A sorption isotherm expresses the relation between ambient relative humidity (RH) and equilibrium moisture content of a material for a specific temperature. The sorption isotherm can generally be divided into a hygroscopic part covering low, medium and high RH levels, and an over-hygroscopic part close to 100% RH. For wood, the hygroscopic part is dominated by water uptake in cell walls and the over-hygroscopic part by capillary condensed water in cell lumina and other voids within the wood structure. Moisture contents in the over-hygroscopic range are required for fungal degradation, and over-hygroscopic sorption isotherms are thus relevant input data for models describing moisture conditions in outdoor structures, i.e. models needed for service life prediction. The over-hygroscopic range is also relevant when modelling wood drying. Measuring over-hygroscopic absorption and desorption isotherms is however time consuming and challenging and a model enabling prediction of over- hygroscopic sorption isotherms would therefore be valuable. In addition, such a model could give a better understanding of how the wood structure affects the capillary condensation of water. This paper presents an approach for modelling over-hygroscopic sorption isotherms based on quantitative wood anatomy, i.e. based on estimates of dimensions and geometries of voids in the wood structure available for capillary condensation. First results are shown for Norway spruce (Picea abies (L.) Karst.).
Keywords: sorption isotherm, over-hygroscopic moisture range, model, wood anatomy
Authors
Fredriksson M.
Department of Building and Environmental Technology, Lund University, Lund
Thybring E. E.
Department of Geosciences and Natural Resource Management, University of Copenhagen, Fredriksberg
Thygesen L. G.
Department of Geosciences and Natural Resource Management, University of Copenhagen, Fredriksberg
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