Filling the gap: modelling sorption isotherms between hygroscopic and water-saturated conditions based on microscopic characterisation of softwood anatomy

The relation between moisture content of a material and the ambient climate is described by sorption isotherms. Several models for prediction of sorption isotherms exist in the hygroscopic moisture range, but models of sorption isotherms in the over-hygroscopic range are less abundant. In wood, water can be present both in the solid cell wall and in the macro voids (e.g. cell lumina, pit chambers) in the wood structure. In the over-hygroscopic range, moisture uptake primarily occurs in the porous structure outside of the cell walls by capillary condensation. That is, condensation occurs at a certain partial pressure which depends on pore size and geometry. Since wood has an ordered structure and distinct geometries this link enables prediction of the relative humidity at which capillary condensation occurs in different macro voids. Here, this was explored by using a mathematical model for prediction of moisture sorption by capillary condensation in the macro void structures of softwoods. Both absorption and desorption isotherms were modelled, and sorption hysteresis was included by the ink-bottle effect. The model was based on wood anatomy data which was determined for softwood material based on quantitative wood anatomy. The modelling results were compared to experimental sorption data. In general, there was a reasonable agreement; two different theoretical criteria for emptying of cell lumina in desorption were tested and, in most cases, the experimental data were in between these two versions of the modelled desorption isotherms. The moisture content at saturation was, however, most often underestimated by the model.