The antioxidant role of vitamin E in polyolefins

Abstract

Polymers are subject to oxidation throughout their lifecycle. Antioxidants are generally incorporated in polymers to inhibit or minimise oxidative degradation. Hindered phenolic antioxidants are important stabilisers for polyolefins. However, hindered phenols undergo chemical transformations while performing their antioxidant function during processing and fabrication. In addition, antioxidants are subject to loss from polymers during processing, or subsequently in-service. Migration of antioxidants is a major concern in applications involving polymers in direct contact with food and human environment. This concern is compounded by the realisation that very little is known about the nature and the migration behaviour of antioxidant transformation products. In this work, the antioxidant role of the biological antioxidant -tocopherol (Vitamin E) , which is structurally similar to many synthetic hindered phenols, is investigated in low density polyethylene (LDPE) and polypropylene (PP). The melt stabilising effectiveness of -tocopherol (Toc) was found to be very high, higher than that of commercial hindered phenol antioxidants, such as Irganox 1076 (Irg 1076) and Irganox 1010 (Irg 1010), after multiple extrusions, especially at very low concentrations. The high antioxidant activity of Toc was shown to be due, at least in part, to the formation of transformation products during processing. The main products formed are stereoisomers of dimers and trimers, as well as aldehydes and a quinone - the relative concentration of each was shown to depend on the processing severity, the initial antioxidant concentration and oxygen availability. These transformation products are shown to impart better, similar or lower melt stability to the polymer than the parent antioxidant. The nature of the products formed from Toc during processing was compared with those formed during processing of Irg 1076 and Irg 1010 with LDPE and a mechanism for the melt stabilisation of Toc was proposed and compared with the stabilisation mechanisms of the synthetic antioxidants Irg 1076 and Irg 1010.