Mechanisms of antioxidant action: Metal complexes as u.v. stabilisers in polyethylene

Abstract

In order to evaluate the importance of u.v. screening, peroxide decomposition and excited state quenching in the mechanism of action of a variety of metal complex stabilisers, screening and additive studies were carried out in high density polyethylene and in model hydrocarbons. All the metal complexes studied were found to be u.v. screening agents when contained in a separate phase between the u.v. source and the polymer and all were rather more effective than a typical commercial u.v. absorber (2-hydroxy-4-octyl-oxybenzophenone, HOBP). However, when used as additives, all the stabilisers had an additional function. In the case of the transition metal dibutyl dithiocarbamates (NiDBC and FeDBC), the additional stabilisation mechanism is consistent with their known peroxide decomposing behaviour. In the case of the 2-hydroxy-acetophenone oxime complexes (NiOx and CuOx), the initial function is a stoichiometric reaction with hydroperoxides to give non-radical products. An amine complex of nickel bis-octylphenol sulphide (NiBOPS) appeared to behave primarily as a screen. Only one of the metal complexes (FeOx) appeared to be capable of functioning as an excited state quencher and this complex is effectively sensitised by triplet carbonyl to give a photo-pro-oxidant effect in cumene. It is concluded that the concept of excited state quenching, although it may occur with some metal complexes, is not a sufficient criterion of u.v. stabilising activity.