Wood and Moisture

Wood is a porous, hygroscopic material that can take up water both within cell walls (cell-wall water) and in the macrovoid structure (capillary water). Therefore, moisture transport in wood occurs through multiple pathways and phases of water, that is, both cell-wall water, liquid water, and water vapor can be transported through the material structure. The amount of water in wood is quantified by the moisture content (mass of water in relation to the dry mass), which changes with the surrounding environmental conditions (relative humidity and temperature). This relation is commonly depicted in sorption isotherms that plot the equilibrium moisture content as function of relative humidity for specific, constant temperatures. How water is taken up by wood changes over the relative humidity range. In the hygroscopic range (0% to 97–98% relative humidity), water is predominantly taken up in cell walls, whereas capillary condensation of liquid water in the macrovoid structure becomes dominant in the over-hygroscopic range (>98% relative humidity). The equilibrium illustrated in sorption isotherms for specific environmental conditions is not singular, but depends on the sorption history; this phenomenon is known as sorption hysteresis. Sorption isotherms are generally modeled using mathematical expressions that are fitted to data in the hygroscopic range. Some of these models include quantities describing the physical reality of wood-water interactions; however, these quantities rarely match up to the experimentally determined reality for wood. The same can be said of the mathematical models describing the kinetics of water uptake in wood cell walls. One of the most well-known concepts concerning water in wood is the fiber saturation point (FSP). It can be found as the threshold moisture content above which physical wood properties do not change significantly with moisture content. The FSP is, however, lower than the maximum moisture content of wood cell walls which occurs in the fully water saturated state. A change in moisture content below the FSP is accompanied by dimensional changes, that is, shrinkage and swelling, which reflect changes in the amount of water within cell walls. While this phenomenon originates at the nanoscale, it is observed on all length scales of wood. Such dimensional changes as well as other physical wood properties and fundamental wood-water interactions can be altered by chemical modification of wood. How these relations are affected depends on the chemical nature of the modification. At the end of this chapter, a broad overview is given of experimental methods for characterizing moisture in wood, including determination of moisture content, sorption isotherms, moisture state and location, and moisture transport in wood.