Photo-oxidative stability and photoyellowing of electron-beam- and UV-cured multi-functional amine-terminated diacrylates: a monomer/model amine study

Abstract

A number of dialkylamine-terminated diacrylate monomers are prepared by a Michael addition reaction and their photo-oxidative stability and photoyellowing are studied after UV and electron-beam curing by IR and UV absorption spectroscopy. Photo-oxidative stability of the resin is controlled more by the structure of the triacrylate monomer used to prepare the dialkylamine diacrylate monomer, while the structure of the latter markedly influences the growth and decay of photoyellowing. Monomer terminated with di- n-butylamine groups gives the greatest amount of photoyellowing whilst alkylethanolamines give the least (as measured by second-order derivative UV absorption spectroscopy at 275 nm). The former, however, is photobleached more rapidly than the latter. UV-cured resins exhibit a more facile photoyellowing than the same electron-beam-cured systems, and this is associated with hydrogen-atom abstraction and oxidation of the alkylamine group by the residual photoinitiator. Model alkylamine-terminated lauryl acrylates exhibit the same yellowing phenomena during both thermal and photochemical oxidation, confirming that the reaction product is a function of the alkylamine group. Possible reaction mechanisms are discussed in some depth to account for the photoyellowing phenomenon. It is suggested that the primary process is oxidation of the methylene groups adjacent to the nitrogen, giving rise to unstable hydroperoxides which dissociate to give α,β-unsaturated carbonyl groups and hence lead to the loss of aldehydes.