Advancing Wood Adhesive Sustainability: Unmodified Technical Lignins as Phenol Substitutes

In this study, lignin-based phenolic adhesives were developed by replacing 80% of the petrochemical phenol with two commercially available kraft softwood (K-SW) and organosolv wheat straw (O-WS) lignins, without any further modifications. The synthesis of lignin-based resins was conducted using an alkaline catalyst with the molar ratio of phenol (lignin 80-phenol 20) to formaldehyde set at 1:3. The chemical, thermal, and mechanical properties of the lignin, lignin-based resins, and final adhesives were evaluated by using appropriate standard test methods. The curing behavior of the formulated resins was investigated through differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). Although the lignin-based resin exhibited lower curing temperatures (110 °C for K-SW LP20F and 105 °C for O-WS LP20F) compared to commercial PF resin (118 °C), the amount of heat generated during exothermic curing by PF resin was three times higher than that of lignin-based resins. After extensive optimization efforts, we were able to significantly reduce the hot curing time of lignin-based resins (e.g., kraft softwood, LP20F) from 11 to 5 min for the first time while achieving acceptable performance. Furthermore, DMA analysis showed a higher degree of curing for lignin-based adhesives (76%) compared to commercial PF (56%) at the end point. The adhesion strengths of the formulated adhesives were also assessed using three-layer plywood samples cured under the recommended press parameters currently used by plywood manufacturers. Although lignin-based adhesives demonstrated dry adhesion strength comparable to that of commercial PF adhesives after 3 min of hot curing and showed an acceptable percentage of wood failure (≥85%), they required a longer press time to meet the mandated wet wood failure. Specifically, it took 5 min for kraft softwood adhesives and 7 min for organosolv wheat straw adhesives, compared to 3 min for commercial PF, to achieve at least 85% wood failure (as a measure of strong adhesive performance) in the boil water test, as required for exterior grade plywood. This is a remarkable achievement, as it enables resin producers to shift their focus toward unmodified, commercially available lignins for phenolic resin formulation, eliminating the need for using modified (phenolated) lignins.