The role of internal stresses, temperature, and water on the swelling of pigmented epoxy systems during hygrothermal aging

The effect of a pigment (titanium dioxide at 20 wt%), temperature and water on the dimensional changes of a model epoxy polymer resin (DGEBA-DAMP) was the focus of this research. Dynamic mechanical analysis was used to quantify in situ dimensional changes of free films, pigmented or not, with a thickness of 120 mu m, at different temperatures (30, 40, 50, and 60 degrees C). The behavior of unpigmented and pigmented systems differed, with pigmented systems swelling less than unpigmented systems. These disparities in swelling were related to the existence of increased internal stresses in the pigmented system because of the inclusion of titanium dioxide. Thermal expansion was investigated, and the coefficient of thermal expansion (CTE) was determined. The CTE of pigmented systems were lower than those of unpigmented systems. Hygroscopic swelling was then deduced and was related to the water content. A relationship between water sorption and hygroscopic swelling was demonstrated and revealed two major points. Firstly, synergistic effects exist between water and temperature leading to a higher swelling. Second, the water/polar group interaction mechanisms govern the swelling kinetics in these systems, a result supported by high enthalpy solubility values. The chemical structure of networks plays a predominant role. Finally, the swellings of free films were compared with those of coated substrates. Due to the presence of residual stresses caused by the formation of the coating/substrate interface, the coatings displayed a greater swelling than the free films. These stresses were calculated utilizing a simplified analytical method and appeared to be comparable for both systems.