Understanding the influence of wood as a substrate on the permeability of coatings by NMR imaging and wet-cup

An important reason to apply coatings on wood is to protect wood against moisture. As a result of regulations and ecological concerns, there has been a shift towards waterborne coatings, which make coatings intrinsically more sensitive to water. As a consequence of the higher sensitivity to water, the durability of both wood and coatings can be negatively affected. In order to use waterborne coatings for woodcare, the main factors influencing transport through these coatings have to be understood. The aim of this study is to elucidate the influence of the wooden substrate on the water permeability of the coating applied to it. Pine sapwood, oak and teak were selected as the wood types, covering a whole range of low to high density wood. Three types of coatings were formulated: a solventborne alkyd, a waterborne alkyd and a waterborne acrylic. NMR imaging was used to measure the moisture distribution and quantify the changes in bound and free water in coated wood during drying. For all wood-coating combinations that were investigated, water transport appeared to be externally (i.e. coating) limited. The loss of bound water started only after evacuation of free water, which showed a local thermodynamic equilibrium associated with bound and free water. We furthermore compared water permeability of free films and wood supported films to understand the influence of the wood-coating interactions. The permeability of free coating films has been determined by the wet-cup method, followed by investigating the solubility and diffusion of water in order to understand the differences in the permeability. We found that the interaction of the coating with the wood has no influence on the water permeability for the considered combinations. Furthermore, the permeability is largely determined by the water solubility.