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.

Surface carbonization, or charring, of wood is a one-sided modification method primarily intended for protection of exterior cladding boards. The heavily degraded surface acts as a barrier layer shielding the interior from environmental stresses, and as such acts as an organic coating. To test the durability of surfaces created in this manner, unmodified, contact charred, and flame charred spruce and birch samples were exposed to the brown rot fungus Coniophora puteana and white rot fungus Trametes versicolor for a period of nine weeks. All sides of the samples except the modified surfaces were sealed to investigate the protective effect of the surface. Mass losses were greatest for unmodified references (up to 60% and 56% for birch and spruce, respectively) and smallest for contact charred samples (up to 23% and 32%). The wood below the modified surfaces showed chemical changes typical of brown rot and simultaneous white rot. The measured glucosamine content revealed fungal biomass in both the modified surface as well as the layers beneath. According to the recorded values, the fungal biomass increased below the surface and was higher for flame charred samples in comparison to contact charred ones. This is likely due to the more intact, plasticized surface and the thicker thermally modified transition zone that restricts fungal growth more effectively in contact charred samples in comparison to the porous, cracked flame charred samples. Scanning electron microscope images verified the results by revealing fungal hyphae in all inspected wood types and species.

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.

Pressurized hot water extraction (HWE) treatment has the benefit of simultaneous extraction of hemicellulose-based carbohydrates and modification of the solid phase, but it does not drastically improve wood durability. However, removing hemicelluloses from the wood by HWE treatment creates water-filled spaces in the cell walls which could be filled with modification agent in order to improve the properties of the wood. Without drying, modification agent can be added into the saturated wood via diffusion. The esterification of wood with citric acid (CA) improves resistance to biological deterioration but increases brittleness. However, combining CA esterification with additional chemicals that form links with CA can mitigate brittleness. This study investigated esterification as a method for modifying HWE treated wood. HWE treatment with CA solution (4% w/v) was applied at 120 °C for 3 h to Scots pine (Pinus sylvestris L.) sapwood specimens. The specimens were further modified by diffusion with CA and starch derivatives followed by curing. The applied method changed the moisture properties and chemical composition of the wood. The results showed successful wood bulking. The investigated method slightly improved decay resistance to Coniophora puteana and Trametes versicolor but did not change resistance to Rhodonia placenta.

The effects of pressurized hot water extraction (HWE) treatment on the mould resistance of wood have not been extensively investigated yet. The activity of the mould fungi is dependent on the availability of nutrients. Therefore, the soluble degradation products produced during HWE treatment could affect the wood’s susceptibility to mould growth. Scots pine (Pinus sylvestris L.) sapwood specimens were treated with HWE at 140 °C for 1–5 h. Afterwards, the degradation products were either removed via leaching or the wood was dried without applying the leaching procedure. The surface layer (1.5 mm) was removed from half of the leached and non-leached specimens. The resistance of the specimens against mould growth was tested in an incubation chamber. HWE treated wood showed a higher susceptibility to mould growth when it was neither leached nor subjected to surface removal. The susceptibility of wood to mould fungi depended on the availability of hemicellulose-based degradation products produced during HWE treatment. These degradation products were removable via a leaching procedure, but also by removing the outermost layer of the wood. The results show the relevance of removing HWE degradation products located on the wood surface in improving resistance against mould growth.