ON A TRANSITION AT 80-DEGREES-C IN POLYPROPYLENE OXIDATION-KINETICS

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