Water vapour sorption and moisture transport in and across fibre direction of wood

Abstract

Water vapour sorption experiments are frequently used to characterise the absorption and desorption of water in wood. To evaluate whether water vapour transport—compared to sorption and sorption related processes—can be neglected in small wood samples, this study investigates the sorption kinetics of Norway spruce (Picea abies) samples with different lengths of transport pathways in and across fibre direction. Water vapour sorption experiments were performed under identical climatic conditions at ambient air pressure and ambient standard temperature. Along the whole tested range of relative humidity sample thickness was shown to have an impact on the sorption kinetics. At low relative humidity, the initial uptake and release of water was considerably influenced by the diffusion of water vapour through the lumen-pit-ray system. Differences between the sorption kinetics for samples in and across fibre direction were thus considerable. With increasing moisture content, the initial uptake and release of water increased for samples across fibre direction, while it decreased for samples in fibre direction. Moisture transport across fibre direction thus seems to be increasing and cell wall processes seem to be more relevant. At high relative humidity, differences between the sorption kinetics for samples in and across fibre direction started to disappear while the impact of sample thickness was still considerable. Therefore, an additional or modified process, which depends on the number of sorption sites but not on the anatomical orientation must be considered at an increased moisture content of wood.