The effect of amines on the UV-curing of epoxy resins

Epoxies are generally cross-linked by the addition of a hardener, most of the time a diamine such as diamine diphenyl sulphone, oxydianiline or methylene diamine, and then thermocured. These formulations are quite often used, particularly in the aerospace industry for making structural materials, prepregs or composites. In this paper we have investigated the cross-linking reactions of a difunctional cycloaliphatic epoxide monomer 3,4-epoxycyclohexylmethyl-3', 4'-epoxycyclohexane ne carboxylate initiated by UV-irradiation and compared the kinetics with N,N-diglycidyl-4-glycidyloxyaniline (a nitrogen-containing monomer with a functionality of three) and 4,4'-methylenebis (N,N-diglycidylaniline) (another nitrogen-containing monomer with a functionality of four). Kinetics is followed using a differential photocalorimetry (DPC) technique. Upon UV irradiation in the presence of cationic photoinitiator, the difunctional cycloaliphatic epoxide monomer shows an exotherm peak whereas for the latter two monomers, no exotherm peaks were observed from the sample as a result of exposure to the UV source. To explain the phenomenon observed, the effect of addition of two amines with wide difference in the basicity of p-nitroaniline and pyridine, has been studied on the UV-curing of epoxy resins. It has been found that the presence of amines does retard the rate of photopolymerization and the extent of retardation is dependent on the basicity of the amine. Of the two amines used, pyridine and p-nitroaniline, the former is a stronger inhibitor, because of the ready availability of the lone pair of electrons. The results explain the non-reactivity of nitrogen-containing epoxy monomers N,N-diglycidyl-4-glycidyloxyaniline and 4,4'-methylenebis (N,N-diglycidylaniline) to cationic polymerization upon exposure to UV-radiation.