Oxidation thickness profiles during photooxidation of non-photostabilized polypropylene

Polypropylene (PP) samples free of photostabilizers were exposed in many accelerated ageing devices equipped with fluorescent or xenon lamps in which the sample temperature ranged from 40 to 70 degrees C and the UV intensity, determined by polycarbonate actinometry, ranged from 7x10(18) to 200x10(18) photon cm(-2) h(-1). The thickness profiles of carbonyl concentration were determined by FTIR measurements on microtomed slices of 20 mu m thickness. The continuous increase in the thickness of the oliidized layer (TOL) with exposure time can be attributed to auto-acceleration effects in the pseudo induction period and to sample cracking shortly after the end of this pseudo induction period. Determinations just before the appearance of cracks show that the 'steady-state' TOL increases with temperature and is practically independent of the light intensity. An attempt is made to explain this peculiar behaviour in terms of oxidation kinetics. (C) 1997 Elsevier Science Limited.