Gas-phase contribution to the spreading of oxidation in polypropylene as studied by imaging chemiluminescence

Oxidation was initiated in stabilised isotactic-polypropylene films using UV-light. The initiation was induced in a localised area of the film and the spreading of the oxidation to the rest of the film at 150 °C in air was followed using imaging chemiluminescence. It was found that the spreading of the oxidation occurred to a large extent via the gas phase over the film surface. A directed air flow was used to distinguish between gas and bulk phase spreading of oxidation. The experiments showed that volatile oxidation products in the gas phase contribute to the spreading of oxidation. To find possible oxidation products responsible for the increased oxidative spreading, a small fraction of the air in the ICL-sample chamber was replaced by each of the following substances: Water, acetic acid, formaldehyde, ethylene and iso-butylene. These substances are all found in the volatile products given off during thermo-oxidative degradation of PP. Both acetic acid and formaldehyde accelerated the oxidative spreading. Water did not effect the oxidative spreading whereas ethylene and iso-butylene decreased the oxidative spreading, possibly due to reaction with radicals formed during oxidation. It was shown experimentally that the accelerating effect on the spreading of oxidation caused by acetic acid was probably due to deactivation of the stabiliser and therefore only formaldehyde was found to increase the spreading rate of oxidation in unstabilised PP.