In this thesis, we investigated the effect of thermal modification at 180 °C and 230 °C on selected physical and mechanical properties of Douglas fir heartwood. Control and thermally modified wood were compared in terms of equilibrium moisture content, density, dimensional stability, colour, surface wettability with water, bending strength, modulus of elasticity, and Brinell hardness. We found that thermal modification reduced the equilibrium moisture content of the wood, with the reduction more pronounced at the higher temperature of modification. Modification also reduced swelling in the radial and tangential directions, indicating improved dimensional stability. Wood density decreased as the temperature of modification increased. Thermal modification caused notable colour changes, as the wood became darker with increasing temperature. Contact angle measurements did not show a consistent effect of modification temperature on surface wettability with water. The impact of thermal modification on mechanical properties depended on the temperature and the specific property studied. More intense modification resulted in reduced bending strength, whereas changes in the modulus of elasticity and hardness did not follow a consistent trend. The results indicate that thermal modification can improve the moisture-related behaviour and dimensional stability of Douglas fir wood and alter its appearance; however, the potential negative impact on mechanical properties must be considered when selecting the degree of modification. Thermally modified Douglas fir is therefore particularly suitable for applications where enhanced dimensional stability, reduced susceptibility to moisture-related changes, and an altered aesthetic appearance are important, but mechanical requirements are not the deciding factor.
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