The Microwave Induced Pyrolysis of Problematic Plastics Enabling Recovery and Component Reuse

Abstract

Recent trends toward the effective utilisation of petroleum derived materials to increase the sustainability of their use (both for economic and environmental reasons), has resulted in an increased interest in the development of recycling methods for plastics including Acrylonitrile- co-Butadiene-co Styrene and Poly Vinylchloride. The recycling of these waste plastics that include mixed monomer compositions and halogens poses a great problem, with their decomposition making them hard to recycle due to loss of their material properties or through the production of problematic compounds e.g. HCl, PCBs, PCDD, and PCDF etc. This work has investigated the microwave induced decompositions of these plastics and explored the potential of a carbon (a microwave absorber) assisted microwave decomposition process. This culminated in the examination of the carbon assisted microwave decomposition of ABS and the potential of a one and two step process for the de-hydrochlorination, then pyrolysis of PVC, which is an untried and novel approach for PVC recycling. . The influence of microwave power, exposure time, along with the effect of the proportion of carbon, was investigated for its influence upon the yields of gases, oils, chars and product components. The proportions of gases, oils and chars were quantified in terms of their product distribution and subsequently analysed for their properties/composition by TGA, FT-IR, GCMS, Py-GCMS and bomb calorimetry. From their analyses product distributions in the oils and gases were derived and decomposition mechanisms evaluated. From these investigations it was found that the microwave decomposition process of both plastics was possible and demonstrated great versatility, with oil yields for ABS of between 2wt.% to 70wt.% and gas yields of 28wt.% to 77wt.% achieved in processing times as little as 3 minutes. From this it was also possible to identify that high quantities of monomer were also able to be recovered, significantly greater than that of a thermal process (39.5%TiC as to 34.5%TiC respectively for styrene monomer). For PVC, it was identified by initial investigations that the de-hydrochlorination of PVC was possible, confirming results of Ito et al., (2006) and Moriwaki et al., (2006). However, the discovery of amplitude dependent heating was of significant interest, not previously identified in any microwave decomposition process.It was also recognized that pyrolysis was not possible after de-hydrochlorination of PVC occurred as a result of the reduction in the materials ability to absorb microwaves (lesser dielectric constant), due to chlorine was removal. Hence it was necessary to investigate the use carbon additive to enable achieving sufficient temperatures to induce the pyrolysis of the remaining polyene structure. The identification of key parameters and ensuing relationships with microwave power, heating rate and temperatures was identified herein, giving the first detailed account of the relationship between specific polymer types and microwaves during a pyrolysis process.Open Acces