Hydroperoxide build-up in the thermal oxidation of polypropylene - A kinetic study

The thermal oxidation of unstabilised polypropylene (PP) was studied at 80 °C under various oxygen pressures: 0.02, 0.5 and 5.0 MPa, and, under 5.0 MPa oxygen pressure at various temperatures: 60, 80, 100 and 120 °C. Hydroperoxides were titrated using a chemical titration method and modulated DSC (taking an enthalpy of −325 kJ mol⁻¹). Starting from a previous kinetic analysis of carbonyl growth in same exposure conditions, we have tried to simulate experimental results by a model based on the classical mechanistic scheme in which initiation results from POOH (mainly bimolecular) decomposition. The model, which takes into account substrate consumption and does not rely on usual simplifying assumptions (steady state for radicals, long kinetic chains, interrelations between termination rate constants), generates kinetic curves with the same shape as experimental ones and predicts well the effect of O₂ pressure and temperature on hydroperoxide and carbonyl concentrations.