible. The dispersal of nanosized fillers into the polymer matrix improves the barrier properties of a polymer film in two ways. The first way is by creating a tortuous path for gas diffusion. NPs are essentially impermeable inorganic

Food Packaging: Polymers as Packaging Materials in Food Supply Chains 1389

Table 3  Commercially available intelligent food packaging systems [94] Trade name Manufacturer Freshpoint Lab, Israel

Integrity indicator

Novas®

Insignia Technologies Ltd., Scotland

Integrity indicator

Ageless Eye®

Mitsubishi Gas Chemical Inc., Japan

Integrity indicator

Freshtag

COX Technologies, USA

Freshness indicator

DSM NV and Food Quality Sensor International, USA

Freshness indicator

®

Sensorq®

Timestrip UK Ltd., UK

Time temperature indicator

Timestrip® PLUS Duo

Timestrip UK Ltd., UK

Temperature indicator

Timestrip Complete

®

Monitormark™

3M , USA

Time temperature indicator

Fresh-Check®

Temptime Corporation, USA

Time temperature indicator

Onvu™

Ciba Specialty Chemicals and Freshpoint, Israel

Time temperature indicator

Checkpoint®

Vitsab, Sweden

Time temperature indicator

™

Cook-Chex

Pymah Corp, USA

Time temperature indicator

Colour-Therm

Colour Therm, UK

Time temperature indicator

Thermax

Thermographic Measurements Ltd., UK

Time temperature indicator

Timestrip®

Timestrip Ltd., UK

Integrity indicators

Novas

Insignia Technologies Ltd., Scotland

Integrity indicators

Easy2log®

CAEN RFID Srl, Italy

RFID

®

Intelligent Box

Mondi Plc, Turkey

RFID

CS8304

Convergence Systems Ltd., Hong Kong

RFID

Temptrip

Temptrip LLC, USA

RFID

crystals, and gas molecules must diffuse around them rather than taking a mean straight line path that lies perpendicular to the film surface. The tortuous path allows the manufacturer to attain larger effective film thickness while  using smaller amounts of polymer (downgauging). The second way that nanofillers influence the barrier properties is by causing changes to the polymer matrix at the interfacial regions. If the polymer–NP interactions are favorable, polymer strands located in close proximity to each NP can be partially immobilized. The result is that gas molecules traveling through these interfacial zones have attenuated hopping rates between free volume and altered density.[102] NP incorporation into biopolymers makes its use feasible, contributing to reduce the dependence on petroleum-­ based materials. However, being a novel technology, there are knowledge gaps that pose questions to the scientific community, especially regarding its toxicity and ecotoxicity. The NPs have a potential to migrate to the foodstuff packed, but migration assays and risk assessment are still not conclusive. The common NPs used in the food packaging industry are nanoclay, nanosilver, and nanosilica, which impart high stiffness and strength. They also act as a nucleating agent in foam formation and as a flame retardant. Nanocomposites are better able to withstand the stress of thermal food processing, transportation, and storage. Bacterial cellulose nanocrystal-reinforced PVA

nanocomposite films exhibiting high thermal stability and mechanical properties is reported.[103] Studies have shown that NPs can be tailored for both controlled release and/or specificity by the action of the active agent. The use of PNCs concept to edible packaging facilitates an effective incorporation of bioactive ingredients to reduce waste disposal. However, edible packaging has to overcome challenges such as consumer acceptance, regulatory requirements, and scale-up concepts for commercial applications. PNCs containing AgNPs are a ­potential broad-spectrum AM activity against Escherichia coli, Vibrio cholerae, Shigella flexneri, and Staphylococcus aureus.[104] AgNP-based nanocomposites offer slower silver ion release into stored foods, which is important for sustained AM activity. Colloidal silver particle coating of 90–150 nm in thickness onto paper using ultrasonic radiation was shown to manifest an excellent AM activity against E. coli and S. aureus, suggesting its potential application as a food packing material for longer shelf life.[105] Other nanocomposites with AgNP exhibiting the AM activity include PE, PVA, PVP, PMMA, PU, PEO, poly(acrylamide), alginate, silicone elastomer, cellulose, and chitosan.[104] AgNP/PNCs have been tested with real food systems; for instance, Fayaz et al.[106] dipped sterilized carrots and pears into alginate solutions containing biosynthesized AgNPs, forming edible antibacterial coating. They found that the treated carrots and pears had

Food Packaging (cont.)–Fuel Cell

Type

O2 Sense™

1390

Food Packaging: Polymers as Packaging Materials in Food Supply Chains

Food Packaging (cont.)–Fuel Cell

less water loss and higher consumer acceptability (judged on the basis of color, texture, and taste) over a period of 10 days. In a similar study, fresh asparagus spears coated with AgNP/PVP nanocomposite films had their shelf life extended to 25 days when stored at 2°C; in addition to less weight loss, greener color and tender texture were maintained, and coated asparagus also had less microbial spoilage.[107] An edible film based on AgNPs dispersed in glycogen has also been reported.[108] AgNP/nanoparticulate TiO2 /PE films used as food storage bags for Chinese jujube fruit maintained the firmness and had less decay, less browning, and slower ripening over a period of 12 days than fruits stored in control materials.[109] Orange juice stored at 4°C in LDPE films incorporating TiO2 and nanosilver mixture exhibited a significant reduction in Lactobacillus plantarum growth. In another application, cellulose pads containing AgNPs generated from silver ions in situ have shown to reduce the microbial levels in beef meat stored under MAP.[110] Fresh-cut melons stored in AgNP-­ containing cellulose pads showed lower microbial counts (mesophiles, psychrophiles, and yeasts) and extended the microbial lag phase.[111] In addition, silver particles catalyze the destruction of ethylene gas; thus, fruits stored in the presence of AgNPs have slower ripening times and extended shelf life. Unlike AgNPs, the AM activity of TiO2 NPs is photocatalyzed, and thus, TiO2-based AMs are only active in the presence of UV light. For instance, TiO2 NPs have been found to be effective against common food-borne pathogens under UV illumination but not in the dark.[112] In principle, food packaging films incorporating TiO2 NPs have the additional benefit of protecting food content from the oxidizing effects of UV irradiation while maintaining good optical clarity as TiO2 NPs are efficient short-wavelength light absorbers with high photostability. Bodaghi et al.[113] developed TiO2-LDPE film by using a blown film extruder and tested on fresh pears packaging. Other nanoscale materials that have been shown to have AM properties are NPs based on magnesium oxide, copper and copper oxide, zinc oxide, cadmium selenide/­telluride, chitosan, and CNTs.[104] Nanotechnology may play an important role in making the world’s food supply healthier and safer. The incorporation of nanofillers such as silicate, clay, and TiO2 to biopolymers not only improves the mechanical and barrier properties but also offers other functions such as AM effect, biosensor, and oxygen scavenging.[9] The bio-nanocomposite can also be a smart food packaging, whereby it can perceive the condition of the packed food such as microbial contamination or expiry date and uses some mechanism to register and convey information about the quality or safety of the food. The nanofillers that are commonly studied for food packaging applications can be classified into NPs, nanofibrils, nanorods, and nanotubes. Researchers used different types of fillers and biopolymers to produce bio-nanocomposite materials. For example,

Kanmani and Rhim [114] studied the AM composite films made from gelatin and AgNPs, and Rafieian et al.[115] studied the bio-nanocomposite films made from wheat gluten matrix and cellulose nanofibrils. Rouhi et al.[116] reported bio-nanocomposite food packaging films made from fish gelatin with zinc oxide (ZnO) nanorods. MMT nanoclay in food contact applications is popular because it occurs naturally, is cost-effective, is proven to result in significant reinforcement (high surface area and aspect ratio) and relative processability, and has improved barrier properties, high stability, and alleged benignity. Lee et al.[117] studied the effect of Cloisite Na+ and Cloisite 10A on the physical properties of sesame seed meal protein bio-nanocomposite for food packaging applications. The most common type of metal studied to produce bio-nanocomposite materials was found to be silver due to its good thermal stability and AM properties, while the most common type of metal oxide is ZnO [116,118,119] due to its deodorizing and antibacterial properties. Usually, low amount of nanofiller (0.05) for ­samples with nisin. In antimicrobial films containing pediocin, the samples with halloysite (PH and PA) are remarkably different (P5 times

[36]

Organometallic polymers

7 times

[37]

Polystyrene

6 times

[38]

Polystyrene

10 times

[18]

Chloromethyl polystyrene

Activity is decreases after second times

[39]

Amino-terminated Tenta Gel resin

≥14 times

[40]

Soluble star polymer with nanoscale dimension

>5 times

[41]

Chloromethylated polystyrene resin

7 times

[42]

Poly(norbornene)

—

[43]

Merrifield resin Wang resin

—

[45]

Core-shell type of polymer

4 times

[46]

The FDU-15 mesopolymer

10 times

[47]

Polystyrene resin

5 times

[48]

Vinyl-modified poly(styrene) beads

5 times

[49]

Poly(divinyl)benzene

>3 times

[50]

Merrifield

4 times

[51]

PEGA resin

—

[52]

Monolithic support or poly(styrene)-divinyl benzene (PS-DVB)

—

[53,54]

Oxanorbornene

7 times

[57]

PEG

5 times

[58]

Ag

Polystyrene

>12 times

[59]

Gd

Chloromethyl polystyrene

>10 times

[60]

Fe

Polystyrene

>10 times

[61]

Rh

Chloromethyl polystyrene

>6 times

[62]

Cu

TEMPO-PEG4000

>6 times

[64]

Laboratory– Membranes

these catalyst systems have problems such as separation and recycling of the catalyst and contamination from ligand residues in products. Therefore, polymer-­supported transition metal catalysts have been an intensive research area for organic chemistry and industrial applications. This heterogeneous catalyst system provides fast recovery and the simple recycling of the catalysts by filtration, and thus there have been less contamination of ligand and environmental pollution caused by residual metals in the waste. Furthermore, they can increase the selectivity.[18,19] Supported catalysts have also been used for rapid production of compound libraries.[20,21] For these r­ easons, chemists started to improve different catalyst frameworks that are more efficient, powerful, and picky. To this end, the immobilization of catalysts like useful NHC ligands on supports facilitates the recycling of these catalysts. To date, the published research

1614

Laboratory Applications: Polymers in

polymer-bound palladium complexes. Pd complexes bearing NHC ligands exhibit an exceptionally high catalytic activity in cross-coupling reactions that are of practical importance in the synthesis of pharmaceuticals, fine chemicals, or natural products.[24] Additionality, the support of Pd–NHC complexes shows high activity in coupling chemistry. Supporting these complexes allows for easy removal of the toxic metal and the possibility of recycling the metal catalysts. Because of these reasons, different supported Pd–NHC complexes are included in the literature. In 2000, Schwarz and coworkers [25] reported the first polymer-supported Pd–NHC complexes (Scheme 5). They used and structurally characterized N-heterocyclic ­dicarbene chelate complexes. The complexes were ­immobilized on a functionalized polystyrene support (Wang resin) through one of the oxygen centers. The ­properties of these catalysts such as high activity, easy accessibility, stability, and recyclability with very high efficiency provide generation of new examples. Qureshi et al.[26] synthesized a polystyrene-supported Pd–NHC complex and characterized it by solid-state 13C NMR (Scheme 6). They used this catalyst for aminocarbonylation of aryl iodides with primary and secondary amines in aqueous medium. The catalytic system was optimized with various reaction parameters to give good yields and desired product. Furthermore, they studied the catalyst recyclability and observed that the recovered

N

N

N Pd

N

catalyst could be reused for four consecutive cycles for ­aminocarbonylation reaction of iodobenzene with aniline. PALLADIUM-CATALYZED C–C COUPLING REACTIONS Since the first laboratory making of a C–C bond by Kolbe in 1845, C–C bond forming reactions have played an important role in chemical synthesis. At first, reactions such as Aldol, Grignard, Witting, and Dies-Alder were used to form carbon–carbon bonding.[27] In the late twentieth century, different studies based on transition metal catalysts were started. Palladium-based catalysts are the most remarkable compounds in these studies. Palladium-catalyzed cross-coupling reaction was discovered in the 1970s by Heck.[28–30] It is the time these reactions have been an attractive field for catalysis, organometallic, and medicinal chemistry. Palladium-catalyzed cross-coupling reactions are often used for synthetic transformations.[31] The use of many of these products obtained through carbon–carbon coupling reactions, such as pharmaceuticals, materials, and optical devices, increases the importance of reactions in organometallic chemistry and catalysis.[32,33] Suzuki–Miyaura, Kumada–Corriu, Stille, Sonogashira–Hagihara, Mizoroki–Heck, Neghishi, and Hiyama are widely used reactions in the carbon–carbon

(CH2)nOH Polymer

X

X

Laboratory– Membranes

O

(CH2)nO

Scheme 5  The first polymer-supported Pd–NHC complexes Source: From Schwarz et al.[25] © 2000, with permission from John Wiley Sons, Inc.

R– – R= H, Me, COCH3

O

I HNR1R2

PS-Pd-NHC, CO Na2CO3,H2O

R– –

NR1R2

R1/R2= H, Ar, Alkyl

N AcO N

N Pd

OAc N

PS-Pd-NHC

Scheme 6  Aminocarbonylation reaction of aryl iodides with primary and secondary amines Source: From Qureshi et al.[26] © 2012, with permission from Elsevier.

Suzuki-Miyaura

Mizoroki-Heck

Hiyama C-C Coupling Reactions

Sonogashira-Hagihara

Negishi Stille

Kumada-Corriu

Scheme 7  Palladium-catalyzed C–C coupling reactions

coupling reactions. In addition, the Heck reaction, the Suzuki–Miyaura coupling, and the Sonogashira reaction have been often used in synthesis and properly studied in recent years. The C–C coupling reactions are shown in Scheme 7. POLYMER-SUPPORTED PD–NHC-CATALYZED SUZUKI REACTION Suzuki reaction is a coupling reaction which was first published in 1979 by Akira Suzuki.[34] The coupling partners are a boronic acid and an organohalide catalyzed by a palladium complex. This reaction is an excellent cross-­ coupling method for the synthesis of olefins, styrenes, and biphenyls. In 1985, a mechanism of this reaction was ­suggested by Akira Suzuki.[35] The Suzuki reaction is one of the most important and robust methods for the synthesis of pharmaceuticals, herbicides, natural products, and ligands for catalysis.[19] Aryl iodides and bromides are generally used as starting materials; however, aryl chlorides have been recently preferred because it is cheaper and more easily available. On the other hand, aryl chlorides are found to be less active than aryl bromides and aryl iodides when the starting materials are compared. The Suzuki reaction has been successfully studied using homogeneous catalysts. However, these catalysts have a few problems such as need to separate and recycle the catalysts and the contamination from ligand residues in

products. Therefore, heterogeneous catalyst systems have been used for these reactions over the past years. Lee and coworkers [36] reported a macroporous polystyrene (MPS)-supported palladium catalyst, and they tested this catalyst for the Suzuki reaction (Scheme 8). They used aryl iodides, aryl bromides, and aryl chlorides as aryl halides. MPS resin has large surface area and high porosity so that it helps the diffusion problem of reagents and solvents. The workers used bulky NHC in the Suzuki reaction. Especially, bulky NHCs increase the catalytic activity of the palladium-catalyzed Suzuki reaction of aryl chlorides because of their steric and electronic properties. Also, in this report, both activated and deactivated aryl chlorides are used as the starting materials. They examined the reusability of MPS-supported Pd NHC complex for the Suzuki reaction of 4-bromoacetophenone. The catalytic activities are very high (94%) after being used five times. Karimi and Akhavan [37] reported several N-substituted main-chain NHC–palladium organometallic polymers (NHC–Pd MCOPs), and they tested these polymers for the Suzuki arylation of aryl chlorides in water (Scheme 9). First, they prepared a variety of NHC ligands bearing different alkyl groups, and they react this precursor with Pd(OAc)2 in dimethylsulfoxide (DMSO) to give the ­corresponding NHC–Pd polymers. The obtained NHC–Pd polymers were tested for the Suzuki reaction of aryl chlorides with arylboronic acids. The reaction conditions were optimized in aqueous media without co-organic solvents. At the end of the reaction, NHC–Pd polymers with N-benzyl and N-dodecyl substituents showed high catalytic activity. The results showed that the recycling was efficiently achieved in seven reaction runs. Mohammadi and Movassagh [38] were synthesized a polystyrene-supported NHC–Pd complex (Scheme 10). The workers employed the complex in the Suzuki reaction of arenediazonium tetrafluoroborate salts with arylboronic acids. They used different functionalized arenediazonium salts and arylboronic acids. The para- and meta-­substituted arylboronic acids showed high catalytic activities in contrast to ortho-substituted arylboronic acids. Withal the polymer-supported NHC–Pd complex was examined for reusability. The results showed that the catalyst activity was not substantially decreased after six reaction cycles.

AcO N X R–

–

X: I, Br, Cl

+

Pd

Cl

N

B(OH)2 DMF/water K2CO3

Scheme 8  The Suzuki reaction of aryl halide, including aryl chloride, with phenylboronic acid Source: From Lee et al.[36] © 2008, with permission from American Chemical Society.

R–

–

Laboratory– Membranes

Laboratory Applications: Polymers in 1615

1616

Laboratory Applications: Polymers in

Pd Br

R N

N R

N R

Br Pd Br

R N

R N

N R

N R

n

Pd

reusability of the polymer-supported NHC–Pd c­ omplex, and finally, the catalyst activity was not ­diminished after ten recycling times. Byun and Lee [39] reported a polymer-supported NHC– Pd complex using chloromethyl polystyrene (CM PS). This complex has water compatibility and was used for the Suzuki reaction of various aryl halides with phenylboronic acid in aqueous media. The workers examined the effect of different solvents, reaction times, and temperatures using 4-­iodobenzene (Scheme 12). Then, they carried out the reaction using various aryl bromides and iodides with phenylboronic acid in DMF/water at 12 h and obtained good yields. Steel and Teasdale [40] reported a polymer-supported pyridyl bis NHC–Pd complex, and the catalyst was studied for the Suzuki reaction. The workers used bromobenzene and iodobenzene with 4-methoxyphenyl-boronic acid as the starting materials. The workers detected that the catalyst can be recycled ≥14 times in the reactions carried out under argon compared to those carried out in air. In both reaction media, the yields were the same (Table 3).

Br Br

Polymer encapsulated Pd nanocluster R

X

R

B(OH)2 X = Br, Cl

Scheme 9  The schematic illustration of polymer-encapsulated Pd nanocluster Source: From Karimi and Akhavan [37] © 2011, with permission from American Chemical Society.

POLYMER-SUPPORTED PD–NHC-CATALYZED HECK REACTION

Laboratory– Membranes

Kim et al.[18] reported a polymer-supported NHC– Pd complex using a poly(1-methylimidazoliummethyl ­styrene)-surface grafted-poly(styrene) resin (Scheme 11). They characterized the polymer-supported NHC ­precursor by scanning electron microscopy (FE-SEM), confocal laser scanning microscopy (CLSM), and infrared microscopy (IR) spectroscopy. The catalyst was employed for the Suzuki reaction of various aryl bromides and iodides in aqueous media. The workers used different bases and solvents to determine the effect of this parameters on the reaction. The isolated yield was up to 90% when Na2CO3 was used as a base and DMF/water was used as a solvent (Table 2). The high yields of desired products were obtained for the Suzuki cross-coupling of aryl iodides and bromides with phenylboronic acid in DMF/H2O at 1 h for aryl iodides and 6 h for aryl bromides. Furthermore, the group tested the

O N N

O N

N

N

Pd

+ – Ar N2 BF4 + Ar' B(OH)2

O O

N N O

N

The Heck reaction is a C–C coupling reaction involving the formation of substituent alkenes in a palladium-­catalyzed aryl or alkenyl halide with a nonfunctional olefin. The reaction progresses in the presence of a base. In 2000, Schwarz and coworkers reported polymer-­ supported Pd complexes using a di-NHC ligand. This is the first report of polymer-supported NHC–Pd complexes.[25] The synthesized homogeneous and heterogenous ­catalysts were tested for the Heck coupling using different aryl b­ romides and chlorides as the starting ­materials (Scheme 13). The catalysts activities were studied with activated and nonactivated aryl halides with styrene or n-butyl acrylate as the vinylic substrate. The immobilized carbene complexes showed high catalytic activity and maintained ­readily accessibility and remarkable stability.

N

N

N

N

O O

PS N

N N

O O N

O

N

N

Pd

N

Pd

Br

R N

N

N N

Ar Ar' O

N

N

O

Scheme 10  The polystyrene-supported NHC–Pd complex Source: From Mohammadi and Movassagh [38] © 2016, with permission from Elsevier.

28 examples up to 98% yield

Laboratory Applications: Polymers in 1617

PF6– HN+ N

(a) Pd(OAc)2 DMSO 50C 4h (b) 100C 30 min

N

N

F6P Pd PF6 N

N

Scheme 11  Preparation of the polymer-supported NHC–Pd complex Source: From Kim et al.[18] © 2005, with permission from American Chemical Society. Table 2  Heterogeneous Suzuki cross-coupling reaction of Ph-I and Ph-B(OH) 2a Entry

Solvent

Base

T (oC)

t (h)

Yieldb (%)

1.

DMF/water (1:1)

Na2CO3

50

1

95

2.

Dioxane/water (1:1)

Na2CO3

50

1

94

3.

THF/water (1:1)

Na2CO3

50

1

72

4.

DMF/water (1:3)

Na2CO3

50

2

94

5.

DMF/water (1:7)

Na2CO3

50

3

94

6.

Water

Na2CO3

50

3

20

7.

Water

Na2CO3

50

12

36

8.

Water

Cs2CO3

50

12

45

9.

Water

KOtBu

50

12

—

Source: Kim et al. [18] © 2005, with permission from American Chemical Society. a Iodobenzene (0.5 mmol), phenylboronic acid (0.6 mmol), NHCPd (1 mol%), and bases (2.5 mmol). b Isolated by column chromatography.

CH2Cl

a

C H2

Cl–H N N+

Pd b

C N H2

N

CM PS resin

Scheme 12  Preparation of imidazolium-loaded polymeric support and formation of polymer-supported Pd–NHC complexes Source: From Byun and Lee [39] © 2004, with permission from Elsevier.

The synthesis of Pd–NHC star polymer catalyst was reported by Bukhryakov et al.[41] in 2015 (Scheme 14). Metal complexes immobilized on nanoscale supports ­provide advantages such as well-defined structures, easily reachable catalytic sites, and recyclability for heterogenous catalysis. The catalyst was used in the Heck reaction and was extremely stable in the air and moisture. The polymer-­ supported catalyst showed high catalytic activity and was recycled several times with no loss of activity. Mohammadi and Movassagh [42] synthesized polymer-­ supported Pd(II)–NHC complex (Scheme 15). To obtain this complex, they used theophylline as an NHC precursor and chloromethylated polystyrene resin. The activity of this complex was investigated for the Heck reaction of some arenediazonium tetrafluoroborate salts with olefinic substrates under aerial conditions

and obtained high yields of products. Furthermore, the h­ eterogeneous catalyst can be easily recovered by simple ­filtration and reused for seven cycles without significant loss of its activity. The synthesis of poly(norbornene)-supported NHC– Pd complexes was reported by Sommer and Weck[43] (Scheme 16). Poly(norbornene) is a soluble support; therefore, it can be easily removed from the reaction media and reused ­simple precipitation methods. The catalyst was employed in the Heck reaction. To optimize the reaction conditions, the workers used triethylamine as a base, and iodobenzene and n-butyl acrylate as the substrates at 120°C in 30 min. At the end of the reaction, 99% of conversion was obtained. But the workers observed some palladium black at the bottom of the flask. So, they repeated the reaction using DMF instead of water and the presence of the QuadraPure poison, and obtained

Laboratory– Membranes

a. 1-methylimidazole, NMP, 80C, 12 h b. Pd(OAc)2, Na2CO3, water/DMF (1:1), 50C, 2 h

1618

Laboratory Applications: Polymers in

Table 3  Suzuki and Heck reactions carried out using the polymer-supported catalyst MeO Ph-X

O NH(CH2)5CONHCH2 TG

Ph

B(OH)2

Cat. (1 mol% Pd) K2CO3, DMA, 165°C

N

OMe

N

N

N Pd N Bu Bu Cl Cat.

Entry

X

Recyclesa

Yieldb

1.

Br

1c

57

2.

Br

4c

55 95

3.

I

1

4.

I

5c

5.

I

>14

c

95 95

d

Source: Steel and Teasdale [40] © 2004, with permission from Elsevier. a Recycles before >5% drop in isolated yield. b Yield of purified product after chromatography. c Reactions carried out in air. d Reactions carried out under Ar.

44% of conversion after 24 h. The lower conversions proposed that the palladium black provided the high catalytic activity instead of the catalyst. Sommer et al. [44] reported a range of palladated PCP pincer complexes bound onto polymeric and ­silica ­supports through either amide or ether linkages (Scheme 17). The synthesized four catalysts were used in the Heck reaction. Catalyst 1 was the standard for all reactions for comparison with the other catalysts. The reaction was

Laboratory– Membranes

HO(CH2)n

N + N

N + N

H

H

p­ erformed using iodobenzene and n-butyl acrylate as the starting materials, distilled triethylamine in DMF at 120°C. The effective conversion was obtained using catalyst 1 within 20 min. Furthermore, the workers investigated the poisoning studies adding poly(vinylpyridine) (PVPy) or mercury into reaction media. When PVPy or mercury was added to the reaction, less than 2% conversion of iodobenzene was observed after 3 h. They observed good conversions within 90 min using catalyst 2. Catalyst 3 provided quantitative conversion within 60 min. Using ­catalyst 4 for the Heck reaction resulted in high c­ onversions of ­iodobenzene in 2 h. A variety of polymer-supported Pd(II)–NHCs based on 3,4,5,6-tetrahydropyrimidin-2-ylidenes were reported by Mayr and Buchmeiser.[45] Merrifield and Wang resins were used as polymer supports (Scheme 18). The catalysts were used for the Heck reaction of butyl acrylate and styrene with various aryl iodides and ­bromides. Conversion in the range of 10%–90% was obtained with different turnover numbers. POLYMER-SUPPORTED PD–NHC-CATALYZED SONOGASHIRA REACTION The palladium-catalyzed Sonogashira reaction is an organic reaction to form a carbon–carbon bond between a terminal alkene and an aryl or vinyl halides with or without the presence of a copper(I) cocatalyst. This reaction is the most important for the synthesis of aryl alkynes and ­conjugated enzymes because of its use for natural ­products, pharmaceuticals, and molecular organic materials.

Polymer

2X (CH2)nOH 1a: n = 3, X = Br 1b: n = 2, X = I

[Pd(OAc)2] DMSO, 50°C, 4 h then 120°C, 1 h

HO(CH2)n

N X

N

N

Pd

N X HO(CH2)n 2a: n = 3, X = Br 2b: n = 2, X = I

O

P o l y m e r

Br O

O(CH2)n

4-(Bromomethyl)phenoxymethyl polystyrene (Wang resin) DMF, N(iPr)2Et, Csl, 24 h

Scheme 13  Reaction scheme of the formation of polymer-supported NHC–Pd complexes Source: From Schwarz et al.[25] © 2000, with permission from John Wiley Sons, Inc.

N X

N Pd

N

N X HO(CH2)n 3a: n = 3, X = Br 3b: n = 2, X = I

Laboratory Applications: Polymers in 1619

(a)

(b) N

Cl

1 equiv + 4 equiv

+

N

Cl Pd Cl N

N O n

n = 90

DVB 1. DMF, 125°C

+

Cl

N3

2. NaN3, DMF, 45°C

10 equiv

PEPPSI-IPr

Cl

P1

1

N

N

N

1. 1, Cul, triethylamine THF, rt

N N N

2. PdCl2, K2CO3, 3-chloropyridine, 90°C

Cl

P3

P2

CMS

N

Cl Pd Cl N

Scheme 14  Synthesis of the Pd−NHC star polymer catalyst Source: From Bukhryakov et al.[41] © 2015, with permission from American Chemical Society.

O N

N

N O

N

H N

Cl CMPS

N

N

Etl CH3CN

N

N

– + Cl N N

N N

DMSO 5 days, 80°C

O

N

O

PS-caff (4)

l– + N

N O

O

N

N O

N O

N

I

O

O

N

Pd(OAc)2 THF 16 h, 40°C

N

O PS-IL (2)

PS-theo (1)

O

N

PS-NHC-Pd(II) (3)

N

DMSO, K2CO3 72 h, 70°C

N

N

Pd

O Pd(OAc)2

THF O 16 h, 40°C

N

N N

N

Cl Pd Cl

N

O

N N

N O

PS-NHC-Pd(II) (5)

Scheme 15  Synthesis of functionalized supported NHC ligands and Pd(II)–NHC complexes Source: From Mohammadi and Movassagh [42] © 2016, with permission from Elsevier.

X +

n-BuO

O

Cat. (5 mol%) NEt3 DMF, 120°C

Cat.

n-BuO

O

y

x O(CH2)7CH3

Ar

O

O (CH2)11 N PdL2 N Mes

x:0, y:50

Scheme 16 The Heck coupling catalytic reaction of ­poly(norbornene)-supported NHC–Pd complex Source: From Sommer and Weck et al.[43] © 2006, with permission from John Wiley Sons, Inc.

The copper-free, heterogenous Sonogashira r­ eaction c­ atalyzed by a core-shell type of polymer-supported NHC– Pd catalyst was reported by Kim et al. (Scheme 19).[46] They used iodobenzene, phenylacetylene, 1 mol% of ­supported NHC–Pd without copper iodide, and Cs2CO3 at ambient atmosphere to form the model reaction. First, they investigated the effect of solvents and found that a ­solution of DMF/H2O (3:1) gave the highest yield. The catalyst also protected its catalytic activity for this reaction after ­recycling four times. Yu et al. reported a FDU-15 mesopolymer-­supported NHC–Pd complex and tested for the Sonogashira ­reaction [47] (Scheme 20). The mesopolymers including pure organic ­structures or pore walls have high surface area, large pore ­volume, high hydrophobicity, and accessibility for ­multifunctionalization. Therefore, these polymers are used for the preparation of supported palladium catalysts in the Sonogashira reaction.

Laboratory– Membranes

O

N

N

O

I

1620

Laboratory Applications: Polymers in

50 O

PPh2

O N H

O

Pd Cl

O

NH

(CH2)11 O

PPh2

1

50 (CH2)11

Pd Cl

Ph2P Pd Cl

Ph2P

PPh2

3

2

O

PPh2

OMe Si (CH2)3 S (CH2)3 O

O OH

PPh2

Pd Cl PPh2

4

Scheme 17  Immobilized palladated pincer complexes evaluated in this work Source: From Sommer et al.[44] © 2005, with permission from American Chemical Society.

Spacer

O N

N Spacer = p–C6H4– Spacer = p–C6H4–OCH2–p-C6H4–

Pd2+ N

N –

(Ag2Br2Cl44 )1/2

O

Spacer

Scheme 18  Structure of polymer-supported Pd catalysts Source: From Mayr and Buchmeiser[45] © 2004, with permission from John Wiley Sons, Inc. Laboratory– Membranes

Ph

n

Ph

PF–6

(1) Pd(OAc)2 DMSO 5 h, 50°C N2

m

N

+

N

(2) 30 min 100°C N2

Ph

n

Ph

m

H2C CH2 N PF6 N Pd N PF6 N Polymer-supported NHC-Pd (1)

Scheme 19  Core-shell type of polymer-supported NHC–Pd catalyst Source: From Kim et al.[46] © 2007, with permission from Elsevier.

In addition, FDU-type mesoporous phenolic resins are suitable for both acidic and basic ­reaction conditions, and showed special hydrophobic property against organic reactants and solvents. The individual properties of FDU-type mesoporous phenolic resins make them useful ligands. The workers investigated the activity of FDU–NHC/Pd(II)

catalyst for the transformation of various aryl halides with substituted phenylacetylene. The reactions between aryl iodides including electron-deficient groups showed sufficient yields, while the aryl iodides including electron-­ donating groups showed low yields. Also, the catalysts activity did not decrease after 10 times.

Laboratory Applications: Polymers in 1621

CH2

OH

OH

OH

CH2 CH2

OH

CH3OCH2Cl

Cl

AlCl3

Cl

CH2

FDU-15

N N toluene

OH

CH2

FDU-CH2Cl

OH

OH + N N Cl–

Pd(OAc)2

–

Cl + N N OH

N

Cl Pd Cl OH

FDU-NHC

N

N

N

FDU-NHC/Pd(II)

Scheme 20  Preparation of the FDU–NHC/Pd(II) catalyst Source: From Yu et al.[47] © 2011, with permission from Elsevier.

N Me

N

SH N

+

Cl

DMF, 100°C, 20 h

N

Me H2N

NH2

N

N

S

[PdCl2(PhCN)2] EtOH, reflux, 15 h N

Me H2 N

N

Cl

N S

Pd

Cl

Scheme 21  The synthesis of 4-amino-5-methyl-3-thio-1,2,4-triazole-functionalized polystyrene resin-supported Pd(II) complex Source: From Bakherad et al.[48] © 2011, with permission from Elsevier.

SYNTHESIS OF POLYMER-SUPPORTED NHC– OTHER METAL COMPLEXES NHCs have been supported on a variety of soluble and insoluble polymers. For instance, Ahmed and ­coworkers [49] reported the first immobilization of the c­ atalysts on an insoluble polymeric support for olefin metathesis (Scheme 22). They used a second-generation Grubbs ­catalyst and vinyl-modified poly(styrene) beads as the ­support. They used the supported catalyst in the ring-­ closing metathesis (RCM) of several common diene reagents.

PCy3

PCy3

Cl Ru PCy3

Cl

Cl Ru

Cl

IMes

Scheme 22  NHCs synthesized by Ahmed et al. Source: From Ahmed et al.[49] © 2000, with permission from Thieme Publisher.

After this report, Jafarpour and Nolan [50] supported second-generation Grubbs catalyst using poly(divinyl) benzene (Scheme 23). In this study, authors showed that they are able to recycle the supported NHC complex up to three times. In 2000 and 2002, Blechert and coworkers[51,52] reported both second-generation Grubbs catalyst supported on insoluble Merrifield, which is a cross-linked polystyrene resin, and second-generation Grubbs–Hoveyda catalyst supported on insoluble acrylamide-polyethylene glycol co-polymer (PEGA) resin (Scheme 24). Use of polystyrene-based Merrifield resin provided easy removal of the catalyst through simple filtration methods, and this compound is tested for RCM. The PEGA resin which supports the second catalyst

Laboratory– Membranes

A polystyrene resin-supported NHC–Pd(II) complex was prepared by Bakherad et al.[48] They used 4-amino-5methyl-3-thio-1,2,4-triazole ligand as an NHC precursor (Scheme 21). Polystyrene is one of the most general polymeric supports used in organic chemistry due to its cheap, easy availability, mechanical robustness, chemical inertness, and simple functionalization. The catalyst was tested for the reactions of terminal alkynes with various aryl iodides and bromides using water as the solvent. Excellent yields were obtained at the end of the reaction, and the activity of catalyst decreased after five recycling times.

1622

Laboratory Applications: Polymers in

PCy3

IMes

SIMes

Cl

Cl

Cl Cl

Ru

Ph

Ru

Cl

PCy3

Cl

Ph

Ru

PCy3

PCy3

N

Ph

N

N

IMes

N

SIMes

Scheme 23  Homogeneous catalysts Source: From Jafarpour and Nolan [50] © 2000, with permission from American Chemical Society.

O

allows for the catalysis to be applied in water or methanol. This system is used for closing metathesis (CM) and RCM. Buchmeiser and coworkers [53,54] reported different catalyst systems using monolithic support or poly (­styrene)-divinyl benzene (PS-DVB). They reported for the first time the use of monolithic supports to ­immobilize the ­second-generation Grubbs catalyst (Compound 1, Scheme  25). They tested this compound for RCM of some extensive allyl malonates with good yields and also the ring-­opening metathesis (ROM) polymerization of ­norbornene and cyclooctene obtaining polymers with molecular weights of up to 45,000 and 2,500 and PDIs as low as 1.2 and 1.7, respectively. This group also synthesized new NHCs and supported onto PS-DVB and monolithic support (Compound 2, Scheme 25). They studied this compound for ring-opening cross-metathesis and obtained good results with high turnover numbers.[55]

SIMes Cl Ru Cl O HN

N N Mes Mes Cl Ru Cl PCy3 Ph : Merrifield resin

O

: PEGA resin

Scheme 24  The compounds synthesized by Blechert et al. Source: From Blechert et al.[51,52] © 2000, 2002, with permission from John Wiley Sons, Inc. and Elsevier, respectively. Laboratory– Membranes

O n m+1

PS-DVB or monolithic support

O

Monolithic support

F3CO(O)C N

N

Mes Cl

Mes

O

Ru

Cl PCy 3 1

F O F

Mes O Ph

F F

C(O)O

N

N

Mes

Ru O

F F 2

Scheme 25  The compounds synthesized by Buchmeiser et al. Source: From Buchmeiser’s group [53,54] © 2002, 2001 with permission from Elsevier and John Wiley Sons, Inc., respectively.

NO2

Laboratory Applications: Polymers in 1623

(CH3)3

Si

(CH3)3

O

O Cl

Ru

SIMes

Cl

Scheme 26  The compound synthesized by Hoveyda’s group Source: From Hoveyda’s group [56] © 2000, with permission from American Chemical Society.

O

O O

Mes

Cl

OiPr

Cl

z

y

x

O

O

O

O

Ru

OiPr

N N

Mes

x:y:z= 1:9:30 n

In 2000, Hoveyda’s group [56] synthesized a new Grubbs– Hoveyda catalyst (Scheme 26). They investigated this catalytic system for RCM, ROM, and CM reactions, and they obtained excellent yields. Also, they were able to recycle their catalyst by simple filtration methods. Blechert and coworker[57] reported a soluble polymer-­ supported Grubbs–Hoveyda catalyst (Scheme 27). This compound was soluble in most organic solvents without hexane and diethyl ether. Also, this catalyst was tested for RCM, ROM–CM, and tandem metathesis ­reactions. The results showed that this catalyst has c­ omprehensive activity. In 2003, Lamaty et al.[58] reported the first PEG-­ supported, second-generation Grubbs–Hoveyda catalyst (Scheme 28). They carried out the RCM of tosyl diallylamine and N-allyl allylglycine using ‘boomerang’-type system and obtained high yields. Li et al.[59] synthesized polystyrene-supported NHC (Compound 3, Scheme 29) and NHC–Ag(I) complexes (Compound 4) in good yields (Scheme 29). They employed these compounds for three-component coupling of aldehyde, alkyne, and amine (A3-coupling). The results showed that PS–NHC–Ag(I) with polystyrene backbone as support was bestowed with a little higher catalytic activity

than their corresponding NHC–Ag(I) analogs. Also, the catalyst was reused at least 12 times with no important loss of its catalytic activity. In 2008, Yoon and coworkers [60] reported polymer-­ supported NHC–Gd catalyst (Scheme 30). This complex was evaluated as a heterogeneous catalyst for the acetylation of alcohols and phenols with acetic anhydride, and exhibited super activity over a wide range of substrates. The catalyst could be reused over 10 times without major loss of its catalytic activity.

Cl Cl

SIMes Ru O

O =PEG

Scheme 28  The complex synthesized by Lamaty et al. Source: From Lamaty et al.[58] © 2003, with permission from American Chemical Society.

Laboratory– Membranes

Scheme 27  The complex synthesized by Blechert et al. Source: From Blechert et al.[57] © 2002, with permission from John Wiley Sons, Inc.

1624

Laboratory Applications: Polymers in

N

P

CH2Cl

N

R

P

Toluene, 80°C

Ag2O

+ Cl

– -

N

P N

CH2Cl2 Ag

N

Cl

R

3

4

N R

Scheme 29  The synthesis of polystyrene-supported NHC and NHC–Ag(I) complexes Source: From Li et al.[59] © 2008, with permission from Elsevier. Polymer-supported Gadolinium Triflate Catalyst (0.5 mol%) R-OH + Ac2O (0.5 mmol) (1.5 mmol) DMSO, RT

N

Cl

‒Cl

1-methylimidazole

1. KOtBu, THF, RT, 1 h 2. Gd(OTf )3, DMF, RT, 2 h

DMF, 80°C, 12 h CMPS

‒OTf N ‒OTf Gd

⊕N Cl‒

CMPS-IM

R-OAc

2+

N

N

N ⊕ N

N ‒OTf Gd+ N N

CMPS-IM-Gd (Supported-NHC-Gd Catalyst)

Scheme 30  Preparation of CMPS–IM–Gd catalyst Source: From Yoon et al.[60] © 2008, with permission from Elsevier.

Cl N

N

CMPS NMP, 24 h

Ionic liquid phase

Laboratory– Membranes

Cl

Cl N

Cl PS-IM

N

N

Metal Chloride (MCln)

N NHC-metal complex

N

Polystyrene bead

N

MCln N

N

PS-NHC-M

Scheme 31  Preparation of polystyrene-supported NHC metal catalysts Source: From Kim et al.[61] © 2013, with permission from Elsevier.

Also, Kim et al.[61] reported polymer-supported NHC– Fe(III) catalysts (Scheme 31). The synthesized polymer-­ supported NHC–metal catalysts were ­successfully applied to the dehydration of fructose into 5-hydroxymethyl-2-furfural. A novel polymer-supported NHC-Rh was prepared by Yan and coworkers [62] in 2006 (Scheme 32). This complex

tested in addition of arylboronic acids to aldehydes and obtained good yields. Köytepe et al.[63] reported a polyimide-supported Ru–NHC complex (Scheme 33) and tested for hydrosilylation reaction. This complex showed high catalytic activity, s­ electivity, and stability in the hydrosilylation of acetophenone.

Laboratory Applications: Polymers in 1625

H2 C

N

N

Cl

Rh

Scheme 32  The synthesized polymer-supported NHC–Rh complex Source: From Yan et al.[62] © 2006, with permission from Elsevier.

N(CH3)2

N

Ru N Cl Cl

O

O

O

O

O

O

N

N N Cl

O

O

N

N

O

O

Ru Cl

N(CH3)2

N

O

O

N

N

O

O

n

HO HO

OH

N

N

Toluene, 80°C

SO2Cl

O N

Cl

Cl

O

N O THF, NaH

O N

CuCl2, t-BuONa

N N

THF, r.t.

O

Cl

NHC-Cu (II) complex

Scheme 34  The synthesis of TEMPO–PEG4000 –NHC–Cu(II) complex Source: From Wang et al.[64] © 2015, with permission from Taylor & Francis.

In 2015, Wang and coworkers[64] prepared a TEMPO– PEG4000 –NHC–Cu(II)   [2,2,6,6-tetramethylpiperidine-1-­ oxyl/polyethylene glycol/N-heterocyclic carbene] complex in a four-step procedure (Scheme 34). This complex was used for one-pot aerobic oxidative synthesis of benzimidazoles from alcohols. At the end of the reactions, the complex was determined highly efficient catalyst because of obtained good yields. Moreover, the reactions were carried out in the presence of water.

CONCLUSIONS NHCs are individual ligands having important effect on catalysis, material, medicinal chemistry and biochemistry. Therefore, various transition metal complexes including these ligands are studied by academia and industry. To develop the catalyst systems, synthesis of new, highly active, and stable organometallic compounds is investigated by many research groups. In this context, supported

Laboratory– Membranes

Scheme 33  The synthesis of polyimide supported Ru–NHC by Koytepe et al. Source: From Köytepe et al.[63] © 2008, with permission from Taylor & Francis.

1626

NHC ligands and their transition metal complexes attract considerable attention because of their great stability, ­reusability, and recyclability. Besides, these catalyst ­systems are easily removed in the reaction media by simple filtration, and thus there have been less contamination of ligand and environmental pollution caused by residual metals in the waste. In addition, they increase the selectivity. Because of these reasons, it will be an encouraging and fast-developing area in the future.

Laboratory Applications: Polymers in

11.

12.

13.

14.

ACKNOWLEDGMENTS

Laboratory– Membranes

The authors would like to thank Inönü University.

15.

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16.

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58. Varray, S.; Lazaro, R.; Martinez, J.; Lamaty, F. New soluble-­polymer bound ruthenium carbene catalysts: ­Synthesis, characterization, and application to ring-closing metathesis. Organometallics 2003, 22 (12), 2426–2435. 59. Li, P.; Wang, L.; Zhang, Y.; Wang, M. Highly efficient three-component (aldehyde–alkyne–amine) coupling ­reactions catalyzed by a reusable PS-supported NHC– Ag(I) under solvent-free reaction conditions. Tetrahedron Lett. 2008, 49 (47), 6650–6654. 60. Yoon, H.J.; Lee, S.M.; Kim, J.H.; Cho, H.J.; Choi, J.W.; Lee, S.H.; Lee, Y.S. Polymer-supported gadolinium t­ riflate as a convenient and efficient Lewis acid catalyst for ­acetylation of alcohols and phenols. Tetrahedron Lett. 2008, 49 (19), 3165–3171. 61. Kim, Y.H.; Shin, S.; Yoon, H.J.; Kim, J.W.; Cho, J.K.; Lee, Y.S. Polymer-supported N-heterocyclic carbene-iron(III) catalyst and its application to dehydration of fructose into

Laboratory Applications: Polymers in

5-hydroxymethyl-2-furfural. Catal. Commun. 2013, 40, 18–22. 62. Yan, C.; Zeng, X.; Zhang, W.; Luo, M. Polymer-supported N-heterocyclic carbene–rhodium complex catalyst for the addition of arylboronic acids to aldehydes. J. Organomet. Chem. 2006, 691 (15), 3391–3396. 63. Köytepe, S.; Seçkin, T.; Yaşar, S.; Özdemir, İ. Polyimide-­ dichloro-1,3-bis(pdimethylaminobenzyl)­ supported benzimidazolidin­-­2- ilidenruthenium (II) as effective catalyst for hydrosilylation reactions. Des. Monomers Polym. 2008, 11, 409–422. 64. Wang, Z.G.; Cao, X.H.; Yang, Y.; Lu, M. Green and ­efficient methods for one-pot aerobic oxidative synthesis of benzimidazoles from alcohols with TEMPO-PEG4000NHC-Cu(II) complex in water. Synth. Commun. 2015, 45, 1476–1483.

Laboratory– Membranes

Marine Applications Asit Baran Samui Institute of Chemical Technology, Mumbai, India

Abstract The polymers exposed in marine atmosphere must be able to sustain saline atmosphere containing suspension of sodium chloride in air, industrial pollutants, UV light, heating in day/cooling at night cycling, and frequent breeze-induced mild erosion. Underwater-exposed polymers experience severe fouling by marine organisms, degrading alkaline environment, and the floating oil. On the other hand, the plastic’s resilience against degradation and the issue of plastic pollution have become a threat to global ecology. The application of polymers in such marine environment and their b­ ehavior are discussed. Various plastics used according to the area of use, such as the non-load-bearing structures in marine vessels, have been deliberated in respect to stability and properties required for the applications. Elastomers used in offshore oil and gas industry facing hostile environment, vibration damping, wave energy harvesting, etc., are discussed. Marine composites are considered while considering their applications for naval use, commercial crafts, wind/underwater turbines, and subsea structures. Fibers are utilized for sails, marine safety apparatus, inflatable crafts, and oil booms and others, which are required to perform without fail. Polymeric foams are discussed in respect of  applications for flotation purposes, transportation of liquefied gas by ship, sound and heat ­insulation, and boat furnishing. Sealant, caulking compound, deck underlay, mastic, adhesives, and putties are described and properties/performance of various types for specific applications is furnished. ­Various fouling and corrosion-resistant coatings are discussed in detail. Detailed deliberation on specific paints, polymers derived from marine sources, deterioration, and degradation of polymers is done.

INTRODUCTION Marine environment can mainly be divided into three areas, such as atmosphere, seawater, and junction of above two called as boot top area or transition zone. Atmosphere Marine aerosols are considered as highly corroding species, which is produced by the interaction of wind and waves. Minute particles of inorganic salts and organic matter are transported by the air and the rain. The exhaust gases from ships pose threat to marine atmosphere as these comprise both conventional pollutants and greenhouse gases. The cruise ships run by diesel engines use high sulfur content fuel oil, which produces sulfur dioxide, nitrogen oxide, and particulates along with carbon dioxide, carbon monoxide, and hydrocarbons. Among the sources for total global air emissions, shipping accounts for 18%–30% of nitrogen oxide and 9% of sulfur oxides.[1,2] Furthermore, 3.5%–4% of all climate change emissions (methane, nitrous oxide, carbon dioxide) are caused by shipping, primarily carbon Encyclopedia of Polymer Applications, First Edition DOI: 10.1201/9781351019422-140000191 Copyright © 2018 by Taylor & Francis. All rights reserved.

dioxide.[2] Natural gas is mostly available along with oil deposit and sometimes without oil. Gas hydrocarbons are also formed as a result of microbial decomposition of organic substances. Substantial portion of these gases are released into the atmosphere. During oil extraction operation, a significant gas is also released in to the atmosphere. The flaring of natural gas on the offshore platforms and on land terminals add to the atmospheric pollution. A wide range of trace gases are released by marine phytoplankton. Dimethyl sulfide is produced by planktonic algae in sea water, which gets oxidized in the atmosphere to form a sulfate aerosol.[3] The diatoms produced bromocarbons are released to the atmosphere as inorganic bromine. Sea Water Sea water contains dissolved materials from Earth’s crust and materials released from organisms. The most important characteristics of seawater are salinity, temperature, dissolved gases (mostly oxygen, and carbon dioxide), nutrients, and alkaline pH. Generally, the seawater in the

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Keywords: Composites; Corrosion prevention coatings; Fenders; Fibers; Foams; Fouling prevention coatings; Plastics; Polymer deterioration and degradation; Rubbers; Sealants.

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world’s oceans has a salinity of approximately 3.5%, or 35 parts per thousand. Thus, 35 g of salts are present in every 1 L of seawater. Apart from sodium chloride there are other salts present in varying low proportions. Although a vast majority of seawater is found in oceans with salinity around 3.5%, seawater is not uniformly saline throughout the world. Interestingly, the relative proportion of sodium chloride and other salts remains constant. Temperature of sea water also varies according to the location and duration of its exposure to sunlight. For example, the tropical areas mostly get regular sunlight during most parts of day, whereas the polar areas get less and are cooler. Both carbon dioxide and oxygen remain dissolved in sea water to certain extent. The interaction of carbon dioxide and water produce buffer action and maintain the pH of sea water in the range of 7.5–8.5. Ocean disposal of society’s waste started long before the Agricultural Age. With the progress of civilization along the coastal zone, the navigation started and oceans  were considered as the ideal place for waste disposal, which comprised food waste, cleaned carcasses, mining waste, sewage carried human waste, etc. With the advent of ­industrialization, chemical waste was added to the list. The sewage water and waste water from toilets and medical facilities, which can contain harmful bacteria, pathogens, viruses, harmful nutrients, etc., are directly discharged in to sea. Solid wastes generated on a ship are either directly discharged in to sea or after treatment, which pose a threat to marine organisms, humans, coastal communities, and industries that utilize marine waters. On a ship, leakage of oil from engine and machinery rooms and also from engine maintenance activities mixes with water in the lowest part of the ship hull (bilge). Cruise ships can generate 1,300–37,000 gal of oily bilge water per day, depending on the size and age of the ship.[4] In ­addition, there are hazards like accidental oil spills. Air/Seawater Interface The junction area of seawater and air is considered as most harsh environment. The area is repeatedly getting wet and dry, which is akin to cycling. This stringent condition is used for testing materials for application in severe conditions. The severity originates from dry/wet cycle, constant exposure to sunlight containing UV radiation and impact of water waves, contact with floating oil, and so on. There is another underwater feature called as “biofouling,” which is the settlement of sea organisms on underwater surfaces. There is slime deposition immediately after the substrate is exposed to sea water. This is followed by deposition of a number of marine organisms. The most menacing are the barnacles. They get settled on the surface, make colonies, and form honeycomb structures having sharp edges, which contains maximum content of calcium carbonate. Figure 1 shows the barnacle settlement on sea-water-exposed panel and ship hull.[5]

Marine Applications

Fig. 1  Fouling on (a) seawater exposed panel (Inset: Barnacle shell) and (b) underwater surface of ship [5]

In light of above description, it can be understood that from air to seawater the conditions are unfriendly. There are chemical, oil pollution inside sea water, which has alkaline pH and plenty of inorganic salts. Under atmospheric condition, the UV radiation, the salt spray, chemicals, deposition of chemicals/salt along with dew during night, and evaporation of water during day make the c­ ondition very severe for any material to perform. Thus, a material required to perform under the conditions needs lot of considerations. A metallic body undergoes severe corrosion under any of the above conditions and is heavily fouled during very short (few months) exposure to sea water. Polymeric materials too have also lot of concerns for application under marine environment. Polymers in Marine Environment The environment faced by polymers can comprise the following: 1. UV radiation 2. Salt spray in air 3. Hazardous gases present in marine conditions and that coming from nearby industries 4. Particulate matters floating in air 5. Seawater having 3.5% salt and pH in the range of 7.5–8.5 6. Floating marine organisms 7. Floating oil 8. Compression at high depth of sea water 9. Exposure to most of the conditions at air/water interface

THERMOPLASTICS Thermoplastics are materials which can be remolded or redissolved in solvent. There are many thermoplastics which have excellent properties and stability in marine environment. They can be used as various components in marine vessels.

Polymethylmethacrylate (PMMA) is a polymer having repeating unit with pendent ester (Fig. 2). The presence of the pendant methyl (CH3) groups does not allow the  polymer chains to have close packing like a crystalline polymer and prevent it from rotating freely around the c­ arbon–­carbon bonds, which make PMMA a tough and rigid plastic. It is used for making boat windows, windshields, boat hatches, stowage for glassware, swimming pool enclosures, etc. It is lightweight and is resistant to salt and cleaning products. It has outstanding optical properties, as can be seen by the transmission of about 92% of visible light through 3-mm thick PMMA sheet. The environmental stability of PMMA is superior to most other plastics such as polystyrene as it does not yellow due to exposure to sunlight and it is therefore the material of choice for outdoor applications. PMMA has impact resistance better than glass and polystyrene. Moreover, this can further be improved by making copolymers with butyl acrylate. PMMA is synthesized by vinyl polymerization of methyl methacrylate monomer using solution, emulsion, bulk, and suspension polymerization methods in the ­presence of ­radical initiator at 60°C–70°C. Processing is done by ­injection, ­compression molding, and extrusion methods. Polycarbonate (PC) is a polymer having carbonate group in the structure. It is also used under marine condition. Its properties are intermediate between commodity plastics and engineering plastics. It has excellent impact resistance, high optical clarity similar to glass and PC window performs excellently during storm. However, the flexibility makes it difficult to seal and the PC windows are prone to leaks. The scratch resistance is poor as salt water spray scratches the PC window. Polycarbonates are polyesters of carbonic acid and are based on 2,2′-bis(4-­ hydroxyphenyl) propane (bisphenol A). It has been synthesized by the reaction of bisphenol A with phosgene or by ester ­interchange with diphenyl carbonate (Fig. 3) It can be processed by molding, thermoforming, ­extruding, etc.

[ CH2

Poly (vinyl chloride) (PVC) is polymer having chlorine atom in the structure and has a wide range of utility (Fig. 4). This plastic product is ideal for outdoor cabinets, marine furniture, signage, and recreational applications. Because of the chlorine content, PVC is inherently fire resistant. It is the only polymer which can produce materials from extreme elasticity to extreme rigidity, producing a versatile range of products from soft rubber like to very hard metal like materials. Mostly PVC is produced by suspension polymerization from vinyl chloride. PVC can form pipes, which are seen everywhere. The polymer can be calendered, extruded into rigid and flexible profiles, injection molded, compression molded, blow molded, rotomolded, and slush molded. High-density polyethylene (HDPE) is a polyethylene thermoplastic made from petroleum (Fig. 5). It has low extent of branching and has high strength-to-density ratio. It is a versatile product having useful characteristics such as low-moisture absorption, excellent impact resistance, high tensile strength, good chemical, and corrosion ­resistance. The material is used for making boat windshields, upholstery, enclosures, etc. Resistance to moisture absorption and decay, superior staple retention, etc., make it ideal for use in marine upholstery, which is extruded sheet plastic with added impact modifiers. Requirement of constant exposure to water without warping makes HDPE the ideal product for swim platforms. Anti-slip attributes are added to make it well-functioning. Radical chain polymerization of ethylene to polyethylene is carried out at high pressures of 120–300 MPa (17,000–43,000 psi) and at temperatures above the Tm of polyethylene. Nylon is chemically known as aliphatic polyamide (Fig.  6). It is extremely corrosion resistant, light weight, vibration damping, easy to machine, and self-lubricating and it has excellent wear resistance, low coefficient of friction. As corrosion resistance is crucial in marine environments, nylon is an ideal material for many marine industry

CH3 C ]n O

C

O CH3

H

H

C

C

H

Cl

n

Fig. 4  Chemical structure of polyvinylchloride

Fig. 2  Chemical structure of polymethylmethacrylate CH2

CH2

CH2

CH2

n

Fig. 5  Chemical structure of polyethylene O O

O

O n

Fig. 3  Chemical structure of polycarbonate

H N O

Fig. 6  Chemical structure of Nylon 66

N H

n

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1632

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applications. Nylon 6 can withstand long-term exposure to  alkalis, dilute acids, and oxidizing agents. Typical marine applications comprise stern tubes, winches, rudder stocks, capstans, and sheaves. The self-lubrication ability is particularly useful in the marine industry, where lubricants or chemical coatings can be removed by constant fresh or saltwater exposure. Nylon can be used for specific applications such as rudder shaft bushings & bearings, deck winch  & capstan bearings, wear pads and plates, door hinges, tube and winch bearings, rollers, etc.[6] Nylon expands by absorbing moisture at a rate of 0.15%–0.20% per 1% absorbed moisture up to 6.5% if completely submerged in water. During designing of articles, this factor needs to be kept in mind. Industrial synthesis is usually done by heating the acids, amines, [7] or lactams to remove water. However, in the laboratory Nylon 6,6 is synthesized by reacting diamine and a diacid chloride through interfacial polymerization using a two-phase system. At the interface of the two phases, the diacid chloride and diamine react to polymerize. The ring opening-polymerization of lactams is a chain-growth polymerization. The strained seven-membered rings, like ε-caprolactam, polymerize easily to make nylon 6. Acrylonitrile butadiene styrene (ABS) combines three unique compounds to make a durable and environmentally friendly material. The proportions of components vary from 15% to 35% acrylonitrile, 5% to 30% butadiene and 40% to 60% styrene. ABS combines the strength and rigidity of acrylonitrile and styrene with the toughness of polybutadiene rubber. The impact and moisture resistance, rigid design, glossy exterior, and electrical insulation properties recommends its use in blocks, bulkheads, stringers, and transoms inserts. ABS has high heat tolerance, which enable it to be processed by thermoforming and sonic welding on wood, epoxy, ceramic, and aluminum. The fiberglass boat builders use it due to its ability to easily adhere to fiberglass and commercial adhesives with no surface preparation. It is synthesized by emulsion, suspension, or bulk copolymerization of styrene acrylonitrile in the presence of a rubber, which may be polybutadiene or SBR.

Marine Applications

CH2

n

Fig. 7  Chemical structure of Novolac

various ratios and catalysts. The prepolymers are cured at higher temperature with/without catalysts. ELASTOMERS Elastomers are polymeric materials in which polymer chains are interlinked by chemical bonds to a cross-linked structure. The extent of cross-links is much lower than other thermoset polymers such as epoxy, PU, phenol-­ formaldehyde, etc. The lower extent of cross-linking allows the rubber to exhibit high elongation on application of stress and quick recovery after releasing the stress. Generally, the elastomers or rubbers are soft, nearly elastic, and incompressible. They are therefore used in ­components, which are designed as deformable and flexible. Processes for Elastomer Formation Curing or Vulcanization It is a process where the elastomer compound is transformed into an elastic material by forming network of bonds between the polymer chains of the elastomer ­material. Sulfur is a commonly used curing agent. •

•

THERMOSETS Phenolic resins are the resins in which aromatic rings are incorporated, which are formed in various forms according the reactants ratio during condensation polymerization (Fig. 7). It is considered as lightweight alternative to wood for construction in the marine industry. Applications include boat windshields, upholstery, enclosures, etc. In addition to their machinability, phenolic exhibits low water absorption, dissipation factors, corrosion resistance to a number of chemicals, and superior electrical characteristics over a wide range of temperatures and humidity. Furthermore, phenolic has superior flame resistance. Synthesis is done by reacting phenol with formaldehyde in

OH

OH

•

Mixing is the process, performed prior to curing, in which the elastomer compounds are mixed with curing agents, accelerators, and other additives including stabilizers. Compression molding is a process in which thoroughly mixed elastomer compound along with curing agent and additives is inserted into a mold cavity and a ­compressive load is applied along with heating to cure. Extrusion is a process in which the elastomer compound along with curing agent and additives is mixed thoroughly and pushed forward by the screw action in the extruder. The extruded parts are forced through a die under pressure and then the heat and pressure are applied for curing.

An elastomer is designed to have required properties in terms of: Tensile strength, elongation at break, tear strength, hardness, compression set, wear resistance, fatigue resistance,

Marine Applications 1633

1. Stress relaxation—Loss of force over time under constant deformation 2. Creep—Increase in deformation with time under constant load 3. Damping—Dissipation of mechanical energy on loading and unloading and its conversion to heat energy 4. Stiffness—Increases with rate or frequency of loading Thus, static seal performance depends on stress relaxation and creep (which affects sealing stress retention or long-term seal integrity). Damping and increasing stiffness with frequency are important for dynamic a­ pplications: flexible hoses, flexible joints, and pulsation bladders. The temperature variation affects the stability and performance characteristics of elastomer, such as at high temperature the mechanical properties decrease and slow degradation occurs. At lower temperature, it becomes stiff and turns to brittle below glass transition temperature (Tg). Elastomers for Fluid Containment in Offshore Oil and Gas Production The elastomer must be able to sustain the conditions comprising oil, sea water, and UV radiation. Furthermore, the seals are exposed to a number of liquid environments in the offshore oil and gas industry. The elastomers can be damaged by exposure to the gases through explosive decompression and chemical damage, respectively. Hostile Liquid Environments A number of liquid environments are encountered by ­elastomers in the offshore oil and gas industry. Methanol Treatment Liquid Excessive swelling of bisphenol A cured fluoroelastomer (FKM I) occurs in methanol. However, when the elastomer is treated with methanol with small percentage of water, the swelling is much lower. Water at High Temperature Water uptake by an elastomer at higher temperatures leads to exceptional behavior. Any FKM I formulation containing magnesium oxide and calcium hydroxide fillers undergo delayed degradation after long periods in water at 150°C and above. Use of lead oxide as filler yields the elastomer which does not undergo degradation.

Corrosion Inhibitors Corrosion inhibitors are known to degrade elastomers in some cases, which lead to permanent decrease in elastomer property. The elastomers, FKM I, acrylonitrile butadiene rubber (NBR) and hydrogenated NBR (HNBR) are susceptible to high pH inhibitors. As always, the best defense against catastrophe is appropriate compatibility testing prior to service. The fluoroelastomer which are resistant to high pH environments is copolymer of ethylene and tetrafluoroethylene (Viton ETP). The most common outcome of interaction between an elastomer and a hostile chemical species is the formation of additional crosslinks within the elastomer structure, which stiffens and may finally become brittle. The opposite type of degradation leads to chain scission resulting in the softening of the material. Well Stimulation Fluids: Acids An acid may chemically attack either ingredients within the elastomer or the elastomer, while both in some cases. Acetic acid, formic acid, and hydrochloric acid/­hydrofluoric acid (HCl/HF) mixtures are the more commonly used  treatment acids used offshore. Butyl rubber, chlorinated polyethylene, cross-linked polyethylene, teflon, viton, and ultra-high molecular weight polyethylene ­perform fare to excellent in terms of stability in the ­presence of most of the organic and inorganic acid. Hostile Gas Environments Hydrogen Sulfide In the oilfield industry, hostile fluids can include hydrogen sulfide (produced in some wells), various amines and amides used as corrosion inhibitors, and certain brines used in well completion. The sealing elastomers that have reactivity with H2S are NBR, HNBR, and several fluorinated copolymers. Only few elastomers which are negligibly affected even at very high temperatures can be named as copolymer of tetrafluoroethylene and propylene (TFE/P) and perfluoroelastomer (FFKM). Oxygen Oxygen at high temperature is aggressive to elastomers. The embrittlement occurs by oxidative attack, which may cause cracking, particularly to NBR elastomers. FKM ­elastomers are oxidation resistant to temperatures up to 200°C. Hydrocarbons Hydrocarbon gases such as methane and ethane are found in production fluids, but are not chemically aggressive to elastomers. Few hydrocarbon-type elastomers exhibit

Laboratory– Membranes

aging resistance, ozone resistance, oil, solvent, chemical resistance, etc. Although elastomers are mostly elastic, the viscous characteristics are associated with frequency or time-­dependent changes. The related characteristics are as follows:[8]

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small swelling. This type of elastomer may undergo explosive decompression damage in seals if it is saturated and the pressure drop is sufficiently rapid. Elastomeric Products in Marine Industry Elastomers being used in shipping yards, boatyards, docks, boat building, and maintenance facilities are listed as follows: Nitrile/PVC Resin Blends (NBR/PVC) NBR/PVC blend is used to construct tube for the toilet and sullage lines in marine applications. The blending of PVC with nitrile is done to provide increased resistance to ozone and abrasion. Significant improvement in solvent resistance is provided by PVC, while maintaining similar chemical and physical properties like nitrile elastomers and high pigment loading capacity. Ethylene-Vinyl Acetate (EVA) EVA is the copolymer of ethylene and vinyl acetate and the weight percentage of latter varies in the range of 10%– 40%. It has good clarity and gloss, low-temperature toughness, stress-crack resistance, hot-melt adhesive, waterproof properties, and resistance to UV radiation. It is used in general purpose vacuum applications. The UV resistance and superflexibility due to the presence of ethylene unit make it suitable for low pressure applications and sleeving of products under marine conditions. Reinforced Rubber Hoses for Offshore Applications

Laboratory– Membranes

Among many important roles, the most common application of rubber is the loading and discharging supertankers offshore through reinforced rubber hoses in the case of non-availability of shore terminal. Also it is used in offshore oil exploration and live crude transfer systems, where high pressures, high temperatures, and sour gases are prevalent. Hose is designed by incorporating more than one rubber compound in its construction. Along with other requirements, one important factor is very high levels of adhesion to maintain high resistance to fatigue. The essential requirements for the hose bore of an offshore oil hose are oil resistance, high resistance to aromatics (up to 50%), stability in the temperature range of −29°C to 52°C, fatigue resistance, and ozone resistance. Acrylonitrile-­ based rubber formulations are mostly selected for oil hose linings. However, where high-abrasion-resistance requirement exist, such as the dredging hose, hard wearing SBR rubber compounds are mostly considered. For live crude hoses, the rubber compound must be formulated so that it resists oil, hydrogen sulfide, and should have sufficient strength to withstand explosive decompression; the rapid release of entrapped gases when bore pressures are reduced.

Marine Applications

The hose comprises three main elements: 1. Elastomeric lining resistant to a variety of hydrocarbons. 2. Elastomer-reinforced carcass with multi-layers of high tensile textile cords and embedded steel wire helix. 3. Fiber-reinforced smooth elastomer cover, which has antiaging characteristics, high tear strength and resistance to abrasion, weathering, sunlight, oil and seawater penetration, respectively. NBR-based elastomer formulation is stable in hydrocarbons with aromatic content up to 60%. For floating hoses, closed-cell foam outer cover—fiber-­ reinforced smooth elastomer cover, such as polyurethane coating is used. Polytetrafluoroethylene (PTFE) corrugated hoses have extraordinary characteristics. PTFE remains non-reactive due to high C–F bond strength, which makes it highly suitable coating used in containers, pipes, hoses, etc., for reactive and corrosive chemicals. It can also act as lubricant as it reduces friction, wear, and energy consumption of machinery. PTFE compositions are non-flammable and maintain their internal diameter even in a constricted radius. Chemical structure of PTFE is given in Fig. 8. Seals and Insulation Ships and boats need high-quality rubber products— including watertight compartment sealing. Parts such as a latch seal on a ship’s deck, a porthole or window seal that is close to the waterline or an access hatch seal for an engine room need proper sealing. Thermal insulation is designed to arrest heat flow from a boat or ship’s engine compartment. Silicone is used for the purpose as it can resist high temperatures while providing damping of acoustic noise from high-decibel diesel engines. Depending on the type of marine engine, the door seal on a fuel tank must be able to resist specific petroleum products such as gasoline and diesel fuel. Expanded neoprene (Fig. 9) serves the purpose as it does not absorb water and is oil, weather and F

F

C

C

F

F

n

Fig. 8  Chemical structure of poly (tetrafluoroethylene)

Cl

n

Fig. 9  Chemical structure of polychloroprene (Neoprene)

heat resistant. It is also used for hatches, cushioning, and ­suppression purposes. Vibration Damping Vibration damping is the dissipation of mechanical energy of a material or system under cyclic stress. Vibrations can cause problems ranging from fatigue failures, reduced passenger comfort to detection of underwater objects (marine vehicle) by SONAR system. For naval system, the last one is extremely important. Vibrations can be minimized by (a) removing or isolating the source of vibrations, (b) changing the mass or stiffness of a structure so that the natural frequency(s) are changed, and (c) absorbing (damping) the vibrational energy. Vibration Isolation vs. Vibration Damping Vibration isolation system lowers the natural frequency of a mechanical system below the excitation frequency. The natural and excitation frequency being “out of sync” reduces the amount of vibration and potential problems. Vibration damping indicates absorption of energy from the system. By damping a reduction in vibration, noise, and the dynamic stresses applied will be done. Active Damping and Passive Damping Active damping: It denotes the dissipation of energy from the system by external means, such as controlled ­actuator, etc. Passive damping: The energy dissipation within the structure can be done by adding damping devices, structural joints/supports, or by internal damping in the structure. The relaxation and recovery of polymer network after it is stressed gives the damping properties. During vibration, the polymer chain segments responding to vibration undergoes motion and when the stress (vibration) is released, the polymer chain try to come back to normal state. During the process, there are frictions among the chains and the vibration energy is converted to heat energy. Thus, the marine vehicles need damping to protect machineries, to minimize other vibrations due to foot movements, and the last one must be prevented from emission from the submarines to keep it camouflaged. Engine anti-vibration mounts, damping mastics, and the final ­barrier are the coating on submarines. Elastomeric materials are well known for damping. However, damping is represented by tanδ value that depends on the ratio of loss modulus (E″) and storage modulus (E′) (E″/E′). Furthermore, for designing any damper, the specific temperature, frequency, and mechanical ­properties need to be taken in to account. The stiffness of ­r ubber being low the extent of loss is limited.[9]

Various blends are designed which have better mechanical properties as well as stability in hostile environments. Polyvinyl chloride/chlorinated polyethylene/epoxidized natural rubber blends, [10] polypropylene/butyl rubber blends, [11] etc., are found useful. Stability under marine conditions was observed with urethane acrylate interpenetrating polymer networks (IPNs)[12] and several elastomeric IPNs, based on nitrile rubber and [13–15] nitrile rubber blends, [16] are found to perform excellently over a wide temperature and frequency range. Synthesis of IPNs uses a strategy by which two monomers are mixed with curing agents, so that both get cured simultaneously (simultaneous IPN). The other method uses one preformed polymer film in which monomer/s is/are soaked and is cured later (sequential IPN). The damping by any elastomer depends on the value of tanδ and it is dependent on the matrix composition. Figure 10 shows tanδ vs. temperature plot for thermoplastic blend of polyaniline-camphor sulfonic acid with polyvinyl chloride-nitrile rubber at various weight ratio.[17] Fiber-reinforced elastomers, laminated on aluminum base panels, exhibit highest vibration damping for softest polyurethane on fixed fiber.[18] For fixed polyurethane elastomer, the stiffest fiber produced maximum damping. Two mechanisms such as high shear deformation of very flexible polyurethane matrices and the loading up of a stiff outer layer of fibers, having constraining effect, are found to be predominant in the damping mechanism. Both the dynamic loss modulus and loss factor of CNT-epoxy nanocomposites and carbon fiber-reinforced-­ CNT nanocomposites show consistent increases with the addition of CNTs, which indicate improved damping performance.[19] In an alternate arrangement of continuous and chopped fibers on the polypropylene honeycomb core, the damping could be improved by splitting the length of fiber into different short lengths so that more energy can be dissipated.[20] For a given fiber volume fraction, the smaller length fibers exhibit enhanced damping. The vibration damping being a must for naval vessels and others, innovative approaches have been made to develop variety of vibration damping systems. Magnetorheological (MR) materials display smart behavior as their rheological properties can be changed continuously, rapidly, and reversibly by applied magnetic fields.[21] The MR suspensions are most common MR material, which are made by dispersing micron-sized or sub-micron-sized magnetizable particles dispersed in liquids. The problems existing in MR suspensions such as particle sediment are eliminated by using a solid matrix such as a polymer. The polymer is cured in the magnetic field. The field-­induced interactions between particles result in the formation of anisotropic ordered chains or three-dimensional structures, which on exposure to magnetic field, results in field dependent performance. The damping ratio of MR elastomer depends on the matrix properties, for example, natural rubber exhibits lowest damping than silicon rubber

Laboratory– Membranes

Marine Applications 1635

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0.5 a

0.4

tanδ

b c

0.3

d

0.2

0.1

0.0 –50.0

–20.0

10.0

40.0 Temp (°C)

70.0

100.0

130.0

Fig. 10  Plot of tanδ versus temperature for PVC-NBR/PANI-CSA blend; (a) 100/0 (w/w); (b) 100025 (w/w); (c) 100/50 (w/w); (d) 100/75 (w/w) Source: © 2006, John Wiley and Sons.[17]

Laboratory– Membranes

and chloroprene rubber, while keeping the same magnetic particle proportion (30% of rubber matrix, 10% of plasticizers, 60% of iron particles). With an increase of iron particles, the damping ratio increases.[21] Free layer damping (FLD) and constrained layer damping (CLD) are two types of damping materials, which are mostly used for attenuating the vibration of the substrate, e.g., submarine deck. In the former, the thick layer of filled elastomeric compound is applied on the substrate and in the latter a thin viscoelastic layer is applied on the  substrate, which is covered by a constraining layer made of thin FRP or metal sheet. FLD dissipates the vibration energy by extension and the CLD by shear. Most of the materials belong to various rubber materials. Nitrile, acrylates, halide, etc., groups present in the polymeric material enhances the damping. For example, nitrile is a good damping material and also it is stable in marine environment. Other materials belong to modified epoxy. Epoxy resin can be modified with polyethylene glycol (PEG). The damping frequency and damping ratio vary with the chain length. Large surface area vibrating/resonating surfaces are better damped by using FLD rather than the CLD as they are more effective on the lower frequencies typically generated by larger surface areas. Figure 11 shows the design of FLD and CLD.[22] Elastomer for Wave Energy Dielectric electroactive polymers are thin films of elastomeric material, coated on both sides with compliant and conductive electrodes, so that large amount of deformation can be made without damage to the compliant electrodes.[23] If this elastomer is used in reverse way to an actuator, it can function as generators, converting mechanical strain

Viscoelastic layer

Constraining layer Viscoelastic layer

Base

Base

Free layer damper

Constrained layer damper

Fig. 11  Design of free layer damper and constrained layer damper[22]

energy into electrical energy using the capacitive behavior of polymer.[24] The natural rubber electromechanical generator is considered to fit in mechanical harvesting systems that are viable and low in cost.[25] Its high fracture energy is expected to make it durable and the combination of excellent mechanical and electrical properties are responsible for high specific energy and power in soft generators. The technology is used to harvest ocean energy to go to the energy grid and power portable devices and remote sensors. Nanoscale fillers enhance permittivity as well as dielectric strength, which can lead to optimized ­r ubber-based materials and improve the performance of ­elastomeric generators.[26] The most promising elastomeric generators have been identified as natural, synthetic rubber, and silicones. [27] Acrylics, having interesting properties, can also be applied. However, the commercial acrylics exhibit excessive ­hysteresis losses. The fatigue behavior of natural rubber was studied in sea water and compared with that in air. [28] The analysis of fatigue life in sea water indicates that antioxidant leaching is not the origin of reduction. The reduction of number of cycles to failure does not occur when natural rubber is used in sea water in fully

Marine Applications 1637

relaxing cycles, as the strain-induced crystallization does not occur in this case.

combination, the strength of the laminate will vary with the direction of the applied force. Resins

Composites, comprising two or more materials having different physical or chemical properties, can be designed with innumerable compositions. The composite composition can be chosen according to the area of use and the properties required. Polymer matrix resins are selected from polyester, isopolyester, vinyl ester, epoxy, and phenolic. The fibers used are normally glass, carbon, aramid, or other reinforcing material. Composite materials have many advantages over metallic structures such as light weight, ease of fabrication of complex structures, and no corrosion problems. Regarding material selection, the type of application dictates the selection criteria. Appropriate matrix resin chemistry must be adopted to minimize the matrix damage. The most important fiber/matrix interface properties are required to be superior for composites used under immersed condition for long periods. Various composite fabrication methods are used • • • • • • • • • • •

Hand lay up Compression molding Bag molding Pultrusion Filament winding Preformed molding compounds Resin transfer molding Injection molding Reaction injection molding Reinforced reaction injection molding Spray up

Various composite materials based on glass, carbon fibers, epoxy, polyester/epoxy resins, and fiber orientation at 90° and ±45° are used and the manufacturing process of infusion, prepreg, and resin transfer molding are adopted to prepare composites for marine application.[29] The bending strength decreases rapidly for glass fiber and shows no change for carbon fiber. Fibers The most used fibers for making composites are fiberglass, aramid, carbon, polyester, and nylon, respectively. The fibers types used are woven (cloth or woven roving), knitted (unidirectional reinforcement), omnidirectional (chopped strand), and unidirectional (no structural reinforcement in the fill direction). The reinforcements (long fibers) in marine composite construction are done by orienting the bundles of fibers in distinct directions. Whether the reinforcements are aligned in a single direction or a

Polyesters are the simplest among the resins and are versatile, most economical, easy to apply, resistant to chemicals. Unsaturated polyesters are synthesized from maleic anhydride or fumaric acid, phthalic anhydride/acid, i­sophthalic acid/tetrahydrophthalic acid, and various glycols. The polyester is diluted with styrene monomer, which participates in curing reaction in presence of fibers. Phthalic (ortho) and isophthalic-based polyester resins are mainly used in marine environment. The ortho-resins have inferior properties than the iso-resin in respect of thermal stability, chemical resistance, and processability characteristics. In the case of use of ortho-resin in the composite, the gel coat (top) or barrier coat in marine laminates are made of isoresin. The rigidity of polyester resins can be decreased by increasing the ratio of saturated to unsaturated acids. Vinyl ester resins are unsaturated resins synthesized by the reaction of a carboxyl containing vinyl monomer, such as methacrylic or acrylic acid, with a bisphenol diepoxide. The resulting polymer is mixed with styrene or styrene derivatives. The vinyl esters are processed similar to polyesters. It exhibits superior corrosion resistance, hydrolytic stability, and excellent impact and fatigue resistance. About 0.5–1.5 mm layer of vinyl ester resin matrix can provide excellent permeation barrier to resist blistering in marine laminates. Epoxy resins constitute a broad family of materials that contain epoxide units in the molecular structure. The resin is cured by reacting with hardeners, which are chemically known as amines, amides, anhydrides, etc. In respect of adhesion, strength, environmental stability, it exhibits the best performance among all the resins used in the marine industry. Thermoplastics are also used in making composites. It is incorporated in composite by melting at high temperature and these can be remolded and redissolved. The advantage of using thermoplastic materials is the absence of exotherm during composite making process, which is known to affect the filament winding of thick sections. Thermoplastic can be named as polyethylene, polystyrene, polypropylene, polyamides, nylon, etc. However, the ­application is mostly in the area of small boats and ­recreational items. Application-Related Problems Blistering in gel-coated FRP structures manifests itself as a localized raised swelling of the laminate in random fashion after a hull is immersed in water for some period of time. Various defects introduced during fabrication and entrapment of liquid can cause blister formation. Even with utmost care during fabrication the blisters can still develop

Laboratory– Membranes

COMPOSITES

1638

Marine Applications

over a period of time, which are due to osmotic water penetration. The water inside the laminate reacts with polymers to form larger molecules. A pressure or concentration gradient develops, which leads to hydrolysis within the laminate. Epoxide and polyurethane resins exhibit better hydrolytic stability than polyester resins. Degradation by UV Exposure is very common with the resins used in composites exposed to sunlight. Epoxies are generally very sensitive to ultraviolet (UV) light and during exposure to UV rays for extended period, the resins will degrade resulting in yellowing and loss of mechanical properties. The vinyl esters also behave similarly with slower rate than epoxy. Polyester is also sensitive to UV degradation. However, it is least sensitive of the three to UV light. The outer surface of most boats is covered with a gel coat. The gel coats, based on ortho or isopolyester resin, are filled with UV protective pigments. Naval Application of Composites Along with lightweight, strong, corrosion-resistant nature, the high acoustic transparency of composites also qualifies them for use in naval communication systems, such as radomes on ships and sonar domes on submarines. Naval application of FRP composites includes:[30] 1. 2. 3. 4. 5. 6. 7. 8.

Mine sweeper Landing craft Personnel boat Sheathing of wood hulls Submarine sonar dome Submarine firs Landing craft reconnaissance Submarine non-pressure hull casing

Laboratory– Membranes

Naval patrol boats are mostly made with an all-­ composite design or a composite hull and aluminum super structure. The FRP patrol boat exhibits excellent corrosion resistance and the lightweight structure provides higher speed and fuel economy. Carbon fiber composites are mostly avoided due to their high cost. The composites are expected to be used in propulsion systems due to its lower cost, reduced weight, lower magnetic signature, better noise damping properties, and superior corrosion resistance. Applications in rudders of ships and control surfaces for submarines, in funnels, bulkheads, decks, watertight doors, machinery foundation, pipes, ventilation ducts, and components for diesel engines and heat exchanger on large war ships are also encompassed. To provide higher speeds to patrol and service boats, engine power can be increased or weight can be reduced. The Kevlar fiber makes the boat light, tough, and more damage-tolerant. It is found highly useful for the construction of hulls, superstructures, bulkheads, etc. The application has also expanded to the area of leisure boats and for competition maxis, sailboats, and powerboats,

respectively. Kevlar is aromatic polyamides with liquid crystalline properties. It is a lightweight material having high strength, stiffness, and impact resistance. Leisure, Sporting, and Commercial Craft GRP-based composite materials are mostly used in leisure and commercial craft. About 95% of all-composite marine craft are constructed with GRP because of its low cost. Defect-free composites are made by using advanced ­fabrication processes, such as resin transfer, resin film intrusion, or auto craving in the construction of hull and decks, respectively. Detailed description about trend in the marine composites, construction, and materials was ­presented by Eric Greene Associates.[31] Construction Single-skin construction. Early fiberglass boat building protocol was the design of single-skin structures with stiffeners to maintain reasonable panel sizes. With increase of strength requirements, fiberglass cloth and woven roving are integrated into the laminate. Sandwich construction was initiated during the early 1970s after it was understood that the sandwich construction technique can yield stiffer and lighter structures. The inner and outer skins are laminated to low-density core, which is made by using linear and cross-linked PVC foam and end-grain balsa. Resin development. Ortho-polyester laminating resins are still being used by the entire boating industry due to their low cost and ease of use. However, high-end crafts are looking for superior resins. The costlier epoxy resins, having better strength properties than polyesters, are used only for specialized applications. Resistance to blistering is better with iso-polyester resin than ortho-polyester resin. Vinyl ester resin exhibits performance better than polyester and ­inferior than epoxy and has excellent blister resistance. The long-term durability and favorable fabrication economics, lower labor costs have triggered the use of composites in commercial marine industry. Smaller crafts are made with composite, whereas nonstructural applications are only made with composite for larger crafts. Composite conduit, electrical connector, electrical box are some of the areas covered by composite application.[32] The most important application of GRP is observed in commercial vessels used in the field of fishery, as it reduces maintenance costs and increases hull life. Resins used are usually non-air-inhibited rigid polyester. Alternating plies of mat and woven roving are used for making composite. In survival craft, the hull and canopy construction utilizes a spray lay-up system using gun roving and fire-retardant polyester resin with isophthalic/neopentyl glycol-based polyester gelcoat finish.

Marine Applications 1639

Various submarine structures are made of composite ­materials, such as periscope fairings on nuclear submarines, the bow domes on combatant submarines, and ­filament-wound air flasks for the ballast tanks. The fairings are made from glass fiber and the resin is changed from earlier used polyester to epoxy resins. A submarine-­ launched missile utilizes a capsule module made of composite. The graphite, wet, filament-wound sandwich construction, metal honeycomb core, and Kevlar reinforcements are used. Torpedoes are also constructed from composite. The nose shell of the torpedo is constructed with syntactic foam core and prepreg skins of carbon and epoxy resin, which ­exhibits reduced noise level. The National Academy of Science report, Use of Composite Materials in Load-Bearing Marine Structures, is followed by several naval goals for future composites:[33] • •

Carbon/PEEK or PPS for dry deck shelter Glass/epoxy prepreg for stern structure

U.S. Navy Osprey Class Mine hunter was constructed using chopped-strand mat stitched to a woven roving (E-glass) and high-grade toughened isophthalic marine polyester resin. One of the most successful Navy composites machinery program involves the development of a standard series of composite centrifugal pumps, which is enclosed in glass-reinforced epoxy (GRE), vinyl ester, or polyester composite housing.[31] The corrosion–erosion performance of the composite is excellent, although the performance under cavitation is poor. The Advanced Enclosed Mast/Sensor (AEM/S) (height: 87′, width: 35′, 40 ton) structure is fabricated in two halves by using E-glass, vinyl ester resin and balsa and foam cores. Shipping containers are made from E-glass/isopolyester pultruded panels up to 48” wide that incorporate 45° off-axis reinforcements, which weighs approximately 42% less than the steel containers. The advantages in respect of corrosion, painting, adhesive bonding for welding, etc., ensure 15-year life as compared to 8–10 years for steel.[34] Offshore Application of Composites Offshore metal structures suffer from heavy corrosion due to saline atmosphere. To prevent corrosion and to minimize maintenance cost, composite structures are found to be very economic. The most common types of composites used are GRP and phenolic composites. Phenolics provide good fire resistance. Pipes made of GRE are commonly used for oil transportation, where resistance to crude oil, paraffin build-up and ability to endure high pressures is required.[35] GRE piping system is also being used on offshore rigs for sea water cooling lines, air vent systems, drilling fluids, fire-fighting, ballasts, and drinking water lines.

The type of resin and hardener used for making GRE decides the chemical resistance and the maximum use temperature in a particular fluid. GRE tubes mostly remain unaffected by the presence of hydrogen sulfide and carbon dioxide.[36] Even though the GRE pipe exhibits the best allround chemical resistance, other types of resins types are also being used. • • •

Isophthalic polyester, for general purpose products Vinyl ester, which often shows corrosion resistance approaching that of epoxy Phenolic (including phenolic/siloxane alloy, PSX), recently developed for fire-critical applications

Due to excellent resistance to saltwater and humidity, FRP vinyl ester piping system is used at offshore installation in the Arabian Gulf. The effective safety factor on GRE pipe is quite high, which indicates that larger diameter high pressure pipes should have high wall thicknesses that are expected to be inconvenient for manufacture. The problem is eliminated by using steel strip laminate (SSL) pipe.[37] SSL pipe is constructed by using conventional glass/epoxy bore and outer layers. The internal load-bearing laminate is the helically wound layers of high tensile steel strip. The advantage is that the steel strip can be operated at a much higher proportion of its UTS than the GRE. Coil tube for oil and gas industries is made by using a thermoplastic liner, over-wound with an epoxy-based structural thermosetting laminate. Flexible thermoset coil tubing can withstand high pressure, typically 500 bar. The reinforcement is typically E-glass or carbon fiber, which depends on the application and economic factors. According to application, the liner materials may be chosen from among polyethylene, cross-linked polyethylene, nylon 11 or PVDF, respectively. Piping and gratings made of glass/phenolic for offshore platforms and drill ships for use of FRP (USCG Policy File Memoranda [PFM] 1–98 and 2–98) got the necessary approval in late 1998.[38] The composite pipe can be designed by using phenolic resins, fire-­ retardant additives, and intumescent coatings, which can ­survive the most extreme fire conditions. Pipe joining in offshore platform is accomplished by wrapping the seam with resin-wet layers of chopped-strand mat and woven E-glass roving, using the same epoxy vinyl ester resin used in the pipe bodies and the curing is accomplished under ambient condition. Fabrication of piping is also done by filament winding, glass roving and carbon tow, wet out with an aminecured epoxy resin. The carbon fiber constitutes about 10% of the hybrid laminate. The carbon fibers remain in contact with each other and maintain end-to-end conductivity and enhance the structural performance.

Laboratory– Membranes

Naval Research and Development Application

1640

Phenolic resin gives off water and formaldehyde during cure and mostly become brittle. This problem can be tackled by using polysiloxane-modified phenolic (PSX) resin matrix. The siloxane addition improves processibility, bond strength and impact resistance of the finished piping. In addition to above properties, heat barrier property is imparted by selected design.

Laboratory– Membranes

The oil and gas industry has expressed great interest in composite pipe systems produced by helical winding process. Nonimpregnated aramid (Kevlar or Twaron) yarn is used, which takes the load.[39] The aramid fiber is helically wrapped right onto the liner. For better handling, fiber yarns are encapsulated in a thermoplastic to form a tape, which is wrapped and welded to the liner and cover. This is the only high strength reinforcement that can be used in the non-impregnated state without damage occurring due to fiber–fiber friction. The lining on carbon steel pipe [40] is done to prevent corrosion and increase the cost-effectiveness of carbon steel flow lines. When the pipe is depressurized there is possibility of liner collapse, which is due to the ­permeation of p­ ressurized gas to void space at the interface between the liner and the pipe inner wall. The liners for carbon steel pipe are normally unreinforced thermoplastic tube ­(polyethylene, PVC, PVDF, etc.). Filament-wound thinner thermoset liners maintain lower permeability and higher modulus. The space between the liner and the pipe wall is injected with a cementitious or polymeric grout. In the oil and gas industry, many operators produce oil from high-pressure high-temperature (HPHT) reservoirs, working with downhole temperatures above 180°C and pressures above 700 bar (10,000 psi).[41] The polymeric materials, incorporated in composite, are exposed to oil field fluids at high pressure, temperature, and hydrocarbons. Engineering thermoplastics such as polyetheretherketone (PEEK), polyoxymethylene (POM) and polyphenylenesulphide (PPS), and thermosets such as bismaleimides and high temperature epoxies are considered suitable under above conditions. In seawater, the moisture absorption and elevated temperature cause plasticization of the thermoplastic matrix, resulting in relaxation of the polymer chains. The weakening of matrix/fiber interface remains the mechanism of failure for the POM, polypropylene, polyamide (PA 11) and polybutylene terephthalate composites with glass. The glass fiber-reinforced POM and PP show only little decline in mechanical properties. The hydrocarbon environment at high temperatures decreases the tensile and flexural property of all the composites. The POM/Carbon and PP/Glass samples disintegrated at elevated temperatures due to deterioration of the matrix/fiber interface. The carbon fiber-reinforced PPS performs well in the hydrocarbon gas condensate. At low concentration H2S gas environment (30 ppm) and elevated temperatures the mechanical property of

Marine Applications

PPS/carbon and PEEK/carbon composites show low extent of aging. Subsea equipment protection structures and thermal insulation systems are employing composites with increasing trend. In drilling, when multiple bridge plugs are used, it becomes a time-consuming process. With composites, it becomes easier and lower cost exercise. The glass or carbon epoxy, thermoplastic resins such as PEEK and PPS are the candidates for their better environmental resistance. Phenolic resin composite ­materials are used to protect subsea fl ­ owline insulation. Structural Applications Blast and Fire Protection Composite materials are used for blast and fire walls on offshore platforms. Composites are also used in combined corrosion and fire protection of load-bearing steel structure, including pipework, risers, and platform legs, respectively. In sandwich configuration fire-resistant core materials such as cross-linked PVC have undesirable properties during fire, such as toxicity or poor fire integrity. Phenolic-­ based syntactics has the most suitable combination of properties.[42] Gratings and Stairways The use of pultruded composite gratings and stairways in offshore structure has started during 1980s. The polyester and vinyl esters are commonly used resins. Also, the successful pultrusion techniques for phenolic resin have enhanced the use of phenolic-based gratings for offshore usage, in locations where fire integrity is important.[43] The phenolic gratings perform during fire and also after that, as it retain sufficient functionality. Kevlar-based concrete strengthening composites, used in construction, ­provide better elongation properties. Series of destructive and fatal fires on board marine vessels in both commercial and defense sectors have drawn the attention for designing composite structures having ability to resist fire. Resin matrices such as polyester, epoxy, vinyl ester, or methacrylate support combustion and emit large quantities of smoke and toxic fume. Phenolic resins are widely used in industry because of its good flame retardance, electrical insulation, dimensional stability, and chemical resistance.[44] Phenolic composites exhibit similar fatigue resistance to marine quality isophthallic polyester and these have equal or better impact strength and fracture toughness than certain polyester, epoxy, vinyl ester, and methacrylate resins.[45,46] The composite strength of phenolics is mostly retained up to 200°C and the heat distortion temperature of glass/

Marine Applications 1641

Renewable Energy Generation Laminated composites are being used in renewable energy devices such as wind turbines [47] and underwater turbines.[48] Lamination is done by bonding carbon fiber/ epoxy composite to glass fiber/epoxy envelope skins around a PVC structural foam core. A single underwater turbine can be installed in a river channel adjacent to a preexisting power plant. In the early stage of composite wind turbine blade building, the technique similar to the boat building was used.[49] Presently, the trend is the application of Automated Tape Layup (ATL) or Automated Fiber Placement (AFP) to reduce labor and improve quality irrespective of use of dry fiber or prepreg tape. Both polyester and epoxy are the resins of choice, while wood or foam cores are still being used in many cases. Skins are made from multiaxial fabrics. The best performance is obtained from unidirectional and biaxial prepreg materials. Carbon fiber is the ultimate choice for its high mechanical properties. Wind turbines blades of modern windmill use Kevlar-based composites to minimize rotational weight and keep the transfer of energy as efficient as possible. Lightweight Structural Composite for Load-Bearing Application The light weight structural composite is used as replacement to wood, which is employed as dock block in ship docking in dry dock. The dock block is required to bear

Fig. 12  Photographs of composite dock block: Single block (a); Dockblock cross-section (b), for use as wood substitute [51]

the load of the marine vessel in stationary condition in dry dock.[50] Concrete or particle board panels, panels for wall, door and windows, blast-proof panels, rail sleepers and shipping pallets are other uses of the composite. The composite comprises an inner core of lightweight panel ­encapsulated by outer layers of polymer material [51] (Fig.  12). The lightweight panel is made by using multiple laminations of corrugated thin metal or fiber-­reinforced plastic sheets held in position by polymeric foam made in-situ and/or by using adhesive, and enclosed in a flat c­ overing of the same material. The whole light weight structure is covered from all sides by using a blend of thermosetting and thermoplastic polymers. The blend is a made from a thermosetting polymer such as phenol-­formaldehyde, melamine-­formaldehyde resin and elastomers such as polychloroprene, nitrile rubber, poly (ethylene-vinyl acetate), polyurethane elastomer, ­styrene-butadiene rubber or vinyl resin, and a thermoplastic polymer such as polyethylene, polypropylene, poly (vinyl acetate) or poly (vinyl chloride). At 5 MPa, a typical blend composition has ­hardness, ­density and compressive strain at 45–49 (Shore D), 0.85 g mL−1 and 0.02, respectively.

FIBERS Fibers are used in functional applications and also in decorative applications in marine industry. The flame retardancy and weight savings are very important requirements, especially in racing craft. The carpets used in ship also need to be appropriately selected. Sails were made of natural fibers in earlier days, which are replaced by nylon and polyester. These fibers are lighter, have greater strength, more rot resistant, absorb minimum water. Some lighter laminated type sails are developed, where film is bonded to the fabric and remains exposed. For racing yachts the weight being crucial, aramids are used for the reinforcing structure. However, aramids has the tendency to degrade in sunlight. Therefore, ultra-high modulus polyethylene yarns (Spectra and Dyneema) and carbon fibers are now used. The new polyethylene yarns are also being used in heavy duty ropes.[52] Marine safety apparatus, such as life rafts, buoyancy tubes, canopies, life jackets and personal flotation devices, are made by coating woven nylon with butyl rubber, natural rubber, polyurethane, and polychloroprene, r­ espectively. Aircraft survival equipment such as life jackets, life rafts

Laboratory– Membranes

phenolic composite remains around 250°C. Phenolic composites are capable of performing similarly as with other composites. Commercial deep sea submersibles are made from foam cored laminates as buoyancy materials. A prototype civilian submarine has been built in Italy for offshore work, which consists of an unpressurized, aramid-epoxy outer hull that has low weight with improved stiffness and impact toughness. This vehicle has higher operational range as compared to glass-based outer hull. PTFE fiber-reinforced composite materials are widely used as engineering components in heavy duty application, instead of the traditional method of using metal parts. An extremely low rate of moisture absorption results in negligible dimensional change, making it ideally suitable for use in corrosive environment & submerged water applications. Application areas include Marine & Shipbuilding, Hydropower, Oil & Gas, Chemical sector. Composite is made by using polyester textile and a high-performance polyester resin with evenly dispersed solid lubricants PTFE and molybdenum disulfide (MoS2) throughout the resin for application as propeller shaft and rudder bushings. This dimensionally stable material is ­considered ideal for replacing bronze in marine environments.

1642

and escape chutes are generally made by using woven nylon coated with polyurethane or a synthetic rubber, while PVC coating is avoided because of toxic gases ­generates during fire. Inflatable crafts are used as life boats and rescue craft, as freight carrying vessels and as pleasure craft and also have several military applications. Polychloroprene, hypalon, polyurethane, and PVC coatings are used over nylon fabric. Polyester’s higher yarn modulus is better than that of nylon. However, it is more difficult to bond rubber coatings to polyester, it is heavier than nylon and degradation of polyester occurs by few compounding ingredients. Aramid fibers may be used, which save significant amount of weight due to its light weight. Oil booms are used to contain accidental oil spillages in rivers and estuaries. They are made from woven nylon base fabric coated with hypalon, polychoroprene, PVC or PVC/nitrile rubber blends.[53] The booms are not required to withstand hostile conditions and are inflated to only low pressure. However, oil resistance is the prime requirement. POLYMERIC FOAMS

Laboratory– Membranes

Polymeric foams are made up of a solid and gas phase dispersed in a matrix. The foam comprises either air bubbles or air tunnels incorporated in it, which are called as closed-cell or open-cell structure. Closed-cell foams are rigid in nature and the open-cell foams are usually flexible. The gas used in the foam is named as blowing agent, which may be either chemical or physical type. Chemical blowing agents are chemicals that take part in a reaction or decompose, giving off gaseous products, which get trapped during the process. Physical blowing agents are gases that are not produced chemically in the foaming process and are blown during the foaming process. A detailed ­description and analysis is given by Sara Black.[54] Polyurethane Foam (PU) Polyurethanes belong to the class of thermoset polymer. The polymer is one of the finest material having excellent properties and stability in the harsh environment. It is made by formation of urethane linkages during reaction of polyol (have multiple –OH group) such as polyester polyol, polyether polyol and others with diisocyanates such as methylene diphenyl diisocyanate (MDI), toluene diisocyanate (TDI), hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), etc. This type of polymer can be used to make film, foam, elastomer, paint, thermoplastic elastomer, tubing adhesives and more. Insulation is mostly preferred in maritime vessels. Depending on the reactants (Polyol & diisocyanate) used for making PU foam; it can be flexible or rigid polyurethane foam. The rigid PU foam insulation, in private

Marine Applications

watercrafts to commercial fishing boats, maintain excellent thermal insulating barrier offering temperature control that improves the temperature control in fish and crab holds, draft reduction, noise reduction, increased stability, etc. The rigid PU foam insulation also provides an air-­ sealing barrier, waterproofing, durable finish, which protect the substrate from the corrosive properties of extreme condensation faced while on water. The closed-cell PU foam, having air and moisture sealing properties does not allow the condensation. Repair and filling of  damaged pontoons, vapor sealing of engine rooms are also done by using PU foam. Rigid PU foam provides boats with insulation from noise, abrasion and tear resistance, plus load-­bearing capacity with only small weight addition. On smaller boats, the noise produced by engines and other gear is a major concern, which is tackled by PU foam having sound absorbing property. The excellent adhesive properties of PU foam insulation provide high insulation with minimum space requirements. The application of foam is done by spraying a liquid, which expands quickly to form a solid insulation by filling contours and even the smallest voids in the available space. The rigid PU foam can combine the elasticity of rubber with the toughness and durability of metal. Considering these properties along with the wide range of hardness, the engineers are able to design parts with a plastic material, such as sleeve bearings, wear plates, sprockets, and rollers required for boat construction. On shore, the closed-cell, high-density, rigid PU foam is used for making clear, lightweight, durable indoor/­outdoor signs and billboards. The builders of recreational watercraft and yachts are replacing wood components with marine PU foams for laminated fiberglass composite structures, which does not absorb water, rot, warp, or delaminate. The increase in strength and stiffness of composite permits the builders to use minimum skin materials, which led to substantial weight reduction. Decreased weight allows higher speed and acceleration, increases cargo capacity, and reduces fuel consumption. Design flexibility, corrosion resistance, ­minimum environmental impact, nonmagnetic nature, radar transparency are the other attributes of the rigid PU foam. Transportation of Liquefied Gas by Ship Large, internally insulated, self-supporting spherical tanks are fitted in cargo ships for the transport of liquefied gases.[55] Liquefied natural gases (LNGs) have the evaporation temperature at −161°C, which makes it difficult to design tankers. In recent years, a number of ­different tank systems have been developed for LNG tankers. These tanks systems can be roughly divided into two main types, such as self-supporting tanks and membrane tanks. Internal insulation has been adopted, which reduced the cost and has greater efficiency. Suitable insulation material used

is polyurethane foam with high density and strength, optionally with reinforcement or a polyurethane foam with orthogonal reinforcement of glass fiber. The insulation is mostly constructed from insulation plate elements which are adhered to the internal wall of the spherical tank. Large ocean going ships and tankers achieve superior mechanical stress properties on their hulls with the sandwich plate system (SPS), where polyurethane foam is injected between the inner and outer hull plates, increasing strength and rigidity. SPS allows a vessel’s hull to absorb impact energy and keep the ship operational. Flotation Foam Nonstructural plastic foam is added as flotation material to small fiberglass boat to prevent it from sinking during damage. The foam is required to withstand the combined effects of petroleum products such as gasoline, bilge ­solvents, fresh, and salt water. The most common and suitable plastic foam used for flotation purposes is based on PU. Using ordinary woodworking tools, the PU foams are easily cut in to sheets and blocks. The foam remains unaffected by resins and are resistant to gasoline and oil. As the low-density foam absorb large quantities of water during long period of time, it is not used below the waterline. Foam is sprayed for insulation and it is covered by protective coating. The insulation forms an air and watertight barrier, which regulates temperature and prevents condensation. PU foam is combustible and it needs modification to make it noncombustible. Fire protection of rigid PU foams is accomplished by using flexible, thermally expandable intumescent mat comprising IM (non-woven glass fiber mixed with exfoliated graphite).[56] The PU foam protected with 2-mm thick IM and filled with 15% of expandable graphite exhibits lowest peak of heat release. ­Amino-9, 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide derivatives (phosphonamidate) are used as flame retardant in PU foam.[57] Urethane-modified polyisocyanurate foams, prepared from chlorine containing polyols, possess reasonable mechanical strength and very high limiting oxygen index.[58] Both isocyanurate and chlorine act in increasing the flame resistance. Flexible PU foams are used for making seat cushions, carpet pads, bedding materials supportive and comfortable and the easy molding process allows the foam to fit into those small, curved, and hard-to-furnish places found on boats. Polystyrene foams are used extensively in sail and surf board manufacture due to its low density, low cost and easy to sand characteristics. However, their high-performance application is limited due to low mechanical properties. Further, in composite structure making, the polyester resin used dissolves the polystyrene foam as styrene is present in the resin. Similarly, as flotation material, its resistance

to gasoline is poor and is extremely flammable. There are special formulations that are solvent-resistant and ­self-extinguishing. An improved type of foam, called styrene acrylonitrile, has negligible water absorption and reasonable solvent resistance. However, the styrene base still creates incompatibility problem with polyester resins. Cross-linked PVC foam: Cross-linked PVC foam provides higher strength and stiffness to weight ratio under both static and dynamic situations, good temperature stability up to 82°C, good impact strength, high fatigue resistance, good fire resistance, good sound and thermal insulation and a closed-cell structure for low resin/water absorption. PVC foam is an IPN of PVC and polyurea. In some cases, the non-compatible nature of the crosslinked PVC with some epoxy prepregs special treatment is required. Cross-linked PVCs find application in decks, superstructures, hull bottoms and sides, bulkheads, and transoms. Linear PVC foam cores are produced for the marine industry. The non-connected molecular structure imparts unique mechanical properties such as significant displacements before failure. In comparison to the cross-linked PVCs, static properties are inferior and impact is better. SAN foams: These foams are based on the thermoplastic styrene acrylonitrile (SAN). High toughness, good impact resistance/energy absorption, good fatigue resistance, and a closed-cell structure are properties of these foams. Comparatively lower strength/stiffness, high temperature problems, susceptibility to styrene attack from unsaturated polyester resin are some of the deficiencies of this foam. The hull bottoms and sides are high impact areas where these foams are successfully used. Polyisocyanurate (PIR) foams: The PIR foams possess good compression strength and moderate physical properties at higher densities. However, having tendency to be friable, or deterioration with time, these foams are applied in non-stressed acoustical and thermal insulation panels. The foams having higher density and less friable are used extensively in transoms and the lower density materials are used as formers or in stringers. The ferry’s hull and superstructure are constructed by using fiberglass-faced sandwich construction cored with PU/PVC foam, which is a high-strength closed-cell p­ roduct having good fatigue resistance and low water uptake.[59] Syntactic foam material is created using pre-formed hollow spheres of glass, ceramic, polymer, or even metal bound together with a polymer. The initial applications of syntactic foams in marine structures were made by considering naturally buoyant behavior along with low moisture absorption and high hydrostatic compressive strength, which provided significant advantage over conventional foams.[60] In insulation for oil pipelines, along with other applications in the oil and gas industry, the foam is used due to its excellent thermal insulation properties. Polyurethane-­ based syntactic foam is employed for the purpose. It is

Laboratory– Membranes

Marine Applications 1643

1644

Laboratory– Membranes

used for buoyancy and thick-walled composites are used for pressure housings in a submarine. Syntactic materials are resistant to the effect of hydrostatic pressure together with long-term exposure stability, which make them ideal for ocean-related projects such as cable and hardball floats and instrumentation support. Because of closed-cell structure, the core is processed easily in all-composite applications including vacuum bag, RTM, and pultrusion without any increase in overall density. Remotely operated vehicles (ROVs) and human-operated vehicles (HOVs) used in deep sea exploration have been constructed using syntactic foams. Application of syntactic foam is made as buoyancy aids for deep sea drilling. Some submersibles have been able to dove to great depths. As an example, Deepsea Challenger was built in 2012 for reaching the bottom of sea (11 km).[61] The foam comprises tiny hollow glass spheres dispersed in epoxy resin, which is able to withstand huge compressive load on the vehicle while occupying about 70% of the submarine’s volume.[62] Cellulose acetate foams (cellular cellulose acetate; CCA) are very light and resist water, fungi, and decay. Extruded foams are produced by generating small gas bubbles in the polymer melt, which is done by using either chemical or physical blowing agents. Honeycomb Various types of manufactured honeycomb cores include aluminum, phenolic resin impregnated fiberglass, polypropylene, and aramid fiber phenolic-treated paper, etc. Thermoplastic polypropylene (PP) honeycomb is tough and damage-tolerant, which is the reason for its better absorption and dissipation of impact. It can offer excellent sound and vibration dampening. As its natural harmonic of 125 Hz matches with the low-frequency vibrations of marine diesel engines, a PP-cored boat hull can damp diesel noise efficiently. Applications of PP foam include boat hulls to decks and bulkheads. Polymethacrylimide foam (PMI Foam) is produced by thermal expansion of a copolymer sheet of methacrylic acid and methacrylonitrile. During the foaming process the copolymer sheet is converted to PMI foam. It exhibits the highest mechanical properties among the foam cores ­available. Excellent high-temperature performance, low-smoke density, and halogen-free chemistry advocate its application in high-speed marine vessels, where fire ­performance is an important issue. Polyethylene terephthalate (PET) foam has many ­possibilities as a core material. PET foam board has ­remarkable ratio of density to stability. It is lightweight, flexible, and stable and has excellent compressive strength and temperature resistance. It is chemically stable, ­UV-resistant and possesses extraordinary resistance to fatigue, and exhibits negligible water absorption. Thermal stability during high temperature processing and post-­curing minimizes after expansion or out-gassing problems. The marine ­application areas with various foam grades include wind energy, small decks, cabin

Marine Applications

sides, bulkheads,  internal components, stringer systems, high-density inserts, and transoms.

SEALANT Sealants are materials which form a watertight and airtight seal between two or more surfaces. Some sealants provide an adhesive bond, while others are used to isolate one surface from another against electrolysis, vibration, or noise, etc. Sealants cure to a tough, flexible, rubbery consistency, and are permanently flexible. Some movement of the bonded surfaces is allowed by sealant without ­sacrificing adhesion. A good marine sealant for deck fittings must be waterproof, and simultaneously it must be flexible, UV resistant, and, ideally, chemical resistant (fuel, bleach, and other solvents). It should not be very strong that the deck hardware cannot be removed if necessary. It must not be very ­tenacious that it leaves a residue, which prevents further application of sealants from adhering. It should not allow dirt to be picked up during use. Selection of sealant depends on the following factors: • • •

• •

Strength—how strong is the mechanical bond strength (some manufacturers give this information)? Flexibility—can the sealant move without breaking apart and can it absorb impact? Compatibility—will it work with the material planned to be bond with? Will it be safe for below the waterline application? Serviceability—how easily it can be removed and replaced, how long does it take to set? Service life—how long does the manufacturer claim their product lasts and is it UV resistant?

Types of Sealants Polyurethane: It provides exceptionally strong adhesion along with maintaining flexibility after it cures. It can be used above or inside water. Polyurethane sealant has excellent stability in water and ultraviolet radiation and it offers a permanent elasticity in all weather conditions. It is ideally used for underwater thru-hull fittings, hull-todeck joints, portholes, and bonding wood to fiberglass. It provides superior adhesion to most substrates, including ceramic, wood, metal, concrete, and most plastics. The flexible seal with excellent cut and tear resistance can handle foot and vehicle traffic. The application may be from either two-component formulation that cure by mutual reaction, or single component that cure by reacting with moisture. Single-pack polyurethane is used to prevent water seepage to unwanted areas in a boat. Polyurethane and epoxy resins are used for sealing boat hulls from water, weather, corrosion, etc. These sealants are effective

Marine Applications 1645

Polysulfide Polysulfide-based sealants are considered as the best general purpose sealants for marine use. The adhesive properties are lower as compared to polyurethane-based sealants. However, they are more elastic in nature. Application areas are both above and below the waterline. Regarding frequent relaying with substrate, it is better performer as compared to PU sealants. It adheres well with teak, which makes it ideal for application with wooden substrates. The main drawback of this sealant is its reactivity with p­ olymers. As an example, it reacts with acrylic, polycarbonate, etc. Thus, its use with plastic is mostly avoided.

self- healing effect, as the sticky tape automatically fills the gap. Compatibility of butyl tape with plastic together with its ­gasket-like application makes it the most reliable sealant for framed portlights. Devoid of cure, there is no time restriction for fitting of hardware on butyl tape. Other important properties are its low cost, stable to UV exposure and long shelf life. However, the material has several drawbacks, such as it is difficult to use it in cold weather, it exhibits creep under compression in tropical heat, the sticky edges capture dirt, it dissolves in petroleum product due to its hydrocarbon nature and poor adhesive strength that rules it out for under water application. Neoprene and reinforcing filler loaded neoprene are also used as sealant. Neoprene is very stable in marine environment.

Siloxane

CAULKING COMPOUND

Siloxane is known as elastic material having high resistant to chemicals and stable to heat. Being nonpolar, its adhesive strength is inferior to that of polysulfide or polyurethane. Minimum thickness of siloxane sealant is used and it functions under compression. It is used in places above water. As it does not contain any toxic component, it can be used in potable water storage systems. In some siloxane curing reaction acetic acid is released, which prevents it from using on metal. Another disadvantage is that during removal of used material it leaves residue. This does not allow fresh sealant to adhere.

Caulking compounds are used to fill gaps or cracks between structures. The main difference between a caulk and a sealant is elasticity. Caulks are mostly rigid after drying whereas, the sealants are elastic in nature. Caulks are used where painting or other coatings are planned to be done afterwards.

Polyether Polyether is one of the sealants suitable for both indoor and outdoor applications, unaffected by oils or cleaners and remains flexible. The polyether sealants have properties much better than urethanes and silicones. It is nontoxic in nature as no aromatic hydrocarbons or isocyanates are incorporated in the composition. Silyl-Terminated Polyether Silyl-terminated polyethers (STPE) are made up of polypropylene oxide polyethers having terminal dimethoxysilyl groups. The moisture in presence of catalysts cures the polymer. STPE sealants have good adhesion characteristics with a wide variety of substrates, which is better than PU and silicones. Butyl Tape Butyl rubber is a copolymer of isoprene and isobutylene with the former being used only 2%. It is being used satisfactorily for years having several advantages. Its adhesive strength is very low and it does not harden. Its ability to remain sticky and pliant throughout use has

Acrylic caulk is very popular as it is applied easily and can be cleaned up easily. This caulk functions very well by filling the gaps and make adherent surface. However, the drawback of acrylic caulking is that it has the ­tendency to shrink and distort over time. Siloxane caulk remains flexible during its lifetime without peeling, cracking, or distorting. Waterproof barrier against moisture is maintained much longer than others. Thus, the siloxane caulking is predominantly used around showers, bathtubs, sinks, and toilets. Additionally, the long service life, its ability to retard moisture while maintaining strength make it an excellent choice for outdoor use. However, the drawback of this caulking compound is that no other overcoat/ paint will adhere to it. Epoxy caulk is well known for its excellent adhesive and mechanical properties. This caulking compound is able to fill gaps efficiently for long and also allow the coating/painting over it. Deck Underlays Deck underlays are used to cover the deck irregularities to a smooth surface. The underlay also prevents the metal deck from undergoing corrosion. Deck covering underlay material mostly consists of a mixture of aggregates and binder that holds the material together and bonds it to the deck. The binder is mostly epoxy and the application is done by using trowel. Synthetic rubber, such as liquid polysulfide rubber filled with microballoons is used as underlay

Laboratory– Membranes

in sloops, catamarans, and speedboats, canoes, kayaks, skiffs and rowboats, respectively.

1646

on ship deck. Application is done by pouring and trawling the rubber mix. Noise Reduction Noise and vibration are the factors that are related to the comfort at sea. As there are noises in fitness areas, restaurants, bars, cinemas, and discos, the passengers have only places to relax are their cabins. Further problems of ­propeller- and machinery noise need sound protection and effective vibration control. The application of low noise resilient mounted engines and propellers making low noise  reduces the structure-born noise and vibration in cruise ships. However, the reduction of noise generation at its source cannot improve the system sufficiently. The ship floor insulation is the effective method for mitigation of noise. Marine-grade epoxy adhesive formulation is used under vinyl, tile, rubber or other materials. Battery Space Deck Coverings The submarine battery compartment mostly remains flooded with dilute acid due to acid spill from the batteries. Therefore, a proper coating is required to protect the ship hull from severe corrosion. Solvent free epoxy with acid stable filler is applied as coating. However, the service life is not adequate. The paint gets damaged within couple of months. More effective rubber (Neoprene) sheathing can also be used to cover entire deck of a submarine battery compartment, including the contours exposed to acid spill. Adhesive is used for fixing the rubber sheet and gaps are blocked with putty. Mastics Laboratory– Membranes

Mastic is a gum derived from plant. It can be used to cover the deck for leveling as well as damping of vibration. However, synthetic counterparts are also called mastics. The most popular is the epoxy resin filled with additives, such as filler used in paint, nanoclay, etc. The epoxy being rigid in nature, the vibration damping performance is poor. Therefore, PEG is reacted to get modified epoxy, which is blended with conventional epoxy to get toughened epoxy.[63] Jeffamin curing agent is used as hardener to get damping quality epoxy mastic. Further, various acrylic resins are also blended to shift the Tg for damping appropriately at the vibration frequency. Adhesives Adhesives are the materials which can bind two or more parts with varying strength and service life. Epoxy adhesives are most common in most of the areas of application. Because of their ability to adhere to most of the materials, their high strength, resistance to chemicals and environments, and their ability to resist

Marine Applications

creep under sustained load epoxies are the mostly used structural adhesive. The properties can be varied by chemically modifying the epoxy or selecting various hardeners. The blending with rubber, ­nanoparticles, etc., can be used to modify the properties. Polyurethane adhesives provide exceptionally strong adhesion and remains throughout the service life. It is used above or below the waterline. The adhesive may be single pack, which cures with moisture or two pack, which cures by reaction of polyol with diisocyanate. It cures fully in few days with no/minimum shrinkage. Methyl Methacrylate adhesives are commonly used as ­structural adhesives due to their excellent strength, impact resistance, thermal shock and fatigue resistance and peel strength. They are based on thermoset adhesives formulations that give excellent structural ­properties to bond a wide—variety of durable substrates. Neoprene adhesive is one of the most versatile solvent-­ based adhesives for rubber and gaskets. It can bond neoprene, reclaimed natural rubber, SBR and butyl rubbers to metal, wood and most plastics. The adhesive possess excellent heat and water resistance, remaining stable up to 148°C. Putty It is a leveling compound for surfaces and glue for metal to metal and metal to GRP and metal to rubber and cured under ambient conditions. Epoxy system is the most preferred resin for making putty formulation. Fillers are added to the resin for making highly viscous putty. After curing, the putty produces a hard, tough, permanent and waterproof bond on a wide range of materials. As an example, the application of putty is made on rubber sheathing on the outside hull of stealth submarine. High-performance putty is made by using toluene diisocyanate capped hydroxyl terminated polybutadiene as one component, which is cured with other component comprising filler, polyol, and diamine. MARINE FENDERS Marine fenders are the marine equipment, which comes in many shapes and are used to prevent boats, ships and other naval vessels from colliding against each other or against docks, wharves and piers.[64] An efficient fender system prevents the ship from external damages arising out of impact between metal hull plates and concrete/metal berth or jetty. In simple term, the marine fenders can be called as a marine bumper. Two types of marine fenders used can be named as follows: •

Ship-to-berth (STB) fendering For port applications, two main types of STB fenders, such as fixed and floating are used.[65] Fixed fenders

Marine Applications 1647

Fender Shapes and Materials •

•

•

•

• • •

•

Foam elastomer fenders are made by encapsulating closed-cell foam core in high strength fiber-reinforced polyurethane skin. Foam elastomer fenders do not deflate. D fenders are commonly used on vessels as well as small jetties. They are compression-molded with steel inserts, as required. Mostly natural rubber or SBR is the construction material. Square fenders are commonly used on vessels as well as small jetties. They are compression-molded fenders generally used on tugs, boats, and ships. Wing fenders are compression molded and are generally used on tugs, boats, and ships. Excellent sea water resistance, ozone resistance, and UV resistance are required for this type of application. Keyhole fenders are the most versatile bow, stem ­fenders used on tugboats and small port craft/ferries. Tugboat fenders are made of high-abrasion-resistance rubber with good resilience properties. Solid rubber fender offers high energy absorption and reaction force, and has low cost, long service life, etc. Floating rubber fender exhibits large compressive deformation energy absorption, low reaction force, easy installation.

The rubber fenders are generally made by using natural rubber, SBR, polybutadiene, nitrile, neoprene, and other general type rubbers. For special requirements, chlorosulfonated polyethylene, EPDM, polyurethane, and other special rubbers are used. EPDM rubber fender has excellent resistance to sunlight, UV, seawater, ozone, impact and has very high compression resilience (90%–95%). For excellent sea water resistance, ozone resistance and UV resistance, HNBR, fluorocarbon, and fluorosilicone rubbers, respectively, are employed. Composite fenders are made from composites of rubber which provides resilience and UHMWPE possessing low friction and wear resistant properties. Foam elastomer fenders are made by encapsulating closed-cell polyethylene foam core in nylon or Kevlar-reinforced PU

skin. PU top layer affords excellent properties against hostile environment. Extruded flexible PVC fenders are also used for marine use. In certain type of fenders, foam is covered with PVC-coated marine-grade fabric. Having chlorine on alternate carbon atoms, PVC has excellent flame retardant property and is highly resistant to oxidation, acid, alkali and almost all inorganic chemicals, and also shows little change in mechanical properties over long use. Having polar groups, the polymer is compatible with many polymers. Although PVC is a viscoelastic material, its creep deformation is very low as compared to other plastics, which originates from limited molecular motion under ambient condition. Extrusion and ­compression molding are the prevalent practice for ­making fenders. In an effort to develop fully polymer-based fender, a series of blends are developed using phenol-­formaldehyde as one component. [67] The other component belongs to one from group comprising polychloroprene, nitrile rubber, poly (ethylene-vinyl acetate), polyurethane elastomer, styrene-butadiene rubber, poly (vinyl chloride), vinyl resin, etc. A typical design of the fender is shown in Fig. 13. Fenders for Buoy A buoy is a floating device used for many purposes, such  as mooring, navigation aid providing a platform for light and radio beacons, sonobuoy, weather buoys, etc. For making lighter, more corrosion resistant, less maintenance, and easier to handle than conventional steel buoys, foam materials are used for construction. The ­abrasion-­resistant urethane skin resists the harshest environments. Impact absorbing closed-cell crosslinked polyethylene foam core is heat laminated into one piece, solid foam core. The impact absorbing foam

Fig. 13  Phenolic blend-based fender for both ship-to-ship and ship-to-berth [67]

Laboratory– Membranes

•

are mounted to the berth structure, which can be named as buckling fenders such as cell fenders, V-type ­fenders,  and non-buckling fenders such as cylindrical fenders.[66] Floating fenders are placed between the berth structure and ship, which incorporates pneumatic ­fenders and foam-filled fenders. Ship-to-ship (STS) Fendering For bunkering operations between two vessels, floating fenders such as pneumatic or foam elastomer fenders are typically used. Various shapes such as cylindrical, arch, cell, cone, etc., shapes are used. Pneumatic and hydro-pneumatic fenders are also used.

1648

Marine Applications

core is covered by a tough thick filament nylon tire cord-­ reinforced elastomer skin. For mooring and pick-up buoy applications, the self-fendering properties of ionomer foam buoys make them a material of choice as a replacement of steel buoys as the latter damage vessels during collisions. These types of foam buoys are lightweight, maintain brightness better than steel or plastic buoys, do not severely damage marine vessels during c­ ollisions, do not require much maintenance, etc.

Oil and Varnish

Nonskid Paint

Today, most varnishes are prepared by blending PU, phenolic, or alkyd polyester. PU is more durable than the others. However, it mostly cracks in marine environments. These varnishes provide excellent moisture resistance as the ratio between oil and resin determines the coating’s hardness and resistance to cracking and peeling. As environmental concern increases, water-based products are being ­developed and marketed.

Laboratory– Membranes

Nonskid paint for boats is an ideal anti-slip floor and deck coating for areas where the presence of water makes the surfaces slippery. Another important application is on the deck of aircraft carrier to prevent sliding during operation in sea. Along with the enhancement of skid resistance, the corrosion resistance must be kept in mind. Nonskid paints are based on coarse fillers and stable resin. Epoxy-based nonskid paints, containing aromatic epoxy resins, are chemical resistant and durable systems. However, degradation of aromatic molecules occurs in sunlight, which is revealed in the form of chalking and loss of profile within few months after the application.[68] Siloxane-based nonskid coating is developed by US Naval Research Laboratory (NRL), which maintain good color and retain profile in sunlight. Lower viscosity of the paint allows it to be applied by using spray painting ­methods.[68] Polyurethane nonskid resilient, protective coating is made by incorporating specially treated rubber granules in a totally flexible polyurethane base, which provides excellent slip resistance, while protecting the surface against corrosion. Acrylic/urethane copolymer blended with recycled crumb rubber, which is the granular foam of tire rubber, [69] as an aggregate. This coating gives anti-skid property and is durable and able to adhere to most substrates.

The oils (tung or linseed) is mixed with varnish, which offers the esthetic advantages of oil but with greater durability than oil. The oil/varnish finish can also be used as a base for a more durable topcoat such as polyurethane. This provides an excellent blend between beauty and longevity. Varnish

Shellac Shellac, a polyhydroxy acid may occur as lactides or internal esters.[70] The unique depth and color of the finish on wood can be achieved with shellac, which is highly water resistant, nontoxic, easy to repair, and have a lovely hue. Epoxy and Varnish Epoxy is superior to every other kind of finish in strength, resilience, and moisture-resistance. It completely seals the underlying wood, which needs to use very dry wood. Epoxy resin coat is applied first, which is covered with two-component PU finishing coat. This way color depth is maintained and protection against degradation of the epoxy film from UV rays is ensured. Thus, the epoxy and PU complement each other, as the varnish protects the epoxy and, in turn, the epoxy maintain stable base for the varnish that does not crack under temperature-linked expansion and contraction.

WOOD FINISHING FOR MARINE USE MICROWAVE ABSORBING MATERIALS Woodworkers and boat builders argue about all kinds of things, but unanimous on wood finishing. The overwhelming diversity of finishing products make many people working on boats surrender either to the marketing ploys of manufacturers or to the old habit of finishing everything with spar varnish. Few wood finishing materials can be described as: Oil Mineral, tung, linseed, walnut, soya, and lemon can be used for finishing wood. The ease of application and the color make it fascinating. Tung oil is the most resilient of the oil finishes and it provides some protection.

Microwave (MW) absorbers have been traditionally used for EMI reduction, antenna pattern shaping, and radar cross-section reduction. More recently with the rise of wireless electronics and the movement to higher frequencies microwave absorbers or “noise suppression sheets” (NSS) are used to reduce electromagnetic interference (EMI) inside of the wireless electronic assemblies. More and more attention has been paid to such materials due to the important applications in microwave technology and radar detection. Electric permittivity (ε) and magnetic permeability (μ) are related to a material’s dielectric and magnetic properties, which are among the most important characteristics of

absorbing materials and are directly associated with their absorbing properties.[71,72] The relative permittivity and permeability are shown as follows: εr = ε ′ − iε ′′

(1)

µr = µ ′ − iµ ′′

(2)

When the material is lossy, part of the incident electromagnetic energy is dissipated. Equations 1 and 2 [71] show the real (ε′, μ′) and imaginary components (ε″, μ″). For magnetic material, losses are produced by changes in the alignment and rotation of the magnetization spin.[73] Radar detection of naval ship superstructure and submarine mast during its stay above water are suppressed by using MW absorbing coating. Various types of NSS have been developed for these types of applications. Magnetic Absorbers These are thin (0.1–3 mm) polymeric films filled with magnetic materials. These materials possess high permeability (magnetic loss properties) and high permittivity (dielectric loss properties), which make these materials very effective in eliminating EMI. For example, barium ferrite, and iron powder combination can reduce the reflection by absorbing substantial amount of radiation. The resin is mostly solid epoxy resin having various loadings of absorbing materials. Foam Absorbers The foam absorbers comprise open celled foam impregnated with a carbon coating. The carbon coating makes the foam lossy at microwave frequencies, which is akin to a free space resistor to incoming electromagnetic energy. These foams thickness range from 3.2 to 6.4 mm for internal cavity applications and can go up to several centimeters of thickness for outdoor applications. Rubber Absorbers Elastomeric absorbers combine the properties of flexibility, MW absorption, protection from UV, and saline environment. The absorbing material is mostly ferrite/iron combination. On board ship there are antennae which pick up the MW frequencies. However, the metallic structure reflects substantial amount of microwave, which damages the sensor. The rubber absorbers, particularly the nitrile, neoprene, bromobutyl rubber, etc., are found to be stable in marine environment. Composite Absorbers Composites can be made using epoxy resin and glass and carbon cloth. The MW frequency band determines the

thickness of the composite which will substantially absorb the radiation. This can be done by selecting the number of cloth layers. The number of carbon cloth can be varied to get the maximum attenuation of MW. By this way, the requirement of carbon cloth will be kept at minimum. Other potential materials for MW attenuation are conducting polymers, such as polyaniline, polyaniline nanocomposites, carbon nanotube (CNT), graphene, carbon black, iron carbonyl, etc. These materials can be f­ ormulated to design various absorbers as mentioned above. A single-layer wave-absorbing coating comprising carbonyl-iron powder (CIP) and carbon black (CB) with epoxy resin as matrix show that at higher thickness, CIP or CB content can make the absorption band shift toward the lower frequency range in the band of 2–18 GHz.[74] By controlling the content of CIP or CB in composites, the absorption can be manipulated for certain frequency range. Chiral molecules having single handed screw sense interact with microwave by showing preferential absorption of one part of circularly polarized light. The other half remain unaffected by the chiral molecules. While coming out of the chiral medium, both the components of circularly polarized light combine to a wave having different characteristics. Thus, the detection by RADAR becomes inconclusive. One- and two-layered chiral microwave absorbing coatings show that using an appropriate complex chiral admittance, the reflection can be minimized and the minimum-reflection-frequency can be controlled.[75] Polyaniline is used as a composite with Fe3O4 along with copper helices as chiral component.[76] The chiral component inclusions to the non-chiral matrix improve both the dielectric and the magnetic losses and obviously, the MW absorption becomes higher. MW absorbing paint formulations consisting of CIP (90% w/w) CIP and polyaniline (15% w/w) dispersed in PU matrix is applied to flat aluminum plates to thickness of 1.10 mm for the first and 1.85 mm for the second.[77] The doped polyaniline powder (17% w/w) was added to two types of silicone rubber, L9000 and RTV630 (GE Silicones) and molded to thickness of 2.8–4.4 mm, respectively. Both paint and rubber matrices attenuate the radiation in the range of 60%–90%. Polyaniline surrounded by a matrix forms conduction paths facilitating the dissipation of energy due to electrical losses. The carbonyl iron in the paints also contributed to the attenuation of microwave energy due to magnetic anisotropy effects, a characteristic of this material for frequencies above 2 GHz. An excellent microwave absorption performance is observed in iron coated graphene dispersed in paraffin wax in the wavelength range of 2–18 GHz, showing reflection loss (RL) of −30 dB.[78] The charge transfer at Fe/graphene interface and the polarization of free carriers in graphene are the reason for such large attenuation. Commercial laminated composite (2.2 mm) consisting of 10 alternating layers of continuous glass fiber and PPS, is coated with nanostructured coating containing

Laboratory– Membranes

Marine Applications 1649

1650

ANTIFOULING COATINGS Marine biological fouling, commonly called as marine biofouling or in short ‘fouling’, is defined as the settlement of marine organisms, plants, and animals on underwater surfaces. First, the species and their attachment time vary significantly depending on the location having specific salinity, pH, temperature, nutrient levels, and solar ­irradiation. Even in the same place seasonal change also affect the fouling.[81,82] The occurrence of biofouling on ships or any marine vessel causes several adverse effects:

Laboratory– Membranes

1. High frictional resistance, due to formation of hard and rough honeycomb structure by the marine organisms, particularly by barnacles. This leads to an increase of weight of the structure, which causes speed reduction and higher fuel consumption. 2. Due to excess fuel consumption increased emissions observed, which increases the pollution.[83] Up to about 40% increased fuel consumption is observed.[84] 3. After the surface is heavily fouled, the dry-docking is required to remove the hard shell of barnacles, prepare the surface, and undertake repainting. This process is costly and time consuming. The vessel is not available for operational purposes for long time. The whole process is frequently done. Naval vessels ­suffer the most due to operational non-availability. 4. The whole process repetitions generate large amount of toxic wastes.[85] In the late 18th century, the development of iron ships really boosted the search for new AF coatings.[86] Various nonmetallic coverings such as felt, canvas, rubber, ebonite, cork, paper, glass, enamel, glaze, and tiles have been attempted for minimizing the fouling. Attempt was also made by placing a nonmetallic layer in between iron surface of ship and copper sheathing, which was aimed to prevent corrosion due to formation of galvanic couple between two metals.[87] Tar soaked cork, rubber, plain brown paper, and felt were employed for the isolation.

Other methods such as wooden sheathing was tried as isolation layer between iron and copper. However, it was not cost efficient. With further attempts, the concept of antifouling (AF) coatings started making the round. During the mid-19th century, toxic compounds entered as filler in antifouling paint. These toxic compounds were selected on the basis of their killing or repelling action toward marine biological organisms. Physically Bound Toxin in A/F Paint Copper oxide, arsenic, and mercury oxide were the three main ingredients added in antifouling paint, which was based on binders that included turpentine oil, naphtha, shellac, tar, and other kinds of resins.[88] The development on the concept resulted in cuprous oxide (Cu2O)-based antifouling paints. As it was applied at ambient temperatures it was called as cold plastic paints. The fouling was successfully reduced during 18 months of exposure, thus widening the gap between two consecutive dry-docking. Furthermore, copper was found effective against most of the fouling organisms. However, the physical mixture suffered from inherent problem of uncontrolled dissolution and poor abrasion resistance (Fig. 14). Two types of paints are developed alongside to solve these problems: a mixture of rosin and a copper compound, and the “hot-plastic paint” composed of copper sulfate with a metallic soap composition.[88] Although these systems were widely adopted as successful antifouling paint in the late 19th century, they were too expensive and had short life span. In 1954 Prof. G. J. M. van der Kerk along with his research team, at Utrecht University, discovered that certain trialkyl tin compounds reduced the growth of typical marine fouling species.[89] Field evaluations showed that these triorganotin-based materials were 10–20 times more effective then cuprous oxide (Cu2O).[90]

Leaching rate (arbitrary unit)

multi-walled CNTs and industrial grade PU [5.0% (w/w)] to an average thickness of 0.15 mm.[79] In the wavelength band of 8–12 GHz approximately 90% energy of the ­incident electromagnetic wave is absorbed. The multi-walled carbon nanotubes (MWCNTs)/­chiralpolyaniline composite, synthesized by in-situ chemical polymerization, exhibits much higher dielectric loss as compared to the magnetic loss in the frequency range of 2–18 GHz.[80] The microwave absorption enhancement of the composites originates from the dielectric loss, rather than magnetic loss within the frequency range of 2.0–18.0 GHz. The peak RL varies in the range of −5 to −15 dB.

Marine Applications

Over protected

Critical leaching rate Under protected Time (arbitrary unit)

Fig. 14  Representative leaching rate variation of a toxin from a biocide dispersed conventional paint formulation

Marine Applications 1651

Controlled leaching from chemically bound toxin Critical leaching rate

Time (arbitrary unit)

Fig. 16  Representative plot for controlled leaching from ­chemically bound toxin vs. time

•

The paint system is able to release the biocide at ­controlled and constant rate by first breaking the bond and then leaching out to sea water. The rate of hydrolysis is maintained constant at minimum rate throughout the service life.

2.0

1.0

R CH

C

R (Bu3Sn)2O

COOH

CH2 C O

Initiator Δ

COSnBu3

R CH2 C n O

COSnBu3

R = H, CH3

Fig. 15  Reaction scheme for synthesis of tri-n-butyltin acrylate/ methacrylate [94]

0.4 0 0

20 40 TBTM content (Mole percent)

60

Fig. 17  Variation of steady state leaching rate with tri-n-­ butyltin methacrylate content in copolymers in seawater at 30°C Source: © 1987, John Wiley and Sons.[92]

Laboratory– Membranes

The variation in steady state leaching rate of TBTO in seawater at 30°C from pure copolymer films as a function of copolymer composition is presented in Fig. 17. The variation in leaching rate with TBTM content in copolymer follows a square relationship, which is due to acceleration of the hydrolysis rate by the acid liberated. To ensure leaching rate above 0.4 µg cm−2 day−1 the TBTM content is required to be about 30 mol% in the copolymer composition. The variation of leaching rate from unpigmented copolymer film and a typical paint formulation having the copolymer as binder are shown in Fig. 18. After 60 days, the steady states were reached in both films and the leaching rates are also comparable. Thus the advantage of self-polishing paint copolymer system can be summarized as follows:[90]

Steady state leaching rate (µg/cm2/day)

TBTO has excellent antifungal and anti-biocidal applications. It is one of the first organotins used in paints for protection over short duration due to its high leaching rate.[91] Antifouling marine buoys are made by incorporating TBTO in chloroprene rubber. TBTO and tributyl tin fluoride inhibits the growth of marine fouling organisms including the most menacing barnacles. To enhance the service life the paint must leach more than the critical leaching rate, which indicates the minimum leaching to prevent fouling. The critical leaching rate of organotin compounds is 1 µg cm−2 day−1, whereas that for cuprous oxide is 10 µg cm−2 day−1, which indicates one order lower leaching from antifouling paint containing the former.[88,89] Lower critical leaching rate of 0.4 µg cm−2 day−1 was confirmed by Deb et al.[92] An antifouling coating must be effective if the leaching is controlled, so that the service life of the coating is reasonably high and is useful. Although organotin containing paint gives better performance than cuprous oxide, the physical leaching is quite high at the initial stage, which diminishes quite fast toward later stages. Thus, the loading of free organotin in paint is not much useful.[93] Attempts were made to chemically link the organotin moiety with a polymeric system so that the leaching is controlled as the chemical bond breaking (hydrolysis) is involved prior to leaching. Thus the service life of paint is likely to be controlled. TBTO can be reacted with acrylic or methacrylic acid to make ester (tri-n-­ butyltin acrylate (TBTA). The synthesis scheme is given in Fig. 15.[94] Tri-n-butyltin methacrylate (TBTM)) is polymerized to get the polymer (poly (tri-n-butyltin acrylate/methacrylate).[95–97] The paint containing chemically linked organotin leaches in water in controlled manner. Once most of the organotin moiety is hydrolyzed and leaches out to surrounding water medium, the top micro layer of paint is left with polymer with carboxyl groups. This layer is highly hydrophilic and the moving sea water erodes the layer to expose the next smooth layer, which starts leaching. Thus controlled leaching coupled with ablative action makes the system self-polishing in nature (Fig. 16).[98] Furthermore, various copolymers have been designed to suit the requirement by having specific mole percentage of TBTA/TBTM.[99,100]

Leaching rate (arbitrary unit)

Tri-n-Butyltin Oxide (TBTO) Based A/F Paint

1652

Marine Applications

1.50 (a)

Steady state leaching rate (µg/cm2/day)

1.25 1.00

(b)

0.75 0.50 0.25 0

0

30

60

90 120 Time (days)

150

180

Fig. 18  Variation of leaching rate with time for copolymer (30 mol% TBTM) film containing 100 pph ZnO and 10 pph DOP (a) and a typical paint formulation (b) Source: © 1987, John Wiley and Sons.[92]

•

•

•

At required mol% of TBTA/TBTM, the life of the coating is proportional to the thickness. This makes it possible to design extended service life of antifouling paint up to five years. The erosion of every microlayer of paint leaves a smooth surface. Therefore, it is called self-polishing paint, which minimizes drag and fuel consumption. Ship/submarine hulls can be repainted directly without the requirement of removal of remaining old paint.

Organotin Polysiloxanes Laboratory– Membranes

Further development in tin-based antifouling technology led to organotin polysiloxanes, which is moisture cured after application on surface. This type of antifouling paint is not ablative in nature and sometimes blended with other resins. The formulations provide highly controlled hydrolysis of organotin ester moiety and release of toxin for long service life.[101] The rate of toxin release from the paint film is in the range of 0.5–1.5 µg cm−2 day−1 and s­ ervice life of three years is ensured without any fouling. Tin-Free Antifouling Thus, tributyltin self-polishing copolymer paints have been considered as the most successful in combating biofouling on ships. Unfortunately, the organotin-based antifouling paint systems is found to adversely affect the environment. TBT is highly toxic for many aquatic organisms, well beyond 4,000 species responsible for biofouling, and its prolonged utilization causes severe damage to aquatic life.[102] The marine environment Protection Committee (MEPC) of the International Maritime Organization (IMO)

has approved a resolution to phase out and finally ban the use of organotin compounds in antifouling paint.[103] In 1999, IMO adopted a resolution that says “After January 1, 2003, the application of organotin-based paint on ship and after January 1, 2008, presence of organotin in antifouling paint on ship is completely banned.” TBT is replaced with surrogate organometal and organic biocides. Zinc pyrithione (zinc complex of 2-mercaptopyridine-­1-oxide, ZnPT) and copper pyrithione (CuPT) mixed with Cu2Obased biocidal coatings are designed to act as main booster ­biocides.[104] Once the most effective antifouling agent organotin is banned the other compounds under consideration belonged to mercury compounds, cuprous oxide, organoarsenics, organoleads, organohalogens, organosulfurs, and zinc compounds. Lead and mercury compounds are not c­ onsidered due to long-term environmental hazards. [105,106] Tin-free antifouling paint can be divided in to three types, such as free association, self- degradation, and self-polishing. [107] In the first type copper and booster biocides are dispersed in resin matrix. Copper compound is not effective against diatoms and algae, which made it mandatory to use secondary biocides. [108] These types of paints protect the surface for short duration. The second type comprise self-degrading type, biologically active components, which are released by hydrolytic degradation of low molecular weight polymers. [109] The third type is self-polishing in nature. Methacrylate or acrylate copolymers are linked to copper, zinc, silyl moiety, and oligomer groups instead of TBT moiety. The details of the molecular structures of copolymers are presented in Fig. 19. Long-chain-modified amides with two carboxyl end groups are used for making soluble organo-copper polymer, which has good film forming property.[110] Zinc methacrylate copolymerized with various comonomers such as 2-methoxyethyl acrylate (2-MTA), ethyl acrylate (EA) and methyl methacrylate (MMA) as hydrophobic monomers studied for self-polishing behavior indicates that leaching rate depends on copolymer composition and molecular weight, respectively.[111] Similarly, the susceptibility of acrylic polymers to hydrolysis at pH 8 can be correlated with erosion behavior of polymer films.[112] Nontoxic, Non-Biocide-Release Antifouling System There are two effective strategies for prevention of fouling are as follows: i. “Detachment of biofoulants” mostly from hydrophobic surfaces (Foul release). ii. “Preventing the attachment” of biofoulants by applying a “hydrophilic coating” which retains a hydration layer.

Marine Applications 1653

H

R

C

C

H

C

H

R

m

C

C

O

H

C

O

O

Cu/Zn

R2

C

O

n O

H

R

C

C

H

C

H

R

m

C

C

O

H

C

O R3

O

O

Si R3

R1

n

R3

R2

O

Where,

R= H, CH3 R1, R2, R3 = Alkyl

Fig. 19  Chemical structures of various tin-free copolymers

Foul release coatings have low surface energy, which does not allow the marine organisms to generate a strong interfacial bond with the surface. The smoothness of the coating lets the organisms to be dislodged after the vessel picks up a velocity beyond a critical velocity, [113] i.e., typically 10–20 knots.[81] In order to create a surface that has marginal adhesion to the adhesives produced by marine ­organisms, it is required to minimize:[114] 1. Mechanical interlocking, by maintaining a smooth and non-porous surface. 2. Wetting of the surface, by choosing appropriate ­surface functional groups. 3. Chemical bonding, by creating a surface without reactive functional groups. 4. Electrostatic interactions, by designing a low polarity surface with or without ionic groups. 5. Diffusion of molecular chains from the marine adhesive into the surface, by producing a surface having closely packed functional groups.

properties alone is not deciding factor regarding the performance of foul release surfaces and that the fracture mechanics plays an important role. It is also observed by others that the adhesive strengths of viscoelastic adhesives cannot be predicted from the surface free energy of the substrate.[116] It is later discovered that fouling release does not depend on only surface energy, but on the square root of the product of surface energy and elastic modulus.[117,118] Table 1 shows the details of surface energy and elastic modulus ­combinations for various fluorinated and siloxane ­polymers.[86] It is also observed that the barnacle release forces decrease with increasing film thickness in accordance with the model developed by Kendall for release of a rigid ­cylinder from a substrate.[117] Foul Release Surface Design From the study of various types of polymer and polymer-­ barnacle adhesion strength, only two groups of materials meet the requirements for foul release, such as siloxanes and fluoropolymers.[119] Fluorinated Coatings

Baier proposed that bioadhesion remains at minimum in a critical surface tension range between 20 and 30 mN m−1.[115] However, it is shown later that the surface

Fluorinated polymers, having very low surface energy, are considered most attractive for application as foul release

Table 1  Physical properties of polymers investigated for AF surfaces [85] No. Polymers Relative adhesion Critical surface free energy (γc) (mN m−1)

Elastic modulus (E) (GPa)

1.

Poly (dimethylsiloxane)

6

23.0

0.002

2.

Poly (hexafluoropropylene)

21

16.2

0.5

3.

Poly (tetrafluoroethylene)

16

18.6

0.5

4.

Poly (vinylidenefluoride)

18

25.0

1.2

5.

Poly (ethylene)

30

33.7

2.1

6.

Poly (methyl methacrylate)

48

41.2

2.8

7.

Poly (styrene)

40

40.0

2.9

8.

Nylon 66

52

45.9

3.1

Source: Nontoxic Reproduced by permission of The Royal Society of Chemistry.

[86]

Laboratory– Membranes

Foul Release Coating

1654

Laboratory– Membranes

coating. PTFE-fluorinated epoxy and poly (urethane) formulations containing PTFE particulates are studied as AF materials.[120] However, the barnacles could strongly adhere to the surface due to irregularities on the surface along with highly polar carbon–fluorine bond. The perfluorinated poly(methacrylates) with perfluorinated chains of different lengths show low surface energy (70 vol%; VOC ≤ 250 g L−1) the requirement is the replacement of high molecular weight polymers, such as epoxy resin and curing agent, by low molecular weight alternatives that have lower viscosity.[202] Further reduction in viscosity can be effected by incorporating mono- or di-functional epoxy-terminated diluents in the formulation, which reduces the solvent requirement.[203] Also, the pigments and fillers with lower oil absorption can be used so that the rise in viscosity is minimized. The curing agent can be changed from conventional polyamide to cyclo-aliphatic amines having lower viscosities. Modern high-performance anticorrosive system usually comprises: • •

Minimum two layers of chemically curing ­two-component epoxy or coal tar epoxy PU/coal tar combinations, which cure at lower ­temperatures than epoxy/coal tar, can also be used [204]

However, the toxicity of the isocyanates makes PU products less preferred. Most anticorrosive coatings are high-solids and sometimes high build materials. Use of epoxy mastics (surface-tolerant-modified high-solids epoxy coating) is increasing. Total DFT of underwater hull and boot top (splash zone) systems ranges from 250 to 400 µm. Very popular vinyl and chlorinated rubber paints, a single component formulation, are slowly disfavored because of their high VOC levels. Due to the ban on coal tar in many European countries, glass flakes added to epoxy are getting popular. Ta’Kuntah of Alaska Tanker Company is a single hull vessel of 350,000 dead weight tonnage (DWT) with 29 cargo tanks (including slop tank) and total cargo capacity of 2.77 million barrels along with tanks for water ballast. The protective coating scheme used for the whole ship can be given as follows:[205] •

• • •

•

Ballast tanks and slop tanks with sacrificial anodes are coated with thick stripe coat followed by epoxy holding primer and coal tar epoxy system (3 nos. coats) Cargo tanks are partially coated (top and bottom: 3.0 and 1.0 m, respectively) Hull topsides are coated with zinc silicate, micaceous iron oxide epoxy, and polyurethane. (4 nos. coats) Hull wind/water area (boot top/splash zone) is coated with epoxy primer, glass flake epoxy, micaceous iron oxide epoxy, and polyurethane. (5 nos. coats) Hull under water is coated with epoxy primer, glass flake epoxy, coal tar epoxy, self- polishing copolymer anti-fouling system. (6 nos. coats)

Marine Applications 1661

•

Deck area is coated with epoxy coating system. (2 nos. coats) Piping system: external area is coated with coal tar epoxy. (2 nos. coats) and internal area is coated with glass flake epoxy. (2 nos. coats)

In addition, impressed current cathodic protection system is used with anodes fitted on both sides of hull in forward/amidships/stern areas. Periodic inspection instills confidence about the anticorrosive performance to ­continue beyond 15 years of expected life. Single component epoxy coating technology presents a high-solids, storage-stable single-pack coating consisting of an epoxy resin, a hydrocarbon compound, a functionalized silicone resin, and a ketimine curing agent.[206] Ketimine, an adduct of acetone and amine, extends the pot life of high-solids two-pack epoxy coatings while reacting with water to hydrolyze back to the original amine and ketone.[207] However, it is very difficult to produce this type of coatings as slight traces of water adsorbed on pigments or picked up from ambient air during processing are enough to cause instability.[206] Further advancement and testing of the technology is done.[208] During accelerated testing at elevated temperature of 49°C viscosity increase occurs with time of exposure. However, the stability of the single pack ensures reasonable shelf life having high level of corrosion resistance and other physical properties such as abrasion resistance, film hardness development, impact resistance, and adhesion, respectively. New systems of paints that combine anticorrosive function and other required properties are also being developed. The advantage is the reduction of the number of coats and the costs associated with its application.[209,210] Another series of coating consisting of various combinations of organic zinc epoxy, inorganic zinc primer, intermediate coat of epoxy, epoxy polyamide, and topcoats of epoxy acrylic, PU, ­fluorocarbon, respectively, are found to perform ­satisfactorily.[211] Cerium tartrate is used as inhibitor pigment in epoxy coatings on aluminum (AA 2024-T3).[212] The release of Ce ions from epoxy coating in to 0.05 mol L−1 NaCl occurs via possible transport of cerium ions through the epoxy coating, which is required for inhibitor pigment. Further, the release is pH dependent. Thus, cerium tartrate ­displayed self-healing effect at the defects in epoxy coatings. The grafting of sol–gel material onto organic polymer backbone substantially enhances the adhesion property and anticorrosive performance. Presence of silane in epoxy polymer backbone at 1–3 wt% concentration improves both anti-corrosive property and adhesion strength. However, at silane concentration of 5 wt%, the protective property of the films declines, which is due to excessive consumption of epoxide groups in epoxy resin by cross-linking reaction with amino and thiol groups present in silane. Phenyl-­amino-propyl triethoxy silane (1 wt%) is added to the hydrolyzed solution of a mixture of glycidoxy-propyl

triethoxy silane and methyl triethoxysilane (1:1, mol mol−1) and the mixture is added to water-based epoxy/­ polyamide mixture to make coating, which exhibits excellent anticorrosive performance in EIS study and salt spray chamber test at 3 wt% concentration.[213] Zinc phosphate epoxy/­micaceous iron acrylic and zinc phosphate epoxy/­ micaceous iron acrylic are also used for brackish or sea water.[214] Shipyards are looking to improve productivity by either speeding up the painting process or reduce the number of steps. The conventional solvent-based longer drying time epoxy coatings are affected by unfavorable weather conditions, thus delaying all the tasks involved. On the other hand, isocyanate-based coatings can be sprayed as a single coat system with shorter production cycles. Ultimately, this reduction in number of coatings results in cost reduction and a rapid return to service. The polyurea and PU coatings system can cure quickly, at a wide range of temperatures in the presence of moisture. They can be sprayed to a desired thickness in single application.[215] However, the adhesion strength and corrosion prevention properties are inferior. Crude oil tanker having the upper and lower areas of her cargo oil tanks, coated with an abrasion-resistant, aluminum pure epoxy coating confirmed excellent performance over 15 years of service.[216] Among the epoxy, chlorinated rubber and coal tar epoxy coatings, applied on MS panels exposed at surf zone (within 50 m away from seawater level), epoxy zinc primer with epoxy finishing gray (90 μm) with and without undercoat can protect up to 15 months of exposure.[217] Chlorinated rubber primer with chlorinated rubber-based weather work paint gray (90 µm) shows excellent corrosion free performance up to 84 months. Similarly, coal tar epoxy and epoxy finishing gray (100 µm) protects MS surface for 84 months. Silicone Alkyd Coatings Alkyd can be modified with siloxane polymer having reactive moiety. Siloxane alkyd coatings are s­ ingle-component systems because their cross-linking occurs at residual alkenes contained within the backbone of the resins in presence of atmospheric oxygen. The pot life is thus very long. Coatings having aminoalkoxysilanes in the composition are employed to generate moderately flexible systems having direct-to-metal adhesion. Hybrid siloxane coatings exhibit better physical properties and external durability than many commercially available coatings, including silicone alkyds. Despite demonstrating better exterior performance compared to straight alkyds, the haze gray silicone alkyds lose their gloss, chalk, and/or color fade to a light gray within 9–12 months after application on Navy ships.[218] The other silicone alkyds formulation with ­low-­solar-absorbing (LSA) pigments (i.e., titanium dioxide, yellow iron oxide, red iron oxide and copper phthalocyanine blue) color-shifts toward a pinkish hue after 6 months

Laboratory– Membranes

•

1662

Marine Applications

of exposure. Two-component polysiloxane topside coatings have been developed during the past decade.[219] These topcoats are hybrid cure coatings comprising resin-­ like epoxy (cured by organic reaction) and moisture-­ curable ­alkoxysilanes that hydrolyze and cross-link to form ­polysiloxane linkages.[220] Haze gray two-­component polysiloxanes demonstrate substantial improvement in color and gloss stability compared to silicone alkyds. Higher cleanability and hydrocarbon ­resistance are also added attributes to the coating. The moisture-curable alkoxysilane-terminated ­N-substituted urea polymers have been developed by NRL for use in single-component polysiloxane topcoats for military assets.[221,222] The coating, as a haze gray topcoat for navy ships, exhibits much higher exterior durability as compared to standard silicone alkyds. Having greater color stability and flexibility compared to two-component polysiloxanes, this coating reduces generation of hazardous waste and is considered a user-friendly system for painting ship. Siloxane-PMMA organic–inorganic hybrid coatings (5.63 μm) are applied on galvanized steel by the dip-­coating sol–gel technique. Hybrid coatings are synthesized from tetraethoxy-silane (TEOS) and methyl methacrylate (MMA), while using 3-methacryloxypropyl-­ trimethoxysilane (TMSM) as a coupling agent.[223] The formation of a compact structure through hydrolysis and cross-linking of TEOS results in excellent performance. Thus, the addition of a higher ratio of TEOS:TMSM at four contributes to the film formation and produce the ­synergistic effect in terms of corrosion prevention.

on the corrosion rate of ferrous alloys (iron, steel and stainless steel) was reviewed by Spinks et al.[226] The proposed mechanisms of corrosion protection is the barrier formation, acting as inhibitor, anodic protection, and the mediation of oxygen reduction. An alternative mechanism proposed is that the conducting polymer becomes polarized through galvanic coupling to the base metal substrate at defects in the coating, which allows the ICP to release an inhibiting anion.[227] Thus, both cathodic reduction of the ­conducting ­polymer and ion exchange with cathodically generated OH−, or both, result in the release of anion dopant. In the case of the anion dopant being a corrosion inhibitor, damage-­responsive corrosion protection occurs. The inhibiting properties are confirmed further with of phosphonate ­dopant [228] and camphor sulfonate anion. [229] Corrosion protection studies of paints comprising styrene butyl acrylate copolymer as binder and PANI-HCl as filler have shown appreciable corrosion protection properties in surf zone at relatively low thickness (80 ± 5 m).[230] Accelerated weathering test, humidity cabinet, salt spray, and underwater exposure study of painted MS panels indicates lower PANI-HCl containing paints (0.1–0.5 parts PANIHCl) protect MS better (Table 2). Water vapor permeability results confirm very good barrier properties. 5 phr PANI–DOPH containing epoxy coating offers protective performance for longer period as compared to other resins.[231] Field exposure study in surf zone shows no sign of ­corrosion after 18 months on PANI–DOPH (5 phr)/ epoxy coated MS panels, whereas all the panels coated with other resins have undergone heavy corrosion much earlier (Table 3).

Conducting Polymer-Based A/C Coating Nanocomposite Coating

Laboratory– Membranes

The emergence of polyaniline (PANI) as an anticorrosive pigment was initiated by Wesseling, as he exhibited that the passivation reaction resulting in oxide film could occur due to catalytic redox reaction of PANI.[224] Later PANI was dispersed in paint and the same oxide film was found to appear, which was also found industrially viable as corrosion prevention primer.[225] The literature describing the effects of conducting polymer coatings

Polyaniline camphorsulfonic acid (CSA) (PANI-CSA/ ZnO) as organic–inorganic hybrid nanocomposites is dispersed in epoxy coating as a corrosion-inhibiting pigment and the coating applied over carbon steel grade ST37.[232] In 3.5 wt% sodium chloride solution, higher corrosion resistance observed with the coating indicates higher barrier properties and corrosion resistance than pure epoxy

Table 2  Results of salt spray exposure of PANI-HCl containing paint Corrosion performance (% area corroded) Parts PANI-HCl in the paint Exposure time (h) 0.0 0.1 0.5 1.0 3.0 5.0

10.0

20.0

210

Blister

—

—

—

Spot

20

5

5

310

Blister

—

—

—

5

30

7

35

410

Blister

—

—

—

15

50

12

55

510

5

Spot

Spot

12

35

75

25

75

610

25

10

10

25

50

85

85

95

710

80

75

70

65

85

95

100

100

Source: © 2003, Elsevier. [230]

Marine Applications 1663

Table 3  Results of field exposure study of PANI–DOPH containing coating on MS panels Exposure time for onset of corrosion (months) Concentration of PANI–DOPH in the coating (phr) Coating Thickness of coating (µm) 0.0 0.1 0.5 1.0 5.0 Silicone alkyd

100 ± 5

3

6

6

6

6

Soya alkyd

--do--

2

5

5

5

5

AC 80

--do--

4

8

8

7

6

Epoxy

--do--

4

15

15

16

No corrosion

PU

--do--

4

10

10

12

Epoxy Red Oxide

--do--

3

—

—

—

12 —

Amerlock 400

--do--

2

—

—

—

—

Source: © 2005, Elsevier. [231]

Surface Tolerant Coating The surface tolerant coatings are mostly used as maintenance coat having anticorrosive property in marine and industrial environments. The challenge of reducing the level of surface preparation without degrading performance is pushing the development resulting in many

new coating products being formulated to accommodate reduced cleaning levels.[238] Active Anticorrosive Pigments Active anticorrosive pigments such as metallic chromates passivate metal surface by forming an oxide film. However, these agents are preferably kept out of the environment due to the toxicity of heavy metals. Metals such as zinc and aluminum are usually employed as anticorrosive additive in epoxy paints, which act as sacrificial coating.[239] As zinc and aluminum remains above iron in the e­ lectrochemical series, it act as anode and iron is protected by remaining as cathode. The extent of permeation through a paint film depends on the structure of the polymer used as binder in the paint. However, the presence of pigments and additives in the coating can alter the porosity and adherence of the films, which modifies their protective action. Pigments within critical pigment volume concentration (CPVC) reduce the porosity of the coatings; the smaller the pigment particles, the lower the porosity. Stimulated Protective Biofilms Some coatings are able to stimulate the formation of protective biofilms. Biogenetically engineered bacteria release corrosion-inhibiting species such as certain polypeptides and polyphosphates.[240] However, the possibility of controlled release is not yet clear. Corrosion protection of Al 2024 in artificial sea water is observed in the presence of a live biofilm.[241] Since bacteria have the ability to coat metals with a regenerative biofilm, it is becoming important for using it for preventing corrosion.[242] Homopolysaccharides show interesting results for the protection of steel. Measurements indicate that it takes some time for ­building up layers of biopolymers on the metal to make it completely protected. The Lactobacillus fermentum Ts is found to produce exopolysaccharides functioning as c­ orrosion inhibitor for mild steel.[243]

Laboratory– Membranes

and epoxy/PANI coatings. Open circuit potential of the conducting paint shifts to noble region and also oxide film is produced in presence of PANI. The barrier and corrosion resistance properties are improved further in presence of ZnO nanorods. Layered structured nanomaterials like graphene oxide offers very high barrier properties against gases and moisture for organic composites.[233,234] SiO2–GO nanohybrids are prepared through two-step sol–gel method using a mixture of tetraethyl orthosilicate (TEOS) and ­3-aminopropyltriethoxysilane (APTS) in water–alcohol solution, which is finally deposited on GO as SiO2 at elevated temperature and pH. that is used for making epoxy composites.[235] Even with 0.2 wt% SiO2–GO hybrids, the barrier performance of epoxy coatings is much improved. The SiO2–GO hybrids can also be prepared by heating GO dispersion in water with TEOS solution in toluene at 70°C–80°C for 24 h.[236] 0.1 wt% SiO2–GO nanohybrid in epoxy coating increases the water contact angle on epoxy coating, adhesion to metallic substrate, and superior corrosion protection of epoxy/SiO2–GO coatings increases with increasing immersion time due to hydrolysis reactions of silane agents through electrolyte diffusion through coating. Conducting PANI/PANI–GO (1% GO) composite is used as a pigment dispersed in epoxy resin, which shows long-term anticorrosion behavior with a corrosion rate of 6.5 9 10 −5 mm year−1, highest impedance modulus ~33 kΩ cm2 and real impedance ~32 kΩ cm2, maximum coating resistance ~14.81 × 103 Ω cm2 and minimum coating capacitance after 96 h of immersion in 3.5% mass NaCl.[237] The best performance is observed with low ­carbon steel.

1664

Corrosion-Sensing Coating Formulations

Laboratory– Membranes

Enormous economic impact of corrosion degradation on metallic structures has prompted the development of concept of active protective coatings. The maintenance costs need to be reduced significantly by avoiding excessive unnecessary preventive operations even if the coating is still able to protect. The answer is active sensing coatings, which will indicate corrosion activity so that it will allow optimization of the maintenance operations of the metallic structures. Clear visual indication of onset of corrosion is the most convenient way. Onset of corrosion is accompanied by generation of alkali and iron ions. These situation leads to color change of pH indicator or color formation due to complexation. Two acrylic-urethane resin-based corrosion-sensing coating formulations (one containing phenolphthalein, a pH indicator, which indicates corrosion with a visible color change, and the other containing fluorescein, which is a pH corrosion indicator through fluorescent changes) exhibit definite indication of corrosion.[244] The coating is clear coat in nature. Phenolphthalein is normally encapsulated into micro-particles, which break during damage and corrosion starts. This is followed by release of the indicator and the color appears. Alkyd resin is chemically modified with a color changing chemical, 1,10-phenanthroline-5-amine, which is used for detection of corrosion. It forms red-colored complex with ferrous ions by sharing lone-pair electron of nitrogen present in its molecular structure.[245] This p­ henanthroline-modified alkyd resin senses under film corrosion effectively. It is generally observed that the number of red color spots and their color intensity increases with the content of phenanthroline moiety in modified alkyd resin. The resin containing 1.90 wt% of phenanthroline moiety shows both intense color and better corrosion resistance. It is known that in hidden shaft area, the bolts tend to corrode, which is not visible. A coating that changes color on the bolt head or nut during onset of corrosion would greatly facilitate the inspection process and increase the safety and reliability of the structure. In both the cases color is visible prior to the appearance of corrosion spot. Smart Paints In an effort to replace hexavalent chromium and maintain lower VOCs in coatings and develop effective and environmental friendly corrosion inhibitors new organic and inorganic corrosion inhibitors have been identified. However, the organic inhibitors are reactive and incompatible with coating systems, while inorganic inhibitors can be both reactive and highly water soluble.[246] This high water solubility can lead to osmotic blistering and premature coating failure. Thus, a new generation of anticorrosion coatings that possesses passive matrix functionality as well as the ability to responds actively during changes in the local environment.[247] Active corrosion protection

Marine Applications

aims at releasing the active and repairing material within a short time after the coating matrix is broken and corrosion of substrate initiated. Inhibitor can be entrapped by complexation of organic molecules by cyclodextrin.[248] Another approach is the use of oxide nanoparticles acting as nanocarriers for corrosion inhibitors. The Ce3+ ions are immobilized on the surface of ZrO2 nanoparticles during the synthesis of a nano-sol by the controlled hydrolyzation of the precursor with a Ce-containing aqueous solution.[249] The inhibition of inorganic ions can also be incorporated by exchangeable ions associated with cation- and ­anion-exchange solids.[250,251] Anion-exchange anticorrosion pigments comprising nitrate, carbonate, and chromate exchanged hydrotalcite (HT) inhibit the propagation of filiform corrosion (FFC) on polyvinyl butyral (PVB) coated AA2024-T3 aluminum alloy.[252] All HT-pigments reduce extensively the rates of coating delamination as compared to unpigmented samples. However, inhibitor efficiency varies in the order carbonate < nitrate < chromate. Encapsulation is another successful approach which isolates the inhibitor from the resin and other ingredients in paints formulations. Using plasma polymerization the corrosion inhibitor can be encapsulated.[253] Layer-by-layer (LbL) assembly approach using polyelectrolytes is also used to form the nanocontainer shell for enclosing the corrosion inhibitor.[254] The polyelectrolytes are pH sensitive, which will act toward release of corrosion inhibitor. Development of drone, capable of remotely inspecting enclosed spaces and ballast water tanks, is considered unique idea for safety in the marine industry and three industry partners joined forces to realize it.[255] For routine maintenance of enclosed spaces and ballast water tanks needs the high risk activity of regular survey, which may be the ideal case for application of drone technology. Powder Coatings Powder coatings are produced as both thermoplastic and thermosetting products and applied directly on substrates. Almost all thin-film powder coatings are applied by electrostatic spray. Other methods include fluidized bed and flame-spray application. Most widely used thermosetting resin is the epoxy powders, used commonly on pipes, rebar, and electrical products. The ultraviolet resistance of epoxy is poor, while hybrid (epoxy-polyester) binders, polyesters, and acrylics exhibit better ultraviolet resistance. The first powder coatings were thermoplastic, but the availability of ground powders was a concern making it difficult to achieve good flow and leveling properties during baking. Vinyl copolymers, polyamides, and fluoropolymers are commonly used polymers. Very low VOC, low toxicity, low flammability, good film built, uniform coating, etc., make it a product for future application. Polyethylene, polypropylene, fluoropolymers, [256] and thermoplastic polyesters can be sprayed by thermal

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12

Tensile strength, MPa

10 8 6 4 2 0

0

15

30

45

60

75

Radiation level, kGy

Fig. 24  Tensile Strength of LDPE-g-MAc with varying radiation doses Source: © 2004, John Wiley and Sons.[257]

Table 4  Corrosion resistance of flame sprayed coatings Polymer Performancea (after 13 weeks) Salt Seawater spray immersion Humidity Polymer LDPE

4

2

4

LDPE-g-MAc

7

7

8

LDPE + Red Iron Oxide

5

7

7

LDPE-g-MAc + red iron oxide

7

9

9

Source: © 2006, Elsevier. [258] a 10, no corrosion; 0, severe corrosion.

this type of coating and its gloss retention are ­guaranteed for 10 years. Correlation between Natural Exposure of Marine Coating Systems and Their Artificial Aging Test Natural atmospheric exposure test and the artificial aging test are correlated for coating used in marine ships by analyzing the surface morphology, gloss, molecular structure, and electrochemical impedance parameters, respectively.[260] The following are the four types of paint schemes: • • • •

High build epoxy anticorrosion paint (150 µm) + gray PU heat reflection paint (80 µm) High build epoxy anticorrosion paint (150 µm) + gray fluorocarbon top paint (80 µm) High build epoxy anticorrosion paint with aluminum red (150 µm) + suboptical gray PU top paint (80 µm) Epoxy anticorrosion paint with iron oxide red (150 µm) + gray PU top paint (80 µm)

The top coats of first one and the last two systems consist of urethane + hydroxyl acrylic resin along with titanium dioxide pigment. The top coat of second system consists of similar resin as used for others with fluorine substitution. The outdoor exposure was performed in a typical marine atmospheric environment. Xenon lamp aging test and natural exposure test show similar behavior, and also the fluorocarbon paint retains high gloss longer than polyurethane paint. The surface morphology changes are also similar in both tests. The rank correlation study indicates that 60, 150, 300, 360, and 450 days of natural exposure results are similar to 228, 443, 841, 1958, and 4013 h of artificial aging test. In spite of appearance of some defects on the coating surface after natural exposure or artificial aging,

Laboratory– Membranes

spray process and are known to provide longer service life to marine structures. For better adhesion, the maleic acid is grafted to LDPE by pre-activation method using ­γ-radiation. Maximum grafting of 2.4% is achieved at 60 kGy dose.[257,258] LDPE, LDPE-g-MAc, and pigmented compositions are applied on grit blasted mild steel surface by flame-spray gun. The powder feed unit and air mixture is used as fuel gas, while compressed air is used as powder carrier gas. Grafting does not affect thermal and mechanical properties of LDPE to appreciable extent (Fig.  24). Grafted LDPE shows improved corrosion resistance compared to pure LDPE resin. Pigmentation with red iron oxide has further improved the corrosion resistance of LDPE-g-MAc (Table 4). The acid-modified polyolefin powder is applied by conventional powder spray techniques to a thickness of 150–200 μm to smooth finish and gloss level 70%. Typical applications include outdoor hand rail and balustrade “warm to touch,” stadium seating, playground equipment, fencing, pipe work for oil, gas, water, and sewage and ­internal hand rail and balustrade, appliances, food shelving, wire work, and fire extinguishers.[259] Metal protection by

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the whole coating systems maintained high impedance, indicating degradation only on the top paint. MISCELLANEOUS APPLICATIONS Thermoplastic Polyurethane (TPU)

Laboratory– Membranes

Another polyurethane material having great use in the maritime industry is thermoplastic polyurethane (TPU). It is an elastic, durable, and easily processable substance, ideally suited for application in wire and cable coatings, engine tubing, drive belts, hydraulic hoses and seals, and even ship molding. As it is paintable and weldable, TPU applications extend beyond the boat, in the form of swim fins, goggles, and possibly the inflatable rafts! For the offshore industry, the application of PU includes seals, protection, and deep water flow line coatings. [261] In the fishing industry selectivity grids are now being proposed in polyurethane, [262] while oceanographic applications include fenders and tubing on underwater vehicles and underwater antenna protection. In response to pressure on fishing stock, various methodologies have been devised to allow the juvenile fish to escape during catching. Fishing technology group of IFREMER developed rectangular metallic grid in the codend, which allows 60% juvenile fish to escape. However, the addition of a grid in the extension reduced the selection range. [263] Various plastics such as polyamide, high modulus polyethylene, polyvinyl chloride, acrylonitrile butadiene styrene, and carbon fiber composites are tried. PU is used, as it can be modified by varying the chemical composition of polyol and diisocyanate to suit the requirement. Easy processing of PU to flexible grid structure is found promising as it is stable in sea water and the juvenile escape is quite satisfactory. POLYMER EXTRACTION FROM MARINE SOURCE Various compounds are currently being isolated from aquatic organisms and proposed as novel products for health-related applications. Marine species have posed as resource for the discovery of novel active pharmaceutical ingredients.[264] The biopolymers synthesized by variety of marine organisms can be grouped in to three main classes: polysaccharides, proteins, and nucleic acids.[265] Polysaccharides are natural macromolecules consisting of osidic monomers and are present in all organisms: microorganisms, plants, and animals.[266] For use in ­industry, the polysaccharides are mostly extracted from plants, algae, and animals. Most of the marine-derived exopolysaccharides (EPS) are bacterial.[267] AGAR is a complex polysaccharide present in the cellular wall of red algae, namely, agarophytes, including species belonging

Marine Applications

to the genera Gelidium and Gracilaria.[268] ALGINATE is extracted from brown algae and the most important polysaccharide from this type of seaweeds is also produced by soil bacteria Azotobacter and Pseudomonas species as an EPS.[269] The major polysaccharide in brown algae is alginate, building up to 45% of the dry weight of these seaweeds. Carrageenan belongs to a family of linear sulfated polymers extracted from some species of red algae ­(Rodophyta—Class Gigartinales), mainly from Chondrus, Eucheuma, Gigartina, and Iridaea genera.[264] Carrageenan accounts up to 60%–80% of its dry weight of red algae, which also can contain proteins (10%–47%), floridean starch, various compounds, and metabolites such as ­phenols, essential oils, and vitamins.[270] Chitin is part of the organic matrix of exoskeletons of arthropods such as crustaceans (e.g., crabs, lobsters and shrimps) and of endoskeleton of mollusks.[271] Chitin is also found as a major polymeric constituent of the cell wall of fungi and algae. The poor solubility of chitin made chitosan, a deacetylated derivative of chitin, to draw more attention. Glycosaminoglycans (GAGs), a linear, complex and polydisperse natural polysaccharides bearing a repeating disaccharide unit, are constituted by hexose and a hexosamine. The sulfated GAG is obtained in various types of marine phyla-like sponges (Porifera)[272] and ­several classes of commercially important species such as sharks, skate, codfish, salmon, and trout.[273] Collagen is formed as a triple helix by three extended protein chains, which are wrapped around one another. Collagen and gelatin are known to be different forms of the same macromolecule, while gelatin is the partially hydrolyzed form of collagen. The main industrial sources of ­collagen and gelatin are bovine and porcine skin. However, attempts have been made to extract collagen and gelatin from marine sources and screen their potential industrial applications.[274] Among various marine origin molecules, algae sulfated polysaccharides have been found to have commercial importance, as observed in their application in food industry and medicine and also, no equivalent is available from terrestrial organisms. Chondroitin sulfate (CS) can be isolated from various natural sources including terrestrial species and marine species.[275] CS consists in a disaccharide basic unit of hexosamine (D-galactosamine) and hexuronic acid (D-glucuronic acid), which are arranged in alternating unbranched sequence containing sulfate ester substituents at various positions. Dermatan sulfate (DS) can be isolated from marine environment, using ray skin (Raja radula).[276] DS is composed of linear polysaccharides assembled as disaccharide basic units containing a hexosamine, N-acetyl galactosamine (GalNAc) or glucuronic acid (GlcA) joined by β-1,4 or -1,3 linkages, respectively.

Heparan sulfate, keratan sulfate, hyaluronic acid, etc., are the other polymers derived from marine sources. DETERIORATION OF POLYMER PROPERTIES As the polymers face hostile marine environment during use, it is bound to undergo deterioration. Few cases are ­discussed in detail. Elastomer In marine energy recovery, wave and tidal energy converters, rubbers are subjected to severe cyclic loadings. The non-relaxing conditions on fatigue lifetime of natural rubber can be significantly reduced when it is used in sea water. In order to understand this new result, the effects of both antioxidant and of minimum strain during the fatigue cycle are investigated. The effect of antioxidant is found to be the same in sea water and air, i.e., an increase of the stabilizer level leads to an increase in fatigue life, indicating that antioxidant leaching is not the origin of the reduction of fatigue life in sea water. The failure is not observed when natural rubber is used in sea water in fully relaxing cycles.[277] It suggests that the strain-induced crystallization, responsible for the beneficial effect of non-relaxing cycles on fatigue resistance, is adversely influenced by presence of sea water. The dynamic bulk modulus of elasticity are measured for 14 different rubbery elastomers (three natural rubbers, five neoprenes, three polyurethanes, and one each of butyl, nitrile, and butadiene types) in the temperature range of −10°C to +40°C, in frequency range from 5 to 3,000 Hz (mostly in 100–1,000 Hz range), at 2.5 MPa pressure.[278] Values of the real (storage) part of the modulus fall within 35% of the mean value of 2.9 GPa for all elastomers, whereas loss moduli are few percent of the storage moduli. The room-temperature aging in artificial sea water decreases the bulk modulus of natural rubbers to maximum extent, intermediate decrease occurs for neoprenes, whereas urethanes exhibit high stability in bulk modulus.

at 60°C.[280,281] The hydrolysis of the epoxy resin is the reason for deterioration of properties. The anhydride hardener does not remain stable over long periods at 60°C. Sea water aging for longer duration shows reduction in quasi-static mechanical properties and a change in flexural mode from compression to tension, which supports the results from the numerical simulations.[282] Static tests after different sea water aging periods exhibit a large reduction (from 40% to 56%) in the quasi-static strengths of the composite materials due to the sea water absorption. As the water absorption increases, the residual failure stresses in b­ ending decrease, particularly in E-glass composite materials, while axial modulus remains unchanged. In seawater, the moisture absorption and elevated temperature plasticizes the thermoplastic matrix, causing relaxation of the polymer chains. The residual stresses present within the composite leads to micro-crack formation and finally enhanced water absorption deteriorates the fiber/matrix interface. The matrix/fiber interface remains the mechanism of failure for the POM, polypropylene, polyamide (PA 11) and polybutylene terephthalate composites with glass. The unaged composite materials undergo brittle failure under tensile load as it is transferred to fiber, while the aged samples undergoing deterioration of matrix/fiber interface cannot transfer the load efficiently. [283] The aging environment of hydrocarbons maintained at three elevated temperatures (100°C, 120°C and 140°C) decreases the tensile and flexural property of all the composites indicting occurrence of aging. The POM/Carbon and PP/Glass samples disintegrate at elevated temperatures and the mechanism of failure is the deterioration of the matrix/fiber interface. The carbon fiber-reinforced PPS perform well in the hydrocarbon gas condensate. Good retention of mechanical property of PPS/carbon, during exposure to low concentration H2S gas exposure (30 ppm) at three elevated temperatures (160°C, 180°C and 200°C), indicates low extent of aging. The PEEK/carbon composite shows similar result. DEGRADATION

Composites The reduction of mechanical properties of marine-grade glass/epoxy, glass/vinyl ester, carbon/epoxy and carbon/vinyl ester composites is higher in the initial stages of seawater immersion, which saturates with increasing durations.[279] The flexural strength and ultimate tensile strength (UTS) reduce by about 35% and 27% for glass/ epoxy, 22% and 15% for glass/vinyl ester, 48% and 34% for carbon/epoxy, 28%, and 21% for carbon/vinyl ester composites, respectively. The water uptake behavior of epoxy-based composites is inferior to that of the vinyl system. Epoxy composites with anhydride hardener show decrease in mechanical properties after 2.5 years in water

The durability of plastics and their diverse applications were anticipated earlier. However, the waste management and plastic debris problems were never predicted.[284] Most of the types of plastics show high resistance to aging and negligible biological degradation.[285] During exposure to the UVB radiation in sunlight under atmosphere atmospheric condition possessing oxidative properties, and seawater having hydrolytic properties, these polymers become embrittled, and break into smaller pieces, which are required to undergo further degradation before becoming bioavailable. The complete degradation process of plastics in the marine environment requires an unknown amount of time.[286] Even the non-petroleum-based polymers degrade

Laboratory– Membranes

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Laboratory– Membranes

very slowly at sea, and hardly at all in the deep ocean, as the marine environment is much colder.[287] Significant amount of plastics have accumulated in the natural environment and in landfills. Plastic is found to be most common material in marine debris and it is turning to be a serious global pollution problem. The presence of particles in aquatic environment stimulates microbial productivity and respiration.[288] The study dealing with the bacterial communities living in the so-called “plastisphere” indicates the presence of a good number of species, such as heterotrophs, autotrophs, predators, symbionts, and also some opportunistic pathogens.[289] The biodegradation of synthetic plastics is a complex phenomenon. Simulation of natural environment is difficult to realize in laboratory as the number of parameters occurring during the biogeochemical recycling is many. Over one-third of commonly used plastic such as polyethylene (PE), polypropylene (PP), poly(vinyl chloride) (PVC), polystyrene (PS), poly(ethylene terephthalate) (PET) and polyurethane (PU) in both the United States and Europe is used for making packaging, eating utensils and trash bags, which are normally discarded within 3 years of their production.[290] PET degradation under marine environment follows the route of photo, photo-oxidative, and hydrolytic degradation, respectively. Photodegradation leads to product with carboxylic acid end group and a vinyl end group directly through cleavage of the ester bond. Alternatively, radical generated first finally form carboxylic acid end group.[291,292] In water PET can undergo hydrolytic degradation, [293] Of course, the rate of hydrolysis is very slow. The potential by-products released into the surrounding environment or into the cytoplasm of the degrading cells following biodegradation of microplastics are studied.[294] FTIR spectra of experimental and control samples show negligible differences, indicating absence of any biodegradation process that releases plastic by-products into the surrounding marine environment or into the cytoplasm of the microorganisms. PU degradation processes in the marine environment include photo-oxidation, hydrolysis, and biodegradation, respectively. Photoinitiation produce radical, which leads to hydroperoxides. Most predominant hydrolytic degradation reaction is the hydrolysis of the ester bond of PU.[295] Also, the urea and urethane bonds degrade by hydrolysis, but at slower rates. Microorganisms degrade polyester segments of PU easily, while polyether segments of PU are hardly affected. Although enzyme degradation is more specific, it occurs only on the surface. An accelerated study of PU degradation was conducted in artificial sea water for periods up to two years at 50°C–100°C.[296] The occurrence of hydrolysis is confirmed from accelerated test results and FTIR analysis. However, the Arrhenius extrapolation indicates that the timescale for 50% property loss at sea ­temperatures is more than 100 years. PE degradation starts by oxidation in the presence of UV radiation. During propagation, the oxygenated low

Marine Applications

molecular weight fragments, such as aliphatic carboxylic acids, alcohols, aldehydes, and ketones, are formed.[297] The oxygenated material becomes brittle and undergoes fragmentation.[298] Microorganisms are able to attack PE at any terminal methyl group.[299] Biodegradation becomes faster below the molecular weight of 500 Da.[297] In the marine environment the degradation is much slower, as the conditions are not optimized for polymer degradation. The time duration for the process will take decades or longer. PP has a lower stability than PE because of the presence of alternate tertiary carbon, which is more vulnerable to abiotic attack than the secondary carbons found in PE.[300] The reaction mechanisms are similar to PE, as the radical reactions lead to both random chain scission and cross-linking. However, the formation of lower molecular weight fragments is predominant.[300] The presence of the tertiary carbon of PP reduces the vulnerability to m ­ icrobial degradation.[301] PS is highly susceptible to outdoor weathering.[302] Cross-linking and chain scission occurs under UV irradiation, which results in the formation of ketones and olefins.[300] As the end-chain scission is major reaction, the main volatile product of degradation is styrene monomers. The degradation pathways in thermo-oxidation and photo-­ oxidation being similar, the known degradation products from thermo-oxidation are also expected in the marine environment by photo-oxidation. However, PS is considered to be the most durable thermoplastic polymer toward biodegradation.[303,304] PVC has very low stability as it has the highest sensitivity toward UV radiation.[305] When exposed to sunlight, dechlorination of PVC occurs initially, which results in the formation of conjugated double bonds and hydrochloric acid together with other products in low yield.[306] The biodegradation of PVC, having chlorine in the chain, is ­negligible.[292] The anthropogenic millimeter-sized polymers have formed a new pelagic habitat for microorganisms and invertebrates.[307] These may modify its habit in future to degrade the polymers. Although biodegradation is very low in marine environment, there is decrease in ­mechanical properties.[308] REFERENCES 1. National Research Council, Committee on Shipboard Wastes. Clean Ships, Clean Ports, Clean Oceans: Controlling Garbage and Plastic Wastes at Sea; National Academy Press: Washington, DC, 1995, 38–39. 2. National Research Council, Committee on Shipboard Wastes. Clean Ships, Clean Ports, Clean Oceans: Controlling Garbage and Plastic Wastes at Sea; National Academy Press: Washington DC, 1995, 126. 3. Charlson, R.J.; Lovelock, J.E.; Andreae, M.O.; Warren, S.G. Oceanic phytoplankton, atmospheric sulfur, cloud albedo and climate. Nature 1987, 326, 655–661.

4. Schmidt, K. Cruising for Trouble: Stemming the Tide of Cruise Ship Pollution; Bluewater Network: San Francisco, CA, 2000. 5. Samui, A.B.; Chavan, J.G.; Swami, B.S. Fouling on (a) Seawater Exposed Panel (Inset: Barnacle Shell); (b) Underwater Surface of Ship, Marine Fouling and its Prevention. Unpublished work, 1991. 6. How To: Marine Apps + Cast Nylon. Onlinemetals.com (Blog), October 24, 2014. Available at http://blog.onlinemetals.com/how-to-marine-apps-cast-nylon/. 7. Ku, P.L. Nylon 66 fundamentals and its processes. Adv. Polym. Technol. 1986, 6 (3), 267–275. 8. Campion, R.P.; Thomson, B.; Harris, J.A. Elastomers for Fluid Containment in Offshore Oil and Gas Production: Guidelines and Review. Research Report 320; MERL Ltd: Hertford, 2005, 5–6. 9. Luo, X.; Chung, D.D.L. Vibration damping using flexible graphite. Carbon 2000, 38 (10), 1510–1512. 10. Yamada, N.; Shoji, S.; Sasaki, H.; Nagatani, A.; Yamaguchi, K.; Kohjiya, S.; Hashim, A.S. Developments of high performance vibration absorber from poly(vinyl chloride)/ chlorinated polyethylene/epoxidized natural rubber blend. J. Appl. Polym. Sci. 1999, 71 (6), 855–863. 11. Liao, F.-S.; Hsu, T.-C.J.; Su, A.C. Enhanced laminate damping via modification of viscoelastic interlayer. J. Appl. Polym. Sci. 1993, 48 (10), 1801–1809. 12. Tung, C.-J.; Hsu, T.-C.J. Vibration damping with urethane/ acrylate simultaneous semi-interpenetrating polymer networks. J. Appl. Polym. Sci. 1992, 46 (10), 1759–1773. 13. Samui, A.B.; Dalvi, V.G.; Patri, M.; Chakraborty, B.C.; Deb, P.C. Studies on semi-interpenetrating polymer network based on nitrile rubber and polymethyl methacrylate. J. Appl. Poly. Sci. 2004, 91 (1), 354–360. 14. Patri, M.; Samui, A.B.; Chakraborty, B.C.; Deb, P.C. Studies on IPNs based on nitrile rubber and polyalkylmethacrylates. J. Appl. Polym. Sci. 1997, 65 (3), 549–554. 15. Patri, M.; Samui, A.B.; Deb, P.C. Studies on sequential IPN based on nitrile rubber and polyvinyl acetate. J. Appl. Polym. Sci. 1993, 48 (10), 1709–1716. 16. Samui, A.B.; Suryavanshi, U.G.; Patri, M.; Chakraborty, B.C.; Deb, P.C. Sequential interpenetrating polymer network based on nitrile-phenolic blend and poly (alkylmethacrylate). J. Appl. Polym. Sci. 1998, 68 (2), 255–262. 17. Samui, A.B.; Manoj, N.R.; Raut, R.; Patankar, A.S. Thermoplastic blend of polyaniline with polyvinyl chloride-­ nitrile rubber. J. Appl. Polym. Sci. 2006, 101, 1217–1222. 18. Sharma, A.; Peel, L.D. Vibration Damping of Flexible and Rigid Polyurethane Composites; SAMPE 2004: Long Beach, CA, 2004. 19. Khan, S.U.; Li, C.Y.; Siddiqui, N.A.; Kim, J.-K. Vibration damping characteristics of carbon fiber-reinforced composites containing multi-walled carbon nanotubes. ­Compos. Sci. Technol. 2011, 71 (12), 1486–1494. 20. Nagasankar, P.; Balasivanandha Prabu, S.; Velmurugan, R.; Paskaramoorthy, R. The effect of the chopped fibers on the damping characteristics of fiber reinforced polymer skins of the polypropylene honeycomb sandwich panel. Adv. Mat. Res. 2014, 893, 245–249. 21. Chen, L.; Gong, X.-L.; Li, W.-H. Damping of magnetorheological elastomers. Chin. J. Chem. Phys. 2008, 21 (6), 581–585.

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38.

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and reliability method. Comp. Sci. Tech. 2001, 61 (14), 2087–2094. Boisseau, A.; Davies, P.; Thiebaud, F. Sea water ageing of composites for ocean energy conversion systems: Influence of glass fiber type on static behavior. Appl. Compos. Mater. 2012, 19 (3–4), 459–473. Roseman, M.; Martin, R.; Morgan, G. Composites in offshore oil and gas applications. In Marine Applications of Advanced Fiber-Reinforced Composites; Graham-Jones, J.; Summerscales, J.; Eds.; Woodhead Publishing, Elsevier: Oxford, UK, 2016, 233–257. Thompson, R.C.; Moore, C.J.; vom Saal, F.S.; Swan, S.H. Plastics, the environment and human health: Current consensus and future trends. Phil. Trans. R. Soc. B 2009, 364, 2153–2166. Moore, C.J. Synthetic polymers in the marine environment: A rapidly increasing, long-term threat. Environ. Res. 2008, 108, 131–139. Andrady, A.L. Plastics in marine environment. A technical perspective. In Proceedings of the Plastic Debris Rivers to Sea Conference, Algalita; Algalita Marine Research ­Foundation: Long Beach, CA, 2005. Wirsen, C. Microbial degradation of organic matter in the deep Sea. Science 1971, 171 (3972), 672–675. Ghiglione, J.F.; Conan, P.; Pujo-Pay, M. Diversity of total and active free-living vs. particle-attached bacteria in the euphotic zone of the NW Mediterranean Sea. FEMS Microbio. Lett. 2009, 299, 9–21. Zettler, E.R.; Mincer, T.J.; Amaral-Zettler, L.A. Life in the “plastisphere”: Microbial communities on plastic marine debris. Environ. Sci. Tech. 2013, 47, 7137–7146. PlasticsEurope. Plastics—The Facts 2014/2015-An Analysis of European Plastics Production, Demand and Waste Data, 2015. Available at http://www.­ plasticseurope.org/Document/plastics-the-facts-20142015. aspx?Page¼DOCUMENT&FolID=2. Gewert, B.; Plassmann, M.M.; MacLeod, M. Pathways for degradation of plastic polymers floating in the marine environment. Environ. Sci. Process Impacts. 2015, 17, 1513–1521. Fagerburg, D.R.; Clauberg, H. In Wiley Series in Polymer Science; Scheirs, J.; Long, T.E.; Eds.; John Wiley & Sons Ltd.: Chichester, 2004, 609–641. Summers, J.W.; Rabinovitch, E.B. Weatherability of vinyl and other plastics. In Weathering of Plastics–Testing to Mirror Real Life Performance; Wypych, G.; Ed.; William Andrew Publishing, 1999, 61–68. ISBN (print) 978-1884207-75-4. ISBN (electronic): 978-0-8155-1958-4. Kumar, M.; Xie, A.; Curley, J. Determining the potential secondary impacts associated with microorganismal biodegradation of microplastics in the marine environment. JESS 2016, 3 (4), jes2s.com. Szycher, M. Szycher’s Handbook of Polyurethanes, 2nd Ed.; Taylor & Francis: Boca Raton, FL, 2013. Davies, P.; Evrard, G. Accelerated ageing of polyurethanes for marine applications. Polym. Degrad. Stab. 2007, 92 (8), 1455–1464. Vasile, C. Practical Guide to Polyethylene; RAPRA ­Technology: Shrewsbury, 2005. Hakkarainen, M.; Albertsson, A.-C. Environmental degradation of polyethylene. In Long Term Properties of

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Polyolefins, Vol. 169; Albertsson, A.-C.; Ed.; Springer: Berlin and Heidelberg, 2004, 177–200. Booma, M.; Selke, S.E.; Giacin, J.R. Degradable plastics. J. Elastomers Plast. 1994, 26, 104–142. Beyler, C.; Hirschler, M. Thermal decomposition of polymers. In SFPE Handbook of Fire Protection Engeniering 2, 3rd Ed., Sec.1, Chapter 7; Scientific Research Publishing: Wuhan, China Delaware, 2002, 111–131. Chanda, M. Plastics Technology Handbook, 4th Ed.; CRC Press/Taylor & Francis Group: Boca Raton, FL, 2007. Singh, D.P.; Dwivedi, S.K. Environmental Microbiology and Biotechnology; New Age International Ltd.: New Delhi, 2004. Faber, M.D. Microbial degradation of recalcitrant compounds and synthetic aromatic polymers. Enzyme Microb. Technol. 1979, 1 (4), 226–232.

304. Mor, R.; Sivan, A. Biofilm formation and partial biodegradationof polystyrene by the actinomycete Rhodococcus ruber. Biodegradation 2008, 19 (6), 851–858. 305. Nicholson, J.W. In Polymer Degradation, The Chemistry of Polymers; RSC Pub.: Cambridge, 2012. 306. Pielichowski, K.; Njuguna, J. Polymers, copolymers and blends. In Thermal Degradation of Polymeric Materials; Rapra Technology: Shawbury, 2005, 62–71. 307. Reisser, J.; Shaw, J.; Hallegraeff, G.; Proietti, M.; Barnes, D.K.; Thums, M.; Wilcox, C.; Hardesty, B.D.; Pattiaratchi, C. Millimeter-sized marine plastics: A new pelagic habitat for microorganisms and invertebrates. PLoS ONE 2014, 9 (6), e100289. 308. Alvarez-Zeferino, J.C.; Beltrán-Villavicencio, M.; Vázquez-Morillas, A. Degradation of plastics in seawater in laboratory. OJPC 2015, 5, 55–62.

Laboratory– Membranes

Medicines: Polymers for Narendra Pal Singh Chauhan Department of Chemistry, Bhupal Nobles University, Udaipur, India

Nirmala Kumari Jangid and Navjeet Kaur Department of Chemistry, Banasthali University, Banasthali, India

Bharatraj Singh Rathore Department of Chemistry, PAHER University, Udaipur, India

Mazaher Gholipourmalekabadi Cellular and Molecular Research Center, Iran University of Medical Sciences, Tehran, Iran; Department of Tissue Engineering & Regenerative Medicine, Faculty of Advanced Technologies in Medicine, Iran University of Medical Sciences, Tehran, Iran

Masoud Mozafari Bioengineering Research Group, Nanotechnology and Advanced Materials Department, Materials and Energy Research Center (MERC), Tehran, Iran

Abstract Polymers are the most important and largest family of materials used in medical technology. With the advent of plastics in the field of medical technology, the use of traditional materials such as glass and metals has been reduced considerably. Various hybrid synthetic biodegradable polymers are used for specific applications because of tunable physical, chemical, and biological properties. A wide range of different polymers are available here. BTHC (butyryl-trihexyl-citrate) and DEHP (di(2-ethylhexyl)phthalate) are used as alternative plasticizer of PVC in blood bags. ePTFE (expanded PTFE Gore-Texs) is used for vascular grafts, surgical meshes, ligament, and tendon repair. PEEK (polyether ether ketone) is useful for hard stable polymer for orthopedic applications or inner lining of catheters. PMMA (poly(methyl methacrylate)) is used as hard methacrylate as bone cement, as intraocular lens, or for dialysis membranes. This entry will include a brief overview about the introduction and developments of polymers in medicine.

INTRODUCTION Polymers are the most important and largest family of materials used in medical technology. With the advent of plastics in the field of medical technology, the use of traditional materials such as glass and metals has been reduced considerably. The utilization of plastics in the medical disposable device sector minimized or avoided the risk. The basic principle of polymers, i.e., multiple assemblies of simple structural units for the formation of a three-­ dimensional construct, has wide distribution in all biological systems. Such natural polymers such as horn, hair, or cellulose have been utilized by human since beginning of manhood, and they have found application in medicine, e.g., as suture material also for long time.[1] Synthetic polymers gained high attraction for ­technical as well as for medical application for various reasons. A wide range of physical and chemical properties can be achieved based on the monomer units, polymerization Encyclopedia of Polymer Applications, First Edition DOI: 10.1201/9781351019422-140000298 Copyright © 2018 by Taylor & Francis. All rights reserved.

reaction, and formation of copolymers consisting of ­different components at adjustable concentrations.[2] These types of polymers mainly fulfill structural and ­mechanical properties. Mechanical self-reinforcement is achieved by integration of oriented fibers of the same material into the matrix.[3] There are also highly advanced mechanical properties, such as shape memory polymers, which can be freely deformed and return to their original shape upon a special stimulus, which can be pH, temperature, magnetic field, or light. They found application in biomedicine in drug delivery devices, vascular stents, sutures, clot removal devices, for aneurysm or ductus arteriosus occlusion, and in orthodontic therapy as reviewed elsewhere.[4] In addition to the mechanical properties, the specific functional characteristics of polymers are also used. Semipermeable membranes of biopolymers (cellulose) or polymers are used for hemodialysis or as drug delivery systems. Swelling or collapsing of pores of the membrane in response to pH, temperature, or other stimuli leads to

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Keywords: DEHP; Different medical applications; ePTFE; PEEK; PMMA.

1680

membranes for responsive drug release.[5] Due to their carbon-­based chemistry, polymers are closer to biological tissue than inorganic materials. This can be used for targeted interaction between the material and the body, but may also cause problems due to an interference of rest-monomers, degradation products, or additives with biochemical pathways. Reactive groups in the polymers usually also offer the possibility for biofunctionalization of the surface, either because they provide reactive groups by themselves, or, e.g., plasma technologies can be used to create such groups for covalent anchorage of molecules on the surface. The surface ­modification ­techniques allow independent optimization of the ­mechanical properties of the bulk and b­ iocompatibility properties of the surface. Functional types of polymers evolved for biomedical applications. Biodegradable polymers ideally stay in the body only as long as they serve their function and then they disappear without the need of a second surgical intervention.[6,7] Orthopedic fixation and ligament a­ ugmentation were the primary motivation for ­biodegradable polymers.[8] Since the 1990s, vascular stents have ­developed as the main target application.[9] These degradable polymers have been further used for the delivery of drugs along with the ­degradation from microcarriers or m ­ acroscopic ­applications.[10] TYPE OF MEDICINE POLYMERS DEHP [Di-(2-EthylHexyl) Phthalate] as Plasticizer in PVC (Poly Vinyl Chloride)

Laboratory– Membranes

For certain medical procedures such as blood t­ ransfusion, hemodialysis, parenteral nutrition, or endotracheal ­tubing, the flexibility of certain parts of a medical device is ­essential. Various substances are used to ensure this flexibility, among which DEHP [di-(2-ethylhexyl) phthalate] is the most frequently used plasticizer in PVC medical devices. DEHP may migrate from the device to the human body, resulting in a certain degree of patient exposure. DEHP is also released when PVC material is heated or comes into contact with certain media. DEHP is not chemically bound into the polymer matrix and therefore can migrate out of the polymer. It is especially likely to migrate out of the polymer in the presence of fatty solutions. Indoor releases of DEHP to the air from plastic materials, coatings, and flooring in home and work environments can lead to higher indoor levels than are found in the outdoor air. [11] Alternative manufacturing processes to create flexible polymers can involve the replacement of DEHP with another plasticizer, or the use of a polymer or other material that does not require the use of a plasticizer to achieve the same characteristics and performance. Compatibility with PVC and any other additives present (ability to create a stable single-phase compound) is a critical factor

Medicines: Polymers for

when considering alternative plasticizers. In addition, the alternative plasticizer should achieve the required level of flexibility (measured as hardness) at a cost that is comparable to that of DEHP-plasticized PVC. In medical device applications, there are additional performance ­criteria for the plasticized polymer. Important considerations include the tendency of plasticizers to migrate out of the ­polymer matrix, and the ability of the plasticized polymer to be s­ terilized by different methods. Additional concerns include: For sheet applications—tensile strength, cold flexibility. For tubing applications—elastic recovery must be ­optimized to assure that tubing does not kink during use. Some hospitals in Europe still recommend medical devices that are not made from PVC mainly because of environmental concerns. However, a new life cycle a­ nalysis that compares a medical device made of PVC with two devices made of alternative plastic materials shows that PVC products based on DEHP plasticizer can be safely used in healthcare products.[12] Expanded Poly(tetrafluoroethylene) (ePTFE) Poly(tetrafluoroethylene) (PTFE), a fully fluorinated ­linear thermoplastic polymer, and in particular the porous form expanded PTFE (ePTFE), has found a widespread use in biomaterial application due to its high toughness, non-­adhesiveness, and hydrophobicity properties (Fig. 1). While it performs ideally for many applications, some challenges have been identified for its use in small d­ iameter vascular grafts and as tissue space-filler for cosmetic reconstructions where the implant interfaces with bone. The first reported biomaterial application of PTFE was as an artificial heart valve. Shortly after, a woven textile graft of PTFE found application as a vascular graft material; however, it was found not to be ideal as it unraveled post implantation.[13] In contrast, ePTFE has proven more favorable as a biomaterial due to its antithrombotic surface and porosity which allow tissue in-growth(e.g., fibrovascular[14] and dermis [15]). Furthermore, it displays enhanced mechanical integrity.[16] PTFE is one of few (if not the only) synthetic polymers which is truly biostable and an in vivo study of ePTFE showed that it is stable for up to 6.5 years (length of study) after implantation.[17] Because of its overall good performance in the human body, PTFE and ePTFE have found numerous biomaterial applications. Disorders of the cardiovascular system are classified into those primarily affecting the blood, heart, or blood vessels. Atherosclerosis is a pathological condition that is central to cardiovascular disorders and is characterized by a build-up of plaque on the interior surface of the coronary arteries. The decrease in luminal diameter that is caused by atherosclerosis results in reduced blood flow and poor cardiac performance. Ultimately atherosclerosis can lead

Medicines: Polymers for 1681

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The fluorine atoms of PTFL prefer their own kind, drawing to each other, while repelling any other kind of molecule, like this water molecule, for example.

H O

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to cardiac ischemia and myocardial infarction. The significance of coronary artery disease is that it is the most common cause of premature mortality in the Western world with total deaths of 750,000 from cardiovascular disease in the USA in 2009. The repair and regeneration of coronary blood vessels as well as restoration of blood flow are the primary focus of biomaterial science and the devices used in these approaches include vascular grafts, stents, and rotary blood pumps. Synthetic vascular grafts are used to provide a bypass of occluded arteries in coronary artery bypass graft surgery as well as the replacement of dilated aortic aneurysms. Commercial ePTFE-based prosthetic vascular grafts with an internal diameter >6 mm have proved to be successful in bypass grafts and are in routine clinical use. The current clinical challenge is the supply of blood vessels with an internal diameter 0, with Gm being the free energy of i mixture and ci the composition of component i, there is only one phase providing a homogenous system. However, between points a′ and b′, where ∂Gm2 / ∂c2 < 0, the system i spontaneously separates into two phases. Points a and b, known as binodal, represent the situation in which the two separated phases are in equilibrium at a desired temperature. Precisely, each one of these points is the locus where the first derivative of Gm to ci is zero, or ∂Gm / ∂c = 0.[11] i The curve defined by points a and b as a function of temperature (T) is called binodal curve. If the temperature of a homogenous system, with composition between points a and b, is changed from T0 to T1, two phases will be formed and, when equilibrium is attained, the final compositions of phases will be a and b. As discussed later, the mechanism from which the phase separation will occur depends on the locus in which the process occurs: spinodal decomposition (between a′ and b′) or nucleation and growth (between a and a′ or b and b′).[12–14] a a΄

ΔGm

b΄ b

(1)

where R is the universal gas constant; T is the absolute ­temperature; ni and φi are mol number and volumetric ­fraction of the i-esim component, respectively. χ12 (u2) is the general function of non-solvent (1) and solvent (2) interaction dependent on the volumetric fraction u2 = φ 2 / (φ1 + φ 2) of a pseudobinary mixture. The solvent (2) and polymer (3) interaction parameter, χ23, is an interaction function and depends on concentration or χ23(φ3). ­Furthermore, the interaction parameter for the non-solvent (1) and p­ olymer  (3), χ13, is considered constant. In fact, determining χ13 is very hard; however, this can be achieved by swelling measurements and literature data of ­mixture heats, or vapor sorption. χ12 can be calculated from a­ ctivity data or literature mixture heats, and also by vapor s­ orption of solvent/non-solvent, spreading light experiments, osmometry or intrinsic viscosity measurements can be employed for obtaining χ23.[17,18] Since the immersion-precipitation is an isothermal ­process, for a ternary system, the phase diagram can be drawn at constant temperature and the binodal and spinodal are now lines instead of points (Fig. 2). Phase diagram is a description of an equilibrium state; it reflects whether the conditions under a multicomponent mixture are stable in a homogeneous single phase or lie in two or more phases. The triangle corners represent the three pure components—polymer, solvent, and non-solvent— whereas each edge is related to the respective binary mixture. Any point inside the diagram (not in the corner or

T T0

T0

Polymer

U

Vitrification line

CP T1

T1

RP M

0



1

Component 1 concentration

0

S

a a΄

Sol-gel transition line

M

b΄ b

Binodal line

1 CP

Component 1 concentration

Fig. 1  Schematic representation of the binary phase diagram (ΔGm versus composition), indicating the stable (S), metastable (M), and unstable (U) regions as well as critical (PC), bimodal, and spinodal points

Solvent

PP

Non-solvent

Tie line

Fig. 2  Binodal and spinodal lines on a ternary phase diagram

Laboratory– Membranes

presented. Hence, some papers describe the variation in the three-component system, also establishing relations between the determining factors of wet-phase inversion and resulting morphology, also described. Finally, recent developments in this area are discussed.

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Membrane: Preparation by Nonsolvent-Induced Phase Inversion

Laboratory– Membranes

in the edge) is related to a specific ternary composition. The ternary phase diagram consists of two regions: a homogeneous (monophasic) region and a heterogeneous (­biphasic) one, where the system separates into two phases, a rich-polymer phase (RP) and a lean-polymer phase (LP), which are in equilibrium.[19–21] As already pointed out, the binodal line represents the border between homogeneous and heterogeneous situations. Moreover, there is a metastable area between the spinodal and binodal lines in very small polymer concentrations. The tie line connects points ­representing the compositions of the two phases in a phase diagram that is in equilibrium. The two compositions (LP and RP) coalesce to an identical composition at the critical point. The volumetric proportion between the two phases is given by the lever rule.[22] From the Flory–Huggins model, it is possible to p­ redict the size and location of biphasic and monophasic regions, which are dependent on the molar component volume and on the interaction parameter.[23] If χ23 has a high value, which corresponds to the low affinity between ­polymer and solvent, the separation region magnitude can be increased, as when there is great compatibility between solvent and non-solvent (small values of χ12). For solvent and non-­solvent mixtures with low compatibility (high values of χ12), there will be huge differences in solvent and non-solvent proportions in the phases in equilibrium. The polymer–non-solvent interaction parameter also affects the area of separation in two phases determination. High values of χ13, or low compatibility between polymer and non-solvent, make the intersection point of the biphasic region with the edge polymer–non-solvent be located in very high polymer concentrations. For a strict binary ­system, the binodal location can also be obtained from transmission light experiments or cloud point measurements.[24] For a non-strict binary system, the cloud point curve and binodal curve can be quite different.[25] The structure of membranes obtained by phase ­inversion is a result of a variation in miscibility ­conditions. Stable solutions, initially stable and homogeneous, are brought to a non-stable state, which occurs when the amount of non-solvent added to the system is sufficient to cause L–L phase separation. This separation happens after the overall composition crosses the binodal line toward the ­heterogeneous area.[26–28] For understanding the L–L phase separation related to polymer and solvent mass fractions, again, the Flory– Huggins model is resorted to this time, from a modified expression by Koningsveld for the free energy mixture:

( )

(

)

( )

∆Gm RT = Wo ln Wo + ∑ M o Mi Wi ln Wi +g

∑ WW i



(2)

o

where ∆Gm is the free energy of mixture, R is the ­universal gas constant, T is the absolute temperature, Wo is the ­solvent fraction, Wi is the mass fraction of polymeric specimens

i, Mo is the solvent molar mass, Mi is the molar mass of polymeric specimens i and g is the interaction parameter that depends on composition, temperature, and pressure. The free energy of the mixture (the two first terms on the right side) is small in comparison with the small molecular weight of substances in the free energy of the mixture, because factor Mo /Mi is small for polymers in solution. A  small free energy of mixture represented by the term g ∑WiWo can, in several cases, be the driven force for the phase separation.[28] There are two different paths for the L–L phase separation to begin: (a) by nucleation and growth and (b) by spinodal decomposition. By nucleation and growth, the initially homogeneous solution separates itself into RP and LP. Entering the metastable region, the area between the binodal and spinodal curves, nuclei are formed in the bulk polymer solution. Each nucleus, together with other nuclei randomly dispersed, will grow until they touch each other and coalesce, situation in which thermodynamic equilibrium is reached or until their growth stops due to viscous effects of the surrounding polymer solution.[29] There are two possibilities for phase nucleation, which depend on the polymer solution initial composition with respect to the critical point: for polymer concentrations above the critical polymer concentration, the nucleus will be composed of a lean polymer phase and, in the other case, the nucleus will be composed of RP. Once the forming solution normally has at least 10 wt% polymer concentration, the L–L phase separation occurs by lean polymer phase nucleation and growth and the RP will be continuous, that is, the forming nucleus grows at the expense of the solvent of the ­surrounding phase, having high solvent ­concentration.[30–33] On the other hand, spinodal decomposition occurs when the system enters an unstable region of the phase diagram (the region given between point a′ and b′ in Fig. 1). For these compositions, the solution is not stable due to infinitesimal fluctuations in concentration. In this region, the concentration fluctuation amplitude grows over time, separating spontaneously (by molecules diffusion) in ­interconnected regions of RP and LP, resulting in a net structure. This separation type would occur if non-solvent could diffuse so fast as to pass the spinodal line before nucleation begins.[27,32,34] Alternatively, there is a second type of phase separation which is gelation. Normally, this separation occurs with highly concentrated solution and forms a three-­ dimensional network, going from the sol state to the gel state. A gel is defined as a three-dimensional extremely viscous lose network, coherent, soft, and elastic structure which contains predominantly liquid. Besides the polymeric chains, a bit of solvent and maybe a bit of non-solvent is immobilized in the gel structure.[35] Gelation can be of the thermoreversible type for amorphous polymer or have microcrystallites linking the whole structure, in the case of crystalline or semicrystalline polymers.[36,37] Generally, skin formation can occur by gelation, the process which

constrains the L–L phase separation by kinetic limitations caused by the high system viscosity, forming a molecularly interconnected net.[26] Membrane solidification can occur by vitrification (amorphous polymer) or crystallization (semicrystalline polymers). By vitrification, the RP that surrounds the nucleus of lean polymer phase solidifies. In the case of crystallization, ordinate agglomerates are formed since, in this situation, the polymer molecules have low Gibbs free energy. Frequently, with crystallization, separation is referred to as solid–liquid phase separation.[27] Kinetics Phase diagram has already been mentioned to represent the description of an equilibrium state. However, in a macromolecular system, true equilibrium is known never to be attained and phase separation to be strongly governed by kinetic parameters, which implies that mass transfer determines the membrane structure, expressed by the solvent exchange rate which goes into coagulation bath and non-solvent and diffuses in the polymer solution, in the interface of the polymer solution and coagulation medium. In polymer systems, where a structural network of polymer chains is gradually formed during phase separation, metastable states are close and the equilibrium between the two phases cannot be held by a precipitation process.[38,39] Mass transfer rate is determined by diffusion coefficients of various low-molecular-mass components.[21] Once more, to facilitate the understanding of the kinetics of the inversion phase process, kinetics of a binary system with limited miscibility can be resorted to. Three states are possible in this system. A stable state with a homogeneous solution in only one phase is kinetically defined by a total diffusion coefficient, D, higher than zero (D > 0). An unstable state, in which an initially stable solution spontaneously separates into two phases in the miscibility region, will go back to equilibrium, and has D  4.8 and pH  4.8, –COOH groups of the grafted AAc polymer side chains are ­ionized or dissociated into carboxylate ions, which results in an expanded conformation due to electrostatic repulsive forces between similar charges causing blocking of the pores of the ­membrane. On the other hand, at pH  pKa.[40,41] The pH of the solution also influences the separation of glucose. At an alkaline pH, PAAc nanobrushes deprotonate and thus swell, which results in a decrease in permeate flux and glucose passage. In an acidic pH (i.e., pH 3.15), AAc moiety exists in collapsed conformation, resulting in greater glucose passage. Pore blocking of the AAm-grafted membrane at higher pH is due to the H-bond formation between AAm (H-atoms connected to N-atoms) and OH– ions in the solution.[42,43] It results in extended chain conformation and pore size reduction due to better swelling behavior for PVDF-AAm membrane.[9] The reversibility in pH-responsive behavior also reflects even after 12 cycles. Apart from AAc and its derivatives, nylong-NIPAAm2-b-PDMAEMA and nylon-g-PDMAEMA2b-NIPAAm membranes also reflect pH response.[44] The partial protonation of pendant amine groups results in more extended conformation at pH  7.5 because of the deprotonation of PDMAEMA, [45] which results in an increase in

In the absence of glucose, the linear graft PAAc chains dissociate and become negatively charged, and ­therefore, the membrane gates are “closed”; however, when glucose concentration increases, GOD takes the role (Scheme 2). Due to the formation of gluconic acid, the pH of the ­solution gets reduced; thus, the grafted carboxylate groups become protonated and gates opened because of the chain shrinkage. Lower rejection of sucrose is observed at higher pH due to the stronger interaction (H-bonding interaction) of sucrose with negatively charged polymer chains, whereas the interaction of glucose is more frequent with the ­neutral PAAc chains. Thus, the rejection of glucose is lower at low pH.[40] pH-dependent permeabilities of vitamin B12 through PE-g-PMAAc [47] and caffeine through PVDF-gPAAc [48] membranes are also observed. At low pH (2.5), the ­dissolution rate of caffeine is accelerated and the release rate is faster because PAAc chains are in collapse ­conformation. The pH-responsive behavior also has its influence on the KCl permeation study. The diffusion coefficient is higher at pH 3.0 compared with that at pH 9.0 for MAAc graft polymer for KCl permeation. The diffusion permeability of KCl [plot of ln (Cf − Ci)/(Cf − Ct) against time t] through the graft membrane decreases dramatically when pH increases from 3 to 9 (Fig. 4).[49] The PMAAc chains are dissociated at higher pH (9.0), and the diffusion of salt is obstructing by the electrostatic repulsion between the dissociated polymer chains and salt ions. Thermo-responsive Grafting of thermo-responsive moieties/gels turns the membranes thermo-responsive in their behavior. Poly-­ NIPAAm (PNIPAAm) is the brightest example in this

Membranes: Graft Modification of Polymers for 1719

Linear grafted PAAc chains

Asymmetric porous membrane structure Glucose [Glucose] pH [Glucose] pH Open pores

Immobilized GOD (Closed pores)

2.5 pH = 3 pH = 9

ln[(Cf –Ci)/(Cf –Ct)]

2.0

D = 22.1590× 10 7cm2 s–1 1.5 1.0 0.5

D = 8.8305 × 10 7cm2 s–1

0.0 0

50

100 150 Time (min)

200

250

Fig. 4  Effect of pH on the diffusional permeation of KCl through the MAAc-grafted membrane prepared in 3 wt% monomer solution [49]

regard. The domination of hydrophilic (>NH, >CO) and hydrophobic (isopropyl) characters makes them thermo-­ responsive. The hydrophobicity of the gel ­surface is drastically different below and above the c­ haracteristic temperature [liquid critical solution temperature (LCST) (32°C)]. The formation of H-bonds between the ­polymer networks and water molecules and the hydrophobic

interactions are the possible reasons for transition.[50] The hydrophobicity–­hydrophilicity swing is the reason for ­temperature dependence of NIPAAm-grafted membrane. The concept is depicted in Scheme 3. The hydrophilic ­solutes pass through the membrane pores, whereas the hydrophobic ones adsorb because of the hydrophobic ­interaction at temperatures above the LCST. However, at temperatures below the LCST, the pore surface is hydrophilic and thus hydrophobic solutes desorb and concentrate in the permeation side.[51] The water flux increases with temperature for PAN-gNIPAAm, poly(tetrafluoroethylene) (PTFE)-g-PGMA/ NIPAAm, and nylon-g-PDEAAm membranes.[52–54] The water flow rate depends slightly on the temperature  butyrylFUdR > pentanoyl-FUdR. The release of butyryl- FUdR and pentanoyl-FUdR from the spheres consisting of low-molecular-weight polymer (Mw = 65,000 kDa) was faster than that from the spheres of higher molecular weight (Mw = 135,000 or 450,000 kDa). The effect of drug content on the release rate was also studied. The higher the drug content in the PHB microspheres, the faster was the drug release. The release of FUdR continued for more than 5 days.[71] Kassab A.C. developed a well-managed technique for the manufacture of PHB microspheres loaded with drugs. Microspheres were obtained within a size of 5–100 µm using a solvent evaporation method by ­changing the initial polymer/solvent ratio, emulsifier concentration, s­ tirring rate, and initial drug concentration. Very high drug l­oading of up to 408 g rifampicin/g PHB was achieved. Drug release rates were rapid: the maximal duration of rifampicin delivery was 5 days. Both the size and drug content of

PHB microspheres were found to be effective in ­controlling the drug release from polymer microspheres.[149] The sustained release of analgesic drug, tramadol, from PHB microspheres was demonstrated by Salman M.A. et al. It was shown that 58% of the tramadol (the initial drug content in PHB matrix = 18%) was released from the microspheres (7.5 µm in diameter) in the first 24 h. Drug release decreased with time. From 2 to 7 days, the drug release was with zero-order rate. The entire amount of ­tramadol was released after 7 days.[167] The kinetics of different drug release from PHB m ­ icroand nanoparticles loaded with dipyridamole, ­indomethacin, and paclitaxel was studied [160,161,164,168,169] (Fig. 4). It was found that the release occurs via two m ­ echanisms, diffusion and degradation, operating simultaneously. Vasodilator and antithrombotic drug, dipyridamole, and anti-inflammatory drug, indomethacin, diffusion processes determine the rate of the release at the early stages of the contact of the system with the environment (the first 6–8 days). The coefficient of the release diffusion of a drug depends on its nature, the thickness of the PHB films containing the drug, the weight ratio of dipyridamole and indomethacin in polymer, and the molecular weight of PHB. Thus, it is possible to regulate the rate of drug release by changing the molecular weight of PHB, for example.[164] The biodegradable microspheres on the base of PHB designed for controlled release of dipyridamole and paclitaxel were kinetically studied. The profiles of release from the microspheres with different diameters present the progression of nonlinear and linear stages. Diffusion kinetic equation describing both linear (PHB hydrolysis) and nonlinear (diffusion) stages of the dipyridamole and paclitaxel release profiles from the spherical subjects has been written down as the sum of two terms: desorption from the homogeneous sphere in accordance with diffusion mechanism and the zero-­ order release. In contrast to the diffusivity dependence on microsphere size, the constant characteristics of linearity

The number of mice

10 8 6 4 2 0

00030

10 µm

0

10 20 Days after tumor transplantation

Control Microparticles 600 mg/kg Paclitaxel 30 mg/kg

30

Microparticles 300 mg/kg Microparticles 1200 mg/kg

Fig. 4  PHB microspheres loaded with paclitaxel for antitumor treatment: (a) electron microscopy photography images of PHB ­m icroparticles loaded with paclitaxel, SEM, ×500; (b) the survival of mice with i.p. LLC cancer model after treatment of microparticles with the effective doses [141]

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(a)

(b)

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NL D4.7 ×250

300 um

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NL D5.1 ×3.0k 30 um

Fig. 5  Growth of rat bone-marrow MSCs on PHB porous microspheres: (a) SEM, ×250; (b) SEM, ×3,000 (Bonartsev, unpublished data)

are scarcely affected by the diameter of PHB microparticles. The view of the kinetic profiles as well as the low rate of ­dipyridamole and paclitaxel release are in satisfactory agreement with k­ inetics of weight loss measured in vitro for the PHB films and observed qualitatively for PHB microspheres. Taking into account the kinetic results, it was supposed that the degradation of PHB microspheres is responsible for the linear stage of dipyridamole and ­paclitaxel release profiles.[160,161,168,169] The biocompatibility and pharmacological activity of advanced drug delivery systems on the base of PHB was studied.[71,148,167–169] It was shown that implanted PHB microspheres loaded with paclitaxel caused the mild tissue reaction. The inflammation accompanying implantation of PHB matrices is temporary and additionally the toxicity relative to normal tissues is minimal.[169] No signs of toxicity were observed after the administration of PHB microspheres loaded with analgesic, tramadol.[167] A single intraperitoneal injection of PHB microspheres containing anticancer prodrugs, butyryl-FUdR and pentanoyl-FUdR, resulted in high antitumor effects against P388 leukemia in mice over a period of 5 days.[118] Embolization with PHB microspheres in vivo at dogs as test animals has been studied by Kasab et al. Renal angiograms obtained before and after embolization and also the histopathological observations showed the feasibility of using these microspheres as an alternative chemoembolization agent.[148] Epidural analgesic effects of tramadol released from PHB microspheres were observed for 21 h, whereas an equal dose of free tramadol was effective for less than 5 h. It was suggested that the controlled release of tramadol from PHB m ­ icrospheres in vivo is possible, and pain relief during epidural a­ nalgesia is prolonged by this drug formulation compared with free tramadol.[167] PHB microspheres loaded with low-molecular drugs (e.g., simvastatin) or bioactive proteins (e.g., BMP-2) that regulate growth and differentiation of MSCs can also be used as carriers for MSCs for tissue engineering (Fig. 5). It was shown that MSCs attached and proliferated on the

surface of porous PHB microspheres with diameter more than 500 μm (Bonartsev, unpublished data). The observed data indicate the wide prospects in ­applications of drug-loaded medical devices and ­microspheres on the base of PHB as implantable and injectable therapeutic systems in medicine for treatment of ­various diseases: cancer, cardiovascular diseases, ­tuberculosis, osteomyelitis, arthritis etc.[1–3] Finally, it can be observed that PHB is a biomaterial of varied and wide range of applications in bioengineering and medicine, which can create multidisciplinary ­network in biomedical science and technology. Nowadays, the study of such multi-applied biomaterials, e.g., bioactive glass, [107,182,183] biodegradable metals, [184] electroconducting polymers, [185] multifunctional proteins and peptides, [5,186] dendrimers, [187] polyplexes, [6] and hydrogels [143,183] develops intensively. But PHB is also a natural bacterial-origin biopolymer with specific functions in humans, [53–65] which requires a more in-depth study of its biomimetic features that should be taken into account in biomedical application of this biomaterial. ACKNOWLEDGMENTS This work was supported by RFBR grant, project No 15-29-04856. REFERENCES 1. Jenkins, M.; Ed.; Biomedical Polymers; University of ­Birmingham: Birmingham, 2007. 2. Manavitehrani, I.; Fathi, A.; Badr, H.; Daly, S.; ­Shirazi, A.N.; Dehghani, F. Biomedical applications of ­biodegradable polyesters. Polymers 2016, 8 (1), 20. 3. Mokhtarzadeh, A.; Alibakhshi, A.; Hejazi, M.; Omidi, Y.; Dolatabadi, J.E.N. Bacterial-derived biopolymers: Advanced natural nanomaterials for drug delivery and ­tissue engineering. Trends Anal. Chem. 2016, 82, 367–384.

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160. Bonartsev, A.P.; Yakovlev, S.G.; Filatova, E.V.; Soboleva, G.M.; Makhina, T.K.; Bonartseva, G.A.; Shaitan, K.V.; Popov, V.O.; Kirpichnikov, M.P. Sustained release of the antitumor drug paclitaxel from poly(3-hydroxybutyrate)-based microspheres. Biomed. Khim. 2012, 6 (1), 42–47. 161. Yakovlev, S.G.; Bonartsev, A.P.; Boskhomdzhiev, A.P.; Bagrov, D.V.; Efremov, Y.M.; Filatova, E.V.; Ivanov, P.V.; Mahina, T.K.; Bonartseva, G.A. In vitro cytotoxic a­ ctivity of poly(3-hydroxybutyrate) nanoparticles loaded with antitumor drug paclitaxel. In Technical Proceedings of the 2012 NSTI Nanotechnology Conference and Expo; NSTI-Nanotech 2012, Santa Clara, CA, June 18–21; NSTI: Santa Clara, CA, 2012, 190–193. 162. Shishatskaya, E.I.; Goreva, A.V.; Voinova, O.N.; Inzhevatkin, E.V.; Khlebopros, R.G.; Volova, T.G. Evaluation of antitumor activity of rubomycin deposited in absorbable polymeric microparticles. Bull. Exp. Biol. Med. 2008, 145 (3), 358–361. 163. Filatova, E.V.; Yakovlev, S.G.; Bonartsev, A.P.; Mahina, T.K.; Myshkina, V.L.; Bonartseva, G.A. ­Prolonged release of chlorambucil and etoposide from poly-3-oxybutyrate-based microspheres. Appl. Biochem. Microbiol. 2012, 48 (6), 598–602. 164. Bonartsev, A.P.; Bonartseva, G.A.; Makhina, T.K.; Mashkina, V.L.; Luchinina, E.S.; Livshits, V.A.; Boskhomdzhiev, A.P.; Markin, V.S.; Iordanskii, A.L. New poly-(3-hydroxybutyrate)-based systems for controlled release of dipyridamole and indomethacin. Appl. Biochem. Microbiol. 2006, 42 (6), 625–630. 165. Coimbra, P.A.; De Sousa, H.C.; Gil, M.H. ­Preparation and characterization of flurbiprofen-loaded poly(3microspheres. hydroxybutyrate-­co-3-hydroxyvalerate) J. Microencapsul. 2008, 25 (3), 170–178. 166. Wang, C.; Ye, W.; Zheng, Y.; Liu, X.; Tong, Z. Fabrication of drug-loaded biodegradable microcapsules for controlled release by combination of solvent evaporation and layer-by-layer self-assembly. Int. J. Pharm. 2007, 338 (1–2), 165–173. 167. Salman, M.A.; Sahin, A.; Onur, M.A.; Oge, K.; Kassab, A.; Aypar, U. Tramadol encapsulated into polyhydroxybutyrate microspheres: in vitro release and epidural analgesic effect in rats. Acta Anaesthesiol. Scand. 2003, 47, 1006–1012. 168. Bonartsev, A.P.; Livshits, V.A.; Makhina, T.A.; Myshkina, V.L.; Bonartseva, G.A.; Iordanskii, A.L. Controlled release profiles of dipyridamole from biodegradable microspheres on the base of poly(3-hydroxybutyrate). Express Polym. Lett. 2007, 1 (12), 797–803. 169. Livshits, V.A.; Bonartsev, A.P.; Iordanskii, A.L.; Ivanov, E.A.; Makhina, T.A.; Myshkina, V.L.; Bonartseva, G.A. Microspheres based on poly(3-hydroxy)butyrate for prolonged drug release. Polym. Sci. Ser. B 2009, 51 (7–8), 256–263. 170. Bonartsev, A.P.; Postnikov, A.B.; Myshkina, V.L.; Artemieva, M.M.; Medvedeva, N.A. A new system of nitric oxide donor prolonged delivery on basis of controlled-release polymer, polyhydroxybutyrate. Am. J. Hypertens. 2005, 18 (5A), A51. 171. Bonartsev, A.P.; Postnikov, A.B.; Mahina, T.K.; Myshkina, V.L.; Voinova, V.V.; Boskhomdzhiev, A.P.; Livshits, V.A.; Bonartseva, G.A.; Iorganskii, A.L. A new in vivo model of

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prolonged local nitric oxide action on arteries on basis of biocompatible polymer. J. Clin. Hypertens. 2007, 9 (Suppl. A, 5), A152. Riekes, M.K.; Junior, L.R.; Pereira, R.N.; Borba, P.A.; Fernandes, D.; Stulzer, H.K. Development and evaluation of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) and polycaprolactone microparticles of nimodipine. Curr. Pharm. Des. 2013, 19 (41), 7264–7270. Bazzo, G.C.; Caetano, D.B.; Boch, M.L.; Mosca, M.; Branco, L.C.; Zétola, M.; Pereira, E.M.; Pezzini, B.R. Enhancement of felodipine dissolution rate through its incorporation into Eudragit® E-PHB polymeric microparticles: In vitro characterization and investigation of absorption in rats. J. Pharm. Sci. 2012, 101 (4), 1518–1523. Zhu, X.H.; Wang, C.H.; Tong, Y.W. In vitro characterization of hepatocyte growth factor release from PHBV/PLGA microsphere scaffold. J. Biomed. Mater. Res A 2009, 89 (2), 411–423. Parlane, N.A.; Grage, K.; Mifune, J.; Basaraba, R.J.; Wedlock, D.N.; Rehm, B.H.; Buddle, B.M. Vaccines ­displaying mycobacterial proteins on biopolyester beads stimulate ­cellular immunity and induce protection against tuberculosis. Clin. Vaccine Immunol. 2012, 19 (1), 37–44. Yilgor, P.; Tuzlakoglu, K.; Reis, R.L.; Hasirci, N.; Hasirci, V. Incorporation of a sequential BMP-2/BMP-7 delivery system into chitosan-based scaffolds for bone tissue engineering. Biomaterials 2009, 30 (21), 3551–3559. Stefanescu, E.A.; Stefanescu, C.; Negulescu, I.I. Biodegradable polymeric capsules obtained via room temperature spray drying: Preparation and characterization. J. Biomater. Appl. 2011, 25 (8), 825–849. Costa, M.S.; Duarte, A.R.; Cardoso, M.M.; Duarte, C.M. Supercritical antisolvent precipitation of PHBV ­m icroparticles. Int. J. Pharm. 2007, 328 (1), 72–77. Errico, C.; Bartoli, C.; Chiellini, F.; Chiellini, E. Poly (hydroxyalkanoates)-based polymeric nanoparticles for drug delivery. J. Biomed. Biotechnol. 2009, 2009, 571702. Althuri, A.; Mathew, J.; Sindhu, R.; Banerjee, R.; Pandey, A.; Binod, P. Microbial synthesis of poly-3-­hydroxybutyrate and its application as targeted drug delivery vehicle. ­Bioresour. Technol. 2013, 145, 290–296. Pouton, C.W.; Akhtar, S. Biosynthetic polyhydroxyalkanoates and their potential in drug delivery. Adv. Drug Deliver. Rev. 1996, 18, 133–162. Ramiro-Gutiérrez, M.L.; Will, J.; Boccaccini, A.R.; DíazCuenca, A. Reticulated bioactive scaffolds with improved textural properties for bone tissue engineering: nanostructured surfaces and porosity. J. Biomed. Mater. Res. A. 2014, 102 (9), 2982–2992. Douglas, T.E.; Krawczyk, G.; Pamula, E.; Declercq, H.A.; Schaubroeck, D.; Bucko, M.M.; Balcaen, L.; Van Der Voort, P.; Bliznuk, V.; van den Vreken, N.M.; Dash, M.; Detsch, R.; Boccaccini, A.R.; Vanhaecke, F.; Cornelissen, M.; Dubruel, P. Generation of composites for bone tissue-engineering applications consisting of gellan gum hydrogels mineralized with calcium and magnesium phosphate phases by enzymatic means. J. Tissue Eng. Regen. Med. 2016, 10 (11), 938–954. Zhang, Y.; Xu, J.; Ruan, Y.C.; Yu, M.K.; O’Laughlin, M.; Wise, H.; Chen, D.; Tian, L.; Shi, D.; Wang, J.; Chen, S.; Feng, J.Q.; Chow, D.H.; Xie, X.; Zheng, L.; Huang, L.;

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Huang, S.; Leung, K.; Lu, N.; Zhao, L.; Li, H.; Zhao, D.; Guo, X.; Chan, K.; Witte, F.; Chan, H.C.; Zheng, Y.; Qin, L. Implant-derived magnesium induces local neuronal production of CGRP to improve bone-fracture healing in rats. Nat. Med. 2016, 22 (10), 1160–1169. 1 85. Kasparkova, V.; Humpolicek, P.; Capakova, Z.; Bober, P.; Stejskal, J.; Trchova, M.; Rejmontova, P.; Junkar, I.; Lehocky, M.; Mozetic, M. Cell-compatible conducting polyaniline films prepared in colloidal dispersion mode. Colloids Surf. B. Biointerfaces 2017, 157, 309–316.

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186. Kuan, S.L.; Wang, T.; Weil, T. Site-selective disulfide modification of proteins: Expanding diversity beyond the ­proteome. Chemistry 2016, 22 (48), 17112–17129. 187. He, X.; Alves, C.S.; Oliveira, N.; Rodrigues, J.; Zhu, J.; Banyai, I.; Tomas, H.; Shi, X. RGD peptide-modified ­multifunctional dendrimer platform for drug encapsulation and targeted inhibition of cancer cells. Colloids Surf. B Biointerfaces 2015, 125, 82–89.

Polymers and Polymeric Membranes Zeenat Arif, P.K. Mishra, and S.N. Upadhyay Department of Chemical Engineering and Technology, Indian Institute of Technology (Banaras Hindu University), Varanasi, India Petroleum–Rapid Prototyping

Rajeev Mehta Chemical Engineering Department, Thapar Institute of Engineering and Technology University, Patiala, India

Abstract Controlled drug delivery (CDD) permits the uniform concentration of drugs and at the same time maintains the plasma concentration within the therapeutic range. This specific property of CDDs minimizes the side effects along with the need for the frequent dosing of drugs. The basic function of controlled release is to alter the pharmacokinetic and pharmacodynamic properties to avoid frequent dosing. Controlled delivery is achieved when polymer is combined with drugs. Polymers are used as matrix or composites for these systems and are being extensively used because of their unique bulk and surface properties. These polymers are broadly classified into natural and synthetic polymers, which are further subdivided into biodegradable and nonbiodegradable polymers. Various mathematical models are used to define the drug release from these systems. The concept of smart polymer provides a platform to deliver drug at an appropriate time at a controlled rate in the stable and biologically active form to the specific site of action. The advent of these polymers permits programmable drug delivery. They are responsive to environmental changes such as ­temperature, pressure, and pH; hence, they are effectively used for targeted drug delivery. Keywords: Biodegradable; Dosage; Pharmacokinetic; Smart polymer; Therapeutic.

NEED FOR CONTROLLED DRUG DELIVERY SYSTEMS The delivery of an active drug molecule into the body and maintaining its level within the therapeutic range by an appropriate technology or approach is called as drug delivery. Conventionally, the drug is either injected or ingested. In conventional systems, drug concentration will fluctuate when administered into the patient’s body. The typical drug concentration profile as shown in Fig. 1 varies in a cyclic manner. The conventional drug delivery methods suffer from several limitations:[1] • • • • •

Only the total mass of the drug delivered to the patient is controlled. It is difficult to attain a steady-state condition. Undermedication or overmedication is caused due to unavoidable fluctuations in the drug concentration. Periodic dosing of drug is required. Adverse effects are caused due to drug concentration fluctuation.

The fluctuation in drug concentration in bloodstreams and tissues produces unwanted toxicity and poor efficiency.[2] Encyclopedia of Polymer Applications, First Edition DOI: 10.1201/9781351019422-140000358 Copyright © 2018 by Taylor & Francis. All rights reserved.

So, it is important to maintain the drug concentration at the required therapeutic level for the effective treatment.[3] Controlled drug delivery (CDD) devices [4] help in overcoming the limitation and improving the ­efficiency of drug therapy. The term “controlled release” implies the predictability and reproducibility of kinetics of release rate, i.e., drug ingredients released from a CDD device should have a rate profile that is predictable kinetically as well as ­reproducible from one to another dose. Controlled Drug Delivery CDD is one of the growing fields in the area of health care and pharmaceutics. It offers numerous advantages such as improved efficacy, reduced toxicity, and improved patient compliance and convenience.[5] This improvement leads to an increase in the therapeutic activity without any increase in the intensity of side effects and eliminates the periodic administration of drug. The concentration profiles of conventional drug delivery versus CDD methods are shown in Fig. 2. It is seen that CDD permits a constant and steady level of drug in contrast to conventional drug delivery where a periodic dosing is required which causes drug ­concentration above and below the therapeutic level.

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Such systems often use polymer-based devices for controlled and effective transfer of drug. In CDD systems, a barrier (quite often a polymeric membrane) is used to ­regulate the rate of drug delivery to the body.

High

MEC

Low

Low Time

Fig. 1  Drug concentration fluctuation with time

Conventional release Plasma drug level

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Concentration

High

Single dose of a conventional tablet Maximum desired level

Controlled release Minimum desired level Time Multiple dosing of a conventional dosage

Fig. 2  Conventional drug versus controlled release drug ­concentration profile [8]

Scope of Polymer System Development in the field of polymer science and technology has resulted in the evolution of various novel drug delivery systems. Release of an active component forms the material in a predesigned manner when a polymer (natural or synthetic) is judiciously combined with an active agent in the CDD system. The characteristics of polymers that make them flexible in drug delivery systems include their wide molecular weight distributions, variation in viscoelastic properties, contraction capability when heated, ­specialized chemical reactivity, variety of d­ issolution times, and ­tolerance to a variety of manufacturing methods. Three key advantages that polymer-based drug delivery systems offer are as follows:[11] • • •

Localized delivery of drug. Sustained delivery of drugs. Stabilization of the drug.

SELECTION OF POLYMER The selection of polymer for designing an efficient drug delivery device is a challenging task and requires a thorough understanding of the polymer bulk property. It is dependent on the specific physicochemical property as well as biocompatibility. The following characteristics, in particular, need to be considered while selecting polymer for CDDs:

The aim of all CDD devices is to improve the effectiveness of drug.[6,7] The major advantages of controlled drug release devices vis-à-vis conventional drug administration methods are as follows:[9,10]

•

Elimination of over- or under-dosing. Maintaining drug levels within the desired range. Avoiding frequent dosing and permitting less dosing. Increased patient compliance. Prevention of side effects. Site-specific targeting. Increase in contact time, hence improved bioavailability.

•

Their major disadvantages are as follows:

Other factors that need to be taken into account are as follows:

• • • • • • •

• • • •

Higher cost per unit dose compared to conventional doses. Dose dumping of administered drug having long half-life. Less accuracy in dose adjustment for the drugs having a narrow therapeutic index. Lower bioavailability under high first-pass clearance.

• •

• • • •

• •

It should be versatile. It should possess a wide range of mechanical, physical, and chemical properties. It should be nontoxic in nature and should possess high mechanical strength. It is expensive and easy to construct. It should be compatible with the environment and inert to the host tissue. It must have a finite molecular weight. It must be compatible with the biological environment. Polymer matrix should provide a good drug polymer linkage.

Use of homopolymers with single monomeric repeating unit. Copolymers with multiple monomer species.

The properties of the selected polymer for the drug delivery system will influence its performance. Large volume of

literatures is available on the physical–chemical characteristics of these polymers, and on their ­biodegradation and release properties. Polymers Used in CDD Techniques including compression, dip coating, and encapsulation have been in use for the past 50 years by the pharmaceutical industry for incorporating the bioactive agents within the polymer.[12] Polymeric delivery systems are mainly developed to achieve temporal or spatial control of drug delivery.[13] Table 1 enlists different types of polymers that have been investigated for drug delivery applications, and these are broadly classified as biodegradable and ­nonbiodegradable polymers. Natural polymers are biodegradable and possess excellent biocompatibility, but variations exist from batch to batch because of difficulties in purification. Synthetic polymers are those that are available easily with readily adjustable properties.[14] Synthetic biodegradable polymers are mainly used in scaffolds for tissue regeneration and as adhesives and hemostats. The advantage of using such polymers is that the release rate is less dependent on the drug properties with a steady-state release rate with time. The disadvantage of nondegradable polymers is that a surgery is required to remove base polymers out of the body once depleted.[11] Due to the inherent limitations of physicochemical characteristics of natural materials, there has been significant

Table 1  General classification of polymers used for the drug delivery systems Classification Polymer Natural polymer Protein-based polymers

Collagen, albumin, gelatin

Polysaccharides

Agarose, alginate, carrageenan

Synthetic polymer Biodegradable Polyester

PLA, PGA, poly(hydroxy butyrate)

Polyanhydrides

Poly(sebacic acid), poly(adipic acid), poly(terephthalic acid)

Polyamides

Poly(imino carbonates), polyamino acids

Phosphorous-based polymers

Polyphosphates, polyphosphonates, polyphosphazenes

Nonbiodegradable Cellulose derivatives

Carboxymethyl cellulose, ethyl cellulose, cellulose acetate

Silicones

Polydimethylsiloxane, colloidal silica

Acrylic polymers

Polymethacrylates, poly(methyl methacrylate), polyhydro(ethylmethacrylate)

research and development activity for developing synthetic materials which can be tailored easily to offer specific properties for specific applications. Table 2 shows the list of few polymers for nonbiodegradable systems based on C–C backbones to degradable heteroatom-containing polymer backbones, while the remaining section of report will include a discussion on a number of biodegradable polymers used in CDDs. Polyester They are the extensively studied and are used in drug delivery applications.[24–26] The path for synthesis of polyester is polymerization of cyclic lactone monomer. However, polymerization is a slow process to produce high-­molecular-weight compounds. The rate of production can be increased by using Zn or Sn as a catalyst with carbonyl ester. The most commonly used catalyst for polymerization reaction is stannous octoate, [27] and it has FDA approval as a food stabilizer. Other salts include salt of Fe (II) as an initiator[28] for ­lactide polymerization above 150°C. Zinc powder and CaH2 are widely used as nontoxic catalysts to form a ­copolymer of poly(lactic acid) (PLA) and poly(ethylene oxide) (PEO). PLA and Poly(glycolic acid), and Their Copolymers PLA, poly(glycolic acid) (PGA), and poly(lactic acid-coglycolic acid) (PLGA), the (copolymer)-based polyester compounds are considered as the best biomaterials with respect to their performance and design. Their chemical structures are shown in Fig. 3. The enantiomers of PLA are poly(d-lactic acid) (PDLA) and poly(l-lactic acid) (PLLA). The physicochemical properties of these two compounds are the same, but racemic PLA has a different property, e.g., PLLA is crystalline, but PLA is completely amorphous; however, their glass transition temperatures, Tg, are 56°C and 57°C, respectively.[29] The PLLA is considered more biocompatible because naturally occurring lactic acid is L (or S). On the basis of crystallinity and steric hindrance, it is said that PLA homopolymers degrade slower than PGA homopolymers. Degradation of PLA, PGA, and PLGA is demonstrated by Vert et al.[30] The formulation of poly(esters) with the selected drug has been reported by many researchers [31] where they highlighted the development of polyester matrices.[32] Poly(ethylene glycol) Block Copolymers Poly(ethylene glycol) (PEG) is also named as PEO but at high molecular weights. One of the important properties of PEG that makes it suitable for application in drug delivery devices arises from its protein resistivity.[33] Its chemical structure is shown in Fig. 4. The hydrophilic nature of PEG and the hydrophobic nature of PLA make a copolymer to

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Table 2  List of some different polymers used in drug delivery devices based on backbone composition S. No. Backbone Polymer name Structure Comment 1

C–C

Zero order achieved by diffusion through matrix

Polyethylene

References [11,15]

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2

Vinyl based

Polypropylene

Ophthalmic drug delivery application

[16]

Polyvinyl chloride

Membrane devices are fabricated to release volatile components to air and nonvolatile to aqueous solution.

[17]

Polyvinyl alcohol

Surface stabilizer in hydrogels

[18,19]

3

C–O

Polyethylene glycol

Used in diffusion limited tablet

[20]

4

C=O

Polycaprolactone

Used in tissue regeneration

[21]

5

Silicon-based Si–O

Polydimethylsiloxane

Treatment of bone infection with cross-linked matrix

[22]

6

Phosphorousbased P=N

Polyphosphazenes

Amino acids’ side chains degrade to amino acids and ammonia by generating flexible material.

[23]

They are used in immunology as vaccine adjuvants.

(b)

(a) O n

O

(c)

O H

O

OH n

HO

O O CH3

H O m

n O

Fig. 3  Structure of (a) PLA, (b) poly(glycolic acid), and (c) PLGA

H

O

O n

H

Fig. 4  Structure of PEG

possess the property of a surfactant. Introduction of PEG in copolymer provides an excellent surface property within the body because it has the ability to repel protein from the aqueous environment.[33] Cannizzaro et al.[34] reported that the PLA-PEG structure could act as the foundation for producing more complex biodegradable materials. Han and Hubbell[35] performed the synthetic utility for PLA-PEG systems by intercalating the acrylate group to form crosslinked systems. Jeong et al.[36] fabricated thermosensitive PLA-PEO hydrogels that possess temperature-­dependent gel–sol transition and can be used for injectable drug ­delivery systems.

Poly(ortho esters) Poly(ortho esters) were developed to inhibit drug release following the diffusion mechanism and permit drug release only after hydrolysis of polymer chain on the surface of the device.[37] Researches on the synthesis of poly(ortho esters) include the addition of polyols to diketene acetals. Heller et al.[37] described the synthesis and application of the 3,9-diethylidene-2,4,8,10-­tetraoxaspiro[5.5] undecane (DETOSU)-based poly(ortho esters). It has an acid labile ortho ester linkage structure. Numbers of modifications have been done to change the diol structure because they suffer from unpredictable degradation kinetics. Many applications are there for poly(ortho ester) compounds formed by the substitution of 1,2,6-hexanetriol for 1,2 hexanediol.[38] This polymer has been applied for the pulsatile delivery of insulin where the drug delivery system consists of the enzyme glucose oxidase.

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In order to develop a device that erodes heterogeneously, the basic requirement is that polymer should be hydrophobic and should contain water sensitive linkages. One such group of polymers is poly(anhydrides) that undergo surface erosion because of high water reactivity of the anhydride bonds on the surface and the hydrophobic nature which prevents water penetration.[39] They are synthesized by melt condensation polymerization. ­Degradation ­tendency depends on the composition of the polymer. Poly(anhydride-imides) Polyanhydride when modified by the introduction of amino acids such as glycine and alanine in order to increase the mechanical property results in poly(anhydride-imides). These amino acids are incorporated with imide bonds at the terminal.[40] This polymer undergoes surface erosion like poly anhydride, [41,42] and the monomers are removed by diffusion from polymer matrix.

processible and are characterized by their high strength and stiffness.[49] They are currently being explored for use in small bone fixation devices such as bone screws and pins.[50] Phosphorus-Containing Polymers Poly(phosphazenes) The special properties of poly(phosphazenes) and their biodegradation kinetics are due to the change in their side chain rather than polymer backbone; hence they provide an interesting approach toward their development.[51] Biodegradable poly(phosphazenes) are water insoluble. On degradation, these polymers form amino acid, ethanol, phosphate, and ammonium salts. A number of researches have been done to develop cross-linked poly(phosphazene) for temporal controlled release. Poly(phosphoesters)

Other modification of polyanhydride has resulted in the development of poly(anhydride esters). Intention of this modification was to develop a polymer that has a degradation profile of two stages. These polymers possess ­potentially beneficial polymer degradation characteristics.

Dahiyat et al. (1993)[52] incorporated a phosphoester group into the poly(urethanes) to produce biodegradable materials and to maintain the mechanical properties of ­poly(urethanes). They also observed that kinetics was influenced by side chains attached to polymer backbone. [53] Their hydrolysis produces phosphates, amines, alcohols, and carbon dioxide. Release mechanism is a combination of swelling, degradation, and diffusion.

Poly(amides)

Poly(N -isopropylacrylamide)

The interesting class in poly(amides) is poly(amino acids) (Fig. 5). Nathan and Kohn [43] have published an excellent article on the history of amino acid-derived polymers. These polymers are used to deliver drugs of low ­molecular weight.[44] They have good biocompatibility, but hydrolytic stability of the amide bond depends on the enzyme for bond cleavage; this dependency results in poor control over drug release. They are hydrophilic where degradation rates depend on the hydrophilicity of the amino acids.[45,46]

It is a temperature-responsive polymer (Fig. 6)and has been extensively used for cancer therapeutics. [54] It was first synthesized in the 1950s via free radical polymerization using N-isopropylacrylamide [55] as a monomer, sodium dodecyl sulfate (SDS) as a surfactant which helps in controlling the size of hydrogels, N-N′-methylene-bis-­ acrylamide as a cross-linker, and potassium persulfate (KPS) as an initiator in the inert (nitrogen) environment at 70°C. It contains hydrophobic (isopropylic) as well as hydrophilic (amide) groups. Polymer is functionalized easily making it suitable for variety of applications in thermoresponsive drug delivery systems, biosensors, etc. [54] Hydrogels based on poly(N-isopropylacrylamide) (­PNIPAAm) can also be used for photodynamic therapy and photo-­thermally driven drug delivery. Though being considered as a smart polymer, its nonbiodegradable

Poly(anhydride esters)

Poly(iminocarbonates) Polymer containing three or more amino acids are highly insoluble, nonprocessible, and antigenic.[47] To overcome this problem, poly(amino acids) were synthesized from tyrosine dipeptide.[48] These polymers are readily

*

H N

O

n *

O

NH

NH2

Fig. 5  Structure of polyamino acid

Fig. 6  Structure of poly(N-isopropylacrylamide)

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nature and possible resultant toxicity is still a matter of concern requiring further research.

O

Poly(2-hydroxyethyl methacrylate)

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It is a hydrophilic polymer (Fig. 7). This polymer is nontoxic and nonantigenic, and exhibits favorable tissue compatibility; in water, it forms a hydrogel. It was explored by Drahoslav Lim and Otto Wichterle [56] for biological use and exhibit suitable mechanical properties. Its medical applications include contact lenses, keratoprotheses, and orbital implants. Presence of hydroxyl and carboxyl group provides compatibility with water and hydrophobic methyl group confer hydrolytic stability to polymer and enhance the mechanical property of polymer matrix.[57] ­Poly(2-hydroxyethyl methacrylate) (pHEMA) gels possess resistivity to high temperature, acid, and alkaline condition. Polypyrrole

O

NH2

N H

N NH2 H

N

N

O

NH2

H N

H N

NH2

O

Fig. 9  Structure of poly(amidoamine)

in immunodiagnostic, and in vitro gene expression applications. They are the substitutes of globular protein and considered as low mammalian toxicity when having attached anionic and neutral groups. These dendrimers have a three-­ dimensional structure exhibiting both nanoscale properties and surface functionality.[59] Polymer Degradation

Polypyrroles (PPys) are organic polymers having structure as shown in Fig. 8. These are synthesized from the polymerization of pyrrole. These are conducting polymers and biocompatible and therefore are used for immobilization of enzymes. The first application of PPys was reported in 1963 by Weiss and coworkers. The drug release from PPy results in an electrically driven change in redox state that produces changes in polymer properties.[58] An important property of PPy in application is that the release rate can be altered according to the demand to fulfill clinical requirements, thus ­providing the opportunity to develop implantable devices. Poly(amidoamine) Poly(amidoamine) (PAMAM) belongs to the class of dendrimer. It is a repetitive subunit of amide and amine functionality. PAMAM dendrimers are sometimes referred to as Starburst (trade name); their chemical structure is shown in Fig. 9. They are considered as unique class of synthetic nanostructure PAMAM dendrimers and are synthesized using divergent methods that involve two steps. PAMAM dendrimers have an approximate diameter equivalent to the ubiquitous lipid bilayer. Commercially, they are used O

O

Polymer degradation is defined as the change in ­properties such as tensile strength and shape under the influence of chemical, heat, or light. It is important to mention that degradation and erosion are two different phenomena. Various routes for degradation of polymers are shown ­schematically in Fig. 10. Degradation is a chemical process, whereas erosion which is a physical phenomenon is dependent on dissolution and diffusion. Erosion process is of two types: surface erosion and bulk erosion as shown in Fig. 11. Water invasion is rapid in the former case, and in the latter case, hydrolysis is fast. Bulk erosion is mostly found in ­biodegradable ­polymers used in drug delivery systems. Several factors that affect polymer biodegradation are Listed as follows:[11] • • • • • • •

OH

•

CH3 n

•

Fig. 7  Structure of pHEMA

• •

N H

Fig. 8  Structure of PPy

•

H N

•

n

• •

Chemical structure and composition. Configuration structure. Presence of chain defects and uncertain units. Existence of low-molecular-weight compounds. Site of implantation. Physicochemical factors (pH, ion exchange, and ionic strength). Physical factors (shape and size). Morphology (amorphous, semicrystalline, crystalline, microstructure, residual stress). Mechanism of degradation (enzymatic, hydrolysis, microbial). Processing conditions and sterilization process. Annealing and storage history. Route of administration and site of action. Mechanism of hydrolysis. Adsorbed and absorbed compounds, e.g., water, lipids, ion. Presence of ionic group.

Polymers and Polymeric Membranes 2083

High molecular weight polymer

Thermodegradati on

Photolysis

Hydrolysis

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Biodegradation

Low molecular weight polymer

Structural weakening

High surface

Brittleness

Fig. 10 Schematic representation of various routes for polymer degradation [60]

Surface Erosion Density of matrix is constant Volume decreases

Bulk Erosion Density of matrix decreases Volume is constant

Large drug particle

Initial matrix material

Small drug particle

Surface eroded matrix material

Bulk eroded matrix material

Fig. 11  Erosion mechanism found in the matrix system

Typical Applications of Polymer Drug Delivery •

•

Polymer in the colon-targeted drug delivery system: Polymer plays an important role in this system as it protects drugs from degradation and promotes ­controlled release in proximal colon.[61] Polymer in the mucoadhesive drug delivery system: Mucoadhesive polymers have the advantages of penetration enhancement, an increase in the residence time of the polymer, site-specific adhesion as well as enzymatic inhibition. These mucoadhesive polymers

•

•

are utilized for the buccal delivery for a variety of ­therapeutic compounds.[62] Polymers for sustained release: Polymers that were used in sustained release are ­prepared using biodegradable microspheres having a new potent osteogenic compound.[63] Polymers in the floating drug delivery system: Such polymers are generally used to target the drug delivery to a specific region in the gastrointestinal tract. Natural polymers which have potential application in the gastrointestinal tract for specific drug delivery

2084

•

Polymers and Polymeric Membranes

include chitosan, pectin, xanthan gum, starch, and husk.[64] Polymers in tissue engineering: Natural origin polymer is potentially useful as carrier compounds with applications in tissue engineering area targeting several biological tissues.[65]

Advent of smart polymer systems leads to accurate and programmable drug delivery. It provides a link between therapeutic need and drug delivery.[74] Table 4 enlists different types of smart drug delivery devices with their applications.

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Mechanism and Types of Controlled Drug Release Using Polymers

Smart Polymers Smart polymers have been investigated as “intelligent” delivery systems.[66–68] These systems have tendency to release drug at the appropriate time and specific site. Biological therapeutics are often limited from short half-lives, poor bioavailability, physical instability (aggregation and precipitation, alteration in highly ordered protein structure), and chemical instability (reactions such as oxidation, hydrolysis, and racemization). Smart polymer provides a platform that can deliver drug at a controlled rate and in biologically active form. They are attracting interest and number of work has been done to advent sensitive macromolecules that can be converted to smart polymers. The major advantages of smart polymer are reduced dosing frequency with prolonged release, simple preparation, maintaining the desired therapeutic concentration, reduced side effects, and improved stability.[69–71] Smart drug delivery device also permits a reduction in cost due to minimization of doses, making medicine universally affordable. Table 3 summarizes various types of smart polymeric drug ­systems with their advantages and limitations. Responses of a stimuli-responsive polymeric solution are initiated by physical or chemical stimuli, and are subjected to a limitation of destruction and the formation of forces that include hydrogen bonding, electrostatic ­interaction, and van der Waals forces.[72,73]

There are three major types of membrane CDD devices. In the first type of devices, the rate of release of drug is controlled by the permeation of the drug from a reservoir. In the second type of devices, water diffuses inside the device cross membrane under osmotic pressure difference to ­actuate a micropump to force the drug out as shown in Fig. 12. In the third type of devices, the drug is impregnated into the membrane matrix that slowly degrades in the body and releases the drug.[60] Some of the basic drug release models are given Table 5. Quite often, the drug delivery is controlled by the synergistic action of diffusion and degradation. These devices may use a single polymer matrix or composite of several layers. A few typical CDDs developed after the 1950s are listed in Table 6. Modeling of Drug Release from Porous Polymers Several mechanistic models have been proposed to express the release rate from CDDs. These mechanisms play an important role in understanding drug release and designing controlled drug release systems.[84] These models have different mathematical functions defining concentration profile.[85] Different types of mathematical models for CDD are defined next.

Table 3  List of various smart polymeric drug deliveries with their advantages and limitations S. No. Stimulus Responsive material Advantages 1

Temperature

Limitations

Poly(N-alkylacrylamide)s

Avoidance of toxic organic solvents

High burst drug release

Poly(N-vinylcaprolactam)s

Deliver of both hydrophilic and lipophilic drugs

Poor mechanical strength

Reduced side effects Sustained and site specific property 2

pH

3

Light

Poly(methacrylic acid)

Suitable for thermolabile drugs

Poor mechanical strength

Biocompatible and biodegradable

Poor mechanical strength

Instantaneous delivery of sol–gel stimuli

Inconsistent and slow response

Pulsative response on changing electric field

Additional equipment is required for external application

Poly(vinylpyridine)s Modified poly(acrylamide)

4

Electric field

Poly(thiophene)s

5.

Ultrasound

Ethylene vinyl acetate

Poly(ethyloxazoline)

Dark toxicity

Difficult to optimize Controlled protein release

Difficult to control release profile Specific equipment required to control release

Polymers and Polymeric Membranes 2085

Table 4  Types of smart polymers and their applications

1

Stimulus

Smart polymer Polymer

Drug

Temperatureresponsive polymer

Outcomes/applications

Ref.

Exenatide

PLGA–PEG–PLGA

Produces a long-acting injectable formulation

[75]

Human mesenchymal stem cells and desferrioxamine

Chitosan-beta

In situ depot for the sustained release of drugs

Glycerophosphate

Treatment of critical limbic

Leuprolide

Polybenzofulvene

Protection of the oligopeptide drug

Alginate and chemically modified carboxymethyl chitosan

Protein drug

Ketoprofen

Poly(acrylamide)-gcarrageenan and sodium alginate

Significant increase in drug release on increase in pH from acid to alkaline

Dexamethasone

Poly(methoxyl ethylene glycol-caprolactone-comethacrylic acid-copoly(ethylene glycol) methyl ethylene methacrylate)

Hydrogel shows a sharp change with change in pH

Treatment of diabetes (type II) [76]

[77]

Treatment of tumors 2

pH-responsive polymer

Hydrogel protects the drug from the harsh acidity

[78]

Used in oral delivery [79]

Colon-targeted delivery [80]

Oral drug delivery Bioresponsive polymer

Methacrylate derivatives of dextran and concanavalin

Response of insulin is irreversible to glucose concentration. Self-regulation of insulin delivery

[81]

N-(2-(Dimethylamino) ethyl)-methacrylamide) and concanavalin A

Quick response of microhydrogel to glucose concentration

[82]

For controlled release of insulin Field-responsive polymer

Poly(2-acrylamido2-methylpropane sulfonic acid-co-nbutylmethacrylate)

[83]

Control of drug release is dependent on the intensity of electric stimulation in distilled deionized water. Delivery of edrophonium hydrochloride

Polymer

(a)

(b) Pump housing Piston Delivery orifice

Semi permeable membrane Drug Time 0

Time t

Fig. 12  Schematic representation of (a) the first type CDD device and (b) osmotic-controlled pump [94]

Drug reservoir

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S. No.

2086

Polymers and Polymeric Membranes

Table 5  List of different drug release models S. No. Physical process Device 1

Diffusioncontrolled delivery

Diffusion reservoir system

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Diffusion matrix systems

Equation Fick’s law

Ref. [93,95]

J = − D ( dC dx ) J = flux of diffusion or amount of substance passing per unit area per unit time (mg/m2 h), D = diffusivity of drug molecule (cm2 /s) Q  2Ca − CP  =  t0.5  DP CP 

0.5

[96]

Cp = solubility of drug in polymer (g/cm3), Ca = initial amount of drug (g/cm3), Dp = diffusivity in polymer (cm2 /s), Q = cumulative amount released (g/s) 2

Membranecontrolled drug release

Nonporous semipermeable membranes

(

)

DkA C2 − C1 δ K = membrane partition coefficient

M=

[94]

δ = thickness, C1 = Concentration inside membrane, and C2 = Concentration outside the membrane, respectively (g/cm3) Porous semipermeable membranes 3

Osmotic-controlled drug release

Deff = Dpore ε τ

[94]

Similar equation as for nonporous can be used here to describe the drug release process by replacing D with an effective Deff. as written in equation where ε and τ are the porosity and the tortuosity factor dMt dV Ak∆πC = C= δ dt dt

[97]

A = membrane area (cm2), C = concentration of drug (g/cm3), K = membrane permeability, Δπ = osmotic pressure differential (atm), δ = membrane thickness (µm) 4

Dissolution-controlled drug release

Noyes–Whitney equation

(

dC DA = C0 − C dt h

[92]

)

C = concentration of drug, (g/cm3), D = diffusion coefficient of drug (cm2 /s), A = area of drug particle (cm2), h = thickness of drug diffusion layer (cm), Co = saturation concentration of drug in diffusion layer (g/cm3) 5

Floating drug release

The system is floating; as a result, drug is released slowly but at the desired rate. Along with buoyancy, minimum floating force is also needed to keep the dosage form reliably on the surface of the meal

[98]

6

Pulsatile drug release

Drug is delivered at a specific time as per the need, hence improving patient therapeutic efficacy

[99]

7

Ion exchange-controlled drug release resins

Ion exchange reaction is expressed as [A+] + [R−][B+] ↔ [B+] + [R−][A+] selectivity coefficient is

[100–102]



 A+  [ B+ ] KBA =  +R  +  BR  [ A ]

[A+], [B+R ], [B+], and [A +R ] are the concentrations of free counter ion, drug bound of resin, drug freed from resin, and counter ion bound to resin, respectively. 8

Biodegradable/erodible delivery

Zero-order release is effectively obtained These systems protect and stabilize the bioactive agents providing long-term administration. Here, the drug is incorporated inside a dissolvable or erodible matrix.

[103]

Polymers and Polymeric Membranes 2087

Table 6  Typical polymer matrix based CDD devices Author Device/membrane Osmotic pump

Folkman and Long[105]

Silicon rubber membrane

Anesthetic cardiovascular drugs

Alza corporation (1974)

Ocusert®

Piliocarbine, dermal delivery of scopolamine, nitriglycerine nicotine, pest control in cats and dogs

Multilayer transdermal patches Monolithic solution in PVC

Zero-Order Delivery Model This model is followed where it becomes important to maintain drug concentration level constant throughout the delivery period.[86] In its simplest form, zero-order release expression is represented as Q = Qo + Ko t

(1)

where Q is the amount of drug released after drug dissolves, Qo is the initial amount of drug, and Ko = zero-­order release constant.

where Q is the amount of drug released per unit area in time t, C is the initial concentration of drug, Cs is the solubility of drug in the matrix, D is the diffusivity of drug molecules, τ is the tortuosity factor, and ℇ is the matrix porosity. Korsmeyer–Peppas Model (the Power Law) Korsmeyer et al. (1983) derived a relation for drug release from a polymeric system using diffusion as the main drug release mechanism as given by At A0 = atn

First-Order Model This model assumes adsorption and desorption of drugs.[87] It is expressed as: dC dt = − KC

(2)

Where K is the first-order rate constant (time−1) and C is the concentration of drug. Hixson and Crowell Model

(6)

where a is the constant (defines structural and geometrical characteristic of dosage), At /A0 is the fraction of drug released at time t, and n is the release exponent, according to Korsmeyer–Peppas.[89,90] n

Diffusion

0.5

Fickian diffusion

0.5–1.0

Non-Fickian diffusion

Weibull Model

It is used to express the rate of dissolution based on the cube root of particle size.[88] The rate equation on size ­particle is: M1/0 3 – M1/t 3 = Kt

(3)

Where M0 is the initial drug amount, Mt is the amount of drug remaining after time t, and K is the proportionality constant.

This model was developed for different dissolution process and is expressed as [91,92]   −(t − T ) b   C = C0 1 − exp    a   

(7)

Higuchi in 1961 developed two models for dissolution:

where C is the amount of dissolved drug as a function of time, C0 is the total amount of drug being released, T is the lag time, a is the scale parameter that describes time dependency, and b is the shape of dissolution curve. This model can be applied to compare the release profile of matrix type drug delivery.

a. For a planar homogenous matrix,

Hopfenberg Model

Higuchi Model

(

)

1/ 2

Q =  D 2C − Cs ⋅ Cs t 

(4)

This is applicable for the drug release from surface eroding polymer such that area remains constant during the process.[93] The mathematical form for cumulative fraction of drug released is given as

(5)

Ct Ci = 1 − K0 t CL a 

and b. For a spherical heterogeneous matrix

(

(

) )

Q = Dε /τ 2C − εCs Cs t

0.5



n

(8)

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Drug

Rose and Nelson [104]

2088

where Ct/Ci is the fraction of drug released at time t, K0 is the zero order rate constant, CL is the initial drug loading, a is the half thickness of system (i.e., radius for a sphere or cylinder), and n is the exponent, whose value varies with geometry: 1 (flat), 2 (cylindrical), and 3 (spherical geometry). Petroleum–Rapid Prototyping

CONCLUSION CDD occurs when a polymer (natural or synthetic) is integrated with a drug or active agent and hence releases an active component from the material in a predesigned manner. This entry presents a brief account of different types of polymers used in drug delivery devices and their properties that are found advantageous in drug delivery. The properties of polymers that make them useful in drug delivery systems include their wide molecular weight distributions, viscoelastic properties, contraction ability when heated, specialized chemical reactivity, and tolerance limits. Polymers enable the development formulating new drug delivery systems which will have improved therapy and treatment. Concept of smart drug delivery helps in reducing cost due to minimal dosage. Mathematical modeling under different mechanisms has helped in the design and development of better devices. REFERENCES 1. Parashar, T.; Soniya, Singh, V.; Singh, G.; Tyagi, S.; Patel, C.; Gupta, A. Novel oral sustained release technology: A concise review. Int. J. Res. Dev. Pharm. L. Sci. 2013, 2, 262–269. 2. Prescott, L.F. The need for improved drug delivery in clinical practice. In Novel Drug Delivery and Its Therapeutic Application; John Wiley &Sons: West Susset, 1989, 1–11. 3. Ratnaparkhi, M.P.; Gupta Jyoti, P. Sustained release oral drug delivery system: An overview. Int. J. Pharm. Res. Rev. 2013, 2, 11–21. 4. Hoffman, A.S. The origins and evolution of “controlled” drug delivery systems. J. Control. Release 2008, 132, 153–163. 5. Uhrich, K.; Cannizzaro, S.; Langer, R. Polymeric systems for controlled drug release. Chem. Rev. 1999, 99, 3181–3198. 6. Langer, R. Drug delivery and targeting. Nature 1998, 392, 5–10. 7. Brouwers, J.R.B. Advanced and controlled drug delivery systems in clinical disease management. J. Pharm World Sci. 1996, 18 (5), 153–162. 8. Huynh, C.; Lee, D. Controlled release. In Encyclopedia of Polymeric Nanomaterials; Kobayashi, S.; ­Müllen, K.; Eds.; Springer: Berlin and Heidelberg, 2014. doi: 10.1007/978-3-642-36199-9-314-1. 9. Bhowmik, D.; Gopinath, H.; Pragati Kumar, B.; Duraivel, S.; Sampath Kumar, K.P. Controlled release drug delivery system. Pharm. Innov. 2012, 1, 2277–7695.

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44.

45.

46.

47.

48.

49.

50.

51.

52.

53.

54.

55.

56. 57.

58.

59.

60.

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93. Shaikh, H.K.; Kshirsagar, R.V.; Patil, S. Mathematical model for drug release characterization: A review. World J. Pharm. Pharm. Sci. 2015, 4, 324–338. 94. Oct.mig.edu. Drug Delivery: Controlled Release, 2006. Available at https://ocw.mit.edu/courses/materialsscienceand-engineering/3–051jmaterials- for biomedical-applicationsspring-2006/lecture-notes/lecture19.pdf, (accessed in February 2016). 95. Tongwen, X.; Binglin, H. Mechanism of sustained drug release in diffusion-controlled polymer matrix-application of percolation theory. Int. J. Pharm. 1998, 170, 139–149. 96. Lyu, S.; Siegel, R.A. Historical survey of drug delivery devices. In Drug Device Combination for Chronic diseases; Lyu, S.; Siegel, R.A.; Eds.; John Wiley & Sons: Hoboken, NJ, 2016, 39–66. 97. Kim, C. Controlled Release Dosage Form Design; Technomic Publishing Co. Inc.: Lancester, 2000, 301. 98. Gopalakrishnan, S.; Chenthilnathan, A. Floating drug delivery systems: A review. J. Pharm. Sci. Technol. 2011, 3 (2), 548–554.

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Polymers and Polymeric Membranes 2091

Polypyrrole: Properties and Application Amir Reza Sadrolhosseini

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Functional Devices Laboratory (FDL), Institute of Advanced Technology, University Putra Malaysia, Serdang, Malaysia; Materials Processing and Technology Laboratory (MPTL), Institute of Advanced Technology, University Putra Malaysia, Serdang, Malaysia

Suraya Abdul Rashid Materials Processing and Technology Laboratory (MPTL), Institute of Advanced Technology, University Putra Malaysia, Serdang, Malaysia

Suhaidi Shafie Functional Devices Laboratory (FDL), Institute of Advanced Technology, University Putra Malaysia, Serdang, Malaysia

Abstract Polypyrrole (PPy) is a type of conducting polymer, which is derived from the oxidation of pyrrole monomer. PPy has inherent conductivity similar to semiconductor in the presence of dopant. PPy and PPy composite are subject of intense research in sensors, biosensors, solar cell, and drug delivery. In this entry, the history, molecule structure, synthesis methods, and physical properties of PPy were presented. The PPy/nanometal composite was considered for sensors and biosensors application. Keywords: Ag nanoparticle; Au nanoparticle; Biosensor; Conducting polymer; Cu nanoparticle; Electrical properties; Optical properties; PPy; Pyrrole.

INTRODUCTION Conducting polymers are the materials based on organic polymers, and they have electrical conductivity such as metallic conductors or semiconductors.[1–5] Polypyrrole (PPy) is a famous and important conducting polymer. It has particular photonic, thermal, and electronic properties related to its high π-conjugated (i.e., system has C=C ­conjugated bonds) length. PPy and PPy composites display unusual electronic properties such as low-energy optical transition, low ionization potentials, and high electron affinities.[6,7] They have nature unusual conducting mechanism and reversible redox doping properties.[8] They have more applications for photonics, energy storage, sensors, biosensors, medicine, electronics, and optics. The composites of conducting polymer are permanent alternative to metallic or inorganic semiconductor components. They have properties for developing flexible or wearable electronics, displays, and other devices. PPys have intrinsic electrical conductivity, [9] which is related to the charge transfer rate and electrochemical redox efficiency. The resistivity of PPy can be controlled by incorporating anions into PPy during synthesis, as doping, the conductivity of PPy is significantly increased within the semiconductor range.[10–12] PPy is an opaque, brittle, and amorphous material. The particular properties, which are influenced by the dopant, [13] are a narrow electronics band and low mobility

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of charge carriers due to a weak intermolecular overlap of electronic orbitals combined with a greater degree of disorder. Therefore, PPys have had serious limitations in specific applications such as transistors and memories.[14] PPy features low oxidation potential, good biocompatibility, and good environmental stability. PPys have become a leading material in various application fields. PPys have also been used to detect some toxic chemicals and ­biomolecules. It can be used for sensing layer as transducer in sensor application. Many articles reported the sensory application of PPy to improve the sensitivity, selectivity, and response time.[15,16] In most cases, a response time is about a few seconds that is enough for some ­application. The sensing mechanisms of PPy involve redox reactions, ion adsorption of molecule, binding the biomolecule, adsorption of toxic chemicals, refractive index and thickness changes, chain conformational changes, or charge transfer and screening. Consequently, PPys have an advantage in achieving high sensitivity and selectivity by virtue of their chemical and structural ­diversities.[17] STRUCTURE OF PPY PPy is a type of conducting polymer that is formed by the polymerization of pyrrole (Py). PPy-related members being polythiophene, polyaniline (PANI), and polyacetylene.[18] Encyclopedia of Polymer Applications, First Edition DOI: 10.1201/9781351019422-140000377 Copyright © 2018 by Taylor & Francis. All rights reserved.

Polypyrrole: Properties and Application 2093

The conductive forms of PPy are prepared by the ­oxidation (“p-doping”) of the polymer as follows:

( C H NH ) 4

N H

Fig. 1  Molecule structure of PPy

PPy can be synthesized through the oxidative polymerization of Py monomer.[18] Figure 1 shows the molecular structure of PPy. Figure 2 depicts the final form of PPy that contains a long conjugated backbone. The PPy has resonance structures that resemble the aromatic or quinoid forms. The natural state of the PPy is not as a conductor and only begins conducting when it is oxidized. The charge associated with the oxidized state is typically delocalized over several Py units and can form a radical cation such as polaron or a bipolaron (Fig. 3), which were reported in 1963 by Weiss and coworkers. The physical form of PPy is usually an intractable powder resulting from the chemical polymerization, and an insoluble thin layer resulting from ­electro-polymerization.[19–21] PPy is prepared by the oxidation of Py, which can be achieved using ferric chloride in methanol:[22,23]

Polymerization is thought to occur via the formation of the pi-radical cation C4H4NH+. This electrophile attacks the C−2 carbon of an unoxidized molecule of Py to give a dimeric cation [(C4H4NH)2]++. The process repeats itself many times.[22,23]

H

N H

–2H+

2 N+

H

H

H

H

–e–

N H

H

H

H H N

H

N H

N+

)

A thin layer of PPy is synthetized using ­electro-polymerization of Py on conducting substrates (platinum, gold, silver, glassy carbon, and indium tin oxide (ITO) glass) as the working electrodes. The standard oxidation potential of Py is about 0.70 V. Therefore, the electro-­polymerization process is stopped, when the oxidation of monomer is occurred. PPy thin layers were fabricated by different electrochemical techniques. Three electrodes were used to prepare PPy thin layer using electro-­polymerization of Py such as working electrode, counter electrode, and reference electrode. A saturated calomel electrode or Ag/AgCl (saturated KCl),

)

–2e–

(

+ xFeCl3 → C4 H2 NH n Clx + xFeCl2

SYNTHESIS OF PPY

nC4 H4 NH + 2FeCl3 → C4 H2 NH n + 2FeCl2 + 2HCl

2

n

PPy can be synthesized by electrochemical methods [23] in the presence of p-dopant. The resulting conductive polymers are peeled off from the anode. Various dopants were used to prepare PPy including iodine, chloride, polystyrene sulfonate, tosylate, and perchlorate. Moreover, the complex biomolecules such as biotin [24] and chondroitin sulfate [25] were used to synthesize PPy. The dopants such as Cl, ToS, and PSS are water soluble, biocompatible, commonly used for biomedical applications, and vary greatly in molecular weight.[26–29] This material is attractive for a wide range of applications, but this flexibility also makes it difficult to select the best ­synthesis parameters for a specific application.

n

(

2

H

–2H+

Fig. 2  The synthesis process and final form of PPy

H

N H

H

N H

H N

H

+

N H

H

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H N

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Polypyrrole: Properties and Application

H N

N H

N H

Aromatic

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H N

N H

N H

Quinoid

+

N H

H N

H N

N H

N H

H N

A– Polaron

N H

+

H N

A–

H N

N H Bioplaron

+ N H

H N

A–

Fig. 3  Aromatic, quinoid, polaron, and bipolaron are polymer units after oxidation

conducting substrate, and graphite rod are used as reference electrode, working electrode, and counter electrode, respectively. PPy layer grows on the surface of working electrode. The thickness of layer is controlled by the time of electrodeposition. [30] Bazzaoui et al. reported the electrodeposition of PPy layer on oxidizable metals such as Al and Fe. [31] The ­formation of homogeneous and strongly adherent PPy layer is prepared using a solution containing ­saccharin and Py.  Similarly, Bazzaoui et al. obtained PPy coating on Fe by the use of an ­aqueous medium of 0.1 M sodium saccharinate and 0.5 M Py. [32] The PPy composite layers were prepared using various techniques with pure m ­ etals, oxides, and anions, and the electrodeposition on the surface of metal is the simplest and usual methods.

PHYSICAL PROPERTIES OF PPY The PPy in pure (un-doped) form is described as electronic insulators. The dopant can change the conductivity of PPy from insulators to conducting material. The conductivity

(σ) of PPy is commensurate to the carrier concentration of electrons (n) and the carrier mobility (μ) as follows:

σ = enµ

(1)

The carrier concentrations of electron charge (n) decrease exponentially with an increase in the bandgap of PPy. The PPys have relatively large bandgap; hence, the carrier concentration of electrons is small in room temperature; consequently, a low value of carrier concentrations of electrons leads to a low value of conductivity of un-doped PPy. Even though PPys have high carrier mobility[33] in doped PPy form, the doping of PPy generates high conductivities by increasing the carrier concentration of electrons. This is performed by the reduction or oxidation with electron acceptors or donors. When the electron removes from Py, the hole generates; it is equivalent to the PPy oxidized by the acceptors. The polaron is the radical cation with lattice distortion around the charge. The polaron has a positively charged hole site. This hole site moves through PPy, and it contributes to the conductivity. This type of PPy is called p-type polymer. The PPy in the form of n-type was achieved

by adding electron to the chain. This process generates polaron with a negative charge. The measurement of Hall effect gives us information about the negatively and ­positively charged carrier of PPy.[34] The doping concentration in PPy is highly compared with organic semiconductors, and sometimes it is about 10% of the final weight of PPy that can measure using analytical technique. PPy doped can be returned to an ­insulating state by the neutralization back to the uncharged state. This return to neutrality is referred as compensation. Exposure of oxidatively doped PPy to electron donors or, conversely, of reductively doped polymer to electron acceptors effects compensation. This ability to cycle between charged and neutral states forms the base for the ­application of ­conducting polymer in rechargeable ­batteries.[35] The amount of charge carrier is generated in PPy during the injection of dopant, and the charge carriers have contribution to conductivity (Eq. 1). The carrier transports in doped conjugated PPy are analogous to a doped semiconductor. Doping creates new electron states within the band gap of polymer. In conductive polymer such as PPy, the total oscillator strength does not increase upon doping, and the generated polaron density of state is created by shifting the band density of state to bandgap. At high doping concentrations of PPy, these states interact strongly with each other; consequently, the overlap of their electronic wave functions yields a band of electronic state within the ­bandgap instead of discrete levels. The mechanism of carrier charge transport in PPy is similar to the amorphous semiconductors (hopping transport) and crystalline semiconductors (band transport). Therefore, polarons (electrons and holes) can move from one site to the other site. Figure 4 illuminates the electronic levels of PPy.[35] This model is semiempirical method that is increasingly doped from its neutral un-doped state. In neutral state, PPys have a very large π–π* (valence → conduction) bandgap of 3.2 eV. Figure 4b shows two polaronic levels emerge in the bandgap, when one electron is removed to form a polaron (radical cation). The lowering of these polaronic levels is half-filled for such partly doped

Normal polymer

PPy (with one positive charge for every four to six monomer units), as confirmed by an electron spin resonance (ESR) signal for the unpaired electron. Further electron removal results in the formation of a spineless bipolaron and the energy levels are shown in Fig. 4c (together with the loss of the ESR signal). These electronic states immingle into bipolaron bands (Fig. 4d) with increasing the dopant (one positive charge for every three Py monomer units). The π–π* bandgap was predicted about 3.6 eV. The conductivity of PPy layer can be measured using four-point probe technique. The sheet resistivity can be calculated using Eq. 2: Rs = 4.5324

V , I

(2)

where V and I are the voltage across the two outer electrodes and the current through the two inner electrodes, respectively.[36] The electrical and thermal conductivities of PPy depend upon the temperature and frequency. The variable range hopping models predict the temperature dependence of the dc conducting polymers such as PPy[37,38] as follows:   T0 1/ 4  σ = σ 0 exp  −     T  and

( )

σ 0 = e2 R2ν ph N EF , T0 =

λα 3 kN EF

( )

where T0 , e, k, R, νph, N(EF), λ, and α are the characteristic temperature, the electronic charge (1.602 × 10 −19 C), the Boltzmann’s constant (8.616 × 10 −5 eV K−1), the average ­hopping distance (cm), the phonon frequency (~1,013 Hz), the density of localized states at the fermi level (cm−3 eV−1), the dimensional constant (~18.1), and the coefficient of exponential decay of the localized states (cm−1), ­respectively.[19]

Polaron orbitals

Bipolaron orbitals

Bipolaron bands

(b)

(c)

(d)

Conduction band

Valence band (a)

(3)

Fig. 4  (a) Neutral PPy, (b) orbitals of polaron, (c) orbitals of bipolaron, and (d) bipolaron bands

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PPY–METAL NANOPARTICLE Recently, the composite based on conducting polymer, such as PPy, is regard for some applications. The main idea and strategy for preparing the conducting polymer ­nanostructures are as follows: Petroleum–Rapid Prototyping

i. The nanostructures are prepared by choosing the appropriate conditions of electrosynthesis at ­simple chemically inert electrodes (template-less synthesis). ii. Nanostructured templates are created on the ­electrode surface, and the electro-polymerization of polymer occurs within the channels, holes, cavities, or related nano-size structural units of a template (template-assisted synthesis). iii. Some molecules having nano-size cavities, and arranged in a distinct way on the electrode surface, can be used. Again, the electro-polymerization of polymer proceeds within these cavities, leading to nanostructures of the resulting polymers (molecular template-assisted synthesis).[39] The main types of nano-size composites of PPy with novel metals are as follows: a. The metal nanoparticles (NPs) can cover with the PPy or other conducting polymer. Hence, these ­composites were synthesized by the chemical or electrochemical polymerization of PPy onto colloid metal ­particles. The final product was the thin layer of PPy metal ­nanocomposite on the conducting substrate [40] (metal core NPs). b. Sometimes, the metal NPs are formed in the PPy matrix using chemical reduction on metal ions. PPy has a sufficiently high reducing power with regard to some noble metal ions with high positive redox potential such as gold, silver, platinum, and copper. The metals can be reduced at a layer of PPy-forming clusters or small particles. As the relatively high metal surface area, these composites are often expected to be of use in various electrocatalytic systems for the electrochemical conversion of solute species.[39,41] Au-Nanoparticle PPy and metal NP composites have been utilized as electrode and electrode-modifying materials, [42–45] contact catalysts, [46] sensor[47] and biosensors, [48–50] and in several other applications.[51] PPy/Au-NPs (gold nanoparticles) composites have unique electrochemical properties, and the research studies in this area are leading to enhance or decrease in their electro-catalytic properties.[52] PPy/Au-NPs have a potential to create appropriate materials for electro-catalysis, chemical sensors and biosensors, microelectronic devices, [53–55] and energy storage.

Polypyrrole: Properties and Application

These nanocomposites were offered as the possible thin layers for DNA sensing, [56] the protection and binding molecules in biosensors, [57,58] and the charge transfer between composite and sensing layer for the immobilization of ­antibodies [59] or cancer treatment.[60] The main objective in PPy/nano-metal research area is the control of particle size and morphology in order to enhance the selectivity of composite in sensor and biosensor applications.[61] They usually depend on the synthesis conditions and dopant. For the PPy/Au-NPs composite, the various dopants could be chosen [62] for different applications. The PPy/Au-NPs composite could be synthesized by different methods, and the synthesis procedures are as follows: a. By forming the micelles of surfactants, where AuCl4 and composites from gold atoms are entrapped and later adding the monomer with instantly occurring polymerization.[63] b. By simultaneously stirring the solution of AuCl4 and Py monomer while obtaining gold–PPy core-shell NPs.[64,65] c. By forming PPy and then decorating with Au-NPs, which are formed in situ in acidic solution by adding hydrochloroauric acid.[66] d. The opposite order could also be applied first producing small Au-NPs (in recent publication they are called gold seeds) and later synthesizing a polymer shell.[67] e. The electrochemical formation of AuCl4 nanocomplexes could also be performed before the addition of Py and collection of PPy/Au-NPs core-shell NPs.[68] The formation of composite nanomaterials in aqueous solution based on the seed-mediated approach is an intense interest to produce ordered morphologies of Au-NPs.[69] In a seed-mediated method, the small metal NPs are synthesized, and then they are used as seeds (nucleation centers) for a systematic growth of larger NPs. To obtain the nucleation condition, arise difficulties such as inhibit additional nucleation [70] weak reducing conditions for a systematic growth of Au-NPs. [71] The formation of Au-NPs might be controlled by the overall molecular weight and relative block length of the block copolymer, if the polymer is used in the reaction solution as a stabilizer. [72,73] Also the stability of the Au-NPs was great, when the block copolymer was used instead of the homopolymer. The size and shape of NPs are controlled by the external factors including reaction duration, ­temperature, and precursor or surfactant concentration. PPy suggests an excellent biocompatibility, [74,75] and it could be doped by various biologically active compounds. Consequently, the development of PPy/Au-NPs composite can be an important strategy and idea in the creation of newly ­targeted drug delivery biosensor and electronic device. [76]

Polypyrrole: Properties and Application 2097

PPy/silver nanoparticles (Ag-NPs) composite obtained to exhibit, enhance, and improve the physical, chemical, and biological properties of some molecules, and they are suitable in a wide range of molecular science.[77] PPy/Ag-NPs composite can be synthesized by simple and efficient methods such as electrochemical, biological, and thermal methods. They can provide tremendous opportunities in the ever-expanding markets of PPy composites.[78] The PPy/Ag-NPs composites have attracted due to their unique properties including high conductivity, thermal stability, and electro-catalytic activity. PPy/Ag-NPs composite was used in sensors, biosensor, electrochemical ­oxidation, and electronic devices.[79–81] Numerous methods were reported for preparing PPy/ Ag-NPs composite. Yang and Lu focused on the preparation of Ag-NPs/PPy core/shell NPs by UV-induced ­polymerization in the presence of polyvinylpyrrolidone (PVP).[82] Tian et al.[83] reported a new method of an electrochemical deposition of silver on PPy thin layer. Kelly et al.[84] synthesized PPy/Ag-NPs composites by depositing the silver on reduced PPy fiber composites using an excess sodium borohydride as a reducing agent at room temperature. Xing and Zhao [85] and Mahesh et al.[86] synthesized PPy/Ag-NPs composite by the polymerization of Py.[85] Chen et al. investigated wet chemical synthesis of silver nano-cable wrapped with PPy in a single-step redox reaction.[87] Dallas et al.’s interfacial polymerization ­technique is applied for the in situ synthesis of PPy/Ag-NPs ­composites in water/chloroform interface to o­ vercome the ­agglomeration of NPs.[88] PPy/Ag-NPs composites are used in health-care applications to protect them from the microbial attack. Gao and Cranston [89] reported that the composite has an antibacterial effect as it contains the spread of disease from bacterial-infected area; contains the development of odor from aspiration, stains, and soil on textile materials; and contains the deterioration of textiles caused by mildew, particularly fabrics made of natural fibers.[90] Cu-Nanoparticels The preparation of PPy/Cu-NPs (copper nanoparticles) composites for catalytical activity[91] was simultaneously card out with the detection and measurement of dopamine and uric acid concentrations, [92] alongside the sensing of H2O2, [93,94] nitrate, and nitrite electroreduction.[95] Electrodeposition is usually used in preparing PPy/Cu-NPs composites.[96] The main UV-visible peak related to PPy/ Cu-NPs was dependent on the size of Cu-NPs. The Raman peaks related to PPy/Cu-NP composite were peaks associated with the vibrations of PPy, [97] which appeared at 943, 1,043, 1,329, and 1,606 cm−1.[97] Moreover, the peaks related to PPy/Cu-NPs composite appeared within the range of 1,329 and 1,606 cm−1, 513, and 602 cm−1.[97] Based on the

results of transmission electron microscopy, the spherical shape was reported for Cu-NPs in the PPy/Cu-NPs. ZnO-Nanoparticles The PPy/ZnO-NPs (zinc oxide nanoparticles) composite was prepared by some researchers using spin coating technique, [98] electrospinning, [99] in situ polymerization, [100,101] wet chemical method, [102] electrodeposition, [103] and biosynthesis via probiotic microbe (Lactobacillus sporogenes) mediated.[104] Zinc oxide has significant physical and chemical properties. Hence, the PPy/ZnO-NPs composite was used to detect gases such as NH3, CH3OH, H2S, [98] NO2, [98,105] and glucose biosensor.[106] The zinc oxide demonstrated an absorption peak around 370 nm, whereas the localized surface plasmon resonance (SPR) peak of PPy appeared around 440 nm. Therefore, two UV-visible peaks related to PPy/ZnO-NPs appeared around 380 and 420 nm.[107] Since the PPy/ZnO-NPs composites are stable components, which can be used to detect the biomaterial, they become a useful composite for sensor and biosensor. APPLICATION OF PPY PPy layers have received a particular attention in cell research based on its biocompatibility in vitro and in vivo, [108] ease of synthesis, [109] low cost, and high electrical conductivity; hence, it was considered a good material for tissue engineering.[110] In vitro studies have proved that PPy supports the adhesion and growth of various kinds of cells, such as neuronal cells, [111] endothelial cells, [112] ­keratinocytes, [113] skeletal muscle cells, [114] and rat ­pheochromocytoma cells.[115] It was obtained that the PPycoated nanowire and nanotube surfaces facilitate in vitro C17.2 neural stem cell line adhesion, proliferation, and differentiation.[116] Lundin et al. have shown that PPy can be tailored to promote the cell survival and maintenance of rat fetal neural stem cells, and the biocompatibility of PPy with neural stem cells based on the counterion incorporated in the polymer chain.[117] A very significant property of PPy is the electrical conductivity for the construction of scaffolds for nerve tissue engineering.[118,119] PPy for Sensor and Biosensor Conductive polymers such as PPy and polythiophene have a potential in designing bioanalytical tools including SPR sensor, electrochemical sensor, and microwave sensor. PPy is an excellent organic material for sensor application. PPys have a tendency to mix with other polymers such as chitosan (Chi), 2-mercaptobenzothiazole (2-MBT), and PANI. The PPy composites were synthesized by an activator material to enhance the sensitivity and selectivity of the sensing layer based on PPy. The chelating mechanism of the sensing layer based on PPy depends on the physical

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and chemical properties of activators. For example, the mechanism of PPy to detect the chemical molecule based on Chi depends on –NH2 and/or hydroxyl (–OH) groups on Chi chains. Hence, the amino group of Chi is the principal group involved in binding metal ions, and it has been widely accepted that a heavy metal ion is immobilized on Chi via four amino groups in a square planar geometry.[119] The degree of deacetylation [120] in Chi is a major factor determining the adsorption capacity of metal ions. Consequently, Chi has free amines in some of its repeated units. These amines can become protonated in dilute acidic media.[121] These protonated amines form multiple bonding sites, which are useful for chelating heavy metals by the following chelating mechanism.[122,123] R-NH2 + H+ → R-NH3+ R-NH2 + heavy metal ion → R-NH2 ( heavy metal ion ) R-NH3 + heavy metal ion → R-NH2 ( heavy metal ion ) + H+ Consequently, PPy can adsorb the chemical and biological molecules. PPy and other conducting polymers are developed for such potential method applications such as transistors [124] and switches, counter electrode in electrolytic capacitors, [125] supercapacitors, [126] sensors [127] and biosensors, [128] chromatographic stationary phases, [129] lightweight batteries, [130] photovoltaic cells, [131] membrane separation, [132] and surface protection.[133] Ratcliffe [134] synthesized PPy for sensing the chemical material. PPy thin layer was prepared electrochemically with a significant reversible increase in resistance with 100 ng cm−3 of ammonia. The layers were also sensitive to low concentrations of hydrazine. It could be speculated that the increase in resistance in PPy when the material was exposed to chemicals with electron-donating capabilities, e.g., ammonia and hydrazine, was possibly due to a reduction in the mobility of, or depletion of, charge carriers owing to the some kind of interaction of lone pairs with polarons/positive charges in the polymer. This thin coating reversibly combines with low concentrations of ammonia or hydrazine with a concomitant reversible increase in resistance; 0.1 pg cm−3 of ammonia could readily be detected with a 1 cm2 area of sensor. De Marcos and Wolfbeis [135] focused on the optical properties of PPy for sensor application. They prepared PPy for measuring the pH based on the optical properties of PPy. PPy thin layer acted as both the matrix and the indicator dye. Its longwave absorption that extended from 600 to >1,100 nm made it compatible with longwave emitting light sources and the optical window of tissue. The dynamic range was from 6 to 12. The sensor had a response to pH that was virtually independent of ionic strength within the range usually found in biological samples. Amitabha et al. prepared the PPy and PPy/ZrO2-NPs composite to detect the 16O ion beam using electrical

conductivity. They used pure conducting PPy, and a set of PPy/ZrO2-NPs composite samples were irradiated using medium-energy, 16O ion beams. It was observed that after irradiation, the electrical conductivity in pure polymer was largely destroyed but the conductivity values of the ­nanocomposite samples remain more or less unchanged.[136] Yu et al.[137] reported a sensing layer based on PPy. PPy– 2-MBT was used to detect the mercury ion using surface plasmon technique. They improved the sensing layer for the detection and the monitoring of the mercury ions using the binding interactions with PPy and 2-MBT. They demonstrated the binding of mercury ions with a PPy thin layer, which is coated on the gold thin layer for i­ mmobilizing the toxic chemicals. The detection limit was improved down to 0.01 ppm with an increase in SPR angle of 20 ± 10 RU. Choi and Jang synthesized PPy-impregnated porous carbon by vapor infiltration polymerization of Py monomers for heavy metal ion adsorbents. They improved the surface of carbon with PPy and enhanced the affinity for heavy metal ions due to the amine group of PPy. Especially, the polymer-impregnated porous carbon had an enhanced heavy metal ion uptake, which is 20 times higher than that of adsorbents with amine functional groups.[138] PPy nanowire arrays were synthesized by electro-­ polymerization in the anodic aluminum oxide (AAO) template, which was fabricated by two-step anodizing process. The sensor was sensitive to ammonia at room temperature and showed relatively high response in low concentration and comparatively short response and recovery time. Two sides of the PPy/AAO were pasted by Si substrates, and the AAO template was etched by hydrofluoric acid. The gold wires were attached on Si substrates, and the sensor came into being.[139] A manganese selective composite cation-exchanger PPy/Sn(IV) phosphate was synthesized. It has good ion-­ exchange capacity (1.04 meq g−1) compared to Sn(IV) phosphate (0.72 meq g−1). This composite material was used to detect Mn(II) in an aqueous solution. The PPy/Sn(IV) phosphate was used as an electro-active component for the preparation of ion-selective membrane electrodes. The limitation of the membrane electrode was in the range of 10 −1 to 10 −6 M, response time of 40 s, and pH range of 4–8. The practical utility was determined as potentiometric sensor for the titration of Mn(II) using ­ethylenediaminetetraacetic acid (EDTA) as a titrant.[140] Omraei et al. used the PPy/SD for removing Zn(II) from aqueous solution in 2011. The optimum conditions of sorption were found to be a sorbent dose of 0.5 g in 100 mL of Zn(II), contact time of 14 min, and pH of 3. The results were well described by the theoretical Freundlich. The kinetic data indicated that the adsorption process was controlled by pseudo-second-order equation. ­Moreover, an increase of temperature had a positive effect on the ­process. The desorption of Zn(II) PPy/SD was studied using several solvents (alkaline, bases, and water), and the maximum desorption efficiency was 75% by using NaOH.

Also PPy/SD was applied for removing Cr(VI), Zn(II), Ni, and COD from plating wastewater that its ability was ­considerable.[141] Bhaumik et al.[142] prepared the PPy–PANI nanofiber for removing Cr(VI) from aqueous solution. PPy–PANI nanofibers were synthesized simultaneously by in situ c­ hemical polymerization technique. The maximum adsorption capacity of the PPy–PANI nanofibers for Cr(VI) was 227 mg g−1. PPy, [143] PPy/Chi, [144] polypyrrole/multiwalled carbon nanotube (PPy/MWCNTs), [145] and PPy/chloride [146] were used to detect the heavy metal ions such as mercury, lead, copper, nickel, zinc, aluminum, and iron. Sadrolhosseini et al.[147] prepared the PPy/Chi on the gold layer for sensing the heavy metals using SPR technique. The limitation of sensors was about 0.1 ppm. The response of SPR sensor depends on the variation of resonance angle. PPy/MWCNTs had the maximum resonance angle shift, and the response time was faster the PPy, PPy/Chi sensing layers. Moreover, PPy/Chi was used to ­measure the corrosiveness of fuel such as biodiesel.

CONCLUSION PPy is a type of conducting polymer, derived from the oxidation of Py monomer. PPy is an insulator in its pure form. The n-type and p-type of PPy depend on dopant. The d­ oping of PPy generates high conductivities by increasing the carrier concentration of electrons. The electronic conductivities of PPy and PPy composite are function of temperature. Au-NPs, Ag-NPs, Cu-NPs, and ZnO-NPs can grow in PPy chain; hence, sometimes, PPy can cover the NPs, and sometimes, the NPs can decorate PPy. ­Consequently, the investigation of PPy/metal-NPs composite showed an intent interest in nanobiomedicine, nano-sensor, nanoelectronics, and solar cell.

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89. 90.

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105. Chougule, M.A.; Dalavi, D.S.; Mali, S.; Patil, P.S.; Moholkar, A.V.; Agawane, G.L.; Kim, J.H.; Sen, S.; Patil, V.B. Novel method for fabrication of room temperature ­polypyrrole–ZnO nanocomposite NO2 sensor. Measurement 2012, 45 (8), 1989–1996. 106. Kong, T.; Chen, Y.; Ye, Y.; Zhang, K.; Wang, Z.; Wang, X. An amperometric glucose biosensor based on the immobilization of glucose oxidase on the ZnO nanotubes. Sensors and Actuators B 2009, 138 (1), 344–350. 107. Chougule, M.A.; Sen, S.; Patil, V.B. Facile and efficient route for preparation of polypyrrole-ZnO nanocomposites: Microstructural, optical, and charge transport properties. J. Appl. Polym. Sci. 2012, 125 (S1), E541–E547. 108. Wang, X.; Gu, X.; Yuan, C.; Chen, S.; Zhang, P.; Zhang, T.; Yao, J.; Chen, F.; Chen, G. Evaluation of biocompatibility of polypyrrole in vitro and in vivo. J. Biomed. Mater. Res. A 2004, 68 (3), 411–422. 109. Leonavicius, K.; Ramanaviciene, A.; Ramanavicius, A. Polymerization model for hydrogen peroxide initiated synthesis of polypyrrole nanoparticles. Langmuir 2011, 27 (17), 10970–10976. 110. Zhang, L.; Stauffer, W.R.; Jane, E.P.; Sammak, P.J.; Cui, X.T. Enhanced differentiation of embryonic and neural stem cells to neuronal fates on laminin peptides doped polypyrrole. Macromol. Biosci. 2010, 10 (12), 1456–1464. 111. Lakard, S.; Herlem, G.; Propper, A.; Kastner, A.; Michel, G.; Vallès-Villarreal, N.; Gharbi, T.; Fahys, B. Adhesion and proliferation of cells on new polymers modified biomaterials. Bioelectrochemistry 2004, 62 (1), 19–27. 112. Garner, B.; Hodgson, A.J.; Wallace, G.G.; Underwood, P.A. Human endothelial cell attachment to and growth on polypyrrole-heparin is vitronectin dependent. J. Mater. Sci. Mater. Med. 1999, 10 (1), 19–27. 113. Ateh, D.D.; Vadgama, P.; Navsaria, H.A. Culture of human keratinocytes on polypyrrole-based conducting polymers. Tissue Eng. 2006, 12 (4), 645–655. 114. Gilmore, K.J.; Kita, M.; Han, Y.; Gelmi, A.; Higgins, M.J.; Moulton, S.E.; Clark, G.M.; Kapsa, R.; Wallace, G.G. Skeletal muscle cell proliferation and differentiation on polypyrrole substrates doped with extracellular matrix components. Biomaterials 2009, 30 (29), 5292–5304. 115. Lee, J.Y.; Bashur, C.A.; Goldstein, A.S.; Schmidt, C.E. Polypyrrole-coated electrospun PLGA nanofibers for neural tissue applications. Biomaterials 2009, 30 (26), 4325–4335. 116. Bechara, S.; Wadman, L.; Popat, K.C. Electroconductive polymeric nanowire templates facilitates in vitro C17.2 neural stem cell line adhesion, proliferation and differentiation. Acta Biomater. 2011, 7 (7), 2892–2901. 117. Lundin, V.; Herland, A.; Berggren, M.; Jager, E.W.H.; Teixeira, A.I. Control of neural stem cell survival by electroactive polymer substrates. PLoS One 2011, 6, e18624. 118. Ghasemi-Mobarakeh, L.; Prabhakaran, M.P.; Morshed, M.; Nasr-Esfahani, M.H.; Baharvand, H.; Kiani, S.; Al-Deyab, S.S.; Ramakrishna, S. Application of conductive polymers, scaffolds and electrical stimulation for nerve tissue engineering. J. Tissue Eng. Regen. Med. 2011, 5 (4), e17–e35. 119. Kofuji, K.; Murata, Y.; Kawashima, S. Sustained insulin release with biodegradation of chitosan gel beads prepared by copper ions. Int. J. Pharm. 2005, 303 (1–2), 95–103.

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Polyurethanes: Biobased Dheeraj Ahuja and Anupama Kaushik Dr. S.S. Bhatnagar University Institute of Chemical Engineering & Technology, Panjab University, Chandigarh, India Petroleum–Rapid Prototyping

Abstract Polyurethane systems containing urethane groups in their backbone structures synthesized from the poly addition polymerization of polyol, polydiisocyanates, and chain extender. They are of immense importance because of their excellent physical and chemical properties and ability to be tailor-made to meet specific requirement of an explicit application with extreme accuracy. This leads to the application of polyurethane in many fields such as footwear, machinery industry, coatings and paints, rigid insulations, elastic fibers, soft flexible foam, and medical devices. However, diminishing petroleum resources and environmental concerns have influenced global intentions towards the development of polymers from renewable sources because of the economic and environmental concerns. Therefore, this study reports the synthesis, properties, and applications of polyurethanes derived from renewable resources. This study is classified into two sections: the first section deals with the introduction of biobased material proceeding with raw material used for the synthesis of biobased polyurethane, and the next section deals with the application of biobased polyurethanes in various fields. Keywords: Biobased; Biomedical; Coatings; Lactide; Lignopolyol; Polyurethanes; Vegetable oil.

INTRODUCTION Polyurethanes, also known as polycarbamates, are the class of polymers that primarily contain urethane groups in their backbone structures and are reaction product of polyol and diisocyanate.[1] Urethane group has characteristic configuration which is discussed next.[2] O >NH

C

O

Functional groups such as esters, ether, urea, and amide may be present in the polymer molecule. Polyurethanes, first developed by Otto Bayer and coworkers in 1937, are most the versatile class of polymeric materials. The chemistry of polyurethanes allows them to have different chemical and morphological properties and display thermoplastic, thermosetting, and elastomeric behavior. Their unique advantage lies in the extensive range of high-­ performance materials that can be tailored with significant accuracy to meet specific requirements of a particular application. Modifications in chemical nature, mixing conditions, and stoichiometry of reactants can produce different end products with extremely varied range of density ranging from 6 to 1220 kg/m3. The properties of the various types of urethane polymers are dependent upon molecular weight, degree of cross-linking, effective intermolecular forces, stiffness of chain segments, and crystallinity.

2104

Polyurethanes are generally obtained by the condensation reaction of polyisocyanates with a wide array of macro-polyols and consist of a unique two-phase structure: semicrystalline and rigid “hard segment”-enriched domains dispersed in a matrix of crystallizable soft ­segments as shown in Fig. 1.[3–5] Hard segment clusters, generally of dimensions 2,000 m2/g) to enhance their gas storage capabilities.

Excess H2 uptake (wt%)

3.0 2.5 2.0 1.5 1.0 0.5 0.0

0

10

20

30 40 50 Pressure (Bar)

60

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Fig. 7  Hydrogen uptake of POP-3 at different temperatures. Square: 77 K; circle: 88 K; triangle: 195 K; diamond: room ­temperature (adsorption: solid point, desorption: empty point) Source: Reprinted with permission from Royal Society of ­Chemistry, © 2010.[23]

Fig. 8  A molecular model of phthalocyanine-based PIM Source: Reprinted with permission from Angewandte Chemie, © 2006.[29] 1.8

Polymers of Intrinsic Microprosity

1.4 H2 uptake/wt%

PIMs are microporous organic materials that have recently been developed.[29] PIMs are amorphous and are derived from microporous polymers. Organic polymers were generally waved off in gas storage as an amorphous structure has low conformity, rotational freedom resulting in efficient packing. This in turn results in lower surface areas compared to crystalline substances. A PIM can be produced as either an insoluble network or a soluble polymer depending on the monomers used.[29] Studies have shown that a network-like structure is not a factor to generate and maintain microporosity in an organic material, instead, it can be maintained and generated from a highly rigid and deformed structure consequently resulting in a high

1.6 1.2 1 0.8 0.6 0.4 0.2 0

0

1

2

3

4 5 6 Pressure/bar

7

8

9

10

Fig. 9 H2 adsorption studies on different PIMs. Circle: PIM-1; triangle: HATN network PIM; square: CTC network PIM Source: Reprinted with permission from Angewandte Chemie, © 2006.[29]

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(a)

Br

Br

Br

TEPA

TBPA

(b)

PPN-1

PPN-2

PPN-3

Fig. 10  Tetrahedral monomers used by Lu et al. and their PPN diamondoid networks Source: Reprinted with permission from ACS Publications, © 2010.[17]

Porous Polymer Networks Porous polymer networks (PPNs) with high surface area are another class of porous polymers which have shown promissing performance in the gas separation and storage applications. One of the great advantages of these structures is their synthesis method which does not require any posttreatment to extract mother liquor or other unreacted compounds. Lu et al.[17] synthesized a series of PPNs by homocoupling of tetrahedral monomers (see Fig. 10). PPN-3 which was formed as a result of 1,5-­cyclooctadiene addition to a mixture of bis(1,5-cyclooctadiene) nickel, 2,2ˊ-bipyridyl and TBPA in dry/toluene solution exhibited a Langmuir surface area of 5,323 m2 /g. Although PPN-1 which was formed by adding TEPM to the solution of finely powdered Cu(OAc)2·H2O in pyridine/ethanol showed the highest gas affinity for H2, CO2, and CH4, mainly due to smaller pore size, PPN-3 gave rise to the maximum H2 uptake at 77 K (4.28 wt%) as a result of high surface area (a Langmuir surface area of 5,323 m2 /g). However, it was also argued that PPN-1 with lowest surface area displayed the best CO2 /CH4 selectivity. The IAST predicted selectivities for CO2 /CH4 as a function of pressure are presented in Fig. 11. In further attempts to improve the structural features and gas uptake capacties, Yuan et al. introduced a series of PPNs with exceptionally high surface area and gas uptake capacities.[30] In particular, PPN-4 was shown to have BET and Langmuir surface areas of 6,461 and 10,063 m2 /g, respectively. In addition, the total hydrogen, methane, and carbon dioxide storage capacities were found to be 158 mg/g (77 K, 90 bar), 389 mg/g (295 K, 55 bar), and

2,121 mg/g (295 K, 50 bar), respectively. The authors perfomed conventional homocoupling reaction at 80°C with DMF solvent which is proven to be efficient for synthesizing PAF-1; however, the reaction yeild was lower and the resulted polymer had low surface area. Accordingly, the optimized Yamamoto homocoupling procedure was utilized, and the reaction was carried out at room temperature to enhance the higher molecular weight polymer yield. Table 2 illustrates the properties of PPN-3, PPN-4, and PPN-5 as a result of this modification and their ­comparison with selected porous artifacts.[30]

7 Selectivity CO2/CH4

Petroleum–Rapid Prototyping

Br

TEPM

6 5 4 3 2 0

10

20

30 40 50 60 Total bulk pressure/bar

70

80

90

Fig. 11  Predicted selectivity of gas uptake in PPNs exposed to an equimolar mixture of CO2 and CH4 as a function of bulk pressure. Black: PPN-1; blue: PPN-2; red: PPN-3 Source: Reprinted with permission from ACS Publications, © 2010.[17]

Porous Polymers: Gas Separation and Storage 2127

Reference

PPN-3

4,221

5,263

6,940

2.67

[30]

PPN-4

6,461

10,063

6,530

3.04

[30]

PPN-5

4,267

6,764

5,881

2.60

[30]

PAF-1

5,600

7,100

6,173

3.05

[21]

UMCM-2

5,200

6,060

4,475

2.32

[31]

PCN-68

5,109

6,033

4,811

2.13

[33]

MOF-210

6,240

10,400

6,497

3.6

[32]

NU-100

6,143

—

5,894

2.82

[65]

Source: Reprinted with permission from Wiley-VCH, © 2011.[30]

The gas adsorption characteristics of the synthesized PPN materials by Yuan et al.[30] are presented in Table 3. On the basis of their results, the authors introduced PPN-4 as an attractive candidate for gas storage applications, particularly in H2, CH4, and CO2 for clean energy purposes. Thanks to exceptionally high surface area, PPN-4 can adsorb 2,121 mg/g CO2 at 50 bar, and a gravimetric methane uptake capacity of PPN-4 (total: 273 mg/g at 295 K and 35 bar) is admirable compared to other selected porous materials such as PCN-14 (total: 188 mg/g at 290 K and 35 bar). Other recently published researches have not added too much to aformentioned significant progresses. Works are mainly focused on the stabilization and functionalization of PPNs to increase the gas capture efficiency and catalytic performance and minimize the process cost.[33–36] Zhang et al.[36] synthesized a novel benzimidazole-incorporated PPN (PPN-101) with mediocre surface area (1,095 m2 /g) based on a synthetically accessible silicon-centered aldehyde monomer (tetrakis(4-formylphenyl)silane) and a commercially available amine monomer. They argued that the presence of benzimidazole units in the framework enhances its CO2 uptake to 226.2 cm3/g at 273 K with a calculated CO2 /N2 selectivity of 200. In addition, stability and low production cost are other merits of this novel artifact.[36] The proposed reaction mechanism for PPN-101 is ­presented in Fig. 12. Recently, Sun et al.[37] introduced a facile nucleophilic substitution reaction for production of cost-effective PPNs (PPN-80 and PPN-81) to be used for CO2 capture. Polymerization reaction at a low temperature of 63°C and in the

absence of catalyst takes advantage of low-cost monomers, namely, 2,4,6-tris(chloromethyl)mesitylene, ethylene diamine, and surfactant template (triblock copolymer P123) that can also be utilized to direct the self-assembly of PPN-81, as schematically shown in Fig. 13. The resulting product contains abundant secondary amine groups which enhance both CO2 uptake and CO2 /N2 selectivity. The application of PPNs for the removal of CO2 has been extensively studied. Verfied by thermal gravimetric results, the authors concluded that the ratio of primary –NH2 to secondary –NH groups and the porosity of two polymers are different. CO2 uptake at 295 K and 1 bar on PPN-81 was higher than that on PPN-80 (84.9 vs. 71.2 mg/g). Unlike CO2, N2 and CH4 were barely adsorbed on the obtained PPNs, reaching high CO2 /N2 and CO2 /CH4 selectivities which were generally higher for PPN-81. PPN-81 renders more accesible amine sites due to possessing mesoporous structure and higher polymerization degree. The presence of a template in the synthesis route can promote a porous structure with enhanced porosity and s­ uperior adsorption performance.[37] One of the most commonly used PPNs is sulfonate grafted PPN-6, which generally has a pore size of 5–10 Å, a range in which CO2 uptake is believed to be suitable. Therefore, it functions in a similar way as a molecular sieve, enabling adsorption of desirable gas specifically. Moreover, the introduction of a functional group such as an aromatic ring or an amine group can greatly affect the adsorption enthalpy for CO2, and this is especially useful as aromatic rings are known to be vulnerable to organic substitutions.[38] A perfect example can be found

Table 3  Gas adsorption for PPNs synthesized by Yuan et al. and comparison with other well-known porous artifacts Total CO2 uptake (mg/g) Total CH4 uptake (mg/g) Material Total H2 uptake (mg/g) PPN-4

158 (80 bar,77 K)

2,121 (50 bar, 295 K)

Refer­ence

389 (55 bar, 295 K)

[30]

PAF-1

120 (48 bar, 77 K)

1,585 (40 bar, 298 K)

—

[21]

UMCM-2

116 (72 bar, 77 K)

—

—

[31]

PCN-68

130 (130 bar, 77 K)

1,393 (55 bar, 298 K)

446 (100 bar, 298 K)

[33]

MOF-210

167 (80 bar, 77 K)

2,396 (50 bar, 298 K)

475 (80 bar, 298 K)

[32]

NU-100

164 (70 bar, 77 K)

2,043 (40 bar, 298 K)

—

[65]

Source: Reprinted with permission from Wiley-VCH, © 2011.[30]

Petroleum–Rapid Prototyping

Table 2  Surface areas of PPNs synthesized by Yuan et al. and comparison with other well-known porous artifacts ALang (m2 /g) ACalc (m2 /g) Vp (cm3 /g) Material ABET (m2 /g)

2128

Porous Polymers: Gas Separation and Storage

N

NH

HN

O

O

Petroleum–Rapid Prototyping

Br

N

CHO

1. n-BuLi, –78 °C Cl Si 2. Cl –78 °C Cl Cl

2M HCl, reflux Hydrolysis

3. R. T.

1.

Si CHO

OHC

4. dil HCl, 0°C - R.T.

H2N

NH2

H2N

NH2

4HCl, –30 °C

2. R. T.

N H

CHO aldehyde monomer tetrakis(4-formylphenyl)silane

N HN

N

N

3. 130°C in O2, 2d

Si

H N

N

NH N

NH

HN

N

Benzimidazole-Incorporated PPN PPN-101

Fig. 12  Reaction mechanism proposed by Zhang et al. [36] for PPN-101 Source: Reprinted with permission from Elsevier, © 2014.[36]

(a)

Cl

HN NH

Cl

Cl

H2N

NH2

Te m pl

N H PPN-80 (b)

n

O O O O O

NH N H

HN NH

at e

HN H N

H N

H N

Mesopore

HN

N H

NH PPN-81

HN

H N

HN

NH

Fig. 13  (a) PPN-80 synthetic route in the absence and presence of template (PPN-81); (b) proposed interaction between template ­molecules and amine groups Source: Reprinted with permission from Royal Society of Chemistry, © 2015.[37]

Br C

Br Br

PPN-6 (PAF-1)

SO3H Br

H2SO4

LiOH

PPN-6-SO3H PPN-6-SO3Li PPN-6-SO3NH4

Fig. 14  Typical synthesis route to functionalize PPNs Source: Reprinted with permission from Royal Society of Chemistry, © 2013.[39]

NH4OH

PPN-6-SO3H

PPN-6

3

10 8

2

6 4

mmol/g

12 295 K CO2 Uptake (wt%)

4

PPN-6-SO3Li

14

1 N2

2 0

0 0.0

0.2

0.4

0.6 P (bar)

0.8

1.0

1.2

Fig. 15 CO2 adsorption/desorption isotherm for PPN and f­ unctionalized PPN Source: Reprinted with permission from ACS Publications, © 2011.[40]

in the s­ ulfonate-grafted-PPN-6 (PPN-6-SO3H and PPN6-SO3Li). The chemical structure of PPN-6 along with the sulfonate grafting process is shown in Fig. 14. Due to reduced pore size and strong affinity to the adsorbate, there is an advantageous adsorption of CO2 at low pressures. The latter (PPN-6-SO3Li) takes this step further by constructing multiple adsorption sites, with Li being a strong site for adsorption of CO2.[39] In another study, the same group obtained CO2 adsorption/desorption isotherms for bare PPN-6 and functionalized PPN-6-SO3H and PPN-6-SO3Li at 295 K and as evident from Fig. 15, the adsorption capacity of CO2 on PPNs was found to be comparable with those of other porous materials that have shown some of the highest capacities reported to date (Table 3). Covalent Organic Frameworks COFs represent an exciting class of crystalline porous polymers comprising organic building units connected by strong covalent bonds to form engineered frameworks. Well-defined crystalline structure and tailored functionalities render them exceptional candidates for gas adsorption and storage applications. Due to their relatively recent discovery, COFs do not have a standard naming convention and are instead named as “COF-X,” where X stands for a number. COFs have very large surface areas (upward of 6,000 m2 /g), large pore volume, and low densities. In addition, they offer very large surface area and pore volume, making them excellent candidates for gas storage. For instance, COF-105 has a surface area of 6,450 m2 /g, and COF-108 shows a pore volume of 5.4 cm3/g. There has been significant progress in the optimization of COFs synthesis routes over the past decade. The prominent strategy for COF synthesis is the utilization of rigid building units with multi-connectivities. The most

prominent work in this field is Yaghi’s exceptional work in the discovery of crystalline POPs [10] through condensation reactions of phenyl diboronic acid (C6H4 [B(OH)2]) and hexahydroxytriphenylene (C18H6 (OH) 6). Highly crystalline products were named (C3H2BO) 6 (C9H12) (COF-1) and C9H4BO2 (COF-5), which showed expanded porous graphite layers in staggered (COF-1) and eclipsed (COF-5) forms. B, C, and O atoms form strong bonds to increase the thermal stability up to 600°C, permanent porosity, and acceptable surface area (up to 1,590 m2 /g). These COFs are essentially 2D networks constructed by dehydration reactions. Ordered open channels in 2D COFs could enable CO2 adsorbtion; however, the dense layer architecture ­creates low porosity, thus low CO2 adsorption.[41] The research works on COFs are mainly integrated with POPs, and in fact, there is no distinct difference between the two.[42] 3D COFs are mainly studied theoritically.[43–45] Choi et al.[43] reported the synthesis of a series of 3D COFs (3D-COF-5 ctn and bor; see Fig. 16) by a combination of ab initio and force-field calculations, which are capable of reversibly adsorbing CO2 at room temperature. The tetrahedral tetra[4-(4-dihydroxyborylphenyl)ethynyl]phenyl methane (TBPEPM) linker used for the synthesis of these COFs contains two benzene rings which form large pores and a central acetylene group which makes the linker planar, and the corner unit is chosen to be HHTP. These 3D COFs exhibit 30% wider surface area and 5% higher percentage of free volume than COF-108. In terms of CO2 saturation uptake at 55 bar, 3D COF-05 (ctn) and COF-05 (bor) offer 9,285 and 8,582 mg/g, which is 7–7.5 times higher than those for COF-108 and COF-102, respectively. In the 3D model, phenyl groups in the linkers can be fully utilized as CO2 adsoption sites and edges and faces of the pore are fully accessible; thus, the CO2 adsorption raises up.[43] In another investigation, hydrogen uptake, diffusion, and adsorption on Li-doped phthalocyanine 3D COFs have been studied by Guo et al.[44] Adsorption and migration energies have been calculated by kinetic Monte Carlo simulations for Li-doped Pc-PBBA COF. Recently, as

B(OH) 2 4

Fig. 16  Structure of TBPEPM linker Source: Reprinted with permission from ACS Publications, © 2012.[44]

Petroleum–Rapid Prototyping

Porous Polymers: Gas Separation and Storage 2129

2130

Porous Polymers: Gas Separation and Storage

materials that maintain significant CH4 –CH4 interactions at higher pressures than current technologies (which remains at

Sensors– Separation

Fig. 10  The sensitivity of D-PDPA/30%Y_H+ [80] towards DCM and DCE at different concentrations at 27°C ± 1°C and 1 atm [80]

2360

Sensors: Zeolite–Polymer Composites for Gas Sensing

80

Response (%)

60

PANI PANI-H-ERI PANI-Cu-ERI PANI-Fe-ERI PANI-Na-ERI

40

20

0

2.5

25 48 Concentration in ppm

100

Fig. 11  Response of PANI and PANI/zeolite erionite composites towards NO2

Sensors– Separation

24 22 20

Response (%)

18 16 14 12 10 8 6 4 2

Na

Fe

Cu

H

PAN

Cation

Fig. 12  Response of PANI and PANI/erionite (Si/Al = 9) composites of various cation types when exposed to 2.5 ppm of NO2 concentration

PANI/H-erionite > PANI. PANI/Na-erionite has the largest response because Na (EN = 0.93) has the smallest EN and large ionic radius. We may note that the EN of Na, Fe, Cu, and H is 0.93, 1.83, 1.9, and 2.1 with corresponding ionic radii 186, 124, 128, and 78 pm, respectively. In case of Na-form because of the small EN and large ionic radius, the gas molecules do not interact with the ion present in the zeolite instead interact with the polymer chain and hence result in large response, whereas in case of H-from, the response is small due to large EN. The gas molecules interact with the ion instead of PANI and hence response

decreases. EN and ionic radius of the cation are the two factors responsible for the adsorption of a gas by the cation present in the zeolite.[85] PANI/Zeolite Erionite (Si/Al = 9) Response to NO2: Effect of NO2 Concentration  Figure 13 shows the response of PANI/zeolite erionite (Si/Al = 9) vs. time when exposed to 100 ppm of NO2 concentration at 28°C ± 2°C. On exposing PANI/zeolite erionite (Si/Al = 9) to 100 ppm of NO2 concentration, the response first increases and reaches to a maximum value of 67.63% up to 4.33 min.

Sensors: Zeolite–Polymer Composites for Gas Sensing 2361

80 PANI-Na-ERI

70

Response (%)

60 50 40 30 20 10

0

1

2

3

4

5

6

7

8

Time (Min.)

The response of the nanocomposite attains a constant value of 67.63% after an NO2 exposure of 8 min. The constant response of PANI/zeolite erionite (Si/Al = 9) after attaining the maximum value of 67.63% is due to reduction in the number of active sites available to which NO2 molecules can get adsorbed and interact with polymer chains. Hence, the response attains a steady state value.[83]

time is the time taken by a sensor within which all the gas molecules get desorbed). In actual practice, there is no loss of sensitivity of such composites with various cyclic intervals. If the cyclic intervals are carried out with a time gap of 5–8 min (recovery time), there is no loss of sensitivity of the composite and its sensitivity remains same with ­various cyclic intervals.[83]

PANI/Zeolite Erionite (Si/Al = 9) Response to NO2: Effect of Cyclic Interval  The response of PANI/ Na-erionite (Si/Al = 9) at 100 ppm of NO2 was found to be 67.63%, 57.41%, 43.32%, and 20% during first, second, third, and fourth cyclic intervals, respectively. This decreasing trend in the sensitivity after repeating the cyclic interval after 2–4 times is because of the fact that during first interval, the gas molecules which get adsorbed do not get desorbed quickly. The gas molecules which get adsorbed during the first interval take some time to get desorbed from the surface of the nanocomposite. Hence, the sensitivity of the composite goes on decreasing with the cyclic interval as shown in Fig. 14. Since the various cyclic intervals were carried out with no time gap between them, the gas molecules adsorbed in the first run do not get sufficient time to get desorbed. Hence, the response decreases. However, if there is a gap maintained between various cyclic intervals, the gas molecules would get sufficient time to get desorbed and there will not be too much loss of response over various cyclic intervals. Keeping in view eight-year warranty by US EPA for sensors, PANI/zeolite erionite composites have a good application for monitoring of gases such as NOx. We here mention that such PANI/zeolite erionite composites lose their sensitivity because of insufficient recovery time provided to them during various cyclic intervals (recovery

PANI/Zeolite Erionite (Si/Al = 9) Response to NO2: Effect of Zeolite Content  The response of PANI/ Na-erionite (Si/Al = 9) towards NO2 at 100 ppm concentration at various zeolite contents is shown in Fig. 15. The response of the nanocomposite increases from 10.40% to 67.63% as the zeolite content increases from 10% to 50% (w/w %), respectively. This increase in the response of the nanocomposite by increasing the zeolite content is due to the availability of active sites in large amount for gas molecules to get adsorbed so that they can penetrate deep inside the nanocomposite and interact with the PANI chains.[18] Beyond 50% of zeolite content, there is no increase in the response of the nanocomposite and attains a steady state. This steady state response of the nanocomposite beyond 50% of zeolite content is due to the reduction in the number of available active sites for NO2 molecules. Therefore, the response attains a constant value. PPY /Zeolite X Response to CO: Effect of Cation Type Figure 16 shows the effect of cation type on the response of the PPy/zeolite X composites towards CO at 5 ppm. For this purpose, four types of cations were selected: (1) Na+, (2) Fe +, (3) Cu+, and (4) H+. In all these composites, the zeolite content was kept at a constant value of 50%. It is clearly evident that the sensitivity of Na+ -X/PPy, H+ -X/

Sensors– Separation

Fig. 13  Response of PANI/erionite (Si/Al = 9) vs. time towards NO2 at 100 ppm [83]

2362

Sensors: Zeolite–Polymer Composites for Gas Sensing

80 PANI-Na-ERI

70

Response (%)

60 50 40 30 20 10

Ist run

2nd run

3rd run

Cyclic interval

4th run

Fig. 14  Response of PANI/Na-erionite (Si/Al = 9) towards NO2 at 100 ppm for various intervals 80

Sensors– Separation

70

PANI-Na-ERI

Response (%)

60 50 40 30 20 10 0

0

10

20

30

40

50

60

70

80

Zeolite content (%w/w)

Fig. 15  Response of PANI/Na-erionite (Si/Al = 9) vs. zeolite content towards NO2 at 100 ppm [83]

PPy, Fe + -X/PPy, Cu+ -X/PPy, and PPy at 5 ppm CO concentrations is 14.43%, 12.93%, 8.86%, 8.2%, and 6.3% respectively.

16 14 Sensitivity (%)

12

PPY /Zeolite X Response to CO: Effect of the Zeolite Content

10 8 6 4 2 0

Na

H

Fe

Cu

Ppy

Cation type

Fig. 16  Sensitivity of PPy and PPy/zeolite X composites of ­various cation types when exposed to 5 ppm of CO

The sensitivity of Na-X/PPy towards CO at a 50 ppm concentration at various zeolite contents is shown in Fig. 17. The sensitivity of the Na-X/PPy composite increases from 25.49% to 50.06% as the zeolite content increases from 10% to 60% (w/w %). This increase in the sensitivity by increasing the zeolite content is due to the increase in the surface area of the composite. Therefore, a large number of active sites become available for gas molecules to be adsorbed so that they can penetrate deep inside the composite and interact

55

Sensitivity (%)

50

-PPy/Na-X -at 50 ppm of CO

45 40 35 30 25 0

10

20

30

40

50

60

Zeolite content(% w/w)

70

80

90

Fig. 17  Sensitivity of Na-X/PPy at 50  ppm for CO vs. zeolite content

with the PPy chains. Beyond 60% of zeolite content, there is no further increase in the sensitivity of the Na-X/PPy composite, and it attains almost a steady state. This steady state sensitivity beyond 60% of zeolite content is because all the CO molecules present have active sites and further increasing the active sites are of no means.[85]

CONCLUSION The procedures given in this review paper for the synthesis of zeolite/polymer composites have been reported and practiced in various articles. There are two types of procedures for the synthesis of composites: one is the dry ­mixing of zeolite and polymer powder by a hydraulic press, and other is the polymerization of the monomer of a polymer in the zeolite framework and then pressing the powder into pellets. Among these two, the second one is best for the synthesis of composites. In this method, the monomer of the polymer fits itself inside the pore of the zeolite and undergoes polymerization. The electrical conductivity sensitivity of zeolite/polymer composites is affected by various variables, and these are discussed here. There occurs an initial increase in the electrical conductivity sensitivity of a zeolite/­polymer composite up to a certain increase in the zeolite content. This increase is because the addition of zeolite into  the composite increases interaction of the target gas and the conductive polymer. But above a certain limit of zeolite content, there is no increase in the conductivity of the composite. The electrical conductivity shows a decrease. This is due to the nonavailability of active sites present in the composite. Zeolites having a higher ion exchange capacity increase the electronic conductivity sensitivity of the composite. Zeolites having a favorable ion position inside the

pore of zeolite also enhance sensitivity. A channel system is more interactive in enhancing sensitivity than a cage system. Even if zeolites Y and 13X have a comparable pore size of 10 and 7 Å, yet they differ in their sensitivities. This is due to their different ion exchange capacities of 0.086 (zeolite Y) and 0.161 (zeolite13X) mol/g. Due to higher amount of Cu2+ in zeolite Y, there is a small space available for CO molecules. T ­ herefore, a small interaction occurs. Further, there is a favorable location of Cu2+ ions in zeolite 13X, which enhances the interaction and hence increases the sensitivity. A lower Si/Al ratio of composite results in lower sensitivity. On increasing the Si/Al ratio of the zeolite/polymer composite, an increase in the electrical conductivity sensitivity occurs. On increasing the Si/Al ratio, a gradual increase in the electrical conductivity sensitivity occurs. CO molecules get adsorbed more strongly on Na+ ions than on Cu+ ions. There occurs a more strong interaction between CO molecules and Cu+ ions. This is because CO molecules are Lewis bases. They interact with Cu+ ions and form coordinate bonds. Hence, Cu+ ions act as active sites in the zeolite for CO adsorption. CO molecules get adsorbed on Cu+ ions and do not interact with the polymer chains, thus resulting in a weak sensitivity of the composite. This is contrary to Na+ ions, in which the CO molecules do not interact with the cation rather than the polymer chains, hence resulting in a strong sensitivity. There will be a difference between the sensitivity of a composite during the first run, second run, and so on. This decrease in the sensitivity is because the vapors adsorbed in the first interval do not get desorbed simultaneously. Hence, it is an irreversible process. The concentration of a cation present in a zeolite/polymer composite affects the sensitivity based on the EN of the cation present in it. Less electronegative cation will not attract gas molecules more towards itself. Therefore, increasing the concentration of a less electronegative cation will lead to more proton mobility and ability to interact with the gas vapor. But the case is opposite for a more electronegative cation present in the zeolite pore. A zeolite having more Cu2+ ion exchange capacity will have a small temporal response. Therefore, a composite from such a zeolite would show a quick response to the gas vapors. A zeolite of comparable pore size is highly sensitive, whereas zeolites having excessively large pore size are less sensitive. The excessively large pore allows the gas molecules to enter and leave with the same ease. So, there occurs a small interaction and hence a lower sensitivity. Electrophilic gas molecules withdraw electrons from the polymer chains, resulting in an increase in the electrical conductivity. On the other hand, a nucleophilic gas molecule gives electrons to the polymer chains. Hence, there occurs a decrease in the number of charge carriers, polarons, and bipolarons, and a decrease in electrical conductivity occurs.

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53. Jiang, L.; Jun, H.K.; Hoh, Y.S.; Lim, J.O.; Lee, D.D.; Huh, J.S. Sensing characteristics of polypyrrole-poly(vinyl alcohol) methanol sensors prepared by in situ vapor state polymerization. Sens. Actuators B 2005, 105 (2), 132–137. 54. Babudri, F.; Farinola, G.M.; Giancane, S.; Naso, F.; Rella, R.; Scarpa, A.; Valli, L. Mater. Sci. Eng. C 2002, 22 (2), 445–448. 55. Bouchet, R.; Rosiri, S.; Vitter, G.; Siebert, E. Sens. Actuators B 2001, 76 (7), 610–616. 56. Graham, S.C.; Fung, S.; Moratti, S.C.; Friend, R.H. Synth. Met. 1999, 102, 1169–1170. 57. Hagen, G.; Dubbe, A.; Retting, F.; Jerger, A.; Birkhofer, T.; Muller, R.; Rlog, C.; Moos, R. Sens. Actuators B 2006, 119, 441–448. 58. Vilaseca, M.; Coronas, J.; Cirera, A.; Cornet, A.; Morante, R.J.; Santamaria, J. Sens. Actuators B 2007, 124, 99–110. 59. Zecchina, A.; Marchese, L.; Bordiga, S.; Paze, C.; Gianotti, E. J. Phys. Chem. B 1997, 101 (48), 10128–10135. 60. Yoshino, K.; Gu, H.B. J. Appl. Phys. 1986, 25 (12), 1064–1068. 61. Collins, G.E.; Buckley, L. J. Synth. Metal. 1996, 78 (2), 93–101. 62. Bhat, A.N.; Gadre, V.P.; Bambole, V.A. J. Appl. Polym. Sci. 2003, 88 (5), 22–29. 63. Vilaseca, M.; Yague, C.; Coronas, J.; Santamaria, J. Sens. Actuators B 2006, 117 (21), 143–150. 64. Phumman, P.; Niamlang, S.; Sirivat, A. Sensors 2009, 9 (10), 8031–8046. 65. Wessling, R.A.; Zimmerman, R.G. Polyelectrolytes from bis-sulfonium salts. US Patent 3, 401, 152, 1968. 66. Murase, I.; Ohnishi, T.; Noguchi, T.; Hiroka, M.; Murakaami, S. Mol. Cryst. Liq. Cryst. 1985, 118 (12), 333–336. 67. Murase, I.; Ohnishi, T.; Noguchi, T.; Hiroka, M. Polym. Commun. 1984, 25 (13), 327–329. 68. Montaudo, G.; Vitalini, D.; Lenz, R.W. Polymer 1987, 28 (5), 837–842. 69. Angell, C.L.; Schaffer, P.C. J. Phys. Chem. 1966, 70 (43), 1413–1418. 70. Zecchina, A.; Bordiga, S.; Lamberti, C.; Spoto, G.; Carnelli, L.; Otero, A.C. J. Phys. Chem. 1994, 98, 9577–9582. 71. Sauer, J.; Ugliengo, P.; Garrone, E.; Saunders, V.R. Chem. Rev. 1994, 94, 2095–2106. 72. Hush, N.S.; Williams, M.L. J. Mol. Spectrosc. 1974, 50 (1–3), 349–368. 73. Murase, I.; Ohnishi, T.; Noguchi, T.; Hiroka, M. Polym. Commun. 1984, 25 (13), 327–329. 74. Nareerat, T.; Ruksapong, K.; Sumonman, N.; Ladawan, W.; Anuvat, S.; Sujitra, W. Materials 2009, 2, 2259–2275. 75. Li, C.Y.; Wen, T.C.; Guo, T.F.; Hou, S.S. Polymer 2008, 49 (4), 957–964. 76. Xu, X.; Wang, J.; Long, Y. Sensors 2006, 6 (12), 1751–1764. 77. Smallwood, I.M. Handbook of Organic Solvent Properties; John Wiley & Sons: New York, 1996. 78. Jose, K.A.; Biju, P.; Ashwin, W.; Vijay, K.V.; Reddy, C.C. Smart Mater. Struct. 2004, 13 (5), 1045. 79. Tharaporn, P.; Anuvat, S.; Darunee, A.; Ladawan, W. Mater. Res. 2013, 16 (5), 1020–1029. 80. Jiang, L.; Jun, H.K.; Hoh, Y.S.; Lim, J.O.; Lee, D.D.; Huh, J.S. Sens. Actuators B 2005, 105 (2), 132–137.

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Separation: Poly(N-isopropylacrylamide) in Lei Yang and Jing Zhang School of Environmental and Biological Engineering, Liaoning Shihua University, Fushun, People’s Republic of China

Xiaoguang Fan College of Engineering, Shenyang Agricultural University, Shenyang, People’s Republic of China

Abstract Poly(N-isopropylacrylamide) (PNIPAAm) is one of the most studied and mainly applied thermosensitive polymers in recent decades, which is attractive and valuable because of its quick response to changes in environmental temperature. PNIPAAm often shows an inverse solubility or wettability across lower critical solution temperature. It can also be easily functionalized to gradually develop and expand its uses. The versatility of PNIPAAm and its derivatives has made them excellent for the applications in drug delivery, cell culture, tissue engineering, biosensors, etc. This review covers the definition, classification, properties, product types, shelf life, testing, and applications for separation.

INTRODUCTION OF POLY(N -ISOPROPYLACRYLAMIDE) Poly(N-isopropylacrylamide) (abbreviated as PNIPAAm) is a typical temperature-responsive polymer that is synthesized from N-isopropylacrylamide (NIPAAm) via free-radical polymerization, and it is readily functionalized making it useful in a variety of applications. PNIPAAm is very attractive because it displays reversible changes in hydration and dehydration during temperature variation and shows LCST at approximately 32°C in water, which is close to the physiological temperature.[1] CLASSIFICATION OF PNIPAAm AND ITS DERIVATIVES PNIPAAm and its derivatives are preliminarily classified into single chains and cross-linked gels from primary structures. Single Chains PNIPAAm with single chains are prepared via bulk polymerization, emulsion polymerization, suspension polymerization, and solution polymerization. One of the simplest and best known kinds of synthetic methods is solution polymerization. Herein, benzoyl peroxide, peroxyacetic acid and other peroxides, or azobisisobutyronitrile and Encyclopedia of Polymer Applications, First Edition DOI: 10.1201/9781351019422-140000359 Copyright © 2018 by Taylor & Francis. All rights reserved.

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Keywords: Cell culture; Controlled drug release; Lower critical solution temperature; Poly(N-isopropylacrylamide); Thermosensitive polymer.

other azo compounds are often used as free-radical polymerization initiators. Water, alcohol, ether, acetone, tetrahydrofuran, chloroform, and benzene are widely applied as solvents. Technology parameters, such as reaction temperature, reaction time, initiator dosage, and monomer ratio, have a major influence on the properties of synthetic polymers.[2] PNIPAAm can also be easily functionalized using of chain transfer agents [3] or copolymerization of functional monomers with double bonds [4] via free-radical polymerization. The former schemes are typically named as chain transfer reactions where the activity of a growing polymer chain is transferred to another molecule to form a derivative polymer with radical initiator end and thermosensitive group end, which allow for the polymer to be applied in more different settings and applications. The latter routines involve the copolymerization of NIPAAm and other monomer species to modify multiple properties of the manufactured polymers to meet specific needs, e.g., to enhance biocompatibility, change transition temperature, control wetting properties, alter mechanical properties, or improve biodegradability. Cross-Linked Gels PNIPAAm-based hydrogels are usually formed by precipitation polymerization, suspension polymerization, membrane emulsification, and so on. Precipitation polymerization is a heterogeneous polymerization process that begins initially

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as a homogeneous system in a continuous phase (usually water), where NIPAAm, initiator (usually ammonium persulfate along with tetramethylethylenediamine), and cross-linking agent (usually N ­ ,N'-methylene-bisacrylamide [MBAAm]) are completely soluble, but upon initiation, the formed cross-linked ­PNIPAAm is insoluble and thus precipitates.[5,6] After precipitation, polymerization proceeds by absorption of NIPAAm and initiator into the PNIPAAm particles. The particle size is often limited to the micron and submicron levels. Suspension polymerization is a heterogeneous polymerization process that begins with a mixture of NIPAAm and cross-linker (usually MBAAm) in oil phase by means of mechanical agitation with the support of a surfactant and then follows by PNIPAAm polymerization to form polymer spheres.[5] The particle sizes are often distributed between 20 μm and 2 mm, [7] which mainly depends on the agitation speed, reactor type, and stirrer style.[8] Membrane emulsification is also a useful technique for producing cross-linked PNIPAAm gels. In this case, PNIPAAm aqueous solution is forced through the pores of a microporous membrane directly into oi phase.[5] The emulsified droplets are formed and then detached individually drop by drop from the end of the pores located at the microporous membrane. Controlled droplet sizes and narrow size distributions can be obtained from this process, and the sizes of uniform droplets can be up to micrometers.[9] The droplets are sequentially transformed into solid particles by polymerization, [10] solvent evaporation, [11] and gelation.[12]

Separation: Poly(N-isopropylacrylamide) in

PNIPAAm and its derivatives are more widely used in various solutions with different concentrations of salt and other species, different pH value, or temperature in practical applications. If the PNIPAAm-containing materials are placed in salt solutions or pH buffers, the particle sizes happen to change. The addition of salts weakens the hydrogen bonds between amide linkages of PNIPAAm and water molecules, resulting in the greater shrinkage of polymers with long chains and hydrogels or the larger aggregation of polymers with short chains in aqueous solutions at the temperature above LCST. Furthermore, the degree of shrinkage or aggregation depends on the valence of the counter ion and the salt concentration. [13,16] The presence of other monomers in PNIPAAm-based copolymers may even more affect the configuration changes and temperature sensitivity in water and salt solutions. [17] PNIPAAms are not pH-sensitive polymers, but if the acidic or alkaline compositions (e.g., acrylic acid and propylacrylic acid) are introduced to the PNIPAAm backbone during the synthesis, the final copolymers display dual thermoresponsive and pH-responsive properties. The copolymer compositions and solution parameters, such as pH value and ionic strength, have a major influence on the hydrated/dehydrated degrees and LCST of copolymers. [18] Furthermore, addition of hydrophilic comonomers increases the LCST up to physiological temperature, [19] whereas the introduction of hydrophobic groups decreases the LCST. [20] Mechanism of Thermosensitivity

PROPERTIES OF PNIPAAm AND ITS DERIVATIVES Thermosensitive Character There have been two distinct thermosensitive characters of PNIPAAm and its derivatives at LCST in purified water. Across LCST, the particle size of the temperature-­ responsive products shows an abrupt change from large to small or generates a sharp transition from small to large. The former is often found in macrogels or microgels presenting swollen-to-collapsed changes [13] and polymers with long linear or branched chains showing coil-to-­ globule transitions.[14] The latter can be observed in linear or block or micelle polymers with short chains exhibiting segment-to-cluster conversions.[15] Although there are two opposite expressions, both collapse and aggregation are caused by the loss of hydrogen bonding between water molecules and amide linkages and by the formation of hydrophobic interactions between isopropyl groups and hydrogen bonding between amide linkages. When the environmental temperature is higher or lower than LCST, there will be an internal structural adjustment between molecules, but the particle sizes are not changed obviously.

The thermoresponsive behavior of PNIPAAm is related to hydrophobic isopropyl groups and hydrophilic amide bonds, as shown in Fig. 1. When the ambient temperature is lower than LCST, the hydrogen bonding between water molecules and amide bonds forms a stable hydration structure around hydrophobic groups. When the temperature rises, the hydrated structure is destroyed, and the hydrogen bonds from amide groups and water molecules gradually convert into amide hydrogen bonds. Then the hydrophobic interactions dominate; thus, bound water is transformed into free water and spreads outward. TYPES OF PRODUCTS BEARING PNIPAAm For ultimate purpose, PNIPAAm and its derivatives are divided into planar films and spatial substrates. Planar Films There are three different strategies for the preparation of planar films: grafting, coating, or both aggregated, as ­displayed in Fig. 2.

Separation: Poly(N-isopropylacrylamide) in 2369

*

H2C

CH

* n

C

O

NH

H

Hydrophilic amide bond

O

CH H 3C

C

CH3

N

Hydrophobic isopropyl group

Grafting Films For insoluble grafting films, it is crucial to immobilize PNIPAAm and its derivatives to substrates or ­supports (such as glass, polystyrene, and polypropylene). The p­ rocesses often require sophisticated and expensive processing means, [21–23] such as electron beam (EB) i­ rradiation, plasma treatment, and ultraviolet (UV) irradiation, or involve a chemical approach to induce the bonding of p­ olymers to substrates.[24,25] Among several techniques for establishing thermoresponsive grafting films, PNIPAAm and its derivatives are grafted to the surfaces of polystyrene culture plates by EB irradiation, which is the most widely used technique. The recrystallized NIPAAm monomers are first dissolved in 2-propanol and added evenly into culture plates, then irradiated by 0.25MGy EB, and finally, the grafted culture plates are repeatedly rinsed by distilled water and dried in

a vacuum oven.[26] Plasma polymerization is another common method for the preparation of temperature-sensitive surfaces, which can be used to produce highly networked thin films without any cross-linking agents.[27] The plasma deposition coatings have excellent physical properties: the films are sterile and voidless, and completely cover and attach to the substrates. However, monomers are vulnerable to ruptures if subjected to plasma treatment; therefore, the active functional chemical structure of monomers will be destroyed, and accordingly, it causes the deficiencies of material characteristics. Grafting polymerization by UV irradiation is also simple and feasible to change the properties of substrate surfaces. Compared with other modification methods, UV irradiation has the following advantages: fast reaction rate, low energy consumption, low cost, simple equipment, and easy industrialization; the reaction occurs only in the shallow surface region near the surface.[28] Because of the weak penetrability of UV rays,

Electron beam irradiation Plasma treatment Ultraviolet irradiation

(a) Grafting

Chemical approach PNIPAAm grafted film

NIPAAm solution

PNIPAAm solution Dying

(b) Coating PNIPAAm preparation

PNIPAAm coated layer preparation preparation

(c) Coating– grafting

Dip coating Spin coating

Preparation of PNIPAAm associated with siloxane and hydroxyl groups

Heating Annealing

PNIPAAm coated layer

PNIPAAm grafted film

Fig. 2  Strategies for the preparation of planar films bearing PNIPAAm, including (a) grafting, (b) coating, and (c) coating–grafting

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Fig. 1  Molecular structure of PNIPAAm

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the substrates are seldom affected by irradiation, so the surface photografting polymerization can only modify and control the surface properties without damaging the main materials. The thicknesses of grafting planar films tend to be determined by reaction parameters, such as irradiation time, temperature, initiator, and concentration. Atom transfer radical polymerization (ATRP) and reversible addition-fragmentation chain transfer (RAFT) polymerization are able to offer special designing plans for the preparation of thermosensitive surfaces.[29–31] ATRP usually uses a transition metal complex (namely, those of Cu, Fe, Ru, Ni, etc.) as the catalyst with an alkyl halide as the initiator, while RAFT polymerization uses thiocarbonylthio compounds as the chain transfer agents, such as dithioesters, thiocarbamates, and xanthates. Typically, ATRP and RAFT polymerization systems are suitable for use under a wide range of reaction parameters, and both of them are known for their compatibility with a variety of functional monomers. But the impurities such as transition metal ions and bipyridine or residual reagents are difficult to remove from the resultant products in the ATRP process, and the introduction of thiocarbonylthio compounds would increase the toxicity of the synthetic polymers in the RAFT polymerization, which cause difficulties for the pretreatment of thermoresponsive planar films and negative influences of cytotoxicity. The abovementioned processing methods can achieve in situ copolymerization of NIPAAm and other monomers, which play an important role in the covalent cross-­linking of polymers as well as grafting of polymers and substrates, to make the end products have good stability and durability, and make the film thickness reach a nanometer level. But owing to random polymerization and grafting sites, in situ copolymerization is inefficient to regulate the structure of polymer films. Nitroxide radical polymerization (NMP) is a remedy that makes use of alkoxyamine initiators to generate PNIPAAm-­containing films with well-defined, functional, and complex m ­ acromolecular architectures.[32]

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adjusting polymer concentration, and changing coating conditions. However, the aid of solvents potentially damages bioactive molecules, the thicknesses of coating films are often difficult to reach nanoscale, and the lack of cross-link of intermolecular chains and further connections between molecular chains and substrates makes the planar films really easy to detach from the substrates or dissolve in the working solution when the ambient temperature is lower than their LCST, namely, poor stability and ­reversibility.[36,37] Coating–Grafting Integrated Films Coating and grafting of PNIPAAm and its derivatives on a substrate have been often used to create PNIPAAm films. But it can be seen from the description above that both methods have their own shortcomings, such as applying expensive instruments and tedious techniques, introducing complex raw materials with unknown toxicity, being difficult to remove impurities or reagents, and having insufficient stability and reversibility of final products. This is not conducive to the promotion and application of ­thermosensitive films. Our laboratory has developed a novel approach to graft PNIPAAm films onto silica-based surfaces.[4] This is to utilize these two strategies simultaneously, which is called coating–grafting two-step film formation. Briefly, the copolymers with siloxane groups and hydroxyl groups are first synthesized, and subsequently, the copolymer solution is evenly spin-coated or dip-coated on the uniform and composite surfaces bearing residual hydroxyl groups. Silyl cross-linking usually starts during film annealing by thermal heating. Condensation reaction between siloxane and hydroxyl groups leads to the methanol removal, and the newly formed bonds provide integration within the film and coupling with the substrate surfaces. The coating– grafting integrated technique used in our studies provides a straightforward and economical approach for creating thermoresponsive surfaces compared with the traditional techniques.

Coating Films Spatial Substrates Unlike chemical attachment, two successive steps would normally be required to produce physically absorbed coating films, including preparation of PNIPAAm polymers and coating of polymer solutions.[33,34] It has been reported that a solution of PNIPAAm or derivatives dissolved in a volatile solvent is first transferred into the wells and treated by natural or manual drying, then the surface is washed to eliminate non-adsorbed polymers, thereby forming a uniform adsorption layer of thermosensitive polymers on the substrate surface.[33,35] The two-step approach is advantageous because of the ease of film coating, commercial availability, modification progress in polymerization by means of adding functional monomers, as well as controllable film thickness regulated by selecting solvent,

The studies on temperature-sensitive planar films based on PNIPAAm and its derivatives have achieved initial results; however, two-dimensional (2D) techniques suffer from the inherent limitation of a flat surface, leading to its unsuitability for scale-up to large quantities in industrial manufacturing. The three-dimensional (3D) systems can not only retain the material and structural basis of microenvironment but also reflect the intuition and controllability. Therefore, more researchers gradually try to introduce PNIPAAm and its derivatives into hydrogels, microcarriers, scaffolds, and other stereoscopic systems and accordingly to form thermosensitive 3D platforms for supporting a wide variety of applications.

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Thermoresponsive Hydrogels

Thermoresponsive Microcarriers In recent decades, scholars have gradually developed thin PNIPAAm layers on the solid surfaces to form various

(a) Grafting to

(b) Grafting from

(c) Grafting through

Fig. 3  Strategies for the fabrication of thermoresponsive microcarriers, including (a) “grafting to,” (b) “grafting from,” and (c) “grafting through”

thermoresponsive microcarriers by different fabrication methods, as shown in Fig. 3. Either way, PNIPAAm and its derivatives should be grafted and distributed on the microsphere surfaces. The “grafting to” approach is to set up multipoint attachment between end-functionalized PNIPAAm and reactive groups located on microsphere surfaces, [6] such as carboxyl-terminated PNIPAAm grafted to aminated glass beads [41] and commercial Cytodex-3(R) microcarriers [42] or amino-terminated PNIPAAm bonded with carboxylate groups on sodium alginate to prepare for the formation of thermoresponsive calcium alginate beads. [43] The second approach is known as “grafting from”; in this case, molecular chains of PNIPAAm and its derivatives grow in situ from the reactive groups (usually polymerization initiators) that have been already immobilized covalently on the surfaces of the microcarriers.[44] This method contributes to the deposition of abundant molecular chains bearing PNIPAAm and its derivatives onto the surfaces compared with the “grafting to” approach, but it provides little control over the thickness of the resultant polymers.[45] Many techniques including ATRP, RAFT, and NMP belonging to surface-initiated controlled/­living radical polymerization processes could be of help to ­overcome the drawback.[6] Another alternative to prepare thermoresponsive ­microcarriers is the “grafting through” approach, which depends on the copolymerization of the surface-tethered monomers with the growing PNIPAAm polymers initially formed in solution via bulk free-radical polymerization. The ­proposed method combines the elements of ­“grafting to” and “grafting from” approaches, and it allows the control of polydispersity, functionality, backbone length, branch length, and reactivity ratio of monomers and ­PNIPAAm.[46,47]

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Hydrogels generated from thermoresponsive ­polymers with a cross-linked network enable themselves to undergo adjustable sol–gel transitions in response to temperature. The thermoresponsive hydrogels are hydrophilic and highly absorbent when the temperature is lower than their LCST, while they undergo phase transition from a swollen hydrated state to a shrinking dehydrated state when heated in water above their LCST. The thermoresponsive hydrogels can mimic native tissue properties, structure, and microenvironment due to their hydrated and interconnected porous structure. ­PNIPAAm-based hydrogels are generally prepared by precipitation ­polymerization, suspension polymerization, and membrane ­emulsification as described in the previous paragraphs. The thermoresponsive hydrogels can also be classified into macroscopic gels, microgels, and nanogels by their size and designed purpose. Interestingly, the driving force and equilibrium extent of swelling are the same for either a microgel (a nanogel) or a macrogel with the same components, however, the dynamic kinetics of swelling are sensitive to the gel size. [38] It is well accepted that it takes longer for macroscopic gels to respond in response to stimuli-triggered factors, which is difficult to accord with the clinical requirements. [39] In addition, the oversized particles fail to be applied in vivo, therefore, the macroscopic gels are generally used in primitive exploration and mechanism study. However, in comparison with macroscopic gels, the acceptable flexibility and improved responsiveness have been shown in the thermoresponsive microgels, which are generally spherical particles with a diameter of 0.1–10 μm. The nanogels composed of synthetic PNIPAAm and its derivatives are usually in tens to hundreds of nanometers in diameter, so the response of nanogels to external stimulus is ­sufficiently smart. The thermoresponsive hydrogels can also be divided into native hydrogels and composite hydrogels (known as hybrid hydrogels) based on their chemical composition. The performance of PNIPAAm hydrogels is relatively monotonous, whereas the properties of composite hydrogels, such as swelling, degradation, and chemical functionality can be elaborately tailored. The thermoresponsive hydrogels can also be categorized into homogeneous hydrogels and heterogeneous hydrogels. The homogeneous hydrogels have the same composition and structure in the whole network, [40] whereas the heterogeneous hydrogels have a different composition and ­structure, such as core–shell and yolk–shell microgels.

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Stability of PNIPAAm-mediated products is mandatory for some biomedical applications, such as wound dressing and large-scale cell culture. Good stability and ­reliability mean that the final products can be used widely and repeatedly, so it is necessary to discuss the stability of PNIPAAm under various working conditions of temperature, pH, and ionic strength in aqueous medium.[40] It is well accepted that high temperature, low pH, and high ionic strength lead to particle aggregation and decrease the stability, whereas the opposite cases stabilize the thermoresponsive ­materials for different applications.[48] It is also demonstrated that the introduction of acrylic acid, dextran, and chitosan into PNIPAAm backbone increases particle stability, and higher amount of cross-linking agent remarkably enhances its stability and lifetime. Furthermore, storage conditions before use are also critical factors affecting the stability of PNIPAAm-mediated products. The storage temperature, humidity, airtightness, time, etc. should be carefully considered. Typically, storage below LCST and in low ­humidity is more conducive to maintain the stability. Degradation Conversely, in other biomedical applications, the trashed PNIPAAm products should be removed or degraded (or biodegraded) after accomplishing their missions, namely, biodegradability is highly desirable. But PNIPAAm, unfortunately, has poor degradability, [49,50] which has limited its practical applications especially in clinical medicine (drug delivery and cell encapsulation). There are currently some strategies to explore biodegradable PNIPAAm-mediated materials, including introduction of biodegradable polymers (such as poly(esters), [51,52] ­poly(caprolactone), [53,54] and poly(ethylene glycol) [PEG] [55,56]), incorporation of degradable cross-linkers (such as acetals and ­disulfides), [57,58] or utilization of natural polymers (such as poly(amino acids), [59] polysaccharides, [60] and proteins [61]). It is also essential to consider the effect of degradative or fractured components on the environment or organisms. PNIPAAm-based systems have to be designed and ­modulated according to specific requirements.

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have been applied to characterize PNIPAAm-based materials. The common physical or chemical testing techniques are listed in Tables 1–3, respectively. Many other robust techniques are equally used, but they will not be covered herein. Identification for PNIPAAm and Its Derivatives Element analysis can be used for the determination of the existing elements and their contents in the PNIPAAm-based materials to be tested. It generally analyzes C, H, O, N, S, and other elements, particularly for analyzing the relative contents of C, H, and O elements. The characteristic peaks Table 1  Identification for PNIPAAm and its derivatives Characterization techniques Measurement data Element analysis

Existing element

FTIR

Functional group

NMR spectroscopy

Molecular composition

GPC

Molecular weight

SEC

Molecular weight

MS

Molecular structure

DLS

Polymer conformation

SLS

LCST

DSC

Phase transition temperature

Table 2  Characterization for thermoresponsive films bearing PNIPAAm Characterization techniques Measurement data XPS

Elemental and chemical composition

SFG

Chemical composition Structural information

ATR-FTIR

Grafting density Grafting amount

TOF-SIMS

Molecular composition

SEM

Surface topology

AFM

Surface topology

SE

Dry thickness

SPR

Wet thickness

Contact angle measurement

Surface wettability

TESTING OF PRODUCTS BEARING PNIPAAm No matter what approach is applied to prepare PNIPAAm-mediated products, it is necessary to determine the chemical composition, thermosensitive character, molecular weight, spatial scale, rheological behavior, swelling properties, grafting density, grafting thickness, surface topography, surface wettability, and other feature parameters of transitional or final products. Many techniques

Table 3  Characterization for thermoresponsive hydrogels bearing PNIPAAm Characterization techniques Measurement data DLS

Swelling behavior

DSC

Swelling behavior

Dynamic rheology

Rheological behavior

Raman microscopy

Pore structure

are assigned to the functional groups of PNIPAAm samples, according to the location and shape of the absorption peaks appearing in the Fourier transform infrared spectroscopy (FTIR). In addition, the ratio of each component in a hybrid complex can also be inferred from peak intensity. The stretching vibration absorption peak of amide bond I and the bending vibration absorption peak of amide bond II detected at 1650 and 1550 cm−1, and the symmetrical deformation vibration absorption peaks of isopropyl bond discovered at 1387 and 1367 cm−1 are the typical absorption peaks of NIPAAm.[62] The precise molecular structure of PNIPAAm compounds can be determined by analyzing the information of chemical shift, peak shape, integral area, and coupling constant obtained from nuclear magnetic resonance (NMR) spectroscopy. The relative molecular weight and molecular weight distribution of polymers can be obtained according to the elution curves of gel permeation chromatography (GPC) or size-exclusion chromatography (SEC).[63,64] In general, the native or composite PNIPAAm has an intermediate polydispersity index (weight average molecular weight/ number average molecular weight). Mass spectrometry (MS) is an analytical technique that ionizes chemical species and sorts the ions based on their mass-to-charge ratio in a high vacuum system, [65] which can be applied to determine the molecular structure and molecular weight of the thermosensitive polymers. Dynamic and static light scattering (DLS and SLS) can predict the interaction between PNIPAAm polymers and can be used to determine the LCST of temperature-­ sensitive systems. Across LCST, the particle sizes of ­PNIPAAm products exhibit an abrupt change from large to small or show a sharp transition from small to large.[66–68] Differential scanning calorimetry (DSC) is a thermoanalytical technique in which the difference in the amount of heat required to increase the temperature of sample and reference is measured as a function of temperature. It can be used to determine the phase transition temperature, crystallization temperature, structural relaxation peak, as well as crystallization enthalpy change of ­PNIPAAm-mediated materials.[69] Characterization for Products Bearing PNIPAAm Characterization for Thin Films Bearing PNIPAAm X-ray photoelectron spectroscopy (XPS) is a surface-­ sensitive quantitative spectroscopic technique that measures the elemental and chemical composition of polymer coatings. XPS spectra are acquired by irradiating a dry material with X-ray beam and simultaneously monitoring the number of electrons which escape from the topmost 10 nm of the material surface.[23] Thus, XPS technique can supply the related messages of chemical composition and coating uniformity of PNIPAAm-based surfaces. XPS with high resolution can also produce the useful information on

chemical structure of polymer coatings according to the chemical states and chemical shift for functional groups.[70] Sum frequency generation (SFG) spectroscopy is another technique that can be applied to deduce the chemical composition, orientation distributions, and structural information at the outermost surfaces or two-phase interfaces. This strategy has the capacity to be sensitive to the monolayer surface and the ability to be conducted in situ and does not cause much damage to the material surface.[71] Attenuated total reflection (ATR) is a reliable sampling technique usually associated with FTIR (ATR-FTIR) that enables samples to be examined directly in the solid or liquid state without further preparation. Thus, ATRFTIR might actually have the capacity of determining the grafting density and grafting amount of thermoresponsive products.[72] For tissue culture polystyrenes (TCPSs), the absorption peak observed at 1600 cm−1 is attributable to monosubstituted aromatic rings. For PNIPAAm coating, the absorption peak of amide bond I is observed at 1650 cm−1. Then the peak intensity ratio of I1650 to I1600 is used to evaluate the grafting density of PNIPAAm or its derivatives on the TCPS surfaces.[72,73] This technique may also be applied to investigate the influence of bond formation and disintegration on the swelling behavior.[1] The time-of-flight secondary ion mass spectrometry (TOFSIMS) is a complementary surface detection technology with high mass resolution of ppm level, which can provide information on the molecular composition of the topmost layer of a thermoresponsive sample.[30,74,75] Scanning electron microscopy (SEM) is a type of electron microscope that produces the information about the surface topography and chemical composition of samples, with the resolution better than 1 nm. Atomic force microscopy (AFM) is a very-high-resolution type of scanning probe microscopy, with demonstrated resolution on the order of fractions of a nanometer. Both of them can be used to analyze the surface structure of temperature-sensitive surfaces [76,77] and further examine the changes of surface morphology as a function of temperature. Spectroscopic ellipsometry (SE) is a very useful measurement technique and have unequaled abilities for characterizing the metrology of thin PNIPAAm-grafted or PNIPAAm-coated films, including composition, roughness, thickness, electrical conductivity, and other material properties.[4,25,29] But routine SE seems to work only for the measurement for dry thickness of grafted layer. For measuring the changes of film thickness with temperature shift in aqueous solutions, special devices are required.[4,78] Fortunately, surface plasmon resonance (SPR) can give the wet thickness of PNIPAAm layer in a liquid environment, taking into account swelling behavior.[64] Surface wettability is another factor that needs to be examined, which is an important parameter affecting protein adsorption or desorption.[25,71,72,79] Contact angle is generally used to quantify the wettability of thermoresponsive surfaces via sessile drop method by optical arrangement,

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and it is measured using a goniometer at a given temperature and pressure. Contact angle is often attributed to surface roughness, surface heterogeneity, and swelling behavior.[1] When the temperature is below LCST, lower contact angle suggests more hydrated thermoresponsive films. As the temperature goes up, the contact angle of thin films is expected to be higher than that in the lower temperature, indicating that the films are hydrophobic in this case. The contact angle analysis is able to demonstrate the thermosensitivity of thin films. Characterization for Hydrogels Bearing PNIPAAm

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Thermoresponsive swelling is the most interesting property of PNIPAAm-based hydrogels. DLS is a powerful technique for swelling evaluation. Nevertheless, the diameters of swollen particles approaching 1 μm generally exceed the sensitive measurement range of DLS, which limits the data accuracy of swelling changes.[38] DSC has also been used to characterize the swelling behavior of PNIPAAm-based hydrogels.[80] Dynamic rheology is a common method to investigate the rheological behaviors of PNIPAAm-based hydrogels, which can be used to determine the transformation of sol–gel with temperature changes. It is reported that the rheological properties of homogeneous PNIPAAm

(a)

microgels are sensitive to temperature, shear rate, and concentration. [81] Raman microscopy is a spectroscopic technique, which can be used to image the pore structure of PNIPAAm macrogels, but low resolution is not suitable for microgel ­characterization.[38] APPLICATIONS OF PNIPAAm FOR SEPARATION PNIPAAm is a thermoresponsive polymer that is widely used in bioengineering applications for separation. Here, we highlight the drug-controlled release system and smart cell culture substrate, as displayed in Fig. 4. Controlled Drug Release System Drug is released slowly by diffusion or upon degradation of polymer network in conventional drug delivery systems, which lead to an early peak of drug concentration in plasma followed by steady, linear release.[82] This is hard to precisely control the local drug concentration and delivery location, thus therapeutic effect is insufficient. Controlled and targeted drug release takes great advantages over traditional formulations.[83] PNIPAAm and its derivatives are likely able to be applied for the challenging purpose, which

Targeted drug

Crosslinked gels T

Noncrosslinked gels

(b)

Adherent cells

Planar films T

Microcarriers

Fig. 4  Applications of PNIPAAm for (a) drug-controlled release system and (b) smart cell culture substrate

can maintain the drug concentration within the desired therapeutic range at a specific location. Over the decades, massive efforts have been devoted to the design and preparation of controlled drug release systems bearing PNIPAAm and its derivatives. Typically, PNIPAAm hydrogel swells in aqueous solutions below its LCST, and it shrinks when the temperature is higher than LCST. To sum up, two situations occur. In the first case, the entrapped drug within the PNIPAAm hydrogel will be squeezed out as the hydrogel is compressed; in the second case, the embedded drug within the PNIPAAm hydrogel will flow spontaneously to the bulk solutions when the hydrogel is expanded. Temperature change can intelligently trigger quick response of PNIPAAm hydrogel, and accordingly motivate on–off switch for drug release. The temperature of pathological site is usually higher than normal physiological temperature (37°C), so the LCST of PNIPAAm-based drug carriers is expected to increase ranging from 37.5°C to 45°C, [83] which can be easily and safely achieved in clinical hyperthermia without inducing potentially harmful side effects in ­surrounding healthy tissue.[84,85] As described previously, addition of hydrophilic monomers such as acrylamide, N-(hydroxymethyl) acrylamide, [86] PEG diacrylate, [87] 2-carboxyisopropylacrylamide, [88] acrylic acid, [89] and poly(2-hydroxyethyl methacrylate) [90] increases the LCST and further accelerates rapid hydration of thermoresponsive materials during temperature reduction, which allows to design different controlled drug release systems for specific pathological sites. Thermosensitivity is monotonous for drug release systems. The copolymerization of PNIPAAm with other monomers containing functional groups will enhance its sensitivity to pH [91,92] and glucose, [62,93,94] which gives us more options in the design of controlled drug release systems. Furthermore, particle size is a key factor in targeted drug delivery systems. The swelling behaviors are mainly associated with the initial sizes of thermoresponsive hydrogels, so size reduction is expected to enhance the response rate. The particles with micron and submicron scale are easily identified in human immune system, so they are less likely to be applied in vivo. Therefore, multi-responsive carriers with lower nanoscale have become new favorite in the controlled drug release systems. Smart Cell Culture Substrate The thermoresponsive PNIPAAm and matching thermal-­ liftoff method is an effective alternative to enzymatic and mechanical pathways. There is already growing evidence that this unique cell culture system not only provides an appropriate growth environment for anchorage-­dependent cells at physiological conditions but also supports mild and efficient cell recovery by reducing temperature. Above LCST, cells readily adhere and grow on the thermoresponsive substrate with weak

hydrophobicity and shrunken structure, whereas below LCST, cells detach  spontaneously from the hydrophilic thermoresponsive surface due to the hydrated and extended PNIPAAm chains. [95] The smart culture ­system based on PNIPAAm mostly takes full advantage of ­surface wettability variations or bulk structure changes with temperature alteration to control cell attachment or detachment. The thermally induced cell detachment can maintain better growth, faster proliferation, and stronger differentiation of therapeutic cells, owing to cell release accompanied by integral membrane proteins and most intercellular ­linkage proteins. The studies on 2D thermoresponsive substrates have made initial achievements in cell culture and recovery. More scholars are looking for ways to facilitate effective adhesion and rapid detachment of adherent cells on thermoresponsive surfaces, such as adjusting reactant ratio, [96] introducing hydrophilic groups,[97] manipulating thickness/density of polymer layer, [98] changing surface roughness, [99] creating patterned surface, [100] setting optimum preparation condition, [101] determining optimal storage condition, [102] selecting special basal material, [103] and providing adjustable driving force.[104] Recently, there has been rapid development from 2D to 3D in thermoresponsive culture platforms involved in microcarriers, [105] scaffolds, [106] hydrogels, [107] hollow fiber membranes, [108] and other stereoscopic systems. The 3D thermosensitive culture systems can not only retain the material and structural basis of the microenvironment in vivo but also reflect the intuition and controllability during cell culture in vitro; facilitate the combination of cell culture techniques and tissue engineering applications; and furthermore, be useful for large-scale expansion of adherent cells, nonenzymatic cell recovery, and tissue repair or organ reconstruction. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21604034), Doctoral Scientific Research Foundation of Liaoning Province of China (20170520391), and General Scientific Research Program of the Department of Education of Liaoning Province of China (LSNYB201619) and (L2015307). REFERENCES 1. Andriola Silva Brun-Graeppi, A.K.; Richard, C.; Bessodes, M.; Scherman, D.; Merten, O-W. Thermoresponsive surfaces for cell culture and enzyme-free cell detachment. Prog. Polym. Sci. 2010, 35 (11), 1311–1324. 2. Ren, Y.R.; Huo, D.Q.; Hou, C.J. Thermosensitive poly(N-isopropylacrylamide) and its application. Mater. Rev. 2004, 18 (11), 54–56. 3. Semsarilar, M.; Ladmiral, V.; Perrier, S. Synthesis of a cellulose supported chain transfer agent and its application

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67. Xie, D.H.; Ye, X.D.; Ding, Y.W.; Zhang, G.Z.; Zhao, N.; Wu, K.; Cao, Y.; Zhu, X.X. Multistep thermosensitivity of poly (N-n-propylacrylamide)-block-poly(N-isopropylacrylamide)- block-poly(N,N-ethylmethylacrylamide) triblock terpolymers in aqueous solutions as studied by static and dynamic light scattering. Macromolecules 2009, 42 (7), 2715–2720. 68. Zheng, S.Q.; Shi, S.X.; Xia, Y.Z.; Wu, Q.J.Y.; Su, Z.Q.; Chen, X.N. Study on micellization of poly(N-isopropylacrylamide-butyl acrylate) macromonomers in aqueous solution. J. Appl. Polymer Sci. 2010, 118 (2), 671–677. 69. Gao, Y.T.; Yang, J.X.; Ding, Y.W.; Ye, X.D. Effect of urea on phase transition of poly(N-isopropylacrylamide) investigated by differential scanning calorimetry. J. Phys. Chem. B 2014, 118 (31), 9460–9466. 70. Cole, M.A.; Voelcker, N.H.; Thissen, H.; Griesser, H.J. Stimuli-responsive interfaces and systems for the control of protein-surface and cell-surface interactions. Biomaterials 2009, 30 (9), 1827–1850. 71. Cheng, X.H.; Canavan, H.E.; Stein, M.J.; Hull, J.R.; Kweskin, S.J.; Wagner, M.S.; Somorjai, G.A.; Castner, D.G.; Ratner, B.D. Surface chemical and mechanical properties of plasma-polymerized N-isopropylacrylamide. Langmuir 2005, 21 (17), 7833–7841. 72. Akiyama, Y.; Kikuchi, A.; Yamato, M.; Okano, T. Ultrathin poly(N-isopropylacrylamide) grafted layer on polystyrene surfaces for cell adhesion/detachment control. Langmuir 2004, 20 (13), 5506–5511. 73. Na, K.; Jung, J.; Kim, O.; Lee, J.; Lee, T.G.; Park, Y.H.; Hyun, J. “Smart” biopolymer for a reversible stimuli-­ responsive platform in cell-based biochips. Langmuir 2008, 24 (9), 4917–4923. 74. Canavan, H.E.; Graham, D.J.; Cheng, X.H.; Ratner, B.D.; Castner, D.G. Comparison of native extracellular matrix with adsorbed protein films using secondary ion mass spectrometry. Langmuir 2007, 23 (1), 50–56. 75. Cole, M.A.; Jasieniak, M.; Thissen, H.; Voelcker, N.H.; Griesser, H.J. Time-of-flight-secondary ion mass spectrometry study of the temperature dependence of protein adsorption onto poly(N-isopropylacrylamide) graft coatings. Anal. Chem. 2009, 81 (16), 6905–6912. 76. Xu, F.J.; Zhong, S.P.; Yung, L.Y.; Kang, E.T.; Neoh, K.G. Surface-active and stimuli-responsive polymer-Si(100) hybrids from surface-initiated atom transfer radical polymerization for control of cell adhesion. Biomacromolecules 2004, 5 (6), 2392–2403. 77. Chen, B.Y.; Xu, F.J.; Fang, N.; Neoh, K.G.; Kang, E.T.; Chen, W.N.; Chan, V. Engineering cell de-adhesion dynamics on thermoresponsive poly(N-isopropylacrylamide). Acta Biomaterialia 2008, 4 (2), 218–229. 78. Tang, Y.; Lu, J.R.; Lewis, A.L.; Vick, T.A.; Stratford, P.W. Swelling of zwitterionic polymer films characterized by spectroscopic ellipsometry. Macromolecules 2001, 34 (25), 8768–8776. 79. Curti, P.S.; de Moura, M.R.; Veiga, W.; Radovanovic, E.; Rubira, A.F.; Muniz, E.C. Characterization of PNIPAAm photografted on PET and PS surfaces. Appl. Surf. Sci. 2005, 245 (1–4), 223–233. 80. Dong, L.; Hoffman, A.S. A novel approach for preparation of pH-sensitive hydrogels for enteric drug delivery. J. Control. Release 1991, 15 (2), 141–152.

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Separation: Polymers in Shahram Mehdipour-Ataei and Samal Babanzadeh Iran Polymer and Petrochemical Institute, Tehran, Iran

Abstract Separation is one of the main branches of chemistry and chemical engineering in which a product is separated from other substrates. The separation of constituents of a mixture to obtain the pure material is of major practical importance in chemistry and industry. Most of the industries such as food, pharmaceutical, water and sewage, chemical, and textile are strongly dependent on the separation techniques for their production. Therefore, various chemical and physical methods have been developed for efficient separation. This entry describes the methods of separation by barriers and solid agents in which polymers are the key part of separation including adsorption, ion exchange, chromatography, as well as modern methods based on membranes such as gas permeation, reverse osmosis, dialysis, electrodialysis, pervaporation, ultrafiltration, and microfiltration. Keywords: Chromatography; Dialysis; Filtration; Osmosis; Pervaporation; Polymeric membrane; Separation. Sensors– Separation

INTRODUCTION In general and for simplicity, all separation techniques (by barriers and solid agents) can be classified based on ­physical or chemical properties, which are summarized in Table 1.[1,2] SEPARATION BY BARRIERS Separation by barriers is a method of separation of a ­mixture containing two or more components through a semipermeable membrane or a barrier. In general, in membrane processes, a mixture is divided into two fractions: retentate and permeate. The fraction of the mixture that does not pass through the membrane is called retentate, whereas the other fraction of the mixture that passes across the membrane is permeate. Typically, both of them are in liquid or gas states. The similar rules apply in the membrane processes with different driving forces for separating liquid mixtures; for instance, in the reverse osmosis, the driving force is a transmembrane pressure difference for solute and/or solvent transport. In electrodialysis and electroosmosis, it is derived from a transmembrane ­electrical–potential ­difference for solute and solvent transport, respectively, and the function of thermal osmosis is based on a ­transmembrane ­temperature difference for solute and ­solvent transport. The membrane that is the heart of this separation may be made of different materials such as polymers, ceramics, or metal materials. Thin polymeric films are among the best candidates, and the processes based on membrane

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separation include reverse osmosis, dialysis, ­electrodialysis, microfiltration, ultrafiltration, p­ ervaporation, and gas ­permeation.[3] Reverse Osmosis One of the widely used separation processes is reverse osmosis in which a semipermeable membrane eliminates low-molecular-weight solutes. An external hydraulic pressure is applied in reverse osmosis as the driving force, and a semipermeable membrane permits the passing of solvent molecules, so that permeate is pure solvent and retentate is solvent-depleted feed. Generally, reverse osmosis is applied for aqueous ­solutions containing a low-molecular-weight solute, which is often a salt. It can also be used for aqueous solutions containing tiny quantities of organic solutes. Actually, osmosis is not a separation process due to the fact that the solvent is transferred in the wrong direction producing a mixture rather than separation. The membrane is almost not permeable to the solute. In reverse osmosis, the transportation of the solvent in the opposite direction is achieved by imposing an external pressure, higher than the osmotic pressure, on the feed side. Desalination of brackish water is one of the most commercial applications of reverse osmosis by a nonporous membrane. High purification via reverse osmosis needs high external pressures, therefore the membrane must be thick enough to tolerate such a large pressure ­difference. ­Therefore, a thick, porous support is covered by ­asymmetric or thin wall composite membranes with a thin, dense skin or layer.[4] Encyclopedia of Polymer Applications, First Edition DOI: 10.1201/9781351019422-140000257 Copyright © 2018 by Taylor & Francis. All rights reserved.

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Table 1  Separation techniques and basis Separation technique Physical or chemical basis Filtration, microfiltration, ultrafiltration, dialysis, gas separation, gel permeation chromatography

Size and dimension of particles

Distillation, membrane distillation, sublimation

Vapor pressure (boiling and melting point) of substances

Crystallization

Freezing point of substances

Extraction, adsorption, absorption, reverse osmosis, gas separation, pervaporation, affinity chromatography

Affinity and interaction of materials

In the above mentioned applications, membranes must have appropriate chemical, mechanical, and thermal ­stability as well as cost-effective to be competitive with other processes.[6] Polymers in Reverse Osmosis

Centrifugation, decantation

Density and gravity of substance

Complexation, transportation

Chemical nature and interaction of substances

Overview of Reverse Osmosis It is a solution-diffusion process containing asymmetric or composite membranes with a thickness of about 150 μm (sublayer of 150 and top layer of 1 μm) and a pore size of below 2 nm. For practical application, pressures of about 15–25 bar and 40–80 bar are needed for desalination of brackish water and seawater, respectively.[5] Applications of Reverse Osmosis Some of the most important applications of reverse ­osmosis are as follows: a. Application in industrial wastewater to eliminate heavy metal ions, nonbiodegradable substances, and other valuable commercial materials b. Application in rinse water from electroplating processes to get a metal-ion concentrate and a recyclable rinse permeate c. Application in wastewater in dyeing processes d. Separation of sulfites and bisulfites from wastes in paper and pulp processes e. Recovery of food value components (lactose, lactic acid, sugars, and starches) from wastewaters in food processing plants f. Application in municipal water to remove inorganic salts, low-molecular-weight organic compounds, viruses, and bacteria g. Application in dewatering of certain food industries such as coffee, soups, tea, milk, orange juice, and tomato juice h. Application in concentration of amino acids and alkaloids

Cellulose Diacetate and Cellulose Triacetate  ­Cellulose esters, especially cellulose diacetate and cellulose triacetate, are an important class of asymmetric reverse osmosis membranes prepared by phase inversion. Their high permeability toward water in combination with a low ­solubility toward the salt is the main feature that makes them very suitable for desalination. However, they suffer from some disadvantages including stability against chemicals, temperature, and bacteria. Also, to prevent hydrolysis of these polymers, it is recommended using these membranes in the pH range of 4.5–7.5 and at a temperature below 30°C. ­Cellulose triacetate is more resistant than cellulose diacetate to hydrolysis because the extent of ­hydrolysis decreases by increasing the degree of acetylation.[8] Aromatic Polyamides  Aromatic PAs are among polymers frequently used for reverse osmosis membranes. They show high selectivity toward salts but their water flux is somewhat low. PAs can be used over a wide pH range from 4 to 11 due to their resistance to hydrolysis. Also, they are resistant to biological attack; however, the main disadvantage is that their reactivity against free chlorine (Cl2) causing degradation of the amide group. Hollow fibers with very small dimensions (thickness of about 20 μm) have been prepared from asymmetric membranes as well as symmetric membranes from these polymers by melt or dry spinning methods.[9,10] Polybenzimidazoles  The main polymer structure used in these membranes is poly[2,2′-(1,3-phenylene)-5,5′-­ bibenzimidazole], which is commonly called PBI. The specific features of this polymer that make PBI membranes suitable for harsh working conditions are the absence of aliphatic weak linkages and the inherent chemical stability of the benzimidazole moiety. They are also deducing resistant to high temperatures, acidic and basic conditions, and several organic solvents. Polycondensation reaction of diphenyl isophthalate with 3,3′, 4,4′-tetraaminodiphenyl results in the preparation of PBI. Membranes can be prepared from PBI by a twostage process: casting to form a film, and annealing to improve membrane rejection and flux properties. High

Sensors– Separation

Ion exchange, electrodialysis, Charge transfer of particles electrophoresis, diffusion dialysis

Different categories of polymers are utilized in reverse osmosis. The main categories are cellulose esters, especially cellulose diacetate and cellulose t­ riacetate, aromatic polyamides (PAs), polybenzimidazoles, p­ olybenzimidazolones, and poly(amide hydrazide)s.[4,7]

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chlorine  tolerance of semipermeable aromatic polybenzimidazole membranes comes from the absence of amide linkages in a polymer backbone, high water absorptivity (water absorbing up to 13 wt%), excellent mechanical ­properties, and thermal and chemical stability over a wide pH range. Interestingly, in the backbone of PBI molecules, the N–H group acts as a hydrogen donor unit while the nitrogen atom with a lone pair behaves as a proton ­acceptor. Additionally, a strong basic constant (pKb = 5.5) has been observed for PBI because it tends to be self-charged in an aqueous media arising from delocalization of the positive charge of the imidazole group by an adjacent benzene ring as shown in Fig. 1. Charged polymer structure induces high water transport property and high hydrophilicity suitable for antifouling of membranes. According to these advantages, this category of polymers has been used as a good membrane for numerous separation purposes.[11,12]

Sensors– Separation

Poly (amide hydrazide)s  It is worth mentioning that the main requirements of polymers for applying as barrier materials in reverse osmosis separations are high molecular weight, proper polarity, and mechanical strength. Nitrogen-containing aromatic polymers inherit most of these useful properties and make it possible to use them

as membranes for a wide spectrum of reverse osmosis separations. PA hydrazide polymers comprising higher percentage of polar units in the polymer backbone are considered capable candidates for reverse osmosis separations (Fig. 2). Other valuable features of these polymers include the following: high solubility in many polar organic solvents, appropriate thermal and thermo oxidative stability, good mechanical and chemical resistance, and also high moisture sorption that would be useful for processing as synthetic membranes. In general, aromatic poly(amide hydrazide)s have been prepared by low-temperature solution polycondensation of aromatic hydrazides with aroyl dichlorides.[13,14] Dialysis Transport by a concentration gradient of small solute ­molecules (crystalloids) through a porous membrane is described as dialysis. Small, insoluble, nondiffusible particles cannot pass through the membrane. In the dialysis membrane-separation process, shown in Fig. 3, the feed is a liquid with pressure P1 containing solvent, solutes of type A and B with different sizes, and insoluble but ­dispersed colloidal substance. A sweep liquid or wash of

H N

N

+

H2O

+

OH

N

N H

n

pKb = 5.5

H N

N

–

N+ H

N H

n

Fig. 1  Resonance structure in polybenzimidazole

O HN

C NH

NH

O

O

C

C n

Fig. 2  Chemical structure of poly(amide hydrazide)s

Separation: Polymers in 2383

pressure P1

Liquid diffusate Osmosis

Solvent

Fast dialysis

Solute A

Solvent

Solute A

Overview of Dialysis

Microporous membrane

Colloids Solute B

Slow dialysis

Liquid dialysate

Solute B

P2 pressure P1 = P2

Sweep liquid

Fig. 3  Dialysis process

the same solvent with pressure P2 is fed to the other side of the membrane. The membrane that consists of microporous thin film designed in such a way that it permits the solutes of type A to pass through by driving force of concentration gradient. As the solutes of type B are larger in molecular size than those of type A, they cannot pass through the membrane or only pass with difficulty. This process, that is, transportation of solute substances through the membrane is called dialysis. Colloids do not pass through the membrane at all. When the pressure ratio of P1 to P2 reaches 1:4, the solvent may also pass through the membrane; nevertheless, the opposite migration can be possible using concentration gradient. In such a system, the transport of the solvent is called osmosis process. By ­elevating P1 above P2, the solvent osmosis can be reduced, and if the difference is higher than the osmotic pressure, the transport of solvent eliminates. The products of a dialysis unit (dialyzer) are divided into two parts: (1) a liquid diffusate (permeate)-containing solvent, the solutes of type A, and little or none of type B solutes; and (2) a dialysate (retentate) including solvent, type B solutes, remaining type A solutes, and colloidal matter. Ideally, the dialysis unit should perfectly be able to separate the solutes of type A and solutes of type B, and any colloidal matter. Nevertheless, at the best condition, even when solutes of type B do not pass through the membrane, only some parts of solutes of type A are recovered in the diffusate. High efficiency for dialysis is attainable when concentration differences for the main diffusing solutes are large and permeability differences between those solutes and the other solute(s) and/or colloids are high.[1,6]

It is a solution-diffusion process containing homogeneous membranes with thickness of about 10–100 μm. The ­concentration differences make difference in the diffusion rate. Applications of Dialysis Some of the most important applications of dialysis can be summarized as follows: a. Removal of mineral acids from organic compounds b. Removal of low-molecular-weight impurities from polymers c. Removal of alcohol from beer to produce a low-­ alcohol beer d. Purification of pharmaceuticals e. Recovery of chromic, hydrochloric, and hydrofluoric acids from contaminating metal ions f. Recovery of sodium hydroxide caustic viscose liquor contaminated with hemicelluloses g. Recovery of sulfuric acid from aqueous solutions ­containing nickel sulfate h. Recovery of nitric and hydrofluoric acids from spent stainless-steel pickle liquor[5] Polymers in Dialysis The main usage of dialysis, in general, is the ­separation of low-molecular-weight components from high-­molecularweight ones. The mechanism of this separation is based on differences in molecular weight as expressed by the Stokes– Einstein equation. To attain satisfactory ­permeation rates, the membrane must be highly swollen which results in decreased selectivity of the membrane. Therefore, an optimization between the diffusion rate and swelling should be established. As well, the membrane should be as thin as possible. Typical microporous-membrane materials (with pore diameters of 15–100 A) used in dialysis are hydrophilic polymers including cellulose, cellulose acetate (CA), various acid-resistant polyvinyl copolymers, and poly(methyl methacrylate) (PMMA).[5] Cellophane  Thin, transparent sheet made from regenerated cellulose is named cellophane. Regenerated cellulose, is manufactured by the conversion of natural cellulose into a soluble cellulosic derivative and subsequent regeneration including forming either a fiber (via polymer spinning) or a film (via polymer casting). It is a kind of semipermeable membrane which operates as a microporous barrier or sieve, widely used as a practical dialysis membrane.[15,16] Cellulose Acetate  Excellent properties of CA make it as the most commonly used material for preparing dialysis membranes. These properties include biocompatibility,

Sensors– Separation

Liquid feed

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good desalting, high flux, maximum uniformity and permselectivity, optimum physical properties such as flexibility, and also relatively low cost. CA is highly comparable to other synthetic polymeric materials and effective in the hemodialysis process. For example, it has been proved that CA membranes develop some features of red blood cell function in hemodialysis patients.[17] Poly (vinyl alcohol)  Poly(vinyl alcohol) (PVA) is a water-soluble synthetic polymer prepared by hydrolysis of poly(vinyl acetate). PVA has superior membrane properties and mechanical strength, and with easily cross-linkable hydroxyl groups, it has been extensively utilized as membrane and biomedical material. Hydrophilic character of cross-linked PVA membrane makes it possible for the suitable diffusion of solutes and the ability for hydrogen bonding with water. Since the degree of crystallinity can be controlled via the variation of cross-linking density, the diffusion nature of solute through the membrane could be managed.[18,19] Sensors– Separation

Poly (methyl methacrylate) PMMA-based ­dialysis membranes are synthetic membranes that reveal good ­solute permeability and a high degree of biocompatibility. They are also unique due to their ability to remove proteins by adsorption as well as permeation. For instance, in the case of patient with uremic blood, a number of s­ olutes exist that may relate to various morbid states. PMMA membranes can remove these solutes and ­similar ­solutes of greater molecular weight, such as the free i­ mmunoglobulin light chains that have molecular weight of about 6,000 Da, which cannot be removed by ­membranes operate by ­permeation alone such as ­polysulfone (PUS) ­membranes. [20] Electrodialysis Electrodialysis system was practically used in the early 20th century, when high rate of dialysis was attained using electrodes and a direct current. By improvement in electrodialysis from the 1940s, it recognized as a complete separate technique. Nowadays, electrodialysis is known as an electrolytic process that separates an aqueous, electrolyte feed into concentrated and diluted or desalted water diluate via an electrical field and ion-selective membranes. Overview of Electrodialysis In this separation technique, a series of alternating cationand anion-selective nonporous membranes (with the thickness of about 100–500 μm) are used with a direct current voltage across an outer anode and an outer cathode to concentrate an electrolyte based on the separation principle of the Donnan exclusion mechanism.[3,21]

Separation: Polymers in

Application of Electrodialysis Some of the most important applications of electrodialysis are as follows: a. b. c. d.

Water desalination Desalination in food and pharmaceutical manufacturing Separation of amino acids Production of ultrapure water for the semiconductor industry e. Salt production [5]

Polymers in Electrodialysis In the electrodialysis process, ions are transported through membrane due to an applied electrical potential difference and as a consequence of a direct electrical current flow. Ion-exchange membranes that are able to either permit the transfer of anions or cations are used in order to make the ions selective membranes. Thus, they are separated into anion-exchange and cation-exchange membranes. In anion-exchange membranes, positively charged groups attached to a polymer backbone, for example, those derived from quaternary ammonium salts. So, positively charged ions (cations) are repelled by the membrane because of similar charge. On the other hand, cation-exchange membranes contain negatively charged groups linked to the polymers such as sulfonic or carboxylic acid groups. In this case, negatively charged anions are repelled by the membrane. Ion-exchange membranes can be ­prepared ­heterogeneously and homogeneously. Heterogeneous membranes are prepared by combining ion-exchange resins with a film-forming polymer and processing them into a film by dry molding or calendering and so on. The disadvantage of such membranes would be the relatively high electrical resistance and poor mechanical strength particularly at high swelling ratios. However, in homogeneous m ­ embranes, ionic groups are introduced into the backbone of a polymer film. The advantage of such membranes would be uniform charge distribution over the membrane and l­ imiting their extensive swelling by cross-linking. An ion-exchange membrane should have high electrical conductivity as well as high ionic permeability. By increasing the ionic charge density, the electrical conductivity can be increased, but the polyelectrolyte may then become highly swollen. One efficient way to overcome this problem is cross-linking of the polymer; by varying the degree of cross-linking and the charge density determining the sorption. Generally, the diffusion coefficient of the ions inside the membrane may differ from 10 −6 cm2 /s for a highly swollen to 10 −10 cm2 /s for a highly cross-linked system. Thus, the enhanced quality of an ion-exchange membrane depends on the high selectivity, high ­electrical ­conductivity, optimized degree of swelling, and high

Separation: Polymers in 2385

Cross-linked Copolymers Based on Divinyl Benzene with Polystyrene  Poly(styrene-co-divinyl benzene) is perhaps the best cation-exchange membrane for electrodialysis. Poly(styrene-co-divinyl benzene) membranes c­ ombine several advantages such as low protonic resistance, good mechanical properties, and high chemical stability. ­Furthermore, poly(styrene-co-divinyl benzene)-based membranes are specifically appropriate for sulfonation since the degree of functionalization on the cross-linked ­polystyrene can be controlled due to the existence of high ­phenyl rings as active sites for the substitution of sulfonic acid groups. Anion-exchange membranes of this ­polymer can be obtained by the substitution of quaternary ­ammonium salts on the phenyl rings, as well.[22–24] Copolymers of Poly(tetrafluoroethylene)  PTFE is a perfluorinated linear C–C main chain polymer with the repeating unit of –(CF2–CF2) n –. PTFE is prepared via free radical polymerization of gaseous tetrafluoroethylene (TFE; CF2 = CF2) in an aqueous medium in the presence of an initiator and other additives using two distinct methods for the commercial polymerization including suspension polymerization and emulsion or dispersion polymerization. PTFE shows an outstanding chemical and thermal ­stability. This polymer is inert to most of the chemicals except to molten alkali metals and turbulent liquid or ­gaseous fluorine. PTFE can be continuously used at about 260°C and its partial degradation begins above 440°C. In electrolysis, copolymers based on perfluorinated sulfonic acid/PTFE with an excellent chemical stability and superior ionic conductance are used. Additionally for improving mechanical and chemical stability, the ­membrane is reinforced with woven PTFE.[25]

Microfiltration Suspended particles with diameters ranging 0.1–10 μm are separated by means of porous membranes in a ­process called microfiltration. It is a low pressure-driven ­membrane separation technique, which has major ­advantages such as low energy consumption, no need for the addition of chemicals, no heat damage to heat-sensitive ­constituents, superior removal of contaminants, easy working, and well-arranged process controls. Recently, there are reports using both hydrophilic and hydrophobic polymeric ­materials to ­prepare the membrane in the microfiltration process. There are generally two types of filtration processes: dead-end (in-line) and cross-flow. In dead-end type, the feed flow is vertical to the membrane surface; in this way, the retained particles gather and make a cake coating on  the membrane surface. The higher the filtration time, the thicker the cake and as a result, the lower the permeation rate. On the other hand, in cross-flow type, the feed flow is along the membrane surface resulting in a partial accumulation of retained solutes. These processes are shown schematically in Fig. 4. As it is necessary to retain particles at the membrane surface in contact with feed solutions, the asymmetric membranes or screen filters with the smallest pores are the only types applied in cross-flow filtration. The operation of cross-flow-type microfiltration systems takes place under a constantly applied transmembrane pressure similar to reverse osmosis and ultrafiltration systems. Since fouling and lose flux of microfiltration systems occurs much faster than reverse osmosis and ultrafiltration, it is difficult to control the operation of this system due to the rapidly reduced flux. In order to avoid this problem, the transmembrane pressure across the membrane is gradually increased in microfiltration to maintain the flow as membrane fouls so that the system can operate as a constant flux one. In most cases, a constant volume pump is employed to set the feed and permeation pressure at atmospheric level

Feed Retentate

Feed

Permeate

Permeate

Dead-end filtration

Cross-flow filtration

Fig. 4  Schematic representation of dead-end and cross-flow filtration

Sensors– Separation

mechanical strength. Accordingly, two common polymer systems for this purpose are cross-linked copolymers based on divinyl benzene with polystyrene and copolymers of poly(tetrafluoroethylene) (PTFE).[1,5]

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Separation: Polymers in

and a valve just below the feed pressure, respectively. Fouling causes reduction of permeability while membranes are in use. Increasing of pressure diving force is an applied method to compensate for the lost permeability. In order to restore the permeability, the module is taken off and cleaned or back flushed whenever the amount of pressure reaches to predetermined values. Table 2 summarizes the advantages and disadvantages of dead-end and cross-flow microfiltration. For already clean solutions, dead-end filtration is generally preferred as a refining operation. Although crossflow filtration needs more complex equipment and is more expensive in comparison with dead-end type, it has longer membrane lifetime and also in cases with high particle contents of water, it is a preferred choice.[3,5] Overview of Microfiltration

Sensors– Separation

It is a sieving mechanism process operates with exerting pressure (less than 2 bar) over porous membranes with symmetric/asymmetric pores (size of 0.05–10 μm) and thickness of about 10–150 μm. Applications of Microfiltration Microfiltration is widely used in industrial processes aiming at the separation of particles bigger than 0.1 μm from liquids. Several common applications of dialysis can be summarized as follows: a. Cold sterilizing of beverages and pharmaceuticals b. Clarity of beverages c. Preparation of ultrapure water for the semiconductor industry d. Wastewater recovery e. Cell harvesting in biotechnology f. Metal recovery g. Continuous fermentation h. Separation of oil–water emulsions i. Analytical applications [26] Table 2  General comparison of dead-end and cross-flow microfiltration Specification of microfiltration type Advantages Disadvantages a. High operation cost (disposable membrane)

Dead-end microfiltration

a. Low asset cost b. Simple operation c. Suitable for dilute solutions

Cross-flow microfiltration

a. Medium operation a. High asset cost b. Complex cost (membrane operation can be cleaned) b. Suitable for concentrated solution

Polymers in Microfiltration Generally, a wide range of materials are employed in the preparation of microfiltration membranes, which are based on organic (polymers) or inorganic materials (ceramics, metals, and glasses). Also, there are various techniques for the preparation of microfiltration materials from polymers including sintering, stretching, track-etching, and phase inversion. All kinds of materials, especially polymers and ceramics, can be used to prepare microfiltration membranes by the aforementioned techniques. Hydrophobic and hydrophilic membranes are two general classifications of synthetic polymeric membranes. The ceramic membranes are mainly prepared from alumina (Al2O3) and zirconia (ZrO2). Principally, it is also possible to apply other materials such as titania (TiO2). A list of various polymers considered as hydrophobic and hydrophilic membranes are represented in the following sections. Microfiltration membranes can be characterized with the pore sizes between 0.1 and 2 μm. Due to the distribution of pore sizes, these membranes are recognized as heterogeneous structures.[5] A number of polymeric materials in microfiltration membranes are described as follows. Polytetrafluoroethylene (PTFE, Teflon)  Since the most important role in membrane processes belongs to membrane material, the higher performance of membranes is needed due to the rapid developments in membrane technology. Some outstanding properties of membranes such as excellent chemical resistance, high permeability, high mechanical strength, and reasonable antifouling performance are specifically demanded. PTFE shows superior properties including high chemical resistance, thermal stability, and hydrophobicity in comparison with other polymeric membranes. It is an ideal material for the application in membrane filtration, in particular, for the separation of strong acid or highly corrosive liquids due to its chemical inertness.[27–29] Poly(vinylidene fluoride)  This hydrophobic polymer is a semicrystalline polymer widely used in the production of porous asymmetric membranes for the fine separation processes including microfiltration and ultrafiltration due to the properties such as acid resistance and chemical inertness. Immersion precipitation is the method of choice to prepare microporous poly(vinylidene fluoride) (PVDF) membranes. They are prepared by an immersion of a formulated cast polymer in a non-solvent bath in which a series of liquid–solid and/or liquid–liquid phase separation procedures are induced. The liquid phase is then removed, followed by a drying process. In fact, the formed pores in the membranes are the spaces that were initially occupied by the liquid phase.[30]

Separation: Polymers in 2387

Polypropylene  Size exclusion or depth filtration is the main process in conventional microfiltration membranes for the separation of colloidal particles. Polypropylene (PP) membranes are one of the main centers of interest according to their specifications such as hydrophobicity, chemical inertness, potential for porosity control, and also their ability for broad applications.[31,32] Cellulose Acetate (CA)  High salt rejection, moderate flux, and renewable source of raw materials are among the priorities of CA membranes in comparison with other types. CA had been widely used for the pressure-driven processes as a polymeric material for the membrane production. However, some disadvantages of this polymer, compared to other polymeric materials, remarkably reduced its extensive usage.[14]

Polycarbonate  Polycarbonates (PCs), which are multipurposed thermoplastic polymers, are considered as a good choice for various applications from biomaterials to high-end engineering. The main half of this polymer is composed of aromatic and/or aliphatic units and has also a repeating carbonate linkage (Fig. 6). In addition to interfacial polymerization, melt ­condensation polymerization is also employed to prepare PCs. The characteristics making PCs a good choice for application as membranes include chemical, heat, and impact resistance; outstanding mold ability; dimensional stability; and also clarity.[14]

Polyacrylonitrile  The chemical structure of polyacrylonitrile (PAN) is shown in Fig. 5. As the chemical structure of this semi-crystalline organic polymer shows, it consists of a combination of nitrile (CN) functional group and polyethylene backbone as the repeat unit structure. Free radical polymerization is the synthetic method for commercial PAN, and therefore, there is no control over their molecular structure. The presence of a lone pair on the nitrogen atom in nitrile group leads it to a suitable acceptor for hydrogen bonding. Additionally, the existence of an electron-deficient carbon atom and electron-rich nitrogen atom produces high dipole moment on the polymer that is suitable for relatively strong attractive interactions. Thus, these strong intermolecular interactions in this backbone make high strength and ­resistance against various organic solvents. The advantages of PAN-based membranes when compared to other conventional polymers such as PUS include high membrane performance, sufficient chemical stability, higher solvent stability, greater hydrophilicity and lesser price. Chemical modification of PAN is another

Polysulfone/Poly(ether sulfone)  Although both hydrophilic and hydrophobic polymeric materials have been employed to prepare the membrane for use in the microfiltration process, the most commercial membranes are made from the hydrophobic polymers such as PUS and poly(ether sulfone) (PES) because of their superior mechanical, ­thermal, and physical properties as well as chemical ­resistance and effective contaminant rejection.[34] Polyimide/Poly(ether imide)  While selecting a microfiltration membrane for specific application, ­different ­properties such as chain interaction and polarity of ­functional groups and also binding affinity must be considered. Various polymers can be selected for such applications due to the wide range of structure–property relations and their diversity. Polyimides (PIs) are g­ enerally insoluble polymers and are easily filled with additives. They exhibit high heat and solvent resistance, ­outstanding mechanical properties, and intrinsic flame resistance as a result of their structure containing imide groups in their backbone and are recognized as high-performance engineering plastics. Because of these unique properties,

H CH2

C N C

n

Fig. 5  Chemical structure of PAN

H3C

C

CH3

O

O C

H2 C

O n

Aromatic polycarbonate

Fig. 6  Chemical structure of aromatic and aliphatic PCs

C H2

H2 C

O

O C

O

n

Aliphatic polycarbonate

Sensors– Separation

specification of this polymer in such a way that the nitrile group can be converted into other functional groups to increase the ­hydrophilicity and subsequently improving antifouling and flux p­ erformance properties of the prepared membrane. The main disadvantage of this membrane is the pore collapse and brittleness of the membrane due to drying of the membrane. Therefore, this membrane should be used carefully due to the breakthrough of filtration as a result of drying.[33]

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widespread applications of these polymers, especially in rough environments and high temperature filtration, have been observed. The basic method to prepare PIs is thermal imidization of diamine and dianhydride at high ­temperatures. Although the backbone structure is rigid, the presence of ether groups in aromatic dianhydride and/or aromatic diamine building blocks induces flexibility and consequently improves processability without reducing the heat stability. The hydrophobicity of PIs may lead to ­fouling while using as filtration membranes.[35,36]

Sensors– Separation

Aliphatic Polyamide  The significance of using aliphatic PAs such as nylon 6, nylon 6,6, and nylon 11 in membrane applications, and especially microfiltration is ­developing in recent years. The presence of methylene group as a flexible aliphatic unit improves the degree of c­ rystallinity in these  polymers. It also plays a key role in corrosion ­resistance against alkaline and alkaline agents. It is expected that the application of these polymers become wider due to their specific mechanical properties and chemical and thermal stability in addition to the presence of polar amide groups and their ability for hydrogen bonding.[37] Ultrafiltration The pore size in this type of filtration is in the range of 10–200 Å and is typically used to separate macromolecules and colloids from a solution, the lower limit being solutes with low molecular weights (a few thousand Daltons). This technique is applied in special tasks such as the separation of low-molecular-weight solutes from higher-­ molecular-weight solutes, i.e., the separation of enzymes from viruses. The nature of membrane in ultrafiltration process is between microfiltration (with the pore size in the range of 0.05 μm) and nanofiltration (with the pore size in the range of 1 nm). Typically, the structure of membrane is more asymmetric in comparison to microfiltration membranes. A thin top layer is supported by a porous sublayer in these asymmetric membranes in which the mechanical resistance to mass transfer is based on the top layer. So, the thickness, pore size distribution, and surface porosity of the top layer are the main characteristic features of an ultrafiltration membrane. In general and in broader extent, the pore diameters are between 20 and 1,000 Å in the top layer of an ultrafiltration membrane. Therefore, the pore sizes are overall too small, so high pressure should be exerted which might destroy the polymeric structure. Ultrafiltration and microfiltration membranes can both be considered as porous membranes, where rejection is essentially controlled by the size and shape of the solutes in relation to the pore size in the membrane and also the carriage of solvent is directly proportional to the practical pressure. The same separation principles with similar membrane processes are ruling on both microfiltration and ultrafiltration by

Separation: Polymers in

this main difference that in ultrafiltration membranes, an asymmetric structure with a much denser top layer exists (membrane thickness is generally less than 1 μm), which means that smaller pore size and lower surface porosity result in a much higher hydrodynamic ­resistance.[1] Overview of Ultrafiltration It is a sieving mechanism process operates with an ­exerting pressure (1–10 bar) over porous and asymmetric membranes with pore size and thickness of about 1–100 μm and 150 μm, respectively.[5] Applications of Ultrafiltration The chief usage of ultrafiltration is summarized in the separation of high-molecular-weight components from low-molecular-weight ones. This process can be applied in numerous industries such as food and dairy, pharmaceutical, chemical, textile, leather, paper, and metallurgy. Application of ultrafiltration in food and dairy industry is much significant due to wider applications such as concentration of milk and egg products, cheese production, recovery of whey proteins, recovery of potato starch and proteins, and clarification of fruit juices and beverages. Several common applications of ultrafiltration are as follows: a. Food industries (potato, starch, and proteins) b. Dairy industries (milk, cheese production, and whey) c. Pharmaceutical industries (separation of enzymes, antibiotics, and pyrogens) d. Textile industry (indigo color) e. Water recovery f. Metallurgy (emulsions of oil–water and recovery of electro painting) g. Automotive industry (electro painting)[5] Polymers in Ultrafiltration As it was mentioned, ultrafiltration process is usually employed for molecular solutions, permitting small molecules, and the solvent to pass through the membrane freely while preventing the large molecules. Modified Loeb– Sourirajan process is applied for manufacturing of most of the recent ultrafiltration membranes. The sublayers of composite membranes applied in nanofiltration, reverse osmosis, gas separation, and pervaporation are composed from these membranes. Currently, the polymers are extensively used in p­ roducing ultrafiltration membranes through phase ­inversion process. Some of the most famous polymers for this application are PUS, PES, sulfonated PUS, PVDF, PAN, cellulosics (CA), PI, poly(ether imide), and aliphatic PAs. As it can be seen, the polymers used in the p­ reparation of microfiltration and ultrafiltration membranes are very similar due to the similarities of the two methods; the

Separation: Polymers in 2389

reader, for study of abovementioned polymer properties, is referred to the previous section.[5]

basis for separation. Since the driving force is a function of vapor–liquid direct equilibrium, it will also have influence on the separation features.[1]

Pervaporation Overview of Pervaporation

Feed

Retentate

Carrier gas

It is a solution-diffusion process based on partial vapor pressure or activity difference of components in which nonporous composite membranes with an elastomeric or glassy polymeric top layer (thickness 0.1 to few μm for top layer) are applied. Applications of Pervaporation The main usage of this process is to separate or remove a part of liquid from a liquid mixture. One advantage of this process is that if highly selective membranes are applied, only the heat of vaporization of the almost pure permeate needs to be provided. This separation process especially applies to azeotropic composition, i.e., the liquid and vapor have the same composition. The azeotropic compositions cannot be separated through ordinary distillation due to the fact that the mixtures of some organic solvents with water make an azeotrope in the composition region of the pure organic solvent. Therefore, using pervaporation technique to separate (dehydrate) these types of mixtures is very valuable. Also, some organic mixtures make an azeotrope, and the most important ones with their ­corresponding azeotropic compositions are collected in Table 3. The most important applications of pervaporation are as follows: a. Chemical reaction and process industries b. Pharmaceutical industries

Feed

Retentate

Carrier gas

Condenser

Vacuum pump

Condenser

Permeate

Permeate

Fig. 7  Schematic drawing of the pervaporation process with a downstream vacuum or an inert carrier gas

Sensors– Separation

The term pervaporation was introduced for the first time in 1917 by Kober when he was working on experimental techniques for separation of water from albumin–toluene solutions. This term comes from the words “permselective” and “evaporation” and denotes a membrane process in which a pure liquid or liquid mixture in atmospheric pressure contacts the membrane on the feed or upstream side, and the current low pressure on the permeate or downstream side leads to its separation as vapor. A carrier gas or a vacuum pump is used to generate the ­fractionally low downstream pressure, and this pressure must be lower than the ­saturation pressure. The schematic process is revealed in Fig. 7. Therefore, there are three essential steps in pervaporation process including (a) selective sorption on the feed side of the membrane, (b) selective diffusion through the membrane, and (c) desorption into a vapor phase on the permeate side. Two phenomena occur in pervaporation process: mass and heat transfer. From the feed to permeate, a phase transition happens in which a barrier layer between the two phases of liquid and vapor exerts by the membrane. So, supplying the vaporization heat for permeates is required. In addition to the two phases of liquid and vapor in the extractive distillation process, the membrane is considered as the third constituent. The main difference between distillation and pervaporation is that in distillation, vapor–­ liquid equilibrium exists, whereas in the pervaporation, the distinction between solubility and diffusivity is the

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Table 3  Liquid azeotropic mixtures (wt%) Water (4.4)/ethanol (95.6) Water (12.2)/isopropanol (87.8) Water (11.8)/tert-Butanol (88.2) Water (18.4)/dioxane (81.6) Water (5.9)/THF (tetrahydrofuran) (94.1) Water (32.0)/HNO3 (68.0) Water (1.7)/H2SO4 (98.3) Ethanol (21.0)/hexane (79.0) Methanol (12.0)/acetone (88.0) Propanol (20.0)/cyclohexane (80.0)

Sensors– Separation

c. Food industries d. Dehydration of organic solvents e. Removal of organic compounds (alcohols, aromatics, and chlorinated hydrocarbons) from water f. Separation of polar/nonpolar organic compounds (alcohols/aliphatics or alcohols/aromatics) g. Separation of saturated/unsaturated organic compounds (C-8 isomers/o-xylene, m-xylene; ethylbenzene/­ styrene)[5] Polymers for Pervaporation Asymmetric and composite membranes are ideal membranes for pervaporation process in which nonporous membranes with an anisotropic morphology are required, that is, an asymmetric structure composed of a dense top layer and an open porous sublayer. These features dictate two basic specifications to the used substructure consist of (a) an open substructure so that minimizing resistance to vapor transport and to avoid capillary condensation, (b) high porosity on the surface with a narrow pore size distribution. Increasing of partial pressure and hence decreasing of flux may be observed if the pressure loss on the permeate side occurs. Capillary condensation may happen if the pores are too small and the pressure loss becomes high. Conversely, when the pores in the support layer are too large, applying a thin selective layer directly upon the support becomes difficult. Also it should be in mind that having high surface porosity is essential. Therefore, a three-layer membrane comprises a highly porous substructure without resistance, with a nonselective i­ ntermediate layer placed on this and after that a dense top layer is recommended. Dip-coating, plasma polymerization, and ­interfacial polymerization are the main methods for ­depositing the thin layer upon a support layer. The choice of the polymeric material in pervaporation process depends strongly on the type of abovementioned applications. However, three key polymers in this area are PVA, PAN, and PES.[5] Poly (vinyl alcohol)  Recently, separation of aromatic/ aliphatic mixtures through pervaporation technique has

Separation: Polymers in

received more attention because of economy, simplicity, and environmental impacts. As it was mentioned earlier, the sorption and diffusion properties of the mixture components in the membrane strongly affect the pervaporation process. Therefore, the membrane material and structure should be selected carefully. It has been proved that the sorption selectivity is more important than the diffusion selectivity of the membrane in pervaporation. However, in order to attain high selectivity for the effective separation of aromatic/aliphatic mixtures, the membrane should have affinity to aromatic components and a rigid structure and sufficient mechanical strength to resist excessive swelling. Aromatic compounds with delocalized π electron usually show higher affinity to polar polymers. PVA due to its polar, hydrophilic, and good membrane-forming properties is one of the ideal polymers for membrane preparation in separating aromatic/aliphatic mixtures. PVA is a semicrystalline compound with closely packed chains because of intermolecular and intramolecular hydrogen bonding. Since PVA molecules lack π electron acceptors, the weak interactions between the membrane and the aromatic ­compounds may occur; therefore, homogeneous PVA membranes often show weak performance in pervaporation. Forming organic–inorganic hybrid ­materials by ­adding inorganic particles to the polymer solution improves the performance of pervaporation.[38] Polyacrylonitrile  PAN is a widely used membrane material because of its good membrane-forming ability, high stability, chemical resistance, and physical strength in general. PAN specifically has unique characteristics including low density, thermal stability, high strength and modulus of elasticity, slow burning and charring, resistance and ­chemical stability against most chemicals and solvents, sunlight, heat, and microorganisms. Additionally, the ­reactivity toward nitrile reagents for the modification of PAN, as well as compatibility with certain aprotic polar solvents, ability to orient, and relatively low ­permeability toward various gaseous converted it to the polymer of choice in chemical industry and some high-tech industry these days.[39] Poly(ether sulfone)  Microporous PES membranes have wide applications in membrane separation processes. This polymer is used as a support layer for the composite membrane due to its high heat and chemical resistance, ­lightweight, good mechanical and film-forming properties. Cross-linked polymers such as PVA matrix as selective layer coated on the top of PES-supporting porous layer are used as composite membranes in pervaporation. Some functionalized PESs also are a good candidate material for the pervaporation process. PESs are generally prepared by the polycondensation via nucleophilic substitution mechanism, as shown in Fig. 8. Polymerization is normally achieved by the reaction between dihalodiphenyl sulfone with bisphenol and

Separation: Polymers in 2391

O

O Cl + KO

S

Cl

S

OK

O

O O O

S

O O

O

S O

x

CH3 NaO

ONa CH3 DMAc/Toluene 160° C, 24h –2x NaCl OR

O Cl

S

Cl

+

O

O CH3

CH3 HO

O

CH3 O

S O

x

OH CH3 K2CO3 DMAc/Toluene 160° C, 24h –2x KCl

its derivatives such as bisphenol salts, which are in turn prepared by the reaction of bisphenol compounds with bases such as sodium hydroxide, potassium hydroxide, and potassium carbonate.[14,40] Gas Separation The separation of gases by thin barriers named as membranes is a progressive and rapidly growing subject. Separation of gases from their mixtures occurs based on the differential permeation of the components through membranes. Porous or dense membranes can be used as a selective membrane for the gas separation, and the mechanism of gas permeation is shown in Fig. 9 for three types of porous Porous membrane

membranes. If membranes with relatively large pore size (from 0.1 to 10 μm) are used, gases permeate the membrane via convective flow without any separation. If the pores size is smaller than 0.1 μm, the pore diameter is smaller than the mean free path of the gas molecules. Diffusion through such pores obeys from Knudsen diffusion and, in this case, the transport rate of a gas is inversely proportional to the square root of its molecular weight (Graham’s law of diffusion). Finally, if membranes with extremely small pore size (in the range of 5–20 Å) are used, then gases are separated by molecular sieving process. Transport through this type of membrane consists of the diffusion in the gas phase as well as the diffusion of adsorbed species on the surface of the pores (surface diffusion).[1,3] Nonporous (dense) membrane

Convective flow (>1000 A)

Solution diffusion Knudsen diffusion (100–1000 A)

Surface diffusion (5–100 A)

Fig. 9  Schematic of the gas permeation mechanism

Sensors– Separation

Fig. 8  Preparation of PESs by polycondensation via nucleophilic substitution

2392

Overview of Gas Separation It is a solution-diffusion process in case of nonporous ­membranes and Knudsen flow process in porous membranes. In this technique, the asymmetric or composite nonporous (or porous  lc, the slope (elastic modulus) decreases, which means a stress-induced softening phenomenon. The softening may be interpreted as a mechanical instability due to the presence of rodlike particle chains. The softening phenomenon is shown in Fig. 13. When the deformation ratio reaches the value 0.9, the ­linear aggregates start to bend under compression, because the polymer chains in the PVA network interact with the magnetic particle as shown in Fig. 13. The ordered structure of the particles and the interaction with the polymer network prevent the particle chains from being destroyed

Wu et al.[30] reported about blank PU, isotropic as well as anisotropic PU MSEs oriented under a MF of 0.9 T during the curing process with different CI contents. The compressive stress–strain curves were investigated, and it was found that, after the incorporation of iron particles, the compression stress was significantly improved. Various weight percent were considered. For example, the compression stress at 10% strain for an anisotropic sample loaded with 70% by weight of particles was 2.02 MPa, around 4.5 times that of blank PU (0.45 MPa). Clearly, compared to the isotropic PU MSEs, the anisotropic one has a higher compression stress, and the reason lies in the orientation of iron particles along the direction of compression. With regard to the tensile properties of such samples (Aniso-50, Aniso-60, and Aniso-70), they showed that the tensile strength decreases with the increasing iron content, but anyway the tensile strength of PU MSEs with 70 wt% iron can still be as high as 8.33 MPa, around 3.5 times that of natural rubber MSEs, which was 2.27 MPa at the same iron content.[89] The deterioration of the mechanical properties of PU MSEs can be related to the fact that (a) the tensile load transfer is poor at high iron content, and (b) the oriented chain-like structure in the PU matrix may act as stress concentration points. Wang et al.[2] demonstrated that the uniform dispersion of iron particles and stronger interfacial interaction between modified iron particles and silicone rubber matrix (a) 6

4

Bending point

Fx

Fx

Gels

(b) 5 σn [kPa] 4

Fx

Fx

Break point

3 2

2

0

Fx

0.6

0.7

1

0.8 λ

0.9

1.0

0

Fx

0.88

0.92

λ

0.96

1.00

Fig. 12  Typical stress–strain behavior of magnetic PVA gels (a) and iron-loaded PDMS elastomers (b) in unidirectional compressions. Three samples are compared having the same amount of filler particles, but with different particle distributions Source: Reprinted with permission.[34]

Smart Polymers: Lightweight Composites and Foams Tailored with Magnetic Field 2411

1 > λ > λc

λ < λc

Fig. 13  Schematic picture of the bending of the magnetic PVA gels under compression Source: Reprinted with permission.[35]

Fig. 14  Structural change of the iron-loaded PDMS elastomer under compression Source: Reprinted with permission.[35]

by compression. The deformation parallel to the structure has a strong influence on the mechanical behavior because of the bending of the ordered structure. Anisotropic Fe3O4loaded PDMS samples showed a similar behavior under compression, but, unlike PVA gels, the compression of the iron-loaded PDMS network results in a break point in the stress–strain curve if the deformation of the sample is parallel to the particles chain structure. Figure 12b shows that the nominal stress increases with the compression up to a deformation ratio of 0.95 in all cases. Increasing the compression above this ratio, the columnar structures of the iron particle are destroyed as Fig. 14 shows because the particles did not interact with the polymer network. I ­polymer network can immobilize the ordered structure, but it cannot prevent the breakup of the ordered s­ tructure under compression. In order to characterize the bending or breaking ­phenomena, an apparent elastic modulus (Ga) was defined as follows:

Figure 15 shows the dependence of the apparent elastic modulus on the quantity D. For the sake of comparison, results of compression measurements performed on both isotropic and non-isotropic samples are shown. The composites contain Fe3O4 filler particles with a concentration of 30 wt%. In the case of the isotropic sample, the apparent elastic modulus increases slightly. For anisotropic sample, the apparent modulus increases significantly under deformation up to the value D = 0.85. Above this value, the modulus does not change notably. Wu et al.[33] studied the mechanical properties of MS PVA hydrogel. In particular, the tensile strength and elongation at break of the blank PVA hydrogel are 1.68 MPa and 366%, respectively. The tensile strength of composites with 50 and 60 wt% iron content is improved to some extent compared with the blank PVA gel. When the iron content increases to 70 wt%, both tensile strength and elongation at breaking decrease. At higher iron contents, the stress concentration and poor load transfer will deteriorate the mechanical properties. For anisotropic PVA MS hydrogels, the tensile strength and elongation at break are lower than those in isotropic systems with the same iron content. The formation of chain-like structures along the

Ga =

(a) ∂σn ∂D

∂σa ∂D

(b)

60 Anisotropic sample

50

30

∂σn ∂D

Anisotropic sample

20

40 Isotropic sample

30

10

20 0.0

0.1

D

0.2

0.3

0 0.0

Isotropic sample 0.1

0.2 D

0.3

0.4

Fig. 15  Change in the apparent elastic modulus due to ­compression: (a) Fe3O4-loaded PDMS; (b) iron-loaded PDMS ­samples. Lines are guides for eyes, and the arrow indicates the breaking point Source: Reprinted with permission.[34]

Smart Polymers–Textiles

λ≈1

Smart Polymers: Lightweight Composites and Foams Tailored with Magnetic Field

thickness direction in the hydrogel may act as stress concentration points, resulting in the decrease in tensile properties. The tensile strength of the PVA MSG with 70 wt% CI can still be as high as ~1 MPa, which is close to that of a natural rubber MS elastomer with the same iron content (~2.27 MPa).[89] It should be mentioned that for MSGs, it is difficult to obtain good mechanical properties compared to rubber- or elastomer-based MS materials. For example, An et al. reported that the shear modulus of styrene ethylene butadiene styrene-based MSG with 30 vol% CI is only ~10 kPa.[90] The improvement in mechanical properties of PVA MSG should be mainly attributed to the presence of the physically cross-linked network structure in which PVA crystallites serve as junction points. The compressive stress–strain curves of the blank PVA gel, and isotropic and anisotropic PVA MS hydrogels are shown in Fig. 16. Compression properties are significantly improved incorporating iron particles (Fig. 16a and b): the compression stress at 10% strain for Iso-70 and Aniso-70 are 56.0 and 83.8 kPa, ~2.3 and ~3.5 times that of blank PVA hydrogel (23.8 kPa), respectively. Compared with the isotropic PVA MS hydrogel, the anisotropic one has a higher compression stress due to the formation of aligned chain-like structures along the direction of compression. Foams

40

20

0

0

80

Iso-2060 Iso-2050

200

Stress (kPa)

Aniso-2050

0.4 Strain (mm/mm)

0.6

0.8

particles exhibited a threefold increase of the elastic modulus when compared with the elastic modulus of the neat PU (Fig. 17). On the contrary, the compressive tests performed along the orthogonal direction to the MF lines showed that the filler only slightly reinforced the matrix, and the resulting stress–strain curve was comparable to that of samples with randomly dispersed particles (Fig. 18). This is ascribed to the low amount of iron particles in all the struts ­orthogonal to the MF direction. As suggested by the modeling of the experimental result by means of theoretical interpretations based on both Halpin–Tsai and Cox–Krenchel models for short fiber composites, the chain-like structures, albeit made of spherical particles, were able to induce a reinforcing effect analogous to that of solid short fibers.[18]

Neat PU AV-Fe5 AV-Fe10 AV-Fe15 AV-Fe20 AV-Fe25

60

400

0.2

Fig. 17  Stress–strain curves in MF direction of the aligned ­particle foam samples in comparison with the unfilled foam Source: Reprinted with permission.[18]

600 Aniso-2060 Iso-2070

Load

Neat PU AV-Fe5 AV-Fe10 AV-Fe15 AV-Fe20 AV-Fe25

60

Aniso-2070

800

Stress/(KPa)

Smart Polymers–Textiles

The addition of iron particles reinforces the foam structure, and both elastic modulus and yield stress increase, as reported in Ref. [18]. Samples filled with randomly ­dispersed particles show an increase of the overall mechanical response with the particle content, when compared with neat PU. Also in foams, the mechanical behavior is strongly influenced by the direction of the developed fibrillar structures. The sample with aligned

80

Stress (kPa)

2412

Load

40

20

Blank PVA gel

0 0

5

10

15 Strain (%)

20

25

30

Fig. 16  Compressive curves of isotropic and anisotropic PVA MS hydrogels with different iron contents Source: Reprinted with permission.[33]

0

0

0.2

0.4 Strain (mm/mm)

0.6

0.8

Fig. 18  Stress–strain curves in the direction orthogonal to MF of the aligned particle foam samples and of the unfilled foam Source: Reprinted with permission.[18]

Smart Polymers: Lightweight Composites and Foams Tailored with Magnetic Field 2413

There are many examples of elastomers with active behavior driven by the MF in mechanics, and electromagnetic and acoustic shielding. In Ref. [3], it was shown that a composite was made of silicone matrix isotropically filled with steel particles (0.15–0.20 mm in size and filling factor pf = 0.4, 0.6, and 0.8, where pf = Vf/Vk which is defined as a ratio of the sum of all the silicon steel particle volumes Vf to the total volume of the composite Vk). The mechanical properties for these composite samples were evaluated in a 2% homogeneous magnetic induction within a range of 0–8 T produced with a Bitter’s magnet and by using an indirect method through a capacitance measurement. A giant magnetostriction, with hysteresis, on the order of 10 −2 was observed. An anisotropic structure produces a more evident effect as shown in Ref. [30], where highly filled poly(tetramethylene ether glycol)-based PU MSEs were prepared under a MF. An SEM image of the composite microstructure showed a chain-like structure of CI, after orientation under a MF of 1.2 T. The aligned chain-like structure of CI in PU greatly enhanced the thermal conductivity, the compression properties, and the magneto-sensitive effect of anisotropic PU MSEs compared to that of the isotropic one. Indeed, magneto-sensitive effect was observed by using a modified dynamic mechanical analyzer[89] at room temperature with the sample moving at a given amplitude and frequency.  The stress in the sample was measured with a load sensor, and the strain was taken as the displacement amplitude. The shear modulus was computed from the data of strain and stress. A MF in the range of 0–1,000 mT was applied to samples during the test. When

(b) 1100 1000 900 800

Iso-80

200

100

0

5

10 Strain (%)

15

Aniso-80-1

700 600 500

Aniso-70-1

400

Aniso-80-2 Aniso-70-2 Aniso-60-2

300

Iso-70 Iso-60 Blank PU foam

0

Elastomers

Stress / KPa

Stress / KPa

(a) 300

ACTIVE BEHAVIOR OF MAGNETO-SENSITIVE SOLIDS

20

200

Aniso-60-1

100 0

0

5

10 Strain (%)

15

Fig. 19  The compressive stress–strain curves of isotropic (a) and anisotropic (b) PU MSF with different iron contents Source: Reprinted with permission.[91]

20

Smart Polymers–Textiles

The compressive response along the foaming direction of MSFs with different magnetic particles, namely iron and barium ferrite particles, was studied in Ref. [22]. The mechanical behavior of samples with randomly ­dispersed particles increased with the particle volume content for both type of reinforcements in accordance with the mechanics of composite materials that predicts an increase of the composite stiffness with the particle content but a major difference between systems reinforced with iron and barium ferrite particles emerged. In fact, the peak performance was exhibited at 25 wt% in iron-reinforced MSFs and at 20 wt% in barium ferrite ones, both for randomly dispersed and aligned particles. This was due to the fact that iron and barium ferrite have different densities (7.8 and 4.6 g/cm3, respectively), and at higher volume content, the foam morphology deteriorates. Barium ferrite systems showed a lesser pronounced increase of the compressive modulus with respect to iron-based ones, as a consequence of the lower aspect ratio of aligned particle aggregates. The compression properties of blank PU foam and isotropic and anisotropic PU MSFs with high weight filling content of CI were studied in Ref. [91] (the results are shown in Fig. 19). It can be seen that the blank PU foam is flexible with low compression stress of 1.2 and 2.6 kPa at 3.9% and 10% strains, respectively. The compression properties are notably improved with increasing the iron content. Moreover, the aggregate structure of CI particles can significantly affect the compression strength. It is interesting that the authors found that the anisotropic PU MSFs they produced presented an anisotropic compression property. When the compression direction is parallel to the magnetic chain structure, the compression stress increases at first until the strain reaches ~3.9%, and then, a dramatic decrease occurs with increasing strain, as a consequence of a loose contact between iron particles.

2414

Smart Polymers: Lightweight Composites and Foams Tailored with Magnetic Field

(a)

(b) 10

Isotropic dispersion (Iso-70) Anisotropic dispersion (Aniso-70)

Shear modulus (MPa)

Shear modulus (MPa)

10 8 6 4 2 0

Anisotropic dispersion (Aniso-50) Anisotropic dispersion (Aniso-60) Anisotropic dispersion (Aniso-70)

8 6 4 2

0

0.2 0.4 0.6 0.8 Magnetic flux density (T)

1

0

0

0.2 0.4 0.6 0.8 Magnetic flux density (T)

1

Fig. 20  Magnetic field-induced shear modulus increment of (a) isotropic and anisotropic PU MSE with 70 wt% carbonyl iron tested at 1 Hz and (b) anisotropic PU MSE with different carbonyl iron contents at 1 Hz [30]

Smart Polymers–Textiles

the test frequency  was 1 Hz, the maximum absolute and relative MS effect of anisotropic PU MSEs with 26 wt% hard segment and 70 wt% CI were ~1.3 MPa and ~21%, respectively, as shown in Fig. 20. Anisotropic properties can be enhanced if geometrical features are introduced during the production of the composite as in Ref. [92], where anisotropic MSEs with 0% and 15% by weight of silicone oil were fabricated under a MF rotated with a 45° angle, as shown in Fig. 21, so that the iron particle alignment inside the MSE was 45° to the direction of flat MSE. Scanning electron microscopic images confirmed the aligned structure of iron particles and showed that the sample with 15% silicone oil contribution resulted in a less volume fraction of iron particles. The MSEs were then tested in an oscillatory pure shear mode at different shear strain amplitudes under different magnetic flux densities to measure their dynamic viscoelastic properties. The testing results showed that the MSE with 15% silicone oil had lower zero-field storage and loss modulus and also had higher maximum magneto-sensitive effect than the MSE with 0% silicone oil. Because of the 45° iron

Electromagnet Nonmagnetic separator

N

MR elastomer Carbonyl iron particles Nonmagnetic mold MF lines

45° S

Fig. 21  Fabrication of MS elastomers in MF with a mold placed at 45° angle [92]

particle alignment, the storage modulus of the MSEs had a higher value in that direction. In Ref. [27], MS elastomers were prepared by mixing CI particles of 3 μm nominal diameter (ISP S-3700), natural rubber or cis-poly(isoprene), and additives including cross-linking agents, antioxidants, and mixing aids in appropriate concentrations on a two-roll mill; the iron volume fraction of the elastomers was roughly 0.27. The MS material was incorporated in a simple resonant structure called a tuned vibration absorber to measure the complex dynamic shear moduli of these materials at high frequencies. It was found that the field-induced modulus increase in MS elastomers is substantial even at kilohertz mechanical frequencies. As in previous measurements at low frequencies, [56] the moduli are generally found to decrease with strain amplitude. Some preliminary measurements of the relatively large elongation of these materials in applied MFs showed butterfly-shaped cycles. A deep analysis of isotropic and anisotropic “soft” MS elastomers was performed in Ref. [61] by using homogeneous MFs of various intensities. Viscoelastic behavior has been studied by three different experimental techniques: elongation, static, and dynamic shears. Both storage and loss moduli of the materials have been measured as functions of material composition and MF intensity at various frequencies of shear oscillations. The MS elastomers under investigation show very high (up to 100-fold) increase not only in storage but also in loss modulus in the MF of up to 300 mT. The appearance of the new effect of pseudo-­ plasticity induced by the MF has been observed leading to a considerable (up to 100-fold) increase in the shear loss modulus of the composites. Two types of magnetic filler have been used. The first one was the powder of iron particles with the average size of 2–4 μm. The second one was the powder of iron particles with a wide size distribution of 2–70 μm. Another type of material studied in this work has been obtained when the polymerization reaction was

(a) 60

0 mT 90 mT 180 mT 240 mT 330 mT

Stress (kPa)

50 40 30 20 10 0

0

0.2

0.4

0.8

0.6

Strain (b) 30

Stress (kPa)

25

0 mT 90 mT 165 mT 245 mT 335 mT

20 15 10 5 0

0

0.2

Strain

0.4

0.6

Fig. 22  Stress–strain dependence for samples 1 (a) and 2 (b) measured in uniform MF of various intensities Source: Reprinted with permission.[61]

carried out in an external homogeneous MF of 40 mT. Magnetic particles formed chain-like structures under the influence of a uniform MF, and the particle aggregates parallel to the field direction were set during the polymerization process. As a result, a highly anisotropic response was obtained. The stress–strain curves obtained for samples 1 and 2 (sample 1 has small microparticles, whereas sample 2 contains larger particles) under elongation at various MF

intensities are shown in Fig. 22a and b, respectively. It was found that the dependence of the stress, σ, on the relative sample elongation, ε, is practically linear in the absence of MF for both samples. The Young’s modulus of the systems, calculated as the slope of the curve σ(ε) at low strains, was equal to 25 kPa for sample 1, and for sample 2, it was equal to 13 kPa. Under the influence of the magnetic field, the linearity of the dependence σ on ε is violated for both samples. With an increase in the field intensity, nonlinear behavior becomes more pronounced; in the range of small elongations (1%–5%), the stress increases faster than that at higher elongations. Similar phenomenon has been observed in Ref. [60]. However, only twofold increase has been observed in the tangential modulus at small deformations up to 4%; at higher deformations, the increase was  even smaller. The dependence of the tangential Young’s modulus, E, on the MF intensity, H, for samples 1 and 2 is shown in Fig. 23. The authors found that the ­functions E(H) were S-shaped, and that the elasticity growth of around 200 mT was due to the structuring of magnetic particles. Saturation occurred in the constitutive relationship E–H as a result of the saturation of particle magnetization, in accordance with the saturation value of iron (400 mT). Similar behavior of elastic modulus has been observed for materials based on other fillers, i.e., magnetite.[61] Shear tests showed similar achievements with an increase of the MF, except that a marked ­residual ­deformation of the samples after unloading was detected. In the presence of the MF, the sample strain after the complete shear stress release remained constant, and its residual value depended on the field intensity. This ­phenomenon was reversible since the deformation of the sample totally ­disappeared once the MF was switched off. Active behaviors have been recorded also for isotropic porous MS elastomers as in Ref. [88], where a new kind of MSE, named porous magneto-rheological elastomer (P-MSE), was prepared by filling with ammonium bicarbonate NH4HCO3. Two groups of samples with different content of NH4HCO3 and CI particles were prepared and

Young’s modulus (kPa)

150

100

50

0

Sample 1 Sample 2 0

50

100

150 200 250 Magnetic field (mT)

300

350

Fig. 23  Dependence of the Young’s modulus of samples 1 (cross) and 2 (squares) calculated for 50% deformations on the intensity of the external uniform MF Source: Reprinted with permission.[61]

Smart Polymers–Textiles

Smart Polymers: Lightweight Composites and Foams Tailored with Magnetic Field 2415

2416

Smart Polymers: Lightweight Composites and Foams Tailored with Magnetic Field

Smart Polymers–Textiles

tested in shear under different magnetic flux densities. The experimental results indicate that the NH4HCO3 had a significant impact on the properties of P-MSE, including the microstructures and dynamic mechanical performance. The increase of NH4HCO3 induced higher porosity in P-MSE and decreased the shear storage modulus. Moreover, P-MSE showed a more pronounced MS effect, promoted by NH4HCO3 and CI contents, with respect to MSE without ammonium bicarbonate. Also, the damping and loss factor were influenced by NH4HCO3. The real-time force and displacement signals were recorded and used to reconstruct the stress–strain relationships in shear mode at a frequency of 30 Hz and 0.97% strain amplitude. Stress–strain curves in elliptical shape were obtained. They were characterized by a hysteresis loop with increasing area with respect to the MF strength, indirectly demonstrating increasing irreversible dissipation energy under cyclic loads. Moreover, the stress–strain relationships showed dependence on NH4HCO3 content under different magnetic flux densities: the stress amplitude of different samples decreases by 68.7% with increasing NH4HCO3 content from 0 to 6 wt%. MS elastomers can show a sort of reverse effect in Ref. [7], where Ni microparticles embedded in a nonmagnetic elastic silicone-based matrix exhibited a magnetic behavior depending on particles’ percentage, temperature, intensity of magnetizing field, and the induced strain. In  particular, the elastic properties of the matrix give nonconventional strain effects on magnetization. When a ­compression is applied and the magnetizing field is high, i.e., near saturation, the effect of the increase in the ­magnetic particle content is clearly evident. When the magnetizing field is below 1/4 of saturation, the effect of elastic matrix deformation on particles’ orientation and the consequent change of the magnetization intensity is prominent. A threshold field can be defined between the two (a)

Plastomers In Ref. [29], a magneto-sensitive plastomer (MSP) concept was investigated as a new kind of soft magneto-­sensitive polymeric composite. This work reports on the large magneto-­deforming effect and high magneto-damping performance of MSPs under a quasi-statical shearing condition. Materials were prepared by adding soft-magnetic CI powder (type CN, 7.2 g/cm3 in density, produced by BASF GmbH, Ludwigshafen, Germany) into a plastic PU matrix (0.986 g/cm3 in density). Different particle concentrations were used, namely, weight ratios of 40%, 50%, 60%, and 70% with respect to the compound mass or volume fractions of 8.4%, 12.0%, 17.0%, and 24.2%, respectively. The samples were identified as MSP-40, MSP-50, MSP-60, and MSP-70, respectively. It is clear from Fig. 24 that the MSP was able to be deformed under the application of an MF (magnetic malleability). The magneto-induced axial (b)

0s

(c)

distinct behaviors. The coupling of experimental results and theoretical modeling demonstrated that even a weak intrinsic magnetoelastic effect can significantly influence the magneto-­elasticity due to coexistence of phases with ­different geometry, ­elastic, and/or magnetic properties. In Ref. [46], MSEs, consisting of a dispersion of micron size (typically 1–5 μm) of iron or nickel particles inside an elastomer, were investigated. During the curing process, the polymer/particle suspension was put under a strong MF to induce chain-like structures. In iron-filled samples, very clear magnetostrictive effects were detected (up to 1.2% at 120 kA/m MF), while in nickel-filled systems, the gap between the particles was much thinner and the small compression during testing reduced the electric resistivity by many orders of magnitude making it a very sensitive pressure sensor, albeit the response to MF was negligible.

30 s

(d)

(e)

Fig. 24  The large deformation of the magneto-sensitive MSP-70 under a uniform 585 kA/m MF. In the upper series, the small sphereshaped MSP-70 at an initial 0 s (a) changes its shape into a shuttle-like shape after 30 s (b). In the lower series, the initial oblate shape of MSP-70 (c) under a magnetic field (d) is changed into a parallel column-like shape (e) by the MF Source: Reprinted with permission.[29]

stress, which drives the shape change of the MSP, can be tuned to a wide range from 0.0 to 55.4 kPa by applying an MF up to 1 T. The capability of dampen mechanical stimuli was in direct relationship with the MF strength, shear rate, CI content, and shear strain amplitude. For an MSP with 60 wt% CI powder, the relative magneto-­enhanced damping effect can reach as high as 716.2% under a ­quasi-statically ­shearing condition.

In Ref. [35], advances in mechanical and swelling behavior of MF-responsive soft materials, including flexible polymer networks and gels, were reported. The elastic modulus of magnetic composites was measured under uniform MF at 293 K. The authors tested the systems by applying the load perpendicular or parallel to the MF, whose strength ranged between 0 and 400 mT (0–100 mT when stress and strain were parallel). Figure 25 shows the effect of uniform MF on the modulus of samples containing randomly distributed magnetite particles. In this case, the dependence on the relative directions of MF field and load was weak, with slight increase in the modulus, in accordance with previous results reported for magnetite (Fe3O4)-loaded PVA hydrogels.[93] The effect of MF on samples with aligned particles was significantly different between the direction perpendicular to the particle chains (Fig. 26, left and center) and parallel to the particle chains (Fig. 26, right), demonstrating that the mutual orientations of MF and loads, the field strength, and the particle arrangement have a major role in the magneto-­ sensitive reinforcing effect. In particular, a small increase was measured when the field was perpendicular to the aligned particles while the elastic modulus increased significantly with the MF parallel to the linear aggregates (Fig. 27, right). Furthermore, at small field intensities (up to 30 mT), a slight increase has been observed while above that value the modulus increased significantly. At higher field ­intensities (from 200 mT), the elastic modulus leveled. In Ref. [96], novel magneto-rheological fluids—­ supramolecular magneto-rheological polymer gels (SMRPGs)—were investigated. Supramolecular ­polymer deposited on the surface of iron particles was suspended in the carrier fluids. The supramolecular network was obtained by metal coordination between terpyridine monomers and zinc ion. SMRPGs exhibit some advantages as controllable off-state viscosity, a reduced iron particle settling rate, and stability. The results of this research indicated that off-state viscosity and particle settling can be controlled

Gels In Ref. [93], the compressive modulus of magnetic fluid containing gels, called ferrogels, in the presence of MF was studied. No hysteresis was shown in their magnetization curve suggesting a super paramagnetic response. The compressive modulus at equilibrium of the ferrogel increased with the MF up to 320 kA/m. The mean change in modulus increased with increasing MF, and saturation occurred above 160 kA/m. The mean and the maximum change in modulus at 320 kA/m were 31 and 71 Pa, corresponding to a variation of 19% and 46% with respect to the values measured without MF, respectively. The same authors [94] showed that MSGs, consisting of carrageenan of polysaccharides and CI particles, can have drastic and reversible changes in dynamic modulus under relatively low MF. The magnetic gel with a volume fraction of 0.34 exhibited a reversible behavior, with the storage and loss moduli increased by 500 and 1,200 times under a 0.5 T MF. In Ref. [23], similar results were found for a magnetic elastomer consisting of PU and CI particles. The magnetic elastomer with a volume fraction of 0.29 exhibited a reversible increase by factors of 277 of the storage modulus and 96 of the loss modulus with a 0.5 T MF. The elastomer showed a high mechanical toughness with a braking strain exceeding 0.8, demonstrating a giant magneto-­ sensitive behavior after half a year without any significant ­performance decrease.[95] (a) G(B) (kPa)

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Fig. 25  Dependence of the MF intensity on the elastic modulus for mPDMS containing different amounts of randomly distributed magnetite particles. The concentrations of the filler particles are indicated in the figure: (a) compression transversal to the magnetic field and (b) compression parallel to the magnetic field Source: Reprinted with permission.[105]

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Smart Polymers: Lightweight Composites and Foams Tailored with Magnetic Field

(a) 32 G(B) (kPa) 28

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Fig. 26  Effect of the MF intensity on the elastic modulus. The iron content of the elastomers is indicated in the figure. The white and black arrows show the direction of the force and the uniform MF, respectively Source: Reprinted with permission.[105]

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Fig. 27  Dependence of the MF intensity on the elastic modulus. The arrangements of the particles in the polymer networks are ­parallel to the applied mechanical stress while the applied uniform MF is parallel (a) or perpendicular (b) to the columnar structure. The ­concentration of the carbonyl iron particles in the mPDMS matrix is indicated Source: Reprinted with permission.[105]

by adjusting the concentration of supramolecular polymer gel. Dynamic yield stress significantly increased with an external MF up to 23.5 kPa under a ­magnetic flux density of 500 mT. In Ref. [97], a series of MSGs consisting of p­ lastic PU matrix swollen by nonvolatile solvent in different weight fractions and CI particles were prepared. Their ­magneto-sensitive properties, both under oscillatory and rotational shear rheometry, were systematically tested. These MSGs could be prepared to have a solid-like behavior at low solvent concentrations (less than 10 wt%) or a liquid-like one (solvent content higher than 25 wt%) by adjusting the particle content. Their behavior can also be influenced by properly managing the applied MF. In Ref. [98], a novel black “Plasticine” was developed by dispersing iron microparticles into the paraffin wax– petroleum jelly composite matrix. Due to the presence of magnetic particles, this Plasticine exhibited magnetic-­ dependent mechanical properties and can be defined as a

typical MSG material. The magnetic Plasticines were malleable, and their mechanical properties were highly influenced by the iron particle content. With increasing of the externally applied MF, the shear storage modulus sharply increased. Under the optimum iron content, the magnetic-­ induced modulus can be increased to 4.23 MPa, and the relative magneto-sensitive effect was 305%. Such system also showed the capability to behave as a fluid above a critical temperature; hence, it could behave as an MSG or an MR depending on the service temperature. Foams In Refs. [17,97], a PU-based MSF was prepared via in situ polymerization and foaming. The anisotropic PU MSFs possesses anisotropic compression behavior with ­compression strength along the aligned particles direction as high as 1,053.5 kPa for the sample with 80 wt% CI content, which is about 878 times that of the blank foam and

Smart Polymers: Lightweight Composites and Foams Tailored with Magnetic Field 2419

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The traditional main industrial applications of magneto-­ sensitive composites are in the field of generation and transformation of electricity in low energy demanding applications (small motors, reluctance motors, and brushless DC motors). Magnetic composites, in fact, can reduce the typical phenomena occurring in devices involving MF (core losses, eddy currents). Replacing the conventional 22.6

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strength (below 200 kA/m) and showed a butterfly-like effect, demonstrating a direct and almost proportional dependence of the stress response on the MF strength. The magnetoelastic behavior in the ­compression of MSFs evidenced the capability of such lightweight composites to continuously follow the MF trend. In fact, the measured stress response of MSFs embedding iron particles was in direct relation with the triangular, sinusoidal, or steplike trend of the MF carrier wave in Fig. 28. Systems with aligned and randomly distributed particles, differently than MS elastomers and gels, showed a peculiar magnetoelastic response at strains higher than the yield strain. In fact, while MSFs with random particles showed the typical reduction of the measured stress under MF, samples with aligned particles exhibited an increased response. This was related to the fact that cell struts and walls in a foam incur in buckling above the yield point. If reinforcing particles are randomly distributed, the typical behavior is not changed. On the contrary, in samples with aligned particles, buckled chain-like structures tend to recover the elongated configuration, hence try to align along the MF lines in turn acting against the forcing load. The macroscopic effect is depicted in Fig. 30, where an increase of the stress response, and hence of the apparent elastic modulus of the foam, was detected.

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Fig. 29  Induced stress with respect to the MF with a “butterfly-­ like” shape Source: Reprinted with permission.[22]

Smart Polymers–Textiles

about 57 times of that at the vertical direction of the same sample. Such PU MSF exhibits a MF controlled shear modulus, which can be adjusted by changing the orientation and content of the CI particle aggregates, test frequency, and MF strength. The maximum absolute and relative MS effects for the anisotropic PU MSF are about 1.07 MPa and about 27.1%, respectively. In Ref. [22], similar MS PU foams were developed. The aligned microparticles induced an anisotropic mechanical behavior, strongly improving the mechanical s­ tiffness and strength along the MF direction compared to unfilled systems. The authors detected a peculiar magneto-sensitive behavior to such systems. In fact, foams showed a direct relationship between the foams’ elastic response and intensity as well as the time profile of the MF applied during their magnetoelastic characterization, as shown in Figs. 28–30. This magnetoelastic behavior has been obtained at low MF

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laminated cores in an electric machine with the new ones can improve the high frequency behavior (tooth ripple losses), because the conductive particles are essentially ­isolated each other by a very thin dielectric polymeric layer. On the other hand, the use of recent functionalized magnetic composites based on polymers and magnetic particles would allow a design freedom not available before, and tailored shapes and material characteristics can be exploited to maximize the MF/function interactions. Very precise and complex shapes can be obtained thanks to the use of small magnetic particles dispersed in the matrix and the large-scale polymer technologies, such as injection molding or powder compaction. Such compounds can be injected or compacted in specific molds at low cost. Smart magnetic composites can replace iron-based alloys at ­similar or lower costs. The use of thermoplastic polymers also potentially allow the recycling of the parts and can ­contribute in the reduction of both production and end-life costs. Due to their recent development, lightweight magnetic composites are still not used in such commercial applications. Same applications employ polymer-based magnetic composites filled with high amount of magnetic (usually iron based) particles, to produce the soft magnetic composites. The use of soft magnetic composites in an electrical machine with segmented armature torus structure, especially as a main drive in hybrid electric or pure electric vehicles, has been investigated in Ref.[99] The authors put in evidence that in devices requiring a three-­dimensional

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Fig. 30  Response to a square waveform MF for aligned and random oriented particle distribution at a pre-strain above the yield point Source: Reprinted with permission.[22]

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Smart Polymers: Lightweight Composites and Foams Tailored with Magnetic Field 2421

Active Noise Control The use of lightweight magnetic foams for the active control of transmission loss was proposed in Ref. [100]. The authors exploited the complimentary advantages of the cellular structure of polymeric foams with the tailored spatial distribution of actuators embedded in it for active noise control. The transmission loss of smart foams using active control strategies was investigated from the experimental and numerical points of views, and good comparisons were found. The active control with broadband and single-frequency primary source inputs demonstrated a good improvement in the transmission loss of the smart foams. Microactuation A very promising application of magneto-sensitive foams is related to their low stiffness and the consequent capability to be deformed with low-intensity MFs. A proof of that was reported by Zhao et al., [101] who prepared a magnetic foam based on a PVA hydrogel. They used phase separation during gelation to remove the solvent (water) from the solution during the orientation of CI particles along the direction of the applied MF. The free water separated from the PVA gel, thus forming elongated pores during drying. The lightweight smart composite foam showed the capability to be remotely controlled by a MF to deliver various biological agents. A smart magnetic composite can also be used to move objects or other substances.[102] Seo et al. used a magnetic composite to guide the motion of water droplets (Fig. 31). They treated the surface of a magnetic elastomer with a superhydrophobic substance and controlled the droplet motion on that surface through local changes in the surface topography created by the magnetic actuation of the MS elastomer device. The direction and the speed of the droplet motion were easily controlled by the motion of magnets. The velocity of the transportation of a water droplet was observed to be higher than 8 cm/s. Xu et al.[59] reported on the production of ultralight magnetic elastomer based on a graphene aerogel homogeneously decorated with Fe3O4 nanoparticles. The presence of graphene rendered electrically conductive the structure, while magnetic particles gave the ability to react with the application of an MF. The main result was that the smart lightweight aerogel exhibited characteristics easily

controlled by using a MF. The aerogel showed the capability to undergo large deformations under MF, thus acting as an actuator with reversible deformability, and it also showed strain-dependent electrical resistance (see Fig. 7). Acoustic Filtering MS elastomers have been applied also as acoustic filters in Ref. [6]. Blom et al. put in evidence that an amplitude dependence of the shear modulus—referred to as the Fletcher–Gent effect—for even small displacements and the appearance of large MS effects can be exploited for acoustic filtering. The material was made out of silicon and natural rubber as matrix and isotropically dispersed particles, at a volume content equal to 33%. Measurements performed in the audible frequency range showed, under an external MF in the range of 0–0.8 T, a strong amplitude dependence and large responses to externally applied MFs, up to 115%. This behavior demonstrates that the presence of iron particles induces a strong, non-negligible, amplitude dependence in the entire frequency range and allows to produce magnetic composites for contactless acoustic noise suppression applications. Sound absorption magnetic composites can also be produced by properly coating, with magnetic fluids, the internal surface of a PU foam.[38] Foams with two cellular morphologies (single and dual porosity) were produced and tested. In such systems, the peak in the absorption curve was tunable by applying the MF, thus potentially allowing the tailoring of the acoustic response to the ­specific needs. EMI Shielding Electromagnetic wave interference (EMI) shielding can be achieved in magnetic composites by a proper selection of the type of particles to avoid degradation of the performances during service. Such need was pointed out by Sedlacik et al., who investigated MS solids to provide EMI shielding performances.[103] In fact, the authors showed that magnetic particles have to be properly protected against oxidation in real applications. They prepared two kinds of MS solids differing in matrix type (silicon elastomer and thermoplastic elastomer) and particle type: CI particles and siloxane-modified CI particles. The difference in magnetic properties after the surface treatment of the microparticles was negligible, while the surface modification significantly increased the anti-acid-corrosion properties. The presence of specific molecular compatibilizers, furthermore, improved the compatibility between particles and silicone matrix. The peculiar distribution of the particles imparted the capability to affect the electromagnetic shielding properties. In particular, the magnetic foams increased the absorption of electromagnetic radiation in ultra-high-frequency band, namely, the frequency range from 700 MHz to 1.6 GHz.

Smart Polymers–Textiles

magnetic flux, it is very difficult if possible to use a conventional laminated steel material and the capability of smart magnetic composites to be manufactured in complex shapes renders their use the only viable choice. They concluded that novel electrical machines with complex magnetic composite parts can be competitive alternatives to the traditional radial flux machines, especially when there are some special demands such as short axial length, high torque density, or high power density.

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52. Hentze, H.-P.; Antonietti, M. Porous polymers and resins for biotechnological and biomedical applications. Rev. Mol. Biotechnol. 2002, 90 (1), 27–53. 53. Wong, S.; Lee, J.W.S.; Naguib, H.E.; Park, C.B. Effect of processing parameters on the mechanical properties of injection molded thermoplastic polyolefin (TPO) cellular foams. Macromol. Mater. Eng. 2008, 293 (7), 605–613. 54. Sorrentino, L.; D’Auria, M.; Davino, D.; Visone, C.; ­Iannace, S. SmartFoams with magneto-sensitive elastic behavior. AIP Conf. Proc. 2014, 1599, 238–241. 55. Olsson, R.T.; Azizi Samir, M.A.S.; Salazar-Alvarez, G.; Belova, L.; Ström, V.; Berglund, L.A.; Ikkala, O.; Nogués, J.; Gedde, U.W. Making flexible magnetic aerogels and stiff magnetic nanopaper using cellulose nanofibrils as ­templates. Nat. Nanotechnol. 2010, 5 (8), 584–588. 56. Ginder, J.M.; Nichols, M.E.; Elie, L.D.; Tardiff, J.L. ­Magnetorheological elastomers: Properties and ­applications. In Proceedings of SPIE 3675, Smart ­Structures and Materials 1999: Smart Materials Technol­ ewport Beach, CA; ogies; Wuttig, M.R.; Ed.; 3–4 March, N 1999, 131–138. 57. Schümann, M.; Günther, S.; Odenbach, S. The effect of magnetic particles on pore size distribution in soft polyurethane foams. Smart Mater. Struct. 2014, 23 (7), 75011. 58. Berkovskii, B.M.; Bashtovoy, V.G. (Eds.) Magnetic Fluids and Applications Handbook; Begell House: New York, 1996. 59. Xu, X.; Li, H.; Zhang, Q.; Hu, H.; Zhao, Z.; Li, J.; Li, J.; Qiao, Y.; Gogotsi, Y. Self-sensing, ultralight, and conductive 3D graphene/iron oxide aerogel elastomer deformable in a magnetic field. ACS Nano 2015, 9 (4), 3969–3977. 60. Jolly, M.R.; Carlson, J.D.; Muñoz, B.C.; Bullions, T.A. The magnetoviscoelastic response of elastomer c­ omposites ­consisting of ferrous particles embedded in a polymer matrix. J. Intell. Mater. Syst. Struct. 1996, 7 (6), 613–622. 61. Stepanov, G.V.V.; Abramchuk, S.S.S.; Grishin, D.A.A.; Nikitin, L.V.V.; Kramarenko, E.Y.Y.; Khokhlov, A.R.R. Effect of a homogeneous magnetic field on the v­ iscoelastic behavior of magnetic elastomers. Polymer 2007, 48 (2), 488–495. 62. Leblanc, J. Rubber–filler interactions and rheological ­properties in filled compounds. Prog. Polym. Sci. 2002, 27 (4), 627–687. 63. Munoz, B.C.; Jolly, M.R. Composites with field responsive rheology. In Performance of Plastics; Brostow, W.; Eds.; Carl Hanser Verlag: Munich, 2001, 553–574. 64. Lokander, M.; Stenberg, B. Performance of isotropic ­magnetorheological rubber materials. Polym. Test. 2003, 22 (3), 245–251. 65. Kordonsky, W.I. Magnetorheological effect as a base of new devices and technologies. J. Magn. Magn. Mater. 1993, 122 (1–3), 395–398. 66. Fuchs, A.; Xin, M.; Gordaninejad, F.; Wang, X.; ­Hitchcock, G.H.; Gecol, H.; Evrensel, C.; Korol, G. Development and characterization of hydrocarbon polyol polyurethane and silicone magnetorheological polymeric gels. J. Appl. Polym. Sci. 2004, 92 (2), 1176–1182. 67. Jun, J.-B.; Uhm, S.-Y.; Ryu, J.-H.; Suh, K.-D. Synthesis and characterization of monodisperse magnetic composite ­particles for magnetorheological fluid materials. Colloids Surf. A 2005, 260 (1–3), 157–164.

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85. Shen, Y.; Golnaraghi, M.F.; Heppler, G.R. Experimental research and modeling of magnetorheological elastomers. J. Intell. Mater. Syst. Struct. 2004, 15 (1), 27–35. 86. Li, Y.; Li, J.; Tian, T.; Li, W. A highly adjustable ­magnetorheological elastomer base isolator for ­applications of ­real-time adaptive control. Smart Mater. Struct. 2013, 22 (9), 95020. 87. Padalka, O.; Song, H.J.; Wereley, N.M.; Filer, II, J.A.; Bell, R.C. Stiffness and damping in Fe, Co, and Ni nanowire-­ based magnetorheological elastomeric composites. IEEE Trans. Magn. 2010, 46 (6), 2275–2277. 88. Ju, B.X.; Yu, M.; Fu, J.; Yang, Q.; Liu, X.Q.; Zheng, X. A novel porous magnetorheological elastomer: Preparation and evaluation. Smart Mater. Struct. 2012, 21, 035001. 89. Chen, L.; Gong, X.L.; Jiang, W.Q.; Yao, J.J.; Deng, H.X.; Li, W.H. Investigation on magnetorheological elastomers based on natural rubber. J. Mater. Sci. 2007, 42 (14), 5483–5489. 90. An, H.; Picken, S.J.; Mendes, E.; Spontak, R.J.; Lin, J.S.; Bukovnik, R.; Zhang, P.Q.; Chen, Z.Y. Enhanced ­hardening of soft self-assembled copolymer gels under homogeneous magnetic fields. Soft Matter 2010, 6 (18), 4497. 91. Gong, Q.; Wu, J.; Gong, X.; Fan, Y.; Xia, H. Smart ­polyurethane foam with magnetic field controlled modulus and anisotropic compression property. RSC Adv. 2013, 3 (10), 3241–3148. 92. Tian, T.; Nakano, M. Fabrication and characterisation of anisotropic magnetorheological elastomer with 45° iron particle alignment at various silicone oil concentrations. J. Intell. Mater. Syst. 2017. doi:10.1177/1045389X17704071. 93. Mitsumata, T.; Ikeda, K.; Gong, J.P.; Osada, Y.; Szabó, D.; Zrnyi, M.; Mitsumata, T.; Ikeda, K.; Gong, J.P.; Osada, Y. Magnetism and compressive modulus of magnetic fluid containing gels. J. Appl. Phys. 1999, 85 (12), 8451–8455. 94. Mitsumata, T.; Abe, N. Magnetic-field sensitive gels with wide modulation of dynamic modulus. Chem. Lett. 2009, 38 (9), 922–923. 95. Mitsumata, T.; Honda, A.; Kanazawa, H.; Kawai, M. ­Magnetically tunable elasticity for magnetic hydrogels consisting of carrageenan and carbonyl iron particles. J. Phys. Chem. B 2012, 116 (40), 12341–12348. 96. Hu, B.; Fuchs, A.; Huseyin, S.; Gordaninejad, F.; ­Evrensel, C. Supramolecular magnetorheological polymer gels. J. Appl. Polym. Sci. 2006, 100 (3), 2464–2479. 97. Xu, Y.G.; Gong, X.L.; Xuan, S.H. Soft magnetorheological polymer gels with controllable rheological properties. Smart Mater. Struct. 2013, 22 (7), 75029. 98. Xuan, S.; Zhang, Y.; Zhou, Y.; Jiang, W.; Gong, X. ­Magnetic plasticineTM: A versatile magnetorheological material. J. Mater. Chem. 2012, 22 (26), 13395. 99. Zhang, B.; Andreas, S.; Doppelbauer, M. Development of a novel yokeless and segmented armature axial flux machine based on soft magnetic powder composites. In European Congress and Exhibition on Powder Metallurgy. European PM Conference Proceedings, 9–13 October, Hamburg, Germany; EPMA, 2016. 100. Kundu, A.; Berry, A. Active control of transmission loss with smart foams. J. Acoust. Soc. Am. 2011, 129 (2), 726–740. 101. Zhao, X.; Kim, J.; Cezar, C.A.; Huebsch, N.; Lee, K.; ­Bouhadir, K.; Mooney, D.J. Active scaffolds for on-demand

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Smart Polymers: Molecularly Imprinted Polymers Mónica Díaz-Bao, Rocío Barreiro, Alberto Cepeda, and Patricia Regal Department of Analytical Chemistry, Nutrition and Bromatology, Faculty of Veterinary Science, University of Santiago de Compostela, Lugo, Spain

Abstract Assuring food safety has historically been the purpose of thousands of research projects all over the world, promoted and supported by government authorities. Dangerous substances in food may include natural toxicants, contaminants, and chemicals and drugs deliberately used to increase the food supply, among others. Analytical methods are crucial to detect and quantify hazardous substances and assure and achieve this way the required safety and quality of food products. In this context, molecularly imprinted polymers (MIPs) have offered a broad range of versatile options to support and complement existing analytical techniques. These “smart” polymers are designed as artificial antibodies, capable of specifically rebinding the template used during their synthesis, or analogue compounds. The target analyte is usually surrounded by a biological matrix that complicates its determination, but these MIPs facilitate enormously the process thanks to their ability to rebind the template within the imprinted sites and isolate it. Several polymerization techniques are available for the analyst, as well as different possible hybridization of MIPs with other materials and supports (magnetite, quantum dots, silica, glass, carbon nanotubes, and so on), all of this in order to obtain the desired device, with the desired characteristics and behavior. In this sense, solid-phase extraction is one of the most popular applications studied for these imprinted materials, while more recent applications include small-sized devices, monoliths, sensors, stationary phases for liquid chromatography, and magnetic sorbents, among others. MIPs have widely demonstrated their ­suitability as supports to extract residues and contaminants in food, and the field is still growing. Smart Polymers–Textiles

Keywords: Chromatography; Contaminants; Food safety; MIP; Residues; SPE.

INTRODUCTION TO MOLECULARLY IMPRINTED POLYMERS IN FOOD SAFETY Food security is a major concern for authorities, and therefore effective methods and analytical technologies have been developed continuously to control food safety and to protect the health of consumers. Feeding a population in a suitable way implies that their countries must provide a constant supply, in sufficient quantity and variety, of safe, good-quality food. Serious food safety incidents during the 1990s urged the European Union and other countries across the world to review their food safety systems and to look for better ways to protect consumers against unsafe food (FAO Web Site). Food contamination due to natural toxic compounds can represent a significant source of foodborne illness, and it poses severe risks to human health. Actually, besides the well-known food contamination due to the presence of living bacterial cells (e.g., enterotoxins from certain strains of Escherichia coli or Staphylococcus aureus), there are other substances such as biocontaminants, for instance, fungal toxins and bacteria recognized by the World Health Organization (WHO), which represent a significant source of foodborne illnesses.[1] ­Dangerous substances in food may

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include natural toxicants, environmental contaminants, and chemicals such as pesticides and veterinary drugs, deliberately used to increase the food supply. Residues of these chemicals can also be present in the processed food and ­potentially affect human health.[2] Modern analytical methods have the sensitivity required for detection and quantification of contaminants in food, but the direct application of these methods to real samples is usually difficult to perform because of the food-matrix complexity. These methods are crucial for the countries to assure and achieve the required safety and quality of their food products, for national products, international trade, and to verify that imported food products meet national requirements. Moreover, increasing consumer demand for safety and nutritional excellence, together with higher market competition, underlines and intensifies the importance of food analysis. The accurate assessment of food safety and quality, the freshness of raw materials and the nutritive values of processed food, as well as the determination of food additives (e.g., food preservatives and food colorants and dyes) are especially important to protect consumers and give them piece of mind. Such a situation forces food  analysts to develop better, less time-­consuming, faster, and more accurate analytical procedures every day. Encyclopedia of Polymer Applications, First Edition DOI: 10.1201/9781351019422-120054090 Copyright © 2018 by Taylor & Francis. All rights reserved.

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These materials are able to mimic natural recognition entities, such as antibodies and biological receptors.[11] In the last decade, MIPs have proved their potential as selective sorbents in SPE (molecularly imprinted solid-phase extraction [MISPE]) or as stationary phases in high performance liquid chromatography (HPLC), demonstrating that may be a highly versatile tool for the selective detection and quantification of natural and synthetic contaminants in food matrices. In fact, this technique seems to be particularly suitable for the extraction of complex matrices such as food products, allowing the achievement of high selectivity and exhibiting good extraction efficiency and reusability.[9,12] DESIGN OF “SMART” POLYMERS: MIPS The challenge of designing and synthesizing an MIP can be discouraging for the uninitiated researcher, mainly because of the large number of experimental variables involved, for example, the nature and amount of template, the wide variety of functional monomers and crosslinkers available, solvents and initiators, as well as the method of initiation and the duration of the polymerization process.[13] All these parameters have to be assessed since they can immensely influence the final morphology, properties, and performance of the polymers. The method of molecular imprinting involves the polymerization of functional monomers around a molecular template, in a crosslinked polymeric matrix, with the subsequent removal of the template to obtain free binding sites for the future target analyte (Fig. 1). In the synthesis of MIPs, the choice of chemical reagents is of primary importance in order to obtain efficient and functional MIPs. Templates, Monomers, Initiators, and Porogenic Solvents Templates The template is of great importance in any molecularly imprinting process because it directs the organization of the functional groups of the functional monomers. A wide range of template molecules such as drugs, amino acids, carbohydrates, proteins, nucleotide bases, hormones, ­pesticides, and coenzymes have been successfully used to obtain MIPs. [14] The template is selected depending on the future target molecule, the type of polymerization, and it should ideally be chemically inert. Usually, the imprinting molecule is the same compound that will be extracted with the obtained polymer, or a very close structural analogue (e.g., a compound from the same chemical family). ­However, when choosing the ­template molecule, the ­a nalyst must take into account at least the ­following aspects: (1) The template must have groups that can polymerize, (2) the template can potentially inhibit or

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However, this is not an easy task since the foodstuffs usually contain a broad range of components, and it is necessary to control all of them, apart from the fact that in some cases those components and hazardous chemicals can be present at trace levels. In the particular case of food safety, the ideal analytical method should combine cost-effectives, one-step isolation, pre-concentration, and quantitative determination of analytes, independently of the complexity of the matrix. In this regard, there is continuous development of methods for the rapid and reliable detection of foodborne pathogens. Improvements in the field of immunology, molecular biology, automation, and computer technology continue to have a positive effect on the development of faster, more sensitive, and more convenient methods in food microbiology. Modern methods are based on molecular biology techniques such as PCR and DNA microarray assay, or in immunological techniques such as ELISA and bio-analytical sensors utilizing enzymes.[3] In the case of chemical contaminants, new methods of sample separation and detection are increasingly being used, including multidimensional and high-speed gas chromatography, ultra-high-performance liquid chromatography (UHPLC), and high-resolution mass spectrometry, among others.[4–6] In this regard, most modern instrumental techniques often require the isolation and pre-concentration of the analytes prior to analyzing them. It is necessary to note that each additional step in the analytical procedure increases the probability of analyte losses, sample contamination, and analytical error. It is therefore necessary to minimize the number of steps in the sample preparation but without decreasing the quality and accuracy of the final analysis.[7] Additionally, despite the high selectivity and specificity of modern systems such as liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) and gas chromatography coupled to tandem mass spectrometry ­(GC-MS/ MS) platforms, it has only been possible to achieve the very low limits required by the legislation for some contaminants through the application of highly selective extraction procedures. To achieve analytical success, it is then necessary to develop effective cleanup procedures to extract the target analytes from the matrix. Many different options have been designed to prepare food samples for their analysis, including liquid extractions, liquid–solid extractions, solid-phase sorbents, QuEChERS (quick, easy, cheap, effective, rugged, and safe) method, molecularly imprinted polymers (MIPs), supercritical fluids, ultrafiltration, etc.[8] Currently, solid-phase extraction (SPE) is one of the most frequently used cleanup solutions in food analysis, as it combines the benefits of being a low-cost option and its easy automatization, along with the wide variety of sorbents available in the market. Molecularly imprinting technology (MIT) is an emerging, powerful tool for sample preparation and chromatography.[9,10] MIT provides the viable synthetic approach to design robust molecular recognition materials that the today’s food analyst demands.

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Polymerization in presence of cross-linker

Removal template

Rebinding

Template Functional monomer Cross-linker

Fig. 1  Schematic representation of molecular imprinting process

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retard a free-radical polymerization (templates with a thiol group or a hydroquinone moiety), and (3) the template must be stable under the selected polymerization conditions (e.g., temperature and UV irradiation). In addition, employing the future target analyte directly as a template may be problematic because the complete removal of the template molecule from the high-affinity sites of the polymer is not always possible. This aspect is very important to consider when MIPs are used for trace analysis, since a potential “bleeding” of residual template during the MISPE process can lead to false positives or to an inaccurate quantification of the analyte in real samples. [9] To solve this problem, the so-called “dummy” templates have been used for the synthesis of MIPs for some analytes. These molecules must mimic as much as possible the shape and size of the target analyte, in order to achieve selective polymers. Ideally, these MIPs will have binding sites with sufficient ­cross-selectivity for the target analyte and even for structurally related compounds. Problems of co-elution of the analyte and the “dummy” template during MISPE can be easily overcome with the employment of accurate ­identification and ­quantification methodologies. [15] Functional Monomers The choice of an appropriate functional monomer is very important in order to create highly specific cavities in the polymer designed for the template molecule.[16] Functional monomers are responsible for binding interactions in the imprinted binding sites. The template and the monomer should be complementary to maximize the complex formation and thus the imprinting effect. The reactivity ratio of the monomers is also important to ensure that copolymerization is feasible when two or more functional monomers are combined in a “cocktail” of polymerization.[17] In general, an excess of the functional monomer relative to the template is required to flavor template–­monomer complex formation and to maintain the integrity of this complex during the entire polymerization process. Typical

functional monomers are carboxylic acids (e.g., methacrylic acid [MAA], acrylic acid), sulfonic acids (e.g., 2-acrylamido-2-methylpropane sulfonic acid), and heteroaromatic bases (e.g., vinylpyridine, vinylimidazole). The extensive use of MAA is justified by its capability to act both as a hydrogen bond and proton donor and as a hydrogen bond acceptor. These characteristics make MAA an excellent candidate for interacting with almost any template. Alternatively, vinylpyridine has been frequently used for designing MIPs for isolating molecules containing acid groups. Moreover, more strong functional monomers have been developed via metal coordination interactions to bind specific amino acid sequences.[18] Generally, acid monomers are more appropriate for basic templates and basic monomers for acidic templates. The structure of the most common functional monomers used for molecular imprinting is shown in Fig. 2. Cross-Linkers The selectivity of any MIP is greatly influenced by the type and amount of the cross-linking agent used for the synthesis. During MIP polymerization, the cross-linker fulfills important functions such as controlling the morphology of the polymeric matrix and also stabilizing the imprinted binding sites and conferring mechanical stability to the polymer, in order to retain its molecular recognition capability.[19] High ratios of cross-linker are generally used in the polymerization mixture, so as to ensure the obtainment of permanently porous (macroporous) materials with adequate mechanical stability. After template removal, the amount of cross-linker should be sufficient to maintain the stability of the recognition sites, the three-dimensional structure, and to maintain the chemical functional groups of the functional monomer in the right position and orientation. Ethylene glycol dimethacrylate (EGDMA) and trimethylolpropane trimethacrylate (TRIM) are the two most commonly employed cross-linkers, according to the literature (Fig. 3).[20] TRIM results in polymers with more rigidity, structure order, and effective binding sites than

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O

O

OH

OH

Methacrylic acid (MAA)

Acrylic acid (AA)

O

O

NH2 Acrylamide (AAm)

OCH3 Methyl methacrylate (MMA)

CF3

O

N

N

4-Vinylpyridine (4-VP)

2-Vinylpyridine (2-VP)

OH

Trifluoro-methacrylic acid (TFMAA)

Fig. 2  List of the most frequently used functional monomers in MIP synthesis, including their chemical structures

O

O OH

O

O

O

O

O

Si

O O

O 2-Hydroxyethyl methacrylate (HEMA)

Ethylene glycol dimethacryla (EGDMA)

Tetraethoxysilane (TEOS)

O

O O

OC2H5 C2H5O

O Trimethylolpropane trimethacrylate (TRIM)

Si CH2CH2CH2NH2 OC2H5

3-Aminopropyltriethoxysilane (APTES)

O O

N

Divinylbenzene (DVB)

2-(Diethylamino) ethyl methacrylate (DEM)

Fig. 3  List of the most frequently used cross-linkers in MIP synthesis, including their chemical structures

EGDMA. Furthermore, it has been observed that the type of cross-linker strongly influences the final size and yield of MIP nanoparticles during precipitation polymerization. In fact, when divinylbenzene (DVB) was used as the crosslinker, polydisperse MIP particles were obtained in low yield, whereas TRIM led to uniform nanoparticles in high yield (90%).[21] Initiators The chemical initiator is responsible for starting the polymerization process. The choice of initiator depends on what method of initiation is to be used, ultraviolet radiation or heat and, in the latter case, the temperature at which polymerization will take place. These parameters affect the time required for polymer solidification. The rate and mechanism of decomposition of an initiator (to form radicals) can be triggered and controlled in a number of ways, including heat, light, and chemical/electrochemical options, depending on the chemical nature of the initiator. Free-radical polymerization is retarded with oxygen to

maximize the rate of monomer propagation and to ensure batch-to-batch polymerization reproducibility. Therefore, the removal of dissolved oxygen from monomer solutions is advisable immediately before polymerization. Although it is known that photoinitiation at low temperatures gives rise to MIPs with better mass transfer properties and superior performance in chromatography, several templates and monomers are incompatible with this technique due to degradation or radical inhibition.[22] In general, MIPs prepared using larger concentrations of initiators possessed a larger surface area. The amount or the ratio of an initiator in the polymerization mixture is always considerably lower than the amount of monomers.[23] Commonly employed initiators are ­azo(bis)-isobutyronitrile, 2,2 ′-azobis(2,4-­dimethylvaleronitrile), and benzoyl peroxide. In contrast, iniferter (initiator-transfer agent-­terminator)type initiators can act simultaneously as an initiator, a chain transfer agent, and a terminator in a polymerization reaction. These molecules generate two free radicals, one of which is able to initiate the polymerization, and the second one is relatively stable and non-active but capable of

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O

O

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terminating the growing polymer chains by a reversible combination. Immobilized iniferter-type ­initiators (normally dithiocarbamates) are useful for obtaining a thin film of polymer grafted on a surface, which controls the degree of polymerization by reaction time.[24,25] ­Iniferter initiators can avoid polymerization in solution and resulting gelation during the ­synthesis of MIP composite ­materials by ­surface imprinting techniques.[24] Porogenic Solvents (Porogens)

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Porogenic solvents play an important role in the formation of the porous structure of MIPs, as they collect all the components (template, functional monomer, cross-linker, and initiator) in a single phase during the polymerization process. The most common solvents used for MIP synthesis are toluene, chloroform, dichloromethane, or acetonitrile. The porogenic solvent is responsible for creating pores within the macroporous polymers, hence the name “porogen.” The nature and the amount of the porogenic solvent determine the strength of the non-covalent interactions and influence polymer morphology that directly affects the performance of the MIP. Moreover, the porogen in a non-covalent imprinting approach should be chosen considering its role in maximizing the likelihood of obtaining a complex between the template and the functional monomer. First of all, the template, functional monomer, cross-linker, and initiator must be soluble in the porogenic solvent. Second, the porogenic solvent should produce large pores to ensure good flow-through properties of the resulting polymer. Low solvent polarity is another important feature that reduces interferences during complex ­formation and ­confers high selectivity to MIPs. Polymerization Approaches and Techniques There are three different approaches to prepare MIPs: covalent (pre-organized approach), non-covalent (­ self-assembly approach), and semi-covalent approaches. The covalent approach involves the formation of reversible covalent bonds between the template and functional monomers before polymerization. Later, the template is removed by the cleavage of the covalent bonds, which will be reformed upon rebinding of the target molecule. This approach leads to a homogenous population of binding sites due to the high stability of the template–monomer interactions. However, it is restrictive since the cleavage of covalent bonds always requires a rather harsh chemical condition. In the ­semi-covalent approach, the template is covalently bound to a functional monomer, but the rebinding is based on non-covalent interactions. Finally, the non-covalent or self-assembly approach is based on the formation of relatively weak non-covalent interactions (e.g., hydrogen bonding, van der Waals forces, electrostatic interactions, hydrophobic interaction, and dipole–dipole bonds) between the template and functional monomers before

Smart Polymers: Molecularly Imprinted Polymers

polymerization. To date, this approach has been the most frequently used method for the preparation of MIPs, due to its simplicity and the availability of different monomers able to interact with almost any kind of template.[26] However, some polymers prepared by this approach can bind the template so strongly that it is challenging to remove all traces of the template (even after repeated washing of the polymer). MIPs can be synthesized in a variety of physical forms, using different methods, depending on their final application. Traditionally, MIPs have been prepared by bulk polymerization because it does not require sophisticated instrumentation and because the reaction conditions can be easily controlled. Although this kind of polymerization is tedious and time consuming, it is the most widely used method for the preparation of MIPs. [14,27,28] Bulk polymerization is popular because it is simple to perform, yet not everything is advantageous in this technique. When the MIP is not intended to be used as a monolith, crushing, grinding, and sieving of the polymer are necessary to obtain the appropriate particle sizes. These latter steps are tedious and time consuming and often produce particles that are irregular in size and shape, resulting in a heterogeneous mixture of imprinted nanoparticles. This kind of particles can cause back-pressure problems when they are packed in columns for their use as stationary phases for chromatography. Also, grinding and sieving can cause a substantial loss of the polymer. Despite these obvious drawbacks, most of the known MIPs have been prepared by bulk polymerization. [29] To overcome these problems, alternative methods have been developed in recent years, enabling the preparation of novel MIP formats, such as MIP beads, membranes, surface-imprinted devices, molecularly imprinted monolayers, and c­ oatings of ­different materials. [30] Regular beads are generally used for packing columns for chromatography or SPE, and they are usually synthesized by suspension polymerization, emulsion polymerization, seed polymerization, and precipitation polymerization. These techniques allow the formation of MIP particles with microspherical shapes of uniform sizes (see Fig. 4). [31] In addition, their polymerization yields are quite high because the tedious grinding and sieving steps are avoided. Furthermore, MIPs obtained by precipitation polymerization and similar techniques have an increased rebinding capacity and a more homogeneous distribution of binding sites than those obtained by bulk polymerization. Suspension polymerization is a different and novel polymerization method, a heterogeneous procedure for the production of spherical beads of a broad size range (from a few micrometers to millimeters). In this process, the organic-based polymerization mixture is suspended as droplets in an excess of a continuous dispersion phase by stirring, and each droplet acts like a mini-bulk reactor. Because the suspension is not stable, it is necessary to add a stabilizer to the dispersion

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Fig. 4  SEM images of MIP particles for dexamethasone extraction obtained by precipitation polymerization: (a) MIP and (b) NIP Source: Figure reprinted with permission.[31]

Combination of MIPs with Other Materials It is of great interest to explore new approaches enabling the modification of MIPs to obtain materials adapted for specific applications. More dedicated and less common applications of polymers have recently been found as a result of combining these imprinted sorbents with

different materials such as magnetite, silica, glass, metals, carbon, polyethylene, and other plastic supports and hybrid materials. Coatings, silane chemistry, and/ or surface-grafting approaches are some of the solutions developed to combine MIPs with fibers, nanoparticles, quantum dots, films or slides, nanotubes, and/or beads of different materials. [33–36] Magnetic Supports Parallel to the developments achieved in molecular imprinting, magnetic materials have been applied increasingly, alone or in combination with MIPs. These materials contain small particles of ferromagnetic material such as magnetic iron oxide or magnetite (Fe3O4). Because of the small size of the magnetite particles, the materials do not aggregate except in a magnetic field. A range of methods have been used to prepare magnetic particles. Solid magnetite or ferrofluids may be incorporated in suspension polymerization protocols, or polymer beads may be “post-magnetized” by the precipitation of iron oxide from solution or inclusion of colloidal magnetite or ferrofluids. [37] In recent years, MIP sorbents have been combined with these different magnetic supports in order to achieve automated methods of extraction. These new combinations are known as magnetic molecularly imprinted polymers (MMIPs). Generally, magnetic polymers are prepared by encapsulating inorganic magnetic particles with organic polymers. The magnetic polymers and the analytes bound to them can be easily collected with the aid of an external magnetic field and without additional centrifugation or filtration (see Fig. 5). This possibility makes separations easier and faster than using traditional sorbents. Because of their large surface area and unique physical and chemical properties, these MMIPs have been widely applied in many fields such as cell separation, drug delivery, and enzyme immobilization. [38] The core–shell MMIPs (e.g., multiple Fe3O4 as a core and SiO2 as a shell rather than a single Fe3O4 nanoparticle as a supporter) have been designed not only to improve the magnetic performance but also to enhance the biocompatibility and stability of MMIPs. [39,40] A novel strategy

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medium. The suspension method is less frequent for MIP synthesis because of the need for a polar (water) or expensive (perfluorocarbon fluid) suspension medium. Two-step or multi-step swelling polymerization can also produce spherical particles of uniform sizes, efficiently controlling size distribution and shape and decreasing material loss. These particles have separation properties comparable to those prepared by bulk polymerization but much better column efficiencies and peak shapes in ­chromatographic applications. Finally, in situ polymerization is another recently developed technique that is very simple and rapid compared to other previously described procedures because it enables the preparation of MIP monoliths within 3 h. [9] The success of the synthesis relies on the presence of both macropores that provide good flow-through properties and selective binding sites. In a typical in situ process, the reaction mixture is poured into a stainless steel tube (sealed at one end), or a physical support with similar characteristics, and then degassed ultrasonically. The second tube end is then sealed, and the (heating) polymerization takes place. After removal of the template, the column containing the MIP monolith inside can be connected, for instance, directly to an HPLC system for online SPE of the target molecules. Moreover, monoliths synthesized in capillaries are potential alternatives to ground monolith columns, polymer bead columns, and silica-based columns. [10] Nevertheless, there is a growing interest in alternative routes for preparing MIPs to better control morphology and thus explore new applications. Additional polymerization methods use different materials such as spherical silica or organic polymers to graft or coat thin films of MIP phases on the surfaces of porous materials. [32]

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Fig. 5  Example of MMIPs that can be collected with the aid of an external magnet (author’s unpublished material)

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to design well-organized, highly magnetic MIP particles was first proposed in 2009, in order to use them as optical sensing phases. [41] Recently, a research group reported its success in the design of well-defined nanospheres with a double-layer core–shell structure consisting of an Fe3O4 nanoparticle core, an inner fluorescent layer, and an outer layer of the MIP. [42] In this regard, for the obtainment of MMIPs, Fe3O4 nanoparticles are usually synthesized by a co-precipitation method. To achieve MMIPs, magnetic materials may be modified by silanization with tetraethyl orthosilicate (TEOS) and 3-methacryloxypropyl trimethoxysilane (MOPTS before imprinting, in order to promote the coating process of the polymer on the magnetic support. [43–46] A silica coating on the surface of magnetic nanoparticles by a sol–gel process (Fe3O4@SiO2 nanoparticles) makes them compatible with MIPs and provides them with a silica-like surface easily modified with various groups for coating purposes, for instance, subsequent modifications with, for instance, MOPTS (Fe3O4@SiO2@MOPTS nanoparticles) or 3-aminopropyl trimethoxysilane (Fe3O4@SiO2@ APTMS nanoparticles). [45,47] Besides, the resulted silica shell provides a hydrophilic surface that prevents the oxidation. Alternatively, oleic acid-modified Fe3O4 nanoparticles have also been used to obtain MMIPs. [38,48] Oleic acid may be easily combined with DVB and styrene polymeric matrixes, which can be obtained easily and cheaply, as it provides adequate hydrophobic shell on the magnetic surface, obtaining stable and magnetic polymers. [38] It is worth mentioning the use of polyvinylpyrrolidone (PVP) as a stabilizer dispersant agent in many of these approaches. [38,49] Another option for obtaining magnetic imprinted materials is the use of magnetic nanocrystals, coated with a thin layer of MIPs. In this case, the combination is successfully achieved using a salt chemistry with the iniferter method. [37]

Silica Silica is another material used often as a physical support in MIT. Different surfaces or imprinting formats have been evaluated and are discussed in the literature. Silica particles containing surface-bound free-radical initiators have been used as supports for the grafting of thin films of MIPs.[36] By confining the initiating radicals to the support surface, a higher density of grafted polymeric chains can be achieved.[50,51] In the absence of chain transfer, the chain will grow mainly from the surface of the support with minimal polymerization occurring in solution. The silica supports can be modified covalently with glycidoxypropyltrimethoxysilane or (3-aminopropyl)triethoxysilane and the azoinitiator 4,4′-azobis(4-cyanopentanoic acid) (Si-GPS or Si-APS) or non-covalently with the diamidine azoinitiator 2,2′-azobis­(N,N′-dimethyleneisobutyramidine). The chromatographic properties of these materials depend on the thickness of the polymeric layer on the silica surface, the solvent, the pore diameter, the cross-linker, and the composition of the mobile phase, among others. A thin film of an MIP can be grafted from preformed spherical porous silica particles using an immobilized i­ niferter-type initiator (inifMIP), and as such this methodology has been reported.[24,52] The procedure involves coupling of an i­ niferter onto the silica, an initiator for free-radical p­ olymerization, which is able to both initiate and terminate the polymerization process. In this manner, polymerization occurs only via the active radical immobilized onto the porous silica surface, and polymerization in solution is avoided. The immobilization of MOPTS onto the silica surface can also provide polymerizable groups for their copolymerization with monomers in solution. MOPTS covalently attached to the surface of Stöber silica particles would result in SiO2MOPTS particles. This method has been recently used to prepare methacrylate-silica particles from the MOPTS

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Glass The combination of MIPs with glass supports is also possible using different approaches as it has been already demonstrated in some studies. To achieve successful MIP coating on glass surfaces, the uniform deposition of silane layer upon glass is the technique of choice. As a prerequisite for subsequent silanization, glass needs to be carefully cleaned, usually applying acidic and/or basic solutions.[56] After rinsing thoroughly with deionized water and drying it under N2 stream, the glass is ready for silane modifications. To date, one of the most frequent silanization reagents for glass supports has been 3-mercaptopropyltrimethoxysilane (MPTS).[57,58] Glass may be used in the form of different formats and devices, such as glass-covered magnets, or even glass slides. The effective coating of glass-covered stir bars has been recently reported, using two types of coating procedures: physical and chemical coating.[58] In the first option, the authors simply used an epoxy adhesive to immobilize the MIP particles, previously synthesized by precipitation polymerization, over a previously conditioned glass stir bar, i.e., etched with the fluorocarbon etchant to provide a rough surface. The silanization of the surface was not necessary in this case. In the second technique, the etched glass surface was submitted to a silanization step before immersing it within the pre-polimerization mixture. Polymerization was performed using an oven, resulting in an MIP-coated stir bar with the shape and the size of the container in which the procedure takes place. The authors concluded that the chemical coating is a better approach when desiring to combine MIPs and glass supports. Besides stir bars, other glass devices or supports have been used to obtain novel MIP-based detection methods. For instance, the combination of colloidal-crystal templating and a molecular imprinting technique resulted in a sensor platform that allowed the efficient detection of the analyte in aqueous solution.[59] The photonic polymer hydrogels are constructed in three steps: preparation of the colloidal-crystal arrays by vertical deposition of silica on glass slides, polymerization of the pre-ordered complex of the template and functional monomers in the interspaces of the crystal, and removal of the template and silica particles form the imprinted matrix (see, e.g., Fig. 6). Finally, to overcome some of the limitations of traditional MIP methodologies such as template bleeding and high binding-site

heterogeneity, the possible synthesis of MIP nanoparticles using a solid-phase approach has been reported. This solution relies on the covalent immobilization of the template onto the surface of glass beads. The obtained MIPs are ­virtually free of template and have demonstrated high affinity for the target molecule.[60] To create these MIPs, the surface of the glass beads is activated by NaOH, and the amino groups are then introduced to the surface by MPTS. The glass beads are used as a solid phase to immobilize the template in the MIP production. Moreover, the r­ eusability of the glass beads–template complex for more than one MIP batch is also an advantage that allows minimizing the amount of template required for the synthesis.[61] Other Materials Materials other than magnetite, silica, and glass may also be combined with MIPs for their application in analytical chemistry. Coatings and surface-grafting-to approaches are also a general form of merging the polymer and its support, which is presented in different forms such as fibers and nanotubes, beads (i.e., quantum dots or gold ­nanoparticles), or films, for achieving hybrid materials. Carbon nanotubes (CNTs) are classified into multiwalled carbon nanotubes (MWNTs) and single-walled carbon nanotubes (SWNTs). MWNTs are considered the ideal nanotube support for MIPs because of high strength, stability under acidic conditions, lack of swelling, and large surface areas. MIT has been applied to the surface of MWNTs through the functionalization of the tube surface, using covalent or non-covalent methods.[34] Besides, various kinds of nanoparticles may be incorporated on MWNTs, modifying the properties of the obtained CNT@ MIP complex.[34] Another strategy that has been used to obtain MIP microspheres with narrow particle size distribution is the core–shell approach. The “core particles” can be prepared by the precipitation of DVB in acetonitrile and used as seed particles in the synthesis of MIP shells by copolymerization of functional and cross-linker monomers in an adequate porogenic environment.[24] Finally, the combination of quantum-dots-encoded microbeads with MIPs has been recently reported, forming a core–shell material with multiplexing and selective recognition abilities.[35] Tips, membranes, or stainless steel columns, among others, can be considered alternative options to classical MIP approaches, being their use much less frequent than the rest of materials.[10,62–65] However, they present some advantages such as miniaturization of procedures and/or simplicity of preparation. For instance, an MIP monolith can be prepared in a micropipette tip, simply by in situ polymerization and with no need of chemical activation of the tip. The monolithic tip can be easily connected to a syringe to perform the MISPE procedure. Monoliths synthetized inside a stainless steel column may be used as online cleanup procedures or even as chromatographic columns.

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precursor and TEOS using a sol–gel process. Then, the silica particles covered with methacrylate groups can be used to prepare two types of MIPs, i.e., SiO2MAA@MIPs and SiO2MA@MIPs.[36] Apart from nanoparticles, polymerization can be performed on the surface of a silica fiber or nanotube. The preparation of imprinted fibers is performed by the silanization step of the surface of silica fibers, which are subsequently immersed in fresh polymerization solution and repeatedly coated until the desired thickness is reached.[40,53–55]

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Fig. 6  SEM images of the macroporous structure obtained using colloidal-crystal templating and molecularly imprinted, before (upper part) and after (lower part) removal of silica particles and template (author’s unpublished material)

APPLICATIONS AND FORMATS OF MIPS IN FOOD SAFETY Food safety is a scientific discipline comprising a number of routine analyses and controls that should be followed to avoid potential health hazards for humans. In this regard, safety considerations include the origins of food, food labeling, hygiene, additives, contaminants, residues, and pathogens, among others. Food contamination refers to the presence in food of harmful chemicals and microorganisms, and it can be a result of a fraudulent practice or non-intentional and unavoidable. In the particular case of chemicals, there is an increasing demand for innovative analytical methods to monitor their presence in food. The use of powerful and modern mass spectrometers coupled to chromatographic separations allows the reliable identification and quantification of hazardous substances that represent a risk for the consumers. However, in virtually all cases, food samples require specific preparative solutions prior to being analyzed using instrumental techniques. Also, chromatographic separations of closely related

compounds are not always achieved in a straightforward manner. Determining a wide range of compounds in different types of matrices/food complicates the procedure calls for highly specific, easy-to-use, and selective options. MIPs have been used effectively as active sorbents in both preparative options and chromatographic separations. Current trends in their use in food safety and ­potential ­directions in this field are outlined later. Extractive Solutions: Solid-Phase Sorbents Nowadays, sample preparation is still the most laborious step of the analytical process because several factors must be considered, including the nature of the matrix and the properties of the analyte. The removal of potential interfering compounds and the pre-concentration of the analyte are two main objectives of sample pretreatment. In this regard, SPE is a frequently used, low-cost, and easily automated method applied to pretreat food samples. A wide variety of sorbents exist in the market, but their main drawback is usually their lack of selectivity for the target

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successful to extract these compounds from food matrices. As for pollutants and environmental contaminants, the molecularly imprinted sorbents are most frequently applied to aqueous samples (such as tap, surface, or waste waters) or for the purification of extracts resulting from the treatment of solid samples (such as soil or sediments).[82] However, the presence of pesticides and other agrochemicals can also be extended to crops, vegetables, and fruits, in which case MISPE may also offer a suitable solution for the ­analysis of their residues.[12] Miniaturization solutions and novel formats and applications, other than classical MISPE, with MIPs alone or in combination with other materials such as magnetite, silica, or glass, are discussed further in the “Novel Formats and Mini-Devices” section. Options for Chromatographic Separations First used for solid-phase extraction, MIPs are also effective chromatographic phases for the separation of isomers and structurally related molecules. Some studies have already demonstrated the selectivity of MIPs prepared by precipitation polymerization as sorbents for SPE and also as packing materials of columns for liquid chromatographic separations of hazardous chemicals that may be present in food, such as antibiotics and fungicides.[54,83,84] Also, MIPs synthesized within the pores of spherical silica particles result in spherical particles with appropriate and homogeneous morphology and therefore optimum characteristics to be used as stationary phases in liquid chromatography (LC). These imprinted polymeric phases have allowed the direct injection of complex samples with the analytes (herbicides), not only becoming separated from each other but also from matrix-interfering compounds.[85] In this regard, the good chromatographic performance of MIP spheres obtained with core–shell technology and/or via iniferter-type initiator techniques was demonstrated in a study on fungicides in fruit and vegetable extracts.[24] As stated by the authors, these materials can be used also in the development of online SPE methods. Monolithic columns are gaining interest as excellent substitutes to conventional particle-packed columns. These columns show higher permeability and lower flow resistance than conventional liquid chromatography columns, providing high-throughput performance, resolution, and separation in short run times.[10] Monolithic imprinting, as one of the methods for preparing MIPs, combines the advantage of monolithic columns and molecular imprinting technology. MIP monoliths can not only concentrate but also selectively separate the target analytes from real samples, which is crucial for the quantitative determination of analytes in complex samples. Time-saving liquid chromatography monolithic columns have been applied to the analysis of residues, contaminants, and food constituents, demonstrating their effectiveness and success for these tasks.[65,74,86–90]

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analytes. MIPs are synthetic materials with recognition sites that specifically bind target molecules in mixtures with other compounds. In contrast to classical SPE sorbents, they are more selective and allow the isolation of analytes from the samples nearly free from co-extracted compounds. The great potential of MIPs as selective SPE sorbents for the analysis of residues and contaminants in food has been extensively demonstrated during the last decade.[2,9,12,66] Immunosorbents offer a highly selective solution for SPE applications, however, the obtainment of antibodies is expensive and time consuming, and they need to be immobilized on adequate supports to preserve their b­ inding capability. MIPs are good alternative immunosorbents, since the cavities obtained after removing the template offer free binding sites, which are complementary in shape, size, and functionality. The specificity of MIPs as receptors is in many cases comparable to monoclonal antibodies.[67–70] MISPE, in its initial and more classical style, has two basic modes: off-line and online extractions. MISPE is typically performed in the off-line mode, being this approach especially useful for multianalyte detections for which MIPs can recognize several structural analogues, for instance, a class of antibiotics that are extracted together. The offline SPE extraction is performed using a small amount of imprinted polymer packed in a cartridge. Subsequently, the common steps of conditioning, loading, washing, and elution are carried out. As the MIP is developed specifically for a target analyte (or a group of structurally related compounds), stronger combinations of solvents may be applied to the cartridges, and the obtainment of cleaner final extracts is then expected. In contrast to off-line, the online mode provides automatic sample loading, interference washing, analyte elution, and analyte separation and detection by directly connecting the MISPE column to an analytical system. Because of the high selectivity of MIPs, extraction, enrichment, separation, and detection of target analytes can be achieved in one step by directly coupling an MIP column in-line with the detection system.[65,71–74] For these online applications, MIP monoliths are usually preferred over particles. In the field of food safety, some of the most frequent chemicals that can be found in food for human consumption are antimicrobial residues. These analytes are present mainly in products of animal origin as a consequence of either a legal veterinary treatment or a fraudulent use. A high number of successful MISPE protocols have been created for the analysis of a­ ntimicrobial agents in food, proving their great potential and usefulness to isolate these compounds.[75–78] Other chemicals that may contaminate food are mycotoxins; environmental contaminants such as pesticides, herbicides, and organic pollutants; veterinary drugs other than antimicrobials; and food adulterants such as melamine. Several studies have already demonstrated the selectivity of MIPs as sorbents for SPE of these hazardous chemicals.[12,31,54,65,79–81] Thus, the imprinted technology has also proven useful and

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Novel Formats and Mini-Devices

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MIP formats have ranged from traditional MISPE columns to disks, micro-columns, fibers and capillaries, membranes, stir bars, magnetic beads, or even needle and micropipette tips.[55] With regard to MMIPs and their application in food safety, they have been one of the most popular solutions for the detection of many different analytes in the past years. Recently, a research group reported its success in the design of well-defined nanospheres with a double-layer core–shell structure consisting of an Fe3O4 nanoparticle core, an inner fluorescent layer, and an outer layer of MIP, for simultaneous separation and recognition of estrogenic disrupting chemicals.[42] A similar approach was reported for the selective enrichment of endocrine disrupting chemicals in water and milk samples.[45] An MMIP has also been employed for the detection of malachite green residue, a parasiticide and antibacterial agent used in freshwater aquaria, in fish samples.[48] An additional example is the development of magnetic dummy molecularly imprinted nanoparticles based on functionalized silica and used as an efficient sorbent for the determination of acrylamide in potato.[47] Acrylamide is a chemical used to obtain copolymers in industrial processes, such as the production of paper, dyes, and plastic. It has been found in fried potatoes due to cooking, and its presence is very dangerous for humans. This kind of polymers can also be useful for determining important natural food components such as vitamins and nutrients. A good example is the application of MMIPs for selective determination of resveratrol in wine.[91] A sensitive and selective electrochemical sensor for metronidazole, an antibiotic and antiprotozoal agent, was developed by attaching a core–shell metronidazole–MMIP to the surface of the magnetic glassy carbon electrode, in order to prepare an electrochemical sensor. The imprinted sensor exhibited high recognition ability and affinity for the analysis of metronidazole in milk samples and honey samples.[92] Silica particles containing surface-bound free-radical initiators have been used also as supports for the grafting of thin films of MIPs, demonstrating its success for the separation of patulin, a mytoxin that can be present in fruit as a consequence of fungal contamination.[36,51] An attractive alternative to particle-based MIPs are imprinted monoliths, which are easy to obtain and also possess great potential for the cleanup and preparation of complex food samples. In situ polymerization inside appropriate supports has allowed the development of several micro-extraction formats, such as needle and pipette tip-based extractions.[10] A pipette tip-based MIP monolith micro-extraction method was developed for the selective extraction of difenoconazole, a triazole fungicide, in tap water and grape juice.[64] Monoliths synthesized inside capillary columns have enabled the online isolation of aflatoxins.[65] Novel selective SPE formats include MIPs

Smart Polymers: Molecularly Imprinted Polymers

grafted to stir bars or to porous polyethylene frits for selective extraction of ­herbicides in food and environmental samples.[24,58] The imprinting of bacteria and viruses, ultimately considered as extremely large molecules, is a very recent application of these MIPs. Literature brings several examples of successful imprinting microorganisms with different structures.[70,93,94] In these strategies, the virus/bacteria surface is imprinted, for instance, on silica nanoparticles as a core material or in the form of micro-gel granules. The application area certainly has the potential to be extended to cover other classes of cells. STATE-OF-THE-ART ON SEPARATION AND PREPARATIVE SOLUTIONS USING MIPS Considering the number or papers found in the literature, it can be concluded that molecular imprinted solid-phase extraction is one of the most popular applications studied for these smart imprinted materials. Novel applications of MIPs include the development of sensors, stationary phases for liquid chromatography, and magnetic sorbents, among others. To meet the high standards required by food safety regulations, the modification of MIPs with different materials (i.e., magnetite, silica, glass, metals, carbon), adapting them for specific applications, has attracted much attention during the last decade. MMIPs have been one of the most versatile for sample preparation in food analysis. Monoliths are also finding their way into separation science and into the solid-phase extraction field. Finally, due to the increasing number of food samples, miniaturization has been the key to achieve the high-throughput capacity of extraction methods. Small-sized devices, fast chromatography, and online extractions are among the major advantages of MIPs, which in addition are able to improve the selectivity of the analytical methods. MIPs have widely demonstrated their suitability as supports to extract residues and contaminants in food, and the field is still growing. REFERENCES 1. Lewis, J.A.; Fenwick, G.R. Natural toxicants in food. In Food Contaminants; Purchase, C.C.; Ed.; Woodhead Publishing: Cambridge, 2004, 1–20. 2. Saini, S.S.; Kaur, A. Molecularly imprinted polymers for the detection of food toxins: A minireview. Adv. Nanopart. 2013, 2 (1), 60–65. 3. Mandal, P.; Biswas, A.; Choi, K.; Pal, U. Methods for rapid detection of foodborne pathogens: An overview. Am. J. Food Technol. 2011, 6 (2), 87–102. 4. Dewulf, J.; Van Langenhove, H.; Wittmann, G. Analysis of volatile organic compounds using gas chromatography. TrAC Trends Anal. Chem. 2002, 21 (9–10), 637–646.

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technique for detection of minute structural differences of proteins, viruses, and cells (bacteria). III: Gel antibodies against cells (bacteria). Electrophoresis 2006, 27 (23), 4682–4687. 94. Iskierko, Z.; Sharma, P.S.; Bartold, K.; Pietrzyk-Le, A.; Noworyta, K.; Kutner, W. Molecularly imprinted polymers for separating and sensing of macromolecular compounds and microorganisms. Biotechnol. Adv. 2016, 34 (1), 30–46.

Smart Polymers–Textiles

Spin Coated Films of Polymeric Materials Ayesha Kausar School of Natural Sciences, National University of Sciences and Technology (NUST), Islamabad, Pakistan

Abstract This entry is an effort to document the worth of spin coating in polymer and nanocomposite thin films. Spin coating is a technique to produce uniform polymer film having thickness of few ­nanometers. Polyamide, polyacrylamide, polystyrene, poly(methyl methacrylate), poly(vinyl chloride), poly(vinyl acetate), poly(3-hexylthiophene), epoxy, blends, and nanocomposite with carbon nanotube, fullerene, and graphene are spin coated for various purposes. Spin-coated polymeric films are employed in microelectronics, transistors, sensors, solar cell, fuel cell, shape memory materials,  electromagnetic shielding, light-emitting diodes, desalination membranes, antireflection coatings, and biomedical fields. Future research may demonstrate an establishment of this state-­of-the-art technology relevant to other thin film methods. Keywords: Graphene; Polymer; Spin coating; Thin film; Transistors.

Spin coating is a principal technique used to form ­uniform thin films of organic materials having thickness of the order of micrometers and nanometers. Initial discovery of this technique is dated back to the 1950s.[1] The c­ oating material, i.e., polymeric, was applied to a substrate in the form of solution. The centrifugal force in spin coating drives the solvent radial outward.[2,3] The solution viscosity and surface tension causes residual thin film to be reserved on the substrate. The film thins by the c­ ombination of ­outward fluid flow and evaporation. The spin coating ­process has been categorized as solution d­ ispersion, spin-up, uniform acceleration, fluid outflow, and evaporation leading to drying. The success of the process depends on parameters such as solution volume, viscosity, concentration, spin speed, film thickness attained, and spin time.[4,5] Spin coating has capability to form progressively uniform thin film as the coating process progresses. This method is also low cost and fast operating. The key disadvantage still associated with spin coating is that large substrates cannot be spun at adequately high rate to allow the film to be uniform and thin.[6,7] Spin coating has been used for processing polymers and polymeric nanocomposites for the several decades. Its potential has been analyzed to some extent in microelectronics, light-emitting diodes (LEDs), transistors, solar cell, fuel cell, sensors, antireflection coatings, and other fields.[8–10] Still there is a lack of up-to-date reviews, recognizing the value of this essential technique in material science. This entry presents a comprehensive review, regarding the identification of the value of spin coating technique in a variety of polymers and Encyclopedia of Polymer Applications, First Edition DOI: 10.1201/9781351019422-140000181 Copyright © 2018 by Taylor & Francis. All rights reserved.

nanocomposite thin films. Various spin-coated polymeric materials, their applications, future perspectives, and challenges have been identified and discussed with reference to essential characteristics. SPIN COATING TECHNIQUE Numerous techniques have been developed to create and monitor new thin film systems. To state an accurate method for producing thin polymer films is definitely challenging.[11–13] However, a proposed solution to several thin film processing problems can be the spin coating method.[14–16] In this technique, initially a polymer solution is prepared and gradually dropped on the preferred surface. Subsequently, the substrate is accelerated to a desired rotation rate.[17,18] Spinning is continued until an equilibrium film thickness is reached (Fig. 1). These steps are sometimes followed by annealing to eliminate any residual solvent. Liquid ejects outward due to the action of centrifugal force. The film continues to thin gradually until it hardens due to viscosity rise and solvent evaporation. The spin coating technique is usually divided into four stages, i.e., deposition, spin-up, spin-off, and evaporation. In this technique, several variables have been defined to control the final film thickness. The solvent used to prepare polymer solution must dissolve the polymer fully. Initially, the solvent was not supposed to affect the final film thickness in this technique. However, the different viscosity and polymer solubility may affect the coating process.[19–21] The film thickness is supposed to decrease with the increase in spin coating speed due to better solution dispersion.

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INTRODUCTION

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Spin Coated Films of Polymeric Materials

The  spinning force is, thus, important. The amount of solution applied to the substrate usually does not affect the thickness because excess may be thrown off the surface by centripetal forces. The concentration-controlled evaporation can be considered to predict the film thickness. Spin coating has been recurrently used to fabricate polymer thin films due to the ease of processing. The polymer film uniformity is usually the function of several parameters such as spin rate, spinning processing steps, substrate temperature, polymer solubility, molecular weight, structure, solvent type, and solution concentration.[22–24] Spin rates of 3,000 rpm can be acceptable for polymer thin films. Initially, spin coating is done below 1,000 rpm for few seconds. The initial few second spinning is necessary for fine adhesion of the polymer solution to the substrate. Afterward, the film is formed by spinning at 3,000 rpm for a few minutes. The success of spin coating techniques for various polymers can be reviewed in the flowing sections of this entry.[25,26] Various technical applications successfully employ spin-coated polymer technology.[27–30] SPIN COATING VS. OTHER THIN FILM METHODOLOGIES

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Few studies have been found regarding the comparison of spin coating method with other thin film fabrication techniques. Light-emitting polymer (LEP) solutions have been processed using spin and dip coating methods.[31] Dip coating is also a simple process for depositing thin film of solution onto a plate-, cylinder-, or irregular-shaped object. The geometry of substrates can be varied as a distinguishing feature of dip coating technique. This process engrosses immersing a substrate in solution reservoir for some time

Spin up

and then withdrawing the substrate from the solution bath. The spin and dip coating processes have been compared in terms of process conditions, thickness, and uniformity of thin LEP films. Modifications of existing models for both spin and dip coating processes were studied to embrace solvent evaporation and the effect of solution viscosity during evaporation. Both models were found to offer an agreement with the major trends for final film thickness as a function of process conditions. The dip coating process was found to generate similar film thickness control as spin coating process. In dip coating, the evaporation process was fundamentally decoupled from the film-forming process. However, in spin coating, the film formation and evaporation occur simultaneously. Spin coating process also possesses a significant difference compared with respect to the solution process.[32] Solution casting is a fundamental process of coating the substrate with a polymer layer. The influence of the macromolecular characteristics of polymers on spin coating process has been analyzed. The average molecular weight of spun-on solution is also an important parameter. The chain entanglements in ­polymer solutions may be considered as the basic phenomenon responsible for the formation of the solid polymer layer. The homogeneously dispersed optimum molecular weight polymer chains are important in the spin cast p­ rocess. However, in solution casting method, the polymer solution is evaporated on a stationary substrate. Denis et al.[33] have prepared homogeneous nanostructured polymer surfaces using colloidal lithography and polymer spin coating. Negatively charged polystyrene (PS) colloidal particles were formed with hemisphere-like protrusions on gold substrate. In spin coating, gold substrata were spin coated with ultrathin PS films. The combination of colloidal lithography and polymer spin coating formed homogeneous surface chemistry

Thinning of liquid film

Solidified region Saturation of solidification

Fig. 1  Schematic representation for spin coating technique

Spin Coated Films of Polymeric Materials 2443

O

OO

OO

OO

OO

conditions has been investigated.[39,40] In spin coating process, a small quantity of solution is dispersed on substrate and accelerated at very high speed to evaporate the solution and solidify a uniform film. It is a preferred method for the application of thin and uniform films on flat substrates, especially in lithography, solar cell, photovoltaic cell, transistors, and several other electronic and energy applications. POLYMER AND POLYMERIC NANOCOMPOSITE FILMS BY SPIN COATING PS-Based Spin-Coated Films Spin coating technique has been established as fine coating method for obtaining polymer films with the range of thicknesses (Fig. 2). The substrate is usually covered with polymer solution and rotated at constant spinning velocity until the solvent evaporates.[41] Particularly, spin-coated films are smooth and uniform in thickness. The spin coating process is, thus, expansively used in several sensitive technical applications for coating planar surfaces. Reiter[42] fabricated thin PS films of 100 nm thickness on silicon substrates. The films underwent dewetting when annealed above glass transition temperature (Tg). The smooth films were broken up by the formation of cylindrical holes. The holes grew to form rims, which became in contact with each other by creating cellular structure. Afterward, the unstable rims decayed into droplets. The effects of film thickness on wetting properties were investigated.

O

O O

CH3

S

S

S

S

H3C

Spin coating Substrate O O

l Po

ym

er

+

ler

Fil

Evaporation of solvent

Solution of precursor

Heat treatment Dense film

Fig. 2  Spin coating method for polymer and composite films

Film

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consistent with pure PS. The surface nanotopography was tuned by changing the particle diameter, concentration, and film thickness. The spin coating method has also been compared with the spray coating technique. Transparent and hydrophobic coatings have been deposited on the glass substrates using spray deposition method, which often generates hierarchical morphology beneficial for several applications.[34,35] Mahadik et al.[36] controlled the wettability and physicochemical properties of TiO2- and SiO2-based coatings fabricated using spray and spin coating methods. The spin coatings were found to be less rough than the sprayed thin films. In other words, the roughness of spin coating was less than the spray-coated materials. Moreover, the spin-coated films had lower contact angles and sliding angles relative to the sprayed coatings. The optical transmittance, thermal stability, and durability of spin-coated films were also better than the sprayed ones. In membrane technology, the precise control on the pore structure of membrane is always highly valued, and so various thin film methods have been tried to achieve that. Immersion–precipitation method or solvent–nonsolvent mass transfer also known as phase inversion may govern the ultimate membrane structure to a large extent. By comparing the phase inversion technique with spin method, the membrane structures can be better regulated through adjusting the spinning conditions (spinning speed, spinning time, and acceleration process). Membranes with homogeneously dispersed pores have been formed using spin coating compared with the phase inversion.[37,38] Studies have critically tested the proposed models for spin coating method. A wide range of film thicknesses and process

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PS/P3OT

PS P3OT

Bilayer formation with continuous phases

Solution casting

PS

PS P3OT

PS

PS

PS

PS P3OT

PS upper perforated phase

Fig. 3  Schematic representation for lamellar morphology ­formation during spin casting of PS/P3OT blends from toluene

1E13

Sheet resistance (Ω / )

Schubert[43] prepared spin-coated PS films using toluene as a solvent. The film thickness was found to depend on the spinning velocity, solution concentration, and molecular weight of PS. Specific scaling exponents were measured. The molecular weight dependence was studied in detail to determine the molecular weight using spin coating. The model molecular weight distributions resulted in the determination of number average (Mn) and weight average (Mw) molecular weight. Mundra et al.[44] studied glass transition temperatures of PS and styrene/methyl methacrylate (S/ MMA) random copolymer films prepared by spin coating method. The Tg was found from the intersection of rubbery- and glassy-state temperature dependences of the integrated fluorescence intensity during cooling. Moreover, the effects of nano-confinement of PS chains on Tg and transition strength were in agreement with the results of probe fluorescence and ellipsometry. Nicho et al.[45] studied the blends of PS with poly(3-octylthiophene) (P3OT) were prepared by the direct oxidation of 3-octylthiophene with ferric chloride (FeCl3) oxidant. The formation of lamellar structure of PS/P3OT blend is given in Fig. 3. Initially, a binary thin film was formed with the continuous phases, having PS as the upper layer and P3OT as the bottom layer. When spin casting is preceded, PS/P3OT solution evaporation led to the formation of upper perforated phase, while P3OT domains remained at the bottom layer. The molecular weight of P3OT polymer was studied using size exclusion chromatography. PS and P3OT were spin coated to form homogeneous films using different polymer concentration and toluene solvent. The doped films were also prepared by an immersion in 0.3 M ferric chloride ­solution in nitromethane. The study of p­ ercolation phenomenon revealed the electrical properties of blends with 5% of P3OT. Surface topographical changes were analyzed by atomic force microscopy (AFM). AFM micrographs showed surface morphology variation as a function of different P3OT content in PS. The films had pitlike and island-like topographies. Figure 4 depicts the sheet resistance of PS/P3OT coatings as a function of P3OT content.

P3OT-PS

1E12 1E11 1E10 1E9 1E8 0

20

40

60

80

100

P3OT Concentration in the PS/P3OT composite (wt%)

Fig. 4  Sheet resistance of PS/P3OT composite films as a ­function of P3OT concentration [45]

A classical percolation phenomenon was observed in the electrical properties of materials indicating physical interaction between P3OT and PS. Thus, spin coating has been found successful in the formation of thin PS films with fine morphology and structural properties. Poly(vinyl chloride)-Based Spin-Coated Films Owing to low cost, chemical stability, biocompatibility, and other physical properties, poly(vinyl chloride) (PVC) has been selected as a matrix for the range of technical applications. Though, poor thermal stability and some properties have led to the employment of filler materials as well as advance processing techniques for the performance enhancement. Carbon nanotube, graphene, graphene oxide (GO), etc. have been incorporated in polymers by spin coating. This method provides better filler interaction with the host polymers. Kim and Urban [46] focused on PVC surfaces for the formation of multilayered thromboresistant thin films of poly(ethylenimine), dextran sulfate, and heparin using spin coating. Dipping and spin coating were used to deposit multilayers. Quantitative analysis revealed that the surface concentration of individual layers was significantly lower for spin-coated sample. Petri[47] explored the morphology of spin-coated PVC films on the basis of competitive interaction between polymer, solvent, and substrate. The films were spin coated on silicon wafers using toluene and tetrahydrofuran (THF) solvent. PVC blend films with PS and poly(vinyl butyral) (PVB) were also spin coated. Tough and segregated films were obtained due to the interaction between polar substrate and solvent. Due to the strong interaction energy between substrate and polymer (relative to substrate and solvent), homogeneous films were obtained. The spin-coated films were characterized using ellipsometry and contact angle measurement. Gupta and Singh [48] reported the fabrication and characterization of Schottky diodes based on PVC–polyaniline

20

(a)

18 16

(b)

C–2(1017cm4F2)

14 12

(c)

10 8 6 4 2 0

–1.6

–1.2

–0.8

–0.4

0.0

0.4

0.8

1.2

1.6

Voltage (V)

Fig. 5  C–V plots for junction of PVC–PANI at different ­temperatures: (a) 303 K, (b) 323 K, and (c) 343 K[48]

(PANI) composite. The composite was prepared using spin coating and was semiconducting in nature. The ­junction parameters have been calculated using temperature-­ dependent current–voltage (I–V) and capacitance–voltage (C–V) methods. The properties of composite were compared with pure PANI. The C–V characteristics of metal/ polymer and metal/composite were studied at different

a

c

t­ emperatures (Fig. 5). The junction was found to have ­specific capacitance due to the space charge in depletion layer. Vadukumpully et al.[49] ­fabricated ultrathin composite films of PVC/graphene nanoflakes. Freestanding composite thin films were prepared by spin coating. Enhancement in mechanical properties of neat PVC was observed with 2 wt% graphene. There was 58% increase in Young’s modulus and 130% improvement of tensile strength. Enhanced thermal stability of the composite films was observed with the increase in glass transition temperature. Moreover, the composite films showed low percolation threshold of 0.6 vol% and high electrical conductivity of 0.058 S/cm at 6.47 vol% graphene loading. Figure 6a shows a tapping mode AFM image of graphene nanoflakes deposited onto the mica sheets using suspension in dimethyl formamide (DMF). AFM micrographs showed small flakes of the order of