(385aj) A Comprehensive Study of the Impact of Polyolefin Structure on Pyrolysis Plastic Oil Composition

Seven types of plastics with differing sources, molecular weights, and polymer structures underwent pyrolysis in a fluidized bed reactor: PCR HDPE, PCR PP, virgin resins of varying molecular weights of HDPE, virgin resins of LDPE, LLDPE, and PP. Each feedstock's structure was characterized using FTIR, GPC, and NMR. The pyrolysis process yielded non-condensable gases (C1-C4), oil phase products (C5-C40), and solids (C40+ and chars). The predominant compounds found in the pyrolysis oils were alkane, alkene, alkadiene, aromatic, and multicyclo-aromatics, with variations in composition observed across different plastic feedstocks.

Among the three factors examined, polymer structure exerted the greatest influence on pyrolysis product distributions, while molecular weight had the least impact. Branches in PE were identified as thermal defects where polymer degradation initiated. DFT calculations revealed that, in the absence of branches or with a methyl group branch, the cleavage of the polymer C-C backbone in HDPE required 365 kJ/mol. Conversely, in polymers like LDPE and LLDPE, where branches contained more than one carbon, branch cleavage from the backbone occurred first, requiring lower energy (305 kJ/mol). Higher branch density in PE resulted in increased concentrations of aromatics, branched alkanes, and alkanes due to more occurrence of secondary reactions. However, PP and PE exhibited distinct degradation mechanisms, with PP requiring less energy for decomposition and producing more pyrolysis oil than PE under equivalent conditions.

Pyrolysis oil derived from PCR HDPE and PCR PP contained a higher proportion of branched compounds compared to virgin plastics. Impurities, such as trace elements present in PCR plastics, could potentially promote the isomerization of linear hydrocarbons to branched forms during pyrolysis. The presence of sand acted as a getter during pyrolysis to remove these impurities. While pyrolysis effectively eliminated most trace elements, additional purification steps may be necessary for the resultant oils.