Effects of water-soluble extractives on the moisture sorption properties of spruce wood hygrothermally treated at 120°C and different humidity levels

Sitka spruce wood specimens were heated at 120°C and different relative humidity during heating (RHh) levels for 2–7 days and their equilibrium moisture contents (EMCs) were measured at 25°C and 11–97% relative humidity (RH) before and after the removal of the water-soluble extractives. By heating at 80% RHh or less, a longer heating at higher RHh resulted in a greater reduction in the EMC; further, the reduction in EMC was greater at higher RH. Conversely, the EMC value of the wood specimens heated at 100% RHh demonstrated a steep increase at high RH levels, exceeding that of the unheated wood at 97% RH. An increase of this nature in EMC was not realized after the removal of the water-soluble extractives. These results indicate that the deliquescent nature of water-soluble extractives, the decomposition residue of hemicelluloses, has significant influence on the moisture sorption of wood when the wood is heated in saturated water vapour.