The thermal degradation of wood is affected by a number of process parameters, which may also cause variations in the resistance against decay fungi. This study compares changes in the chemical composition, water-related properties and decay resistance of Scots pine sapwood that was either thermally modified (TM) in dry state at elevated temperatures (≥ 185 °C) or treated in pressurized hot water at mild temperatures (≤ 170 °C). The thermal decomposition of easily degradable hemicelluloses reduced the mass loss caused by Rhodonia placenta, and it was suggested that the cumulative mass loss is a better indicator of an actual decay inhibition. Pressurized hot water extraction (HWE) did not improve the decay resistance to the same extent as TM, which was assigned to differences in the wood-water interactions. Cross-linking reactions during TM caused a swelling restraint and an effective reduction in moisture content. This decreased the water-swollen cell wall porosity, which presumably hindered the transport of degradation agents through the cell wall and/or reduced the accessibility of wood constituents for degradation agents. This effect was absent in hot water-extracted wood and strong decay occurred even when most hemicelluloses were already removed during HWE.

The wettability of a freshly cut wood surface will change over time, which is denoted as natural ageing. Water contact angle measurements indicated that the thermal modification of European beech reduces its wettability, but does not affect the rate at which the contact angle increases within a 4-week period. A plasma treatment of fresh wood surfaces enhanced the wettability and equaled the wettability properties of unmodified and thermally modified wood surfaces. The contact angle on plasma-treated wood surfaces increased faster than on unmodified wood surfaces, but a reduction in contact angle by the plasma treatment was still evident after ageing for 4 weeks.

The efficiency of hot water extraction (HWE) is dependent on the size of treated wood. While previous research regarding this size-effect has focused on HWE treating sawdust and wood chips, this study investigated its effect on wood blocks with precise dimensions and a broad range of treatment conditions. Scots pine (Pinus sylvestris L.) sapwood samples with dimensions of 10 × 10 × 20 mm 3 and 25 × 25 × 50 mm 3 (R × T × L) were HWE treated at 130-170°C for 40-200 min using liquid-to-solid ratios of 4-20. Our results showed that wood mass loss, which was primarily caused by the decomposition of hemicelluloses, was larger when using small samples. This was mainly assigned to a higher quantity of acetone-soluble decomposition products that remained within the large samples, due to longer distances for diffusion and mass transfer from the wood blocks to the extraction liquid. In line with wood mass loss differences, the amount of dissolved compounds (i.e., carbohydrates) in the extraction liquid at different treatment severities was dependent on the wood size, while the liquid-to-solid ratio had only modest effects. However, composition changes of the extraction liquid, in particular changes in the proportion of poly-and monocarbohydrates with increasing treatment severity, were similar for both sample sizes.

Heat treatments reduce the strength and ductility of wood, but the extent depends on the direction of load and the treatment conditions applied. The tensile behavior of wood is very sensitive to heat treatments, but there is a lack of understanding how this is related to different heat treatment conditions. In this study, we treated homogeneous micro-veneers under different time-, temperature-, and moisture-environments and compared the effect on the tensile behavior of the treated veneers based on their chemical composition changes. The results confirmed the adverse effect of the preferential hemicellulose removal on the strength and toughness of wood. However, chemical composition changes could not fully explain the tensile behavior of dry heat-treated wood, which showed an additional loss in maximum load and work in traction at the same residual hemicellulose content compared to wet heat-treated wood. The scission of cellulose chains as well as the enhanced cross-linking of the cell wall matrix under dry heat conditions and elevated temperatures was discussed as additional factors. The enhanced cross-linking of the cell wall matrix helped in preserving the tensile properties when testing the veneers in water-saturated state, but may have also promoted the formation of cracks that propagated across the cell wall during tensile loading.

Hot water extraction (HWE) treatment changes the physicochemical properties of the wood, including hygroscopic properties. HWE treatment decreases the hydroxyl accessibility of the wood, but the relevance of other mechanisms that change hygroscopic properties are not fully understood. This study investigates the effect of drying on the hydroxyl accessibility and sorption properties of wood. Pressurized hot water extraction (HWE) treatment was applied at 140 °C for 1–5 h to Scots pine (Pinus sylvestris L.) sapwood samples in order to remove increasingly more hemicellulose from the cell wall matrix. Following HWE treatment, half of the wood samples were oven-dried and then re-soaked, while the other half was kept in a fully saturated state. The samples were investigated by applying a new approach that was based on the deuteration of accessible hydroxyl groups, which was followed by the measurement of mass loss due to re-protonation. Sorption properties of the wood samples were studied by measuring moisture content, sorption isotherms and dimensional changes. The present results showed that accessible hydroxyl group content decreased only due to hemicellulose removal during the HWE treatment and was unaffected by oven-drying. However, oven-drying enhanced the effect of HWE treatments in reducing the water-saturated dimensions and the moisture content of wood. Therefore, the additional reductions in hygroscopicity and water-saturated dimensions were not related to changes in sorption site density.