Chemical and mechanical aspects of HMR primer in relationship to wood bonding

The mechanism by which hydroxymethylated resorcinol (HMR) primer improves the durability of various adhesives to wood has been hypothesized as covalent chemical links between the adhesive and primer and possibly between the primer and wood. The present work presents experiments to test this hypothesis. In the first test, some resorcinol was displaced by 2-methylresorcinol at various levels to decrease crosslinking but retain hydroxymethyl groups on the active primer for reaction with adhesive and possibly with wood. Increased substitution for resorcinol produced lower durability, which indicates that reduced crosslinking in the primer leads to performance loss. In the second test, HMR-primed wood was stored at elevated temperature and/or long times (up to 3 months) before being bonded with an epoxy adhesive. These extreme storage conditions were expected to allow any remaining hydroxymethyl groups to react and be consumed, if possible, before adhesive was applied. This treatment had no effect on bond durability. The results indicate that the mechanism for durability enhancement likely lies not in chemical reactions with the adhesive, but in some mechanical stabilization of the wood surface. In addition, the results demonstrated that HMR primed wood has excellent bondability to epoxy resin and does not require that the surface be replaned, even after 3 months of storage.

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