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The properties of the transverse section surface of the wood are crucial when using it as an adhesive in finger joints. Contact angles of three test liquids sessile drops - water, diiodomethane and ethylene glycol - with known characteristics of surface free energy were measured on wood surface with the grain orientation 45 ° against the plane. Acid–base approach was used to calculate surface free energy and its characteristics of Scots pine (Pinus sylvestris L.) with moisture content of 21% and Norway spruce (Picea abies L. Karst) with moisture content of 10% wood samples from values of the contact angle. The wetting behavior of the wood samples was examined with the contact angle method (goniometer technique) in the Laboratory of the Department of Chemistry of Latvia University of Agriculture at the beginning of 2013. The aim of this study was to verify possibility of determining the contact angle values of the wood and calculate the surface free energy and its components of wood from the obtained contact angle values using acid-base theory. At the end of this study it was concluded that the acid–base approach is a suitable method to calculate surface free energy and its characteristics of wood from the values of the contact angle. Nevertheless, indirect methods of liquid drop contact angle value estimation must be used because direct determination is not feasible on wood surfaces with open capillaries.

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