Work of adhesion as criterion for determination of optimum surface tension in adhesives

The article presents a new method for determination of the optimum surface tension in adhesives (glues and wood varnishes). The criterion employed is maximum work of adhesion. In view of the fact that work of adhesion depends, among other things, on the free surface energy of wood and the surface tension of the adhesive, and that the free surface energy changes in relation to roughness, this experiment was carried out on three groups of specimens of beech wood with radial surface. One group of specimens thus had a sanded surface, the second group consisted of planed pieces, while those in the third group had a surface worked out with a steel hand scraper. Work of adhesion can be good criterium for determination of optimum surface tension because it take into consideration adhesiv, adherent and their interaction. The roughness of wood sutface was measured and expressed as the average arithmetic deviation of the surface roughness profile Ra. The wetting angle Θ was also meusured for nine liquids with different levels of surface tension. On the basis of the wetting angle and surface tension the critical surface energy of wood (γc) was calculated, as well as the work of adhesion (Wa), the optimum surface tension of the liquid wetting the wood surface in question (γL,G,opt) and the maximum work of adhesion (Wa,max).