Development of a novolak-based hydroxymethylated resorcinol coupling agent for wood adhesives

A hydroxymethylated resorcinol (HMR) coupling agent was previously shown to enhance the exterior durability of bonds between wood and epoxy or other thermosetting wood adhesives, as well as bonds between phenol-resorcinol-formaldehyde and southern pine treated with chromated copper arsenate. When originally developed, HMR had no shelf life, so the resorcinol and formaldehyde components had to be mixed and reacted for several hours before being ready for application. Now we have developed an HMR coupling agent in the form of a liquid novolak prepolymer that remains chemically stable in storage. The novolak HMR is deficient in formaldehyde in its prepolymer state, which makes it stable. With the addition of formaldehyde, the reaction resumes. In the most effective formulation, one-quarter of the formaldehyde was incorporated into the novolak, and three-quarters of the formaldehyde was added to the novolak when it was ready for use. In this study, the reactivated HMR was usable immediately and up to at least 7 hours after the final addition of formaldehyde. The new form of HMR is as effective a coupling agent as the original HMR (based on ASTM D 2559), and capable of producing epoxy bonds to wood that easily pass the delamination requirements (5% maximum for softwoods) of ASTM D 2559 after exposure to the severe accelerated aging test. Differential scanning calorimetry and carbon-13 nuclear magnetic resonance were used to determine the reactivity and changing chemical structure of the novolak HMR.

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