Smart construction of metal chelating for enhancing waterborne polyurethane multi-crosslinked soybean meal adhesives

Developing low-cost renewable bio-based adhesives with excellent properties remains a challenge. Soybean meal (SM) adhesives have gained widespread attention for being green. However, it still has the problems of poor water resistance, low toughness, and easy to mildew. Herein, a waterborne polyurethane (TWPU) with carboxyl side chains was synthesized from tartaric acid (TA). It was used as chelating ligands to form the metal chelating to synergistically modify (SM) adhesives. This overall improved the adhesives bonding strength, toughness and water resistance. The coordination interactions between the polyurethane molecules and the protein molecules of SM were formed by multi-crosslinking via metal chelating, covalent bonding, and hydrogen bonding to enhance the adhesive's toughness, bonding strength and water resistance. As a result, the wet bond strength and toughness of the modified SM adhesives were 166.7% and 275.3% higher than blank sample, respectively, reaching 1.29 MPa and 0.61 J. Considering of the used raw materials and synthesis method in our process are simple and widely available, this study provides an eco-friendly and effective method for green, low-cost, and high-performance wood-based composite adhesives.