Humidity dependence of carbon dioxide generation during photodegradation of biaxially oriented polypropylene in oxygen

UV generation of CO₂ gas from a range of biaxially oriented polypropylene (BOPP) films exposed in oxygen gas has been measured by in situ infrared spectrometry and compared with the development of absorption at 1713 cm⁻¹ due to carbonyl formation in the same films. As in studies of other polyalkene films, the CO₂ measurements, which take only 3 h, correlate well with carbonyl development measurements that require hundreds of hours.Upon UV irradiation of BOPP in dry oxygen an induction time of ∼60 min preceded a linear increase of photogenerated CO₂. In wet oxygen, no noticeable induction time preceded the linear increase of CO₂. The rate of CO₂ increase was greater when the oxygen was humidified. This pattern was observed for four different types of film and for films of different thickness. The increase in the rate of CO₂ photogeneration with increased humidity was greater for thicker films. The correlation between the amount of CO₂ generated and the strength of the carbonyl absorption developed under standard conditions was better for BOPP oxidation in dry than in humidified oxygen. The inclusion of recycled polymer caused an increase in the rate of photodegradation.Although measurements on each subset of films demonstrated a clear increase in the photogeneration of CO₂ at high humidity, the mechanism by which the CO₂ generation from BOPP is enhanced remains unclear and further work is necessary to resolve this puzzle.