Effect of duration of storage of MUF resin on the wetting angle on beechwood

If high-quality gluing of wood is to be achieved, it is necessany that the surface of the wood is suitably wetted and that the adhesive penetrates adequately into the wood. What degree of wetting is achieved depends mainly on the structure of the wood and the preparation of its surface, as well as on the type, composition and viscosity of the adhesive resins. One of the important factors relating to the surface of the wood is its geometry. Since the viscosity of adhesive resins increases with the length of time for which they are stored, longer storage times are also apt to cause a decrease in their wettability during gluing. Measurements of the wetting angle were performed on test specimens of beech (Fagus sylvatica L) in the form of peeled veneer of thickness 1,5 mm and in the form of solid wood of thickness 18 mm. Melamine-urea-formaldehyde (MUF) resins having different degrees of condensation and viscosity were used. In the research it was found that the influence of the surface of the wood was evident from the different initial and final wetting angles, and that ageing of the resin causes a greater increase in the wetting angle because of changed viscosity than that caused by under-condensation of the resin.