Preparation and characterization of tannin-composite soy protein-based bio-adhesives

Developing sustainable wood adhesives from renewable biomass is essential for reducing dependence on petroleum-derived adhesives and promoting environmentally friendly wood manufacturing. In this study, an environmentally friendly tannin composite soy protein adhesive was successfully prepared by adding tannin-based resin (TR) and silica (SiO₂) to improve its water resistance and wet shear strength. The structure, curing characteristics, thermal properties, and adhesion performance of the obtained soy protein adhesives were determined by Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and wet shear strength. When TR was 10%, the soy protein adhesive without adding SiO₂ had a wet shear strength of 0.84 MPa at 63 °C. After further adding SiO₂, the adhesive achieved a wet shear strength of 1.59 MPa at 93 °C, attributed to its densely cross-linked network comprising Schiff base, inorganic-organic hybrid interactions, and non-covalent hydrogen bonding. This work not only overcomes the water resistance issue of traditional soy protein adhesives, but also provides a feasible method for the development of biomass adhesives. It has the potential to replace petroleum-based products and is particularly suitable for the field of wood composite materials with high weather resistance requirements.