Plastics Waste - Stream

Additives often form a problem in recycling processes. Material recycling is often not possible or only with a considerable loss of quality. Plastics recycling is notoriously difficult due to the mixed composition of the plastics waste stream. The recycled material can only be used in certain applications that do not demand a pure material. Recycling of the additives themselves is theoretically possible only for metals, but in practice this type of recycling is not feasible. The metals occur only in low concentrations. Recovery from fly ash and bottom ash is possible, but expensive in view of growing scarcity problems it may become a viable options for at least some metals. 

Mechanical recycling is the preferred route for homogeneous and relatively clean plastics waste streams. It is assumed that the cushion vinyl floor covering will be mechanically recycled [11].1... 

Feedstock or chemical recycling is seen as complimentary to mechanical recycling and seems appropriate for cost effective treatment of mixed and contaminated plastic waste streams (115). A progress report, on potential technologies for high PVC content mixed plastic waste streams, is available (21). Promising developments, which look technically and economically viable, are ... 

The PVC industry is actively involved in the development of recycling solutions for contaminated mixed plastics waste streams with a high PVC content. Potential technologies for achieving this goal are tested at present at pilot scale. This paper provides an overview of the projects under investigation and the results obtained so far, while development work continues. 9 refs. 

The major disadvantage of chemical depolymerization is that it is almost completely restricted to the recycling of condensation polymers, and is of no use for the decomposition of most addition polymers, which are the main components of the plastic waste stream. Condensation polymers are obtained by the random reaction of two molecules, which may be monomers, oligomers or higher molecular weight intermediates, which proceeds with the liberation of a small molecule as the chain bonds are formed. Chemical depolymerization takes place by promoting the reverse reaction of the polymer formation, usually through the reaction of those small molecules with the polymeric chains. Several resins widely used on a commercial scale are based on condensation polymers, such as polyesters, polyamides, polyacetals, polycarbonates, etc. However, these polymers account for less than 15% of the total plastic wastes (see Chapter 1). 

Hydrolytic treatments can serve not only as a PET degradation method, but may simultaneously enable the separation of hydrolysable and non-hydro-lysable polymers present in the plastic waste stream. Thus, Saleh and Wellman64 have proposed the separation of PET and polyolefin mixtures by treatment with water from about 200 °C up to the critical temperature of water under autogenous pressure. The resulting liquid phase contains the hydrolysis products, TPA and ethylene glycol, whereas the solid phase is formed by the non-reacted polyolefins. 

This section describes the chemical recycling, via chemolysis, of certain condensation polymers which, although being produced in significantly lower amounts than polyesters and polyurethanes, are used in important applications, and so also contribute to the plastic waste stream. 

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. 

This section reviews the different aspects of the thermal conversion of those polymers which are the main components of the plastic waste stream polyethylene, polypropylene, polystyrene, PVC and PET, although the thermal degradation of other polymers is also commented on. The discussion focuses on mechanistic and kinetic factors, as well as on the type of products derived from the thermal decomposition of each individual polymer. The thermal degradation of plastic mixtures, which reflects more accurately the phenomena taking place in the thermal conversion of plastic wastes, is analysed and discussed in the next section. 

Polypropylene is a polyolefin found in high concentrations in the plastic waste stream. Of the different types of PP, isotactic polypropylene is the one most widely used on a commercial scale and so is the type predominant in plastic wastes. Compared to PE, the backbone of the PP molecule is characterized by the presence of a side methyl group at every second carbon. This fact implies that half of the carbons in a PP chain are tertiary carbons and so, as a consequence of their higher reactivity, PP is thermally degraded at a faster rate than PE. Thus, as can be seen in Figure 4.7, the PP weight loss in TGA measurements starts at a lower temperature compared to both HDPE and LDPE. 

Mechanism of pyrolytic degradation is complex involving a large set of reactions, even for a single class of plastic. Generic degradation reactions such as chain scission, H-transfer, unzipping, disproportionation, and combination occur in the process. Typical mix of products from mixed plastic waste streams with different pyrolysis conditions are shown in Table 9.3. 

Table 2.3 Composition of Miami, Florida Aiiport Plastic Waste Stream [Peritz, 1990]...

COMPATIBILIZATION OF HETEROGENEOUS POLYMER MIXTURES FROM THE PLASTICS WASTE STREAMS... 

The observations discussed on the preceded pages will be illustrated using the melt mixed blends of HDPE with LDPE (see Table 2). This system was selected as typical light fraction of the plastics waste stream. The necessity to mix different types of PE s is one of the main problems in polymer recycling. The material parameters of PE and process variables determine the morphology and performance. 

Compatibilization of Heterogeneous Polymer Mixtures from the Plastics Waste Streams H. J. Radusch, J. Ding and G. Akovali... 

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