Improving End-To-End Grain Butt Joint Gluing of White Pine

By pretreating and heating the mating surfaces of butt joints before gluing, tensile strengths approaching the strength of clear eastern white pine (Pinus strobus L.) were achieved. Resorcinol, polyvinyl, urea, and DFTA-cured epoxy resins were used in three glueline thicknesses. Five flexibilizers were added at various levels. Two isomeric phenols, resorcinol and catechol, and the amino analog of resorcinol, m-phenylene diamine, performed very well as accelerators and without significant differences in the pretreatment. Joints made with 5-mil-thick gluelines were more variable and lower in strength than comparable joints with 15- and 30-mil gluelines. Although the tensile strength of adhesive free films decreased as concentrations of flexibilizer increased, the strength of the butt joints was often higher than that of the adhesive in free film. As much as 20 parts of flexibilizer were added to the adhesive without affecting the strength of the butt joints even though the strength of the film declined.

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