SORPTION OF MOISTURE BY WOOD WITHIN A LIMITED RANGE OF RELATIVE HUMIDITIES

Information on wood sorption behavior provides desorption and adsorpti on curves from the green and fully dry initial conditions, respectivel y. Wood in service, however, is exposed to only a narrow range of rela tive humidities (RH) and, therefore, should exhibit intermediate isoth erms that are different from those of the full isotherms. This paper p resents the results of a study performed to establish the intermediate desorption curves at 30 C for yellow poplar wood samples subjected to RH ranges of 92% to 0%, 75% to 0%, 53% to 0%, and 32% to 0%. The desi ccator method was used to establish the full desorption curve, while a high-vacuum system was employed to obtain the full adsorption and the four intermediate desorption curves. The desired relative humidities were maintained by saturated salt solutions. The generated intermediat e desorption curves all fall within the full adsorption and desorption isotherms, with the isotherms originating from 92%, 75%, 53%, and 32% RH tracing progressively lower curves. These isotherms exhibit relati vely short crossovers; a 40% RH change being needed for an intermediat e desorption curve to cross over from the full adsorption isotherm to the full desorption isotherm. The intermediate curve from 92% RH has t he characteristic sigmoidal shape, but those from 75%, 53%, and 32% RH are concave towards the X-axis. All the intermediate curves exhibit h ysteresis, even at low RH levels and narrow RH ranges.