Reaction Mechanism of Hydroxymethylated Resorcinol Adhesion Promoter in Polyurethane Adhesives for Wood Bonding

Hydroxymethylated resorcinol (HMR) coupling agent is a low condensation resorcinol-formaldehyde resin mix of low molecular weight oligomers. The mechanism that appears to link the hydroxymethylated resorcinol coupling agent to polyurethane adhesives for the reported enhancing of their wood bonding performance was shown to depend on (i) the initial formation of urethane linkages between the methylol groups of HMR and the still active isocyanate groups of the polyurethane adhesive, followed by (ii) slower formation of urethane linkages between the resorcinol phenolic hydroxyl groups and the isocyanate groups of polyurethane adhesive. This mechanism is in addition to the adhesion promoting effect that HMR has on the wood substrate as already reported by other authors.