On a transition at 80°C in polypropylene oxidation kinetics

The thermal oxidation of isotactic polypropylene homopolymer (PP) in air has been studied between 50 and 150 degrees C by means of infra-red spectrophotometry and gravimetry. The induction period, t(i), and oxidation rate in the steady state, r(s), were determined from isothermal tests. The Arrhenius plot of r(s) is linear over the whole temperature range, whereas for t(i) it displays a slope change at about 80 degrees C. This transition has been discussed within the framework of 'closed-loop' kinetic schemes in which initiation is only due to POOH decomposition. The hypotheses involving a molecular mobility change that influences propagation and termination rate constants can be rejected. Only the initiation step is involved. The transition can be explained by the coexistence of many modes of POOH decomposition. Several hypotheses are compared. (C) 1997 Elsevier Science Limited.