The thermal degradation kinetics of polypropylene: Part I. Molecular weight distribution

A kinetic model for the thermal degradation of polypropylene was developed and fit to molecular weight distribution data obtained by high-temperature size-exclusion chromatography. In a series of ampoule experiments, reaction temperatures of 275 to 315 °C were examined with reaction times of up to 48 h. A single-parameter version of the model, containing an apparent rate constant, was found to provide excellent fits of all molecular weight distributions. Values of the parameter varied with both temperature and reaction time. The variations with temperature provided Arrhenius plots at each time. A lower-than-expected overall activation energy of 123.8 kJ/mol was attributed to the temperature range examined and the presence of ‘weak links’ due to oxidized moieties in the polymer. The ‘weak links’ were below the detectability limit of Fourier transform infra-red spectroscopy applied to the reacted samples. However, other data on heavily oxidized polypropylene and a recent study using thermal gravimetric analysis¹ where an activation energy of 98.3 kJ/mol was determined for similar temperatures, did provide further support for the hypothesis.