Thermal modification is a well-established commercial technology for improving the dimensional stability and durability of timber. Numerous reviews of thermally modified timber (TMT) are to be found in the scientific literature, but until now a review of the influence of cell wall moisture content during the modification process on the properties of TMT has been lacking. This paper reviews the current state of knowledge regarding the hygroscopic and dimensional behaviour of TMT modified under dry (cell wall at nearly zero moisture content) and wet (cell wall contains moisture) conditions. After an overview of the topic area, the review explores the literature on the thermal degradation of the polysaccharidic and lignin components of the cell wall, as well as the role of extractives. The properties of TMT modified under wet and dry conditions are compared including mass loss, hygroscopic behaviour and dimensional stability. The role of hydroxyl groups in determining the hygroscopicity is discussed, as well as the importance of considering the mobility of the cell wall polymers and crosslinking when interpreting sorption behaviour. TMT produced under wet processing conditions exhibits behaviour that changes when the wood is subjected to water leaching post-treatment, which includes further weight loss, changes in sorption behaviour and dimensional stability, but without any further change in accessible hydroxyl (OH) content. This raises serious questions regarding the role that OH groups play in sorption behaviour. Graphical abstract

There is a complex link between the water sorption behavior and the presence of accessible hydroxyl groups in the wood cell wall, which can be altered by heat-treatment (HT). This study analyses the effect of changes in the cell wall ultrastructure caused by two HT techniques on the hydroxyl accessibility, water vapor sorption and dimensional changes of Scots pine (Pinus sylvestris L.) sapwood. HT of wood in pressurized hot water at 120-170 °C was applied to cause the preferential bond cleavage, whereas HT of wood in oven-dry state in superheated steam at 180-240 °C was performed to create additional covalent cross-links within the cell wall matrix. Removal of cell wall polymers by HT and water leaching reduced the oven-dry dimensions of wood and enhanced the cellulose aggregation during drying. Cellulose aggregation restricted the cell wall shrinkage in circumferential direction, resulting in inhomogeneous shrinkage of the cell wall with only little changes in lumen volume by HT. Cellulose aggregation also reduced the water-saturated dimensions, but a decrease in swelling was only achieved when additional cross-links were formed by HT in dry state. Additional cross-links in the cell wall matrix also resulted in an additional reduction in water sorption at 25 °C and 93 % RH. However, this was not caused by a further reduction in the hydroxyl accessibility. Instead, cross-linking was shown to reduce the amount of accessible OH groups that are simultaneously active in sorption, which was explained based on the concept of sorption of water dimers at hydroxyl group pairs at high RH levels.

Elevated wood moisture contents during the thermal modification process have been shown to adversely affect the improvement in dimensional stability and hygroscopicity. This study tested the hypothesis that the effect of elevated wood moisture content is based on the impact of water on chemical reactions which determine the cell wall matrix stiffness. Samples of Scots pine sapwood (Pinus sylvestris L.) were thermally modified in saturated water vapor at different peak temperatures and durations starting either in oven-dry or in water-saturated state. For a given mass loss caused by the modification process, the improvement in maximum swelling and equilibrium moisture content was stronger for oven-dry samples. After removal of water-soluble degradation products, which caused a cell wall bulking effect, the maximum swelling even increased after modification in water-saturated state. Based on dynamic vapor sorption measurements, it was evidenced that the modification in oven-dry state increased the cell wall matrix stiffness which improved dimensional stability and hygroscopicity. Enhanced bond formation in the polymeric network, i.e., via condensation and cross-linking reactions during the treatment of oven-dry wood, is suggested as a cause for this increase in matrix stiffness. In contrast, the modification in water-saturated state enhanced the flexibility of the cell wall matrix, which increased the cell wall swelling and limited the improvement of hygroscopicity to the reduction in OH groups by removal of hemicelluloses. This enhanced matrix flexibility was potentially caused by predominant hydrolytic cleavage of bonds in case of water-

Wood and other cellulosic materials are highly sensitive to changes in moisture content, which affects their use in most applications. We investigated the effects of moisture changes on the nanoscale structure of wood using X-ray and neutron scattering, complemented by dynamic vapor sorption. The studied set of samples included tension wood and normal hardwood as well as representatives of two softwood species. Their nanostructure was characterized in wet state before and after the first drying as well as at relative humidities between 15 and 90%. Small-angle neutron scattering revealed changes on the microfibril level during the first drying of wood samples, and the structure was not fully recovered by immersing the samples back in liquid water. Small and wide-angle X-ray scattering measurements from wood samples at various humidity conditions showed moisture-dependent changes in the packing distance and the inner structure of the microfibrils, which were correlated with the actual moisture content of the samples at each condition. In particular, the results implied that the degree of crystalline order in the cellulose microfibrils was higher in the presence of water than in the absence of it. The moisture-related changes observed in the wood nanostructure depended on the type of wood and were discussed in relation to the current knowledge on the plant cell wall structure.

Loss in strength and ductility is a major drawback for the heat-treatment of solid wood. Previous studies focused mainly on the de-polymerization of cell wall constituents as a cause and the importance of the preferential removal of hemicelluloses. This study tested the hypothesis that the mechanical behavior of wood is additionally affected by re-polymerization reactions within the cell wall matrix during heat-treatment. This was achieved by comparing changes in chemical composition, FT-IR spectra, and mechanical properties of Scots pine sapwood that was heattreated in either dry state in superheated steam or in wet state using pressurized hot water. Although preferential de-polymerization of hemicelluloses was evident for both heat-treatment techniques, the analysis of the chemical composition and FT-IR spectroscopy indicated additional re-polymerization reactions within the cell wall matrix of dry heat-treated wood. The consequent formation of covalent bonds and cross-links increased the resistance against compression loads and hindered inelastic deformation during bending. This resulted in an additional reduction in bending strength and strain energy density of dry compared to wet heat-treated wood. Repolymerization reactions during heat-treatments of wood in dry state were suggested as the main cause for the brittle failure under bending loads, while the effect of hemicellulose-removal on brittleness was much smaller than stated previously.