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

Abstract

The antioxidant efficiency of the biological antioxidant α-tocopherol during melt extrusion of polypropylene (PP) was investigated. The melt stabilising effect of α-tocopherol was found to be much higher than that of the commercial synthetic hindered phenol antioxidant Irganox 1010 especially at very low concentrations. The excellent stabilising performance of α-tocopherol was maintained even under severe extrusion conditions. The colour stability of PP processed in the presence of α-tocopherol was shown to be as good as that of Irganox 1010 when the former was used at low concentrations. However, higher concentrations of α-tocopherol imparted greater discoloration to the polymer than Irganox 1010 examined at the same concentrations. The incorporation of a phosphite antioxidant or a polyhydric alcohol (non-antioxidant) was shown to reduce drastically polymer discolouration concomitant with higher levels of retention of the tocopherol in the polymer. This was attributed partly to the reduction of the overall concentration of transformation products, formed from tocopherol as a result of its antioxidant function, and partly to interactions of the co-additives with coloured transformation products and regeneration of tocopherol. The transformation products were shown to be more coloured than the parent antioxidant. A mechanism is proposed for the co-operative interactions between α-tocopherol and phosphite U-626 which result in the improvement in melt stability and reduction in colour of PP.