Influence of internal stresses on the physicochemical and mechanical properties evolution of pigmented epoxy systems during hygrothermal ageing

This study focused on the influence of a pigment (titanium dioxide at rates of 10 wt% and 20 wt%) on the hygrothermal ageing of a model epoxy polymer resin (DGEBA-DAMP). Firstly, FT-IR, DSC, DMTA and mechanical tests were carried out in order to obtain thermal, physicochemical and mechanical properties. Specific mechanical tests performed under DMA were developed to determine the internal stresses (type II and III) present within these systems and linked to microstructural heterogeneities. Results of the pigmented free films were compared to those of the resin alone to better understand the impact of the titanium dioxide. They show that the addition of titanium dioxide does not influence the physicochemical properties while the mechanical properties are modified. Then, free films were immersed in pure water at 30 °C to investigate water influence on the system behaviors through measurements of the properties at each step of ageing. In the wet state, the physicochemical and mechanical properties of the pure resin show a plasticization of the system by absorption of water. Concerning pigmented samples, a decrease in Tg is observed whereas Young's modulus is not modified and internal stresses increase. Indeed the evolution of Young's modulus is modified by presence of titanium dioxide. This is due to the presence of the additional internal stresses of type II. The latter are therefore predominant on plasticization effect onto the evolution of mechanical properties. After a desorption step, the results of thermal properties show that plasticization seems to be reversible. This result is in contradiction with the fact that additional internal stresses induced by water ingress are not recovered during the desorption step. This suggests that plasticization is not totally reversible. Finally swelling of free films during hygrothermal ageing is modified because of internal stresses due to introduction of titanium dioxide.