Different degradability of two similar polypropylenes as revealed by macromolecule scission and crosslinking rates

The photodegradation of bars made from two nominally similar polypropylenes has been monitored during prolonged UV exposure by deriving the scission and crosslink concentrations at different depths using computer-aided analysis of GPC molecular mass distributions. The degradation in one of the polypropylenes (MPP) was much faster than in the other (PP). A steep degradation profile was observed near to the exposed surface; it remained in a fixed position with PP but moved progressively away from the surface in MPP. This correlated with the tensile failure behaviour observed with the samples. The scission/crosslink ratio fell steeply from 6 to 8 near the surface to ∼3 in the interior. The steepest part of the transition was at about 0.3 mm with PP for all exposures whereas with MPP it moved progressively away from the exposed surface (∼0.3 mm after 16 weeks exposure; ∼0.7 mm after 49 weeks). Stabilized PP and MPP showed quite different degradation profiles to the unstabilized polymers.