Gluing Hardboard to Fir Plywood and Phenolic Resin Adhesives

Phenol-formaldehyde resins were tested for their suitability in producing a boil-proof bond between fir plywood and hardboard. The test used was a modification of the conventional knife test, which causes failure in tension rather than shear. This test is more critical of adhesion than existing shear test procedures. (See Forest Prod. J. 4(8)). The results were that any phenolic plywood glue will bond any standard grade of hardboard to fir plywood in a satisfactory manner. However, most of the treated hardboards (tempered, high surface resin content, etc.) cannot be glued satisfactorily without planing or sanding the surface of the hardboard. These exterior grades of hardboard can be easily glued with many of the interior plywood glues, which are of course unsuited for outside exposure. The reason treated boards cannot be satisfactorily glued is the presence of oxidizing oils or of phenolic resin concentration on the surfaces. Either almost completely prevents the adhesion of the phenolic glue films to the surface of the hardboard. On many of these hardboards, sanding, planing, or wire brushing is not even adequate treatment to enable gluing. Since, wet-process treated hardboards always have the screen impression on one side, that would be the side which would be planed off prior to gluing. The screen impression on standard grade wet-process boards does not seem to prevent good adhesion. Dry-process boards appear to differ in surface character relative to the position in the press. One side may be harder and more difficult to glue with phenolic resin adhesives than the other side of the same board. The less dense side is easier to glue, and can be identified by the scotch tape test, which consists of pulling tape from the surface of the hardboard with a right angle pull. The side showing less wood fiber on the scotch tape is almost invariably the bottom-manufactured side, which is the side most difficult to glue. The high density of hardboard has little affect on adhesion except when the density results because of a high resin content. Hardboards transmit heat more readily than wood, so hot pressing is not difficult. Fiberboards and particle boards appear to glue equally well, but chip boards sometimes reflect the character of the raw material from which they are made to a greater degree. Phenolic resin glues have the least expensive plywood adhesives offering a boil-proof bond. Glue costs are no greater for hardboard faced panels than for normal plywood because the rates of glue application are the same. There may be a larger amount of permanent compression when hotpressing phenolic resin glues since the termperature required is higher than for some of the protein hot-press glues. Assembly-time tolerances are better for phenolic resin glues than protein, but poorer than urea.

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