Urea formaldehyde resin extended with 0,50,100 and 150 percent of wheat flour based on dry resin weight was used in the manufacture of three-ply 3/16″ thick plywood with civit veneer. Veneer moisture contents of 6 and 12 percent and specific pressures of 150, 200 and 250 psi, applied during hot pressing, were the other variables. The quality of glue bond was evaluated by the dry plywood shear test and warm-water plywood shear test.

The dry test shows that with wheat flour extension, reduction in the bond strength is statistically significant. However, even the extended glue mixes produce adequate bond. The wet test shows that upto 50 percent extension, there is no significant reduction in the bond strength but 150 percent extension produces a very poor bond.

In the dry shear test, a significantly (1% level) higher bond strength is found at 6 percent than at a 12 percent moisture content level although in the wet tests no difference can be detected. This phenomenon can be well explained in the light of the “Water-Monolayer Theory of adhesion”. According to this theory, an adhensive adheres to the wood through a layer of water and the optimum bond takes place when a monolayer is present in the substrate. This monolayer condition is attained at about ten percent equilibrium moisture content for most of the wood species. Wood, conditioned to six percent equilibrium moisture content, attains the monolayer condition by absorbing water from the glue-mix. This is the reason of higher bond strength of plywood made with veneer conditioned to six percent moisture content as revealed by the dry test. However, during the wet tests, plywood absorbs more water and thereby destroys this monolayer condition as a consequence of which no difference is detected in the bond strength between the two moisture content levels.

For adequate water resistance, urea formaldehyde resin may be extended with wheat flour upto 50 percent though for less exacting purposes an extension upto 150 percent may be employed. A specific pressure of 200-250 psi should be used for hot pressing civit plywood. The moisture content of veneer for making plywood using urea formaldehyde resin should be about six percent for optimum gluing.