Comparison of the UV-degradation chemistry of polypropylene, polyethylene, polyamide 6 and polybutylene terephthalate

The photodegradation mechanism of polymers highly depends on the type and concentration of chromophores present. This influence is studied by making a comparison between the UV-degradation of PE, PP, PA6 and PET and the thermooxidative degradation of Pf at a comparable temperature. The degradation processes are followed by determining the oxygen uptake, CO, CO2 and peroxide formation. During UV-degradation of PP, PE, PA6 and PET the oxygen uptake is linear in time without an induction period. The oxidation rate of PP is higher than that of PET, which is slightly higher than the oxidation rate of PA6, while that of PE is the lowest. The determined amount of peroxides depends on the ageing time. For the UV-degradation this does not lead to the expected increase of the oxidation rate, as is the case with thermooxidation of PP. Only a limited proportion of the peroxides formed during the UV-degradation decomposes into radicals. The oxidation rate is mainly determined by other radical forming reactions. In the case of PE and PP this is probably initiation by polymer-oxygen Charge Transfer Complexes (CTCs) and for PA6 and PET direct photolysis of the amide or the ester linkage, respectively. Thickness degradation profiles show that the UV-degradation processes of PP and PET are heterogeneous. For PP this is due to oxygen diffusion limitation, while in the case of PET it is the result of absorption of the UV-light. (C) 1999 Elsevier Science Ltd. All rights reserved.