DMTA characterization of moisture aged epoxy

A commercial epoxy resin used as a matrix for carbon fiber reinforced composites for structural rehabilitation in civil infrastructure, was cured at room temperature. The cured samples were then aged in ambient conditions, then immersed in deionized water, salt solution or alkali solution for two years. Moisture absorption and thermomechanical properties were investigated using gravimetric measurements and dynamic thermal mechanical analysis (DMTA), respectively. When aged in ambient conditions (23&degC, 30% humidity) for two years, epoxy samples did not reach moisture uptake saturation, and the moisture diffusion curve was essentially sigmoidal. When immersed in deionized water, salt solutions or alkali solutions, the moisture absorption behaviour followed Fick's law. The competing effect between post-curing and water plasticization is believed to be responsible for the variation in the glass transition temperature (T_g) as well as the variation in the dynamic modulus. Aged in ambient conditions, the postcuring effect was dominant compared to the plasticizing effect due to the moisture uptake. As a result, T _g and the glassy storage modulus increased with the ageing time. On the contrary, aged in deionized water and the solutions, the plasticizing effect surpassed the postcuring effect, leading to a lower T_g and relatively unchanged glassy storage modulus. The split of T_g of the epoxy aged in the deionized water or solutions was attributed to a part of the epoxy network structure dried during DMTA testing.