The permeability of moisture through the paint film on the wood surface seems to differ from that in the case of an isolated paint film. The main difference is that the swelling of the wooden basis caused by the adsorption of moisture expands the paint on the wood surface, and that subsequently the expansion of the film gives rise to high permeability of moisture through it. In this paper discussion is made on the method to determine the permeability of the paint film on the wood surface on the measurement of the rate of moisture adsorption and desorption of wood coated with four kinds of paint, i.e. aminoalkyd resin paint (AA), nitrocellulose lacquer (NC), polyester resin paint (PE) and polyurethane resin paint (PU).
From the adsorption and desorption curves for the painted wood (Fig. 1 and 2), the adsorption and desorption curves for the wooden basis are plotted in Fig. 4 and 5, assuming distribution of moisture within the paint film and the wooden basis which is schematically shown in Fig. 3. The rate of moisture permeation is obtained from inclination of the curves in Fig. 4 and 5, and is plotted against the average moisture content of the wooden basis in Fig. 6. The coefficient of moisture permeation, P, has been calculated, using eq. 5, where Q/tA is the rate of moisture adsorption or desorption from the unit area of the surface, l the thickness of the paint film, ua the equilibrium moisture content for wood at the surrounding humidity, and ub the moisture content of the wooden basis. The coefficient, P, is plotted against the average moisture content of the wooden basis in Fig. 7. The figure shows that the coefficient, P, is slightly increasing with the increasing moisture content of the wooden basis.
The comparison of the measured P values with those calculated by eq. 6 from diffusion coefficient, D, density, ρ, and k-value of the isolated paint film shown in Table II indicates that the nature of moisture permeation of the paint film on the wood surface differs from that in the case of an isolated paint film.
