Low-Cost and High-Strength Soybean Meal Adhesives Modified by Tannin–Phenol–Formaldehyde Resin

Wood adhesive is very important to the quality of wood-based panels in the forest product industry. Researchers are currently focused on developing green and environmentally friendly adhesives using biomass materials. Soybean meal (SM), a cheap and high-protein byproduct from soybean processing, is considered a green biomaterial resource for adhesive production. In this study, SM was modified using urea and sodium hydroxide as denaturants, and two cross-linking agents were prepared to incorporate into the soybean meal adhesive for bonding reactions. Sodium hydroxide added to soybean protein caused structural damage to the protein structure, which allowed the peptide chains to unfold extensively, forming a low-molecular-weight peptide mixture with a good size distribution and exposing numerous active functional groups for cross-linking with the tannin–phenol–formaldehyde resin (TR) and wood. Adding the epoxy resin CA and TR enabled the cross-linking agents to react with the active functional groups on proteins, forming a cross-linked network structure. As a result, the adhesive’s strength, residual rate, and solid content were improved.