Preparation of wood formaldehyde-free adhesive by concentrated water delignification black liquor

Traditional lignin-based adhesives are difficult to widely use due to their poor mechanical properties and high modification costs. In this study, an aldehyde-free lignin-based adhesive (LAs) with excellent performance was developed by blending unmodified lignin, crosslinking agent 1,4-butanediol diglycidyl ether, and curing agent polyamide. The hydroxyl group of lignin, the epoxy group of the crosslinking, and the amino group of the curing agent cross-linked and solidified to form a three-dimensional macromolecular structure, which greatly enhanced the adhesive's mechanical strength and chemical stability. The highest dry board bonding strength, wet board bonding strength, and weather resistance strength of lignin-based adhesives were 5.55 MPa, 2.05 MPa, and 1.7 MPa, respectively, which met the Chinese national standard GB/T 9846-2015 (>= 0.7 MPa). Additionally, the VOC test showed that the total organic volatile content of the adhesive was 30.5 g/L, which met the Chinese standard GB 18583-2008 (<= 350 g/L). This formaldehyde-free preparation method without modifying lignin or synthesizing other resins opens up a new way for lignin-based adhesives and helps achieve high-value lignin utilization.