Wood to Wood Bonds with Epoxide Resins–Species Effect

The quality of bond in wood to wood joints using epoxy resins is examined. Information on the effect of the species on epoxy resin bonds is necessary to assess the usefulness of this type of adhesive. The experiments involved red oak, white oak, hard maple, yellow-poplar, Douglas-fir, southern yellow pine, spruce, redwood, lignum vitae. Bakelite C-8 epoxy, ERL 3794 with Versamid 125, ERL 2793, and ZZL 0812 curing agents. The control group used Koppers Penacolite G 1124-a resorcinol resin. The tests were carried out on cross-lap type specimens in a Tinius-Olsen universal testing machine. Load was applied at a rate of 0.05 inch per minute and maximum load with area of wood failure recorded. Strength values were obtained for the various combination of woods and adhesives; curing temperatures were also included in the evaluation. The temperature-adhesive interaction was found to be significant for all species except redwood, but in a comparison between species and bond strength due to adhesives, the influence of curing temperature was insignificant. A study of the relationship between wood failure, adhesive, and curing temperature indicates that wood failure is dependent on the properties of the species rather than on the particular adhesives used. Some species develop high wood failures with any temperature-adhesive combinations whereas other show uniformly low wood failure values. Failures may depend on the strength of the adhesive or wood and on the quality of adhesion. The latter is determined by the occurrence of separation between adhesive layer and wood. In most instances the type of failure is difficult to establish, since it is usually a combination of the three. Thus a comparison between bond strength and the accompanying wood failure gives a good measure for formulating the ideal adhesive for a particular species. Water resistance may often be an additional criterion to strength. The limited amount of tests indicate that the epoxy formulations used do not approach the water resistance of resorcinol resins. Curing above room temperature improves on their water resisting qualities, but does not make them waterproof.

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