Epoxy-Resin Adhesives for Gluing Wood

Fifteen commercial epoxy-resin adhesives and 23 laboratory modifications of one commercial resin were evaluated as wood glues by conducting block shear tests with hard maple (in accordance with ASTM D 906/49). Shear tests of plywood bonded with commercial epoxy resins included both shear strength and estimated wood failure when tested dry, after soaking 48 hours, after a boil, dry, boil cycle and after heating to 160?F for 30 minutes. For the commercial epoxy resins (none of which were formulated for wood gluing) only one exceeded the urea and the phenol-resorcinol resin glues (included for comparison) when compared on a basis of normal thickness joints. In thick glue line tests (1/20-inch thick) five of the 15 epoxy resins were superior to the urea and phenol resorcinol glues. The difference was attributed in part to confinement of the glues by shims used to control glue line thickness. These confined the epoxies which tended to squeeze out in the normal glue line tests. Twenty-three formulations of one commercial epoxy resin were tried in attempts to reduce the tendency of epoxies to produce starved joints. The formulation containing titanium dioxide and lacquer thinner behaved best and gave the highest “joint quality” (combination of shear strength and wood failure). Although its pot life was short (37 minutes before setting in a paper carton) it required about 6 hours to reach maximum joint strength as compared with at least 12 hours for urea or phenol-resorcinol glues. This formulation and two commercial epoxy resins were used to fabricate 3-ply and 5-ply birch plywood for durability tests. The plywood was subjected to such accelerated aging as the boil cycle test, a 120-hour boiling test with 5 cycles of soaking and drying, as well as being exposed outdoors for continuing evaluation. All three epoxies performed well. The boil-cycle test values would easily meet the requirements of commercial standard CS 35-56 for Type I bonds. The authors concluded that the indicated durability of epoxy resins and their promise in thicker than normal glue lines should encourage further work on formulations for wood gluing.

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