Plasticizers Derived from Post-consumer PET: Research Trends and Potential Applications 0323462006, 9780323462006

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Plasticizers Derived from Post-consumer PET: Research Trends and Potential Applications
 0323462006, 9780323462006

Table of contents :
Cover
PLASTICS DESIGN LIBRARY (PDL) PDL HANDBOOK SERIES
PLASTICIZERS DERIVED FROM POST-CONSUMER PET
Copyright
Preface
1 -
Assessment of Traditional Plasticizers
1.1 General Characteristic of Plasticizers
1.2 Definitions
1.2.1 Internal Plasticization
1.2.2 External Plasticization
1.2.3 Polymerisable Plasticizer
1.2.4 Plastisol
1.2.5 Rigisol
1.2.6 Organosol
1.2.7 Plastigel
1.2.8 Dry Blend
1.2.9 Gelation
1.2.10 Plasticization
1.3 Role of Plasticizers in Plastic Processing
1.3.1 Theories Explaining Plasticization
References
2 -
Classification of Plasticizers
2.1 Introduction
2.1.1 Classification Based on Type of Plasticization
2.2 Monomeric Plasticizers
2.2.1 Phthalate Esters
2.2.2 Phosphate Ester
2.2.3 Dibasic Acid Ester
2.2.4 Trimellitate Ester
2.2.5 Citrate Esters
2.3 Extenders
2.4 Polymeric Plasticizers
2.5 Other Plasticizers
References
3 -
Essential Quality Parameters of Plasticizers
3.1 Basic Quality Parameters
3.1.1 Compatibility of the Plasticizer With the Polymer
3.1.1.1 Interaction Parameter
3.1.1.2 Solubility Parameter
3.2 Specific Quality Parameters
3.2.1 Volatility of Plasticizers
3.2.2 Plasticizer Efficiency
3.2.3 Permanence
3.3 Antiplasticization
References
4 -
Research Trends in Plasticizer Production
4.1 Petrochemical Alternatives
4.1.1 Adipates
4.1.2 Sebacates
4.1.3 Azelates
4.1.4 Trimellitate Esters
4.1.5 Phosphoric Acid Esters
4.1.6 Carboxylates
4.1.7 Benzoates
4.1.8 Sulfonate Esters
4.2 Plasticizers Manufactured from Renewable Raw Materials
4.2.1 Epoxidized Vegetable Oils and Epoxidized Fatty Acids
4.2.2 Esters of Succinic and Citric Acid
References
Further Reading
5 -
Methods of PET Recycling
5.1 Chemical Recycling
5.1.1 Hydrolysis
5.1.1.1 Alkaline Hydrolysis
5.1.1.2 Alkaline Hydrolysis in the Presence of a Phase Transfer Catalyst
5.1.1.3 PET Hydrolysis in a Nonaqueous Alkaline Solution
5.1.1.4 Acid Hydrolysis
5.1.1.5 Neutral Hydrolysis
5.1.2 Glycolysis
5.1.3 Alcoholysis
5.1.4 Aminolysis
5.2 Mechanical Recycling
5.3 Energetic Recycling
References
6 -
Synthesis of Plasticizers From Postconsumer PET
6.1 Introduction
6.2 Synthesis of Monomeric Plasticizers
6.3 Synthesis of Oligomeric Plasticizers
References
7 -
Application of New, Synthesized Plasticizers
References
Index
Back Cover

Citation preview

PLASTICS DESIGN LIBRARY (PDL) PDL HANDBOOK SERIES Series Editor: Sina Ebnesajjad, PhD ([email protected]) President, FluoroConsultants Group, LLC Chadds Ford, PA, USA http://www.FluoroConsultants.com The PDL Handbook Series is aimed at a wide range of engineers and other professionals working in the plastics industry, and related sectors using plastics and adhesives. PDL is a series of data books, reference works and practical guides covering plastics engineering, applications, processing, and manufacturing, and applied aspects of polymer science, elastomers and adhesives. Recent titles in the series Phthalonitrile Resins and Composites, Derradji, Jun & Wenbin (ISBN: 9780128129661) Chemical Resistance of Thermosets, Baur, Ruhrberg & Woishnis (ISBN: 9780128144800) Recycling of Polyurethane Foams, Thomas et al. (ISBN: 9780323511339) Thermoplastics and Thermoplastic Composites, Biron (ISBN: 9780081025017) Dielectric Polymer Materials for High-Density Energy Storage, Dang (ISBN: 9780128132159) Recycling of Polyethylene Terephthalate Bottles, Thomas et al. (ISBN: 9780128113615) Plastics to Energy, Al-Salem (ISBN: 9780128131404) Electrical Conductivity in Polymer-based Composites, Taherian & Kausar (ISBN: 9780128125410) Hydraulic Rubber Dam, Thomas et al. (ISBN: 9780128122105) Introduction to Plastics Engineering, Shrivastava (ISBN: 9780323395007) The Effect of Sterilization Methods on Plastics and Elastomers, 4e, McKeen (ISBN: 9780128145111) Polymeric Foams Structure-Property-Performance, Obi (ISBN: 9781455777556) Technology and Applications of Polymers Derived from Biomass, Ashter (ISBN: 9780323511155) Fluoropolymer Applications in the Chemical Processing Industries, 2e, Ebnesajjad & Khaladkar (ISBN: 9780323447164) Reactive Polymers, 3e, Fink (ISBN: 9780128145098) Service Life Prediction of Polymers and Plastics Exposed to Outdoor Weathering, White, White & Pickett, (ISBN:9780323497763) Polylactide Foams, Nofar & Park (ISBN: 9780128139912) Designing Successful Products with Plastics, Maclean-Blevins (ISBN: 9780323445016) Waste Management of Marine Plastics Debris, Niaounakis, (ISBN: 9780323443548) Film Properties of Plastics and Elastomers, 4e, McKeen, (ISBN: 9780128132920) Anticorrosive Rubber Lining, Chandrasekaran (ISBN: 9780323443715) Shape-Memory Polymer Device Design Safranski & Griffis, (ISBN: 9780323377973) A Guide to the Manufacture, Performance, and Potential of Plastics in Agriculture, Orzolek, (ISBN: 9780081021705) Plastics in Medical Devices for Cardiovascular Applications, Padsalgikar, (ISBN: 9780323358859) Industrial Applications of Renewable Plastics, Biron (ISBN: 9780323480659) Permeability Properties of Plastics and Elastomers, 4e, McKeen, (ISBN: 9780323508599) Expanded PTFE Applications Handbook, Ebnesajjad (ISBN: 9781437778557) Applied Plastics Engineering Handbook, 2e, Kutz (ISBN: 9780323390408) Modification of Polymer Properties, Jasso-Gastinel & Kenny (ISBN: 9780323443531) The Science and Technology of Flexible Packaging, Morris (ISBN: 9780323242738) Stretch Blow Molding, 3e, Brandau (ISBN: 9780323461771) Chemical Resistance of Engineering Thermoplastics, Baur, Ruhrberg & Woishnis (ISBN: 9780323473576) Chemical Resistance of Commodity Thermoplastics, Baur, Ruhrberg & Woishnis (ISBN: 9780323473583) Color Trends and Selection for Product Design, Becker, (ISBN: 9780323393959) Fluoroelastomers Handbook, 2e, Drobny (ISBN: 9780323394802) Introduction to Bioplastics Engineering, Ashter (ISBN: 9780323393966) Multilayer Flexible Packaging, 2e, Wagner, Jr. (ISBN: 9780323371001) Fatigue and Tribological Properties of Plastics and Elastomers, 3e, McKeen (ISBN: 9780323442015) Emerging Trends in Medical Plastic Engineering and Manufacturing, Scho¨nberger & Hoffstetter (ISBN: 9780323370233) Manufacturing and Novel Applications of Multilayer Polymer Films, Langhe & Ponting (ISBN: 9780323371254) To submit a new book proposal for the series, or place an order, please contact Edward Payne, Acquisitions Editor at [email protected]

PLASTICIZERS DERIVED FROM POST-CONSUMER PET Research Trends and Potential Applications

Ewa Langer Krzysztof Bortel Sylwia Waskiewicz Marta Lenartowicz-Klik

William Andrew is an imprint of Elsevier The Boulevard, Langford Lane, Kidlington, Oxford, OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States Copyright © 2020 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN: 978-0-323-46200-6 For information on all William Andrew publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Matthew Deans Acquisition Editor: Edward Payne Editorial Project Manager: Peter Adamson Production Project Manager: Sruthi Satheesh Cover Designer: Matthew Limbert Typeset by TNQ Technologies

Preface Postconsumer waste, including synthetic packaging waste in particular, poses a major environmental problem. What inspired us to begin research on poly(ethylene terephthalate) (PET) recycling was the current focus on this matter among research centers worldwide. A factor that encouraged us to write this book included the positive results of several years of work on the synthesis of oligomeric plasticizers from waste PET. The book presents a classification of plasticizers with regard to their chemical structure and properties. Theories explaining the plasticization processes and the role of plasticizers in PET processing are discussed. The main quality parameters of plasticizers, which determine their suitability for polymer plasticization, i.e., compatibility, plasticizing efficiency, and volatility, have been reviewed. The antiplastification effect has been explained, and current trends in the substitution of phthalate plasticizers are also discussed. PET recycling methods are the subject of one of the book’s chapters. The results of synthesis of plasticizers from postconsumer PET are presented. The synthesis method proposed by us makes it possible to significantly reduce the amount of by-products generated during the process, which distinguishes this solution from other methods described in the literature. Considering the possibility of using a wide range of dicarboxylic acids and glycols in synthesis, the presented method can be considered universal as it makes it possible to obtain products with the desired properties. The possibilities relating to the practical application of the synthesized products, including their use on an industrial scale, have been illustrated by the results of studies on poly(vinyl chloride) compositions, in which the new plasticizers were used. During synthesis, many solid products were obtained, which have not been characterized in this book, but can surely be used for modifying thermoplastic polymers. The book is intended for students of technical universities, polytechnics, and other schools of higher education, for experts specializing in oligoester synthesis, in the modification and recycling of polymeric materials, and for technologists working in the plastics processing industry.

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1 Assessment of Traditional Plasticizers 1.1 General Characteristic of Plasticizers Plasticizers are an important class of compounds widely used as additives in the polymer industry to improve the properties and processing characteristics of polymers. The definition of plasticizers adopted by the International Union of Pure and Applied Chemistry (IUPAC) in 1951 is still generally accepted: a substance incorporated in a material (usually a plastic or elastomer) to increase its flexibility, workability, or distensibility. IUPAC defines a plasticizer as a substance or material incorporated in a material (usually a plastic or elastomer) to increase its flexibility, workability, or distensibility [1]. These compounds are intended to reduce the tension of deformation, hardness, density, viscosity, and electrostatic charge of a polymer, while increasing the polymer’s chain flexibility by lowering the glass transition temperature (Tg) and resistance to fracture [2]. Plasticizers are used in many polymers, but about 90% of global plasticizer production is used to produce flexible poly(vinyl chloride) (PVC) [3]. They are typically used in the range of 10e50 phr, except in PVC, where the range is about 30e100 phr. In flexible PVC, approximately 50 phr plasticizer is used. Higher quantities can change the hard rigid unplasticized PVC to a soft rubbery material [4]. The plasticizer should be compatible with PVC; be stable; have good permanence, low volatility, low odor, low toxicity and low color; be cost-effective; and not cause negative interaction with other essential ingredients comprising the formulation [5]. The plasticizers used contain polar and nonpolar groups, and their ratio determines the miscibility and compatibility of a plasticizer with the original polymer. Long-chain molecules such as aliphatic compounds play an important role in lowering the Tg. The polar groups present in plasticizer molecules, which usually consist of ester groups, interact with the polar sites of the polymer molecules, namely chlorine atoms, causing PVC chains to spread. In this case, the softener’s polarity should be too high or too low, and it may exudate from the plasticizer, a process often

Plasticizers Derived from Post-consumer PET https://doi.org/10.1016/B978-0-323-46200-6.00001-5 Copyright © 2020 Elsevier Inc. All rights reserved.

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referred to as sweating out. In other words, a plasticizer should create a stable mixture with the original PVC to form a soft product after the gelation process [6]. The use of plasticizers in the manufacture of plastic products to modify polymer characteristics began in the 1800s. It all began from the introduction of natural camphor and castor oil by manufacturers of celluloid or celluloid lacquers to plasticize the final product. This method, however, was unsatisfactory for many end uses. In 1912, triphenyl phosphate was tested to substitute camphor oil, marking the beginning of the ester plasticizer era. Phthalic acid esters were used for the first time as plasticizers in 1920 and came to represent the largest class of plasticizers. In 1930, di(2-ethylhexyl) phthalate (DEHP), also known as dioctyl phthalate, which belongs to this group of plasticizers, was introduced and has been the most widely used plasticizer in the industry ever since [5,7]. Unfortunately, one flaw of low molecular weight additives (monomeric plasticizers) is their tendency to migrate from the PVC item toward the medium it is in contact with (gas, liquid, or solid). Thus, migration limits the utilization or commercialization of plasticized PVC articles, especially in medical products, food packaging materials, and children’s toys, due to their possible toxicity. Numerous phthalates are classified as toxic for reproduction, with 13 of them featuring in the REACH (Registration, Evaluation, Authorization and Restriction of Chemicals) candidate list. Four of them (DEHP, dibutyl phthalate, benzyl butyl phthalate, and diisobutyl phthalate) also appear in the authorization list (Annex XIV of REACH) [8]. Currently, there are specific regulations in Europe, which restrict the use of reproductive toxic phthalates in toys, cosmetics, and food contact materials. In 2005, the European Union approved a ban on reproductive toxic phthalates in all toys and childcare articles [9]. Moreover, recent studies have shown that exposure to certain phthalates results in profound and irreversible changes in the development of the male reproductive system [10].

1.2 Definitions 1.2.1 Internal Plasticization Plasticization resulting from modification of the polymer molecules. It is usually achieved by copolymerization, e.g., by the use of vinyl acetate in PVC or sometimes by chemical modification of the obtained polymer.

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1.2.2 External Plasticization Plasticization resulting from the use of a plasticizer as an additive. It is the usual method of plasticization, but it can suffer from a lack of permanence due to the loss of plasticizer through volatilization or extraction during use of a plasticized polymer.

1.2.3 Polymerisable Plasticizer A plasticizer that may be polymerized. It enables a rigid product to be manufactured from a liquid PVC plastisol because on heating, the plasticizer can polymerize to rigisol. The most common polymerisable plasticizer is diallyl phthalate.

1.2.4 Plastisol A stable dispersion of fine particles (about 1 mm diameter) of emulsion PVC in a plasticizer, which is a viscous fluid. Plastisols may be shear thinning or shear thickening, depending mostly not only on PVC particle size, size distribution, and shape but also on plasticizer type and other additives used. A liquid dispersion consisting of very small particles of resin in a plasticizer that can be molded, cast, or made into a continuous film by application of heat. The conventional melt processing method includes dipping, spreading, low-pressure injection molding, rotational molding, and casting. A plastisol’s rheological behavior is of great importance in these processes and can be very complex.

1.2.5 Rigisol A plastisol that contains a polymerisable plasticizer, which polymerizes during gelation, creating a rigid product after gelation, as opposed to being soft and flexible in a normal plastisol product.

1.2.6 Organosol A plastisol to which an organic solvent has been added to lower viscosity. On gelation, the solvent is lost by evaporation.

1.2.7 Plastigel A plastisol to which a thickening agent has been added to produce a material of putty-like consistency.

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1.2.8 Dry Blend Powder compounds resulting from the blending of primarily suspension PVC resin together with plasticizers, stabilizers, lubricants, processing aids, and other additives at different temperatures.

1.2.9 Gelation During heating of a plastisol, usually at 180e200 C (depending on the formulation of the plastisol) for a few minutes, the plasticizer is absorbed into the particles and solvates them so that they fuse together to produce a homogeneous mass. After the sol is cooled, a homogeneous gelled mass is obtained usually in the form of a film produced as a result of pouring, coating, or dipping. The fusion process is referred to as gelation. The gelling of PVC compositions in the form of dry blends consists in destroying the morphological structures of the PVC grain and part of the primary crystallites by the simultaneous action of shear forces, heat, and pressure applied during processing (e.g., extrusion, rolling, injection). As a result, a homogeneous polymer melt is obtained with the additives, which is formed into the desired shape. The processing temperatures of plasticized PVC compositions usually do not exceed 200 C and depend on the composition of the blend.

1.2.10 Plasticization Usually refers to the softening and increase in flexibility of a polymer brought about by the incorporation of a plasticizer. In the context of polymer melt processing, the term is also used to include softening of a polymer by the action of heat, shear forces, and pressure. Thus, in general, the term refers to an increase in deformability, whether permanent (i.e., as plasticity) or not. Occasionally, as with PVC, small amounts of plasticizer may act as antiplasticizers. Plasticization is sometimes brought about by the use of a flexibilising comonomer, in which case it is referred to as internal plasticization, rather than the more usual external plasticization resulting from the use of a plasticizer as an additive. Phr is defined as part by weight of ingredient per 100 parts of PVC resin [4,11e14].

1.3 Role of Plasticizers in Plastic Processing Plasticization, in general, refers to a change in the thermal and mechanical properties of a given polymer which involves lowering its

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rigidity at room temperature; lowering the temperature, at which substantial deformations can be effected when little force is applied; increase of the elongation at break at room temperature; and increase of the toughness (impact strength) down to the lowest temperature of serviceability. These effects can be achieved (1) by mixing a given polymer with a plasticizer or with another polymer (external plasticization) and (2) by chemically modifying the original polymer with a comonomer, which reduces crystallinity and increases chain flexibility (internal plasticization) [15,16]. To be effective, the plasticizer must be mixed and fully incorporated into the polymer matrix. In the process of obtaining a homogeneous mixture of polymer and plasticizer, the following mixing steps can be distinguished: (1) Mixing the plasticizer with PVC resindadsorption step; (2) Penetration of the plasticizer and swelling of the resin particlesdadhesion step; (3) Freeing of the polar groups in the PVC resin from each otherdabsorption step; (4) Interaction of the plasticizer’s polar groups with the polar groups of the resindintermolecular plasticizing step; (5) The structure of the resin is reestablished, with full retention of the plasticizerdintramolecular plasticizing step. The course of step 2 depends on the plasticizer’s viscosity, degree of branching, resin pore size, and free volume and particle size. The processes in steps 3 and 4 depend on the chemical properties of molecular polarity, molecular volume, and molecular weight of the plasticizer. However, if the plasticizer is not retained in the final product, the product will be rendered useless [14]. Homogenization of the plasticizer with the polymer matrix during processing is assisted by temperature, pressure, and shear forces. The material being plasticized is then formed into a defined shape and is cooled down. However, the plasticizer’s action in a polymer has not yet been fully understood. Many attempts have been made over the years to explain the process resulting in a series of theories.

1.3.1 Theories Explaining Plasticization The lubricity theory was developed by Kilpatrick [17], Clark [18], Houwink [19], and others [14]. According to this theory, the function of

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the plasticizer is to reduce intermolecular friction between the polymer molecules. It holds that the plasticizer in PVC acts as a molecular lubricant, allowing the polymer chains to move freely over one another when a force is applied to the plasticized polymer. In this model, one segment of the plasticizer is strongly attracted to the polymer, whereas other segments are not. The former acts as a solvent for the polymer, while the latter acts as a lubricant (Fig. 1.1) [1]. The gel theory of plasticization was developed by Aiken and others [14,20]. This theory extends the lubrication theory, where the plasticizer’s function is to reduce the number of loose attachments between the polymer chains that have formed a three-dimensional honeycomb or gel structure. The polymer molecules might be loosely tied together by such interactions as van der Waals forces or crystalline domains. The plasticizer separates the polymer chains allowing the polymer molecules to move more freely (Fig. 1.2). The free-volume theory extends the above ideas and also allows a quantitative assessment of the plasticization process. The free volume is a measure of the internal space available within a polymer matrix. When the free volume increases, so does the freedom of movement of polymer chains. A polymer in the glassy state has an internal structure with little

Figure 1.1 Plasticizer polymer response based on the lubricity theory. PVC, poly(vinyl chloride).

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Figure 1.2 Plasticizer polymer response based on the gel theory.

free volume and, as a result, the molecules cannot move easily, making the material stiff and hard. When small plasticizer molecules are added to the formulation and the polymer is heated above the glass transition temperature, the thermal energy increases and the polymer chains separate creating more free volume. For the plasticized resin, free volume can arise from 1. Motion of the chain ends; 2. Motion of the side chains; 3. Motion of the main chain. These motions can be increased in a variety of ways, including 1. Increasing the number of end groups; 2. Increasing the length of the side chains; 3. Increasing the possibility of main chain movement by the inclusion of segments of low steric hindrance and low intermolecular attraction; 4. Introduction of a lower molecular mass compound that imparts the above properties; 5. Raising the temperature [14]. Kinetic or mechanistic theories of plasticization (also referred to as solvationedesolvation equilibrium) see the association between the polymer and plasticizer and between the plasticizer and plasticizer as transient and everchanging. Associations form, disappear, and then reform.

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Some plasticizers form stronger associations with the PVC polymer than others. At low plasticizer loadings in the PVC, plasticizerepolymer associations predominate. At high plasticizer levels, plasticizereplasticizer associations predominate [14]. Mathematical models for plasticization, such as those developed by Mauritz and Storey [21], attempt to predict the Tg of a plasticized PVC from the glass transition temperatures of the polymer and the plasticizer. A plasticizer’s efficiency at reducing this glass transition temperature is based on its structural features such as length and branchiness of the side chains. The permanence of a plasticizer in a flexible PVC compound/the mobility of plasticizers within the PVC depends on three major factors related to both the polymer and the plasticizer, which include structure, molecular weight/viscosity, and polarity. All the PVC plasticization models suggest the existence of some form of chemical interaction between the plasticizer and the polymer. The major factors determining the solvation of polymers by plasticizers are the chemical structure and the polarity of their molecules. The polarity of the plasticizer must be higher than of the forces between the polymer chains. However, if the forces between the plasticizer molecules become stronger than the plasticizerepolymer interactions, then no plasticization occurs. Lack of such plasticizerepolymer interaction would lead to the self-association of plasticizers in plasticized PVC, causing them to form larger micropools within the PVC resulting in their exudation. These interactions allow the plasticizer in a finished flexible PVC to solvate the amorphous part of the polymer, but not its tightly selfassociated crystalline part. In flexible PVC, such crystalline cross-links between polymer molecules play a role similar to that of the crosslinks in elastomers or in thermoplastic olefins. Plasticizers increase the modulus of elasticity of the polymer composition. The polarity and flexibility of plasticizer molecules determine their interaction with the polymer PVC, every repeating unit of which contains a polarized carbonechlorine bond. Plasticizers that have a polar aromatic group, e.g., phthalic acid ester, are characterized by the presence of a polarizable aromatic ring, the behavior of which is similar to dipolar molecules. Plasticizers belonging to this group are easily introduced into the polymer matrix and are characterized by a good facility to produce gelation. Plasticizers containing polar aliphatic groups, e.g., ester aliphatic acids and aliphatic alcohols, interact with the polar sites on the polymer molecules. On the other hand, the aliphatic part of the plasticizer may

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Figure 1.3 The polarity and flexibility of plasticizer molecules interaction with the polymer poly(vinyl chloride).

screen these polar sites, reducing thus the extent of intermolecular interactions between two neighboring polymer chains (Fig. 1.3) [6]. Furthermore, as the aliphatic chain in the plasticizer molecule becomes longer, the following performance changes occur: decrease of solvation power, efficiency and volatility; improvement of low temperature flex and low temperature brittleness; and a decrease of compatibility [22,23]. Branched plasticizers are more permanent than their equivalent but linear counterparts because branching tends to hinder movement or entangle the plasticizer within the polymer matrix making it more difficult for it to migrate or to be removed by volatilization or extraction. However, linear structures provide less permanence, but they do yield a better lower Tg [24]. A comparison of linear versus branched chains demonstrates that: (1) Linear chains have a faster dry blend time; (2) Branched chains have a faster fusion time; (3) Branched chains are more compatible; (4) Linear chains have better low temperature brittleness and flexibility; (5) Linear chains are more efficient; (6) Linear chains are less volatile; (7) Branched chains have better electrical properties; (8) Linear chains exhibit better UV stability and long-term permanence; (9) Linear chains provide higher thermal stability [22].

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The compatibility between the polymer and the plasticizer also depends on the molecular weight of the plasticizer. A smaller molecular weight results in its increased volatility and diffusion. A greater molecular weight, on the other hand, results in a larger and longer molecule, which makes it more difficult for the plasticizer to diffuse from the PVC molecular structure. Low volatility plasticizers constitute compounds with average molecular weights between 300 and 600 g/mol, under which the volatility increases. The ideal plasticizer must meet four principal requirements, namely compatibility, low volatility, performance, and efficiency. In addition, it should be odorless, tasteless, nontoxic, nonflammable, and temperature stable. For a plasticizer to be compatible with the original polymer, it should not exudate even after extended usage of the plasticized material. Incompatibility can also be manifested by poor physical properties that appear after some period of usage [16].

References [1] P.H. Daniels, A brief overview of theories of PVC plasticization and methods used to evaluate PVC-plasticizer interaction, J. Vinyl Addit. Technol. 15 (2009) 219e223. [2] C.S. Brazel, S.L. Rosen, Fundamental Principles of Polymeric Materials, third ed., Wiley, 2012 (Chapter 7 and Chapter 18). [3] Chemical Economics Handbook, Plasticizers, IHS Markit, 2015. [4] M. Alger, Polymer Science Dictionary, third ed., Springer, 2017. [5] C. Wilkes, J. Summers, C. Daniels, PVC Handbook, HANSER, 2005 (Chapter 5: Plasticizers).  epek, H. Daoust, Additives for Plastics, Springer, 1983, pp. 7e32. [6] J. St [7] M.G.A. Vieira, M. Altenhofen da Silva, L. Oliveira dos Santos, M.M. Beppu, Natural- based plasticizers and biopolymer films: a review, Eur. Polym. J. 47 (2011) 254e263. [8] Phthalates Which Are Toxic for Reproduction and EndocrineDisrupting e Proposals for a Phase-Out in Sweden. Report from a Government Assignment, Swedish Chemicals Agency, Stockholm, 2015. [9] European Commission, Restrictions on the marketing and use of certain dangerous substances and preparations (phthalates in toys and childcare articles) directive 2005/84/EC, Off. J. Eur. Union (2005) 40e43. L344.

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[10] A.J. Martino-Andrade, I. Chahoud, Reproductive toxicity of phthalate esters, Mol. Nutr. Food Res. 54 (2010) 148e157. [11] W.F. Carroll, R.W. Johnson, S.S. Moore, R.A. Paradis, M. Kutz (Eds.), Applied Plastics Engineering Handbook, Processing, Materials, and Applications, second ed., Plastics Design Library (PDL), Elsevier, 2016 (Chapter 4). [12] S.G. Patrick, Practical Guide to Polyvinyl Chloride, Practical Guide to Polyvinyl Chloride, Rapra Technology Limited, UK, 2005 (Chapter 3). [13] G. Wypych (Ed.), Handbook of Plasticizers, ChemTec Publishing, Toronto-New York, 2004. [14] D.F. Cadogan, C.J. Howick, in: Kirk-Othmer (Ed.), Encyclopedia of Chemical Technology, John Wiley and Sons, New York, 1996 (Chapter: “Plasticizers”). [15] E.H. Immergut, H.F. Mark, in: N.A.J. Platzer (Ed.), Plasticization and Plasticizer Processes, American Chemical Society, Washington, 1965 (Chapter 1: Principles of Plasticization). [16] R.O. Ebewele, Polymer Science and Technology, CRC Press, New York, 2000 (Chapter 9). [17] A.J. Kilpatrick, Some relations between molecular structure and plasticizing effect, J. Appl. Phys. 11 (1940) 255e261. [18] F.W. Clark, Plasticizers Soc. of Chem. Ind. e J. Chem. Ind., vol. 60, 1941, pp. 225e228. [19] R. Houwink, Proc. XI Cong. Pure Appl. Chem., London, 1947, pp. 575e583. [20] W. Aiken, T. Alfrey, A. Janssen, H. Mark, Creep behavior of plasticized vinylite VYNW, J. Polym. Sci. 2 (1947) 178e198. [21] K.A. Mauritz, R.F. Storey, B.S. Wilson, Efficiency of plasticization of PVC by higher-order di-alkyl phthalates and survey of mathematical models for prediction of polymer/diluent blend Tg’s, J. Vinyl Technol. 12 (1990), 165e1173. [22] Fourth print and online ed., in: H.F. Mark (Ed.), Encyclopedia of Polymer Science and Technology, vol. 3, John Wiley and Sons, 2014, pp. 498e524.  [23] K. Bortel, Srodki pomocnicze stosowane w przetwo´rstwie tworzyw polimerowych, Przetwo´rstwo Tworzyw 2 (2008) 148e153. [24] A. Marcilla, S. Garcı´a, J.C. Garcı´a-Quesada, Study of the migration of PVC plasticizers, J. Anal. Appl. Pyrolysis 71 (2004) 457e463.

2 Classification of Plasticizers 2.1 Introduction There exist several possible classifications that are based on the variability of a plasticizer’s chemical structure and molecular weight or type of plasticization of the original polymer (Fig. 2.1).

2.1.1 Classification Based on Type of Plasticization Internal plasticizers are plasticizers incorporated into a resin during the polymerization process. As a result, compounds with a flexible segment become part of the basic polymer chain. Internal plasticization is achieved by copolymerization or grafting of the original polymer. Monomers based on a polymer that demonstrates good low-temperature properties are usually chosen for this process. Internally plasticized systems consisting of simple random copolymers, with flexible segments incorporated regularly or irregularly between and inflexible segments, have an unsatisfactorily narrow temperature range of application because they soften more sharply than analogous externally plasticized systems or polyblends.

Figure 2.1 Classification of plasticizers. Plasticizers Derived from Post-consumer PET https://doi.org/10.1016/B978-0-323-46200-6.00002-7 Copyright © 2020 Elsevier Inc. All rights reserved.

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The most widely used internal plasticizer monomers are vinyl acetate and vinylidene chloride. Another type of internal plasticization achieved by means of grafting methods consists in reducing crystallinity and glass transition through reduction of intermolecular forces due to the presence the side chains [1e3]. External plasticizers are low-volatile substances, which are added to an original polymer. In other words, external plasticization consists in the physical mixing of a plasticizer with a polymer. In this case, plasticizer molecules interact with polymer chains but are not chemically attached to them by primary bonds and can, therefore, be lost by evaporation, migration or extraction. External plasticizers act as solvents for amorphous regions of polyvinyl chloride (PVC). The PVC chains in the amorphous regions become solvated at elevated temperatures during processing [3]. Primary and secondary plasticizers make up the two recognized groups of external plasticizers. The terms are related to the plasticizers’ compatibility with polymers. Primary plasticizers are compounds containing polar groups and are characterized by high solvation capabilities with PVC. They have the ability to efficiently increase polymer flexibility. A primary plasticizer is used as the sole plasticizer and is generally miscible with the polymer in all proportions. They are expected to gel the polymer rapidly in the normal processing temperature range and should not exude from the plasticized material. Primary plasticizers include adipates (e.g., dioctyl adipate), citrates, sebacates (e.g., dioctyl sebacate), azelainates, trimellitates, phosphoric acid esters or epoxy softeners; however, phthalates are the most common. Primary plasticizers demonstrate very good miscibility with the polymer at one part plasticizer to one part polymer by weight. In special cases, up to 140e150 phr of primary plasticizer can be added into PVC formulations for extremely soft products [4]. Secondary plasticizers contain groups that are less polar and have therefore limited compatibility with the polymer. They demonstrate limited solubility and compatibility, which is why they are often used in mixtures with primary plasticizers. Such mixtures show not only a reduced tendency to migration but only increased strength at reduced temperatures and resistance to precipitation [5e7]. However, for secondary plasticizers, the miscibility limit with regard to polymers is 1:3, whereas for extenders it is 1:20. Secondary plasticizers include aliphatic and aromatic chlorinated hydrocarbons, e.g., chlorinated paraffins, and epoxidized esters of unsaturated fatty acids derived from

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plants, e.g., epoxidized butyl esters and n-hexyl unsaturated fatty acids of soybean oil. Some secondary plasticizers can function as primary plasticizers when used at relatively low levels [8]. Extenders are a subset of secondary plasticizers. A substance, which although relatively incompatible with the original polymer, extends the material or fills out its bulk at a low cost without causing a significant loss of flexibility as would occur if using a solid filler as an extender. Many plasticizer extenders are chlorinated paraffins and therefore also have a flame-retardant effect. Examples of extenders include naphthenic hydrocarbons, aliphatic hydrocarbons, chlorinated paraffins (fire resistant), and others [9,10]. Plasticizers are commonly classified based on their chemical composition, as it is easier to understand the influence of structural elements on the properties of plasticizers and their effect on materials, which contain them (Table 2.1). Plasticizers have been used in the production of flexible PVC for over 50 years. Phthalates, according to Michael P. Malveda, director of specialty chemicals at the market research firm IHS Chemical, made up 70% of the more than 8 million metric tons of plasticizers sold worldwide in 2014 [12]. During the past 20 years, phthalates have come under considerable legislative scrutiny because of their proven toxicity and influence on the development of living organisms. As a result, a number of health and environmental controls have been imposed [13]. The greatest drawback of these compounds is the lower resistance of the bond line to heat and migration. There have been many studies on the phenomenon of migration of plasticizers, but few on reducing it. The migration process may depend on the polymer properties (i.e., type of PVC), the nature and amount of plasticizer, the plasticization process and the homogeneity of the compound and the surrounding media, i.e., nature, compatibility with the plasticizer, effect on the polymer, and obviously on the temperature, contact area and other conditions [14]. Various methods have been proposed to reduce the migration of plasticizers and other additives from PVC materials [15,16]: 1) Surface modification: surface cross-linking, modification of surface hydrophilicity/lipophilicity; surface coating; surface extraction. 2) Use of polymeric plasticizers and oligomers. 3) Use of alternative plasticizers. 4) Alternative polymer.

Table 2.1 Chemical Structures, Properties and Application of Some Commercial Plasticizers Used in Polyvinyl Chloride (PVC) [1,11].

Plasticizer Type Phthalate esters (dialkylphthalate)

Chemical Structure O C C O

General Specification

Example O

OR OR

C C

O O

O

Di(2-ethylhexyl) phthalate (DEHP)

- Plasticizers most abundantly produced worldwide - Good plasticizing efficiency

2014 Estimates of Global Plasticizer Consumption [%] [3]

Application

75

- Medical devices - Food wrap - Building materials - Packaging

- Good fusion rate

- Automotive parts

- Good viscosity

- In cosmetics: Hair spray, nail varnish, lotion, shampoo, soap

- Low volatility - Water resistant - Competitive price

- School supplies: notebooks, binders - Raincoats - Boots - Handbags - Soft plastic shoes such as flip-flops (Continued )

Table 2.1 Chemical Structures, Properties and Application of Some Commercial Plasticizers Used in Polyvinyl Chloride (PVC) [1,11]. (Continued )

Plasticizer Type Dibasic acid ester (adipates, sebacates, azelates, glutarates, oleates)

Chemical Structure O RO

General Specification

Example O

C(CH2)nC

O

OR

O

O

C(CH ) C

O

Di(2-ethylhexyl) adipate (DEHA)

- Used in some demanding flexible PVC applications where superior lowtemperature performance is required/improve low temperature properties

2014 Estimates of Global Plasticizer Consumption [%] [3]

Application

3

- Automobile parts - Aircraft interiors - Vinyl cable insulation

- Resistance to UV light Epoxy plasticizers

Epoxidized soybean oil (ESBO) Epoxidized fatty acid esters

- Lower cost - Improve plasticizer permanence (ESBO) - Improve PVC stabilization (ESBO)

3

- Cables - Electrical tapes - Footwear - Hoses and tubing (Continued )

Table 2.1 Chemical Structures, Properties and Application of Some Commercial Plasticizers Used in Polyvinyl Chloride (PVC) [1,11]. (Continued )

Plasticizer Type

Chemical Structure

Example

General Specification

2014 Estimates of Global Plasticizer Consumption [%] [3]

Application

- Worse compatibility with PVC (ESBO) - Epoxy esters more compatible but too volatile as primary plasticizers DEHP - Heat and light stability of PVC Glycol derivatives (glycol ethers and their esters; dibenzoates, benzoates

O R

O

C(OCH CH(R')) OC

Monoester

R

Dipropylene glycol dibenzoate

- Glycol ethers and their esters: Low viscosity, low temperature flexibility - Dibenzoates: stain resistance, low migration

2

- Flooring and film - Adhesives and sealants - Automotive

(Continued )

Table 2.1 Chemical Structures, Properties and Application of Some Commercial Plasticizers Used in Polyvinyl Chloride (PVC) [1,11]. (Continued )

Plasticizer Type

Chemical Structure

General Specification

Example

2014 Estimates of Global Plasticizer Consumption [%] [3]

Application

- Good compatibility with PVC - High solvating plasticizers for PVC Trimellitates (trialkyl trimellitate)

O

O

OR RO

OR O

O

O O

O O

O

Tris(2-ethylhexyl) trimellitate

- Low volatility and migration - Improve thermal stability - High permanence for PVC compounds subjected to elevated temperatures

2

- High-temperature PVC wire and cable insulation - Gaskets - Automotive interiors

- Very low fogging (Continued )

Table 2.1 Chemical Structures, Properties and Application of Some Commercial Plasticizers Used in Polyvinyl Chloride (PVC) [1,11]. (Continued )

Plasticizer Type

Chemical Structure

Example

General Specification

2014 Estimates of Global Plasticizer Consumption [%] [3]

Application

- UV resistance - Electric insulation - Plasticizing effectiveness and compatibility are similar to phthalates Phosphate ester (aryl, alkyl, and mixed phosphate ester)

- Flame retardant

O R3O P OR1 O R2

- Low smoke Tricresyl phosphate

- Aryl plasticizers have low volatility and good extraction resistance, but they decrease low temperature flexibility

1

- In the hospital sector - Packaging - Cables - Floor and wall coverings

(Continued )

Table 2.1 Chemical Structures, Properties and Application of Some Commercial Plasticizers Used in Polyvinyl Chloride (PVC) [1,11]. (Continued )

Plasticizer Type

Chemical Structure

General Specification

Example

2014 Estimates of Global Plasticizer Consumption [%] [3]

Application

- Alkyl phosphates are good low temperature plasticizers but less effective as flame retarding additives - Accelerates thermal degradation of PVC - Not suitable for low temperature and food-contact applications Citrates

O

OR

RO

OR O

OH

O

O

OH C O

C O

C

O

Triethyl citrate

O

O

- Considered safe in food, pharmaceutical, and medical applications - Some do not support fungal growth

CHeCl group in PVC. Oligomeric plasticizers seem to be a more adequate term for some plasticizers with an intermediate relative molecular weight. This is because their properties vary significantly with the removal of one or a few of the units. Oligoester plasticizers have an average molecular weight

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ranging from less than 1000 to about 25,000 g/mol and are viscous liquids to semisolids at room temperature [18]. The higher molecular weight of these plasticizers contributes to their decreased volatility and diffusion within the polymer matrix compared with monomeric plasticizers. PVC products containing these types of plasticizers are also characterized by their resistance to migration and extraction by organic solvents, oil, fat and detergents [8]. Polymeric plasticizers are generally used as additives to conventional polymers such as acrylonitrile butadiene rubbers and nitrileePVC blend, but they are commonly used with PVC when permanence is a critical parameter [1]. Polymeric plasticizers are not as popular and their use does not exceed 5% of global production. This is usually because they significantly increase the viscosity of PVC plastisol and provide lower plasticizing efficiency than traditional plasticizers, plus they are too expensive to be used on their own. The properties of plasticized PVC compounds are dependent on the plasticizer type, content and molecular weight. To ensure satisfying PVC plasticization, the dipole forces between the polymerepolymer, plasticizereplasticizer, and polymereplasticizer molecules must be of the same magnitude. This is the reason why a certain similarity in the structure and polarity between the polymer and plasticizer is required. This is when the compatibility and permanence of the plasticizer as well as its good distribution in the polymeric phase are considered to be optimal [26]. Miscibility between two polymers was for a long time considered a rare exception to the rule that polymers are immiscible as a consequence of minimal combinatorial entropy change when two high molecular weight compounds are blended. However, if the heat of mixing is small enough, it can compensate for the inherent low combinatorial entropy change, resulting in a negative free energy of mixing and polymerepolymer miscibility. When two polymers are mixed, their attraction toward each other will determine the extent of phase separation and the adhesion between the phases. Phase behaviour in a polymer blend depends on the extent of interpolymeric interactions and self-association. If strong interactions (such as hydrogen bonds) are present within a polymer, there must be a driving force strong enough for breaking these to form interpolymeric interactions instead. In light of these rules, polyesters seem be perfectly suitable to be used as plasticizers. Hydrogen bonding between the carbonyl group of the ester and the a-hydrogen next to the chlorine atom in PVC and the dipoleedipole interaction between the carbonyl group of the ester and the chlorine atom in PVC constitute intermolecular

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forces that contribute to an effective plasticization of PVC and its miscibility with these compounds [27]. Among polymeric or oligomeric plasticizers, the most commonly used ones are saturated polyesters obtained from a reaction between a DA and a diol [28e31]. They were introduced to the vinyl industry in the early 1940s [32]. Their synthesis usually involves the use of DAs, such as adipic, azelaic, glutaric, o-phthalic or sebacic acid, and glycols (G) which have two to five carbon atoms. These plasticizers are usually terminated by monocarboxylic acid (Ac) or alcohol (Al). The general structure of polyester plasticizers is as follows: - acid-terminated polyestersdpolyesters produced with an excess of glycol and terminated by an addition of carboxylic acid Ac½OR1 OOCR2 COn OR2 OAc - alcohol-terminated polyestersdpolyesters produced with excess DA and terminated by an addition of alcohol Al½OCR2 COOR1 On OCR3 COAl The choice of DA, glycol and termination groups/end caps is very important to achieve good compatibility with PVC. A lack of termination groups, and the resulting blocking of hydroxyl groups, can generate problems with compatibility, especially under high-humidity conditions, whereas blocked free acid groups impact both compatibility and heat stability. Alcohol end caps are usually in the C8eC10 range. Acetic anhydride is typically introduced to eliminate hydroxyl groups; however, saturated fatty acids are also used [11,24]. The higher concentration of hydroxyl groups of a given polyester fraction contributes to its quicker exudation from the PVC than in the case of blocked polyesters or ones with a very low hydroxyl number [32]. As a result, if the molecular weight of the plasticizer is high, then the influence of unprotected, free end groups on its properties in much smaller. In the case of aromatic polyester polyols containing terephthalic and ethanediyl units, certain unfavourable physicochemical properties develop due to their ability to take on an aggregate form by association. These include storage instabilitydsolid products at room temperature separate from the solution over timedand high viscosity. As mentioned earlier, the molecular weight affects the performance of plasticizers. However, when assessing the usability of a polymeric

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plasticizer, it is worth drawing attention to the number of ester group carbon atoms versus the number of carbon atoms in the segments between them, as well as on the types of esters which may contain aliphatic and/or aromatic segments. Understanding the constituents of a polymeric plasticizer will help in understanding performance. Howick found that as the polar centre of the plasticizer interacts with the polar centres of the PVC molecule, this interaction can impact the ability of the alkyl chains of the plasticizer to create free volume. This effect determines the efficiency of the plasticizer, and, as such, alkyl side chains yield a greater free volume contribution during PVC plasticization [33]. It has been observed that the polarity of the plasticizer molecules decreases with the length of the aliphatic chain between the ester bonds. A relation between miscibility and the CHx/COO ratio of the polyester has been proposed. A CHx/COO ratio of approximately 3e4 is suggested as the lower limit for miscibility. At lower ratios, the polyester backbone becomes too rigid, and the free rotation of the chain is impaired. At CHx/ COO ratios higher than 10e12, the concentration of interaction centres is too low to induce miscibility. Linear polycaprolactone (PCL) and poly(butylene adipate) (PBA) have CHx/COO ratios equal to 5 and 4, respectively, and because of their documented miscibility with PVC, they are frequently used in the development of new polymeric plasticizers for PVC. According to literature data, it is accepted that the miscibility of polyester/PVC blends is favoured by a polyester CH2/C]O ratio of 4e10, in which flexible films of PVC and polyester are obtained [31,34]. Higher viscosity, on the other hand, provides for greater permanence. The viscosity of oligomeric esters is strictly correlated with their molecular weight and the presence of branches in their structure. A higher molecular weight of the plasticizer leads to more chain entanglements, as a result of which the tensile strength of the material increases, whereas elongation is reduced [27]. Rushton and Salomons [32] showed that low molecular weight polyesters exuded faster from PVC sheets than higher molecular weight analogues and that acetylation of hydroxyl groups reduced exudation. The molecular weight of a plasticizer also influences the processing of the material being modified. This is because the material usually becomes more difficult to process as the molecular weight of the plasticizer increases. A high molecular weight of a polymeric plasticizer requires it to be heated up before dry blending and needs a higher process temperature [8,35]. Therefore, selection of the right molecular weight needs to satisfy the conflicting requirements of increased plasticizer retention and decreased manufacturing compatibility, and processability poses a manufacturing dilemma. Their lower

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efficiency and higher cost compared with monomeric plasticizers is often eliminated by using these plasticizers in a mixture with low molecular weight plasticizers. Linear saturated polyesters are used in flexible PVC formulations as nonmigrating alternative plasticizers because they exhibit good miscibility with PVC. Moreover, they are able to improve the mechanical properties of PVC, such as abrasion and fatigue resistance. The majority of polyester plasticizers are based on adipic acid. A good balance of efficiency, compatibility and low-temperature flexibility are an advantage of this acid. The longer the acid chain, the worse the compatibility and plasticizing effectiveness of PVC. Among glycols, 1,3-butanediol is commonly used to prepare polymeric plasticizers. The branch helps to improve compatibility with vinyl and lends a lower freezing point to the polyester. The following glycols yield similar properties and are used in the synthesis of these types of plasticizers: propylene glycol, neopentyl glycol and 2-methyl-1,3-propanediol. Although linear glycols, such as 1,4-butanediol, diethylene glycol (DEG) and triethylene glycol (TEG), will support low-temperature flexibility in vinyl, this is done at the price of a higher plasticizer freezing point and lower compatibility. Increasing the length of glycol chains could decrease the viscosity of the oligoester, but too large a number of methylene units in the glycol chains may cause crystallization of the product [36]. Phthalic acid in the backbone can reduce this effect and improve compatibility. In recent years, a growing interest has also been shown in the application of branched and hyperbranched polymers as substitutes to phthalate plasticizers for PVC [37e40]. In view of the concept which advocates the positive influence of free volume formation in the course of PVC plasticization, these compounds, due to their spherical and bulky structure, should also constitute very efficient and durable plasticizers. Polymeric plasticizers composed of branched structures are more resistant to diffusivity losses than those based on linear isomeric structures; on the other hand, they are more susceptible to oxidative attack [8]. The first polyester polymeric plasticizer, Paraplex G-25, was developed in the early 1940s in the United States. Paraplex G-25 is a polyester sebacate and propylene glycol with an approximate average molecular weight of 10,000 g/mol. It is the gold standard by which all of the other polymeric and monomeric plasticizers are measured and has an excellent compatibility in PVC resins where no tack or exudation is evident even when exposed to high temperatures and humidity. It is also compatible with chlorinated rubber, epichlorohydrin, nitrocellulose and nitrile. The plasticizer is recommended for products such as coated

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fabrics, coaxial cables, electrical tape, high-temperature applications, refrigerator gaskets and upholstery [24]. One of most commonly used polymeric PVC plasticizers is PBA and PCL and their derivatives. Shah and Shertukde [35] used poli(butylene adipate) with an Mn ¼ 1200 g/mol in an amount of 40 phr for PVC plasticization. This system is characterized by a higher viscosity of the molten compound and a final melt temperature slightly higher by 4 C compared with PVC plasticized using DEHP. Most importantly, the plasticizer made it possible to obtain a PVC with greatly improved permanence properties. The plasticizer loss after 7 days amounted to approximately 0.05%, and its extraction resistance is superior to DEHP, butyl benzyl phthalate and isodecyl diphenyl phosphate, which it owes to its molecular weight. Castle et al. [41] conducted studies on the migration of PBA with varying chain lengths (3e11) from PVC film into olive oil. The highest migration was observed for trimer and the lowest for fractions of 7e11 units of oligoesters. The migration and hydrolysis of this polyester is also influenced by pH and temperature. The gas chromatography-mass spectrometry (GC-MS), Fourier-transform infrared spectroscopy (FTIR), weight loss, differential scanning calorimetry (DSC) and thermogravimetry analysis (TGA) showed that the polyadipate was stable during 70 days of ageing at 37 C and 56 days of ageing at 70 C in water. Low molecular weight polyadipate oligomers, adipic acid and 1,4butanediol were the main migration chemicals found in the water ageing solution. However, the product is unstable after 7 days in a low pH buffer solution (pH ¼ 1.679) at 37 C. Here, the solution contained more adipic acid and 1,4-butanediol, as well as traces of oligomerics [42]. In a series of papers by Castle et al. the migration of PBA from PVC films to food was studied [41,43]. It was shown that the low molecular weight species with molecular weights between 300 and 1100 [g/mol] migrated 90 times faster than the bulk plasticizer with an average molecular weight of 4060 [g/mol]. Compounds of this group are available for sale under the brand name of Palamoll by BASF. They comprise a range of different molecular sizes. Their fields of application include food contact materials and technical products such as wires and cables, as well as decorative film for automotive applications. In 2003, Bayer Chemicals started producing a new polymeric plasticizer, Ultramoll VP SP 51022, which is a low-viscosity polymeric plasticizer based on phthalic acid [44]. The newest literature also provides application examples of aliphatic polyester PVC plasticizers belonging to this group: poly(1,2-propylene adipate) [45], poly(1,3-butylene adipate) (Reoplex) [46], poly(butylene succinate) [47e49] and poly(butylene adipate-co-terephthalate) (trade name Ecoflex) [50,51].

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The possibility of using PCL as a plasticizer for PVC was reported in the 1960s. It is well known that PCL produces miscible blends with PVC in all proportions, and it was reported to be a very effective phthalatereplacing eco-friendly plasticizer for PVC. PCL is deemed attractive as a ‘green’ plasticizer for PVC due to the following: (1) biodegradation behaviour and nontoxicity of PCL; (2) its low glass transition temperature (Tg) (around 60 C) that provides low temperature flexibility, which is enhanced by using oligo(caprolactones) with a molecular weight of about 600e1100 g/mol; (3) the miscibility of PCL with PVC, which facilitates processing and mechanical performance and (4) the relatively simple synthesis and low cost of PCL make it possible for large-scale production [52]. The PVCePCL blends are tougher and more extensible than those prepared with conventional plasticizers, with better drape, softness and higher resistance to extraction by oil and water. In blends containing 40%e100% PCL, PVC reduces the crystallinity of PCL, producing a twophase system, namely a crystalline PCL and amorphous PVC/PCL blend, and in blends containing 0e30% PCL, only the amorphous PVC/PCL phase forms. In the amorphous phase, PCL acts as a polymeric plasticizer for PVC, but at very low concentration, it produces “antiplasticization,” which has been explained as “pseudocrosslinking” of the PVC [53]. However, the plasticizing efficiency of PCL is significantly lower than that of phthalate, which is a challenge to be overcome for the practical application of PCL plasticizers [54]. The introduction of branches in the plasticizer structure came as quite a surprise. Controlling the architecture of additives in linear matrices, for example, the number, the length and the distance between branches, enables control over material properties, such as plasticizer migration and surface character. Branched polymers have a higher density of chain ends compared with that of linear polymers of the same molecular weight and therefore also provide more free volume and mobility at the same molecular weight. Using a branched polymeric plasticizer would thus increase the mobility of the system when compared with a system plasticized with a linear polymer, as the branched polymer’s high molecular weight and bulkiness significantly reduce its volatility and diffusivity. A branched and hyperbranched polyester used as a polymeric plasticizer in PVC provides better miscibility, excellent migration stability, improves the mechanical properties of the material and makes the blend easier to process than blends containing a linear polymeric plasticizer. Improvement in the processability of the material results from the reduction of the melt viscosity by this plasticizer type. A high concentration of short- or medium-length chain branches may reduce crystallinity significantly [55,56].

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A highly branched polyester plasticizer based on DAs, monocarboxylic acids, diols and triols was patented by Huber et al. in 1967 [57]. The polyester showed good miscibility with PVC, the migration of the plasticizer was significantly reduced and the thermal stability was improved when both diols and triols were used. A lot of research using linear and branched PBA as PVC plasticizers was conducted and published by Lindstro¨m and Hakkarainen, who studied the effects of the chemical structure, molecular weight, end group functionality and chain architecture on plasticizing efficiency and durability. The number average molecular weights of the PBA they used ranged from 700 to 10,000 g/mol, while the degrees of branching ranked from very low to hyperbranched. They used either trimethylolpropane or 1,2,4-butanetriol to synthesize the branched PBA polyesters. The branching frequency ranged from 0.19 to 1.23. Miscibility was evaluated by the existence of a single glass transition temperature and a shift of the carbonyl group absorption band. Desirable mechanical properties were achieved in flexible PVC films containing 40 wt% of polyester plasticizer. Methyl estereterminated polyesters with a low degree of branching and an intermediate molecular weight enhanced the plasticizing efficiency, as shown by greater elongation, good miscibility and reduced surface segregation. A more migration-resistant polymeric plasticizer was obtained by combining a low degree of branching, hydrolysis-protecting end groups and higher molecular weight of the polyester. Films plasticized with a slightly branched polyester showed the best durability and preservation of material and mechanical properties during ageing. A high degree of branching resulted in partial miscibility with PVC, poor mechanical properties and low migration resistance. The thermal stability of polyester-plasticized films was higher than that of films containing a low molecular weight plasticizer, and the stabilizing effect increased with increasing plasticizer concentration [27,38,58]. Choi and Kwak [37] experimented with the use of hyperbranched poly(ε-caprolactone) (HPCL) as a plasticizer in PVC. They found that PVC formulations containing hyperbranched poly(ε-caprolactone)s with a large number of branches showed excellent migration stability and a plasticization quality as good as the PVC products with DEHP. However, the synthesis of HPCL was complicated and included the synthesis of macromonomers via protectionepolymerizationedeprotection steps and polycondensation of the macromonomers, which impedes their large-scale production at a low cost. Irvine et al. [59] successfully prepared highly branched PCLs (hbPCLs) using 4,4-bioxepanyl-7,7-dione as a branching agent and benzyl alcohol/tin 2-ethylhexanoate as the initiator/catalyst system in branching ring-opening polymerization (ROP) bulk copolymerization (solvent free).

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On the other hand, Kwak and his team developed a method of synthesizing (hbPCLs) via Sn(Oct)2-catalyzed bulk copolymerization of CL and glycidol in a single pot, the core of which consisted of 1,4-benzenedimethanol. Afterwards, the obtained products with a molecular weight of 1400e5360 g/mol with free hydroxyl groups were esterified by butyric anhydride and used as PVC plasticizers. After esterification, a plasticizer with an average length of its branched segment of 5.8 and a molecular weight of 1960 [g/mol] showed an excellent PVC plasticizing effect, which is comparable to that of DEHP, of PVC/DEHP [54]. To replace or partially replace conventional plasticizers, other polymers are also used as polymeric plasticizers. These include ethylene-vinyl acetate copolymer [60,61], polyester urethane [62], ethylene-acrylic copolymer [63], nitrile rubber [61,64,65], thermoplastic polyurethane [66,67], epoxidized natural rubber [65,68], carboxylated nitrile rubber [65] and chlorinated polyethylene [69]. The behaviour of blends derives from the individual components in which a more favourable balance between the inherent advantages are attainable. Various modifications in the properties of PVC by polymer alloy combination are presented in the diagram below (Fig. 2.6).

EVA (ethylene-vinyl acetate copolymer)

CPE

(chlorinated polyethylene) Im p Pr act oc es resis sa bil tanc ity e

High elasticity, Abrasion resitation, Flexibility

Freeze resistance, Elasticity, Oil resistance

PVC Hi Fl gh Fr exib elas Ab eez ility tici ty ra e r sio es i n re stan sis c ta e nc e

Processability, Cost

TPEE (thermoplastic polyester elastomer)

Processability Heat resistance Impact resistance

TPU (thermoplastic polyurethane)

e e i t y nc c i d ta an f l u is ist h es s i g r re ce H at ct e an H pa rd Im ta re re Fi

(acrylonitrilebutadienestyrene)

Processability Antimigration Impact resistance Freeze resistance

ABS

Weatherability Im Cost, Processability Pr pac oc t re es sa sista bil ity nce

NBR (acrolonitrylebutadiene rubber)

MBS (methacrylatebutadienstyrene)

Acrylic resin

Figure 2.6 Property modification of polyvinyl chloride (PVC) by polymer alloy.

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The Lanxess Group offers a wide range of products to replace phthalate-containing additives with safer alternatives. In addition to the already established Mesamoll plasticizer, there is Mesamoll TP LXS 51067, which is used by many companies as a substitute for phthalatebased plasticizers. The sales of Mesamoll have been growing by around 15% a year, much faster than the plasticizer market itself. Mesamoll TP LXS 51067, a phthalate-free plasticizer, is intended for the production of PVC products, particularly floor and wall coverings. The active substance present in Mesamoll, i.e., alkyl sulphonic acid ester containing phenol with 11e15 repeating units, was combined with another plasticizer. This additive may replace, among others, DBP and BBP. Both of these plasticizers are considered to have a toxic effect on the reproductive system, and therefore significant restrictions have been imposed on their use. Recently, there has been increasing interest in the use of natural-based plasticizers such as epoxidized triglyceride vegetable oils from soybean oil, linseed oil, castor oil, sunflower oil and fatty acid ester. These plasticizers are described in Chapter 4.

2.5 Other Plasticizers Many other alternative plasticizers are being developed as replacements for harmful phthalates. Ionic liquids (ILs) constitute one such novel alternative. ILs are molten salts that melt below 100 C. There also exist ionic liquids, which melt at temperatures below room temperature (20 C) and are referred to as ‘low-temperature ionic liquids’. They are built exclusively of a cation and an anion. The cation is large and has a limited nature and an asymmetric structure, which is defined by the length of the alkyl substituents. Alkyl imidazoleic, alkyl pyridinic and alkyl phosphonyl cations are used most often. The anions are smaller and may be of an organic (CH3COO, CF3COO, (CF3SO2)2N) or inorganic nature (BF4  , PF6  , AlCl4  , Br, NO3  ) and typically consist of a bulky inorganic cation and an anion. Many of the ILs are liquid over a wide temperature range (often more than 300 C). Many of them are nonflammable, nonexplosive and have high thermal stability. They are also recyclable, which can be helpful in reducing landfill waste. All these advantages point to the fact that ILs hold prospects as alternative plasticizers in the rapidly growing plastic industry. Tests have shown, however, that some ILs may lead to PVC degradation. The poor thermal stability of IL-plasticized PVC samples may have been due to the IL taking part in the catalytic degradation of PVC, similar to the well-known autocatalytic

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degradation of PVC by HCl given off as PVC degrades. Another issue in the case of these compounds is their high cost. However, thanks to the tunability of IL structures (possibility of forming numerous ILs with different combinations of anions and cations), it is possible to design ILs with desired characteristics to be used as plasticizers for specific applications [70]. Pentaerythritol esters are a type of ‘miscellaneous’ plasticizers that impart both low volatility and diffusivity. Pentaerythritol and dipentaerythritol are tetra and hexa alcohols, respectively, and are made into plasticizers via esterification with aliphatic monocarboxylic acids having a straight fatty acid chain length of between 6 and 10 carbon atoms. Hercoflex 600 is the pentaerythritol tetraester, and 707 is a mixture of tetra and hexa esters, using a mixture of pentaerythritol and dipentaerythritol. Their molecular weights are approximately 600 and 750, respectively, which contributes to both low volatility and diffusivity [6].

References [1] G. Wypych (Ed.), Handbook of Plasticizers, third ed., ChemTec Publishing, Toronto-New York, 2004. [2] R. Sothornvit, J.M. Krochta, Plasticizers in Edible Films and Coatings, in: J.H. Han (Ed.), Academic Press, London, 2005. [3] C.S. Brazel, S.L. Rosen, Fundamental Principles of Polymeric Materials, third ed., Wiley, 2012 (Chapter 7 and Chapter 18). [4] U. Zucchelli, Pvc Flame Retardant Compositions, Patent: WO 2014013284 A1. [5] M.G.A. Vieira, M. Altenhofen da Silva, L. Oliveira dos Santos, M.M. Beppu, Natural-based plasticizers and biopolymer films: a review, Eur. Polym. J. 47 (2011) 254e263. [6] C. Wilkes, J. Summers, C. Daniels, PVC Handbook, HANSER, 2005 (Chapter 5: Plasticizers). [7] M. Chanda, S.K. Roy, Plastic Polymers Handbook, Marcel Dekker, Inc., New York, 1986 (R. Sothornvit, J. M. Krochta, Plasticizers in edible films and coatings). [8] R.F. Grossman (Ed.), Handbook of Vinyl Formulating, second ed., Wiley, 2008.  [9] K. Bortel, Srodki pomocnicze stosowane w przetwo´rstwie tworzyw polimerowych. Cz.2, Przetwo´rstwo Tworzyw (2008) 148e153. [10] M.A. Da Silva, M.G.A. Vieira, A.C.G. Mac¸umoto, M.M. Beppu, Polyvinylchloride (PVC) and natural rubber films plasticized with

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a natural polymeric plasticizer obtained through polyesterification of rice fatty acid, Polym. Test. 30 (2011) 478e484. H.F. Mark (Ed.), Encyclopedia of Polymer Science and Technology, 4th Print and Online Editions, John Wiley and Sons, 2014. A.H. Tullo, Plasticizer makers want a piece of the phthalates pie, Chem. Eng. News 93 (25) (2015) 16e18. S. Benjamina, S. Pradeepa, M.S. Josha, S. Kumarb, E. Masaic, A monograph on the remediation of hazardous phthalates, J. Hazard Mater. 298 (2015) 58e72. A. Marcilla, S. Garcıa, J.C. Garcıa-Quesada, Study of the migration of PVC plasticizers, J. Anal. Appl. Pyrolysis 71 (2004) 457e463. M. Rahman, C.S. Brazel, The plasticizer market: an assessment of traditional plasticizers and research trends to meet new challenges, Prog. Polym. Sci. 29 (2004) 1223e1248. M.O. Boussoum, N. Belhaneche-Bensemra, Reduction of the additives migration from poly(vinyl chloride) films by the use of permanent plasticizers, J. Geosci. Environ. Prot. 2 (2014) 49e56. L.G. Krauskopf, Plasticizer structure/performance relationships, J. Vinyl Addit. Technol. 15 (3) (1993) 140e147. M. Kutz (Ed.), Applied Plastics Engineering Handbook, Processing, Materials, and Applications, second ed.Plastics Design Library (PDL), PDL Handbook Series, William Andrew is an imprint of Elsevier, 2017. Directive 2005/84/EC of the European Parliament and of the Council of 14 December 2005. Amending for the 22nd time Council Directive 76/769/EEC on the approximation of the laws, regulations and administrative provisions of the Member States relating to restrictions on the marketing and use of certain dangerous substances and preparations (phthalates in toys and childcare articles), Off. J. Eur. Union L 344 (40). K. Larsson, C.H. Lindh, B.A.G. Jo¨nsson, G. Giovanoulis, M. Bibi, M. Bottai, A. Bergstro¨m, M. Berglund, Phthalates, non-phthalate plasticizers and bisphenols in Swedish preschool dust in relation to children’s exposure, Environ. Int. 102 (2017) 114e124. D.F. Cadogan, C.J. Howick, in: Kirk-Othmer (Ed.), Encyclopedia of Chemical Technology, John Wiley and Sons, New York, 1996 (Chapter: Plasticizers). J.T. Renshaw, Plasticizers for vinyl chloride polymers, 1983. US Patent 4,423,178,.

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[23] N. Gil, I. Negulescu, M. Saska, Evaluation of the effects of bio-based plasticizers on thermal and mechanical properties of poly(vinylchloride), J. Appl. Polym. Sci. 102 (2006) 1366e1373. [24] E.J. Wickson (Ed.), Handbook of PVC Formulating, WileyInterscience, New York, 1993 (Chapter 5: Plasticizers, A.D. Godwin, L.G. Krauskopf, (Chapter 8): Specialty Plasticizers, W.D. Arendt, M. Joshi). [25] https://www.hallstar.com/webfoo/wp-content/uploads/hallstar-highperformance-ester-plasticizer.pdf. [26] M. Kisbenyi, Study of the efficiency of plasticizers in poly(vinyl chloride) by dielectric spectroscopy, J. Polym. Sci. Part C 33 (1) (1971) 113e122. [27] A. Lindstro¨m, M. Hakkarainen, Environmentally friendly plasticizers for poly(vinyl chloride) - improved mechanical properties and compatibility by using branched poly(butylene adipate) as a polymeric plasticizer, J. Appl. Polym. Sci. 100 (2006) 2180e2188, and references therein. [28] L. Yan, W. Changming, W. Guojian, Q. Zehua, Application of the long-chain linear polyester in plastification of PVC, J. Wuhan Univ. Technol. Mater. Sci. Ed. 23 (1) (2008) 100e104. [29] A. Lindstro¨m, M. Hakkarainen, Migration resistance polymeric plasticizer for poly(vinyl chloride), J. Appl. Polym. Sci. 104 (4) (2007) 2458e2467. [30] A. Jime´nez, L. Torre, J.M. Kenny, Thermal degradation of poly(vinyl chloride) plastisols based on low-migration polymeric plasticizers, Polym. Degrad. Stabil. 73 (3) (2001) 447e453. [31] J.J. Ziska, J.W. Barlow, D.R. Paul, Miscibility in PVC-polyester blends, Polymer 22 (7) (1981) 918e923. [32] B.N. Rushton, N.S. Salomons, An investigation of factors relating to the exudation of polyester plasticizers from poly(vinyl chloride), J. Appl. Polym. Sci. 13 (1969) 2341e2358. [33] N.J. Clayden, C. Howick, Effect of the processing temperature on the interaction between plasticizer and poly(vinyl chloride) as studied by solid state n.m.r. spectroscopy, Polymer 34 (1993) 2508e2516. [34] A. Lindstro¨m, Environmentally Friendly Plasticizers for PVC e Improved Material Properties and Long-Term Performance through Plasticizer Design, KTH Fibre and Polymer Technology, Stockholm, 2007.

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[35] B.L. Shah, V.V. Shertukde, Effect of plasticizers on mechanical, electrical, permanence, and thermal properties of poly(vinyl chloride), J. Appl. Polym. Sci. 90 (2003) 3278e3284. [36] C. Zhang, S. Feng, Effect of glycols on the properties of poliester polyols and of room-temperature-curable casting polyurethanes, Polym. Int. 53 (2004) 1936e1940. [37] J. Choi, S.Y. Kwak, Hyperbranched poly(ε-caprolactone) as a nonmigrating alternative plasticizer for phthalates in flexible PVC, Environ. Sci. Technol. 41 (2007) 3763e3768. [38] A. Lindstro¨m, M. Hakkarainen, Designed chain architecture for enhanced migration resistance and property preservation in poly (vinyl chloride)/polyester blends, Biomacromolecules 8 (2007) 1187e1194. [39] A. Sunder, R. Mu¨lhaupt, R. Haag, H. Frey, Hyperbranched polyether polyols: a modular approach to complex polymer architectures, Adv. Mater. 12 (3) (2000) 235e239. [40] C. Gao, D. Yan, Hyperbranched polymers: from synthesis to applications, Prog. Polym. Sci. 29 (2004) 183e275. [41] L. Castle, A.J. Mercer, J. Gilbert, Migration from plasticized films into foods. 5. Indentification of individual species in a polymeric plasticizer and their migration into foods, Food Addit. Contam. 8 (1991) 565e576. [42] Q. Wang, B.K. Storm, Migration of additives from poly(vinyl chloride) (PVC) tubes into aqueous media, Macromol. Symp. 225 (2005) 191e203. [43] L. Castle, A.J. Mercer, J. Gilbert, Migration from plasticized films into foods. 4. Use of polymeric plasticizers and lower levels of di-(2ethylhexyl)adipate plasticizer in PVC films to reduce migration into foods, Food Addit. Contam. 5 (1988) 277e282. [44] New plasticizer joins Bayer’s range, Addit. Polym. 2 (2003) 7e8. [45] X. Li, Y. Xiao, B. Wang, Y. Tang, Y. Lu, C. Wang, Effects of poly(1,2propylene glycol adipate) and Nano-CaCO3 on DOP migration and mechanical properties of flexible PVC, J. Appl. Polym. Sci. 124 (2) (2012) 1737e1743. [46] C. Oriol-Hemmerlin, Q.T. Pham, Poly 1,3-butylene adipate reoplexÒ as high molecular weight plasticizer for PVC-based cling filmsdmicrostructure and number-average molecular weight studied by 1H and 13C NMR, Polymer 41 (12) (2000) 4401e4407. [47] S. Chuayjuljit, P. Chaiwutthinan, S. Samutthong, O. Saravari, A. Boonmahitthisud, Effects of poly(butylene succinate) and calcium

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carbonate on the physical properties of plasticized poly(vinyl chloride), J. Met. Mater. Miner. 24 (2) (2014) 15e21. S. Chuayjuljit, C. Siraprapoj, A. Boonmahitthisud, Effects of poly (butylene succinate) and ultrafine wollastonite on the physical properties of plasticized poly(vinyl chloride), J. Vinyl Addit. Technol. (2014) 220e227. A. Lindstro¨m, Poly(butylene succinate) and Poly(butylene adipate) Quantitative Determination of Degradation Products and Application as PVC Plasticizers, KTH Fibre and Polymer Technology, Stockholm, 2004. M.N.M. Rahim, N.A. Ibrahim, J. Sharif, W.M.Z. Wan Yunus, Mechanical and thermal properties of poly(vinyl chloride)/ poly(butylene adipate-co-terephthalate) clay nanocomposites, J. Reinf. Plast. Compos. 29 (21) (2010) 3219e3225. N.A. Ibrahim, N.M. Rahim, W.Z. Wan Yunus, J. Sharif, A study of poly(vinyl chloride)/poly(butylene adipate-co-terephthalate) blends, J. Polym. Res. 18 (2011) 891e896. G. Shi, D.G. Cooper, M. Maric, Poly(ε-caprolactone)-based “green” plasticizers for poly(vinyl choride), Polym. Degrad. Stabil. 96 (2011) 1639e1647. R.D. Deanin, Z. Zheng-Bai, Polycaprolactone as a permanent plasticizer for poly(vinyl chloride), J. Vinyl Technol. 6 (1) (1984) 18e21. K.W. Lee, J.W. Chung, S.-Y. Kwak, Highly branched polycaprolactone/glycidol copolymeric green plasticizer by one-pot solvent-free polymerization, ACS Sustain. Chem. Eng. 6 (7) (2018) 9006e9017. A.-C. Albertsson, I.K. Varma, Aliphatic polyesters: synthesis, properties and applications, Adv. Polym. Sci. 157 (2002) 1e40. Y.H. Kim, O.W. Webster, Hyperbranched polyphenylenes, Macromolecules 25 (1992) 5561e5572. C.F. Huber, H.C.J. Foulks, R.D. Aylesworth, Polyesters from Dibasic Acids, Monobasic Acids, Glycols, and Trihydric Alcohols as Plasticizers for Vinyl Resins, Patent US 3,331,802A, 1967. A. Lindstro¨m, M. Hakkarainen, Miscibility and surface segregation in PVC/polyester blends - the influence of chain architecture and composition, J. Polym. Sci. B Polym. Phys. 45 (13) (2007) 1552e1563. N.T. Nguyen, K.J. Thurecht, S.M. Howdle, D.J. Irvine, Facile onespot synthesis of highly branched polycaprolactone, Polym. Chem. 5 (2014) 2997e3008.

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[60] D.S. Marathe, P.S. Joshi, Characterization of highly filled wood flourPVC composites: morphological and thermal studies, J. Appl. Polym. Sci. 114 (1) (2009) 90e96. [61] S.N. Pal, A.V. Ramani, N. Subramanian, Studies on poly (vinylchloride)-based polymer blends intended for medical applications. Part II: mechanical properties, J. Polym. Eng. Sci. 32 (13) (1992) 845e853.  [62] K. Pielichowski, B. Swierz-Motysia, Influence of polyesterurethane plasticizer on kinetics of poly(vinyl chloride) decomposition process, J. Therm. Anal. Calorim. 83 (1) (2006) 207e212. [63] J.R. Pena, M. Hidalgo, C. Mijangos, Plastification of poly(vinyl chloride) by polymer blending, J. Appl. Polym. Sci. 75 (10) (2000) 1303e1312. [64] H. Li, L. Wang, G. Song, Z.H. Gu, P. Li, C.H. Zhang, L. Gao, Study of NBR/PVC/OMMT nanocomposites prepared by mechanical blending, Iran. Polym. J. 19 (1) (2010) 39e46. [65] M.C. Sunny, P. Ramesh, K.E. George, Use of polymeric plasticizers in polyvinyl chloride to reduce conventional plasticizers migration for critical applications, J. Elastomers Plastics 36 (1) (2004) 19e31. [66] C.S. Ha, Y. Kim, W.K. Lee, W.J. Cho, Y. Kim, Fracture toughness and properties of plasticized PVC and thermoplastic polyurethane blends, Polymer 39 (20) (1998) 4765e4772. [67] V.J.R.R. Pita, E.E.M. Sampaio, E.E.C. Monteiro, Mechanical properties evaluation of PVC/plasticizers and PVC/thermoplastic polyurethane blends from extrusion processing, Polym. Test. 21 (5) (2002) 545e550. [68] C.T. Ratnam, K. Zaman, Stabilization of poly(vinyl chloride)/ epoxidized natural rubber (PVC/ENR) blends, Polym. Degrad. Stabil. 65 (1) (1999) 99e105. [69] N.L. Thomas, Alloying of poly(vinyl chloride) to reduce plasticizer migration, J. Appl. Polym. Sci. 94 (5) (2004) 2022e2031. [70] M. Rahman, C.S. Brazel, Ionic liquids: new generation stable plasticizers for poly(vinyl chloride), Polym. Degrad. Stabil. 91 (2006) 3371e3382.

3 Essential Quality Parameters of Plasticizers 3.1 Basic Quality Parameters A measure of a plasticizer’s suitability for plasticizing poly(vinyl chloride) (PVC) is the physicochemical, physicomechanical and functional properties it provides a plasticized polymer with. They depend mainly on the properties of the plasticizer or mixture of plasticizers used. These properties include miscibility with PVC, plasticizing efficiency, plasticizer volatility, limited susceptibility to migration and extraction. Considering the functional properties of plasticized products, some of the essential parameters also include resistance to low and high temperatures, resistance to UV radiation, flammability, electrical properties and environmental impact, especially on living organisms. Plasticizing generally leads to changes in the thermal and mechanical properties of a plasticized polymer, which include decreased glass transition temperature, a lower stiffness of the material at room temperature; reduced force necessary to induce deformation and increased elongation at break and impact resistance. The incorporation of a plasticizer into a specific polymer matrix significantly changes its susceptibility to processing by lowering the viscosity of the melt being processed and shortening of the gelation time in the case of PVC blends [1]. These effects can usually be achieved by mixing a particular polymer with a low molecular weight compound or with another polymer or by incorporating a comonomer into the initial polymer resulting in a reduction in its crystallinity and increase in chain elasticity. The miscibility of PVC with plasticizers is one of the most important factors determining their suitability for polymer plasticization. The miscibility of PVCeplasticizer systems is usually associated with the ability to form homogeneous systems, for which mutual solubility is one of the most important parameters. As a basis for the determination of miscibility, only physicochemical phenomena determining the mutual solubility of components forming a homogeneous thermodynamically balanced system can be taken into consideration. For binary and more complex systems, in which no chemical reactions take place, their phase balance is usually taken into account. Plasticizers Derived from Post-consumer PET https://doi.org/10.1016/B978-0-323-46200-6.00003-9 Copyright © 2020 Elsevier Inc. All rights reserved.

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Good miscibility of the polymer with a plasticizer makes it possible to obtain a homogeneous material with good functional properties. However, there have been cases where the discussed systems mixed well at higher temperatures during processing, but once cooled, the plasticizer precipitated from the material. Primary plasticizers show good miscibility with PVC in a wide range of concentrations and rarely exudate intensely from the polymer matrix (e.g., in the form of a thin plasticizer film on the surface of the material). These plasticizers are characterized by adequate miscibility with the polymer both during processing and final use of the products. Secondary plasticizers, which are usually incompatible with the polymer or whose miscibility is limited, are referred to as extenders. Chlorinated hydrocarbons and alkyl aromatic hydrocarbons demonstrate the desired dispersion force values and sufficiently high solubility parameters, but their molecules lack polarity and the ability to form hydrogen bonds and thus are unable to act as primary plasticizers. A polymer’s miscibility with a plasticizer is influenced by many factors, some of which include pressure, temperature, humidity, UV radiation and the presence of other components in the blend. As the temperature rises (energy enters the system), the forces of interaction between the polymer and the plasticizer are weakened. The incorporation of a plasticizer into the PVC composition leads to an increase in the free volume of the system. Previous studies have shown a relationship between changes in free volume and in polymer chain mobility. It has been shown that the increase in free volume can be caused by Ref. [2]: - an increase in temperature; - reduction of the molecular weight (either by the addition of a polymer of a lower molecular weight or additives, e.g., a plasticizer); - increase in chain mobility (fewer end groups e lower polarity, weaker interaction between chains e hydrogen bonds); - increase in the hydrodynamic volume (e.g., numerous, long side chains). The space occupied by the plasticizer depends on the shape and volume of its molecule. The size of the plasticizer molecule affects the free volume, plasticization efficiency, susceptibility to processing of the polymer composition and the preservation of the material properties

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(its permanence). The increase in the free volume caused by plasticization correlates with the glass transition temperature Tg of the polymer/ plasticizer mixture according to the following equation: Vf ¼ V$½ 0:025 þ af $ðT  Tg Þ where Vf e average free volume in polymer; V e total volume of sample; af e thermal expansion coefficient; T e sample temperature; Tg e glass transition temperature. The free volume of the PVC/plasticizer composition depends on the size of the plasticizer molecule and its conformation. The glass transition temperature, Tg, is one of the basic parameters characterizing polymereplasticizer systems. The glass transition temperature Tg of the plasticized polymer depends both on the amount and the efficiency of the plasticizer and can be determined using the following dependency: Tg ¼ Tg2  kw1 where Tg e glass transition temperature of polymereplasticizer(s) mixture; Tg2 e glass transition temperature of unplasticized polymer; k e plasticizer efficiency parameter; w1 e weight fraction of plasticizer. The above dependency does not apply to the antiplasticization range characteristic for each plasticizer. The basic parameters determining the usefulness of plasticizers in modifying polymers include good miscibility with the polymer matrix, low volatility of the plasticizer and susceptibility to migration as well as high plasticization efficiency. These parameters determine its durability in the polymer composition. The above dependencies are shown graphically below in Fig. 3.1.

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Figure 3.1 Influence of essential plasticizer properties: miscibility, efficiency and volatility on the permanency of the plasticizer in the polymer matrix.

3.1.1 Compatibility of the Plasticizer With the Polymer In the case of polymer plasticization, compatibility should be interpreted as the ability to create a homogeneous system of a modified polymeric matrix with a plasticizer. Homogenization of the PVCeplasticizer system takes place as soon as the two substances are mixed. The subsequent intensive process, assisted by thermal and mechanical (shear) energy, takes place at the processing stage. What is important is for the obtained polymer melt, after gelation process and cooling, to be thermodynamically stable. Regardless of whether the plasticizer is a monomeric, oligomeric, polymeric substance or a polymer, the suitability for a modified polymer matrix to be plasticized is also determined by the plasticizer’s miscibility with the polymer, which is determined by its chemical structure, polarity of its molecule and its molecular weight. The structure of chemical substances determines their polarity, this also applies to polymers. To achieve a high degree of compatibility between the plasticizer and the polymer, the plasticizer and polymer should have

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approximate polarity values. In terms of decreasing polarity, the polymers can be ranked as follows: Nylon 6/6, Nylon 6, cellulose acetate, NBR (50% ACR), polyurethane, NBR 40% ACR, nitrocellulose, epoxy, polycarbonate, acrylic (PMMA), poly(vinyl acetate), NBR (30% ACN), acrylate elastomers, poly(vinyl butyral), chlorosulfonated polyethylene, poly(vinyl chloride), cellulose acetate butyrate, polystyrene, polychloroprene, NBR (20% ACN), nitrocellulose, EPR, fluorinated polymers and silicone. In turn, plasticizers can be ranked as follows in terms of their decreasing polarity: aromatic sulfonamides, aromatic phosphate esters, alkyl phosphate esters, dialkylether aromatic esters, polymeric plasticizers, dialkylether diesters, polyglycol diesters, tricarboxylic esters, polyester resins, aromatic diesters, aromatic triesters (trimellitates), aliphatic diesters, epoxidized esters, chlorinated hydrocarbons, aromatic oils, alkylether monoesters, naphthenic oils, alkyl monoesters and paraffinic oils [3]. A lack of or limited compatibility of the polymereplasticizer system will be apparent at the processing stage. In such cases, the compositions are characterized by much longer gelation times or they do not gel altogether and delaminate. Signs of system instability also include excessive and rapid exudation of the plasticizer, which leads to a deterioration of the mechanical properties of the product. The exudation of the plasticizer and its migration to the surface of plasticized products may be caused not only by the lack of compatibility of the plasticizer with the polymer matrix. Limited thermodynamic stability can also be associated with the maximum amount of plasticizer that can be incorporated into the polymer matrix. When composing formulations, an acceptable level of compatibility in polymereplasticizer systems should be determined. In the case of plasticized PVC compositions, in which polymers polymerized in a suspension or in bulk are used, the absorbency of the plasticizer determined for a particular polymer is used as an indicator. The interactions between a polymer and solvents or plasticizers are the basis for understanding the compatibility of systems with a complex phase composition. It has been shown that the individual properties of polymers and plasticizers determine their mutual interaction, and in consequence, their miscibility or lack of affinity [4].

3.1.1.1 Interaction Parameter The polymereplasticizer interaction force values determine the plasticizer’s suitability for physically modifying the polymer. The forces of interaction between the plasticizer molecules and polymer macromolecules should be greater than the forces of interaction between the plasticizer

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molecules themselves, otherwise the plasticizer molecules would become associated, leading to their migration from the polymer matrix. In the case of PVC, its repeating monomer units contain polarized CeCl bonds, thanks to which the polymer is of a dipolar nature. This PVC property resulting from its structure enables polar elements of plasticizer molecules (e.g., aromatic rings, ester bonds) to interact with the polymer via van der Waals forces and dipoleedipole interaction. This sort of interaction takes place in the amorphous structures of the polymer outside the tightly packed crystalline spaces. The FloryeHuggins theory plays an important role in assessing the mutual miscibility of the polymer and the plasticizer. The so-called FloryeHuggins c parameter of mutual interaction is the criterion defining the miscibility of PVC with plasticizers. It allows for a quantification of the affinity of the plasticizer with the polymer. The dimensionless c parameter determines the difference between the energy of the plasticizer molecules immersed in a pure polymer and the energy of interaction of the same molecule in a pure plasticizer and characterizes the energy of interaction related to 1 mole of a solvent [5]: c ¼

ZDw12 RT

where c e FloryeHuggins interaction parameter; Z e coordination number of the crystalline network model; Dw12 e mutual interaction energy between polymer and solvent molecules; R e universal gas constant; T e absolute temperature. A good plasticizer or solvent should have a low c value. Plasticizers for which the c values are 0.55 or higher are generally considered to be immiscible, those characterized by values between 0.55 and 0.3 demonstrate moderate miscibility, while values below 0.3 are characteristic of plasticizers with good miscibility [6]. It is assumed that the critical c value, which is sufficient in ensuring miscibility between a high molecular weight polymer and a plasticizer, is 0.5. The FloryeHuggins c interaction parameter constitutes a popular miscibility criterion. Solubility data for selected plasticizers are shown in Table 3.1. For calculation purposes, the molar value for a repeating unit was used for four of the polymeric plasticizers [7].

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OF

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51

Table 3.1 Solubility Data for Individual Plasticizers [7]. Plasticizer

tm [ C]

V1 [cm3 molL1]

c

Octyl-diphenyl phosphate

88

338.91

0.41

Diisobytyl phthalate

88

280.81

0.13

Dibutyl phthalate

92

290.39

0.10

Dioctyl phthalate

115

424.84

0.10

Diisononyl phthalate

123

464.03

0.03

Diisodecyl phthalate

127

499.12

0.01

C911P Linear phthalate [di(linear C9, C11)]

130

503.08

0.05

Diisoundecyl phtalate

135

537.21

0.11

Dioctyl adipate

136

435.99

0.29

Diundecyl phtalate

140

543.71

0.21

Diisotridecyl phthalate

144

610.64

0.22

Diisooctyl sebacate

148

511.39

0.43

URAPLAST RA10

146

215.83

0.74a

URAPLAST S5561

159

199.54

0.87a

URAPLAST RA11

159

199.54

0.87a

URAPLAST S5640

159

199.54

0.87a

a

Calculated using polyester repeat unit.

Table 3.2 contains c values calculated by Doty and Zable for the PVC-selected plasticizer system. According to these authors, miscibility between PVC and the plasticizer can be expected when c  0:55, miscibility in all the ratios occurs only when c  0, whereas values of c ¼ 0:2 to 0:4 are characteristic for so-called neutral solvents, for which the heat of mixing is equal to zero. When analyzing the dependence between c and the molecular weight of di-n-alkyl phthalates, Doty and Zable found that the correlation curve reached a minimum referring to ester from the butyl to hexyl. It should therefore be assumed that the phthalates of these alcohols have the highest miscibility with PVC in this group. The miscibility of esters of lower and higher alcohols with PVC is limited.

52

P LASTICIZERS D ERIVED

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Table 3.2 c Values for a Poly(vinyl chloride)eSelected Plasticizer System [8]. c Plasticizer

53 C

76 C

Tributyl phosphate

0.65

0.53

Dihexyl phthalate

0.13

0.09

Ditetrahydrofurfuryl sebacate

0.11

0.10

Dibutyl phthalate

0.04

0.01

Dioctyl phthalate

0.01

0.03

Triethyl phosphate

0.13

0.15

Diethyl sebacate

0.17

0.18

Dihexyl adipate

0.19

0.23

Diamyl sebacate

0.24

0.24

Dimethyl sebacate

0.34

0.34

Dihexyl sebacate

0.35

0.36

Tricresyl phosphate

0.38

0.38

Diethyl phthalate

0.42

0.40

Dibenzyl sebacate

0.56

0.53

Methyl acetyl ricinoleate

0.56

0.56

Di(2-ethylhexyl) sebacate

0.59

0.57

Butyl acetyl ricinoleate

0.65

0.67

Butyl ricinoleate

1.22

1.20

Octyl laurate

1.38

1.41

Dilauryl phthalate

1.75

1.41

Butyl palmitate

1.73

1.66

Ethyl stearate

2.60

2.80

Table 3.3 contains c values and parameter of polymer/solvent interaction energy (B) for the PVC-selected plasticizer system. The plasticizer usually acts on the unorganized amorphous morphological structures of the polymer. PVC is a polymer with a poor crystalline

Table 3.3 Characteristic Values for Poly(vinyl chloride)eEster Systems [8]. Ester

tm,o C

c

B, kal/cm2

Dimethyl phthalate

109e112

0.52

2.3

Diethyl phthalate

98e104

0.34

1.2

Di-n-propyl phthalate

93e96

0.10

0.3

Di-n-butyl phthalate

90e94

0.05

0.1

Di-n-pentyl phthalate

98e101

0.05

0.1

Di-n-hexyl phthalate

106e108

0.04

0.1

Di-n-heptyl phthalate

109e112

0.11

0.2

Di-n-octyl phthalate

116e118

0.03

0.1

Di-2-ethylhexyl phthalate

116e118

0.03

0.1

Di(1-propylpentyl) phthalate

141e143

0.40

0.7

Di(2-ethyl-4-methylpentyl) phthalate

124e127

0.30

0.2

Di-n-nonyl phthalate

118e122

0.09

0.1

Di-n-decyl phthalate

135e137

0.19

0.3

Di-n-undecyl phthalate

139e141

0.20

0.3

Di-n-dodecyl phthalate

152e155

0.49

0.7

Diethyl isophthalate

87e89

0.16

0.5

Di-n-butyl isophthalate

95e100

0.01

0.0

Di-2-ethylhexyl isophthalate

135e137

0.10

0.6

Diethyl terephthalate

82e85

0.31

0.3

Di-n-butyl terephthalate

95e100

0.01

0.0

Di-2-ethylhexyl terephthalate

144e146

0.46

0.9

Diethyl fumarate

94e99

0.38

1.6

Di-n-butyl fumarate

109e111

0.20

0.8

Di-2-ethylhexyl fumarate

148e150

0.56

1.2

Dimethyl maleate

125e130

0.02

5.3

Diethyl maleate

125e130

0.64

3.9

Di-n-butyl maleate

111e113

0.32

1.0

Di-2-ethylhexyl maleate

135e137

0.36

0.8

Didecyl maleate

148e152

0.53

1.0

P LASTICIZERS D ERIVED

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FROM

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arrangement. It was found that the stereochemical configuration of this polymer significantly determines its physical properties [9]. It was also discovered that the syndiotacticity level increases with a decrease of polymerization temperature, and for commercial PVC suspensions, it varies from 54% to 65% when their crystallinity reaches a value of approximately 10% [10]. For polymers and low molecular weight substances, the balance in two-component systems applies to both the amorphous and crystalline phases of the components. For PVC, the equilibrium status relating to both arrangement structures should be taken into account. Explanation of the plasticization mechanism takes into account the interactions between the plasticizer and the PVC macromolecules. It is assumed that the plasticizer molecules are not permanently bonded with the PVC macromolecules. The interactions between the polymer and solvents or plasticizers are the basis for understanding the compatibility of systems characterized by a complex phase structure. It has been shown that the individual properties of polymers and plasticizers determine their mutual interaction and in consequence, their miscibility or lack of affinity [4]. When evaluating miscibility, it is important to determine the interaction between the components being mixed. It is essential, however, to assess compatibility by means of a numerical thermodynamic criterion in relation to other external factors, e.g., pressure or temperature. Most importantly, the polymer and the plasticizer should demonstrate complete miscibility at the molecular level. Thermodynamic criteria regarding the miscibility of auxiliaries with polymers are similar to those for mixtures of common liquids [11]. In the FloryeHuggins theory, it was assumed that the process of mixing a polymer with a solvent, e.g., a plasticizer, can be determined using enthalpy and entropy. In cases when two liquid substances form a stable solution, then according to the classic thermodynamic approach, the DGm Gibbs free energy of mixing assumes a negative value [11]. The overall correlation can be presented as follows: DGm ¼ DHm  TDSm where DGm e Gibbs free energy of mixing; DHm e enthalpy change; TDSm e entropy change.

3: E SSENTIAL Q UALITY PARAMETERS

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55

The Gibbs free energy of mixing presented in the equation below describes this process [12e15]:   DG V2 ¼ x1 lnf1 þ x2 lnf2 þ c1 f1 f2 x1 þ x2 RT V1 where DG e Gibbs free energy of mixing; x1, x2 e mole fractions of plasticizer and polymer, respectively; f1, f2 e volume fractions of plasticizer and polymer, respectively; c1 e Huggins interaction parameter. In this theory, it is assumed that the c1 parameter is independent of the concentration but has not been confirmed experimentally either. This parameter was introduced by Flory and Huggins into the equation for determining plasticizer activity, as an extension of the theory of athermal to nonathermal mixing processes: ln a1 ¼

Dm1 ¼ lnð1  f2 Þ þ f2 þ c1 f22 RT

where a1 e plasticizer activity; m1 e chemical potential of plasticizer. Hildebrand and Scatchard proposed combining the internal energy of mixing with the solubility parameters of the solvent and solute [15]: ¼ ðx1 V1 þ x2 V2 Þ

1 2  1 # DE2 2 f1 f2  V2 =

DU

DE1 V1

¼ ðx1 V1 þ x2 V2 Þðd1  d2 Þ2 f1 f2 where DUm e internal energy of mixing; x1, x2 e molar fractions of components; f1, f2 e volume fractions of components; V1, V2 e molar volumes of components.

=

" m

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Gibbs free energy of mixing, DGm, can be calculated using the equation: " 1  1 # DE1 2 DE2 2 f1 f2  TDSid ¼ ðx1 V1 þ x2 V2 Þ  V1 V2 =

=

DGm

¼ Vðd1  d2 Þ2 f1 f2  TDSid The DSid entropy change is calculated using Gibbs equation that is based on mixing ideal gases and always assumes a positive value. The interactions between a polymer and solvents or plasticizers are the basis for understanding the compatibility of systems demonstrating a high level of phase complexity. It has been shown that the individual properties of polymers and plasticizers determine their mutual interaction and in consequence their miscibility or lack of affinity [4]. Thermodynamic theories and interaction parameters are used in all quantitative descriptions of interactions occurring in polymer solutions. We often use phase diagrams defining regions of compatibility and incompatibility of polymereplasticizer systems, one of which is shown below. The diagram presented in Fig. 3.2 describes an amorphous polymereplasticizer system and covers three areas: I e complete mutual mixing, II e phase segregation and III e metastability (an area, in which incompatible phases can coexist for a longer period of time without becoming segregated). The high viscosity of polymereplasticizer solutions in segregation regions is usually due to the low speed of this process. 1,3 (Binodal) boundary curves separate homogenous regions from heterogeneous ones, while boundary curve 2 (spinodal) separates metastable and unstable regions. Typically, when the temperature of the system increases, the incompatibility decreases and the system becomes monophasic at the TAC temperature. The temperature of this transition is dependent on the composition of the system and reaches its maximum at T1c, also referred to as the UCST when both components are compatible in any proportion.

3.1.1.2 Solubility Parameter The plasticizer usually acts on the unorganized amorphous structures of the polymer. Explanations of the plasticization mechanism take into account the interactions between the plasticizer and the PVC

3: E SSENTIAL Q UALITY PARAMETERS

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Figure 3.2 Representation of a phase diagram with two critical solution temperatures. 1,3 e binodals, 2 e spinodal; I e complete mutual mixing, II e phase segregation, III e metastability. T1c e upper critical solution temperature, UCST; T2c e lower critical solution temperature, LCST [4].

macromolecules, which assumes that the plasticizer molecules are not permanently bonded with polymer. In the case of plasticizers with much lower solvation ability, the swelling and dissolving effect of the morphological structures of the polymer are believed to be considerably lower. In amorphous regions, PVC chains or fragments thereof are dissolved at elevated temperatures during processing. The plasticization mechanism itself is not fully understood. For many years, various attempts were made to explain the process, resulting in several theories. Sears and Darby made a theoretical review of this process and described plasticity using three theories. Furthermore, they discussed in detail the mixing of PVC with plasticizers [16e19] and analyzed a number of parameters relating to the compatibility of the PVCeplasticizer system covering solubility parameters, polarity parameters, the dielectric constant and FloryeHuggins interaction parameters. The solubility parameter was considered as one of the most important factors in polymereplasticizer interaction. Knowing the parameter makes it possible to determine the miscibility of the polymer and plasticizer.

P LASTICIZERS D ERIVED

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The theory of solubility parameters for low molecular weight compounds was introduced by Small, Hildebrand and Scott [20] and Van Krevelen [21] in the 1950s, but later studies showed its many other useful implications that can be used to describe surface phenomena (characteristics of adsorption and wettability properties) and in particular macromolecular interaction, based on which it is possible to determine the solubility of a substance in solvents as well as the affinity and mutual miscibility of a substance as per the principle of “like dissolves like”. The molar energy of cohesion (E) characterizes intermolecular cohesion. It is a measure of the intermolecular interactions in 1 mole of a chemical compound. The molar energy of polymer cohesion is disproportionately high compared with the value for a low molecular weight compound. The molar cohesion energy is proportional to the energy of intermolecular interaction (w) and to the molar volume (V): EwwV

E=Vww

The E/V ratio is the cohesion energy density. Regardless of the particle size, the cohesion energy density is a measure of intermolecular contact energy. According to the Hildebrand-Scatchard concept, the strongest attraction between solvent and solute molecules occurs when the cohesion energy of both components is the same (E1/V1 ¼ E2/V2). The solubility parameter, also referred to as the cohesion energy parameter, was defined by Hildebrand [15] as the square root of cohesion energy density. As this measure  represents the energy (heat) of evaporation per unit of liquid volume DE V , the solubility parameter d expressed in MPa1/2 is related to the cohesion energy density expressed in the equation: rffiffiffiffiffiffiffi DE d ¼ V where V e molar volume [m3/mol]; DE e molar evaporation energy [J/mol] in absolute temperature. The thermodynamic condition for spontaneous dissolution to take place, described by a change of free energy, is based on the relationship: DG ¼ DH  T$DS

3: E SSENTIAL Q UALITY PARAMETERS

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is a negative or zero mixing energy value. According to Hildebrand’s and Scatchard’s proposal, the DH mixing enthalpy can be expressed by the equation: DH ¼ 41 $42 $Vm $ðd1  d2 Þ2 where 41, 42 constitute volume share values of the polymer and solvent fractions, respectively, while Vm is the mixture volume. The square of the solubility parameter difference ðd1  d2 Þ2 is referred to as the b compatibility parameter and can be applied to any solventepolymer or polymerepolymer pair, etc., when the d solubility parameters are known. Hansen [22e24] assumes that the total evaporation energy of liquids (E) contains at least three components. They come from nonpolar (atomic) dispersion forces (ED), permanent dipole (molecular) forces (EP) and hydrogen bond forces (EH). The last of the components (EH) is more generally referred to as the energy of electron exchange. E ¼ ED þ EP þ EH The total cohesion energy (E) can be determined on the basis of the heat of evaporation, i.e., a process that destroys all types of cohesive bonds. This became the basis for the description of the so-called Hansen solubility parameters (HSPs) as depicted by means of the following formula: E ED EP EH ¼ þ þ V V V V 2 2 2 2 d ¼ dD þ dP þ dH

HSP ¼

where the dD , dP and dH partial parameters characterize dispersion, polar and hydrogen bond interactions and d is the Hildebrand solubility 1 E 2 parameter V . As the solubility parameter d is related to cohesion energy, it is an effective descriptor values  0:5  0:5 of intermolecular interactions. It assumes for nonpolar substances up to 23 MJ m3 for from 12 MJ m3 water [25]. Knowing the d values of various solvents and of the substance being dissolved, we can predict which solvent the polymer will not dissolve in. For example, polyisobutylene with an d in the range   14e16 0:5 (MJ/m3)0.5 does not dissolve in solvents with a d ¼ 20  24 MJ m3 .

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P LASTICIZERS D ERIVED

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  0:5 A polar polymer with a d ¼ 18 MJ m3 does not dissolve in   3 0:5 . solvents whose d ¼ 14 or d ¼ 26 MJ m Knowledge of the d solubility parameter value makes it possible to limit the range of substances that qualify as plasticizers (solvents) for a given polymer. Good miscibility of the plasticizer with the polymer is usually observed when these substances have similar HSP values [26]. As shown in Fig. 3.3, the CED (cohesive energy density) values depend on the temperature. Because dCED/dT assumes different values for different materials, there may be situations in which the polymer/ plasticizer composition will be compatible at a given temperature but incompatible in another. The solubility parameters for selected plasticizers and polymers are included in Tables 3.4e3.6, respectively. It should be noted that the calculation methods commonly used for oligomers or polymers are significantly flawed. The higher the molecular weight, the less accurately the solubility parameter can be calculated based on the structural formula. In the case of polymers, solubility parameter values can be calculated on the basis of a solubility assessment or swelling rate, using polymer softening point values or intrinsic viscosity measurements. A valuable technique that can be applied to both oligomeric compounds and polymers is inverse gas chromatography (IGC). Determination of the properties by IGC involves placing the test material in a chromatographic column and injection of carefully selected test substances into the column, which are then transferred through the

Figure 3.3 Typical variation of Pi or cohesive energy density (CED) with temperature [27].

3: E SSENTIAL Q UALITY PARAMETERS

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Table 3.4 Hansen Solubility Parameters for Selected Plasticizers [28].

Plasticizer

V1, kmol/ m3

d, (MJ/ m3)0.5

dD , (MJ/ m3)0.5

dP , (MJ/ m3)0.5

dH , (MJ/ m3)0.5

Dodecane

228.5

16.0

16.0

0.0

0.0

Diethyl malonate

151.8

19.5

15.5

4.7

10.8

Diethyl oxalate

135.4

22.5

15.5

5.1

15.5

Dioctyl adipate

399.0

18.2

16.7

6.2

3.5

Tributyl acetylcitrate

384.0

17.1

15.4

4.1

6.2

Dimethyl phthalate

163.0

21.9

18.6

10.8

4.9

Diethyl phthalate

198.0

20.5

17.6

9.6

4.5

Dibutyl phthalate

266.0

19.0

17.8

8.6

4.1

Dioctyl phthalate

377.0

16.8

16.6

7.0

3.1

Benzyl butyl phthalate

335.0

22.4

19.1

11.3

3.1

Trimethyl phosphate

116.7

25.2

16.7

15.9

10.2

Triethyl phosphate

169.7

22.2

16.7

11.4

9.2

Tricresyl phosphate

316.0

23.1

19.0

12.3

4.5

Trioctyl phosphate

469.0

17.7

16.2

6.2

3.7

Transformer oil

300.0

17.1

17.0

0.4

0.6

Nitrile of oleic acid

312.0

16.4

16.0

2.9

1.8

column by means of a carrier gas. Substances injected into the column are low molecular weight organic solvents with defined properties. The measured reference substance retention parameters depend on the type and value of intermolecular interactions between the solvent and the test substance [30]. An advantage of IGC is the ease in making determinations and the small amount of test substance used. Thanks to the IGC technique, FloryeHuggins parameters can be determined at infinite dilution using the equation [22,31e33]: # " 273; 15R P1 ðB11  V1 Þ 1  cN 12 ¼ ln 0 0 RT P1 Vg M1

62

P LASTICIZERS D ERIVED

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Table 3.5 Hansen Solubility Parameters for Selected Polymers [28].

Polymer

d, (MJ/ m3)0.5

dD , (MJ/ m3)0.5

dP , (MJ/ m3)0.5

dH , (MJ/ m3)0.5

Polyamide 66

22.8

18.5

5.1

12.2

Polyacrylonitrile

25.1

18.2

15.9

6.7

Polybutylmethacrylate

20.1

18.1

8.4

3.1

Polyethylene, sulfonated

20.8

20.0

7.5

4.3

Polyethyleneterephthalate

21.6

19.5

3.5

8.6

Polymethylmethacrylate

20.2

17.7

5.7

7.8

Polyphenyleneoxide

19.7

18.7

3.5

5.1

Polystyrene

19.8

19.7

0.9

2.0

Polytetrafluoroethylene

14.0

14.0

0.0

0.0

Polyviny lacetate

21.4

18.7

10.0

3.1

Polyvinylalcohol

26.4

16.0

8.8

19.1

Polyvinyl butyral

22.4

17.3

8.8

11.2

Polyvinyl chloride

21.4

18.7

10.0

3.1

Butadiene nitrile rubbers: (18% acrylonitrile) (26% acrylonitrile) (40% acrylonitrile)

19.2 19.6 20.2

17.9 18.1 17.7

3.5 4.7 6.8

6.2 6.0 7.2

Butadiene styrene rubber

19.3

18.5

3.7

3.9

Butadiene rubber

17.9

17.7

1.2

2.5

Butyl rubber

17.9

17.5

3.1

2.4

Cellulose nitrate

23.3

17.0

12.5

9.9

Epoxydiane resin

21.5

17.5

10.3

7.2

Natural rubber

18.1

17.4

3.1

4.1

Neoprene

19.6

18.0

6.2

4.5

Nitrocellulose

25.9

19.0

12.2

12.2

Polyisoprene

18.0

17.3

3.1

3.1

3: E SSENTIAL Q UALITY PARAMETERS

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Table 3.6 Vapor Pressure of Selected Plasticizers at Various Temperatures (Pa). Vapor Pressure, Pa Plasticizer

80 C

100 C

140 C

180 C

250 C

Dibutyl phthalate

2.0

8.2

87.0

608.0

8667.0

Di-2-ethylhexyl phthalate

0.1

3.9

57.1

2075.0

Dioctyl adipate

0.1

2.3

28.2

969.7

The following equation is used to describe the interaction between a composite material (mixture of polymers, a polymer with a plasticizer, a composite with a filler) and a solvent: cN 1m

!     P01 273; 15$R r1 V1  $ðB11  V1 Þ þ ln ¼ ln  1 $42 RT rm V2 P01 Vg M1   V1 $43  1 V3

where rm

e

mixture density;

42 , 43 e volumetric fractions of mixture components. Lazar and Miklusowa have compiled calculated values of the solubility parameters d for some important polymers. Knowledge of these values is very helpful in the use of plasticizers and determining the interaction of polymers with plasticizers [29]. FloryeHuggins parameters are determined for polymers, fillers, pigments, their mixtures and composites. Solubility parameters can be determined using the following equation [21e23]: 

d21 cN  12 RT V1



 ¼

 2d2 d2 d1  2 RT RT

64

P LASTICIZERS D ERIVED

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If we draw a correlation diagram for the left side of the equation versus d1 , we will obtain a straight line with a 2d2 =ðRTÞ slope. The stationary phase solubility parameter can be calculated based on the inclination of the straight line.

3.2 Specific Quality Parameters 3.2.1 Volatility of Plasticizers The functional qualities of products manufactured from plasticized polymers mainly depend on the properties of the plasticizer used. One of the basic quality parameters of plasticizers that determine their suitability for softening the polymer is their volatility. The volatility of a liquid substance is its tendency to evaporate and is directly related to the vapor pressure of a given substance. At a certain temperature, a substance with a high vapor pressure evaporates more easily than one whose vapor pressure value is lower. Volatility is expressed as a percentage change in the weight (%wt.) of the plasticizer sample caused by its evaporation into the atmosphere. This property is closely related to the chemical structure of the plasticizer and its molecular weight, which in turn determine its boiling point and vapor pressure. Other factors such as polarity and the presence of hydrogen bonds also have an effect on the vapor pressure of the plasticizer. The vapor pressure and volatility of a plasticizer decreases with an increase in its molecular weight, providing for greater stability and preservation of the properties of the plasticized PVC composition. Knowing the boiling point or the vapor pressure of a plasticizer allows for a quick assessment of its suitability for plasticizing a polymer. For practical reasons, it is important for the processors of plasticized products to determine the volatility and vapor pressure of the plasticizer at a temperature close to the processing temperature of the polymer composition. Some researchers believe that the boiling point of a plasticizer should exceed 200 C (at 4 mm Hg), while Reed [34] provided a higher temperature for the same conditions, i.e., 225 C. Information on the volatility, boiling point and vapor pressure of a plasticizer are very important. However, the behaviour of a plasticizer in a composition with a polymer, especially its evaporation from the material, depends not only on its properties but also on other factors, such as the properties of the polymer, the additions used and the conditions in which value determinations are made. In practice, the plasticizer is dispersed in a polymer matrix in which its molecules interact with the

3: E SSENTIAL Q UALITY PARAMETERS

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polar groups of the polymer. Therefore, the tendency of the plasticizer to escape from the plasticized material will also depend on the properties of the polymer itself. In spite of this, the vapor pressure of the plasticizer is an important parameter determining its volatility and technological suitability. Tables 3.6 present a series of plasticizer vapor values. For technological reasons, it is advisable to have the vapor pressure values of the plasticizer determined at different temperatures, as such information makes it possible to predict losses of the plasticizer when preparing the polymer/softener/additive composition, processing the composition and using the plasticized product. In Table 3.6, examples of the vapor pressure values of plasticizers at various temperatures are given [19]. It is shown that the vapor pressure of a plasticizer increases with temperature, and therefore the higher the processing temperatures, the higher plasticizer losses can be expected as early as in the processing stage. A correlation between the vapor pressure of di-2-ethylhexyl phthalate (DEHP) and temperature is shown in Fig. 3.4. Because of restrictions on the use of phthalate plasticizers, a number of alternative plasticizers are used in some applications. Selected alternative plasticizers are summarized in Table 3.7 [37].

Figure 3.4 Changes in vapor pressure of di-2-ethylhexyl phthalate (DEHP) depending on temperature [36].

Table 3.7 Physicochemical Properties of Common Phthalate Plasticizers and Alternative Plasticizers, Taken From the European Chemicals Agency Database [37]. Density, g/cm3 at 20 C

Vapor Pressure, Pa at 25 C

Solubility in Water, mg/L at 25 C

0.98

7.59  104c

2.49  103

Log Kow at 25 C

66

Substance

Molar Weight, g/mol

Melting Point,  C

Phthalate plasticizers Di-2-ethylhexyl phthalate

390.56

6a

446.66

0.96

4.91  10

Diisononyl phthalate

418.61

0.97

5.17  106a 6a

2.20  10

48.0

9.52a

54.0

2.24  10

9.46

a

45.0

4.33a

32.4

8.94

67.8

1.74  105a

1.84  10

Dibutyl adipate

258.35

0.96

0.02

35.00

Diethylheptyl adipate

370.57

0.92

4.27  104a

5.45  103a

4.41  10

4a

5a

4a

Alternative plasticizers Adipates

Diisononyl adipate

398.62

0.92

3.98  10

6a

2.50  10

9.24

a

65.0

10.08

a

diisodecyl phthalate (DIDP) > diisotridecyl phthalate (DTDP) [52]. Moreover, the number of branches and atoms in the alcohol chain has a significant impact on the efficiency of the plasticizer. Linear plasticizers demonstrate a greater effectiveness compared with ones with a branched structure. Esters of phthalic and adipic acid and a given alcohol demonstrate higher softening efficiency than the ester of trimellitic acid and the same alcohol. Generally speaking, an increase in the length of the ester chain of the plasticizer results in the following changes to its properties: decreased solvation power, efficiency, volatility and compatibility (miscibility) with the polymer matrix but improved resistance to low temperatures. Positive features of linear plasticizers compared with their branched varieties include greater plasticizing efficiency, decreased volatility, shorter dry blend preparation times and increased resistance to UV radiation. Branched plasticizers, despite their worse compatibility and plasticizing

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effectiveness, shorten the gelation time of polymer compositions and provide them with better electric properties. When determining the content of a plasticized polymer composition, it is necessary to take into account various plasticizer properties and not limit oneself only to a single parameter such as plasticization efficiency, regardless of how crucial this property may be.

3.2.3 Permanence A potential restriction on the use of plasticizers in plasticized PVC is their retention from the PVC material into the surrounding environment. The consistency of the composition of flexible PVC materials after the manufacture of certain products is one of the most important requirements these materials have to fulfil. The permanence of the plasticizer, i.e., its tendency to remain in the plasticized material, depends on its miscibility with the polymer, the size of the plasticizer molecule, its boiling point and diffusion rate in the polymer matrix. Plasticizers with large molecules are less volatile because they have a lower vapor pressure and therefore exhibit greater permanence. For these reasons, some polymeric plasticizers, e.g., polyesters, are often used in the production of plasticized products despite their high price. Large alkyl substituents in plasticizers, such as high molecular weight phthalates, can lead to increased hydrophobicity [53]. Polarity, the external environment and the possibility of hydrogen bonds appearing between the plasticizer and the polymer all affect the volatility and solubility of plasticizers and consequently their durability in the polymer composition. In the case of plasticizers exhibiting a high effectiveness as determined by their rapid diffusion into the polymer matrix, their permanence can be expected to be lower due to their diffusion from the polymer [54]. A PVC blend that includes a polymer and various stabilizers, lubricants, plasticizers, fillers, pigments and auxiliaries should remain unchanged after processing. The choice of a plasticizer usually involves finding a compromise between good dissolution ability, compatibility, efficiency and permanence, as all of the mentioned characteristics cannot be satisfied simultaneously. Among all the components contained in PVC blends, mainly plasticizers exhibit a tendency to escape from the flexible polymer matrix. It can escape in various ways [55]: (1) Evaporation from the PVC surface into the air; (2) Extraction from PVC to the water it is in contact with;

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(3) Migration from PVC to the solid it is in contact with; (4) Exudation under pressure. The loss of plasticizer from the polymer matrix during use of the product is one of the most unfavourable features of plasticized PVC [56]. As a result of the process, the products become susceptible to deformation, lose their flexibility and a series of other useful properties. Plasticizer migration can be defined as the movement of the plasticizer molecules within the polymer matrix and from the surface of the PVC composition to another medium with which the composition is in contact [57]. The loss of plasticizer from a plasticized material is accompanied by two other simultaneous plasticizer transfer processes, namely plasticizer diffusion from the polymer matrix to the surface and its evaporation into the surrounding gaseous atmosphere, which usually consists of air [58]. The rate of evaporation of the plasticizer from the surface of the material depends on its vapor pressure in the specific environmental conditions (pressure, temperature), which the plasticized material is located in. On the basis of PVC tests, it has been shown that within certain limits, the vapor pressure of the plasticizer depends on its volume in the polymer composition. If the plasticizer concentration exceeds the antiplasticization range, it can evaporate easily. In this case, the amount of plasticizer lost mainly depends on the surface of the sample [59]. Plasticizer loss through evaporation from the sample surface is determined by its thickness. Under certain conditions, the percentage loss of plasticizer is inversely proportional to the thickness of the sample [60]. According to Wilson, the loss approximately doubles with every 10 C increase [19]. It is obvious that the evaporation of the plasticizer from the plasticized material will proceed more quickly when the evaporated plasticizer is removed from the sample surface (air flow) or when the product is used in vacuum conditions. The diffusion rate is related to the size and shape of the plasticizer molecule and the permeability of the PVC matrix. Permeability is strongly dependent on the amount of plasticizer in the polymer composition. The diffusion rate is higher for small amounts of plasticizer than in materials containing large amounts of the substance. The plasticizer can escape from the polymer matrix via extraction in a process occurring at the plasticized material/liquid interface. Extraction of the plasticizer from the product takes place when the plasticized material is immersed in a liquid or is exposed to an atmosphere highly saturated with the vapor of a liquid, e.g., in high humidity conditions.

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Most PVC plasticizers exhibit hydrophobic properties and are characterized by low solubility in water (from 0.1% to 1.0%). However, in some cases, the association between the polymer macromolecules and water molecules may induce increased extraction. The presence of surfactants in water leads to the dispersion and removal of the plasticizer film from the surface, which increases its diffusion toward the surface, limiting the permanence of the plasticized product. In the case of very high surfactant concentrations, the rate of plasticizer escape from the surface may be greater than its diffusion to the surface. In such cases, the diffusion rate will determine the permanence of the plasticized product. An important qualitative feature of plasticizers that determine their suitability for polymer modification is their susceptibility to migrate from the polymer matrix. Many parameters affect the established migration values. They include type of medium with which the sample is in contact (solid, liquid), temperature and sample size (especially its thickness). In the case of migration into a liquid, the ratio of the sample volume to the simulating fluid volume is an essential factor. Migration is defined as the diffusion of additives from the polymer material to another material with which it remains in contact with [61e65]. Often, when determining plasticizer migration, the following limiting factors and assumptions are taken into account [66e68]:  the plasticizer diffusion coefficient is independent from the concentration;  the surrounding medium has an unlimited volume;  phenomena occurring in the boundary layer can be disregarded;  migration may involve more than one of the composition’s ingredients;  diffusion includes plasticizer loss through extraction;  migration involves diffusion to the surface and sorption from the surface;  diffusion can be described using Fick’s second law of diffusion;  it is assumed that the samples consist of very thin flat plates and migration occurs mainly through the surface with the omission of the sample’s edges.

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Three main processes accompany the migration of plasticizers from the polymer matrix: - diffusion of the plasticizer from the material toward the surface; - phenomena taking place at phase boundaries; - sorption to the surrounding environment. Based on experimental investigations of DEHP migration from PVC film to isopropanol and isooctane at 60 C, the diffusion coefficient was determined using the following equation [66]:  1 2 Mt Dt ¼ 2 pl2 MN =

where Mt e total amount of plasticizer desorbed over time t; MN e total amount of plasticizer desorbed after an infinite time of extraction; D e diffusion coefficient; t e time; l e half of sample thickness. It was found that the diffusion coefficient value grows linearly with the concentration of the plasticizer in the plasticized PVC material. The size and speed of migration of plasticizers into liquid media depends on the type of medium surrounding the sample, mainly its molecular weight and polarity. The diffusion rate depends on the molecular structure of the plasticizer and the permeability of the PVC material. The molecular structure of the plasticizer is defined by its molecular weight and linearity. The mobility of a plasticizer’s molecules is one of the main factors affecting its susceptibility to diffusion from the polymer matrix. For most plasticizers, the mobility of plasticizer molecules, their volatility and their tendency to migrate from the plasticized polymeric material decreases with increasing molecular weight [69]. The rate of diffusion of plasticizer molecules in the polymer matrix determines the migration rate and, as a result, the permanence of the plasticizer. In most cases, however, even though a high diffusion coefficient of the plasticizer provides greater plasticization efficiency, it will

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lead to decreased plasticizer permanence. The polymer matrix into which the plasticizer has been introduced also determines the diffusion rate of the plasticizer molecules in the polymer. Auxiliaries and other additives present in the PVC composition also affect the migration rate and volume of the plasticizer from the plasticized material. The diffusion of the plasticizer, its deposition and accumulation on the surface of the material are conditioned by the compatibility of the plasticizer with the polymer matrix, the energy at the liquid/solid interface, and the volatility of the plasticizer. The plasticizer desorption rate from the surface of the material is determined using the Herz equation [70]: p 1

ð2pMkTÞ

=

W ¼

2

where p e partial pressure of plasticizer at temperature T; M e molecular weight of plasticizer; k e Boltzmann constant; T e temperature. The evaporation rate of plasticizers is usually 10e100 times slower than their diffusion rate. Plasticizers should be nontoxic, odourless (i.e., have low vapor pressure) and exhibit low volatility. However, if the polymer and plasticizer are not compatible, then plasticizer syneresis from the polymer matrix can occur [71]. If the plasticizer exhibits good compatibility and wetting properties, especially of solid material surfaces, then the migration of its molecules in the polymer matrix will be limited, as will its tendency to exudate on the surface of the material. A plasticizer that has not been properly selected for the polymer matrix will exudate on the surface of the material, leading to increased hardness and decreased elasticity of the material. Blooming is the result of the conditions that prevail at the solid/gas phase interface. Deposition of the plasticizer or other components of the PVC composition on the surface of the material is caused by their migration and results mainly from the lack or limited miscibility of the plasticizer with the polymer matrix.

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For this reason, processors, who use higher molecular weight plasticizers in PVC compositions that exhibit lower volatility, limited migration, and as a result, decreased compatibility with the polymer, should take into account that the processing capability of these compositions will be compromised. In normal conditions of use, a plasticizer exhibiting favourable properties will also diffuse on the surface of the product. However, this phenomenon cannot be treated as typical blooming. Plasticizer blooming is manifested by the appearance of an oily layer on the surface of the material, which results from the separation of the plasticizer. When this occurs, the dynamic balance of absorption and desorption between the plasticizer and the polymer is disturbed. Testing the extraction and migration of plasticizers from PVC products is essential for many reasons such as preservation of the physical, mechanical and functional properties of PVC products as well as determining the negative impact they may have on materials or media they come in contact with. In practice, the permanence of a plasticized material is usually determined by specifying the percentage weight loss of the sample after it is aged in a given environment, temperature and time. The conditions in which those samples are aged can vary. They are selected according to the intended purpose of the material. Extensive research on mass changes of geomembrane samples (containing various plasticizers) caused by plasticizer volatility and water extraction was carried out by Stark et al. The results of these studies are presented in Table 3.12. The permanence of plasticized polymer compositions is determined in different conditions usually determined by the environment the product is normally used in. Weight changes of PVC compositions containing 50 phr plasticizer after ageing at 100 C for 7 days are shown in Fig. 3.7. The migration of plasticizers from products is a cause of concern in a number of fields, mainly in the medical and food packaging industries. Most migration studies concern these areas of application. The low molecular weight phthalic acid esters commonly used to date are relatively easy to extract from plasticized materials. That is why they can be easily found in the natural environment and in living organisms [74e76]. Many alternative plasticizers have been developed, but it is difficult to find equivalents that fully provide the wide range of properties that phthalic acid esters have to offer. Alternative commercially available plasticizers include phosphates, aliphatic dibasic esters, trimellitates, pyromellitates and polymeric plasticizers [77].

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30 26,5

Weight loss, %

25

22,3 20

20 15,9 15 10 5 0,8 0

DOA

DOTP

1,2

DINP TOTM Plasticizer

ATBC

LBMollA

Figure 3.7 Weight changes of plasticized PVC samples with selected plasticizers, determined after ageing at an elevated temperature (7 days at 100 C, plasticizer concentration ¼ 50 phr) (acetyl tributyl citrate, LBMoll A e low temperature plasticizer, LEBA) [73].

In practice, reducing the migration of plasticizers from polymeric products can be achieved by  use of polymeric plasticizers [65];  use of nanocomposites and lamellar fillers [61];  cross-linking of the surface of products by means of plasma treatment [78], photocrosslinking [79] or cross-linking of bulk polymeric materials [63];  grafting low permeability materials [80] or application of low permeability coatings onto the surface [64];  complexation with additives capable of forming bonds with plasticizers [81];  annealing (changing the arrangement of the polymer matrix) [68]. The abovementioned measures limiting the migration of the plasticizer are often done at the expense of other properties, such as flexibility, thermal stability, surface properties and appearance.

3.3 Antiplasticization Many polymers tend to increase their crystalline order when mixed with a small amount of plasticizer. If small amounts of plasticizer are

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incorporated into the polymer, its molecules can be completely immobilized in the polymer matrix through various types of interactions. These compositions then exhibit low mechanical energy absorption due to the limited freedom of the polymer macromolecules. The system becomes more rigid, fragile and is characterized by lower relative elongation at break. This phenomenon is termed antiplasticization. Typically, the introduction of small quantities of plasticizers into the polymer neither causes a drop in breaking strength nor an increase in elongation of the modified material. On the contrary, an increase in the hardness and fragility of such systems occurs instead. Antiplasticization was found to occur among many polymers, ranging from completely amorphous ones to polymers of high crystal order, such as polycarbonate, polysulfone, polyphenylene ether, polyesters, PVC and polyamide 66 [27,82e85]. The issue of antiplasticization of a PVC composition containing less than 15% plasticizer was first described by Brous and Semon, who failed to obtain good quality test samples for polymer blends containing up to 15% plasticizer [86]. Robeson states that this peculiarity of the PVC-softener system stems from the fact that when the softener fills the free volume, it reduces the mobility of the polymer segments [87]. Another theory assumes that this phenomenon is caused by strong intermolecular interactions between the polymer and the plasticizer, which is often referred to as PVC pseudocrosslinking [88]. Guerrero, on the other hand, attributes the PVC antiplasticization effect to the change in PVC crystallinity caused by the presence of the plasticizer in certain amounts [89]. Antiplasticization induced by the presence of small quantities of plasticizers in the polymer matrix is accompanied by an increase in the orderliness of its structures but does not necessarily lead to an increase in the crystallinity of the polymer. Based on current research, it has been shown that antiplasticization is caused by  the addition of small amounts of plasticizer to the polymer, which increases the mobility of its chains, but leads to increased crystallinity and reduced free volume;  polymereplasticizer interactions, involving hydrogen bonds and van der Waals forces that restrict movement of chain segments by increasing spatial hindrance;  immobilization of plasticizer molecules.

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Typically, the introduction of a low molecular weight solvent (plasticizer) into the polymer results in an increase in free volume, therefore a decrease in the glass transition temperature is observed, which varies depending on the molecular weight and chemical structure of the liquid additive molecule [90]. However, in the presence of small quantities of plasticizers in the polymer composition, they exhibit an antiplasticizing effect by increasing the stiffness of the polymeric material and its Tg [88,90e92]. According to Kinjo, the ability of a plasticizer to induce PVC antiplasticization decreases in reverse order to it softening capability [88]. Small quantities of plasticizers lead to a change in the Tg of the two-component system, which is usually a linear function with respect to the 4s solvent mass fraction [93]. Tg ¼ Tg;p  l0 4s where l0 e softener efficiency parameter; Tg,p e glass transition temperature of the polymer; 4s e solvent’s mass fraction. Mauritz linked this linear relationship with the diffusion theory and used it to define a modification model of plasticized PVC using dialkyl phthalates [94,95]. The above equation does not apply to higher plasticizer quantities (over 40%), where the Tg correlation with the solvent concentration is nonlinear [96,97]. The concept of plasticization and antiplasticization of the polymer, according to Sears and Darby, is shown below in Fig. 3.8 [16]. Highly polar plasticizers exhibit a particularly marked antiplasticization effect. Their molecules can become a spatial hindrance, restricting the forces of interaction existing between the PVC chains. The maximum antiplasticization effect of the PVC composition depends on the structure of the plasticizer (aliphatic/aromatic acid, linear/branched alcohol) and generally occurs at a plasticizer content of up to 15%. A weaker antiplasticization effect is observed with emulsion PVCs as compared with suspension PVC polymers. Some elastomers, such as nitrile butadiene rubber, which soften rigid polymer matrices, do not display any antiplasticization effect. Most of the commonly used plasticizers were found to induce an antiplasticization effect in PVC compositions when used in amounts of up

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Figure 3.8 Polymer plasticization concept explaining the phenomena of antiplasticization and plasticization [16].

to 15% of the blend during the processing of PVC [98,99]. Much work has been devoted to characterizing the properties of polymer compositions modified with small amounts of plasticizers. Creep tests conducted in the process of PVC stretching using small amounts of dioctyl phthalate (DEHP) and dioctyl adipate (DOA) at room

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temperature revealed that peak antiplasticization effects occurred at a 5% content of plasticizer in the composition [100]. In the linear viscoelastic region, only minor differences in the degree and nature of antiplasticization between the two types of blends (PVCeDEHP and PVCeDOA) were observed. It has also been found that DOA molecules, despite their lower PVC miscibility, exhibited stronger intermolecular interactions with the polymer compared with DEHP. X-ray diffraction has shown that small amounts of DEHP (up to about 15%) introduced into PVC lead to a gradual increase in the crystallinity of the polymer. For greater amounts of plasticizer in the PVC/DEHP composition, the ordering degree has been shown to decrease. In these cases tensile and flexural strength values are usually highest, while impact values are lowest at a plasticizer content as mentioned above. An increase and decrease of system crystallinity leads to an analogous increase and decrease of the modulus value [101]. It has also been found that the refractive index of the PVCetricresyl phosphate (TCP) system goes through its maximum value when the TCP content of the composition increases [101]. This is attributed to an increase in material density caused by a more compact polymer chain structure that occurs after the addition of a small amount of plasticizer. X-ray diffraction investigations have revealed the formation of crystalline zones [101,102]. This explanation has been challenged by others, who suggest that the increase in material brittleness should rather be attributed to polymer solvation by the plasticizer [103]. At a plasticizer content of 4%e15%, the degree of ordering is the highest, while some mechanical parameters of the composition such as tensile strength at break and bending strength reach maximum values and others, which include impact strength and elongation at break, attain minimum values. For example, plasticized PVC, containing 10% DEHP by weight, is characterized by reduced (50%) impact strength for notched samples [69]. Introducing more plasticizers into the PVC composition in amounts exceeding the 15% threshold reduces and ultimately eliminates the crystallinity of the polymeric material. Therefore, when using plasticizers in polymer compositions, we must be aware that plasticization of the polymer occurs only after a critical amount of plasticizer is exceeded. This quantity is usually higher for aromatic plasticizers, which most often demonstrate higher polarity, than for aliphatic linear plasticizers with a lower polarity [102]. With higher temperature, a decrease in the critical amount of plasticizer at which a “normal” plasticization effect occurs is observed.

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Tensile strength

92

DOS

A B w of plasticizer

DOP TCP

C

Figure 3.9 Tensile strength of poly(vinyl chloride) compound as a function of plasticizer concentration. Critical concentration (A) DOS (diisooctyl sebacate); (B) DEHP (di-2-ethylhexyl phthalate); (C) TCP (tricresyl phosphate) [104].

Fig. 3.9 shows the correlation between the tensile strength of a PVC composition containing three different plasticizers and the amount of plasticizer. It is observed that diisooctyl sebacate is the most effective plasticizer among the three and for this reason the critical concentration determined for it is the lowest. It has also been shown that the amount of plasticizer in the polymer composition also influences the abrasion resistance of the material [105]. PVC compositions containing monomeric (DEHP) and polymeric (Priplast 3157 e polyadipate ester, Croda Inc.) softeners showed the lowest abrasion resistance at a 20% plasticizer content in the composition (Fig. 3.10) [105]. Small quantities of plasticizer in the composition (less than 20%) facilitate PVC processing but usually have a negative effect on a number of properties of the end material. The solvation power of a plasticizer has a direct impact on the brittleness of the composition. The brittleness of the material increases with the plasticizer content (in the antiplasticization region) [107]. Therefore, to improve the processing properties of rigid PVC compositions, it is advisable to incorporate lubricants instead of plasticizers in small quantities into the formulation.

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Figure 3.10 Loss of sample mass depending on the content of monomeric (di-2-ethylhexyl phthalate, DEHP) and polymeric (Priplast 3157) plasticizer in the poly(vinyl chloride) composition, after 400 and 500 revolutions. Determined using the Taber test [105,106].

References [1] E.H. Immergut, H.F. Mark, Principles of Plasticization, Polytechnic Institute of Brooklyn, Brooklyn, N.Y, 2015. http://pubs.acs.org. [2] G. Wypych, Handbook of Plasticizers, ChemTec Publishing, Toronto, 2004, p. 333. [3] The Function and Selection of Ester Plasticizers, 2018. https://www. hallstar.com/webfoo/wp-content/uploads/ester-plasticizers-forelastomers.pdf. [4] G. Wypych, Handbook of Plasticizers, ChemTec Publishing, Toronto, 2004, pp. 121e123. [5] P.J. Flory, Principles of Polymer Chemistry, Cornell University Press, Ithaca, New York, 1953. [6] PVC Technology, fourth ed., Elsevier Applied Science Publishers Ltd, London, New York, 1984, p. 128. [7] J.T. van Oosterhout, M. Gilbert, Interactions between PVC and binary or ternary blends of plasticizers. Part I. PVC/plasticizer compatibility, Polymer 44 (26) (2003) 8081e8094. [8] P. Doty, H.S. Zable, Determination of polymer-liquid interaction by swelling measurements, J. Polym. Sci. 1 (2) (1946) 90e101. [9] J.D. Roberts, M.C. Caserio, Chemia Organiczna, PWN, Warszawa, 1969.

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[10] M. Gilbert, Crystallinity in Poly(vinyl chloride), J. Macromol. Sci.: Rev. C34 (1) (1994) 77e135. [11] W.J. Moore, Physical Chemistry, Prentice-Hall, Englewood Cliffs , NJ, 1972. [12] P.J. Flory, Thermodynamic properties of solutions of long-chain molecules. Part I, J. Chem. Phys. 9 (1941) 660. [13] P.J. Flory, Thermodynamic properties of solutions of long-chain molecules. Part II, J. Chem. Phys. 10 (1942) 51. [14] M.L. Huggins, Solutions of long chain compounds, J. Chem. Phys. 9 (1941) 440. [15] J.H. Hildebrand, R.L. Scott, Solubility of Non-electrolytes, Dover Publications, New York, 1964. [16] J.K. Sears, J.R. Darby, The Technology of Plasticizers, Wiley, New York, 1982. [17] L.F. Ramos DeValle, M. Gilbert, PVC/plasticizer compatibility: evaluation and its relation to processing, J. Vinyl Technol. 12 (4) (1990) 222e225. [18] S.V. Patel, M. Gilbert, Effect of processing on the fusion plasticized PVC, Plast. Rubber Process. Appl. 5 (1) (1985) 85e93. [19] A.S. Wilson, Plasticizers: Principles and Practice, Institute of Materials, London, 1995. [20] P.S. Small, Some factors affecting the solubility of polymers, J. Appl. Chem. 3 (2) (1953) 71e80. [21] D.W. Van Krevelen, Properties of Polymers, Elsevier, Amsterdam, 1997. [22] C.M. Hansen, Hansen Solubility Parameters, CRC Press LLC, USA, 2000. [23] C.M. Hansen, The three dimensional solubility parameter - key to paint component affinities I. - solvents, plasticizers, polymers, and resins, J. Paint Tech. 39 (505) (1967) 104e117. [24] C.M. Hansen, 50 Years with solubility parameters - past and future, Prog. Org. Coating 51 (1) (2004) 77e84. [25] W.L. Archer, Industrial Solvents Handbook, Marcel Dekker, Inc., New York, 1996. [26] J.K. Sears, J.R. Darby, Mechanism of Plasticizer Action, John Wiley & Sons, New York, 1982. [27] L.F. Ramos DeValle, Plasticization of Poly(vinyl Chloride): PVC/ plasticizer Compatibility and its Relationship with Processing and Properties of Plasticized PVC (Ph.D thesis), Loughborough University of Technology, 1988.

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[28] G. Wypych, Handbook of Plasticizer, ChemTec Publishing, Toronto, New York, 2004. [29] M. Lazar, D. Mikulasova, Synteza a Vlastnosti Makromolekulovych Latek, Alfa, Bratislava (Czechoslovakia), 1976. [30] E. Andrzejewska, A. Voelkel, M. Andrzejewski, R. Maga, Odwro´cona chromatografia gazowa w badaniach powierzchnii polimero´w: w1asciwosci kwasowo-zasadowe i dyspersyjne poli(dimetakrylano´w), Polimery 39 (7e8) (1994) 464e467. [31] S. Zhao, F. Zhang, W. Zhang, B. Shi, Determination of the solubility parameter of cellulose acrylate using inverse gas chromatography, Chin. Sci. Bull. 52 (2007) 3051e3055. [32] E. Ferna´ndez-Sa´nchez, A. Ferna´ndez-Torres, J.A. Garcı´a-Domı´nguez, E. Lo´pez de Blas, Thermodynamic characterization of Superox 20M by inverse gas chromatography, J. Chromatogr. A 655 (1) (1993) 11e20. [33] A. Voelkel, J. Fall, Inverse gas chromatography. Relationship between mass activity coefficient and Flory-Huggins interaction parameter in the examination of petroleum pitches, Chromatographia 44 (3e4) (1997) 197e204. [34] M.C. Reed, Survey of plasticizers for vinyl resins, J. Polym. Sci. 2 (2) (1947) 115e120. [35] D. Kali nska, K. P1ochocka, Zmie˛kczanie Tworzyw Sztucznych, WNT, Warszawa, 1965, p. 22. [36] European Union Risk Assessment Report Bis(2-ethylhexyl) phthalate (DEHP) CAS No: 117-81-7 EINECS No: 204-211-0 Risk Assessment, vol. 80, Final Report, 2008. [37] T.T. Bui, G. Giovanoulis, A.P. Cousins, J. Magne´r, I.T. Cousins, C.A. de Wit, Human exposure, hazard and risk of alternative plasticizers to phthalate esters, Sci. Total Environ. 541 (2016) 451e467. [38] United States Environmental Protection Agency, Washington, DC, USA. https://www.epa.gov/tsca-screening-tools/epi-suitetm-estimationprogram-interface. [39] V.W. Saeger, O. Hicks, R.G. Kaley, P.R. Michael, J.P. Mieure, E.S. Tucker, Environmental fate of selected phosphate-esters, Environ. Sci. Technol. 13 (7) (1979) 840e844. [40] R.P. Schwarzenbach, Environmental Organic Chemistry, John Wiley & Sons, Inc., Hoboken, New Jersey, 2005. [41] R.S. Boethling, J.C. Cooper, Environmental fate and effects of triaryl and tri-alkyl aryl phosphate-esters, Residue Rev. 94 (1985) 49e99.

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[42] A. Merz, Determination of volatility and migration behavior of plasticizers with high molecular weight, Kunststoffe 47 (2) (1957) 69e73. [43] S.C. Lelczuk, W.I. Sedlis, Hur. Vrjlm. yjnjj. 31 (1958) 887. [44] Submission of Information on DIDP CAS: 68515-49-1, EC: 271-091-4 as an Alternative to DEHP. Applications for Authorisation, ExxonMobil Petroleum and Chemical B.V.B.A., January 7 , 2014. [45] Eastman 168 Non-phtalate Plasticiser for Vinyl Plastisols and Vinyl Compounds, Eastman Chemical Company, Brochure, 2016. [46] European Union Risk Assessment Report 1,2-benzenedicarboxylic acid,di-C9-11-branched alkyl esters, C10-RICH and di-“isodecyl” phthalate (DIDP) CAS: 68515-49-1 and 26761-40-0 EINECS: 271-091-4 and 247-977-1 Risk Assessment, vol. 6, 2003, p. 39. [47] European Union Risk Assessment Report 1,2- benzenedicarboxylic acid,di -C8-10- branched alkyl esters, C9-RICH and di-“isononyl” phthalate (DINP) CAS: 68515-48-0 and 28553-12-0 EINECS: 271-090-9 and 249-079-5 Risk Assessment, vol. 35, 2003, p. 45. [48] https://echa.europa.eu/documents/10162/83a55967-64a9-43cd-a0fad3f2d3c4938d. [49] R.F. Grossman, Handbook of Vinyl Formulating, second ed., John Wiley & Sons, Hoboken, New Jersey, 2008. [50] C.E. Wilkes, J.W. Summers, C.A. Daniels, PVC Handbook, Hanser Garderer Publisher, Munich, 2005. [51] E. Langer, K. Bortel, Internal IMPiB research report, unpublished, 2017 [52] L. Krauskopf, W. Arndt, in: J. Edenbaum (Ed.), Plastics Additives and Modifiers Handbook, Van Nostrand Reinhold Co., Inc., New York, 1992. [53] G. Wypych, Handbook of Plasticizers, ChemTec Publishing, Toronto, 2004, p. 603. [54] J.H. Han, Innovations in Food Packaging, Elsevier Academic Press, Oxford, 2005, pp. 12e23. [55] W. Titow, PVC Technology, Elsevier App. Sci., NY, 1986. [56] M.L. Marı´n, J. Lo´pez, A. Sa´nchez, J. Vilaplana, A. Jime´nez, Analysis of potentially toxic phthalate plasticizers used in toy manufacturing bull, Environ. Contam. Toxicol. 60 (1998) 68e73. [57] M. Hakkarainen, Migration of monomeric and polymeric PVC plasticizers, Adv. Polym. Sci. 211 (2008) 159e185. [58] J.K. Sears, J.R. Darby, The Technology of Plasticizers, John Wiley & Sons, 1982, p. 1166.

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[59] Y.R. Shashoua, Inhibiting the Deterioration of Plasticized PVC e a Museum Perspective (Ph.D. thesis), Danish Polymer Centre DTU, September 2001, p. 27. [60] P.A. Small, The diffusion of plasticisers from polyvinyl chloride, Journal of Society of Chemical Industry 66 (1) (1947) 17e19. [61] G. Chen, K. Yao, J. Zhao, Montmorillonite clay/) hybrid resin and its barrier property to the plasticizer within poly(vinyl chloride) composite, J. Appl. Polym. Sci. 73 (3) (1999) 425e430. [62] G.E. Zaikov, K. Z Gumargalieva, S.A. Semenov, O.A. Zhdanova, Int. Polym. Sci. Technol. 25 (2) (1998) 72e74. [63] C. Lambert, M. Larroque, J.C. Lebrun, J.F. Gerard, Food-contact epoxy resin: co-variation between migration and degree of crosslinking, Food Addit. Contam. 14 (2) (1997) 199e208. [64] A. Jayakrishnan, M.C. Sunny, M.N. Rajan, Photocrosslinking of azidated poly(vinyl chloride) coated onto plasticized PVC surface: route to containing plasticizer migration, J. Appl. Polym. Sci. 56 (10) (1995) 1187e1195. [65] N.L. Thomas, R.J. Harvey, PVC/nitrile rubber blends, Prog. Rubber Plast. Technol. 17 (1) (2001) 1e12. [66] C.D. Papaspyrides, S.G. Tingas, Comparison of isopropanol and isooctane as food simulants in plasticizer migration tests, Food Addit. Contam. 15 (6) (1998) 681e689. [67] M. Hamdani, A. Feigenbaum, Migration from plasticized poly(vinyl chloride) into fatty media: importance of simulant selectivity for the choice of volatile fatty stimulants, Food Addit. Contam. 13 (6) (1996) 717e730. [68] C.D. Papaspyrides, S.G. Tingas, Effect of thermal annealing on plasticizer migration in poly(vinyl chloride)/dioctyl phthalate system, J. Appl. Polym. Sci. 79 (10) (2001) 1780e1786. [69] J. Stepek, H. Daoust, Additives for Plastics, Springer-Verlag, New York, 1983, pp. 28e29. [70] G.E. Zaikov, S. Rakovsky, D.A. Schiraldi, Diversity in Chemical Reactions: Pure and Applied Chemistry, Nova Publishers, New York, 2006, p. 118. [71] J.K. Sears, J.R. Darby, Mechanism of plasticizer action, in: The Technology of Plasticizers, vol. 3, Wiley-Interscience, New York, NY, USA, 1982, pp. 35e77. [72] T.D. Stark, H. Choi, P.W. Diebel, Influence of plasticizer molecular weight on plasticizer retention in PVC geomembranes, Geosynth. Int. 12 (2) (2005) 1e12.

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[73] http://www.leba.com.tr/performance. [74] R.J. Jaeger, R.J. Rubin, Migration of phthalate ester plasticizer from PVC blood bags into stored human blood and its localization in human tissues, N. Engl. J. Med. 287 (1972) 1114e1118. [75] G. Latini, C. De Felice, A. Verrotti, Plasticizers, infant nutrition and reproductive health, Reprod. Toxicol. 19 (1) (2004) 27e33. [76] H.M. Koch, H. Drexler, J. Angerer, An estimation of the daily intake of di (2-ethylhexyl)phthalate (DEHP) and other phthalates in the general population, Int. J. Hyg Environ. Health 206 (2) (2003) 77e83. [77] L.G. Krauskopf, How about alternatives to phthalate plasticizers? J. Vinyl Addit. Technol. 9 (4) (2003) 159e171. [78] J.L. Audic, F. Poncin-Epaillard, D. Reyx, J.C. Brosse, Cold plasma surface modification of conventionally and nonconventionally plasticized poly(vinyl chloride)-based flexible films: global and specific migration of additives into isooctane, J. Appl. Polym. Sci. 79 (8) (2001) 1384e1393. [79] S. Lakshmi, A. Jayakrishnan, Photo-crosskicking of dithiocarbamatesubstituted PVC reduces plasticizer migration, Polymer 39 (1) (1998) 151e157. [80] D. Ruckert, F. Cazaux, X. Coqueret, Electron-beam processing of destructurized allylureaestarch blends: immobilization of plasticizer by grafting, J. Appl. Polym. Sci. 73 (3) (1999) 409e417. [81] K. Sreenivasan, Effect of blending b-cyclodextrin with poly(vinyl chloride) on the leaching of phthalate ester to hydrophilic medium, J. Appl. Polym. Sci. 59 (13) (1996) 2089e2093. [82] R. Boughalmi, J. Jarray, F. Ben Cheikh Larbi, A. Dubault, J.L. Halary, Molecular analysis of the mechanical behavior of plasticized amorphous polymer, Oil Gas Sci. Technol. 61 (6) (2006) 725e733. [83] Y. Maeda, D.R. Paul, Effect of antiplasticization on gas sorption and transport. III. Free volume interpretation, J. Polym. Sci., Part B: Polym. Phys. 25 (5) (1987) 1005e1016. [84] S.E. Vidotti, A.C. Chinellato, G.H. Hu, L.A. Pessan, Effects of low molar mass additives on the molecular mobility and transport properties of polysulfone, J. Appl. Polym. Sci. 101 (2) (2006) 825e832. [85] S.Y. Soong, R.E. Cohen, M.C. Boyce, W. Chen, The effects of thermomechanical history and strain rate on antiplasticization of PVC, Polymer 49 (2008) 1440e1443. [86] S.L. Brous, W.L. Semon, Koroseal a new plastic some properties and uses, Ind. Eng. Chem. 27 (6) (1935) 667e672.

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[87] L.M. Robeson, The effect of antiplasticization on secondary loss transitions and permeability of polymers, Polym. Eng. Sci. 9 (4) (1969) 277e281. [88] N. Kinjo, T. Nakagawa, Antiplasticization in the slightly plasticized poly(vinyl chloride), Polym. J. 4 (1972) 143e153. [89] S.J. Guerrero, Antiplasticization and crystallinity in poly(vinyl chloride), Macromolecules 22 (8) (1989) 3480e3485. [90] D.J. Plazek, K.L. Ngai, Physical Properties of Polymers Handbook, AIP Press, Woodbury, NY, 1996, p. 139. [91] J.S. Vrentas, J. Duda, H.C. Ling, Antiplasticization and volumetric behavior in glassy polymers, Macromolecules 21 (5) (1988) 1470e1475. [92] B.J. Cauley, C. Cipriani, K. Ellis, A.K. Roy, A.A. Jones, P.T. Inglefield, B.J. McKinley, R.P. Kambour, Glass transition and dynamics of phosphate esters dissolved in two glassy polymer matrices, Macromolecules 24 (2) (1991) 403e409. [93] E.B. Stukalin, J.F. Douglas, K.F. Freed, Plasticization and antiplasticization of polymer melts diluted by low molar mass species, J. Chem. Phys. 132 (8) (2010). [94] K.A. Mauritz, R.F. Storey, S.E. George, A general free volume-based theory for the diffusion of large molecules in amorphous polymers above the glass temperature. I. Application to di-n-alkyl phthalates in PVC, Macromolecules 23 (2) (1990) 441e450. [95] K.A. Mauritz, R.F. Storey, B.S. Wilson, Efficiency of plasticization of PVC by higher-order di-alkyl phthalates and survey of mathematical models for prediction of polymer/diluent blend Tg’s, J. Vinyl Technol. 12 (3) (1990) 165e175. [96] T.M. Martin, D.M. Young, Correlation of the glass transition temperature of plasticized PVC using a lattice fluid model, Polymer 44 (2003) 4747e4754. [97] K.J. Beirnes, C.M. Burns, Thermal analysis of the glass transition of plasticized poly(vinyl chloride), J. Appl. Polym. Sci. 31 (8) (1986) 2561e2567. [98] S.Y. Soong, R.E. Cohen, M.C. Boyce, Polyhedral oligomeric silsesquioxane as a novel plasticizer for poly(vinyl chloride), Polymer 48 (2007) 1410e1418. [99] S.Y. Soong, R.E. Cohen, M.C. Boyce, A.D. Mulliken, Ratedependent deformation behavior of POSS-filled and plasticized poly(vinyl chloride), Macromolecules 39 (8) (2006) 2900e2908.

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[100] H. Bertilsson, J.F. Jansson, Transition from approximately linear to marked nonlinear viscoelasticity in antiplasticized PVC Journal of Macromolecular Science, Part B Physics 14 (2) (1977) 251e263. [101] R.A. Horsley, in: P. Morgan (Ed.), Progress in Plastics, Iliffe and Sons, London, 1957, pp. 77e88. [102] P. Ghersa, Effect of small quantities of plasticizers in poly(vinyl chloride) compounds, Mod. Plast. 36 (2) (1958) 135e136. [103] O. Fuchs, H.H. Frey, Effect of small plasticizer concentrations cause embrittlement, Kunststoffe 49 (5) (1959) 213e216. [104] L. Mascia, The Role of Additives in Plastics, Edward Arnold, London, 1974. [105] K. Bortel, Influence of the amount and type of plasticizer on abrasive wear of plasticized PVC, Przetwo´rstwo Tworzyw 4 (2012) 304e308 (in Polish). [106] ISO 9352:1995 Plastics: Determination of Resistance to Wear by Abrasive Wheels. [107] L. Bohn, Embrittlement of polyvinyl chloride of low plasticizer content, Kunststoffe 53 (11) (1963) 826e830.

4 Research Trends in Plasticizer Production It is predicted that the global demand for all plasticizers will increase to around 9.75 million tonnes in 2024, with an average growth rate of around 3%e3.5% for its various types. In spite of the strong pressure to phase out phthalate plasticizers, their market share is still high. Bis(2-ethylhexyl) phthalate (DEHP) was still the most commonly used plasticizer in 2016 (consumption of 3.07 million tonnes). The phthalates DINP (diisononyl phthalate) and DIDP (diisodecyl phthalate) accounted for around one-third of the plasticizer market. Analysts from Cerasana predict an annual increase of demand for DINP/DIDP at 3.6% and 3.2%, respectively, whereas DEHP is set to continue to lose market share [1]. On the other hand, the MarketsandMarkets (M&M) report provides a forecast for nonphthalate plasticizers, predicting a compound annual growth rate (CAGR) of 8.7% in 2017e22 for this sector [2]. Plastic products constitute the main area of application for plasticizers e about 87% of all plasticizers were consumed in this sector in 2016. The largest quantities of plasticizers are used in polyvinyl chloride (PVC) compositions. In 2016, 2.17 million tonnes of plasticizers were used in the production of films, cables and profiles made from this polymer. The demand for plasticizers in the production of rubber products, paints, varnishes and adhesives is much lower [1]. Problems concerning the impact of phthalate plasticizers of low molecular weight on humans and the environment have exerted pressure on the industry to expand the application of other groups of plasticizers and to search for innovative alternatives. The key factor driving the search for phthalate plasticizer substitutes was the implementation of rigorous regulations regarding their use. Accordingly, there is an increasing demand for alternative plasticizers that show less migration from polymers and which exhibit lower toxicity. The lack of covalent bonds between plasticizers and polymers is one of the main reasons for their susceptibility to elimination from the polymer matrix to the surrounding environment [3]. Wittassek et al. demonstrated that the toxic potential of phthalate esters is largely the result of metabolic transformation into more toxic metabolites. Phthalates entering the human body are rapidly metabolized by Plasticizers Derived from Post-consumer PET https://doi.org/10.1016/B978-0-323-46200-6.00004-0 Copyright © 2020 Elsevier Inc. All rights reserved.

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phase II reactions (hydrolysis and further oxidation reactions) followed by Phase II metabolism and excretion through urine as monoesters or glucuronide conjugates [4]. According to Hauser and Calafat, relatively polar and low molecular weight phthalates are mostly metabolized to their stable hydrolytic monoesters, whereas high molecular weight phthalates with 8 carbons in the alkyl chain are metabolized to their hydrolytic monoesters, which are in turn extensively transformed by u-, (u1) and b-oxidation to oxidative products (alcohols, ketones and carboxylic acids) [5]. However, it is most likely that phthalate monoester and secondary oxidation metabolites exhibit biological activity [6,7]. For these reasons, it is best for alternative plasticizers to produce metabolites that exhibit less severe consequences for human health and the environment. Based on literature and patent analyses, two main trends in the synthesis of new plasticizers can be distinguished. Currently, there are many different alternatives to phthalate plasticizers, however, two basic groups, which are currently in use, need mentioning. These include petroleum-based phthalates, which are used to a limited extent or in specific applications, and plasticizers made from renewable raw materials, e.g., vegetable oils and fats. The first group includes adipates, benzoates, citrates, cyclohexane dicarboxylic acids, glycerol acetylated esters, phosphate esters, sebacates, terephthalates and trimellitates. Often the term “alternative plasticizer” is used as a synonym for nonphthalate plasticizers. We are aware that not all of the “alternative plasticizers” listed here are substances that have recently entered the market or are produced solely to replace phthalate plasticizers. Therefore, all nonphthalic chemicals that can be used as plasticizers and thus constitute an “alternative” can be considered as alternative plasticizers. Raw materials of petrochemical origin and those derived from natural sources (vegetable oils, polysaccharides, fats) may be used as primary materials in the production of plasticizers. Therefore, in some cases, the obtained plasticizers may be classified as petro- or bioplasticizers. Mixed source plasticizers, i.e., ones involving the use of, e.g., acids made synthetically from petroleum-based products and alcohol from renewable raw materials in ester production, are also available on the market. Generally speaking, the following alternative plasticizer groups can be distinguished:  aliphatic dicarboxylic and dibasic acid esters (e.g., adipates, sebacates, azelates, glutarates, oleates);  phosphate ester (aryl, alkyl and mixed phosphate ester);

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 trimellitates;  epoxy plasticizers (methyl, amyl and soyate, methyl, amyl and nonyl epoxy stearate);  glycol derivatives (glycol ethers and their esters; dibenzoates, benzoates);  sulfonate esters;  citrates and many others.

4.1 Petrochemical Alternatives The requirement to replace phthalate plasticizers in the modification of polymers led to an increased interest in the use of nonphthalate plasticizers produced from petrochemical raw materials. In many cases, limited miscibility, the processing requirements imposed on plasticized compositions and the performance parameters of the products being produced have forced processors to use plasticizer mixtures in the production process. Most examples of petrochemical plasticizers constituting an alternative to their phthalate counterparts are presented in Chapter 3. Selected examples of alternative petro-based plasticizers are shown below.

4.1.1 Adipates A few advantageous properties of alkyl adipate esters include lower viscosity compared with their phthalate equivalents, good plastisol storage stability and favourable low-temperature flexibility properties. Dioctyl adipate (DOA), dibutyl adipate and diisobutyl adipate demonstrate good miscibility with PVC, polyvinyl acetate resins and various rubbers. DOA in PVC features flexibility at low temperatures, good electrical properties, good resistance to weathering and good stability to heat. DOA is used to produce clear films for food packaging applications. Bis(2-(2-butoxyethoxy)ethyl) adipate (DBEEA), on the other hand, demonstrates good miscibility with natural and synthetic rubber, low volatility and retains plasticity at low temperatures. It is used in plasticized products with increased heat resistance. Fig. 4.1 shows the structure of DBEEA. Di(2-ethyl hexyl) adipate e a plasticizer used in the production of plasticized PVC products, such as toys, gloves, tubing, artificial leather, shoes, sealants and carpet backing. It is also used in the production of films for food packaging materials, fillers, paint and lacquers, adhesives,

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O O

O O

O O

CH3 O

CH3

O

Figure 4.1 Structure of bis(2-(2-butoxyethoxy)ethyl) adipate.

plastic in concrete and rubber products. It is anticipated that the use of this plasticizer will increase in products manufactured for the hospital sector, in printing inks and other PVC products [8,9]. The adipate plasticizer group also includes diisononyl adipate, diisodecyl adipate and ditridecyl adipate.

4.1.2 Sebacates Linear esters of sebacic acid and azelaic acid have low viscosity and provide good flexibility to products at low temperatures. The most widely used esters in this group include di-2-ethylhexyl sebacate (DOS) and di-2-ethylhexyl azelate (DOZ). They exhibit good miscibility with PVC and with synthetic rubber. Dibutyl sebacate (DBS) e an odourless plasticizer for products used in low temperatures. Fig. 4.2 presents the structure of dibutyl sebacate. Bis(2-ethylhexyl) sebacate e a low volatility plasticizer with good migration resistance and favourable electrical properties; ensures higher resistance to low temperatures compared to DBS. Fig. 4.3. shows the structure of bis(2-ethylhexyl) sebacate. O O

O O

Figure 4.2 Structure of dibutyl sebacate. H3C H3C

O O

CH3

O O

Figure 4.3 Structure of bis(2-ethylhexyl) sebacate.

CH3

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4.1.3 Azelates DOZ e characterized by low volatility, provides products with high resistance to low and high temperatures; a plasticizer for vinyl plastics, polystyrene and cellulosic. The chemical structure of bis(2-ethylhexyl) azelate is presented in Fig. 4.4. Di-n-hexyl azelate (DNHZ) is a plasticizer with low volatility and good compatibility used in plasticizing cellulose and vinyl resins. It is approved for use in articles that come in contact with food [10]. The chemical structure of DNHZ is shown in Fig. 4.5.

4.1.4 Trimellitate Esters Trimellitic acid esters exhibit excellent thermal properties and are resistant to extraction. The most widely used of these includes tri-2ethylhexyl trimellitate, which, in spite of its lower plasticizing efficiency than the corresponding phthalate, ensures superior thermal stability and extraction resistance. These properties can be improved still further by using trimellitate esters produced from mixed linear alcohols, e.g., L79 trimellitate and L810 trimellitate [11]. An example of a plasticizer belonging to this group also includes triisononyl trimellitate with a high molecular weight. It is compatible with PVC and exhibits the lowest volatility in the group. The plasticizer is suitable for use in wire and cable applications. It is a high molecular weight (Mw ¼ 588 g/mol) monomeric plasticizer. Similar properties are characteristic of plasticizers with even higher molecular weights, O

O CH3

O

O

H 3C

CH3

CH3

Figure 4.4 Chemical structure of bis(2-ethylhexyl) azelate.

O

O O

Figure 4.5 Structure of di-n-hexyl azelate.

O

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O

O O O

Figure 4.6 Structure of triisononyl trimellitate.

namely triisodecyl trimellitate (Mw ¼ 630 g/mol) and triisotridecyl trimellitate (Mw ¼ 757 g/mol). The chemical structure of triisononyl trimelliate is shown in Fig. 4.6.

4.1.5 Phosphoric Acid Esters Numerous alternatives to phthalic acid esters have been identified in industrial practice. This group also includes phosphoric acid esters. They are widely used as plasticizers in polymer compositions with limited flammability [12]. The main advantage of using phosphate esters as plasticizers for PVC is their low volatility and their ability to impart fire-retardant properties to a PVC formulation. Selected examples of phosphoric acid esters used in plasticizing polymers are shown below. Tributyl phosphate (TBP) e a plasticizer that mixes well with synthetic resins and rubbers. It exhibits high resistance to light and to low temperatures. The chemical structure of TBP is shown in Fig. 4.7. Tris(2-ethylhexyl) phosphate (TOP) e imparts PVC and polyvinyl acetate and synthetic rubber with resistance to low temperatures. TOP shows good compatibility with PVC and also ensures good lowtemperature performance in addition to good fire retardancy.

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O O

P O O

Figure 4.7 Structure of tributyl phosphate.

Triphenyl phosphate (TPP) e a low volatility plasticizer exhibiting good compatibility with various synthetic resins and PVC. Products containing TPP are resistant to oils and show limited flammability due to their high phosphorus content. The plasticizer is used as a flame retarding plasticizer for phenolic resin, epoxy resin, various engineering plastics, acetate plastics and synthetic rubber. Tricresyl phosphate (TCP) e a plasticizer with high thermal resistance and used in compositions with PVC, phenolic resin, epoxy resin and various engineering plastics as a flame-retardant substance. Trixylenyl phosphate e a low volatility softener; provides products with increased resistance to flame and water. Area of application is similar to TCP. Cresyl diphenyl phosphate (CDP) e effectively influences the process of PVC gelation and provides products with resistance to low temperatures. It demonstrates lower viscosity and better flame-retarding properties than TCP. The chemical structure of CDP is shown in Fig. 4.8. Di(2-ethylhexyl) phosphate (DEHPA) e a plasticizer mainly used for its flame-retarding properties in PVC compositions. It is also employed as a plasticizer in PVC products used in hospitals, in PVC packaging materials, cables and floor and wall coverings [8]. Tri(2-ethylhexyl) phosphate e area of application similar to DEHPA, also used in fillers, paint, lacquers and adhesives [8].

O O

P

O O

Figure 4.8 Structure of cresyl diphenyl phosphate.

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O O O

C9H19

H2

O O

catalyst

C9H19

O

C9H19 C9H19

O

DINP

DINCH

Figure 4.9 Diisononyl phthalate (DINP) hydrogenation to di(isononyl) cyclohexane-1,2-dicarboxylate (DINCH).

4.1.6 Carboxylates Carboxylates are an alternative to phthalate plasticizers, in particular di(isononyl) cyclohexane-1,2-dicarboxylate (DINCH) [13,14]. This plasticizer is obtained by the hydrogenation of the benzene ring that is present in o-phthalates. The value of practical efficiency of this plasticizer was estimated at 1.11 [15]. A schematic description of this process is presented in Fig. 4.9 [16]. The properties of this plasticizer proved it to be suitable for the production of medical devices, toys and products that come in contact with food [17]. More examples of alternative plasticizers belonging to this group that are available on the market include di(2-ethylhexyl) terephthalate (DEHT) plasticizers [18], whose chemical structure is shown in Fig. 4.10.

H

H

O

O

O

O

Figure 4.10 Chemical structure of di(2-ethylhexyl) terephthalate.

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DEHT is used in plasticized PVC products such as toys, bottle caps and closures, coatings for cloth, electric connectors, flexible films, pavements, striping compounds, walk-off mats, sheet vinyl flooring, other vinyl products and PVC/VA copolymer resins [19]. It should be noted that dialkyl terephthalates with side chains containing more than eight carbon atoms have limited compatibility with PVC. On the other hand, dialkyl terephthalates, in which the side chains contain less than eight carbon atoms, show increased volatility. Nontoxic polymeric PVC plasticizers also include polyphthalates. They are considered to be stable and have no negative impact on the environment [20].

4.1.7 Benzoates This group of plasticizers has been used in the industry for many years, but as with other alternative plasticizers, their importance in industrial applications is set to grow. Benzoates are one of many groups of chemical substances that are being studied to determine if they can be used more widely in the plasticization of polymers. As an example, 1,2propylene glycol dibenzoate, which demonstrates a high polarity, has very good plasticizing properties with respect to PVC and other polar polymers [21]. Dibenzoate plasticizers have strong solvating properties and render it possible to apply lower processing temperatures and shorter gel times. In addition, they offer superior resistance to extraction by solvents such as kerosene, cottonseed oil and soapy water. From a health and environmental perspective, the European Chemical Agency listed dibenzoate plasticizers as a preferred alternative to phthalates in May 2009. Dibenzoate plasticizers are compatible with many polymeric materials, including ethylene-vinyl acetate, PVC, styrene-butadiene rubber, ethyl cellulose, nitro cellulose, cellulose acetate butyrate, polyurethane (PU) and acrylics (PMMA) [22]. The chemical structures of diethylene glycol dibenzoate and dipropylene glycol dibenzoate are shown in Fig. 4.11 [23]. Dibenzoate plasticizers are often used in blends with other plasticizers. Several diester compounds are currently being produced, e.g., diol dibenzoates used as substitutes for phthalates due to their structural similarity to phthalate plasticizers, as well as their higher biodegradation rates, lower toxicity (for succinates and dibenzoates) and comparable plasticizing properties to DEHP. This group includes several diol dibenzoates [24] and even numbered n-alkyl succinates and maleates [25e27].

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O O

O

O

diethylene glycol dibenzoate (DEGDB)

O

O O

O

O

dipropylene glycol dibenzoate (DPGDB)

Figure 4.11 Chemical structures of two commercial dibenzoate plasticizers: diethylene glycol dibenzoate (DEGDB) and dipropylene glycol dibenzoate (DPGDB).

4.1.8 Sulfonate Esters Sulfonate ester plasticizers for PVC typically consist of aryl esters of a C13eC15 alkane sulfonic acid. They are relatively efficient and easily processable plasticizers with good extraction properties [11]. The general chemical structure of alkylsulfonic acid phenylester (ASE) has been presented in Fig. 4.12. Hyperbranched poly(ε-caprolactone) is also considered to be an alternative plasticizer to phthalates. Its main applications include PVC plasticization. It is also used in coating resins, polymer additives, adhesive agents and in processing aids. According to Choi and Kwak, this plasticizer does not migrate from PVC even under harsh conditions such as high temperatures [28]. Among the many innovative plasticizers offered on the market, pentaerythritol tetravalerate (PETV) deserves special attention [29]. This plasticizer is characterized by good miscibility with PVC, has excellent resistance properties, very high plasticizing efficiency, good resistance to oil, chemicals, water solutions and UV radiation and

O O

S n

O

n + m = 7 - 15 m

Figure 4.12 General chemical structure of alkylsulfonic acid phenylesters.

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H3C

O O

O O

O H3C O

CH3

O O

CH3

Figure 4.13 Chemical structure of pentaerythritol tetravalerate.

demonstrates fire and smoke resistance. PETV is recommended for the production of flooring in hospitals, schools and public buildings, as well as other close-to-consumer applications, e.g., automotive interiors, toys, moulded parts, wall coverings and coated fabrics [30]. The chemical structure of PETV is shown in Fig. 4.13. Some of the alternatives to phthalic acid esters may be produced partly from raw materials of natural origin (e.g., alcohols used during synthesis), in which case we are dealing with plasticizers of a partially petrochemical origin. By analogy, you can find plasticizers made mainly from renewable raw materials, but their synthesis also involves the use of acids or alcohols obtained from petrochemical raw materials. In such cases, we can refer to them as plasticizers made from mixed raw material sources. Only selected groups of alternatives to monomeric phthalate plasticizers have been presented in this chapter, which is why examples of polymeric plasticizers are not given.

4.2 Plasticizers Manufactured from Renewable Raw Materials In recent years, many new plasticizers have appeared on the market. What is most noticeable is that many of them are based on renewable resources. The ban on the use of phthalate plasticizers in PVC products and the increase in consumer awareness was one of the reasons why the

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use of renewable raw materials in the synthesis of bioplasticizers has increased. The application of chemicals based on natural oils is probably one of the most important trends in the polymer industry. The use of natural oils has further extended the range of renewable raw materials available on the market, which can be used for synthesis. Because of their lack of compatibility, unmodified oils or fatty acids cannot be used as plasticizers for polar polymers, such as, for example, PVC. Modification of oils or fatty acids, e.g., by epoxidation, increases their polarity and their compatibility with the polymer matrix. To meet the requirements of current regulations and standards, plasticizer manufacturers focus their efforts on introducing safer and nontoxic alternatives to phthalate plasticizers. Bioplasticizers are made from recycled plant materials and constitute an alternative to plasticizers of petrochemical origin, which are widely used in PVC processing. The raw materials used for the production of bio-based plasticizers most often include soybean oil, linseed oil, sunflower oil, castor oil, corn oil, canola oil, rapeseed oil, palm kernel oil, cottonseed oil, peanut oil, coconut oil, palm oil, tung oil and safflower oil. Because of their relatively low cost, easy accessibility and biodegradability, vegetable oils are an attractive alternative for the synthesis of biopolymers and bioplasticizers [31e33]. Vegetable oils mainly encompass triglycerides, the glycerol esters of fatty acids. Fatty acids are a product of the hydrolysis of triglycerides derived from five basic fatty acids with a chain length from 16 to 18 carbon atoms containing 0 to 3 double bonds. These include palmitic, stearic, oleic, linoleic and linolenic acids. The structures of these triglycerides and fatty acids are shown in Fig. 4.14 [34]. The fatty acid composition and degree of unsaturation for some common vegetable oils is presented in Table 4.1 [35e39]. Acids and alcohols derived from natural renewable products constitute the primary raw materials for the production of plasticizers of biological origin. Starch contained in corn, wheat, potatoes, peas and other vegetable products constitutes a primary raw material for the production of esters from isosorbides and fatty acids. The starch contained in corn, potatoes and wheat is used increasingly more often as a raw material in alcohol production, especially in Europe. As we all know, short-chain alcohols, e.g., ethanol and butanol, can be produced from biomass in the fermentation processes. Long-chain alcohols such as octanol, decanol and dodecanol can be obtained using Escherichia coli and potassium salts [40].

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O O

O O O

triglyceride

O

O HO palmitic acid(C16:0) O HO stearic acid (C18:0) O HO O

oleic acid (C18:1)

HO linoleic acid (C18:2) O HO linoleic acid (C18:3)

Figure 4.14 Example of triglyceride and fatty acid structures.

According to Challener, synthesis processes employing biological materials are more environmentally friendly than traditional petrochemical processes. They require less energy, generate less waste and reduce our dependence on oil [41]. Until recently, the cost of production of carboxylic acids made using biocatalysts has stood in the way towards their commercialization. However, efficient biotechnological solutions have made it possible to reduce the production costs of effective bioplasticizers based on raw materials of natural origin [41e44]. The industrial use of vegetable oils is based mainly on the chemical modification of carboxyl groups or on the unsaturation of carbon groups found in fatty acids [45e47].

4.2.1 Epoxidized Vegetable Oils and Epoxidized Fatty Acids In recent years, a lot of research has been devoted to the synthesis and the possibilities of using vegetable oilebased plasticizers such as epoxidized safflower oil, epoxidized neem oil, epoxidized linseed oil, epoxidized soybean oil (ESBO) [48e54], soybean oil fatty acid methyl ester [49],

114

Table 4.1 Selected Properties and Fatty Acid Compositions of Common Vegetable Oils [35e39]. Saturated

Unsaturated Oleic

Linoleic

Linolenic

Double Bondsa

Iodine Value

Canola

4

2

61

21

9

3.9

110e126

Cottonseed

22

3

19

54

1

3.9

90e119

Corn

11

2

25

60

1

4.5

102e130

Linseed

5

4

22

17

52

6.6

168e204

Olive

14

3

71

10

1

2.8

75e94

Palm

44

4

39

10

e

1.8

44e58

Peanut

11

2

48

32

e

3.4

80e106

Soybean

11

4

23

53

8

4.6

117e143

Sunflower

6

4

42

47

1

4.7

110e143

Note: Because of the rounding and/or the presence of small amounts of other fatty acids (not included in the statement), the sum may not be 100%. a Average number of double bonds per triglyceride.

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benzyl ester of dehydrated castor oil fatty acid [50], soybean oilebased polyol [51] and epoxidized broccoli oil [52]. Epoxidized vegetable oils and epoxidized fatty acids can be obtained through the oxidation of vegetable oils and fatty acids with peroxide acids. Epoxidized vegetable oils are a bio-based class of plasticizers produced by the esterification of vegetable oils with polyols. A typical example of a plasticizer belonging to this class commonly used in industrial practice is ESBO produced in the esterification of ESBO using glycerin [53]. The main advantages of this plasticizer include its migration resistance (resulting from its high molecular weight and structure), high temperature stability and PVC compatibility [54]. It should be noted that the initial versions of soybean oil plasticizers exhibited limited compatibility with PVC and played the role of secondary plasticizers in polymer compositions; however, newly developed compounds may be used as primary plasticizers as well [53]. Epoxidized linseed oil (ELO) has been proven suitable as a plasticizer in PVC processing [55]. Its structure is shown in Fig. 4.15. Dow Chemicals is particularly active as far as the synthesis of plasticizers from vegetable oils is concerned. The company focuses on using acids derived from soybean oil [56,57]. Many universities and research teams have patented their own solutions, e.g., the University of Minnesota patented a product obtained in the epoxidation of palm oil [58]. The application of ESBOs has been known for a long time, but their role in plastics processing is constantly growing. In addition to the popular ESBO, methyl and alkyl epoxy soyates are of industrial importance. Good PVC plasticization properties are also demonstrated by epoxidated stearates, e.g., iso-amyl epoxy stearate (bio-based content: 100%, MW: 365 g/mol) and 2-ethylhexyl epoxy stearate (bio-based content 68%, MW: 407 g/mol) [59]. The chemical structure of 2-ethylhexyl epoxy stearate is shown in Fig. 4.16. In addition to epoxidation, other methods that render natural oils or derivatives of fatty acid esters more compatible with polar polymers such O O

O

O O O

O O

O

O

O

O

Figure 4.15 Structure of epoxidized linseed oil.

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O O

O

Figure 4.16 Chemical structure of 2-ethylhexyl epoxy stearate.

as PVC have been patented. One example includes acetylation. Acetylated castor oil or acetylated stearate esters have been patented by Dow as plasticizers [60]. Products displaying plasticizing properties are obtained in reactions of oils with polyols and acid anhydrides [61]. For example, acetylated castor oil is a commercial product used for plasticizing polymers. In addition, it is also used on an industrial scale in the production of sebacates and citrates [62].

4.2.2 Esters of Succinic and Citric Acid New technological solutions in the synthesis of acids and alcohols from renewable raw materials are the basis for the production of bioplasticizers that constitute an alternative to their phthalate counterparts. These substitutes, among others, include esters of succinic and citric acid [63]. Citrates have been known and used as plasticizers for many years, but the push to replace phthalic acid esters resulted in a greater appreciation of this group of plasticizers, especially those made from natural raw materials. Citric acid used for the production of biocitrates is obtained chiefly in the fermentation of the constituents that make up maize [64]. Esters of citric acid, with the exception of acetyl tributyl citrate, are commonly used as additives for cosmetics. They are also used in the production of food packaging and medical products, e.g., medical blood bags, devices and pharmaceutical tablet coatings. Acetyl tributyl citrate (ATBC) is an alternative to phthalates used in children’s goods [65,66]. According to Wilson, despite the growing popularity of citrates as substitutes for phthalate plasticizers, the technical benefits resulting from their use are rather negligible [67]. According to the results of the work presented in the Danish Environmental Project, ATBC and butyryl trihexyl citrate (BTHC) are effective replacements for DEHP in PVC compositions, including those used in the manufacture of products used in medicine [68]. However, in some specific applications, the migration of citrates from plasticized products may be a problem. The chemical structure of ATBC and BTHC is shown in Figs. 4.17 and 4.18.

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O O O O O O

O O

Figure 4.17 Chemical structure of acetyl tributyl citrate. O

O

O

O O

O

O O

Figure 4.18 Chemical structure of butyryl trihexyl citrate.

Other examples of citric acid esters which demonstrate a plasticizing effect also include triethyl citrate, tributyl citrate, acetyl triethyl citrate, acetyl trihexyl citrate and acetyl trioctyl citrate. According to Stuart, diesters of succinic acid (derived from a biobase) are an underestimated substitute for phthalate plasticizers. Succinates with longer alkoxy chains (dioctyl succinate and dihexyl succinate) were shown to be more efficient than dioctyl phtalate in plasticizing PVC on a mass percent basis [63]. For example, diesters of dicarboxylic acids with different amounts of methylene groups and different lengths of their alkoxy groups, have been recognized as substances exhibiting plasticizing properties. According to Van Veersen, differences in the chemical structure determine the compatibility with a polymer, plasticization capacity, and affect mechanical properties [69]. Stuart et al. investigated the effectiveness of plasticizing PVC with biobased succinate esters, dioctyl succinate dihexyl succinate, dibutyl succinate and diethyl succinate. They showed that succinates with longer alkoxy chains dioctyl succinate and dihexyl succinate were more efficient than dioctyl phtalate (DOP) in plasticizing PVC. The succinates with shorter alkoxy chains like dibutyl succinate and diethyl succinate did not appear to achieve a comparable amount of plasticization on a mass percent basis. However, similar results were obtained with higher concentration levels. These results establish all four succinates as potentially bio-based sustainable alternatives to phthalate plasticizers.

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Succinates with shorter alkoxy chains dibutyl succinate and diethyl succinate showed lower plasticization efficiency. Based on the test results, it was found that the succinates mentioned above are an alternative to dioctyl phthalate [63]. The chemical structures of selected succinates are presented in Fig. 4.19 (dioctyl succinate) and in Fig. 4.20 (dihexyl succinate). By way of an oligomerisation reaction of monomeric unsaturated fatty acids, dimerized fatty acids are obtained, which are used as polyethylene terephthalate (PET) plasticizers [70]. The structure of a typical dimer acid is shown in Fig. 4.21. An example of a by-product, which is suitable for making new plasticizers, is glycerol, which is obtained in the production of biodiesels [71,72]. Plasticizers can be prepared from glycerol by esterification with acids to yield triglycerides. In contrast to natural triglycerides, which are a part of natural oils, there is a greater choice of side chains for the production of synthetic triglycerides. The properties of the plasticizer and its compatibility with the polymer are modified by choosing a specific side-chain structure. It has been found that the product of esterification of glycerol with short-chain acids, e.g., glycerol triacetate (“triacetin”), can be used as a plasticizer for some polymers, e.g., thermoplastic PUs. Such a product has been patented by BASF [73]. O O

O O

Figure 4.19 Chemical structure of dioctyl succinate. O H 3C

O

O O

Figure 4.20 Chemical structure of dihexyl succinate. O OH HO

O

Figure 4.21 A typical dimer acid structure.

CH3

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Carbohydrates are another group of compounds of biological origin used in the synthesis of new plasticizers. Dianhydrohexitol diesters of 2-ethylheptanoic acid, which are suitable in the plasticization of PVC, can serve as examples [74]. Isosorbide diesters are bio-based plasticizers produced from the reactions between fatty acids of vegetable origin (e.g., n-octanoate) and isosorbide produced by dehydration of sorbitol, a glucose derivative [75]. This plasticizer group is promoted as a nontoxic, biodegradable, bio-based alternative to phthalates for PVC applications. Furan and its derivatives can be produced from both renewable raw materials and from crude oilebased raw materials. The use of furan derivatives and 2,5-dicarboxylates of furan and tetrahydrofuran in particular as effective plasticizers of PVC and other polymers was patented by BASF [76], Dow [77] and Evonik [78], to name a few. Plant-based resins are a source of terpenes that, according to some patent applications, can also be used as plasticizers. Modification of myrcene and faresene with, for example, maleic anhydride as a dienophile in the DielseAlder reaction produces forms of myrcene and faresene that exhibit increased compatibility with polar polymers [79]. The largest producer of bio-based 1,3-propanediol is DuPont. They use it to obtain polytrimethylene ether glycol esters, which can be used in various polymer bases [80]. For example, other bio-based low molecular weight products patented as plasticizers include polyhydroxyalkanoates [81], lactide oligomers [82] and lacton-based hyperbranched polymers [83]. It has been found that bio-based plasticizers can be used in many applications that cannot contain phthalates, such as soft toys, food packaging and carpet underlay.

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[67] A.S. Wilson, Plasticizers: Principles and Practice, The Institute of Materials, Great Britain, 1995. [68] B.S. Nielsen, D.N. Andersen, E. Giovalle, M. Bjergstrøm, P.B. Larsen, Alternatives to Classified Phthalates in Medical Devices, The Danish Environmental Protection Agency, 2014. Environmental Project No. 1557. [69] G.J. van Veersen, A.J. Meulenberg, Relation between the chemical structure and the efficiency of plasticizers, Kunststoffe 57 (1967) 561e566. [70] WO2012/080163, Polymer Composition Containing a Polymer, Which Polymer Contains Monomer Units of a Dimerised Fatty Acid. [71] WO2010/110911, Process for Making Triglyceride Plasticizer From Crude Glycerol. [72] WO2010/074737, Tryglyceride Plasticizer and Process of Making. [73] WO2011/141408, Thermoplastic Polyurethane Comprising Glycerol, Esterified with at Least One Aliphatic Carboxylic Acid, as Plasticizer. [74] US20140088226, Dianhydrohexitol Diesters of 2-Ethylheptanoic Acid. [75] Roquette Co. FIRST Pilot Production of Isosorbide Diesters Polysorb idÒ. http://www.roquette.com/delia-CMS/t1/article_id-4979/topic_ id-1291/roquettefirst-pilot-production-of-isosorbide-diesters-polysorbid-r-.html. [76] WO2015/032794, Tetrahydrofuran Derivatives and Their Use as Plasticizers. [77] WO2014/193634, Dialkyl 2, 5-Furandicarboxylate Plasticizers and Plasticized Polymeric Compositions. [78] WO2012/113607, C11-C13 Dialkyl Esters of Furandicarboxylic Acid as Softeners. [79] WO2012/158250, Plasticizers. [80] WO2015/023714, Plasticizers Comprising Poly(Trimethylene Ether) Glycol Esters. [81] WO2013/156930, Composition Comprising at Least One Biodegradable Polymer and at Least One Plasticiser. [82] WO2012/131252, Use of a Biodegradable Lactide Oligomer as Plasticizer. [83] WO2012/121553, Environmentally-Friendly Nanobrush-Type Alternative Plasticizer Compound and Method for Manufacturing Same Using One-Pot Process.

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Further Reading [1] P.K. Gamage, A.S. Farid, Migration of novel epoxidized neem oil as plasticizer from PVC: experimental design approach, J. Appl. Polym. Sci. 121 (2011) 823e838. [2] O. Fenollar, D. Garcia-Sanoguera, L. Sa´nchez-Na´cher, J. Lo´pez, R. Balart, Characterization of the curing process of vinyl plastisols with epoxidized linseed oil as a natural-based plasticizer, J. Appl. Polym. Sci. 124 (3) (2012) 2550e2557. [3] C. Bueno-Ferrer, M.C. Garrigo´s, A. Jime´nez, Characterization and thermal stability of poly(vinyl chloride) plasticized with epoxidized soybean oil for food packaging, Polym. Degrad. Stabil. 95 (2012) 2207e2212. [4] P. Karmalm, T. Hjertberg, A. Jansson, R. Dahl, Thermal stability of poly(vinyl chloride) with epoxidised soybean oil as primary plasticizer, Polym. Degrad. Stabil. 94 (12) (2009) 2275.

5 Methods of PET Recycling Poly(ethylene terephthalate) (PET) is one of the most commonly used engineering plastics and owes its popularity to a series of mechanical properties it possesses including excellent tensile and impact strength, appropriate thermal stability, chemical resistance, clarity, low O2 and H2O permeability and good rigidity. PET was first synthesized in DuPont’s laboratories in 1929 by Carothers. Its use has increased significantly in recent years since its introduction as a material for the production of beverage packaging. Moreover, it is widely used in the textile industry and in the production of high strength fibres and photographic films. The global PET packaging market was worth almost 16 million tonnes in 2014 according to a new market report from Smithers Pira. Demand for PET packaging is expected to increase by an average of 4.6% annually over the next 5 years and will amount to 19.9 million tonnes by 2019 [1]. PET itself is not directly hazardous for the natural environment but because it is a nondegradable plastic in normal conditions, it does make up a considerable volume of all the municipal waste ending up in landfills. Because of its high resistance to weathering and biological agents, it does not erode, and for this reason, it is considered hazardous for the environment [2]. The increase in the amount of PET waste has led to economical and environmental problems. One of the best ways to process PET waste is recycling. The PET bottle was patented in 1973, and 4 years later i.e., in 1977, the first PET bottle was recycled and turned into a bottle base cup [3]. Currently, PET waste is one of the easiest materials to recycle and is second only to aluminium in terms of the scrap value for recycled materials [4]. In 2015%, 59% of PET bottles were collected and recycled in Europe [5]. Manufacturing products from recycled plastics can save up to 50%e60% of energy as compared with making the same product from virgin resin [6]. There are three main factors to consider when recycling PET: first is the collection of postconsumer PET, second is recycling itself, and third is the question of whether there is a market for the final product of the recycling process [7].

Plasticizers Derived from Post-consumer PET https://doi.org/10.1016/B978-0-323-46200-6.00005-2 Copyright © 2020 Elsevier Inc. All rights reserved.

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PET recycling can be carried out in many different ways. Four main approaches have been proposed [6]: 1. Primary recycling (preconsumer industrial scrap) Primary recycling to ‘in-plant’ recycling of scrap material of a controlled history: This type of recycling is the most popular as it is easy to carry out and is relatively inexpensive. The material to be recycled is clean, free from contamination and not mixed with other types of materials. It is blended with virgin material in quantities and ways that ensure adequate quality of the manufactured products. It can also be used as a second-grade material. 2. Mechanical recycling (secondary recycling) The polymer material is separated from impurities and is converted into granular form by conventional melt extrusion. Mechanical recycling consists of the following stages: sorting, removal of impurities, grinding, melt filtration and reforming of the plastic material. The main disadvantage of this method is that the product properties tend to deteriorate in each subsequent cycle, which is caused by a reduction of the molecular weight of the recycled polymer due to a polymer chain reaction initiated by the presence of water and acid impurities. Techniques used to prevent adverse reactions include intensive drying, reprocessing with a degassing vacuum, use of chain extender compounds, etc. [6] 3. Tertiary recycling Tertiary recycling is defined as a process leading to complete depolymerization of PET into monomers and/or oligomers. Only chemical recycling conforms to the principle of sustainable development because it leads to the formation of raw materials or monomers from which PET is originally made. 4. Quaternary recycling Energy recovery refers to the recovery of the plastic’s energy content. This is currently the most effective method in reducing the volume of organic materials. Although polymeric materials are a valuable source of energy, this method is currently widely accused of being ecologically unacceptable, owing to the health risks from airborne toxic substances such as dioxins [7]. In addition to the methods mentioned above, direct reuse of PET seems to be an interesting solution. In many countries it is a common practice to refill and reuse PET bottles. Bottles made of plastic, however, absorb

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Table 5.1 Minimum Requirements for Recycled Poly(ethylene terephthalate) Flakes for Reprocessing [8,11]. Property

Value

Viscosity coefficient [h]

>0.7 dL/g

Melting point Tm

>240 C

Water content

Mnþ2 > Coþ2 > Pbþ2) partly overlapped with the results obtained by Baliga et al. [46]. According to the authors, this difference in the activity of manganese salts may result from the form of PET waste, as Beliga investigated PET degradation in flake form, while the activity stated by Ghaemy and Mossaddegh is for PET in fibre form. This was also

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confirmed by waste PET degradation studies using 1 wt% zinc acetate, where the yield from PET flakes for the same volume of EG is higher than that obtained from PET fibres. The difference in the yield was explained as the result of a delay in the penetration of enough EG into the polymer caused by the molecular order in PET fibres [48]. The main products of PET waste depolymerization by glycolysis carried out in the best possible conditions included BHET monomer with a yield of about 75% and dimers. However, these observations do not always correspond with other results, where PET waste in flake or fibre form was also degraded. Such experiments were conducted, e.g., by Troev’s team (Table 5.4) [42]. Mishra and Goje reported that the depolymerization degree of PET with EG, whose molar ratio was 1:6.6, is inversely proportional to the particle size of PET. The optimal reactant size is recorded as 127.5 mm, above which the degree of depolymerization decreases. The reaction was conducted at 0.2 MPa and 220 C for 40e120 min and was catalyzed by lead acetate and zinc acetate. The particle size of PET waste powder was in the range of 50e512.5 mm [65]. Differences in the results of the experiments carried out by the different teams may be related to different molar ratios of PET:EG; the average molecular weights of PET waste and the amounts of degraded raw materials that authors often do not provide. Summarizing the research on metal acetates as catalysts, it is clearly stated that the glycolysis reaction rate of PET by EG and the efficiency of BHET monomer correlated with temperature and the concentration of glycol as well as the type and concentration of the catalyst (Table 5.4). An interesting solution is the use of xylene solvent in a reaction catalyzed by zinc acetate. The entire process is carried out in a dispersion. Such a process helps reduce the amount of EG (EG:PET ¼ 3:1) and produces BHET with a yield of 80% after 3 h at 220 C. The advantages of this method include obtaining a glycolysis product with higher solubility, especially BHET monomer at high temperature, which shifts the equilibrium of the reverse reaction toward depolymerization. EG is slightly soluble in xylene and thus significantly improves the purification of the products from unreacted EG. A disadvantage, however, is the hazard the organic solvent poses to the environment [12,51]. Unfortunately, zinc and lead acetate contain toxic heavy metal cations that tend to accumulate in living organisms. For this reason, researchers are looking for catalysts that would be equally effective in the degradation of PET via glycols. Shukla’s group was the first to successfully apply safe and mild alkalies such as sodium carbonate and sodium bicarbonate, thus obtaining similar BHET yield to that of zinc acetate in the same

Table 5.4 Various Reaction Parameters for PET Glycolysis.

Catalyst Zn(OOCCH3)2

Zn(OOCCH3)2

Field of Main Products, %

Glycol/PET Molar Ratio

Mainly: PG-TPA-PG (BHPT),

1.5:1

PG-TPA-EG,

2.5:1

EG-TPA-EG

4.2:1 PG:PET (fibre waste)

Main product is BHET

1.9:1

79.2 BHET

Pb(OOCCH3)2

77.3 BHET

Mn(OOCCH3)2

77.3 BHET

Co(OOCCH3)2

75.0 BHET

Zn(OOCCH3)2

80.0 BHET

Zn(OOCCH3)2

62.5 BHET

PET, g

Catalyst, wt%

Temperature,  C

Time, min.

References

Range 18,000e 20,000

n/a

0.5

200

480

[49]

Range 18,000e 20,000

n/a

0.5

200

480

[50]

4:1 EG:PET (bottle waste)

28,000

n/a

0.5

190

480

[46]

3:1 EG:PET (bottle waste) xylene

18,000

n/a

1.0

220

180

[51]

3.1:1 5.2:1 EG:PET

Zn(OOCCH3)2

PET Average Molecular Weight, g/mol

Pb(OOCCH3)2

61.6 BHET

Na2CO3

61.5 BHET

NaHCO3

61.9 BHET

Zn(OOCCH3)2

67.6 BHET

Pb(OOCCH3)2

65.9 BHET

Na2CO3

65.4 BHET

NaHCO3

66.2 BHET

Titanium (IV)-phosphate

95.3 BHET

n/a

n/a

0.5

190

480

6:1 EG:PET (fibre waste)

n/a

n/a

0.5

190

480

2.77:1 EG:PET (fibre waste)

30,000 (fibre waste)

25

0.05

190e200

195

96.5 BHET

0.10

180

95.5 BHET

0.20

160

97.5 BHET

0.30

150

0.50

155

66.7 BHET

0.3

105

62.9 BHET

0.5

110

95.3 BHET

Zn(OOCCH3)2

6:1 EG:PET (bottle waste)

62.8 BHET

61.1 BHET

2.77:1 EG:PET (bottle waste)

2.77:1 EG:PET (fibre waste)

0.3

0.3

190e200

[52]

[42]

150

105 (Continued )

Table 5.4 Various Reaction Parameters for PET Glycolysis. (Continued )

Catalyst

Field of Main Products, %

Glycol/PET Molar Ratio

PET Average Molecular Weight, g/mol

PET, g

Catalyst, wt%

Temperature,  C

Time, min.

220

12

2.77:1 EG:PET (bottle waste) Titanium (IV)-phosphate

91.2 BHET 90.4 BHET 64.2 BHET 58.0 BHET

Titanium (IV)-phosphate

93.6 BHPT 93.3 BHPT

2.77:1 DEG:PET (fibre waste)

51,000 (bottle waste)

0.3 0.5

13

2.77:1 DEG:PET (bottle waste)

0.3

8

0.5

8

2.77:1 PG:PET (fibre waste)

0.3 0.5

180e188

480 530

References

52.8 BHPT 47.7 BHPT

2.77:1 PG:PET (bottle waste)

0.3

285

0.5

400

Zn(OOCCH3)2

75.0 BHET

8.5:1 EG:PET (fibre waste)

29,000

10

1.0

198

480

[48]

Zn(OOCCH3)2

85.6 BHET

18.5:1 EG:PET (bottle waste)

28,000

n/a

1.0

196

180

[45]

Zn(OOCCH3)2

66.0 BHET

n/a

n/a

0.5

190

Na2CO3

64.0 BHET

NaHCO3

65.0 BHET

6:1 EG:PET (bottle waste)

35 [53] (microwave irradiation)

Zn(OOCCH3)2

64.0 BHET

n/a

30

1.15

196

60

[54]

Na2CO3

65.0 BHET

NaHCO3

50.0 BHET

7.6:1 EG:PET (scraps PET waste)

Na2SO4

10.0 BHET

0.24

K2SO4

1.0 BHET

0.26

CH3COOH

62.4

196

480

[55]

LiOH

63.5

Na2SO4

65.7

K2SO4

64.4

6:1 EG:PET (fibre waste)

0.23 0.23

n/a

n/a

0.5

(Continued )

Table 5.4 Various Reaction Parameters for PET Glycolysis. (Continued ) PET Average Molecular Weight, g/mol

PET, g

6:1 EG:PET (bottle waste)

n/a

n/a

59.4 BHET

12.4:1 (virgin pellets PET)

26,280 (Mn)

[bmim]FeCl4

76.4 BHET

12.4:1 (virgin pellets PET)

[bmim]OH

71.2 BHET

[bmim]Cl

57.1 BHET

[bmim]ZnCl3

84.9 BHET

[bmim]MnCl3

83.3 BHET

Catalyst

Field of Main Products, %

Glycol/PET Molar Ratio

bzeolite Y zeolite

66.0 BHET 65.0 BHET

[bmim]Cl

Zinc chloride

Temperature,  C

Time, min.

References

1.0

196

480

[56]

5.0

13.8

180

480

[57]

26,300

5.0

3.8

150

240

[58]

31:1 (bottle waste)

18,000

2.0

5.0

190

120

[59]

34:1 (bottle waste)

18,100

2.0

5.0

190

480

[60]

6:1

n/a

Catalyst, wt%

1.25

120

5.0

120

0.5

196

480

[61]

60.5 BHET 73.2 BHET

10:1

62.9 BHET

6:1

59.5 BHET

10:1

Didymium chloride

66.4 BHET

6:1

71.6 BHET

10:1

Magnesium chloride

66.1 BHET

6:1

55.7 BHET

10:1

Ferric chloride

57.6 BHET

6:1

56.3 BHET

10:1

ZnO/silica nanoparticles

85.0 BHET

11.3:1 (virgin PET pellets)

38,400 (Mw)

0.3

Mn3O4/silica nanoparticles

90.0 BHET

e

20.0 BHET

4:1 (fibres waste)

18,000e 20,000

n/a

Lithium chloride

EG:PET (bottle waste)

18,000e 20,000 (Mn)

1.0

e

300 0.1 MPa

80

[62]

240 (0.62 MPa)

60

[47] (Continued )

Table 5.4 Various Reaction Parameters for PET Glycolysis. (Continued )

Catalyst

Field of Main Products, %

Glycol/PET Molar Ratio

e

47.0 BHET

2.48:1 (fibres waste)

e

45.2 BHET

e

25.1 BHET

e

22.3 BHET

e

96.0 BHET

PET Average Molecular Weight, g/mol

PET, g

Catalyst, wt%

Temperature,  C

Time, min.

References

13,500

778

e

220 (0.15e 0.18 MPa)

120

[63]

e

220 (0.17e 0.19 MPa)

e

240 (0.16e 0.20 MPa)

180

230 (0.15e 0.20 MPa)

300

450 (15.3 MPa)

35

18,000 1:1 (fibres waste)

13,500 (Mn)

1057

18,000 51.6:1 (bottle waste)

59,100 (Mn)

0.3

e

BHET, bis(2-hydroxyethyl) terephthalate; BHPT, bis(hydroxypropyl terephthalate); EG, ethylene glycol; PG, propylene glycol; TPA, terephthalic acid.

[64]

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PET R ECYCLING

149

reaction conditions. These compounds do not pose a threat to the environment at relatively low amounts. An additional advantage is their lower price compared with conventional metal acetates [52]. However, using microwave radiation instead of a conventional heating source made it possible to drastically reduce the reaction time to 35 min when using the aforementioned catalysts [53]. The same research group also used glacial acetic acid, lithium hydroxide, sodium sulphate and potassium sulphate as catalysts in the process. The influence of catalyst quantity (0.3, 0.4, 0.5, 0.6 or 1.0 wt%) and the reaction time on its yield was investigated. The best effects were achieved after introducing 0.5 wt% catalyst after 8h, regardless of its type [55]. An interesting solution employed by this research group was the application of natural zeolites as catalysts demonstrating a large mesoporous surface and high hydrothermal stability; bzeolite and Y zeolite were used in amounts of between 0.25 and 1.50 wt%. The yield of the BHET monomer increased from 48% to 70% for both the zeolites as the weight ratio of EG to PET rose from 4:1 to 9:1. The EG:PET ratio of 6:1 was considered as optimum; over this ratio, the yield of BHET increased only marginally. It was also observed that the time which passed until complete dissolution of PET became increasingly shorter as the PET:EG ratio increased, and as we know, the glycolysis reaction occurs predominantly in the liquid phase. The yield of the reaction increased with the amount of catalyst, reaching a maximum at 1% catalyst content, after which it decreased. The authors explain this result as favouring transesterification over depolymerization of PET [56]. Research on metal salts was also carried out by the Gutie´rrez-Ortiz team, which used sodium carbonate, sodium bicarbonate, sodium sulphate and potassium sulphate. The researchers succeeded in achieving a similar BHET yield to Shukla’s group, without using microwaves, after a longer reaction time of 1 h, but with half the Na2CO3 and NaHCO3 catalysts used at 0.23 wt% and at a molar ratio of EG:PET ¼ 7.6:1 at 196 C. Unfortunately, Na2SO4 and K2SO4 showed reduced activity, which the authors explain is the result of their reduced solubility in EG compared to carbonate salts [54]. Further attempts to improve the efficiency of this process involved the use of a metal halide catalyst such as didymium chloride, lithium chloride, magnesium chloride, ferric chloride and zinc chloride. Didymium chloride, due to its ecofriendliness, is considered to be a very promising substance. The best results were obtained by introducing 0.5 wt% of the catalyst at a molar ratio EG: PET ¼ 10:1 after 8 h and at T ¼ 196 C. Such excess EG needs to be recovered not only for economic reasons but also not to impair the properties of the product obtained. The excess unreacted

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glycol can be removed via salting out, distillation and pervaporation separation [61]. In 2003, Troev and Gitsov [42] used a new catalyst synthesized through a reaction of TiCl4 with triethyl phosphate in the glycolysis of PET. The optimal concentration of titanium (IV)-phosphate was 0.3 wt%. The depolymerization of PET fibre proceeds faster in the presence of titanium (IV)-phosphate compared with compounds traditionally used in this process such as zinc acetate. Using EG, 97.5% BHET was obtained after 150 min by maintaining the temperature in the range of 190e200 C. In the case of DEG, 91.2% BHET was obtained after 12 min at a process temperature of 220 C, whereas for propylene glycol (PG), 93.6% bis(hydroxypropyl terephthalate) (BHPT) was obtained after 480 min in a process temperature range of 180e188 C. Equally high yields of about 90% BHET, but in a much shorter glycolysis reaction time of 80 min, were achieved by Imran et al. Their process involved the use of 1 wt% of manganese and zinc oxides supported on silica nanoparticles and an 11-fold excess of EG as well as a very high process temperature of 300 C. Such high efficiency of this type of catalyst is explained by its high surface area and the presence of numerous active sites on the surface of the nanocomposite catalyst [62]. Although homogeneous catalysts are highly effective in the glycolysis of PET, they present several drawbacks. Given that most of these catalysts dissolve in glycol, their separation from the reaction mixture is difficult. This results in the need to introduce an additional unit operation (distillation) into the chemical process [48]. An interesting solution is the use of ionic liquids, the removal of which from the reaction mixture is simpler, i.e., by vacuum evaporation at 70 C. The recovered ionic liquid after being stored in a vacuum oven for 48 h could be efficiently reused several times [66]. Their first reported use was in 2009 and involved the successful application of 1-butyl-3-methylimidazolium bromide ([bmim]Br), 1-butyl3-methylimidazolium chloride ([bmim]Cl), (3-amino-propyl)-tributylphosphonium glycine and (3-amino-propyl)-tributyl-phosphonium alanine, achieving a 100% conversion of PET. The yield of BHET increased with the amount of catalyst used and then with the reaction temperature but decreased with the duration of the glycolysis process. The use of ionic liquids helped reduce the reaction temperature to 180 C, but in order to achieve a high PET conversion rate, it was necessary to use a large amount of [bmim]Cl at 13.8 wt% and catalysts at amounts of 3.85 wt% at a relatively long reaction time for the other cases [57]. However, the magnetic ionic liquid 1-butyl-3-methylimidazolium tetrachloroferrate ([bmim]FeCl4) exhibited higher catalytic activity in PET glycolysis than

5: M ETHODS

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151

ionic liquids. The yield of BHET catalyzed by [bmim]FeCl4 at 150 C is comparable to that of catalysis carried out by traditional catalysts at higher reaction temperatures, but the PET conversion rate achieved was only 16.5%. Increasing the temperature to 178 C helped reach a 100% PET conversion rate under the same reaction conditions, where the yield of BHET was 59.2% [58]. The use of basic ionic liquids, 1-butyl-3methylimidazolium hydroxyl ([bmim]OH) at 5 wt% at 190 C with a molar ratio of EG:PET ¼ 31:1, resulted in the production of BHET 71.2% after a 2 h reaction at a 100% PET conversion rate. This basic ionic liquid [bmim]OH exhibits higher catalytic activity than [bmim]Cl or [bmim]Br at the same process conditions for which PET conversion amounted only to 28.9% and 10.7%, respectively, and at minimal amounts of BHET produced [59]. Lewis acidic ionic liquids such as [bmim]ZnCl3 and [bmim] MnCl3 showed higher catalytic activity in the glycolysis of PET than [bmim]Cl. The conversion of PET and the BHET yield amounted to 100% and 84.9% for [bmim]ZnCl3 at 1.25 wt% and 100% and 83.3% for [bmim] MnCl3 at 5 wt%. The [bmim]ZnCl3 catalyst after being reused up to five times presented a similar yield to is fresh version [60]. Most studies on PET glycolysis were, however, carried out under atmospheric pressure. The first trials of this process under high pressure without adding catalysts was carried out by Lin’s groups in 1991. The reactions were carried out at a temperature in the range of 190e240 C, under a pressure in the range of 0.10e0.62 MPa at an EG molar ratio of PET ¼ 4:1, 3:1, 2:1 and 1:1. It was observed, as in many other experiments, that a low EG:PET ratio results in a higher oligomer molecular weight, which reaches a value of around 2000 g/mol, corresponding to n ¼ 10 [47]. In 2004, Grzebieniak and Weso1owski also conducted experiments of water PET glycolysis by EG at a temperature of 220e240 C, a pressure of 0.14e0.22 MPa and without using catalysts. The product thus obtained was used in the synthesis of degradable polyesters of ethylene terephthalate and L-lactic acid without separating the oligomer fraction first. The process yield ranged from 86.1% to 97.4% after a relatively short reaction time of 60e240 min. Depending on the conditions in the postreaction mixture, BHET appeared in amounts ranging from 18.0% to 47.0%. A greater yield of the oligomer fraction was recorded at an EG:PET molar ratio of 1:1 than at 2.48:1 [63]. A completely new approach to PET glycolysis was the use of EG in its supercritical state (Tc ¼ 446.7 C, Pc ¼ 7.7 MPa). Virgin PET in the form of powder and pellets, mixed waste and soft drink bottle PET waste was degraded. PET waste was completely decomposed after being depolymerized at 450 C and 15.3 MPa for about 30 or 35 min, producing BHET with a maximum

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yield of 96.0% in the case of soft drink PET bottle. Virgin PET in powder form degraded slightly faster, though yielding BHET in an amount of 93.5%. With an increase in reaction time from 35 to 45 min, the weight percentage of the BHET monomer decreases as BHET dimer, oligomers, DEG and TEG are formed. Compared with the subcritical process (350 C and 2.49 MPa or 300 C and 1.1 MPa), the supercritical optimum reaction time was achieved later in the first case, i.e., after 75 and 120 min, respectively, to the conditions provided. However, BHET yield was equally high (>90%) [64]. Apart from EG, other glycols are also used in PET degradation. They include DEG [67,68], PG [49,69], dipropylene glycol [67], 1,4-butanediol (BD) [70] and poly(ethylene glycol) [71]. Investigations of the influence of a glycol’s chemical structure on the glycolysis of PET (being a solid in the reaction temperature) revealed that the overall reactivity is not only a function of the chemical reactivity of the glycol but also of its physical and chemical properties, i.e., the ability to depolymerize solid polymers through solvolytic chain cleavage and the polarity of the reacting mixture [72,73]. Zinc acetate as a catalyst in PET degradation with PG [49,69] was first used by Vaidya and Nadkarni. The degree of PET degradation also depends on the PG:PET molar ratio and rises with the glycol level, as demonstrated by the increasing hydroxyl value (OHV) and decreasing average molecular weight number (Mn) determined by vapour pressure osmometry of the reaction mixture after removal of unreacted glycol. Hydroxyl-terminated BHPT monomer or monomers and oligomers with EG or PG end group dominance were obtained. These products were then used to prepare unsaturated or saturated polyesters in a reaction with maleic anhydride, phthalic anhydride or adipic acid, respectively. The depolymerization of waste PET using BD, at a molar ratio of 1:1.8 and 0.5 wt% zinc acetate under reflux for 10 h, mostly produced bis(hydroxybutyl) terephthalate monomer and dimer. In contrast, the use of TEG at a PET/TEG molar ratio of 1:0.85 or 1:0.55 under the same reaction conditions produced mainly TEG-(TPA-TEG)n products where n ¼ 1e3. A smaller amount of TEG resulted in a mixture of products with a higher acid number of 12.38 mg KOH, which the authors explain is caused by the formation of the same glycolyzed products with carboxyl end groups [70]. Experiments with 1,4-butadiol were also performed by the Patel team where they used a reagent molar ratio of PET:1,4-butanodiol ¼ 1:8.7 at 210 C and 1.7 wt% zinc acetate. The synthesis lasted until all the solids disappeared. The average molecular weight of the product was 400 g/mol with a dispersion index

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of 1.08. This oligoester diol is used for the synthesis of solvent-free polyurethane dispersions [74]. More complex research on the effect of the chemical structure of glycols such as DEG and DPG was conducted by Pardal and Tersac. The authors aptly noted that despite many publications and patents, it is difficult to compare the reactivity of different glycols because the conditions of glycolysis described differ significantly from one another. The global reactivity of both the glycols depends on their chemical reactivity and their physical and chemical properties. Regardless of whether the reaction was catalyzed (in this case with titanium (IV) n-butoxide) or not, the reactivity of DEG was always greater than of DPG, which is most likely because DEG possesses good solvating properties. However, DPG has a very weak chemical reactivity in uncatalyzed conditions. It was also observed that the PET dissolution time is considerably shorter if PET, and DEG are heated to 220 C before mixing [75]. Apart from typical glycols, glycol glycoside was also used. It was obtained by glycosylation of starch and EG using 0.5 wt% of zinc acetate as a catalyst and 5, 15 or 25 wt% of PET at 200e220 C for 6e7 h. The obtained depolymerized oligomers were then esterified with dehydrated castor oil and coco fatty acid to produce novel polyester polyols, polyurethane adhesives and polyurethane coatings [76]. The team also conducted PET depolymerization in identical conditions using PEG with a molecular weight of 200, 400 or 600 g/mol. The products obtained were transesterified with castor oil to obtain hydroxyl functional polyester polyols [71]. The aromatic polyesterols, including those obtained from waste PET, possess certain unfavourable physical and chemical properties, such as storage instability manifested by the tendency to precipitate at ambient temperature and high viscosity, which may be related to the association between terephthalic and ethylene units. Furthermore, these polyols, which are incorporated into polyurethane foams and contain a high content of free low molecular weight glycols, lead to the formation of an irregular, three-dimensional network. These shortcomings can be eliminated through the introduction of monoethylene glycol after glycolysis or via chemical modification by ethoxypropoxylation or esterification with dicarboxylic aliphatic/aromatic acids [77,78]. The polyurethane foam based on polyols derived from PET glycolysis demonstrates low flammability and their decreased manufacturing costs. Tersac et al. conducted PET glycolysis using a mixture of EG, DEG and DPG glycols and a titanium (IV) butoxide catalyst at 200e240 C [67]. The reactivity order in these conditions presented itself in the order of

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FROM

P OST- CONSUMER PET

DEG > DPG > EG. The polyols thus obtained underwent esterification with adipic acid and phthalic anhydride. Investigations of the physical and chemical properties of the polyols confirm that their storage stability is enhanced by lowering the amount of ethylene moieties. Polyols with a OHV of