A heterogeneous model for the thermal oxidation of solid polypropylene from chemiluminescence analysis

From an analysis of chemiluminescence (CL) curves from the isothermal oxidation of polypropylene powder and film, it is proposed that the usual homogeneous kinetic analysis of the sigmoidal increase in apparent oxidation rate with time is inappropriate and should be considered instead as a statistical accumulation of rapidly oxidizing centres. Over the temperature range 90–150°C it is found that the CL curves up to the maximum emission intensity may be reduced to a single sigmoidal master curve by plotting the data after the induction period in relative coordinates. Identical activation energies are observed for the initial emission and the maximum emission intensities, which suggests that throughout the oxidation there is no change in the CL mechanism, which requires the occurrence of a degenerately branched chain reaction within the oxidizing centres. The induction period shows a different temperature dependence and in the heterogeneous model is interpreted as the time required for the oxidation to spread from the initially localized centres. This model is able to explain the observations of volatile formation in the earliest stages of oxidation and the rapid crack formation at the end of the induction period as well as other anomalies which cannot be rationalized in a homogeneous kinetic model.