Characterizing phenol–formaldehyde adhesive cure chemistry within the wood cell wall

Adhesive bonding of wood using phenol–formaldehyde remains the industrial standard in wood product bond durability. Not only does this adhesive infiltrate the cell wall, it also is believed to form primary bonds with wood cell wall polymers, particularly guaiacyl lignin. However, the mechanism by which phenol–formaldehyde adhesive integrally interacts and bonds to lignin within the cell wall remains unclear. We used recently developed solubilization methodologies in conjunction with two-dimensional 1H–13C solution-state NMR spectroscopy of ball-milled pine earlywood and latewood bonded assemblies to characterize the chemical modification of wood cell wall polymers after phenol–formaldehyde curing at various cooking times. The results showed that the highly alkaline resin at 140 °C decreased the frequency of the principal arylglycerol-β-aryl ether interunit linkage by eighty percent in earlywood and by twenty percent in latewood. The presence of newly formed diarylmethanes between guaiacyl lignin units and phenolic methylols was confirmed via NMR spectra of the aliphatic methylene and aromatic regions. The phenol–formaldehyde cure chemistry showed that o–p methylene bridges dominated in both earlywood and latewood cell walls, but the propensity of p–p substitution is higher in the latewood cell wall. Our results provide evidence for a simultaneous wood polymer degradation and guaiacyl unit C5 bond formation that occurs during phenol–formaldehyde curing. This competition may be necessary for developing good bond durability between the adhesive and wood.