Hygroscopicity of wood

Hygroscopicity is the property of wood to attract moisture from surrounding atmosphere and to hold it in the form of liquid water or vapor. The moisture of living trees varies from about 30 to 300 %. In softwoods, heartwood has a lower moisture content than sapwood (heartwood about 55 % and sapwood about 149 %).

The basic reason for moisture entering into the mass of wood is the attraction of water molecules by the hydroxyls of its chemical constituents, mainly cellulose. As a result, a monomolecular layer of water is formed and held by these hydroxyls with strong hydrogen bonds. Formation of this layer results in pushing apart chains of cellulose molecules in the amourphous regions and between the crystallites of the microfibrils, so that wood starts swell. Under the effect of secondary attractive forces, more water molecules enter and form a polymolecular layer. An additional part may enter by capillarity condensation in cell-wall voids and pit features. After saturation of the walls, liquid water also enter cell cavities. The distinction is made between water held in cell walls (bound water) and cell cavities (free water). The condition at which the walls are saturated but the cavities are empty is called fiber saturation point. Fiber saturation point (FSP) may be calculated from the relationship:

FSP=\left(\frac{1}{\rho_{k}}-\frac{1}{\rho_{0}}\right)\cdot\rho_{water}          (%)

Moisture content of wood (w) may be calculated from the relationship:

w=\frac{m_{w}-m_{o}}{m_{0}}\cdot100        (%)

where

mw - weight of wet wood

m0 – weight of dry wood

Maximum moisture content of wood (wmax) varies depending on species and may be calculated from the relationship:

w_{max}=\left(\frac{1}{\rho_{k}}-\frac{1}{\rho_{s}}\right)\cdot100          (%)

where

ρk – basic density (g.cm-3)

ρs – density of wood substance ( 1.50 g.cm-3)

The quantity of moisture contained in wood exposed to the atmosphere is not constant but subject to continuous change. Loss of moisture is called desorption, and gain is called adsorption. Wood exposed to constant conditions of ambient relative humidity and temperature for sufficient time, desorbs and adsorbs moisture to be at equilibrium with the ambient atmosphere. This moisture content is called equilibrium moisture content (EMC). Equilibrium moisture content values for various combinations of temperature and relative humidity may be estimated from figure x. EMC shows differences when wood loses and gains moisture. Equilibrium is greater in desorption than in adsorption. This phenomenon is called hysteresis, and is a characteristic property of all cellulosic materials.

The moisture content of wood may be found by different methods, which may be distinguished into direct (gravimetric method) and indirect methods (moisture meters). Measurement of moisture with electric moisture meters is based on the change of electric properties of wood with change of its moisture. There are two types of instruments based on

(a) electric resistance to passage of direct current and

(b) dielectric properties of wood in a high-frequency electrical field.