High Molecular Weight and Polymer-Bound Antioxidants

Abstract

Studies in model system and in polypropylene, of a series of sulphur antioxidants of increasing molecular weight and containing the phenolic moiety have furnished evidence for the importance of both the intrinsic antioxidant activity of the additives as well as those of their physical attributes. The latter include: compatibility, solubility and volatility phenomena. While thermal oxidation studies in a liquid substrate (decalin) showed the insignificant influence of the additives’ molecular weight over their stabilising activity, studies in polypropylene subjected to enclosed and flowing environments demonstrated markedly different behaviour. Furthermore, the photostabilising performance of the antioxidants and their processing melt behaviour showed considerable variation which were primarily attributable to the physical characteristics of the additives. Analogous studies were also carried out on a second series of sulphur antioxidants of varying molecular weight but containing the 2-hydroxybenzophenone moiety. Despite their inherent outstanding uy. screening property and peroxidolytic activity, their limited solubility in polypropylene proved to be their main shortcoming as polymer stabilisers. Synergistic studies of selected synthesised phenolic (Sulphur) antioxidants with a widely-used uv. stabiliser showed superior synergistic activity in relation to those of commercial phenolic antioxidants. This was rationalised on the basis of the powerful peroxide decomposing activity of the sulphur antioxidants. Further improvement of antioxidant substantivity and compatibility was investigated on the basis of the possible polymer-binding reactions of selected antioxidants and the simultaneous formation of higher molecular weight derivatives during the melt processing operations. A uv-initiated grafting of 3, 5-di-tert—butyl—4—hydroxy—benzyl mercaptan to polypropylene films was investigated and its application as a polymeric antioxidant masterbatch for virgin polypropylene was also carried out.