Photo-induced scission and crosslinking in LDPE, LLDPE, and HDPE

The molecular degradation characteristics of three different polyethylenes were determined by deriving chain scission and crosslinking concentrations from gel permeation chromatography molecular weight distributions obtained after 3 weeks and 6 weeks laboratory ultraviolet exposure. Injection-molded bars (3 mm thick) made from a low-density polyethylene (LDPE), a linear low-density polyethylene (LLDPE), and a high-density polyethylene (HDPE) were used and all showed strong depth variations in degradation. Degradation was rapid near the exposed surfaces but very little change occurred in the bar centers, due to oxygen starvation. The most rapid rises in scission and crosslink concentrations were observed with LDPE, for which the concentrations after 6 weeks exposure were approximately double those measured after 3 weeks. With LLDPE and HDPE the scission and crosslink concentrations after 6 weeks exposure were very much greater than twice those after 3 weeks. Scission dominated over crosslinking at all depths and for all materials the scission/crosslink ratio was always >= 3, with a value of similar to 9 recorded for HDPE near the exposed surface after 6 weeks exposure. (c) 2005 Society of Plastics Engineers.