**2. Pyrolysis of poly(bisphenol A carbonate)**

Pyrolysis is a thermal process taking place typically in the temperature range of 300-1000°C in absence of oxygen. It decomposes organic molecules in gaseous and carbonaceous products. After cooling, the vapors give place to condensed mixtures (the so called tars). Uncondensed products are typically CO, H2, CH4 and other hydrocarbons with low

Poly(bisphenol A carbonate) Recycling: High Pressure Hydrolysis Can Be a Convenient Way 119

Small amounts

Some researchers (Chiu et al., 2006) have also studied thermal degradation in presence of an active catalyst. The expected advantage is to lower the required temperature, to shorten the degradation time, to increase the extent of the degradation, reduce the portion of solid residue and to narrow the product distribution (Ali et al., 2002). In their paper, Chiu et al. report experiments related to the use of ten different metal chlorides. They found SnCl2 and ZnCl3 to be catalytically active for degradation. They can efficiently perform the expected improvements in thermal treatment of PC and, therefore, some benefits can be obtained. In particular by using these catalysts, liquid degradation products are reduced from 20 to

CH2

CH3

3CH

CH3

O

O

OH

*(360°C)* 

amounts

amounts

O

CH3

3CH

CH3

3CH

O

OH

CH3

Large amounts Significative

Large amounts Present

Large amounts Present

Significative

*Phenolic products Lee(475°C) Davis-Golden* 

3CH Large amounts Present

OH

OH

OH

OH

OH

Table 3. main phenolic products obtained from pyrolysis.

O

O

OH

Fig. 2. Examples of xanthone structures.

CH3

CH3

3CH OH

3CH

3CH

2CH

3CH

respectively 10 and 8.

3CH

CH2

molecular weight. The ratio among solid, liquid and gaseous products depends on the pyrolysis conditions in terms of temperatures and residence times. As mentioned before, pyrolysis is one of possible routes for PC recycling (Jang & Wilkie, 2004). The idea is to transform plastic wastes to fuels or valuable products. Several papers related to the study of PC degradation are reported into the literature (Day et al., 1999; McNeil & Rinchon, 1991; McNeil & Basan, 1993; Montaudo & Puglisi, 1992; Montaudo et al., 2002; Oba et al., 2000; Puglisi et al., 1999). Also the pyrolysis in presence of active catalysts has been studied (Ali et al., 2002; Chiu et al., 2006). Figure 1 shows some thermogravimetries of PC from three different research groups. Decomposition occurs in the temperature range of about 300- 500°C. The remaining residue is in the range of 20-30 wt%.

Fig. 1. fractional weight loss during thermal degradation of PC.

At 360°C, gas fraction is composed (wt%) by: 95% CO2, 3% CO and 2% CH4. At higher temperature CO and CH4 increase their amount (Davis & Golden, 1968).

The condensed pyrolysis product has been found to be constituted by resins and a crystalline part. The latter contains BPA which amount depends on the operating conditions (time and temperature). In table 3 phenolic products obtained by different researchers are reported. Non phenolic products are present in minor amounts.

Authors report also the presence of xanthone units, which examples of structures are reported in figure 2. Dibenzofurane and fluorenone are also present in the residue, together with ethers and products coming from crosslinking, substitutive addition and branching reactions. This brief information regarding products obtained from pyrolysis of PC shows that thermal decomposition is not offering a good solution for PC recycling because of the difficulty of separation and low selectivity.

molecular weight. The ratio among solid, liquid and gaseous products depends on the pyrolysis conditions in terms of temperatures and residence times. As mentioned before, pyrolysis is one of possible routes for PC recycling (Jang & Wilkie, 2004). The idea is to transform plastic wastes to fuels or valuable products. Several papers related to the study of PC degradation are reported into the literature (Day et al., 1999; McNeil & Rinchon, 1991; McNeil & Basan, 1993; Montaudo & Puglisi, 1992; Montaudo et al., 2002; Oba et al., 2000; Puglisi et al., 1999). Also the pyrolysis in presence of active catalysts has been studied (Ali et al., 2002; Chiu et al., 2006). Figure 1 shows some thermogravimetries of PC from three different research groups. Decomposition occurs in the temperature range of about 300-

500°C. The remaining residue is in the range of 20-30 wt%.

Fig. 1. fractional weight loss during thermal degradation of PC.

reported. Non phenolic products are present in minor amounts.

difficulty of separation and low selectivity.

temperature CO and CH4 increase their amount (Davis & Golden, 1968).

At 360°C, gas fraction is composed (wt%) by: 95% CO2, 3% CO and 2% CH4. At higher

The condensed pyrolysis product has been found to be constituted by resins and a crystalline part. The latter contains BPA which amount depends on the operating conditions (time and temperature). In table 3 phenolic products obtained by different researchers are

Authors report also the presence of xanthone units, which examples of structures are reported in figure 2. Dibenzofurane and fluorenone are also present in the residue, together with ethers and products coming from crosslinking, substitutive addition and branching reactions. This brief information regarding products obtained from pyrolysis of PC shows that thermal decomposition is not offering a good solution for PC recycling because of the


Table 3. main phenolic products obtained from pyrolysis.

Some researchers (Chiu et al., 2006) have also studied thermal degradation in presence of an active catalyst. The expected advantage is to lower the required temperature, to shorten the degradation time, to increase the extent of the degradation, reduce the portion of solid residue and to narrow the product distribution (Ali et al., 2002). In their paper, Chiu et al. report experiments related to the use of ten different metal chlorides. They found SnCl2 and ZnCl3 to be catalytically active for degradation. They can efficiently perform the expected improvements in thermal treatment of PC and, therefore, some benefits can be obtained. In particular by using these catalysts, liquid degradation products are reduced from 20 to respectively 10 and 8.

Fig. 2. Examples of xanthone structures.

Poly(bisphenol A carbonate) Recycling: High Pressure Hydrolysis Can Be a Convenient Way 121

especially at the beginning of de-polymerization. The selection of the appropriate solvent will depend on the evaluation of the efficiency of reaction, separation and purification

Pyrolysis of PC in subcritical and supercritical water was studied by Tagaya et al. (1999). Water has the advantage to be in-expensive and non-toxic. In these conditions the decomposition reaction is more selective and the products are reduced to phenol, bisphenol

Tagaya et al. (1999) decomposed PC in the temperature range from 230°C to 430°C. No decomposition occurred below 230°C even for a reaction time of 24 h. Production of phenol, BPA, p-isopropenylphenol and p-isopropylphenol begin starting from 240°C. At 250°C a significant decomposition starts after 2 h, while at 300°C, after 2h, 38.1% of the former products were obtained. However estension of reaction time resulted ineffective for increasing the yield. NaCl and CH3COOH addition was no effective for decomposition reaction. The latter results to be accelerated by the addition of Na2CO3. At 300°C no BPA was detected in the product because of its transformation into phenol catalysed by Na2CO3. Phenol reached in these conditions, after 10 h, a yield of more than 30% indicating that subcritical water is also an advantageous medium for decomposing polymeric compounds. On the contrary supercritical water, in decomposing polymeric materials, has the disadvantage to cause rapid corrosion of equipments at very severe operating conditions. Chen et al. (2004) observed that high yields of BPA and dimethylcarbonate (DMC) were obtained using methanol. DMC is a valuable byproduct, a nontoxic and environmentally benign carbonylating and methylating agent that can replace hazardous chemicals like for instance phosgene. It is also a raw material useful for producing again polycarbonate resins. Also Dongpil et al. (2009) studied PC methanolysis mechanism in the temperature range 160-220 °C and with reactions time until 180 min. The increase of molar ratio between methanol and PC has been found to improve both DMC and BPA yields but, of course, it

Hu et al. (1998) studied alkali catalysed methanolysis using a mixed solvent of methanol and toluene. They found that by using more than a stoichiometric amount of methanol, DMC was produced in quantitative yields, highlighting the importance of adopting also an

Water 373.94 22.064 Methanol 239.48 8.097 Ethanol 240.76 6.148 1-propanol 263.62 5.175 Acetone 234.94 4.700 Benzyl alcohol 441.84 4.300

Table 4. critical parameters of water and alcohols.

A, p-isopropenylphenol and p-isopropylphenol.

demands larger capital and energy costs.

optimized ratio between PC and methanol.

Critical temperature [°C] Critical pressure [MPa]

process.

No chlorinated products were formed. Main obtained liquid products are phenol, p-isopropylphenol, diphenil carbonate and BPA.

Other researchers (Mitan et al., 2007) proposed to co-pyrolyse at 450°C PC wastes (as for instance DVDs and CDs) with vegetable cooking oil (VCO). They found an increased yield in liquid fraction, at the expense of solid residue, not coming just from the sum of the liquid and solid yields of individual components. This fact was also confirmed by the analysis of degradation rates that results increased by the addition of VCO. Metal contained into DVDs and CDs became 2.5 more concentrated into the solid residue compared to that deriving from thermal treatment of individual reagents.

Grause et al. (2009) studied the pyrolysys of PC with steam in the presence of earth –alkali oxides and hydroxides at temperature between 300°C and 500°C. The experiments were carried out in a semibatch reactor, in steam atmosphere, in the presence of MgO, CaO, Mg(OH)2, Ca(OH)2. All these catalysts accelerated the decomposition, in comparison with the hydrolysis of PC alone, MgO and Mg(OH)2 resulting more effective that CaO and Ca(OH)2. BPA were obtained as the main product at 300°C with a yield of 78% with MgO. Grause et al. (2009) suggested then to further increase the temperature at 500°C in order obtain high yields of phenol and isopropenylphenol and a drastic reduction of the residue.

#### **3. Ester exchange reaction and hydrolysis of poly(bisphenol A carbonate)**

Obviously pyrolysis presents some disadvantages such as non-uniform heat flux distribution, low yield of the preferred products and excessive char and gas formation, even if some mitigations towards these aspects have been shown to be possible by using catalysts or by co-pyrolysis. Degradation in liquid solution has been proposed to overcome some of these problems. In this method, the polymer is dissolved in a solvent and degraded thermally in a single phase. Yoshiki et al. (2005) obtained high yields of oil but low selectivity of BPA using tetralin, decalin and ciclohexanol as solvent. Also methylene chloride with ammonia or phenol in combination with an alkali catalyst were applied successfully for PC decomposition (Fox & Peters, 1989; Shafer, 1994). In these cases, however, a complicated product separation is required in addition to environmental problems related to the toxicity of the solvents. Supercritical and subcritical fluids are attractive solvents. They have unique properties, many of their physical and transport properties are intermediate between liquid and gas. The diffusion limited reactions are enhanced because of the increased solubilities and diffusivities. Polymers can therefore be degraded to low molecular weight products by thermolysis in supercritical fluids with high degradation rates. Water and alcohols were mainly used as sub- and supercritical fluids in depolymerization processes of plastics (Goto, 2009). In most of the case the role of the subor supercritical fluids is to act both as a solvent reaction medium and as a reactant. In table 4 are reported the critical parameters of water and some simple oxygenated compounds. The critical temperature of methanol is lower than that of water. Properties of water, such as dielectric constant and ion product, change drastically around the critical point (Kruse & Dinjius, 2007). Thus, special effect of water can be expected. When the de-polymerized products are not sufficiently stable, and solvent and supercritical condition are required, alcohols may be better solvents than water, because of their lower critical temperature. During the de-polymerization process, plastic phase is often solid or melt status. In that case, dissolution of solvent into solid or melt phase is essential to enhance the reaction rate,


especially at the beginning of de-polymerization. The selection of the appropriate solvent will depend on the evaluation of the efficiency of reaction, separation and purification process.

Table 4. critical parameters of water and alcohols.

120 Material Recycling – Trends and Perspectives

No chlorinated products were formed. Main obtained liquid products are phenol, p-iso-

Other researchers (Mitan et al., 2007) proposed to co-pyrolyse at 450°C PC wastes (as for instance DVDs and CDs) with vegetable cooking oil (VCO). They found an increased yield in liquid fraction, at the expense of solid residue, not coming just from the sum of the liquid and solid yields of individual components. This fact was also confirmed by the analysis of degradation rates that results increased by the addition of VCO. Metal contained into DVDs and CDs became 2.5 more concentrated into the solid residue compared to that deriving

Grause et al. (2009) studied the pyrolysys of PC with steam in the presence of earth –alkali oxides and hydroxides at temperature between 300°C and 500°C. The experiments were carried out in a semibatch reactor, in steam atmosphere, in the presence of MgO, CaO, Mg(OH)2, Ca(OH)2. All these catalysts accelerated the decomposition, in comparison with the hydrolysis of PC alone, MgO and Mg(OH)2 resulting more effective that CaO and Ca(OH)2. BPA were obtained as the main product at 300°C with a yield of 78% with MgO. Grause et al. (2009) suggested then to further increase the temperature at 500°C in order obtain high yields of phenol and isopropenylphenol and a drastic reduction of the residue.

**3. Ester exchange reaction and hydrolysis of poly(bisphenol A carbonate)** 

Obviously pyrolysis presents some disadvantages such as non-uniform heat flux distribution, low yield of the preferred products and excessive char and gas formation, even if some mitigations towards these aspects have been shown to be possible by using catalysts or by co-pyrolysis. Degradation in liquid solution has been proposed to overcome some of these problems. In this method, the polymer is dissolved in a solvent and degraded thermally in a single phase. Yoshiki et al. (2005) obtained high yields of oil but low selectivity of BPA using tetralin, decalin and ciclohexanol as solvent. Also methylene chloride with ammonia or phenol in combination with an alkali catalyst were applied successfully for PC decomposition (Fox & Peters, 1989; Shafer, 1994). In these cases, however, a complicated product separation is required in addition to environmental problems related to the toxicity of the solvents. Supercritical and subcritical fluids are attractive solvents. They have unique properties, many of their physical and transport properties are intermediate between liquid and gas. The diffusion limited reactions are enhanced because of the increased solubilities and diffusivities. Polymers can therefore be degraded to low molecular weight products by thermolysis in supercritical fluids with high degradation rates. Water and alcohols were mainly used as sub- and supercritical fluids in depolymerization processes of plastics (Goto, 2009). In most of the case the role of the subor supercritical fluids is to act both as a solvent reaction medium and as a reactant. In table 4 are reported the critical parameters of water and some simple oxygenated compounds. The critical temperature of methanol is lower than that of water. Properties of water, such as dielectric constant and ion product, change drastically around the critical point (Kruse & Dinjius, 2007). Thus, special effect of water can be expected. When the de-polymerized products are not sufficiently stable, and solvent and supercritical condition are required, alcohols may be better solvents than water, because of their lower critical temperature. During the de-polymerization process, plastic phase is often solid or melt status. In that case, dissolution of solvent into solid or melt phase is essential to enhance the reaction rate,

propylphenol, diphenil carbonate and BPA.

from thermal treatment of individual reagents.

Pyrolysis of PC in subcritical and supercritical water was studied by Tagaya et al. (1999). Water has the advantage to be in-expensive and non-toxic. In these conditions the decomposition reaction is more selective and the products are reduced to phenol, bisphenol A, p-isopropenylphenol and p-isopropylphenol.

Tagaya et al. (1999) decomposed PC in the temperature range from 230°C to 430°C. No decomposition occurred below 230°C even for a reaction time of 24 h. Production of phenol, BPA, p-isopropenylphenol and p-isopropylphenol begin starting from 240°C. At 250°C a significant decomposition starts after 2 h, while at 300°C, after 2h, 38.1% of the former products were obtained. However estension of reaction time resulted ineffective for increasing the yield. NaCl and CH3COOH addition was no effective for decomposition reaction. The latter results to be accelerated by the addition of Na2CO3. At 300°C no BPA was detected in the product because of its transformation into phenol catalysed by Na2CO3. Phenol reached in these conditions, after 10 h, a yield of more than 30% indicating that subcritical water is also an advantageous medium for decomposing polymeric compounds. On the contrary supercritical water, in decomposing polymeric materials, has the disadvantage to cause rapid corrosion of equipments at very severe operating conditions. Chen et al. (2004) observed that high yields of BPA and dimethylcarbonate (DMC) were obtained using methanol. DMC is a valuable byproduct, a nontoxic and environmentally benign carbonylating and methylating agent that can replace hazardous chemicals like for instance phosgene. It is also a raw material useful for producing again polycarbonate resins. Also Dongpil et al. (2009) studied PC methanolysis mechanism in the temperature range 160-220 °C and with reactions time until 180 min. The increase of molar ratio between methanol and PC has been found to improve both DMC and BPA yields but, of course, it demands larger capital and energy costs.

Hu et al. (1998) studied alkali catalysed methanolysis using a mixed solvent of methanol and toluene. They found that by using more than a stoichiometric amount of methanol, DMC was produced in quantitative yields, highlighting the importance of adopting also an optimized ratio between PC and methanol.

Poly(bisphenol A carbonate) Recycling: High Pressure Hydrolysis Can Be a Convenient Way 123

By cooling the reactor, the resulting content (at room temperature) is constituted only by condensed phases. The liquid and solid internal material was discharged, identified and

The solid sample obtained from the tests was characterized by Shimadzu FTIR: IRAffinity-1 in the 500 - 4000 cm-1 range (KBr disc). Gas chromatographic analyses of solid products were carried out (using methanol as solvent) on a GC-FID HP 6890plus with SLB-5ms capillary SUPELCO column (30 m length, 1 μl injection volume, split ratio 1:10, Helium carrier 1.4 mL/min, constant flow). The temperature was held at 65°C for the first 2 minutes, then increased at 255°C at a heating rate of 10°C/min and kept at this temperature for 15 minutes. The main products were identified and quantified by comparing the retention time

Figure 3 shows a typical gas chromatographic analysis of the condensed phase discharged at the end of a test at high conversion. As it can be observed BPA is the main product. Gas chromatographic analyses of evolved gas were performed with a column 0.53 mm ID, Molecular Sieve 5A as stationary phase, GC-T as detector, isothermal at T=25°C. Permanent

Fig. 3. Typical gas chromatographic analysis of solid product obtained by the tests.

Figure 4 shows FT transmittance plot showing the FTIR analyses of commercial poly(bisphenol A carbonate): (A) starting material obtained from a CD crash, (B) low conversion depolymerization solid discharged material, (C) medium conversion material, (D) solid material discharged at complete depolymerization. The FTIR spectra of reagent grade BPA, p-isopropylphenol and phenol are also reported. The figure 4 D shows the same

quantified by FTIR and GC.

with standard compounds.

gases and CO2 were detected.

shape as pure BPA.

**4.1.3 Some analytical aspects** 

Piñero et al. (2005) used methanol-water mixtures and developed (Piñero et al., 2006) a shrinking particle model to describe the reactive dissolution of the BPA-PC particles in semicontinuous de-polymerization of polycarbonate. Jie et al. (2006) studied decomposition in ethanol that has a critical point practically at same temperature and a lower pressure than that of methanol, allowing lower operating temperatures and pressures. Comparing the use of methanol and ethanol as solvents, it is reported that PC completely decomposes in supercritical methanol at 290°C and 9.96 MPa, in supercritical water above 374°C and pressure higher than 22.1 MPa, while in ethanol this is possible above 243 °C and 6.38 MPa producing BPA and diethylcarbonate (DEC). The mechanism consists in the random reaction along the polymer chain of the ester linkage with the solvent , that produces two smaller polymer chains, which can still react by ester exchange reaction until the polymer is completely converted to BPA and another product depending on the solvent.

In all the reported examples of alcoholysis yields of 90% of BPA (in terms of weight of obtained BPA divided the weight of initial PC) were obtained. The activation energy deduced from experimental data are resumed in table 5 (paragraph 6).
