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

In a previous study on the strength and durability of a new class of wood a dhesives called one-part polyurethanes, four commercial one-part polyuretha nes, 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 ess entially 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 l ook at improving the durability of polyurethane bonds to wood by priming be fore bonding with a hydroxymethylated resorcinol (HMR) coupling agent that is known to enhance adhesion of all thermosetting wood adhesives. The HMR d ramatically increased the wet wood failure and resistance to delamination o f polyurethanes that were found deficient in the previous study, to levels comparable with the highly durable resorcinol adhesive. One-part polyuretha ne adhesives met the strength and durability requirements of the most rigor ous of tests, specifically American Society for Testing and Materials (ASTM ) standard D 2559 (3), when wood surfaces were primed with the HMR coupling agent.