Factors Affecting the Quality of An Epoxy Resin Wood-To-Wood Bond

Epoxy resins have a chain of carbon atoms. During the resin formation the oxygen rings open, leading to the formation of linear polymers. The presence of a third reactive group in the epoxy compound, or in other added ingredients, leads to crosslinking and the resultant thermosetting properties. The properties of epoxies are: a minimum of shrinkage during hardening, high adhesion to most materials, practically no elimination of fugitive materials, high mechanical strength, resistance to most common chemicals, and good dielectric properties. The hardener used to cure the resin affects the properties of the cured resin because it completely enters into the final reaction. Thus each resin-hardener combination will yield a cured adhesive with different properties. One of three epoxy formulations was selected in pilot tests with wood for a detailed study; namely, Hysol 2020 with hardener “C”. Glueline shear stress and percent wood failure were evaluated in terms of the following variables: curing temperature: room to 190??, assembly time: 0 to 30 minutes, spread: 40 to 60 lbs/1,000 ft. , and age: 7 or 14 days. Glueline shear strength increased with temperature, but the effect varied with curing time. Shear values for specimens of both age classes cured at 160?F. and 190?F. were higher than the published shear value for hard maple (2,540 psi) at 8 percent moisture content. At room temperature and at 100?F., the glue lines were only partially cured. Aging of these glue lines increased shear strength; however, after 14 days, shear strength of the glue line still had not reached the strength of the wood. The authors concluded that a temperature of 160?F. for 1 hour cured the glue line. Maximum shear strength was secured with spreads of 40 to 50 pounds per 1,000 ft. of the epoxy formulation used in the tests. The length of assembly time had no significant effect on shear strength. The percentage of wood failure increased with an increase in curing temperature for both 7-day and 14- day aging times. Wood failure as a percentage of glued area ranged from about 10 for room-temperature curing to 40 for specimens cured at 190?F. It was observed that none of the specimens soaked 15 days in water during delamination tests satisfied the requirements for a water-resistant glue bond. The failure was attributed to a breakdown between the epoxy and the wood.

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