The Lifshitz-van der Waals, acid-base, and total surface free energies of various wood species were calculated from contact angle measurements. For spruce (Picea abies) and meranti (Shorea spp.) the following three methods were compared: capillary rise in wood powder columns (based on the Washburn equation), dynamic contact angle measurements (according to the Wilhelmy-plate principle), and sessile drop measurements along and across the grain of the wood. The capillary rise method was limited to nonswelling solvents, which means that only the Lifshitz-van der Waals component could be measured. With the dynamic contact angle measurement, the wettability during the first immersion was decreased compared to that of the sessile drop. This was probably due to reduced capillary penetration, but with the second immersion the presence of an adsorbed solvent layer increased the wettability and hence affected the surface energy data. The sessile drop measurements were highly dependent on the direction of measurement. Increasing the wood moisture content decreased the Lifshitz-van der Waals component and increased the basic surface energy parameter of the wood. All of the wood species tested were characterized as having low-energy surfaces with a dominant Lifshitz-van der Waals component. Measurement of acid and base parameters of wood surfaces seemed not to be very reliable because of its strong dependence on the measuring conditions. With respect to this, it should be noted that thermodynamic equilibrium conditions assumed by Young's equation are generally not fulfilled with wood surfaces because of chemical heterogeneity, surface roughness, and the adsorption of the test solvent.
The sorption of moisture by spruce and meranti coated with both waterborne and solventborne coatings was studied during controlled conditions. Experimental variables included: coating ®lm thickness, temperature and gradients in relative humidity (33±98, 65±98 or 33±75%), or exposure to liquid water. Changes in moisture content of the wood and the tangential or the radial dimensions were recorded as a function of time. Apparent moisture diffusion coef®cients were calculated from the initial slope of the fractional weight increase following Fick's second law for unsteady state conditions. The apparent moisture diffusion coef®cients were clearly in¯uenced by both coating and wood species. The fastest moisture adsorption was observed for uncoated spruce which could be explained by the rapid capillary uptake of water. Moisture diffusion in coated samples of spruce or with coated or uncoated meranti was much lower. With the exception of liquid water, moisture diffusion during desorption was faster than during adsorption. The measured moisture diffusion coef®cients should be considered as apparent because they were dependent on the initial and ®nal moisture content. Moisture diffusion was found to be strongly dependent on temperature with activation energies between 55±76 kJ mol )1 . The rate of dimensional change was described by a two-parameter asymptotic regression model that included one constant for the rate of dimensional change and an asymptotic constant given by the dimensions at equilibrium wood moisture content as an asymptote. The differences in rate constant were inuenced by the same factors as those for moisture diffusion. The correlation between rate constant and moisture diffusion coef®cient was good except for very high moisture diffusion coef®cients that were controlled by capillary water uptake.
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