Influence of moisture present during polymerisation on the properties of a photocured epoxy resin

The cationic photopolymerisation of a bisphenol A diglycidylether-based adhesive is strongly retarded by the humidity of the surrounding atmosphere. Due to the side reaction with water the hardness of the formed polymer decreases with increasing humidity of the atmosphere. Although the polymerisation is retarded by moisture, the epoxide groups are completely turned over because the cationic polymerisation of epoxides is a living polymerisation. Depending on the irradiation time and the humidity of the surrounding atmosphere the epoxide consumption can take up to some days. For adhesive connections with a large open surface of the adhesive the side reaction with the moisture of the atmosphere is an obvious problem, because the mechanical properties are depending on the extent of the reaction with the water. For adhesive connections with low contact surface between the adhesive and the atmosphere, no influence of moisture adsorbed on the surfaces of the parts to be joined could be detected for adhesive thicknesses down to 1.5 μm.