Water transport in multilayer organic coatings

Protective multilayer coatings are designed with each layer providing distinctive features to reduce the transport of water, ions, and oxygen from the environment to the substrate. The topcoat provides the barrier against the environment whereas the basecoat/primer provides adhesion to the substrate and additional protection such as galvanic or passivation. The change in the coating capacitance during the ingress or egress of water is associated with the change in the dielectric property of the coating due to the large difference between the relative dielectrics of water and coating. The coating capacitance during wetting and drying can be monitored using the electrochemical impedance spectroscopy response of a high frequency signal applied to the coating. The evolution of water uptake during wetting and water loss during drying associated with a two-layer coating is presented. Embedded electrodes were used to access the primer and a hydrophilic room temperature ionic liquid was used to simulate drying while maintaining a non-aqueous electrical contact to the coating. The wetting and drying behaviors of epoxy, urethane, and epoxy primer/urethane topcoat coatings were experimentally measured. Mathematical models were developed based on Fick's second law for water transport in single-layer and two-layer coatings. Comparison of measured and simulated water uptake demonstrated that the assumptions used in the single-layer model were appropriate while the assumptions used for transport across the interface between the layers in the two-layer coating were inadequate.