GC-MS OF POLYETHYLENE AND POLYPROPYLENE THERMAL CRACKING PRODUCTS

Abstract

Low-density polyethylene and polypropylene were thermally decomposed individually in a batch reactor at 450°C as a recycling route from the aspect of the production of petrochemical feedstock. The resulting transformation liquid products were analysed by high resolution gas chromatographic-mass spectrometric analysis. 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 C5-C23 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-C8 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 main decomposition products of polypropylene. In the liquid fraction up to n-C8 84 peaks including around 40 acyclic octenes were identified. Other 149 GC peaks were analysed in the C9-C25 fraction with quantitative ratio of alkane : alkene : alkadiene equal to 1 : 17 : 4. High resolution GC allowed more detailed and more reliable analysis of multicomponent polyalkenes cracking products.