A novel green polyurethane adhesive based on castor oil and tannic acid with excellent water, salt, acid, alkali and antibacterial properties

In recent decades, solvent-free polyurethane adhesives from renewable sources are increasingly needed for engineering and biomedical applications. However, the significant drawbacks persist in bio-based polyurethane adhesives, including low moisture resistance, high sensitivity to variations in pH and salt, a sophisticated synthesis route, and costly adhesive building blocks. Inspired by the catecholic properties of mussel foot proteins, we used tannic acid as a crosslinker to strengthen the capabilities of castor-oil-based adhesives through a facile yet robust strategy. After directly mixing at room temperature, tannic acid was bonded with polyurethane prepolymer to construct a dual-network of noncovalent interaction and covalent crosslinks, hence improving the crosslinking density and cohesive connections. In addition, the adhesive and substrate interface interactions were strengthened. Owing to the diverse bonding abilities, the dry shear strength rose from 1.1 to 9.5 MPa. After immersing in water for 200 h, the largest wet shear strength was 5.2 MPa. Furthermore, this adhesive strength also increases at pH 2,12 and ionic strength of 0.2 M NaCl, demonstrating excellent adhesiveness (similar to 11.8 MPa). The easy-to-make adhesive also has outstanding bacterial resistance. The utilization of tannic acid and the formation of a dual network are a new strategy for developing green high-performance adhesives with biomass feedstock.