Moisture content of the coating determines the water permeability as measured by 1D magnetic resonance imaging

In addition to the desired aesthetical properties, a coating is applied to protect against weathering. A coating prevents moisture accumulation in wood by reducing the water uptake into the wood by its barrier function. The studies in the coating permeability has gained interest with the shift towards waterborne coatings, which make coatings intrinsically more sensitive to water. This paper presents the results of a Nuclear Magnetic Resonance (NMR) Imaging study on the influence of a coating's moisture content on the permeability of a coating on wood. In this work, pine sapwood, oak, and teak were selected as wood types covering a whole range of low to high density wood types. Three transparent coatings were formulated: a solventborne alkyd, a waterborne alkyd and a waterborne acrylic. The aim of this study is to investigate how sensitive the permeability of coatings on wood against moisture during liquid water uptake and subsequent drying below Fiber Saturation Point. During both water uptake and subsequent drying, the coating limited transport was observed for the studied wood-coating combinations. The NMR profiles are used to calculate the water permeability of coatings on wood. We have demonstrated the direct relation of the permeability with the average water activity inside the coating, which is connected to the activities of both sides of the coating. We observed reasonably well correlation between the sorption isotherm of the coatings and the permeability, which indicates that the permeability variations are due to the amount of water present in the coating. Finally, we have shown that the permeability is not about the type of water vapour or liquid present at one side of the coating, it is all about the local moisture content in the coating.