Over the past four years, the Forest Products Research Institute has been conducting wide-ranging investigations into the water sorption properties of wood, natural fibres and cellulose. More recently, this work has been extended to modified wood. The work reported in this paper presents data on the water sorption properties of thermally modified wood, densified wood and a combination of the two. The paper presents new findings concerning the sorption isotherms and sorption kinetics behaviour. The sorption kinetics is analysed in terms of the parallel exponential kinetics (PEK) model which has recently been introduced to the wood science community as a better alternative to the often used Fickian models, at least where cell wall sorption behaviour is considered. The PEK model comprises two exponential sorption kinetic terms which are termed fast and slow sorption processes. The PEK model is then interpreted by using a relaxation-limited kinetics model consisting of two series-coupled Kelvin-Voigt elements. Such a model may be used to provide insights into the sorption process and in particular to the phenomenon of sorption hysteresis. Wood modification has been used as a means of altering the substrate in a controlled manner in order to advance understanding of sorption behaviour. A model for sorption is given in which sorption onto a glassy material below the glass transition temperature can explain hysteresis. Furthermore a link between sorption kinetics and hysteresis is shown in which considerations of molecular relaxation processes within the cell wall can be linked. It will also be shown that the sorption behaviour is governed less by the availability of hydroxyl groups and more by matrix stiffness of the cell wall.
Keywords: Thermal modification, water vapour sorption, kinetics, rheology
Authors
Hill C. A. S.
Forest Products Research Institute, Edinburgh Napier University, Edinburgh
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