Colour development and luminescence phenomena in epoxy glasses

Abstract

The process of colour development in aromatic-amine cured epoxy resin glasses following exposure to oxygen has been studied using electron spin resonance (ESR) and thermoluminescence (TL). Even short duration exposure to oxygen was found to produce species in glassy epoxy resins which gave rise to TL when heated from room temperature to a temperature just below the cure temperature. In some epoxy samples, oxygen exposure over longer periods was found to produce sample colouration. UV-visible spectrophotometry revealed chromophores which were believed to arise from the diaminodiphenylmethane structural unit found in epoxy systems containing the resin, N,N,N′,N′-tetraglycidyl-4,4′-diaminodiphenylmethane (TGDDM) or the curing agent, 4,4′-diaminodiphenylmethane (DDM). The chromophore produced in TGDDM-based resins appeared to be simlar to the 'Wurster's salt'-type structure produced by photoionization of these systems. A distinctive ESR spectrum was also noted for samples exposed to oxygen. These results are indicative of both peroxide group formation on the methylene groups as well as the formation of amine centred radical cations, and are consistent with the oxidative sensitivity of these epoxy resins. The TL is believed to result from the thermally induced recombination of the 'Wurster's salt'-like cations with peroxy anions leading to the production of an electronically excited diaminobenzophenone structure