Influence of pigment and internal stresses on water uptake in model epoxy: a thermodynamic approach

This study focused on the influence of a pigment (titanium dioxide at rates of 10 and 20 wt%) on the hygrothermal ageing of a model epoxy polymer resin (DGEBA–DAMP). An ageing cycle composed of sorption and desorption steps was applied at different temperatures. Gravimetric study was carried out on free films. Results of pigmented samples were compared with those of the resin alone to better understand impact of the titanium dioxide. TiO₂ had no major influence on gravimetric curves: water content curves exhibited a pseudo-Fickian behaviour during sorption and Fickian one during desorption. Moreover, results pointed out that pigment did not absorb water. Curve treatment allowed to obtain diffusion coefficient and water content at equilibrium for each step of ageing. Values of diffusion coefficients indicated that titanium dioxide did not have major influence on the beginning of diffusion, whereas it slowed down the end of water uptake. During sorption, water content at equilibrium was dependent on TiO₂ amount. After desorption, pigmented systems contained residual water for ageing cycles at high temperature. Finally, results of diffusion coefficients showed that diffusion phenomena were heat dependent. A thermodynamic approach was then used to have access to enthalpy of diffusion ΔH and pre-exponential factor D ₀. These parameters were impacted by the presence of titanium dioxide. This was mainly due to the presence of additional internal stresses of Type II.