**4. Chemical recycling of polystyrene**

### **4.1 Introduction**

Polystyrene (PS) is widely used in the manufacture of many products due to its favorable properties such as good strength, light weight, and durability and is the material of choice for packaging various electronics and other fragile items. In general, PS accounts for about 9-10% of the plastic waste in municipal solid waste (MSW). In the past several years, PS has received much public and media attention. Polystyrene has been described by various environmental groups as being nondegradable, nonrecyclable, toxic when burned, landfillchoking, ozone-depleting, wildlife-killing, and even carcinogenic. These misconceptions regarding PS have resulted in boycotts and bans in various localities. Actually, PS comprises less than 0.5% of the solid waste going to landfills.

Recent Advances in the Chemical Recycling

solvent is vacuum-evaporated and re-used.

**4.3.2 Chemical recycling of PS** 

presence of the catalysts (Fe–K/Al2O3).

of Polymers (PP, PS, LDPE, HDPE, PVC, PC, Nylon, PMMA) 19

that solvent. In this sense a naturally occurring compound, i.e. limonene (occurring in citrus fruits) has been successfully used to dissolve EPS (Achilias et al., 2009). This solvent has the ability to dissolve EPS in large amounts safely and with negligible degradation of the polymer's performance properties. Conventional melt separation methods cause a large drop in the polymer's molecular weight due to thermal degradation. Consequently dissolved PS can be precipitated through the addition of a non-solvent in the mixture. The

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

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.

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.

#### **4.2 Types of polystyrene accepted for recycling**

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 recycling (Vilaplana et al., 2006).

#### **4.3 Recycling methods for polystyrene products**

**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 pulverizing volume reduction (pulverized).

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 recycled resource.

#### **4.3.1 Recycling using the dissolution technique**

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 that solvent. In this sense a naturally occurring compound, i.e. limonene (occurring in citrus fruits) has been successfully used to dissolve EPS (Achilias et al., 2009). This solvent has the ability to dissolve EPS in large amounts safely and with negligible degradation of the polymer's performance properties. Conventional melt separation methods cause a large drop in the polymer's molecular weight due to thermal degradation. Consequently dissolved PS can be precipitated through the addition of a non-solvent in the mixture. The solvent is vacuum-evaporated and re-used.
