Water vapor transport in cataphoretic coating and its associated multilayer system

The water transport properties of an automotive paint system were characterized. First, we focused on epoxy-based cataphoretic free films and the corresponding multilayer paint system, consisting of a clearcoat, two basecoats, and the cataphoretic coating. The temperature dependence of diffusivity was highlighted by measuring water uptake at 30 °C, 50 °C, and 70 °C. The activation energy was found to range from 52 kJ/mol to 26 kJ/mol, depending on relative humidity. Water concentration at saturation was measured as a function of relative humidity across the temperature range. Sorption isotherms exhibited a linear behavior consistent with Henry’s law up to 50% relative humidity, followed by a positive deviation beyond this threshold. This deviation was attributed to the formation of water clusters, whose size was subsequently quantified. The cluster size increased significantly beyond 50% relative humidity ( i.e. , 0.5 water activity) and appeared to be only slightly influenced by temperature. • Full characterization of water transport properties of cataphoretic coating. • Assessment of water solubility and clustering as a function of relative humidity. • Water transport contribution of each layer/coating to a multilayer coating system.