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Wood is a natural polymeric material widely used in construction, joinery, furniture, etc. Many applications of wood require that the material is previously dried to a defined level below the fibre saturation point. During drying, the structure and properties of wood are modified. The objective of this research was to investigate the mechanisms that occur during drying, and to evaluate their effects on the structure and properties of wood. Understanding the effects of such mechanisms may provide a basis for developing wood drying methods that improve the performance of dried wood products. According to the results, prolonged exposure of wood to elevated temperatures caused thermal degradation of its structural components. Temperature levels for thermal degradation considerably decreased when wood was heated in moist conditions, as in wood drying. Mass loss due to thermal degradation impaired the mechanical properties and reduced the hygroscopicity of wood. The hygroscopicity was further reduced by the irreversible hydrogen bonding (hornification) that occurred within the wood structure during drying. Increasing drying severity enhanced the occurrence of hornification. There appeared to be competing effects between mass loss and hornification in terms of the strength and stiffness of the wood. Microscopic damage due to anisotropic drying shrinkage of cell wall layers may be minimized by conducting a slow high-temperature drying process. However, any benefits on the mechanical properties of wood induced by stress relaxation during slow hightemperature drying appeared to be offset by thermal degradation of structural components. Enhancing the mechanical performance of dried wood might be achieved by conducting a rapid high-temperature drying process up to high dryness, thus avoiding thermal degradation whilst favouring hornification. Additionally, the decreased hygroscopicity of wood may improve other physical properties such as dimensional stability. Irreversible hydrogen bonding during high-temperature drying was manifested through the closure of macropores in the earlywood. Stress relaxation during slow high-temperature drying appeared to decrease the extent of microcracks in the earlywood, resulting in a lower nonfreezing water (NFW) content. The NFW content in wood was found to be lower than previously indicated, considering the effect of phase change on the heat capacity of water.

Mechanisms affecting the structure and properties of heat-treated and high-temperature dried Norway spruce (Picea abies) wood