This study tests the hypothesis that thermal modification of wood influences the effectivity of air plasma treatment. Micro-veneers of European beech, Scots pine and Norway spruce were thermally modified at two different temperatures and subsequently plasma-treated for 1 and 3 s. The veneer surfaces were characterized in terms of morphology, wetting behaviour and surface chemistry. No severe changes in the veneer surfaces due to plasma treatment were observed by scanning electron microscopy. Plasma treatment increased surface free energy and wettability by water and urea–formaldehyde adhesive; it was more effective on thermally modified wood than on unmodified wood. X-ray photoelectron spectroscopy revealed a similar distribution of oxygen-containing functional groups on the wood surface after plasma treatment of thermally modified and unmodified beech wood. It is suggested that enhanced wettability through plasma treatment is due to the generation of carboxyl groups within the lignin network, which contribute to the polar part of the surface free energy. The high effectiveness of plasma treatment on thermally modified wood might thus be explained by its high relative proportion of lignin.
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