The antioxidant role of alpha-tocopherol in polymers II. Melt stabilising effect in polypropylene

The antioxidant efficiency of the biological antioxidant alpha-tocopherol d uring melt extrusion of polypropylene (PP) was investigated. The melt stabi lising effect of alpha-tocopherol was found to be much higher than that of the commercial synthetic hindered phenol antioxidant Irganox 1010 especiall y at very low concentrations. The excellent stabilising performance of alph a-tocopherol was maintained even under severe extrusion conditions. The col our stability of PP processed in the presence of alpha-tocopherol was shown to be as good as that of Irganox 1010 when the former was used at low conc entrations. However, higher concentrations of alpha-tocopherol imparted gre ater discoloration to the polymer than Irganox 1010 examined at the same co ncentrations. The incorporation of a phosphite antioxidant or a polyhydric alcohol (non-antioxidant) was shown to reduce drastically polymer discolour ation concomitant with higher levels of retention of the tocopherol in the polymer. This was attributed partly to the reduction of the overall concent ration of transformation products, formed from tocopherol as a result of it s antioxidant function, and partly to interactions of the co-additives with coloured transformation products and regeneration of tocopherol. The trans formation products were shown to be more coloured than the parent antioxida nt. A mechanism is proposed for the co-operative interactions between alpha -tocopherol and phosphite U-626 which result in the improvement in melt sta bility and reduction in colour of PP. (C) 1999 Elsevier Science Ltd. All ri ghts reserved.