Effect of oxygen pressure on the oxidation kinetics of unstabilised polypropylene

The thermal oxidation of unstabilised polypropylene films at 80 °C and various oxygen pressures ranging from 0.02 MPa to 5 MPa has been studied by FTIR spectrophotometry (carbonyl growth). The induction time decreases and the maximum oxidation rate increases quasi-hyperbolically when the oxygen pressure increases. The asymptotic behaviour (corresponding to the regime of oxygen excess) is not reached at the highest pressure under study. A kinetic model derived from a classical mechanistic scheme but free of simplifying hypotheses, has been used to simulate the observed behaviour and to determine the elementary rate constants. It is shown that a good simulation of kinetic curves of carbonyl build-up in the whole pressure interval under study can be obtained with a set of physically reasonable rate constant values. The “inverse problem” cannot be, however, totally solved because certain constants are interdependent so that some rate constant values have to be arbitrarily chosen or taken from the literature.