Adsorption boundary curve influenced by step interval of relative humidity investigated by Dynamic Vapour Sorption equipment
E Tang Engelund, M Klamer, T Mark Venås
The adsorption of water vapour from dry conditions by Norway spruce sapwood has been investigated using Dynamic Vapour Sorption (DVS) instrumentation. This equipment allows a fast and easy data acquisition as well as enables detailed studies of sorption properties using very small sample masses. In this study, particular focus was paid to the effect of step size on the sorption isotherms. Furthermore, the influence of relaxation of swelling stresses was investigated. This was done by having relative humidity (RH) histories with different RH step sizes and by introducing prolonged periods of conditioning at constant climate. The adsorption isotherms constructed on the basis of acquired sorption data was not significantly influenced by the differences in relative humidity (RH) histories. Thus, for practical purposes a stability criterion of 0.002 %/min was found to be adequate for acquiring wood adsorption isotherms using sample masses around 20-45 mg. The different RH histories did, however, affect the sorption kinetics. The sorption proceeds slower if the sample was conditioned at constant climate for a prolonged period before being exposed to another RH level. This indicates that relaxation of swelling stresses affects the sorption kinetics. During the initial phase of adsorption after changing RH, the moisture uptake was found to be linear with the square-root of time. From sorption and swelling kinetic theory the diffusion coefficient of the wood cell wall could be estimated based on data from the initial phase of the adsorption processes. The diffusion coefficient was found to decrease with increasing RH and to be independent of step size, as expected.