Effect of pigment and temperature onto swelling and water uptake during organic coating ageing

Abstract

Free films and coatings of a model epoxy system, with or without pigment (TiO2), were aged in saline solution at different temperatures (30°C, 40°C and 50°C). The water uptake was evaluated using gravimetry for free film and using electrochemical impedance spectroscopy (EIS) for coatings. In parallel, the swelling of coatings was in situ monitored using scanning electrochemical microscopy (SECM).It was shown that the temperature has no effect on the water uptake values for unpigmented systems and a slight effect onto swelling. For pigmented systems, the water uptake values slightly increase likely due to additional water absorption at the pigment/binder interphase. The swelling of pigmented coating was found to be lower than that of unpigmented coatings for all ageing temperatures. This result was explained by particular internal stresses that develop in pigmented system.Water uptake values obtained from gravimetry and EIS, using the Brasher and Kingsbury equation, were different for all the temperatures for unpigmented and pigmented coatings. Using a modified Brasher and Kingsbury equation that considers the experimental swelling obtained by SECM, the same water uptake values were obtained for all systems and all temperatures. Finally, it appears that the coating swelling is a key parameter to evaluate water uptake in organic coatings, especially when different coatings are compared.