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.