Effect of Transition Metal Compounds on the UV Degradation of Polymers

Abstract

The effects of typical nickel complex light stabilisers as UV screens and as additives were compared in model systems initiated by hydroperoxides or carbonyl compounds. Oxygen absorption studies showed the nickel oxime and nickel dithiocarbamate complexes to impart a stability additional to UV screening in the hydroperoxide initiated photo-oxidation and only screening in the triplet initiated oxidant. On the other hand, cyasorb UV(1084) was found to stabilise photo-oxidation only by screening of ultra violet radiation. The order of effectiveness of the additives is Ni(DBDC)2 > Ni (OX)2 > cyasorb UV(1084). The stability of these complexes to UV radiation was compared by measuring the changes in the UV absorption spectra of these complexes. Their stabilities to UV radiation was found not to be effected by the presence of carbonyl compounds. The determination of the extinction coefficients of typical UV stabilisers in the range 230 mm - 400 mm enabled a comparison to be made of their theoretical screening ability. These values correlated well with their screening ability in the above tests. The reactions of Ni (DBDC)2 and Ni (0X)2 with cumene hydroperoxide were examined kinetically, and by subsequent product analysis some of the kinetic parameters were determined. Ni (DBDC)2 decomposed hydroperoxides by a first order ionic reaction to give phenol and acetone, whereas the reaction of Ni (0X)2 with hydroperoxide was rather slower and produced the corresponding alcohol in a stoichiometric reaction. An analysis of the products of this reaction by TLC, IR and GLC produced evidence to suggest that the first step in the stabilising mechanism is the attack of hydroperoxide on a Ni(OX)molecule giving rise to O-hydroxyacetophenone and salicylic acid as the major oxidation products. An attempt was made to correlate the results obtained on the above solution studies with the behaviour of same stabilisers as screens and additives in high density polyethylene, low density polyethylene and polystyrene. It was found that all three complexes retarded the breakdown of hydroperoxide to ketone, when used as screens when used as an additive Ni(DBDC), a typical peroxide decomposer completely eliminated the formation of hydroperoxide derived carbonyl. Ni (0X)2 and cyasorb UV(1084), which have previously been reported to quench excited states retarded but did not eliminate hydroperoxide derived carbonyl formation. On the basis of the above results it is concluded that the stabilising activity of these nickel complexes is primarily due to a screening of the UV radiation. In addition, interaction with hydroperoxides is considered to be an auxiliary stabilising mechanism, No evidence was found that excited state quenching was an important stabilising mechanism.