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POLYMER SCIENCE AND TECHNOLOGY

POLYETHYLENE TEREPHTHALATE USES, PROPERTIES AND DEGRADATION

No part of this digital document may be reproduced, stored in a retrieval system or transmitted in any form or by any means. The publisher has taken reasonable care in the preparation of this digital document, but makes no expressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of information contained herein. This digital document is sold with the clear understanding that the publisher is not engaged in rendering legal, medical or any other professional services.

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POLYMER SCIENCE AND TECHNOLOGY

POLYETHYLENE TEREPHTHALATE USES, PROPERTIES AND DEGRADATION

NAOMI A. BARBER EDITOR

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Copyright © 2017 by Nova Science Publishers, Inc. All rights reserved. No part of this book may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic, tape, mechanical photocopying, recording or otherwise without the written permission of the Publisher. We have partnered with Copyright Clearance Center to make it easy for you to obtain permissions to reuse content from this publication. Simply navigate to this publication’s page on Nova’s website and locate the “Get Permission” button below the title description. This button is linked directly to the title’s permission page on copyright.com. Alternatively, you can visit copyright.com and search by title, ISBN, or ISSN. For further questions about using the service on copyright.com, please contact: Copyright Clearance Center Phone: +1-(978) 750-8400 Fax: +1-(978) 750-4470 E-mail: [email protected].

NOTICE TO THE READER The Publisher has taken reasonable care in the preparation of this book, but makes no expressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of information contained in this book. The Publisher shall not be liable for any special, consequential, or exemplary damages resulting, in whole or in part, from the readers’ use of, or reliance upon, this material. Any parts of this book based on government reports are so indicated and copyright is claimed for those parts to the extent applicable to compilations of such works. Independent verification should be sought for any data, advice or recommendations contained in this book. In addition, no responsibility is assumed by the publisher for any injury and/or damage to persons or property arising from any methods, products, instructions, ideas or otherwise contained in this publication. This publication is designed to provide accurate and authoritative information with regard to the subject matter covered herein. It is sold with the clear understanding that the Publisher is not engaged in rendering legal or any other professional services. If legal or any other expert assistance is required, the services of a competent person should be sought. FROM A DECLARATION OF PARTICIPANTS JOINTLY ADOPTED BY A COMMITTEE OF THE AMERICAN BAR ASSOCIATION AND A COMMITTEE OF PUBLISHERS. Additional color graphics may be available in the e-book version of this book.

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Published by Nova Science Publishers, Inc. † New York

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CONTENTS Preface Chapter 1

Chapter 2

Chapter 3

vii Poly(Ethylene Terephthalate): Synthesis and Physicochemical Properties Marija V. Pergal and Milica Balaban The Applications of Polyethylene Terephthalate for RF Flexible Electronics Tzu-Hsuan Chang, Yei Hwan Jung, Dong Liu, Hongyi Mi, Juhwan Lee, Jiarui Gong and Zhenqiang Ma Progress in Polyethylene Terephthalate Recycling Adel Elamri, Khmais Zdiri, Omar Harzallah and Abdelaziz Lallam

Index

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PREFACE Polyethylene terephthalate (PET) is an aliphatic-aromatic and semicrystalline thermoplastic polyester of prime commercial and industrial importance. Namely, PET is a very important industrial polymer due to its excellent properties such as processability, chemical resistance, high tensile impact strength, high thermal stability and clarity. Chapter One summarizes the synthesis and physicochemical properties of PET. In Chapter Two, the authors review the frequency-dependent parameters of the PET substrate, developments of the flexible RF electronics on PET substrate, and the challenges and potentials of RF applications using flexible electronics fabricated on PET films. Chapter Three presents a background of the current state of knowledge with respect to PET recycling. Chapter 1 - This chapter summarizes the synthesis and physicochemical properties of thermoplastic polyester, poly(ethylene terephthalate) (PET). PET, along with poly(butylene terephthalate) (PBT) is an aliphatic-aromatic and semicrystalline thermoplastic polyester of prime commercial and industrial importance. Namely, PET is a very important industrial polymer due to its excellent properties such as processability, chemical resistance, high tensile impact strength, high thermal stability and clarity. PET is synthesized from ethylene glycol and terephthalic acid or dimethyl terephthalate by a two-stage

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polycondensation process. High molecular weight of PET can be achieved by solid-state polymerization. This chapter presents different types of nanoparticles, such as organoclays, carbon nanotubes and carbon black, which are used for the improvement of the physical, mechanical, thermal and barrier properties of PET nanocomposites. It also highlights the recent developments in PET/layered silicate nanocomposites. The blending of PET with other polymers, as an excellent method of preparing materials with enhanced property/cost performance, is described. Processing, recycling and degradation of PET are also presented. This chapter discusses the surface modification of PET by physical treatment, chemical treatment and grafting polymerization in order to modify its surface properties, for enhanced surface wettability, adhesion activities and biocompatibility improvement. Moreover, this chapter also surveys the most relevant aspects related to the preparation and characterization of thermoplastic copolyester elastomers, especially PET and PBT copolyesters. The application potential of PET is discussed and selected examples of commercially available PET are given. Future trends in PETbased material synthesis and design are also discussed. Chapter 2 - Polyethylene terephthalate (PET) has been one of the most reliable and cost-efficient candidates in recent development of flexible device applications. Flexible electronics have rapidly evolved into a variety of applications including displays, e-papers, solar cells, sensors, wearable electronics, etc. In particular, high-performance electronics such as radiofrequency devices have been designed and fabricated on flexible substrates, such as PET, with operating frequencies of up to the giga-hertz (GHz) regime, which covers current major portable electronics, wireless communication, and transmission units. The flexible electronics that operate at the GHz regime are generally composed of high-performance flexible microwave active and/or passive devices. Typically, they have stringent requirements for substrate choice, in terms of mechanical properties, thermal properties, and electrical parameters, which have a significant impact on the performance at high frequencies. In this chapter, the authors review the frequency-dependent parameters of the PET substrate, developments of the flexible RF electronics on PET substrate,

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and the challenges and potentials of RF applications using flexible electronics fabricated on PET films. Chapter 3 - In the last decade, an increasing interest has been focused on the recycling of plastic wastes, especially on the polyethylene terephthalate (PET). PET polymer is already being recycled and numerous applications for recycled polyesters can be explored depending on the properties of the resin. However, the common problem faced during processing of recycled PET is degradation. Thus, many solutions have been proposed in literature to undermine this problem. This chapter presents a background of the current state of knowledge with respect to PET recycling. In the first section, a brief theoretical background is presented about virgin PET synthesis, thermal transitions, processing and applications. The second section deals with the PET recycling process with a focus on contaminations and ways to increase the molecular weight of recycled PET (RPET). It serves as an introduction to Section Three where the authors’ process to improve the RPET properties is described. Finally, Section Four covers the effect of blending virgin PET (VPET) with recycled PET on thermal and rheological behaviors.

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In: Polyethylene Terephthalate Editor: Naomi A. Barber

ISBN: 978-1-53611-991-6 © 2017 Nova Science Publishers, Inc.

Chapter 1

POLY(ETHYLENE TEREPHTHALATE): SYNTHESIS AND PHYSICOCHEMICAL PROPERTIES Marija V. Pergal1,* and Milica Balaban2 1

Institute of Chemistry, Technology and Metallurgy (ICTM) – Centre of Chemistry, University of Belgrade, Belgrade, Serbia 2 Faculty of Science, University of Banja Luka, Banja Luka, Bosnia and Herzegovina

ABSTRACT This chapter summarizes the synthesis and physicochemical properties of thermoplastic polyester, poly(ethylene terephthalate) (PET). PET, along with poly(butylene terephthalate) (PBT) is an aliphaticaromatic and semicrystalline thermoplastic polyester of prime commercial and industrial importance. Namely, PET is a very important industrial polymer due to its excellent properties such as processability, *

Correspondence to M. V. Pergal; e-mail: [email protected].

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Marija V. Pergal and Milica Balaban chemical resistance, high tensile impact strength, high thermal stability and clarity. PET is synthesized from ethylene glycol and terephthalic acid or dimethyl terephthalate by a two-stage polycondensation process. High molecular weight of PET can be achieved by solid-state polymerization. This chapter presents different types of nanoparticles, such as organoclays, carbon nanotubes and carbon black, which are used for the improvement of the physical, mechanical, thermal and barrier properties of PET nanocomposites. It also highlights the recent developments in PET/layered silicate nanocomposites. The blending of PET with other polymers, as an excellent method of preparing materials with enhanced property/cost performance, is described. Processing, recycling and degradation of PET are also presented. This chapter discusses the surface modification of PET by physical treatment, chemical treatment and grafting polymerization in order to modify its surface properties, for enhanced surface wettability, adhesion activities and biocompatibility improvement. Moreover, this chapter also surveys the most relevant aspects related to the preparation and characterization of thermoplastic copolyester elastomers, especially PET and PBT copolyesters. The application potential of PET is discussed and selected examples of commercially available PET are given. Future trends in PET-based material synthesis and design are also discussed.

Keywords: poly(ethylene terephthalate), thermoplastic polyester, twostage polycondensation, physicochemical properties, surface modification, PET blends, PET nanocomposites, applications

1. INTRODUCTION This chapter summarizes the synthesis, physicochemical properties and applications of the main member of the thermoplastic polyester family, poly(ethylene terephthalate) (PET), which is the most widely recognized in the scientific community and which has widespread commercial and industrial applications. Polyesters are polymers containing at least one ester linking group per repeating unit. They are obtained from the chemical resources found mostly in petroleum and are manufactured in films, fibers and objects with a simple or complex shape [1]. Namely, commercial thermoplastic polyester is produced in the reaction including dimethylester of terephthalic acid rather than terephthalic acid, owing to the faster rate of

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transesterification compared to direct polycondensation from acid, and the fact that the diester can easily be purified and has better solubility characteristics [1-3]. Poly(ethylene terephthalate) and poly(butylene terephthalate) (PBT) are linear aliphatic-aromatic thermoplastic polyesters that are commercially available engineering plastics [3]. PET and PBT possess high mechanical strength, excellent processing characteristics and electrical properties, and good resistance to chemical attack for a broad range of applications [1, 2]. These characteristics make them suitable for use in fibers, films, textiles, bottles, molding components and as housing for home appliances [1-5]. The modern history of thermoplastic polyester goes back to 1929 with the pioneering research of Carothers, which was based on the aliphatic polyesters produced from aliphatic dicarboxylic acids and diols [6-10]. These aliphatic polyesters had low melting temperatures (Tm = 80-100°C), were sensitive to hydrolysis, and thus, were not suitable for commercial applications. Moreover, they could not compete against aliphatic polyamide fiber (nylons) which was discovered by Carothers at DuPont in 1930 [11]. In order to increase the melting temperature of polyester and get the thermomechanical properties obtained with nylons, it was required to stiffen the polyester chain by using rigid aromatic monomers instead of flexible aliphatic monomers [1]. The first aromatic polyester with a high melting temperature, PET, was prepared between ethylene glycol (EG) and terephthalic acid (TPA), by J.R. Whinfield and J.T. Dickson in the early 1940s [12-14]. Subsequently, in 1941, they developed the first aromatic polyester fibers called Terylene and which were initially manufactured by Imperial Chemical Industries (ICI). Around the same time Schlack had commenced the synthesis of polyester from 1,4-butanediol (BD) and TPA and patented it in 1949 [15]. Schlack found that PBT was less suitable to be used as a fiber material in contrast to PET, which was easily melt-spun into a fiber. PET was produced commercially in 1953 as fiber for the textile industry (Dacron) by DuPont using modified nylon technology, while PBT was commercialized by the Celanese Corporation in 1969, under the trade name Celanex [1]. DuPont’s polyester research rapidly led

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to a wide range of trademarked products such as a strong polyester film, called Mylar, however PET bottles came much later (in the 1970s). PET retains good mechanical properties, toughness and fatigue resistance up to 150-170°C, as a result of a high melting temperature (270°C) and rigid polymer chains. PET also exhibits a resistance to solvents, chemicals and hydrolytic degradation under the usual temperatures of application. The disadvantage of PET is a relatively slow rate of crystallization, which increases the price of the processing (extended cycle due to slow cooling) and requires nucleating agents (such as talc, magnesium oxide, zinc-stearate, calcium-silicate, etc.) for extrusion and injection-molding applications [2]. Conversely, PBT owes its success to its fast crystallization rate compared to PET, which makes it suitable for injection molding applications [3]. Due to the lower melting temperature of PBT compared to PET, PBT can be molded at a comparatively lower molding temperature. This permits fast processing and rapid production cycles. However, larger production volumes and lower prices make PET a serious competitor to PBT for many varied applications. Hence, PBT is often used as substitute for PET if a higher crystallization rate is necessary, but the maximum temperature of application for PBT is slightly lower as compared to PET at 120-140°C. PET has also an advantage over polyamides due to its much lower moisture absorption, allowing the material to maintain excellent dimensional stability through extremes of temperature and high humidity. Due to its low cost, superior aesthetic appearance and better handling, PET is usually preferred over polycarbonates. Nowadays, a broad range of pure and modified PET grades are available, and are found in many applications such as fibers, parts in the automotive and electric/electronic industries, architectural glazing and window films [2]. Due to the excellent mechanical and barrier (carbon dioxide permeation), PET is used for food and beverage packaging applications, even replacing traditional materials such as metals or glass. PET is also used as a recyclable polymer, and the markets for recycled PET (R-PET) are growing by the year [16, 17]. Commercial PET grades can be found under the trade names of Arnite (DSM), Crastine (DuPont),

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Dacron (Invista), Hostaphan Films (Mitsubishi), Impet (Ticona), Melinex (DuPont), Mylar (DuPont Teijin Films), Terylene (ICI), Tergal (Tergal Industries), Rynite (DuPont), Raditer (Radici Group), Petra (BASF), Pocan (Bayer). In considering the great versatility of PET, it is obvious time and again that the single most important factor is the significant contribution made by its chemical structure. This attribute, due to the foresight of PET’s inventors, has some noteworthy properties which affect its processing behavior and also the products obtained from it. Firstly, as previously mentioned, PET is a polymer that is crystallizable, but is distinguished by slow crystallization kinetics [2, 18]. Because of this combination of properties, PET can be obtained in various states of order, such as amorphous (transparent), “oriented mesomorphic” (transparent), spherulitically crystallized (opaque), and oriented crystallized (transparent) from strain-induced crystallization [1, 18]. Mechanical properties, storage stability, gas barrier, dye ability and transparency are the properties controlled by the nature of the degree of ordering in finished articles made from PET. Contrary to PBT which occurs almost in partially crystalline form, PET can also be processed into amorphous molded bodies with high transparency; on heating to 70-100°C this transparency is lost due to postcrystallization. The production of PET has increased rapidly and during the two last decades PET has become the material of choice in various applications [2]. Currently, the annual production of PET is close to 60 million tons, registering average annual growth of over 7% [19]. PET is mainly used to produce fibers in textile applications, and the use of PET represents 80% of the worldwide consumption of synthetic fibers. The extensive use of PET for food and beverage packaging and its high resistance to atmospheric and biological agents, and accumulation in the waste stream, are of serious concern for the environment. This issue of environmental pollution could be overcome by applying some of the many methods which have been developed, such as mechanical recycling, non-destructive methods, or chemical recycling, which represent some of the great advantages of PET [1, 2, 20, 21].

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2. SYNTHESIS OF PET 2.1. Conventional Two-Stage Polycondensation Reaction Two methods are used for the industrial manufacturing of PET. PET can generally be produced by reacting ethylenediol with an aromatic diester (dimethyl terephthalate – DMT) or diacid (terephthalic acid), in the presence of a polyesterification catalyst [5]. Although polycondensation to high conversions requires the stoichiometric balance of reacting groups, the industrial methods for manufacturing polyesters such as PET involve the initial use of excess ED, which is later removed and recycled in the process. The original industrial synthesis of PET is based on a relatively expensive two-step melt polymerization reaction between dimethyl terephthalate and excess ethylenediol of approximately 30-50%, in the presence of a catalyst. The polymerization of dimethylterephthalate (DMT) and ethylene glycol (EG) to PET proceeds in two steps: transesterification and polycondensation [1-3, 5, 22]. The first, the transesterification step, involves the conversion of the methyl ester groups of DMT into bis(2hydroxyethyl)terephthalate (bis-HET) and a small amount of oligomers are formed in this step of the reaction. This reaction is performed at atmospheric pressure, in an inert atmosphere to prevent oxidative side reactions, and in a temperature range from 150 to 210°C. During the first step methanol is distilled off and hydroxyethyl–terminated terephthalate oligomers are produced [19]. If only one terephthalate residue exists per molecule (x = 1 in Figure 1), the diester is usually called bis(2-hydroxyethyl)terephthalate). In reality, the transesterification step produces not only bis-HET, but also a decreasing number of 2hydroxyethyl-terminated oligomers containing 2,3,4,... terephthalate residues. The first step is finished when the methanol stops distilling off. In the second step of the method i.e., polycondensation, the temperature increases to 270-280°C (well above the melting temperature of PET) and a high vacuum is applied (10-50 Pa) [2]. bis-HET, and other low molecular weight oligomers, formed during the transesterification step,

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further react by polycondensation between the two hydroxyethyl end groups to result in a PET homopolymer. ED is formed and is eliminated as a by-product (Figure 1). PET with a degree of polymerization of approximately 100 is usually obtained. In order to favor the polycondensation, which is an equilibrium reaction, it is necessary to efficiently remove the excess of ED. The evaporation of ED from the reaction mixture is accelerated by the application of the high vacuum and by intensive mixing of the melt [2]. During the polycondensation the viscosity of the reaction mixture dramatically increases with the molecular weight of the polymer formed. As a result, the mass transfer of the volatiles out of the reaction mixture becomes a rate limiting process. Heating above 300°C leads to thermal degradation of the polymer, since the thermal stability of PET is limited at such high temperatures. Temperature control and the choice of catalyst are very important in performing polycondensation. The reaction time including the two steps of the reaction is long (between 5 and 10 h) and can be decreased by a high temperature and the addition of a metal catalyst [1]. In the production of PET, a combination of two different catalysts is used in each of the two polymerization phases. Catalysts such as acetates of manganese, zinc, calcium and magnesium are used in the first step of the reaction, while antimony trioxide is the most popular catalyst for the second stage of the reaction, although antimony pentoxide, tetrabutoxy titanate and germanium dioxide have also been used [3]. The monomers used for the direct polyesterification reaction for PET synthesis are terephthalic acid and ethylenediol [1-3]. The process based on TPA is very similar to the DMT-based route to PET, which has been previously described in this section. In the first step, TPA esterifies with an excess of ED, distilling water in order to shift the esterification equilibrium towards the product, i.e., hydroxyethyl end functionalized oligo-esters of PET (Figure 2). However, in the case of the TPA-based route, the reaction medium is heterogeneous because it is difficult to dissolve TPA in ED at the temperatures which are applied for the melt polymerization. The use of a small monomer feed ratio and a high reaction temperature are important to improve the solubility. Water formed during the reaction was collected

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to estimate the conversion of the reaction. The addition of a catalyst is not required since the acid group of TPA self-catalyzes the reaction. However, when the concentration of acid groups decreases, metal catalysts could be used to maintain the reaction rate [19].

Figure 1. Synthesis of PET from DMT as the starting reactant: (a) Formation of bisHET and other hydroxyethyl–terminated terephthalate oligomers by transesterification of DMT with ED and (b) polycondensation of bis-HET and hydroxyethyl–terminated oligomers resulting in PET.

One of the main advantages in production from DMT, compared with production from TPA, is that no environmentally aggressive chemicals are used (such as bromides or acetic acid), which eliminates the need for expensive, high corrosion resistant reaction vessels. Moreover, at the beginning of production of terephthalate-based polyesters, DMT was predominantly used for their synthesis because it was relatively easily purified in comparison with TPA. Yet, since the 1960s purified TPA has become available due to the development of new technologies and as a

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consequence, it has gained a lot of importance as a monomer in polyester synthesis [2]. Currently, the most frequently adopted method for the purification of TPA is commercialized by Amoco. In both methods for producing PET, the secondary reactions that occur during polycondensation can alter the stoichiometric ratio, and thus terminate polycondensation or confer undesirable properties on the end product [19, 23]. Diethylene glycol can be generated during the polycondensation reaction and incorporated in the polymer. Other important secondary reactions include the dehydration of ethylene glycol to form acetaldehyde, ester pyrolysis which in the case of PET generates carboxyl groups and olefins. Commercial PET prepared by polycondensation contains a small amount of low molecular weight cycle structures with high molecular weight linear macromolecules. The cyclic oligomers can create problems during the processing of PET but the removal of cyclic oligomers is not currently practiced in the industry.

Figure 2. Synthesis of PET from TPA as the starting reactant.

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At the industrial level, PET can be made by using both batch and continuous processes [3]. Early commercial processes were DMT-based batch processes, which were typically run in at least two reactors. In the batch processes, after the first step is completed (when no more methanol is distilled off), the reaction mixture transfers to the second reactor for the polycondensation step, where a vacuum is applied (< 1 mbar) at increased temperatures (between 270°C and 280°C), well above the melting temperature of PET, in order to strip off the excess ED. Nowadays, batch processes are usually replaced by continuous processes, which involve a series of reactors in which the pressure is gradually reduced. Finally, so-called finishing reactors, which create a high surface area combined with a deep vacuum, are applied to increase the molecular weight even further. Batch processes are mainly used to produce specialty PET grades.

3. SOLID STATE POLYMERIZATION Solid state post-polycondensation is widely used in order to increase the average molecular weight of polyesters [2]. The process is particularly important for achieving large values of Mn in the case of crystalline condensation polymers of high melting temperature, when thermal degradation takes place in the melt. PET of a very high molecular weight (Mn above 100000 g/mol) can be obtained by the solid state postpolymerization of melt-synthesized material. PET has both hydroxyl and carboxylic acid groups at the chain ends, as well as the active residual catalyst and as a result PET is still capable of reacting, and the molecular weight may be increased by solid state polymerization. Solid state polymerization is carried out around 220-230°C, i.e., at a temperature between the glass transition (Tg) and melting temperature of PET, for 1030 h in order to enhance the polycondensation reaction. PET obtained by solid state polymerization has a higher melting temperature and higher crystallinity compared to conventional PET [19]. To promote the removal of low-molecular weight products and thus increase the molecular weight

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of polymer chains, the polymer chips or pellets are heated in a vacuum or in a steam of inert gas at a temperature approximately 20-50°C below the melting temperature. PET of higher molecular weight can be used for articles made by blow molding and for shock absorber elements.

4. PHYSICAL PROPERTIES OF PET 4.1. Thermal and Mechanical Properties of PET Depending on its processing and thermal conditions, PET may exist both as amorphous and as semicrystalline polymer. In its purest form, PET is an amorphous glass-like material. Under the influence of modifying additives or by heat treatment of the polymer melt, PET develops crystallinity. The degree and quality of crystallinity have long been recognized as having a dominant influence upon the properties of PET and they are highly dependent on processing conditions like processing temperature, cooling rate, stretching process etc. At glass transition temperature of 72°C, the semicrystalline PET changes from a rigid glasslike state into a rubbery elastic form. The polymer molecular chains in the rubbery state can be stretched and aligned in either one direction to form fibers, or in two directions to form films and bottles. Only a limited amount of crystallization can occur during cooling after injection molding of PET due to its rather high transition temperature. However, when the PET melt is cooled quickly, while still held in the stretched state, the chains are frozen, wherein their orientation remains intact. The resulting material is extremely tough and suitable for the fabrication of bottles. At temperatures above 72°C, the stretched PET slowly crystallizes and the material gradually starts to become opaque, more rigid and less flexible. This so-called crystalline PET or CPET is capable of withstanding higher temperatures and can be used for trays and containers capable of withstanding moderate oven temperatures. The crystallization of PET has been widely investigated. Jabarin [24, 25] studied the crystallization behavior of PET under isothermal and

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dynamic cooling conditions by differential scanning calorimetry (DSC), as a function of molecular weight, polycondensation catalyst system and polymerization conditions. Kinetic parameters of crystallization were calculated for both cases by utilizing Avrami-type expressions. The analysis of the crystallization kinetic data indicated that the same mechanism was operative under isothermal and dynamic cooling crystallization conditions. The dynamic cooling crystallization method could establish the minimum cooling rate required to produce PET without detectable crystallinity. The results showed that the cooling requirements for producing noncrystalline PET were dependent on the molecular weight of the resin, but more importantly, were dependent on the catalyst system used in the polycondensation step. These results were in good agreement with the results obtained under isothermal conditions. The fabrication of PET into fibers, films and containers usually involves molecular orientation caused by molecular strain, which may lead to stress- or strain-induced crystallization (SIC). Jabarin [26] also investigated the SIC of PET by the methods of birefringence, density, thermal analysis, light scattering and wide-angle X-ray scattering (WAXS). The results indicated that the SIC of annealed, stretched PET could proceed in three different paths depending on the residual degree of orientation. At a low degree of residual orientation, as indicated by the birefringence value, annealing of stretched PET led only to molecular relaxation, resulting in a decrease of birefringence. At intermediate orientation levels, annealing caused an initial decrease in birefringence followed by a gradual increase and finally a leveling off of birefringence after a fairly long period of time. At higher orientation levels, annealing caused a rapid increase in birefringence before leveling off. Alves et al. [27] determined the relaxation times of the cooperative conformational rearrangements of the amorphous phase in semicrystalline PET and compared them with those calculated in amorphous PET. The thermograms measured in the samples with low crystallinity clearly showed the existence of two amorphous phases with different conformational mobility, Phase I and Phase II. Phase I contained polymer chains with a mobility similar to that in the purely amorphous polymer,

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while Phase II showed a much more restricted mobility, probably corresponding to conformational changes within the intraspherulitic regions. The model simulation allowed to determine the temperature dependence of Phase II relaxation times, which were independent from the crystallinity fraction in the sample and around two decades longer than those of the amorphous polymer at the same temperature. The crystalline phase may contribute highly anisotropic behavior to the bulk polymer due to the orientation of chains which accompanies preferential alignment of the crystallites themselves. Such preferential alignment may be accomplished through careful processing operations. The noncrystalline phase is also often anisotropic, prompting investigators to decompose this component into isotropic and oriented constituents [28]. In 1994, Fu et al. [29] proposed that, beside the crystalline phase and the randomly oriented amorphous phase, there is an intermediate phase in semicrystalline PET fibers. The chains of the intermediate phase are partially oriented with respect to the fiber axis. The intermediate phase is mainly present between the fibrils, whereas the crystallites are separated in the fiber direction mainly by a more random amorphous phase. Recognition of this third phase permitted the development of a simple model for structure-insensitive properties such as modulus, density and orientation. PET is a typical example of polymer with complex melting behavior. On heating, it can show three main fusion endotherms with sizes and positions related to its thermal history. The melting behavior of PET, crystallized and/or annealed under elevated pressure, was studied using DSC at atmospheric pressure. The melting point of the sample crystallized from the melt by slow cooling under elevated pressure was lower than that of the sample crystallized at atmospheric pressure, although the former sample had a slightly thicker lamella than the latter one. This implied that the fold surface energy was much larger in the elevated pressure crystallized sample. The atmospheric pressure melting point increased greatly by annealing under elevated pressure. In particular, a remarkable increase in melting point was observed at the early stage of the annealing for the elevated pressure crystallized sample, which may be due mainly to

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the decrease in fold surface energy in addition to lamella thickening. An electron micrograph of the sample annealed for a long period after the melt crystallization under elevated pressure showed a morphology of a band structure composed of extended-chain-like crystal [30]. Medellin-Rodriguez et al. [31] studied the melting behavior of isothermally crystallized PET using linear heating by DSC. Variables such as crystallization temperature, crystallization time, heating rate and average molecular weight were the main focus of the study. On the basis of several experimental techniques, a correlation of the melting behavior of PET with the amount of secondary crystallization was found to exist. It was observed that the triple melting of PET was a function of programmable DSC variables such as crystallization temperature, crystallization time and heating rate. However, in testing the hypothesis that there is a correlation between melting endotherms and secondary crystallization inside spherulites, it was found necessary to use a DSC-independent variable in order to enhance the observed effects. Therefore, on the basis of a crystallization model that involved secondary branching along the edges of parent lamellar structures, it was speculated that an increase in the average molecular weight could affect the triple melting of PET due to an increase of rejected portions of the macromolecules. It was found that the second melting endotherm increased, apparently, at the expense of the third one as the average molecular weight was increased. The second melting endotherm was also found to correlate proportionally with the amount of secondary crystallization inside spherulites. The results support a model of crystallization which basically consists of parent crystals and at least one population of secondary, probably metastable, crystals. This latter structural component must involve excluded portions of the macromolecules that did not crystallize during the isothermal crystallization period of the parent crystals. An increase of molecular weight gives rise to a higher entanglement density which in turn increases the fraction of initially rejected chain sections and therefore the amount of secondary crystallization. Reorganization of semicrystalline polymers on heating is a fast process. For PET heating rates of several thousand Kelvin per second are

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needed to prevent reorganization of unstable crystals. Utilizing a thin film vacuum gauge as a fast calorimeter, Minakov et al. [32] extended the scanning rate range of commercial DSC’s (μK/s to 10 K/s) to rates as high as 10 000 K/s on heating and cooling. Because of the fast equilibration time, isothermal experiments can be performed after scanning at several thousand Kelvin per second. The dead time after such a quench is in the order of 10 ms and the time resolution is in the order of milliseconds. These ultra-fast calorimeters allow the study of the kinetics of extremely fast processes in semicrystalline polymers like reorganization. For the PET crystallized at 130°C, reorganization needed less than 40 ms between 150°C and 200°C. Isothermal reorganization at 223°C was about two orders of magnitude faster than isothermal crystallization from the isotropic melt at the same temperature. The melt memory for the remaining structures needed for reorganization is removed 25°C above the equilibrium melting temperature of PET. In isothermally crystallized samples, a small endotherm is often observed during heating about 10-30°C above the crystallization temperature. Song [33] used the differential of the reversing heat capacity and nonreversing heat flow signals were used to analyze the behavior of the glass transition and the low temperature melting endotherm. With increasing annealing time, the increment of the heat capacity at the glasstransition temperature decreased and the increment of the heat capacity at the annealing temperature increased. It was suggested that the origin of the low temperature melting endotherm mainly resulted from the transition of the rigid amorphous fraction for the PET used. The glasslike transition of the rigid amorphous fraction occurred between the glass transition and melting. Recently, Di Lorenzo et al. [34] have presented the quantitative thermal analysis of PET including details about the coupling between the crystal and amorphous fractions. Isothermal crystallization at 190°C followed by cooling to room temperature provides a three-phase structure composed of a mobile amorphous, a crystalline and a rigid amorphous fraction. A close connection between multiple melting and devitrification of the rigid amorphous fraction in PET is revealed by conventional (DSC) and temperature-modulated calorimetry (TMDSC). Rearrangements of

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PET crystals at high temperatures involve recrystallization/ annealing/crystal perfection following partial melting, which can occur only if the amorphous chain portions coupled to the crystal/melt phase boundary have sufficient mobility. Such mobility can be achieved above the glass transition of the amorphous chain segments coupled with the justmelted crystals. Combined analysis of the reversing heat capacity monitored during quasi isothermal modulation with the thermal properties of the resulting structure suggested that annealing at temperatures below 210°C did not result in considerable reorganization and perfection of the crystal phase. The temperature of 210°C seems to be the point at which the rigid amorphous fraction coupled with the crystal phase attains sufficient mobility to allow development of crystals with increased perfection and thus higher thermal stability. In this study the combination of DSC, TMDSC and fast scanning calorimetry (FSC) has proven to be a powerful calorimetric technique to study not only crystals and amorphous polymers but also the multiphase structures with intermediate, coupled nanophases. The dynamic mechanical properties of PET have been studied by a number of authors [31, 35-38]. PET exhibits at least two mechanical loss peaks in the temperature range -180 to 200°C. One peak occurs at about 40°C ( dispersion) and the other is near 100°C ( dispersion). The  dispersion has been reported to be related to the degree of crystallinity, while the  dispersion of PET has been associated with the glass transition. In their early work, Illers and Breuer [35] studied unoriented films of the PET samples in which the position of the  peak shifted to higher temperatures for degree of crystallinity up to 30%. At higher degree of crystallinity, the position of this peak shifted toward lower temperatures. The variations of the  loss modulus peak indicated the onset of segmental motion within the amorphous regions with increasing degree of crystallinity. This behavior has been attributed to the effect of crystal size on the amorphous regions. At low to medium degree of crystallinity, there would be many small crystallites which would act like crosslinks and restrict the motion of segments in the amorphous regions, while at high degree of crystallinity, the crystallites would be larger and fewer in number, which would allow the segments in the amorphous regions more

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freedom. However, in all samples a very broad  peak occurs at approximately 65°C (at a frequency of 1 Hz). Increasing crystallinity causes a slight shift of the  peak to higher temperatures in the first stage, followed by a lowering for the higher degree of crystallinity. This behavior is similar to that described above the  peak. The relationship between crystallinity and crystal size parameters and the position of the  dispersion peak, which was developed by Dumbleton and Murayama [39] for polyamides, has been successfully applied to PET. Matsuo and Ishimuro [37] have reported a detailed study on effects of moisture and solid phase polymerization on melt viscoelastic properties of PET. They found a drying condition and an experimental procedure which did not induce any variation of either molecular weight or molecular weight distribution in samples during periods of sample preparation and a short time run of viscoelastic measurements. For a long time run, η’ and G’ increased with time and especially the increase of the latter was remarkable. The similar behavior was observed for nylon-66 but not for PBT. It is suggested that highly branched and/or cross-linked substances were produced in the samples besides linearly polymerized ones. Thermal and micromechanical properties of amorphous and semicrystalline PET are found to be dependent on ageing effects occurring by thermal treatments below Tg. These effects are studied using two complementary techniques: DSC and dynamic mechanic analysis (DMA) at low frequencies (1-10-4 Hz). Experimental results can be described through a physical model assuming diffusion and annihilation of “quasipoint defects”. The distribution in the mobility of these defects is able to take into account all the phenomena observed after physical ageing, in particular the effects of low-temperature ageing. The ageing effects in the semicrystalline material cannot be deduced from those observed for the amorphous one by a single two-phase rule. Changes in the correlation factor of molecular movements and changes in the distribution of defects are necessary to describe all the observed effects, putting in evidence the role of the crystallites to reduce the segmental mobility [38]. Thermal analysis techniques such as DSC and DMA have proved to be suitable techniques for the quality assessment of recycled PET. Thermo-

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mechanical degradation causes deep changes in the microstructure and properties of PET under recycling. A dramatic increase in the degree of crystallinity is observed during consecutive extrusion cycles, leading to significant embrittlement of the recycled material and the complete loss of its plastic deformation properties after four reprocessing cycles. Chain scission reactions induced under thermo-mechanical degradation may result in a heterogeneous distribution of polymeric chain lengths in the melt state, altering the subsequent amorphous and crystalline microstructure in recycled PET after chain rearrangement during cooling [40]. Also, the thermal properties (glass transition, melting point and crystallinity) and mechanical properties (Young’s modulus, elongation at break and impact strength) of post-consumer PET bottles were compared with those of the virgin resin. There are two types of scraps of recycled PET: one arising from homogeneous deposits of bottles and the other of heterogeneous deposits soiled by contaminants such as poly(vinyl chloride), PVC, and adhesives. The presence of contaminants and residual moisture coming in the shape of scraps facilitates the crystallization of recycled PET compared to virgin PET and induces cleavages of chains during the melt processing. This leads to a reduction in intrinsic viscosity and consequently in molecular weight, and these decreases are more significant when the recycled resin is soiled. Virgin PET exhibited a ductile behavior (>200% of elongation at break), whereas post-consumer PET bottles exhibited a brittle one (500 h), the crystallinity only increases to about 5%. The implications of these results are discussed in terms of assessing the degradation and hydrolysis of the polymer through physical and mechanical analysis in relation to actual polymer breakdown [73]. The hydrolysis of PET (Mw = 27 kg/mol) in boiling water has been studied by steric exclusion chromatography. It appears that both the average molar mass and the polydispersity index decrease in a pseudohyperbolic way and tend towards asymptotic values, respectively Mn∞→2 kg/mol and I∞→1.50. This behavior indicates that hydrolysis is homogeneous at dimension scales higher than lamellae thickness (Lc).

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Hydrolysis occurs in the amorphous phase and at chain folds at the crystal surface, whereas crystals are unaffected due to their impermeability to water. The chain length decreases to tend, thus, towards Lc. Embrittlement is expected to occur when the entanglement network is destroyed in the amorphous phase. However, despite the above mentioned, PET hydrolysis can be considered as a random chain scission process in kinetic studies. [74]. Hosseini et al. [67] studied the hydrolytic degradation which occurred at the drying of PET before processing and the loss of weight and mechanical properties in textile materials during washing. Hydrolytic conditions were used to expose fiber-grade PET chips in water at 85°C for different periods of time. Solution viscometry and end-group analysis were used as the main methods for determining the extent of degradation. The experimental results show that PET is susceptible to hydrolysis. Also, as the time of retention in hydrolytic condition increased, the molecular weight decreased, while the rate of chain cleavage decreased to some extent, at which point there was no more sensible degradation. The effects of PET resin moisture content and temperature exposure have been investigated in terms of material changes resulting from the injection molding process. Two resins with initial carboxyl contents of 10 µeq/g PET and 20 µeq/g PET have been analyzed. Preforms processed at different resin moisture contents and processing temperatures of 280, 290 and 300 °C were evaluated in terms of carboxyl end-group concentration using a titration method. Mathematical models describing the relationships of carboxyl end-group concentration and intrinsic viscosity to the processing conditions were generated from the experimental data. Carboxyl end-groups formed were compared for both resins and shown to be dependent on initial carboxyl content in the resin. Reducing the initial carboxyl content in the resin was shown to increase its hydrolytic stability. The hydrolytic effect on the overall molecular weight drop was separated from the thermal/thermal-oxidative degradation and shown to be dependent on both the processing temperature and the resin moisture content [75].

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Thermal and hydrolytic degradation of PET (Partly Oriented Yarn (POY), Fully Drawn Yarn (FDY) and granule) at temperatures above and below Tg were carried out using a water bath and an electrical oven to illustrate the extent of hydrolytic degradation of PET caused by warm water and also to separate the influence of moisture, temperature, and orientation in the degradation process. The results obtained from different analysis, including determination of the moisture content, viscometric analysis, carboxylic end group titration, and X-ray diffraction, showed that the major portion of degradation is carried out by both moisture and heat mutually. Degradation at lower temperatures from Tg was less prominent and was increased noticeably above Tg. Crystallinity played a significant role in preventing hydrolytic degradation as the extent of degradation was increased from FDY to granule to POY. X-ray diffraction analysis showed that crystallinity was increased from POY to granule to FDY [76]. The kinetic studies of polymer microwave depolymerization have received much attention because of the non-thermal effects that are observed. The facts that the polymer depolymerization could be accelerated by microwave irradiation, and that the depolymerization mechanism was a combination of regular and random chain scission have generally been accepted. The kinetic processes of PET hydrolytic depolymerization under microwave irradiation were studied in detail. The microwave depolymerization reaction of PET occurred simultaneously in the interior and on the exterior of PET. More than one kinetic process occurred because of the non-thermal effects of the microwave irradiation. The chain scission during PET depolymerization under microwave irradiation involved regular, then random and then again regular processes. Microwave irradiation made PET depolymerize with a higher rate of random chain scission events, resulting in the observed non-thermal effects of microwave irradiation. The reaction rate of PET depolymerization increased more quickly as the temperature rose. In this experiment, the depolymerization process was divided into two stages, that is, the stage before random chain scission and the stage after random chain scission. The corresponding activation energies were 142 and 378 kJ/mol,

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respectively, which were much higher than that of the thermal depolymerization of heating [77].

5.3. Thermal and Thermo-Oxidative Degradation of PET The mechanism of PET thermal degradation has been the subject of several studies [70, 72, 78]. It is believed that the thermal degradation of PET occurs through random chain scission of the ester linkages, wherein the methylene group represents the most likely point at which the degradation begins. The quantitative estimation of the thermal degradation extent has been done in terms of products formed such as acetaldehyde, rate of change of molecular weight and rate of change in the concentration of end groups. The reactions of thermo-oxidative degradation are additionally complicated by the participation of oxygen. It has been suggested that the process starts with the formation of a hydroperoxide at the methylene group, followed by homolytic chain scission. It has been concluded that hydroperoxide groups not only play an important role in inducing the thermal and photo-oxidative degradation of PET, but are also important intermediates in such reactions [79]. When PET is maintained in a molten state under nitrogen at about 280°C, it decomposes slowly to gaseous and to solid, low molecular weight products. The functional groups react with one another and the polymer becomes colored. The composition of gaseous products varies with temperature, whereby acetaldehyde is always the major product. A white “dust”, formed during melt spinning of PET, consists primarily of terephthalic acid and acidic oligomers. Cyclic oligomers (mainly the trimer) are also formed during the thermal degradation of PET. A kind of equilibrium between the cyclic oligomers and the polymer seems to exist. After extracting these oligomers from PET and heating the extracted polymer for about one hour at 280°C, the same amount of cyclic oligomer is formed again. During thermal degradation of PET, the concentration of hydroxyl groups tends to fall and the carboxyl group content of the

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polymer increases. Anhydride groups can be formed and they begin to accumulate when most of the hydroxyl end groups have been consumed. The anhydride groups are formed by the reaction of vinyl end groups with carboxyl groups and by dehydration of two carboxyl groups [72]. Discoloration is a serious problem in the production of PET since the consumer of white polyester goods demands a brilliant white color. As PET degrades, its color changes first to yellow, then to brown, and finally to black. Discoloration has assumed the formation of polyenaldehydes from acetaldehyde. On the other hand, Zimmermann and Kim [70] showed that unsaturated, color forming molecules are formed from polyvinyl esters. Edge et al. [80] suggested that the colored species in melt degraded PET arise from hydroxylation of the terephthalate ring, formation of unsaturated-ester and quinonoid species. Jabarin and Lofgren [78] studied the kinetics of thermal and thermaloxidative degradation of PET as a function of melt temperature, melt residence time, melt environment and drying environment. Rates of thermal and thermal-oxidative degradation were measured in terms of weight loss of volatile degradation products, decreasing inherent viscosity and increasing of the carboxyl end groups concentration. Thermaloxidative degradation was also investigated by DSC. The DSC results showed that thermal-oxidative degradation of PET is an exothermic reaction, with an apparent activation energy of 117 kJ/mol. The authors concluded that the time and temperature of melting play major roles in determining the extent of PET degradation as exemplified by weight loss, color formation, final inherent viscosity and carboxyl end group concentration. At equivalent experimental (time and temperature) conditions, significantly more degradation occurs when a material is melted in an air rather than in a nitrogen environment. When melting in air, the amount of degradation increases rapidly with increasing temperatures, especially at temperatures above 300°C. Drying conditions of PET pellets (temperature, time and atmosphere) can affect weight loss during melting as well as resultant inherent viscosity and carboxyl end group concentration. The amount of degradation in the melt is larger (at a given melt temperature and time) when the material was previously dried in air

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atmosphere rather than in a vacuum or inert atmosphere. The amount of degradation of the PET samples dried in air followed by vacuum drying is the same as for the samples dried in air atmosphere alone. Botelho et al. [81] conducted a comparative study of the thermooxidative degradation of PET and PBT. Degradation of the polymer films and model compounds, ethylene dibenzoate (EDB) and butylene dibenzoate (BDB), was carried out in an oxygen atmosphere at 160°C. On the basis of the compounds identified by GC/MS (gas chromatographymass spectrometry), a mechanism is proposed for the degradation of the model compounds that involves the oxidation at the -methylene carbon with formation of unstable peroxides and carboxylic acids. From the studies performed under nitrogen at 160°C, it could be concluded that benzoic acid and esters are products of the thermal degradation, while benzoic and aliphatic acids, anhydride and alcohols are due to thermooxidative degradation. In contrast to the thermo-oxidative degradation of other polymers, for PET and PBT, especially at the beginning, thermal degradation plays an important role. The results clearly showed that PET is more stable towards degradation than PBT. 1 H NMR and MALDI-TOF MS measurements were used to study the thermo-mechanical and thermo-oxidative degradation mechanisms of bottle-grade PET (btg-PET). In the thermo-oxidative degradation, the concentration of low molar mass compounds increased with time and the main products were cyclic and linear di-acid oligomers. In the thermomechanical degradation, the main-chain scission reactions affected the stability of the cyclic oligomers. One of the most important bottle-grade PET co-monomers is diethylene glycol (DEG), which is a “reactive” site in the thermal degradation of btg-PET. The DEG co-monomer was shown to be the precursor to color changes in btg-PET, owing to the attack by molecular oxygen on the methylene protons adjacent to the ether oxygen atoms of DEG. This behavior was observed in the thermo-oxidative degradation process in which the degradation of DEG caused the release of hydroxyl radicals in the polymeric matrix, thereby producing mono- and di-hydroxyl substituted species. This was also observed in the thermomechanical degradation process [82].

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De A. Freire et al. [83] investigated two PET samples under severe heating conditions and analyzed oligomers and volatile substances as potential migrants into foods. The samples were tested for migration into water, 3% acetic acid and 15% ethanol solution for 1 hour at 95°C. Overall migration and the specific migration of TPA, EG and DEG were all very low. The plastics were heated at 150°C, 260°C and 270°C, for 5 minutes, 30 minutes and 60 minutes. Oligomer analysis by LC/MS (liquid chromatography-mass spectrometry) showed that the concentration of the second series alicyclic oligomers increased up to 15-fold on heating whereas the major oligomer fraction, the cyclic trimer, tetramer, pentamer and hexamer showed only minor concentration changes with heating. Volatiles evolved by the samples were trapped on a Tenax trap and identified by GC/MS. They were few in number and low in concentration and none merited migration tests. It is concluded that even when tested up to melting point, PET plastics of this type have good temperature stability and are well suited for high-temperature food contact applications. The thermal oxidative degradation kinetics of the PET copolymers modified with poly(lactic acid) (PLA) were investigated by TGA. The thermal properties of the modified products were also determined by DSC technique. PETW (P100) obtained from postconsumer water bottles was modified with a low-molecular-weight PLA. The PET/PLA weight ratio was 90/10 (P90) and 50/50 (P50) in the modified samples. The thermal oxidative degradation kinetics of the modified samples was compared with those of the P100 sample. The segmented block and/or random copolymer structure of the modified samples formed by a transesterification reaction between the PLA and PET units in solution and the length of the aliphatic and aromatic blocks were found to have a great effect on the degradation behavior. On the basis of the results of the degradation kinetics determined by Kissinger method, the degradation rate of the samples decreased in the order of P50 > P90 > P100, depending on the amount of PLA in the copolymer structure. However, the degradation activation energies of the samples decreased in the order of P100 > P90 > P50. The authors concluded that the degradation rate and mechanism were affected

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significantly by the incorporation of PLA into the copolymer structure [84]. Yuan et al. [85] studied thermal stability of the PET nanocomposite based on the fibrous silicates (PT) organically modified by water-soluble polyvinylpyrrolidone (PVP). The thermal degradation behavior of PET and PET/PT nanocomposite was investigated by thermogravimetric analysis (TGA) under non-isothermal conditions at various heating rates in air and nitrogen, respectively. The apparent activation energies of the samples were evaluated by Kissinger and Flynn-Wall-Ozawa method. It is suggested that, during thermal decomposition in nitrogen, the clay as a mass-transport protective barrier can slow down degradation of polymer, but the catalytic effect of metal derivatives in clays may accelerate the decomposition behavior of PET. The combination of these two effects determines the final thermal stability of nanocomposite. However, in air atmosphere, the oxidative thermal stability of PET/PT nanocomposite was obviously superior to that of pure PET.

6. PROCESSING OF PET 6.1. PET Bottles Production of PET bottles requires injection molding of preforms, i.e., PET granules, and subsequent stretching and blowing of these into bottles. The combination of the two operations is called injection-stretch-blow molding and was developed by Wyeth and Roseveare at Du Pont [86]. Both operations can be combined in one machine (the single-stage process) or in two (the two-stage process). In the single-stage process, the temperature remains constantly high for the whole process of injection molding and blow molding, the material should continue being in an elastic form. This method saves a lot of energy as the material has to be heated only once, that is when it is injected into the cavity to produce the preform. The single-stage method is commonly used in small or medium PET production companies.

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In the two-stage process, the preform is injected into shape on the first machine and then it is reheated and blown on the second machine. The two-stage method is not very efficient as the heat lost is considered as a loss of energy, but this machines are fully automated and about 200% more efficient than the one-step machine [87]. When PET is stretched, it exhibits strain-hardening properties, which are temperature and strain-rate dependent. This provides a self-leveling effect on the stretching preform, which is important for forming a bottle of uniform wall thickness. It is also well known that this behavior is temperature and strain-rate dependent. At any given strain, increasing the temperature reduces the strain hardening properties and vice versa. In contrast, at a fixed temperature, increasing the strain rate causes the polymer to strain-harden, above all at large strain values. To take advantage of the strain-hardening properties of PET, the operating conditions during blow molding, as well as the bottle and preform designs, must be considered in order to achieve bottles of desirable quality. Pham et al. [88] characterized experimentally two grades of PET using biaxial tests. A visco-hyperelastic model is used to describe the stretching behavior for the polymer. A biaxial characterization method is employed to determine the model parameters using a robust nonlinear curve-fitting program. This model can represent adequately well the stretching behavior of PET. Based on this model, the membrane finite element formulation is developed to simulate the stretch blow molding process. Two bottles of different designs, produced based on the single-stage injection blow molding process, are used to validate the model. Good agreement with the bottle thickness profile is observed. Simulations of various stretch blow and blow moldings of axisymmetric PET bottles have been carried out using ABAQUS software. A creep constitutive model with material data developed for a thermoforming process was used in the finite element analysis. Simulations using shell and solid elements were compared with experimental moldings. The creep material model, when combined with solid elements and a very high coefficient of friction, provided the best predictions for bottle side wall thickness, strains, blowing pressure, and general material movement.

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It was found that the predicted wall thickness distribution of the material in an injection blow molded bottle agreed well with the values obtained using commercial process conditions [89]. Also, Jang et al. [90] proposed a non-isothermal finite element (FE) model for the injection stretch-blow molding (ISBM) process of PET bottles. The constitutive behavior of PET is modeled by the physically based Buckley glass-rubber model in form of UMAT in ABAQUS. The heat transfer between the stretch rod, the preform and the mold was modeled. Extensive FE simulations were carried out to model ISBM of a 20 g - 330 ml bottle made in plant tests. Comparisons of numerical results with the measurements demonstrated that the model could satisfactorily predict the bottle thickness and material distributions. Significant nonlinear differentials were found in strain, temperature and temperature reduction rate in both bottle thickness and length direction during the process.

6.2. PET Fibers Textile yarns, used for the production of finished fabrics, are of two categories: filament yarns and spun yarns. Filament yarns are made of continuous filaments which are kilometers long, while spun yarns are composed of cut staple fibers which are a few centimeters long and are twisted or spun to hold them together in the form of strands. For filament and fiber applications, PET with inherent viscosity in the range 0.4 to 0.98 dL/g is used [91]. The most effective approach for the production of PET fibers with improved mechanical properties so far is the utilization of high molecular weight polymers, which can be obtained by solid-state polymerization. Due to extremely high viscosity, fiber formation of high molecular weight PET can be accomplished by solution spinning or spinning with a plasticizer [42]. However, the melt process, without the added complexities of chemical reaction and/or mass transfer, is the simplest, conceptually and economically, for producing fibers suitable for textile and industrial end uses. In the melt spinning technique, the molten polymer is extruded

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through a die, called the spinneret, which has multiple capillaries to form a slander and cylindrical jet of molten polymer. After passing a certain distance (frost line) from the spinneret, the molten polymer jet becomes solidified. Lin et al. [92] studied fiber melt spinning of PET via modification of threadline dynamics. Several techniques were implemented in the highspeed spinning process for the judicious control of threadline dynamics. This included a thermal conditioning zone (TCZ) for controlling the threadline temperature profile and a hydraulic drag bath (HDB) for controlling the threadline spinning stress. Through controlled threadline dynamics, key factors affecting the structure development such as temperature, tensile stress and crystallization time, were manipulated to favor formation of a highly oriented and transversely uniform structure in the spun fibers. The attenuation profile of the threadline is observed to be dependent of TCZ temperature, residence time in the HDB, temperature of the HDB, and take-up speed. It is believed that for the melt spinning process with the TCZ and the HDB, the threadline dynamics is changed from the one controlled by inertia and air drag forces to the one controlled by the imposed hydraulic drag. Bicomponent melt spinning technology is mainly used to make the new fibers. Wool is a natural bicomponent fiber, and the side-by-side bicomponent fibers were developed to imitate the crimp property of wool. Hwan Oh [93] studied the crimp contraction and the thermal shrinkage in the bicomponent fiber consisting of two different PET. Regular PET and modified PET were selected to make a latent crimp yarn. The modified PET was synthesized to increase thermal shrinkage. The crimp contraction was mainly dependent on drawing conditions such as draw ratio, heat-set temperature, and drawing temperature. Difference in shrinkage between the PET and the modified PET caused the self-crimping of bicomponent fibers. Although changing the heat-set temperature and the drawing temperature could not affect dimensional change, the crimp contraction varied with those variables. As the heat-set temperature and the drawing temperature decreased, the crimp contraction increased. Difference in elongation also affected the crimp contraction in the effect of draw ratio.

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When the modified PET with neopentyl group was used for highly shrinkable part, the crimp contraction was greater in comparison with modified PET with dimethyl isophthalate.

6.3. PET Sheets and Films Both, PET sheets (thicker than 180 μm) and films can be produced in a melt polymerization line or by a secondary processing step from preform. The unoriented PET sheets are made by extruding an appropriate molecular weight PET melt through a slot die and quenching over a chill roller. The amorphous, transparent sheets are used mainly in thermoforming, where a sheet of polymer (50-1200 μm), is reheated (softened) and then deformed by vacuum into a mold where it cools. The products include clear cups, trays and blister packages for pharmaceutical tablets [87]. A highly crystalline, but opaque PET trays are produced from a polymer with very high inherent viscosity (1 dL/g) and high melting point (260°C). Due to the high crystallinity, the trays can be heated with food in a microwave oven and can withstand temperatures up to 220°C. For their production, the transparent thermoformed PET article is transferred to a heated mold to crystallize. Spherulite crystallization takes place because the article formed by the initial thermoforming did not undergo much strain-induced crystallization and it may be promoted by addition of nucleation additives incorporated to promote fast thermal crystallization [87]. Biaxially oriented films are much thinner (5-180 μm) than the unoriented sheets. Their applications include video and magnetic tapes, floppy disks, photographic products, and capacitors. A high degree of purity is required, and clean room conditions similar to those used to manufacture electronic or medical components are often applied. A sequential drawing process for the production of biaxially oriented films consists of four steps: melt extrusion through a slot die and quenching to form an amorphous precursor film, on-line drawing in the extrusion

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direction, on-line drawing in the transverse direction and on-line heatsetting. A thick biaxially oriented PET films (75-180 μm) are used in various photographic applications and are produced by extrusion and chill casting. Biaxially oriented films can be produced also by the tubular film blowing process. The melt is extruded in the form of a tube from an annular die and is inflated by air pressure. Kang et al. [94] reported on double bubble-tubular film extrusion of PET. In the second bubble, the film is passed through a hot air where it undergoes a significant increase in inflation level. The film produced by the first bubble is largely amorphous/glassy (5 to 7% of crystallinity), while that produced in the second bubble can have significant levels of crystallinity (7 to 45%) dependent upon process conditions. Crystallinity notably increases with machine direction draw down ratio. The crystallinity levels are similar to those of biaxially stretched films. Subsequent annealing increases and perfects crystallinity. The surface roughness of double bubble tubular films is similar to that of glassy polymers produced from single bubble operations and is much smaller than for polyolefins. Biaxiality of orientation increases with the second blowup ratio.

7. RECYCLING OF PET WASTE As already stated, due to its excellent mechanical properties and chemical resistance, today PET is an irreplaceable material with numerous applications. On the other hand, a continuous increasing consumption along with non-biodegradability of poly(ethylene terephthalate) waste (PETW) has led to serious problem of environmental and economic importance, wherein management of PETW has become a global social issue. It is considered that recycling processes are the best way to economically reduce PETW [95]. Recycling of PET represents one of the most successful and widespread examples of polymer recycling. This material is fully recyclable and may be used for manufacturing new

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products in many industrial fields such as packaging for detergents, cosmetics, carpets, foils, car parts or back into PET bottles [96]. The various methods of PET recycling include primary, secondary (mechanical), tertiary (chemical) and quaternary methods [97]. Primary recycling refers to the in-plant recycling of the scrap material of controlled history, that is, reuse of products in their original structure. This method is easy to perform and low cost, but dealing only with clean, uncontaminated, single-type waste.

7.1. Mechanical Recycling of PETW Currently, the dominant form of PETW recycling worldwide is mechanical recycling which means the reprocessing of waste plastics by physical means, like cutting, shredding, washing, and so on, into plastic products. Mechanical recycling requires homogenous plastics and relatively clean material. It is especially important to remove paper labels and label adhesives which cause the PET to discolor and lose clarity, as well as PVC liners in bottle caps. PVC and PET have almost the same density and are difficult to separate from each other. PVC releases hydrochloric acid during PET reprocessing, reducing the commercial value of recycled PET. Also, the presence of colored PETW imparts an undesirable gray color to the recycled PET [98]. The general process of mechanical recycling involves a six-stage process: collection/segregation, cleaning and drying, chipping/sizing, coloring/agglomeration, pelletizing/ extrusion and manufacturing of the end product [99]. However, the powder obtained by mechanical recycling, due to the degradation which occurs during each cycle, shows a decrease in physical properties compared to virgin PET and it is not suitable for bottle products. The molecular weight of the recycled polymer is reduced because of chain-scission reactions caused by the presence of water and traces of acidic impurities. Through mechanical recycling, waste PET bottles can be found in films, sheets, strapping packaging, and fiber used for sacking and their usage for insulation and for floor covering has also been studied [100, 101].

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Strategies for maintaining the high average molecular weight of polymer during reprocessing are also studied and include intensive drying, reprocessing with vacuum degassing and the use of chain extender compounds [102]. Mechanical recycling, however, does not prevent the accumulation of PET in landfills, because the lifetime of the secondary product is limited, and it will eventually be discharged into the landfill. The recycled PET is chemically heterogeneous due to the presence of trace metals from the catalyst residues and additives (such as antimony, cobalt and manganese) which may affect the melt rheology behavior from batch to batch [98]. Despite this fact, mechanical recycling is an inexpensive way to reduce the buildup of waste PET into the landfills after they have been consumed. Also, the reprocessed material can be blended with virgin PET to improve physical properties and obtain superior results [97].

7.2. Chemical Recycling of PETW Chemical or feedstock recycling (tertiary recycling) involves transformation of polymer chains. It is defined as the process leading to the total or partial degradation of the polymer backbone to the monomers (i.e., depolymerization) or to oligomers or larger chain fragments (i.e., random chain scission) and other chemical substances, which are mainly gaseous products. The chemical recycling is carried out either by solvolysis or by pyrolysis. The solvolysis occurs through degradation by different solvents, while the pyrolysis occurs through degradation by heat in absence of oxygen or air or in a vacuum. Chemical recycling yields monomers, petroleum liquids and gases [21]. Monomers are purified by distillation and drying, and they could subsequently be repolymerized to regenerate the original polymer [97]. Chemical recycling of PET has been the subject of increased interest as a valuable feedstock for different chemical processes. The solvolytic chemical recycling by chemical degradation of waste PET is usually divided in several groups according to the reaction mechanism involving methanolysis, glycolysis, hydrolysis, ammonolysis,

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aminolysis and other processes, whereby methanolysis and glycolysis are mainly applied on a commercial scale [103]. The techniques of tertiary recycling include hydrogenation, gasification, chemical depolymerization, thermal cracking, catalytic cracking and reforming, photodegradation, ultrasound degradation and degradation in a microwave reactor. Some other technologies are also developed in order to produce oligomeric intermediate products from waste PET of specialized components for the chemical industry. Only chemical recycling conforms to the principles of sustainable development because it leads to the formation of the raw materials from which PET is originally made. In a large collection of researches of the chemical recycling of PET, the primary objective is to increase the monomer yield while reducing the reaction time and/or carrying out the reaction under mild conditions. Continuous efforts of researchers have brought about great improvements in the chemical recycling processes [104].

7.3. Methanolysis Methanolysis of PET consists of the degradation by methanol at relatively high temperatures (170-280°C) and pressures (20-40 atm). The main products of PET methanolysis are DMT and EG, which are the monomers for the PET synthesis [105, 106]. The Kodak Co. possesses a patent describing a process of PET methanolysis and its optimum properties [107]. Another approach to methanolysis was presented in a patent concerning a continuous, two stage process whose main product is TPA [108]. Sako et al. [109] proposed an improved procedure of methanolysis by treating PET with supercritical methanol, whereby PET was completely depolymerized to DMT, EG and oligomers above 300°C at 11 MPa for 30 min without a catalyst. The solid products of the PET methanolysis with supercritical methanol mainly composed of dimethyl terephthalate and small amounts of methyl-(2-hydroxyethyl) terephthalate, bis(hydroxyethyl)terephthalate, dimers and oligomers, and the liquid products composed of ethylene glycol and methanol. It was found that both

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the yield of dimethyl terephthalate and the degree of PET depolymerization were seriously influenced by the temperature, weight ratio of methanol to PET, and reaction time, whilst the pressure has insignificant influence when it is above the critical point of methanol [110]. PET methanolysis involves both random and chain-end scission, and is interesting from a theoretical point of view because of the secondary reactions of the chain-end scission products. Goto et al. [111] reported the population balance equation that governs the PET molecular weight distribution solve with the moment method. Their work demonstrated that conventional kinetics can be combined with distribution kinetics to analyze complex macromolecular reactions. The methanolysis is used by large PET manufacturers such as Hoechst and Eastman as well as lesser manufacturers [21]. The main advantage of this method is that an installation of methanolysis can be located in the polymer production line, where waste PET arising in the production cycle is used and the monomers recovered can be re-used in the manufacture of a full value polymer. Disadvantages of the method include the high cost associated with the separation and refining of the mixture of the reaction products (glycols, alcohols and phthalate derivatives). The methanolysis process can tolerate a wide range of contaminants. However, water does perturb the process and poisons the catalyst to form various azeotropes. The main disadvantage is associated with the trend of all of the new PET production processes to use TPA instead of DMT as the raw material. The conversion of the DMT produced by hydrolysis to TPA adds considerable cost to the methanolysis process [21]. Misra and Goje [112] determined optimal reaction conditions for the methanolysis of PETW including reaction time, reactant particle size, percentage recovery of monomeric products, methanolysis rate constant, condensation rate constant, equilibrium constant, Gibbs free energy, enthalpy and entropy of reaction using zinc acetate in the presence of lead acetate as the catalyst at 120, 130 and 140°C. Methanolysis reactions were carried out below 150 °C to avoid oxidation/carbonization during the reaction. Optimal percentage conversion of PETW into DMT and EG was 97.8% (at 120 °C) and 100% (at 130 and 140 °C) for the optimal reaction

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time of 120 min. Yields of DMT and EG were almost equal to PET conversion. A kinetic model was developed and the experimental data show good agreement with the kinetic model. Rate constants, equilibrium constant, Gibbs free energy, enthalpy and entropy of reaction were also evaluated at 120, 130 and 140 °C. The methanolysis rate constant of the reaction at 140 °C (10.3 atm) was 1.4 × 10−3 g PET mol-1 min-1. The activation energy and the frequency factor for methanolysis of PETW were 95.31 kJ/mol and 107.1 g PET mol-1 min-1, respectively. Kurokawa et al. [113] studied the use of aluminium triisopropoxide (AIP) as a catalyst for the methanolysis of PET. The methanolysis at 200°C in methanol with an AIP catalyst gave DMT and EG in 64% and 63% yields, respectively. The yields were increased by using a toluene/methanol mixed solvent containing 20-50 vol.% toluene. Maximum yields, 88% for DMT and 87% for EG, were obtained at 20 vol.% toluene. These results indicate that the rate of methanolysis strongly depends on the solubility of PET. The results of gel permeation chromatography (GPC) suggest that the methanolysis of PET in the absence of the catalyst includes three steps. In the first step, the depolymerization occurred at a tie molecule connecting PET crystals and the chain length was shortened to about 1/3. The shortened chain was depolymerized to oligomers in the second step. The GPC curve of the oligomers tailed to low molecular weight, clearly indicating that the depolymerization took place at random positions on the polymer chain. The third step, the depolymerization from the oligomers to the monomers, was promoted only in the presence of the AIP catalyst.

7.4. Glycolysis The simplest method of PET depolymerization, which is widely used on a commercial scale in many companies worldwide such as DuPont, Goodyear, Shell Polyester, Zimmer, Eastman Kodak and other, is the glycolysis reaction [98]. The glycolysis represents the molecular degradation of PET polymer by glycols, in the presence of typical

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transesterification catalysts, mainly metal acetates, where ester linkages are broken and replaced with terminal hydroxyl groups, either under pressure or at atmospheric pressure [114-116]. The method involves a transesterification reaction of PET with an excess of glycol at temperatures in the range of 180-240°C, promoting the formation of bis-HET. The bisHET monomer and a low molecular mixture of oligomers from a partial hydrolysis of the PETW with EG can be re-circulated to the PET synthesis process or used in the synthesis of various polymers such as unsaturated polyesters, polyurethanes, vinyl esters, epoxy resins and polymer concretes [117]. Glycolysis of PETW proceeds through at least three stages: oligomers, dimer and monomer. The glycol diffuses into the polymer, causing the polymer to swell up, thus increasing the diffusion rate. The glycol subsequently reacts with an ester bond in the chain and degrades the PET into lower fractions [118]. Baliga and Wong reported on glycolysis of PETW from post-consumer soft-drink bottles in excess of EG at 190°C in the presence of a metal acetate catalyst. After 8 h the glycolyzed products consisted mostly of the monomer, bis-HET and the dimer. After prolonged glycolysis, equilibrium was established between the monomer and the dimer and EG, with an equilibrium constant lying between 1.13 and 1.53. Of the four metal acetates (lead, zinc, cobalt, and manganese) tested, zinc acetate was found to be the best in terms of the extent of depolymerization, that is, the relative amount of monomer formed [119]. Also, PETW was depolymerized with ethylene glycol in the presence of different catalysts, two conventional metal catalysts (zinc acetate and lead acetate) and two alkalies (sodium carbonate and sodium bicarbonate). The results show that the qualitative and quantitative yields of the monomer obtained with alkalies as catalysts were most comparable with the conventional heavy metal catalysts, thus providing a further advantage for the recycling of polyester waste for the cause of environmental pollution abatement [120]. Controlled glycolysis of PET was carried out under nonaqueous alkali conditions to obtain an enhanced hygroscopicity. This treatment considerably enhanced the hygroscopicity of the treated PET as indicated by instant wicking and contact angle measurement. The effectiveness of

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three alkali catalysts was directly related to their alkali strength, i.e., sodium hydroxide > sodium carbonate > sodium bicarbonate. Both losses of weight and strength increased at higher temperatures, but with sodium hydroxide they leveled off at 120°C. At 2 h of treatment, the treated PET showed instantaneous wetting in most cases. The morphological changes that substantiated strength loss of the fabrics were confirmed by FTIR analyses, and the effect was greater with sodium hydroxide. DSC analysis indicated that melting peaks of the glycolyzed PET were relatively consistent except at high temperature (120°C), which showed a significant conformational change. These results showed that the glycolysis treatment of PET fabric at 80°C for 1 h could be an optimal condition to obtain the balance between high hygroscopicity and desired physical properties [121]. Various experimental factors include temperature, pressure, reaction time, as well as stirring speed, particle size, a choice of solvent and its ratio [122]. López-Fonseca et al. [123] studied the kinetics of glycolysis of PETW to give highly pure bis-HET in a batch reactor using EG in excess at atmospheric pressure. The reaction was carried out in the presence of sodium carbonate as active catalyst. A kinetic model considering the reaction rate to be first order with respect to PET, bis-HET and sodium carbonate concentration satisfactorily described the kinetics of the glycolysis with an excess of EG. The effect of particle size (0.14–3 mm) and stirring rate (50–800 rpm) on conversion was examined. The selected PET particle size and stirring rate were 0.25 mm and 600 rpm, respectively. Using PET:catalyst molar ratio of 100:1 about 80% bis-HET yield was attained at 196°C after 1 h. The type and amount of used glycol have a significant impact on the properties of the final products. For the glycolysis of PETW can be used different glycols, such as EG, diethylene glycol (DEG), propylene glycol (PG), poly(ethylene oxide) (PEO), neopentyl glycol (NPG), 1,4-butanediol, and hexylene glycol. Also, it is possible to use the mixture of some glycols, but EG is more suitable and brings about remarkable result in PET depolymerization [124]. An increasing of glycol/PET ratio increases the rate of decomposition which leads to producing lower weight oligomers, but the high glycol/PET ratio, which is suitable for achieving lower

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molecular weight of oligomers, can cause problems in the mixing process [122]. G. Güçlü et al. [125] reported the use of xylene as the solvent for the products of glycolysis, wherein PET and EG were insoluble in xylene, even at high temperatures. During the multiphase reaction, depolymerization products transferred to the xylene medium from the dispersed PET/glycol droplets, shifting the equilibrium to glycolysis. Best results were obtained from the EG reaction at 220°C, which yielded 80 mol. % bis-HET monomer and 20 mol.% dimer fractions in quite pure crystalline form. Other advantages of the use of xylene in glycolysis of PET were improvement of mixing at high PET/EG ratios and recycling possibility of excess glycol, which separates from the xylene phase at low temperatures. Besides metal acetates, there are various catalysts which can be used in PET glycolysis, such as ionic liquids [126-129], urea [130], metal oxide nanocomposites [131, 132] or hydrotalcit [133]. Wang et al. [127] reported on the chemical recycling of PETW by EG catalyzed by different kinds of ionic liquids at atmospheric pressure. It was found that the purification process of the products in the glycolysis catalyzed by ionic liquids was simpler than that catalyzed by traditional compounds, such as metal acetate. Qualitative analysis showed that the main product in the glycolysis process was the bis-HET monomer. Thermal analysis of the glycolysis products was carried out by DSC and TGA. The influences of experimental parameters, such as the amount of catalyst, glycolysis time, reaction temperature, and water content in the catalyst on the conversion of PET, selectivity of bis-HET, and distribution of the products were investigated. The results show that reaction temperature was a critical factor in this process. The conversion of PET increased with increasing amount of catalyst, glycolysis time and reaction temperature, but decreases with the addition of water in the catalyst. The selectivity of bis-HET is enhanced by increasing amount of catalyst and reaction temperature with a maximum value for an optimum value of glycolysis time, but decreases with the increase of water content in the ionic liquid. The glycolysis of PET was studied using 1-butyl-3-methylimidazolium acetate [Bmim][OAc] as a catalyst. The effects of temperature, time, ethylene glycol dosage, PET

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amount and [Bmim][OAc] dosage on the glycolysis reaction were examined. The results revealed that the [Bmim][OAc] has a PET conversion of 100% and bis-HET yield of 58.2% under the optimum conditions of 1.0 g of [Bmim][OAc] with 20 g of ethylene glycol in the presence of 3.0 g of PET at 190°C after 3 h of glycolysis. The ionic liquid could be reused up to six times with no apparent decrease in the conversion of PET or yield of bis-HET. The pH plays a major role in explaining the proposed mechanism of glycolysis using the Lewis base ionic liquid [Bmim][OAc]. The kinetics of the reaction was first-order with an activation energy of 58.53 kJ/mol [128]. The degradation of PET bottles has been successfully achieved using hydrotalcite as catalyst and dimethyl sulfoxide (DMSO) as solvent. The reaction was carried out at boiling point of DMSO (190°C) and degradation was complete in 10 min. The oligomer (tetramer) obtained was treated with NaOH at room temperature in methanol to get DMT and EG. The hydrotalcite can be recycled again. The oligomer (tetramer) has been found to be pure [133].

7.5. Aminolysis Aminolysis is the reaction of PET with different primary amine aqueous solutions such as methylamine, ethylamine, ethanolamine and anhydrous n-butylamine in the temperature range of 20-100°C to yield the corresponding diamides of TPA and EG [98]. PET in the form of waste fibres and disposable soft drink bottles was subjected to depolymerization through aminolysis using excess of ethanolamine in the presence of glacial acetic acid, sodium acetate and potassium sulphate as catalysts. The product bis(2-hydroxyethylene) terephthalamide (BHETA) obtained was in its pure form with sufficiently high yields with all the catalysts. The maximum yields of BHETA under optimized conditions were 91.1% from PET fibrous waste and 83.2% from bottle waste. In the absence of any catalyst, the yield was only 52%. This difference in the yields was attributed to the molecular weight and its distribution in PET fibrous waste and the bottle waste. For the fibre grade PET, the molecular weight is

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lower and the molecular weight distribution is narrower than that for the bottle grade PET in order to attain higher viscosity for the latter, necessary for the blow-molding process [134]. The aminolysis reaction can be also run successfully as uncatalyzed reaction. Soni et al. [135] reported on the uncatalyzed degradation of PETW by hydrazine monohydrate at ambient temperature and pressure. The end product was characterized as terephthalic dihydrazide (TPD) and further used in PVC compounding as secondary plasticizer. The complete degradation of PETW was achieved after 45 days in the case of a 1:10 weight/volume ratio of PET to amine, but with a 1:2 weight/volume ratio of PET to hydrazine monohydrate, the degradation time was considerably reduced to 24 h at ambient conditions. Mittal et al. [136] studied the degradation properties of the PETW flakes by aqueous methylamine and aqueous ammonia, respectively, at room temperature in the presence and absence of quaternary ammonium salt as a catalyst for different periods of time. PET degraded more quickly in the presence of the catalyst, where the long polymeric chains in the semicrystalline PET were reduced to monodisperse rods before full degradation into the end products, and the fissures on the surface of PET were found to deepen with time. The amorphous portion was removed at a faster rate, and there was a marked increase in the crystallinity of the residue toward the completion of the reaction. The product of aminolysis, BHETA, has the potential for further reactions to obtain useful products. There are few reports on usage of recycled BHETA from PETW to synthesis of novel biodegradable polyurethanes based on polycaprolactone diol, which showed excellent chemical, thermal and mechanical properties [137]. Also, the suitability of BHETA obtained from the aminolyzed PET waste was assessed for use as an ingredient in the anticorrosive paint formulations for the protection of steel structures [138]. The synthesized BHETA possessed high hardness and stiffness, good resistance to weathering, creep strength, and high dimensional stability. The addition of organic BHETA into the paint formulation barely affected the physical and mechanical properties of the paint films.

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7.6. Ammonolysis Ammonolysis is the reaction of ammonia with PET to produce a terephthaldiamide (TPDA). This can be converted to terephthalonitrile, and further, to other chemical substances. Very good results were obtained from the ammonolysis of PET waste from postconsumer bottles. The process was carried out under a pressure of about 2 MPa in a temperature range of 120-180°C for 1-7 h. After the reaction was completed, the amide produced was filtered, rinsed with water, and dried at a temperature of 80°C. The TPDA amide is produced by the action of anhydrous ammonia on PET in an ethylene glycol environment. This can be converted into terephthalic acid nitrile and further to p-xylylenediamine or 1,4bis(aminoethyl)cyclohexane [97, 139].

7.7. Hydrolysis Hydrolysis is a method of PETW recycling in which PET is reacted with water in acidic, basic or neutral condition, leading to complete depolymerization and resulting in its monomers, TPA and EG. The hydrolysis reactions require the use of high temperature (200-250°C), as well as relatively high pressure and also long time needed for complete depolymerization. Commercially, hydrolysis is not widely used, because of the cost associated with purification of the recycled TPA. Hydrolysis of PET can be carried out as alkaline, acid and neutral hydrolysis [21]. Alkaline hydrolysis of PET is usually carried out with the use of an aqueous alkaline solution of NaOH or KOH, in the concentration of 4-20 wt. % [140]. The reaction products are EG and the disodium or dipotassium terephthalate salt. The mixture is heated up to 340°C to evaporate and recover the EG by distillation. Pure TPA can be obtained by neutralization of the reaction mixture with a strong mineral acid (e.g., H2SO4) [98, 103]. Kumar and Guria [141] studied hydrolytic decomposition of PETW in the presence of an aqueous potassium hydroxide solution and developed a modified shrinking core model with a

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depleting product layer of waste PET under finite solution volume conditions. This model assumes the first order depolymerization kinetics with respect to the concentration of potassium hydroxide solution. The conversion of PETW is determined after filtration of the unreacted PET left after depolymerization. The conversion of PET is also compared with the amount of terephthalic acid obtained after acidification of the filtrate. The factors which have a great effect on the kinetics of the alkaline depolymerization of PET in NaOH solution are temperature, time and alkali concentration, as well as the choice of solvent [142]. The main advantage of alkaline hydrolysis in comparison with the other methods of the chemical recycling is that it can tolerate highly contaminated, postconsumer PET such as magnetic recording tape, metallized PET film or photographic film (X-ray film). The process is relatively simple and less costly than methanolysis [98]. Karayannidis et al. [143] studied the reaction alkaline hydrolysis of PET in an autoclave with a nonaqueous solution of KOH in an ether alcohol, methyl Cellosolve. The ether part of methyl Cellosolve led to swelling of the PET solid and the alcoholic part supported the action of the OH- groups in destroying the chemical structure of PET during depolymerization. The reaction temperature was 120°C and the reaction time was 2.5 h. Acid hydrolysis is performed most frequently using concentrated sulfuric acid, although other concentrated mineral acids (e.g., phosphoric or nitric acid) can be used. Acid hydrolysis of PET in sulfuric acid at different temperatures and solution concentrations was reported [140]. Neutral hydrolysis is carried out with the use of water or steam in the presence of alkali metal acetates as transesterification catalysts. Low purity of TPA is the major drawback of this method. Hydrolysis is comparatively slow because water is a weak nucleophile [97, 98].

7.8. Enzyme Catalyzed Hydrolysis Recently, the recycling methods based on enzyme catalysis have been provided with many advantages compared to classical chemical processes.

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Enzyme technology could be applicable in large scale such as, in term, improving green industries especially in dealing with production (such as detergents) or recycling and detoxifying of daily products. Advantages of enzymatic methods include the fact that their action under mild conditions with low energy input without the need for expensive machinery might be helpful in PET degradation [122]. PET is a non-natural substrate for enzymatic reactions and therefore classically is considered as recalcitrant to biodegradation, which results in slow enzymatic rate toward this polymer. Eberl et al. [144] reported on hydrolysis of PET and bis(benzoyloxyethyl) terephthalate (3PET) endo-wise by a lipase from Thermomyces lanuginosus and cutinases from Thermobifida fusca and Fusarium solani and bis(benzoyloxyethyl)terephthalate (3PET) endo-wise as shown by MALDI-Tof-MS, LC-UVD/MS, cationic dyeing and XPS analysis. Due to interfacial activation of the lipase in the presence of Triton X-100, a seven-fold increase of hydrolysis products released from 3PET was measured. In the presence of the plasticizer N,N-diethyl-2phenylacetamide (DEPA), increased hydrolysis rates of semi-crystalline PET films and fabrics were measured both for lipase and cutinase. The formation of novel polar groups resulted in enhanced dye ability with additional increase in color depth by 130% and 300% for cutinase and lipase, respectively, in the presence of plasticizer. Recently, Yoshida et al. [145] isolated a novel bacterium, Ideonella sakaiensis 201-F6, that is able to use PET as its major energy and carbon source. When grown on PET, this strain produces two enzymes capable of hydrolyzing PET and the reaction intermediate, mono(2hydroxyethyl)terephthalic acid. Both enzymes are required to enzymatically convert PET efficiently into its two environmentally benign monomers, TPA and EG. The assimilation of PET by I. sakaiensis bacteria may be advantageous for removing this plastic material from the environment. However, if the terephthalic acid could be isolated and reused, this could provide huge savings in the production of new polymer without the need for petrol-based starting materials. To establish such a process, it may be possible to integrate the PETase/MHETase pair into

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common production strains via metabolic engineering or the use of enzyme cascade systems. Further research in this area will hopefully provide concepts and solutions for the degradation and recycling of other degradation-resistant plastic materials that are currently used and disposed [146].

7.9. Quaternary Recycling Energy recovery or quaternary recycling refers to the recovery of energy that is contained in a polymer, and this is currently the most effective way to reduce the volume of organic materials together with considerable yield of energy. However, in some aspects this method is ecologically unacceptable owing to the health risk from airborne toxic substances [147]. Apart from the aforementioned methods, direct reuse of PET bottles could be considered as a zero-order recycling techniques. However, this should be done with a great care since plastic bottles are more likely than glass to absorb contaminants, which could be released into the contents (especially food) when the bottle is refilled. Moreover, refilling a PET bottle with a drink of high alcoholic content may lead to the degradation of the macromolecular chains with unpredictable consequences.

8. THERMOPLASTIC COPOLYESTER ELASTOMERS Thermoplastic copolyester elastomers, poly(ester-ether)s (TPEEs) are multiblock copolymer containing crystalline, hard segments and amorphous, soft segments [148-150]. The thermoplastic and elastic behavior of TPEEs can be explained by their two-phase microstructure, which is a consequence of the chemical nature and incompatibility of the hard and soft segments, which are built into the polymer chains. The hard phase acts as physical crosslinks for the amorphous, rubbery regions, imparting the material properties as an elastomer [151-154]. TPEEs

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possess the mechanical properties of chemically crosslinked elastomers, e.g., elastic recovery and good flexibility at low temperatures, and of thermoplastics such as high modulus and impact strength. Moreover, TPEEs can be processed as conventional thermoplastic materials above the melting temperature of the hard segment and conventional methods for processing are extrusion and injection-molding. TPEEs behave as crosslinked elastomers at room temperature and as linear polymers at high temperatures. Over the past four decades, the commercial growth of TPEEs has been increasing constantly due to their good physical and mechanical properties. The properties of TPEEs depend on their molecular weight, the type and molecular weight of the soft segment and the hard/soft segment content. PET and PBT are very suitable for use as hard crystallizable segments in multiblock copolymers, which belong to the class of thermoplastic copolyester elastomers. The hard, crystalline PET or PBT segments are dispersed in an amorphous matrix usually at lower hard segment content, which contains the soft polyether segments and the non-crystalline parts of the hard segments. In a molten state, TPEEs with relatively short hard segments typically form a homogeneous mixed phase of hard and soft segments. Upon crystallization, hard segments are ordered into domains, creating a physical network, which provides dimensional stability and minimizes the cold flow of the polymeric material [155-157]. It was shown that the crystallization process is the driving force for the phase separation causing the formation of a co-continuous two-phase morphology [158, 159]. The structure of TPEEs after crystallization can be presented by a twophase model: a crystalline polyester phase and a homogeneous amorphous phase of the polyether mixed with the non-crystalline polyester part. Both phases are considered to be continuous (Figure 5). Gabriëlse et al. [160] reported that the crystalline structure of TPEEs is typically a lamellar or pseudo-lamellar microstructure. The lamellar hard domains in TPEEs are regular in size and shape, similar to the structure observed for PBT or PET homopolymers. Transmission electron microscopy (TEM) images have shown that the dimensions of these lamellae are 10 nm in thickness and up

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to several hundred nanometers in length. The crystalline hard domains appear as white regions, while the amorphous soft segments are visible as black regions owing to their absorption of the staining agent. The crystalline lamellae are probably interconnected by tie molecules as they are in crystalline PBT homopolymer. In 1954, Coleman [161] reported the first synthesis of thermoplastic poly(ester-ether) elastomers based on PET as the hard segment and PEO with up 30 wt.% as the soft segments. It was shown that the inclusion of PEO segments into polyester chains decreased crystallinity and enhanced hydrophilicity, which could improve the dyeability of the PET fibers. In 1959, Charch and Shivers [162] revealed that when the PEO content as the soft segments was between 40 and 70 wt.%, then the elastic properties of TPEEs could be observed.

Figure 5. TEM image of poly(ester-ether) films based on PBT and PTMO: (a) 35 wt.% of PTMO soft segment and (b) 60 wt.% of PTMO soft segment [160]. Similar morphology was observed for PET-based poly(ester-ether)s with same soft segment content.

Commercial thermoplastic poly(ester-ether) elastomers, based on PBT as the hard segment and poly(tetramethylene oxide) (PTMO) as the soft segment were developed by DuPont and introduced to the market in 1972, under the trade name of Hytrel® [163]. Lately, a large number of studies of

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segmented copolymers based on PBT hard, rigid segments and different new soft, flexible segments, such as polyethers, aliphatic polyesters, polyolefin, polyamide, dimerized fatty acids, and polydimethylsiloxanes, have been reported [164-176]. TPEEs are mostly synthesized from PET or PBT by step-growth polycondensation in the molten state with ED or BD, polyether macrodiol, DMT or terephthalic acid in two or three stages (Figure 6). These stages are: (a) transesterification of DMT or the condensation of terephthalic acid with hydroxyl groups of both polyether and low molecular weight diol, (b) low-pressure melt polycondensation at a high temperature (250-300°C), and (c) post-polycondensation in a solid state when a copolyester of higher molecular weight is required.

Figure 6. Synthesis of PET-based poly(ester-ether)s by conventional two-stage polycondensation.

Contrary to PBT, which generates TPEE based on PTMO with a phase-separated microstructure, PET is not known to produce such elastomers without additional modification of the polymer [157]. The synthesis and properties of PET block copolymers based on PEO [177-

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180], PTMO [181], and poly(ε-caprolactone) (PCL) [182-184] segments have been described. The addition of 10 wt.% of polyether segments in PET-based TPEEs hampers is known as the “cold crystallization” of PET, and these block copolymers can be considered to be semicrystalline. A reduction the melting temperature due to the presence of polyether segments in block copolymers [185] was observed. Moreover, the enhanced modulus and thermal properties are of great interest in developing PET based block copolymers [3]. The introduction of any flexible molecular repeat units, such as long-chain aliphatic diols or diacids, large hydrocarbon phases, such as C36 dimer acid would be expected to impart macroscopic flexibility to PET copolymers. Hence, the primary goal of the copolymerization of PET with modifying substances is to either accelerate or delay the ultimate degree of crystallization, to either increase or decrease the modulus and tensile properties, to bring about higher or lower glass transition temperature and melting temperature values for the copolymer, and to modify the dynamic properties, such as oxygen and carbon dioxide permeation rates – all while essentially retaining the balance of the properties and the low manufacturing costs which are associated with PET homopolymer. The employees at AlliedSignal have developed a block copolymer based on PET and PCL, which have been commercialized for use in ‘loadleveling’ seat belt fabrics [184]. These block copolymers were obtained by reacting a caprolactone monomer with PET a homopolymer, in the presence of a tin catalyst, in a twin screw extruder. The reactive extrusion produced a block copolymer of PET and PCL with domains of poly(caprolactone) which grew from the hydroxyl end groups of the starting PET core, with minimal randomization of the monomeric units. It has been found that this bock copolymer in fiber form is resistant to creep under normal usage, but it irreversibly deforms when elongated at high strain rates, such as those encountered in an automobile accident. It is shown that the block copolymers of PET and PEO, synthesized by the copolymerization of terephthalate, ED and PEO macrodiol, crystallize into two phases. Crystallization of the PEO blocks in these block copolymers are constrained by the microstructure of the PET phases [186].

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Due to the significant increase in equilibrium moisture content in the block copolymer with an increasing content of PEO, it has been suggested that these block copolymers are good candidates for biodegradation, especially in the presence of lipase enzymes. Employees at Eastman Chemical have reported that the incorporation of 1–45 mol.% of C3–C8 aliphatic acids, such as succinic acid or adipic acid, can lead to a significant decrease in carbon dioxide permeability versus the PET homopolymer [187]. This improvement can be explained by a decrease in the glass transition temperature, which leads to less supercooling of the polymer at room temperature (the temperature of interest in measuring the loss of carbonation in soft drinks in polyester bottles) and lower gas solubility in the copolymer and therefore decreased gas permeation [188]. With lower glass transition temperature values and decreased gas permeation, the copolyesters of PET based on aliphatic diacids are also more prone to creep under the pressure of carbonated beverages, rendering them unsatisfactory this application. TPEEs can replace rigid plastics when improved shock absorption, impact resistance, flexural and compressive properties, and spring characteristics or silent operations are required, for example for industrial and automotive hydraulic tubing, hoses, gaskets and bellows and also for the jacketing of electrical and fiber-optic cable. Interestingly, TPEEs can also be used as biodegradable materials. It has been reported that the inclusion of PEO soft segments into polyester (PET or PBT) chains has resulted in an enhancement of the hydrolytic susceptibility that leads to the degradation of these poly(ester-ether)s within a few months. Gilding and Reed [179, 189] reported that the synthesis and biodegradation of block copolymers based on PET and PEO, are suitable for temporary biomedical application. Namely, biodegradable TPEEs based on PBT and PET – commercialized under the trade name of PolyActive® and first developed by HC Corporation in the Netherlands – could be used in tissue engineering scaffolds, wound dressings, artificial skin, bone replacement and as drug delivery systems, due to good biocompatibility and mechanical properties which are similar to native cartilage [190].

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Thermoplastic copolyester elastomers are produced by various manufactures under the trade names: Arnitel® (DSM, Netherlands), Ecdel® (Eastman Kodak, USA), Elitel® (Elana, Poland), Pelprene® (Toyobo, Japan) and Lomed® (General Electric, USA).

9. PET BASED BLENDS AND NANOCOMPOSITES 9.1. PET Based Blends The blending of two or more polymers is an excellent strategy for obtaining a specified portfolio of physical properties. From the standpoint of commercial applications, polymer blending represents one of the fastest growing aspects of polymer technology to obtain new and tailored materials with improved physical and processing properties and optimal cost performance [1]. The primary requirement for achieving a useful set of properties through blending is either miscibility or compatibilization (which is frequent) of the immiscible components [2]. In the majority of cases, the criterion of polymer-polymer miscibility is not satisfied and the blends phase separate under such conditions, with the formation of undesirable product morphology. In order to achieve the desired property profile and stabilize the blend morphology, the control of compatibility at the interface between the component polymer phases is the most important factor. The interfacial properties of an immiscible polymer blend are suitably modified using the compatibilization method, and the desired morphologies and properties are obtained. Two methods that find practical application are following: (a) incorporation of a separate chemical compatibilizer into an immiscible polymer blend during melt compounding and (b) “reactive” compounding to form in situ a compatibilizer. Compatibilizers for blends are low molecular weight block or graft copolymers which are used separately or generated in situ in blends by reactive blending. A copolymer compatibilizer usually consists of correspond chain segments that will interact with the component polymers of the blends, leading to decreased interfacial tension and increased

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interfacial adhesion. Since thermoplastic polyesters possess carboxyl and hydroxyl functional groups at the chain ends, in situ or reactive compatibilization is an effective approach for obtaining engineering thermoplastic blends from these materials. Also, since transesterification is often difficult to control, reactive compatibilization is normally practiced in a modern blend system. The most general industrial method of the preparation of blends is melt blending in a mixer or a twin-screw extruder [2]. However, the rheological properties of the blend components, the conditions of the melt blending, and the method of stabilization of the morphology, e.g., by controlled cooling, crystallization and chemical reaction as well as optimum processing conditions, are important parameters for obtaining the desired properties in the blends. Generally, thermoplastic copolyester elastomers are used as modifiers, i.e., the minor phase components, and rarely as continuous phase or matrix of blends [1]. They can be blended with different polymers or with other TPEEs, due to their low melt viscosity and melt stability. Most of the blends based on TPEEs and other polymers are physical mixtures. Moreover, during the blending of TPEEs with condensation polymers, such as polyesters in the melt or solid state, chemical reactions can take place at the interface of the condensation polymers. Blends of PET with other polymers may offer an attractive balance of processability as well as mechanical and barrier properties. Primarily, the motivations for obtaining blends based on PET and other polymers are to: a) enhance the solvent resistance and processability of amorphous polymers (such as polycarbonate); b) improve toughness; and c) reduce the mold shrinkage of PET connected with its crystallization [191]. Therefore, the development of PET blends has mainly been market driven, motivated by the desire to extend the applications of PET into areas where improved solvent resistance and processability are required, while still retaining good mechanical properties. Recent developments have resulted in blends of PET with PBT, polyamide (PA), polycarbonate (PC), poly(propylene) (PP) and acrylonitrile-butadiene-styrene copolymer (ABS), to improve impact resistance at low temperatures and other mechanical properties. There are

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many possibilities for blending PET with other polymers, some of which are described below.

9.1.1. PET/PBT Blends Blends of PET with PBT constitute an important category of commercial blends. A commercial grade of PET and PBT were melt blended over different composition using a twin-screw extruder [1]. There was a transesterification reaction during the melt processing, resulting in the formation of block copolymers in the initial stage and a random copolymer at the end of the reaction. PBT (Tm  220°C) and PET (Tm  250°C) are miscible in the melt, but crystallize as separate phases upon cooling [2]. Because of the complete miscibility of PBT and PET, only a single glass transition temperature was observed, and the mobility of PBT molecules decreased, while that of PET molecules increased at a particular crystallization temperature. Consequently, the crystallization rate of PBT was reduced in the presence of PET, and on the other hand, PET could crystallize much faster due to enhanced mobility in the presence of PBT, compared to the neat polymer. Therefore, the processability of PET increases in the presence of PBT [192]. PET slightly improves the mechanical properties of PET/PBT blends. It was observed that these blends showed enhanced crystallinity, high elongation and impact strength compared to a PET- or PBThomopolymer. The good cost/performance of PET/PBT blends results from the incorporation of PET, which is much cheaper than PBT. PET/PBT blends can also be impact-modified and reinforced with glass fibers [193]. The typical properties of unfilled PBT and PET/PBT blends containing glass fibers as reinforcements (Commercial Pocan® PBT grades) are given in Table 2 [194], while the properties of PET are presented in Table 1. Impact-modified blends contain a third component for the enhancement of impact strength. Commercially interesting modifiers include acrylonitrile-butadiene-styrene elastomers, acrylic elastomers and other styrene copolymers [193]. The Valox 800 series from GE Plastics and the Celanex 5000 series from Hoechst-Celanese are commercial examples of PET/PBT blends, which may be impact-modified

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with elastomeric substances such as polyacrylates. PET/PBT blends, which are commercially available in glass reinforced, injection molding grades, find applications in household appliances [2]. The properties of these blends – such as excellent surface appearance and gloss, high resistance to warpage for glass-reinforced grades, high strength and high temperature performance – are better compared to glass-reinforced PBT grade. Table 2. Typical properties of PBT and reinforced PET/PBT blends (Commercial Pocan® PBT grades) [194]

Property

1300 220 2700 90 2600 85

PBT/PET blends, 15% glass fibers (KU1-7313) 1430 225-250 300 6500 170 5500 25

PBT/PET blends, 15% glass fibers (T 7331) 1550 225-250 300 10500 220 9000 50

2.10 2.10

110 3.0 0.30 1.10

130 2.5 0.30 0.90

PBT

Density, kg/m3 Melting temperature,°C Melt viscosity (260°C), Pa s Tensile modulus, MPa Flexular strength, MPa Flexular modulus, MPa Izod Impact strength at 23°C, kJ/m2 Stress at break, % Stain at break, % Mold shrinkage (parallel)a, % Mold shrinkage (across)a, % a Plaque 150x105x3 mm.

9.1.2. PET/Polyamide Blends The reactive blending of various nylons with PET has been generally applied as a method for the toughening of polyamides. These blends exhibit two glass transition temperatures, but with a slight shift of both glass transition temperatures supporting the opinion that good interfacial grafting has been obtained in this two-phase system, which is a result of the transesterification between the components. The PET/PA blends possess good mechanical properties and sometimes better than those of

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pure polyamides [195-197]. The target applications of PET/polyamide blends are for automotive parts, since these blends offer an excellent combination of mechanical and impact strength with thermal and chemical resistance [2]. Blending PET with polyamide overcomes the drawback of PET’s poor impact strength and helps expand its application potential. Considerable efforts have been directed toward the compatibilization of PET/polyamide blends because the non-compatibilized blends exhibit brittle fracture and inferior impact strength [198]. It is known that PET/polyamide blends are immiscible at ambient temperature [199]. In addition to compatibilization, the blending of PET with polyamide requires some degree of toughening, in order to meet the stringent conditions for their use in automobiles [2]. The reason being that the faster crystallizing polyamide acts as a nucleating agent in the blend to enhance the crystallization rate and the degree of the crystallinity of PET, resulting in a higher modulus, and also inadequate impact strength of the blends. Huang et al. [199] reported that low molecular weight bisphenol A epoxy resin was successfully used as a reactive compatibilizer in PET/polyamide 6 (PA6) blends. They found that only 5 wt.% of the epoxy compatibilizer was used for blending, resulting in a significant improvement of the notched impact strength and flexural strength of the blends. PA6 domains were uniformly distributed within the PET phase in the presence of the epoxy resin compatibilizer for the blending of PET with PA6. The grafting during melt mixing onto PA6 by the epoxy resin was thought to impart good compatibility to the blend components. They observed that a crosslinking reaction took place during the blending at a high level of epoxy, leading to the improvement of both the impact toughness and flexural rigidity. The WAXS analysis revealed that PET and PA6 crystallized separately without the formation of any cocrystallites. The DSC results showed that the crystallization of the blend was slowed down by the presence of the epoxy resin. The use of other reactive couplers, such as phosphoryl azide, polyhydroxyether of bisphenol A or phenoxy resin, ethylene glycidyl methacrylate grafted on butylacrylate-blockmethylacrylate copolymer, has also been reported for PET/polyamide blends [2].

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A new type of polymer/polymer composite from PET/polyamide blends (so-called microfibrillar composites, MFCs) have been developed [200]. The MFCs were obtained by reactive extrusion ensuring an adequate ester-amide interchange reaction between the PET and the polyamide, followed by cold drawing of the extrude and annealing of the drawn blend. Bundles of highly oriented microfibrils of a diameter of about 1-2 μm, upon drawing, were formed from the minor, higher-melting PET component, as well as from the major, lower-melting polyamide. The microfibrillar structure of the PET was later stabilized in an isotropic, unoriented material of polyamide via annealing, which was performed at a temperature between the melting temperatures of the two polymer components. The prepared microfibrils significantly enhanced the mechanical properties, such as the tensile modulus and tensile strength of the PET/polyamide blend.

9.1.3. PET/PC Blends Blends of PET with amorphous engineering thermoplastic, such as bisphenol A polycarbonate represent an important group of materials. The motivation for choosing an amorphous engineering polymer in PET-based blends is to increase the dimensional stability and reduce mold shrinkage [2]. In the case of glass fiber-reinforced grades, the blending approach helps to reduce warpage. PC provides good impact strength in addition to dimensional stability and freedom from shrinkage or warpage, while crystalline PET offers good mechanical properties, chemical resistance, and ease of melt processing due to low viscosity above its melting temperature. Blends of PET with PC prepared in the melt with no exchange reaction between the components, showed partial miscibility through a shift in the glass transition and melting temperatures [1]. For compositions containing PET higher than 70 wt.% and less than 5 wt.%, the blends are transparent, with only one glass transition temperature [201]. One of the most important commercial examples of PET/PC blends is Makroblend from Mobay/Bayer. The blends are impact-modified with high-density polyethylene (HDPE) or ABS, and are normally available in injection

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molding grades as unfilled (Makroblend UT-1018) or reinforced (Makroblend DP-4-1350) compounds [2]. Due to the superior toughness, thermal properties and resistance to chemicals of PET/PC blends, these blends have widespread applications, such as: (a) automotive - car bumpers, mirror housings and brackets, rear quarter panels, (b) telecommunications - radio housings, speaker grills, instrument housings, (c) outdoor power equipment - tractor shrouds and consoles, lawn mower decks, (d) sports goods - protective helmets, ski boot and binding components.

9.1.4. PET/PP Blends Recycling PET and PP wastes and designing novel fiber materials are important factors for the development of PET/PP blends [2]. Blends of PP with PET offer some advantages in comparison with the pure components. PET can improve the stiffness of PP at higher temperatures, whereas the PP can facilitate the crystallization of PET. It is known that PET as a polar polymer is immiscible with PP, which is a nonpolar polymer. Noncompatibilized and compatibilized PP/PET blends can be prepared. Appropriate compatibilization is required to enhance the impact strength and other mechanical properties when the dispersed phase concentration exceeds 5-10 wt.% [202]. There are two approaches to the compatibilization of PET with PP: (a) the addition of a suitable block or graft copolymer containing reactive functionalities such as maleic anhydride (MA) or glycidyl methacrylate (GMA) in styrene/ethylenebutylene/styrene (SEBS) block copolymer, and (b) blending of a suitably functionalized PP, such as maleic or acrylic acid-grafted PP [203]. Graft copolymers formed in situ by the reactions between the functionalities of the compatibilizer and the end-groups of PET assist in blend compatibilization. In uncompatibilized PET/PP blends, the strength and modulus increase with increasing PET content almost linearly, however very poor impact strength results due to incompatibility. Heino et al. [204] reported that PP/PET blends with GMA-functionalized SEBS compatibilizer showed superior impact strength compared to those with MA-functionalized SEBS. These blends were on a rotating twin-screw

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extruder at 275°C. Reactions between the functionalized SEBS and PET favored the good mixing of PP and PET phases, leading to smaller dispersed phase domains. The improved interfacial adhesion in compatibilized blends was also reflected by the shift in the glass transition temperature of the PET phase toward that of the PP phase, which is demonstrated by dynamic mechanical analysis (DMA). Rudin et al. [205] observed that sheath/core filaments of PP/PET could not be oriented because of the poor adhesion between PP and PET. However, melt-blended compositions of 70/30 and 50/50 PET/PP produced oriented monofilaments with useful properties. It was revealed that the blended monofilaments have good mechanical properties and better recycling and reuse value compared to the blended products.

9.1.5. PET/ABS Blends The motivation for obtaining blends based on PET and tough amorphous thermoplastic ABS is to combine the high impact strength of ABS with the solvent resistance, good mechanical properties and ease of processability of PET [2]. These blends can replace the more expensive ABS/PC blends that are used in automotive applications. Makroblend formulations based on PET/PC/ABS are available from Mobay/Bayer. PET and ABS are immiscible and need compatibilization [206]. The grafting of maleic anhydride-grafted ABS (ABS-g-MA) particles onto the PET matrix lead to compatibilization, resulting in stabilization of morphology and the desired mechanical properties [207]. Uncompatibilized blends quenched from the melt showed good tensile properties, but these deteriorated during storage at room temperature due to the debonding of the ABS particles. Impact-modified PET is glass fiber-reinforced and can find applications in injection molded automotive and industrial components such as windshield wiper blade supports, industrial pump housings and impellers, gears and bearings. Cook et al. [208] prepared a PET/ABS blend in an extruder without a compatibilizer or with a commercially available compatibilizer, namely, maleated SEBS triblock copolymer. They observed that the uncompatibilized blends were immiscible and consisted of four phases: styrene-acrylonitrile copolymer (SAN), grafted polybutadiene, amorphous

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PET, and small amounts of crystalline PET. They observed that the two major phases, SAN and amorphous PET, interpenetrated to result in cocontinuous structures over the compositional range of 30-70 wt.% PET. The flexural modulus and yield stress increased in an almost linear fashion with the increase of the PET content in the blend. However, the notched Izod impact strength and toughness reached the highest levels at 50 wt.% PET.

9.2. PET Nanocomposites The development of nanocomposites with different kinds of nanofillers has become an attractive subject in material science in recent years. PET nanocomposites reinforced with layered silicates have been intensively investigated as promising materials for packaging applications. Among the diverse types of layered silicates, montmorillonite (MMT) is particularly attractive as a reinforcement for polymer/clay nanocomposites [209-212]. It is a naturally occurring 2:1 phyllosilicate with a high surface area and its crystal lattice is composed of two silica tetrahedral sheets and an octahedral alumina sheet between the tetrahedrons [213]. The production of organically modified organoclays by exchange of intragallery cations with alkylammonium ions has attracted great interest in the last decade [214]. As a result, the surface energy of MMT decreases, the wetting properties of polymer–clay interface is enhanced, and the basal spacing expands causing an easing in the intercalation of the polymers [215, 216]. Upon the loading of a small amount of clay, PET/organoclay nanocomposites exhibit significantly improved mechanical, thermal, flame resistant and physicochemical performance, compared to pure polymer or conventional composites. The high aspect ratio (70–150) [217], large surface area (750 m2/g) [217] and high strength and stiffness of nanoclays are reasons why these materials have such an influence. In order to obtain polymer/clay nanocomposites (NCs), aggregation of the nanolayers and tactoids of a few nanolayers must be prevented.

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Incorporation of organoclay can have a significant influence on the applications of PET, potentially improving flame resistance for textiles, decreasing oxygen permeability in food packaging and enhancing the modulus in reinforcement and packaging products. The main approaches for the preparation of PET-based nanocomposites are in situ polymerization [218-222], solvent casting [223], and the melt intercalation method [224, 225]. In most cases, intercalated PET-based nanocomposites were prepared. PET/clay nanocomposites prepared by in situ polymerization result in high levels of dispersion and improved physical properties. However, a more commercially viable approach with conventional melt processing leads to relatively poorly dispersed clay in the polymer matrix. This phenomenon is attributed to the low decomposition temperature (250°C) of the ammonium salt modifier bound to the clay surface. In addition, the presence of thermally stable organoclay can overcome the problem of melt compounding and processing at high temperatures. Imai et al. [226] prepared intercalated PET/clay nanocomposites with different types of cationic surfactants as compatibilizers with a two-step polymerization: a melt polymerization of bis(2-hydroxyethyl)terephthalate followed by a solid state polymerization. The higher tensile strength of nanocomposites was observed, compared to those prepared without compatibilizers. Davis et al. [227] compounded PET/montmorillonite clay nanocomposites by melt blending in a corotating mini twin-screw extruder. They prepared PET/organoclay nanocomposites with an intercalated structure. Nanocomposites compounded with 1,2-dimethyl-3- Nhexadecylimidazolium treated montmorillonite had higher levels of clay dispersion and delamination within the PET matrix compared to those prepared with a typical quaternary ammonium salt as the modifier. Abdallah et al. [228] carried out surface modification of the purified bentonite with phosphonium salts which resulted in thermally stable organoclays that can be used in the melt processing of PET. The compatibility of the organoclay with the polymer matrix was the main factor in the intercalation and dispersion of the organoclay within the PET matrix. They found that the alkyl based organoclay exhibited better

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dispersion and thus, higher tensile strength and elongation at break in the PET/organoclay/elastomer ternary nanocomposites than the aryl-based organoclay did. Upon compounding PET with alkyl and aryl phosphonium organoclays, the onset decomposition temperature of PET increased from 413°C to 420°C and 424°C, respectively. The DSC results showed that the use of organoclays did not significantly promote the nucleation process in PET based nanocomposites, whereas the elastomer decreased the level of crystallinity. There are reports in literature of PET nanocomposites incorporating carbon nanotubes (CNTs), such as single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). CNTs are a good candidate for use as nanofillers for high strength polymer nanocomposites because of their excellent mechanical, thermal and electrical properties, high aspect ratio, and nanometer scale diameter [229, 230]. An important increase of the tensile modulus and yield strength of polymers has been reported after the random dispersion of CNTs. It is known that the addition of CNTs to PET not only improves the mechanical and electrical properties of PET, but also increases the rate of crystallization of PET by acting as a nucleating agent [231]. The chemical modification of CNTs is a good method for overcoming the problems related to the poor dispersion of CNTs into the polymer matrix due to strong van der Waals interactions between the CNTs [232]. Recently, various approaches toward enhancing the properties of PET/CNT nanocomposites using the surfactant and chemical functionalization of CNTs have been demonstrated, including the functionalization of CNTs through covalent and noncovalent reactions with organic molecules and polymers [230]. The other methods for improving the dispersion of CNTs within the PET matrix are solution processing techniques, in situ polymerization and melt processing [230, 233]. Gómez-del Río et al. [234] observed that, due to their large aspect ratio and mechanical properties, SWCNT are considered to be unique candidates for making PET-based nanocomposites. They prepared nanocomposites of PET reinforced with small quantities of carbon nanotubes by in situ polymerization. The results showed that the Young’s modulus increases

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continuously with the volume fraction of SWCNT added, while the Poisson ratio decreases. Regarding the plastic properties of PET and its nanocomposites, yield stress exhibits a minor increase with the SWCNT content, but the failure strain is dramatically reduced with the presence of SWCNT. Despite the superior mechanical performance of SWCNTs, MWCNTs are the most common variety of CNTs studied and a wide variety of surface functionalization has been investigated. Yoo et al. [229] prepared PET nanocomposites by melt-extruding mixtures of PET and functionalized MWCNTs with some interaction with PET molecules. Isocyanate-attached CNTs were obtained through the covalent bonding of the isocyanate group on the molecule and CNT. They observed that PET nanocomposites with MWCNT-phenyl and MWCNT-benzyl showed better dispersion of nanotubes in the PET matrix because of the increased interaction between the PET chains and CNTs compared to the nanocomposites of pristine MWCNT and MWCNT-COOH. The PET nanocomposites containing isocyanate groups showed improved mechanical properties, including tensile strength and tensile modulus, compared to those with pristine and acid-treated MWCNTs. These improvements were ascribed to π–π interactions between the aromatic rings of the PET molecules and the isocyanate group in MWCNTs. The crystallinity of the PET/functionalized MWCNT nanocomposites was significantly higher than those of the pristine and acid-treated MWCNTs. Fourier transformed infrared spectroscopy (FTIR) results indicated that the presence of MWCNTs induced the trans-conformation of PET chains, and trans-conformation was particularly dominant in the PET composites incorporating MWCNT-phenyl. Yesil and Byram [235] prepared PET-based conductive composites by using functionalized CNT by melt mixing with a twin screw extruder. The surfaces of MWCNTs were functionalized by treatment with a strong acid mixture (purification) followed by modification with sodium dodecyl sulfate, PEO, and diglycidyl ether of Bisphenol A (DGEBA). The amount of carboxylic acid groups on the CNT surface increased after acid treatment, but decreased with surface modification due to the consumption

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of these groups during the chemical reactions between the surface modifiers and the CNT surface. FTIR and nuclear magnetic resonance analyses of the composites revealed an increase in the interactions between the PET and CNT surface after treatment with PEO and DGEBA. The mechanical strength of the composites prepared with modified CNT was higher than that of the untreated CNT-filled composite owing to the enhanced interactions between the PET and CNT. Heeley et al. [236] observed that uniaxial deformed PET/MWCNT films displayed improved mechanical properties compared to unfilled PET films. The PET homopolymer was physically blended with low loadings of MWCNTs by weight (1 and 2 wt.%), with the intention of improving the electrical and mechanical properties of the PET. SAXS/WAXS data revealed a well oriented lamellar structure for unfilled PET films. In contrast, the PET-MWCNT composites revealed a nanohybrid shish-kebab (NHSK) morphology, with reduced orientation and crystallinity. The MWCNTs in the PET based composite increase nucleation events, increasing the rate of crystallization of PET, but inhibit the growth of the crystallite structure and reduce the overall crystalline orientation. The development of these PET/CNT composite materials could potentially lead to further industrial applications, e.g., in fuel cells and bipolar plates. There is also a growing body of work on PET nanocomposites incorporating carbon black (CB) nanoparticles. In the case of conductive PET composites reinforced with carbon black, their properties depend on the features of both, polymers (such as the degree of crystallinity, melt viscosity, surface tension) and CB (such as species, surface area, chemical groups, and dibutyl phthalate (DBP) absorption values). The properties of PET based composites are also affected by the distribution of the CB particles in the material, as well as the interaction between CB and polymers and especially, by the percolation threshold [237, 238]. The percolation threshold is the critical amount of CB required to build originally continuous conductive networks, at which the drastic insulator conductor transition appears. A reduction of the percolation threshold of conductive polymer composites is of significance for better mechanical properties, easier processing and a decrease in cost. Many authors [239-

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242] have reported that the CB particles were first localized in the minor phase (i.e., in PET), and then the conductive CB/PET masterbatch was elongated in situ to form conductive in situ microfibrils in the polyethylene (PE) matrix during melt extrusion processing. After compression molding, a conductive three-dimensional microfibrillar network was obtained, resulting in a relatively low percolation threshold (6.0 vol.%). Dai et al. [243] successfully fabricated electrically conductive in situ microfibrillar CB/PET/PE composite with the selective distribution of CB particles on the surfaces of the PET microfibrils. The percolation threshold of the in situ microfibrillar conductive PET composite was only ca. 3.8 vol.%, showing a considerable decrease of percolation compared to that of the common CB/PE composite. They found that interfacial tension, viscosity and chemical groups on the surface of the CB particles and the processing order, were in favor of the migration of the CB particles from the PE matrix to the dispersed PET phases. The morphological observation indicated that, depending on the mixing time, the CB particles gradually migrated from the PE matrix to the surfaces initially, and then to the center of the PET phases. They concluded that the anisotropic distribution of the CB particles, the tunneling conduction, the geometry of the electrical network, and the complex structure of the microfibrillar composite, are likely to make conducting composites.

10. SURFACE MODIFICATION OF PET PET is one of the most commonly applied polymers for textile and biomedical applications [244]. The polyester fibers have interesting physicochemical and mechanical properties. However, the major problems associated with PET are related to its hydrophobic character i.e., poor hydrophilicity, low moisture regain (0.42%) and lack of softness [245248]. Enhancement of the hydrophilicity of PET fibers is an important requirement for many applications, ranging from textile production to applications in the biomedical field.

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In the textile field, increased hydrophilic properties improve comfort in wear with better moisture management due to increased wettability and wicking; it also improves adhesion to other materials (coating for example), and dyeing [244]. Several approaches can be adopted to increase the surface energy and hence the hydrophilicity of PET fibers: physical treatment, chemical treatment and grafting polymerization [244, 249-251]. Surface treatments have been developed to obtain specific surface properties, such as: enhanced surface wettability, adhesion activities, surface functionalization, molecules immobilization, biocompatibility improvement, non-fouling coating and barrier surface coating. The surface properties of PET can be modified both chemically and physically after its surface has been exposed to physical treatments such as plasma, corona discharge, UV irradiation or electron and neutron beam irradiation [252]. Plasma treatments were primarily developed in order to modify surface characteristics of PET, such as: hydrophilicity and adhesive properties [1]. The modification of the surface properties of PET films is of great interest in the packaging industry in order to enhance wettability or printability, to enhance optic characteristics and in biomedical applications to improve biocompatibility. Plasma treatments have the advantage of being eco-friendly by keeping the bulk properties unchanged, compared to conventional chemical treatment cold plasma techniques. The gases used for plasma treatments include NH3 and O2, H2O vapor, He and Ar, O2 and CF4, SF6, and air. These plasma treatments can make the PET fabric surface hydrophilic by introducing highly reactive species (radicals, ions, electrons, excited molecules, photons, etc.). Plasma treatment using the energetic ions and radiation generated in the plasma, leads to disruption of the polymer chains as well as to the introduction of the functional groups on the PET surface [253]. Interestingly, double plasma treatment has been used to graft glycopolymers onto PET fibers in order to obtain a surface which is compatible with the biological material [254]. The first step is based on activation of the surface of the PET using argon plasma treatment leading to the creation of radicals by scission of the chemical bonds. Furthermore, the PET fibers are exposed to the air with the aim of forming

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hydroperoxide functions. Then, the activated fibers are dipped in glycomonomers solutions and dried. The second plasma treatment is used to polymerize the monomers adsorbed on fibers. Yang et al. [247] successfully modified PET film surfaces using a glow discharge plasma treatment. The plasma treatment incorporated polar functional groups onto the surface of the PET film (as shown by XPS analysis) causing a decrease in the contact angle and an increase in the surface free energy. This is due to the existence of a mass of reactive particles (electrons, ions, radicals, etc.) in the discharge area, while in the afterglow area and remote area there are only radicals. The AFM and weight loss results showed moderate increased roughness in the afterglow area and the properties of the PET film surfaces were more outstanding in the afterglow and the remote area. The above changes in the PET surfaces made the films more hydrophilic and increased the surface free energy leading to PET being suitable for industrial applications. Takke et al. [244] demonstrated that immobilizing a hydrophilic oligomer like PEO immediately after plasma treatment would perhaps yield a more durable hydrophilic treatment. PEO can be used for surface modification because of its unique properties such as hydrophilicity, flexibility, and nontoxicity. These authors activated PET fabrics with atmospheric plasma treatment before immobilizing PEO on the polyester fabric surface by padding. Air-plasma treatment of the PET fabrics before PEO coating further increases the hydrophilicity of the inner fabric fiber surface (i.e., soil-release properties) and improves adhesion of PEO-1500 to the plasma-treated PET woven fabric surface. Jingrun et al. [255] modified the surfaces of PET by oxygen plasmainduced and ultraviolet (UV)-assisted acrylic acid (AAc) grafting polymerization, and using the –COOH as reacting sites, the molecules of gelatin and bovine serum albumin (BSA) were further co-immobilized on the PET surface. Immobilization of albumin and gelatin markedly decreased the extent of platelet adhesion. Activation compared to the untreated PET surface immobilization of albumin and gelatin molecules which could improve the anticoagulation (i.e., decrease the extent the platelet adhesion and activation on PET surface) of the PET surface, and

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may enhance the endothelialization (i.e., increase endothelial cell attachment on the PET surface) than onto that of the control PET. Many methods of the surface modification of PET by chemical treatment have been described [1]. Generally, these methods involve chemical breaking of the ester bonds by reaction with low molecular weight compounds containing hydroxyl or amine groups in order to generate reactive functional groups on the surface. These reactions enhance the hydrophilicity of the PET surface and generate anchored functionalities (when difunctional compounds are used) available for subsequent reactions. The aminolysis of PET fibers with various diamines (such as 1,2diaminoethane, 1,6-diaminohexane or hydrazine) can introduce amino groups onto the PET surface [251]. This approach leads to the formation of a new amide group and the presence of free alcohol and amine groups on the PET surface. This reaction is effective because amine groups are hydrophilic; thus, aminated PET fabric exhibits excellent hydrophilicity. The amine groups are also reactive; thus, other functional compounds can be grafted onto the PET fabric to endow the PET macromolecules with various functions. One of the disadvantages of this chemical treatment is finding optimal conditions that do not lead to degradation and the loss of the mechanical properties of the PET films or fibers. Thus, the development of a new method to aminate PET fabric while maintaining its tensile strength and weight is very much required. Bech et al. [251] developed a technique based on aminolysis and sugar grafting through which polymeric biomaterials can easily be obtained. They grafted different diamines onto PET fibers by aminolysis reaction and observed that factors such as temperature, solvent, diamines concentration, and reaction time significantly affect the grafting yield. The introduction of amino groups onto PET fibers cannot only modify the hydrophilicity but also provide the active sites for immobilization of biocompatible molecules. Oligosaccharides such as maltose, maltotriose, and maltohexaose were successfully grafted onto the PET surface by reductive amination and amidation.

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Zhou et al. [256] developed a facile aminolysis method involving the gradual concentration of diluted ethylenediamine in order to aminate PET fibers. The results showed that the hydrophilicity of the PET fabric substantially improved and that reactive NH2 groups were introduced onto the PET fibers. The aminated PET fabrics retained the weight loss, breaking strength and elongation, crease recovery angle, stiffness and whiteness of the original PET fabrics. They reported that the aminolysis of PET fibers via the gradual concentration of dilute ethylenediamine is a promising method for the efficient, rapid, and eco-friendly modification of PET fibers. Grafting from approaches can be applied for the surface modification of PET-based materials [1]. In this approach, the polymerization is initiated from initiator-functionalized surfaces. Living polymerization methods are satisfactoty for obtaining polymer brushes following the grafting from approach. These overall methods for the preparation of polymer brushes provide precise control over brush thickness, architecture and composition [257]. Bech et al. [258] demonstrated the preparation of polymer brushes on the surface of PET films, fibers and fabrics. In the first step, a primary amine such as 1,2-diaminoethane is used to anchor the amino groups onto the PET surface by aminolysis reaction. In the second step, atom transfer radical polymerization initiators were grafted by reaction with bromoisobutyryl bromide and this reaction was carried out in bulk using styrene and the corresponding catalyst in the presence of a sacrificial initiator. Then, the polymer brushes were prepared with good control of polymerization.

11. APPLICATIONS OF PET According to the Merchant Research and Consulting Ltd. website [259], the global annual production of PET in 2012 stood at over 28 million tonnes. Asia-Pacific held about half of the overall production, with China ranking first both in the region and throughout the globe. Europe and North America followed after, each accounting for around 14% of the

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installed capacity of PET production worldwide. During 2004-2011, the world PET production increased from nearly 11.3 million tonnes to around 18.6 million tonnes, registering an average annual growth of over 7%. In 2012, it surpassed the 19.8 million tonnes mark. The dominant companies engaged in the global PET industry are Indorama Ventures PCL, DAK Americas, M&G Group, Jiangsu Sanfangxiang Group Co, China Resources Packaging, JBF Industries Ltd, SINOPEC, Octal Holding & Co, Shahid Tondgooyan Petrochemical Co, Lotte Chemical, Far Eastern Textile Ltd and Nan Ya Plastics Corp. The world PET consumption was on an upward trend during 2004-2012. In 2012, it stood at over 19.8 million tonnes. Asia was the leading PET consumer in the world, followed by Europe and North America. The polyester fiber industry is the major enduser sector for PET - over 60% of the annual supply volume was used in this application in 2012. The main use of PET is in the textile industry, to manufacture video and audio tapes, X-ray films, food packaging, as well as water and softdrink bottles. Besides, PET is used in thermoforming applications, as well as engineering resins often in combination with glass fiber. It also used for microwave food trays and food packaging films. In its semicrystalline form, PET is made into a high-strength textile fibre marketed under such trademarked names as Dacron (DuPont) and Terylene (Imperial Chemical Industries Ltd.). PET is the most important of the man-made fibers in weight produced and in value. The stiffness of PET fibers makes them highly resistant to deformation, what imparts excellent resistance to wrinkling in fabrics. They are often used in durable-press blends with other fibers such as rayon, wool, and cotton, reinforcing the inherent properties of those fibers while contributing to the ability of the fabric to recover from wrinkling. PET is also made into fiber filling for insulated clothing and for furniture and pillows. When made in very fine filaments, it is used in artificial silk, and in large-diameter filaments it is used in carpets. Among the industrial applications of PET are automobile tire yarns, conveyor belts and drive belts, reinforcement for fire and garden hoses, seat belts (an application in which it has largely replaced nylon), nonwoven fabrics for stabilizing drainage ditches, culverts, and railroad

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beds, and nonwovens for use as diaper top sheets and disposable medical garments [260]. At a slightly higher molecular weight, PET is made into a highstrength plastic that can be shaped by all the common methods employed with other thermoplastics. Recording tapes and magnetic films are produced by extrusion of PET film (often sold under the trademarks Mylar and Melinex). Molten PET can be blow-molded into a transparent container of high strength and rigidity that also possesses good impermeability to gas and liquid. In this form PET has become widely used in carbonated-beverage bottles and in jars for food processed at low temperatures. PET is used in the packaging sector in the form of films, trays or bottles, its principle use being in bottles. In 2010, almost 70% of all bottled water and soft drinks sold globally was supplied in PET bottles. Due to its excellent mechanical and optical properties, good oxygen and carbon dioxide barrier properties, as well as consumer trend favoring healthier beverage options, lately the carbonated soft drink sector has been growing more rapidly than other applications. Of the few polymers that are potentially suitable for bottles, PET is the only plastic with a balance of properties such as transparency (near 100% light transmission in a bottle), gloss, lightweight and resistance to carbon dioxide permeation. This has resulted in the nearly full replacement of glass in Europe for all but the most demanding applications that require both an oxygen barrier and UV resistance to protect the contents. Recently, growing interest has been observed in the utilization of chemically recycled PET products as raw materials for the preparation of a rather different class of polymers such as unsaturated polyester resins, polyurethanes, epoxy resins, vinyl esters, and alkyd resins [124]. Also, PET is a commercially available fabric suitable for many biomedical applications. Due to its ability to retain mechanical strength and fatigue characteristics for a long period of time in the body, as well as considerable biocompatiblity, PET has attracted considerable attention for many applications such as vascular graft, sewing cuffs of mechanical heart valves, sutures, etc. Under trademark Dacron®, PET has been used

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successfully as a material for blood vessel tissue engineering in treating the pathology of large diameter arteries [261]. The first tissue-engineered blood vessel substitute was created by Weinberg and Bell in 1986 [262]. They generated cultures of bovine endothelial cells, smooth muscle cells and fibroblasts in layers of collagen gel supported by a Dacron® mesh. Although physiologic pressures were sustained for only 3-6 weeks, they did demonstrate the feasibility of a tissue-engineered graft with human cells. Dacron® is most commonly used for aortic replacement and to a lesser extent as a conduit for femoropopliteal bypass surgery [263]. Numerous following studies have focused on the improvement of the existing and development of new PET based biomaterials. Ma et al. [264] reported on the modification of conventional polymer used in vascular graft, which was processed into non-woven nanofiber mat (NFM) via electrospinning. To overcome the chemical and biological inertness of the PET surface, gelatin was covalently grafted onto the PET NFM surface to produce a new kind of material for blood vessel tissue engineering. Also, hybrid materials combining PET and different types of cells (endothelial and osteoblastic cells) have been developed thanks to the covalent grafting of different densities of rat genome database (RGD) containing peptides onto the polymer surface. The results demonstrate that various peptides, grafted to PET surface, act to enhance osteogenic differentiation and mineralization of pre-osteoblastic cells, which has a potential in developing engineered biomaterials for bone regeneration [265]. Kidane et al. [266] developed a simple two-step procedure for covalent grafting of poly(ethylene oxide) (PEO) onto the surface of Dacron® fabric. This surface modification substantially reduced platelet adhesion on the PET polymer. To improve mechanical properties of collagen, a novel porous scaffold for bone tissue engineering was prepared with collagen sponge reinforced by polypropylene/poly(ethylene terephthalate) (PP/PET) fibers. Incorporation of PP/PET fibers significantly enhanced the compressive strength of the collagen sponge. Proliferation and osteogenic differentiation of mesenchymal stem cell in collagen sponges reinforced with PP/PET fibers incorporation were significantly enhanced compared with collagen sponge without PP/PET incorporation. The incorporation of

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PP/PET fibers not only improves the mechanical properties of collagen sponge, but also enables mesenchymal stem cells to positively improve their proliferation and differentiation [267].

CONCLUSION AND FUTURE TRENDS Since the mid-twentieth century PET has been attracting a great deal of interest due to its excellent mechanical properties, high thermal stability and chemical resistance, as well as processability and optical clarity. Nowadays, a broad range of pure and modified PET grades are available, and are found in many applications such as fibers, films and food and beverage packaging applications. The use of PET as a recyclable polymer has opened numerous new markets, which are in constant growth. Through mechanical recycling, waste PET bottles can be found in films, sheets, strapping packaging and fiber used for sacking, insulation and floor covering. In the field of mechanical recycling, development of the strategies for maintaining the high average molecular weight of PET during reprocessing is of great importance. Also, the improvement of the chemical recycling processes will certainly still be attracting great interest, both in terms of obtaining the monomers for the re-synthesis of PET, and of producing chemically recycled PET products as raw materials for the preparation of different class of polymers such as unsaturated polyester resins, polyurethanes, epoxy resins, vinyl esters and alkyd resins. Among various polyesters, PET and also PBT are very suitable for use as the hard crystallizable segments in thermoplastic copolyester elastomers, whose commercial importance has constantly been increasing in the last four decades. One of the future trends could be the inclusion of hyperbranched polyesters or dendritic segments into PET-based copolymers. Moreover, some prospect has been obtained in inclusion of liquid crystalline sequences into the backbone or as side chains. In addition, numerous studies have so far focused on the blends of PET with other polymers in order to achieve an attractive balance of

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processability and mechanical and barrier properties of the final products. The blends based on PET and other, mostly amorphous, polymers often have enhanced the solvent resistance and processability in comparison with amorphous component of the blends. On the other hand, the presence of other polymer can improve toughness and reduce the mold shrinkage of PET connected with its crystallization. Various type of nanofillers such as organoclay, carbon nanotubes and carbon black are usually used in improving physical, thermal and mechanical properties of PET. Incorporation of nanofillers can have a significant influence on the applications of PET, potentially improving flame resistance for textiles, decreasing oxygen permeability in food packaging and enhancing the modulus in reinforcement and packaging products. Future trend in development of PET-based materials would be their reinforcement with different nanomaterials, such as organoclays, nanoparticles and nanofibers. Due to the wide applications of PET in many fields, it can be expected that the volume of its production will be further enhanced in the future.

ACKNOWLEDGMENTS This work was financially supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia and by the Ministry of Science and Technology of the Republic of Srpska.

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In: Polyethylene Terephthalate Editor: Naomi A. Barber

ISBN: 978-1-53611-991-6 © 2017 Nova Science Publishers, Inc.

Chapter 2

THE APPLICATIONS OF POLYETHYLENE TEREPHTHALATE FOR RF FLEXIBLE ELECTRONICS Tzu-Hsuan Chang, Yei Hwan Jung, Dong Liu, Hongyi Mi, Juhwan Lee, Jiarui Gong and Zhenqiang Ma Electrical and Computer Engineering, University of Wisconsin-Madison, Madison, US

ABSTRACT Polyethylene terephthalate (PET) has been one of the most reliable and cost-efficient candidates in recent development of flexible device applications. Flexible electronics have rapidly evolved into a variety of applications including displays, e-papers, solar cells, sensors, wearable electronics, etc. In particular, high-performance electronics such as radiofrequency devices have been designed and fabricated on flexible substrates, such as PET, with operating frequencies of up to the gigahertz (GHz) regime, which covers current major portable electronics, wireless communication, and transmission units. The flexible electronics

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Tzu-Hsuan Chang, Yei Hwan Jung, Dong Liu et al. that operate at the GHz regime are generally composed of highperformance flexible microwave active and/or passive devices. Typically, they have stringent requirements for substrate choice, in terms of mechanical properties, thermal properties, and electrical parameters, which have a significant impact on the performance at high frequencies. In this chapter, we review the frequency-dependent parameters of the PET substrate, developments of the flexible RF electronics on PET substrate, and the challenges and potentials of RF applications using flexible electronics fabricated on PET films.

1. INTRODUCTION Radio-frequency (RF) stands for the electromagnetic wave frequencies that are used for radio stations, radar, satellite transmission, and other telecommunication applications. During the last few decades, the focus of RF applications has been placed on the general frequency spectrum which has the lowest transmission attenuation in air. The outer shell of the atmosphere mainly contains nitrogen (N2), oxygen (O2), carbon dioxide (CO2), water vapor (H2O), and ozone (O3). Due to different resonant vibration frequencies of each individual gas molecules in the atmosphere, a significant amount of electromagnetic wave energy is absorbed at certain wavelengths. For example, H2O absorbs the electromagnetic wave with wavelengths in the range of 5.5-7.0 μm and larger than 27 μm. CO2 primarily contributes to absorptions in the mid and far infrared portions of the absorption spectrum. O3, which absorbs energy strongly in the ultraviolet (UV) range, heavily covers the atmosphere and participates in signal transmission in satellite communication. In contrast to the absorption spectrum, the transmission spectrum shows the fraction of electromagnetic energy that can pass through the atmosphere and propagate at a given distance. Figure 1 shows the absorption/transmission spectrum of the atmosphere. The intermediate bands in between the high absorption region, open the windows for the electromagnetic waves with certain wavelengths to transmit through the atmosphere in wireless applications.

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Figure 1.1. Absorption/Transmission spectrum at which electromagnetic radiation will penetrate the Earth's atmosphere. Chemical notation (CO2, O3, etc.) indicates the gas responsible for blocking sunlight at a wavelength. Copyright 2016 NASA.

RF electronics, as indicated by the term, are electronics that operate at high frequencies ranging from 3kHz to 300 GHz [1]. Pursuing higher frequency in RF applications leads to the need for a broader bandwidth and higher speed in telecommunication applications. As engineers pursue the discovery of higher performances in RF applications, another branch of RF electronics has gained popularity in the development of flexible electronics, which cover anything from flexible displays, e-papers, wearable electronics, flexible RFIDs for warehousing, to implantable or bandage-like in situ biomedical monitoring electronics. Typically, lowspeed flexible electronics are based on organic materials [2-4] or noncrystalline semiconductors (e.g., amorphous-Si) [5] or metal oxide materials [6], which are enabled by related process developments including large area printing, coating, and deposition techniques that are compatible with the flexible substrate. On the contrary, RF flexible transistors can extend flexible electronics applications toward wireless data transmission and wireless power transfer with wider signal handling capability, allowing circuits to operate with significantly lower power consumption, higher signal-to-noise ratio, and faster operating response [7]. Nevertheless, such organic or non-crystalline semiconductors are unsuitable for RF electronics

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fabrications due to its poor intrinsic electrical properties. Thus, semiconductors with superior intrinsic carrier mobility, such as inorganic semiconductors, must be utilized. In parallel, the substrates in which the active electronic components adhere to must possess certain specifications to minimize RF loss, be compatible with electronics fabrication, and maximize reliability. Table 1. Available candidate plastic substrates

Tg, °C Transmission (400-700 nm), % Moisture absorption, % Young’s Modulus, Gpa Tensile strength, Mpa Density, gcm-2 Refractive index Birefringence, nm

PET (Melinex) 78

PEN (Teonex) 121

PC (Lexan) 150

PES (Sumilite) 223

PI (Kapton) 410

89

87

90

90

Yellow

0.14

0.14

0.4

1.4

1.8

5.3

6.1

1.7

2.2

2.5

225

275

-

83

231

1.4 1.66 46

1.36 1.5-1.75 -

1.2 1.58 14

1.37 1.66 13

1.43 -

Table 2. Comparison of Organic/Inorganic Materials for Flexible Electronics Organic P3HT Pentacene C60 Inorganic Si Ge GaAs

Mobility (cm2/V*s) 10-5 ~ 10-1 ~8.9 2~4.9 Mobility (cm2/V*s) 1400 3900 8500

Bandgap (eV) 2.0 0.97 2.3 Bandgap (eV) 1.12 0.6 1.39

Young’s Modulus (GPa) 1.3 1.0 8-20 Young’s Modulus (GPa) 185 103 85.5

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References [8, 9] [10, 11] [12, 13] References [7, 14] [15, 16] [17]

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Plastic materials are perfect for flexible electronics as they are bendable in nature. Among a variety of plastics, the quest to find the most suitable substrate starts with comparing the physical properties. Ideally, the substrate must be able to withstand high temperature, repel moisture, have good mechanical strength, and have high transparency The properties of the common candidate plastics for flexible electronics fabrication are shown in Table 1. Polyethylene terephthalate (PET – e.g., Melenix® from DuPont Teijin Films) [18-20], polyethylene naphthalate (PEN, e.g., Teonex® Q65 from DuPont Teijin Films) [19, 21, 22], poly carbonate (PC, e.g., Lexan® from GE) [20, 23], polyethersulfone (PES, e.g., Sumilite® from Sumitomo Bakellite) [20, 24], and polyimide (PI, e.g., Kapton® from DuPont) [25, 26] comprise the candidate substrates. Polyimide has excellent glass transition temperature for the process capability; nevertheless, it absorbs visible light which is not feasible for electronics applications that require the substrate to be transparent. Engineers have been investigating clear plastic substrates with high process temperature for the replacement of glass with the conventional electronics fabrication process [27, 28]. Also, high moisture absorbing materials like PC and PES are unsuitable for electronics fabrication as the processes generally require chemical solvents and organic compounds. PET and PEN can repel moisture, while at the same time exhibit high mechanical strength and relatively high glass transition temperature (enough for electronics processing). Out of the two, PET is the material of choice for many applications due to its high-abundancy and low-cost. The main challenges in the development of RF flexible electronics on PET are: (1) the lack of high quality active channel materials with sufficient mobility and simultaneous mechanical flexibility, and (2) difficulties in designing a fabrication procedure that is thermally compatible with the PET substrate [7, 14, 29, 30]. Table 2 summarizes the mainstream organic/inorganic materials used in flexible electronics. Organic materials typically feature a relatively low Young’s modulus that is close to the modulus of PET film (2~2.7 GPa) compared with other flexible substrate candidates. The high bendability and ease-of-fabrication have enabled the organic materials-based applications to prevail in the

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early development of flexible applications, but the intrinsic low carrier mobility of the organic materials pose an inherent barrier to achieve highspeed electronics devices that are comparable to those of its rigid counterparts. As an alternative, scientists and engineers have been searching for solutions to shape bulk inorganic semiconductor materials that are superior to organic in terms of carrier mobility into flexible formats and produce high-performance RF electronics in practical flexible applications. Even though inorganic materials are rigid in their bulk formats due to a high Young’s modulus, the scaling law developed in microelectromechanical systems (MEMS) explains that flexural rigidity decreases as the thickness of the bulk materials decrease. As shown in Figure 1.2a, a 22 nm thick Si nanomembrane can be bent without fracturing and thus is highly flexible.

Figure 1.2. a) SEM image of a 22 nm Si nanomembrane shows high flexibility. b) Flexural rigidity decreases linearly with thickness and a 2 nm Si nanomembrane (dash line) shows a ~1015 times smaller flexural rigidity than a 200μm Si bulk wafer. The blue line represents calculations for silicon nanomembranes bonded to sheets of polyimide at room temperature, and then heated to 300°C [31].

In Figure 1.2b, the flexural rigidity of a 2 nm-thick silicon thin-film is on the order of ~1015 times smaller than that of a 200 μm thick Si bulk wafer. Meanwhile, the energy release rate also decreases linearly with thickness, enabling inorganic nanomembranes to conformably and robustly bond onto the flexible substrates. Figure 1.3 summarizes the development on the fast (high-speed) flexible electronics within the last decade.

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Researchers have been working on replicating those rigid electronics based on the flexible infrastructure. Significant efforts to maximize frequency responses of RF transistors on flexible substrate have been made, with the majority of such developments being made on PET films. These advances are known for the ability to transfer print numerous inorganic semiconductors on flexible plastic films, where certain conventional electronics fabrication processes may be utilized. Other active channel materials with exceptional properties, such as Ge, III-V group, and complex epitaxy heterogeneous structures, have been introduced into flexible electronics and the function of the flexible electronics has moved from single flexible RF transistors to functional integrated circuits. In the following sections, a generic fabrication process for building RF devices using thin-film nanomembranes on PET films are introduced.

Figure 1.3. A short development history of flexible RF active devices (and related passive devices) on plastic substrates using transferrable nanomembranes. Update from [14].

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2. FABRICATION PROCEDURE OF PET FOR FAST FLEXIBLE RF ELECTRONICS Although many approaches based on organic semiconductors have been demonstrated with low production cost and high flexibility that can take advantage of the flexibility of the PET substrate, these organic materials generally have relatively low mobility in the range of below 1 to 10 cm2/Vs for electronic devices, respectively. Thus, the typical transient response from these devices is around the order of mili-seconds that are only suitable for the flexible display applications, but not for RF electronics. To adapt the mature rigid applications into flexible applications and explore its potentials, researchers have been looking for reliable methods to prepare the large scale single crystal semiconductor nanomembrane. The nanomembrane is thin enough to maintain the flexibility. On the other hand, it still possessess its intrinsic mobility from several hundreds to thousands cm2/Vs, which is generally two orders of magnitude higher than the organic semiconductors. In the following paragraphs, the current mainstream approaches in preparing the highquality free-standing single-crystalline nanomembranes are covered, which render high speed flexible electronics possible.

2.1. Thin Film Membrane Released from Anisotropic Etching of Bulk Wafer The initial approach to prepare thin film Si nanomembrane is derived from the intuitive idea of releasing the thin-film from the bulk singlecrystalline substrate. Bulk Si substrates have been the fundamental building blocks in the current semiconductor industry and can be grown in large-scale at low-cost. The direct release of the thin-film membrane from the bulk substrates can significantly reduce the production cost and the bulk substrate can be later polished and recycled.

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Figure. 2.1.1. Schematic process flow of single-crystal silicon ribbon fabrication. a) SF6 plasma etch trenches in a (111) Si surface. b) Thermal oxidation and angled evaporation of Ti ∕Au passivate the sidewalls. c) Hot KOH ∕IPA ∕ H2O solution undercuts the Si ribbons. d) Cross-sectional SEM image of partially undercut ribbons [32].

Because the target thin film Si nanomembrane has similar chemical properties as the bulk Si substrate, this approach is not unveiled until the anisotropic wet etching of Si is discovered [33]. The atoms of single crystalline Si are arranged in a diamond cubic lattice and the periodicity of the lattices are organized in three orientation planes: (100), (110), and

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(111). By viewing the Si lattices from these three directions, the atomic density of the three directions are different. The plane where Si has the lowest density, (100), is more vulnerable to chemicals such as KOH(aq) and TMAH. Si (110) and especially Si (111), which has the highest density of Si atoms, are not etched until the surrounding Si atoms are attacked; thus, the etching rate is much slower compared to Si (100). This phenomenon is identified as the anisotropic etching of Si and has been widely used in the micro-electro-mechanical-system (MEMS) to fabricate reliable vibration/gauge sensors. To release the thin Si nanomembrane, Si (111) substrate is adapted as shown in Figure 2.1.1. Before immersing the Si (111) substrate into the anisotropic KOH etchant, a trench is formed in the Si substrate by plasma SF6 etching and the design of the trench is to define the final thickness of the released Si nanomembrane. After etching, thermal oxidation is applied and a Ti/Au protection layer is deposited around the membrane area. SiO2 has high selectivity in KOH etching and prevents the KOH from attacking the sidewalls of the membrane. The Ti/Au layer is used as the hard-etching mask when removing the oxide in the trench and the protection layer through the KOH wet etching process. After immersing the structures in hot KOH/IPA/H2O solution, the exposed Si will be etched inwards along the horizontal Si (111) plane that is protected within the oxide. The front edge of the etching will stop at the Si (110) plane and move towards the two sides until the etching from both sides meet. The released Si nanomembrane is then rinsed with KI2 metal etcher to remove the gold and is then ready for the post-transfer process.

2.2. Thin Film Membrane Released from Si on Insulator (SOI) Substrate Different from the anisotropic wet releasing approach, this method has much higher controllability over the uniformity of the released nanomembrane. The thickness of the released membrane could range from a few nanometers to a couple of micrometers and the surface roughness can be controlled within ~1 nm. To adapt this approach, special Si on

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Insulators (SOI) substrates have been prepared by the SmartCut process. The target Si nanomembrane is embedded on top of the handling Si substrate with a thick SiO2 sacrificial layer as illustrated in Figure 2.2.1a. Similar to the anisotropic releasing approach, hydrofluoride (HF) solution was employed as the wet etchant to selectively remove the SiO 2 sacrificial layer.

Figure 2.2.1. Generic process for Si nanomembrane release from SOI and transfer. (a) Use SOI as the starting material; (b) Patterning top Si template layer into strips or meshed NM and partially expose BOX; (c) Immersing SOI into aqueous HF to undercut BOX; (d) Si template layer falls as BOX is fully undercut and gets registered on the handling substrate. Two transfer routes exist: direct flip transfer (e1)–(f1) and stamp-assisted transfer (e2)–(g2). (e1) Flexible substrate with adhesive coating is attached to Si nanomembrane. (f1) Peel off the plastic substrate with NM transferred. (e2) Si nanomembrane is first picked up by the elastomeric stamp. (f2) Bring the stamp into contact with a (adhesive-coated) new host substrate. (g2) The stamp is slowly peeled off, leaving the NM attached/transferred to the new host [14].

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In the approach, the top Si layer is patterned with an array of holes or strips to expose the SiO2 layer underneath, which is later removed by HF solution. The membrane becomes suspended and falls to the top of the bottom handling substrate due to capillary force when the sacrificial layer is removed, as shown in Figure 2.2.1b-2.21d. The free-standing Si nanomembrane is then picked up by the PDMS stamp and printed on to the flexible substrate for further processing. The printing process can be classified as direct-flip transfer, by flipping the released membrane upside down, and stamp-assisted transfer, which maintains the original surface direction. The high yield and high reproducibility of the release/print top Si layer SOI has boosted the rapid advancement in high performance flexible electronics. The success of this approach relies heavily on the discovery of the PDMS releasing and printing methods [34, 35]. As shown in Figure 2.2.2, the soft PDMS stamp is used to pickup (Figure 2.2.2a) and print back (Figure 2.2.2b) the target membrane. The stamp is peeled away from the substrate at a steady-state speed υ, which is similar to the steady-state propagation of a crack at the same velocity. The thin film and substrate are both elastic, while the stamp is viscoelastic. The critical energy release rate for the film/substrate interface is denoted by , which is considered a material property of the interface and is independent of the peeling velocity υ since the film and substrate are elastic, while , the energy between the stamp and the film, is a monotonically increasing function of the release rate υ. In other words, adjusting the release rate of the PDMS stamp at the speed above Vc which corresponds to the Gcrit will attach the film onto the stamp, as illustrated in Figure 2.2.2c. After the success of releasing the Si nanomembrane with the sacrificial SiO2 layer, researchers have discovered approaches to transfer Ge [36], GaAs [37], InP [38], GaN [39, 40], and even complex epitaxy heterostructures [41, 42].

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Figure 2.2.2 Schematic diagrams of a) the pickup and b) printing of thin film. c) Schematic diagram of critical energy release rates for the film/substrate interface and for the stamp/film interface. The intersection of the horizontal line in the middle with the curve representing the critical peel velocity for kinetically controlled transfer printing. The horizontal lines at the bottom and top represent very weak and very strong film/substrate interfaces, respectively, corresponding to pickup only and printing only [43].

2.3. Thermal Compatibility Issues High speed RF electronics on PET substrate requires distinctly different fabrication procedures from the procedure on rigid bulk Si substrate. Unlike Si substrate with a melting point of up to 1414°C [44], the reported melting point of PET film is as low as 254°C (Dupont Mylar). This will prohibit most standard processes commonly used for rigid applications, such as furnace oxidation (750°C ~1150°C), dopant implantation, dopant activation annealing (750°C ~900°C), and PECVD oxide/nitride (250°C ~ 350°C). Besides the low melting point, PET film will also generate shape distortion and internal tensile strain even at lower temperatures. For example, the basic photolithography and plasma etching process could cause the PET film to deform and curl up, which would significantly compromise the resolution of the lithography, featurealignment in following processes, and the device yield.

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To resolve the distortion issues during fabrication, PET is placed on a hot plate at 115°C for pre-process conditioning for 10-15 minutes. The film is tightly attached to the hot plate with a backside vacuum to distribute the heat uniformly and maintain the flatness of the film. PET film will experience a shrinking of around 1-2% in dimension in the first few minutes and the shape of the film will gradually become stable afterwards. To peel off the PET film from the hot plate, the temperature of the hot plate needs to decrease slowly to below 50°C before removing the film. In this way, the PET film will maintain strain balance on both sides of the film. Even though the pre-baking can greatly reduce the misalignment caused by the distortion of PET film, fabricating devices with feature sizes smaller than 1 μm will still require alternative procedures: transferring the pre-fabricated electronic devices onto rigid substrates. In this approach, the devices are pre-fabricated on rigid substrates with releasable structures. The thermal processes are not limited only to lithography related processes, the preparation of dielectric layer, metallization, and the postprocess capping process should all be taken into consideration. The dielectric layer, for example, needs to be formed by a lower temperature procedure, such as evaporation, sputtering, and atomic layer deposition to preserve the performance of the devices. Multiple high performance flexible RF transistors have been demonstrated using the thermal compatible process of PET film and will be introduced in a later section of the chapter.

2.4. Dielectric Constant and Dissipation Factor Another consideration of designing flexible RF transistors is the dielectric constant and the dissipation factor of the flexible substrate. The dielectric constant characterizes the strength of a material under the electric field, while the substrate with a higher dielectric constant can withhold a higher breakdown voltage and qualify for high-power electronics. However, there exists a trade-off between having a high dielectric constant and device response, as the substrate will result in a crosstalk delay due to

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the parasite capacitance. Hence, for the flexible substrate with a lower dielectric constant can reduce the loss from the embedded electronics [40]. Different from the dielectric constant that represents the ability of storing the energy under the applied electric field, the dissipation factor characterizes of loss-rate of energy of a mode of oscillation electric field, which is one of the critical parameters in designing high speed flexible RF electronics. The dissipation constant can be interpreted by the phase angle δ of the resistive (lossy) component and lossless (reactive) component of an electromagnetic field in the complex plane, usually quantized by tangent δ and named tangent loss of a material. If the tangent loss of a substrate is large, the majority of the electromagnetic energy transport in this material will dissipate as localized heat, thus significantly reducing the efficiency of the RF devices. In the following paragraph, we will briefly introduce the state-of-the-art methods of accurately measuring the dielectric constant and tangent loss of a flexible substrate.

Figure 2.4.1. a) The schematic structure of microstrip transmission line. b) Optical image of fabricated microstrip transmission line based on the PET film c) Equivalent circuit model of the transmission line. The parasitic elements L, C and R were included in this model [45].

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The characterization of these two factors have been an important issue in designing the high speed flexible electronics, since the existing approaches cannot precisely retrieve the two parameters of PET at high frequency. The transmission line method (TLM) approximation with a single measurement was recently developed for characterizing microwave dielectric properties of thin films and is applicable in a wide frequency range [46-48]. The schematic of the TLM structures is shown in Figure 2.4.1a and the PET embedded transmission line is shown in Figure 2.4.1b. In this method, thin-film is adopted as the substrate for microstrip transmission lines. The dielectric properties of the thin film can be extracted from measured scattering parameters (S-parameters). This method is employed to characterize the microwave dielectric properties of flexible substrates in the frequency range of 1 - 10 GHz, which covers the wireless communication bands: the L-, S-, C- and the lower part of the Xbands. The transmission (signal) line, made of thick copper foil tape, was made to cover the entire length of the flexible PET film, and is designed to be long enough for several guided wavelengths, in order to reduce the influence of the SMA connectors and parasitic effects between the connector and transmission line, on overall measurement accuracy. An equivalent circuit for this microstrip transmission line was built using Agilent/Keysight Advanced Design System (ADS). This circuit also included the presence of an edge mounted SMA connector on each side and illustrated parasitic effects between the connectors and microstrip transmission line. The circuit structure is shown in Figure 2.4.1c. The ground plane of the microstrip transmission line was made by covering the backside of the PET substrate with conducting copper foil tape. A flat copper sheet was then brought into contact with the copper foil tape to facilitate mounting of the SMA connectors. Two SMA connectors were finally soldered to the structure. As shown in Figure 2.4.1c, a series inductance (L), a shunt capacitance (C), and a series resistance (R) on each side between the line and connectors were included in the model to represent the transition between coaxial connectors and the transmission line, and assembly imperfections. Fabrication imperfections exist at the

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two connection points, such as an air gap between the substrate edge and the side of the SMA connector.

Figure 2.4.2 a) Measured and simulated transmission coefficients (S21) of the PET based microstrip transmission line. b) Extracted microwave dielectric properties (dielectric constant and loss tangent) [45].

Table 3. Comparison of Dielectric Constant & Tangent Loss of Flexible Substrate and Si

Si PET Kapton

Dielectric Constant 11.7 3.1 3.7

Tangent Loss (1kHz) 0.005 0.008 0.002

Tangent (10 GHz) 0.015 0.013 0.005

Figure. 2.4.2a shows the extracted s-parameters from the designed TLM circuits, and the average dielectric constant and loss tangent for both the PET films are extracted from the simulation (blue line) to fit the experimental data [48, 49]. The PET film shows an average dielectric constant close to 3.1 and an average loss tangent between 0.008 at 1 kHz and 0.013 at 10 GHz. 10 GHz is beyond the current prevailing 4G standard Wi-Fi applications and more than enough for flexible applications. PET showing an improved tangent loss over Si at high frequency will facilitate the development of high frequency flexible electronics. Table 3 lists most

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of the research representative flexible substrates and the related extracted parameters.

3. APPLICATION OF HIGH SPEED FLEXIBLE ELECTRONICS ON PET Among many types of flexible electronics, high-performance devices are especially challenging to fabricate on plastic substrates, requiring advanced fabrication techniques that typically involve high temperatures and organic compounds [50]. RF electronics usually comprise of such high-performance components as they deal with the generation, acquisition, and manipulation of high-frequency signals, mostly in the gigahertz range. For active RF flexible components, like transistors and diodes, inorganic semiconductors in nanomembrane forms must be utilized, since these materials have superior mobility and wide bandgaps compared to its organic counterparts. To fabricate one, a semiconductor nanomembrane is usually transfer printed onto the targeted plastic substrate and goes through all the advanced fabrication processes, such as photolithography, plasma etching, chemical etching, metal and dielectric deposition, etc. In conventional electronics fabrication, the substrate is typically the semiconductor itself, which is generally not affected by the abovementioned processes. For flexible electronics, however, the plastic substrate typically encounters problems as it can expand or contract during lithography, chemically react in plasma, dissolve in chemicals, and melt at high temperatures. In most cases, the manufacturing tools are not designed for such plastic substrates and the expensive tools also become damaged from the adverse reactions in a process. Only a handful of plastic types are compatible with all processes, and the excellent mechanical properties in conjunction with the high-abundance and low-cost of PET films have enabled various classes of RF components on the film for flexible RF electronics. The fabrication of flexible electronics using inorganic semiconductor

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nanomembranes on PET films starts with a generic process of transfer printing the nanomembrane using rubber stamps as shown in Figure 3.1. Using this process, different types of vital RF components such as transistors, diodes, and switches are fabricated on the PET film.

Figure 3.1. Illustration of Si nanomembrane transfer methods: a,b,c,d) for PDMS assisted transfer and a,b’,d) for direct flip transfer. a) Strips are released in situ on source substrate. b) PDMS was applied to the source substrate and the strips were picked up. b’) A PET substrate coated with an adhesive layer was applied to the source substrate and lifted up the released strips. c) The strips on PDMS were print transferred to PET. d) The transferred strips were fabricated into transistors/circuits using either PDMS or direct flip transfer method. e) Strips are picked up by a PDMS stamp. f) Strips were picked up by PET substrate coated with SU-8 [29].

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Figure 3.2. a) Cross-section illustration of the TFT on PET with the channel length and the overlap distances between gate and source/drain shown. b) Microscope image a two-gate-finger TFT. c) Zoomed-in view of accurate gate alignment realized with the local gate alignment procedure. d) Image of a TFT array on a bent PET substrate. e) Illustration of bending test for the TFT on PET. (f) Measured fT and fmax variations under different bending radii [29].

Of most significance, transistors are the vital components in an RF circuit. Transistors can amplify or switch electronic signals and electrical power, and the ability to do so at high frequencies has outperformed traditional low-frequency transistors in terms of power consumption and signal processing capability. Also, the high-frequency transistors allow wireless communication and power transfer capability. As flexible electronics are mainly advantageous in mobile electronics, like wearable or implantable devices, the wireless functionalities would dramatically expand the applications of flexible electronics. Whereas low-frequency transistors were readily fabricated on PET films using either organic or inorganic semiconductors [6, 32, 35, 51, 52], one of the most important demonstrations that proved PET films can also be utilized for highfrequency transistors is shown in Figure 3.2 [29]. Figure 3.2a represents the cross-section schematic illustration of the device. The transistor is fabricated with silicon (Si) nanomembrane, a high-mobility material mostly comprised of conventional electronics devices, as the active

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material, and silicon monoxide, an insulator with a high dielectric constant, as the gate dielectric material, all on the PET substrate. In addition, the gate length of the device of only 1 μm to maximize the speed of the device was achieved on the PET substrate for the first time as shown in Figure 3.2b and 3.2c. Thus, a record-breaking maximum oscillation frequency of 12 GHz was achieved in flexible format. This is a significant achievement as it indicates that the transistor can be utilized on flexible substrates for amplification of RF signals typically used in wireless communications. Finally, the finished images of the flexible transistors are shown in Figure 3.2d. With this successful entry of a high-speed flexible transistor, many improvements have been made to maximize the speed, such as the strained transistor and nanotrench transistor, all on the same PET film as the substrate [7, 53]. The semiconductor nanomembranes from transfer and printing methods can further integrate different fabrication techniques to boost the device performance by several times. The traditional lithography method (Figure 3.3b) has limits on both the length and thickness of the channel. Due to dopant diffusion, the length of the channel is limited at the microscale, which lower the maximum-oscillation frequency. To reduce the resistance and maintain the mechanical strength of the device, the thickness of the channel is also limited to several hundred nanometers, setting constraints on the on-off speed of the transistor. As an alternative approach, nano-imprinting lithography, as shown in Figure 3.3a, can create a sharp-and-deep-submicron channel on the Si nanomembrane, allowing to overcome the previously mentioned limits posed by the traditional fabrication. Instead of the traditional lateral structure, a vertical PN structure on the Si membrane was built, which helped to overcome the problem of dopant diffusion, making it possible to narrow down the channel length to mechanical limit. The channel length can be narrowed down to 100 nm, while the thickness is only 20 nm (Figure 3.4b). As a result, the carriers of the active channel are highly confined only in the local thin bridge-like structure, as shown in Figure 3.4c, and this unique structure significantly improves the overall DC characteristics of the transistors, as shown in Figure 3.4d and 3.4e. The maximum oscillation

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frequency of such a device reaches an amazing 38 GHz in practice (Figure 3.4f), making it possible to realize the same performance on flexible substrates as the best transistor on rigid substrates [7].

Figure 3.3. Comparison of the device structures (cross-sectional view) and fabrication processes between a) 3-D nano trench Si nanomembrane flexible RF TFTs, and b) conventional 2-D TFTs. The effective channel lengths (Lch) are marked in red in (a3, b3). The smallest Lch of the nano trench TFT can reach down to 50 nm via NIL and that of the conventional TFT can only reach down to about 1.5 µm. (a1) Blanket phosphorous ion implantation and thermal anneal. (a2) Nano trench formation via nanoimprint. (a3) Final structure of nano trench TFT where the channel length Lch is defined by nanoimprint. (b1) Photolithography to define S/D regions for ion implantation. (b2) Selective ion implantation and thermal anneal. (b3) Final structure of conventional TFT where Lch is limited by gate electrode and dopant out-diffusion during ion implantation and thermal anneal [7].

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Figure 3.4. a) Optical image of arrays of the bent TFTs on a PET substrate. b) Defining a nano trench on a phosphorus implanted p- SOI substrate using NIL. c) Simulated current density with the 250 nm deep trench (70 nm of the trench depth extends into the p- layer: 20 nm p- layer remains as the active channel) forms a very strong fieldeffected channel without a leakage current. d) Drain current versus drain voltage. e) Drain current versus gate voltage. f) Measured (solid lines) and simulated (dashed lines) RF characteristics of the trench TFT [7].

Along with the transistors, diodes play a critical role in an RF integrated circuit, as it can rectify RF signals to DC signals, as well as switch, mix, and regulate high-speed signals. A diode is a simple twoterminal element that conducts in one direction only, allowing current to flow in one direction and blocks from the other direction with high resistance. Diodes utilizing numerous types of materials were demonstrated on the PET film for many DC or low-frequency applications. These are relatively simple to fabricate as it involves rough doping of one terminal and eliminates the need of low contact resistance, which is problematic on plastic substrates. On the other hand, the high-frequency ones are not so easy to fabricate. High-doping concentrations in the top layer are essential to ensure low sheet resistance and low contact resistance. Therefore, careful ion implantation, followed by hightemperature diffusion is carried out during fabrication [54]. Figure 3.5 represents the RF diodes, as well as RF switches made using the diodes, fabricated on a flexible PET film. As shown in Figure 3.5a, the crosssection schematic illustration shows that the diode utilizes a heavily doped

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Si nanomembrane connected with a thin-film metal on top for low resistance. Figure 3.5b shows the final device bent using hands. Figures 3.5c and d show the optical-microscope images of the finished diodes and a shunt-series single-pole single-throw switch containing the two diodes, respectively. Further demonstrations of RF diodes incorporating other semiconductor materials like germanium on PET films have been reported [36]. These demonstrations of high-speed switches on PET films prove that high-frequency diodes have enabled RF functionalities that were not possible with only using low-frequency diodes on flexible substrates.

Figure 3.5. a) Schematic cross-section of lateral Si nanomembrane PIN diodes on a flexible PET substrate. b) Optical image of finished PIN diode and switch arrays on a bent PET substrate. c) Optical microscope image of a finished Si nanomembrane PIN diode. Shown in the inset is the diode circuit diagram. d) Optical microscope image of a finished shunt-series PIN diode SPST switch. The circuit diagram of the SPST RF switch is shown in the inset. e) Illustration of the mechanical bending direction for the SPST PIN diode switch under bending test [54].

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Figure 3.6. Illustration of the fabrication process for integrated flexible spiral inductors and MIM capacitors. a) M1 was evaporated on a PET substrate to form the bottom electrode of MIM capacitors and the center lead metal of inductors. b) A 200 nm SiO layer was evaporated on top of the bottom electrode as the capacitor high-k dielectric. M2 was evaporated on top of SiO to form the top electrode for capacitors. The two layers were lifted together to form a self-aligned structure. c) A layer of SU-8 was spun on to act as the intermetal low-k isolation layer. Via holes were opened with lithography and SU-8 was cured to cross link. d) M3 was evaporated to form the spiral metals of inductors and interconnects. An optical-microscope image of e) a 4.5-turn spiral inductor, f) an MIM capacitor, and g) the finished inductor and capacitor arrays on a bent PET substrate. Measured (h) L values and (f) Q values of a 4.5-turn spiral inductor as a function of frequency under flat and bending conditions. The spiral metal line width is 15 µm and the metal line spacing is 4 µm. The inset of (h) shows the zoomed-in graph of L values in the low frequency range. The fres is indicated by the zero L and zero Q values. The two figures have the same X-axis scale [29].

Electronic components, like the transistors and the diodes, require semiconductor materials, whereas some passive components, which are components that do not require an external source of power to perform, like inductors and capacitors, do not necessarily require semiconductors. These two elements are the devices that store energy rather than dissipate it and play important roles in many circuits. At high frequencies, inductors and capacitors are extremely significant as they serve diverse purposes, such as matching, storing, attenuating, and passing of signals. Successful fabrication of RF inductors and capacitors on PET films would allow the

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abovementioned transistors and diodes to be monolithically integrated in a circuit. As the PET allows for electronics fabrication processing, layers of metals and insulators can be lithographically defined and deposited to form spiral inductors and metal-insulator-metal capacitors as presented in Figure 3.6a-f [29]. Figure 3.6g presents the image of the devices fabricated on the PET film. Inductors and capacitors fabricated this way can reliably operate up to 10 GHz even with bending [30]. Other than using lithography techniques, these relatively simple devices are also demonstrated on PET with other techniques like the screen printed passive devices, as presented in Figure 3.7 [55]. As such, different approaches may be used to fabricate RF devices, although using one common technique to fabricate all the devices, including transistors and diodes is ideal, as the devices ultimately need to be merged into a circuit. While the demonstrated RF devices on PET films already perform well at a few to a couple of tens of gigahertz, it is highly desirable that these devices could perform at even higher frequencies. Because of the lack of available bandwidths at low frequencies and the demand for a larger bandwidth, the technology of RF devices is motivated to advance towards higher frequencies for search of unexplored bandwidths. In addition, devices that are capable of operating at higher frequencies are more efficient at lower frequencies. With battery and heat issues associated with flexible electronics becoming prominent, efficiency must be improved as well. On the other hand, the devices operating at a certain frequency on a conventional wafer substrate typically performs at a lower frequency on a plastic substrate like PET. This is due to the intrinsic electrical properties that plastics suffer from, such as low thermal conductivity and higher RF losses. Thus, there are a lot of advancements to be made to the RF electronics devices on PET films. Nevertheless, the remarkable accomplishments made on the flexible RF electronics fabricated on PET films have undoubtedly created a practical and promising path for flexible electronics research.

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Figure 3.7. (a) Photograph of screen-printed RLC circuit using a series combination of 8 μ H inductor and 0.8 nF capacitor, in parallel with a 25 kΩ resistor. (b) Model of the circuit including inductor and capacitor series resistances. (c), (d) Impedance magnitude (c) and phase (d) of the circuit [55].

CONCLUSION In conclusion, we have discussed the applications of RF flexible electronics on the plastic film, PET, as one of the applicable candidates in terms of cost, process temperature (high glass transition temperature, Tg), mechanical flexibility, solvent resistance, and thermal stability. Despite of numerous studies regarding the low temperature process of conventional TFT fabrication on plastic films, the current fabrication technology still limits the flexible electronics application that requires high speed RF elements. With the introduction of the alternative fabrication approach such as membrane transfer technique on PET film, this chapter summarized various applications available for high speed flexible RF electronics on flexible substrates.

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Releasing and transferring thin film from bulk Si wafer or silicon on insulator (SOI) wafer onto plastic film enable high speed RF electronics in flexible form by using active semiconductor materials with high mobility and high saturation velocity, which is also less demanding in regards to processing temperature during preparation of single crystal nanomembrane. While the silicon process temperature is much higher than 1000°C, the PET film has a relatively low Tg which requires heat-treated pre-process conditioning to avoid significant distortions in shape. The new method can accommodate the rest of the RF element fabrication process with high yield as well as produce various elements for RF electronic circuitry. Successful characterization of passive elements including capacitors, inductors, and microstrip transmission line in both low and high frequency microwave circuits using PET as a dielectric layer has been investigated and even active devices have been fabricated on PET film with outstanding flexibility, while still maintaining high performance and achieving a record-breaking maximum oscillation frequency. Other flexible substrates for flexible RF electronic applications have also been introduced, such as PEN, PC, PES, and PI, that can replace PET as a flexible RF electronics substrate, which enable the devices to be wirelessly rechargeable and promote the wireless communication application of portable and wearable electronics. In summary, the unique and exceptional achievements made on RF electronics in flexible form on PET films indeed extend the applications of flexible electronics to portable consumer electronics, wearable sensors, and even nanoscale implantable medical devices in the near future.

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BIOGRAPHICAL SKETCHES Tzu-Hsuan Chang Affiliation: University of Wisconsin-Madison Education: B.S. in Computer and Electrical Engineering, National Taiwan University M.S. in Computer and Electrical Engineering, National Taiwan University Ph.D. in Computer and Electrical Engineering, University of Wisconsin-Madison Business Address: 1415 Engineering Dr., B640 Madison, WI 53706

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Research and Professional Experience: Chang was a research assistant in National Taiwan University majoring in enhance the efficiency in automata and later pursuing the Ph.D. degree in UW-Madison, U.S.A. In the Ph.D. degree, Chang continued his research assistant appointment with research topic such as self-assembly sub-10nm nanofabrication, high power fast flexible electronics, green electronics, 2D materials, and novel renewable energy from heterogeneous tandem solar cell and photonic applications. Chang is currently a post-doctoral fellow in UW-Madison and continuing his research in high power k-band flexible electronics, solar cell, and high speed UV detectors. Professional Appointments: Post-Doctoral Fellow, University of Wisconsin-Madison Honors: Harold A. Peterson Best Dissertation Award 2016. Publications Last 3 Years: 1) T.-H. Chang, K. Xiong, D. Liu, M.-Y. Wu, H. Mi, H. Zhang, Z. Xia, S. H. Park, M. S. Arnold, J. Han and Z. Ma, “Fast solar-blind AlGaN/GaN 2DEG UV detector with transparent graphene electrode,” Government Microcircuit Applications and Critical Technology Conference (GOMACTech), Reno, NV, March 20-23, 2017. 2) T.-H. Chang, S. Xiong, R. M. Jacobberger, S. Mikael, C.-C. Liu, D. Geng, X. Wang, M. Arnold, Z. Ma, P. F. Nealey, “Directed selfassembly of block copolymer films on atomically-thin graphene chemical patterns,” Scientific Reports, 6, 31407, 2016. 3) T.-H. Chang, W. Fan, S. Liu, D. Liu, Z. Xia, L. Menon, H. Yang, J. Berggren, M. Hammar, Z. Ma, W. Zhou, “Selective Release of InP Heterostructures from InP Substrates,” JVSTB, 34, 041229, 2016. 4) H. Mi, C. Liu, C. Yao, T.-H. Chang, J. Seo, H. Zhang, S. Cho, S. Gong, N. Behdad, Z. Cai and Z. Ma, “Microwave dielectric

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characterization of biogradable cellulose nanofibrils (CNF) thin film for flexible microwave applications,” Cellulose, 2016. 5) Z. Ma, Y. H. Jung, T.-H. Chang, J.-H. Seo, Z. Cai and S. Gong, “Green Microwave Electronics for the Coming Era of Flexible Electronics,” 2016 IEEE/MTT-S International Microwave Symposium, San Francisco, CA, May, 2016. 6) Z. Ma, Y. Jung, J. Seo, J. Lee, S. Cho, T.-H. Chang, H. Zhang, S. Gong and W. Zhou, “Radio-frequency flexible and stretchable electronics,” Proceedings of CSTIC, Shanghai, China, Feb 13-17, 2016. 7) D. Liu, Z. Xia, S. Cho, D. Zhao, H. Zhang, T.-H. Chang, X. Yin, M. Kim, J. Seo, J. Lee, X. Wang, W. Zhou and Z. Ma “Cavity enhanced 1.5 m LED with silicon as hole injector,” SPIE Photonics West, San Francisco, CA, Feb, 2016. 8) Y. H. Jung*, T.-H. Chang*, H. Zhang, C. Yao, Q. Zheng, V. W. Yang, H. Mi, M. Kim, S. J. Cho, D.-W. Park, H. Jiang, J. Lee, Y. Qiu, Z. Cai, W. Zhou, S. Gong, Z. Ma, “High-Performance Green Flexible Electronics Based on Biodegradable Cellulose Nanofibril Paper,” Nature Communications, 6, 7170, 2015. 9) T.-H. Chang, K. Xiong, S. H. Park, H. Mi, H. Zhang, S. Mikael, Y. H. Jung, S. Gong, J. Han, Z. Ma, “High power fast flexible electronics: Transparent RF AlGaN/GaN HEMTs on Plastic Substrates, “ 2015 IEEE/MTT-S International Microwave Symposium, Phoenix, Arizona, 2015. 10) Y. H. Jung, T.-H. Chang, S. Gong and Z. Ma, “Towards HighPerformance Green Flexible Electronics Using Nanocellulose Materials,” 2015 MRS Fall Meeting & Exhibit, Boston, MA, December, 2015. 11) K. Xiong, H. Mi, T.-H. Chang, M. Wu, S. Gong, W. Zhou, M. Arnold, H. Yuan, Z. Ma, “AlGaAs/Si dual-junction tandem solar cells fabricated by epitaxial lift-off and print transfer-assisted bonding,” 42nd

*

Equal contribution.

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IEEE Photovoltaic Specialists Conference (2015), New Orleans, June 14-19, 2015. 12) J.-H. Seo, T.-H. Chang, J. Lee, R. Sabo, W. Zhou, Z. Cai, S. Gong, Z.Ma, “Microwave Flexible Transistors on Cellulose Nanofibrillated Fiber Substrates,” Applied Physics Letters, 106, 2015. 13) M.-Y. Wu, J. Zhao, F. Xu, T.-H. Chang, R. M. Jacobberger, Z. Ma, M. S. Arnold, “Highly Stretchable Carbon Nanotube Transistors Enabled By Buckled Ion Gel Gate Dielectrics,” Applied Physics Letters, 107, 053301, 2015. 14) D.-W. Park, S. Mikael, T.-H. Chang, S. Gong and Z. Ma, “Bottomgate coplanar graphene transistors with enhanced graphene adhesion on atomic layer deposition Al2O3,” Applied Physics Letters, 106, 102106, 2015. 15) J.-H. Seo, T.-H. Chang, R. Sabo, Z. Cai, S. Gong, Z. Ma, “RadioFrequency Flexible Transistors on Cellulose Nanofibrillated Fiber (CNF) Substrates,” 15th Topical Meeting on Silicon Monolithic Integrated Circuits in RF Systems, Jan. 26-28 2015, Omni San Diego Hotel, San Diego, California, 2015. 16) F. Xu, M. Wu, N. Safron, S. S. Roy, R. M. Jacobberger, D. J. Bindl, J.H. Seo, T.-H Chang, Z. Ma, M. S. Arnold, “Highly stretchable carbon nanotube transistors with ion gel gate dielectrics,” Nano Letters, 14 (2), 682-868, 2014.

Yei Hwan Jung Affiliation: University of Wisconsin-Madison Education: B.S. in electrical engineering, University of Illinois at UrbanaChampaign M.S. in electrical engineering, University of Wisconsin-Madison

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Business Address: 1415 Engineering Dr., B642 Madison, WI 53706 Research and Professional Experience: From 2010 to 2012, Jung was a Research Associate with the University of Illinois at Urbana-Champaign, where he focused on wireless implantable medical devices for brain optogenetics and high-performance flexible thin-film silicon transistors. Since 2012, he has been a Research Assistant and since 2015, a Fellow with the University of WisconsinMadison, focusing on flexible and stretchable microwave electronics, transparent electronics and optoelectronics for neural applications, and micro-structured implantable scaffolds. Professional Appointments: HHMI Predoctoral Fellow, University of Wisconsin-Madison Honors: Jung was a recipient of the 2015 HHMI International Student Research Fellowship and the 2012 Wisconsin Chancellor’s Opportunity Award and has won numerous innovation competitions including the Grand First Prize of the 2014 Qualcomm Innovation Prize and Runner-Up Prize of the 2014 G. Steven Burill Business Plan Competition. Publications Last 3 Years: 1) Y. H. Jung, J. Lee, Y. Qiu, N. Cho, S. J. Cho, H. Zhang, S. Lee, T. J. Kim S. Gong and Z. Ma, “Stretchable Twisted-Pair Transmission Lines for Microwave Wearable Electronics,” Advanced Functional Materials 26, 4635-4642 (2016). 2) Y. H. Jung, H. Zhang and Z. Ma, “Wireless Applications of Conformal Bioelectronics,” a chapter in “Stretchable Bioelectronics for Medical Devices and Systems,” edited by J. A. Rogers, R. Ghaffari and D.-H. Kim, Springer (2016).

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3) Y. H. Jung, J.-H. Seo, W. Zhou and Z. Ma, “High Speed, Flexible Electronics by Use of Si Nanomembranes,” a chapter in “Silicon Nanomembranes: Properties and Applications,” edited by J. A. Rogers and J.-H. Ahn, Wiley-VCH (2016). 4) Y. H. Jung, Y. Qiu, S. Lee, T.-Y. Shih, Y. Xu, R. Xu, J. Lee, A. A. Schendel, W. Lin, J. C. Williams, N. Behdad and Z. Ma, “A Compact Parylene-Coated WLAN Flexible Antenna for Implantable Electronics,” IEEE Antennas and Wireless Propagation Letters 15, 1382-1385 (2016). 5) Y. Qiu, Y. H. Jung, S. Lee, J. Lee, T.-Y. Shih, Y. Xu, R. Xu, W. Lin, J. C. Williams, N. Behdad and Z. Ma, “Flexible Capacitively Loaded Antenna with Parylene Conformal Coating,” Microwave Journal 59 (4), 134-142 (2016). 6) S. J. Cho, Y. H. Jung and Z. Ma, “X-band Compatible Flexible Microwave Inductors and Capacitors on Plastic Substrate,” IEEE Journal of the Electron Devices Society 3 (5), 435-439 (2015). 7) Y. H. Jung, T.-H. Chang, H. Zhang, C. Yao, Q. Zheng, V. W. Yang, H. Mi, M. Kim, S. J. Cho, D.-W. Park, H. Jiang, J. Lee, Y. Qiu, W. Zhou, Z. Cai, S. Gong and Z. Ma, “High-Performance Green Flexible Electronics Based on Biodegradable Cellulose Nanofibril Paper,” Nature Communications 6, 7170 (2015). 8) J. Kim, M. Lee, H. J. Shim, R. Ghaffari, H. R. Cho, D. Son, Y. H. Jung, M. Soh, C. Choi, S. Jung, K. Chu, D. Jeon, S.-T. Lee, J. H. Kim, S. H. Choi, T. Hyeon and D.-H. Kim, “Stretchable Silicon Nanoribbon Electronics for Skin Prosthesis,” Nature Communications 5, 5747 (2014). 9) Y. Qiu, Y. H. Jung, S. Lee, T.-Y. Shih, J. Lee, Y. H. Xu, R. Xu, W. Lin, N. Behdad and Z. Ma, “Compact Parylene-C-coated Flexible Antenna for WLAN and Upper-band UWB Applications,” Electronics Letters 50 (24), 1782-1784 (2014). 10) T.-I. Kim, M. J. Kim, Y. H. Jung, H. Jang, C. Dagdeviren, H. A. Pao, S. J. Cho, A. Carlson, K. J. Yu, A. Ameen, H.-J. Chung, S. H. Jin, Z. Ma and J. A. Rogers, “Thin Film Receiver Materials for Deterministic

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Assembly by Transfer Printing,” Chemistry of Materials 26 (11), 35023507 (2014). 11) G. Park, H.-J. Chung, K. Kim, S. A. Lim, J. Kim. Y.-S. Kim, W.-H. Yeo, R.-H. Kim, S. S. Kim, J.-S. Kim, Y. H. Jung, T.-I. Kim, V. Kumar, C. Yee, J. A. Rogers and K.-M. Lee, “Immunologic and Tissue Biocompatibility of Flexible/stretchable Electronics and Optoelectronics,” Advanced Healthcare Materials 3, 515-525 (2014). 12) J. G. McCall, T.-I. Kim, G. Shin, X. Huang, Y. H. Jung, R. AlHassani, F. G. Omenetto, M. R. Bruchas and J. A. Rogers “Fabrication and Application of Flexible, Multimodal Light-Emitting Devices for Wireless Optogenetics,” Nature Protocols 8 (12), 2413-2428 (2013). 13) T.-I. Kim, Y. H. Jung, H.-J. Chung, K. J. Yu, N. Ahmed, C. Corcoran, J. S. Park, S. H. Jin and J. A. Rogers, “Deterministic Assembly of Releasable Single Crystal Silicon-Metal Oxide Field-Effect Devices Formed from Bulk Wafers,” Applied Physics Letters 102, 182104 (2013). 14) T.-I. Kim, J. G. McCall, Y. H. Jung, X. Huang, E. R. Siuda, Y. Li, J. Song, Y. M. Song, H. A. Pao, R.-H. Kim, C. Lu, S. D. Lee, I.-S. Song, G. C. Shin, R. Al-Hassani, S. Kim, M. P. Tan, Y. Huang, F. G. Omenetto, J. A. Rogers and M. R. Bruchas, “Injectable, Cellular-Scale Optoelectronics with Applications for Wireless Optogenetics,” Science 340, 211-216 (2013).

Dong Liu Affiliation: University of Wisconsin-Madison Education: B.S. in electrical engineering, University of Electronics Science and Technology of China Ph. D in electrical engineering, Tsinghua University

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Research and Professional Experience: From 2009 to 2014, Dong was a Research Assistant with the University of Illinois at Tsinghua University, where she focused on III-V semiconductor based optoelectronic devices, including light emitting diode, laser, modulator and photodetectors. Since 2014, she has been a Research Associate with the University of Wisconsin-Madison, focusing on high performance electronics and optoelectronics devices based on heterogeneous materials for applications in silicon photonics, deep ultraviolet light emitting diodes and high response photodetectors. Honors: Dong was a recipient of the 2014 Best Doctoral Thesis upon Ph.D graduation in Tsinghua University. Publications Last 3 Years: Dong Liu, Changzheng Sun, Bing Xiong, and Yi Luo, Nonlinear dynamics in integrated coupled DFB lasers with ultra-short delay, Optics Express, 2014, 22(5): 5614-5622. Dong Liu, Changzheng Sun, Bing Xiong, and Yi Luo. Suppression of chaos in integrated twin DFB lasers for millimeter-wave generation, Optics Express, 2013, 21(2): 2444-2451. Changzheng Sun, Dong Liu, Bing Xiong, and Yi Luo, Modulation Characteristics Enhancement of Monolithically Integrated Laser Diodes under Mutual Injection Locking. IEEE Journal of Selected Topics in Quantum Electronics, 2015, 21(6): 1802008. Cho Minkyu, Jung-Hun, Munho Jaeseong, Dong Liu, Weidong Zhou,and Zongfu Yu,and Zhenqiang Ma,, Resonant cavity germanium photodetector via stacked single-crystalline nanomembranes, Journal of Vacuum Science & Technology B, 2016, 34: 040604.

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Dong Liu, Zhenyang Xia, SangJune Cho, Huilong Zhang, Tzu-Hsuan Chang et al., 1.5um InGaAsP edge-emitting laser with silicon hole injector, SPIE Photonics West, 2016, 9767-34. Zhenyang Xia, Ming Zhou, Munho Kim, Tzu-Hsuan Chang, Dong Liu, et al., Ultra-thin single crystal Germanium nanomembrane photodetecting MOSFET. CLEO2016. Dong Liu, Changzheng Sun, Bing Xiong, and Yi Luo. Resonance Frequency Enhancement by Mutual Injection Locking of Monolithically Integrated Laser Diodes, Conference on Lasers and Electro-Optics, CLEO2014, SF2G.4. Dong Liu, Changzheng Sun, Bing Xiong, and Yi Luo, Locked and Unlocked Behavior of Mutually Coupled Lasers with Ultra-short Delay, 24th IEEE International Semiconductor Laser Conference, ISLC 2014, TP7. Dong Liu, Changzheng Sun, Bing Xiong, and Yi Luo. A Novel Method for Photonic Generation of Millimeter-wave Signals with Optical Injection Locked Microring Laser, Asia communications and photonics conference, ACP2013, AW3B.4. Dong Liu, Changzheng Sun, Bing Xiong, and Yi Luo, Suppression of Chaos in Integrated Coupled DFB Lasers for Millimeter-wave Generation, Conference on optical fiber communication, OFC, 2013, JTh2A.53.

Hongyi Mi Affiliation: Department of Electrical and Computer, the University of Wisconsin-Madison. Education: Ph.D, Department of Electrical and Computer, the University of Wisconsin-Madison. Business Address: B640 Engineering Hall, 1415 Engineering Dr., Madison, WI 53706.

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Research and Professional Experience: Research in electrical and opto-electrical devices and materials.

Juhwan Lee Affiliation: University of Wisconsin-Madison Education: B.S. in electrical engineering, University of Illinois at UrbanaChampaign Business Address: 1415 Engineering Dr., B642 Madison, WI 53706 Research and Professional Experience: From 2011 to 2013, Juhwan was a Research Associate with the University of Illinois at Urbana-Champaign, where he focused on stretchable batteries with self-similar serpentine electrodes and integrated wireless recharging systems. Since 2013, he has been a Research Assistant and since 2015, a Fellow with the University of Wisconsin-Madison, focusing on flexible and stretchable microwave electronics, electrocardiogram monitoring with graphene electrodes and microstructured implantable scaffolds. Professional Appointments: Research Assistant, University of Wisconsin-Madison Honors: Juhwan has been selected as one of the finalists in the 2016 Wisconsin Alumni Research Foundation Innovation Awards

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Publications Last 3 Years: 1)

2)

3)

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Y. H. Jung, J. Lee, Y. Qiu, N. Cho, S. J. Cho, H. Zhang, S. Lee, T. J. Kim, Shaoqin Gong, and Z. Ma, “Stretchable Twisted-Pair Transmission Lines for Microwave Frequency Wearable Electronics,” Advanced Functional Materials, 26, 4636-4642 (2016). Y. Qiu, Y. H. Jung, S. Lee, T.-Y. Shih, Y. Xu, R. Xu, J. Lee, A. A. Schendel, W. Lin, J. C. Williams, N. Behdad, and Z. Ma, “A Compact Parylene Coated WLAN Flexible Antenna for Implantable Electronics,” IEEE Antennas and Wireless Propagation Letters, 15, 1382-1385 (2016). Y. Qiu, Y. H. Jung, S. Lee, J. Lee, T.-Y. Shih, Y. Xu, R. Xu, W. Lin, J. C. Williams, N. Behdad, and Z. Ma, “Flexible Capacitively Loaded Antenna with Parylene Conformal Coating,” Microwave Journal, 59 (4), 134-142 (2016). Y. H. Jung, T.-H Chang, H. Zhang, C. Yao, Q. Zheng, V. W. Yang, H. Mi, M. Kim, S. J. Cho, D.-W. Park, H. Jiang, J. Lee, Y. Qiu, Z. Cai, W. Zhou, S. Gong, and Z. Ma, “High-Performance Green Flexible Electronics Based on Biodegradable Cellulose Nanofibril Paper,” Nature Communications, 6, 7170 (2015). DOI: 10.1038/ncomms8170. Y. Qiu, Y. H. Jung, S. Lee, T.-Y. Shih, J. Lee, Y. H. Xu, R. Xu, W. Lin, N. Behdad, and Z. Ma, “Compact parylene-c-coated flexible antenna for WLAN and upper-band UWB applications,” IET Electronics Letters 24 (50), 1782-1784 (2014), DOI:10.1049/el.2014.3647. Y. Zhang, S. Xu, H. Fu, J. Lee, J. Su, K.-C. Hwang, J. A. Rogers and Y. Huang, “Buckling in serpentine microstructures and applications in elastomer-supported ultra-stretchable electronics with high areal coverage,” Soft Matter 9, 8062-8070 (2013). S. Xu, Y. Zhang, J. Cho, J. Lee, X. Huang, L. Jia, J. A. Fan, Y. Su, J. Su, H. Zhang, H. Cheng, B. Lu, C. Yu, C. Chuang, T.-I. Kim, T. Song, K. higeta, S. Kang, C. Dagdeviren, I. Petrov, P. V. Braun, Y. Huang, U. Paik, and J. A. Rogers, Stretchable Batteries with Self-

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Jiarui Gong Affiliation: Department of Physics University of Wisconsin-Madison, Madison, WI, US Education: B.S. Physics, Peking University, Beijing, China, 2015 Business Address: B640 1415 Engineering Drive Madison, WI 53706 Professional Appointments: Teaching Assistant, University of Wisconsin-Madison

Zhenqiang (Jack) Ma Affiliation: Department of Nuclear Engineering and Engineering Physics Department of Materials Science and Engineering Madison, WI, USA Education: Ph.D. Electrical Engineering, University of Michigan, Ann Arbor, Michigan, 2001

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Business Address: 3445 EH 1415 Engineering Drive Madison, WI 53706 Research and Professional Experience: Zhenqiang Ma received the B.S. degree in applied physics and the B.E. degree in electrical engineering from Tsinghua University, Beijing, China, in 1991, the M.S. degree in nuclear science and the M.S.E. degree in electrical engineering from the University of Michigan, Ann Arbor, in 1997, and the Ph.D. degree in electrical engineering from the University of Michigan, in 2001. From 2001 to 2002, he was a Member of the Research and Development Team, Conexant Systems and later its spin-off Jazz Semiconductor, Newport Beach, CA. In 2002, he left Jazz to join the faculty of University of Wisconsin–Madison, as an Assistant Professor with the Department of Electrical and Computer Engineering. He is currently the Lynn H. Matthias Professor of Engineering and the Vilas Distinguished Achievement Professor with affiliated appointments in four other engineering departments/institutions/programs. His current research interest includes high-speed electronics, optoelectronics, and nanophotonics, semiconductor materials processing and heterogeneous integration, flexible electronics, flexible optoelectronics, and flexible photonics, energy conversion semiconductor devices, including power devices, solar cells, and light emitting devices, bioelectronics, and biomimetics, and semiconductor device physics. He has authored or coauthored over 400 peer-reviewed technical papers and book chapters related to the above areas and holds about 34 U.S. and international patents. He has been featured in MIT’s Technology Review for his research innovations four times since 2004. He was a recipient of the Presidential Early Career Award for Scientists and Engineers and the DARPA Young Faculty Award. He serves on the editorial boards and as a Reviewer of 78 international journals.

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Professional Appointments: Professor Lynn H. Matthias Professor in Engineering and Vilas Distinguished Achievement Professor Department of Electrical and Computer Engineering University of Wisconsin-Madison Honors: H. I. Romnes Fellowship, 2012. Presidential Early Career Award for Scientists and Engineers (PECASE) 2007 DARPA Young Faculty Award, 2008. Publications Last 3 Years: 1) T.-H. Chang, S. Xiong, R. M. Jacobberger, S. Mikael, H. S. Suh, C.-C. Liu, D. Geng, X. Wang, M. S. Arnold, Z. Ma, P. F. Nealey, “Directed self-assembly of block copolymer films on atomically-thin graphene chemical patterns,” Scientific Reports, In Press (2016). 2) M. Kim, J.-H. Seo, Z. Yu, W. Zhou, Z. Ma, “Flexible germanium nanomembrane metal-semiconductor-metal (MSM) photodiodes,” Applied Physics Letters, In Press (2016). 3) D.-W. Park, S. K. Brodnick, J. P. Ness, F. Atry, L. Krugner-Higby, A. Sandberg, S. Mikael, T. J. Richner, J. Novello, H. Kim, D-H. Back, J. Bong, S. T. Frye, S. Thongpang, K. I. Swanson, W. Lake, R. Pashaie, J. C. William, Z. Ma, “Fabrication and utility of a transparent graphene neural electrode array for electrophysiology, in vivo imaging,and optogenetics,” Nature Protocols, In Press (2016). 4) M.Cho, J.-H. Seo, D.-W. Park, W. Zhou, Z. Ma, “Capacitance-voltage characteristics of Si and Ge nanomembrane based flexible metal-oxidesemiconductor devices under bending conditions,” Applied Physics Letters, 108, 233505 (2016).

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5) M. Kim, S. C. Liu, J. Lee, T. Kim, J.-H. Seo, W. Zhou, Z. Ma, “Light absorption enhancement in Ge nanomembranes and its optoelectronic application,” Optics Express, 24(15): 16894-16903 (2016). 6) D. Liu, W. Zhou, Z. Ma, “Semiconductor nanomembrane-based lightemitting and photodetecting devices” (Invited Review), Photonics, 3(2), 40 (2016). 7) S. Mikael, J.-H. Seo, A. Javadi, S. Gong, Z. Ma, “Wrinkled Bilayer Graphene with Wafer Scale Mechanical Strain,” Applied Physics Letters, 108, 183101 (2016). 8) L. Menon, H. Yang, S. J. Cho, S. Mikael, Z. Ma, W. Zhou, “Heterogeneously integrated InGaAs and Si membrane four color photodetector arrays,” IEEE Photonics Journal, 8(2), 6801907 (2016). 9) J.-H. Seo, T. Ling, S. Gong, W. Zhou, L. J. Guo, Z. Ma, “Fast Flexible Thin-Film Transistors with a Nanotrench Structure,” Scientific Reports, 6, 24771, (2016). 10) Y. H. Jung, Y. Qiu, S. Lee, T.-Y. Shih, Y. Xu, R. Xu, J. Lee, A. A. Schendel, W. Lin, J. C. Williams, N. Behdad, Z. Ma, “A Compact Parylene-Coated WLAN Flexible Antenna for Implantable Electronics,” IEEE Antennas and Wireless Propagation Letters, 15, 1382-1385 (2016). 11) J.-H. Seo, H. Wu, S. Mikael, H. Mi, G. Venkataramanan, J. P. Blanchard, S. Gong, W. Zhou, D. Morgan, Z. Ma, “Thermal Diffusion Doping of Single Crystal Natural Diamond,” Journal of Applied Physics, 119, 205703 (2016). 12) F. Wang, J.-H. Seo, G. Luo, M. Starr, Z. Li, D. Geng, X. Yin, S. Wang, D. Fraser, D. Morgan, Z. Ma, X. D. Wang, “Nanometre-thick single-crystalline nanosheets grown at the water-air interface,” Nature Communications, 7, 10444 (2016). 13) D. Zhao, S. Liu, H. Yang, Z. Ma. C. R. Hedlund, M. Hammar, W. Zhou, “Printed Large-Area single mode photonic crystal bandedge surface-emitting lasers on Silicon,” Scientific Reports, 6, 18860 (2016).

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14) H. Yang, D. Zhao, S. Liu, Y. Liu, J.-H. Seo, Z. Ma, W. Zhou, “Transfer Printed Nanomembranes for Heterogeneously Integrated Membrane Photonics,” Photonics 2(4), 1081-1100 (2015). 15) H. Mi, S. Mikael, C.-C. Liu, J.-H. Seo, G. Gui, A. L. Ma, P. F. Nealey, Z. Ma, “Creating periodic local strain in monolayer graphene with nanopillars patterned by self-assembled block copolymer,” Applied Physics Letters, 107, 143107 (2015). 16) J.-H. Seo, T.-H. Chang, J. Lee, R. Sabo, W. Zhou, Z. Cai, S. Gong, Z.Ma, “Microwave Flexible Transistors on Cellulose Nanofibrillated Fiber Substrates,” Applied Physics Letters, 106, 262101 (2015). 17) Y. H. Jung, T.-H Chang, H. Zhang, C. Yao, Q. Zheng, V. W. Yang, H. Mi, M. Kim, S. J. Cho, D.-W. Park, H. Jiang, J. Lee, Y. Qiu, Z. Cai, W. Zhou, S. Gong, Z. Ma, “High-Performance Green Flexible Electronics Based on Biodegradable Cellulose Nanofibril Paper,” Nature Communications, 6, 7170 (2015). 18) M. Kim, H. Mi, M. Cho, J.-H. Seo, W. Zhou, S. Gong, Z. Ma, “Tunable biaxial in-plane compressive strain in a Si nanomembrane transferred on a polyimide film,” Applied Physics Letters, 106, 212107106 (2015). 19) M. Cho, J.-H. Seo, J. Lee, D. Zhao, H. Mi, X. Yin, X. Wang, W. Zhou, Z. Ma, “Ultra-thin distributed Bragg reflectors via stacked singlecrystal silicon nanomembranes,” Applied Physics Letters, 106, 181107 (2015). 20) S.-C. Liu, D. Zhao, J.-H. Seo, Y. Liu, Z. Ma, W. Zhou, “Athermal Photonic Crystal Membrane Reflectors on Diamond,” IEEE Photonics Technology Letters, 27, (10) 1072-1075 (2015). 21) J.-H. Seo, J. Li, J. Lee, J. Lin, Hx. Jiang, and Z. Ma, “A simplified method of making flexible blue LEDs on a plastic substrate,” IEEE Photonics Journal, 7(2) 8200207 (2015). 22) D.-W. Park, S. Mikael, T.-H. Chang, S. Gong and Z. Ma, “Bottomgate coplanar graphene transistors with enhanced graphene adhesion on atomic layer deposition Al2O3,” Applied Physics Letters, 106, 102106 (2015).

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23) G. Qin, K. Zuo, J.-H. Seo, Y. Xu, H.-C. Yuan, H. Liu, Z. Huang, J. Ma, and Z. Ma, “On the bending characterization of flexible radiofrequency single-crystalline germanium diodes on a plastic substrate,” Applied Physics Letters 106, 043504 (2015). 24) G. Gui, D. Morgan, J. Booske, J. Zhong, and Z. Ma, “Local strain effect on the band gap engineering of graphene by a first-principles study,” Applied Physics Letters 106, 053113 (2015). 25) L. Menon, H. Yang, S. J. Cho, S. Mikael, Z. Ma, W. Zhou, “Transferred flexible three-color silicon membrane photodetector arrays,” IEEE Photonics Journal, Vol.7, No.1, 6800106 (2015). 26) Y. Liu, A. Chadha, D. Zhao, J. R. Piper, Y. Jia, Y. Shuai, L. Menon, H. Yang, Z. Ma, S. Fan, F. Xia, W. Zhou, “Approaching total absorption at near infrared in a large area monolayer graphene by critical coupling,” Applied Physics Letters, 105, 181105 (2014). 27) D.-W. Park, A. A. Schendel, S. Mikael, S. K. Brodnick, T. J. Richner, J. P. Ness, M. R. Hayat, Farid Atry, S. T. Frye, R. Pashaie, S. Thongpang, Z. Ma, J. C. Williams, Graphene-based carbon-layered electrode array technology for neural imaging and optogenetic applications. Nature Communications, 5:5258 (2014). 28) D. Zhao, H. Yang, J.H. Seo, Z. Ma, W. Zhou, “Design and Characterization of Photonic Crystal Membrane Reflector Based Vertical Cavity Surface Emitting Lasers on Silicon,” Reviews in Nanoscience and Nanotechnology, 3 (2), 77-87 (2014). 29) G. Qin, T. Cai, H. C. Yuan, J. H. Seo, J. Ma, Z. Ma, “Flexible radiofrequency single-crystal germanium switch on plastic substrates,” Applied Physics Letters, 104, 163501 (2014).

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In: Polyethylene Terephthalate Editor: Naomi A. Barber

ISBN: 978-1-53611-991-6 © 2017 Nova Science Publishers, Inc.

Chapter 3

PROGRESS IN POLYETHYLENE TEREPHTHALATE RECYCLING Adel Elamri1, Khmais Zdiri1, Omar Harzallah2,* and Abdelaziz Lallam2 1

Unité de Recherche Matériaux et Polymères Textiles, Ecole Nationale d’Ingénieurs de Monastir, Monastir, Tunisie 2 Laboratoire de Physique et Mécanique Textiles (EA 4365), Université de Haute Alsace, Mulhouse, France

ABSTRACT In the last decade, an increasing interest has been focused on the recycling of plastic wastes, especially on the polyethylene terephthalate (PET). PET polymer is already being recycled and numerous applications for recycled polyesters can be explored depending on the properties of the resin. However, the common problem faced during processing of recycled PET is degradation. Thus, many solutions have been proposed in literature to undermine this problem. This chapter presents a background *

Corresponding Author address: Email:[email protected].

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Adel Elamri, Khmais Zdiri, Omar Harzallah et al. of the current state of knowledge with respect to PET recycling. In the first section, a brief theoretical background is presented about virgin PET synthesis, thermal transitions, processing and applications. The second section deals with the PET recycling process with a focus on contaminations and ways to increase the molecular weight of recycled PET (RPET). It serves as an introduction to Section Three where our process to improve the RPET properties is described. Finally, Section Four covers the effect of blending virgin PET (VPET) with recycled PET on thermal and rheological behaviors.

Keywords: PET, recycling, blends, thermal and rheological

1. INTRODUCTION The production of plastic materials in its various declinations comes mainly from petroleum. However, we know that the depletion of fossil resources is inevitable. The economy of the main material, in this case oil, becomes a goal in itself. This objective can be achieved both through the use of renewable materials, or recycling of materials, if operations are technically and economically feasible. In this work, the treatment is focused on the technical aspect of thermomechanical recycling of PET waste from mineral water bottles. The thermomechanical recycling presents the advantage of being fast and cheap. In contrast, thermomechanical recycling leads to loss of important mechanical properties by thermal degradation mechanism. It is possible to overcome the loss of these properties, either by recycling under vacuum, which requires heavy and expensive equipment, or by the addition of virgin material to the primary material for recycling. In this chapter, it is the second procedure which is described.

2. VIRGIN PET Virgin PET has come to be considered as one of the most important engineering polymers in the past two decades. It is regarded as an excellent

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material for many applications and is widely used for making liquid containers (bottles). It has excellent thermomechanical and chemical properties [1]. Many companies produce virgin PET giving it different trade names [2]. Some of the common trade names of commercially available PET are summarized in Table 1. Table 1. Trade names of common PET and their manufacturer Trade name Rynite Diolen Eastapac Arnite Mylar Melinex

Manufacturer Du Pont de Nemours & Co. ENKA-Glazstoff Eastman chemical company DSM Engineering Plastics E. I. Du Pont de Nemours & Co. Imperial Chemical Industries Ltd.

Commercial PET has a wide range of intrinsic viscosity [η] that varies from 0.45 to 1.2 dL.g-1 with a polydispersity index generally equal to 2. The PET repeating unit is shown in Figure 1.

Figure 1. PET repeating unit.

2.1. PET Synthesis PET production process involves two different starting reactions. The first starting reaction is an esterification reaction (Figure 2a) where terephthalic acid (TPA) reacts with ethylene glycol (EG) at a temperature between 240°C and 260°C and a pressure between 300 and 500 kPa. The second reaction is a trans-esterification reaction (Figure 2b) where dimethyl terephthalate (DMT) reacts with EG at 180 – 210°C and 100 kPa

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[3]. Trans-esterification is the much preferred process due to easier purification. The output of both these processes is bis(hydroxyethyl) terephthalate (BHET). The pre-polymerisation step follows, in which BHET is polymerised to a degree of polymerization (DP) of up to 30 (Figure 2c). Pre-polymerisation reactions conditions are 250–280°C and 2– 3 kPa. The third stage is the polycondensation process where the DP is further increased to 100. The polycondensation process conditions are 280–290°C and 50–100 Pa. Up to this stage, PET is suitable for applications that do not require high molecular weight (Mw) or intrinsic viscosity [η] such as fibers and sheets. A solid state polymerization (SSP) step might be required when a high Mw PET is produced. SSP is used to increase the DP to 150, and also increasing Mw. SSP operating conditions are 200–240°C at 100 kPa and 5–25 h [4]. Bottle grade PET that has an [η] of 0.7–0.85 dL.g-1 is normally produced by SSP at 210°C for around 15–20 h [5-6]. Some virgin PET manufacturers have tended in recent years to produce PET co-polymer; such as isophthalic acid modified PET, rather than homopolymer PET. Bottles are then made from co-polymer PET because of its lower crystallinity, improved ductility, better process ability and better clarity [7].

(a)

(b) Figure 2. (Continued).

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(c) Figure 2. PET synthesis reactions (a) esterification of TPA with EG (b) transesterificationof DMT with EG and (c) polymerization.

The PET chain is considered to be highly stiff above the glass transition temperature (Tg) unlike many other polymers. The low flexibility of the PET chain is a result of the nature of the short ethylene group and the presence of the p-phenylene group. This chain inflexibility significantly affects PET structure-related property. The standard physical and chemical properties of commercial PET are shown in Table 2. Table 2. Physical and chemical properties of PET [8] Property Molecular weight (of repeating unit) Weight-average Mw Density Glass transition temperature Melting Temperature Breaking strength Tensile strength (Young’s modulus) Yield strain Water absorption (after 24h)

Test method -

Value (Unit) 192 (g.mol-1)

GPC DSC DSC Tensile -

30,000 – 80,000 (g.mol-1) 1.41 (g.cm-3) 69-115 (°C) 265 (°C) 50 (MPa) 1700 (MPa)

Tensile -

4 (%) 0.5 (%)

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2.2. PET Applications PET is used broadly in products such as bottles, electrical and electronic instruments, in different sectors of automotive industry, housewares, lighting products, power tools and material handling equipment. PET films and fibers are the oldest applications of PET. Films are produced by biaxial orientation through heat and drawing. PET films are used in photographic applications, x-ray sheets and in food packaging. PET films are also reported to be used in electrical and dielectrics applications due to the severe restriction of the electric dipole orientation at room temperature that is well below the transition temperature [9]. PET fibers are another important application of PET and are produced by forcing molten PET through small holes in a die. Fiber strength is achieved by applying tension to align the chains through uniaxial stretching. Bottle production requires the use of PET with high molecular weight. This is explained in part by the manufacturing process (bistretching, blow - extrusion), and by the need to obtain sufficient barrier properties for this application, often related to beverage packaging. PET chains with high molecular weight will allow a 'mesh' at most ends of the chains after bi-stretching which will limit the diffusion of gaseous molecules from the outside to the content and vice versa [10]. Table 3. Intrinsic viscosity of PET depending on its application [11]

Fibers

Bottles

PET Textiles Techniques bi-oriented Thermoforming for water for soft drinks

[η] (dL.g-1) 0.40 – 0.70 0.72 – 0.98 0.60 – 0.70 0.70 – 1.00 0.70 – 0.78 0.78 – 0.85

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Virgin PET is produced at different specifications because different applications require various properties. Examples of PET’s [η] with respect to the required application are shown in Table 3.

2.3. Thermal and Crystallization Behaviors Commercial PET has a melting temperature (Tm) between 255 and 265°C and for more crystalline PET it is situated between 255 and 270°C [4]. The Tg of virgin PET varies between 67 and 140 C. The thermal transitions and crystallization of virgin PET with a focus on reversing crystallization and melting have been analyzed by several researchers [12, 13]. An interesting phenomenon was reported in which the virgin PET experiences multiple endothermic transitions during thermal analysis [14]. It was reported that this phenomenon is attributable to morphological and structural re-organization. As the temperature increases, better crystal structures are achieved because of the re-organization of the less perfect crystals. Virgin PET is well known for having a very slow crystallization rate. The highest crystallization rate takes place between 170°C and 190°C. Cooling PET rapidly from the melt to a temperature below Tg can produce an amorphous, transparent PET. Semi-crystalline PET can be obtained by heating the solid amorphous PET to a temperature above Tg where 30% crystallinity can be achieved [15]. The rate of crystallization of virgin PET depends greatly on temperature and reaches its maximum at a temperature of 150–180°C. The rate of crystallization also depends on other factors such as Mw, the presence of nucleating agents, the degree of chain orientation, the nature of the polymerization catalyst used in the original production of PET and the thermal history.

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Adel Elamri, Khmais Zdiri, Omar Harzallah et al.

3. RECYCLED PET Due to its increasing consumption and non-biodegradability, PET waste disposal has created serious environmental and economic concerns. Thus, management of PET waste has become an important social issue. In view of the increasing environmental awareness in the society, recycling remains the most viable option for the treatment of waste PET. On the other hand, as the price of virgin PET remains stable, new and cheaper technologies for recycling PET give an added value to the PET recycling industry by providing industry with relatively cheaper PET. Many researchers reported that in order to achieve successful PET recycling, PET flakes should meet certain minimum requirements [16, 17]. Examples of the minimum requirements for the post consumer PET (POSTC-PET) flakes are summarized in Table 4. Table 4. Minimum requirements for POSTC-PET flakes to be reprocessed Property Intrinsic viscosity [η] Melting temperature Tm Water content Dye content Yellowing index Metal content PVC content Polyolefin content

Value >0.7 dL.g-1 >240 °C