HETEROGENEOUS AND HOMOGENEOUS KINETIC ANALYSES OF THE THERMAL-OXIDATION OF POLYPROPYLENE

The evidence from chemiluminescence analysis for the heterogeneous model for the oxidation of polypropylene powder and him is re-examined. In this model, the polymer contains a small number of localized zones in which oxidation occurs at a high rate and from which it then spreads. Stabilizer, if present, prevents the spreading for a period of time (the induction period) and also lowers the rate of subsequent spreading through the polymer. By considering a model of progressive 'infection' of the unoxidized polymer fraction a sigmoidal chemiluminescence growth curve is predicted and depends on several parameters including the initial oxidized fraction and the spreading coefficient. It is shown that a homogeneous kinetic analysis based on an autoaccelerating free radical reaction initiated by trace hydroperoxides fails to provide a good fit to the observed CL data. The activation energies for the chemiluminescence at the onset of oxidation, the maximum in the sigmoidal profile, and the induction period are reinterpreted with consideration given to the activation energy for the chemiluminescence quantum yield, as well as the influence of instrumental sensitivity on measured values. From these results and the observation that the extent of oxidation at the end of the induction period is not a constant, independent of temperature, it is concluded that a homogeneous kinetic model cannot be applied.