UV AND THERMAL HYDROLYTIC DEGRADATION OF POLY(ETHYLENE-TEREPHTHALATE) - IMPORTANCE OF HYDROPEROXIDES AND BENZOPHENONE END-GROUPS

The preconditioning of amorphous poly(ethylene terephthalate) sheet and biaxially oriented bottle material by heating for 48 h in a nitrogen atmosphere at 60 and 90-degrees-C improves both their thermal hydrolytic and UV stabilities. This stabilising effect is due to the decomposition of active hydroperoxides into inactive products. Hydroperoxide formation during hydrolytic degradation of bottle material is found to be both temperature and humidity dependent. There is an initial rapid growth followed by an induction period and then a rapid autocatalytic rise in concentration. UV irradiation shows a similar trend which is more likely associated with a photo-thermal effect. Polyester bottle material prepared with 1% w/w of 4,4'-dicarboxy and 4,4'-dihydroxybenzophenones is markedly UV stabilised with the latter having the greater effect. Hydrogen atom abstracting carbonyl species are therefore concluded to be unimportant in the UV degradation of polyester material compared with initiation by hydroperoxides. The enhanced stabilising effect of the hydroxyl end groups is associated with their ability to minimise chain end unzipping of terephthalic acid groups normally observed during UV degradation. Some UV absorption is also concluded to play a role in stabilisation. Such an effect is observed by doping the polyester with poly(ethylenenaphthalate) at the same concentration.