Vitamin E: an effective biological antioxidant for polymer stabilisation

The bio-active form of vitamin E, a-tocopherol, has been shown to be an extremely effective antioxidant for plastics and rubbers, e.g. under melt processing conditions in polyolefins, under long term storage conditions in polybutadiene, PBD, and under fatiguing conditions in natural rubber, NR. The concentration of tocopherol needed to stabilise polymers, and in particular plastics, is very small and a concentration as low as 100-300 ppm would be sufficient to impart an excellent melt stabilisation to polyethylene, PE, and polypropylene, PP, at a level lower than is required by some of the best synthetic hindered phenol antioxidants traditionally used for this purpose. The colour stability of polyolefins, e.g. PP, processed in the presence of a-tocopherol is as good as that of Irganox 1010 when the former is used at low concentrations, though higher concentrations of the former impart greater discoloration to the polymer. The incorporation of a phosphite antioxidant drastically reduces polymer discolouration concomitant with higher levels of retention of the tocopherol in the polymer. Examination of the nature and distribution of oxidative transformation products formed from a-tocopherol during melt processing of polyolefins has confirmed that polymer discoloration is, at least in part, due to the formation of coloured products from tocopherol.