Wetting-induced changes on the surface of thermally modified Scots pine and Norway spruce wood

The process of thermal modification can increase the dimensional stability and fungal resistance of wood, thus improving its performance in outdoor applications. This paper investigates the chemical and structural changes in thermally modified and unmodified Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.) wood caused by long-term soaking, with a focus on the dynamics of the timber surface properties. Fourier transform infrared spectroscopy, pH measurements and microscopy analysis were conducted in 2-week intervals for 14–20 weeks. The main chemical degradation of the wood, which was attributed to the changes in hemicelluloses and cellulose, was identified by the decreased carboxyl signal and the increased intensity of C–O stretching after soaking. The crystallinity and hygroscopicity of the wood were also affected by soaking. The pH level of the modified wood did not change markedly during soaking. However, the initially low pH values of the thermally modified wood, combined with the effect of water, probably facilitated the wood degradation processes, thus leading to an increased number of cracks and holes, especially in the outer parts of the cell walls.