The Effects of the Molecular Weight and the Environment of Antioxidants on their Efficiency

Abstract

Following the commercial exploitation of synthetic high polymers, a considerable amount of effort has been expended in seeking additives for these materials which would suppress their oxidative deterioration. The majority of available information centres upon the search for antioxidants which will provide the optimum chemical efficiency in preventing these oxidation processes. However, it has become apparent over recent years that other factors, physical rather than chemical in nature, are of primary importance in determining the activity of antioxidants. The present work has involved an investigation of the effects of these physical phenomena on the efficiency of antioxidants in polypropylene. In particular, it is demonstrated that the loss of antioxidants from polypropylene by volatilisation is of mejor importance in determining their efficiency, It is shown that although this loss can be curtailed by increasing the molecular weight of phenolic antioxidants, the direct oxidation of the antioxidant itself, to more volatile products, can limit the success of such an approach. Furthermore, it is demonstrated that mechanism of action of sulphur antioxidants markedly reduces the persistence of these stabilisers. The effects of the compatibility and mobility of antioxidants in polypropylene upon their efficiency are discussed and it is demonstrated that low compatibility and low mobility do not necessarily preclude useful stabilising efficiency. Furthermore, indications are obtained that high compatibility may be detriments1 to the efficiency of an antioxidant. A sensitive technique is introduced for assessing the progress of oxidation reactions in a polymer and the stabilising activity of antioxidants, based upon the measurement of the torsional properties of the polymer.