The mode of action of corn starch and a prooxidant system in LDPE: Influence of thermooxidation and UV-irradiation on the molecular weight changes

The mode of action of corn starch and a pro-oxidant system consisting of LDPE, styrene-butadiene copolymer (SBS) and manganese stearate (denoted as MB hereafter) as a system creating a (bio)degradable LDPE was analysed in line with ongoing research about degradable LDPE. Pure LDPE, LDPE with corn starch and LDPE with MB were subject to thermo-oxidation (65 and 95 degrees C) and UV-irradiation. The degradation rates in the different environments are discussed based on the molecular weight changes measured by SEC. In thermo-oxidation at both temperatures, LDPE with corn starch demonstrated the lowest degradation rate, even lower than that of pure LDPE, in contrast to the situation in UV-irradiated samples where pure LDPE was the most stable material. Addition of corn starch to LDPE thus has a stabilizing effect on the thermo-oxidation mode while the same additive destabilizes the LDPE in, e.g. UV-irradiation. This property would diminish the risk of degradation during the processing of such materials. Decreasing values of molecular weight were observed in thermo-oxidized LDPE-MB while the UV-irradiation resulted instead in increasing molecular weight in the LDPE-MB. Large amounts of radicals are formed during UV-irradiation which increases the possibilities for chain elongation reactions. (C) 1997 Elsevier Science Limited.