Discoloration of polymers by phenolic antioxidants

Part of colour development in aged polyolefins stabilized with phenolic antioxidants can be attributed to the formation of conjugated diene compounds, arising as a consequence of sacrificial trapping of alkylperoxy radicals by phenolics. The discoloration depends on the structure and concentration of the phenol transformation products. Peroxycyclohexadienones are formed transiently in low concentrations and themselves do not discolour the polymer matrix. However, some products of their thermolysis are discolouring and accumulate slowly in the polymer matrix. The principal contribution to polymer discoloration is due to formation of quinone methides. Their discolouring effect is reduced in propionate-type phenolics, as a consequence of intramolecular rearrangement of a part of the primarily formed quinone methide, and is due to oxidative dimerization accounting for nonconjugated dimeric quinone methides. Attention was paid to transformations of octadecyl 3-(3,5-di- tert-butyl-4-hydroxyphenyl)propanoate and related quinone methides and 4-hydroxycinnamates.