Depolymerization Mechanism of Poly(ethylene terephthalate) in

Apr 20, 2005 - The kinetics of poly(ethylene terephthalate) (PET) depolymerization in supercritical methanol was investigated to develop a chemical re...
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Depolymerization Mechanism of Poly(ethylene terephthalate) in Supercritical Methanol Minoru Genta, Tomoko Iwaya, Mitsuru Sasaki, Motonobu Goto,* and Tsutomu Hirose Department of Applied Chemistry and Biochemistry, Kumamoto University, Kumamoto 865-8555, Japan

The kinetics of poly(ethylene terephthalate) (PET) depolymerization in supercritical methanol was investigated to develop a chemical recycling process for postconsumer PET bottles. PET with a high molecular weight (IV value (intrinsic viscosity) ) 0.84) was depolymerized in a batch reactor at temperatures between 543 and 573 K under estimated pressures of 0.1-15 MPa. In addition to PET with high molecular weight, PET with low molecular weight, such as its oligomer (trimer), bis-hydroxyethyl terephthalate (BHET), and methyl-(2-hydroxyethyl) terephthalate (MHET), was used as a model reactant to clarify the depolymerization pathway of poly(ethylene terephthalate) in supercritical methanol. The reaction products were analyzed with size-exclusion chromatography, high-performance liquid chromatography, and highperformance liquid chromatography-mass spectrometry. The main products of each reaction were the monomers, dimethyl terephthalate (DMT) and ethylene glycol (EG). The depolymerization of high molecular weight PET to its oligomer was faster than that of the oligomer to its monomer. PET was depolymerized into DMT and EG through MHET. The kinetics study showed that the depolymerization of PET proceeds consecutively, where the step of the oligomer to its monomer would be a rate-determining step. MHET is a relatively stable intermediate in the depolymerization. The rate constants were estimated with simple reaction models. This kinetic model of PET depolymerization in supercritical methanol was proposed. Introduction The amount of plastic production has been increasing significantly year by year, with uses including fiber, packing, container, parts for machines and electronic devices, building materials, etc. Plastics offer a tremendous convenience for our life, and daily life would be quite different without plastics. However, the proliferation of plastic uses has raised waste disposal issues. Poly(ethylene terephthalate), commonly known as PET, a common form of plastic, is no exception. PET has excellent characteristic features such as thermal stability, clarity, strength, and moldability. The amount of PET production has been rising each year, used for producing fibers, textiles, film base for audio and video recording, packaging and containers, etc. As a result of recycling laws promulgated in many nations, there has been a growing need for development of postconsumer PET goods to improve the environment and increase resource conservation. Chemical recycling through depolymerization of PET into its monomer is an ideal recycling method because, in a theoretical sense, PET could be recycled permanently. It is most suitable for a recycling-oriented society. Various chemical recycling methods for PET, such as methanolysis in liquid or vapor methanol, glycolysis in liquid ethylene glycol, and hydrolysis under the existence of an alkali, have been developed on the commercial or pilot scale.1- 4 However, these methods have problems such as slow reaction, use of catalyst, separation of catalyst from product stream, and purification of produced monomer, respectively. * To whom correspondence should be addressed. Tel.: +81-96-342-3664. Fax: +81-96-342-3679. E-mail: mgoto@ kumamoto-u.ac.jp.

Supercritical fluids have been focused on for depolymerization of PET because of their reactivity. The supercritical fluid over its critical point has high density, such as in a liquid state, and high kinetic energy as in a gas molecule. Therefore the reaction rate is expected to be higher than the reaction under liquid state conditions. PET is depolymerized in supercritical fluids quickly by solvolysis. PET, having an ether bond between terephthalic acid and ethylene glycol (EG), is easily decomposed to its monomers by solvolysis in supercritical water, supercritical methanol, or supercritical ethanol.5-14 Sako et al. reported that the methanolysis in supercritical methanol produced both monomers, dimethyl terephthalate (DMT) and EG, with almost 100% yield in 30 min without a catalyst.7 To commercialize the recycling process, the reaction mechanism of PET depolymerization in supercritical methanol should be revealed. However, many existing reports focus mainly on qualitative issues. Mechanism analysis of PET depolymerization in supercritical methanol is challenging work due to severe reaction conditions under high pressure (P > 8.09 MPa) and high temperature (T > 512.6 K) and due to the limitations of the methods used to monitor the intermediate product and to interpret the experimental data. Continuous kinetics is one of the procedures for analyzing the system dynamics that are distributed in a property, such as molecular weight (MW).15-18 Polymer degradation is such a dynamic system, because the molecular-weight distribution (MWD) changes with time owing to chain cleavage. We have reported the continuous kinetics analysis of PET depolymerization in supercritical methanol.13 The evolution of the MWD of depolymerization products and monomers was simulated by the continuous kinetics model. We determined the PET depolymerization mechanism in supercritical metha-

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Figure 1. Schematic diagram of type 1 test apparatus: (a) reactor; (b) electrical furnace; (c) sc MeOH phase; (d) PET.

nol using this model of the MWD of depolymerization products. In the present work, to adapt PET depolymerization in supercritical fluid to the commercial recycling process, we investigated the influence of mass transfer during PET depolymerization in supercritical methanol and the kinetics and rate-determining step in the progress of PET depolymerization in supercritical methanol and confirmed our previously reported depolymerization mechanism with model PET compound. Experimental Procedures The two types of experimental apparatus used, made by AKICO Co., Japan, are shown in Figures 1 and 2 (type 1 and type 2, respectively). The type 1 apparatus, consisting of a batch reactor (about 5 cm3 inner volume) and a heating furnace, was used for analyzing the MWD of depolymerization products and monomers. The type 2 apparatus, consisting of a batch reactor (about 9 cm3 inner volume) and a heating furnace, was used for the other experiments. The type 2 apparatus has the special function of swinging the reactor to reduce the influence of mass transfer (swing span ) 2 cm; frequency of swing ) 60 Hz). The depolymerization reactions were carried out at temperatures between 543 and 603 K under the estimated pressure of 0.1-15 MPa for a reaction time of 3-60 min. Approximately 0.2-0.5 g of PET was charged into the reactor and purged with argon. The reactor charged PET was placed in the electric furnace. It took 2 min to reach a desired temperature for the type 1 reactor and 15 min for the type 2. After the reactor reached a given reaction temperature, a specified volume of methanol was injected into the reactor with a plunger pump. After injection of methanol, the temperature of the reactor inside dropped about 40 K quickly and rose to a desired temperature within 1 min. The amount of injected methanol was calculated using the Peng-Robinson equation of state to get a desired pressure. PET and methanol were charged into the reactor at the same time for the MWD analysis.

During the process, the reactor was kept under the reaction conditions, after which the reactor was cooled quickly and the products were taken out. The products were dissolved in hexafluoroisopropyl alcohol (HFIP) (Central Glass Co., Ltd., Japan) or tetrahydrofuran (THF) (Wako Pure Chemical Industries, Ltd., Japan), and depolymerized products were analyzed by a sizeexclusion chromatograph (SEC), a high-performance liquid chromatograph (HPLC), and a gas chromatograph (GC-FID). SEC (Tosoh, TSKgel Super HM-M) using HFIP as a mobile phase with a UV detector (Waters 996 UV detector) was used to analyze the evolution of polymer and oligomer MWD produced by PET depolymerization in supercritical methanol. A HPLC (Masis, Grand 120-STC18-5) using THF (50%) in water (50%) as a mobile phase and a UV detector (Shimazu LC-10) were used to analyze the components derived from terephthalic acid (TPA). The GC-FID (Varian, CP-PoraBOND U), using He as a carrier within the FID detector (Shimazu GC-14), was used to analyze the components derived from EG. GC-MS was used for identification and quantification of low molecular weight components. The PET was provided by Mitsubishi Heavy Industries, Ltd., Japan. Its instinct viscosity is 0.84. To clarify the depolymerization scheme of PET in supercritical methanol, PET oligomer (trimer) provided by Mitsubishi Heavy Industries, Ltd., bis-hydroxyethyl terephthalate (BHET; Tokyo Kasei Kogyo Co., Ltd., Japan), and methyl-(2-hydroxyethyl) terephthalate (MHET; Tokyo Kasei Kogyo Co., Ltd., Japan) were depolymerized under the above reaction conditions. The PET oligomers (trimer), bis-hydroxyethyl terephthalate (BHET), and methyl-(2-hydroxyethyl) terephthalate (MHET) served as the model compounds of PET with a high polymerization degree. They consisted of an ester bond between TPA and EG. These samples are shown in Figure 3. Dehydrated methanol (Wako Pure Chemical Industries, Ltd., Japan) with a purity higher than 99.7 was used as a solvent. Results and Discussion The main products of PET depolymerization were DMT and EG, as shown in Figure 4. Amounts of MHET, BHET, diethylene glycol (DEG), and 2-methoxythanol (ME) were also produced in the present reactions. The yield of monomers is defined as

yield (mol %) )

moles of specific products × 100 moles of PET units (1)

These products were produced by the methanolysis of

Figure 2. Schematic diagram of type 2 test apparatus: (a) reactor; (b) electrical furnace; (c) sc MeOH phase; (d) PET; (e) SUS ball; (f) valve; (g) plunger pump; (h) messcylinder.

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Figure 3. Reactants used for depolymerization study.

Figure 5. Comparison of PET depolymerization in supercritical methanol and in methanol vapor. Reaction conditions were 573 K, Sample/methanol ) 1/5 (weight base), apparatus type 1, reaction pressure of (9) 14.7 or (0) 0.9 MPa, and reactant PET.

Figure 4. Main reaction of PET depolymerization in supercritical methanol

the ester bond between TPA and EG. DEG might be produced by intermolecular dehydration of EG as a side reaction or by methanolysis of the ester bond between TPA and DEG, which is contained as an impurity in PET. ME may be formed by the side reaction. In the present work, we focused on components derived from TPA (such as DMT, MHET, etc.), which were more expensive components than EG from an economic point of view. In a series of our experiments, the yield of EG was 10-20% less than the yield of DMT. This lower yield of EG may have resulted from the side reaction of EG as we previously reported13 or from the existence of PET impurities, such as DEG. In the present work, we did not investigate its cause in detail. At reaction temperature higher than 603 K, a side reaction such as thermal decomposition might proceed during the sample heating process as reported.19,20 In the present work, we investigated the kinetics of PET depolymerization in supercritical methanol at temperature between 543 and 573 K to reduce the influence of such side reactions. In the series of our experiments, overall material balance was between 60% and 120%. Comparison with Conventional Technology. Several existing reports7,21,22 focus mainly on PET depolymerization in supercritical methanol and in nearsupercritical methanol, in vapor methanol, or in liquid methanol, respectively. Few reports have investigated directly the comparison between PET depolymerization in supercritical methanol and PET depolymerization in low-pressure vapor methanol under the same temperature with the same reaction apparatus. Figure 5 shows the dependency of the DMT yield on the reaction time in supercritical methanol (14.7 MPa) at 573 K and in vapor methanol (0.9 MPa) at 573 K. The yield of DMT at 14.7 MPa sharply increased with the passage of

reaction time, reaching 85% and 98% at the reaction times of 10 and 30 min, respectively. PET is almost completely depolymerized within a reaction time of 30 min in supercritical methanol. In contrast, the yield of DMT at 0.9 MPa increased gradually, reaching 40% and 60% at a reaction time of 40 and 100 min, respectively. These results clearly show that the advantage of using supercritical fluid is a reduced reaction time without using a catalyst. Influence of Mass Transfer during PET Depolymerization in Supercritical Methanol. The influence of mass transfer during PET depolymerization in supercritical methanol was also investigated. PET and PET oligomer (trimer) were depolymerized in supercritical methanol at 573 K at 14.7 MPa with or without a swing of reactor. The swing was carried under a 2 cm span and at a 60 Hz frequency, with several SUS balls (diameter 5 mm) used as a stirring rod to reduce the influence of mass transfer. Figure 6 shows the dependency of the DMT yield on the reaction time. For all conditions, the yield of DMT increased with the passage of reaction time within 20 min. The DMT yield of PET depolymerization with the swing was more quickly increased than that of PET depolymerization without the swing. The difference between the DMT yield of PET depolymerization with the swing and that without the swing was not observed definitively for PET oligomer (trimer) depolymerization. Mass transfer influences the depolymerization of PET into PET oligomer more strongly than the depolymerization of PET oligomer into monomers. These results suggest that the depolymerization of PET into PET oligomer is a heterogeneous reaction in supercritical methanol and that the depolymerization of PET oligomer into monomer is a homogeneous reaction in supercritical methanol. Molecular-Weight Distribution for PET Depolymerization Production in Supercritical Methanol. To observe the evolution of the MWD for depolymerization products of PET during the reaction, SEC elution curves for depolymerization products at various

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Figure 7. SEC elution curve of PET depolymerization product. Reaction conditions were 573 K, 14.7 MPa, sample/methanol ) 1/5 (weight base), apparatus type 1, and reactant PET or PET oligomer (trimer).

Figure 6. Influence of mass transfer during PET depolymerization in supercritical methanol. Reaction conditions were 573 K, 14.7 MPa, sample/methanol ) 1/5 (weight base), apparatus type 2, and reactant (top) PET or (bottom) PET oligomer (trimer) (9) with swing (swing span ) 2 cm; frequency of swing ) 60 Hz) or (0) without swing.

reaction times were measured. The SEC chromatograms of the depolymerization products of PET and PET oligomer (trimer) are shown in Figure 7. For both samples, the molecular weight decreased with the progress of the reaction. After 5 min passed, SEC elution curves of each sample became similar shapes. Furthermore, at the reaction time of 10 min, the depolymerization proceeded to DMT for all samples. These results suggest that the depolymerization of PET with high polymerization degree into PET with low polymerization degree such as dimer, BHET, or MHET would proceed in a short time. In other words, the depolymerization of PET with low polymerization degree into its monomers would be a rate-determining step. Therefore, the investigation of the depolymerization behavior in this step is important for the clarification

Figure 8. Relationship between the yields of products and the reaction time on depolymerization of PET in supercritical methanol. Reaction conditions were 573 K, sample/methanol ) 1/5 (weight base), apparatus type 2, reactant PET, swing span ) 2 cm, and frequency of swing ) 60 Hz for (9) DMT, (0) MHET, or (2) BHET.

of the reaction mechanisms of PET depolymerization in supercritical methanol. Reaction Pathway. Figure 8 shows the yield of DMT, MHET, and BHET for PET depolymerization as a function of reaction time at 573 K and 14.7 MPa. The

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PET + 2nCH3OH f MHET MHET + CH3OH f DMT + EG

(2)

To confirm the existence of these reaction pathways, PET oligomer (trimer), BHET, and MHET were depolymerized in supercritical methanol. Figure 9 shows the yield of DMT, MHET, and BHET as a function of reaction time at 543 K and 14.7 MPa. For the depolymerization of the PET oligomer (trimer) and BHET, the yield of MHET increased initially and then decreased. Meanwhile, the yield of DMT increased with a decrease in MHET yield. The behavior of the DMT yield is coincident with depolymerization of PET. The results suggested that the depolymerization of PET would proceed consecutively when the reactant molecule became small. Based on our experimental results, the whole mechanism of PET depolymerization could be represented by the reaction pathway shown in Figure 10. Kinetics. Sako et al. reported that the PET degration reaction in supercritical methanol was first-order for PET concentrations.7 However, our results showed that the PET degration reaction in supercritical methanol would be consecutive reactions, as shown in eq 2. A modification of reaction kinetics, which was reported by Sako et al., would be necessary. On the basis of our results, a simple kinetic model was constructed. We assumed that each reaction showed first-order kinetics. The depolymerization of a PET oligomer (trimer) is described by the following consecutive reaction.

The production rates of MHET and DMT are given by eqs 4 and 5.

Figure 9. Relationship between yields of products and reaction time in depolymerization of PET oligomer (trimer), BHET, and MHET. Reaction conditions were 543 K, sample/methanol ) 1/5 (weight base), apparatus type 2, swing span ) 2 cm, and frequency of swing ) 60 Hz for (9) DMT, (0) MHET, or (2) BHET.

yield of DMT increased with reaction time and reached about 80 mol % in 10 min. The yield of MHET increased to 36 mol % in 5 min and then decreased. The yield of BHET was a trace amount, and the behavior of BHET yield was similar to that of the MHET yield. The yield of DMT increased with a decrease in MHET yield. These results lead to the following reaction pathway in the reaction of PET depolymerization into DMT, as reported in our previous paper.13

dCMHET ) k1CPET_oligmer - k2CMHET dt

(4)

dCDMT ) k2CMHET dt

(5)

Equations 4 and 5 are integrated with the initial conditions that at t ) 0, CPET_ologomer ) CPET_ologomer,0, CDMT ) CMHET ) 0; then CDMT and CMHET are expressed by eqs 6 and 7.

k1 (e-k2t - e-k1t) CMHET ) CPET_oligmer,0 k1 - k 2

(6)

CDMT ) CPET_oligmer,0(1 - e-k1t) - CMHET

(7)

The values of k1 and k2 were determined from the comparison of observed time variation of CPET_ologomer, CDMT, and CMHET, depicted in Figure 11. The determined values are k1 ) 0.0033 (s-1) and k2 ) 0.0008 (s-1) for a curve fitting with the least-squares method.

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Figure 10. Reaction pathway of depolymerization of PET in supercritical methanol.

The degration kinetics of BHET and MHET in supercritical methanol were studied in the same manner. These reactions are described in eqs 8 and 9. CDMT and CMHET are described in eqs 10, 11, and 12 at t ) 0, CBHET ) CBHET,0, CDMT ) CMHET ) 0 for BHET degration and at t ) 0, CMHET ) CMHET,0, CDMT ) 0 for MHET degration.

Figure 11. Result of fitting. Reaction conditions were 543 K, 14.7 MPa, and sample/methanol ) 1/5 (weight base). Table 1. Rate Constants for Each Reactanta reactants

k1 (s-1)

k2 (s-1)

PET oligomer (trimer) BHET MHET

0.0033 0.0029

0.0008 0.0006 0.0010

a Reaction conditions: 543 K; sample/methanol ) 1/5 (weight base); apparatus ) type 2; swing span ) 2 cm; frequency of swing ) 60 Hz.

value of k2 is larger than that of k1 at 448 K (k1 ) 4.24 L/(mol min) and k2 ) 13.8 L/(mol min)).23

For BHET degration,

k1 CMHET ) CBHET,0 (e-k2t - e-k1t) k1 - k2 -k1t

CDMT ) CBHET,0(1 - e

) - CMHET

(10) (11)

and for MHET degration,

CDMT ) CMHET,0(1 - e-k2t)

(12)

The determined values k1 and k2 for each reactant are shown in Table 1. Regardless of the reactants, the values of k1 (PET oligomer (trimer), k1 ) 0.0033 s-1; BHET, k1 ) 0.0029 s-1) and the values of k2 (PET oligomer (trimer), k2 ) 0.0008 s-1; BHET, k2 ) 0.0006 s-1; MHET, k2 ) 0.0010 s-1) almost coincide. The value of k1 is larger than that of k2. These results suggest that the reaction of MHET into DMT was a slow reaction and MHET would be a relatively stable intermediate in the PET depolymerization in supercritical methanol. Furthermore, these results indicate that the reaction pathway in supercritical methanol is the same regardless of the reactants. In contrast, Peebles et al. reported that at the initiation of PET polymerization shown in eq 13, the

The degration of MHET is shown to be a critical reaction path in the depolymerization of PET in supercritical methanol. Thus, MHET is a key component in both polymerization and depolymerization. Kinetic Model of PET Depolymerization. PET has more than 200 ester bonds in its molecule. Clarification of the decomposition sequence of so many bonds in supercritical methanol is difficult and significant. In our previous reports,13 the continuous kinetics model was constructed on the assumption that in the depolymerization of PET oligomer two reaction paths may represent the reactions to produce monomeric species from the polymer. One is the purely random degradation (random scission), represented by the binary scission of bonds at any position along the chain. The other is a specific reaction (specific scission), which releases the monomeric species of the polymer by scission at the chain end. The results in the current work indicate that random scission would be a faster reaction than specific scission. Furthermore, our results on mass-transfer influence indicate that the random scission proceeds predominantly in the heterogeneous phase during the initial

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Figure 12. Kinetic model of PET depolymerization in supercritical methanol.

stage of PET depolymerization in supercritical methanol, and specific scission proceeds predominantly in the homogeneous phase during the final stage. This kinetic model is depicted in Figure 12. By adding this reaction model to our previous continuous kinetics model, it is possible to interpret the kinetic behavior of PET depolymerization in supercritical methanol more accurately. These results could complete the recycling process with PET depolymerization in supercritical methanol. Conclusions Mass transfer influences the depolymerization of PET into PET oligomer more strongly than the depolymerization of PET oligomer into monomers. The results of the kinetic study suggested that the depolymerization of PET with a low polymerization degree into its monomers would be a rate-determining step and that the depolymerization of PET would proceed consecutively through MHET. First-order rate constants were determined in the consecutive reaction. MHET would be a relatively stable intermediate in the PET depolymerization in supercritical methanol. The kinetic model of PET depolymerization in supercritical methanol, that the random scission proceeds predominantly in the heterogeneous phase during the initial stage of PET depolymerization in supercritical methanol and the specific scission proceeds predominantly in the homogeneous phase during the final stage, was proposed. Acknowledgment The financial supports of a Grant-in-Aid for Scientific Research (No. 14350420) from the Ministry of Education, Science, Sports and Culture, Japan, and Mitsubishi Heavy Industries, LTD, are gratefully acknowledged. Literature Cited (1) Grunschke, H.; Hammerschick, W.; Naucheim, B. Process for Depolymerising Poly(Ethylene Terephthalate) to Terephthalic Acid Dimethyl Ester. U.S. Patent 3403115, 1968. (2) Baliga, S.; Wong, W. T. Depolymerization of Poly(Ethylene Terephthalate) Recycled from Post-Consumer Soft-Drink Bottles. J. Polym. Sci., Part A: Polym. Chem. 1989, 27, 2071-2082. (3) Delattre, J.; Raynaud, R.; Thomas, C. A Process for Converting a Bis (diol) Terephthalate to Dimethyl Terephthalate. U.S. Patent 4163860, 1979.

(4) Datye, K. V.; Vaidya, A. A. Chemical Processing of Synthetic Fibers and Blends; John Wiley and Sons: New York, 1984; p 132. (5) Genta, M.; Yano, F.; Kondo, Y.; Matsubara, W.; Oomoto, S. Development of Chemical Recycling Process for Post-Consumer PET Bottle by Methanolysis in Supercritical Methanol; Technical review, 40, Extra No.1; Mitsubishi Heavy Industries, Ltd.: Tokyo, Japan, 2003; pp 1-4. (6) Genta, M.; Uehara, R.; Yano, F.; Kondo, Y.; Matsubara, W. Development of Chemical Recycling Process for Post-Consumer PET Bottle by Methanolysis in Supercritical Methanol. 6th ISSF, Proc. 2003, 1381. (7) Sako, T.; Sugeta, T.; Otake, K.; Takebayashi, Y.; Kamizawa, C.; Tsugumi, M.; Hongo, M. Kinetic Study on Depolymerization of Poly(ethylene terephthalate) with Methanol at High Temperature and Pressure. Kobunshi Ronbunshu 1998, 55, 685-690. (8) Yamamoto, S.; Aoki, M.; Yamagata, M. Recovery of Monomers from Poly(ethylene terephthalate) by Hydrolysis under Pressure. R-D Kobe Steel Eng. Rep. 1996, 46 (1), 60-63. (9) Adschiri, T.; Sato, O.; Machida, K.; Saito, N.; Arai, K. Recovery of Terephthalic Acid by Decomposition of PET in Supercritical Water. Kagakukogaku Ronbunshu 1997, 23 (4), 505511. (10) Sako, T.; Sugeta, T.; Otake, K.; Nakazawa, N.; Sato, M.; Namiki, K.; Tsugumi, M. Depolymerization of Polyethylene Terephthalate to Monomers with Supercritical Methanol. J. Chem. Eng. Jpn. 1997, 30, 342-346. (11) Sako, T.; Okajima, I.; Sugeta, T.; Otake, K.; Yoda, S.; Takebayashi, Y.; Kamizawa, C. Recovery of Constituent Monomers from Polyethylene Terephthalate with Supercritical Methanol. Polym. J. 2000, 32, 178-181. (12) Goto, M.; Koyamoto, H.; Kodama, A.; Hirose, T.; Nagaoka, S. MWD Analysis in Decomposition of PET in Supercritical Methanol. Proc. 1st Int. Symp. Feedstock Recycl. Plast. 1999, 255258. (13) Goto, M.; Koyamoto, H.; Kodama, A.; Hirose, T.; Nagaoka, S.; McCoy, B. J. Degradation Kinetics of Poly(ethylene terephthalate) in Supercritical Methanol. AIChE J. 2002, 48 (2), 136-141. (14) Goto, M.; Genta, M. Supercritical Methanol for Chemical Recycling of PET Bottle. Proc. 2nd Int. Symp. Supercrit. Fluid Technol. Energy Environ. Appl. 2003, 64-73. (15) Sako, T.; Kamizawa, C.; Sugeta, T.; Otake, T.; Tugumi, M.; Shudo, H. Recovery of monomers from polyethylene terephthalate. Japan Patent JP-11209317, 1999. (16) Wang, M.; Smith, J. M.; McCoy, B. J. Continuous Kinetics for Thermal Degradation of Polymer in Solution. AIChE J. 1995, 41 (6), 1521-1533. (17) Kodera, Y.; McCoy, B. J. Distribution Kinetics of Radical Mechanisms: Reversible Polymer Decomposition. AIChE J. 1997, 43 (12), 3205-3214. (18) McCoy, B. J.; Wang, M. Continuous-Mixture Fragmentation Kinetics: Particle Size Reduction and Molecular Cracking. Chem. Eng. Sci. 1994, 49 (22), 3773-3785. (19) Goto, M.; Koyamoto, H.; Kodama, A.; Hirose, T.; Nagaoka, S. Depolymerization of polyethylene terephthalate in supercritical methanol. J. Phys.: Condens. Matter 2002, 14, 11427-11430. (20) Jabarin, S. A.; Lofgren, E. A. Thermal stability of polyethylene terephthalate. Polym. Eng. Sci. 1984, 24, 1056-1063. (21) Kim, B.-K.; Hwang, G.-C.; Bae, S.-Y.; Yi, S.-C.; Kumazawa, H. Depolymerization of Poly ethylene terephthalate in Supercritical Methanol,” J. Appl. Polym. Sci. 2001, 81, 2102-2108. (22) Yang, Y.; Xiang, H.; Yang, J.; Xu, Y.; Li, Y. Methanolysis of Poly(ethylene terephthalate) in Supercritical Phase. Chin. J. Process Eng. 2001, 1 (1), 71-75. (23) Peebles, L. H., Jr.; Wagner, W. S. The Kinetic Analysis of Distilling System and Its Application to Preliminary Data on The Transesterification of Dimethyl Terephthalate by Ethylene Glycol. J. Phys. Chem. 1959, 63, 1206.

Received for review December 6, 2004 Revised manuscript received February 15, 2005 Accepted March 8, 2005 IE0488187