Transformation products of hindered phenolic antioxidants and colour development in polyolefins

Several quinoidal oxidation products of 2,6-di-t-butyl-phenol-based antioxidants have been evaluated for UV/visible absorption characteristics in methylene chloride solutions and for contributions to Yellowness Index (YI) values in polyolefins. The YI values were determined in polypropylene (PP), high density polyethylene (HDPE) and linear low density polyethylene (LLDPE). The quinoidal compounds with conjugated π-electrons exhibited the highest extinction coefficients in the visible spectrum and also contributed the most colour to polymer formulations. 3,3′,5,5′-Tetra-tert-butyl-stilbene-4,4′-quinone had the highest extinction coefficient in the visible spectrum and caused the highest YI values in the polyolefins—its presence at 5 ppm in the polyolefins was already discernible visually. Colour development during use of polymeric articles containing hindered phenolic antioxidants is attributed to the formation of conjugated quinoidal products as a consequence of radical trapping by the hindered phenols. It is a natural consequence of the stabilization process which can be ameliorated through appropriate choice of antioxidants and through the use of trivalent phosphorus coadditives.