The antioxidant role of alpha-tocopherol in polymers III. Nature of transformation products during polyolefins extrusion

The nature of oxidative transformation products of the biological antioxida nt alpha-tocopherol formed during melt extrusion of polyethylene (PE) and p olypropylene (PP) was investigated. The distribution of the different produ cts in the polymers, when processed under different conditions and in the p resence of co-additives such as a phosphite and a polyhydric alcohol, was a lso examined. It is shown that similar transformation products were formed in both PP and PE and these were found to be mainly based on quinonoid-type structures including different dimers and trimers as well as aldehydes. Th e reduction in discolouration of PP observed, when using combinations of th e biological antioxidant with either a phosphite antioxidant or a polyhydri c alcohol (non-antioxidant), was attributed to a reduction in the overall c oncentration of transformation products formed, as well as to a more favour able distribution of these products in the polymer leading to much lower co ncentrations of the most highly coloured products, e.g. aldehydes. Based on the nature of the products formed a mechanism is proposed for the antioxid ant action of alpha-tocopherol in PP melt. (C) 1999 Elsevier Science Ltd. A ll rights reserved.