Halogen-Free Flame-Retardant Polymers: Next-generation Fillers for Polymer Nanocomposite Applications [1st ed. 2020] 978-3-030-35490-9, 978-3-030-35491-6

Table of contents :
Front Matter ....Pages i-xv
Introduction (Suprakas Sinha Ray, Malkappa Kuruma)....Pages 1-4
Polymer Combustion and Flame Retardancy (Suprakas Sinha Ray, Malkappa Kuruma)....Pages 5-9
Flame-Retardancy Testing (Suprakas Sinha Ray, Malkappa Kuruma)....Pages 11-14
Types of Flame Retardants Used for the Synthesis of Flame-Retardant Polymers (Suprakas Sinha Ray, Malkappa Kuruma)....Pages 15-45
Flame-Retardant Polyurethanes (Suprakas Sinha Ray, Malkappa Kuruma)....Pages 47-67
Melt-Dripping and Char Formation (Suprakas Sinha Ray, Malkappa Kuruma)....Pages 69-82
Polymer Nanocomposites for Fire Retardant Applications (Suprakas Sinha Ray, Malkappa Kuruma)....Pages 83-109
Back Matter ....Pages 111-113

Citation preview

Springer Series in Materials Science 294

Suprakas Sinha Ray Malkappa Kuruma

Halogen-Free Flame-Retardant Polymers Next-generation Fillers for Polymer Nanocomposite Applications

Springer Series in Materials Science Volume 294

Series Editors Robert Hull, Center for Materials, Devices, and Integrated Systems, Rensselaer Polytechnic Institute, Troy, NY, USA Chennupati Jagadish, Research School of Physical, Australian National University, Canberra, ACT, Australia Yoshiyuki Kawazoe, Center for Computational Materials, Tohoku University, Sendai, Japan Jamie Kruzic, School of Mechanical & Manufacturing Engineering, UNSW Sydney, Sydney, NSW, Australia Richard M. Osgood, Department of Electrical Engineering, Columbia University, New York, USA Jürgen Parisi, Universität Oldenburg, Oldenburg, Germany Udo W. Pohl, Institute of Solid State Physics, Technical University of Berlin, Berlin, Germany Tae-Yeon Seong, Department of Materials Science & Engineering, Korea University, Seoul, Korea (Republic of) Shin-ichi Uchida, Electronics and Manufacturing, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki, Japan Zhiming M. Wang, Institute of Fundamental and Frontier Sciences - Electronic, University of Electronic Science and Technology of China, Chengdu, China

The Springer Series in Materials Science covers the complete spectrum of materials research and technology, including fundamental principles, physical properties, materials theory and design. Recognizing the increasing importance of materials science in future device technologies, the book titles in this series reflect the state-of-the-art in understanding and controlling the structure and properties of all important classes of materials.

More information about this series at http://www.springer.com/series/856

Suprakas Sinha Ray Malkappa Kuruma •

Halogen-Free Flame-Retardant Polymers Next-generation Fillers for Polymer Nanocomposite Applications

123

Suprakas Sinha Ray Centre for Nanostructures and Advanced Materials, DSI-CSIR Nanotechnology Innovation Centre, Council for Scientific and Industrial Research Brummeria, Pretoria, South Africa Department of Chemical Sciences University of Johannesburg

Malkappa Kuruma Centre for Nanostructures and Advanced Materials, DSI-CSIR Nanotechnology Innovation Centre Council for Scientific and Industrial Research Brummeria, Pretoria, South Africa

Johannesburg, South Africa

ISSN 0933-033X ISSN 2196-2812 (electronic) Springer Series in Materials Science ISBN 978-3-030-35490-9 ISBN 978-3-030-35491-6 (eBook) https://doi.org/10.1007/978-3-030-35491-6 © Springer Nature Switzerland AG 2020 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Dedicated to our parents

Preface

This monograph presents an overview of recent academic and industrial research efforts on halogen-free flame-retardant (FR) polymers and their nanocomposites. The synthesis methods of various types of halogen-free FR polymers and their nanocomposites are summarized and their flame-retardant behavior, toxic gas evolution during combustion, and inhibition methods are critically reviewed. The FR activities of different FR compounds containing polymers, their FR mechanisms, and fire toxicant releasing and inhibition methods are covered in this book, with special emphasis on polyurethanes (PUs). The significance of thermal stability and melt dripping on the FR activity of PUs is explained. The FR activities of various P-, N-, and P-N-based FRs-containing PUs are critically analyzed. The synergetic effect between different FR compounds and their importance in FR activity, char formation, and improving char strength are systematically summarized. The importance of metal oxide nanoparticles, nanoclay, and graphene in flame inhibition is discussed. In summary, the various fundamental concepts described in this book are essential to understand the FR activity of various polymers and their nanocomposites, and they would be of great aid in developing efficient environmentally friendly FR polymers and their nanocomposites for a wide range of applications. We hope that graduate students and researchers at universities and industry alike will find this monograph enjoyable and stimulating, as well as helpful for their future work. Pretoria/Johannesburg, South Africa Pretoria, South Africa

Suprakas Sinha Ray Malkappa Kuruma

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Acknowledgements

The authors would like to thank the Department of Science and Innovation and the Council for Scientific and Industrial Research, South Africa, for financial support. We express our sincerest appreciation to all colleagues, postdoctoral fellows, and students for their valuable contributions as well as the reviewers for their critical evaluation of the proposal and chapters. We also thank the authors and publishers for their permission to reproduce their published works. Our special thanks go to Zachary Evenson at Springer Nature for his encouragement, cooperation, suggestions, and advice during various phases of preparation, organization, and production of this book. Finally, we would like to thank our CEO, T. Dlamini, for his continued support and encouragement. Pretoria/Johannesburg, South Africa Pretoria, South Africa

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1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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2 Polymer Combustion and Flame Retardancy 2.1 FR Polymers . . . . . . . . . . . . . . . . . . . . . 2.1.1 Gas-Phase FRs . . . . . . . . . . . . . . 2.1.2 Endothermic FRs . . . . . . . . . . . . . 2.1.3 Solid-Phase FRs . . . . . . . . . . . . . 2.1.4 Intumescent FRs . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . .

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4 Types of Flame Retardants Used for the Synthesis of Flame-Retardant Polymers . . . . . . . . . . . . . . . . . . . . . . 4.1 Nitrogen-Based FRs . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Nitrogen and Phosphorus-Based Salts as FRs . . . . . . . . 4.3 Phosphorus-Based FRs . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1 DOPO-Based Phosphorus FRs . . . . . . . . . . . . . 4.3.2 Phosphorus-Based FRs with Different Oxidation States . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.3 Phosphazene-Based FRs . . . . . . . . . . . . . . . . . . 4.3.4 Phosphorus-Based Spiro Cyclic FRs . . . . . . . . . 4.4 UV-Curable FRs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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5 Flame-Retardant Polyurethanes . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Thermal Degradation and Evolution of Components During PU Combustion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Importance of Polyols in Flame-Retardant PUs . . . . . . . . . . . . 5.3 Phosphorus-Based Polyols for Flame-Retardant Polyurethanes . 5.4 Importance and Reactivity of Diisocyanates in FR PUs . . . . . . 5.5 Effect of Segmental Separation in PUs on Their FR Activity . 5.6 Interactions Between P,N-Based FRs and PUs for FR Activity References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Melt-Dripping and Char Formation . . . . . . . . . . . . . . . . . . . . . 6.1 Relationship Between the Melting Dripping Characteristics of Polymer Films and Fire Properties . . . . . . . . . . . . . . . . . . 6.2 The Relationship Between Char Formation and Morphology Development in Fire Retardant Activities . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Polymer Nanocomposites for Fire Retardant Applications 7.1 FR Polymer Nanocomposites Based on Various Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 Clay-Based Flame-Retardant Polymer Nanocomposites . 7.3 Graphene-Based FR PNCs . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abbreviations

AM-APP AMLR AO APP BCPPS BDP CA CC CE CFA CMA DAEEP DB DDE DDM DDP DDPPU DETA DGEBA DICY DOPO DOPO-HEA and DOPO-AC DOPO-POSS DPPA DPPMA DTA

A novel single macromolecular intumescent flame retardant Average mass loss rate Antimonyoxide Ammonium polyphosphates Bis(1-oxo-4-hydroxymethyl-2,6,7-trioxal-1-phosphabicyclo [2.2.2] octane) phenylphosphine sulfide Bis(diphenyl phosphate) Carbonizing agent Cone colorimetry Cyanate ester Charring forming agent 2-carboxyethyl-(phenyl) phosphinic acid melamine salt Di(acryloloxyethyl)ethyl phosphate Decabromodiphenyl ether 4,4’-diaminodiphenyl ether 4,4’-diaminophenylmethane Ditrimethylolpropane-di-N-hydroxyethyl phosphoramide Di(2,2-dimethyl-1,3-propanediol phosphate) urea Diethylene triamine Diglycidylether of bisphenol 4, 4’-diaminodiphenyl sulfone (DDS); dicyanodiamide 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO)-containing unsaturation bonds; DOPO-containing a novel polyhedral oligomeric silsesquioxane Diphenyl phosphinic acid Melamine-diphenyl phosphinic acid salt DOPO-based triazol

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EBAPP EDA EDA-AP EPC-DDS FGO FPUF FR GDPPO GO GOTP HBPA HCTP HFFR HPHPCP HS IFR LDH-rGO LOI MCAPP MEL MHPA MoS2 MP MPOA MPP MPyP N-PBAAP NPHE NPHE-PU ODA OPS PA6 PA-APP PBT PDBPP PDCP PDEPD PER PET PETBP PHRR

Abbreviations

A novel monomer containing phosphorus and nitrogen Ethylenediamine Ethylenediamine-modified ammonium polyphosphate Epoxy resin of 4,4’-diaminodiphenyl sulfone Unique hybridized graphene oxide Flame-retardant flexible polyurethane foams Flame retardant Glycidyloxy diphenyl phosphine oxide Graphene oxide Graphene oxide hexachlorocyclotriphosphazene Hyperbranched polyamide Hexachloro triphosphazene Halogen-free flame retardants Hexa(phosphite-hydroxyl-methyl-phenoxyl)-cyclotriphosphazene Hard segment Intumescent flame retardant MgAl-layered double hydroxide loaded reduced graphene oxide Limiting oxygen index Microencapsulated ammonium polyphosphate Melamine Melamine hypophosphite Molybdenum disulfide nanoflowers Melamine phosphate Melamine phosphite Melamine polyphosphate Melamine pyrophosphate A new UV-reactive monomer piperazine-N,N’-bis (acryloxyethylaryl-phosphoramidate) UV-reactive phosphazene acrylate UV-reactive phosphazene acrylate-polyurethane 3,4’-oxydiamide Octaphenyl polyhedral oligomeric silsesquioxane Polyamide 6 Piperazine-modified ammonium polyphosphate Poly(butylene terephthalate) Poly(4,4-diaminodiphenylmethane-bicyclicpentaerythritol phosphoryl phosphate) Phenyl dichlorophosphate Novel nanocomposite containing of a polymeric flame retardant Pentaerythritol Polyethylene terephthalate Poly(ethylenediamine-1,3,5-triazine-o-bicyclic pentaerythritol phosphate) Peak heat releasing rate

Abbreviations

PLA PN6 POM PP PPCTP PP-EP PPMS PPPZ PPSPB PSZ PTFE PTPA PUs PVFA rGO SAPP SMB SPUA SS TAEP TCPP Tg TGIC TGICA THR Tm TPP TPU TSP TSR UL-94 ZrP

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Poly(lactic acid) Hexakis-[P-(hydroxyl methyl)phenyl] cyclotriphosphazene Polyoxymethylene Polypropylene (4-diethoxyphosphoryloxyphenoxy)-(4-glycidoxyphenoxy) cyclotriphosphazene Polypropylene modified with ethylene-propylene rubber Poly(2-phenylpropyl) methylsiloxane 4-diphenylphosphoryloxyphenoxy-(4-glycidoxyphenoxy) cyclotriphosphazene Poly(piperazine spirocyclic pentaerythritol bisphosphonate) Polyphosphazene Polytetrafluoroethylene Poly[N4-bis(ethylenediamino)-phenyl phosphonic-N2, N6-bis (ethylenediamino)-1,3,5-triazine-N-Phenyl phosphonate] Polyurethanes Polyvinyl formal acetal Reduced graphene oxide Soluble ammonium polyphosphate Hydrated sodium metaborate Star polyurethane acrylate Soft segment Tri(acryloyloxyethyl) phosphate Tris(chloroisopropyl) phosphate Glass transitional temperature 1,3,5-triglycidyl isocyanate Triglycidyl isocyanurate Total heat releasing rate Melting temperature Triphenyl phosphate Thermoplastic polyurethanes Total smoke production Total smoke release Underwriters laboratory-94 Zirconium phosphate

Chapter 1

Introduction

In modern society, plastics are considered as most versatile materials for wide range of applications, such as coatings, paintings, adhesives, tough elastomers, composites, soft flexible foams, rigid insulation materials, packaging, electronic, commodity, furniture, construction, insulation, and many more [1–3]. However, one of the major drawbacks of polymeric materials is rapid flammability and release of large amounts of smoke and toxic gases during combustion. Thus, they are considered as materials prone to fire hazards, which limit their use in some specific applications. In the last two decades, literally hundreds of test methods have been developed to assess the response of plastic materials to fire and quantify flame retardance. The cone calorimeter test is one of the most extensively used bench-scale methods for studying the fire-retardant properties of polymeric materials. Fire-relevant properties such as the heat release rate (HRR), heat peak HRR, smoke production, and carbon dioxide yield are vital to the evaluation of the fire safety of materials. However, most regulatory tests [4] are not forced combustion testing, as is cone calorimeter testing, nor do they measure HRR. Therefore, the relationship between cone calorimeter test results and regulatory test results continues to be undefined. The nature of the fire products from a polymeric material is dependent on the polymeric material chemical composition and the conditions under which the burning process occurs. Smoke, in particular, is a combination of complete and incomplete combustion species, whereas solid residue is mostly carbon and ash. Within a closed compartment, a fire can go through several phases of growth. First, the fuel source is ignited and undergoes sustained combustion. The fire grows if adequate fuel and oxygen are available, causing the continuous increase of room temperature. Flashover occurs when all of the combustible items in the compartment are engulfed in fire. When the heat release rate and temperature are at their peak, the fire is fully developed [5, 6]. To inhibit, suppress, or delay the production of flames to prevent the spread of fire, filler, commonly known as flame retardant (FR), is added to polymer during processing. A FR is defined as a material that can inhibit or delay flame © Springer Nature Switzerland AG 2020 S. Sinha Ray and M. Kuruma, Halogen-Free Flame-Retardant Polymers, Springer Series in Materials Science 294, https://doi.org/10.1007/978-3-030-35491-6_1

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propagation. The main advantage of using FRs is that they limit the release of toxic gases and noxious smoke; further, they can eliminate polymer dripping during burning and spreading of fire. FRs are broadly classified into halogenated and non-halogenated FRs. Over the years, halogenated FRs (HFRs) are the most commonly used FRs in the plastic industry. HFRs are said to act in the vapor phase that means they actually interfere with the chemistry of the flame. Chlorinated and brominated are both used in this role, but brominated FRs have been the most effective and the most common one is hexabromocyclododecane. However, HFRs generate a large amount of toxic gases and smoke. Moreover, HFRs can cause environmental pollution as they release large amounts of smoke and toxic corrosive gases. Therefore, environmental concerns and governmental regulations restrain the use of HFRs in several countries. Thus, it is necessary to develop novel environment-friendly and efficient FRs to achieve a balance between FR performance and environmental issues. There are several factors that affect polymer flammability, but the most important and crucial factor is the chemical structure of the polymer. From literature, it is clear that aromatic, hetero aromatic, and non-burning hetero atoms, such as N-, P-, S-, Si-, and B-containing derivative compounds, when incorporated into the polymer main chain or copolymerized, efficiently reduced flammability [7–12]. The thermal stability of a polymer also plays an important role in controlling its FR activity. Moreover, the inherent mechanical properties, melt viscosity, melting temperature (Tm), and glass transition temperature (Tg) of a polymer are also important in determining its FR activity [13, 14]. It has been found that polymers with superior mechanical, thermal, and rheological properties show excellent FR activity. Generally, to improve the inherent properties of polymeric materials, different types of nanofillers are incorporated in polymer matrices to prepare polymer nanocomposites (PNCs) [15, 16]. However, to improve the FR activity, these nanofillers are surface-modified with P- and/or N-containing surfactants in order to improve their compatibility with polymeric matrices as well as to improve their FR activity [17–20]. Heat release and toxic gas inhalation are the main reasons for death in fire accidents; in fact, the number of such deaths is higher than that of deaths occurring due to fire burns. Therefore, it is necessary to develop strong fire-toxicant inhibitors; in the past few decades, immense research efforts have been carried out in the area of polymeric FRs and smoke suppressants. The findings of these studies revealed that graphene and nanoclay-functionalized metal oxides exhibit excellent smoke toxicity and heat-release inhibition compared to pristine metal oxides owing to their synergetic action [21–24]. C, N, and Si-based inorganic network substituents exhibited flame retardant activity due to increasing char formation and reduction in the heat-release rate. In this monograph, we critically review the main results of the academic and industrial research on halogen-free FRs such as P-, N-, and P, N-based salts and surface-modified nanofillers containing different types of P, N-based organic compounds with respect to their synthesis, FR activity, and FR mechanism. In particular, toxicant evolution and inhibition, FR mechanisms, synergetic effects,

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importance of char formation, and morphological transformation are discussed. In addition, this monograph gives a conclusion and an outline of future prospects of environmentally–friendly FR polymers and their nanocomposites.

References 1. A.K. Mishra, S. Chattopadhyay, P. Rajamohanan, G.B. Nando, Effect of tethering on the structure-property relationship of TPU-dual modified Laponite clay nanocomposites prepared by ex-situ and in-situ techniques. Polymer 52, 1071–1083 (2011) 2. C. Xiang, W. Lu, Y. Zhu, Z. Sun, Z. Yan, C.C. Hwang, J.M. Tour, Carbon nanotube and graphene nanoribbon-coated conductive Kevlar fibers. ACS Appl. Mater. Interfaces 4, 131– 136 (2011) 3. M.M.A. Nikje, A.B. Garmarudi, A.B. Idris, Polyurethane waste reduction and recycling: from bench to pilot scales. Des. Monomers Polym. 14, 395–421 (2011) 4. P. Kiliaris, C.D. Papaspyrides, Polymer/layered silicate (clay) nanocomposites: an overview of flame retardancy. Prog. Polym. Sci. 35, 902–958 (2010) 5. S.V. Levchik, Introduction of Flame Retardancy and Polymer Flammability (Wiley, New Jersey, 2007) 6. A.B. Morgan, Flame retarded polymer layered silicate nanocomposites: a review of commercial and open literature systems. Polym. Adv. Technol. 17, 206–217 (2006) 7. E. Çakmakçı, A. Güngör, Preparation and characterization of flame retardant and proton conducting boron phosphate/polyimide composites. Polym. Degrad. Stabil. 98, 927–933 (2013) 8. H.t. Pu, L. Qiao, Liu Qz, Z.L. Yang, A new anhydrous proton conducting material based on phosphoric acid doped polyimide. Eur. Polym. J. 41, 2505–2510 (2005) 9. L. Meng, C. Xu, T. Liu, H. Li, Q. Lu, J. Long, One-pot synthesis of highly cross-linked fluorescent polyphosphazene nanoparticles for cell imaging. Polym. Chem. 6, 3155–3163 (2015) 10. L. Gu, G. Chen, Y. Yao, Two novel phosphorus–nitrogen-containing halogen-free flame retardants of high performance for epoxy resin. Polym. Degrad. Stabil. 108, 68–75 (2014) 11. C. Gao, L. Wang, Z. Lei, L. Yang, X. Xu, X. Guo, Property of intrinsic flame retardant epoxy resin cured by functional magnesium organic composite salt and diethylenetriamine. Fire Mater. 41, 180–192 (2017) 12. S. Levchik, A. Balabanovich, O. Ivashkevich, P. Gaponik, L. Costa, Thermal decomposition of tetrazole-containing polymers. V. Poly-1-vinyl-5-aminotetrazole. Polym. Degrad. Stabil. 47, 333–338 (1995) 13. L.J. Qian, L.J. Ye, G.Z. Xu, J. Liu, J.Q. Guo, The non-halogen flame retardant epoxy resin based on a novel compound with phosphaphenanthrene and cyclotriphosphazene double functional groups. Polym. Degrad. Stabil. 96, 1118–1124 (2011) 14. M. Thirumal, D. Khastgir, N.K. Singha, B. Manjunath, Y. Naik, Effect of foam density on the properties of water blown rigid polyurethane foam. J. Appl. Polym. Sci. 108, 1810–1817 (2008) 15. C.Y. Su, A.Y. Lu, Y. Xu, F.R. Chen, A.N. Khlobystov, L.J. Li, High-quality thin graphene films from fast electrochemical exfoliation. ACS Nano 5, 2332–2339 (2011) 16. X. Wang, L. Song, H. Yang, W. Xing, H. Lu, Y. Hu, Cobalt oxide/graphene composite for highly efficient CO oxidation and its application in reducing the fire hazards of aliphatic polyesters. J. Mater. Chem. 22, 3426–3431 (2012) 17. H. Ma, L. Zhao, J. Liu, J. Wang, J. Xu, Functionalizing carbon nanotubes by grafting cyclotriphosphazene derivative to improve both mechanical strength and flame retardancy. Polym. Compos. 35, 2187–2193 (2014)

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18. L. Xu, L. Xiao, P. Jia, K. Goossens, P. Liu, H. Li, C. Cheng, Y. Huang, C.W. Bielawski, J. Geng, Lightweight and ultrastrong polymer foams with unusually superior flame retardancy. ACS Appl. Mater. Interfaces 9, 26392–26399 (2017) 19. S. Pappalardo, P. Russo, D. Acierno, S. Rabe, B. Schartel, The synergistic effect of organically modified sepiolite in intumescent flame retardant polypropylene. Eur. Polym. J. 76, 196–207 (2016) 20. H. Lu, C.A. Wilkie, Study on intumescent flame retarded polystyrene composites with improved flame retardancy. Polym. Degrad. Stabil. 95, 2388–2395 (2010) 21. S.D. Jiang, Z.M. Bai, G. Tang, L. Song, A.A. Stec, T.R. Hull, Y. Hu, W.Z. Hu, Synthesis of mesoporous silica@ Co–Al layered double hydroxide spheres: layer-by-layer method and their effects on the flame retardancy of epoxy resins. ACS Appl. Mater. Interfaces 6, 14076– 14086 (2014) 22. Z. Wang, P. Wei, Y. Qian, J. Liu, The synthesis of a novel graphene-based inorganic–organic hybrid flame retardant and its application in epoxy resin. Compos. Part B: Eng. 60, 341–349 (2014) 23. W. Wang, H. Pan, Y. Shi, Y. Pan, W. Yang, K. Liew, L. Song, Y. Hu, Fabrication of LDH nanosheets on b-FeOOH rods and applications for improving the fire safety of epoxy resin. Compos. Part A: Appl. Sci. Manuf. 80, 259–269 (2016) 24. Q. Kong, T. Wu, H. Zhang, Y. Zhang, M. Zhang, T. Si, L. Yang, J. Zhang, Improving flame retardancy of IFR/PP composites through the synergistic effect of organic montmorillonite intercalation cobalt hydroxides modified by acidified chitosan. Appl. Clay Sci. 146, 230–237 (2017)

Chapter 2

Polymer Combustion and Flame Retardancy

Most polymers are derived from petroleum hydrocarbon sources and hence are extremely flammable. However, for any material to catch fire, oxygen, heat, and external energy are essential. During combustion, polymer particles become airborne; when a polymer is exposed to high heat energy, a significant amount of degradation occurs. During this process, airborne particles are evolved along with combustible volatile substances and mix with atmospheric oxygen. If the ambient temperature is close to or above the ignition temperature (flash point temperature), then these airborne particles will ignite. Some of the heat is fed back to the polymer for further degradation, as shown in Fig. 2.1; in a similar way, volatile substrates and flames will spread in a cyclic process until the polymer completely burns to produce pyrolytic components containing heat, smoke, fumes, and toxic gases, as depicted in Fig. 2.1. This scheme helps us to understand the process of polymer combustion; based on this concept, different types of fire retardants (FRs) have been designed and developed by various researchers to decrease or delay the flammability of polymers. These FRs are used to synthesize FR polymers; hence, in the presence of a FR, the flammability of polymers is decreased. Moreover, the flame toxicity is reduced. In the last two decades, literally hundreds of test methods have been developed to assess the response of plastic materials to fire and quantify flame retardance. The cone calorimeter test is one of the most extensively used bench-scale methods for studying the fire-retardant properties of polymeric materials. Fire-relevant properties such as the heat release rate (HRR), peak HRR, smoke production, and carbon dioxide yield are vital to the evaluation of the fire safety of materials. HRR is one parameter to assess the FR activity of polymers because in the presence of heat, polymer degradation is accelerated, resulting in an increase in the amount of airborne components, which are responsible for flame propagation. When exposed to flame, a FR releases flame inhibitors either in the form of radicals or acidic components. FR polymers are synthesized using FR additives and these additives adopt various chemical pathways (such as condensed-phase and gas-phase mechanisms) to exhibit FR activity. In the gas-phase mechanism, FR activity is attributed to © Springer Nature Switzerland AG 2020 S. Sinha Ray and M. Kuruma, Halogen-Free Flame-Retardant Polymers, Springer Series in Materials Science 294, https://doi.org/10.1007/978-3-030-35491-6_2

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Polymer Combustion and Flame Retardancy

Fig. 2.1 Schematic representation of a polymer combustion cycle

the release of non-flammable gases and radicals, such as –Cl, –Br, and –OH, which interrupt flame propagation. In the condensed-phase FR mechanism, acid compounds, such as phosphorus and sulfur-containing acids are formed; they dehydrate the polymer and form a char layer on the surface of the polymer. This char layer can prevent heat and oxygen from coming into contact with the polymer and thus its flammability is reduced. Based on these phenomena, FR activity is classified into various categories, which will be discussed in the following sections.

2.1

FR Polymers

Since the end of the last decade, different types of FRs have been used for the synthesis of FR polymers either as chain extenders or monomers. When a FR polymer is exposed to flame, it inhibits or suppresses the initial stages of flame by releasing non-flammable gases or by forming char during burning. Hence, the synthesis of easy-to-use FR polymers is extremely important for different applications. The synthesis of FR materials gained much attention in the USA during World War II; in particular, FR military clothing and aerospace coatings were considered to be of paramount importance. A FR does not burn but prevents or delays the flammability of the material in which it is incorporated. Overall, the best FRs are expected to exhibit three key properties, viz (i) increasing the limiting oxygen index (LOI) value, (ii) decreasing the peak HRR, and (iii) suppress smoke. The basic FR mechanism varies depending on the type of FR used, chemical

2.1 FR Polymers

7

structure, and interactions between the polymer and FR. Crosslinking can also improve FR activity; the functional groups of a FR may be capable of increasing the degree of crosslinking. Based on their mechanism, FRs are classified into four categories—gas phase, endothermic, solid phase, and intumescent FRs.

2.1.1

Gas-Phase FRs

These FRs show FR activity by releasing non-flammable gases into the flame zone area to decrease the heat and oxygen levels, which are responsible for flame propagation. The quantity of airborne components is low because FRs limit the heat released into the flame zone area. When polymers containing mineral fillers as FRs are exposed to heat or flame, the mineral fillers start to decompose and release non-flammable gases, such as CO2, water, SO2, and some acids, resulting in a reduced oxygen content and cooling the flame zone area [1–4].

2.1.2

Endothermic FRs

Endothermic FRs show FR activity by forming endothermic radicals that can absorb heat energy and reduce flammability by decreasing the amount of heat in the flame zone area. For example, hydrated fillers when incorporated in polymers decompose endothermically and decrease the temperature to a value less than that required for polymer degradation or combustion. Metal hydroxides, halogens, phosphorus derivatives of low oxidation levels, and antimony-containing polymers fall under this category. Antimony is normally used as a synergistic agent along with halogen-based FRs to initiate halogen radical formation and transfer into the flame zone, which results in flame interruption.

2.1.3

Solid-Phase FRs

Most phosphorus and sulfur derivatives act as solid-phase FRs. When exposed to flame, they form the corresponding acids, which are capable of fast dehydration, and form an insulating char layer on the surfaces of polymer materials. The char layer can protect the substrate from oxygen attack and heat transfer, which is necessary for flame propagation [5–9].

2.1.4

Intumescent FRs

Intumescent FRs are one of the most important type of FRs because they are capable of forming highly dense expandable char layers, which can efficiently

8

2

Polymer Combustion and Flame Retardancy

protect the substrate from oxygen attack. Intumescent FR research was started in the 19th century using organic polymers. In 1821, Gay-Lussac started his research on FR cellulosic materials and he recommended the use of ammonium phosphate to improve the FR properties of polymeric materials. Later on, Tramm et al. described an intumescent coating and patented it in 1930. Intumescent coatings have attracted much attention due to their anti-dripping property, halogen-free composition, low smoke emission, and low amount of toxic gases. Presently, intumescent FRs are used commercially in many surface-coating applications, for example on wood, plastics, and metals, to provide fire protection. Most N-based salts and N,P-based salts show intumescent flame retardant (IFR) activity. Some metal oxides containing graphene and clay also exhibit IFR activity. Intumescent FRs lead to the formation of a dense char layer; therefore, char-forming agents, such as ammonium polyphosphates (APP) and melamine polyphosphate (MPP), are introduced into polymers to prepare such FRs. Currently, research is focused on the development of FR PUs due to their multitude of applications in various domains, such as furniture, electronic devices, vehicles, surface coatings, and foams [4, 10]. However, all commercially available PUs using for various applications are flammable. Therefore, a number of FRs are introducing into PUs through chemical reactions. If FR compounds are part of the monomer or polymer, the corresponding polymer is inherently flame retardant. There are two commonly used methods to achieve flame retardancy in polymers, namely additive-type and reactive-type methods.

2.1.4.1

Additive Method

In this method, FR polymers are prepared via physical mixing. Additive FRs are incorporated in the polymer matrix without any chemical bonding between the additives and polymer. The FR additives are dispersed evenly throughout the polymer matrix via physical interactions, such as H-bonding, van der Waals forces, and ionic interactions. Most additive FRs are not compatible with polymers and may experience phase separation and leaching. As there are no covalent bonds between the polymers and additive FRs, this method is not effective in improving the FR properties of polymers. Therefore, in order to increase the FR activity, large quantities of FR additives should be incorporated, which adversely affects the mechanical properties of the polymers.

2.1.4.2

Reactive Method

Using this method, FR polymers are synthesized via the introduction of reactive FR additives into the polymer in the form of a monomer or polymer precursor. Consequently, reactive FRs are more effective in improving the FR activity of polymers compared to additive-type FRs owing to the presence of covalent bonds between the FR compounds and polymers; under such conditions, the FR additives are neither phase-separated nor leached out. However, they do not increase the

2.1 FR Polymers

9

thermal stability of or plasticize polymers [11]. Compounds containing phosphorus, nitrogen, halogens, boranes, silicones, and combinations of phosphorus-silica, phosphorus-nitrogen, etc., belong to this category. These compounds can be used as chain extenders or co-monomers during polymer synthesis. Of these, phosphorusbased compounds are more effective and compatible with thermoplastic and thermosetting polymers. In some specific FRs, a synergetic effect occurs when combinations of P-N, P-Si, and N-Si compounds are used.

References 1. G. Camino, L. Costa, M. Luda, Mechanistic aspects of intumescent fire retardant systems, in: Macromolecular Symposia (Wiley, 1993), pp. 71–83 2. M. Le Bras, S. Bourbigot, B. Revel, Comprehensive study of the degradation of an intumescent EVA-based material during combustion. J. Mater. Sci. 34, 5777–5782 (1999) 3. K. Wazarkar, M. Kathalewar, A. Sabnis, Reactive modification of thermoplastic and thermoset polymers using flame retardants: an overview. Polym. Plast. Technol. Eng. 55, 71–91 (2016) 4. D.K. Chattopadhyay, K. Raju, Structural engineering of polyurethane coatings for high performance applications. Prog. Polym. Sci. 32, 352–418 (2007) 5. R. Kidder, Handbook of Fire Retardant Coatings and Fire Testing Services (1994) 6. Y. Khanna, E.M. Pearce, Synergism and flame retardancy, in: Flame-Retardant Polymeric Materials (Springer, 1978), pp. 43–61 7. B.K. Kandola, A.R. Horrocks, Complex char formation in flame-retarded fiber/intumescent combinations: physical and chemical nature of char1. Textile Res. J. 69, 374–381 (1999) 8. A.R. Horrocks, D. Price, D. Price, Fire Retardant Materials (Woodhead Publishing, 2001) 9. D. Aslin, The design and development of intumescent coatings for structural fire protection. J. Oil Colour Chem. Ass. 72, 176–190 (1989) 10. G. Camino, L. Costa, M.L. di Cortemiglia, Overview of fire retardant mechanisms. Polym. Degrad. Stabil. 33, 131–154 (1991) 11. S.M. Lomakin, G.E. Zaikov, Ecological Aspects of Polymer Flame Retardancy (VSP, 1999)

Chapter 3

Flame-Retardancy Testing

Standard FR activity testing methods and procedures have been developed to evaluate the FR activity of polymers [1]. Generally, the FR activity of a material can be evaluated on the basis of some specific parameters, such as ease of ignition, flame spread rate, HRR, smoke-production rate (SPR), and ease of fire extinction. All these properties depend on the chemical structure of the polymer, type of FR used, and nature of the degradation components. When heated, a polymer degrades and evolves combustible and non-combustible components. For example, flammable hydrocarbon gases, such as methane and ethane, may be released and they can be used to monitor the rate of flame propagation. Similarly, non-combustible components, such as water, carbon dioxide, NH3, and CO may also be released and they decrease the flammability of the material by reducing the heat at the flame zone. Therefore, HRR is an important parameter to analyze polymer degradation. Hence it is important to measure the HRR and total heat-releasing rate (THR) to assess the susceptibility of polymers to fire. According to the target application, the flammability of polymers can be measured using various established methods. Some commonly used methods are described below.

3.1

Cone Calorimeter

The cone colorimetry test method (ASTM E1354) was developed in the United States; in this method, polymer flammability is determined by measuring the volume of oxygen in a mixture of combustion gases evolved from the sample. Figure 3.1 shows below is a schematic of Cone. A sample is placed below a “cone” shaped radiant heater. Typically the samples are exposed to an external flux form the heater of 35 kW/m2. However, for more fire resistant materials the heater frequently increased to 50 kW/m2. Once enough pyrolysis products are generated, ignition occurs. The combustion product travel through the Cone heater and through an instrument exhausted pipe. The values measured/calculated that are © Springer Nature Switzerland AG 2020 S. Sinha Ray and M. Kuruma, Halogen-Free Flame-Retardant Polymers, Springer Series in Materials Science 294, https://doi.org/10.1007/978-3-030-35491-6_3

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3 Flame-Retardancy Testing

Fig. 3.1 Schematic drawing of cone. Reproduced from NIST, USA website

typically important include, but are not limited to, the time to ignition, the mass loss rate during combustion, time to and the value of the maximum amount of heat released during combustion, total amount of heat released during the test. The HRR is determined by measuring the volume of consumed oxygen. Further, the THR, effective heat of combustion, smoke development, mass loss rate, and release of CO and CO2 can also be evaluated. During measurement, the sample is placed atop a load cell and combustion is triggered by an electric spark; polymer flammability is evaluated based on the mass loss in the sample during the measurement period.

3.2

Limiting Oxygen Index (LOI)

The LOI indicates the minimum oxygen concentration (vol.%) required at ambient temperature in an inert gaseous medium for the material to continue burning after ignition; the LOI test is one of the most important methods to analyze the flammability of polymers. The flammability of a polymer depends on its nature,

3.2 Limiting Oxygen Index (LOI)

13

chemical structure, and thermal stability among other factors. Hence, these values can vary depending on the FR and polymer being tested, as we shall describe in the following sections. This method, first introduced by Fenimore and Martine in 1966, is now standardized in the USA (ASTM D2863) and internationally as well (ISO 4589). The LOI value generally defined as the ratio between consumed oxygen volume divided by total volume of oxygen and nitrogen gases in atmosphere, i.e., LOI = [O2]/[O2] + [N2]  100. On the basis of the LOI values, we can predict the flammability of a material. Usually, ambient atmosphere contains 21% oxygen. Therefore, materials with LOI values less than 21% are considered combustible, while those with LOI > 21% are categorized as self-extinguishing, because these materials cannot undergo combustion atmospheric temperature without any external energy, such as temperature or a light spark. Therefore, a high LOI value indicates better FR activity. In the ISO 4589 method, the specimen is placed at the center of the top of the chimney. A mixture of gases flows upstream via this chimney and is homogenized by passing through layers of glass beads for purging the column over 30 s; the top of the specimen is ignited like a candle and the consumed oxygen volume is measured.

3.3

Underwriters Laboratory-94 (UL-94 V)

This test is useful in investigating and classifying the FR level of different materials. It is based on time taken by the flame to spread and the ability of the material to resist dripping. In particular, it is critical to understand the FR activity of plastic materials in applications where flame resistance is necessary. The UL-94 test consists of a set of tests approved by the Underwriters Laboratory and materials are classified as V-0, V-1, V-2, etc. depending on simple vertical combustion; if a material burns slowly or self-extinguishes without dripping, then it is assigned the highest ranking in the UL-94 test. This test method is most commonly used for evaluating the ignitability and flame spread of a vertical sample exposed to a small flame. Flame is applied to the bottom of a sample vertically hung from the top of the burner and the minimum sample size is 10 mm. The sample is exposed to the flame for 10 s and removed; the after-flame time, t1, which is the time period required for the flame to terminate or stop is measured. After the flame dies down, the sample is exposed to a flame once again for another 10 s and the after-flame time t2 is noted together with the after-glow time, t3 (the time required for the glow to disappear). The distance between the burner and specimen should be constant during the experiment. The standard specifies that five specimens must be tested and based on t1, t2, and t3, the specimens can be categorized as V-0, V-1, and V-2 as shown in Table 3.1 [2].

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3 Flame-Retardancy Testing

Table 3.1 Criteria and classification of materials rating in UL-94 V testing Sample classified based on following criteria

V-0 (s)

V-1 (s)

V-2 (s)

After flame time for each individual specimen t1 or t2 Total after flame for any set (t1 + t2 for 5 specimens) After flame/glow time for each specimen after second application (t1 + t3) After flame glow of any specimen up to clamp Cotton indicator ignited by flaming drips

 10  50  30

 30  250  60

 30  250  60

No No

No No

No Yes

References 1. J. Troitzsch, International Plastics Flammability Handbook: Principles-Regulations-Testing and Approval (Hanser, 1983) 2. P. Patel, T.R. Hull, C. Moffatt, PEEK polymer flammability and the inadequacy of the UL-94 classification. Fire Mater. 36, 185–201 (2012)

Chapter 4

Types of Flame Retardants Used for the Synthesis of Flame-Retardant Polymers

Currently, a variety of chemical compounds are being used as Flame-retardants (FRs) and they can be classified according to the type of element present, such as phosphorus, nitrogen, silicon, boron, or halogens. Halogenated FRs are not environmentally friendly as they tend to release toxic corrosive gases and dense smoke while burning. Therefore, halogenated FRs have been banned in the USA, Europe, and several other countries. As mentioned earlier, fire retardants containing phosphorus or nitrogen are being focused upon nowadays and numerous efforts are concentrated towards developing P-, N-, and P & N-containing FRs for the synthesis of effective FR polymers. A brief summary and applications of these halogen-free FR polymers is included in the following sections.

4.1

Nitrogen-Based FRs

Nitrogen-containing FRs are one of the most important groups of halogen-free FRs and are widely used for the synthesis of FR polymers. Organic nitrogen-based compounds, such as melamine, guanidine, and their derivatives are widely used for various applications. For example, melamine is used in flexible PU cellular plastics, melamine cyanurate is applied in unreinforced nylons, guanidine sulfamate is used as a FR in PVC wall coverings, and guanidine phosphate is used as a FR for textile fibers [1, 2]. Melamine is a type of hetero cyclic nitrogen-rich compound; it is crystalline and thermally stable with a high melting temperature of 345 °C. At high temperatures, melamine decomposes endothermally, which means that it acts as a heat sink during combustion and releases non-combustible ammonia gas. Further, it also forms thermally stable condensed compounds, including melam, melem, and melon, which produce a strong char structure [3]. Therefore, most nitrogen derivative-containing polymers exhibit FR activity as they act like heat sinks and evolve non-flammable gases, such as N2, NH3, CO2, and H2O into the flame zone area, which cool it down and dilute oxygen concentration. Su et al. [4] synthesized © Springer Nature Switzerland AG 2020 S. Sinha Ray and M. Kuruma, Halogen-Free Flame-Retardant Polymers, Springer Series in Materials Science 294, https://doi.org/10.1007/978-3-030-35491-6_4

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a novel oligomeric charring agent (PTCA) from nitrogen-based compounds, such as cyanuric trichloride, diphenyl amine, and ethylene diamine, via nucleophilic reaction. Using a combination of PTCA with APP, they prepared FR polypropylene (PP) composites and studied their thermal and FR activities. They observed that 20 wt% PP composites exhibited a high LOI value with significantly low PHRR, THR, and TSR and also passed UL-94 with V-0 rating due to the synergetic effect between PTCA and APP. The high thermal stability of these composites was attributed to the formation of a stable highly graphitized aromatic char structure, which was confirmed by FTIR analysis. Similarly, Xu et al. [5] synthesized highly efficient FR PTPA and used it as a IFR in combination with APP for PPs. They achieved excellent FR activity with a high LOI value and V-0 rating in UL-94 testing owing to the synergetic effect between PTPA and APP. Lv et al. [6] synthesized oligomeric FR and smoke suppression agents consisting of poly(melamineethoxyphosphinyl-diisocyanate) (PMPC) and used it a stand-alone FR for epoxy resins; epoxy/PMPC composites with 20 wt% FR loading showed the highest LOI value and achieved a V-0 rating. Most studies suggest that for acceptable FR activity, more than 25 wt% IFR is required. However, such high loadings are detrimental to the mechanical and physical properties of the polymer and increase the cost of production. Combinations of N- and P-containing FRs, exhibit efficient FR activity even at low loadings owing to a synergetic reaction between nitrogen and phosphorus. Wang et al. [7] used several nitrogen-based compounds, such as adenine, cytosine, uracil, and guanine, obtained from bio-based gas sources along with an IFR to improve the fire-retardancy of PP. They observed that PP containing 17 wt% of IFR and 1 wt% of cytosine or uracil showed self-extinguishing ability with high LOI values and passed the UL-94 test; on the other hand, PP containing same percentages of adenine or guanine did not pass the UL-94 test. This indicates that cytosine and uracil compounds are efficient in improving the FR activity of polymers due to the formation of a stable cellular structure, which acts as a barrier to prevent the exchange of gas and heat between the resin and outside environment. Hu et al. [8, 9] synthesized a novel FR containing triazine (CA) and used it in combination with APP in low-density polyethylene (LDPE); they observed that a 11% APP and 4% CA-containing system showed high FR activity and thermal stability due to the synergetic effect between APP and CA. Similarly, Li et al. [10] synthesized a char-forming agent (CFA) from cyanuric chloride, ethylene diamine, and ethanolamine according to the reaction shown in Scheme 4.1. The char forming agent (CFA) is a type of triazine-containing polymer, which can be used as a FR in combination of APP for PP. The authors observed that 64% APP, 32% CFA, and 4% zeolite containing system exhibited efficient IFR activity with high LOI value 30.2% and passed UL-94 test. However, they assumed IFR activity without given proper mechanism of FR activity. Wang et al. [11] synthesized an IFR agent, PEPAPC, from the nitrogen-based compounds, cyanuric chloride, PEPA (2,6,7-trioxa-1-phosphabicyclo[2.2.2] octane4-methanol), and piperazine, by nucleophilic substitution, as shown in Scheme 4.2. They used PEPAPC with APP to prepare FR polypropylenes at different ratios; at 20% FR loading (APP:PEPAPC = 3:1), the highest LOI value was obtained

4.1 Nitrogen-Based FRs

17

Scheme 4.1 Synthesis of macromolecular triazine polymer as a char-forming agent for intumescent systems. Reproduced with permission from [10]. Copyright 1006, Elsevier Science Ltd

and the PP specimen passed the UL-94 test with V-0 rating and resulted in a high char residue. Compared to CFA, even at low percentages of APP, these PP systems showed strong FR activity due to the presence of P and N elements in the PEPAPC structure. The degradation characteristics of melamine alone and when in combination with MPP and PER were studied by TG-FTIR. It was observed that in the presence of MPP, the degradation and sublimation of melamine are highly inhibited whereas when it was used in combination with MPP and PER, sublimation does not occur. Furthermore, in this case, the evolution of volatile combustible substrates is limited [12].

Scheme 4.2 Synthesis of P- and N-containing IFR-PEPAPC. Reproduced with permission from [11]. Copyright 2017, Elsevier Science Ltd

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4.2

4 Types of Flame Retardants Used for the Synthesis of …

Nitrogen and Phosphorus-Based Salts as FRs

Generally, nitrogen-based salts and nitrogen and phosphorus-based salts, such as ammonium melamine phosphates, show IFR activity. A number of research articles report that melamine salt-containing polymers exhibited excellent IFR, such as melamine phosphate (MP), melamine polyphosphate (MPP), melamine salts of pentaerytropolyol phosphoric acid, dicyclic phosphorous-melamine derivative compounds, and bis(pentaerythritol phosphate) phosphoric acid compounds [13–15]. At high temperatures, the presence of phosphorus induces the formation of char and at the same time, the presence of nitrogen induces the generation of non-flammable gases. This results in an expandable char layer on the surface of the polymer, which can protect it from heat and oxygen transfer. Recently, IFRs have drawn much attention due to low toxic gas and smoke evolution and high efficiency [16–21]. To exhibit IFR activity, the system must contain an acid source (the dehydration catalyst for char formation), carbon source (the carbonization agent), and gas source (the blowing agent) [22–24]. In the presence of acid catalysts, the carbonizing agent forms a char layer, which further expended by releasing blowing gases [24]. The FR mechanism mainly depends on the type of additives and their interactions with the polymer. Intumescent PU coatings may be used to protect various substrates, especially plastic substrates. APPs are considered to be highly effective IFRs for different types of polymers [25, 26] and in particular, for PU polymers [27]. Duquesne et al. [28] prepared FR-PU coatings using different percentages of APP as FR additives and they investigated the IFR mechanism. They observed that with increasing APP content, the LOI values increased. At 40 wt% APP, the PUs exhibited the highest LOI value of 44%. However, high APP loadings can adversely the mechanical properties; hence, to decrease the loading amount required for high IFR activity, nanocomposites were prepared. Yuan et al. synthesized rigid PU foams/ expandable graphite (RPUF/EG) using a combination of phosphorus and nitrogenbased polyols (BHPP and MADP) and demonstrated the synergetic effect of these compounds towards FR activity [29]. A phosphorus tungstic acid (PTA)-catalyzed MPP system showed remarkably high FR activity without disturbing the mechanical properties when compared to non-catalyzed MPP [30]. This is because PTA acts as an effective synergist and decreases the reaction temperature as well. Lubczak et al. [31] prepared MPP-modified PUF and reported high thermal stability, LOI values, and self-extinguishing character. Semi-rigid PUFs were prepared using soluble ammonium polyphosphate (SAPP) and water as the blowing agent; these systems exhibited excellent mechanical, thermal, and FR properties [32]. Further, in these systems, a strong dripping phenomenon was observed before char formation due to a strong synergetic effect. Combinations of APP/PER act as efficient FRs for polyolefin-based formulations [33–35] and especially for ethylene-butyl acrylate maleic anhydride formulations [36]; all these FR formulations achieved high LOI values and V-0 rating in the UL-94 test [37]. Bourbigot et al. [34] systematically studied intumescent char formation and protection mechanism at high temperatures

4.2 Nitrogen and Phosphorus-Based Salts as FRs

19

in APP/PER-containing polymer systems using IR and solid-state nuclear magnetic resonance (NMR) studies. It was found that instead of water formation, phosphoric acids were formed. The polyethylene links between poly aromatic species were rearranged, which enhances the mechanical strength of the intumescent coatings and also provides strong barrier properties. Liu et al. [38] synthesized FR PP composites with different percentages of PP/MP/PER and PP/MP/PER/TPU-containing polymers and they observed that the presence of TPU improved the FR activity and led to high LOI values and V-0 rating in the UL-94 test. These observations can be attributed to the following causes. (i) The charring agent PER reacts TPU during melt-blending instead MP (PER) reacts with MP and (ii) prevention of the leaching of PER from the nonpolar PP polymer matrix. Additionally, it has been noticed that MPP works more effectively in the reactive mode rather than in the additive mode; reactive-mode MPP-containing epoxy resins showed high LOI values and more char yield [39]. Tang et al. [40] synthesized TPUs using different percentages of APP and APP/DPER (dipentaerythritol). In the case of APP, they observed that with increasing APP content the FR activity of the TPU improved but there was a marked decrease in its thermal and mechanical properties owing to the catalytic action of APP and the accelerated decomposition of TPU. However, when APP was combined with DPER in the TPU, with increasing APP/DPER content, there was a significant improvement in the FR properties and no adverse impact on the mechanical and thermal properties. The important observation here is that at high temperatures, esterification takes place between APP and DPER, resulting in an improvement in the thermal, mechanical, and FR properties. Similarly, Lv et al. [41] synthesized a series of PP polymers containing MP and MP/PER and MP/DPER and MP/TPER and characterized them at the same FR loading. The FR activity of the polymers increased with high LOI values and all of them, except PP/MP, achieved a V-0 rating in the UL-94 test. These results clearly indicate that in the presence of PER, FR activity increases, while THR, PHRR, and smoke production decrease. To improve FR activity, APP modified with different chemical components was used to prepare polymer composites; the results showed improved FR activity at high loadings, but the mechanical properties were disturbed [42]. Therefore, it is necessary to modify and develop efficient APP derivatives. The modified chemical functionalities should be able to improve FR activity either by increasing the crosslinking density or releasing non-flammable gases. Shao et al. [43] prepared a series of PP polymers with piperazine-modified APP (PA-APP) and carried out flammability analysis; they observed that PP/PA-APP (piperazine-modified ammonium polyphosphate) led to superior FR activity compared to PP/APP composites. In the presence of PA-APP, flame propagation in PP and the SPR were reduced by 86.2% and 78.2%, respectively. The same research group synthesized epoxy polymers with multifunctional groups using organic and inorganic hybrid DETA (diethylenetriamine)-APP at different loading levels and evaluated the FR properties of the resultant polymers. A large number of functional groups led to a greater crosslinking density, which conferred strong barrier properties helpful in preventing the evolution of combustible substrates and heat transfer into the flammable zone. The researchers also noticed that DETA-APP is an efficient curing agent for epoxy polymers. With increasing

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4 Types of Flame Retardants Used for the Synthesis of …

DETA-APP content in the resin, the LOI values increased and excellent smoke suppression was observed [16]. Chen et al. [13] synthesized FR FPUF (flexible polyurethane foams) using a single component, 2-carboxyethyl-(phenyl) phosphinic acid melamine salt (CMA), and studied its flammability; at high CMA loadings, FPUF showed self-extinguishing ability but its mechanical strength was reduced. Further, there occurred the problem of poor compatibility at high CMA loadings. Similarly, Rao et al. [44] prepared FPUF using DPPMA (melaminediphenylphosphinic acid salt) as a FR additive and studied the FR properties of the synthesized FPUF; they observed that even at low loadings of DPPMA, FPUF showed high mechanical strength and a self-extinguishing property, which indicates high FR activity with high LOI values. This is mainly because DPPMA follows a gas-phase FR mechanism. Some structures of P- and N-containing salts are shown in Scheme 4.3.

Scheme 4.3 Some P- and N-containing salts used as flame retardants. Reproduced with permission from [11, 44]. Copyright 2017, Elsevier Science Ltd

4.2 Nitrogen and Phosphorus-Based Salts as FRs

21

Chen et al. [45] synthesized phosphorus-containing melamine salts (carboxyethyl (phenyl) phosphinic acid melamine salt, CMA; melamine phosphite, MPOA; melamine pyrophosphate, MPyP; melamine hypophosphite, MHPA) of different valencies and structures and used them to prepare FR PU foams. They observed that with an increase in the P-oxidation level in the melamine salt, the FR activity decreased; this is because lower oxidation states in P favored gas-phase FR activity, while higher oxidation states favored condensed-phase mechanism and intermediate oxidation states showed a combination of gas- and solid-phase FR activity (Scheme 4.4).

Scheme 4.4 Schematic diagram of the proposed thermal degradation mechanism and FR activity model of CMA. Reproduced with permission from [45]. Copyright 2014, Elsevier Science Ltd

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4 Types of Flame Retardants Used for the Synthesis of …

Most nitrogen- and phosphorus-based salt-containing polymers exhibit IFR activity. Unfortunately, these IFR-active substances face some problems, such as moisture sensitivity and poor compatibility with polymers, even though APP is soluble in water. Therefore, to eliminate these limitations, a microencapsulation technique was adopted. Microencapsulated di-ammonium hydrogen phosphate (DAHP)-containing PUs were coated on textiles and it was found that they exhibited improved compatibility and IFR activity; further, flame dripping was decreased compared to virgin PU coatings [46, 47]. Microencapsulated APP/PU resins showed significant improvement in thermal and FR activity compared to normal APP/PU resins [48]. Microencapsulated ammonium polyphosphate (MCAPP) was introduced into PUs at different percentages and it was found that 30% MCAPP led to a V-0 rating in the UL-94 test and no dripping [49]. Using microencapsulation, microscopic amounts of additives can be incorporated in polymers. Polyoxymethylene (POM) is considered an excellent engineering plastic material as it exhibits high mechanical strength as well as self-lubrication and high wear resistance. It is therefore used in several applications, but it faces the problem of high flammability (low LOI value of 15%). Typically, FR polymers can be prepared by the addition of FR additives, but this strategy is not applicable for POM polymers as they exhibit a highly crystalline structure, which renders them incompatible with FR additives. Many American and Japanese companies are involved in the preparation of POMs with different FR additives, such as APP, MP, and triazine; they could achieve high LOI values along with self-extinguishing ability, but the mechanical properties of the resultant POMs are compromised [50, 51]. Studies on TPU spherical particles, which are compatible with POM (through formation of H-bonds) have been undertaken to overcome these limitations [52–55]. From the above discussion, it is clear that it is necessary to develop methods to improve FR compatibility to improve the FR activity of POM polymers.

4.3

Phosphorus-Based FRs

The possibilities for the application of phosphorus-containing FRs are versatile because elemental phosphorus can exhibit flexible oxidation states; however, most FR phosphorus compounds are in the 0, +3, and +5 oxidation states. Most phosphorus-based FRs exist in the form of organic and inorganic compounds, such as phosphines, phosphonium compounds, phosphine oxides, phosphonates, elemental red phosphorus, phosphites, and phosphates. Likewise, most inorganic phosphorus FRs contain phosphates in their structures. Among them, red phosphorus with an oxidation state of zero has a unique structure and is often used as a FR additive in polymers. Compared to normal red phosphorus, the stabilized form of coated red phosphorus is effective with oxygen-containing polymers, such as PUs, polyesters, and polyamides. In 1965, Piechota, for the first time, synthesized FR PUs using red phosphorus and observed excellent FR activity [56]. Later, Peters found that red phosphorus can show FR activity even in oxygen-containing

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polymers [57]. However, they did not analyze smoke and toxic gas evolution. Other types of basic phosphorus, such as white phosphorus, are pyrophoric materials, which can show FR activity; they can undergo structural reconfiguration from one shape to another. Unconstrained organophosphorus FRs have attracted great attention in polymer chemistry. These phosphorus-based FRs are classified into two types, reactive type and non-reactive type. Red phosphorus is the best example of additive FRs; it is non-reactive in nature and is widely used for FR polymer synthesis [58, 59]. Reactive phosphorus-based FR compounds are incorporated in polymers as chain extenders or monomers and hence they become the part of the polymer. Reactive organophosphorus FRs exists in different forms based on their structure and phosphorus oxidation states.

4.3.1

DOPO-Based Phosphorus FRs

Polymer materials are being used in a variety of applications; however, most polymers are highly flammable and the fire hazards associated with them can lead to loss of life and property. Consequently, it is required to develop new synthesis methods and techniques for low-cost highly efficient FRs to enhance the FR activity of the synthesized polymers. Normally, FR polymers can be synthesized by introducing phosphorus oxide FR-derivative compounds in their formulations, which decreases the flammability of the polymer and toxic gas evolution. Therefore, many research institutes, primarily American, are focusing on the reactivity of different phosphorus oxides to develop efficient FRs for aircraft applications. Organic phosphorus compounds are popularly used for FR applications; these FRs exist in different structural forms, but the synthesis of P,C-containing compounds is difficult and expensive compared to P,O-containing compounds. Most of the existing phosphorus-based FR compounds are P–O bond-containing compounds and different organic moieties are attached to the oxygen. Sanko Chemical Co. Ltd. developed and patented several novel manufacturing methods for cyclic organophosphorus compounds in 1972 [59]. In general, these organophosphorus compounds were obtained by heating and condensation of orthophenyl phenol derivatives and PCl3. 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) was synthesized by reacting stoichiometric ratios of orthophenylphenol and PCl3 in the presence of ZnCl2 or AlC3 at 210 °C [60]. Among the various types of phosphorus-oxygen containing compounds, cyclic structured DOPO, which was first synthesized by Saito [61], has the best flame retardant properties. In recent years, the development of phosphorus-based FRs has generated much research interest, especially DOPO and its derivatives. Further, several methods have been developed for the synthesis of DOPO and its derivatives, with an aim to increase the phosphorus content and aromatic groups in the structure; some of these structures are shown in Scheme 4.5. Because DOPO is a phosphinate oxide compound, it contains P–H bonds and in solution easily converts P-OH bonds via

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tautomerization [62, 63]. Due to tautomerization, the phosphorus atom in DOPO is highly reactive towards nucleophilic and electrophilic compounds. A reaction between DOPO and P-benzoquinone leads to the formation of mono substituted P-benzoquinone, which is subsequently tautomerized to yield DOPO-DQ as the product [60, 64, 65]. Similarly, DOPO reacts with 1,4-naphthoquinone to form naphthal-DOPO, as shown in Scheme 4.5. Xiong et al. [67] synthesized DOPO derivatives, such as 4-[(phenylamino) methyl] phenol (P-Ph) and DOPO-containing mannich base type (P-DDS-Ph) (Scheme 4.5); using these FRs, they developed FR epoxy (EP) resins. It was found that the LOI values increased drastically when P-Ph was replaced with P-DDS-Ph in the epoxy resins. Jeng et al. [70] prepared different epoxy polymers with aromatic and aliphatic phosphorus-containing compounds and reported that the aromatic groups increased the LOI values and thermal stability of the epoxy resins. LOI values, vertical burning test results, and cone colorimeter test results are reflective of the FR activity of a polymer and these are highly dependent on the chemical structure of the used phosphorus-based FRs. One particular DOPO-derivative FR is DOPO-PEPA, which is synthesized by the reaction between DOPO and 1-oxo-4-hydroxymethyl-2,6,7-trioxa-1-phosphabicyclo[2.2.2] octane (PEPA) [71]. The epoxy/DOPO-PEPA system exhibited efficient FR activity

Scheme 4.5 Structures of some DOPO derivative-based flame retardants containing hydroxyl and amine functionalities. Reproduced on the basis of information available in [58, 66–69]

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compared to epoxy/DOPO and epoxy/PEPA, owing to the synergism between DOPO and PEPA. These FR compounds can be incorporated in the polymer structure as chain extenders or as co-monomers. DOPO was introduced into polymeric diols and reacted with diisocyanates under suitable conditions to prepare PUs for FR applications [72]. Phosphorylation of poly(epichlorohydrin) (PECH) was conducted by reacting the P–H bond of DOPO with the chloromethyl group of PECH; it resulted in the formation of hydroxyl-containing PECH as the product. These hydroxyl groups are capable of reacting with diisocyanates to form phosphorus-containing PUs, as shown in Scheme 4.6. FR-PU polymers were synthesized using DOPO-phosponamidates and their FR activity was compared with that of various commercial available FRs, such as TCCP (tris (chloroisopropyl) phosphate) and DOPO. They observed that the thermal and FR properties of DOPO-phosponamidates are superior compared to those of commercial FRs [73]. Wang et al. [74] synthesized triazole-containing DOPO derivatives and used them as curing agents for epoxy resins; the resultant epoxy resins exhibited high thermal and FR properties with high LOI values and a V-0 rating in the UL-94 test. Similarly, different curing agents based on DOPO derivatives containing –OH and –NH2 functionalities were synthesized and used either as chain extenders or curing agents to prepare FR epoxy resins. The resulting epoxy resins exhibited a high Tg value, thermal stability, low thermal expansion, and excellent FR properties [66, 75–78]. Liu et al. [79] prepared a series of PU foams with different DOPO derivatives along with various P-oxides as FR additives; the DOPO-derivativecontaining PU foams displayed high thermal stability and FR activity with low smoke production and high char residue formation. DOPO in combination with hyper-branched polyamide (HBPA) exhibited excellent IFR activity as shown in Fig. 4.1; the ·PO radicals released by DOPO can interrupt coating degradation, while HBPA induces the formation of a strong crosslinked char layer and APP (APP incorporated along with DOPO and HBPA) releases non-flammable gases. HBPA exhibits good expandability and plays an important role as a carbonization and blowing agent as it contains large amounts of C and N atoms, which can increase the IFR activity by improving the quality of the char layer.

4.3.2

Phosphorus-Based FRs with Different Oxidation States

Most phosphorus derivatives and phosphorus-containing polymers show FR activity by virtue of the presence of phosphorus. During burning, they can form phosphorus and phosphoric acid; phosphoric acid further esterifies and dehydrates the polymer, resulting in the formation of a protective carbonaceous char layer on surface of the polymer substrate. This char layer acts as a heat-resistant layer at high temperatures and it also protects the substrate from oxygen and heat transfer [81]. Previous studies have shown that phosphorus-based FRs show efficient FR activity

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Scheme 4.6 Synthesis of DOPO-based polyurethanes. Reproduced with permission from [72]. Copyright 2002, John Wiley Ltd

via a condensed-phase mechanism; a char layer is produced during polymer combustion. Char formation and crosslinking are attributed to the strong phosphonic and phosphoric acids formed during initial pyrolysis. The studies also reported that the strength of the acids also influenced the FR activity (apart from the formation of char residue); phosphoric acid esterifies faster and forms a carbonaceous char layer more rapidly compared to phosphonic acid, due to it being a strong acid. Price et al. [82] studied the thermal and FR properties of polymers with different substituents containing phosphate and phosphonic derivatives; the FRs are linked to the polymer via covalent bonds. They observed that the derivatives of phosphorus acid-containing copolymers exhibit stronger FR activity than derivatives of phosphoric acid-containing copolymers. Lorenzetti et al. [83] studied the FR mechanism of PU foams containing phosphorus oxides of various oxidation states. Lower oxidation-state P-oxides exhibited a combination of gas-phase and solid-phase FR activity, whereas high oxidation-state P-oxides exhibited only the solid-phase mechanism. Further, Yang et al. [84] systematically investigated the thermal and FR properties of polyesters containing melamine P-oxides of various oxidation states; with a decrease in the oxidation state, the thermal stability of the polyesters decreased, but their FR activity increased. At lower oxidation states, P-oxides exhibit a combined gas-phase and solid-phase mechanism by diluting the combustible gases and releasing non-flammable gases, such as CO2, CO, and NH3. Several aryl groups containing different P-based compounds of different oxidation states are shown in Scheme 4.7. Some aromatic substituent aryl phosphate and phosphonate derivative-containing polymers show high FR and char residue formation compared to aliphatic substituent methyl phosphates and phosphonatecontaining polymers, as the extra crosslinking possible with aryl groups restricts the release of volatile products.

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Fig. 4.1 A schematic of the intumescent flame-retardant mechanism of PP/DP7/A10H. Reproduced with permission from [80]. Copyright 20177, Elsevier Science Ltd. PVFA, polyvinylformal acetal; DOPO, 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide; HBPA, hyperbranched polyamide

Braun et al. [87] studied the FR activities of epoxy resins containing phosphorus oxides of different oxidations states; the order of FR is as follows: EPC (epoxy resin composite)-Ar3PO > EPC-Ar2PO2 > EPC-ArPO3 > EPC-PO4. Therefore, on the basis of bond strength, EPC-Ar3PO shows better FR activity than EPC-PO4. Phosphorus atoms are released faster from EPC-Ar3PO compared to EPC-PO4; further, EPC-Ar3PO exhibits gas-phase FR activity. However, the researchers did not consider the effect of aromaticity or crosslinking on the FR activity. They also studied the FR activity of 4,4′-diaminodiphenyl sulfone-containing epoxy resins (EPC-DDS). The LOI of EPC-PO4 is lower than that of EPC-DDS due to the presence of –SO2 groups, which accelerate decomposition in the condensed phase and dilute the fuel available in the flame zone. Cheng et al. [88] prepared epoxy resins with glycidyloxy diphenylphosphine oxide (GDPPO) as the phosphoruscontaining chain extender. As the GDPPO content increased in the epoxy resins, their thermal stability and FR activity decreased, even though the phosphorus content in the resins increased. Owing to a high gel fraction and less crosslinking,

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Scheme 4.7 Phosphorus-containing (phosphine, phosphonate, and phosphate oxide-based) hydroxyl compounds and amine-containing structures. Redraw on the basis of information available in [85–87]

the formation of char also decreased. This clearly indicates that crosslinking density affects the FR activity. Mariappan et al. [89] synthesized PUs and epoxy resins containing phosphorus oxides and phosphorus oxide derivatives of different oxidation states (Scheme 4.8). Their thermal properties and FR activity mechanisms were studied by TGA and cone colorimetry, respectively. They found out that phosphate-containing PUs show better FR activity, while phosphite oxide compounds are more suitable for epoxy resins. This is because phosphites react with epoxy resins during combustion via transesterification and form ester products; however, in the case of polyurea, such kinetic reactions are not possible. Therefore, in the case of polyurea, phosphorus oxides of high oxidation states show good FR activity. Unsaturated polystyrene (PS) resins were modified with different a phosphorus-containing monomer (PDPA); when the PDPA content in the resin increased, the char yield and LOI values increased with a decrease in the HRR and THR.

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Scheme 4.8 a Synthesis of polyurea and epoxy resins with aryl phosphorus compounds of various oxidation states. b Triphenylphosphite (TPPi), c triphenylphosphate (TPPa), and d triphenylphosphine oxide (TPPO). Reproduced with permission from [89]. Copyright 2013, Elsevier Science Ltd

4.3.3

Phosphazene-Based FRs

Phosphazenes contain P and N in their structure and hence exhibit good FR activity; furthermore, they can possibly exhibit a synergetic effect to enhance the FR activity. Therefore, phosphazenes have been utilized for the synthesis of FR polymers and copolymers; they are introduced into polymer backbone via chemical bonding. These phosphazenes exhibit good physical properties, which favor an increase in the FR activity, such as high flexibility [90], thermal stability [91, 92], high LOI values [93–95], low smoke-releasing property [96], and flame resistance [93, 95, 97, 98]. However, these compounds are not much commercialized due to their high cost compared to halogenated FRs. The chemical structure of phosphazene includes – P=N– repeating units, either in a linear or cyclic form. Therefore, it shows high thermal stability, FR activity, and self-extinguishing ability. Allcock [99, 100], in their review on the synthesis and applications of poly(phosphazene) and their derivative compounds, reported that poly(phosphazene) compounds are generally synthesized from hexachloro cyclotriphosphazenes by ring-opening polymerization to form linear, branched, or cyclic structures; the chlorine atoms in these structures are highly susceptible to nucleophilic substitution. Therefore, –Cl can be replaced with a variety of substituents such as –OH, –NH2, aryl, and vinyl groups; some examples are shown in Scheme 4.9.

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Scheme 4.9 Structures of cyclophosphazenes containing –OH, –NH2, aryl, and vinyl groups. Redraw on the basis of information available in [108, 110, 114, 116]

The compounds shown can also be used to synthesize FR PUs, as shown in Scheme 4.10 [101–103]. Different types of phosphazene-containing PUs were synthesized, which exhibited high FR activity and thermal stability [104–106]. Mathew et al. [101] synthesized various cyanate derivatives from hydroxyl phenyl cyclotriphosphezenes and further prepared cross-linked network polymers with varying percentages of phosphazene and triazine. The resulting polymers displayed strong FR activity due to the high crosslink density and char residue formation. PUs have found major applications as fire-proofing additives in textiles, plastics, foams, synthetic fabrics, and rubbers. Dez et al. [107] synthesized PUs from derivative compounds of cyclotriphosphazene-containing hydroxyl groups and diisocyanates as shown in Scheme 4.10, and they observed that the prepared PUs exhibited high thermal stability and heat resistance. Most of the reported studies indicate that cyclotriphosphazene derivative-containing polymers exhibit excellent FR activity without dripping; however, Dez et al. [107] did not study the FR activity. Chen-Yang et al. [108] synthesized a reactive flame retardant EPPZ [(4-diethoxyphosphoryloxyphenoxy) (4-hydroxyphenoxy) cyclotriphosphazene] and used it as a FR to prepare FR PUs. The results indicate that with increasing EPPZ content in the PUs, the tensile strength, LOI values, and char yield improved. Further, the EPPZ-containing PUs showed excellent anti-dripping and flameextinguishing ability. Similarly, Yuan et al. [109] prepared diphenylphosphoryloxyphenoxy)(4-hydroxyphenoxy)cyclotriphosphazene (PPPZ)-containing PUs and studied their thermal, mechanical, and FR properties. The results revealed that the LOI values significantly increased with increasing PPPZ content in the PUs due to the aromatic groups in PPPZ containing cyclophosphazenes. Although most phosphorus-containing FR PUs are thermally less stable than phosphorus-free PUs, PPPZ-PUs are thermally more stable than the corresponding normal PUs. A novel

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FR, hexa(phosphazene aminophenoxyl)cyclotriphosphozene (HPAPC) was synthesized, which contains phosphazene combined with DOPO, and it showed efficient FR activity. Polylactide (PLA) FR composites were prepared by melt blending with different percentages of HPAPC and their thermo-mechanical, FR, and other properties were evaluated [110]. It was observed that even at low weight percentages (5%), PLA FRs showed high FR activity with high LOI values and V-0 rating in the UL-94 test. Reactive phosphazene-containing epoxy FR resins (PPCPT) were prepared and cured with different amine compounds, including 4,4′diaminodiphenylsulfone (DDS); dicyanodiamide (DICY); 3,4′-oxydianiline (ODA); 4,4′-diaminodiphenylmethane (DDM). The LOI values of 4-diethoxyphosphoryloxyphenoxy)(4-glycidoxyphenoxy)cyclotriphosphazene (PPCPT) with DICY were found to be high, owing to the synergetic effect of P and N in DICY and PPCPT [111]. DOPO-based triazole (DTA) was synthesized and used to prepare FR epoxy polymers and it was observed that even at low weight percentages (4 wt%), the DTA-containing epoxy resins showed a high LOI value of 34.8% and achieved a V-0 rating in the UL-94 test [74]. Because of the blowing-out effect, the addition of DTA drastically changed the combustion behavior of the resins; flame dripping disappeared completely, the ignition time decreased, and the resins exhibited self-extinguishing ability, as illustrated in Chap. 6. Yang et al. [112] synthesized rigid PU foams using reactive HPHPCP (hexa (phosphite-hydroxylmethyl-phenoxyl) cyclotriphosphazene) as a FR additive and they observed a great improvement in the FR and thermal properties of the resultant foams. Because HPHPCR contains a large number of functional groups, it increases the crosslinking density and hence the FR activity. Liu et al. [113] prepared DGEBA/PN-EPC thermosets cured with different FR hardeners, such as DDM, DICY, and MeTHPA. In the presence of DICY, the epoxy resins showed high FR activity compared to other systems due to a possible synergetic effect. Melt behavior and crystallinity also play important roles in controlling the char formation and dripping. Similarly, in the presence of PN6 and PPPMS, poly(ethylene terephthalate) (PET) showed improved melt viscosity, char yield, and anti-dripping nature during combustion [114]. Mathew et al. [101] synthesized different cyanato derivatives from hydroxyl phenyl cyclotriphosphazenes and further used them to prepare cross-linked polymer networks with varying percentages of phosphazene and triazine. The resulting polymers showed high FR activity with increasing crosslink density and char residue in the condensed phase. Huang et al. [115] synthesized NHPE from hexachloro phosphazene and HEMA and prepared FR-PU coatings using a UV-curable technique; these coatings exhibited high thermal stability and FR activity.

4.3.4

Phosphorus-Based Spiro Cyclic FRs

Pentaerythritol (PER) is considered as one of the best char-forming agents and it has been used in the synthesis of P-based caged bicyclic compounds, as shown in Scheme 4.11; these compounds are later used in the synthesis of FR-PUs. Because

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Scheme 4.10 Synthesis and chemical structures of cyclophosphazene-containing PU polymers. Reproduced with permission from [107]. Copyright 1999, Elsevier Science Ltd

compounds containing –OH and –NH2 groups are capable of reacting with isocyanates to form FR-PUs, they become part of the polymer main chain. Reaction between POCl3 and PET results in the formation of a basic caged bicyclic compound PEPA [117], as shown in Scheme 4.11. Similarly, Ou and his team [118] synthesized a caged spirocyclic structure of tri(2,6,7-trioxa-1-phosphabicyclo [2,2,2] octane-1-oxo-4-methanol)phosphate (trimer of PEPA), as shown in Scheme 4.11. The high phosphorus content and symmetrical caged structure offer excellent thermal stability and FR activity. Li and Ou [119] reported that the trimer showed superior FR activity than PEPA. They used the trimer as a FR additive for polyolefins and ethylene vinyl acetate copolymers (EVA) and found that the trimer-containing polymers exhibited excellent FR activity due to the high phosphorus content and structural symmetry. Phosphorus-containing carbonizing agents (CA) (Scheme 4.11) were synthesized from PEPA and they exhibited excellent IFR activity via a condensed-phase mechanism. Some studies reported that the presence of metal-containing compounds improved the IFR activity of polymers [120–122]. Xie et al. [123] briefly studied the FR activity of polyethylene using different types of metal chelating agents and the correlation between the metal compounds and IFR activity of polyethylene. They found out that metal chelates convert the more stable alkyl peroxy radicals, strongly accelerating the formation of char. It was also confirmed that the presence of aromatic chelating compounds improved the FR activity, but the effect of specific metals on the FR activity was not analyzed. DDP was synthesized from ditrimethylolpropan (Di-TMP), POCl3 and 2-amino ethanol [124], and used as a FR additive for FR fabric coatings; the coated fabrics exhibited high FR activity compared to uncoated fabrics. P,N-containing caged bicyclic phosphate (P-N IFR) was synthesized and used as a FR additive to prepare FR

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Scheme 4.11 Structures of some caged bicyclic phosphorus, phosphate, and phosphonate oxide derivative compounds for flame retardant applications. Redraw on the basis of information collected from [130, 131]

polybutylene terephthalate (PBT) [125]; the resultant polymers showed V-1 rating in the UL-94 test and anti-dripping characteristics. At the same FR weight loading, PU polymers achieved V-0 rating in the UL-94 test. FTIR analysis revealed that P–N present in the char improve intumescent char strength and formation at high temperatures. Similarly, benzonitrile-containing bicyclic phosphates (PDN) were synthesized and used as FR additives for epoxy polymers; they exhibited strong FR

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34

activity and self-extinguishing ability. Lai et al. [126] synthesized PETBP and used it alone as an IFR to prepare FR-PP polymers with varying contents of PETBP. They observed that 25 wt% PETPB-containing PP showed a high LOI value and a V-0 rating in the UL-94 test; further, this polymer exhibited low THR and PHRR. A novel phosphorus and sulfur-containing caged bicyclic FR compound, BCPPS, was synthesized and used with PP polymers; a high FR activity with a high LOI value and V-0 rating in the UL-94 test was observed [127] as the presence of sulfur led to an increased FR activity. Fullerenes (C60) are reported to be good flame retardants acting via a gas-phase mechanism; they delay the thermal oxidative degradation of PP polymers [128]. Song et al. [129] synthesized a novel P–N containing IFR, where PDBPP was introduced along with fullerenes into the polymer. They observed that C60-d-PDBPP-containing PP exhibited reduced thermal degradation, high initial ignition temperature, and a significantly low PHRR compared to neat PP. The researchers also commented on the dual-phase activity of the FR additive; fullerenes (C60) exhibited a gas-phase mechanism at the same time as PDBPP displayed a condensed-phase mechanism, which induced excellent FR activity.

4.4

UV-Curable FRs

UV curing technology is an emerging technology in polymer science; in this process, the unsaturated parts of the monomers are polymerized in the presence of incident UV radiation. Compared to traditional solvent-cured films, radiation-cured polymer resins exhibit certain advantages, such as requiring less energy, rapid and efficient curing, selective part curing, and no environmental pollution, which means that no solvent is eluted during curing. Hence, this technology is being used increasingly in many fields, such as the coating industry, microelectronics, and for the protection of different material surfaces. However, there are several disadvantages including the need for a UV source and oxygen inhibition; molecular oxygen is highly active in terminating polymerization. However, a number of methods have been developed to utilize oxygen scavengers and high radiation intensity to overcome these drawbacks [132]. During UV curing, only the unsaturated parts of acrylic oligomers are involved in polymerization. The acrylic derivative compounds treated using this method are polyester acrylates, epoxy acrylates, polyether acrylates, silicon acrylates, and urethane acrylates [133–136]. Urethane acrylate oligomers exhibit excellent properties, such as high impact and tensile strength, abrasion resistance, toughness, and durability. Therefore, UV curable PUs are used in many applications, including composite wood, plastics, and ceramics. Some acrylate oligomer structures are shown in Scheme 4.12; these structures can be used to synthesize FR PUs. Chen et al. [137] synthesized UV curable FR-PUA coatings by introducing different percentages of N-PBAAP (Scheme 4.13) and studied their thermal and FR properties. It was observed that increasing the N-PBAAP content in PUA coatings significantly decreased the THR and HRR values but increased the

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Scheme 4.12 Structures of some FR additives used to prepare UV-curable FR polymers. Redraw on the basis of information collected from [115, 138, 140, 143, 144, 153]

Scheme 4.13 Preparation of UV-curable FR PUA-PBAAP polymer films. Reproduced with permission from [137]. Copyright 2010, Elsevier Science Ltd. A new UV-reactive monomer piperazine-N,N′-bis(acryloxyethylaryl-phosphoramidate), N-PBAAP

char residue when compared to pure PUA coatings. Another important observation was that lower percentages of N-PBAAP in PUA coatings favored a gas-phase FR mechanism, while higher percentages favored a solid-phase FR mechanism. This is

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because high percentages of N-PBAAP in PUA coatings induce high crosslinking density, which are responsible to inhibit the formation of volatile substrates into the gas phase but they did not discuss about LOI values. Further, Chen et al. prepared EA/N-PBAAP UV curable resins with varying N-PBAAP contents and observed a significant IFR activity with high char residue even at high temperature. This result indicates that N-PBAAP shows greater synergetic and intumescent activity with EA compared to PUA. Similarly, Qian et al. [138] prepared acrylate resins containing different percentages of EBAPP and observed that 20 wt% EBAPP-containing resins showed higher LOI values than 30 wt% EBAPP-containing resins. The 20 wt% EBAPP resin exhibited a combination of gas-phase and solid-phase FR activity, whereas the 30 wt% EBAPP resin followed only the condensed-phase mechanism. However, increasing the EBAPP content in the resin drastically reduced the THR and HRR due to an increase in the quantity of non-flammable gases released into the flammable zone resulting in a dilution of the hot atmospheric zone and cooling it. NPHE-PU acrylate films were synthesized at different NPHE contents; with increasing NPHE content, the LOI value increased (the highest achievable LOI value was 27%) and the resins exhibited excellent thermal, mechanical, and FR properties [115]. UV-curable IFR resins were prepared at different weight loadings of N-PBAAP content and it was observed that increasing the N-PBAAP content in the resin highly decreased the THR and HRR; on the other hand, the char residue formation increased [139]. Further, these resins exhibit a strong synergetic effect, which contributed to their high IFR activity. EA/DOPO-HEA epoxy acrylate resins containing different weight loadings of DOPO-HEA were prepared and it was found that increasing the DOPO-HEA content in the polymer resin decreased the THR and PHRR and increased the LOI value and char residue [65]. Most DOPO derivatives follow the gas-phase FR mechanism and hence display a high FR activity. Chen et al. [140] synthesized a novel phosphorus skeleton compound (PEPA), which showed a high photo response and polymerized to form a cured film. These cured films exhibited high FR activity; their LOI value was 39%, while the char yield was 53% at 600 °C. Liang et al. [141] prepared UV-curable FR polymer films from two different phosphorus acrylate monomers containing TAEP and DAEEP and they systematically demonstrated the correlation between thermal stability and degradation mechanism and the FR activity. Crosslinking density, phosphorus content, and thermal degradation mechanism are three important factors that influence the FR activity of polymer resins. TAEP films show high thermal stability and higher LOI values than DAEEP even at lesser P contents due to the higher density of crosslinking and lesser evolution of small volatile products into the flame zone. Another important observation in the case of DAEEP curable films is that during thermal degradation, small volatile products (C2H4) are generated from P-O-C2H5 and are highly flammable. Therefore DAEEP showed lesser FR activity and lower LOI values compared to TAEP curable polymer films [142, 143].

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Phosphorus-containing epoxy resins are thermally less stable, whereas phosphorus/silicon-containing epoxy resins exhibit high thermal stability and a synergetic FR activity; therefore, they are mainly used in advanced electronic and electrical applications. Epoxy resins containing different percentages of phosphorus and silica were prepared and they were found to exhibit a high char yield. This is because phosphorus enriches char formation and silica protects the char from thermal degradation [144, 145]. The highest LOI value achieved with such epoxy resins as 42.5%. Neisius et al. [146] prepared a series of FR PU foams by introducing different substituted phosphoramidate FR additives and studied the relationship between the structure and FR activity of these PU forms. They observed that methyl-substituted phosphoramidates show high FR activity compared to phenyl-substituted phosphoramidates, while mono-substituted phosphoramidates showed higher FR activity than di- and tri-substituted phosphoramidates. The quantity of phosphorus is greater in methyl-substituted phosphoramidates and hence they showed higher FR activity than other phosphoramidates. Chen et al. [147] synthesized a novel high UV reactive DGTH and polymer films cured with DGTH showed excellent FR activity with a high LOI value 48%, which is much higher than the LOI values of cured films containing phosphorus alone. This is because there is a synergetic effect between P and N and also P and Si. Further, Chen et al. prepared DGTH-containing SPUA curable films and studied their FR properties in terms of their LOI values and using cone calorimetry [148]. They observed that 2.5 wt% DGTH-containing films displayed a high LOI value of 41% and a further increase in DGTH content beyond this value decreased the LOI values. Films with high DGTH contents, during burning, resulted in a poor char yield owing to the generation of a large amount of nitrogen volatiles. However, the researchers did not investigate the FR activity mechanism of these resins. Phosphorus and silica-containing polymers displayed a synergetic FR activity through a condensed-phase mechanism [149, 150]. Therefore, at increasing DGTH content, it may be possible that the resins exhibit a condensed-phase mechanism and the LOI values decreased even though there was a synergetic effect. Various methods have been developed for the synthesis of phosphorus-containing PUs and PEA through the grafting of phosphorus groups onto the oligomer backbone [139, 151, 152]. Unsaturated phosphorus was included on skeleton compounds and used to synthesize UV curable resins with high IFR activity, thermal stability, and char yield. Phosphorus-containing acrylate DOPO derivatives (DOPO-AC and DOPO-HEA) were cured by UV; they exhibited both gas-phase and condensed-phase FR activity with high LOI values compared to pure EA [65, 139]. Hyper-branched UV-cured FR PUs were prepared and they showed high mechanical and thermal stability owing to a high crosslinking density, which is also responsible for the improved FR activity [151].

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121. J. Holcik, M. Kosik, A. Benbow, C. Cullis, The oxidative thermal degradation of polypropylene and the influence of transition metal chelates. Eur. Polym. J. 14, 769–772 (1978) 122. A. Benbow, C. Cullis, H. Laver, Effects of metal chelates on the oxidation of polyolefins at high temperatures. Polymer 19, 824–828 (1978) 123. F. Xie, Y.Z. Wang, B. Yang, Y. Liu, A novel intumescent flame-retardant polyethylene system. Macromol. Mater. Eng. 291, 247–253 (2006) 124. W. Jiang, F.L. Jin, S.J. Park, Synthesis of a novel phosphorus-nitrogen-containing intumescent flame retardant and its application to fabrics. J. Ind. Eng. Chem. 27, 40–43 (2015) 125. F. Gao, L. Tong, Z. Fang, Effect of a novel phosphorous–nitrogen containing intumescent flame retardant on the fire retardancy and the thermal behaviour of poly(butylene terephthalate). Polym. Degrad. Stab. 91, 1295–1299 (2006) 126. X. Lai, S. Tang, H. Li, X. Zeng, Flame-retardant mechanism of a novel polymeric intumescent flame retardant containing caged bicyclic phosphate for polypropylene. Polym. Degrad. Stab. 113, 22–31 (2015) 127. H.Q. Peng, D.Y. Wang, Q. Zhou, Y.Z. Wang, An S- and P-containing flame retardant for polypropylene. Chin. J. Polym. Sci. 26, 299–309 (2008) 128. Z. Fang, P. Song, L. Tong, Z. Guo, Thermal degradation and flame retardancy of polypropylene/C60 nanocomposites. Thermochim. Acta 473, 106–108 (2008) 129. P.A. Song, H. Liu, Y. Shen, B. Du, Z. Fang, Y. Wu, Fabrication of dendrimer-like fullerene (C60)-decorated oligomeric intumescent flame retardant for reducing the thermal oxidation and flammability of polypropylene nanocomposites. J. Mater. Chem. 19, 1305–1313 (2009) 130. J.-P. Hu, D. Li, Y. Qin, X.-Y. Wang, Promotion effect of melamine on flame retardancy of epoxy resins containing caged bicyclic phosphate. Chin. J. Polym. Sci. 25, 581–588 (2007) 131. Y. Halpern, D.M. Mott, R.H. Niswander, Fire retardancy of thermoplastic materials by intumescence. Ind. Eng. Chem. Prod. Res. Dev. 23, 233–238 (1984) 132. K. Studer, C. Decker, E. Beck, R. Schwalm, Overcoming oxygen inhibition in UV-curing of acrylate coatings by carbon dioxide inerting, Part I. Prog. Org. Coat. 48, 92–100 (2003) 133. H. Kim, M.W. Urban, Molecular level chain scission mechanisms of epoxy and urethane polymeric films exposed to UV/H2O. Multidimensional spectroscopic studies. Langmuir 16, 5382–5390 (2000) 134. V. Kumar, Y. Bhardwaj, N. Goel, S. Francis, K. Dubey, C. Chaudhari, K. Sarma, S. Sabharwal, Coating characteristics of electron beam cured Bisphenol A diglycidyl ether diacrylate-co-aliphatic urethane diacrylate resins. Surf. Coat. Technol. 202, 5202–5209 (2008) 135. J.T. Yeh, Y.C. Shu, Characteristics of the degradation and improvement of the thermal stability of poly(siloxane urethane) copolymers. J. Appl. Polym. Sci. 115, 2616–2628 (2010) 136. F.S. Yen, L.L. Lin, J.L. Hong, Hydrogen-bond interactions between urethane–urethane and urethane–ester linkages in a liquid crystalline poly(ester–urethane). Macromolecules 32, 3068–3079 (1999) 137. L. Chen, Q. Tai, L. Song, W. Xing, G. Jie, Y. Hu, Thermal properties and flame retardancy of an ether-type UV-cured polyurethane coating. Express Polym. Lett. 4, 539–550 (2010) 138. X. Qian, Q. Tai, L. Song, R. Yuen, Thermal degradation and flame-retardant properties of epoxy acrylate resins modified with a novel flame retardant containing phosphorous and nitrogen. Fire Saf. Sci. 11, 883–894 (2014) 139. L. Chen, L. Song, P. Lv, G. Jie, Q. Tai, W. Xing, Y. Hu, A new intumescent flame retardant containing phosphorus and nitrogen: preparation, thermal properties and application to UV curable coating. Prog. Org. Coat. 70, 59–66 (2011) 140. X. Chen, Y. Hu, C. Jiao, L. Song, Thermal and UV-curing behavior of phosphate diacrylate used for flame retardant coatings. Prog. Org. Coat. 59, 318–323 (2007) 141. H. Liang, W. Shi, Thermal behaviour and degradation mechanism of phosphate di/triacrylate used for UV curable flame-retardant coatings. Polym. Degrad. Stab. 84, 525–532 (2004)

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142. G. Matuschek, Thermal degradation of different fire retardant polyurethane foams. Thermochim. Acta 263, 59–71 (1995) 143. L. Jiao, H. Xiao, Q. Wang, J. Sun, Thermal degradation characteristics of rigid polyurethane foam and the volatile products analysis with TG-FTIR-MS. Polym. Degrad. Stab. 98, 2687– 2696 (2013) 144. Y.L. Liu, Y.C. Chiu, C.S. Wu, Preparation of silicon-/phosphorous-containing epoxy resins from the fusion process to bring a synergistic effect on improving the resins’ thermal stability and flame retardancy. J. Appl. Polym. Sci. 87, 404–411 (2003) 145. Y.L. Liu, C.S. Wu, Y.S. Chiu, W.H. Ho, Preparation, thermal properties, and flame retardance of epoxy–silica hybrid resins. J. Polym. Sci. Part A: Polym. Chem. 41, 2354– 2367 (2003) 146. M. Neisius, S. Liang, H. Mispreuve, S. Gaan, Phosphoramidate-containing flame-retardant flexible polyurethane foams. Ind. Eng. Chem. Res. 52, 9752–9762 (2013) 147. X. Chen, Y. Hu, C. Jiao, L. Song, Preparation and thermal properties of a novel flame-retardant coating. Polym. Degrad. Stab. 92, 1141–1150 (2007) 148. X. Chen, Y. Hu, L. Song, Thermal behaviors of a novel UV cured flame retardant coatings containing phosphorus, nitrogen and silicon. Polym. Eng. Sci. 48, 116–123 (2008) 149. G.H. Hsiue, Y.L. Liu, J. Tsiao, Phosphorus-containing epoxy resins for flame retardancy V: synergistic effect of phosphorus–silicon on flame retardancy. J. Appl. Polym. Sci. 78, 1–7 (2000) 150. C.S. Wu, Y.L. Liu, Y.S. Chiu, Epoxy resins possessing flame retardant elements from silicon incorporated epoxy compounds cured with phosphorus or nitrogen containing curing agents. Polymer 43, 4277–4284 (2002) 151. S.-W. Zhu, W.-F. Shi, Flame retardant mechanism of hyperbranched polyurethane acrylates used for UV curable flame retardant coatings. Polym. Degrad. Stab. 75, 543–547 (2002) 152. T. Randoux, J.C. Vanovervelt, H. Van den Bergen, G. Camino, Halogen-free flame retardant radiation curable coatings. Prog. Org. Coat. 45, 281–289 (2002) 153. Z. Bai, L. Song, Y. Hu, R.K. Yuen, Preparation, flame retardancy, and thermal degradation of unsaturated polyester resin modified with a novel phosphorus containing acrylate. Ind. Eng. Chem. Res. 52, 12855–12864 (2013)

Chapter 5

Flame-Retardant Polyurethanes

In 1937, the German scientist Otto Bayer and his team discovered poly addition reactions [1–3]; the poly addition of diols and diisocyanates forms PUs under suitable conditions and completely avoids the formation of undesired by-products. PUs can basically be prepared by two methods, of which the most often used method is the reaction between polyols and diisocyanates under appropriate conditions (Scheme 5.1). In a single-step process, the two monomers, polyol and diisocyanate, react together to form PU. Using this method, it may be possible to obtain linear PUs, given that the starting compounds are linear. Under similar conditions, in the presence of excess diisocyanates, a pre-PU polymer is formed. Subsequently, in the next step, a calculated amount of chain extender is added for the complete formation of PUs. Chain extenders contain hydroxyl, amine, or other functional groups capable of reacting with isocyanate groups [4, 5]. PUs are considered versatile polymeric materials and their physical and chemical properties depend on the choice of starting materials; further, their properties can be tailored by varying the composition of the monomers and monomer compounds. Accordingly, depending on the application, the required chemicals for PU synthesis can be chosen. Unfortunately, most PU polymers are highly flammable and release toxic smoke while burning. Therefore, it is important to study parameters that can improve the FR activity of PU polymers for customized applications. In this respect, melting temperature (Tm), glass transition temperature (Tg), viscosity, and degree of crosslinking are the important factors affecting the FR activity of a polymer. Most of the early established studies on segmented PUs suggest that segmental phase separation increases Tg and Tm; further, the Tg values of hard segments (HS) and soft segments (SS) are different [6–8]. The responsible factors are the –NCO/–OH ratio, the size of HS and SS domains, and reaction conditions [4, 9–13].

© Springer Nature Switzerland AG 2020 S. Sinha Ray and M. Kuruma, Halogen-Free Flame-Retardant Polymers, Springer Series in Materials Science 294, https://doi.org/10.1007/978-3-030-35491-6_5

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Scheme 5.1 General scheme of PU synthesis and representation of HS-SS blocks in the structure of PU

5.1

Thermal Degradation and Evolution of Components During PU Combustion

Even though a vast number of reports are available on the thermal decomposition, FRs, and smoke inhibition strategies of PU polymers, the relationship between thermal decomposition, FR activity, and toxic gas evolution is still not completely understood. However, such an understanding is necessary to enhance the FR activity and decrease toxic smoke evolution during PU combustion. Thermal stability and decomposition are important factors in analyzing the FR activity of a given polymer; thermal degradation reflects the break-down of the polymer into small airborne particles and gases when the material is exposed to flame or increased temperature. These degraded flammable components enter into the atmosphere, owing to which the material catches fire. However, when the tested polymers are thermally more stable and do not decompose even at high temperatures, no airborne particles are generated, which minimizes the flammability of the material. In some cases, polymers degrade rapidly to form a char layer, which can prevent further polymer degradation and prevent oxygen from coming into contact with the substrate. Thermo-gravimetric analysis (TGA) is a useful technique to evaluate the thermal stability of polymers. Many researchers reported that PUs start to degrade at around 180–200 °C in the presence of nitrogen and the degradation

5.1 Thermal Degradation and Evolution of Components During PU …

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temperature varies with PU structure [14]. For example, PUs used in industrial thermal insulating applications start to degrade at temperatures above 250 °C. A number of investigations have been carried out to understand the thermal degradation and stability of PUs in terms of various parameters, such as different polyols [15–21], diisocyanates [13, 17, 22–27], chain extenders [28–32], hard and soft segments [13, 23, 27, 33–35], –NCO/–OH ratio [31, 36–40], and crosslinking density [16, 18, 27]. Crosslinking density depends mainly on the molecular weight of the polyol, –NCO/–OH ratio, and number of reactive functional groups in the polyol and diisocyanate. A long carbon chain between hydroxyl groups indicates an increasing molecular weight of the polyol in PUs, which in turn increases the thermal stability of the synthesized PU [15]. It has also been reported that a large number of reactive functional groups containing polyol or diisocyanates in the PU polymer increases its thermal stability because of the high crosslinking density [41, 42]. A well-known and efficient method to enhance the thermal stability of PUs is to introduce hetero atom-containing cyclic compounds and chemical cross linkers, which can form 3D graphite-like structures, which prevent thermal degradation [43, 44]. A number of studies have been carried out on the structure of PU and it has been reported that during PU synthesis, micro-phase separation occurs between its hard and soft segments; this phenomenon also enhances the thermal stability of the polymer [45–50]. The aggregation of hard segments in the PU polymer represents the HS domains, which confer an ordered or semi-crystalline structure and stiffness to the PUs. Kuruma et al. [27] synthesized PUs from hydroxyl-terminated polybutadiene (HTPB) and HTPB-DNB (HTPB functionalized at the terminal carbon atoms with dinitrobenzene) with different diisocyanates and demonstrated an instantaneous increase in the tensile strength and elongation in HTPB-DNB-PUs. Strong hydrogen bonding interactions between the –NO2 groups of DNBs and the soft segments of the PU backbone resulted in a highly ordered crystalline structure and also a “fibrous-assembly” morphology, which is responsible for enhancing the thermo-mechanical properties of the synthesized PUs. Rao et al. [13] studied the nature and formation of HS domains in different modified HTPB-PUs using SAXS. They observed that domains of different sizes are produced in different PUs due to the differences in electrostatic interactions between the various triazine substrates and the PU backbone. Diols and diamines are used as chain extenders during the synthesis of PUs and polyurea, respectively. Compared to urethanes, the structure of urea is more polar and hence capable of forming more hydrogen bonds, resulting in an increase in the rigidity of the polymer, which promotes phase separation and higher thermal stability in polyureas compared to PUs [34, 35, 51]. Butane diol is widely used as a chain extender (CE) as it enhances phase separation; this is because it contains an even number of carbons. Further, it has been noticed that increasing branching in the CE structure reduces phase separation and the thermal stability of PUs [30, 52–54]. During PU synthesis, if the ratio of –NCO/–OH is greater than one; it means that in the presence of excess isocyanates, it is possible to form side products such as allophanate and biuret. In addition, isocyanate compounds can react with each other to form dimer uretdione and trimer isocyanurates [55, 56]; these isocyanurate linkages are capable of providing additional thermal

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stability to the PUs [40, 57–59]. Many studies indicate that compared to solvent-based PUs, water-dispersible PUs show less thermal stability due to the presence of thermally labile urethane and urea linkages [21, 60]. Kuruma et al. [61] reported that with increasing hard segment content in water-dispersible PUs, thermal degradation is accelerated in a particular temperature region due to the large number of labile urethane and urea linkages. The thermal stability of PUs also depends on various characteristic features of CEs, such as their polarity, +I effect, and presence of electron-donating groups [21, 62–64]. Coutinho et al. [21] synthesized PU dispersions using two different CEs, ethylene diamine and hydrazine, and noticed that hydrazine-containing PU dispersions exhibited high thermal stability than their counterparts containing ethylene diamine owing to the more polar nature of hydrazine. Qin et al. [64] synthesized PUs using two different aromatic diamines as chain extenders; the CEs contained sulfone and diphenyl ethers as functional groups. It was observed that the PUs prepared using sulfones exhibited higher thermal stability due to the more polar nature of sulfone groups. Chattopadhyay et al. [65] systematically evaluated the relationship between the thermal decomposition of different types of PUs and flammability. However, they did not discuss toxic gas evolution from PUs and halogenated FRs. The FR activity of polymers also depends on their Tm and Tg. Therefore, polymers with lower Tg values melt easily and catch fire easily but those with higher Tg values do not catch fire easily, which implies higher FR activity. Liu et al. [66] synthesized phosphorus-containing aryl alkyl novolac (Ar-DOPO-N) blended with phenol formaldehyde novolac and melamine-modified phenol formaldehyde novolac and used them as curing agents for o-cresol formaldehyde novolac epoxy resins; the resulting cured epoxy resins showed increased Tg values in both cases but in the case of melamine-modified phenol formaldehyde novolac, higher FR activity was observed. Tirumal et al. [67] evaluated the relationship between Tg, thermal stability, and density of PU foams. Thermal stability decreases with a decrease in PU foam density but the Tg increased. To increase the thermal stability of the polymers, PNCs with varieties of nanoparticles (NPs) have been prepared. However, transition metals are more advantageous in improving the thermal stability and FR activity of PUs. In their exhaustive investigation, Moroi et al. [68] analyzed the thermal decomposition of various transition metal ion-containing composite PU polymers. Generally, polymer combustion is accompanied by the release of CO and CO2 and if the polymer contains nitrogen, NO, NO2, NH3, and HCN may also be released. Chlorinated plastic materials can produce HCl and the hazardous gas phosgene (COCl2), while fluorinated plastics release HF [70]. Combustion of most polymer materials yields only a small quantity of HCN, but some polymers such as PUs, acrylates, polyamides, nitrocellulose, and other nitrogen-containing plastics produce high quantities of HCN when subjected to flame exposure. Under normal conditions, the toxicity of HCN by itself is not high, but when combined with other toxic gases, it is considerable [70–72]. During combustion, PU polymers may emit CO, CO2, NO, HCN, isocyanuric acid, isocyanates, hydrocarbons, amines, and other potential hazardous components [72–75]. The evolution of these degradation components and gases and their structures can be determined by TGA-Fourier

5.1 Thermal Degradation and Evolution of Components During PU …

51

transform infrared spectroscopy (FTIR) or TGA-mass spectrometry (MS). TGA-MS provides information on evolved gas fragment ions with their m/z values and intensities. Meanwhile, TGA-FTIR provides information on the functional groups in the decomposed components; further, any rearranged reaction intermediate compounds can also be analyzed. In general, PUs exhibit good physical properties, such as durability, abrasion resistance, chemical resistance, and self-lubrication and hence are considered industrially attractive polymers. Therefore, it is necessary to know the properties of the produced smoke and toxic gases when PUs are exposed to flame. Chlorinated phosphate esters are widely used in the preparation of FR PU foams but their disadvantage is that their usage may lead to high smoke and toxic corrosive gas content. Hence, it is necessary to study the FR mechanism of PUs in the presence of various additives and FRs, which can inhibit or decrease smoke and toxic gas evolution. Some additives effectively trap the volatile isocyanates evolved during the combustion of PU foams, which decreases smoke and toxic gas content. Price et al. [76] reported that smoke and toxic gas evolution from PU foams can be reduced using melamine. At high temperatures, melamine can react with the released isocyanates and suppresses smoke and toxic gas evolution [77]. McKenna and Hull et al. [78] systematically analyzed the release of toxic components from PU foams during combustion as well as the methods employed to assess their flame toxicity. They further studied the decomposition of PUs; at temperatures greater than 600 °C, volatile fragments of PUs release nitrogen-rich “yellow smoke,” which contains volatile nitrogen compounds, non-volatile polyols, polymerized isocyanates, and isocyanate droplets. The yellow smoke further decomposes into small volatile components, such as HCN, CH4, CO2, CO, and NO2 [78, 79]. From the practical viewpoint, a consideration of the associated fire hazards is important because they affect both human lives and property. PUs are highly flammable and release large amounts of smoke, heat, and toxic combustible gases; oxygen depletion, which occurs during burning threatens human lives. The two main causes of fire-related deaths are inhalation of toxic smoke and burns [80, 81]. Smoke development in flames is highly dependent on the structure of the gases, which act as the fuel, and the ratio of fuel to oxidant. Liu et al. [69] evaluated the thermal decomposition and smoke and toxic component evolution from PU materials (Fig. 5.1). They also studied the factors affecting smoke and toxic gas suppressants. Polymers consisting of only aliphatic structural units generate relatively less smoke compared to polymers containing aromatic groups in their backbone, but aliphatic polymers are more flammable than aromatic compounds. To understand smoke and toxic gas generation, one must know the chemical structure of the polymer and its thermal degradation characteristics. The FR activity mechanism of a polymer can be either gas-phase or condensed-phase, which can be determined using TG-FTIR and gas chromatography–mass spectrometry (GC–MS) [82]; these techniques can also be used to determine rearranged reaction intermediates and mechanisms [83, 84]. On the basis of the obtained results, it may possible to reduce smoke and toxic gas formation during polymer burning. A number of reports are available on smoke and toxic gas

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Fig. 5.1 Evolution of components and toxic gases during PU combustion [69]. Reproduced with permission from Elsevier Science Ltd

evolution from polymer subjected to combustion [85, 86], but the inclusion of new analytical methods, such as TGA-FTIR and TGA-MS, is not yet widely used. Recently, a number of studies have been carried out to understand the mechanism of smoke inhibition using various smoke inhibitors and FR additives [87–89]. Hastie et al. [90] studied smoke suppression and inhibition using various metal oxides and metal derivative compounds as additives. They found out that Ba, Sr, Mo, and W are highly efficient smoke inhibitors. Some metals, such as Fe and Mn, exhibit smoke inhibition activity in the form of carbonyl complexes; further, ferrocene is also considered a highly efficient smoke inhibitor [91, 92]. The surface area of NPs is important in smoke suppression because NPs with a large surface area can absorb large amounts of smoke and toxic gases. In this context, aluminum trihydroxide (ATH)-containing TPU composites and PU foams showed significantly high smoke suppression [93, 94]. Generally, red phosphorus is used as a FR for PU polymers [95], but it releases highly toxic phosphine in the presence of water during polymer combustion. This can be suppressed by using metal oxides, such as ZnO2, CdO, and CuO, as stabilizers in conjunction with red phosphorus; in the presence of these metal oxides, phosphine is converted into phosphoric acid, which activates char formation in the condensed phase [96, 97]. Levin et al. [97, 98] prepared flexible PU foams by adding 0.1% of copper dust, copper sulfate, and cupric oxide and they noticed a drastic reduction in HCN content by 80–90%; further, the toxicity of the resulting smoke reduced by 40–70%. Melamine PU foams release 10% more HCN than PU foams without melamine, but in the presence of Cu2O, HCN generation from melamine PU foams could be reduced by 90% [97, 98]. It was suggested that

5.1 Thermal Degradation and Evolution of Components During PU …

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due to the catalytic effect of Cu2O, HCN was oxidized and converted into CO2, N2, H2O, and NO2. This clearly indicates that the fire toxicity of FR PUs is extremely dependent on the FR used. Similarly, organic acid compounds also improve smoke suppression and limit the release of toxic gases. Doerge et al. synthesized PU foams using various organic acid compounds, such as fumaric acid, maleic acid, citric acid, and oxalic acid, and found that they act as excellent smoke suppression agents; they also decreased toxic gas evolution [99]. The evolution of smoke and toxic fumes depends on the polymer structure, its chemical and physical composition, and decomposition. Nitrogen-containing compounds, such as melamine, dicyandiamide, and urea seem to be the best FRs for PU foams. According to the burning rate and oxygen value, urea is a better FR additive for urethane foams compared to melamine and dicyandiamide [100]. Owing to the greater polarity of urea, it may lead to greater crosslinking density and thus decrease flammability. Studies indicate that APP (ammonium polyphosphate) or EG (expandable graphite) highly reduced the production of HCN, CO2, and CO during the thermal decomposition of PU polymers. Compared to PU/APP, PU/EG produced a high percentage of toxic gases with less char yield [101]. Jiang et al. [102] prepared epoxy composites with ZnS-decorated graphene sheets and studied their thermal decomposition behavior and composition of the evolved materials. They observed that composites containing 2 wt% ZnS/graphene sheets showed significant inhibition in toxic gas evolution when compared to individual ZnS and graphene sheet-containing epoxy composites and pure epoxy resin. This is particularly true in the case of CO gas formation; this phenomenon is attributed to the synergetic effect between ZnS and graphene nano sheets. On the basis of the above discussion, it can be concluded that in the presence of FR additives, the evolution of smoke and toxic gases, such as CO, CO2, and HCN, is reduced when compared to the case of blank polymers. Further studies are required on the production of smoke and toxic gases to optimize the conditions necessary to eliminate these aspects, especially because more people die due to toxic gas and smoke inhalation than burns.

5.2

Importance of Polyols in Flame-Retardant PUs

High-molecular-weight materials with hydroxyl groups at the terminal positions are called polyols; these are often used as raw materials for PU synthesis. These polyols with high molecular weight, high viscosity, and large number of reactive functional groups improve the FR activity of the resulting PUs. The properties of the synthesized PUs mainly depend on the molecular weight of the starting polyols, viscosity, and degree of crosslinking. These polyols may also contain different functionalities along with hydroxyl groups, such as esters, ethers, amides, acrylic, or other functionalities [103]. Papageorgiou et al. [104] synthesized aromatic and aliphatic ester copolymer diol-containing PUs. They noticed that the aliphatic part confers elasticity to PU; conversely, the tensile strength and Young’s modulus of the polymer decreased. Kashiwagi et al. [105] studied the effect of molecular weight

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of polyols on the FR activity of polymers and they observed that high-molecular weight-containing poly(methyl methacrylate) (PMMA)/silica nanocomposite resins showed high FR activity; furthermore, the melt viscosity increased with an increase in the molecular weight of PMMA. Poly(1,3-phenylene phenyl phosphonate) (PPP)-containing polyethylene terephthalate (PET) and polybutylene terephthalate (PBT) polymers of different molecular weights were also investigated; however, there was not much difference in their FR activities. The limiting oxygen index (LOI) values of low and high molecular weight PPPs were different; it may also be assumed that there were significant changes in the physical properties of high-molecular weight PPP [106].

5.3

Phosphorus-Based Polyols for Flame-Retardant Polyurethanes

Polyols are one of the main components for PU synthesis and the incorporation of phosphorus functionalities into polyols via covalent bonding [65, 107] offers several advantages, such as permanent attachment, homogeneous distribution, low smoke production, and FR efficiency at small quantities [36, 108–110]. FR PUs were prepared using phosphorylated polyols and the effectiveness of phosphorylated polyols as reactive additives for coatings was investigated. It was observed that at low phosphorus contents of *0.54 wt%, polyol PUs showed high LOI values (*30%) and the ignition time was delayed [111]. Phosphorus-based polyols were synthesized by a reaction between castor oil and tris(m-hydroxyl phenyl) phosphate (THPP) [112–115] using an ester exchange method; this polyol was further used to prepare FR-PU coatings with various diisocyanates, including aromatic and aliphatic diisocyanates [116]. The prepared coatings were used as coatings on mild steel surfaces and their FR properties, such as LOI values and UL-94 test performance were analyzed. Aromatic diisocyanate-containing PUs exhibited high LOI values and V-0 rating in the UL-94 test. However, it was not clear why aromatic PUs exhibited better FR activity compared to aliphatic PUs. Further, castor oil-based PU blends were prepared by introducing different percentages of THPP; the LOI values increased with THPP content and they could also achieve a V-0 rating in the UL-94 test because of the presence of aromatic rings. Polyester polyol PUs are well-known, but to prepare FR PUs, organo-phosphorus functionalities are introduced into the polyol structure. Polyester polyols were prepared from bisphenol-A and POCl3 at suitable reaction conditions [117] and used to synthesize polyester polyol PUs with different diisocyanates, as shown in Scheme 4.11, and the thermo-mechanical and FR properties of these coating materials were studied [118]. It was observed that TDI-based polyester PUs showed high LOI values and achieved a V-0 rating in the UL-94 test due to their aromatic nature [119]. Zhang et al. [120] prepared a castor oil-based FR polyol (COFPL) via the epoxidation of castor oil and ring opening reaction with triethyl phosphate. They prepared a series of PUs with varying percentages of COFPL and found that

5.3 Phosphorus-Based Polyols for Flame-Retardant Polyurethanes

55

the polymers exhibited FR activity even at low P contents (3%); the corresponding PU exhibited a high LOI value 24.3%. LOI values mainly depend on the crosslink density and P content. An increase in the crosslink density can improve the modulus and stability of the polymer, which also influence the FR activity. FR PU sealants were prepared from ricinoleic acid-containing P and N polyols and methylene diphenyl diisocyanate. The results indicated a great improvement in the thermal stability and FR activity along with high char yield and strong barrier properties; this is because the PHRR, THR, and toxic gas evolution decreased [121]. A variety of phosphorus-containing polyol FR polymers have been synthesized and they exhibited good FR properties [107, 110, 120, 121] (Scheme 5.2).

5.4

Importance and Reactivity of Diisocyanates in FR PUs

Isocyanates are one of the most important organic components in PU synthesis; they contain cumulative double bonds with N, C, and O and their structure is R–N=C=O; here, R is an aromatic, aliphatic, cyclic, or acyclic compound. Because

Scheme 5.2 General structure of phosphorus-based polyester polyurethanes with different diisocyanates. This scheme redraw on the basis of information available in [118]

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of the industrial importance of PUs, it is necessary to study diisocyanate chemistry to understand their properties and reactivity. Due to the presence of cumulative double bonds, isocyanates are highly reactive in forming urethane groups without any by-product formation. Most of the existing isocyanate compounds containing either two or more –NCO functional groups per one molecule are called as diisocyanates and polyisocyanates, respectively. An industrially important monoisocyanate compound is methyl isocyanate (MIC), which is primarily used for pesticide manufacture, but this isocyanate is extremely hazardous and is held responsible for the 1984 Bhopal disaster. Diisocyanates exist as either liquids or solids and are highly reactive through the double bond –N=C– of the –NCO group. Aromatic diisocyanates are more reactive than aliphatic diisocyanates because of the possible resonance structures and decreased electron density on the central carbon of isocyanates, which plays an important role in controlling their reactivity. Hence, aromatic diisocyanates are more reactive than their aliphatic counterparts and can exhibit a high degree of crosslinking, which is also an important factor in improving the FR activity of polymers. Therefore, the reactivity of diisocyanates also plays an important role in FR PU synthesis; diisocyanates can lead to the formation of dimers and trimers and can also undergo self-condensation [56]. In addition, a number of crosslinking reactions may take place depending on the reactivity of the monomers and reaction conditions, such as temperature, catalysts, and the structures of the participating alcohols, amines, and isocyanates. The commonly used diisocyanates are toluenediisocyanate (TDI), 4,4′-dicyclohexylmethane diisocyanate (H12MDI), 4,4′-methylene diphenyl diisocyanate (MDI), isophorone diisocyanate (IPDI), and hexamethylene diisocyanate (HDI) [65]. The isocyanates used most often industrially are TDI and IPDI; however, compared to TDI, IPDI has some advantages as it is not toxic, has average reactivity, and is aliphatic in nature. IPDI is widely used for the preparation of light-stable PU coatings. The two isocyanate groups of IPDI exhibit different reactivities because of differences in steric hindrance and chemical environment. Different types of diisocyanates contribute to PU properties in different ways; for example, compared to cycloaliphatic or aliphatic diisocyanates, aromatic diisocyanate and polyisocyanate-containing PUs show high FR activity [119].

5.5

Effect of Segmental Separation in PUs on Their FR Activity

PUs are used in a number of products we use in our day-to-day life, such as shoe leather, rubbers, adhesives, seat cushions, insulation for walls and roofs, automotive structural foams, refrigerators, automotive paints and coatings, textile coatings, and surface coatings due to their good physical properties such as flexibility, durability, impact resistance, and abrasion resistance [103]. PUs are a type of block

5.5 Effect of Segmental Separation in PUs on Their FR Activity

57

copolymers, composed of alternating hard segments and soft segments; the soft segments provide flexibility and hard segments provide strength to the derived PUs. Most of the soft segments are amorphous and exhibit Tg values lesser than room temperature, which results in polymer softening at room temperature; these soft segments are responsible for the elasticity of the PU polymer. All the hard segments are crystalline in nature and their Tg value is higher than the room temperature; they are responsible for the stiffness of the PUs [45]. Owing to the presence of HS and SS, PUs exhibits thermodynamic immiscibility and micro-phase separation. The properties of the polymer can be tailored by changing the concentrations and lengths of the hard and soft segments [55–57, 122–124]. Hydrogen bonding is also an important factor in PU synthesis, especially with relation to phase separation and phase morphology. If there exist a large number of hydrogen bonds between the two phases, the degree of phase mixing will be high. When the number is low, hydrogen bonds exist only in the hard segments, which increases crystallization and phase separation. Clough and Schneider [9] first introduced and reported the two-phase morphology of PUs using small angle X-ray scattering (SAXS). Later, Koutsky et al. [125] discovered two-phase microstructure establishment in polyesters and polyethers containing PUs using transmission electron microscopy (TEM), but their results were not conclusive. Subsequently, Chen-Tsai et al. and Serrano et al. clearly illustrated the two-phase morphology of PUs containing different percentages of hard segments by SAXS and TEM analyses [7, 126, 127]. Therefore, it can be concluded that PUs show a combination of properties; they behave like glassy and soft elastomers at the same time, which allows the use of thermoplastic-processing techniques. HS and SS phases are schematically represented in Fig. 5.2; phase separation and phase mixing are reversible functions of temperature. When a polymer is heated above the melting temperature, its

Fig. 5.2 Representation of the segmental domains in PUs at different temperatures

58

5 Flame-Retardant Polyurethanes

crystallinity is destroyed due to the formation of a viscous melt, which can be processed by conventional thermoplastic techniques, such as extrusion and injection molding, for conventional material preparation [127].

5.6

Interactions Between P,N-Based FRs and PUs for FR Activity

Commonly, phosphorus compounds are considered to accelerate char formation in hetero atom (oxygen and nitrogen)-containing polymers. This is because at high temperatures during polymer burning, they can form thermally stable 3D char networks with P–O and P–N compounds. This char network acts like a thermoset material and protects the polymer substrate from the flame. The formation of a stable char structure mainly depends on the interactions between the polymer and P-based FR derivatives at high temperatures. In the case of polyurethanes (PUs) containing –PO4 functionalities in their structure, the formation of phosphoric acid is accelerated, which further interacts with PU to yield isocyanate and alcohol, as shown in Scheme 5.3. These isocyanates recombine with phosphoric acid to form complex crosslinked structures, which can easily convert into azophosphonates at high temperatures. This chain propagation reaction continues to complete degradation of PUs, which finally lead to the formation of crosslinked char, CO2, and NO2. Overall, the inclusion of phosphorus-nitrogen functionalities in the polymer structure improves its FR activity to a great extent. The introduction of FRs into thermoplastic PU (TPU) polymers through covalent or non-covalent bonds can delay the degradation of the polymers and improve the FR activity with increasing char residue and release of non-flammable gases. Literature indicates that polymer nanocomposites (PNCs) exhibit high thermal stability with low HRR values; they accelerate the formation of char residue but do not self-extinguish. Further, they do not pass the UL-94 test. Therefore, for a polymer to exhibit these properties; it is necessary to prepare PNCs with a combination of 2 or 3 FRs, either additive FRs or reactive FRs. Toldy et al. [129] prepared a series of TPU composite polymers using a combination of various FRs and comprehensively studied their FR properties (Table 5.1). The advantage of using a combination of FRs in polymer composites is that they might exhibit a synergetic effect in improving the properties of the polymer. TPU composites containing different additive and reactive FRs displayed strong FR activity due to the synergetic effect of the incorporated additives. An increase in the phosphorus content in the TPU increases the LOI value, but the polymer specimens could not pass the UL-94 test because most P-based FRs cannot prevent dripping by themselves. However, the combination of P-containing FRs in TPUs with other FRs such as clays, boron derivative compounds, and hindered amines increased the LOI values and the specimens could pass the UL-94 test without melt dripping.

5.6 Interactions Between P,N-Based FRs and PUs for FR Activity

59

Scheme 5.3 The FR mechanism of phosphorus-based FR-containing polyurethanes. The scheme redraw on the basis of information collected from [128]

TPU %

100 80 75 65 73 73 65 65 85 80 75 70 72.5 95 90 80 84 69 74 77 74 74

S. No:

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

APP %

– – – – – 5 5 – – – – – – – – – – – – – – –

Reogard %

– 20 25 35 25 20 30 30 – – – – – – – – – – – – – –

– – – – – – – 5 – – – – – – – – – – – – – –

PER % – – – – 1 1 – – – – – 2.5 1.25 – – – – – – – – –

OMMT % – – – – 1 1 – – – – – 2.5 1.25 – – – – – – – – –

SEP % – – – – – – – – – – – – – – – 5 5 10 5 2 – –

OSEP % – – – – – – – – 15 20 25 25 25 – – – – – – – – –

OP1230% – – – – – – – – – – – – – – – 10 10 20 10 10 10 5

OP550% – – – – – – – – – – – – – – – – – – 10 10 15 20

APP 101/ ME% – – – – – – – – – – – – – – – – – – – – – –

ZnB % – – – – – – – – – – – – – – – – – – – – – –

MelB % – – – – – – – – – – – – – 5 5 5 1 1 1 1 1 1

HALS % 23 24 25 32 27 29 32 32 25 25 26 34 32 19 20 22 21 19 22 27 22 20

UL-94

HB V-2 V-2 V-2 V-2 V-2 V-0 V-0 HB V-2 V-2 HB HB V-2 V-2 V-2 HB HB V-2 V-0 V-2 V-2 (continued)

LOI

Table 5.1 Formulations of various additive and reactive FRs-containing TPU nanocomposites. Some specific combinations of FRs led to a synergetic effect in the polymer composites

60 5 Flame-Retardant Polyurethanes

TPU %

79 69 69 64 64

S. No:

23 24 25 26 27

APP %

– – – – –

Reogard %

– – – – –

Table 5.1 (continued)

– – – – –

PER % – – – – –

OMMT % – – – – –

SEP % – – – – –

OSEP % – – – – –

OP1230% – 5 5 5 5

OP550% – 20 20 25 25

APP 101/ ME% – – 5 – 5

ZnB % 20 5 – 5 –

MelB % 1 1 1 1 1

HALS % 22 28 26 24 24

LOI V-2 V-0 V-0 V-0 V-0

UL-94

5.6 Interactions Between P,N-Based FRs and PUs for FR Activity 61

62

5 Flame-Retardant Polyurethanes

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111. M.M. Velencoso, M.J. Ramos, R. Klein, A. De Lucas, J.F. Rodriguez, Thermal degradation and fire behaviour of novel polyurethanes based on phosphate polyols. Polym. Degrad. Stab. 101, 40–51 (2014) 112. R. Patel, H. Patel, Property modification of conventional castor oil based polyurethane using novel flame retardant polyurethanes. Pragna J. Pure Appl. Sci. 15, 66–76 (2007) 113. A. Kaushik, P. Singh, Synthesis and characterization of castor oil/trimethylol propane polyol as raw materials for polyurethanes using time-of-flight mass spectroscopy. Int. J. Polym. Anal. Charact. 10, 373–386 (2005) 114. M. Kimura, G. Salee, R.S. Porter, Blends of poly (ethylene terephthalate) and a polyarylate before and after transesterification. J. Appl. Polym. Sci. 29, 1629–1638 (1984) 115. K. Dai, L. Song, S. Jiang, B. Yu, W. Yang, R.K. Yuen, Y. Hu, Unsaturated polyester resins modified with phosphorus-containing groups: effects on thermal properties and flammability. Polym. Degrad. Stab. 98, 2033–2040 (2013) 116. R.H. Patel, M.D. Shah, H.B. Patel, Synthesis and characterization of structurally modified polyurethanes based on castor oil and phosphorus-containing polyol for flame-retardant coatings. Int. J. Polym. Anal. Charact. 16, 107–117 (2011) 117. R. Patel, H. Patel, Studies on flame retardant polyurethanes based on bisphenol-A monophosphate. Pragna J. Pure Appl. Sci. 14, 70–78 (2006) 118. R.H. Patel, K.S. Patel, Synthesis and characterization of polyesterurethanes and their applications to flame-retardant coatings. Int. J. Polym. Anal. Charact. 17, 85–92 (2012) 119. R. Patel, M. Shah, H. Patel, Synthesis and characterization of structurally modified polyurethanes based on castor oil and phosphorus-containing polyol for flame-retardant coatings. Int. J. Polym. Charact. 16, 107–117 (2011) 120. L. Zhang, M. Zhang, L. Hu, Y. Zhou, Synthesis of rigid polyurethane foams with castor oil-based flame retardant polyols. Ind. Crops Prod. 52, 380–388 (2014) 121. H. Ding, J. Wang, C. Wang, F. Chu, Synthesis of a novel phosphorus and nitrogen-containing bio-based polyols and its application in flame retardant polyurethane sealant. Polym. Degrad. Stab. 124, 43–50 (2016) 122. A. Szymczyk, J. Nastalczyk, R. Sablong, Z. Roslaniec, The influence of soft segment length on structure and properties of poly (trimethylene terephthalate)-block-poly (tetramethylene oxide) segmented random copolymers. Polym. Adv. Technol. 22, 72–83 (2011) 123. L.F. Wang, Effect of soft segment length on the thermal behaviors of fluorinated polyurethanes. Eur. Polym. J. 41, 293–301 (2005) 124. L. Rueda-Larraz, B.F. d’Arlas, A. Tercjak, A. Ribes, I. Mondragon, A. Eceiza, Synthesis and microstructure–mechanical property relationships of segmented polyurethanes based on a PCL–PTHF–PCL block copolymer as soft segment. Eur. Polym. J. 45, 2096–2109 (2009) 125. J.A. Koutsky, N. Hien, S.L. Cooper, Some results on electron microscope investigations of polyether‐urethane and polyester‐urethane block copolymers. J. Polym. Sci. Part C Polym. Lett. 8, 353–359 (1970) 126. M. Xu, W. MacKnight, C. Chen-Tsai, E. Thomas, Structure and morphology of segmented polyurethanes: 4. Domain structures of different scales and the composition heterogeneity of the polymers. Polymer 28, 2183–2189 (1987) 127. M. Serrano, W.J. MacKnight, E.L. Thomas, J.M. Ottino, Transport-morphology relationships in segmented polybutadiene polyurethanes: 1. Experimental results. Polymer 28, 1667–1673 (1987) 128. C. Kloock, Synthesis of potential phosphorus-nitrogen containing flame retardants. Honors Thesis, University of Dayton, 2015 129. A. Toldy, G. Harakály, B. Szolnoki, E. Zimonyi, G. Marosi, Flame retardancy of thermoplastics polyurethanes. Polym. Degrad. Stab. 97, 2524–2530 (2012)

Chapter 6

Melt-Dripping and Char Formation

This chapter reports the fundamental understanding of melt-dripping and how this characteristic of polymeric materials link with fire properties.

6.1

Relationship Between the Melting Dripping Characteristics of Polymer Films and Fire Properties

Dripping is also considered as an important parameter that needs to be analyzed while evaluating the FR properties of polymers; in the UL-94 test, polymers are characterized based on their dripping behavior [1, 2]. Dripping may lead to flame propagation due to spreading of flames on the surface of the polymer and surroundings. Many researchers carried UL-94 test to quantify the dripping behavior during combustion [3–6]. If a polymer melts easily, it obviously exhibits high dripping, which is one of the main reasons for the spreading of fire. The melt flow and dripping of polymers mainly depend on the polymer viscosity. At particular temperatures, solid polymer films melt, where there is no solid phase, is called melting point or melting temperature. This is a physical phenomenon and phase change occurs from solid to liquid, which in turn affects the efficiency of heat diffusion, particle size, and heating rate. Crystalline polymers exhibit a sharp melting point and at this temperature they lose their ordered structure. The melting of some compounds or polymers may be accompanied by simultaneous decomposition, similar to sublimation [7]. It is well known that the Tg, polymer degradation, and viscosity of the polymer melt affect its melt dripping behavior [8]. Generally, the dripping of polymers depends on their melting behavior, which is dependent on the Tg and melting temperature. Zhang et al. [9] studied the effect of Tg and melting point on the flammability of polymers and they observed that the degree of melt flow or melt drip is directly proportional to Tg. Therefore, polymers with low Tg values melt easily, while those with higher Tg do not. In case the Tg of a © Springer Nature Switzerland AG 2020 S. Sinha Ray and M. Kuruma, Halogen-Free Flame-Retardant Polymers, Springer Series in Materials Science 294, https://doi.org/10.1007/978-3-030-35491-6_6

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polymer is around the ambient temperature or less, we could expect pool-like fire. Polymers with Tg values >100 °C do not melt significantly and do not catch fire easily. The melting temperatures, Tg values, and decomposition temperatures can be determined by differential scanning calorimetry (DSC) and TGA. During melting, solid-viscous liquid phase change occurs, while during decomposition, bond breakage occurs. In between the melt and decomposition temperatures, the viscosity of a polymer decreases and it loses its mechanical integrity; furthermore, significant changes may occur in other associated properties. The relationship between melt dripping and melt viscosity of polymers and nanocomposites has been studied [10, 11]. It has been shown that even though polymer degradation occurred to a high extent, if the viscosity is high, then the FR activity of the polymer is high; this is because viscosity is a parameter dependent on temperature. In the presence of nanoparticles, the viscosity of a polymer increases, due to which the melt flow and dripping decrease. The dripping and viscosity of polymer composite resins depend on interactions with different FR hardeners and decomposition pathways. Schartel et al. [2] prepared epoxy resins with different DOPO-based hardeners and the effect of different hardeners on the dripping properties and FR properties was evaluated. They also studied reactive and non-reactive FR additive-containing epoxy resins, such as DGEBA/DDS, DGEBA/DDS/DOPO analogs (non-reactive) and DGEBA/ DDS/DOPO-based diamine (reactive) composites, and evaluated their FR activity. They noticed that compared to non-reactive epoxy resins, the LOI values of reactive epoxy resins increased from 13% to 17% and the UL-94 test results also improved from HB to V-1 rating. Pawlowski and Schartel [1] synthesized polycarbonate/ acrylonitrile-butadiene-styrene (PC/ABS) blends with three different types of aryl phosphates (TPP, BDP, and PTFE) and systematically conducted comparative studies on their FR activity and melt dripping behavior. PC/ABS with BDP achieved a V-2 rating in the UL-94 test and exhibited high deformation due to BDP acting as a plasticizing agent. Meanwhile, a combination of BDP with PTFE in the PC/ABS system led to a remarkable FR activity and V-0 rating in the UL-94 test due to a synergetic effect between BDP and PTFE. In the presence of PTFE, the condensed-phase action of BDP is enhanced with accelerating char formation. Therefore, it is clear that PTFE strongly affects the dripping property and polymer melt viscosity. Therefore, the PC/ABS system with PTFE achieved an HB rating in the UL-94 test because it burns completely without dripping. One of the most important phenomena in FR polymer synthesis is selfextinguishability or the “blowing-out effect,” which indicates the release of phosphorus radicals and non-flammable gases. This mainly involves the formation of protective highly thermally stable carbonaceous char layers, which hinder further polymer combustion, but at the same time, allow fast accumulation of nonflammable gases inside the char layers. A schematic model of the “blowing-out effect” is shown in Fig. 6.1 in the initial stages, the polymer can burn plentifully and subsequently a thermally stable char layer is formed. Inside this char layer, non-flammable gases are accumulated, which weaken the strength of the flame. After some time, pressure due to the non-flammable gases increases and breaks down the char structure; once eluted, the non-flammable gases immediately extinguish the

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flame, as shown clearly in Fig. 6.1. FR epoxy resins were synthesized using DOPO either in combination with OPS or alone. When 5 wt% of OPS or DOPO was used alone for EP composites, the obtained specimens did not achieve any rating in the UL-94 test, but a 5 wt% mixture (2.5 wt% OPS and 2.5 wt% DOPO) led to a V-1 rating in the UL-94 test due to a strong “blowing-out effect” and also possible synergism between OPS and DOPO [12]. The same research group studied the FR properties of epoxy resins using two different amide curing agents, including aliphatic oligomeric poly amide (PA 650) and aromatic 4,4′-diaminodiphenylsulphone (DDS). The UL-94 test results revealed that DGEBA/DDS with DOPO-POSS exhibited a stronger “blowing-out effect” than DGEBA/PA650, which means that emission of pyrolytic gases is very less in the case of aromatic amides compared to aliphatic amides even at low percentages. It is assumed that aromatic DDS leads to the formation of a strong crosslinked structure and dense char [13]. Further, EP resins were synthesized using a combination of various FRs such as PEPA, APP, and DOPO with or without OPS and flammability studies were conducted. A clear difference could be observed in the combustion behavior of the resultant resins due to differences in the chemical structures of the FRs. Organic phosphorus compounds, PEPA and DOPO, showed better FR activity than inorganic phosphorus compounds, such as APP, AP, and MPP [14]. Qian et al. [15] synthesized TGIC-DOPO, which contains phosphaphenanthrene and triazine-trione as the FRs. They observed that the 12% TGIC-DOPO/DGEBA/DDS system showed the highest LOI value of 33.3% and achieved a UL-94 V-0 rating. Further, they also observed that TGIC-DOPO exhibited a dual-phase FR mechanism (gas phase and solid phase) during combustion. The phosphaphenanthrene groups are responsible for the condensed-phase FR activity and the triazine-trione group is responsible for the gas-phase FR activity. Similarly, Trif-DOPO was also synthesized and used as a FR for epoxy resins; it was observed that the systems containing very little phosphorus such, as 1.2% Trif-DOPO/DGEBA/DDS exhibited a V-0 rating in the UL-94 test and showed a high LOI value of 36%. However, the researchers did not analyze why Trif-DOPO showed high efficient FR activity even at low weight percentages compared to TGIC-DOPO. Based on literature, it can be assumed that Trif-DOPO with more aromatic groups in its structure induces higher crosslinking density and might participate in a synergistic reaction [16]. Li et al. [17] synthesized PET/PN6/PPPMS composite polymers and studied their flammability properties. They observed that PET/10%PN6/10%PPPMS showed the highest LOI value of 30.9%, V-0 rating in the UL-94 test, and no dripping. The melt dripping of polymer composites mainly depends on the melt viscosity during polymer combustion; upon the addition of PPPMS to PET/PN6, its viscoelastic properties are drastically changed. PET/10%PN6/10%PPPMS behaved like a completely elastic solid at all test frequencies and hence it exhibited excellent anti dripping properties. Wang and Cai [18] synthesized triazole-containing DOPO derivatives (DTA), as shown in Scheme 6.1, and used it as a co-curing FR agent for epoxy resins. In the presence of DTA, melt dripping was completely inhibited and a V-0 rating could be achieved in the UL-94 test. It was observed that 4 wt% DTA-containing EP showed the highest LOI value, low average combustion time,

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Fig. 6.1 Schematic of the blowing-out effect [13]. Reproduced with permission from Elsevier Science Ltd

and achieved a V-0 rating in the UL-94 test. Meanwhile, neat EP did not pass the UL-94 test and burns vigorously for a longer time period with dripping until clamping. This observation clearly indicates that DTA exhibited efficient FR activity and was remarkable in decreasing flame dripping and combustion time; furthermore, auto extinguishability was observed in EP/DTA resins. In the initial stage, flame propagation slowly increased up to 0.5 s then slowly decreases the flame and at 3 s, the flame was extinguished as shown in Fig. 6.2, due to the “blowing-out effect,” which means that in the initial stage, non-flammable gases are accumulated inside the char layer and once released, they extinguished the flame. These observations indicate that FR activity in such cases depends on the fast accumulation and release of non-flammable gases as well as strong char formation. From literature, it can be inferred that that triazole and tetrazole-containing DOPO derivative compounds and condensed compounds of p-di-substituted amine derivatives with DOPO show a blowing out effect which contributes to their FR activity; the synthesis of these compounds is illustrated in Scheme 6.1. In addition, some phosphazene derivative compounds also exhibited this behavior. In such compounds, the P-moieties aid in the formation of a char layer, whereas N-moieties release non-flammable gases. During polymer burning, with time, the volume of non-flammable gases increases and at a certain pressure, the char layer breaks and the gases are released into the flame zone and immediately extinguish the flame. Therefore, new synthesis methods and techniques should be developed for efficient P and N-containing FRs, which are inexpensive and are capable of fast accumulation and release of high volumes of non-flammable gases and strong char formation. Matzen et al. [19] systematically studied the effect of dripping on the FR activity of different thermoplastic polymers, such as PBT, PP, PP-EP, and PA6, in different thermal conditions with different flame retardants. They reported that a high melt viscosity is imperative to improve the dripping property; in addition, dripping can be reduced by high crosslinking

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Scheme 6.1 Synthesis of DTA and PDAP from 1,4-dibenzene [18, 20]. Reproduced with permission from Elsevier Science Ltd

densities, inclusion of hardeners with large number of reactive functional groups, and a high decomposition temperature. Although there are some intensive studies on the relationship between flame retardants and dripping behavior, there is still room for development in improving the anti-dripping and FR activity of polymers and nanocomposites.

6.2

The Relationship Between Char Formation and Morphology Development in Fire Retardant Activities

The analysis of char morphology is also important as morphological features allow the prediction of FR mechanisms. The FR activities of epoxy resins with and without FR are evaluated. In the presence of FRs, the resins exhibited strong char formation, which hinders further flammability and prevents the release of combustible components. The char layer also acts as a barrier to oxygen attacking the underlying polymer substrate, consequently weakening the flame, as schematically

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Fig. 6.2 Screenshot images of EP/DTA6 (with) and EP (without) flame retardant-containing epoxy thermoset polymers during a UL-94 test [18]. Reproduced with permission from Elsevier Science Ltd

represented in Fig. 6.3. In the case of pure EP resin, the evolution of combustible volatile components and flammable gases is very high; when the pressure inside the char layer reaches a critical point, the char structure weakens and breaks down, resulting in an open porous morphology (Fig. 6.3), whereas in the case of FR-containing EP resins, there is no porous morphology due to the formation of a strong char layer, which can inhibit the evolution of combustible components. The morphology of the char layer and the relationship between the chemical components of the char layer and FR activity of epoxy resins was also investigated. It was found that the char structure of pure EP showed a highly open porous morphology and with increasing FR in the EP resins, the porosity gradually decreased and completely disappeared (like a honey comb-sealed structure) and a strong dense char was formed [21]. Zhang et al. [22] reported that EP, EP/DOPO, EP/PEPA, and EP/DOPO-PEPA resins exhibited morphologies of broken char residue, crosslinked char, cracks and porous char, and a compact char structure, respectively. The compact char in the DOPO-PEPA resin could be attributed to the compatibility and synergetic effect of EP/DOPO-PEPA resin, which led to an enhanced FR activity and mechanical strength. Similarly, other studies clearly demonstrated the differences in the char morphologies of various polymers with and without FR; in the presence of a FR, a strong sealed and structured char morphology was observed, whereas in the case of pure polymers, open and fractured char morphologies were observed but there may be slight differences due to differences in the chemical structures of the used FRs and their interactions with the polymers. This discussion clearly indicates that the char structure and morphology play an important role in controlling the FR activity. In the case of IFRs, most of the reported char morphologies exhibited good adhesion and a solid shield structure, which acted as a strong barrier to prevent further burning [23]. In some cases, during the process of burning, non-flammable

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Fig. 6.3 A schematic representing the combustion process and morphologies of pure EP and FR-containing EP resins [21]. Reproduced with permission from Elsevier Science Ltd

gases are accumulated inside the char layer. At a certain pressure point, narrow pores are formed in the char layer and these gases slowly diffuse out. These gases, being non-combustible in nature, instantly extinguish the flame. A porous morphology is mainly observed in the case of FRs which act via the “blowing-out” effect [12, 13, 15, 16]. Some studies reported specific morphologies, such as a nacre morphology and nanobricks morphology, which conferred excellent FR properties via the creation of FR functionalities [24]. In a recent report, Kuruma et al. [25] reported the development of new-generation 2D-molybdenum sulfide (MoS2) nanosheet-containing polyurethane (PU) composite materials with improved thermo-mechanical stiffness, thermal stability, and fire retardation property. The surface of 2D-MoS2 nanosheets was modified with melamine (M-MoS2), and then PU composites with varying M-MoS2 loadings were synthesized using an in situ polymerization method. During polymerization, 3amino-propyl-trimethoxy silane was introduced to create silicate functionality on the PU chains, which further improves the compatibility between PU and M-MoS2. Microscopy studies confirmed the distribution of highly intercalated and agglomerated M-MoS2 nanosheets in the PU matrix. The fire-properties of neat PU and various composites were studied using cone-calorimetry and the composite containing 5 wt% M-MoS2 showed improved fire retardation properties, with 45 and 67.5% decrease in the peak heat and total heat release rates, respectively, as compared with those of pure PU. To understand the mechanism of fire-retardant activity of composites, the authors extensively studied the structural morphology and nature of the residual char, because the entire

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mechanism of FR activity depends on the char structure. Figure 6.4 shows digital photographs of the char residues for neat PU and different PU/MoS2 and PU/ M-MoS2 composites. From Fig. 6.4a, it is observed that pure PU polymer film is highly flammable and burns vigorously. On the other hand, the presence of MoS2 and M-MoS2 improved the char structure and char residues of PU composites. During combustion, MoS2 catalyzes the char formation, the silica improves the char residue and insulating properties, and melamine in the PU chain helps to form crosslinked char. Also, with increasing M-MoS2 content in the PU matrix, there is increased intumescent char formation due to the increased melamine content. In the case of PU/MoS2-5%, the amount of formed char residue increases as compared to neat PU, but it is not strong enough to allow char expansion. Hence, the formed char is porous and contains holes, as shown in Fig. 6.4b, whereas in the case of PU/ M-MoS2 composites the char is stronger and capable to expand to avoid a porous texture, as shown in Fig. 6.4c, d. After the burning test in cone calorimetry, the obtained char was investigated by SEM. From the morphologies shown in Fig. 6.5, pure PU burned efficiently with less char residue, due to the very poor fire resistance of neat PU polymer. The corresponding morphology of char layers is presented in Fig. 6.5a1, a2. The morphology of PU/MoS2-5%, shown in Fig. 6.5b1, b2, is relatively more compact, although porous and open voids formed in the char during combustion.

Fig. 6.4 Photographs of char residues of a PU, b PU/MoS2-5%, c PU/M-MoS2-3%, and d PU/ M-MoS2-5% composites after cone calorimetry test [25]. Reproduced with permission from Wiley-VCH

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Therefore, this char layer is not strong enough to expand during combustion. The open voids are also responsible for evolution of toxicants and heat. However, with M-MoS2 in the PU composites, the formation of char is improved significantly, and the char layer is strong enough to inhibit the formation of cracks and voids in it during combustion. The morphology of the outer and inner char residues of PU/M-MoS2-3% and PU/M-MoS2-5% are shown in Fig. 6.5c1, c2 and Fig. 6.5d1, d2, respectively. A noteworthy observation is that, upon increasing the M-MoS2 content in the PU matrix, the char layer becomes stronger with fewer holes and cracks on the surface. Moreover, the inside of the char is also much smoother and more crosslinked, so that it could inhibit the volatile component evolution and oxygen transfer. As we explained in the earlier section, M-MoS2 contains melamine groups that are either functionalized on the surface of MoS2 nanosheets or intercalated into the sheets. Melamine is capable of forming hydrogen bonds with urethane urea linkages and silicate functionalities of the PU chains, and increasing the content of M-MoS2 content in the polymer matrix increases the hydrogen bond formation. Hence, the organic/inorganic phase mixing is more thorough, resulting in improved chars structure and strength to provide strong barrier properties. From Fig. 6.5c1, c2, the char contains bubbles due to the released ammonia and volatile components. These bubbles decreased when there was more M-MoS2 in the polymer composite, as seen from Fig. 6.5d1, d2. A high percentage of M-MoS2 content causes a more crosslinked char, which could inhibit the bubble formation in char layer and provide a stronger barrier. Hence, the PU/M-MoS2-5% composite displayed better FR properties. To explain the FR activity and thermo-mechanical properties of PU/M-MoS2 composites, the schematic model proposed in Fig. 6.6. These M-MoS2 nanosheets are randomly aligned in the polymer matrix by formation of strong interfacial hydrogen bonds with PU chains, which is important to improve the mechanical and

Fig. 6.5 FE-SEM micrographs of outer and inner char residues after cone calorimetry test for neat PU (a1, a2), PU/MoS2-5% (b1, b2), PU/M-MoS2-3% (c1, c2), and PU/M-MoS2-5% (d1, d2) [25]. Reproduced with permission from Wiley-VCH

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Fig. 6.6 Schematic model to explain FR activity of PU/M-MoS2 composites [25]. Reproduced with permission from Wiley-VCH

FR property of resulting PU composites. Owing to its high aspect ratio of MoS2 nanosheets, it acts as strong physical barrier to lower the transfer of combustible pyrolysis components and accelerate the polymer degradation with increasing surface temperature [26–28]. Therefore, in presence of M-MoS2 nanosheets accelerate the polymer degradation in early stage to forms high crosslinked char and also it release non-flammable gases such as NH3 to the gas phase and dilutes the combustible gases. Moreover, the silica segment can provide thermal insulating property to the char even at high temperature. Hence, the resulting char capable to provide strong barrier properties and improved the FR activity. The same authors also established the relation between the fire properties and char formation in the case of organically modified montmorillonite (OMMT) containing polylactide (PLA) nanocomposite [29]. The char morphologies of PLA and two different OMMTs containing PLA nanocomposites after cone calorimetric tests are shown in Fig. 6.7. PLA-0 (neat polymer) completely burns and there is no char residue remaining, as shown in Fig. 6.7a, d. However, the addition of OMMT to the PLA greatly improved the flammability with increasing intumescent char residues, as shown in Fig. 6.7b, e and Fig. 6.7c, f. The char residues obtained from PLA-5 (nanocomposite containing 5 wt% OMMT) were highly intumescent compact char because less non-combustible gas escaped, resulting in greater height and strength of the char, as shown in Fig. 6.7b, e. As we explained using SAXS and

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Fig. 6.7 Digital photographs of burning char residues after cone calorimetry test PLA-0 (a, d), PLA-5 (b, e) and PLA-8 (c, f). 1st row for top view and 2nd row for side view [25]. Reproduced with permission from Wiley-VCH

TEM analyses, in PLA-5, a greater degree of delamination of silicate layers occurs during the melt-blending process. We also believe at this specific composition, there may be strong interactions between the OMMT-1 (nanocomposite containing 1 wt% OMMT) and MP. Hence, while burning with the clay, the high phosphorous containing compounds migrate to the polymer surface and enrich the char residue and char strength. Forming a continuous, highly compact char layer on the substrate results in the excellent FR activity of the PLA-5 nanocomposite system. Another explanation is a possible synergetic effect at this specific composition, because such an improvement in char residue was not observed in the other compositions, PLA-3 and PLA-4. Porous or loose char residues were obtained from PLA-8, as shown in Fig. 6.7c, f, and it is clearly visible that there was less expansion and a lower height of the char. Voids and cracks in the char layer were observed, and the char was not strong enough to inhibit the evolution of volatile components. Therefore OMMT-1containing PLA composites exhibited better FR activity than OMMT-2-containing PLA composites. The SEM images of the residual char of PLA-0, PLA-5, and PLA-8 are shown in Fig. 6.8. Figure 6.8a clearly shows that pieces of char layers are present. During combustion, a broken char layer and inferior char quality cannot form an effective intumescent char layer to prevent the underlying material from further degradation. Hence, serious melt drippings form and there is no char residue in the case of PLA-0. During combustion, the first stage of MP decomposition releases non-combustible components like NH3 and water along phosphoric acid, which can

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Fig. 6.8 SEM images of the outer surface of char residues of PLA-0 (a), PLA-5 (b), and PLA-8 (c) [25]. Reproduced with permission from Wiley-VCH

be further dehydrate the polymer chains to form char as we observed in the case of nanocomposite samples. In the OMMT-1, phosphonium derivatives are formed from the surfactant during combustion at high temperatures, creating the possibility for the formation of aryl phosphine, phosphite oxides, and also graphitic crosslinked char, resulting in highly compact and strong char, which inhibits the volatile components and toxic gases. The char morphology of PLA-5 is shown in Fig. 6.8b, and it clearly exhibits a highly compact and strong char layer with bubbles, due to the release of pyrolytic components and ammonia gases accumulating inside. These chars can prevent further degradation and melt drippings. Therefore, PLA-5 displayed high FR activity with decreased HRR, THRR, and TSR. The residual char morphology of PLA-8 is shown in Fig. 6.8c. It clearly shows thin char, with some cracks and holes. The presence of aliphatic phosphorous compounds and it cannot form the graphitic strong char, hence prevent the formation of graphitic char and the char is unstable. Because of these cracks and holes in the char, volatiles easily escaped. Overall, this indicates that the presence of aromatic phosphorous derivatives contributes to the FR activity, forming a highly compact and graphitic char layer at high temperatures.

Fig. 6.9 Schematic model to explain synergetic charring formation. Drawing is not based on proper scale [25]. Reproduced with permission from Wiley-VCH

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Based on the above analyses from TGA, SAXS, rheology, and SEM images of obtained residual chars after cone calorimetric test, it can be found that there is an existence of strong synergetic effect between OMMT-1 and MP. During combustion along with MMT platelets, MP also migrates to the polymer surface results high crosslinking which can improve the viscoelastic properties of the nanocomposite. Therefore, in case of PLA-5, a strong synergistic effect is responsible for superior FR activity which is schematically presented in Fig. 6.9.

References 1. K.H. Pawlowski, B. Schartel, Flame retardancy mechanisms of triphenyl phosphate, resorcinol bis (diphenyl phosphate) and bisphenol A bis (diphenyl phosphate) in polycarbonate/acrylonitrile–butadiene–styrene blends. Polym. Int. 56, 1404–1414 (2007) 2. B. Schartel, U. Braun, A. Balabanovich, J. Artner, M. Ciesielski, M. Döring, R. Perez, J. Sandler, V. Altstädt, Pyrolysis and fire behaviour of epoxy systems containing a novel 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide-(DOPO)-based diamino hardener. Eur. Polym. J. 44, 704–715 (2008) 3. Y. Wang, F. Zhang, X. Chen, Y. Jin, J. Zhang, Burning and dripping behaviors of polymers under the UL94 vertical burning test conditions. Fire Mater. 34, 203–215 (2010) 4. L. Karlsson, A. Lundgren, J. Jungqvist, T. Hjertberg, Influence of melt behaviour on the flame retardant properties of ethylene copolymers modified with calcium carbonate and silicone elastomer. Polym. Degrad. Stab. 94, 527–532 (2009) 5. Y. Wang, J. Jow, K. Su, J. Zhang, Development of the unsteady upward fire model to simulate polymer burning under UL94 vertical test conditions. Fire Saf. J. 54, 1–13 (2012) 6. Y. Wang, J. Jow, K. Su, J. Zhang, Dripping behavior of burning polymers under UL94 vertical test conditions. J. Fire Sci. 30, 477–501 (2012) 7. S.K. Bhattacharia, B.L. Weeks, C.C. Chen, Melting behavior and heat of fusion of compounds that undergo simultaneous melting and decomposition: an investigation with HMX. J. Chem. Eng. Data 62, 967–972 (2017) 8. B. Kandola, D. Price, G. Milnes, A. Da Silva, Development of a novel experimental technique for quantitative study of melt dripping of themoplastic polymers. Polym. Degrad. Stab. 98, 52–63 (2013) 9. J. Zhang, T. Shields, G. Silcock, Effect of melting behaviour on upward flame spread of thermoplastics. Fire Mater. 21, 1–6 (1997) 10. A.B. Morgan, C.A. Wilkie, G.L. Nelson, Fire and Polymers VI: New Advances in Flame Retardant Chemistry and Science. ACS Symposium Series (ACS Publications, 2012) 11. C. Ma, S. Qiu, B. Yu, J. Wang, C. Wang, W. Zeng, Y. Hu, Economical and environment-friendly synthesis of a novel hyperbranched poly (aminomethylphosphine oxide-amine) as co-curing agent for simultaneous improvement of fire safety, glass transition temperature and toughness of epoxy resins. Chem. Eng. J. 322, 618–631 (2017) 12. W. Zhang, X. Li, L. Li, R. Yang, Study of the synergistic effect of silicon and phosphorus on the blowing-out effect of epoxy resin composites. Polym. Degrad. Stab. 97, 1041–1048 (2012) 13. W. Zhang, X. Li, R. Yang, Blowing-out effect in epoxy composites flame retarded by DOPO-POSS and its correlation with amide curing agents. Polym. Degrad. Stab. 97, 1314– 1324 (2012) 14. W. Zhang, X. He, T. Song, Q. Jiao, R. Yang, The influence of the phosphorus-based flame retardant on the flame retardancy of the epoxy resins. Polym. Degrad. Stab. 109, 209–217 (2014)

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15. L. Qian, Y. Qiu, N. Sun, M. Xu, G. Xu, F. Xin, Y. Chen, Pyrolysis route of a novel flame retardant constructed by phosphaphenanthrene and triazine-trione groups and its flame-retardant effect on epoxy resin. Polym. Degrad. Stab. 107, 98–105 (2014) 16. L. Qian, Y. Qiu, J. Liu, F. Xin, Y. Chen, The flame retardant group-synergistic-effect of a phosphaphenanthrene and triazine double-group compound in epoxy resin. J. Appl. Polym. Sci. 131, 39709 (2014) 17. J. Li, F. Pan, H. Xu, L. Zhang, Y. Zhong, Z. Mao, The flame-retardancy and anti-dripping properties of novel poly (ethylene terephthalate)/cyclotriphosphazene/silicone composites. Polym. Degrad. Stab. 110, 268–277 (2014) 18. P. Wang, Z. Cai, Highly efficient flame-retardant epoxy resin with a novel DOPO-based triazole compound: thermal stability, flame retardancy and mechanism. Polym. Degrad. Stab. 137, 138–150 (2017) 19. M. Matzen, B. Kandola, C. Huth, B. Schartel, Influence of flame retardants on the melt dripping behaviour of thermoplastic polymers. Materials 8, 5621–5646 (2015) 20. P. Wang, F. Yang, L. Li, Z. Cai, Flame retardancy and mechanical properties of epoxy thermosets modified with a novel DOPO-based oligomer. Polym. Degrad. Stab. 129, 156–167 (2016) 21. T. Ma, C. Guo, Synergistic effect between melamine cyanurate and a novel flame retardant curing agent containing a caged bicyclic phosphate on flame retardancy and thermal behavior of epoxy resins. J. Anal. Appl. Pyrolysis 124, 239–246 (2017) 22. Y. Zhang, B. Yu, B. Wang, K.M. Liew, L. Song, C. Wang, Y. Hu, Highly effective P-P synergy of a novel DOPO-based flame retardant for epoxy resin. Ind. Eng. Chem. Res. 56, 1245–1255 (2017) 23. X. Li, Z. Zhao, Y. Wang, H. Yan, X. Zhang, B. Xu, Highly efficient flame retardant, flexible, and strong adhesive intumescent coating on polypropylene using hyperbranched polyamide. Chem. Eng. J. 324, 237–250 (2017) 24. M.J. Nine, D.N.H. Tran, T.T. Tung, S. Kabiri, D. Losic, Graphene-borate as an efficient fire retardant for cellulosic materials with multiple and synergetic modes of action. ACS Appl. Mater. Interfaces 9, 10160–10168 (2017) 25. M. Kuruma, S.S. Ray, N. Kumar, Enhanced thermo-mechanical stiffness, thermal stability, and fire retardant performance of surface-modified 2D MoS2 nanosheet-reinforced polyurethane composites. Macromol. Mater. Eng. 304, 1800562 (2019) 26. K. Zhou, S. Jiang, Y. Shi, J. Liu, B. Wang, Y. Hu, Z. Gui, Multigram-scale fabrication of organic modified MoS2 nanosheets dispersed in polystyrene with improved thermal stability, fire resistance, and smoke suppression properties. RSC Adv. 4, 40170–40180 (2014) 27. J.R. Potts, D.R. Dreyer, C.W. Bielawski, R.S. Ruoff, Graphene-based polymer nanocomposites. Polymer 52, 5–25 (2011) 28. K. Zhou, G. Tang, R. Gao, S. Jiang, In situ growth of 0D silica nanospheres on 2D molybdenum disulfide nanosheets: towards reducing fire hazards of epoxy resin. J. Hazard. Mater. 344, 1078–1089 (2018) 29. M. Kuruma, J. Bandyopadhyay, S.S. Ray, Thermal degradation characteristic and flame retardancy of polylactide-based nanobiocomposites. Molecules 23, 2648 (2018)

Chapter 7

Polymer Nanocomposites for Fire Retardant Applications

Recently, nanostructured materials have drawn significant attention for increasing the fire safety of polymer materials and also to overcome several drawbacks of pure polymer materials, such as their inadequate thermal and mechanical properties. Even at low loadings, nanocomposites can achieve efficient FR activity without disturbing the mechanical properties of the polymers [1–5]. Numerous methods have been developed for the synthesis of various functionalized surface-modified nano fillers, such as nanoparticles, layered double hydroxides, nanoclay, and graphene, and these fillers have been used to prepare fire retardant (FR) polymer nanocomposites [6–10]. Inorganic metal oxides are majorly used to prepare polymer nanocomposites for FR applications [1, 2, 11–13]. Inorganic metal oxides are expected to improve the FR activity due to their catalytic effect or by participating in synergetic reactions; further, the formed char exhibits superior FR properties compared to neat polymers. The main advantages of nanofillers are that they can accelerate char formation, increase char strength, and improve the graphitization of char residue during polymer combustion, which consequently prevents oxygen transfer to the polymer substrate.

7.1

FR Polymer Nanocomposites Based on Various Nanoparticles

Nanomaterials that consist of only one element are defined as metal nanoparticles (NPs). They can exist as individual atoms or clusters of many atoms. In addition to existing in neutral forms (i.e., Ag(0), Au(0)), Au and Ag particles can exist in various nanocluster forms, such as Au8, Au11, Au13, Au18, Au25, Au38, Au55 and Ag2 to Ag8, and Ag25, with characteristic electronic transitions [14]. Due to their excellent luminescent properties, such NPs are important for bio-labelling applications, producing luminescent patterns and fluorescence resonance energy transfer. © Springer Nature Switzerland AG 2020 S. Sinha Ray and M. Kuruma, Halogen-Free Flame-Retardant Polymers, Springer Series in Materials Science 294, https://doi.org/10.1007/978-3-030-35491-6_7

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Commonly synthesized NPs include Ag, Au, Cu, Pd, Pt, Re, Ru, Zn, Co, Al, Cd, Pb, Fe, and Ni; among them Fe and Ni are highly reactive and explosive. Further, metal NPs can also include the category of bimetallic NPs (i.e., Pt–Pd, Cu–Ni), which often exist as core–shell and alloy structures. Bimetallic NPs have better properties or efficiency than their single-metal NP counterparts [15]. Metal NPs are produced in the form of colloidal solutions or solid particles by simple techniques, such as bio-assisted synthesis, hydrothermal synthesis, microwave-assisted synthesis. They have shown interesting characteristics, such as localized surface plasmon resonance (LSPR), high reactivity, and broad absorption in the electromagnetic spectrum. Due to their advanced optical, optoelectrical, catalytic, anti-microbial/-cancer/-viral properties, metal NPs are highly interesting materials for numerous practical applications. Metal oxides are considered one of the most stable naturally occurring compounds. They are formed by reaction between electronegative oxygen and electropositive metal. They have polar surfaces due to presence of anionic oxygen and are insoluble in most organic solvents due to strong bonding between the metal and oxygen. The formation of metal oxides is the lowest free energy states for the metals in the oxidative nature of Earth and among other compounds of periodic table. Presently, they are widely used nanomaterials due to their high natural abundance, high chemical stability, tunable bandgap/band edge positions, and excellent thermal/electrical conductivity. Their applications include semiconductors, superconductors, and even insulators. With growing industrial interest, various types of versatile metal oxides, such as Al2O3, TiO2, Fe3O4, Fe2O3, SiO2, ZnO, and CeO2 have been synthesized for application in the water purification, cosmetics, bio-medical, energy, and environmental remediation fields. These metal oxides can be easily modified by doping, resulting in hetero-structures and mixed oxides, to further meet the stringent demands of excellent properties and efficiency. (Layered) metal hydroxides are an interesting category of inorganic nanomaterials with flexible properties achieved by tailoring the structure and composition. These materials occur in two forms: (1) hydroxides with neutral layers, without intercalated molecules (i.e., b-Ni(OH)2 and b-Cu(OH)2); (2) hydroxides with a cationic layer and with intercalated molecules (i.e., a-Ni(OH)2 and a-Cu(OH)2) [16]. Due to their high surface area, high thermal stability, and excellent ion exchange capability, these materials have attracted attention for catalysis, supercapacitors, fuel cells, flame retardants, sensors, and pollutant removal. To improve the FR activity of polymers, various metal oxide NPs, such as Cu2O, MoO3, Sb2O3, Bi2O3, Co3O4, and SnO2 have been used to prepare polymer nanocomposites [17–19]. These metal oxides can catalyze the formation of char residue and strengthen it during polymer combustion. It has been reported Cu dust and Cu2O are excellent smoke suppressants for all polymers, especially for PU polymers [20–22]. In the initial stages, Cu2O nanoparticles catalyze the formation of poly phosphoric acid from EDA-APP, subsequent dehydration of EP, and accelerate the formation of intumescent char as shown in Fig. 7.1. This char layer acts as a barrier to heat and O2 transfer. Further, it limits the supply of flammable gases to the flame zone, resulting in a reduced smoke and toxic gas evolution. Due

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to the strong synergism between Cu2O and EDA-APP, the formation and compactness of the intumescent char layer increase. Additionally, CO, which is a toxic gas, can be oxidized to CO2 according to the redox cycle in which Cu+2 is converted to Cu0 (Fig. 7.1); this results in reduced smoke toxicity. This conversion is reversible in the presence of oxygen, which acts as fuel for polymer combustion. This conversion reaction presents two advantages, including a reduction in oxygen concentration and toxic gas CO concentration. Further, it was observed that in the presence of Cu2O, the HRR, THR, TSP, and COP of the polymer system are reduced through intumescent char formation. EP nanocomposites with various metal oxides, microencapsulated ammonium polyphosphate (MAPP), and APP were prepared and a strong synergetic reaction was observed between Cu2O and MAPP. Hence MAPP/Cu2O containing system showed strong FR activity with high LOI values and achieved a V-0 rating in the UL-94 test [23]. Xu et al. [24] synthesized PU nanocomposites with MoO3-GNS, Cu2O-GNS, and GNS alone. There was a clear difference in the FR activities of the resultant nanocomposites; those containing a metal oxide-GNS combination showed better FR activity with strong smoke suppression owing to a strong synergetic effect. These combination FRs presented several advantages, such as catalytic charring due to metal oxides and a strong physical barrier due to GNS, which can prevent the release of combustible gases. Cu2O-TiO2-graphene oxide (GO) was synthesized by a hydrothermal method and used to prepare polyester composites. The results revealed that even at low loadings, the FRs significantly decreased pyrolytic toxic gas evolution and HRR compared to either Cu2O-TiO2 or TiO2-GO nanocomposites [25].

Fig. 7.1 Illustration of a possible synergetic effect in EDA-APP and the mechanism of flame retardation and smoke suppression [26]. Reproduced with permission from Elsevier Science Ltd

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Some specific compounds, such as zinc borate, can improve the melt viscosity of polymers in the condensed phase during the initial stages of polymer composite burning. Wang et al. [27] designed efficient halogen-free composite flame retardants (CFR) using a nano engineering pathway; the CFRs consisted of a brucite core and fine zinc borate Zn6(OH)(BO3)3. They found out that Zn6(OH)(BO3)3 accelerates the formation of a specific char structure, which can prevent contact between stored combustible gases and oxygen. Several studies reported that zinc borate may enhance the rigidity of the char layer and form a glassy structure, which can enhance the FR activity and mechanical properties of the composite [28, 29]. Guo et al. [30] coated Mg–Al LDH on wood surfaces and observed improved mechanical and FR properties compared to uncoated wood. Shao et al. [31] prepared PP nanocomposites using APP modified with ethylene diamine via an ion exchange reaction (MAPP); they observed that 40 wt% MAPP-containing PP showed higher LOI values and V-0 rating in the UL-94 test compared to the neat polymer. MAPP showed better FR activity than APP, especially in inhibiting the fire rate growth (FRG) and SPR. The rate of heat release is an important factor affecting the flammability of a polymer. When the HRR increases, the degradation of a polymer is accelerated, resulting in the formation of combustible components. Therefore, an analysis of HRR and THR is necessary to evaluate FR activity. Some specific compounds highly inhibit HRR and THR. Different types of metal phenyl phosphonates were synthesized and used to prepare 2 wt% PS nanocomposites. The nanocomposites exhibited excellent thermal stability and low HRR due to high graphitization and char formation, which can prevent heat release and act as a thermal insulating agent [32]. Polyamides and polyimines are mostly used as FRs and they can efficiently decrease dripping. Imine compounds in combination with APP exhibited excellent intumescent characteristics. At lower temperatures, APP can decompose into NH3, which is helpful for the formation of intumescent char [33–35]. Similarly, Jin et al. [36] synthesized AM-APP and used it to prepare polyamide 11 with TiO2 nanoparticles at different loadings. The results revealed that 22% AM-APP and 3% TiO2 containing polymer nanocomposites exhibited a high LOI value, V-0 rating in the UL-94 test, and no dripping. In this case, TiO2 migrates at high temperatures to the surface of the polymer and aids the formation of a strong char structure. Further, it accelerates the decomposition of APP, resulting in the release of non-flammable gases. Tan et al. [37] used hyper branched polyimide (PI)-modified APP to prepare EP nanocomposites and observed an enhancement in the FR activity with excellent smoke suppression, high Tg values, and V-0 rating in UL-94 test. Liu et al. [38] synthesized epoxy resins with and without thermal cross-linkers and observed that in the presence of azobenzene and phenyl acetylene, the epoxy resins showed efficient FR activity. This is because azo functionality-containing FRs led to the release of N2 gas. In addition, the formation of thermally crosslinked intermediate products improves the intumescent properties of the composite and char compactness. The same research group synthesized different azo benzene and acetylene derivatives for EP resins and confirmed that they effectively enhanced the FR activity by the formation of crosslinked graphitic

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compounds at high temperatures (Scheme 7.1). These compounds are present in the char layer and improve its barrier properties, which means that it can inhibit toxicant evolution and protect the substrate from oxygen attack. Crosslinking occurs between unsaturated double and triple bonds at high temperature conditions, for e.g., the p-p cyclo addition in condensation products (Scheme 7.1). The crosslinking reaction occurs before maximum polymer degradation and promotes the aromatization and carbonization of polymers. Thus, it increases the thermal stability and strength of the char residue, which plays an important role in enhancing the FR activity of polymers. Similarly, a number of symmetrical and asymmetrical azo alkenes have been reported as strong FRs or FR synergists for polyolefins [39, 40]. Sheet-type metal oxides improve the FR activity as their sheet structure can prevent toxic gas evolution and heat release and accelerate char formation. Layered and nano sheets of MoS2 can be synthesized by different methods such as a hydrothermal method [41–43], liquid-phase exfoliation [44], mechanical exfoliation [45, 46], intercalation exfoliation [47], and chemical vapor deposition [48]. To improve their FR activity, nanoparticle surfaces are modified with different phosphorus and nitrogen-containing compounds and these nanoparticles can be used to prepare FR polymer nanocomposites. MoS2 nanoflowers were modified with polyphosphazene (PZS) by a hydrothermal method as shown in Fig. 7.2. EP nanocomposites were prepared using modified and unmodified MoS2 nanoparticles and their FR properties were investigated. Modified MoS2-containing EP nanocomposites showed a significant increment in FR activity and decreased

Scheme 7.1 Homo and hetero crosslinking between azobenzene and phenyl acetylene groups for aromatization and co-crosslinking [38]

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Fig. 7.2 A schematic of the surface modification of MoS2 nano spheres used to prepare flame-retardant polyurethane nanocomposites [49]. Reproduced with permission from Elsevier Science Ltd

HRR. The MoS2 sticking on the surface of the uniformly formed PSZ micro sphere, which acts as a template for MoS2 growth in different shapes like nano-sheets and nano-flowers (Fig. 7.2). MoS2 was modified with various metal oxides using a hydrothermal method and used to prepare polyethylene (PE) nanocomposites. The results implied good FR activity with decreased CO evolution compared to pristine PE [50]. From literature, it can be understood that most transition metal oxides inhibit CO evolution, with Cu oxides being the best. Cai et al. [51] modified MoS2 nano sheets with PEI and DOPO and prepared PU nanocomposites. These nanocomposites exhibited excellent thermal, FR, and mechanical properties even at low filler loadings. Polymer nanocomposites were prepared with different modified MoS2 nano sheets and they showed excellent FR activity with decreased toxic gas evolution [52, 53]. PU nanocomposites were prepared using zinc hydroxyl stannate in combination with APP and they showed excellent FR activity and achieved a V-0 rating in the UL-94 test without dripping unlike pure PU, which failed the UL-94 test and burned with dripping [54]. It was also observed that the evolution of toxic gases, such as CO and HCN, was strongly inhibited. ZrP is an inorganic layered compound first synthesized in 1964 by Clearfield and Stynes [55]. This structure can vary depending on the reaction conditions, such as temperature and phosphoric acid concentration [56]. The layered structure of ZrP

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resembles the montmorillonite (MMT) clay structure; in the layered structure, the Zr atoms are connected to the oxygen atoms of phosphate groups. The ZrP layered structure was modified with different types of organic compounds and organo-phosphorus compounds similar to MMT clays to prepare polymer nanocomposites for FR applications. Zhang et al. [57] modified ZrP with 1,2,2,6,6-pentamethyl-4-vinylidiethoxysiloxy piperidine (PMVP) to synthesize intercalated ZrP (F-ZrP), as shown in Fig. 7.3. Subsequently, they prepared nanocomposites of liquid silicon rubbers (ALSR) with ZrP and F-ZrP and obtained a mixer of intercalated and exfoliated nanocomposites. FR activity studies revealed that 4 wt% F-ZrP composites of ALSR showed strong FR activity with high LOI values and a V-0 rating in the UL-94 test. Similarly, Liu et al. [58] modified ZrP with organo-phosphorus compounds and prepared PLA nanocomposites. The composites exhibited an intercalated morphology and displayed efficient FR activity and passed the UL-94 test. ZrP modified with melamine and hexachloro cyclotriphosphazene (HCCP) was used to prepare poly (vinyl alcohol) nanocomposites, which exhibited high FR activity [59]. ZrP modified with different kinds of N, P-containing compounds was used for polymer nanocomposite preparation. These composites exhibited improved FR activity [60–63].

Fig. 7.3 Preparation of ALSR/F-ZrP nanocomposite. a Synthesis of PMVP, b intercalation process of F-ZrP, and c preparation of ALSR/F-ZrP nanocomposites [57]. Reproduced with permission from Elsevier Science Ltd

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Recently, Kuruma et al. [64] reported the development of new-generation PU composite materials containing surface-modified MoS2 nanosheets. Nanostructured MoS2 was synthesized using a hydrothermal method. To improve their compatibility with the PU matrix, the synthesized MoS2 nanosheet surface was modified with melamine, an eco-friendly and non-corrosive organic compound containing a high nitrogen number. In a typical synthesis process, the poly(ethylene glycol) (PEG) polyol (18.4 g) was first fed into a round bottom flask and stirred for 1 h under N2 atmosphere at 85 °C. After that, excess methylene diphenyl diisocyanate (MDI) was dissolved in 30 mL of solvent mixture dimethyl acetamide (DMAc): tetrahydrofuran (THF) = 3:2 v/v ratio) separately, and then slowly added to the polyol together with 3 drops of dibutyl tindilaurate (DBTDL) catalyst. Then, the reaction was continued for 2 h to obtain the “–NCO” terminated PU polymer. Afterwards, a calculated amount of 3-amino-propyl-trimethoxy silane (APTMS) (0.9 mL) was added to the mixture to react with the remaining “–NCO” by forming the urethane urea bonds. The stoichiometric ratio –NCO/–OH of 1 was maintained in all cases. The reaction was allowed to continue until “–NCO” was completely converted to urethane, as monitored by IR spectra until the complete disappearance of –NCO peak at 2270 cm−1 [65]. Into the reaction mixer, M-MoS2 at a calculated weight percentage with respect to the polyol was added, and the reaction continued for another 3 h. Finally, the obtained homogeneous and viscous solution was poured into a Teflon mold and kept in an oven overnight (14 h) at 80 °C, and then kept in a vacuum oven for one day (24 h) at 60 °C for complete drying. The same procedure was carried out for the synthesis of PU/MoS2 composite, except using varying weight percentages of MoS2 nanosheets. The reaction conditions and step-by-step chemical reactions are schematically presented in Fig. 7.4. The formation of MoS2, melamine-functionalized MoS2, and their PU composites was confirmed by Fourier-transform infrared (FT-IR) spectroscopy. X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) were used to study the structure and morphology of the various samples. Cone calorimetry tests were carried out to evaluate the flammability of pure PU and its MoS2-containing composites. Figure 7.5 reports the heat release rate (HRR) (Fig. 7.5a), total heat release rate (THR) (Fig. 7.5b), total smoke release (TSR) (Fig. 7.5c), and mass loss rates (MLR) (Fig. 7.5d) with time. In addition, important parameters for the complete analysis of FR activity, such as the time to ignite (TTI), peak heat releasing rate (PHRR), total smoke release (TSR), time to PHRR (tPHRR), fire growth index (FGI), and fire performance index (FPI), were calculated from Fig. 7.5 and summarized in Table 7.1. After the pristine PU ignition, it exhibited PHRR and THR values of 477.4 kW m−2 and 16.9 MJ m−2, respectively. After the addition of MoS2 and M-MoS2 to the PU, the TTI values of the composites slightly decreased, since the MoS2 nanosheets catalyze polymer degradation as explained in the previous section. Moreover, MoS2 nanosheets in the polymer matrix increase the viscosity and rapidly increase the surface temperature with the physical barrier effect of the nanosheets, and therefore, the polymer decomposes sooner than neat PU. In other words, it burns in the early stage to form

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Fig. 7.4 Schematics of the synthesis of PU/M-MoS2 composite

a thermally insulating char, which can prevent heat and oxygen from contacting the polymer underneath, resulting decreased flammability. Therefore, compared to neat PU polymer, the MoS2-containing PU composite burns slowly and the PU/ M-MoS2-5% composite exhibits better FR activity. The PHRR and THR values of pure PU decreased to 259.5 kW m−2 and 10.1 MJ m−2 after composite formation with M-MoS2 (5 wt%), and the relative reductions are 45% and 40%, respectively.

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Fig. 7.5 Cone calorimetry plots of PU and PU/M-MoS2 composites with different weight percentages: a HRR, b THRR, c TSR, and d mass loss plots

Table 7.1 Cone calorimetry data of PU/M-MoS2 composites with different weight percentages Sample

PHRR (kWm−2)

TTI (s)

tPHRR (s)

THR (MJm−2)

FGI (kWm−2 s−1)

FPI (m2 s/kW)

TSR (m2/m2)

Residue (wt%)

PU

477.4

41.0

108.4

16.9

4.4

0.09

644.3

15.2

PU/MoS2-5%

468.5

38.8

72.3

15.2

6.3

0.08

841.8

18.3

PU/MMoS2-1%

277.5

35.4

81.3

11.9

3.4

0.13

465.1

22.1

PU/M-MoS2-3%

308.1

32.8

108.0

12.3

2.8

0.10

413.2

26.1

PU/M-MoS2-5%

259.5

32.5

108.6

10.1

2.4

0.12

360.2

28.3

FGI = PHRR/tPHRR and FPI = TTI/PHRR

Such remarkable reduction in HRR and THR indicates that the more organic structural part in the PU composites are involved in the carbonization process, with some of it remaining in the condensed phase and some is converted to non-flammable fuel in the gas phase. Moreover, with increasing M-MoS2 percentage in the PU composite, the formed char is stronger with a high degree of crosslinking, which improves the FR activity.

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To completely analyze fire hazard properties of the PU composites, we calculated the fire growth index (FGI) and fire performance index (FPI) from HRR plots, and the obtained values are also reported in Table 7.1. The FGI value allows the estimation of flame spread rate, and a lower FGI value indicates delayed flashover. The FGI value is significantly decreased from 4.4 kW/m2s for PU to 2.4 kW/m2s for PU/M-MoS2-5%. However, for PU/MoS2-5% the FGI value increased to 6.3 kW/m2s. The reason is that the combustion of PU/MoS2 forms a porous and unstable char, as reported below. Similarly, the FPI values of PU, PU/MoS2-5%, and PU-M-MoS2-5% are 0.09, 0.08, and 0.12 m2 s/kW, respectively. These values clearly show that PU composites containing M-MoS2 exhibit strong FR activity with decreased fire risks. Hence, we conclude that surface modification of M-MoS2 nanosheets improves the FR activity of PU/MoS2 composites, since the abundant – NH2 groups in melamine mean strong interactions with silicate and urethane functionalities of PU chains, and the extra hydrogen bonds cause more extensive interfacial mixing of organic and inorganic phases. Therefore, during combustion, the PU/M-MoS2 composites form a strong and highly crosslinked char layer, which is responsible for the better FR properties. Figure 7.5c reports the total smoke release (TSR) plots of all the PU composites, and the corresponding data is included in Table 7.1. Clearly, the TSR values decreased from neat PU to PU/M-MoS2 composites, and it also decreased with increasing M-MoS2 content in the PU composite, owing to the formation of highly crosslinked char. Generally, smoke can develop from the release of incompletely burned volatile components during combustion. If the formed char is porous and loose, then more such volatile components will be released and condense to form a dense smoke. Figure 7.5d and Table 7.1 report the percentage of char residues after cone calorimetry test. From neat PU to PU/MoS2-5% and PU/M-MoS2-5%, the ratio of char residues significantly increased, with the respective percentages of 15.2%, 18.3%, and 28.3%. Thus, the PU/M-MoS2 composite produces a significantly increased amount of residual char.

7.2

Clay-Based Flame-Retardant Polymer Nanocomposites

Nano clays are layered mineral silicates and based on the arrangement of the silicate structure and chemical components, nano clays are classified into several types, such as MMT, kaolinite, bentonite, and halloysite. Generally, polymer nanocomposites can be prepared by the introduction of silicate layers into the polymer matrix. Many studies have been conducted on the polymer layered silicate nanocomposites for different applications. The presence of silicate layers in the polymer matrix can enhance its thermal, mechanical, barrier, and flammability properties owing to their high aspect ratio, large surface area, and nanoscale dispersion. Most of the aforementioned properties depend on the quality of dispersion;

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hence, it is important to improve the dispersion of nano clays in the polymer matrix. Inorganic clays are modified with organic compounds by exchanging interlayer cations with organic cationic surfactants, resulting in a decrease in the surface energy of the inorganic clay. This improves the compatibility of the nano clay with the polymer matrix. Organic cationic compounds can increase the interlayer spacing due to the presence of long alkyl chains. Ray et al. [66] reviewed and studied the surface modification of different types of clays and polymer nanocomposites. Typically, the FR activity and other properties are dependent on the dispersion quality in the polymer matrix and the organic modifier. Polymer-clay nanocomposites show FR activity due to the following reason. During polymer burning, the viscosity of the molten polymer composite decreases, which increases the migration of nano clay to the polymer surface and creates FR active sites on the polymer surface. This accelerates char formation and reduces flammability, similar to other surface-modified metal oxides and carbon-based nanocomposites. Therefore, clay surface modification plays an important role in improving the compatibility with the polymer as well as in increasing the FR activity. Different types of nitrogen and phosphorus-containing organic compounds are used for the surface modification of clays to prepare FR polymer nanocomposites and also to meet special requirements [67, 68]. It is assumed that the synergism between P and N improves the FR activity of polymer nanocomposites. Commonly, polymer nanocomposites are prepared by three processes, namely melt blending, solvent casting, and in situ polymerization. Compared to other methods, in situ polymerization has certain advantages. The monomer participates in interfacial interactions with the clays, which helps in enhancing the properties of the composites. Tai et al. [69] prepared PDEPD/clay composites via in situ polymerization; DDE was intercalated into the clay platelets followed by the addition of PDCP. The monomer was polymerized inside the clay and during the polymerization process, HCl was eliminated (Fig. 7.6). Further, PS and PU clay nanocomposites were prepared at varying nano clay percentages and they were found to exhibit high FR activity. PU composites showed higher FR activity compared to PS due to the occurrence of rearrangement reactions involving PDEPD during the combustion of PUs and increased char density. Zhu et al. [70] prepared PMMA/clay nanocomposites via in situ polymerization with MMT modified with different quaternary ammonium salts and they observed exfoliated structure formation with unsaturated quaternary ammonium salts. In the case of saturated ammonium salts, an intercalated structure was formed. Many studies have indicated that exfoliated polymer nanocomposites exhibited better properties than intercalated structures. Si et al. [71] prepared PMMA/clay nanocomposites using a combination of traditional DB and AO flame retardants. They observed that those nanocomposites containing a combination of the three components showed excellent FR activity with self-extinguishing ability. In this case, clay helps in flame quenching, which means that it accelerates char formation, increases FR dispersion, and catalyzes chain reactions. Similarly, Song et al. [72] prepared PU/clay nanocomposites with and without MPP as a FR; they observed that the inclusion of MPP induced excellent FR activity with high LOI values compared to other nanocomposites owing to a synergetic effect.

7.2 Clay-Based Flame-Retardant Polymer Nanocomposites

95

Fig. 7.6 In-situ polymerization of P,N-containing PDEPD/clay nanocomposites [69]. Reproduced with permission from Elsevier Science Ltd

Nano fibrillated cellulose (NFC) nanocomposite films were prepared with dispersed monolayer clay nano platelets; films with 50% loading showed high transparency, very high LOI values of *96.5%, and self-extinguishing ability [73]. Barrier formation is also important for FR activity, because it prevents heat and oxygen transfer, thus inhibiting flame propagation. Clay addition to a polymer matrix drastically improves its barrier properties by creating maze-like or tortuous paths that hinder gas molecule diffusion through the polymer matrix. Thus, barrier formation mainly depends on the interactions between the clay and polymer. Alonso et al. [74] prepared PU/clay nanocomposites using different types of clays, including Cloisite®10A (C10A), Cloisite®20A (C20A), and Cloisite®30B (C30B) and investigated the effect of clay structure on the barrier properties and FR activity. The barrier properties follow the order of C30B > C10A > C20A. C10A contains benzene rings in the clay structure and polymer chains cannot penetrate the clay gallery, whereas C30B contains two hydroxyl groups of tallows, which help the penetration of polymer chains into the clay gallery. They also form hydrogen bonds with the PU chains. Based on the interactions between the organically modified clay and polymer, the formation of clay nanocomposite structures is classified into three types, high-interactions exfoliation, low-interactions intercalation, and very low-interactions aggregation or delamination. Kuruma et al. [75] prepared water-dispersible PU-clay nanocomposites using two different organically modified clays and they observed three different clay nanocomposite structures; the exfoliated structures showed a high improvement in the composite properties.

96

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MMT clay was organically modified to prepare polymer nanocomposites with melamine phosphate by a cation exchange process in water; the organic compound was intercalated with MMT clay platelets and resulted in an increased d-spacing, as shown in Fig. 7.7. During polymer combustion, the non-combustible gases evolving from the decomposition of blowing agent MA-MMT/MPP contribute to the IFR activity and anti-dripping properties. Some photographs of the tested specimens are shown in Fig. 7.7; it can be clearly seen that in the presence of MA-MMT and MPP, excellent intumescent char formation occurred and it resulted in a strong anti-dripping characteristic compared to other composites due to the high melt viscosity of the polymer in the presence of MMT clay [76, 77]. PA6-3 (MA-MMT/MPP:1/24) showed the highest LOI value and achieved a V-0 rating in the UL-94 test, whereas PA6-6 (MA-MMT/MPP:5/20) showed a low LOI value and no rating in the UL-94 test. The reason behind this observation is not clearly known but it may be assumed that at particular compositions of MA-MMT/MPP, there occurs a strong synergetic reaction between the clay and MPP; furthermore, the PA6-3 system contained a higher quantity of phosphorus than the PA6-6 system. Phosphorus can move to the surface along with the MMT clay and enrich and strengthen the char layer [78]. Shan et al. [79] prepared TPU nanocomposites using combinations of an IFR (APP + PER) and NaNiP and studied the importance of composition and PER in determining the FR activity. They noticed that TPU nanocomposite systems containing TPU3 (APP + PER/NaNiP: 19/1) showed a V-0 rating in the UL-94 test, high LOI value of 32%. With increasing NaNiP content in TPU5 (APP + PER/NaNiP: 15/5), the LOI value decreased and the composite achieved a V-2 rating in the UL-94 test. Literature clearly indicates that the synergetic effect and phosphorus content play important roles in such cases. At particular compositions, one can observe a strong synergetic effect and the presence of NaNiP induces dense intumescent char formation, thus leading to strong IFR activity. A variety of P,N-based compounds and metal oxides have been used for clay surface modification to prepare polymer nanocomposites with different IFRs (Table 7.2). It is clear that the FR activity significantly improved with a combination of IFRs at specific compositions. In some cases, increasing the modified clay content decreased the IFR activity because there was no synergetic effect. In addition, phosphorus content also influences the IFR activity (Fig. 7.7).

7.3

Graphene-Based FR PNCs

Graphene has a layered structure and the carbon atoms are arranged in monolayers similar to a honeycomb network structure; graphene has a large surface area and strong absorption capacity. Owing to its structural features, graphene exhibits high thermal stability, mechanical strength, strong barrier properties, and toxic gas absorption capacity. GO exhibits good compatibility and strong interactions with polymers and can be used to form 3D polymer network structures. Hence, graphene has attracted the attention of polymer chemists to prepare polymer nanocomposites

7.3 Graphene-Based FR PNCs

97

Fig. 7.7 Schematic of various stages. a Intercalation of organic compounds into MMT clay platelets. b Melt-blended PA6/clay composites. c Evolution of non-combustible gases from the blowing agent during thermal decomposition. d Char residue on MMT clay platelets after combustion. e Polymer film specimens of PA6 and clay nanocomposites after LOI testing. f Polymer specimens after the UL-94 vertical burning test [78]. Reproduced with permission from Elsevier Science Ltd

with excellent mechanical and FR properties. In addition, GO contains oxygen groups, which may undergo decomposition and dehydration even at low temperatures, which cools down the surroundings during combustion and decreases oxygen concentration. Therefore, graphene can effectively decrease the HRR and improve char formation during polymer combustion compared to layered silicate clays and carbon nanotubes. Surface modification of graphene is an important objective in the preparation of polymer nanocomposites. Due to the occurrence of p-p interactions and van der Waals forces, it is difficult to disperse graphene evenly in the polymer matrix. Therefore, surface modification is necessary to avoid such problems. Graphene and surface-modified GO have been widely used to prepare FR polymer nanocomposites. FR properties typically depend on the nature of the surface modifier and its interactions with the polymer matrix. Graphene modified with various metal oxides was used to prepare polymer nanocomposites; it acts as an excellent smoke suppressant [1, 23, 89, 90]. Xu et al. [91] synthesized MgAl-layered double hydroxide loaded graphene (RGO-LDH) via co-precipitation and introduced CuMoO4 on the surfaces of RGO-LDH to obtain hydrides of combination metal oxides, as shown in Fig. 7.8. Further, they used these compounds to prepare EP nanocomposites with different loadings and observed a significant improvement in FR activity and smoke suppression. Combinations of metal oxides with graphene lead to better properties; during burning, Cu2O and MoO3 are generated from RGO-LDH/CuMoO4. Cu2O is well known to suppress of CO,

98

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Polymer Nanocomposites for Fire Retardant Applications

Table 7.2 The IFR activity of polymer clay nanocomposites with various types of surface-modified clays and combinations of IFRs Sample

THR (MJ/m2)

HRR (kW/m2)

LOI

UL-94

References

PP

136

1390

17

NR

[16]

PP/IFR

127

759

26.5

V-2

PP/IFR/Co-MMT-4%

112

503

32.1

V-0

PP

–

–

17

NR

PP/IFR

–

–

21

NR

PP/Fe-OMMT-(6%)

–

–

29

V-0

PP/Na-MMT-(4%)

–

–

24

NR

PI

11.5

92.8

44.6

–

PI/G5/M10

10.7

52.5

55

–

PU

–

–

22

NR

PU/IFR

–

–

29

V-1

PU/IFR/rGO-2%

–

–

34

V-0

PBS

23

NR

PBS/IFR

31

V-1

PBS/IFR/GNS-2%

33

V-0

TPU

78.2

1031

–

–

TPU/CAHPI-20%

73.0

622

–

–

TPU

–

–

21

NR

TPU/IFR

–

–

27

NR

TPU/IFR/NaNiP-3%

–

–

30

V-0

TPU/IFR/NaNiP-5%

–

–

28

V-2

TPU

71.8

1013.7

22

V-2

TPU/APP

49.9

247.8

25.5

V-2

TPU/LRAPP-0.5

44.1

170.6

25.5

V-0

TPU/LRAPP-1

49.8

215.6

25

V-2

PA-11

125

943

22.2

NR

PA-11/APP-AM

56

249

28.3

V-2

PA-11/APP-AM/TiO2-3%

44

177

29.2

V-0

PU-(I)

2561

741

–

–

PU-(I)/O-MMT

918

344

–

–

PU-(II)

2254

637

–

–

PU-(II)/O-MMT

641

363

–

–

PU

90

523

–

–

PU/c-MMT-10%

88

436

–

– –

PU/Na-MMT-4%

86

410

–

PP

–

841

17.6

NR

PP/IFR

–

247

30.5

NR

PP/IFR/O-Clay-2%

–

214

28.8

V-0

[80]

[81] [82]

[83]

[84] [79]

[85]

[36]

[86]

[87]

[88]

7.3 Graphene-Based FR PNCs

99

HCN, and other toxic gases and further, it catalyzes char formation and compactness. It may be expected that such hybrid RGO-LDH/CuMoO4 compounds are efficient in preparing FR-PU PNCs. In the presence of Cu2O, the released HCN from PU combustion can be oxidized and converted into CO2, N2, H2O, and NO2; additionally, toxic CO is converted to CO2. Similarly, iron lignosulfonate was employed to modify graphene via non-covalent interactions and PU nanocomposites were prepared; they exhibited significantly improved thermal, mechanical, and FR properties [92]. ZnS/GNS hybrids were successfully prepared by a hydrothermal method and further used to develop EP nanocomposites. Strong inhibition of toxic gas evolution was observed along with a decrease in the HRR. Graphene sheets act as barriers and inhibit heat and toxic gas evolution, while ZnS exerts a catalytic effect [93, 94]. Generally, phosphorus-based FRs are considered as halogen-free FRs, but they are not suitable for heat-resistant resins as they adversely affect the thermal stability and toughness. Therefore, to overcome this problem, surface-modified GO composites containing different functionalities were prepared. Zhang et al. [95] prepared cyanate ester (CE) composites with modified and unmodified GO such as FGO/CE and GO/CE. At the same loading levels, FGO/CE composites showed stronger FR activity than GO/CE composites. Sodium metaborate is used as a cross-linker in polymers; it imparts high adhesiveness, antibacterial activity, and is considered a FR additive due to its heat-sink nature [96–100]. GO reduction can be performed using NaBH4 in a simple aqueous solution and during reduction, sodium metaborate (SMB) is formed, which shows FR activity. A number of studies proved that a

Fig. 7.8 Schematic of a GO hybrid with a combination of metal oxides [91]. Reproduced with permission from Elsevier Science Ltd

100

7

Polymer Nanocomposites for Fire Retardant Applications

combination of SMB with graphene leads to a strong FR activity, as this combination generates various types of fire-protecting functionalities [101–104]. rGO/ SMB was coated on the surface of cellulosic materials, which subsequently displayed excellent FR activity and self-extinguishing ability. This is because of the strong barrier properties and synergetic effect between GO and SMB. Moreover, inside graphene layers the specific structural morphology formation such as “nacre-like structural morphology” [101]. Similarly, to enhance the abrasion resistance and FR properties of surface coatings for industrial applications, multi-functional graphene nanocomposites, which can provide “Swiss-Knife” or “3 in 1” protective properties, such as abrasion resistance, FR activity, and antibacterial activity, were prepared. A schematic model of such “3 in 1” coatings is shown

Fig. 7.9 Preparation of highly adhesive graphene protective coatings for “3 in 1” FR coatings. a Schematic of the synthesis of multifunctional graphene by reduction. b Physical structure of rGO/SMB composites. c Enhanced abrasion resistance, FR activity, and antibacterial activity of “3 in 1” coatings. d Proposed model for the fire-retardant mechanism of rGO/SMB layers [104]. Reproduced with permission from Elsevier Science Ltd

7.3 Graphene-Based FR PNCs

101

in Fig. 7.9a, b, c; multi functionalities were created by GO reduction with NaBH4. The growth of SMB crystals in the GO layers leads to the formation of a “nacre-like” structure, which exhibits multiple modes of fire protection. Industrial GO/SMB composite wood coatings exhibited a strong intumescent character, non-flammable nature, and self-extinguishability. Therefore, it has been reported that the combination of rGO and SMB shows IFR activity, similar to nitrogen and phosphorus-based salts. In the presence of hydrated SMB which is present in between rGO layers, swelling is possible. At the same time, rGO leads to the formation of char. rGO/SMB starts releasing water at high temperatures, which increases the volume of char. During combustion the evaporated water molecules inside char come out through impermeable graphene layers, which lead to the formation of intumescent char formation. This characteristic prevents the evolution of combustible and toxic gases as shown in Fig. 7.9d; further, it prevents oxygen attack on the material [104]. Tang et al. [105] prepared FR systems of DDPPU in combination with traditional FRs. They observed that in the case of DDPPU combined with H3BO3, there was a significant improvement in the LOI values (46%) due to a synergetic effect and char formation. An intumescent flame retardant, PPSPB, was covalently grafted on the surface of GO and used to prepare EVA nanocomposites. The results showed increased FR activity for the resulting nanocomposites and increase in Tg and modulus due to surface modification with PPSPB, which help in the uniform dispersion of graphene in polymer matrix. In addition, the THR, PHRR, and AMLR decreased [106]. Generally, phosphonamidates decrease dripping and increase the melt viscosity at high temperatures. Guo et al. [107] synthesized hybrids of phosphonamidates/rGO and incorporated them into EP resins; the resulting nanocomposite resins displayed simultaneous improvement in mechanical and FR properties. Nanocomposite with 4 wt% filler loading showed remarkable FR activity, achieved a V-0 rating in the UL-94 test without dripping, and significantly reduced THR and PHRR values compared to neat EP resins. Jin et al. [108] modified GO with P,N-containing dendrimers (PND-GO) and used them to prepare PU nanocomposites. The formed exfoliated structures exhibited strong FR activity with a reduction in PHRR and increased ignition time compared to neat PU. GO surfaces were also modified with DOPO via covalent bond formation and the modified GO particles were used to prepare EP resins with excellent mechanical properties and FR activity. DOPO modification improved the dispersion quality and at high temperatures, uniform sheet-like char layers were formed, which hindered flame propagation [89, 109]. GOTP foams were prepared by mixing GO solution and hexachlorocyclotriphosphazenes (HCTP); the foams showed excellent FR activity compared to traditional polymers and their nanocomposite [110]. In the presence of flame, HCTP-functionalized GO spontaneously converted into a 3D graphene network structure, which improved the melt viscosity of the polymer. It is well known that metal oxides improve char formation owing to a catalytic effect. Studies revealed that DOPO in combination with rGO showed excellent FR activity due to a synergetic effect between DOPO and rGO; further, a combined gas-phase and solid-phase FR activity was observed [111]. Ethylene-vinyl acetate thermoset rubbers (EVM) are

102

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Polymer Nanocomposites for Fire Retardant Applications

mainly used in electronic wires and cables and hence it is important to improve their FR activity. Lu et al. [112] prepared EVM rubber composites with expandable graphite (EG) and APP; they noticed effective FR activity and reduced dripping due to excellent intumescent char formation. Co3O4 nanoparticles were decorated on graphene nano sheets synthesized by a hydrothermal method and were used to prepare TPU nanocomposites, which exhibited significant improvement in their thermal, mechanical, FR properties. In such systems, the fire toxicity is reduced drastically. Because these hybrid nano sheets contain different elements, a catalysis effect (cobalt) as well as absorption of toxicants (graphene) could be observed [113]. Similarly, a hyper branched FR was synthesized from N-aminoethyl piparazine and phosphonates; it was subsequently used to prepare PS nanocomposites, which exhibited low flame toxicity and high FR activity. Hyper branched FRs contain more functional groups and hence contribute to high FR property improvement [114]. TPU nanocomposites were prepared by the introduction of g-C3N4/AHPi into the TPU structure and the resulting nanocomposite films showed high fire safety [84]. g-C3N4 plays important role here by accelerating the decomposition of AHPi to form char; further, it promotes the release of free radicals from AHPi. PS nanocomposites were prepared with g-C3N4/DAHPi and similar results were observed—a high FR activity and suppression of pyrolysis gas components compared to neat PS [115]. However, the PS and TPU systems were not compared with each other for FR activity. The potential of fire hazards can be reduced with g-C3N4, but there are some problems, such as poor dispersion in the polymer matrix and chemical corrosion leading to partial structural scission of g-C3N4. To counter such problems, Shi et al. [116] prepared CPDCPAHPi and CBPODAHPi by esterification and salification and introduced them into PS to synthesize PS nanocomposites. They observed that the resultant PS nanocomposites exhibited reduced smoke releasing rate and PHRR; OAHPi releases non-flammable gases and strengthens the char structure. Cai et al. [117] prepared TPU nanocomposites with non-covalently modified multi-functional FR graphene nano sheets and they observed that 4 wt% FGNS showed much decreased PHRR and THR compared to neat PU because of conjugation between FGNS and the attached FR. Red phosphorus is also used widely as a FR; isocyanate-based polyimide (PI) foam nanocomposites have been prepared with red phosphorus (RP)-hybridized graphene (2.2 wt%). The foams showed a remarkable FR activity with a high LOI value 39.4%, thermal insulating character, and high mechanical properties [118]. The development of new methods is necessary to prepare surface-modified GO with various organic and inorganic compounds for FR polymer nanocomposite preparation.

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Index

A Acid compounds, 6 Aerospace coatings, 6 3-amino-propyl-trimethoxy silane, 75 Ammonium polyphosphates, 8 Auto extinguishability, 72 B Bentonite, 93 Bis(pentaerythritol phosphate) phosphoric acid, 18 Blowing-out, 75 Brominated, 2 C Carbonaceous char layer, 25 Carbonization and blowing agent, 25 Carbonization of polymers, 87 Char formation, 3 Char-forming agents, 8, 16 Char morphology, 73 Chlorinated, 2 CO2, 7 Combustion, 1 Condensed-phase FR mechanism, 6 Cone calorimeter test, 5 Cone colorimetry, 11 Crosslinking, 26 Cyanate ester, 99 Cyanuric trichloride, 16 D Degradation mechanism, 36 Degree of crosslinking, 47

Di-ammonium Hydrogen Phosphate (DAHP), 22 Dibutyl tindilaurate, 90 Dicyclic phosphorous-melamine derivative compounds, 18 4-diethoxyphosphoryloxyphenoxy, 30 Differential scanning calorimetry, 70 Diphenyl amine, 16 Ditrimethylolpropan, 32 E Elemental red phosphorus, 22 Engineering plastic material, 22 Epoxy FR resins, 31 Epoxy resins, 27 Ethylene-butyl acrylate maleic anhydride formulations, 18 Ethylene diamine, 16 Evolves combustible and non-combustible components, 11 Excellent FR properties, 25 Extinguish the flame, 75 F Fire accidents, 2 Fire hazards, 1 Fire rate growth, 86 Fire-retardancy, 16 Fire retardants, 1, 5 Flame propagation, 5 Flame resistance, 29 Flame retardance, 1 Flame-retardants, 15 Flammability, 1, 12

© Springer Nature Switzerland AG 2020 S. Sinha Ray and M. Kuruma, Halogen-Free Flame-Retardant Polymers, Springer Series in Materials Science 294, https://doi.org/10.1007/978-3-030-35491-6

111

112 Form an insulating char layer, 7 FR military clothing, 6 Fumes, 5 G Glass transition temperature, 2, 47 Glycidyloxy diphenylphosphine oxide, 27 Graphene, 2, 96 Graphitization, 83 H Halloysite, 93 Halogenated, 2 Halogenated FRs, 15 Halogens, 7 H-bonding, 8 Heat peak HRR, 1 Heat release rate, 1 Heat-releasing rate, 11 Hexachloro cyclotriphosphazenes, 29 Hexamethylene diisocyanate, 56 High chemical stability, 84 High flammability, 22 High wear resistance, 22 I Ignition temperature, 5 Initial ignition temperature, 34 Intumescent activity, 36 Intumescent coatings, 19 Intumescent flame retardant, 8 Ionic interactions, 8 Isophorone diisocyanate, 56 K Kaolinite, 93 L Limiting oxygen index, 6, 12 Low-density polyethylene, 16 Low thermal expansion, 25 M Mechanical properties, 2 Melamine cyanurate, 15 Melamine-diphenylphosphinic acid salt, 20 Melamine, guanidine, 15 Melamine hypophosphite, 21 Melamine phosphite, 21 Melamine polyphosphate, 8 Melamine pyrophosphate, 21 Melting dripping characteristics, 69 Melting temperature, 2, 47 Melt viscosity, 2

Index Metal hydroxides, 7 4,4′-methylene diphenyl diisocyanate, 56 Methyl isocyanate, 56 Microencapsulated ammonium polyphosphate, 22 M-MoS2, 77 Monoisocyanate compound, 56 Mono substituted P-benzoquinone, 24 Montmorillonite (MMT), 93 Morphological transformation, 3 N Nanoclay-functionalized metal oxides, 2 Nanofillers, 2, 83 Nanostructured materials, 83 Nitrogen and phosphorus-based salts, 18 Nitrogen-based compounds, 16 Nitrogen-based salts, 18 Non-combustible, 75 Non-flammable, 6 Non-flammable gases, 15, 70, 86 Non-halogenated, 2 Nucleophilic reaction, 16 O Organic phosphorus compounds, 23, 71 Orthophenyl phenol derivatives, 23 P Pentaerytropolyol phosphoric acid, 18 Phase separation and leaching, 8 Phosphaphenanthrene, 71 Phosphaphenanthrene groups, 71 Phosphate and phosphonic derivatives, 26 Phosphates, 22 Phosphazene and triazine, 30 Phosphazene includes, 29 Phosphazenes, 29, 31 Phosphine oxides, 22 Phosphites, 22 Phosphonates, 22 Phosphonium compounds, 22 Phosphoric acid esterifies, 26 Phosphorus, 6 Phosphorus-based compounds, 9 Phosphorus-based FRs, 23 Phosphorus-containing carbonizing agents, 32 Phosphorus-containing chain extender, 27 Phosphorus-containing epoxy resins, 37 Phosphorus-containing monomer, 28 Phosphorus-containing polymers, 25 Phosphorus derivatives, 7, 25 Phosphorus oxide, 23 Phosphorus skeleton compound, 36

Index Phosphorus tungstic acid, 18 Piperazine-modified ammonium polyphosphate, 19 Piperazine-modified APP, 19 Plastic materials, 1 P-, N-, and P & N-based salts, 2 Polybutylene terephthalate, 33 Poly(ethylene glycol), 90 Poly(melamine-ethoxyphosphinyldiisocyanate), 16 Polymer degradation, 87 Polymer degradation or combustion, 7 Polymer degrades, 11 Polymer flammability, 2 Polymeric matrices, 2 Polyolefin-based formulations, 18 Polyols and diisocyanates, 47 Polyoxymethylene, 22 PU cellular plastics, 15 PU foams displayed high thermal stability, 25 PU/MoS2composites, 93 PU nanocomposites, 88 R Reactive phosphazene, 31 S Self-extinguishability, 70 Self-extinguishing property, 20 Self-lubrication, 22 Small angle X-ray scattering, 57 Smoke, 1, 2, 5 Smoke-production rate, 11 Smoke-releasing property, 29 SO2, 7 Solid-state nuclear magnetic resonance, 19 Soluble ammonium polyphosphate, 18

113 Strong barrier properties, 19 Sulfur-containing acids, 6 Surface-modified GO composites, 99 Surface-modified nanofillers, 2 T Thermal decomposition, 48 Thermal degradation, 34 Thermal stability, 25 Thermoplastic and thermosetting polymers, 9 Total smoke release, 93 Toxic gases, 1, 5 Toxic gas inhalation, 2 Toxicity, 2 Toxic smoke evolution, 48 Transmission electron microscopy, 57 Triazine, 31 Triazine-containing polymer, 16 Triazine-trione, 71 Triazine-trione group, 71 2,6,7-trioxa-1-phosphabicyclo[2.2.2] octane-4-methanol, 16 Tunable bandgap, 84 U UL-94 test, 13 Underwriters Laboratory, 13 V Van der Waals forces, 8 Viscosity, 47 Volatile combustible substrates, 17 W Water, 7 Water-dispersible PUs, 50