Photosensitized degradation of polyolefins. II

The oxydative photodegradation of polyethylene sensitized by aromatically substituted dienes such as 1,6-diphenyl-1,3,5-hexatriene (DPH) and 1,4-diphenyl-1,3-butadiene (DPB) has been investigated. The rate of degradation has been monitored by following the change of the mechanical properties of the polymer during accelerated indoor aging tests. It was found that both additives are efficient sensitizers of the photodegradation of polyethylene. The mechanism of photo-oxidation has been investigated by ESR and IR spectroscopy and it has been interpreted on the basis of a complex mechanism which includes excited singlet oxygen (1O2) and the products of photodegradation of the additives. Singlet oxygen, produced by DPB and DPH and/or by impurities present in the polymer, reacts with the additives themselves and, directly, with polyethylene. In turn, the products of photodegradation of the additives react with the polymer via hydrogen atom abstraction, further promoting the photodegradation. Irradiation carried out in absence of air seems to rule out the possibility of direct hydrogen atom abstraction by DPB and DPH.