Hydroxymethylated resorcinol (HMR) and novolak-based HMR (n-HMR) primers to enhance bond durability of eucalyptus globulus glulams

The effectiveness of the hydroxymethylated resorcinol (HMR) and the novolak-based HMR (n-HMR) primers to enhance bond durability of Eucalyptus globulus glulams was evaluated according to EN 391 (Delamination) and EN 392 (Shear strength) for outdoor load-bearing timber structures (Service Class 3). All glulams prepared under increasingly more stringent conditions with both primers, surpassed the requirements for delamination and shear strength. Specifically, when used with a one-component polyurethane adhesive: lamellas did not need to be planed before bonding; the primer amount could be reduced to 50%; and the adhesive could be applied to only one of the bonding surfaces. Additionally, lamellas primed with the n-HMR were effective for 2 weeks prior to bonding. The novolak-based primer overcomes the main limitations in using the original HMR primer, offering more flexibility for the same efficiency, and consequently, it is a more attractive option for manufacturing Eucalyptus globulus glulams.