Predicting water transport in multilayer coatings

Water transport in polymer coatings is a key process in their failure. Mostly, such systems are composed of multiple layers to meet the required specifications. Understanding water transport requires a model that relates the kinetics with the properties of the layers. In this article a theoretical model for water transport in two-layered coatings is formulated and verified. The model considers systems consisting of a hydrophobic top coat and a hydrophilic base coat. It describes the process on the basis of the top coat permeability and the base coat sorption isotherm, which define the timescale and the driving force of transport. The systems of interest had a base coat sorption isotherm, which indicated that sorption is a process of clustering of water. The non-linear isotherm shape resulted a timescale for uptake and drying of days and hours, respectively. The model is verified with NMR imaging of water transport and gravimetry. Practically, the results mean that it is sufficient to measure the top coat permeability and the base coat sorption isotherm to understand water transport kinetics in such systems.