Scanning or desorption isotherms? Characterising sorption hysteresis of wood

Water influences most physical wood properties such as mechanical properties and dimensional stability, and additionally plays an important role in wood degradation processes. The interaction between wood and water has therefore been widely studied, and this is often done by measuring sorption isotherms. These depict the equilibrium moisture content of a material as a function of ambient relative humidity (RH) at constant temperature. Typically, the path to equilibrium, i.e. whether equilibrium is obtained through an increase (absorption) or a decrease (desorption) in moisture content, influences the moisture content at given climatic conditions; desorption to equilibrium results in a higher moisture content than absorption to equilibrium with the same ambient climate conditions (Masson and Richards 1906 ). This phenomenon is termed sorption hysteresis and can be observed in many chemically and structurally different materials, e.g. polymers (Watt 1980 ), cement based materials (Espinosa and Franke 2006a ), food (Wolf et al. 1972 ) and wood (Pidgeon and Maass 1930 ). Several mechanisms have been proposed to explain sorption hysteresis such as the ink-bottle or pore-blocking effect (McBain 1935 ), differences between absorption and desorption in principal radius of condensation (Cohan 1938 ), or free volume in swelling polymeric materials (Vrentas and Vrentas 1996 ). Sorption isotherms initiated from non-extreme conditions (neither dry nor saturated state) are generally referred to as scanning isotherms (Espinosa and Franke 2006a , b ; Peralta and Bangi 1998 ; Velasco et al. 2016 ). Scanning isotherms connect the desorption and absorption isotherms and describe the moisture content of a material when it is exposed to alternate absorption and desorption.