Edinburgh Napier University

  The water vapour sorption properties of several commercial tropical hardwoods and temperate softwood species as well as thermally modified wood and acetylated wood has been studied. A dynamic vapour sorption apparatus was used for this investigation to give data on the sorption isotherms under equilibrium conditions as well as sorption kinetic behaviour. Small differences in the sorption isotherms were found between the wood species, but modification resulted in substantial changes to the sorption isotherms. The sorption isotherms were analysed using the Hailwood-Horrobin model. Sorption kinetics behaviour was found to be accurately described using the parallel exponential kinetics (PEK) model. This model has only recently been applied to wood and this work represents a much more comprehensive study of the applicability of the model. Until now the favoured approach has been based upon Fickian models. The conventional interpretation of the PEK model relies upon the idea of different types of sorption sites, but his work has shown (partly based upon Hailwood-Horrobin analyses of the isotherms) that this interpretation is not applicable. Instead, an interpretation based upon relaxation limited kinetics has been adopted. These ideas are commonly employed in the polymer science literature, but have hardly been used in wood science. The model employed was that involving two Kelvin-Voigt elements in series, from which cell wall moduli and viscosities have been determined. The values of the moduli appear sensible, but the model is only tentative at this stage. The kinetic data has also been used to determine activation energies, entropies and Gibbs free energy of sorption. This is the first time that the entropy and Gibbs free energy of sorption have been studied. The purpose of this work was to understand the phenomenon of sorption hysteresis and in particular to examine if there was a link between sorption hysteresis and sorption kinetics.