Thermosetting Exterior-Plywood Type Adhesives from Bark Extracts

Extracts of western hemlock bark were prepared as a source of components for adhesives. The evidence points to the fact that the tannin extracts are a polyphenolic polymer system. It was found that polymethylophenol performed well as the cross-linking and copolymer agent with the bark extract. Laboratory plywood tests, based on percent wood failure and wet shear strength, showed that a formulation approaching 60 percent bark extract and 40 percent polymethylophenol reagent seemed to provide the best balance of properties. A trial run of commercial plywood was made under production conditions. The adhesive formulation used was 49 percent bark extract, 34 percent polymethylophenol, and 17 percent filler. The results of the tests for wet shear strength and percent wood failure showed that the adhesive produced an acceptable exterior-grade plywood.

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