The water vapour sorption properties of thermally modified and densified wood

The water vapour sorption behaviour of Scots pine (Pinus sylvestris L.) and Scots pine that was densified, thermally-modified, or subjected to a combination of thermal modification and densification has been investigated. It was found that all modifications resulted in a decrease in the equilibrium moisture content of the wood samples throughout the hygroscopic range. The water vapour sorption isotherms were reproducible for the unmodified wood samples, but changed between the first and subsequent sorption cycles for the densified, thermally-modified and for wood subjected to a combination of the two treatments. This is the first time that changes in the sorption isotherm between the first and subsequent cycles have been reported for thermally-modified wood. Irrespective of the wood treatment the difference between the adsorption and desorption isotherm loops (sorption hysteresis) was the same and greater than that observed for the unmodified wood sample. After the first sorption cycle, the hysteresis decreased to the values observed for the unmodified wood, even though the isotherms were different. The sorption kinetic behaviour was also investigated and found to be accurately described using the parallel exponential kinetics (PEK) model. The PEK model describes the dynamic sorption behaviour in terms of a fast and slow kinetic process and this has been interpreted in terms of two Kelvin-Voigt elements coupled in series (i.e. relaxation-limited kinetics).