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 top coat provides the barrier, whereas the base coat/primer provides adhesion to the substrate as well as galvanic or passivation protection. 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 and can be monitored using the electrochemical impedance spectroscopy response of a high-frequency signal applied to the coating. The wetting and drying behaviors of epoxy and urethane single-layer coatings and epoxy primer/urethane top-coat two-layer coatings were experimentally measured. A hydrophilic room-temperature ionic liquid was used to simulate drying while maintaining a nonaqueous electrical contact to the coating. Embedded electrodes were used to access the top-coat-primer interface in the two-layer coating. 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.