Moisture transport in coated wood. Part 1: Analysis of sorption rates and moisture content profiles in spruce during liquid water uptake

The sorption of liquid water in coated spruce was determined by measuring the change in the overall moisture content of the wood as a function of time, and by determination of the moisture content profiles after various time intervals. The coatings studied included a solventborne alkyd coating and two waterborne acrylic coatings with different layer thickness. Uncoated wood was used as a reference. The measured data were analysed according to different models, which were based on capillary flow or on diffusion with a constant or a changing surface concentration. The apparent diffusion coefficients based on sorption rates ranged between 7.65 and 98.16 x 10(-11) m s(-2) depending on the surface treatment. These diffusivities were however, not suitable to predict the moisture content profiles in the coated wood, which showed a strong increase close to the surface. The most accurate prediction of the moisture content profile was based on the changing surface concentration and the diffusion coefficient from sorption data of uncoated wood. In general, diffusion seemed to be the most important factor in the transport of water. Capillary flow of water only influenced the sorption very close to the surface of the wood. This aspect became more important for uncoated wood.