Kristiina Laine

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).

Cupping behaviour of surface densified Scots pine wood: the effect of process parameters and correlation with density profile characteristics

Surface densification is a process by which the mechanical properties of the wood surface can be improved, increasing the attractiveness of low density wood for applications such as flooring. The purpose of this study was to investigate the development of undesired cupping deformations in surface densified solid wood. The effects of process parameters (compression ratio, temperature, holding time, closing time, and initial moisture content) on cupping were studied, and correlation analysis was used to determine the strength of association. Correlation analysis was also performed to explore the potential dependence of cupping on the density profile characteristics of surface densified wood. Correlations of moderate strength were found between cupping and the process parameters, as well as between cupping and the density profile characteristics. Compression ratio was found to be the most influential process parameter: samples with a high compression ratio showed distinctly different cupping behaviour than samples with a low compression ratio. Density profile characteristics were considered a probable contributor to cupping, but the influence of other effects was also considered likely. Interestingly, certain combinations of parameters were found to result in very small cupping deformations. Therefore, the results of this study indicate that cupping can be minimised by optimisation of process parameters.

Water vapour sorption behaviour of thermally modified wood

Abstract The work presented in this paper reports on the water sorption properties of thermally modified wood, densified wood and wood modified using a combination of the two methods. 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.

Analysing density profile characteristics of surface densified solid wood using computational approach

Abstract In this study, solid Scots pine was surface densified in an open press using different moisture contents (9·6 and 12·4%), temperatures (150 and 200°C), press closing times (0·5 and 5 min), holding times (1 and 10 min) and compression ratios (6·7 and 25%). The characteristics of the formed density profiles were defined and their correlations to the process parameters and Brinell hardness and elastic recovery were analysed. Compression ratio and closing time had the strongest effect on the formation of the density profile, as well as the hardness and elastic recovery. The amount of localised deformation (peak area) did not have significant effect on the Brinell hardness or elastic recovery, whereas, the highest density achieved (peak density) and its location (peak distance) dominated the effect. Brinell hardness and elastic recovery were found to correlate well with each other.