Diffusion Coefficients of Softwoods By Steady-State and Theoretical Methods

The effect of moisture content and temperature on the diffusion coefficient of wood was determined by a modified steady-state method and compared with Stamm’s theoretical analysis. There was good agreement between the mean transverse diffusion determined theoretically and experimentally. The coefficient increased with increasing wood moisture content at all temperatures. The longitudinal coefficient decreased with increasing moisture content. Both transverse and longitudinal diffusion coefficients increased with increasing temperature. The degree of increase indicates that the moisture movement is an activated process with activation energies comparable to those obtained from the change in vapor pressure of water with temperature. Theoretical considerations of diffusion paths to moisture movement through softwoods show the path of vapor movement through cell lumens in series with bond water movement in the cross walls as the most important path in all directions. The importance increases with increasing moisture content but decreases with increasing specific gravity. Vapor movement transversely through lumens in series with pit pores is important at low moisture content and higher temperature but is negligible at high moisture content and low temperature. The effect is more pronounced in longitudinal movement.

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