KINETICS OF MASS CHANGES IN OXIDATION OF POLYPROPYLENE

Isothermal ageing of isotactic polypropylene films was studied in air, in the temperature range 50-150 degrees C, by using gravimetry. The c urves display a nonmonotonic shape owing to the occurrence of mass gai n by oxidation and mass loss by evaporation of small molecular fragmen ts. We have tried to construct a kinetic model of these mass changes u sing a 'closed-loop' model in which initiation is only due to bimolecu lar decomposition of hydroperoxides and volatile formation results dir ectly from the initiation or/and termination reactions. This model giv es better results than the corresponding one with unimolecular POOH de composition but it needs a temperature variation of the initial POOH c oncentration, which seems physically unrealistic. Certain slightly mor e complex schemes could be the solution in mathematical terms, but the y carry some ambiguity and are presumably more difficult to check. (C) 1997 Elsevier Science Limited.