High Temperature Moisture Relations of Grand Fir

The apparatus for this research included: 1) a high-pressure container or capsule, 2) a controlled-temperature, hot-oil bath in which the pressure capsule was submerged, 3) an evacuation system, and 4) a system for continuously recording vapor pressure within the capsule. The capsule consisted of two flat plates of stainless steel bolted together. A cavity 7-1/2 inches square and 1/4 inch deep was milled into the center of the inner face of the bottom plate. Adsorption data were collected for whole wood of grand fir (Abies grandis Lind.) at four experimental temperatures (100, 130, 150, and 170?C.). Specimens were prepared by crosscutting wafers 1/8 inch thick so water vapor had ready access to all cell lumens. The wafers were air-dried and then ovendried at 105?C. For adsorption measurements, a precise quantity of water was injected into the preheated capsule cavity and the vapor pressure in the system was recorded continuously until a steady state was reached. The adsorption isotherms show that, from zero to approximately 0.6 rvp, the equilibrium moisture content of wood decreased with each successive increase in temperature. Above approximately 0.6 rvp, on the other hand, the hygroscopicity of wood increased markedly with each increase in temperature. Under the influence of combined high temperature and high moisture content the wood was considered to have been swelled “inordinately” and its hygroscopicity (accessibility) increased. Fiber saturation points obtained at 100, 130, 150, and 170?C for grand fir were approximately 28, 38, 45, and 46 percent moisture content, respectively. From these data it is theorized that sorption is monomolecular until the second inflection of the isotherm, where multilayer adsorption and inordinate swelling both occur. Adsorption isosteres converge at 298?C and 1200 psi pressure. Sorption-desorption hysteresis persists at high temperatures.

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