Ind. Eng. Chem. Res. 2009, 48, 685–691
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Kinetics of Polycarbonate Glycolysis in Ethylene Glycol Dongpil Kim,† Bo-kyung Kim,† Youngmin Cho,† Myungwan Han,*,† and Beom-Sik Kim‡ Department of Chemical Engineering, Chungnam National UniVersity, 220 Gung-dong, Yuseong-gu, Daejeon 305-764, Korea, and New Chemical Research DiVision, Korea Research Institute of Chemical Technology, 100 Jang-dong, Yuseong-gu, Daejeon 305-600, Korea
Depolymerization of polycarbonate waste by glycolysis using ethylene glycol without catalyst was explored in order to get the monomer bisphenol A (BPA). The depolymerized products were identified by GC/MS and FTIR spectroscopy. The effects of operation variables such as reaction time, reaction temperature, ethylene glycol/polycarbonate (EG/PC) weight ratio, and the kinetics of glycolysis were studied. A maximum yield of BPA of 95.6% was achieved at a reaction temperature of 220 °C for 85 min with an EG/PC weight ratio 4. It was found that the depolymerization reaction has two different activation energies, indicating that the reaction occurs in series. A new model was proposed to explain the depolymerization reaction which consists of a series of reactions: random scission from high molecular weight PC to its solid oligomer, dissolution from the solid oligomer to liquid oligomer, and homogeneous degradation from the liquid oligomer to its monomer, BPA. The activation energies were found to be 98.9 kJ/mol for the random scission reaction, 32.7 kJ/mol for the dissolution, and 355.8 kJ/mol for the homogeneous reaction, respectively. The predicted values by the proposed model were shown in good agreement with the experimental ones. 1. Introduction PC (polycarbonate) is one of the widely used engineering plastics. PC has found wide applications in the electronics, automobile, optical material, and other fields. The amount of PC production has been rising recently, leading to a continuous increase in the generation of the plastic waste. Disposal of the waste plastic has posed serious environmental as well as economic problems. Chemical recycling of PC waste is becoming an increasingly important method for the conversion of waste PC into valuable chemicals. Several studies regarding decomposition of polycarbonate have been reported in recent years. Some organic solvent systems such as mixed solvents of phenol and methylene chloride in combination with an alkali catalyst have been used for decomposition.1-3 Alkali-catalyzed methanolysis also has been performed with a mixed solvent of toluene and methanol to give high yields of bisphenol A (BPA) and dimethyl carbonate (DMC).4 However, they can cause an environmental safety problem due to the use of highly toxic organic solvents, which makes a complicated subsequent product separation necessary. Other methods such as hydrolysis and alcholysis in supercritical or near critical conditions have been reported.5-8 Hydrolysis of polycarbonate in subcritical and supercritical water was reported to offer low yield (67%) at 300 °C for 24 h in a batch process.5 Methanolysis in supercritical or near critical conditions showed that high yields of BPA and DMC could be achieved.6,7 The severe operating conditions, high temperature and pressure, may lead to operational difficulties and a very high capital cost for the apparatus. Glycolysis using ethylene glycol takes place at a reasonably high temperature for the decomposition but at a relatively low pressure, which gives high reaction yields without using toxic solvents or severe reaction conditions. Very few studies of PC depolymerization kinetics have been reported.4,7,8 Hu et al.4 studied the characteristics of methanolysis of PC to its monomer components BPA and DMC by * To whom correspondence should be addressed. E-mail: mwhan@ cnu.ac.kr. † Chungnam National University. ‡ Korea Research Institute of Chemical Technology.
the use of a catalytic amount of alkali-metal hydroxide. They suggested that the formation of DMC is controlled under firstorder kinetic equation. Jie et al.8 studied depolymerization of polycarbonate in supercritical ethanol. They used continuousdistribution kinetics to describe the mechanism of polymer degradation and obtained the energy of activation for random scission of PC. Continuous distribution kinetics can provide a framework for analyzing the system dynamics that are distributed in a property, such as molecular weight. Recently, Genta et al.9 proposed a kinetic model of PET depolymerization which consists of consecutive reactions: PET into PET oligomers and PET oligomers into its monomer. They maintained that the reaction from PET to PET oligomers is heterogeneous, while the reaction from PET oligomers into monomer is a homogeneous reaction. Pardal and Tersac10,11 studied the kinetics of noncatalyzed glycolysis of PET by diethylene glycol and reported that the process was composed of multiple reaction steps involving depolymerization to oligomers and depolymerization of the oligomers to monomers. In this study, a method for depolymerization of polycarbonate waste in ethylene glycol was studied to get BPA without a catalyst. The characteristics of depolymerization of PC in ethylene glycol were investigated. A new kinetic model for PC glycolysis which consists of consecutive reactions was developed and the mechanism of the glycolysis was analyzed considering a shrinking core model and continuous distribution kinetics. 2. Experimental 2.1. Materials and Reagents. PC wastes coming from CDs were collected and broken into pieces with 3 mm × 3 mm size for recycling tests. Standard samples of BPA used were purchased from Aldrich Co. Commercially available reagentgrade ethylene glycol was used without purification as a solvent for the depolymerization. 2.2. Apparatus and Procedure. The experimental apparatus is shown in Figure 1. A batch-type reactor was used for the depolymerization experiments. The reactor was made of stainless steel and had an inner volume of 18 mL. The reaction
10.1021/ie8010947 CCC: $40.75 2009 American Chemical Society Published on Web 12/09/2008
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Figure 2. GC chromatogram of reaction mixture.
Figure 1. Experimental apparatus.
temperature was considered to be equal to that of the oil bath in which the reactor was immersed. Experimental procedures are as follows. The PC wastes and ethylene glycol (EG) were charged in the reactor. The glycolysis was started by placing the reactor in oil bath. The reactor was agitated by shaker at 100 rpm with small zirconia balls in the reactor. After a specified reaction time, the reactor was quenched in cold water to stop the reaction. The reaction mixture was poured into distilled water and allowed to stand at a room temperature over 20 h to crystallize BPA. The crystallized BPA was filtrated and dried. The product was determined by high-performance liquid chromatography (HPLC) and checked by FTIR. The reaction mixture was analyzed by gas chromatography equipped with a mass spectrometer (GC/MS). We investigated the effect of operation variables such as reaction time, reaction temperature, EG/PC weight ratio, and the kinetic of glycolysis. In all experiments, 2.7 g of PC waste was used. The BPA yield was calculated as the ratio of the actually produced amount of BPA and the maximum amount of BPA obtained theoretically as the following: BPA yield(wt %) )
(g BPA)exp × 100 (g BPA)max
(1)
In this case, the maximum amount of BPA which can be obtained theoretically was 2.42 g. 3. Results and Discussion 3.1. Reaction Mixture Analysis. Reaction products were qualitatively analyzed by gas chromatography (GC; Agilent 6890N) equipped with a mass spectrometer (Agilent 5973N) with a DB-1HT column. Identified products were BPA, EC (ethylene carbonate), and 4-tert-buthylphenol. Figure 2 shows GC chromatogram of the reaction mixture. On the basis of the analysis results, we proposed the reaction mechanism illustrated in Figure 4a. Ethylene glycol attacks the carbonate bond and creates smaller polymer chain. The polymer chain reacts again with ethylene glycol until the BPA and EC are produced. Then, the BPA and EC decompose into phenol compounds including 4-tert-buthylphenol, CO2, and others. Oku et al. reported that products of catalyzed glycolysis consist of monohydroxyethyl ether of BPA (MHE-BPA, 42%), bishydroxyethyl ether of BPA (BHE-BPA, 11%,) and BPA (42%) due to further reaction of BPA with EC.12 However, only BPA was found in the products
Figure 3. FTIR spectroscopy of reaction mixture.
of noncatalyzed glycolysis used in our experiments. This seems to be because the EC does not react again with BPA and is rapidly decomposed to linear carbonate, creating CO2. Figure 3 shows IR spectroscopy of the reaction mixture. The absorption bands of ethylene carbonate (1803 cm-1) and linear carbonate (1776 cm-1) were confirmed.13 Figure 4b gives the simplified glycolysis reaction equation, neglecting the reactions which produce byproduct. 3.2. Temperature Effect. To investigate the effect of reaction temperature on the rate of depolymerization of PC, the reaction temperature was varied from 180 to 220 °C with an EG/PC ratio of 4. Figure 5 provides the relationship between the yield of BPA and the reaction temperature. High temperature increases the rate of depolymerization and decreases the induction time. The BPA yield reached 95.6% when the reaction temperature was 220 °C for 85 min. 3.3. Kinetic Analysis. The depolymerization was initially assumed to be represented by one single reaction. The depolymerization reaction rate was assumed to be proportional to the concentration of polycarbonate repeating unit d[PC] ) -k[PC] (2) dt [PC] means the concentration of the polycarbonate repeating unit [PC] ) [PC]o(1 - X)
(3)
The concentration of EG is also assumed to be constant during the reaction. dX ) k(1 - X) dt
(4)
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Integration of the equation against time yields
( 1 -1 X ) ) kt
ln
(5)
where X is the yield of BPA and k is a pseudo first-order rate constant. The rate constants of depolymerization at 180, 190, 200, 210, and 220 °C, which were obtained from eq 5 and Figure 6, were 0.002, 0.015, 0.032, 0.041, and 0.004 min-1, respectively. Figure 7 shows an Arrhenius plot for the PC degradation reaction. For an increase in temperature, the observed activation energy falls from 355.8 to 32.7 kJ/mol. A change in the observed activation energy with temperature indicates a shift in the controlling mechanism of reaction for multiple reactions.14 Thus, the drop of activation energy for an increase in temperature indicates that the reactions take place in series. This finding leads to the following reaction mechanism which consists of three consecutive reaction steps. Ethylene glycol penetrates into the PC polymer particle so that the particles are swollen. The PC is depolymerized in the solid state by the
Figure 6. Kinetic expression for the formation of BPA.
Figure 7. Arrhenius plot for the formation of BPA.
Figure 4. Reaction pathway.
Figure 5. Temperature effect on the yield of BPA.
diffused EG. Random scissions of PC take place to lower the average molecular weight until the resulting oligomer can be dissolved in the bulk EG solution but retains solid state. We assumed that the degradation time to the dissolvable oligomer is almost constant and depends only on reaction temperature. The solid oligomer dissolves in EG solution, and the size of the PC particle shrinks as the dissolution proceeds, which is a heterogeneous reaction. The dissolved oligomer continues to be depolymerized with EG in the bulk solution to produce its monomer, BPA, which is a homogeneous reaction. This reaction mechanism is illustrated in Figure 8. The reaction steps from polycarbonate to monomer BPA can be summarized as follows: (1) EG penetration into the PC particle (swelling) and random scission to dissolvable solid oligomer (induction) (2) Dissolution of the solid oligomer into bulk EG solution (shrinking) (3) Conversion from the dissolved oligomer to bisphenol A. Figure 9 shows the surface of the PC particle at different reaction times of glycolysis. Formation of cracks appear on the surface of the PC particle at a reaction time of 80 min when dissolution is supposed to be on process. The formation of cracks not only increases the effective surface area for dissolution but also promotes penetration of ethylene glycol into the solid PC particle, increasing the random scission reaction. The dissolution rate should be proportional to the reaction surface area and concentration of the solid oligomer at the surface. The surface area for the dissolution decreases as the PC particle shrinks due to the dissolution reaction. However, the concentration of the solid oligomer at the surface of the particle increases as the reaction goes by. This is because, for the swollen polymer, the random scission reaction to dissolvable solid oligomer occurs
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Figure 8. Dissolution of the polycarbonate particle. Table 1. Reaction Parameters Determined from Experiments k1 (min-1) 190 200 210 220
°C °C °C °C
0.0087 0.0104 0.0124 0.0146
k10
k2 (min-1)
45.7
0.0207 0.1459 0.9466 5.6928
k20 2.5 × 1038
θ (min) 85 45 25 17
not only at the surface but also inside the particle so that the solid oligomer concentration on the surface of the PC particle increases, as the particle shrinks. The two effects can be canceled out by each other so that the generation rate of the liquid oligomer may very roughly be assumed to be constant, irrespective of progress of the dissolution process. The expression for the oligomer generation has the following form: d[solid oligomer] ) -k1S(t - θ) dt
(6)
where θ is the degradation time necessary to be the dissoluble oligomer during which EG penetration into the PC particle and random scission take place. S is a step function, defined as S(t - θ) )
{
0 teθ 1 t>θ
(7)
The reaction rates for polycarbonate, the solid oligomer, liquid oligomer, and BPA can be written as the following: d[PC] d[solid oligomer] ) ) -k1S(t - θ) dt dt
(8)
d[liquid oligomer] ) k1 - k2[liquid oligomer]:[PC] present dt (9) )-k2[liquid oligomer]:[PC] absent
(10)
d[BPA] ) k2[liquid oligomer] dt
(11)
where [PC] is the fraction of unconverted polycarbonate fraction to liquid oligomer, [solid oligomer] is the fraction of solid oligomer from polycarbonate, [liquid oligomer] is the fraction of liquid oligomer from polycarbonate, and [BPA]is the fraction of BPA converted from polycarbonate. On the basis of the model, the model parameters k1, k2, and θ were determined from the experimental data. Having obtained activation energies E1 and E2 from the Arrhenius plot, we could calculate frequency factors for the rate constants, k10 and k20, as shown in Table 1. Reactions with high activation energies are very temperature sensitive; reactions with low activation energies are relatively less temperature sensitive. The reaction with small activation energy was assumed as the dissolution step of oligomer into EG bulk solution; the reaction with large activation energy was assumed as the decomposition from the oligomer to BPA.
Figure 9. SEM photographs of the PC particle at 200 °C: (a) 60; (b) 80 min.
Figure 10 shows the distribution of PC, liquid oligomer, and BPA, from which we can see that the model predicts the BPA yield very well. Here, PC denotes polycarbonate polymer with molecular weights undissolvable in EG solution. As the reaction temperature gets higher, the concentration of oligomer becomes smaller. This means that the homogeneous reaction from oligomer to monomer BPA becomes more active than the heterogeneous reaction from PC to oligomer as the reaction temperature increases, depleting the oligomer in the mixture as shown in Figure 10b. This result confirms that the reaction from oligomer to BPA is more temperature sensitive and has higher activation energy than dissolution reaction. Continuous distribution kinetics8 provides a framework for explaining the degradation of PC by random chain scission to the solid oligomer. The polymer, A(x), is considered to be a mixture of a large number of homologous molecules with molecular weight (MW) x as a continuous variable. Polymer degradation through random scission can be written: kr
A′(x) 98 A(x) + A(x′ - x)
(12)
The degradation of PC includes random scissions, p′(x) f p(x) + p(x′ - x)
(13)
where kr is the rate coefficient for random scissions. The rate coefficient for the PC degradation could be obtained by the moment operation. The moment operation is defined p(n)(t) )
∫
∞ n
0
x p(x, t) dx
(14)
The zeroth moment, p(0)(t) is the molar concentration of the polymer (mol/L) and the first moment, p(1)(t) is the mass concentration (g/L) of the polymer. Number- and weight-
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where Mn(0) is the initial number-averaged molecular weight and Mn(θ)is the number-averaged molecular weight at induction time. Mn(0) ( Mn(θ) )
C ) ln
Figure 10. Product distribution for the PC degradation reaction at reaction temperatures: (a) 190; (b) 220 °C.
(19)
The number average molecular weight of the dissolvable oligomer was assumed to be the same independent of the reaction temperature. The time necessary for the PC particle to reach the molecular weight of the oligomer can be expressed as induction time θ. The induction time varies with reaction temperature. The average molecular weight of the oligomer is unknown so that the reaction rate coefficient cannot be determined. However, if the molecular weight of the oligomer is assumed to have a certain constant value as we assumed, the rate coefficients for random scission can be calculated from induction times determined from experimental results. The activation energy for the degradation reaction can be calculated from the Arrhenius plot shown in Figure 11 since the activation energy can be determined from the slope of reaction rate coefficients against temperature. The activation energy for random scissions (Er) of PC is calculated to be 98.9 kJ/mol. In summary, the depolymerization of PC using ethylene glycol consists of multiple steps: random scission, dissolution, and homogeneous reaction. The activation energies for the series reaction were 98.9 kJ/mol for the random scission, 32.7 kJ/mol for dissolution of the oligomer, and 355.8 kJ/mol for the reaction from the oligomer to BPA, respectively. The last homogeneous reaction step is most activated among the three reaction steps as the reaction temperature goes higher. 3.4. Effect of EG/PC Weight Ratio. The effect of the amount of EG involved in the reaction on the BPA yield was
Figure 11. Arrhenius plot for random scission reaction.
averaged MWs are defined as Mn ) p(1)/p(0). The moment expression with constant rate coefficient when n ) 0 is8 dp(0) ) krp(0)(t) dt
Figure 12. Effect of EG/PC weight ratio on the percent yield of BPA.
(15)
Integration with the initial gives p(0)(t) ) exp(krt) p(0) 0
(16)
The ratio of average molecular weight between the oligomer at time θ, Mn(θ), and polycarbonate at time zero, Mn(0), can be expressed as Mn(0) ) exp(krθ) Mn(θ)
( Cθ )
ln kr ) ln
(17) (18)
Figure 13. Effect of EG/PC weight ratio on the rate constant of PC depolymerization at 210 °C for 110 min.
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ratio of 4. A depolymerization model was proposed for better understanding of the PC glycolysis which consists of three reaction steps: random scission from high molecular weight PC to its solid oligomer, dissolution from the solid oligomer to liquid oligomer, and homogeneous degradation from the oligomer to its monomer, BPA. A shrinking core model was considered for the dissolution and continuous distribution kinetics model for the random scission reaction. The activation energies for the series reaction were 98.9 kJ/mol for the random scission, 32.7 kJ/mol for the dissolution of oligomer, and 355.8 kJ/mol for the reaction from oligomer to BPA, respectively. The effect of operation variables on the yield of BPA were analyzed on the basis of the proposed model. The predicted values by the proposed model were shown in good agreement with the experimental ones. Figure 14. Effect of agitation for the PC degradation reaction at 200 °C.
Acknowledgment investigated. The depolymerization reaction was carried out at reaction temperature 210 °C for 110 min. The BPA yield increases as the amount of EG increases as shown in Figure 12. The diffusion of ethylene glycol, random scission, and dissolution of the polycarbonate particle appear to be a little dependent on the amount of ethylene glycol since the reactions are heterogeneous. However, when the reaction from the oligomer to BPA occurs in homogeneous phase, the concentration of ethylene glycol takes a great effect on the reaction rate. Figure 13 shows that the plot of reaction rate coefficient vs EG/ PC ratio gives a straight line, indicating that the reaction rate constant from oligomer to its monomer BPA can be expressed as k2 ) k2 ′ [EG]
(20)
3.5. Effect of Agitation. The effect of agitation on the yield of BPA in the reaction is shown in Figure 14. The reaction was carried out at reaction temperature 200 °C. Nonagitated reaction was performed in the furnace at the same temperature. Agitation of the reaction mixture increases the BPA yield and decreases induction time significantly, but BPA generation rate does not change so much. Agitation is generally used to increase mass transfer. The depolymerization of PC into PC oligomer is a heterogeneous reaction and the depolymerization of PC oligomer into its monomer is a homogeneous reaction. Mass transfer influences the heterogeneous reaction more strongly than the homogeneous reaction. Major mass transfer processes involved in the depolymerization are swelling and dissolution. The agitation promotes diffusion of ethylene glycol in the liquid phase into the solid PC (swelling) and accelerates random scission reaction in the solid phase, decreasing induction time for the depolymerization reaction significantly. However, the agitation does not appear to affect diffusion of solid oligomer into the liquid phase (dissolution) and the depolymerization reaction from the oligomer to monomer, since the BPA generation rate does not give much difference between the agitated process and the nonagitated process. 4. Conclusion Chemical recycling of polycarbonate waste by noncatalyzed glycolysis using ethylene glycol was studied in this paper. The depolymerization process can be considered as a green process from the viewpoint of using neither toxic solvents nor alkalicatalyst. The maximum yield of BPA of 95.6% was achieved at reaction temperature 220 °C for 85 min with an EG/PC weight
This work was supported by Korea Ministry of Environment as “The Eco-technopia 21 project”. Nomenclature [PC] ) concentration of polycarbonate repeating unit (mol/L) [EG] ) concentration of ethylene glycol (mol/L) k ) pseudo first-order rate constant (min-1) X ) the yield of BPA θ ) degradation time (min) S ) step function [solid oligomer] ) fraction of solid oligomer from polycarbonate [liquid oligomer] ) fraction of liquid oligomer from polycarbonate [BPA] ) fraction of BPA converted from polycarbonate A(x) ) polymer molecule having molecular weight x kr ) rate coefficient for random scission (min-1) k1 ) rate coefficient for dissolution (min-1) k2 ) rate coefficient for the reaction from oligomer to monomer (min-1) k10 ) frequency factor for dissolution k20 ) frequency factor for the reaction from oligomer to monomer E1 ) activation energy for dissolution (kJ/mol) E2 ) activation energy for the reaction from oligomer to monomer (kJ/mol) Er ) activation energy for random scissions (kJ/mol) p(0)(t) ) molar concentration of the polymer (mol/L) p(1)(t) ) mass concentration of the polymer (g/L) Mn(0) ) initial number-averaged molecular weight Mn(t) ) number-averaged molecular weight at induction time C ) number average molecular weight of the dissolvable oligomer
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ReceiVed for reView July 17, 2008 ReVised manuscript receiVed September 27, 2008 Accepted October 26, 2008 IE8010947
