High resolution gas chromatographic–mass spectrometric analysis of polyethylene and polypropylene thermal cracking products

Abstract

Low-density polyethylene and polypropylene were thermally decomposed individually in a batch reactor at temperatures up to 450 °C as a recycling route from the aspect of the production of petrochemical feedstock. More detailed separation of polyalkenes splitting compounds was achieved using high resolution GC (efficiencies up to 490,000 plates) than before. The GC–MS as well as the dependence of homomorphy factors and isopropyl group increments on the number of carbon atoms of alk-1-enes, alk-2-enes, alka-α,ω-dienes and alkanes were used for identification of separated analytes. Thermal cracking of LDPE products in the range C 5–C 23 were characterized by quintets of peaks in the chromatogram which were assigned to n-alkanes, alk-1-enes, ( E)-alk-2-enes, ( Z)-alk-2-enes and alka-α,ω-dienes with average quantitative ratio 1:1.2:0.07:0.05:0.08. In fraction up to n-C 8 140 GC peaks were separated and identified, including around 30 acyclic octenes. In contrast to the polyethylene thermal cracking which yields products with straight-chain hydrocarbon structure, polypropylene cracking is characterized by the formation of compounds with branched and high prevailingly unsaturated hydrocarbon structure. The pretention of this analytical problem lies in stereoisomerism and corresponding multicomponentity of branched alkenes, alkadienes and alkanes in a broad range of carbon atoms number as characteristic decomposition products of polypropylene. Partial separation of all four diastereoisomers of 2,4,6,8-tetramethylundec-1-ene, which is the most abundant compound except for 2,4-dimethylhept-1-ene, was obtained. In the liquid fraction up to n-C 8 84 peaks including around 40 acyclic octenes were identified. Other 149 GC peaks were analysed in the C 9–C 25 fraction with quantitative ratio of alkane:alkene:alkadiene equal to 1:17:4.