**4.3.2 Chemical recycling of PS**

18 Material Recycling – Trends and Perspectives

Polystyrene is used in solid and expanded forms both of which can be recycled. Solid PS components such as coffee cups, trays, etc. can be recycled back into alternative applications such as videocassette cases, office equipments, etc. Expanded PS (EPS) foam waste loses its foam characteristics as part of the recovery process. The recovered material can be re-gassed but the product becomes more expensive than virgin material. Instead it is used in solid form in standard molding applications. Both expanded and solid PS wastes have been successfully recycled in extruded plastic timber-lumber. Recycled PS is used to produce plant pots and desk items such as pen, pencils, etc. As with other types of plastic materials, PS recycling takes place after consideration by the industry of a number of issues including eco-efficiency, availability, corporate social responsibility, product quality=hygiene aspects, and traceability. More than a thousand tones of PS foam worldwide is being disposed off into environment as MSW. The amount is increasing every year. The booming development of electronic products has sharply increased the quantities of Waste from Electrical and Electronic Equipment (WEEE), amplifying the problem of their disposal. The solution can be found only through a modern Design For Environment (DFE) with a big attention to recycling and disassembly.

Expanded polystyrene (EPS) foam packaging, which is the familiar white material, custom molded to cushion, insulate and protect all types of products during transportation, can be recycled. EPS insulation boards used for housing and commercial construction, foodservice products like cups, plates, trays, etc. that are made of PS resin foamed to provide a unique insulating quality and loosefill packaging are accepted for recycling. Non-Foam Polystyrene products also called high impact polystyrene (HIPS), oriented polystyrene (OPS), post consumer products, post industrial products, and styrofoam (A Dow Chemical Company brand trademark for a PS foam thermal insulation product) have also been accepted for

**Before recycling,** the recyclable materials should be rinsed off for the removal of any food or dirt particles, the caps of the plastic bottles and glass jars should be thrown away and the oversized materials like cartons, milk jugs, etc. should be crushed so that they can fit into the bin and into the truck more easily. The volume of EPS is reduced by methods such as solvent volume reduction (dissolved using solvent), heating volume reduction, and

The processed EPS is used in its reduced state as an ingredient for recycled products or it is burnt to generate heat energy. A large amount of expanded PS is discharged after use at wholesale markets, supermarkets, department stores, restaurants and shops, such as electrical appliances stores, as well as at factories of machinery manufacturers. It is collected through the in-house collection of companies or by resource recycling agents and becomes a

A rather easy way of recovering polymers from a mixture of different plastics is by using an appropriate solvent to selectively dissolve the polymer and then recovering it by removal of

**4.2 Types of polystyrene accepted for recycling** 

**4.3 Recycling methods for polystyrene products** 

pulverizing volume reduction (pulverized).

**4.3.1 Recycling using the dissolution technique** 

recycling (Vilaplana et al., 2006).

recycled resource.

One of the attractive chemical recycling processes is the catalytic degradation [Kim et al., 2003] of polystyrene. This process enables to get styrene monomer (S) at relatively low temperature with a high selectivity. Modified Fe-based catalysts were employed for the catalytic degradation of EPS waste, where carbanion may lead to high selectivity of S in the catalytic degradation of PS. The yield of oil (YOil) and S (YS) were increased in the presence of Fe-based catalysts and with increasing reaction temperature. YOil and YS were obtained over Fe–K/Al2O3 at the relative low reaction temperature (400oC) 92.2 and 65.8 wt. %, respectively. The value of Ea (activation energy) is obtained as 194 kJ/mol for the thermal degradation of EPS. However, the Ea was decreased considerably to 138 kJ/mol in the presence of the catalysts (Fe–K/Al2O3).

Bajdur et al., 2002 have synthesized sulfonated derivatives of expanded PS wastes, which may be used as polyelectrolytes. Modification was conducted by means of known methods and products having various contents of sulfogroups in polymer chain were obtained. They have found that the polyelectrolytes have good flocculation properties similar to those of anionic commercial polyelectrolytes. The effect of a base catalyst, MgO, on the decomposition of PS was studied through degradation of both a monodisperse polymer and a PS mimic, 1,3,5-triphenylhexane (TPH), to determine the potential of applying base catalysts as an effective means of polymer recycling [Woo et al., 2000]. The presence of the catalyst increased the decomposition rate of the model compound but decreased the degradation rate of PS as measured by evolution of low molecular weight products. Although the model compound results suggest that the rate of initiation was enhanced in both cases by the addition of catalyst, this effect is overshadowed for the polymer by a decrease in the 'zip length' during depropagation due to termination reactions facilitated by the catalyst. Due to the small size of the model compound, this effect does not impact its observed conversion since premature termination still affords a quantifiable low molecular weight product. A decrease in the selectivity to styrene monomer in the presence of MgO was observed for both PS and TPH. They have discussed the reconciliation of their results with those of Zhang et al., 1995 based on differences in the reactor configuration used.

Degradation of PS into styrene, including monomer and dimer, was studied by Ukei et al., 2000 using solid acids and bases viz. MgO, CaO, BaO, K2O, SiO2/Al2O3, HZSM5 and active carbonas catalysts. They have found that solid bases were more effective catalysts than solid acids for the degradation of PS into styrene. This was attributed to differences in the degradation mechanisms of PS over solid acids and bases. Among the solid bases employed, BaO was found to be the most effective catalyst and about 90 wt. % of PS was converted into styrene when thermally degraded PS was admitted to BaO powder at 350oC.

Recent Advances in the Chemical Recycling

**4.3.3 Mechanism of polystyrene cracking** 

Fig. 2. Mechanism of PS cracking.

had almost no effect on the thermal decomposition of EPS.

to chain end unsaturation and neighboring ring protonation (Figure 2).

of Polymers (PP, PS, LDPE, HDPE, PVC, PC, Nylon, PMMA) 21

butylcumyl peroxide was less effective than DCP. On the other hand, di-tert-butyl peroxide

Radical depolymerization of neat PS samples produces large quantities of monomer (styrene) and chain-end backbiting yields substantial amounts of dimer and trimer. Polymer decomposition proceeds by entirely different processes when a catalyst is present. The formation of the primary PS catalytic cracking volatile products can be explained by initial electrophilic attack on polymer aromatic rings by protons. Protons preferentially attack the *ortho* and *para* ring positions because the aliphatic polymer backbone is an electron-releasing group for the aromatic rings. Most volatile products can be derived from mechanisms beginning with ring protonation. Thermal decomposition of *ortho*-protonated aromatic rings in the polymer chain (1) can lead directly to the liberation of benzene, the primary catalytic cracking product, or may result in chain shortening. Benzene cannot be obtained directly from *para*-protonated aromatic rings in the polymer. However, *para*-protonated rings can react with neighboring polymer chains to yield the same chain scission products that are formed by *ortho*-protonation. The macro cation remaining after benzene evolution (2) may undergo chain shortening β-scission to produce (3) and an unsaturated chain end, rearrange to form an internal double bond and protonate a neighboring aromatic ring (either by intra- or intermolecular proton transfer), cyclize to form an indane structure, or abstract a hydride to produce a saturated chain segment. The substantial quantities of indanes obtained by PS catalytic cracking suggests that cyclization of (2) to form indane structures is a favored process. A consequence of chain unsaturation resulting from (2) might be the formation of conjugated polyene segments that may subsequently cyclize to form naphthalenes. Decomposition of (3), which might be formed from (1) or (2), can result in the formation of styrene or may lead

Koji et al., 1998 have succeded to obtain PS foam, which can be recycled into styrene by mixing a PS with a basic metal oxide being a catalytic decomposition catalyst and foaming the mixture with an inert blowing agent. When it is wasted, it can be recycled into styrene by decomposing it by heating to 300–450oC in a nonoxidizing atmosphere. The basic oxide is Na2O, MgO, CaO or the like and among them CaO is desirable. When the basic metal oxide carried by porous inorganic filler is used, it can exhibit improved effectiveness desirably. The blowing agent used is a nitrogen gas, a chlorofluorocarbon, propane or the like.

Lee et al., 2002, have studied several solid acids, such as silica-alumina, HZSM-5, HY, mordenite, and clinoptilolite as catalysts and screened their performances in the catalytic degradation of PS. The clinoptilolite catalysts (HNZ, HSCLZ) showed good catalytic performance for the degradation of PS with selectivity to aromatics more than 99%. Styrene is the major product and ethylbenzene is the second most abundant one in the liquid product. The increase of acidity favored the production of ethylbenzene by promoting the hydrogenation reaction of styrene. Higher selectivity to styrene is observed at higher temperatures. An increase of contact time by reducing nitrogen gas flow rate enhanced the selectivity to ethylbenzene. Thus a designed operation including acidity of catalyst, reaction temperature, and contact time will be necessary to control the product distribution between styrene monomer and ethylbenzene.

Ke et al., 2005 studied the degradation of PS in various supercritical solvents like benzene, toluene, xylene, etc. at 310–370oC and 6.0MPa pressure. It was found that PS has been successfully depolymerized into monomer, dimmer, and other products in a very short reaction time with high conversion. Toluene used as supercritical solvent was more effective than other solvents such as benzene, ethylbenzene, and p-xylene for the recovery of styrene from PS, though the conversions of PS were similar in all the above solvents. The highest yield of styrene (77 wt%) obtained from PS in supercritical toluene at 360oC for 20 min.

Subcritical water is a benign and effective media for polymer degradation. Suyama et al., 2010 have found that on subcritical water treatment in the presence of an aminoalcohol, unsaturated polyesters crosslinked with styrene were decrosslinked, and a linear polystyrene derivative bearing hydroxy-terminated side-chains was recovered. After modification of the hydroxy groups with maleic anhydride, the polystyrene derivative was re-crosslinked with styrene to form a networked structure again. The resulting solid was degradable by subcritical water treatment in the presence of the aminoalcohol to give another polystyrene derivative bearing hydroxy groups. These processes could be repeated successfully, demonstrating the applicability as a novel recycling system of thermosetting resins. The polystyrene derivative was also re-crosslinked again on heating with an alternative copolymer of styrene and maleic anhydride due to the formation of linkage between the hydroxy groups and carboxylic anhydride moieties.

In order to reduce the consumption of energy and get oligostyrene of several thousands of molecular weight, which can be used as a kind of fuel oil, the thermal decomposition of EPS with a-methylstyrene as a chain-transfer agent was studied by Xue et al., 2004, at a temperature about 200oC. Three kinds of organic peroxides were used as radical accelerators. They found that the addition of dicumyl peroxide (DCP) enhanced the thermal decomposition of EPS even at lower temperature, about 140 oC, but the addition of tertbutylcumyl peroxide was less effective than DCP. On the other hand, di-tert-butyl peroxide had almost no effect on the thermal decomposition of EPS.

#### **4.3.3 Mechanism of polystyrene cracking**

20 Material Recycling – Trends and Perspectives

Koji et al., 1998 have succeded to obtain PS foam, which can be recycled into styrene by mixing a PS with a basic metal oxide being a catalytic decomposition catalyst and foaming the mixture with an inert blowing agent. When it is wasted, it can be recycled into styrene by decomposing it by heating to 300–450oC in a nonoxidizing atmosphere. The basic oxide is Na2O, MgO, CaO or the like and among them CaO is desirable. When the basic metal oxide carried by porous inorganic filler is used, it can exhibit improved effectiveness desirably.

Lee et al., 2002, have studied several solid acids, such as silica-alumina, HZSM-5, HY, mordenite, and clinoptilolite as catalysts and screened their performances in the catalytic degradation of PS. The clinoptilolite catalysts (HNZ, HSCLZ) showed good catalytic performance for the degradation of PS with selectivity to aromatics more than 99%. Styrene is the major product and ethylbenzene is the second most abundant one in the liquid product. The increase of acidity favored the production of ethylbenzene by promoting the hydrogenation reaction of styrene. Higher selectivity to styrene is observed at higher temperatures. An increase of contact time by reducing nitrogen gas flow rate enhanced the selectivity to ethylbenzene. Thus a designed operation including acidity of catalyst, reaction temperature, and contact time will be necessary to control the product distribution between

Ke et al., 2005 studied the degradation of PS in various supercritical solvents like benzene, toluene, xylene, etc. at 310–370oC and 6.0MPa pressure. It was found that PS has been successfully depolymerized into monomer, dimmer, and other products in a very short reaction time with high conversion. Toluene used as supercritical solvent was more effective than other solvents such as benzene, ethylbenzene, and p-xylene for the recovery of styrene from PS, though the conversions of PS were similar in all the above solvents. The highest yield of styrene (77 wt%) obtained from PS in supercritical toluene at 360oC for 20 min.

Subcritical water is a benign and effective media for polymer degradation. Suyama et al., 2010 have found that on subcritical water treatment in the presence of an aminoalcohol, unsaturated polyesters crosslinked with styrene were decrosslinked, and a linear polystyrene derivative bearing hydroxy-terminated side-chains was recovered. After modification of the hydroxy groups with maleic anhydride, the polystyrene derivative was re-crosslinked with styrene to form a networked structure again. The resulting solid was degradable by subcritical water treatment in the presence of the aminoalcohol to give another polystyrene derivative bearing hydroxy groups. These processes could be repeated successfully, demonstrating the applicability as a novel recycling system of thermosetting resins. The polystyrene derivative was also re-crosslinked again on heating with an alternative copolymer of styrene and maleic anhydride due to the formation of linkage

In order to reduce the consumption of energy and get oligostyrene of several thousands of molecular weight, which can be used as a kind of fuel oil, the thermal decomposition of EPS with a-methylstyrene as a chain-transfer agent was studied by Xue et al., 2004, at a temperature about 200oC. Three kinds of organic peroxides were used as radical accelerators. They found that the addition of dicumyl peroxide (DCP) enhanced the thermal decomposition of EPS even at lower temperature, about 140 oC, but the addition of tert-

between the hydroxy groups and carboxylic anhydride moieties.

The blowing agent used is a nitrogen gas, a chlorofluorocarbon, propane or the like.

styrene monomer and ethylbenzene.

Radical depolymerization of neat PS samples produces large quantities of monomer (styrene) and chain-end backbiting yields substantial amounts of dimer and trimer. Polymer decomposition proceeds by entirely different processes when a catalyst is present. The formation of the primary PS catalytic cracking volatile products can be explained by initial electrophilic attack on polymer aromatic rings by protons. Protons preferentially attack the *ortho* and *para* ring positions because the aliphatic polymer backbone is an electron-releasing group for the aromatic rings. Most volatile products can be derived from mechanisms beginning with ring protonation. Thermal decomposition of *ortho*-protonated aromatic rings in the polymer chain (1) can lead directly to the liberation of benzene, the primary catalytic cracking product, or may result in chain shortening. Benzene cannot be obtained directly from *para*-protonated aromatic rings in the polymer. However, *para*-protonated rings can react with neighboring polymer chains to yield the same chain scission products that are formed by *ortho*-protonation. The macro cation remaining after benzene evolution (2) may undergo chain shortening β-scission to produce (3) and an unsaturated chain end, rearrange to form an internal double bond and protonate a neighboring aromatic ring (either by intra- or intermolecular proton transfer), cyclize to form an indane structure, or abstract a hydride to produce a saturated chain segment. The substantial quantities of indanes obtained by PS catalytic cracking suggests that cyclization of (2) to form indane structures is a favored process. A consequence of chain unsaturation resulting from (2) might be the formation of conjugated polyene segments that may subsequently cyclize to form naphthalenes. Decomposition of (3), which might be formed from (1) or (2), can result in the formation of styrene or may lead to chain end unsaturation and neighboring ring protonation (Figure 2).

Fig. 2. Mechanism of PS cracking.

Recent Advances in the Chemical Recycling

and photochemical oxidant precursors.

PS and commercial products appear in Table 12.

was found to have no effect on PS degradation rate.

styrene.

of Polymers (PP, PS, LDPE, HDPE, PVC, PC, Nylon, PMMA) 23

A swirling fluidized-bed reactor (0.0508m ID and 1.5m in height) has been developed to recover the styrene monomer [Lee et al., 2003] and valuable chemicals effectively from the PS waste, since it can control the residence time of the feed materials and enhance the uniformity of the temperature distribution. To increase the selectivity and yield of styrene monomer in the product, catalyst such as Fe2O3, BaO, or HZSM-5 have been used. It has been found that the reaction time and temperature can be reduced profoundly by adding the solid catalyst. The swirling fluidization mode makes the temperature fluctuations more periodic and persistent, which can increase the uniformity of temperature distribution by reducing the temperature gradient in the reactor. The yields of styrene monomer as well as oil products have increased with increasing the ratio of swirling gas, but exhibited their

The thermal degradation of real municipal waste plastics (MWP) obtained from Sapporo, Japan and model mixed plastics was carried out at 430oC in atmospheric pressure by batch operation (Bhaskar et al., 2003). The resources and environmental effects assessed over the life of each of the packaging, includes fossil fuel consumption, greenhouse gas emissions,

Achilias et al. 2007 investigated catalytic and non-catalytic pyrolysis experiments in a fixed bed reactor using either model polymer or commercial waste products as the feedstock. The liquid fraction produced from all the pyrolysis experiments consisted mainly of the styrene monomer and this was subjected to repolymerization without any further purification in a DSC with AIBN initiator. A basic (BaO) and an acidic commercial FCC catalyst were examined in relation to the yield and composition of gaseous and liquid products. Aromatic compounds identified in the liquid fraction of the thermal and catalytic pyrolysis of model

Results show that this product can be polymerized to produce a polymer similar to the original PS. However, it was found that other aromatic compounds included in this fraction could act as chain transfer agents, lowering the average molecular weight of the polymer produced and contributing to a lower Tg polymer. Therefore, it seems that the polymer can be reproduced but with inferior properties compared to a polymer prepared from neat

A general model for polymer degradation by concurrent random and chain-end processes was developed by Sterling et al., 2001 using continuous distribution kinetics. Population balance equations based on fundamental, mechanistic free radical reactions were solved analytically by the moment method. The model, applicable to any molecular weight distribution (MWD), reduces to the cases of independent random or chain-end scission. Polystyrene degradation experiments in mineral oil solution at 275–350 oC supported the model and determined reaction rate parameters. The degradation proceeded to moderate extents requiring a MW-dependent random scission rate coefficient. Polystyrene random scission activation energy was 7.0 kcal/mol, which agrees well with other thermolysis investigations, but is lower than that found by pyrolysis due to fundamental differences between the processes. Magnesium oxide, added as a heterogeneous catalyst in solution,

Thermal and thermo-oxidative degradation of PS in the presence of ammonium sulfate [Zhu et al., 1998] was studied with thermogravimetry and FTIR. TGA results indicated that

maximum values with increasing the total volume flow rate of gas.

Hydride abstraction by (3) would result in a saturated chain end. The lack of significant styrene production from any of the PS-catalyst samples suggests that β-scission of (3) to form styrene is not a dominant decomposition pathway at low temperatures. Chain end unsaturation derived from (3) may result in formation of indenes, which were detected in substantial amounts only when HZSM-5 catalyst was present. The restricted volume of the HZSM-5 channels apparently inhibits hydride abstraction pathways for (3), which results in increased production of indenes and styrene for PS-HZSM-5. Protonation of aromatic rings adjacent to methyl-terminated chain ends (4) can result in the formation of alkyl benzenes, propene, and benzene, depending on how the macro cation decomposes.

#### **4.3.4 Thermo-chemical recycling of PS**

Thermochemical recycling techniques such as pyrolysis are usually applied. Thus, PS can be thermally depolymerized at relatively low temperatures in order to obtain the monomer styrene with a high selectivity.

Arandes et al., 2003 have studied the thermal cracking of PS and polystyrene-butadiene (PS-BD) on mesoporous silica which has no measurable acidity. Although the content of PS in domestic plastic wastes is approximately 10wt. %, less attention has been paid to the cracking of dissolved PS than to the cracking of dissolved polyolefins. The kinetic characteristics of PS cracking described by Arandes et al. are different to those of polyolefins and the ideal aim of its valorization is the recovery of the styrene monomer. Bockhorn et al., 1999 and Kruse et al., 2001 made an analysis of the reactions involved in the mechanism of PS cracking and developed a detailed mechanistic model for the polymer degradation. Faravelli et al., 2003 presented a detailed kinetic model for the thermal degradation of PE – PS mixtures.

The technology for thermal cracking that has been more widely studied and that has been tested at larger scale is that based on a fluidized bed, in which the plastics are fed in the solid state and sand is used for helping fluidization [Scheirs and Kaminsky, 2006; Milne et al., 1999; Westerhourt, 1997]. The design of fluidized beds used at laboratory or pilot plant scale has been carried out on the basis, that the kinetics of pyrolysis of plastics is subjected to great uncertainty caused by factors such as heterogeneity of the material, synergy in the cracking of different constituents, and limitations to heat and mass transfer. These factors prevent obtaining kinetics that is reliable for the design of the reactor at temperatures of industrial interest (above 450oC) [Mehta et al., 1995]. Furthermore, this strategy is suitable for its development in a refinery by using the existing equipment and by optimizing the possibilities of incorporating the products either into the market (subsequent to fuel reformulation) or into the production process itself (subsequent to monomer purification). An additional problem in the cracking of PS is the rapid deactivation of the catalyst caused by the coke formed on the acid sites, which is favored by the aromatic nature of styrene and its high C/H ratio.

A conical spouted bed reactor (CSBR) has been used for the kinetic study of PS pyrolysis in the 450–550oC range [Aguado et al., 2003] and the results have been compared with those obtained by thermogravimetry (TGA) and in a microreactor (MR) of very high sample heating rate. The comparison proves the advantages of the gas–solid contact of this new reactor for the kinetic study of pyrolysis of plastics at high temperature, which stem from the high heat transfer rate between gas and solid and from the fact that particle agglomeration is avoided.

Hydride abstraction by (3) would result in a saturated chain end. The lack of significant styrene production from any of the PS-catalyst samples suggests that β-scission of (3) to form styrene is not a dominant decomposition pathway at low temperatures. Chain end unsaturation derived from (3) may result in formation of indenes, which were detected in substantial amounts only when HZSM-5 catalyst was present. The restricted volume of the HZSM-5 channels apparently inhibits hydride abstraction pathways for (3), which results in increased production of indenes and styrene for PS-HZSM-5. Protonation of aromatic rings adjacent to methyl-terminated chain ends (4) can result in the formation of alkyl benzenes,

Thermochemical recycling techniques such as pyrolysis are usually applied. Thus, PS can be thermally depolymerized at relatively low temperatures in order to obtain the monomer

Arandes et al., 2003 have studied the thermal cracking of PS and polystyrene-butadiene (PS-BD) on mesoporous silica which has no measurable acidity. Although the content of PS in domestic plastic wastes is approximately 10wt. %, less attention has been paid to the cracking of dissolved PS than to the cracking of dissolved polyolefins. The kinetic characteristics of PS cracking described by Arandes et al. are different to those of polyolefins and the ideal aim of its valorization is the recovery of the styrene monomer. Bockhorn et al., 1999 and Kruse et al., 2001 made an analysis of the reactions involved in the mechanism of PS cracking and developed a detailed mechanistic model for the polymer degradation. Faravelli et al., 2003

The technology for thermal cracking that has been more widely studied and that has been tested at larger scale is that based on a fluidized bed, in which the plastics are fed in the solid state and sand is used for helping fluidization [Scheirs and Kaminsky, 2006; Milne et al., 1999; Westerhourt, 1997]. The design of fluidized beds used at laboratory or pilot plant scale has been carried out on the basis, that the kinetics of pyrolysis of plastics is subjected to great uncertainty caused by factors such as heterogeneity of the material, synergy in the cracking of different constituents, and limitations to heat and mass transfer. These factors prevent obtaining kinetics that is reliable for the design of the reactor at temperatures of industrial interest (above 450oC) [Mehta et al., 1995]. Furthermore, this strategy is suitable for its development in a refinery by using the existing equipment and by optimizing the possibilities of incorporating the products either into the market (subsequent to fuel reformulation) or into the production process itself (subsequent to monomer purification). An additional problem in the cracking of PS is the rapid deactivation of the catalyst caused by the coke formed on the

presented a detailed kinetic model for the thermal degradation of PE – PS mixtures.

acid sites, which is favored by the aromatic nature of styrene and its high C/H ratio.

A conical spouted bed reactor (CSBR) has been used for the kinetic study of PS pyrolysis in the 450–550oC range [Aguado et al., 2003] and the results have been compared with those obtained by thermogravimetry (TGA) and in a microreactor (MR) of very high sample heating rate. The comparison proves the advantages of the gas–solid contact of this new reactor for the kinetic study of pyrolysis of plastics at high temperature, which stem from the high heat transfer rate between gas and solid and from the fact that particle

propene, and benzene, depending on how the macro cation decomposes.

**4.3.4 Thermo-chemical recycling of PS** 

styrene with a high selectivity.

agglomeration is avoided.

A swirling fluidized-bed reactor (0.0508m ID and 1.5m in height) has been developed to recover the styrene monomer [Lee et al., 2003] and valuable chemicals effectively from the PS waste, since it can control the residence time of the feed materials and enhance the uniformity of the temperature distribution. To increase the selectivity and yield of styrene monomer in the product, catalyst such as Fe2O3, BaO, or HZSM-5 have been used. It has been found that the reaction time and temperature can be reduced profoundly by adding the solid catalyst. The swirling fluidization mode makes the temperature fluctuations more periodic and persistent, which can increase the uniformity of temperature distribution by reducing the temperature gradient in the reactor. The yields of styrene monomer as well as oil products have increased with increasing the ratio of swirling gas, but exhibited their maximum values with increasing the total volume flow rate of gas.

The thermal degradation of real municipal waste plastics (MWP) obtained from Sapporo, Japan and model mixed plastics was carried out at 430oC in atmospheric pressure by batch operation (Bhaskar et al., 2003). The resources and environmental effects assessed over the life of each of the packaging, includes fossil fuel consumption, greenhouse gas emissions, and photochemical oxidant precursors.

Achilias et al. 2007 investigated catalytic and non-catalytic pyrolysis experiments in a fixed bed reactor using either model polymer or commercial waste products as the feedstock. The liquid fraction produced from all the pyrolysis experiments consisted mainly of the styrene monomer and this was subjected to repolymerization without any further purification in a DSC with AIBN initiator. A basic (BaO) and an acidic commercial FCC catalyst were examined in relation to the yield and composition of gaseous and liquid products. Aromatic compounds identified in the liquid fraction of the thermal and catalytic pyrolysis of model PS and commercial products appear in Table 12.

Results show that this product can be polymerized to produce a polymer similar to the original PS. However, it was found that other aromatic compounds included in this fraction could act as chain transfer agents, lowering the average molecular weight of the polymer produced and contributing to a lower Tg polymer. Therefore, it seems that the polymer can be reproduced but with inferior properties compared to a polymer prepared from neat styrene.

A general model for polymer degradation by concurrent random and chain-end processes was developed by Sterling et al., 2001 using continuous distribution kinetics. Population balance equations based on fundamental, mechanistic free radical reactions were solved analytically by the moment method. The model, applicable to any molecular weight distribution (MWD), reduces to the cases of independent random or chain-end scission. Polystyrene degradation experiments in mineral oil solution at 275–350 oC supported the model and determined reaction rate parameters. The degradation proceeded to moderate extents requiring a MW-dependent random scission rate coefficient. Polystyrene random scission activation energy was 7.0 kcal/mol, which agrees well with other thermolysis investigations, but is lower than that found by pyrolysis due to fundamental differences between the processes. Magnesium oxide, added as a heterogeneous catalyst in solution, was found to have no effect on PS degradation rate.

Thermal and thermo-oxidative degradation of PS in the presence of ammonium sulfate [Zhu et al., 1998] was studied with thermogravimetry and FTIR. TGA results indicated that

Recent Advances in the Chemical Recycling

depended on the degradation temperature of the plastics.

of Polymers (PP, PS, LDPE, HDPE, PVC, PC, Nylon, PMMA) 25

Thermal degradation of PS has been investigated in the presence of water [Beltrame et al., 1997] under subcritical conditions (hydrous pyrolysis). The experiments were carried out in closed systems under inert atmosphere, in the temperature range 300–350 oC, at pressures up to 18 MPa, for 1–120 h. The products obtained, separated as gases, volatiles, and heavy compounds. The results showed that the presence of water increases the yields of the volatile products, mainly in the first steps of the pyrolytic process, and leads to higher yields of monomer. This latter observation suggests a lowering of the secondary reactions extent. The catalytic degradation of waste plastics such as HDPE, LDPE, PP and PS over spent fluid catalytic cracking (FCC) catalyst was also carried out at atmospheric pressure with a stirred semi-batch operation at 400 oC [Lee et al., 2002]. The objective was to investigate the influence of plastic types on the yield, liquid product rate, and liquid product distribution for catalytic degradation. The catalytic degradation of waste PE and PP with polyolefinic structure exhibited the liquid yield of 80–85% and the solid yield of below 1%, whereas that of waste PS with polycyclic structure produced much more liquid, solid products, and much less gas products. Accumulative liquid product weight by catalytic degradation strongly

In accordance with the option of recycling plastics into fuels by dissolving them in standard feedstocks [Puente et al., 1998] for the process of catalytic cracking of hydrocarbons, FCC, and various acidic catalysts (zeolites ZSM-5, mordenite, Y, and a sulfur-promoted zirconia) were tested in the conversion of PS dissolved into inert benzene at 550 oC in a fluidized- bed batch reactor. Experiments were performed with very short contact times of up to 12 sec. Main products were in the gasoline range, including benzene, toluene, ethylbenzene, styrene, and minor amounts of C9–12 aromatics and light C5– compounds. Coke was always produced in very significant amounts. Even though sulphur promoted zirconia is highly acidic, the low proportion of Brönsted-type acid sites does not allow the occurrence of secondary styrene reactions. It was shown that most favorable product distributions (higher yields of desirable products) are obtained on equilibrium commercial FCC catalysts. PS can be recycled into styrene monomer in association with some other aromatics, from which styrene can be converted to biodegradable plastic such as polyhydroxyalkanoates (PHA). Recently scientists have achieved 10% yield of PHA from PS [Ward et al., 2006]. The chain length of PHA produced was 10. The yield and composition of oils and gases derived from the pyrolysis and catalytic pyrolysis of PS has been investigated. The pyrolysis and catalytic pyrolysis was carried out in a fixed bed reactor. Two catalysts were used, zeolite ZSM-5 and Y-zeolite and the influence of the temperature of the catalyst, the amount of catalyst loading, and the use of a mixture of the two catalysts was investigated. The main product from the uncatalyzed pyrolysis of PS was oil consisting mostly of styrene and other aromatic hydrocarbons like toluene and ethylbenzene. In the presence of either catalyst an increase in the yield of gas and decrease in the amount of oil production was found, but there was significant formation of carbonaceous coke on the catalyst. Increasing the temperature of the Y-zeolite catalyst and also the amount of catalyst in the catalyst bed resulted in a decrease in the yield of oil and increase in the yield of gas. Oil derived from the catalytic pyrolysis of PS contain aromatic compounds such as single ring compounds like benzene, toluene, styrene, m-xylene, o-xylene, p-xylene, ethylmethylbenzene, propenylbenzene, methylstyrene; two ring compounds like indene, methylindene, naphthalene, 2-

ammonium sulfate accelerated thermal degradation in nitrogen but delayed thermooxidative degradation of PS in air. IR analysis of tetrahydrofuran extracts, from the samples degraded at 340οC and of residues after thermal treatment at 340οC in a furnace, showed that the acceleration of thermal degradation and the suppression of thermo-oxidative degradation were due to sulfonation and oxidation of ammonium sulfate and its decomposition products, and formation of unsaturated structures in the PS chain.


Table 12. Aromatic compounds identified in the liquid fraction of the thermal and catalytic pyrolysis of model polystyrene and commercial products based on polystyrene (wt.-% on liquid produced) (Achilias et al., 2007).

ammonium sulfate accelerated thermal degradation in nitrogen but delayed thermooxidative degradation of PS in air. IR analysis of tetrahydrofuran extracts, from the samples degraded at 340οC and of residues after thermal treatment at 340οC in a furnace, showed that the acceleration of thermal degradation and the suppression of thermo-oxidative degradation were due to sulfonation and oxidation of ammonium sulfate and its

BaO

<sup>X</sup> 2.0 0.3 5.0 2.5 0.2

Catalytic FCC

63.9 69.6 45.1 53.3 70.0

2.0 2.4 5.0 5.6 2.5

0.5 1.1 - 1.9 1.5

2.1 2.6 6.3 5.9 2.3



2.2 0.7 0.9 2.1 0.8

1.1 0.4 0.5 1.4 0.4

0.6 0.6 0.5 2.8 0.8

0.7 0.4 1.1 - 0.7

Plastic container

Plastic glass (EPS)

decomposition products, and formation of unsaturated structures in the PS chain.

2,4-Diphenyl-1-butene (**dimer**) 14.0 18.4 1.9 11.9 9.0

2,4,6-Triphenyl-1-hexene(**trimer**) 2.2 1.8 0.3 3.5 5.0

Other aromatic compounds 8.7 1.7 14.9 8.9 6.4 Table 12. Aromatic compounds identified in the liquid fraction of the thermal and catalytic pyrolysis of model polystyrene and commercial products based on polystyrene (wt.-% on

Compounds, chemical formula Thermal Catalytic

Styrene (**monomer**)

Ethylbenzene

Xylene

Cumene

a-Methylstyrene

Indane, Ιndene, etc.

1,2-Diphenylethane

1,3-Diphenylpropane

1,1'-Diphenylpropene

1-methyl-1,2Diphenylethan

liquid produced) (Achilias et al., 2007).

Benzene, Toluene ;

Thermal degradation of PS has been investigated in the presence of water [Beltrame et al., 1997] under subcritical conditions (hydrous pyrolysis). The experiments were carried out in closed systems under inert atmosphere, in the temperature range 300–350 oC, at pressures up to 18 MPa, for 1–120 h. The products obtained, separated as gases, volatiles, and heavy compounds. The results showed that the presence of water increases the yields of the volatile products, mainly in the first steps of the pyrolytic process, and leads to higher yields of monomer. This latter observation suggests a lowering of the secondary reactions extent.

The catalytic degradation of waste plastics such as HDPE, LDPE, PP and PS over spent fluid catalytic cracking (FCC) catalyst was also carried out at atmospheric pressure with a stirred semi-batch operation at 400 oC [Lee et al., 2002]. The objective was to investigate the influence of plastic types on the yield, liquid product rate, and liquid product distribution for catalytic degradation. The catalytic degradation of waste PE and PP with polyolefinic structure exhibited the liquid yield of 80–85% and the solid yield of below 1%, whereas that of waste PS with polycyclic structure produced much more liquid, solid products, and much less gas products. Accumulative liquid product weight by catalytic degradation strongly depended on the degradation temperature of the plastics.

In accordance with the option of recycling plastics into fuels by dissolving them in standard feedstocks [Puente et al., 1998] for the process of catalytic cracking of hydrocarbons, FCC, and various acidic catalysts (zeolites ZSM-5, mordenite, Y, and a sulfur-promoted zirconia) were tested in the conversion of PS dissolved into inert benzene at 550 oC in a fluidized- bed batch reactor. Experiments were performed with very short contact times of up to 12 sec. Main products were in the gasoline range, including benzene, toluene, ethylbenzene, styrene, and minor amounts of C9–12 aromatics and light C5– compounds. Coke was always produced in very significant amounts. Even though sulphur promoted zirconia is highly acidic, the low proportion of Brönsted-type acid sites does not allow the occurrence of secondary styrene reactions. It was shown that most favorable product distributions (higher yields of desirable products) are obtained on equilibrium commercial FCC catalysts.

PS can be recycled into styrene monomer in association with some other aromatics, from which styrene can be converted to biodegradable plastic such as polyhydroxyalkanoates (PHA). Recently scientists have achieved 10% yield of PHA from PS [Ward et al., 2006]. The chain length of PHA produced was 10. The yield and composition of oils and gases derived from the pyrolysis and catalytic pyrolysis of PS has been investigated. The pyrolysis and catalytic pyrolysis was carried out in a fixed bed reactor. Two catalysts were used, zeolite ZSM-5 and Y-zeolite and the influence of the temperature of the catalyst, the amount of catalyst loading, and the use of a mixture of the two catalysts was investigated. The main product from the uncatalyzed pyrolysis of PS was oil consisting mostly of styrene and other aromatic hydrocarbons like toluene and ethylbenzene. In the presence of either catalyst an increase in the yield of gas and decrease in the amount of oil production was found, but there was significant formation of carbonaceous coke on the catalyst. Increasing the temperature of the Y-zeolite catalyst and also the amount of catalyst in the catalyst bed resulted in a decrease in the yield of oil and increase in the yield of gas. Oil derived from the catalytic pyrolysis of PS contain aromatic compounds such as single ring compounds like benzene, toluene, styrene, m-xylene, o-xylene, p-xylene, ethylmethylbenzene, propenylbenzene, methylstyrene; two ring compounds like indene, methylindene, naphthalene, 2-

Recent Advances in the Chemical Recycling

(Al-Salem *et al,* 2009).

**5.2.1 Pyrolysis** 

**5.2 Thermolysis schemes and technologies** 

thermal processes (Al-Salem *et al*, 2009).

of Polymers (PP, PS, LDPE, HDPE, PVC, PC, Nylon, PMMA) 27

radical centre (β-scission) to regenerate an abstracting radical and to produce a molecule with a double bond (a molecule with a double bond involving the carbon atom that had been the radical centre). Sample size and surface area to volume ratio of the melt have a significant influence on the rate and relative importance of the various mechanisms of polymer degradation. In pyrolysis, which is normally done on micro-scale, only random initiation and intermolecular transfer were reported to be important. Conversely, on milligram scale of polyethylene charges and samples, intermolecular transfer of hydrogen atoms via abstraction by free radicals was considered to be the predominant transfer mechanism to produce volatiles. There is also a growing interest in developing value added

The development of value added recycling technologies is highly desirable as it would increase the economic incentive to recycle polymers. Several methods for chemical recycling are presently in use, such as direct chemical treatment involving gasification, smelting by blast furnace or coke oven, and degradation by liquefaction. The main advantage of chemical recycling is the possibility of treating heterogeneous and contaminated polymers with limited use of pre-treatment. Petrochemical plants are much greater in size (6–10 times) than plastic manufacturing plants. It is essential to utilize petrochemical plants in supplementing their usual feedstock by using plastic solid wastes (PSW) derived feedstock

Thermolysis is the treatment in the presence of heat under controlled temperatures without catalysts. Thermolysis processes can be divided into advanced thermo-chemical or pyrolysis (thermal cracking in an inert atmosphere), gasification (in the sub-stoichiometric presence of

Thermal degradation processes allow obtaining a number of constituting molecules, combustible gases and/or energy, with the reduction of landfilling as an added advantage. The pyrolysis process is an advanced conversion technology that has the ability to produce a clean, high calorific value gas from a wide variety of waste and biomass streams. The hydrocarbon content of the waste is converted into a gas, which is suitable for utilisation in either gas engines, with associated electricity generation, or in boiler applications without the need for flue gas treatment. This process is capable of treating many different solid hydrocarbon based wastes whilst producing a clean fuel gas with a high calorific value. This gas will typically have a calorific value of 22–30 MJ/m3 depending on the waste material being processed. Solid char is also produced from the process, which contains both carbon and the mineral content of the original feed material. The char can either be further processed onsite to release the energy content of the carbon, or utilized offsite in other

The main pyrolysis units and technologies on an industrial scale include PYROPLEQ (rotary drum), Akzo (circulating fluidized bed), NRC (melt furnace), ConTherm technology (rotary drum), PKA pyrolysis (rotary drum), PyroMelt (melt furnace), BP (circulating fluidized

air usually leading to CO and CO2 production) and hydrogenation (hydrocracking).

products such as synthetic lubricants via PE thermal degradation.

methylnaphthalene, 1-methylnaphthalene, biphenyl, methylbiphenyl, dimethylnaphthalene, trimethylnaphthalene, tetramethylnaphthalene, ethylbiphenyl; three ring compounds like phenanthrenes and four ring compounds like pyrenes and chrysenes.

### **4.4 Future prospectus**

Some future prospectus of PS recycling include (Maharana et al., 2007).

