Plastics recycling depolymerization

Pyrolysis treatments are interesting regarding the aforementioned plastic refuse makeup. Other successful treatments for feedstock recycling of condensation polymers (PET, ABS, etc.), that allows for the depolymerization and recovery of their constituent monomers (e.g. hydrolysis, alcoholysis, methanolysis, etc.), cannot be applied for polyolefin plastics recycling. In contrast, pyrolysis of polyolefins yields valuable hydrocarbon mixtures of... 

M. W. Meszaros, Advances in plastics recycling, thermal depolymerization of thermoplastic mixtures, ACS Symposium Series, 609, 170-182 (1995). 

O.A Schematic illustration of the chemical recycling routes for waste plastic. The depolymerization unit uses fossil-fuel-type techrxilogy (t rogenation, cracking, pyrolysis, gasification) to produce syncrude (a mixture of hydrocarbons of low molecular mass) which enters the refinery, where it is recycled with incoming cmde oil into fuel or nxmomer feedstock. 

There is also uncertainty in the regulatory status of tertiary recycling when it does not result in the direct production of monomers suitable for polymerization into new plastic. The European Commission has at times supported the chemical recycling (depolymerization) of condensation polymers such as polyethylene terephthalate back to monomer (e.g., dimethyl terephthalate) as recycling for the purpose of government-mandated plastics recycling rate calculations, but not the liquefaction of polyolefin plastics back to petrochemical feedstocks for reprocessing in a refinery. Discussions around these types of definitional issues, and their environmental and economic implications, are likely to continue for many years to come. 

In those jurisdictions where money is specifically collected from consumers to underwrite the costs of recycling plastic containers, funds may be available to offset some costs of depolymerization. If bottles are provided at no cost at the sorting/baling facility, an economically attractive venture can be contemplated, but still at a large scale. Securing the feed on a long-term basis at a favorable price will require significant co-operation. 

Depolymerization reduces inherently valuable long-chained molecules to inherently less valuable smaller molecules. If mechanical recycling can utilize collected plastic materials profitably, it is highly unlikely that depolymerization will achieve greater profitability. 

This idea becomes even more pointed when we look at polymerization. Polyvinyl chloride is the familiar plastic PVC and is made by reaction of large numbers of monomeric vinyl chloride molecules. There is, of course, an enormous decrease in entropy in this reaction and any polymerization will not occur above a certain temperature. Some polymers can be depolymerized at high temperatures and this can be the basis for recycling, low... 

Strode K. S. and Demianiw, D. G. Advanced Recycling of Plastics A Parametric Study of the Thermal Depolymerization of Plastics, Final Report to the American Plastics Council, 1995. 

In the liquefaction step the plastic waste is cracked nnder relatively mild thermal conditions. This depolymerization results in a synthetic heavy oil and a gas fraction, which in part is condensable. The noncondensable fraction is nsed as a fuel in the process. The process is very comparable to the cracking of vacnnm residues that originate from oil recycling process. 

N. Brand, Depolymerization of polymethylmethacrylate (PMMA), In Recycling and Recovery of Plastics, J. Brandrup, M. Bitmer, W. Michael and G. Menges (eds), Hanser Press, Munich 1996, pp. 488-493. 

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). 

Chemical depolymerization of polyesters has been mainly applied to polyethylene terephthalate (PET), the most common polyester on the market. Chemo-lysis of PET by a variety of methods has been known for many years. In fact, the chemical depolymerization of PET can be considered the starting point of plastic chemical recycling. 

Different PET chemolysis methods have been developed aimed at the production of TPA, DMT or BHET, all of them being possible monomers for the reconstruction of fresh polyesters. The exact monomer formed by PET depolymerization depends on the type of chemical agent used to break down the polymeric chains. In certain processes, the final product of PET chemolysis is a mixture of polyols, useful in the formulation of other polymers such as unsaturated polyesters, polyurethanes and polyisocyanurates. This is an interesting case of chemical recycling because the breakdown of one polymer leads to the raw materials for the preparation of a quite different class of plastics. 

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. 

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