Waste - American Chemical Society

Review pubs.acs.org/IECR

Recent Developments in the Chemical Recycling of Postconsumer Poly(ethylene terephthalate) Waste Neena George* and Thomas Kurian Department of Polymer Science and Rubber Technology, Cochin University of Science and Technology, Kochi 22, Kerala, India ABSTRACT: Global production and consumption of poly(ethylene terephthalate) (PET) products has increased dramatically over the past few decades. World consumption of PET has exceeded 13 million tonnes, of which about 1.5 million tonnes is exclusively consumed by the packaging sector itself. However, this tremendous increase in PET consumption has resulted in the accumulation of an enormous quantity of waste, the disposal of which is complex day by day. Among different PET recycling methods, chemical recycling (chemolysis) is the most successful method to convert PET into its monomers/oligomers. This review presents in detail recent developments in the chemical recycling (glycolysis and aminolysis) of PET. The wide spectrum of depolymerizing agents used, the reaction conditions, catalysts, products of depolymerization, and their potential applications are described.

1. INTRODUCTION Polyethylene terephthalate (PET) is one of the most commonly used consumer plastics in the world by virtue of its light weight, durability, excellent wear, and chemical resistance coupled with its low market price. Its widespread use, particularly in the textile and packaging sectors, generates tonnes of PET waste to the environment. PET accounts for 8% by weight and 12% by volume of the global solid waste.1 Thus, the current scenario demands a simple, ecofriendly, and economical route for recycling PET wastes that may otherwise disrupt the balance of the ecosystem due to their nonbiodegradable nature. PET can be recycled practically by mechanical, thermal, and chemical recycling methods and has the number “1” as its recycling symbol. Nevertheless, postconsumer PET wastes and industrial plant scraps are commonly recycled either mechanically or chemically. Mechanical recycling (material recycling) of PET involves a number of treatments and operations: separation of PET from other plastics, washing to remove dirt and other contaminants, grinding and crushing to reduce the PET particle size, extrusion by heat, pelletizing, and reprocessing into new PET products.2 Compared to chemolysis recycling routes, mechanical recycling of PET is relatively simple, requires low investments, utilizes established equipment, is flexible in terms of feedstock volume, and has little adverse environmental impact. Despite the positive incentives to recycle PET mechanically, there are a number of barriers, and these include:3 (i) Paper labels and label adhesives (based on polyvinyl acetate/ethylene vinyl acetate) cause the PET to discolor and lose clarity. (ii) PET containing residual moisture degrades readily when reprocessed, if not dried. (iii) Thermal and oxidative degradation products cause yellowing and diminish the mechanical properties of PET. (iv) Collection, sorting, and separation costs are high because of the low-bulk density of PET bottles and the stringent requirement to have well-sorted feedstock. © 2014 American Chemical Society

(v) PVC liners in bottle caps are problematic to the PET recycling process. 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. (vi) The presence of colored PET wastes imparts an undesirable gray color to the recycled PET. (vii) Trace metals such as antimony, cobalt, and manganese (from the catalyst residues and additives used for PET production) present in postconsumer PET wastes promote transesterification and polycondensation reactions in the recycled PET. This makes the recycled PET chemically heterogeneous and may affect the melt rheology behavior from batch to batch. Thus, it is very difficult to recycle complex and contaminated PET wastes mechanically. Recycled PET obtained from mechanical recycling plants exhibits relatively low and heterogeneous intrinsic viscosity values. This characteristic has prevented recycled PET from being directly used to produce bottle-grade PET, films, and superior industrial fibers. It is usually used for applications such as textile fibers, carpets, non-food-contact containers, and injection-molded household products, all of which do not always claim superiority in their properties.4 Solid state polymerization (SSP) processing can be done with mechanically recycled PET pellets to convert them to a desired high molecular weight polymer of uniform and consistent intrinsic viscosity.5 The undesirable side reactions and the levels of byproducts are also considerably reduced. SSP is usually done at a high temperature (below the melt temperature of the polymer) in the absence of oxygen and water, by means of either vacuum or purging with an inert gas to drive off the byproducts of reactions. For PET, the SSP Received: Revised: Accepted: Published: 14185

May 17, 2014 August 18, 2014 August 28, 2014 August 28, 2014 dx.doi.org/10.1021/ie501995m | Ind. Eng. Chem. Res. 2014, 53, 14185−14198

Industrial & Engineering Chemistry Research

Review

as polyamides, polyacetals, and polycarbonates, too, can be subjected to chemical depolymerization reactions.2,8 2.1. Methanolysis. PET methanolysis is based on the treatment of PET with methanol at relatively high temperatures (180−280 °C) and pressures (20−40 atm), which leads to the formation of DMT and EG as the main products, which are the raw materials necessary for the production of this polymer.9−12 The reaction is catalyzed by typical transesterification catalysts such as zinc acetate, magnesium acetate, cobalt acetate, and lead dioxide; however, zinc acetate is the most commonly used catalyst. The reaction products of methanolysis of postconsumer PET comprise an extensive mixture of glycols, alcohols, and phthalate derivatives in addition to DMT. The separation and refinement of these make methanolysis a rather costly process. The methanolysis process can tolerate a wide range of contaminants; however, water does perturb the process and poisons the catalyst to form various azeotropes. Currently the cost of methanolysis-derived DMT from postconsumer PET bottles is approximately double that of virgin DMT.13 2.2. Hydrolysis. Reaction of PET with water under neutral, acidic, or basic conditions at high temperature and pressure breaks the polyester chains into TPA and EG. Acid hydrolysis is performed most frequently using concentrated sulfuric acid (minimum 87 wt %), although the application of other concentrated mineral acids (e.g., phosphoric or nitric acid) is permissible.14,15 Alkaline hydrolysis is usually carried out with the use of an aqueous solution of NaOH of a concentration of 4−20 wt %.16,17 Neutral hydrolysis is carried out with the use of water or steam in the presence of alkali metal acetates as transesterification catalysts.18−20 Low purity of TPA is the major drawback of this method. Hydrolysis is comparatively slow because water is a weak nucleophile. 2.3. Glycolysis. Glycolysis is the simplest and oldest method of PET depolymerization. It is a commercial PET recycling method practiced by renowned companies worldwide such as DuPont, Goodyear, Shell Polyester, Zimmer, Eastman Kodak, etc.3,21 The first patents on PET glycolysis were filed more than 40 years ago.22−28 Glycolysis is considered to be a versatile chemical recycling method in view of the fact that besides monomer formation specialized oligomeric products such as α,ω-dihydroxy materials (polyols) are also produced, which can be further utilized for the synthesis of polymers such as unsaturated polyesters, polyurethanes, vinyl esters, epoxy resins, and polymer concretes.29,30 Glycolysis is a preferred recycling option when the incoming PET feed source is of known history and high quality. It is not suited to remove low levels of copolymers, colorants, or dyes. It is best suited to the recovery of postindustrial scrap.3 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 BHET (a substrate for PET synthesis). Different glycols, such as EG, diethylene glycol (DEG), propylene glycol (PG), polyethylene glycol (PEG), 1,4-butanediol, and hexylene glycol, are used for the glycolysis of PET.31,32 Because the process is sluggish in the absence of any catalyst, transesterification catalysts are usually employed.32 Glycolysis of waste PET proceeds through at least three stages: oligomers, dimer, and monomer (Figure 2). 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.2

byproducts are ethylene glycol and acetaldehyde. SSP is frequently applied with high contaminant PET loads after melt polymerization for enhancing the mechanical and rheological properties of PET before injection blow molding or extruding. Chemical recycling, on the other hand, is an accepted PET recycling method that follows the principles of “sustainable development”.6 The fact that chemically recycled PET is well suited for food-contact applications has increasingly attracted researchers to the various chemolysis possibilities. Chemical recycling methods are opening newer pathways for using PET waste as a precursor in generating pure value-added products for various industrial and commercial applications.7 However, chemically recycled PET is more expensive than virgin PET because of its raw material cost, capital investment, and scale of operation. It has been calculated that for PET chemolysis facilities to be economically viable, they require a minimum throughput of 1.5 × 104 tonnes per annum. The specific minimum size may vary with the technology used. This review discusses recent research done in the area of PET chemical recycling with emphasis on glycolytic and aminolytic depolymerization of PET wastes. Applications of the glycolysis and aminolysis products are also discussed.

2. CHEMICAL RECYCLING OF PET PET, a thermoplastic polyester, is formed by the condensation reaction between terephthalic acid (TPA) and ethylene glycol (EG) or through a transesterification reaction between dimethyl terephthalate (DMT) and EG. Chemical recycling of PET totally depolymerizes it into monomers such as TPA, DMT, bis(hydroxylethylene) terephthalate (BHET), and EG or partially depolymerises it to oligomers or other chemical substances. Thus, depolymerization is the reverse reaction of the polymer formation. There are different depolymerization routes such as methanolysis, glycolysis, hydrolysis, ammonolysis, aminolysis, and hydrogenation depending on the chemical agent used for PET chain scission.2 Figure 1 summarizes the different routes for PET chemolysis, as well as the type of products derived thereafter. Other condensation polymers such

Figure 1. Different methods of PET chemolysis and the value-added products derived therefrom.2 14186

dx.doi.org/10.1021/ie501995m | Ind. Eng. Chem. Res. 2014, 53, 14185−14198


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catalysts. Pingale et al.44 showed that chlorides of different metals such as zinc, lithium, didymium, magnesium, and iron can catalyze the glycolytic depolymerization of PET bottle waste with EG to obtain the pure monomer BHET with an equivalent performance as the acetate salts. 2.3.2.2. Nontoxic Metal Salts. Even though heavy metal salts are the common catalysts used in the glycolysis of PET wastes with high conversion of PET and selectivity of BHET, there are some drawbacks, such as severe reaction conditions, slow reaction rates, and difficult recycling of the catalyst. Many sincere efforts have been put forward by researchers to find an efficient and green catalyst for the glycolytic depolymerization of PET. In this regard, glycolysis of PET bottle waste with EG was conducted using ecofriendly salts such as sodium carbonate and sodium bicarbonate as catalysts to substitute for the conventional zinc acetate catalyst.45 It was observed that under identical conditions of catalyst concentration and PET:EG ratio, the yield of BHET (61%) was nearly the same as that obtained earlier by conventional catalyst (63%). Lopez-Fonseca et al.46 also examined several ecofriendly simple salts, namely, sodium carbonate, sodium bicarbonate, sodium sulfate, and potassium sulfate, for the glycolytic degradation of PET. Among these salts sodium carbonate and sodium bicarbonate depolymerized PET wastes almost as efficiently as zinc acetate. 2.3.2.3. Recoverable Catalysts. There have been numerous studies to develop catalysts for the chemical recycling of PET via glycolysis. However, only a few have attempted to recover and reuse the catalysts. Wang et al.47 used urea as a green, lowpriced, efficient, and reusable catalyst for the glycolysis of PET wastes under mild conditions. Experimental and DFT studies show that the hydrogen bonds (H-bonds) formed between ethylene glycol and urea play a key role in enhancing the glycolysis of PET. Ionic liquids, being considered green solvents, have the potential to substitute for traditional organic solvents in the degradation of PET. The degradation of PET using ionic liquids proceeds easily under relatively lower pressure and temperature, with no emission of toxic substances. Another attractive feature of this reaction is that the products are easily separated from the ionic liquid by the addition of water followed by filtration, enabling the ionic liquid to be reused. The purification of the glycolysis products catalyzed by ionic liquids was simpler than that catalyzed by traditional compounds, such as metal acetates. Wang et al.48 investigated the potential of acidic, basic, and neutral ionic liquids as novel catalysts in depolymerizing PET wastes using EG. Basic ionic liquids and neutral ionic liquids accelerated the glycolysis process, but the synthesis of basic ionic liquids was quite complex and expensive, whereas the acidic ionic liquids were found to be unstable at temperatures above 180 °C. Therefore, neutral ionic liquids having good catalytic effects and appropriate price were preferred. Even though a better yield of about 98.7% was obtained with 1-butyl3-methylimidazolium bromide as catalyst, 1-butyl-3-methylimidazolium chloride ([bmim]Cl) was chosen as the ideal catalyst due to its high stability.49 No chemical reaction occurs between PET and the ionic liquid, but the process proceeds through the breakage of the chemical bonds. The kinetic study indicates that the degradation of PET in the neutral ionic liquid, 1-butyl3-methylimidazolium chloride, is a first-order reaction with an activation energy of about 232.79 kJ mol−1. Yue et al.50 conducted the glycolysis of PET using several basic ionic liquids as catalysts. Among different basic ionic

Figure 2. Three different stages of PET depolymerization with EG.33

2.3.1. Kinetics of PET Glycolysis. The glycolysis of PET has been found to depend on the reaction parameters such as glycolysis time, glycolysis temperature, catalyst concentration, and glycol concentration. The strength of the reaction parameters over the course of the glycolysis reaction of PET follows the order catalyst concentration > glycolysis temperature > glycolysis time.34−36 Glycolysis products with significantly different functionalities and molecular weights could be prepared by changing the glycol concentration, keeping the other parameters constant.37,38 Studies on the glycolysis of PET waste powder with EG under variable external pressure, temperature, particle size,39 reaction time,39 and catalyst type/concentration40 showed that the reaction is of first order. The rate of the glycolysis reaction of PET was found to be proportional to the polymer surface area. Thus, by reducing the size of the PET waste to small particles by grinding or cutting, the glycolysis rate can be substantially increased. Optimum particle size was found to be 127.5 μm. Pardal and Tersac41 made an attempt to study the isothermal kinetics of PET glycolysis by DEG, dipropylene glycol (DPG), glycerol, and mixtures of these glycols by two experimental procedures: uncatalyzed at 220 °C and catalyzed at 190 °C using titanium(IV) n-butoxide (TBT) as catalyst. The effect of the TBT catalyst on the chemical reactivity was far more intense for DPG than for DEG. A synergic effect was found with the mixture of DPG and glycerol in which the PET is digested more quickly than in pure DPG or glycerol in the presence of 0.5/100 w/w TBT catalyst. They also studied the influence of various parameters such as temperature (from 190 to 220 °C), catalysis, and PET morphology on the kinetics of PET glycolysis by DEG.42 2.3.2. Catalysts of Glycolysis. 2.3.2.1. Heavy Metal Salts. Heavy metal acetates are the conventional transesterification catalysts used for the glycolysis of PET. Troev et al.43 introduced titanium(IV) phosphate as a novel catalyst for the glycolysis of PET. The depolymerization of PET fibers proceeds at a faster rate in the presence of this catalyst in comparison with traditional heavy metal acetate salts as 14187



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zeolite is more active because it has high Si/Al ratios and large mesopore surface for solubilization of PET. Bartolome et al.55 used superparamagnetic γ-Fe2O3 nanoparticles having an average size of 10.5 ± 1.4 nm and surface area reaching 147 m2 g−1 as a reusable catalyst for PET glycolysis. After glycolysis, γ-Fe2O3 nanoparticles were easily recovered by simple magnetic decantation. The glycolysis reaction carried out at 300 °C and 1.1 MPa pressure with a 0.05 catalyst/PET weight ratio required only an hour to achieve BHET yield >90%. The high catalytic activity of the superparamagnetic γ-Fe2O3 nanoparticles may be attributed to its ability to catalyze the glycolysis through redox reactions, high surface area rendering more active sites, thermal stability, and good crystallinity. Metal oxide-doped silica nanoparticles were recently used as a recoverable transesterification catalyst for the glycolysis of PET to obtain BHET at 300 °C and 1.1 MPa with an EG:PET molar ratio of 11 and a PET:catalyst weight ratio of 0.01. The oxides of zinc, manganese, and cerium were deposited on silica nanospheres with diameters ranging from 60 to 750 nm employing ultrasonic irradiation.56,57 The reaction reached equilibrium after 80 min, and the highest BHET yield reached >90% for a manganese oxide-doped silica nanoparticle based system. The distribution of the catalysts on the support becomes better as the size of the support becomes smaller. This could be due to the higher chances of contact between the catalyst and the support because of the higher surface area to volume ratio for smaller supports. The better distribution of the catalysts resulted in higher catalytic activity. The optimum glycolysis conditions of PET with EG in the presence of various catalysts under ambient pressure conditions are summarized in Table 1.

liquids, 1-butyl-3-methylimidazolium hydroxyl exhibited higher catalytic activity with 100% conversion of PET. Yue et al.51 showed that the Lewis acidic ionic liquid ([bmim]ZnCl3) possesses high catalytic activity as compared to the neutral ionic liquid ([bmim]Cl) in the glycolysis of PET. Significantly, the conversion of PET was achieved at 100% with low catalyst ([bmim]ZnCl3) loading (0.16 wt %). The mechanism of the glycolysis of PET catalyzed by [bmim]ZnCl3 (Figure 3) suggests that Lewis acidity of the ionic liquid affected its catalytic activity in the PET glycolysis.

Table 1. Optimum Conditions of PET Glycolysis Reactions with EG in the Presence of Various Catalysts

Figure 3. Mechanism of glycolysis of PET catalyzed by Lewis acidic ionic liquid, ([bmim]ZnCl3).51

PET:EG ratio (w/w)

Zhu et al.52 synthesized three series of recyclable solid acid catalysts including sulfated oxides of zinc/titanium (SO42−/ ZnO, SO42−/TiO2), and their binary oxide (SO42−/ZnO− TiO2) by precipitation/coprecipitation method. The glycolysis of PET into the monomer BHET was conducted over these solid catalysts. Relationships between the catalyst’s textural properties, the surface acidity, and the catalytic activity have been investigated. SO42−/ZnO−TiO2 presented the highest catalytic activity due to its high surface area and predominance of Lewis acid sites. However, the corrosive nature of these catalysts resulting in severe pollution cannot be ruled out. Fukushima et al.53 conducted organocatalytic glycolysis of PET with EG using 1.0 mol % of a commercially available guanidine catalyst, 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD). Their computational and experimental studies revealed that TBD and EG activate PET through hydrogen bond formation/ activation to facilitate its glycolytic depolymerization reaction. TBD exhibited catalytic efficiency comparable to that of the common metal acetates/alkoxide catalysts used for the glycolytic depolymerization of PET. 2.3.2.4. High Surface Area Catalysts. Natural and environmentally safe large-pore aluminosilicates having considerably large void spaces (β-zeolite and Y-zeolite) were found to substitute effectively for the conventional heavy metal catalysts for the glycolysis of PET bottle waste.54 Of the two zeolites, Y-

reflux time (h)

reaction temp (°C)

catalyst (w/w of PET)

yield of BHET (%)

ref

1:5

3

196

1% zinc acetate

85.6

33

1:6

8

196

0.5% zinc acetate 0.5% sodium carbonate 0.5% sodium bicarbonate

63 61 61

45

1:18 1:14 1:7.6 1:4 1:6

6 8 8 3 8

190 197 196 180 196

87.0 74.0 70.0 78.0 65.0

58 44 59 47 54

1:10 1:4 1:11 0.96:5

2 8 2 3.5

190 180 190 190

1% zinc acetate 0.5% zinc chloride 1% sodium carbonate 10% urea 1% Y-zeolite/ 1% β-zeolite 5% [bmim]OH 80% [bmim]Cl 1.25% [bmim]ZnCl3 0.7% TBD

71.2 78.0 84.9 78

50 48 51 53

2.3.3. Supercritical Glycolysis. Supercritical glycolysis has a considerably shorter reaction time with high throughput due to the high solvent density, solubility, high kinetic energy, and high diffusion and reaction rates of the supercritical ethylene glycol. Imran et al.60 investigated the glycolysis of PET with EG (Tc = 446.70 °C, Pc = 7.7 MPa) under supercritical (at 450 °C and 15.3 MPa) and subcritical (at 350 °C and 2.49 MPa; and 14188



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300 °C and 1.1 MPa) conditions with a PET:EG ratio of 1:16.67. Supercritical fluids in a reaction can replace the catalysts, which are difficult to separate from the reaction products. Compared to the subcritical process, the BHET− dimer equilibrium was achieved much earlier for the supercritical process: a maximum BHET yield of 93.5% was reached in a mere 30 min, whereas the yield of the side products (DEG, TEG, BHET dimer, oligomers) was suppressed. 2.3.4. Microwave-Assisted Glycolysis. Microwave heating is a unique and valuable tool in the hands of organic chemists to revolutionize organic syntheses. The use of microwave heating offers many advantages over conventional heating such as instantaneous and rapid heating with high specificity without contact with the material to be heated.45 Thus, extremely short reaction time coupled with substantial energy conservation can be attained. The increased efficiency of the microwave-assisted glycolysis has been attributed to the high microwave absorption capacity of glycols, which results from their high loss factor. PET could be glycolyzed in the presence of lower or higher glycols; however, the reactivity of glycols was found to decrease with increase in the molecular weight.61 Microwave-assisted glycolytic recycling of PET bottle waste was conducted in ethylene glycol using zinc acetate, sodium carbonate, and bicarbonate as catalysts.45 The reaction time drastically reduced from 8 h to 35 min, and the yield of BHET increased using microwaves as the heating source. The comparative yields of BHET using different heating sources are presented in Figure 4.

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. Major reported applications of the PET glycolyzed products are briefly discussed below. 2.3.5.1. Unsaturated Polyester Resins. The products of PET glycolysis can be utilized as a lower cost source of raw material for the production of unsaturated polyester (UP) resins. For the past two decades, this segment of recycling has seen vast developments as awareness and concern for environmental protection, preservation of petrochemical sources, and conservation of energy have increased. Depending on the type of glycol used for depolymerization, and the nature of dicarboxylic acid used for subsequent polycondensation, the polyester obtained may be saturated or unsaturated. PET-based polyols and residual glycol are reacted with maleic anhydride at a fixed ratio of 1:1 for the hydroxyl to carboxyl groups to form the polyester. The reaction was stopped when the acid value reached 29−34 mg KOH/g. The esterification step may also include phthalic anhydride63/adipic acid.64 These were subsequently mixed with styrene (35−45 wt %) and cured using different initiator/catalyst systems such as benzoyl peroxide (BPO)/amine,63 BPO/methyl ethyl ketone peroxide (MEKP),65 MEKP/cobalt 2-ethylhexanoate,66 and MEKP/ cobalt octoate.67 In the preparation of UP resin, glycolysis and polyesterification can proceed in the same reaction vessel by a two-stage reaction. A scheme for the synthesis of UP resin from glycolyzed PET oligomers is shown in Figure 5. UP resins obtained from recycled PET would be applicable as matrices in fiber-reinforced plastics, sheet molding compounds, bulk molding compounds, gel coats, polymer concretes, and mortars as they offer versatility in processing and enhanced properties.68 For UP resins, propylene glycol (PG) is preferred over EG or DEG as a glycolytic agent. The reason is that PET/PG-derived UP resins are compatible with styrene and increase the activity of the anhydrides before starting the reaction, whereas PET/EG- or PET/DEG-derived resin glycolysis products are not fully compatible with styrene.69 PG is of low cost and possesses good hydrolytic resistance, which imparts excellent physical and chemical properties to the product. Suh et al.68 studied the influence of glycol compositions on the cure characteristics and the mechanical properties of the cured UP resin based on glycolyzed PET using PG, DEG, and their mixtures as depolymerization agents. The gelation time of UP was delayed with increasing DEG content. The tensile modulus decreased, and the toughness of the UP resins was greatly enhanced on increasing the percentage of DEG. DEG also imparts good flexural properties to the resin, because its long chains with ether linkage improve flexibility.70 The cured resin based on the glycolyzed PET has a tensile strength similar to that of the typical hand lay-up mat laminate polyester resin. Duque-Ingunza et al.66 conducted polyesterification of the recovered monomer BHET- instead of PET-derived oligomers with maleic anhydride to obtain UP resin. Raheem and Uyigue71 converted PET wastes into a thermosetting polyester resin employing two different chemolysis routes: glycolysis (using DEG) and hydrolysis− glycolysis (using DEG/water) with potassium acetate as catalyst. They deduced 4 and 3 h, respectively, as the maximum reaction times for the glycolysis and hydrolysis−glycolysis of the PET waste.

Figure 4. Comparative yield of BHET using different heating sources: catalyst concentration, 0.5% w/w; PET:EG ratio, 1:6; time, 8 h for conventional heating (black bars) and 35 min for microwave heating (gray bars).45

Benes et al.62 used castor oil (CO) as a renewable alternative to petrochemical-based reagents, for example, glycols for the chemical depolymerization of waste PET under microwave irradiation using zinc acetate as catalyst. The optimum decomposition of PET was observed at a relatively narrow temperature interval from 230 to 240 °C. A maximum number of six repeating monomeric units of PET was found in the product, which confirmed practically the complete depolymerization of PET chain and good reactivity of the acyl ester hydroxyl groups of CO. The expected polyol products have the following formula: H−(E−T)x−TG, where H is hydrogen, E is the ethylene glycol unit (−O−CH 2 CH 2 −), T is the terephthalic unit (−O−CO−C6H4−CO−), and TG is the triglyceride unit of CO. The index x indicates the number of repeating PET units. 2.3.5. Applications of the PET Glycolyzed Products. Recently, growing interest has been observed in the utilization 14189



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Figure 5. PET glycolysis and synthesis of UP resin from the glycolyzed PET oligomers.32

Tahvildari et al.72 prepared several unsaturated polyester resins through catalytic glycolysis of PET wastes with DEG and triethylene glycol (TEG) in the presence of zinc acetate as a catalyst. The products were then reacted with two different anhydrides, namely, maleic anhydride (MA) and phthalic anhydride (PA), along with EG and PG to obtain PET-based UP resins. DEG and TEG imparted flexibility, whereas EG and PG added at the polycondensation step increased the spacing between the double bonds incorporated by MA and PA. Synthesis of the UP resin using the longer chain glycolyzed products led to higher molecular weight (Mn) and improved flexibility of the UP resin. Limpiti and Potiyaraj67 tried to improve the mechanical properties of cured UP resin prepared from glycolyzed PET by mixing it with a commercial UP resin at various ratios. The hardness and cross-link density of UP resin prepared from glycolyzed PET were slightly higher than those of the cured commercial resin. The addition of commercial resin did not

affect the hardness of mixed resins. The impact strength and flexural strength increased as the amount of commercial resin increased. However, at higher loading of the commercial resin (>60 wt %), the improvement was leveled off. Abdelaal et al.29 glycolyzed waste PET bottles with different glycols, namely, PG, DEG, TEG, and a mixture of DEG with PG or TEG in equal amounts. Among all of the glycol-based matrices, PGbased UP showed a higher overall rate of cross-linking. Abdullah and Ahmad73 prepared a coconut fiber reinforced composite by blending the synthesized UP resin from waste PET with 0.3 vol % of coconut fiber via in situ interactive polymerization. A phosphate type of flame retardant (Dricon) was also incorporated into the matrix. Coconut fiber before incorporation was first treated with 5% (w/v) sodium hydroxide followed by washing and drying. The coconut fiber was then soaked in a 0.5% silane solution for an hour and finally dried at room temperature. The treated fiber composite showed better mechanical and thermal properties as compared to the 14190



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untreated fiber composite. The well-dispersed Dricon enhanced the flame retardancy of coconut fiber/polyester composites without sacrificing their mechanical properties. 2.3.5.2. Polyurethanes. The oligoesters derived from glycolyzed PET can be further reacted with aliphatic diacids to form polyester polyols used as a starting material in the polyurethane industry for applications such as adhesives, elastomers, and foams. Glycolyzed PET oligoesters can also be reacted directly as a polyol component with diisocyanate compounds to build up a urethane group. Mercit and Akar74 synthesized new urethane oil varnishes from glycolyzed PET waste. The glycolyzed products of PET were transesterified with soybean oil and glycerin. The product of this step was reacted with toluene diisocyanate to yield urethane varnish oil that was diluted with white spirit to about 60% solid. Saravari et al.75 obtained urethane oils at hydroxyl to isocyanate ratios from 1:1 to 1:0.7, with and without methanol acting as a blocking agent. A lower diisocyanate content and the presence of a blocking agent resulted in higher viscosity, higher molecular weight, and shorter drying time. The properties of urethane oil in both liquid and dry film were comparable to those of commercial urethane oil. The films exhibited good hardness and adhesion and excellent water and acid resistance but only fair alkali resistance. However, the prepared urethane oils showed lower flexibility and poor wear resistance compared to those of the commercial urethane oil. Desai et al.31 made an attempt to synthesize polyester polyols using starch, PET waste, and vegetable oil based fatty acids. The starch was glycolyzed with EG to synthesize a novel polyhydric compound (glycol glycoside), and further depolymerization of PET waste was conducted using this polyhydric compound. Depolymerized oligomers were esterified with dehydrated castor oil fatty acids to give polyester polyols. Polyurethane adhesives were synthesized using these polyols for joining SBR substrates. The amount of PET content in polyurethane determines the properties such as adhesion, flexibility, and chemical resistance. Adhesion and flexibility improved as the amount of PET increased in the polyol. Synthesized polyurethanes were found to possess good resistance against cold and hot water but moderate resistance against acid and alkali. Chaudhary et al.61 prepared polyurethane foams utilizing the PET-derived polyester polyols with aromatic diphenylmethane diisocyanate. The compressive strength of the polyurethane foams was found to be inversely proportional to the molecular weight of the glycolyzed polyol used for its preparation. 2.3.5.3. Polymer Concrete. Tawfik and Eskander64 synthesized a fast-curing polymer concrete (PC) by mixing styrenated polyester and marble wastes as fillers. UP was prepared from the reaction of glycolyzed PET oligomers with maleic anhydride and adipic acid. The UP was then mixed with the styrene monomers at a ratio of 60:40% by weight to obtain the styrenated polyester used for the synthesis of the PC. The synthesized PC exhibited acceptable physical properties, good mechanical integrity, enhanced chemical resistance, good heat, and flame resistance. Kim et al.65 reacted PET waste flakes with EG, DEG, and 2-methyl-1,3-propanediol mixtures at a PET/ glycol weight ratio of 250:142. The obtained PET oligomers were then reacted with maleic anhydride and dicyclopentadiene to synthesize UP resin that could be used for the synthesis of polymer concrete upon mixing with styrene. 2.3.5.4. Epoxy Resin. Atta et al.76 formulated two-pack epoxy coating systems using saturated polyester polyols as base component and melamine formaldehyde resins as hardener

component. Polyester polyols were obtained through a transesterification reaction between castor oil/jatropha oil and the glycolyzed oligoesters of PET wastes with poly(propylene glycol) of molecular weight 2000. Their results clearly indicate that the coating properties were influenced by the amount of PET used for glycolysis and the type of oil used for transesterification. Castor oil-based system formed a dense coating with an excellent corrosion resistance.77 2.3.5.5. Vinyl Esters. Atta et al.1,78 prepared epoxy resin by the reaction of glycolyzed products of PET with epichlorohydrin. New diacrylate and dimethacrylate vinyl esters were then synthesized by the reaction of the terminal epoxy groups with acrylic and methacrylic acid. These vinyl esters were used as cross-linking agents for preparing UP resin-based coating on steel. 2.3.5.6. Alkyd Resin. Atta et al.79 glycolyzed waste PET using pentaerytheritol in the presence of manganese acetate as catalyst and m-cresol as solvent at 220 °C to produce suitable hydroxyl oligomers for alkyd resin application. Alkyd resins for coating applications on steel were then prepared using phthalic anhydride, glycerin, PET-derived oligomers, sunflower oil/ linseed oil, and ethylene glycol in the presence of butylhydroxytin oxide as catalyst. The data reported indicated that all cured alkyd resins based on sunflower oil have superior adhesion and mechanical properties with steel. They showed excellent chemical resistance, too, as organic coatings among other cured resins. Torlakoğlu and Gücļ ü80 prepared two short oil alkyd resins of high acid values (30−40 mg KOH/g) utilizing the glycolyzed products of waste PET. Phthalic anhydride (PA), glycerin, and coconut oil fatty acids were mixed with glycolyzed products of waste PET to synthesize PET-based alkyd resins. 2.3.5.7. Acrylic and Allylic Monomers. Aguilar et al.81 utilized BHET obtained by glycolysis of postconsumer PET with boiling EG for synthesizing an acrylic and two novel allylic bifunctional monomers, which find application as cross-linking agents. The bifunctional acrylic monomer (bis(2-(acryloyloxy)ethyl) terephthalate) was obtained from acryloyl chloride, whereas the allylic monomers (2-(((allyloxy)carbonyl)oxy) ethyl (2-hydroxyethyl) terephthalate and bis(2-(((allyloxy)carbonyl)oxy)ethyl) terephthalate) were obtained from allylchloroformate. Karayannidis et al.70 methacrylated the oligoesters obtained on glycolysis of PET waste flakes to synthesize a value-added monomer for preparing a UV-curable acrylic copolymer with styrene. Thermal polymerization of this monomer was carried out at 80 °C in the presence of BPO as initiator. Nanoparticles of silicon dioxide were dispersed into the copolymer matrix as reinforcing agents, but their incorporation resulted in slightly lower Young’s modulus, perhaps due to the presence of nanoparticle agglomerates and their inefficient dispersion into the copolymer matrix. 2.3.5.8. Textile Dyes. Shukla et al.82 converted PET fiber waste into useful products, such as hydrophobic disperse dyes for synthetic textiles. For this, the glycolyzed product (BHET) was converted partly to p-amino benzoic ester and partly to a benzothiazole derivative, the latter when coupled with N,Ndiethylaniline produced a bright yellow disperse dye. Similarly, coupling of p-amino benzoic ester with N,N-diethylaniline yielded an orange-colored disperse dye. These dyes when applied onto polyester fabric by conventional method gave promising results. 14191



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2.3.5.9. Plasticizers. Thavornsetawat et al.83 directly esterified the TPA obtained from glycolysis of PET with 2ethyl-1-hexanol at 160 °C for 6 h to yield a clear, slightly yellowish liquid plasticizer, dioctyl terephthalate (DOTP), with a specific gravity of 0.936, a refractive index of 1.472, and an acid value of 0.0044 mg KOH/g. Their investigations on the possibility of using DOTP as a plasticizer in PVC found that the tensile strength of DOTP-plasticized PVC was comparable to that of DOP-plasticized PVC, but the DOTP-plasticized PVC possessed a higher modulus than DOP-plasticized PVC. On the other hand, Dutt and Soni84 obtained a polyester polymeric plasticizer with an average molecular weight in the range of 450−900 g/mol on depolymerizing PET waste directly with 2-ethyl-1-hexanol. Nitrile rubber and nitrile−PVC blend sheets were prepared using the synthesized polymeric plasticizer, and a comparative study was conducted with the conventionally used dioctyl phthalate plasticizer. The sheets prepared by incorporating the synthesized polymeric plasticizer provided excellent tensile properties and aging resistance for high-performance applications as compared to sheets prepared using dioctyl phthalate. 2.4. Aminolysis. Aminolysis of PET waste has been scarcely studied so far. There are no known papers concerning the utilization of this process on a commercial scale for PET recycling. Chemical depolymerization of postconsumer PET through an aminolytic chain cleavage yields corresponding diamides of terephthalic acid (TPA) and ethylene glycol. The reaction is usually carried out using primary amine aqueous solutions such as methylamine, ethylamine, ethanolamine, and anhydrous n-butylamine in the temperature range of 20−100 °C.2 2.4.1. Kinetics of Aminolysis. Goje et al. have made significant contributions in the studies on the kinetics and thermodynamics of aminolytic recycling of waste PET. PET waste powder on reaction with boiling EG at 470 K followed by treatment with hydrazine monohydrate (HMH) and hydroxylamine hydrochloride gives terephthalic dihydrazide (TDH)4,85 and terephthalohydroxamic acid,4,86 respectively. A one-step aminolysis of PET waste with HMH at 339 K also recovered TDH with significant yields.87 Depolymerization of PET waste was found to be proportional to the reaction time (3 h) and inversely proportional to the particle size of PET waste (optimal reactant size ∼ 127.5 μm). The activation energy (Ea) was recorded as 62.4 kJ/mol, and the Arrhenius constant (A) was recorded as 12,173 L mol−1 min−1. 2.4.2. Uncatalyzed Aminolysis. Soni et al. have contributed much in the area of aminolysis of PET and developed an ecological and economical methodology for the recycling of PET waste at ambient temperature and pressure. They studied the degradation reaction of PET waste with various amines. namely. methylamine, ethylamine, and n-butyl amine. at ambient temperature and pressure so as to obtain N,N′dialkylterephthalamide.88 The complete degradation of PET waste 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 HMH, the degradation time was considerably reduced to 24 h at ambient conditions.89 Hoang and Dang90 obtained bis(2-aminoethyl) terephthalamide (BAET, or trimer) and α,ω-aminoligo(ethylene terephthalamide) (AOET, or oligomers) through the reaction of PET waste with an excess amount of ethylenediamine (EDA). 2.4.3. Catalyzed Aminolysis. Shukla and Harad91 aminolyzed PET fiber waste with excess ethanolamine (EA) so as to

obtain bis(2-hydroxyethylene) terephthalamide (BHETA) with 91% yield. The catalytic activities of glacial acetic acid, sodium acetate, and potassium sulfate were compared. Tawfik and Eskander92 investigated the catalytic activity of dibutyl tin oxide for the aminolytic degradation of PET waste with EA at 190 °C and under atmospheric pressure. The next year, Tawfik et al.93 came up with an environmentally friendly degradation route for aminolyzing PET flakes with EA utilizing sunlight as a renewable source of energy. The catalysts used were dibutyl tin oxide, sodium acetate, and cetyltrimethylammonium bromide. The complete degradation of PET was achieved after 60 days of exposure to sunlight. Fukushima et al.94 have reported an organocatalyzed aminolytic depolymerization of waste PET using 1,5,7triazabicyclo[4.4.0]dec-5-ene (TBD) as catalyst (Figure 6),

Figure 6. Organocatalysis of the aminolytic depolymerization of waste PET using TBD as catalyst.94

producing a broad range of crystalline terephthalamides having great potential as building blocks for high-performance applications with attractive thermal and mechanical properties. Their computational study established insight into selfcatalyzed and organocatalyzed aminolysis of terephthalic esters, suggesting that the bifunctionality of TBD is capable of activating the carbonyl group of PET. This differentiates TBD from other organic bases. The surface morphology of the aminolyzed PET waste flakes with HMH95 and aqueous methyl amine96 at different intervals of time during the aminolysis reaction, both in the presence and in the absence of catalyst, was investigated. PET degraded more quickly in the presence of catalyst; 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. SEM photographs of PET flakes subjected to aminolytic degradation with aqueous methyl amine in the presence of quaternary ammonium salt as catalyst are given in Figure 7. 2.4.4. Microwave-Assisted Aminolysis. In the field of PET aminolysis also, microwave irradiation offers an economical and convenient technology for producing degradation products with little compromise in the yield in comparison to the conventional heating method. In this regard, Pingale and Shukla97 tried to depolymerize PET with excess EA to synthesize BHETA under microwave irradiation in the presence of cheap and nontoxic catalysts such as sodium acetate, sodium bicarbonate, and sodium sulfate. The use of microwave energy gave >90% yield of BHETA in a considerably shortened reaction time of 4 min. Parab et al.98 carried out aminolytic depolymerization of PET bottle waste with HMH under microwave irradiation using sodium acetate and sodium sulfate as catalysts. TDH with a 14192



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aminolyzed PET bottle waste flakes using 3-amino-1-propanol under conventional (5 h) and microwave heating (7 min) methods in the presence of sodium acetate/potassium sulfate as catalyst. The product, bis(3-hydroxy propyl) terephthalamide, was converted into bis-oxazin by cyclization with thionyl chloride. Bis-oxazin is useful in polymer syntheses as a chain extender or a cross-linking agent. Some reported aminolysis reactions of PET are shown in Figure 9. 2.4.5. Applications of PET Aminolyzed Products. 2.4.5.1. Secondary Plasticizer in PVC Formulations. TDH obtained on PET aminolysis with HMH finds application as a secondary plasticizer in PVC compounding.89 The mechanical properties PVC sheets were improved with the incorporation of TDH as secondary plasticizer as compared with DOP (conventional plasticizer) alone. The thermal stability of the PVC sheets also improved by 5−15%, and the stability increased as the dosage of TDH was increased in the formulations. 2.4.5.2. Acrylic Oligomer. Soni et al.102 synthesized a novel acrylic aromatic amide oligomer (N,N′-bis(2-propenamido) benzene-1,4-dicarboxamide) with the reaction of acryloyl chloride with TDH (depolymerized end product of PET waste with HMH). This oligomer can be used as an adhesion promoter on metal/glass surface along with other acrylate monomers in UV-curable formulations. The oligomer with excellent hydrogen bonding capacity also finds application as an alternative to urethane acrylates in radiation curable formulations. 2.4.5.3. Antibacterial Drugs. Palekar et al.103 synthesized a novel series of bis-substituted thiadiazoles and thiazolidinone derivatives from TDH derived from PET. Bis-substituted oxadiazoles and terephthalohydrazides were also synthesized from TDH by cyclization with various aromatic acids and aldehydes. All of the synthesized compounds were screened for their antibacterial activities against various bacterium and fungus strains. Most of these compounds showed antibacterial activity comparable to those of commercial drugs. 2.4.5.4. Textile Dyes. Palekar et al.104 reacted TDH with 4aminobenzoic acid in the presence of polyphosphoric acid to obtain a cyclic compound (4,4′-[5,5′-(1,4)-phenylene] bis(1,3,4-oxadiazole-5,2-diyl) dianiline) having a heterocyclic moiety. Diazotization of this compound followed by coupling with various N,N-disubstituted anilines resulted in a series of novel disazo disperse dyes. Application of these dyes on polyester and nylon fabrics using high-temperature dyeing methods gave brilliant yellowish red hues with fair to moderate light fastness and very good to excellent wash fastness and sublimation fastness. 2.4.5.5. Corrosion Inhibitors. 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.93 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. The platelet-shaped BHETA provided a reinforcing effect and reduced the water and gas permeability, imparting good anticorrosive properties and a special appearance to the paint film. The paint film with a pigment/binder ratio of 2.233 containing 10% BHETA exhibited the best adherence to the steel substrate and corrosion resistance, and the results were comparable to that of the high-built control.

Figure 7. SEM photographs of (a) PET flakes before degradation, (b) PET flakes after 7 days of degradation, (c) PET flakes after 21 days of degradation, and (d) product obtained after degradation with aqueous methyl amine in the presence of quaternary ammonium salt as catalyst.96

yield of about 86% was obtained with a PET:HMH molar ratio of 1:6 with a 1% w/w concentration of either of the two catalysts. The reaction time was reduced significantly from 4 h to 10 min with microwave irradiation as heating source. The mechanism of aminolytic depolymerization of PET with HMH in the presence of sodium salt as catalyst is given in Figure 8.

Figure 8. Mechanism of aminolytic depolymerization of PET with HMH in the presence of sodium salt as catalyst.98

Parab and Shukla99 conducted microwave-assisted expeditious synthesis of medicinally important 2,5-disubstituted-1,3,4oxadiazoles possessing antibacterial properties in a single step from TDH in a very short reaction time of 20 min. Later they explored the catalytic activity of heterogeneous and recyclable β-zeolite acid catalyst and montmorillonite KSF clay catalyst in the aminolytic depolymerization of PET waste with ethanol amine.100 The product, BHETA (85−88% yield), undergoes a cyclization reaction on refluxing with polyphosphoric acid to give the product, 2,2′-(1,4-phenylene)-bis(2-oxazoline) (PBO), which finds applications in polymer synthesis as a chain extender/chain coupling agent or a cross-linker. Shah et al.101 14193



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Figure 9. Aminolysis reactions of PET.86−91

Abd El-Hameed105 evaluated the performance of BHETA prepared from waste PET as corrosion inhibitors for carbon steel (in HCl corrosive medium) by checking weight loss, open circuit potential, and potentiodynamic polarization. The polarization curves indicate that these compounds can act as mixed-type initiators. The inhibition efficiency imparted by the adsorbed BHETA on carbon steel increased with increase in the concentration of inhibitors and decreased with increase in the temperature. The values of activation energy (Ea) and free energy of adsorption (ΔGads) indicate adsorption by both physical and chemical process, whereas the decrease in inhibition efficiency with increase in temperature indicated predominate physisorption of the inhibitor. The adsorption followed a Langmuir adsorption isotherm without modifying the mechanism of corrosion process. 2.4.5.6. Epoxy Emulsion Hardener and Polyol Component for Polyurethane Foams. Spychaj et al.106 aminolyzed PET waste flakes by diethylenetriamine, triethylenetetramine, and their mixtures, as well as mixtures of triethylenetetramine (TETA) and p-phenylenediamine/triethanol amine (TEA) at 200−210 °C with a molar ratio of the recurrent polymer unit to amine of 1:2. The reaction products were tested as water-borne self-emulsifying epoxy resin hardener and a polyol component for rigid polyurethane foams. The glass transition temperature (Tg) of the resin hardened using PET recycled product was

higher than the Tg of the resin hardened with a conventional cross-linker (TETA). The compression strength and thermal conductivity of the polyurethane foams made with the polyol component derived from PET were similar to those of the foams made with a commercial polyol. 2.5. Ammonolysis. The ammonolysis of PET waste flakes is usually carried out with liquor ammonia either in the presence or in the absence of catalyst at temperatures between 70 and 180 °C, under pressure.2 Usually, zinc acetate is used as the catalyst for the process. The main degradation product obtained is 1,4-benzene dicarboxamide, commonly known as terephthalamide. Ammonolysis has not been studied extensively for PET chemical recycling. 2.6. Hydrogenation. Most of the methods to chemically depolymerize PET convert the polymer to the corresponding terephthalates. Ruthenium(II) PNN pincer complexes have recently been used as an effective catalyst for hydrogenative depolymerization of postconsumer PET bottles to furnish value-added chemicals such as EG and 1,4-benzene dimethanol (BDM) rather than a phthalate.107 To increase the PET solubility a 50:50 mixture of anisole and THF was used as the solvent, and the reaction system was exposed to 13.6 atm of hydrogen gas for 48 h at 120 °C. Moreover, an equivalent of the catalyst (1 mol %) was activated with 2 equiv of potassium tert-butoxide (KOtBu) to ensure complete PET depolymeriza14194



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tion. The catalyst system was found to be tolerant to impurities such as pigments and additives in postconsumer PET bottles. Typical ruthenium(II) PNN pincer complexes used for hydrogenation of PET are shown in Figure 10.

Figure 10. Ruthenium(II) PNN pincer complexes used for hydrogenation of PET.107

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3. CONCLUSION The review of the reaction process, kinetics, and broad range of catalysts employed in the glycolytic and aminolytic degradation of postconsumer PET waste shows that the reaction parameters such as reaction time, temperature, catalyst concentration, and PET:reagent ratio have great significance on the efficient depolymerization of PET. The strength of the reaction parameters follows the order catalyst concentration > reaction temperature > reaction time. The degradation products of PET after glycolysis and aminolysis find potential application as plasticizers, cross-linking agents, chain extenders, corrosion inhibitors, and precursors in the generation of value-added products such as UP resins, polyurethanes, textile dyes, antibacterial drugs, epoxy resins, and vinyl esters. A microwave heating method considerably reduces the reaction time from several hours to a few minutes. The rapidly growing nanotechnology provides efficient, ecofriendly, and reusable catalysts to increase the yield of PET degradation products. Thus chemica recycling of PET, besides providing a partial solution to the solid waste problem, contributes to the conservation of raw petrochemical products and energy.

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MA = maleic anhydride MEKP = methyl ethyl ketone peroxide PA = phthalic anhydride PC = polymer concrete PET = poly(ethylene terephthalate) PG = propylene glycol PVC = poly(vinyl chloride) SBR = styrene butadiene rubber TBD = 1,5,7-triazabicyclo[4.4.0]dec-5-ene TBT = titanium(IV) n-butoxide TDH = terephthalic dihydrazide TEG = triethylene glycol TETA = triethylenetetramine TEA = triethanol amine TPA = terephthalic acid UP = unsaturated polyester

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AUTHOR INFORMATION

Corresponding Author

*(N.G.) E-mail: [email protected]. Tel: +91-4842575723. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the University Grants Commission (UGC), India, for the research fellowship. ABBREVIATIONS AIBN = azobis(isobutyronitrile) BHET = bis (hydroxyethylene) terephthalate BHETA = bis(2-hydroxyethylene) terephthalamide BPO = benzoyl peroxide CO = castor oil DEG = diethylene glycol DMT = dimethyl terephthalate DOTP = dioctyl terephthalate DPG = dipropylene glycol EG = ethylene glycol HMH = hydrazine monohydrate 14195



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