chemical recycling of pet

Chapter 27

Chemical Recycling of PET: Methodsand Products

T. Spychaj

1. Introduction

Poly(ethylene terephthalate) (PET) is a polymer used in large amounts bythe textile industry, for the production of audio and video tapes and trans-parent films, as well as in the manufacture of various types of packaging,mainly bottles for soft drinks. An annual rise of the polymer consumption,in the immediate future, is evaluated on ca. 10% [I].

The increasing worldwide usage of PET, along with economic and eco-logical pressures, has caused post-consumer or waste polymer recycling tobe the subject of continuous interest for many scientists and industrial com-panies for ude as a valuable feedstock for chemical processes. PET can berecycled by practically all known recycling methods. Among these, chemi-cal recycling is of great interest because of the wide spectrum of degrading(depolymerizing) agents and the large variety of products (Table 1). Chem-ical recycling can result in total polymer degradation to monomers or in apartial chain cleavage to oligomers.

The molecular weight of the post-consumer PET, more commonly rep-resented by its equivalent intrinsic viscosity, may vary in a range of 0.60-0.85 dL/g; the highest values stand for the material from bottles for carbon-ated drinks or industrial type cord while the lowest ones are characteristicof recording tapes [2].

It is also worth mentioning that the bottle grade PET is in fact apolyester modified by low levels (total up to ca. 3 wt% [3]) of other glycols

Handbook of Thermoplastic Polymers: Homopolymers, Copolymers, Blends, and CompositesEdited by Stoyko Fakirov

Copyright © 2002 WILEY-VCH Verlag GmbH, WeinheimISBN: 3-527-30113-5

Chemical Recycling of PET: Methods and Products 1253

(such as diethylene glycol or 1,4 bis(hydroxymethyl) cyclohexane) or diacids(usually isophthalic acid) rather than a homopolyester of terephthalic acid(TA) and ethylene glycol (EG). This fact is important because the productof post-consumer bottle PET recycling may contain, in addition to TAand EG, some other monomers. Compared to the homopolymer of TA andEG, modified PET exhibits lower crystallinity, improved ductility, betterprocessability, and increased transparency.

In this chapter, chemical recycling methods of post-consumer or wastePET are discussed together with applications of the obtained products.Compared to our earlier work [4], this presentation is a more comprehen-sive review of the state of the art in the field of polyester chemical recycling.Greater emphasis is placed on recent developments and industrially impor-tant techniques. The reader should also consult the specialized chaptersin recent monographs [2,5,6] and Chapters 26 and 28, related to polyesterrecycling, or should consider reactive modification (Chapter 18).

Poly (ethylene terephthalate), like other polyesters, can undergo variousdegradation processes [6]: (i) thermal degradation under the influence ofheat alone, (ii) oxidative (oxygen and heat) and photo-oxidative (oxygenand light) degradation, (iii) hydrolytic degradation in the presence of mois-ture, (iv) radiochemical degradation under the action of ionizing radiation,and (v) chemical degradation in the presence of various solvolytic agents.

2. Theoretical basis of ester bond cleavage

PET and other polyesters are polymers synthesized in reversible reactions.In the forward direction polymerization occurs and in the reverse directionpolymer degradation (depolymerization) takes place by hydrolysis or byother solvolytic reactions.

C-OH + nHOCH2CH2OH ;=M

solvolysis

O

HO+C—(( ))- COCH2CH2O^H + (2n-l)H2O

O

The solvolytic reactions of polyesters consist of the cleavage of C-O bondsof the polymer backbone. The polymer chain is degraded according toScheme 2

I I]—O—C wv +YZ » ΛΛΛΛΟ—OZ +Y-CWV /9\

Il I Il I (2)

O O

1254 T. Spychaj

where YZ is the solvolytic agent molecule, e.g., water, alcohol, acid, alkali,or amine.

The moisture content, temperature, and type of solvolytic agent are thefactors affecting most strongly the rate of solvolysis of ester bonds in thePET chain. Moisture content as low as 0.02wt% still causes PET molec-ular weight reduction by hydrolysis [7]. The rate of hydrolysis is higherby several orders of magnitude than that of thermal breakdown. Heatingof PET at elevated temperatures in the absence of moisture can lead tothe formation of carboxylic acid groups; terminal carboxylic groups cancatalyze hydrolysis [5].

COCH2CH2OH

O

C-OH +CH3CHO

(3)

Poly(ethylene terephthalate) is quite prone to chemical degradation. De-pending on the type of the solvolytic agent, the PET degradation reactionsare as follows:

Hydrolysis

COCH2 CH2 O^ΛΛ/v

O

H 2 O

COH + ^ΛΛΛOCH2CH2OH

O

Alcoholysis— with monohydroxylic compounds

COCH2 CH2 O^ΛΛ/v +ROH

OvVS^C

Il \^/O O

— with glycols or polyhydroxylic compounds (glycolysis)

^ΛΛΛOCH2CH2OH (5)

COCH2 CH2 O^ΛΛ/v +HOROH

OCOROH+ ^ΛΛΛOCH2CH2OH (6)

O


Acidolysis— with monocarboxylic or polycarboxylic compounds

COCH2CH2OWVV +ACOOH

OCOH +WVVOCH2CH2OC-A (7)

O O O

Ammonolysis— with ammonia under anhydrous conditions

COCH2CH2OWVV + NHs

O

O O

Aminolysis— with primary or secondary amines

/R1

- COCH2CH2OWVv +HN —\R2

WVVOCH2CH2OH(9)V '

O O

The hydrolysis of PET can obey various mechanisms of polyester chaindegradation in alkaline, neutral, or acidic media [8].

pH < 7

:—ovw

OH+O

+

IOH (10)

— OWV + H2O ^ ws/vC=O + H+ + HOvw

OH

P H > 7

ΛΛΛ/vC — OVWIlO

OH~

OH

OHI— OVW ^^

I0~

— OH +

0

(U)

-O" + HOvw

O

1256 T. Spychaj

In practice, the chemical degradation processes in post-consumer bot-tles or polyester waste are associated with mixed chemical mechanisms,depending on the PET additives or contaminants and on the reaction con-ditions (see also Chapter 1).

3. Methanolysis

This process consists in the degradation of PET by methanol at high tem-peratures under high pressures. Methanolysis of PET results in the stoi-chiometric formation of dimethyl terephthalate (DMT) and EG that areraw materials for the production of this polymer.

COCH2 CH2 O^ΛΛ/v + CH3OH —>

COCH3 + HOCH2CH2OHI (12)

DMT EG

The process is performed at temperatures of 160-30O0C [4,5] and pres-sures up to 7MPa [5], in the presence of transesterification catalysts, andusually involves two stages: methanolysis itself to give DMT and EG andpurification of the resulting raw DMT by crystallization and distillation.Distillation removes all physical impurities and yields a high purity product.Methanolysis is currently applied to PET waste, arising in the productioncycle, as well as to the polymer scrap. Typical feedstocks for methanolysisinclude used films, plant waste, fiber waste, and post-consumer bottles.

Methanolysis of PET was first reported in patents issued in the late1950s [9-11]. The process is used by the principal PET manufacturers,such as Hoechst [12], Eastman-Kodak [13], and DuPont [14], as well asby smaller-scale manufacturers [4]. The monomers recovered are used inthe production of a full value polymer. A schematic representation of theinstallation for PET methanolysis is shown in Figure 1.

The reaction is conducted most frequently at pressures of 2-4 MPa andtemperatures of 180-28O0C for ca. Ih [15-20], and is usually catalyzed bytypical transesterification catalysts, such as zinc acetate (most commonlyused), magnesium acetate, or cobalt acetate. Examples are known of us-ing arylsulfonic acid salts as catalysts for the methanolytic degradationof PET [1O]. After the completion of the reaction, the catalyst should bedeactivated; otherwise, in subsequent stages of the process, possible DMTlosses could result from transesterification with EG. The DMT obtained isprecipitated from the cooled post-reaction mixture and is then centrifuged,crystallized and, optionally, distilled.

Both continuous and batch methods can be applied in methanolysis.The principal elements of the installation used in the batch method are


PET

Methanol

PEwa

A

l·

]Tste[ ~

Methanoltank

^

A

r

EG tanki. A

Methanol

Depolymerization ^

^

Separation

PET waste

^

DMT,

PETproduction

A ^

DMTtank

^

Figure 1. Flowchart of the methanolysis process [4]

autoclave, crystallizer, centrifuge, and a system for the melting and distil-lation of the DMT obtained [15]. The continuous method requires a muchmore complex apparatus because of the necessity to continuously supply apressurized reactor with raw materials. In a two-stage continuous processdeveloped for Hoechst [16], PET waste, melted at 265-2850C, is collectedin the liquid state in a tank and then fed to a first reactor. Methanol, pre-heated to the reaction temperature, e.g., 190-21O0C, is introduced into theautoclave, equipped with a mixer, at a weight ratio to PET of 4:1. The av-erage residence time of the reagents in the first reactor is 7-13 min and theconversion is 70-90%. The reaction mixture is then fed to the bottom partof the second autoclave at a slightly lower temperature and is slowly raisedtoward the outlet placed at the top of the reactor. The denser impuritiessettle at the bottom and are removed, the reaction stream leaves the secondautoclave, and its pressure is reduced to 0.3MPa. It is then directed to amixer where it is cooled to about 10O0C. After further pressure reductionand cooling, DMT is precipitated and then purified.

In accordance with another continuous depolymerization technology,premelted (by superheated steam) PET waste is solidified and sputteredinto dust of 1 mm particle size. The dust is introduced into the reactor in theform of an aerosol with an inert gas (e.g., nitrogen) and methanol vapors[17]. The temperature of the reactor is maintained at 250-30O0C. It isimportant for the reaction to proceed in an oxygen-free atmosphere and theturbulent flow of the substrate through the reactor should be maintained.

Eastman-Kodak Co. is the owner of patents describing improvedmethanolysis processes [21,22]. According to [21], the PET methanolysis isrealized in a three-stage installation, consisting of a dissolver, a depolymer-ization reactor, and a rectifying column. The polyester stream is directedinto the dissolver where, under the action of the molten polymer from thereactor and the liquid from the rectifying column, the polyester chain length

1258 T. Spychaj

is reduced. PET is then degraded in the reactor by superheated methanol.The degradation products are divided by rectification into a gaseous phasecontaining the monomer components and a liquid oligomerized phase. Thesecond method [22] is based on mixed glycolytic/methanolytic PET degra-dation. This process, as well as the mixed chemical recycling by successivemethanolysis and hydrolysis [19], are described in Section 9 where examplesof the hybrid chemical recycling methods of PET are also given.

A combination of high-temperature PET methanolysis with the esteri-fication of TA or with the products of para-xylene oxidation is proposed in[U]. Polyester waste is introduced at a weight fraction of 20-30% (basedon TA) which results in an increased process yield.

Because of the increased interest in the production of polymer blendscontaining PET, the need to develop methods for the chemical recycling ofsuch systems appeared. This is particularly justified in the case of poly-condensation polymer blends. Sato and Sumitani [23] have patented amethod of methanolysis of polymer blends containing the segment alkylene-2,6-naphthalene dicarboxylate and the segment alkylene terephthalate.Dimethyl esters of the corresponding dicarboxylic acid and glycol are thusrecovered.

The reaction products of methanolysis of post-consumer PET com-prise a complex mixture of glycols, alcohols, and phthalate derivatives.For this reason, the yield of DMT usually does not exceed 90% [15]. Ithas been found that after methanolysis and DMT separation, dissolvedmethyl(hydroxyethyl) terephthalate (MHET) remains in the filtrate. De-pending on the reaction parameters, the filtrate usually contains 11-22 wt%MHET [15]. This TA derivative can be almost quantitatively transformedinto DMT. The presence of MHET is disadvantageous during subsequentdistillation of the EG formed.

Substantial amounts of waste EG usually contaminated by degradedpolyester are formed during PET depolymerization and may be recov-ered by distillation in recycling installations and fed back into the system.The greasy and environmentally harmful residue from the glycol rectifi-cation contains about 80wt% BHET, 5wt% EG, diethylene glycol, andpoly(ethylene glycol)s, as well as other derivatives of TA and EG. A methodhas been developed for the utilization of this residue in PET methanoly-sis processes [24] through its introduction into the reactor along with anappropriate transesterification catalyst. This method causes an increase byabout 10% in the yield of the PET methanolysis process and significantlyincreases the size of the DMT crystals, thus facilitating the separation ofthe latter from the reaction mixture. The essential features of methanolysisas a process for the chemical recycling of PET are: (i) it allows the use oflower quality PET feed than glycolysis because of the easier purificationof the final DMT product; (ii) it is a more expensive than glycolysis, buttolerates higher levels of contamination so that higher chemical processingcosts are offset by relatively low feed-stock costs [2]; (iii) it is rather sensi-


tive to the presence of water and causes problems associated with catalystpoisoning, formation of azeotropes, etc. [25]; (iv) the costs of DMT recov-ery are usually higher than that of virgin DMT [25], but new technologies,developed by Eastman-Kodak and DuPont, are economically more advan-tageous than conventional processes [2].

4. Glycolysis

4.1. Process characteristics

The degradation of high molecular weight PET to short-chain oligomeric oreven low molecular weight, hydroxy-terminated products can be achievedby glycolysis. These products are valuable raw materials for the preparationof unsaturated polyester resins, polyurethanes (especially rigid foams) orbis(hydroxyalkyl) terephthalates, which are the substrates for PET andpoly(butylene terephthalate) (PBT) syntheses.

Glycolysis is performed by heating PET waste with a glycol (ethy-lene glycol, diethylene glycol, propylene glycol, dipropylene glycol, 1,4-butanediol, etc.) under normal or higher pressure at 180-25O0C in thepresence of a catalyst. Typical catalysts are amines, alkoxides, BHET,or metal salts of acetic acid. Usually, glycolysis proceeds for 3-8 h (de-pending on the glycol applied) at ca. 20O0C under reflux, the PET/glycolweight ratio ranging from 1:2 to 1:3. The reaction is carried out under acontinuous nitrogen purge to inhibit degradation of the resulting polyols.Number-average molecular weight, Mn, and hydroxyl number characterizethe polyol products.

PET glycolysis products

with ethylene glycol

HOCH2CH2O-C

.O

BHET for n = I

with 1,4-butanediol

COCH2CH20+H

O(13)

HOCH2CH2CH2CH20--C-

.0

- COCH2CH2CH2CH2O-^H

O

BHBT for n = I

Glycolysis of PET was first described in Polish and US patents in 1964and 1965 respectively [26,27]. Since then, this method of waste polyester

1260 T. Spychaj

recycling has been a subject of permanent interest from both a scientificand an applied viewpoint. PET degradation is often carried out with EG[28-41]. The result of deep polymer glycolysis by EG is primarily BHET,which, similarly to DMT, is a substrate for PET synthesis. Some oligomers(with n = 2 — 10) are obtained in parallel [2,41]. Since BHET is a waxysolid with a relatively high melting point, it cannot be easily purified bydistillation. Purification of BHET is generally achieved by melt filtrationunder pressure to remove physical impurities. An additional treatment withactivated carbon is usually conducted to remove impurities responsible foroxidative degradation/discoloration. Glycolysis of PET with 1,4-butanediolis performed in order to obtain bis(4-hydroxybutyl) terephthalate, whichis a substrate for the production of PBT [2]. Zimmer AG has developeda two-stage process — PET glycolysis takes place in an extruder wherebis(4-hydroxybutyl) terephthalate is formed and then PBT is synthesizedin a polycondensation reactor. Commercial grade EG and tetrahydrofuran(by-product from 1,4-butanediol) are recovered by distillation [2].

Other frequently used glycolytic agents include diethylene glycol (DEG)[34,35,37,42], propylene glycol (PG) [5,39,40,43,44,], and dipropylene gly-col [37,44]. The variables affecting the rate of PET glycolysis with var-ious glycols have been studied by Baliga and Wong [32], Johnson andTeeters [36], Campanelli et al. [38], Vaidya and Nadkarni [43], and Leeet al. [45]. Souza et al. [46] have investigated PET glycolysis products, us-ing EG oligomers (PEG) and PG oligomers (PPG), as solvolytic agents.By means of proton nuclear magnetic resonance (1H NMR) analysis, theseauthors have found that the products obtained in the process carried out at19(K270°C contain incorporated oligomers Jn the range 77-91% for PEGof Mn - 400g/mol, 80-99% for PPG of Mn ~ 220g/mol, and 39-50%for PPG of Mn ~ 1000g/mol. Research on the glycolysis process has beenmainly conducted from the point of view of the utilization of the oligomericproducts. The reactions performed to obtain polyols designated for unsat-urated polyesters and polyurethanes are widely described in the literature[32,35,40,42,43,47-5O].

A new method of PET glycolysis in xylene using EG or PG has been re-cently developed [39]. The process performed in the temperature range 170-2450C is characteristic in that it initially proceeds in PET/glycol dropletssuspended in xylene. PET dissolves in EG at the reaction temperature,whereas EG is almost immiscible with the solvent. The reaction products(oligomeric diols) are gradually transferred into the xylene phase. The pro-cess has some advantages over conventional glycolysis, such as shorter du-ration, improved stirring, and higher purity products.

Another extremely efficient method of polyester glycolysis has been de-veloped by Krzan [51,52]. By microwave irradiation, complete PET degra-dation with glycols takes place in a few minutes, depending on the radia-tion power and the amount and type of the energy-absorbing additive. Thecumulative parallel wave action and heating are responsible for the high


effects of the process.Glycolysis is widely used on a commercial scale by large, medium

and small enterprises to produce unsaturated polyester resins (e.g., Ash-land Chemical, USA, Chemical Works Pustkow and Chemical WorksOrganika-Sarzyna, both from Poland) and polyurethane/polyisocyanuraterigid foams (e.g.. The Chardonol Division of Cook Composites and Poly-mers, USA).

4.2. Unsaturated polyesters from PET-derived polyols

PET glycolyzates find application in the manufacture of unsaturatedpolyester resins [26,40,42-44,47,50,53-59]. For unsaturated polyester resins(UPR), PG is preferred over EG or DEG as a glycolytic agent. The reasonis that PET/PG-derived UPRs are compatible with styrene, whereas thosebased on PET/EG or PET/DEG glycolysis products are not fully compat-ible [50]. In the preparation of UPR, glycolysis and polyesterification canproceed in the same reactor by a two-stage reaction. A schematic diagramof PET glycolysis and use of the glycolyzate for UPR production is givenin Figure 2.

One of the first methods for the synthesis of UPRs, using a product ofpartial PET glycolysis, was developed by Ostrysz et al. [26]. After dissolv-ing the synthesized polyester alkyd in styrene, UPR was obtained. Later,the Industrial Chemistry Research Institute in Warsaw developed a tech-

PET waste

Propylene glycol

(First stage)

Partial glycolysis(First stage) 18O0CPolycondensation

(Second stage) 15O0C

Maleic anhydride(Second stage)

Styrene

Unsaturated polyesterresin

Figure 2. Scheme of PET glycolysis and use of the glycolyzate as a substrate forunsaturated polyester resin

1262 T. Spychaj

nology for the production of UPR with built-in segments of oligo(ethyleneterephthalate) by partial PET waste glycolysis at PG/PET molar ratiosof 0.25-1.0, 20O0C, using a reaction time of 2h [42,47]. Because of diffi-culties in producing a glycolyzate with reproducible properties, a new typeof unsaturated polyester has been prepared, containing ethylene-diethylenediester obtained as a result of PET degradation by DEG as its terephthalicpart. This resin was used in the production of polyester molding compounds[59].

Generally, glycolysis occurs at a temperature higher than that ofpolyesterification— ca. 18O0C vs. ca. 15O0C (Figure 2). After cooling, thePET-based polyol and residual EG are treated with maleic anhydride toform a polyester. The esterification step may also include phthalic anhy-dride and/or isophthalic acid [5]. The reaction of maleic anhydride with PGand PET-based polyols is faster than that with PG and phthalic anhydrideor isophthalic acid. The PET-derived resins require about 12 h to forman unsaturated polyester alkyd with a number-aver age molecular weightof 2000-2500 g/mol and acid value of 25-30 mg KOH/g [5]. Conventionalresins require production times of ca. 20 h. The kinetics of polyesterificationhave been reported by Vaidya and Nadkarni [33,43].

The growing interest in the manufacture and usage of UPRs, utilizingPET waste, results from the comparable or higher strength, stiffness, andtoughness offered by terephthalic acid-based resins in comparison with con-ventional unsaturated polyesters [54-57]. UPRs derived from PET-basedpolyols offer various application opportunities, e.g., for gel coat, polymerconcretes and mortars, sheet molding compounds (SMC) used in the auto-motive industry, etc.

4.3. Polyurethane/polyisocyanurate foams from PET-derivedpolyols

The methods of synthesis of saturated (sometimes branched) polyestersor rather oligomeric estrodiols based on PET glycolyzates as substrates forpolyurethanes (PUR) developed in the last two decades have been reviewedrecently [60,61].

Rigid polyurethane foams are the most important materials pro-duced on this basis because the insulation is invisible to the con-sumer. Milgrom [5] has evaluated that more than 50% of all rigidpolyurethane/polyisocyanurate (PUR/PIR) insulation boards in the USA(in 1990-1992) used polyols made from waste X-ray and lithographic filmsor PET soft drink bottles. The leading American company (Cook Compos-ites and Polymers, Port Washington, WI) converted 11.3-11.5 thousandtons of waste PET (especially colored) into aromatic polyols designated forPUR/PIR insulations [5].

PET-based polyols can also be used in flexible PUR elastomers andfoams, typically as modifiers combined with other commercial polyols. The


addition of PET-derived polyols provides significant improvements in tearstrength, elongation, and indentation-load deflection of flexible PUR foams[5].

Probably the first patents claiming PET-based polyols designated forthe synthesis of polyurethanes were those of Ostrysz and Penczek [62,63].PET was reacted with polyhydroxylic alcohols and a saturated polycar-boxylic acid (or its anhydrides, or hydroxy acids) at temperatures in therange 120-16O0C. The oligoestrols formed had hydroxyl number values of50-200 mg KOH/g and viscosities of 500-700 mPa-s at 550C [62]. Thesepolyols were used for PUR foam synthesis [63].

In the 1980s, numerous methods for the synthesis of PET-derived satu-rated polyols and their usage for PUR/PIR materials have been reported,e.g., [28,29,32-34,49,64-73]. Research on the application of PET-basedpolyols for PUR synthesis has been continued during the last decade (see,e.g., [35-37,45,74-77]). In order to get polyols designated for PUR/PIRsyntheses, the following hydroxyl group-bearing solvolytic agents are usedfor PET glycolysis: DEG [5,36,64,72,73,77], PG [36,72,73], tri- and poly-hydroxylic compounds, such as glycerol, sorbitol, saccharose [68], mixturesof glycols with £eri-alkanolamines [69] or with the products of oxyalkenyla-tion of alkylphenols [71], amines and/or amides [70], or PET-derived poly-ols [65]. However, the products of PET glycolysis with DEG are the mostimportant for this purpose. Adipic acid most often serves as an acidic com-ponent in the synthesis of saturated oligoestrols [4].

Compared to other methods of polyester chemical recycling, the mainfeatures of glycolysis are: (i) it does not lead to the formation of well de-fined chemical species (as in the cases of methanolysis or hydrolysis) andthe mixture of glycolysis products is difficult to separate by conventionaltechniques of crystallization and distillation; (ii) it can be easily adaptedto the required scale of PET-derived polyols production, but it cannot pro-vide the removal of dyes, copolymers, and other contaminants from thefinal products; (iii) the application options for the final glycolysis productsare numerous.

5. Other transesterification methods

In recent years, numerous processes have been developed based on new,specific trends of PET waste chemical recycling. Most of these processesare for low-tonnage production of substrates for the plastics and coatingsindustry. It is possible to obtain valuable products with prices that are com-petitive to their equivalents manufactured through conventional synthesismethods. These technologies are safe, low-waste or practically waste-free,and sometimes less complex than the traditional ones.

1264 T. Spychaj

5.1. Plasticizers

The degradative transesterification of PET may be realized with long-chainaliphatic alcohols [78-82], and the respective products serve as plasticizersof poly (vinyl chloride) (PVC). PET alcoholysis, aiming at obtaining dioctylterephthalate (DOTP), is most often carried out using 2-ethylhexanol [78-81]. The actual product is a mixture of DOTP and small amounts of2-ethylhexyl-2-(hydroxyethyl) terephthalate and terephthalate oligoesters[81]. The plasticizer obtained in this way is at least as good as a commer-cial DOPT from direct synthesis [80] and can replace the most commonlyused dioctyl phthalate [80,81].

Bathe [82] described a method of synthesis of a plasticizer of PVC in thereaction of PET waste with a poly(ester-ether) synthesized from trimelliticacid, Ci2-Cis fatty acids, and a mixture of DEC and EG oligomers.

5.2. Coating materials

The use of PET chemical recycling products for the manufacture of coat-ing materials is reported more and more frequently. Polyester-based paintshave excellent properties, such as hardness, tensile strength, and chemi-cal resistance [2]. PET degradation products can be used for alkyd resinsyntheses; polyhydroxylie alcohols or esters serve as solvolytic agents.

PET transesterification can be performed by esters of higher glycolsand carboxylic acids as well as by esters of polyhydroxylic compounds andfatty acids. The first technology to obtain coating materials using the PETdegradative transesterification products was claimed in 1985 by DainipponToyo Co. [83]. The polymer waste was dissolved in the reaction product ofpentaerythritol with fatty acids at temperatures of 200-30O0C and the in-termediate was esterified with polycarboxylic acids; the final product withMn = 2000-8000 g/mol was dissolved in a mixture of organic solvents (xy-lene/butanol). The lacquer obtained had an outstanding shelf-life stabilityand gave a glossy coating after drying [83].

PET waste can be used in the production of terephthalic electroinsu-lating lacquers [84]. For this purpose, PET is heated in a mixture of trioland glycol at temperatures of 230-26O0C to obtain a homogeneous solu-tion. The process is continued under reflux until the polyester softeningtemperature is established below 6O0C (l-4h). The next stage consists ofthe addition of a transesterification catalyst, e.g., zinc chloride, heating at220-25O0C, and distilling off the low molecular weight products of degrada-tion, until the required softening temperature of the polyester (60-10O0C)is achieved.

The products of transesterification of PET or PBT waste with alkane-diol esters or dicarboxylic acids at 30O0C are low-melting polyesters, findingapplication as adhesives or coating materials [85]. For instance, a mixtureof PET and PBT waste was subjected to a reaction with the product ofpolycondensation of adipic acid and 1,4-butanediol at 28O0C. The polyester


obtained was applied as a powder coating for steel plates, using a fluidizedbed technique at 35O0C. The coating exhibited excellent adhesion to themetal substrate [85].

In the patent specification [86], a hot-melt adhesive based on PET isclaimed. It is designated for paper, aluminum, and polyethylene foils. PETwaste is subjected to degradative transesterification using the product ofthe reaction of trimellitic anhydride and poly(ethylene glycol) at tempera-tures of 230-25O0C, under pressures of 0.1-0.3MPa.

Khan and Chandra [87] described PET degradation with glycerol ormonoglycerides to obtain intermediates for alkyd resins designated for coat-ings. A mixture of PET waste and glycerol (1:1 by wt) refluxed at 23O0Cfor 12 h yielded a clear solution that could be used for alkyd resin produc-tion. The polyol prepared in the first stage of reaction is capable of reactingwith acidic species such as phthalic anhydride and unsaturated fatty acidsaccording to Schemes 14 and 15.

PET

COCH2CH2OWv+HOCH2- CH-CH2OH

OGlycerol

OHI

COCH2CH2OH + HOCH2-CH-CH2 OC-

O O

(14)

cOCH2CH2Ow

O

Polyols

^ Phthalic anhydride Unsaturated fatty acid

= CVWCH3

Oil-modified alkyd resin(15)

Alternatively, polyester waste can be processed with monoglyceridesprepared from glycerol and linseed oil. The costs of the surface coatings

1266 T. Spychaj

based on PET-derived polyols are reduced by up to 25% as compared tothe conventional glycerol/phthalate-based type [2].

The products of PET degradation by trimethylolpropane and pentaery-thritol can be used in the manufacture of high-solids paints [88] . In the firststage, PET waste is depolymerized by trimethylolpropane and pentaery-thritol at temperatures of 230-24O0C for 45 min. The final paint composi-tions contain 30-50 wt% of PET degradation products.

PET waste was used in the manufacture of alkyd resins for water-thinnable paints. The products obtained from the reaction of PET witha mixture of fatty acids, linoleic acid, and trimethylolethane were used inthe preparation of water-dispersible coating compounds [89].

As described in [90], trimethylolpropane was reacted with PET at 23O0Cfor 45 min. The resulting mixture of oligomers was subjected to a reactionwith isophthalic and fatty acids and then with trimellitic anhydride. Thereaction took place without the addition of a catalyst since the catalystspresent in commercial PET ensured a satisfactory reaction rate [9O].

6. Hydrolysis

The hydrolysis of PET relies on the use of aqueous systems at elevatedtemperatures and pressures to obtain terephthalic acid and ethylene glycol.

COH + HOCH2CH2OH

O

TA EG

Each cleavage of the polymer chain consumes one molecule of water andcreates two functional groups, carboxylic and hydroxylic, at the scissionplace. The growing interest in this method is associated with the develop-ment of the PET synthesis directly from EG and TA, which eliminates toxicmethanol from the technological cycle. Hydrolysis of PET waste can be per-formed as an acid-catalyzed, base-catalyzed, or neutral process, and is usedon a commercial scale. However, this method is not as widely applied asmethanolysis and glycolysis since the cost associated with the purificationof the recycled TA is rather high [2] . PET hydrolysis was first described inpatents issued in the years 1959-1962 [91-93].

6.1. Acid hydrolysis

Acid hydrolysis of PET is performed most frequently using concentratedsulfuric acid [94-97] although the application of other concentrated mineral


acids, e.g., phosphoric [98] or nitric acid [99], is possible. If concentratedsulfuric acid (at least 87wt%) is used, PET degradation can be performedunder normal pressure at temperatures below 10O0C for times shorter than30 min [94-96]. The process can be carried out without an external energysupply [95] or may require heating of the reaction mixture [94,96]. Thereaction takes place for up to 5 min under atmospheric pressure. In thefirst stage, PET waste is mixed with sulfuric acid of a concentration of atleast 87wt% at temperatures of 85-9O0C [96], 60-930C [94], or at roomtemperature [95]. As a result of dissolution and PET degradation to TAand EG, an oily viscous liquid is obtained. It is introduced into an aqueoussolution of sodium hydroxide in order to neutralize TA and raise the pHto 7.5-8 [94]. According to Pusztaszeri [95], the post-reaction mixture wasfirst diluted with cool water, and then alkali was added to obtain a pH levelof 11. The solution had a dark coloration and contained TA in the form ofits water-soluble sodium salt, sodium sulfate, EG, and sodium hydroxide,as well as a small amount of insoluble impurities which were filtered offby conventional methods. The coloration of the filtrate could be removedby means of ion-exchange columns. The next stage of the process was theacidification of the solution to pH 0-3 [95], 2.5-3 [94], or 6-6.5 [96], usingan acid (e.g., sulfuric or hydrochloric) in order to reprecipitate TA. Afterfiltering, washing with water, and drying, TA of purity > 99% was obtained.

EG was recovered from the remaining filtrate through extraction withorganic solvents, e.g., trichloroethylene [94]. Another method for EG re-covery is based on the introduction of sodium sulfate into the filtrate so asto obtain a saturated solution; EG salting-out takes place, forming a sep-arate organic layer. Calcium oxide in quantities equivalent to the sodiumsulfate/sulfuric acid content is added and simultaneously mixed into theaqueous layer obtained in the first variant. The precipitated calcium sul-fate is separated by filtration or centrifugation, and the remaining aqueoussolution of sodium hydroxide can be reused. A flowchart of the installationbased on this concept is shown in Figure 3.

Substantial drawbacks of PET hydrolysis by concentrated sulfuric acidare the high corrosivity of the reaction system and the generation of largequantities of waste inorganic salts and aqueous wastes. Yoshioka et al. [97]tried to minimize the last two inconveniences. Dilute solutions of ^SO4

(< 67.7wt%) were used to this purpose; the sulfuric acid can be recoveredand reused in the process. However, this requires prolonged reaction timesof 1-6 h and much higher temperatures (~ 15O0C); also, the pressurizedapparatus used should have a large volume because of the necessity ofusing an excess of dilute acid. After the reaction, PET residue and TA werefiltered off, the deposit obtained was treated with 5M NH4OH in order totransform TA into a soluble salt and then PET was filtered off. TA wasprecipitated with the previously obtained filtrate containing ^SO4.

A hydrolysis process in which PET soft-drink bottles are digested in ni-tric acid (7-13M) at temperatures in the range 70-10O0C, at atmospheric

1268 T. Spychaj

NaOH solution

^r

Reactionmixer

-+ Filtration

Hydrolysis Neutralization

NaOH solution for the cycle

Lime Calcium sulfate

Figure 3. Flowchart of an installation for acidic hydrolysis of PET [94]

pressure for 72 h, was described by Yoshioka et al. [99]. The reaction prod-ucts were TA and EG, and the EG was simultaneously partially oxidizedto oxalic acid. This approach allows one to obtain a product (i.e., oxalicacid) which is more expensive than TA and EG. The yield of oxalic acidwas 40% at 7O0C after 72 h and the yield of EG was 60.8%. TA was formedquantitatively since it is stable to oxidation by nitric acid [99]. As alreadymentioned, corrosion of the equipment and the separation of EG from thewaste acid are the critical problems in the acidic processes.

6.2. Alkaline hydrolysis

Alkaline hydrolysis of PET is usually carried out by the use of an aque-ous solution of sodium hydroxide [91,100-106] or potassium hydroxide[100,104,107,108], or with an aqueous ammonia solution [109,11O]. Accord-ing to literature data, PET waste containing impurities up to 40wt% canbe processed by hydrolysis with NaOH solutions [2].

Sodium hydroxide-catalyzed hydrolysis processes are performed in al-kaline solutions of a concentration of 3-20 wt% under pressures of 1-2MPa(or under atmospheric pressure) at temperatures of 100-25O0C. However,in order to obtain reaction rates applicable to industrial processes, hydroly-sis should be conducted at a temperature above the polymer melting point[111,112]. Catalysts may also be used in the alkaline hydrolysis of PET,e.g., amines with dissociation constant K > 10~5 [113]. Representativeexamples of PET hydrolysis using alkaline solutions are given below.

Pitat et al. [91] have patented a method of PET alkaline hydrolysis inan 18 wt% solution of NaOH. The most advantageous results are achievedat a PET/NaOH weight ratio of 1:20 at about 10O0C for 2h. The sodium


salt of TA formed has relatively good solublility in aqueous solutions ofalkaline hydroxides; however, by maintaining the NaOH concentration at aconstant level of 18 wt%, it is possible to achieve its complete precipitation.After separation, the salt is dissolved in a small amount of water so as toobtain a nearly saturated solution. After acidification, TA is precipitatedfrom the solution, filtered off, rinsed, and dried. EG formed during thereaction remains in the aqueous phase and is fed back into the process afteraddition of NaOH. The EG content in the solution increases and, therefore,its recovery by vacuum distillation becomes feasible. The process can berun under either high or atmospheric pressure as well as under conditionsusing lower hydroxide concentrations [91].

Lazarus et al. [100] described a process allowing the recovery of TA andother monomeric components from PET/polyamide-6 blends. In the firststage, the mixture is heated in an aqueous solution of sodium or potassiumhydroxide; most favorable results are achieved at temperatures of 210-25O0C under autogenic pressure. Weight ratios of the polymer blend towater of 1:2 to 1:3 are technologically most advantageous. When hydroxidesolutions of concentrations of 3-10 wt% are used, the reaction time amountsto 3-5 h. The quantity of alkalies used depends on the polyester content ofthe polymer blend. After the completion of the reaction, the mixture isfiltered in order to remove the insoluble residue and a strong mineral acidis added in order to separate the dicarboxylic acid formed. The generatedcaprolactam and EG are separated by distillation or are salted-out usingNaCl.

Another relatively simple process has been developed in which PETcontaining up to 40 wt% of impurities can be recycled to starting monomers[2]. High pressure equipment is not required and the process uses just arotary kiln, a condenser, and a centrifuge, thus being less capital intensiveand less costly to run than methanolysis. After degradation of the PETwaste in aqueous NaOH solution to EG and disodium terephthalate, themixture is heated up to 34O0C to evaporate and recover EG by distillation.This treatment also reduces the organic impurities to CÜ2 and water. TAis isolated and purified under normal pressure at 10O0C. The process wascommercialized in the USA under the trade name UnPET™ and in Franceunder the trade name RECOPET™ [2].

An interesting approach is the alkaline hydrolysis of a mixture of PETwaste and methyl benzoate formed as a by-product of the oxidation ofpara-xylene to TA [102]. In the first stage, PET is treated with methylbenzoate at temperatures of 190-20O0C. The mixture obtained undergoeshydrolysis by an aqueous solution of alkali-metal hydroxide with a concen-tration of 2-7 wt% for 30 min at temperatures of 95-10O0C. The processallows the recovery of TA and benzoic acid in yields of 87-95% and 84-89%,respectively.

Yoshioka et al. [105] described an alkaline hydrolysis process, involvingthe conversion of post-consumer PET to TA and oxalic acid in a con-

1270 T. Spychaj

centrated NaOH solution. The optimal conditions for this base-catalyzedoxidation were determined to be NaOH concentration of 27 M, temperatureof 25O0C, reaction time of 5h, and oxygen partial pressure of 5MPa. Theprocess can effectively oxidize even green PET waste, giving a colorlessproduct mixture. A concept of EG oxidation in the mixture with disodiumterephthalate (which is stable to oxidation under the reaction conditions)has also been used in the acidic PET hydrolysis method [99].

In a kinetic study of PET alkaline hydrolysis by thermal methods, Kaoet al. [107] have found that potassium hydroxide is more active than sodiumhydroxide. Experiments performed with KOH solutions (molar ratio ofPET repeat units to KOH of 1:4) under autogenic pressure at 16O0C for30min showed complete polymer disintegration [108].

Using ammonium hydroxide instead of sodium hydroxide solutions forPET hydrolysis, it is possible to eliminate the impurities present in post-consumer PET bottles. Ammonium hydroxide is more expensive thanalkali-metal hydroxides or carbonates used in typical alkaline degradationprocesses. However, the maximum obtainable pH of the reaction mediumis below the level at which aluminum bottle cups or other extraneous ma-terials are attacked, thereby eliminating the need to remove the cups orlabels before processing.

The process according to Lamparter et al. [110] is performed preferablyat temperatures of 190-23O0C under pressures of 1.4-3.1 MPa for maximum3h, with the amount of NH4OH being higher by a factor of about 3 thanthe stoichiometric amount with respect to the formed terephthalic aciddiammonium salt. The reaction mixture containing water-soluble diammo-nium terephthalate is filtered while still hot. The filtrate is then acidifiedwith sulfuric acid to precipitate TA. The latter is isolated by filtration,washed with water, and dried. The secondary residual filtrate contains EGand ammonium sulfate. Ammonium sulfate is reacted with calcium oxide(or hydroxide) to remove gaseous ammonia; gypsum is formed as a by-product [UO]. EG is separated from the aqueous solution by distillation(as a bottom residue). A similar process was reported by Datye et al intheir review article [109].

Namboori and Haith [101] have compared reactivities of PET with vari-ous solutions, namely aqueous NaOH, sodium tert-butoxide in £eri-butanol,sodium isopropoxide in isopropanol, sodium methoxide in methanol, andsodium ethoxide in ethanol. It has been demonstrated that sodium ethoxidein ethanol is the most reactive solution and an aqueous solution of sodiumhydroxide is the least reactive.

Treatments based on partial PET alkaline hydrolysis are widely usedin the polyester fiber industry. The effect of such processes on the me-chanical properties of fibers [101,114,115], oligomer content, and change inthe molecular weight distribution [103,115], or loss of fiber mass [103,114]has been investigated. Collins and Zeronian [103] have demonstrated thatNaOH solutions in methanol react with PET significantly faster than anal-


Neutral hydrolysis [122]:30O0C, H2O/PET weight ratio 1:1,conversion 18.5%

(B)

MeltingpET zoneflakes :

Reaction zone

Qi CO

tTA+ EG

TA/Na salt+ EG

Solid NaOH

Basic hydrolysis [118]:100-20O0C, conversion 97% (A)

Figure 4. Hydrolytic PET degradation in extruder

ogous aqueous solutions.Tindall et al. [116,117] have claimed methods of PET hydrolysis using

substantially anhydrous three-component mixtures comprising a hydroxide(or alkoxide), alcohol or glycol, and a polar aprotic solvent. The most effec-tive reaction conditions are as follows: sodium or potassium hydroxide (orthe alkoxide tetralkylammonium hydroxide), methanol or EG, dimethylsul-foxide or N-methyl-pyrrolidone as polar aprotic solvents, at temperaturesof 90-15O0C and pressures from atmospheric to 1.5MPa.

In the recycling of PET to terephthalates of alkali metals or alkaline-earth metals, the process described by Benzaria et al. [118] may be crucial.The depolymerization is carried out in a mixer-extruder with the use ofsolid NaOH at temperatures of 100-20O0C. After distillation of EG fromthe post-reaction mixture under reduced pressure, the respective salt ofTA is obtained in powder form. Here, the necessity of separating the glycoland water mixture is eliminated, which is undoubtedly the essential ad-vantage of the process. The degree of polyester saponification reaches ca.97%. Figure 4A shows a scheme of the extruder system used for PET wastedegradation according to this concept.

6.3. Neutral hydrolysis

During the last decade, the hydrolysis of PET with water at neutral pHand temperatures of 200-30O0C has been intensively studied [119-124],although the first reports on such degradative polyester reactions have beenknown since the 1960s [92,93]. Despite the fact that the process is conductedin the absence of acidic or alkaline catalysts, the pH of the reaction mixture

1272 T. Spychaj

reaches values of 3.5-4.0, which, according to Michalski [120], is because ofthe formation of TA monoglycol ester. The process is usually carried outat pressures of 1-4MPa and temperatures of 200-30O0C [112,119-121,123-126]. The weight ratios of PET to water vary from 1:2 to 1:12. It has beenconfirmed that PET hydrolysis proceeds significantly faster in the moltenstate than in the solid state [111,112]; therefore, it is advantageous for therecycling to be carried out at temperatures higher than 2450C.

The application of common transesterification catalysts is possible, butthe recommended ones are alkali-metal acetates [102]. Michalski [127] hasstudied the influence of the catalysts contained in commercial PET, i.e., ac-etates of calcium, manganese or zinc used in the first stage of PET synthesis(transesterification), antimony trioxide added in the second stage (poly-condensation), and the effect of the stabilizers blocking transesterification(added between the first and second stage), i.e., phosphorus compounds.Michalski's investigations have proved the accelerating action of the trans-esterification catalysts. No inhibition by stabilizers was observed and insome cases they had an accelerating effect. Campanelli et al. [112,121]have described the catalytic effect of zinc catalysts at temperatures of 250-2650C. The rate constant was found to be higher by about 20% than inthe uncatalyzed system. The catalytic effect of zinc and sodium salts is at-tributed to the electrolytic destabilization of the polymer/water interfacein the hydrolysis process.

During PET hydrolysis, a monoester of glycol and terephthalic acid isformed as a by-product. It dissolves well in water at 95-10O0C; at thesetemperatures TA is practically insoluble and its separation from the post-reaction mixture does not create any problems [127]. It has been provedthat the appropriate control of the process parameters limits the amountof monoester formed to less than 2% [127,128].

An effective five-stage process of neutral hydrolysis of PET to EG andTA of purity required for the synthesis of this polymer has been patentedby Tustin et al. [123]. PET is hydrolyzed at temperatures of 200-28O0C.After cooling the post-reaction mixture to 70-10O0C, the solid product isfiltered and dried at temperatures of 25-1990C. EG is recovered from thefiltrate as a result of two-stage distillation. The solid product of hydrolysisis heated with water at temperatures of 310-37O0C and, after cooling, TA isobtained. The purity of the recovered TA and EG allows their application inthe production of high quality homo- and copolymers and does not excludetheir use in the manufacture of bottles and fibers.

The neutral hydrolysis method lacks the primary drawbacks character-istic of acid or alkaline hydrolysis. The formation of substantial quantitiesof inorganic salts that are hard to dispose of is avoided; corrosion problemsdue to the use of concentrated acids and alkalies are also avoided. Anotherundoubted advantage of neutral hydrolysis is that the process is environ-mentally friendly and, therefore, growing interest in this technology can beexpected. Its main drawback is that all mechanical impurities present in


the polymer are left in the TA and, consequently, a much more sophisti-cated purification process is required. Possible product contaminations areremoved by filtration of the caprolactam solution of TA or of its aqueoussodium hydroxide solution [127]. The crystallization of TA from caprolac-tam makes it possible to obtain a product with purity of at least 99%[129]. During the hydrolysis of PET, a substantial volume of diluted EG isgenerated, which can be recovered through extraction or by distillation.

Kamal et al [122] have presented an efficient process of continuoushydrolysis, using a twin-screw extruder as a reactor and resulting in PEToligomers containing 2-3 repeat units. The products with carboxyl end-groups have higher melting temperatures than virgin PET, while thosewith one carboxyl and one hydroxyl group or with two hydroxyl groupshave lower melting points. It has been demonstrated that the utilization ofcold or even hot water in the process does not give a satisfactory degreeof depolymerization. The situation changes when high-pressure saturatedsteam of temperatures close to that of molten PET is injected into thereaction zone of the extruder. The maintenance of adequate high pressuresrequires the application of suitable throttling systems, allowing the controlof back- leakage of the post-reaction mixture from the extruder. A scheme ofthe extruder setup for neutral hydrolysis of PET waste is shown in Figure4B.

7. Ammonolysis and aminolysis

7.1. Ammonolysis

The action of anhydrous ammonia on PET causes degradation of the poly-mer, resulting in terephthalamide [130,131]. Terephthalamide can be con-verted into terephthalic acid nitrile and further to para-xylylene diamineor l,4-bis(aminomethyl) cyclohexane [13O].

COCH2CH2OVX,I , -EGO O O

Terephthalamide ^ '

H2NH2C-

p-Xylylene diamineN=C-(( ))-C=N 52 ,.

100-20O0C vcatalysts \ . . (18)

Terephthalonitrile H2NH2C-(^J-CH2NH2

l,4-Bis(aminomethyl) cyclohexane

1274 T. Spychaj

Very good results were obtained during the ammonolysis of PET wastefrom post-consumer bottles; the process was carried out under a pressureof about 1 MPa in a temperature range of 120-18O0C for 1-7 h [13O]. Afterthe reaction was completed, the amide product was filtered, rinsed withwater, and dried at a temperature of 8O0C. The product had a purity of atleast 99%, in a yield of more than 90%.

A low-pressure method of PET ammonolysis, in which the degradationagent is ammonia in ethylene glycol medium is also reported [131]. Theprocess, conducted at a temperature of 7O0C and a ratio of PET/NH3 of1:6, is catalyzed by zinc acetate in a quantity of 0.05 wt%. Terephthalamideis produced in a yield of about 87%.

7.2. Aminolysis

The processes of a partial surface modification of PET fibers or powders byaminolysis have been the subject of extensive research [114,132-139] andfind industrial-scale application. Such modification processes improve thedyeability and other technical and end-use parameters of the fibers.

In most cases, the aminolytic modification of PET fiber surfaces is con-ducted using primary amines in aqueous solutions [114,132-139] or, lessfrequently, in gaseous form [138]. Most frequently used are methylamine[133-136], ethylamine [114,136], and butylamine [114,137]. Ethanolamine[136], ethylene diamine [139], and triethylene tetramine [138] are also used.The purpose of this aminolytic PET fiber modification is a selective degra-dation, allowing control of the fiber morphology. The amorphous regionsof a semicrystalline polymer undergo rapid degradation, whereas the crys-talline regions are much more resistant to attack by amines [132]. Popoola[135] put forward the mechanism of aminolytic degradation of PET, basedon the example of n-butylamine.

Carduner et al. [140] have investigated the PET fiber degradation mech-anism caused by sulfonamides and thiurams present in nitrile rubber com-posites used for cord tyres. These rubber additives (applied in curing com-positions) diffused into the PET fiber, resulting in a uniform aminolyticdegradation of the polymer surface.

There is one earlier report on deep aminolytic PET degradation [141].Other references concerning this process and the respective reaction prod-ucts have started to appear over the last few years [142-147].

Aminolytic degradation of PET waste with allylamine [142,143] pro-duced Ν,Ν'-bisallyl terephthalamide. The degradation occurred at a tem-perature of 17O0C under pressures of 1.5-2.OMPa for 2 h with the aminein excess. After removing the residual allylamine by distillation and EG bywashing with water, a high purity powdery product, melting at 217-2190C,was obtained [142]. The product may be considered as a high-temperaturesolid crosslinking agent for unsaturated polyesters.

Sulkowski et al. [144] have recently investigated products of PET


aminolysis with morpholine and hydrazine. Conditions of the degradationreaction were not given. These authors found that morpholine-degradedPET contains practically exclusively N-bismorpholine terephthalamide,whereas the product of aminolysis with hydrazine contains a mixture ofhydrazides of TA and hydrazide derivatives of ΤΑ/EG oligomers.

Polyamines (diethylenetriamine and triethylenetetramine (TETA))have been applied for PET aminolysis in order to obtain aminoamidederivatives of TA, designated for epoxy resin hardening [145-147]. Thechemical degradation of PET with polyamines, at a molar ratio of the re-current polymer unit to polyamine of 1:2, results mainly in low molecularweight products and oligomers.

COCH2CH2OVW +H2NCH2CH2(NHCH2CH2)2NH2

20°-210°c--EG

O

H2N(CH2CH2NH)2CH2CH2HNC-/QV CNHCH2CH2(NHCH2CH2)2NH2

O OSymmetrical TA amide

I y /CH2CH2NH2

H2N(CH2CH2NH)2CH2CH2HNC-(C))- CNIl v^V Il X(CH2CH2NH)2H (19)

Asymmetrical TA amide/imide

H2NCH2CH2 i ι /CH2CH2NH2

/ N C ^ O ) V - CNH(HNCH2CH2)/ Il V^y Il N(CH2CH2NH)2H

Symmetrical TA imide

NMR spectra of the PET/TETA aminolysis products showed the pres-ence of symmetrical terephthalamide (47%), symmetrical terephthalim-ide (21%), and asymetrical amide/imide TA derivatives (32%) [146]. ThePET/TETA product is a viscous liquid (~ 5Pa-s at 6O0C) that is compat-ible with liquid epoxy resins. Some characteristics of the epoxy materialshardened with PET/polyamine aminolysis products have been reported[145-147].

8. Aminoglycolysis

This process concerns the solvolytic reactions performed by alkanolamineswith tert-nitrogen atom. In addition to alkali solutions [113] or glycols [70],solvolysis of the ester bond in polyesters is sometimes realized in the pres-ence of amines. Primary and secondary amines in such reaction systems canact either as catalysts or as active aminolytic agents. Examples of mixtures

1276 T. Spychaj

of glycols and aminoalcohols used in PET solvolytic degradations can befound, e.g., in [69]. Similar mixed substances are applied in the chemicalrecycling of PUR/PIR foams [148].

Research on PET chemical degradation with triethanolamine (TEA)was undertaken in the author's laboratory [142,145-147,149]. The chemicaldegradation of PET with TEA is considered to be glycolysis catalyzed bya tertiary amine. For this reason, the process is termed "aminoglycolysis".The solvolytic activity of £eri-N-alkanolamines with respect to PET esterbonds is a function of the hydroxyl group content and of the amine basicity[146].

Aminoester derivatives of TA and TEA are formed during the PETdegradation with TEA in excess (molar ratio of 1:2).

COCH2CH2OVW +HOCH2CH2N/CH2CH2OH

XCH2CH2OH

COCH2CH2NCH2CH2OH

'CH2CH2OH

(20)

CH2CH2OHI

COCH2CH2NCH2CH2OC

O O

The product of PET aminoglycolysis with TEA (PET/TEA) at a tem-perature of 2050C is a multicomponent mixture of TA ester derivatives[146]. This product is a viscous liquid (~ 13mPa-s at 6O0C) with hy-droxyl number in the range of 100-400 mg KOH/g, depending on the reac-tion conditions [60,146,149]. It is miscible with epoxy resins, polyols desig-nated for rigid polyurethane foams, and water. Results of the applicationof PET/TEA as a polyol component for rigid polyurethane foams are de-scribed in [60,149]. However, the most promising results have been obtainedwith the PET/TEA product used as epoxy resin hardener [145-147,15O].

The main features of the product obtained by PET degradation withtriethanolamine and applied as epoxy resin hardener are: (i) advantageoustechnological properties, i.e., low viscosity in the compositions with liquidepoxy resins at elevated temperatures, long pot-life at ambient temperatureand at 8O0C [146,147]; (ii) relatively wide range of the epoxy resin/hardenerweight ratios [147]; (iii) high values of the mechanical properties; the si-multaneously demonstrated flexural, tensile and impact strengths of thePET/TEA-hardened epoxy materials [146,147] place them on the level ofepoxy materials modified with special additives, i.e., rubber or thermoplas-


tic resin modifiers [151-153]; (iv) ability to emulsify liquid epoxy resins inwater and to be used in water-thinnable paints [15O].

Research on PET aminoglycolysis with other alkanolamines is inprogress, and some new degradation products are considered as promis-ing additives to reactive polymeric systems [154].

9. Hybrid chemical recycling methods

Some hybrid solvolytic methods of PET degradation are described inthe literature, e.g., [2,19,22,155]. The combined processes of methanoly-sis/hydrolysis and glycolysis/hydrolysis are most important, and some in-dustrially relevant examples are described below.

A continuous process for the recovery of TA and EG from PET wasteby hybrid methanolysis/hydrolysis, resulting in the production of fiber-grade terephthalic acid, was claimed by Socrate and Vosa [19]. The processcomprises the following steps: (i) decomposition of the polymer to its con-stituent monomers with superheated methanol (230-25O0C) introduced tothe molten PET (26O0C) at atmospheric pressure; (ii) continuous removalof methanol and monomer vapors and condensation of EG and DMT mix-ture; (iii) removal of EG from the mixture of condensed monomers by dis-tillation; (iv) hydrolysis of the distillation residue at a temperature above15O0C (e.g., 27O0C) under pressure (e.g., 8MPa) for a certain time (e.g.,30min); (v) recovery of TA by filtration; (vi) optionally, crystallization ofTA.

A two-step hybrid glycolysis/methanolysis process [22] involves glycol-ysis as a first stage to obtain bis(hydroxyethyl) terephthalate (BHET) andhigher ΤΑ/EG oligomers. As a result of methanolysis of these products,DMT is obtained. The method allows inconveniences associated with high-pressure methanolysis to be minimized. The PET feed is dissolved in aglycolyzed PET product to achieve considerable energy savings. Next, thesuperheated methanol vapor is added. The reaction mixture is held at pres-sures lower than that necessary to keep methanol in the liquid state becausethe DMT formed is removed as a vapor. The removal of esters and alcoholsas vapors allows the use of more contaminated PET than that accepted byconventional liquid-phase methanolysis.

The process developed by West [2] allows the conversion of significantlycontaminated plastics by a combination of glycolysis and hydrolysis intohigh-quality substrates for PET synthesis. This technology was used forTA and EG recovery by Innovations in PET, West Footscray, Victoria,Australia. The scheme of the production line is shown in Figure 5 [2], andthe process includes several steps: (i) shredding of the PET bottles at thecollection point; (ii) washing of the flakes in a "float/sink" water tank toseparate paper and polyolefins; (iii) drying of the PET flakes; (iv) partialglycolysis with boiling EG to obtain PET in a brittle state (performed

1278 T. Spychaj

PET bottles

EG & contaminants

GranulationDrier

Glycolysis EG Hindered settling

Screening

EG, water J | paper fiber PVC, PE, PP,Waste Adsorbents PaPer> metal

Figure 5. Scheme of hybrid glycolysis/hydrolysis process for recycling contami-nated post-consumer PET bottles [2]

in a screw conveyor for ca. 50min); (v) crushing of the brittle flakes toparticles of less than 1 mm in size; (vi) screening (to < 700 μηι) to removeuncrushed paper, PVC, aluminum fragments, etc., and hindered settlingto eliminate the remaining small particles, such as paper fibers and glue;(vii) glycolysis of PET crumb to BHET and adsorption of other organiccompounds, pigments, etc., on activated carbon or clays; (viii) filtrationin order to eliminate submicron particles; (ix) BHET hydrolysis at 20O0C;(x) recovery of TA from the boiling water (impurities remain dissolved);(xi) distillation of EG.

With the exception of the aforementioned impurities, the modifiers andpigments added to commercial PET are also effectively removed in thishybrid process. PET produced using TA and EG obtained from the processis of similar purity to that of the parent polyester [2].

Penczek and Ostrysz [155] have developed a process, combining par-tial hydrolysis of PET with subsequent condensation of the product intophenol/formaldehyde resins, that can also be considered as a chemical re-cycling method. Unlike the typical hydrolysis processes, only the first stageof the solvolytic reaction is carried out in order to achieve partial PETdegradation. PET waste is dissolved in phenol, and a polymer solution ofhigh viscosity is obtained. In order to reduce the viscosity, a small amountof water and a catalyst are added to the system, and subsequent heatingresults in partial PET hydrolysis. The next step consists in the condensa-tion of phenol with formalin, under conditions characteristic of standardnovolac resin synthesis. PET waste contaminants, including the mechanicalimpurities, do not pose problems because powdery fillers are subsequently


added into the final resin for use in the production of molding compounds[155].

10. Other methods

There are known processes of chemical degradation of PET waste whichdo not belong to the methods described so far (see Chapters 26 and 28).

Wang et al [156] have investigated the kinetics of PET degradationusing bisphenol A (phenolysis?) as the solvolytic agent. The reaction wasperformed at temperatures of 190-23O0C under autoclave conditions. Theinfluence of the process parameters and the reaction mechanisms of thisprocess were investigated.

An interesting method of recycling PET waste is based on polymerdegradation in a coal-tar pitch environment, and the products obtainedare utilized in the manufacture of polyurethane of an inferior quality, des-ignated for the production of putties and sealing compounds for construc-tion [157]. Pulverized foil waste or PET fibers are mixed in batches withcoal-tar pitch and heated at temperatures of 130-33O0C. The most ad-vantageous effects are obtained in the case of mixtures containing at least30 wt% of coal-tar pitch. Products with softening temperatures within therange 30-23O0C are obtained.

Post-polymerization of PET waste, after a special treatment, can beconsidered as a separate method of chemical recycling. This process leadsto higher molecular weight PET. For instance [158,159], PET waste is dis-solved in a suitable organic solvent or solvent mixture (benzyl alcohol,phenol/tetrachloroethane 60/40 by wt, ort/io-chlorophenol, trifluoroaceticacid, trifluoroacetic acid/dichloromethane 50/50 by vol, or nitrobenzene)and then precipitated with methanol so as to develop a large surfacearea in the material. The latter is subjected to solid-state polyconden-sation under reduced pressure at 23O0C for 8h. A molecular weight ofabout 60000g/mol is achieved, i.e., threefold higher than the starting one(Mn = 20300g/mol).

Chemical reactions during blend preparation are described in Chapters17, 20, and 28, and chain extension is discussed in Chapter 18.

11. Summary and conclusions

PET waste originates from various sources (soft-drink bottles, photographicand X-ray films after removal of the silver compounds, spent textile materi-als, industrial wastes), differing in shape (flakes, pellets), color, and degreeof contamination. It is considered as a feed-stock for chemical processing.

PET waste recycling represents the most successful and widespreadexample of polymer reuse. The chemical recycling of PET can be per-formed with various solvolytic agents, and useful products for many appli-

1280 T. Spychaj

cations may be obtained. The important advantages of the chemical recy-cling methods related to post-consumer and waste PET are: (i) removal ofany impurities associated (physically or chemically) to the polymer chainby purification of the monomers and oligomers; (ii) the selection of themost appropriate recycling method depends on the quality of the availablefeed-stock and on the type of the targeted product.

However, some drawbacks of the PET chemical recycling processesshould also be listed: (i) necessity to operate with well-sorted feed-stocks;(ii) consistency of the batch-to-batch quality; (iii) separation of coloredPET wastes is often required; (iv) contaminants, such as PVC, paper, glue,should be reduced to a minimum; (v) in addition to TA and EG, the re-cycled PET product contains a number of comonomers that increase itschemical heterogeneity.

Among the chemical methods of polyester recycling, glycolysis,methanolysis, and hydrolysis are commercially important. In addition tomonomers for PET or PBT synthesis, the former can also offer polyol sub-strates for various applications, especially for unsaturated polyester resinsand foamed polyurethanes. The products of methanolysis and hydrolysisare monomers for virgin PET synthesis. A comparative evaluation of thethree most commercially important chemical recycling methods is given inTable 2.

Some hybrid methods, i.e., combinations of the aforementioned im-portant chemical degradation processes, find industrial-scale applications.These recycling techniques are economically viable at a facility throughputof at least 15000 tons per year [2].

In recent years, an increasing tendency to use less common chemicalsfor PET degradation is observed. These trends relate to the special trans-esterification reactions, ammonolysis, aminolysis, or aminoglycolysis meth-ods used in order to obtain the components or additives for polymeric orresinous materials.

The economic viability of a given method of chemical recycling is deci-sive for its application. This is related to the following factors: (i) process-ing capability of the installation; (ii) specific parameters of the technologyused (temperature, pressure); (iii) nature of the agents used for solvolysisand as auxiliary solvents, neutralizers, etc. (corrosive and/or toxic); spe-cial labor safety requirements associated with the above chemical agents;(iv) methods of the products separation (if necessary); (v) purity and fur-ther application of the products; (vi) waste generation; (vii) contaminationof the recycled PET; (viii) cost of the recycled polymer; (ix) continuity ofthe PET recycled material supply. All these factors influence the final priceof the product obtained from PET recycling.

It seems, however, that chemical recycling processes offering medium orlow tonnage specialized products for paints, adhesives, plasticizers, harden-ers, etc., are economically plausible with a throughput limit substantiallylower than that given above for large-scale methods.


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Acknowledgement

This work was supported by the State Committee for Scientific Research(KBN), Poland.

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