ULTRA-VIOLET DEGRADATION OF POLYPROPYLENE .1. DEGRADATION PROFILE AND THICKNESS OF THE EMBRITTLED SURFACE-LAYER

The formation and consequences of the embrittled surface layer in artificially ultra-violet (u.v.) degraded polypropylene (PP) samples were investigated. The condition of the surface was studied by visual inspection, infra-red spectrometry and mechanical testing. The degradation profile was obtained by determining the carbonyl content of thin (14-mu-m) slices, which were microtomed from the surface. This profile mainly arises from an oxygen diffusion limitation and, as in previous findings, it shows a plateau region at the surface and a sharp decrease ('shoulder') at some distance from the surface. However, in this research the shoulder shifts to deeper layers in the course of time. This is probably due to cracks in the degraded layer, which facilitate oxygen diffusion. Tensile tests performed on the microtomed slices showed a good correlation between the carbonyl content and embrittlement: as the carbonyl content decreases, the strain at failure rises. The degraded layer reached a thickness of about 400-500-mu-m. The presence of u.v. stabilizer (Tinuvin 326) produced relatively small differences in the shape and depth of the degradation profile. Injection-moulded material tended to degrade slower than compression-moulded material, resulting in a somewhat smaller depth of degradation.