Catalytic Degradation of Waste Polymers

Abstract

Plastics have become an integral part of our lives. However, the disposal of plastic waste poses an enormous problem to society. An ideal solution would be to break down a polymer into its monomer, which could then be used as the building-blocks to recreate the polymer. Unfortunately, the majority of plastics do not degrade readily into their monomer units. Thermal degradation of polymers usually follows a radical mechanism (which is of high energy and requires high temperatures) and produces a large proportion of straight chain alkanes, which have low relative octane number (RON) and so cannot be used in internal combustion engines. However, a suitable catalyst can help to branch straight alkane chains and so give high RON fuels that can be blended into commercial fuels.

An extensive thermogravimetric study of polymer-catalyst mixtures was undertaken and produced dramatic reductions in the onset temperature of degradation and significant changes in the activation energy, suggesting a change to a desirable Brønsted- or Lewis-acid catalysed degradation mechanism in many cases. For example, GC-MS analysis of low-density polyethylene (LDPE) degraded with Fulcat 435 clay showed the polymer forming a large number of C6-C7 single-branched alkanes of intermediate RON value. In comparison, degradation of LDPE in the presence of a ZSM-5 zeolite (280z) resulted in the production of a large aromatic content (41% of Total Mass at 450ºC) together with branched C6-C8 hydrocarbons (40%). This formation of a large proportion of high RON components from polyethylene and other polymers could move us one step closer to tackling the enormous problem of plastic waste disposal that the world faces today.