Durability of one-part polyurethane bonds to wood improved by HMR coupling agent

In a previous study on the strength and durability of a new class of wood adhesives called one-part polyurethanes, four commercial one-part polyurethanes, along with a resorcinol-formaldehyde adhesive representing a standard of performance, were compared in bonds to yellow birch and Douglas-fir in a series of industry-accepted tests (7). The polyurethanes all performed essentially the same: high in dry and wet strength but low in wet wood failure and resistance to delamination. In the study described in this paper, we look at improving the durability of polyurethane bonds to wood by priming before bonding with a hydroxymethylated resorcinol (HMR) coupling agent that is known to enhance adhesion of all thermosetting wood adhesives. The HMR dramatically increased the wet wood failure and resistance to delamination of polyurethanes that were found deficient in the previous study, to levels comparable with the highly durable resorcinol adhesive. One-part polyurethane adhesives met the strength and durability requirements of the most rigorous of tests, specifically American Society for Testing and Materials (ASTM) standard D 2559 (3), when wood surfaces were primed with the HMR coupling agent.

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