Bio-Based Wood Adhesive from Camelina Protein (a Biodiesel Residue) and Depolymerized Lignin with Improved Water Resistance

The aim of this study was to improve water resistance of camelina protein (CP) for wood adhesives with depolymerized lignin. Kraft lignin was depolymerized by H2O2-induced oxidation in the presence of ultrasound (US) irradiation to reduce lignin's particle size and thermal stability and increase the hydroxyl group. Coupling with depolymerized lignin camelina protein exhibited increased hydrophobicity. Fluorescence spectroscopy analysis revealed that the oxidation treatment of lignin further stimulated the hydrophobization effect of the protein-lignin copolymer due to the increased reactivity of depolymerized lignin to camelina protein. Accordingly, the water resistance of CP-lignin adhesives was significantly improved. When copolymerized with US-induced oxidized lignin, the camelina protein had increased wet shear adhesion strength from 0.28 to 1.43 MPa, with wood panels passing the three-cycle water-soaking test. The CP resin, with depolymerized lignin as an economical, green, and bio-based hydrophobic enhancer, provided an alternative to the petroleum-based and other edible protein-based adhesives, such as soy protein.