Hydrostatic pressure and temperature dependence of wood moisture sorption isotherms

By expressing wood moisture content data as a function of adsorption energy, an interesting scaling capability is obtained, wherefrom the general hydrostatic pressure and temperature dependence of wood moisture content is determined. The scaling law is fully consistent with the thermodynamics of swelling. It can be used to transform room condition sorption isotherms to other temperatures and hydrostatic pressures, provided that the wood matrix is not irreversibly modified. A special procedure is suggested for the case of an irreversibly changing wood matrix, as in thermal modification and thermo-hydro-mechanical treatments. Using the present scaling theory, several fundamental aspects of wood moisture sorption are explained, such as the absence of a significant quantity of strongly bound wood moisture, the internal stress generation by sorption hysteresis in the wood cell wall, and the reason for the reversible disappearance of the sigmoid shape of the sorption isotherm at higher temperature. The results of this research may be useful (a) for transformation of known sorption data to other conditions, notably where in situ moisture measurements are difficult to perform and (b) to quantify the effects of internal stresses in the ultrastructure of the cell wall on moisture content.