Chemical Recycling of PET Research Articles

Chemical Recycling of Poly(ethylene terephthalate) (PET) by Hydrolysis and Glycolysis* Daniela Carta 1, Giacomo Cao 1"" and Claudio D'Angeli 2

1Dipartimento di Ingegneria Chimica e Materiali, Unitfi di ricerca del Consorzio Interuniversitario Nazionale La Chimica per l'Ambiente e Centro Interdipartimentale di Ingegneria e Scienze Ambientali, Universitfi di Cagliari, Piazza D'Armi, 1-09123 Cagliari, Italy

2INCA International S.p.A., A Subsidiary of The Dow Chemical Company, Via Patroclo 21, 1-20151 Milano, Italy

** Corresponding author (cao@visnu.dicm.unica.k)

low molecular-weight products which can be purified and reused as raw materials for the production of high-quality chemical products. Depending upon the depolymerizing agent, the chemical process applied in PET recycling can be divided into five groups: methanolysis, glycolysis, hydroly- sis, aminolysis, ammonolysis.

In this work, our attention is confined to the hydrolysis (neu- tral, acid and alkaline) and glycolysis processes of PET chemi- cal recycling. A schematic diagram of each of these proc- esses is shown in Fig. 1.

Introduction

Poly(ethylene terephthalate) (PET) is an important thermoplas- tic polyester, used in the form of fibers, sheets and films, whose consumption is dramatically increasing in the world as a whole. Such a level of consumption generates considerable amounts of waste and scrap that could be recycled and/or recovered.

The recycling of PET can be carried out in several ways. A very attractive strategy is the 'material recycling' which is characterized by the collection, disintegration, and granula- tion of waste polymer which is recirculated into production [1,2]. Unlikely, this recycled PET cannot be used to make products that have to meet very high quality standards.

An interesting method of PET recycling, called 'chemical recycling', is based on the concept of depolymerizing the condensation polymer through solvolytic chain cleavage into

* Part of this work was presented at the '4 ~ Convegno Consorzio Interuniversitario Nazionale La Chimica per I'Ambiente (INCA)', Santa Marghedta Ligure (Italy), 25-28 February 2001

Schematic diagram of PET acid hydrolysis process

PET ~ I > excessacid concentrated sulfuric acid P= 0.1 MPa

NaOH T=298-363 K > EG > Na2SO.

calcium oxide or hydroxide ~ CaSO,

Schematic diagram of PET alkaline hydrolysis process

PET ~ > TPA NaOH ~J> P= 1.4 - 2 MPa

wetting agent or surfactants T= 483-523 K > EG calcium oxide or hydroxide > CaSO,

H2SO4 > sulfate salts

Schematic diagram of PET neutral hydrolysis process

PET >1 P= 1 - 4 MPa > TPA T= 473-573 K

water or steam Transesterification catalyst (alkali metal acetates) > EG

Schematic diagram of PET glycolysis process

PET > P= 0.1 - 0.6 MPa > BHET

> T= 463-513 K >- oligomers

EG zinc acetate > EG

Fig. 1 : Schematic diagrams of PET hydrolysis and glycolysis processes

1 Chemical Recycling Methods

1.1 Hydrolysis

Neutral hydrolysis. Neutral hydrolysis is typically carried out contacting scrap PET with water or steam at a pressure of 1-4 MPa and temperatures of 473-573 K. The main prod- ucts formed are ethylene glycol (EG) and terephthalic acid (TPA). TPA is separated from the post reaction mixture by filtration, while a substantial volume of diluted EG is recov- ered through extraction or by distillation [3-5].

390 ESPR - Environ Sci & Pollut Res 10 (6) 390 -394 (2003) �9 ecomed publishers, D-86899 Landsberg, Germany and Ft. Worth/TX �9 Tokyo ~ Mumbai �9 Seoul * Melbourne ~ Paris

Research Articles Chemical Recycling of PET

Despite the apparent simplicity of its basic mechanism, the neutral hydrolysis of PET is a complex process which is not fully understood. There is a common agreement that the neutral hydrolysis has an autoaccelerating character. The hydrolysis is typically considered as a non-diffusion con- trolled process where the water concentration is constant. Nevertheless, two different kinetic models have been pro- posed, and are summarized as follows along with the rel- evant main assumptions:

1) Half-order kinetic model �9 Autocatalytic reaction mechanism �9 Catalytic effect of the carboxyl end-groups are taken

into account �9 Specific chain-end process

2) Classic second-order kinetic model �9 No autocatalysis �9 Crystallinity and hydrophilicity changes induced by

chain scission are taken into account �9 Simple second-order random scission process

Three main sources of difficulty for the investigation of neu- tral hydrolysis have to be considered:

1) The role of chain ends 2) Morphological changes induced by the degradation 3) Type of scission of chain (random or specific chain-end

process)

The effect of chain ends was either neglected [6-7] or taken into account through different approaches [8-12].

A systematic study by Ravens et al. [8] in the early 60s of the hydrolytic degradation of PET revealed that the hydro- lytic scission of polyester chains above their glass transition temperature was an autocatalytic reaction and the process was a rate-controlled chemical reaction. Each chain scission uses up one water molecule and creates one carboxylic end group. Hence, the extent of reaction can be followed by measuring the number of carboxyl ends present when vary- ing the hydrolytic degradation time. The built-up of termi- nal acid groups is responsible of autocatalytic phenomena that can explain the autoaccelerating character of the reac- tion. According to this interpretation, the reaction is catalyzed by hydrogen ions produced by the carboxyl end groups.

Some authors [13] suggested that autocatalysis is not needed to explain the behavior of PET during its hydrolysis. It was sug- gested that the physical effects linked to the increase in cristallinity and hydrophilicity could explain this autoaccelerating charac- ter. It has been observed by several authors that the crystallinity increase during the hydrolysis can be attributed to a chemi- crystallization process. It was estimated that about 5-6 monomer units enter the crystalline phase per chain scission.

PET is a semicrystalline thermoplastic polyester, thus built of amorphous and crystalline regions. Water absorption and thus chain scission are only possible in the amorphous phase, ow- ing to the impermeability of crystals. The chain segments pre- viously entrapped in the amorphous phase are then liberated and become sufficiently mobile to enter into the crystalline phase. The drastic increase in the crystallinity cannot be ig- nored in a kinetic study of hydrolysis. Allen et al. [14], ex- plored the degradation of PET materials under environmental conditions, exposing bottles to 45-100% relative humidity.

They observed a difference in the degradation rates associated with differences in the initial crystallinity of the materials. The strong influence of crystallinity on the rate of hydrolytic deg- radation of PET is believed to be due to the fact that crystallites act as barriers to moisture and oxygen diffusion. The initial crystallinities of PET determine the degradation rates as well as the nature of the degradation conditions.

The other problem related to the hydrolysis is the type of chain scission. Launay et al. [13] stated that the hydrolysis is a second-order, random-chain scission process, while Ballara and Verdu [15] proposed that the rate of a specific chain end process is higher than the random one. They ob- served the existence of a weight loss of PET during the reac- tion that suggested that the rate of hydrolysis is higher at terminal than at internal ester groups. The weight loss is linked to the extraction of the smaller molecular fragments resulting from a hydrolysis reaction near the chain ends. The major extractable products are monomeric TPA and/or EG. Larger fragments would have a lower solubility in water and a lower diffusivity in PET matrix.

Several research studies were conducted on the reaction ki- netics for the neutral hydrolytic depolymerization of PET at low temperatures (i.e. less than 473 K) [7-9,13-15].

However, the effort of those authors was mainly focused on measuring the degradable properties of the material so that the conversion of PET hydrolysis were rather low. Thus, their results were not applicable to the practical depolymeri- zation process in which the PET is depolymerized into the original feedstock (TPA and EG). PET hydrolysis below its melting range (518-538 K) is an extremely slow reaction, hardly perceivable by ordinary analytical methods limited by physical factors such as diffusion and crystallinity. Physi- cal factors are less important for the case of hydrolysis above the melting range which is therefore more effective. In fact, whereas the cleavage of esters groups of polyesters in the melt state is an extremely rapid reaction, hydrolysis in the solid state turns out to be a complex process which highly depends on chain mobility and permeability. Because hy- drolysis proceeds significantly faster in the molten state than in the solid state, it has been found advantageous to carry out hydrolytic recycling at a temperature above the melting point of PET. In practice, the process is usually run at a pressure of 1-4 MPa and temperature of 473-573 K with a ratio by weight of PET to water ranging from 1:2 to 1:12.

Although the process for the hydrolytic depolymerization of PET at high temperatures has been revealed in a number of patents, the investigations on the corresponding reaction kinetics for this kind of process are scarce in the literature [10,12,16,17].

Kao et al. [12] studied the kinetic of the depolymerization of molten PET in water in a stirred batch reactor at tem- peratures of 508,523 and 538 K.

The reaction extent for kinetic study is defined as moles of carboxylic acid per gram of PET used.

The conversion of PET hydrolysis was fast at temperatures higher than 523 K and was high even in a short time interval. Kinetics plots show that a second-order model cannot describe the reaction rate of PET and it is proposed that the hydrolysis

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possesses an autocatalytic mechanism induced by the carboxyl group, as has already been observed for PET hydrolysis with low conversions at temperatures below the melting point.

An autocatalytic model of half-order fits the experimental data better than the second-order model. The activation energy for PET hydrolysis calculated from the slope of the Arrhenius plot was 123 KJ/mol.

The autocatalytic model with half-order in carboxylic acid concentration suggests that the hydrogen ion formed by the dissociation of the carboxylic group of TPA (pKi=3.51 and pK2=4.82) may catalyze the depolymerization of PET. In agree- ment with the autocatalytic model of the carboxyl group, the concentration of hydrogen ion is assumed to be proportional to the square root of the carboxylic acid concentration.

It is then suggested that a typical mechanism of nucleophilic addition-elimination induced by the hydrogen ion is respon- sible of PET hydrolysis, as indicated below:

-C6H4-CO-O-C2H4-O- + H + ~-~ C 6H4-C(OH +)-o-C2H 4-0- (1)

-C6H4-C(OH§ ~ + H20 ~C6H4-C(OH)2-(OH+)-C2H4-O- (2)

~C6H4-C(OH)2-(OH+)-C2H4-O- <-0 ~C6H4-C-(OH+)-OH + HO-C2H4-O- (3)

-C6H4-C-(OH+)-OH ~ -C6H4-COOH + H + (4)

Campanelli et al. [16], suggested that essentially complete depolymerization could be obtained within two hours at 538 K for an initial reactor loading of 5.1 water/g PET. However, it would be desirable to reduce the reaction time through the use of appropriate catalysts, in order to facilitate the development of an economic depolymerization process.

It is known that metal salts are able to catalyze the hydroly- sis of some simple esters in aqueous solutions. Zinc acetate has been found to be a good catalyst for thermal and glyco- lytic degradation of PET.

Campanelli et al. [17] studied the effect of zinc catalysts in the hydrolytic depolymerization of PET melts in excess wa- ter at temperatures of 523-538-553 K. The addition of the zinc salts resulted in a little increase in the rate of the hy- drolysis reaction. The hydrolysis reaction apparently occurs at the interface of a PET in water emulsion; the catalytic effect of zinc salts is attributed to the electrolytic destabili- zation of the polymer-water interface during hydrolysis. The stability of emulsions is influenced by the zinc salts that af- fect the surface charge of the disperse phase droplets. In particular, electrostatic repulsion between disperse phase droplets are weakened by the electrolyte, so that the rate of coalescence increases. Less thermal and mixing energy is required to form an interface than in absence of electrolytes, since they decrease the stability of PET-water interface. The result is an increase of the rate of hydrolysis. However, be- cause the hydrolysis rate constant increase only about 20% using salt catalysts, their effect on monomer production is prob- ably not crucial. In fact, most of the processes regarding neu- tral hydrolysis are carried out without a catalyst.

Acid hydrolysis. Acid hydrolysis is performed by contacting scrap PET with concentrated sulfuric acid (minimum 87% by weight). This acid levels allowed the process to take place in a pressureless apparatus, typically without requiring external energy supply. The products obtained are TPA and EG [18,19].

The latest studies about acid hydrolysis are related to the kinet- ics of hydrolysis PET powder in sulfuric and nitric acid [20,21].

In both studies, PET powder from waste bottles was degraded at atmospheric pressure but in the former case the concentra- tion of sulfuric acid was 3-9 M, the temperature 423-463 K and the hydrolysis time 12 hours; in the latter one, the concen- tration of nitric acid was 7-13 M, the temperature 343-373 K and the hydrolysis time was 72 hours. In both cases, the kinet- ics of the hydrolysis could be explained by a modified shrinking core model under chemical reaction control [22] and the reac- tion temperature is below the melting range of PET; thus, the reaction proceeds at the solid-liquid interface and the effects of the changing effective surface area of PET on the reaction has to be taken into account. Using HNO3, as well as H2SO4, it was observed that the rate of hydrolysis increases with: 1) The increase of acid concentration (this suggests that

hydrogen ions may catalyze the hydrolysis) 2) The decrease of particle size (with decreasing particle

size, the surface area available for the reaction increases) 3) The increase of temperature (as expected when a chemi-

cal reaction is rate determining).

It can be stated that hydrolysis catalyzed by the hydrogen ion is rate-determining on the PET surface.

Although all these similarities, there are some differences between the two acid media for hydrolysis, regarding the analysis of the process.

The kinetics of the hydrolysis of PET in HNO 3 could be ex- plained by a modified shrinking core model under chemical reaction control, where the effective surface area is proportional to the degree of unreacted PET, which is in turn affected by the deposition of the product TPA on the surface of unreacted PET particles. The particle size decreased with reaction time, but no cracks were observed on the surface of the particles. It was suggested that the rate of hydrolysis is suppressed by both the deposition of TPA and the decrease in surface area. The modi- fied shrinking-core model scheme for the degradation of PET particles by HNO 3 is shown in Fig. 2 [21].

The kinetics of the hydrolysis of PET in H2SO 4 could be explained by a modified shrinking-core model under chemi- cal reaction control, where the effective surface area is pro- portional to the degree of hydrolyzed PET, which is in ad- diction affected by the formation and growth of pore and crack on PET powder [23].

Fig. 2: Acid Hydrolysis with nitric acid: Modified shrinking-core model. Adapted from Yoshioka T, Okayama N, Okuwaki A (1998) Ind Eng Chem Res 37, 336-340

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Research Articles Chemical Recycling of PET

PET particle

Cracks

,/ O O

0 0

0 / Terephthalic Acid

Fig. 3: Acid Hydrolysis with sulfuric acid: modified shrinking-core model. Adapted from Yoshioka T, Motoki T, Okuwaki A (2001): Ind Eng Chem Res 40, 75-79

The modified shrinking-core model scheme for the degrada- tion of PET particles by H2SO 4 is shown in Fig. 3 [20]. The formation of pores and cracks increase the effective surface area. In fact, the degree of degradation of PET using H2SO 4 reached 100% at 3 M, 463 K, 1 hour, whereas it reached a maximum of 91.3% at 13 M, 373 K, 24 hours using HNO 3.

Furthermore, the activation energy was 101.3 K J/tool using H-NO 3 and 88.7 KJ/mol using H2S04, respectively. Thus, patents regarding chemical recycling of PET by acid hydroly- sis make use of H2SO 4.

Alkaline hydrolysis. Alkaline hydrolysis is typically carried out contacting PET scrap with caustic soda aqueous solutions which are characterized by concentration levels of 4--20 wt%. The saponification reaction takes place in autoclave reactors at 483- 523 K and at 1.4-2 MPa for about 3-5 hours. Once the reac- tion is over, the solution is filtered to remove the insoluble im- purities and it is then acidified with a strong acid (typically sulfuric acid) to precipitate TPA. The latter is filtered off, washed with fresh water and dried. On the other hand, EG formed during the reaction remains in the aqueous phase, and can be separated by extraction or distillation [24,25].

There are very few studies about the kinetics of alkaline hydrolysis. In particular, Ramsden and Phillips [26] investi- gate the effect of solvent on the kinetics of alkaline depoly- merization of PET. They found that the process is first-order with respect to hydroxide ion concentration and the activa- tion energy for that reaction in water is 69+_13 KJ/mol. This value is lower than the reported activation energy for the neutral solid-state heterogeneous hydrolysis (94.5+2 KJ/mol [7]; 101.6 KJ/mol [8]). The alkaline hydrolysis may there- fore offer advantages over the neutral hydrolytic route.

1.2 Glycolysis

Glycolysis is conducted contacting PET scrap with EG in a wide range of temperatures (453-523 K) during a time period of 0.5-8 hours. The main product of deep glycolysis by EG is the monomer of PET, bis(hydroxyethyl)terephthalate (BHET) that can be polymerized after purification to produce PET [27,28].

Most of the studies regarding the chemical recycling of PET by glycolysis are related to applications of glycolized prod- ucts to produce polyols for unsatured polyester or poly- urethane [29,30]. Very few studies regarding the kinetics of the glycolysis reaction are available.

Two different kinetic models have been proposed for glyco- lysis by EG:

1) The rate of depolymerization is proportional to the square of EG concentration. These results suggest that EG acts as both a reactant as well as a catalyst in the glycolysis reaction [31].

2) Kinetic model of first-order in both EG and ethylene di- ester concentrations, for small reaction times. These re- sults suggest that EG does not play a significant role as an internal catalyst in the glycolysis reaction [32].

Chen et al. [31] studied the glycolysis by EG under pressure (0.1-0.6 MPa; T= 463-513 K). The mechanism of pressu- rized depolymerization of PET can be divided in two steps:

PET+EG ~ BHET + oligomers (5)

BHET ~ oligomers + EG (6)

The products resulting from glycolysis consisted of PET, dimer and trimer. The first step (chain scission) is very fast; an equilibrium reaction exists between BHET and oligomers after the scission reaction of the polymer chains is completed. In the case of a high EG/PET ratio, oligomers higher than trimer were not detectable, because the scission reaction occurs quickly and randomly. Oligomers having higher mo- lecular weight (n=10) are detected for lower EG/PET ratios.

Thus, the EG/PET ratio can be used to regulate the degree of polymerization of a product. In order to depolymerize PET to a short-chain length, a sufficient amount of EG is required.

The depolymerization rate is dependent on pressure, tem- perature and EG/PET ratio.

In particular, under a constant pressure, temperature and PET concentration, the depolymerization rate is assumed to be proportional to the square of the EG concentration.

As mentioned above, a different kinetic model is proposed by Campanelli et al. [32] that studied the reaction of PET melts with EG a T=518-548 K in a pressure reactor.

The rate of glycolysis can be written as:

-d[EG]/dt = kc[EG ] [EDE] (7)

where lEG] and [EDE] are the concentration of ethylene glycol and ethylene diester groups, respectively.

It was found that the activation energy was 92 KJ/mol.

In this work, the effect of zinc salts has also been studied.

The electrolytic destabilization of the water-PET interface was considered to be responsible of this catalytic activity. It was observed that zinc salts increase the reaction extent be- low 518 K, while it does not promote any further increase in the glycolysis rate at 538 K.

Below 518 K, the glycolysis reaction occurs between solid PET and liquid EG.

At temperatures greater than 518 K the glycolysis reaction occurs in a single liquid phase. The observation that zinc salts do not have catalytic activity on the glycolysis of the melts can be explained by the fact that the influence of the interfacial properties of zinc salts are not important in a single-phase reaction. In an earlier work, zinc compounds were found to catalyze the hydrolytic depolymeri- zation of PET melts. This difference in catalytic activity between hydrolysis and glycolysis can be due to the differences in solubility of PET and its oligomers in water and EG; EG is known to be a better solvent for PET oligomers than water. Also in the work of

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Chen et al. [33] it was found that the activation energy for gly- colysis with EG with addition of zinc acetate (85 KJ/mol) was lower than the one without addition of a catalyst (108 KJ/mol).

2 Conclusions

In the present work, we confine our attention to the hy- drolysis (neutral, acid and alkaline) and glycolysis processes of PET chemical recycling. In particular, we review and analyze the relevant kinetic mechanism assumed to provide a qualitative as well as a quantitative description of the above processes. Along these lines, the main results obtained so far can be summarized as follows.

The neutral hydrolysis has an auto-accelerating character. Two different kinetic models have been proposed: a half-order ki- netic model and a classic, second-order kinetic model. The main sources of difficulty for the investigation of neutral hydrolysis are: the role of chain ends, morphological changes induced by the degradation and the type of scission of a chain. The use of salt catalysts increase hydrolysis rate constant only by about 20%, thus most of the processes regarding neutral hydrolysis are carried out without catalyst.

The kinetics of PET powder hydrolysis in HNO 3 could be explained by a modified shrinking core model under chemi- cal reaction control, where the effective surface area is pro- portional to the degree of unreacted PET, which is also af- fected by the deposition of the product TPA on the surface of unreacted PET particles. No cracks were observed on the surface of the particles.

On the other hands, the kinetics of the hydrolysis of PET in HzSO 4 could be explained by a modified shrinking-core model under the chemical reaction control, where the effective surface area is proportional to the degree of hydrolyzed PET, which is also affected by the formation and growth of pore and crack on PET powder. The degree of degradation of PET using H2SO 4 reached 100% at 3 M, 463 K, 1 hour, whereas it reached a maximum of 91.3% at 13 M, 373 K, 24 hours using HNO 3.

The process of alkaline hydrolysis is first-order with respect to hydroxide ion concentration and the activation energy for that reaction in water is lower than the reported activa- tion energy for the neutral solid-state heterogeneous hydroly- sis. The alkaline hydrolysis may therefore offer advantages over the neutral hydrolytic route.

To describe glycolysis by EG, two different kinetic models have been proposed: in the first one, the rate of depolymerization is proportional to the square of EG concentration (EG acts as both a reactant as well as a catalyst in the glycolysis reaction); in the second one, the kinetic model is first-order in both EG and ethylene diester concentrations (EG does not play a sig- nificant role as an internal catalyst in the glycolysis reaction).

Zinc salts increase the glycolysis extent below 518 K, while they do not promote any further increase in the glycolysis rate at 538 K. The result that zinc salts have no catalytic activity on the glycolysis of the melts can be explained by the fact that the interfacial properties of zinc salts are not important in a single phase reaction. However, zinc compounds catalyze the hydro- lytic depolymerization of PET melts; this difference in catalytic activity between the hydrolysis and glycolysis of PET melts can be due to the fact that EG is a better solvent for PET than water.

It may be concluded that further experimental and theoretical investigations are required to shed light on the promising proc- esses of PET chemical recycling considered in this work.

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Received: May 8th, 2001 Accepted: November 28th, 2001

OnlineFirst: December 31st, 2001

394 ESPR - Environ Sci & Pollut Res 10 (6) 2003