Thermal modification (TM) has been widely used to improve the dimensional stability and durability of wood. However, the performance of thermally modified wood (TMW) in condi-tions where it must endure continuous changes in ambient moisture content are not entirely clear. This thesis investigated the chemical components, cellular structure, and physical prop-erties of thermally modified Scots pine, Norway spruce, and European ash wood exposed to long-term water contact condition, different temperature and relative humidity condition and natural weather condition.
The results showed that increase in TM intensity reduced the equilibrium moisture content (EMC) and improved the dimensional stability of wood mostly in a tangential direction. TM did not affect Brinell hardness, while increase in EMC decreased wood hardness. Prolonged exposure to water mainly changed hemicelluloses and cellulose and increases the hygrosco-picity of both modified and unmodified wood. In addition, the initial higher acidity of TMW tends to promote the degradation of the cell-wall compounds, resulting in faster degradation in TMW than in unmodified wood during water contact exposure.
Degradation of lignin and leaching of the degradation products during the weathering ex-posure leaves wood with a grey hue and surface with higher relative cellulose and hemicellu-lose content. TMW presented less changes in lignin structure and color due to its condensed lignin structure and lower hygroscopicity compared to unmodified wood. The lower EMC and fiber saturation point (FSP) value of TMW compared to unmodified wood indicates that TM can limit water absorption during weathering. Therefore, TMW showed less cupping than unmodified wood in wet conditions. Brinell hardness was slightly decreased in all specimens due to cell wall degradation and increase in EMC. Additionally, increase in the TM intensity improved weathering performance of wood by reducing the surface chemical changes, water accessibility and cell wall porosity.