Influence of treatment temperature on wettability of Norway spruce thermally modified in vacuum

Wettability of Norway spruce modified by a new vacuum thermal modification procedure was studied. The mass loss caused by this process ranged from almost 0% when treated at 140 °C to 4.3% at 210 °C. Apparent contact angles of water, formamide, and diiodomethane were measured by the Wilhelmy plate method on the specimens taken from the centers of the thermally modified wood samples. For the treatment at the highest temperature, the contact angle of water was significantly higher when compared to untreated spruce (96.6° vs. 83.6°); lower treatment temperatures, however, did not result in a clear correlation between treatment temperature and contact angle. Formamide yielded lower contact angles for treated spruce compared to the untreated one, but without clear influence of the treatment temperature and diiodomethane always gave perfect wetting. Surface energy calculations according to the Owens, Wendt, Rabel, and Kaelble method revealed that the thermal modification process in vacuum increased the surface free energy and lowered the polarity of wood significantly only at the highest applied temperature (210 °C); the treatment had only limited effect at lower temperatures of modification. These results indicate that adequate wetting and adhesion can be achieved on the surfaces of Norway spruce thermally modified in vacuum.