Depolymerization of Poly( carbonate) in Subcritical and Supercritical Toluene On-line Number 9029 Zhiyan Pan1,2, Zhen Bao2, Liwei Huang2 , Yinxiu Liu2, Yingxu Chen1 1 Department of Environmental Engineering, Zhejiang University, Hangzhou 310027, P.R. China 2 College of Biological and Environmental Engineering, Zhejiang University of Technology, Hangzhou 310032, P.R. China

ABSTRACT The depolymerization of bisphenol-A (BPAPC) in supercritical toluene was experimentally investigated. The operation parameters which influence the depolymerization reaction were investigated, such as temperature(570~633K), reaction pressure (4.0~7.0MPa), residence time(5~60min), and weight ratio of toluene to PC(3~11). The reaction products were examined by GC, GC/MS and FT-IR spectrometry. It was found that the main product of the depolymerization reaction was bisphenol-A. BPA production occupied over 55.7% amount of the depolymerization products at reaction temperature of 613K, pressure of 5.0~6.0MPa, residence time of 15 min and Toluene/PC weight ratio of around 7. Activation energy of depolymerization is 61.41kJ/mol in supercritical toluene for first order reaction

KEYWORDS depolymerization, poly(bisphenol a carbonate), supercritical toluene

INTRODUCTION

The degradation of bisphenol-A polycarbonate(BPAPC) has been received continued attention, since this is an important engineering thermoplastic material with outstanding characteristics, such as stability, good machinability, great flexibility and strength, good electrical insulativity, which is called “glassy metal”. Due to its outstanding properties, polycarbonate has widely used in many fields from household to industrial processes. It was estimated that the production ability of the global polycarbonate was 2 million tons per year at present, and that the global demand of polycarbonate would increase near 8%~10% per-year in the future (Wei, et al., 2003) . Due to the large amount of the PC production, the the scrap polycarbonate becomes an important issue in recent years. But simple regeneration may result in the decrease of its qualities. The chemical recycling of waste has been gaining greater attention as a means of obtaining valuable products from waster (Tadahiro, 1993; Giridhar, 1997; Masura, 1999; Pan, 2002). Tagaya studied the decomposition of polycarbonate in subcritical and supercritical water. They reported that the polycarbonate was decomposed into phenol, bisphenol A, p-isopropenylphenol, and p-isopropylphenol by the reaction at 503K to 703K in water(Tagaya, et al., 1999). They also investigated the addition of

Na2CO3 which can accelerate the decomposition reactions, and the yields of identified products reached 67% even in the reaction at 593K for 24h. Madras studied the kinetics of degradation of polycarbonate in

1 supercritical and subcritical benzene at various temperatures (523-618K) at 5.0 MPa. They reported that the degradation rate coefficients at the supercritical conditions were an order higher than the rate coefficients at subcritical conditions, indicating enhanced degradation at supercritical conditions(Madras et al., 2002). In this work, the depolymerization of polycarbonate resin in supercritical toluene is studied. The aim of this work is to recover the useful chemical products from depolymerization of scrap polycarbonate.

EXPERIMENT ARRANGERMENT A 0.5-liter batch autoclave with stirrer (FYXL0.5) was employed for the experiment. A kind of colorless-transparent bisphenol-A polycarbonate with molecular weight of 8439- 16667 was used as the sample, and toluene (A.R. 99.5%) was used as the supercritical fluid. When a certain amount of PC sample and toluene were put into the batch autoclave, the pressure in batch autoclave and reaction temperature was adjusted to the predetermined value to start the experiment. After depolymerization reaction for a certain time, the batch autoclave was cooled down to room temperature to analyze the depolymerization products. The Solid products were analyzed by a FTIR spectrometer. Liquid products were determined by both GC and GC/MS.

RESULT AND DISCUSSION

Characterizations of the depolymerization products Both solid products and liquid products were observed after the depolymerization of bisphenol-A polycarbonate in supercritical toluene. The solid products were divided into two kinds of compounds: one was a kind of yellow matter, and the other was a kind of viscount black matter. It was confirmed by FT-IR that the yellow matter was the unreacted polycarbonate resin swollen in toluene by compared with the standard pattern of polycarbonate resin. The black matter was regarded as tar or char. The liquid products were analyzed by GC/MS as showed in the Fig. 1. The GC/MS pattern shows that over 35 species of products were produced, which included bisphenol-A, 1-tert-butyl phenol, p-(1- methyl-1-phenyl) ethanyl phenol, 2, 4-di-(phenylethyl) phenol, and a few m-(1-methyl)ethyl phenol, biphenyl methane, and p-(1-methyl-1-phenyl) ethyl phenol. But the main product was bisphenol-A. Fig. 2 gives the MS-pattern of bisphenol-A after deploymerization. : . TIC 5194 0 Abundance 16. 24 22.83 Abundance Scan 2949(16.269 min);519-4.D 2000000 650000 213 1800000 600000 1600000 550000 5. 45 500000 1400000 450000 1200000 9.04 400000 1000000 350000 13.12 300000 800000 250000 0. 494.32 600000 10. 95 200000 228 3.41 10.74 11.56 400000 11.74 150000 7.76 14.52 119 5.59 19.15 100000 200000 5.75 9.28 12.0114.24 19.45 23.95 91 18.02 19.98 50000 65 75 99107 135 152 166 181 3945 55 71 84 113 128 141 158 171 187197207 Time 4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00 20.00 22.00 24.00 0 Time 30 50 70 90 110 130 150 170 190 210 230

Fig.1. GC of depolymerization products Fig.2. MS pattern of depolymerization product of at 603K,5.5MPa bisphenol-A

2 Effect of toluene/PC ratio on polycarbonate depolymerization

Fig. 3 shows the results of the PC resin depolymerization in supercritical toluene at a changing toluene/PC ratio (in weight) from 3 to 11. The reaction condition is at temperature of 603K, pressure of 5.0~5.2 MPa, and residence time 30 minutes. It was found that the production of bisphenol-A increased with the increase of toluene/PC ratio. On the other hand, the depolymerization rate decreased with the increase of toluene/PC ratio.

100 100 90

80 80 bisphenol A depolymerization rate 70 bisphenol A 60 depolymerization rate 60 50 40 40 30

20 20 depolymerization rate(%) bisphenolA in solution(%) depolymerization rate (%) bisphenol A in solution (%) solution A in bisphenol 10 0 0 0 102030405060 24681012 PC/toluene weight ratio resident time (min) Fig. 3. The influence of PC/toluene on Fig. 4. The influence of residence time on depolymerization rate and yield of bisphenol-A depolymerization rate and yield of bisphenol-A

Effect of reaction time on polycarbonate depolymerization

The effect of reaction time on the deploymerization rate of PC resin in supercritical toluene was investigated at reaction temperature 603K, pressure of 5.0~5.2MPa, and toluene/PC ratio of 7, the result was shown in Fig. 4. It was found that the depolymerization rate of PC resin increased rapidly with the increase of the residence time from 10 to about 15 minutes. At the same time, the production of bisphenol-A in liquid phase was almost stable.

Effect of temperature on polycarbonate depolymerization

Fig.5 shows the effect of temperature on polycarbonate depolymerization and bisphenol-A production in supercritical toluene. Other operation parameters were pressure of 5.0~5.2MPa, reaction time 20 minutes. It was found that the depolymerization seemed difficult under the temperature of 583K. When the reaction temperature was increased to about 593K, PC was almost completely depolymerized. It was also observed that an increase in reaction temperature leaded to the increase of liquid products. The product of bisphenol-A in liquid increased with the increase of the reaction temperature from 583K to 593K. When the reaction temperate increased above 623K, the production of bisphenol-A in liquid rapidly decreased. It was believed that the yield of tar or char might be responsible for the decrease of bisphenol-A.

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110 100 100 90 90 bisphenol A 80 80 depolymerization rate 70 70

60 60 50 50 40 40 30 30 20 bisphenol A 20 depolymerization rate bisphenol A in solution(%)bisphenol A bisphenol A in solution(%) 10 depolymerization rate (%) depolymerization rate (%) rate depolymerization 0 10

-10 0 300 310 320 330 340 350 360 4.0 4.5 5.0 5.5 6.0 6.5 7.0 reactor temperature (degree) reaction pressure( MPa) Fig.5. The influence of temperature on Fig. 6. The influence of reaction pressure on depolymerization rate and yield of bisphenol-A depolymerization rate and yield of bisphenol-A

Effect of pressure on polycarbonate depolymerization The influence of pressure on the deploymerization rate of PC and yield of bisphenol-A was shown in Fig.6. It was found that the depolymerization rate of PC increased with increasing the pressure to reactor, and the increase of bisphenol-A production was also seen with the increase of the pressure from 4.0 to 6.5MPa. However, continuing increasing the pressure might result in the decrease of the production of bisphenol-A. This could be explained that the product of bisphenol-A might be decomposed further to small molecular compounds.

DEPOLYMERIZATION MECHANISM The depolymerization of PC is a radical chain mechanism, where initiation, propagation and termination reactions are relevant reaction classes. Initiation reactions determine a C-C bond cleavage of chains to form radicals including random scission and chain-end scission. Propagation step consists of the sequence of H-abstraction and β-decomposition or unzipping reactions. Termination reaction includes recombination reactions and disproportionation reacytion of radicals. In the experiment, the major components of liquid products recovered from PC were measured by GC/MS, FT-IR. The main liquid products were bisphenol A, 4-ethyl-phenol, 4-(1-methylethyl)-phenol, m-tert-butyl-phenol, bibenzyl, deiphenylmethane etc. According to the analysis results, mechanisms of random scission is the main depolymerization path in supercritical fluids. The depolymerization mechanism of PC in supercritical toluene is shown in Fig. 7.

KINETICS MODEL OF DEPOLYMERIZATION RATE In general, the deploymerization rate for solid wastes can be formulated as: dα = k(T)(1−α) n (1) dt

4 Where, dα/dt is the instantaneous reaction rate, α=(W(t)-Wi)/Wi, W(t) means the mass of PC during the reaction time(t), Wi is the initial mass, n is the apparent order ,and k is the reaction rate constant dependent on reaction temperature(T). Also, the reaction rate constant, k, can be expressed by the following Arrhenius’s equation: k=Ae-Ea/RT. or ln(k)=ln(A)-Ea/RT (2) Where, Ea represents the active energy of depolymerization reaction, and A represent pro-exponential factor. Reaction rate constant k was tested in different experiment tempera ture. Table 1 shows the result. Equation 3 can be deduced from equation 1 by Table 1 reaction rate constant in PC integralation. depolymerization[min-1] dα Temperature k = k(T)dt (3) No. -1 ∫(1−α)n ∫ [K] [min ] 1 583 0.0247 It can be deduced from equation 3 that ln(1-α) 2 593 0.0430 and time yield a straight line whose slope is k. The 3 608 0.0536 result tallies with the literatures. 4 613 0.0583 According to the equation 2, relationship 5 623 0.0801 between lnK and 1/T can be shown. We obtained an activation energy, Ea, of 61.41kJ/mol in supercritical while Ea becomes 155.96 kJ /mol in subcritical.

OH

O O C OH CH 2 CH 2 CH 2 CH CH 3

CH 3 O . C C OH

CH 3

CH 3 O [ C O C O ]n CH 3

CH 3 O [ O C ] O C n CH 3

CH 3 O [ C O C O ]n CH 3

CH 3 CH 3 . C OH C OH CH 3 CH 3

CH 3 CH 3 CH 3 CH 3 C OH C OH C OH C H 3 CH 2 CH 3

+

CH 3 CH 3 HC OH HO C OH

CH 3 CH 3 Fig.7 PC depolymerization mechanism in supercritical toluene

5 CONCLUSION

This experiment showed that the polycarbonate could be depolymerized in supercritical toluene. It was found that PC was almost completely depolymerized above the temperature of 593K under the pressure of 5.0MPa. It was confirmed that the main product of the depolymerization reaction was bisphenol-A, and the production of bisphenol-A increased with the increase of toluene/PC ratio and depolymerization temperature. We obtained an activation energy, Ea, of 61.41kJ/mol in supercritical while Ea becomes 155.96 kJ /mol in subcritical for first order reaction. The depolymerization mechanism of PC in supercritical toluene was proposed.

ACKNOWLEDGMENTS

The financial support for this work provided by Zhejiang Natural Science Foudation is gratefully acknowledged.

REFERENCE

Giridhar M, Chung G Y. “Molecular weight effect on the dynamics of ”, Ind. Eng. Chem. Res, 36,2019-2024(1997) Hideyuki Tagaya and Kazuya Katoh, “Decomposition of polycarbonate in subcritial and supercritical water”, and Stability, 64(2), 289-292(1999) Masaru W, Hideyuki H, Shuhei S. “ conversion in supercritical water”, J. of Supercritical Fluids, 13(1-3), 247-252(1998) Madras, Giridhar, Sivalingam, G. “Kinetics of degradation of polycarbonate in supercritical and subcritical benzene. Industrial and Engineering Chemistry Research, 41( 22), 5337-5340(2002) Pan Zhiyan Hu Zhiwei,” Depolymerization of polystyrene in supercritical xylene”, J of Chem. Eng. of Chinese Universities, 16(2), 227-231(2002) Tadahiro M, Yasunobu S. “Solvent effect on thermal degradation of PS and poly-α methylstyrene”, Polymer, 34(7), 1436-1439(1993) Wei Dongwei, Cui Jinhua, Li Fusheng et al.; “Production and market analysis of polycarbonate”, Modern Chemical Industry, 23(10), 54-56(2003)

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