Effect of physical ageing onto the water uptake in epoxy coatings

This study aims to evaluate the effect of physical ageing onto the water uptake process in epoxy coatings during immersion in saline solution. DGEBA/Jeffamine230 stoichiometric polyepoxyde system was prepared as coatings onto aluminum Q-panels. Two different curing protocols were applied in order to obtain two fully cured polyepoxyde systems, with different amounts of physical ageing. DSC measurements allowed to measure the enthalpy associated to the physical ageing and the glass transition temperature.

Coated samples were immersed in sodium chloride solution (30 g L⁻¹) at different temperatures (30 °C, 40 °C, 50 °C and 60 °C) and the water uptake was evaluated by electrochemical impedance spectroscopy (EIS), using the Brasher and Kingsbury equation. In parallel, the swelling of the coatings was in situ monitored using scanning electrochemical microscopy (SECM).

At different solution temperatures, it was found that the water uptake values at equilibrium for coatings with physical ageing was lower than the values obtained with coatings without physical ageing. The swelling of coatings with physical ageing was also lower than the swelling of the coatings without physical ageing. A thermodynamic analysis of the diffusion parameters allowed to show that the densification of the polymer matrix induced by the physical ageing is the main factor that governs the water uptake process. This was explained by more interactions between polar groups of the polymer chains that decrease the number of polar groups for water diffusion.