Photosensitized degradation of polyolefins

The effect of aromatically substituted dienes such as 1,1,4,4-tetraphenylbutadiene (TPB) as sensitizers of the accelerated oxidative photodegradation of polyethylene has been studied. The mechanism of photosensitization has been investigated by ESR and IR spectroscopy, while the degradation rate has been monitored by following the change of the mechanical properties during accelerated indoor and outdoor weathering tests. It was found that TPB is an efficient sensitizer for the oxidative degradation of polyethylene. The degradation of the polymer takes place via 1O2 formation, and it was interpreted on the basis of two simultaneous processes. In the first, excited singlet oxygen (1O2) is produced by TPB itself or by ketonic impurities present in the polymer. In the second, 1O2 reacts with TPB giving benzophenone, peroxides, and other oxygenated products which in turn can promote the degradation of polyethylene. Irradiation carried out in the absence of air seems to rule out the possibility of a photodegradation mechanism via excitation of TPB to an upper excited state capable of hydrogen abstraction from the polymer.