Comparison of UV-degradation depth-profiles in polyethylene, polypropylene and an ethylene-propylene copolymer

Macromolecule scissions and crosslinks have been found to have uneven distributions over the cross-section of 3 mm thick polymer test bars. Depth localization and time development of the degradation processes have different character in polyethylene and polypropylene samples. In polyethylene macromolecule scission and crosslinking both progress approximately proportional to the exposure time. On the contrary, in polypropylene the degradation processes have been found to accelerate significantly at the illuminated surface, with the change in scission and crosslinking between 3 and 6 weeks of exposure much greater than that present after 3 weeks. The scission concentration at the surface after 6 weeks was 70 times larger than that in the sample interior for polypropylene whereas for polyethylene this ratio was approximately 3. The degradation of an ethylene-propylene copolymer was rather similar to that of the (low-density) polyethylene in the early stages and was very similar to that of polypropylene after 6 weeks of UV exposure. The scission/crosslink ratio showed a clear maximum just below the surface with polypropylene and the ethylene-propylene copolymer whereas it varied much less with polyethylene.