Waterborne Lignin-Based Epoxy Resin: A Green and Effective Chemical Cross-Linker to Form Soy Protein-Based Wood Adhesive with High Performance

Waterborne lignin-based epoxy resin emulsions (WLEPs) bearing poly(ethylene glycols) (PEGs) as hydrophilic and soft segments were successfully prepared without using any solvent, and then the WLEPs were cured with polyamide (PA) and the soybean protein isolate (SPI) to form wood adhesives SPI/WLEP/PA with good bonding performance and water resistance. The WLEPs were uniformly dispersed in the SPI matrix, and the multiple interactions between WLEP, PA, and SPI resulted in a more stable and compact cross-linked network, which was conducive to enhancing the cohesion strength, water tolerance, and thermal stabilities of the SPI-based adhesives. In comparison to the pristine SPI adhesive, the dry and hot water (63 °C for 3 h) shear strengths of the plywood formulated with SPI/WLEP/PA (WLEP: 16 wt %) remarkably increased by 137% (3.94 MPa) and 134% (0.75 MPa), respectively, satisfying the demand of interior-use plywood type II. Especially, when the WLEP loading reached 21 wt %, the plywood demonstrated outstanding boiling water strength of 0.77 MPa, higher than the China national standard requirement for exterior-grade plywood type I (≥0.70 MPa). These results suggested that the WLEPs could serve as a green and effective cross-linker to give SPI-based wood adhesives with high water tolerance and bonding performance.