Plastics and Rubbers Waste

The reuse of once used plastics, may not be always possible and has certain limitations, because of impurities and health hazards of the chemicals involved. Hence proper effective recycling , although it still has a number of limitations, remains a promising approach. This option is still under development. One of the most difficult issues in plastics recycling still remains the recycling of composite plastics and laminates. 

In recycling, there are three main methods employed physical recycling, chemical recycling and incineration . 

In physical recycling, used plastics are reprocessed directly back to produce new (generally inferior quality) materials, for less demanding applications. The waste material already had certain additives from its previous history, and during its re-processing, new additives 

Using special reactions, it has also been shown that, a degradable polymer can be produced as well i.e., by transesterification, new biodegradable polyesters can be synthesised from polybutylene adipate-co-succinate and polyethylene terephthalate [21]). 

Already there are certain restrictions for certain non-biodegradable plastic packaging materials (the US Plastics Pollution Research and Control Act of 1987, Public Law 100-220 and the Annex of the MARPOL (marine pollution) Convention - the International Convention for the Prevention of Pollution from Ships which prohibit the disposal of plastics at sea, which allowed the US Navy promote the development of aquatic biodegradable plastics at sea. 

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Fifty percent of the total energy needed to produce plastic materials is used in the polymerization process. Thus at first sight, it seems sensible to re-use plastic and rubber waste materials (8 ). But Re-use processes have a negative influence on the macro-molecular structure which is mainly a chain depolymerisation effect (9 ). Therefore Re-use by itself is not a satisfactory process. In addition,a large proportion of the plastic waste is polluted and mixed with other types of waste so that Re-use is impossible. 

A wide variety of procedures and treatments have been investigated for the feedstock recycling of plastic and rubber wastes. For the purposes of this book, these methods have been classified into the following categories (Figure 1.12) ... 

While currently most plastic and rubber wastes are still disposed of in landfills, it is forecast that in the next few years the significance of other alternatives will have to be enhanced, as the number of landfill sites progressively decreases in many countries. There will have to be an increase in both mechanical and feedstock recycling and, when they are not feasible, energy recovery may be the most suitable option. 

In addition to gasification, other oxidative treatments of plastic and rubber wastes, excluding total combustion, are described in this chapter. These methods, although relatively unknown, may be of great interest in the future for the chemical degradation of polymeric wastes. 

Most of the processes so far proposed for the gasification of polymeric wastes have been directly derived from earlier processes developed for the conversion of coal, natural gas and heavy petroleum fractions. However, certain details must be taken into account when processing plastic and rubber wastes in the gasification units, for instance the heterogeneity of the starting material, the problem of feeding the highly viscous melted plastics, and the possible formation of corrosive compounds, mainly HC1 from PVC. 

One of the major advantages of gasification for the conversion of wastes is that it requires little pretreatment or sorting of the starting materials. Consequently, in many gasification processes plastic and rubber wastes are converted simultaneously with other solid wastes. This section describes some of the processes currently used for the gasification of mixed solid wastes. 

In spite of the above mentioned background on polymer oxidation, only a few works have been published on the subject of obtaining commercially valuable products by partial oxidation of plastic and rubber wastes. Several of these works are described below. 

Other possible oxidative treatments for the feedstock recycling of plastic and rubber wastes include partial oxidation with organic peroxides and decomposition by reaction with oxygen by thermooxidation or under supercritical water conditions. However, these latter alternatives have so far not been widely investigated. 

Thermal degradation of plastic and rubber wastes in inert atmospheres has been extensively studied in the past. It is widely accepted that it takes place through radical mechanisms, two main pathways having been proposed depolymerization by end-chain cracking and random chain scission. In the first case, high concentrations of the starting monomer are obtained, but this mechanism is predominant only in the thermal degradation of a few polymers, such as PS and... 

A large number of processes and reactors have been developed for the thermal conversion of plastic and rubber wastes stirred tanks, rotary kilns, fluidized beds, circulating bed reactors, screw extruders, etc. Many of the studies carried out in recent years have been based on sand fluidized or circulating bed reactors. Likewise, several works have recently appeared on plastic degradation in the presence of solvents. 

The last chapter highlights the main conclusions and establishes a comparative study of the various alternatives for the feedstock recycling of plastic wastes. The final conclusion is that feedstock recycling of both plastic and rubber wastes has a high potential for growth in the next few years, although to be commercially successful a number of technical and economic aspects still have to be addressed. 

PartB Characterization of Plastic and Rubber Waste in a Hot Glovebox. 710... 

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