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

Giulia Bozzano, Mario Dente and Renato Del Rosso *Politecnico di Milano Italy* 

#### **1. Introduction**

114 Material Recycling – Trends and Perspectives

Zammarano M., Séverine B., Jeffrey W.G., Franceschi M., Frederick L. Beyer, Richard H.

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Harris, Sergio Meriani, *Delamination of organo-modified layered double hydroxides in* 

*polypropylene nanocomposites based on oligomerically-modified clay*, Polymer

Polycarbonates are polymers characterized by a carbonate group. Their molecular structure can be of various kinds depending on the unit (G) that is connected with the carbonate. Table 1 reports some of the most used polycarbonates.


Table 1. most diffused polycarbonates.

Among them, the poly(bisphenol A) carbonate (PC in the following) is the most diffused. It is commonly referred as polycarbonate because of its vast number of industrial applications. It is a lightweight, high-quality plastic. It is well known and appreciated for its transparency, its excellent resistance to impact and its ability to withstand high temperature during the lifespan of the final article. Generally speaking, materials based on polycarbonates are resistant, rigid till 140°C and not fragile under –20°C. They are amorphous and have excellent mechanical properties and dimensional stability. Some restrictions in their use consist in a limited resistance to chemicals and to scrapes, and to color changes after exposition to UV ray. These problems can be solved by means of the proper additives or making use of mechanical mixing with other polymers. The main physical properties are resumed in table 2.

Poly(bisphenol A carbonate) Recycling: High Pressure Hydrolysis Can Be a Convenient Way 117

Mechanically recycled PC is less resilient, because it has decreased impact resistance when compared with newly manufactured polycarbonate. The addition of fillers and pigments can also decrease the plastic's resilience. This problem can be addressed by blending with other materials to modify impact resistance in recycled polycarbonate (Elmaghor et al.,

Thermal degradation or pyrolysis is intrinsically characterized by a low selectivity. This is due to the prevailing radical mechanism of the thermal decomposition that is substantially acting randomly over the backbone of the polymer macromolecules and over the obtained monomer, then producing their further degradation. It is well known that in this kind of radical reactions, all the H active positions can be attacked by metathetical transfer reaction of H and then giving place to different products. Moreover additive reactions can occur on aromatic rings, producing more and more condensed molecules, which are char precursors. Of course, the most active H positions are those of the two methyl groups. It is worthwhile to mention that these kind of attacks are active also on the formed BPA. Moreover the simultaneous crosslinking elementary process, and polycondensation reactions on aromatic rings, give place to the formation of char precursors. These aspects make the simple thermal degradation a not particularly suitable process because of the low selectivity in restoring

Several studies of possible depolymerization processes, based on the use of chemicals, have been reported in the literature for decomposing PC to its original monomer, BPA. They are based, for instance, on the use of solvent systems (Pan et al., 2006; Piñero et al., 2005) such as methylene chloride with ammonia, phenol in combination with an alkali catalyst, or via trans-esterification (alcoholysis) in super- or sub- critical conditions. These processes can require a complicate product separation in addition with environmental safety problems related to the use of more or less toxic organic solvents. Also decomposition of PC in sub-

The chapter firstly resumes and shortly analyzes most of the proposed processes. Then, the results obtained by adopting a PC recycling process based on hydrolysis using subcritical liquid water are reported (Bozzano et al., 2010). In this study both pure PC and CDs wastes are used. The driving concept came from an analogy with the fats hydrolysis producing fatty acids and glycerol in the soap production field or with the hydrolysis of oils (see for instance Khuwijitjaru et al., 2004 and Pinto & Lanças, 2006;). A concerted path depolymerization mechanism is proposed. A characterization of the process kinetics is presented and compared with lab-scale experimental data. The results show that this process can be a valuable alternative for BPA recovery mainly for its simplicity and absence of toxic agents or non-desired byproducts. It is of interest to mention that similar hydrolysis mechanisms can take place in other fields, like for instance the production of bio-oils from

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

and super- critical water has been taken into account by Tagaya et al. (1999).

2004).

biomasses.

monomer and the large amount of byproducts.

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


Table 2. PC physical properties.

PC also shows a high limiting oxygen index (LOI=27), and can produce a large amount of char on combustion conditions. PC is widely used in mixture with other polymers with the aim of enhancing resistance to external factors. Typical mixtures include PC with poly(butylene-terephthalate) or with ABS. These latter exhibit effective flame retardant properties upon the addition of conventional halogen and/or non halogen flame retardant agents, and this supports their large use in electrical appliances. Its properties make it appropriate for durable goods applications. PC is used in the construction of many everyday products, including CDs and DVDs, dinnerware, computer casings, medical equipments, bicycle helmets, automotive parts, packaging, sports and optical materials. Other applications are for paintings and covertures of buildings.

Two industrially significant syntheses for PC are mainly adopted. They were developed in 1960s . The first one was developed by Bayer in 1962 and consists in a two phase reaction (Schnell et al., 1962). In this process bisphenol A (BPA) is added first to the reactor in methylene chloride (with a monohydroxylic phenol to control molecular weight). Subsequently, phosgene is added to the reactor, along with aqueous sodium hydroxide (HCl scavenger), to produce a biphasic liquid-liquid system. This process allows obtaining high molecular weight polymer with excellent optical clarity and color. Major disadvantage is the use of phosgene and the generation of a large amount of wastewater and methylene chloride to be treated or disposed. The second process, developed by GE in 1964, is a melt transesterification between diphenil carbonate and BPA (Fox, 1964a, Fox 1964b). This results in intermediate molecular weight product with phenol as a condensation byproduct. This route is solvent free and avoids the direct use of phosgene. The high viscosity of the melt limits however the final molecular weight of the polymer.

According to the research "Polycarbonate: 2009 World Market Outlook and Forecast" (http://www.reportlinker.com) the PC market has been shown in not favourable perspectives in recent times. In facts the application of PC in optical media segment shrank as a result of lower demand for CDs and DVDs. Moreover a health concern raised over Bisphenol A (BPA) has negatively influenced the demand as well. PC could disappear from the food and beverage container market in the future. Notwithstanding, the global demand is expected to grow of 6-7% annually, driven by Asia and China in particular (2.4 Mt were produced in 2004). Demand would catch up with production capacity and the market would strengthen. This fact suggests that PC recycling will cover in the future more and more importance. It is therefore necessary to optimize and develop processes for PC wastes treatment. Because it is not a suitable alternative for waste treatment to landfill or incinerate wasted PC products, it is important to find resourceful recycling processes both for environmental protection and for economical benefits purposes. PC recycling can be performed in three main different ways: direct recycle (mechanical recycling or blending with other materials), pyrolysis and chemical treatment.

Traction resistance 70-80 N/mm2 Impact resistance 60-80 kJ/m2 Maximum temperature 125 °C Density 1.2 g/cm3

PC also shows a high limiting oxygen index (LOI=27), and can produce a large amount of char on combustion conditions. PC is widely used in mixture with other polymers with the aim of enhancing resistance to external factors. Typical mixtures include PC with poly(butylene-terephthalate) or with ABS. These latter exhibit effective flame retardant properties upon the addition of conventional halogen and/or non halogen flame retardant agents, and this supports their large use in electrical appliances. Its properties make it appropriate for durable goods applications. PC is used in the construction of many everyday products, including CDs and DVDs, dinnerware, computer casings, medical equipments, bicycle helmets, automotive parts, packaging, sports and optical materials.

Two industrially significant syntheses for PC are mainly adopted. They were developed in 1960s . The first one was developed by Bayer in 1962 and consists in a two phase reaction (Schnell et al., 1962). In this process bisphenol A (BPA) is added first to the reactor in methylene chloride (with a monohydroxylic phenol to control molecular weight). Subsequently, phosgene is added to the reactor, along with aqueous sodium hydroxide (HCl scavenger), to produce a biphasic liquid-liquid system. This process allows obtaining high molecular weight polymer with excellent optical clarity and color. Major disadvantage is the use of phosgene and the generation of a large amount of wastewater and methylene chloride to be treated or disposed. The second process, developed by GE in 1964, is a melt transesterification between diphenil carbonate and BPA (Fox, 1964a, Fox 1964b). This results in intermediate molecular weight product with phenol as a condensation byproduct. This route is solvent free and avoids the direct use of phosgene. The high viscosity of the melt

According to the research "Polycarbonate: 2009 World Market Outlook and Forecast" (http://www.reportlinker.com) the PC market has been shown in not favourable perspectives in recent times. In facts the application of PC in optical media segment shrank as a result of lower demand for CDs and DVDs. Moreover a health concern raised over Bisphenol A (BPA) has negatively influenced the demand as well. PC could disappear from the food and beverage container market in the future. Notwithstanding, the global demand is expected to grow of 6-7% annually, driven by Asia and China in particular (2.4 Mt were produced in 2004). Demand would catch up with production capacity and the market would strengthen. This fact suggests that PC recycling will cover in the future more and more importance. It is therefore necessary to optimize and develop processes for PC wastes treatment. Because it is not a suitable alternative for waste treatment to landfill or incinerate wasted PC products, it is important to find resourceful recycling processes both for environmental protection and for economical benefits purposes. PC recycling can be performed in three main different ways: direct recycle (mechanical recycling or blending

Other applications are for paintings and covertures of buildings.

limits however the final molecular weight of the polymer.

with other materials), pyrolysis and chemical treatment.

*Physical properties* 

Table 2. PC physical properties.

Mechanically recycled PC is less resilient, because it has decreased impact resistance when compared with newly manufactured polycarbonate. The addition of fillers and pigments can also decrease the plastic's resilience. This problem can be addressed by blending with other materials to modify impact resistance in recycled polycarbonate (Elmaghor et al., 2004).

Thermal degradation or pyrolysis is intrinsically characterized by a low selectivity. This is due to the prevailing radical mechanism of the thermal decomposition that is substantially acting randomly over the backbone of the polymer macromolecules and over the obtained monomer, then producing their further degradation. It is well known that in this kind of radical reactions, all the H active positions can be attacked by metathetical transfer reaction of H and then giving place to different products. Moreover additive reactions can occur on aromatic rings, producing more and more condensed molecules, which are char precursors. Of course, the most active H positions are those of the two methyl groups. It is worthwhile to mention that these kind of attacks are active also on the formed BPA. Moreover the simultaneous crosslinking elementary process, and polycondensation reactions on aromatic rings, give place to the formation of char precursors. These aspects make the simple thermal degradation a not particularly suitable process because of the low selectivity in restoring monomer and the large amount of byproducts.

Several studies of possible depolymerization processes, based on the use of chemicals, have been reported in the literature for decomposing PC to its original monomer, BPA. They are based, for instance, on the use of solvent systems (Pan et al., 2006; Piñero et al., 2005) such as methylene chloride with ammonia, phenol in combination with an alkali catalyst, or via trans-esterification (alcoholysis) in super- or sub- critical conditions. These processes can require a complicate product separation in addition with environmental safety problems related to the use of more or less toxic organic solvents. Also decomposition of PC in suband super- critical water has been taken into account by Tagaya et al. (1999).

The chapter firstly resumes and shortly analyzes most of the proposed processes. Then, the results obtained by adopting a PC recycling process based on hydrolysis using subcritical liquid water are reported (Bozzano et al., 2010). In this study both pure PC and CDs wastes are used. The driving concept came from an analogy with the fats hydrolysis producing fatty acids and glycerol in the soap production field or with the hydrolysis of oils (see for instance Khuwijitjaru et al., 2004 and Pinto & Lanças, 2006;). A concerted path depolymerization mechanism is proposed. A characterization of the process kinetics is presented and compared with lab-scale experimental data. The results show that this process can be a valuable alternative for BPA recovery mainly for its simplicity and absence of toxic agents or non-desired byproducts. It is of interest to mention that similar hydrolysis mechanisms can take place in other fields, like for instance the production of bio-oils from biomasses.
