Influence of degradation products from thermal wood modification on wood-water interactions

Thermal modification is a widely used technology for enhancing the dimensional stability and durability of wood. However, thermal degradation reactions in wood are complex and depend on the process conditions applied. Degradation products from thermal wood modification are expected to affect the wood chemistry and the wood–water interactions of the thermally modified wood. In this paper, we investigated the impact on wood chemistry and wood–water interactions of retaining or evaporating degradation products while thermally modifying beech and Scots pine in a closed thermal treatment process. Wood–water interactions were studied by LFNMR and DSC. Additionally, the presence of extractable degradation products was determined based on water and multi-solvent extraction, and pH measurements and ATR-FTIR analyses were performed to determine differences in wood chemistry. Light microscopy images of xylem cross-sections were taken to determine the lumen areas for interpretation of the LFNMR results. We found that thermal treatment with a cooling step at atmospheric pressure allowed some degradation products to evaporate, in the case of beech, resulting in a less hydrophobic end product. However, for Scots pine, evaporating degradation products during the thermal modification process did not have an effect on the pH and the amount of residual extractives, and as the impact on wood–water interactions was not in line with the findings on wood chemistry, the results are inconclusive. Our results demonstrate that degradation products can have an impact on the wood–water interactions of thermally modified wood in the cooling step and that the results are wood species dependent.