Recycling of Polymers by Thermal Degradation

The main types of feedstock recycling processes can be summarised as chemical depolymerisation, gasification and partial oxidation, thermal degradation, catalytic cracking and reforming, and hydrogenation [852663]. The current advanced thermal treatments are as follows [a.372]. 

Pyrolysis is becoming an important route for treating the increased amounts of waste plastics - mainly LDPE, HDPE, PP, PS, PVC, PU, PA and PET that account for more than 90% of total plastic wastes 888003) [a.373]. The process parameters that have the largest influence on the pyrolysis products are the temperature and heating rate 886353. The oil produced has a high heating value and may be combusted directly or refined for the recovery of speciality chemicals. The production of a liquid product has advantages in that it is easier to handle, store and transport and hence the product does not have to be used at or near the pyrolysis process plant. 

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Contaminants in recycled plastic packaging waste (HDPE, PP) were identified by MAE followed by GC-MS analysis [290]. Fragrance and flavour constituents from first usage were detected. Recycled material also contained aliphatic hydrocarbons, branched alkanes and alkenes, which are also found in virgin resins at similar concentration levels. Moreover, aromatic hydrocarbons, probably derived from additives, were found. Postconsumer PET was also analysed by Soxhlet extraction and GC-MS most of the extracted compounds (30) were thermally degraded products of additives and polymers, whereas only a few derived from the original contents... 

Recently the pyrolysis of polymer mixtures has become a focus of interest due to the increasing role of plastics recycling. Many researchers have investigated the thermal decomposition of various polymers in the presence of PVC. Kniimann and Bockhom [25] have studied the decomposition of common polymers and concluded that a separation of plastic mixtures by temperature-controlled pyrolysis in recycling processes is possible. Czegfny et al. [31] observed that the dehydrochlorination of PVC is promoted by the presence of polyamides and polyacrylonitrile however, other vinyl polymers or polyolefins have no effect on the dehydrochlorination. PVC generally affects the decomposition of other polymers due to the catalytic effect of HCI released. Even a few per cent PVC has an effect on the decomposition of polyethylene (PE) [32], HCI appears to promote the initial chain scission of PE. Day et al. [33] reported that PVC can influence the extent of degradation and the pyrolysis product distribution of plastics used in the... 

Thermal processes are mainly used for the feedstock recycling of addition polymers whereas, as stated in Chapter 2, condensation polymers are preferably depolymerized by reaction with certain chemical agents. The present chapter will deal with the thermal decomposition of polyethylene, polypropylene, polystyrene and polyvinyl chloride, which are the main components of the plastic waste stream (see Chapter 1). Nevertheless, the thermal degradation of some condensation polymers will also be mentioned, because they can appear mixed with polyolefins and other addition polymers in the plastic waste stream. Both the thermal decomposition of individual plastics and of plastic mixtures will be discussed. Likewise, the thermal coprocessing of plastic wastes with other materials (e.g. coal and biomass) will be considered in this chapter. Finally, the thermal degradation of rubber wastes will also be reviewed because in recent years much research effort has been devoted to the recovery of valuable products by the pyrolysis of used tyres. 

The processes of feedstock recycling of plastic wastes considered in this chapter are based on contact of the polymer with a catalyst which promotes its cleavage. In fact, plastic degradation proceeds in most cases by a combination of catalytic and thermal effects which cannot be isolated. As was described in Chapter 3, the use of catalysts is also usual in chemolysis processes of plastic depolymerization. However, there are two main differences between catalytic cracking and chemolysis there is no chemical agent incorporated to react directly with the polymer in catalytic cracking methods, and the products derived from the polymer decomposition are not usually the starting monomers. 

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