Influence of earlywood/latewood and ring position upon water vapour sorption properties of Sitka spruce

Variation in the water vapour sorption kinetics of the stemwood of Sitka spruce (Picea sitchensis (Bong.) Carr.) has been studied, with respect to possible variations between earlywood and latewood and between different annual ring positions from pith to bark. The sorption kinetics was analysed using the parallel exponential kinetics (PEK) model that has recently been introduced by this laboratory for studies with wood. Significant differences in kinetic parameters were found between ring positions, corresponding to differences between juvenile wood and mature wood that contrast with previously reported work in the literature. Differences were also found in the sorption kinetics of earlywood and latewood in agreement with the literature. Thus far, there has been no satisfactory explanation as to the physical interpretation of the PEK model and in this paper it is argued that the kinetic behaviour is consistent with what would be expected for sorption with a viscoelastic material. Analysis of the PEK data using the Kelvin-Voigt model has proved that it is possible to obtain values for the wood cell wall stiffness and viscosity that are in broad agreement with previously published values in the literature.