Degradable plastics recovery/recycling

As a partial solution to the global issue of plastic waste, in recent years much interest has been devoted to the formulation of environmentally degradable plastic materials. In particular the use of natural polymers presents several advantages such as biodegradability, utilizing of renewable resources, recyclability. At the same time water sensitivity and degradability of natural polymers limit their possible applications. Consequently bioplastics cannot replace synthetic plastics in every application but they can result appropriate in specific products especially for those applications in which recovery of plastics is not economically feasible, viable and... 

Plastics are recycled but during the recovery stage, the polymers (particularly in the ease of mixed plastics such as composites) lose quality and texture due to partial degradation, which invariably occurs during the recovery stage. 

The changes in mechanical properties due to degradation and/or blending, contamination, antioxidant consumption and general use (e.g. oxidation processes) of the polymeric material during first service life are some of the characteristics that determine if the material should be recycled mechanically or disposed of in another way (e.g. energy recovery). A successful recycling is strictly dependent on the purity and uniformity of the plastic waste. 

Unsaturated polyesters (UPs) crosslinked with styrene are often used as a matrix of fiber reinforced plastics. Several reports treated the degradation of the crosslinked UPs with high temperature treatment in water (1,2), acetic acid (5), alcohols including glycols (4,5), and amines (6), often in the presence of catalysts. In these literatures, recovery of polymeric materials from the crosslinked UPs was not a main objective. However, in case we can hydrolyze polyester chains selectively, linear polystyrene derivatives can be obtained as recycled materials. 

W. Kaminsky and H. Sinn, Pyrolytic techniques, In Recycling and Recovery of Plastics, J. Brandrup et al. (eds), Hanser Publishers, New York, pp. 435-444, 1996. W. Kaminsky, B. Schiesselmann and C. M. Simon, Thermal degradation of mixed plastic waste to aromatics and gas, Polymer Degradation and Stability, 53, 189-197 (1996). 

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. 

Recovery of plastics from MSW can be as a material resource (by material recycling), a feedstock (by thermal or chemical degradation), or as energy (by incineration or biological treatment). Different technical methods are available for each of... 

Thermoplastic copolyesters elastomers can be recycled by the conventional methods used for plastics, such as mechanical recycling, chemical degradation and sometimes by energy recovery, i.e. incineration. 

The prodegradant structural inhomogeneities and nonpolymeric impurities may negatively affect the recyclate lifetime. With the formation of new pro-oxi-dative functional moieties a substantial part of the stabilizers added for the first lifetime are simultaneously consumed. As a consequence the mechanical properties of commingled plastic recyclates generally do not meet material expectations. Increase in resistance to degradation by proper restabilization and the recovery of... 

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