Preparation and characterization of a soy protein-based high-performance adhesive with a hyperbranched cross-linked structure

Although eco-friendly soybean meal-based adhesives can potentially replace formaldehyde-based ones to eliminate indoor pollution, the practical applicability of the former is limited by their poor water resistance and low bond strength. Herein, we prepared a high-performance soy protein-based adhesive by oxidation of soybean soluble polysaccharide (SSPS, the major component of soybean meal) followed by polyamide grafting to obtain hyperbranched aminated SSPS (A-SSPS). The thus obtained A-SSPS was mixed with soy protein (SPI) and bio-based triglycidylamine (TGA) to afford a novel adhesive with a hyperbranched cross-linked structure. Characterization of the above polysaccharide and adhesive allowed the A-SSPS grafting rate to be evaluated as 87.4%, and incorporation of 3 wt% A-SSPS into the SPI/TGA adhesive resulted in a 72% viscosity reduction, a 42% moisture uptake reduction, and a 10% residual rate improvement of the cured adhesive relative to the corresponding parameters of the SPI-only adhesive. Consequently, plywood bonded by the above A-SSPS-containing adhesive exhibited a wet shear strength (1.07 MPa) exceeding that of plywood bonded by the SPI-only adhesive control by 168% and was therefore suited for interior use. These improvements were rationalized as follows. The incorporation of A-SSPS allowed both proteins and polysaccharides to participate in cross-linking to create a hyperbranched cross-linked structure in the adhesive, which resulted in increased cross-link density and promoted compact cross-section formation in the cured adhesive, thereby preventing moisture intrusion. Therefore, this work provides an important reference for future research, since the developed approach can potentially be used to enhance the performances of other bio-based materials and adhesives.