Bio-crosslinking of oxidized hardwood kraft lignin as fully bio-based adhesives for wood bonding

Lignin's low reactivity and crosslinking challenges limit its applications. To address this, many synthetic crosslinkers have been used, but they often involve hazardous chemicals, raising environmental concerns. In particular, it is also true for hardwood kraft lignin (HKL) being burned or wasted in kraft pulping mills. This study reports the successful chitosan bio-crosslinking of oxidized HKL with sodium periodate rather than toxic and environmentally harmful crosslinkers. Both low oxidation (LO) and high oxidation (HO) levels enhance the reactivity of HKL by introducing aldehyde groups, thereby facilitating the formation of imine and amide bonds with chitosan, leading to higher glass transition temperature (Tg), higher viscosity, and greater adhesion strength. The results indicate that the crosslinking of acetone soluble HKL (ASHKL) at LO level with chitosan exhibits excellent dry adhesion strength (1.15 ± 0.2 MPa) for plywood, which meet the required adhesion level of Korean Standard (0.6 MPa) and European Norm 314–2 (1 MPa). These results reveal that chitosan is an outstanding polysaccharide-based crosslinker for the bio-crosslinking of HKL owing to its sustainability, biocompatibility, functional properties, and capability to form covalent bonds.