Performance of soybean protein adhesive cross-linked by lignin and cuprum

As a substitute for aldehyde adhesives, soybean protein adhesive has drawn wide interest in addressing formaldehyde pollution in human living rooms. However, during the preparation process, the addition of a cross-linking agent from petroleum resources is necessary to improve the strength and cause the problems of high viscosity and brittleness . Inspired by the adhesion structure of catechol in mussels, lignin with a catechol structure was constructed by one-step demethylation in this study. Lignin, copper ions , and soybean protein isolate were mixed to produce a biobased adhesive exhibiting a triple network structure without adding a petroleum-based epoxy crosslinking agent. The quinone in the lignin and the amino group in the soybean protein underwent a cross-linking reaction to ensure the bond strength and water resistance of the adhesive. The catechol structure built a sacrificial network by forming hydrogen bonds to improve the toughness and coating performance of the adhesive. Multifunctional copper ions formed multiple interface coordination bonds, reducing the viscosity of the adhesive. The wet shear strength of the adhesive with the triple cross-linked network structure reached 1.29 MPa, which was 92.5% higher than that of the pristine adhesive. The viscosity decreased to 24340 mPa s, reflecting a reduction of 84%, and the toughness and coating performance improved. Notably, the prepared adhesive showed mildew resistance (120 h), antibacterial properties (antibacterial zone = 6.8 mm), flame retardancy , and low VOCs release (6.86 × 106 mg/m2·h). • Demethoxy-treated lignin formed catechol structure (UAL), and mixed with SPI and Cu2+ to obtain adhesive. • The adhesive with triple network has good toughness, suitable viscosity and bonding performance. • The adhesive exhibited good antibacterial, flame-retardant, and mildew resistance. • Good strength was maintained after the hot-pressing temperature was reduced by 13°.