Influence of veneer moisture content on shear strength of plywood adhesive bond

In the formation of wood-adhesive bond numerous factors are involved, and the influence of some of them is still not sufficiently explained. In this frame, another important factor is also wood moisture content (MC) during the gluing process. It influences a number of processes, which take place during adhesive curing, among others also penetration depth and thus mechanical adhesion or strength of adhesive bond. The article presents the results of a research in which the influence of MC of peeled beech veneer during the gluing process of plywood board adhesive bond shear strength was studied. Melamine-urea-formaldehyde adhesive was used for hot gluing. Veneer moisture content during the gluing process ranged between 4 and 14%. Shear strength was determined by using test specimens prepared according to the EN 314-1 and EN 314-2 standards. The results show that (1) veneer moisture content within the range studied did not influence shear strength of dry (exposed to a standard climate) tested specimens, that (2) shear strength of test specimens decreased after the boiling as veneer moisture content increased and that (3) variation coefficients for shear strength of specimens prepared according to the boiling method are on the average much higher than in the case of specimens prepared in standard climate.