Photooxidative stability of electron-beam and u.v.-cured di- and triacrylate resins. Influence of resin/initiator structure

The post-cured photooxidative stabilities of electron-beam and u.v.-cured di- and triacrylate based resins have been examined using i.r. and second order derivative u.v. spectroscopic techniques. Using hydroxyl index at 3450 cm¹ as a measure of the rate of photooxidation of the resin films, their stabilities were found to be strongly dependent on the curing process and structure of the residual photoinitiator. The structure of the resin also played a major role in influencing the development of potentially active intermediate chromophores. Resins containing a terminal secondary amine group developed a strong absorption band at 250–300 nm during photooxidation, which paralleled the production of i.r. bands at 1680 and 1605 cm¹. The development of this chromophore is strongly influenced by the nature of the curing process and residual photoinitiator and is shown to be produced through an oxidative mechanism. The chromophore is associated with an α,β-unsaturated carbonyl species and is considered to be a key intermediate in the process of photoyellowing. Resins with no terminal secondary amine groups exhibited no such characteristic spectral changes. Possible mechanistic schemes are proposed to account for the observations.