Water diffusion and swelling stresses in ionizing radiation cured epoxy matrices

In this work a DGEBF epoxy monomer was cured by electron beam radiation in the presence of an iodonium salt and the obtained system was hydrothermally aged as such and also after a thermal treatment, in order to obtain two systems having different uniformity in the cross-linking degree. On both systems, the transient stress field arising from swelling was measured and monitored by an optical Photoelastic technique and the results were commented with reference to a thermally cured epoxy system containing the same monomer and already discussed in a previous work. Beam samples with identical dimensions, obtained from the irradiated systems, have been aged at 80 °C in water, and characterised by Gravimetric and DMTA tests. The results are compared also with already reported swelling behaviour of similar thermally cured systems. It is observed that the different curing techniques (radiation curing, radiation curing followed by thermal curing and thermal curing) determine a different network structure and a different water chemical affinity, which influence the amounts of absorbed/desorbed water, and the relative amounts of bonded/free water. Such differences affect the swelling behaviour, and then the transient stress field. Photoelastic Stress Analysis has allowed to evaluate the evolving stress field, providing a different point of view on the investigation of the material transformations associated to water diffusion.