Fundamental Analysis of Wood Adhesion Primers

Hydroxymethyl resorcinol (HMR) is an effective adhesion promoter (primer) for wood bonding; it dramatically improves adhesion and enhances bond durability against moisture exposure. In an effort to improve understanding of the HMR mechanism of action, this work compared HMR with two other chemical treatments investigated as wood primers: alkyl-HMR (a-HMR), an HMR variant having reduced crosslink density, and a 5% solution of polymeric methylenebis(phenylisocyanate) in N-methylpyrrolidone (solution referred to as "pMDI"). The experimental system was red oak (Quercus rubra) bonded with a moisture-cure polyurethane adhesive (PUR). The objective was to document wood rheological changes induced by the three primers, and determine if these changes correlated to primer efficacy in PUR-bonded red oak. Adhesion was tested in mode-I (opening) fracture using dual cantilever beam specimens. HMR and a-HMR proved to be highly effective primers for PUR-bonded red oak; both primers dramatically improved bondline toughness and durability. Relative to HMR, the reduced crosslink density in a-HMR did not impair primer efficacy. In contrast, the pMDI primer was ineffective; it reduced bondline toughness and durability. Solvent-submersion, torsional dynamic mechanical analysis (DMA) was conducted on primer-treated red oak (with specimens immersed in dimethylformamide). Using all three grain orientations, the lignin glass/rubber transition was carefully studied with attention directed towards primer-induced changes in stiffness (storage modulus), the glass transition temperature (Tg), the associated damping (tan ° maximum intensity), and the breadth of tan ° transition. It was found that primer effectiveness correlated with a reduction in damping intensity, and also with a Tg increase greater than 5°C. Determination of these correlations was complicated by grain dependency, and also by rheological changes caused by solvent treatments that were used as primer control treatments.