WEIGHT-LOSS MECHANISM IN THE PHOTOOXIDATION OF POLYPROPYLENE

The photooxidation (fluorescent lamp; lambda(max) = 365 nm; intensity = 7 x 10(18) photons cm(-2) h(-1); temperature: 40 degrees C) of photo chemically unstabilised polypropylene films (thickness 100 microns) wa s studied by gravimetric and LR spectrophotometric methods. After a ps eudo-induction period, the sample mass increases in a sigmoidal way, r eaches a maximum and then decreases abruptly. From IR spectrophotometr ic data, we tried to estimate the weight gain due to oxygen incorporat ion (hydroperoxides and carbonyl groups). From the weight balance, the weight loss kinetics were established. It was first supposed that wei ght loss results from a random chain scission with evaporation of the small fragments. A simple kinetic model reveals, however, that experim ental data are inconsistent with this hypothesis. Heterogeneity of oxi dation cannot account for the observed behaviour. Finally, on the basi s of previously published analytical data on volatile products, it is suggested that weight loss could essentially result from a chain 'back biting' process capable of propagating along the macromolecule as a zi p depolymerisation.