**7.2 Recycling techniques**

42 Material Recycling – Trends and Perspectives

Polycarbonate plastics, C16H14O3 (Fig. 12) are polyesters known for their excellent mechanical properties. Featuring high‐impact resistance, UV resistance, and flame retardancy as well as excellent electrical resistance, polycarbonates are used in a wide variety of materials. Polycarbonates do not have their own recycling identification code and therefore fall under the #7 "other" classification. Polycarbonates may be made a variety of ways, the most popular of which from Bisphenol‐A (BPA) feedstock. BPA use is highly controversial, and the FDA has recently decided to reopen an inquiry on the safety of BPAs.

This is following an approval in 2008. Nalgene Outdoor Products, the pre-eminent manufacturer of reusable plastic water bottles, is transitioning from polycarbonate bottles to other plastics as well as metal alternatives in the wave of negative consumer perception of

Due to its excellent properties, polycarbonate (PC) is widely used in the manufacture of compact disks, bullet proof windows, food packaging and soft-drink bottles. With the rapid increase in the production and consumption of PC, the chemical recycling of waste PC to obtain valuable products has received greater attention in recent years. Waste PC can be depolymerised through a chemical treatment to produce monomers that can be used to reproduce virgin PC products. Various methods for the chemical recycling of waste PC to recover raw materials have been reported; these methods include thermal pyrolysis [Yoshioka

Fig. 11. Veba Combi Cracking process (Sas, 1994).

**7. Chemical recycling of polycarbonate** 

Fig. 12. Structural unit of polycarbonate.

**7.1 Introduction** 

[Jawad et al., 2009].

BPA.

With the rapid increase of production and consumption of PC, the chemical recycling of waste PC has been gaining greater attention in recent years to obtain valuable products. Methanolysis is one of the most important method to recover pure monomer BPA and dimethyl carbonate (DMC). However, due to the insolubility of PC in methanol, the reported methanolysis methods require high temperature and pressure and in presence of a lot amount of concentrated bases or acids. The acid or base catalysts used in traditional methods cannot be reused and result in other disadvantages such as equipment corrosion, tedious workup procedure and environmental problem. Although supercritical method can overcome some of above-mentioned shortcomings, it has its own disadvantages such as severe conditions, so its application is limited. According to a study polycarbonate could be completely decomposed into its monomer, BPA with high pressure (not atmospheric pressure) high temperature steam (300 °C) in five minutes reaction time. It is known that PC can be decomposed into monomer in alkaline alcohol or aqueous solutions. However, the monomer BPA yield has been reported as to be relatively low due to BPA instability in that condition. To develop a high-effective process of PC recycling, a reactive atmosphere must be provided that preserves the stability of BPA and get has high reactivity for PC. To determine the optimum conditions for recycling PC, it is important to know the stability or reactivity of BPA, as well as the decomposition rate of PC.

Alkali-catalysed depolymerization of polycarbonate wastes by alcoholysis in supercritical or near critical conditions has been also studied by other researchers in order to recover the essential monomer BPA and DMC as a valuable by-product (Liu et al., 2009). Some works aimed to develop continuous process and possible scale-up for decomposition of both PC plastic wastes using methanol as solvent/reagent and NaOH as alkali catalyst. Total depolymerization of PC has been achieved working at a temperature range of 75–180 °C and pressures from 2 to 25 MPa.

However, due to the insolubility of PC in water, the aqueous depolymerizations require severe conditions such as long reaction times, high temperatures and pressures. Therefore, instead of using aqueous systems, organic solvent systems such as methylene chloride in combination with ammonia, a mixed solvent of phenol and methylene chloride in

Recent Advances in the Chemical Recycling

BPA formation.

al., 2011].

**7.2.3 Hydrolysis with high temperature steam** 

of Polymers (PP, PS, LDPE, HDPE, PVC, PC, Nylon, PMMA) 45

The alkali catalysed methanolysis also studied by Liu et al., 2009 but the reaction took place in a reactor with a stirrer and a refluxing condenser. The results did not differ much because of the use of refluxing condenser. The temperature, on the other hand effected the efficiency of methanolysis in both studies with the temperature of 60οC presenting the greater rate of

Fig. 14. Methanolysis mechanism for PC in the presence of ionic liquid [Bmim][Ac] [Liu et

Watanabe et al. 2009, found that polycarbonate was rapidly hydrolyzed in high pressure high temperature steam around the saturated pressure of water at 573 K. For 300 s (5 min) reaction time, PC completely decomposed into bisphenol A and the maximum yield of BPA was around 80%. In liquid water phase at 573 K, PC still remained even for 3000 s (50 min). The high yield of bisphenol A in high pressure steam was due to its high stability. The amount of water required for degradation was drastically reduced and thus the high pressure high temperature steam process was energetically and economically preferable.

combination with an alkali catalyst is also used. An environmentally friendly strategy for methanolysis of polycarbonate to recover bisphenol A and dimethyl carbonate was recently developed in which PC could be methanolyzed in an ionic liquid without any acid or base catalyst under moderate conditions (Liu et al., 2011).

#### **7.2.1 Methanolysis in the presence of ionic liquids**

The methanolysis of polycarbonate using ionic liquid [Bmim][Ac] as a catalyst was studied recently by Liu et al., 2011. The effects of temperature, time, methanol dosage and [Bmim][Ac] dosage on the methanolysis reaction were examined. They concluded that methanolysis of PC to obtain its starting monomers, BPA and DMC, could occur in the presence of ionic liquid [Bmim][Ac] under moderate conditions without an acid or base catalyst. The methanolysis conversion of PC was nearly 100% and the yield of BPA was over 95% under the following conditions: m([Bmim][Ac]):m(PC) = 0.75:1, m(methanol):m(PC) = 0.75:1, a reaction temperature of 90 ◦C and a total time of 2.5 h. The ionic liquid [Bmim][Ac] could be reused up to 6 times without an apparent decrease in the conversion of PC and yield of BPA. This strategy could overcome the shortcomings associated with the traditional methods, such as the infeasibility of reusing the catalyst, equipment corrosion, tedious workup procedures and environmental problems. Moreover, the investigation on kinetics indicated that the methanolysis of PC in [Bmim][Ac] was a first-order reaction and the activation energy was 167 kJ/mol. The reaction formula was as follows:

$$\mathbf{H} \cdot \mathbf{C} \mathbf{O} - \mathbf{C} - \mathbf{O} \mathbf{C} \mathbf{H} + \mathbf{H} \mathbf{O} - \sum\_{\mathbf{q} \in \mathbf{Q}} \mathbf{C} - \sum\_{\mathbf{q} \in \mathbf{Q}} \mathbf{C} - \mathbf{O} \mathbf{H} \mathbf{q} \xrightarrow[\mathbf{H} \mathbf{O} \text{ or } \mathbf{C} \to \mathbf{I}]{} \mathbf{H} \cdot \mathbf{C} \mathbf{O} - \mathbf{C} \mathbf{I} - \begin{bmatrix} \mathbf{O} & \mathbf{C} \mathbf{O} \\ -\mathbf{C} \mathbf{O} - \mathbf{C} \sum\_{\mathbf{j}} \mathbf{C} - \sum\_{\mathbf{C}} \mathbf{O} - \mathbf{O} \\ \mathbf{C} \mathbf{O} - \mathbf{C} \sum\_{\mathbf{j}} \mathbf{C} - \mathbf{O} - \mathbf{O} \end{bmatrix} \mathbf{H}$$

Fig. 13. Methanolysis of PC.

A mechanism for the methanolysis of PC in the presence of ionic liquid [Bmim][Ac] is suggested in Figure 14. After PC was dissolved or swelled in the ionic liquid, it reacted with methanol to form oligomers under ionic liquid catalysis. Then, the resulting oligomers reacted with methanol further to produce the final products, BPA and DMC.

#### **7.2.2 Alkali catalyzed methanolysis**

Alkali-catalyzed methanolysis of poly[2,2-bis(4-hydroxyphenyl)propane carbonate] in a mixed solvent of methanol and toluene or dioxane was studied by the team of Oku et al., 2000 at Kyoto Institute of Technology. Treatment of PC pellets in MeOH with a catalytic amount of NaOH at 608C for 330 min yielded only 7% BPA. However, in a mixed solvent of MeOH and toluene, the analogous treatment for 70 min completely depolymerized PC to give free bisphenol A (96%) in a solid form and dimethyl carbonate (DMC) (100%) in solution.

The characteristic feature of the present methanolysis is that PC can be depolymerized to its starting monomer components BPA and DMC by the use of a catalytic amount of alkalimetal hydroxide under mild reaction conditions. The monomers can be obtained almost quantitatively in very pure states and they can be recycled as the monomers of PC and epoxy resins.

combination with an alkali catalyst is also used. An environmentally friendly strategy for methanolysis of polycarbonate to recover bisphenol A and dimethyl carbonate was recently developed in which PC could be methanolyzed in an ionic liquid without any acid or base

The methanolysis of polycarbonate using ionic liquid [Bmim][Ac] as a catalyst was studied recently by Liu et al., 2011. The effects of temperature, time, methanol dosage and [Bmim][Ac] dosage on the methanolysis reaction were examined. They concluded that methanolysis of PC to obtain its starting monomers, BPA and DMC, could occur in the presence of ionic liquid [Bmim][Ac] under moderate conditions without an acid or base catalyst. The methanolysis conversion of PC was nearly 100% and the yield of BPA was over 95% under the following conditions: m([Bmim][Ac]):m(PC) = 0.75:1, m(methanol):m(PC) = 0.75:1, a reaction temperature of 90 ◦C and a total time of 2.5 h. The ionic liquid [Bmim][Ac] could be reused up to 6 times without an apparent decrease in the conversion of PC and yield of BPA. This strategy could overcome the shortcomings associated with the traditional methods, such as the infeasibility of reusing the catalyst, equipment corrosion, tedious workup procedures and environmental problems. Moreover, the investigation on kinetics indicated that the methanolysis of PC in [Bmim][Ac] was a first-order reaction and the

A mechanism for the methanolysis of PC in the presence of ionic liquid [Bmim][Ac] is suggested in Figure 14. After PC was dissolved or swelled in the ionic liquid, it reacted with methanol to form oligomers under ionic liquid catalysis. Then, the resulting oligomers

Alkali-catalyzed methanolysis of poly[2,2-bis(4-hydroxyphenyl)propane carbonate] in a mixed solvent of methanol and toluene or dioxane was studied by the team of Oku et al., 2000 at Kyoto Institute of Technology. Treatment of PC pellets in MeOH with a catalytic amount of NaOH at 608C for 330 min yielded only 7% BPA. However, in a mixed solvent of MeOH and toluene, the analogous treatment for 70 min completely depolymerized PC to give free

The characteristic feature of the present methanolysis is that PC can be depolymerized to its starting monomer components BPA and DMC by the use of a catalytic amount of alkalimetal hydroxide under mild reaction conditions. The monomers can be obtained almost quantitatively in very pure states and they can be recycled as the monomers of PC and

bisphenol A (96%) in a solid form and dimethyl carbonate (DMC) (100%) in solution.

catalyst under moderate conditions (Liu et al., 2011).

**7.2.1 Methanolysis in the presence of ionic liquids** 

Fig. 13. Methanolysis of PC.

epoxy resins.

**7.2.2 Alkali catalyzed methanolysis** 

activation energy was 167 kJ/mol. The reaction formula was as follows:

reacted with methanol further to produce the final products, BPA and DMC.

The alkali catalysed methanolysis also studied by Liu et al., 2009 but the reaction took place in a reactor with a stirrer and a refluxing condenser. The results did not differ much because of the use of refluxing condenser. The temperature, on the other hand effected the efficiency of methanolysis in both studies with the temperature of 60οC presenting the greater rate of BPA formation.

Fig. 14. Methanolysis mechanism for PC in the presence of ionic liquid [Bmim][Ac] [Liu et al., 2011].

#### **7.2.3 Hydrolysis with high temperature steam**

Watanabe et al. 2009, found that polycarbonate was rapidly hydrolyzed in high pressure high temperature steam around the saturated pressure of water at 573 K. For 300 s (5 min) reaction time, PC completely decomposed into bisphenol A and the maximum yield of BPA was around 80%. In liquid water phase at 573 K, PC still remained even for 3000 s (50 min). The high yield of bisphenol A in high pressure steam was due to its high stability. The amount of water required for degradation was drastically reduced and thus the high pressure high temperature steam process was energetically and economically preferable.

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and the yield of bisphenol A was over 94%.

220 °C for 85 min with an EG/PC weight.

**7.2.5 Noncatalyzed glycolysis of PC in ethylene glycol** 

Fig. 16. PC glycolysis reaction pathway (Kim et al., 2009**).** 

of Polymers (PP, PS, LDPE, HDPE, PVC, PC, Nylon, PMMA) 47

Also, with the increasing of amount of H2O, the yield of BPA gradually increased and a maximum yield was obtained when using ratio of PC:H2O close to 1.5:1. When the amount of water was more than this ratio the BPA yield decreased. Moreover, under the conditions of reaction temperature 100 oC, m(PC):m(H2O) = 1:0.7, m(PC):m(NaOH) = 10:1, reaction time 8 h and using 1,4-dioxane as solvent, the hydrolysis conversion of PC was almost 100%

Kim et al. 2009 explored the depolymerization of polycarbonate waste by glycolysis using ethylene glycol without catalyst in order to get the monomer bisphenol A. This process can be considered as a green process from the viewpoint of using neither toxic solvents nor alkali catalyst. The maximum yield of BPA of 95.6% was achieved at reaction temperature

This reaction mechanism is illustrated in Fig. 16. Ethylene glycol penetrates into the PC polymer particle so that the particles are swollen. The PC is depolymerized in the solid state

Fig. 15. Apparatus hydrolysis with high temperature steam (Grause et al., 2009).

The reaction mechanism of polycarbonate hydrolysis in high pressure high temperature steam seems to be both surface and bulk erosion.

Another study of polycarbonate hydrolysis has been done by Grause et al., 2009 at Tohoku University. They studied the pyrolytic hydrolysis in the presence of earth-alkali oxides and hydroxides as catalysts. The experiments were carried out in a steam atmosphere in the presence of MgO, CaO, Mg(OH)2 or Ca(OH)2. All of these catalysts accelerated the hydrolysis of PC drastically, with MgO and Mg(OH)2 being more effective than their Ca counterparts. The differences between oxides and hydroxides were negligible indicating the same mechanism for both, oxides and hydroxides. BPA was the main product at 300 oC, with a yield of 78% obtained in the presence of MgO. At 500 oC, BPA was mainly degraded to phenol and isopropenyl phenol (IPP). It can be shown that a combined process involving PC hydrolysis at 300 oC and BPA fission at 500 oC leads to high yields of phenol and IPP and the drastic decrease of residue. The apparatus of the experimental process is shown in Fig. 15.

#### **7.2.4 Hydrolysis in other solvents but water**

Alkali-catalyzed hydrolysis of PC in a solvent in which it can substantially dissolve such as N-methyl-2-pyrrolidone, 1,4-dioxane, tetrahydrofuran or DMF were studied by Liu et al., 2009. The results showed that hydrolysis of PC could take place under moderate conditions. No BPA was detected when hydrolysis of PC was carried out under given conditions in water without co-solvents. However, when the hydrolysis was carried out under the same conditions in presence of such a solvent, the rate of hydrolysis was significantly accelerated.

Fig. 15. Apparatus hydrolysis with high temperature steam (Grause et al., 2009).

steam seems to be both surface and bulk erosion.

**7.2.4 Hydrolysis in other solvents but water** 

process is shown in Fig. 15.

The reaction mechanism of polycarbonate hydrolysis in high pressure high temperature

Another study of polycarbonate hydrolysis has been done by Grause et al., 2009 at Tohoku University. They studied the pyrolytic hydrolysis in the presence of earth-alkali oxides and hydroxides as catalysts. The experiments were carried out in a steam atmosphere in the presence of MgO, CaO, Mg(OH)2 or Ca(OH)2. All of these catalysts accelerated the hydrolysis of PC drastically, with MgO and Mg(OH)2 being more effective than their Ca counterparts. The differences between oxides and hydroxides were negligible indicating the same mechanism for both, oxides and hydroxides. BPA was the main product at 300 oC, with a yield of 78% obtained in the presence of MgO. At 500 oC, BPA was mainly degraded to phenol and isopropenyl phenol (IPP). It can be shown that a combined process involving PC hydrolysis at 300 oC and BPA fission at 500 oC leads to high yields of phenol and IPP and the drastic decrease of residue. The apparatus of the experimental

Alkali-catalyzed hydrolysis of PC in a solvent in which it can substantially dissolve such as N-methyl-2-pyrrolidone, 1,4-dioxane, tetrahydrofuran or DMF were studied by Liu et al., 2009. The results showed that hydrolysis of PC could take place under moderate conditions. No BPA was detected when hydrolysis of PC was carried out under given conditions in water without co-solvents. However, when the hydrolysis was carried out under the same conditions in presence of such a solvent, the rate of hydrolysis was significantly accelerated. Also, with the increasing of amount of H2O, the yield of BPA gradually increased and a maximum yield was obtained when using ratio of PC:H2O close to 1.5:1. When the amount of water was more than this ratio the BPA yield decreased. Moreover, under the conditions of reaction temperature 100 oC, m(PC):m(H2O) = 1:0.7, m(PC):m(NaOH) = 10:1, reaction time 8 h and using 1,4-dioxane as solvent, the hydrolysis conversion of PC was almost 100% and the yield of bisphenol A was over 94%.

#### **7.2.5 Noncatalyzed glycolysis of PC in ethylene glycol**

Kim et al. 2009 explored the depolymerization of polycarbonate waste by glycolysis using ethylene glycol without catalyst in order to get the monomer bisphenol A. This process can be considered as a green process from the viewpoint of using neither toxic solvents nor alkali catalyst. The maximum yield of BPA of 95.6% was achieved at reaction temperature 220 °C for 85 min with an EG/PC weight.

This reaction mechanism is illustrated in Fig. 16. Ethylene glycol penetrates into the PC polymer particle so that the particles are swollen. The PC is depolymerized in the solid state

Fig. 16. PC glycolysis reaction pathway (Kim et al., 2009**).** 

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**8. Chemical recycling of nylon** 

**8.1 Introduction** 

them made from nylon 6.

chemical and thermal recycling.

of Polymers (PP, PS, LDPE, HDPE, PVC, PC, Nylon, PMMA) 49

recycling of production waste or recyclates is therefore possible only to a very limited extent. When recycling polycarbonate residues, production wastes, remainders, recyclates and similar polycarbonate compositions, it is therefore desirable and essential to increase the molecular weight to a sufficient level for the projected new use. So, for example, lowmolecular production scrap from PC production for Compact Discs could be increased to the molecular weight range required for injection molding. Or the average molecular weight of PC recyclate from the de-lamination of Compact Discs should be increased sufficiently to

It was found that, surprisingly, it is possible to condense polycarbonates from waste by simple melting in a vacuum, optionally with bisphenols or suitable oligocarbonates with OH terminal groups, to produce, directly, polycarbonates of higher molecular weights.

Nylon is one of the early polymers developed by Wallace Carothers in 1935, at DuPont's research facility. Today, nylon is one of the most commonly used polymers. Nylons, also known as polyamides, can be produced by the reaction of a diamine with a dicarboxylic acid, condensation of the appropriate amino acid, ring opening of a lactam, reaction of a diamine with a diacid chloride, and reaction of a diisocyanate with a dicarboxylic acid. Nylon is a crystalline polymer with high modulus, strength, impact properties, low coefficient of friction, and resistance to abrasion. A variety of commercial nylons are available including nylon 6, nylon 11, nylon 12, nylon 6,6, nylon 6,10, and nylon 6,12. The most widely used nylons are nylon 6,6 and nylon 6. Polyamides are used most often in the form of fibers, primarily nylon 6,6 and nylon 6, although engineering applications are also of importance. Nylon 6,6 is prepared from the polymerization of adipic acid and

Nylon recycling has increased substantially in the last several years. Most recycling efforts have focused on recovery of carpet. According to the U.S. Department of Energy, about 3.5 billion lb of waste carpet are discarded each year in the United States, with about 30% of

Processing of recyclables is necessary to transform the collected materials into raw materials for the manufacture of new products. In general there are two categories for nylon recycling,

• **Chemical recycling.** Involves breaking down the molecular structure of the polymer, using chemical reactions. The products of the reaction then can be purified and used

• **Thermal recycling.** Also involves breaking down the chemical structure of the polymer. In this case, instead of relying on chemical reactions, the primary vehicle for reaction is heat. In pyrolysis, for example, the polymer (or mixture of polymers) is subjected to

hexamethylenediamine, while nylon 6 is prepared from caprolactam.

again to produce either the same or a related polymer.

allow the material to be used, as a component in the production of PC/ABS blends.

by the 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. 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.

### **7.3 Pyrolysis of PC based polymers**

Achilias et al., 2009, investigated pyrolysis of PC and PC based Waste Electric and Electronic Equipment as a means of chemical recycling of this polymer. A laboratory-scale fixed bed reactor was used and the appropriate pyrolysis temperature was selected after measuring the thermal degradation of model PC by Thermogravimetric analysis. After pyrolysis a large amount of oil was measured, together with a smaller amount of gaseous product, leaving also a solid residue. For both samples (model PC and a compact disc based on PC), the gaseous fraction consisted mainly of CO2 and CO, whereas in the liquid fraction a large amount of different phenolic compounds, including the monomer bisphenol A, was measured. It seems that recycling of used CDs by pyrolysis is a very promising technique having the potential of producing useful high-value chemicals, which may find applications in the petrochemical industry.

#### **7.4 Problems of PC recycling**

Recycled polycarbonate is usually less resilient, have 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 the use of chemicals to modify impact resistance in recycled polycarbonate. Up to 15% recycled material can safely be added to the virgin resin without significantly altering properties of the virgin material.

The nature of the compact disc (CD) does not allow it to be easily recycled. The disc is a multi-layer product consisting of PC substrate and three coatings. These coatings, aluminium, lacquer and printing, respectively make up only a small portion of the entire disc. These materials should be separated or recovered in order to recycle the polycarbonate. There are a variety of methods for the removal of paint or plating from engineering plastics, ranging from the chemical to physico-mechanical procedures. Such techniques include chemical stripping or chemical recovery (high-temperature alkaline treatment), melt filtration, mechanical abrasion, hydrolysis, liquid cyclone, compressed vibration, cryogenic grinding, dry crushing and roller pressing.

The disadvantages of PC include high melt viscosity and notch sensitivity. Used PC usually suffered from crazing caused by light, radiation and chemicals present in the service environment, which make the problem of notch sensitivity even worse.

As with other thermoplastics, the level of mechanical and physical properties of polycarbonate depends on the molecular weight. However, production waste, recyclates etc. frequently do not, or no longer, possess the required molecular weights. Direct material recycling of production waste or recyclates is therefore possible only to a very limited extent. When recycling polycarbonate residues, production wastes, remainders, recyclates and similar polycarbonate compositions, it is therefore desirable and essential to increase the molecular weight to a sufficient level for the projected new use. So, for example, lowmolecular production scrap from PC production for Compact Discs could be increased to the molecular weight range required for injection molding. Or the average molecular weight of PC recyclate from the de-lamination of Compact Discs should be increased sufficiently to allow the material to be used, as a component in the production of PC/ABS blends.

It was found that, surprisingly, it is possible to condense polycarbonates from waste by simple melting in a vacuum, optionally with bisphenols or suitable oligocarbonates with OH terminal groups, to produce, directly, polycarbonates of higher molecular weights.
