Development of rigid toughened photocurable epoxy foams

This study present a fast and efficient method for preparing photocurable rigid epoxy foams. A system based on the anionic photopolymerization of diglycidyl ether of bisphenol A (DGEBA) combined with a thiol-ene photopolymerization, was used. A tertiary diamine curing agent functionalized with four allyl groups was used in conjunction with a multifunctional thiol. The presence of several basic species, like tertiary amines, thiolates, polythioethers and alkoxide groups induced the anionic ring opening polymerization of the epoxy groups. At the same time the double bonds of the curing agent reacted with the multifunctional thiol generating polythioethers. The flexibility of the polythioethers modified the properties of the epoxy polymers, improving their toughness. Benzenesulfonyl hydrazide as foaming agent and zinc oxide as activator were added to DGEBA to produce the epoxy foams. The photopolymerization kinetics showed that conversion of the epoxide groups reached 80% in 600 s. Thermal curing studies performed by DSC showed that the main curing exotherm were determined at 84-89 degrees C, which match the temperature at which the photocuring was carried out in the UV light chamber (85 degrees C). It was found that the viscosity of the formulations plays a key role in the pore size of the foamed polymer, the higher the viscosity the larger the pore sizes. It was also determined that the impact resistance of the produced foams increased with increasing concentration of the thiol-ene system, as a consequence of a greater amount of polythioethers present in the co-network.