Principles of Moisture Movement in Wood

Study of the movement of moisture in wood is complicated by the fact that wood is a heterogeneous material consisting of a relatively orderly arrangement of cell walls and cell cavities. The combined movement of bound water and water vapor below the fiber saturation point has been studied, but these studies have not dealt with the complexities of liquid movement in the void structure. Fickian diffusion is the principle that the rate of transfer (F) is equal to the product of the diffusion coefficient and the concentration gradient.If two opposite boundaries of a piece of wood are maintained at different relative humidities but at equal total pressures, a constant rate of moisture movement will develop. However, if the pressures are unequal, the moisture movement is termed hydrodynamic rather than Fickian. Hydrodynamic flow is important in treating wood with preservatives and in high temperature drying. Fickian diffusion is complicated by another rate-controlling factor, the time dependent deformation of the cellulose-lignin matrix. The exact relationship between these two phenomena in unsteady state situations remains to be worked out. Attempts to describe the movement of moisture in wood as parallel and series combinations of bound water and water vapor diffusion have met with problems in estimating component factors. In determining bound water diffusion, measurements of tangential flow require substantial correction for the gross anatomy of the wood. More accurate measurements of the role of minute anatomy, especially fibril angle, upon rate of flow of bound water, are needed. While permeability has an appreciable effect on drying time above the fiber saturation point, it has much less effect on diffusion below saturation. There is a strong need for greater precision in every phase of the study of wood moisture movement.

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