Process Analysis of Controllable Polycarbonate Depolymerization in Ethylene Glycol

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1 Process Analysis of Controllable Polycarbonate Depolymerization in Ethylene Glycol Process Analysis of Controllable Polycarbonate Depolymerization in Ethylene Glycol Baixue Li 1, Feng Xue 1*, Jian Wang 1, Enyong Ding 1 and Zhen Li 2 1 College of Material Science and Engineering, South China University of Technology, Guangzhou , China 2 College of Mechanical and Electrical Engineering, Shihezi University, Shihezi , China Received: 10 July 2014, Accepted: 26 December 2014 SUMMARY A simple and convenient method was explored for glycolyzing polycarbonate (PC). Different from the harsh conditions in traditional methods, this method can be performed under moderate condition by using 1, 4-dioxane as the solvent and zinc acetate as the catalyst. The recycling of PC has been extensively studied for the recovery of bisphenol-a, whereas in this study, the controllable glycolysis of PC could be achieved at various reaction conditions. The glycolysis products with different molecular weight can be used to copolymerize with other monomers to prepare copolymers. This simple chemical technology will certainly facilitate the cyclic utilization of PC. Keywords: Controllable depolymerization, Polycarbonate, Ethylene glycol, Viscosity number, Recycling INTRODUCTION Polycarbonate (PC) is well known for its good transparency, high-impact resistance, and attractive thermal stability. Featuring excellent mechanical properties, PC is widely used in variety of materials, such as compact discs, construction materials, and software media. Due to the rapid increase of production and consumption of PC, the large amounts of used PC become a threat to environment. The recycling of waste PC has received more and more attention in recent years. Correspondence author: Feng Xue. psfxue@scut.edu.cn, Tel.: , Fax: Smithers Information Ltd., 2017 Progress in Rubber, Plastics and Recycling Technology, Vol. 33, No. 1,

2 Baixue Li, Feng Xue, Jian Wang, Enyong Ding and Zhen Li In order to recycle waste PC, various methods for chemical recycling to recover bisphenol-a (BPA) have been reported [1], such as pyrolysis [2-5], hydrolysis [6-9], glycolysis [10-13], and photo-degradation [14-17]. The thermal degradation of PC containing methylphenyl-silicone additive via thermogravimetry (TG), infrared (IR) and pyrolysis gas chromatography mass spectrometry (PY-GC- MS) was studied by Wenjun Zhou and co-workers [4]. Because of the low selectivity of the molecular and the large amount of byproducts, the catalytic pyrolysis was a promising alternative for the PC treatment. As water is green solvent, acid and base catalyzed hydrolyses of aliphatic PC were performed by Jung s research group [9]. Kim et al. [12] studied the glycolysis of PC in ethylene glycol (EG), the maximum yield of BPA was 95.6% which was achieved at 220 C. Owing to the PC was often subjected to weathering conditions, and the photodegradation of PC was investigated by Diepens s research group [15-17]. Huang et al. [18] focused on the degradation of PC in supercritical ethanol and recover 90% BPA in the supercritical region. Some novel processes have been developed to depolymerize the solid waste PC. For example, the use of different lipases, especially the porcine pancreas (PPL) as biodegrading regent in THF was performed by Artham and co-workers [19]. Fusheng-Liu and co-workers have reported on the methanolysis of PC by using ionic liquid as the catalyst [10, 20]. Among majority of the reported methods on recycling of waste PC, the depolymerization of PC require severe conditions such as high temperature, long degradation time, even under supercritical condition. Furthermore, almost all the methods on waste PC recycling was for the recovery of BPA, the controllable depolymerization of PC was rarely reported. Therefore, the objective of the present study was to investigate a simple method for controllable glycolysis of PC under moderate condition. The aim of the depolymerization was to recover the useful chemical products with various uniform molecular weights for the preparation of condensed polymers. It not only realizes cyclic utilization of waste PC, but also offers a practicable way to obtain the new type of PC. EXPERIMENTAL Materials PC sample (Makrolon 2407) was supplied by Bayer and dried before being used. Commercially available analytical-grade EG, 1, 4-dioxane, and zinc acetate were used without further purification. 40 Progress in Rubber, Plastics and Recycling Technology, Vol. 33, No. 1, 2017

3 Process Analysis of Controllable Polycarbonate Depolymerization in Ethylene Glycol Instrumentation Fourier transform infrared (FTIR) spectra were recorded on a Tensor 27 spectrometer (Bruker, GER) in the range of 4000 to 400 cm -1, by using KBr plate containing ca. 1 wt% of product. Differential scanning calorimetry (DSC) analysis was carried out in a DSCAQ20 apparatus (TA, USA). The samples were put into a pan while another empty pan was used as reference. The glass transition temperatures (T g ) of degradation products of PC were determined at a heating rate of 10 C/ min under nitrogen atmosphere at a purging rate of 25 ml/min. The temperature range was from 30 to 200 C with the sample weight of about 5 to10 mg. Scanning electron microscope (SEM) was used to observe the surface morphologies of the solid depolymerization products, with a Nova Nanosem 430 apparatus (NL). Viscometry analysis was performed at 25 ± 1 C in dichloromethane solutions, by using an ubbelohde viscosimeter (Shanghai, China). The viscosity number (VN) can be calculated by the following formula (1): Where t is the flow time of sample solution, t 0 represents the flow time of solvent, and c is the concentration of the solution. (1) General Procedure The recycling processes were performed in a round bottom glass reactor equipped with a stirrer, a thermometer and a refluxing condenser. Certain proportion of PC and 1, 4-dioxane were charged into the reactor, then the mixture was heated at 90 C for about 1 h to dissolve the PC completely. The molar ratio of PC repeat unit/eg was 2. According to the mixture ratio, the quantitative EG and catalyst were put into the mixture at prescribed temperature, after the special reaction was finished, the mixture was poured into the ice water to precipitate. The obtained solid products were dried in vacuum for further characterization. Progress in Rubber, Plastics and Recycling Technology, Vol. 33, No. 1,

4 Baixue Li, Feng Xue, Jian Wang, Enyong Ding and Zhen Li RESULTS AND DISCUSSION Effects of Reaction Conditions on the VN of Depolymerization Products It can be seen from Table 1, the temperature had a remarkable effect on the VN. When the temperature was 70 C, the VN was under the certain conditions. With the increasing of temperature, the VN decreased gradually. When the temperature was increased to 90 C under the same condition, the VN decreased to Moreover, with the prolonging of reaction time, the VN declined slowly. When the reaction time was prolonged from 1 to 3 h, the VN decreased from 9.89 to Furthermore, with the increasing of amount of catalyst, the VN fell initially. When the amount of catalyst was increased from 0.5 to 2 wt%, the VN decreased from to The catalytic mechanism of depolymerization was put forward in Scheme 1 [21, 22]. However, the VN increased with the increasing of solvent dosage, which was attributed to the decrease of the concentration of EG and catalyst. On the premise of PC dissolving in solvent completely, the less dosage of solvent is the better. Therefore, the optimal dosage seems to be 35 ml. All these results could be concluded that the VN of degradation products decreased with increasing temperature, prolonging reaction time, as well as increasing amount of catalyst. Table 1. Effects of conditions on degradation results Entry Temperature Reaction m(cat.)/ Dosage of VN ( C) time (h) m(pc) solvent (ml) : : : : : : : : : Structure of Depolymerization Products Figure 1 shows the expanded IR spectra between 4000 and 400 cm -1. It can be seen that the main bands of the degradation products were similar 42 Progress in Rubber, Plastics and Recycling Technology, Vol. 33, No. 1, 2017

5 Process Analysis of Controllable Polycarbonate Depolymerization in Ethylene Glycol to those of pure PC. The peak at 667 cm -1 corresponded to the aromatic ring C=C out of plane bending, the peak at 830 cm -1 to the out-of-plane aromatic of CH wagging. The band at about 1230 cm -1 was the aryl-o-aryl C-O stretching and the 1405 and 1505 cm -1 bands belonged to the aromatic semicircle stretching. The peak at 1775 cm -1 matched to the carbonate C=O stretching, the 2875 and 2970 cm -1 paralleled the CH 3 symmetric and asymmetric stretching, respectively. The peak at 3000 to 3200 cm -1 assigned to aromatic CH stretching. Figure 1. The IR spectra of pure PC and the degradation products at various temperatures. (a) pure PC; (b) 70 C; (c) 80 C; (d) 90 C There was no hydroxyl peak observed at cm -1 in the spectrum of pure PC, however, the spectrum of glycolysis products had a strong hydroxyl peak with different content at 3500 cm -1. It demonstrated that with the increasing of temperature, the molecular weight of the products decreased gradually, the hydroxyl peak became stronger simultaneously. It was further verified by using integrated intensity, one of infrared quantitative methods. The peak of 3058 cm -1 whose intensity remained the same comparing to the other peaks, was reference peak. The peak area ratio of OH and benzene ring, namely A 3500 /A 3058, was revealed in Table 2. When the temperature was increased from 70 to 90 C, the value of A 3500 /A 3058 became bigger correspondingly. All the results illustrated that the structure of depolymerization products had the same repeat unit with PC, and carried OH terminal group. The reaction pathway was proposed in Scheme 1. Progress in Rubber, Plastics and Recycling Technology, Vol. 33, No. 1,

6 Baixue Li, Feng Xue, Jian Wang, Enyong Ding and Zhen Li Table 2. Variation of the integral area of OH/benzene ring under different temperatures Temperature ( C) A 3500 /A Scheme 1. Catalytic mechanism of PC depolymerization in EG and the reaction pathway Thermal Analysis of Depolymerization Products It is well known that PC is one of amorphous thermoplastics; it doesn t have melting temperature (T m ) yet has glass transition temperature. Moreover, T g is related to the molecular weight of product, sc. T g will move to the low temperature with the decreasing of molecular weight. Figure 2 presents the DSC curves of pure PC and the depolymerization products with various temperatures. Table 3 shows the T g results collected 44 Progress in Rubber, Plastics and Recycling Technology, Vol. 33, No. 1, 2017

7 Process Analysis of Controllable Polycarbonate Depolymerization in Ethylene Glycol from the DSC analysis. The T g of PC in this work is C. The remarkable decrease of T g can be observed in Figure 2, by comparing the pure PC with the degradation products. When the temperature was increased from 70 to 90 C, the T g decreased from to 78.2 C. In terms of the results, the molecular weight of products decreased with increasing of the temperature, which was consistent with the analysis of IR spectrum. Figure 2. DSC curves of pure PC and depolymerization products under various temperatures. (a) 85 C; (b) 80 C; (c) 75 C; (d) 70 C; (e) pure PC Table 3. T g of pure PC and depolymerization products in Figure 2 Temperature ( C) Pure PC T g ( C) Surface Morphology of Depolymerization Products Surface SEM micrographs of solid products obtained from depolymerization of PC at different reaction time are presented in Figure 3. The surface of the PC which dissolved in solvent completely was compact and smooth without adding the EG and catalyst as shown in (a). PC depolymerized in EG at 80 C for 1 h processing the reaction between EG and PC, the formation of holes exhibited the dissolution of PC as illustrated in (b). A completely different morphology was observed when the reaction time ran to 2 h, in which amounts Progress in Rubber, Plastics and Recycling Technology, Vol. 33, No. 1,

Baixue Li, Feng Xue, Jian Wang, Enyong Ding and Zhen Li of pores distributed over the surface of product, it revealed that the EG diffused into the PC,

From the SEM photograph, it can be obtained that EG and catalyst penetrates in the PC bulk to induce the random chain scission reaction, which leads to the

SEM photograph of the depolymerized products at 80 C (a) 0 h; (b) 1 h; (c) 2 h Kinetics of Degradation of PC There is a discrepancy between the degradation

However, the molecules are free to react in solution, which proceeds random fission.

8 Baixue Li, Feng Xue, Jian Wang, Enyong Ding and Zhen Li of pores distributed over the surface of product, it revealed that the EG diffused into the PC, result in decreasing of the VN, as displayed in (c). From the SEM photograph, it can be obtained that EG and catalyst penetrates in the PC bulk to induce the random chain scission reaction, which leads to the depolymerization of PC. Figure 3. SEM photograph of the depolymerized products at 80 C (a) 0 h; (b) 1 h; (c) 2 h Kinetics of Degradation of PC There is a discrepancy between the degradation in solution and pyrolysis. The pyrolysis has some drawbacks such as nonuniform heat transference, bad molecular mobility which leads to chain-end process. However, the molecules are free to react in solution, which proceeds random fission. Furthermore, different from traditional kinetic [18, 23, 24], this paper explored the reaction kinetic in another form i.e. molecular weight with regard to reaction time, which will have a broad application. On the basis of these principles, the degradation of PC in EG solution via random scission was assumed to be first-order reaction. The dynamical equation was illustrated as follows: (2) 46 Progress in Rubber, Plastics and Recycling Technology, Vol. 33, No. 1, 2017

9 Process Analysis of Controllable Polycarbonate Depolymerization in Ethylene Glycol Where M h0 is the initial viscosity-average molecular weight, M ht is the viscosityaverage molecular weight at induction time, and k d is the depolymerization rate coefficient. The Equation (2) could be rewritten as: The k could be defined as follows: (3) Therefore, the Equation (3) was represented as: (4) The intrinsic viscosity from a single point measurement can be obtained in short time. The empirical equation was proposed as: (5) Where h r is the relative viscosity, h sp is the specific viscosity, [h] is the intrinsic viscosity. The viscosity-average molecular weight (M h ) was calculated by using Mark Houwink equation: (6) Where K is the constant of proportionality, which relate to the temperature, and α is the expansion factor, which connect with the solvent system. K= ml/g and α=0.82 were employed to calculate the viscosity-average molecular weight [25]. The kinetic data is shown in Figure 4 based on the formula (5). The rate constants are listed in Table 4. It can be noticed that all the linear correlative coefficients were more than or equal to 0.96, which is an indication that M h0 / M ht -1 was proportional to the reaction time at various temperature, conforming that the degradation of PC in EG was a first-order reaction. (7) Progress in Rubber, Plastics and Recycling Technology, Vol. 33, No. 1,

10 Baixue Li, Feng Xue, Jian Wang, Enyong Ding and Zhen Li Figure 4. Kinetic behavior of the PC depolymerization in EG Table 4. Linear regression data in Figure 4 Temperature ( C) Regressive equation Linear correlative coefficient 70 y= x y= x y= x y= x y= x As illustrated in Table 4, the activation energy (Ea) could be gained with Equation (8): Where A is the pre-exponential factor, R is the gas constant and T is the Kelvin temperature. The Ea for depolymerization of PC, which obtained from the slope of Arrhenius plot in Figure 5, was 36 kj/mol. (8) 48 Progress in Rubber, Plastics and Recycling Technology, Vol. 33, No. 1, 2017

11 Process Analysis of Controllable Polycarbonate Depolymerization in Ethylene Glycol Figure 5. Arrhenius plot of the degradation of PC in EG CONCLUSIONs This work showed that PC could achieve controllable depolymerization in EG. It demonstrated that the VN of products decreased with increasing temperature, reaction time, and amount of catalyst. The appropriate dosage of solvent was 35ml. By setting different experiment parameters, the products with various molecular weights could be obtained to copolymerize with the other monomers. Furthermore, the investigation on kinetics revealed that the depolymerization of PC was a first-order reaction and the activation energy was 36 kj/mol. REFERENCES 1. Antonakou E.V., and Achilias D.S., Waste and Biomass Valorization, 4, (2013) Chiu S.J., Chen S.H., and Tsai C.T., Waste Management, 26, (2006) Jang B.N., and Wilkie C.A.A., Polymer Degradation and Stability, 86, (2004) Zhou W.J., Yang H., and Zhou J., Journal of Analytical and Applied Pyrolysis, 78, (2007) Zhao W., Li B., and Xu M.J., Journal of Macromolecular Science, Part B: Physics, 51, (2012) Progress in Rubber, Plastics and Recycling Technology, Vol. 33, No. 1,

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