Penetrability of Glue Lines in Douglas-Fir Plywood By Preservative Solutions

The internal bond strength of particleboard was observed to vary with several factors including type of glue, how it was spread, and how surfaces were prepared. To evaluate these variables, a flake-type Douglas-fir particleboard 1/2-inch in thickness and having a density of 40 pounds and 8 percent urea resin was cut into one hundred twenty 2- by 2-inch specimens. Half these blocks had 1/32 inch removed from each face by sanding and the others were unsanded. The steel loading blocks were sanded and wiped with acetone. One-half were glued without further treatment and the other half were etched in an acid bath prior to gluing. Three types of adhesive, an epoxy, a polyvinyl, and a hot-melt, were used. The hot melt was applied in a heavy spread to heated loading blocks and allowed to cool until the cement became slightly tacky. The epoxy and polyvinyl glues were applied three ways: as a heavy single spread, a slight double spread, or a heavy double spread. In all cases the results showed higher loads for sanded than unsanded particleboard. However, with the double-spread epoxy the results were similar for sanded and unsanded blocks. The authors conclude that the surface of this particular particleboard is the weakest part and that the double-spread epoxy glue reinforced it, while the polyvinyl and hot-melt glues did not. There was no difference in results with etched and unetched loading blocks. The hot-melt adhesive applied at 285?F. caused deterioration of the particleboard and low internal bond strength. The authors conclude that the adhesive and gluing conditions must be carefully selected for the board being evaluated.

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