Water Uptake of Thermally Modified Norway Spruce

Abstract

Thermal modifi cation of wood has been commercially available for almost twenty years but the complete mechanism of improved durability is still not completely understood. It is known that the temperature and duration of the modifi cation infl uences the properties of the fi nal products. There are several potential reasons for the increased durability of the modifi ed wood. In recent research in particular, water exclusion effi ciency has been identifi ed as one of the key mechanisms. In order to elucidate this presumption, specimens made of Norway spruce heartwood were thermally modifi ed at 6 different temperatures (160 °C, 180 °C, 190 °C, 200 °C, 210 °C and 230 °C) for three hours according to the Silvapro® procedure. Control specimens were left unmodifi ed. Three sets of tests were performed: (a) samples were soaked in water for 4 days and then positioned on load cells and allowed to dry until a constant mass was achieved; (b) short term water uptake was determined with a tensiometer and (c) wood-water interactions were verifi ed using constant gravimetric moisture measurement during outdoor exposure. As expected, the degree of modifi cation was refl ected in the moisture content of the wood during testing. Short and medium term water uptakes correlated quite well with the performance of wood in outdoor applications. On the other hand, long term tensiometer measurements were not in line with either short term water uptake or outdoor measurements.