Preparation of a high-performance soy-based adhesive based on biomimetic construction of organic-inorganic hybrid structure

Bio-based soy protein adhesive has the potential to solve the problems of formaldehyde-based adhesive dependence on petrochemical resources and formaldehyde gas release. However, the existing cross-linked modified soy protein adhesives still suffer from problems such as high adhesive brittleness, poor mold resistance and flame retardancy. Therefore, it is of great significance to develop a soy protein adhesive with excellent comprehensive properties such as adhesive toughness, mildew resistance, and flame retardancy. In this study, soy protein was combined with Polyethylenimine (PEI), highly active epoxy crosslinker (TGA), and various inorganic metal compounds to construct an organic-inorganic hybrid structure inspired by oyster adhesion. The results indicated that in comparison with unmodified soy protein adhesive, SPI/PEI/TGA/AlCl 3 adhesive had the highest water resistance bonding strength, increased by 76.4 % to 0.97 MPa (≥0.7 MPa), meeting the requirements of the indoor Class II plywood standard, and the wet/dry bonding toughness increased by 44.8 % and 10.2 %, respectively, to 1.39 MJ/m² and 1.62 MJ/m². This is attributed to the organic-inorganic hybrid structure in the adhesive system, inorganic materials can effectively bear and transfer external loads in the adhesive, thereby enhancing the overall strength of the adhesive; soy protein as an organic part, dissipate energy during the fracture process and improve the toughness of the adhesive. In addition, hydrogen bonds and ionic bonds break and recombine during the breaking process, further dissipating energy and enhancing the toughness of the adhesive. Compared with SPI adhesive, the anti-mildew time of the SPI/PEI/TGA/ZnCl₂ adhesive was extended from 3 days to over 20 days, and the limiting oxygen index (LOI) increased by 16.5 % to 27.5 %, indicating improved anti-mildew and flame retardancy of the adhesive. This modification method can effectively improve the comprehensive performance of soy protein adhesive, which helps promote the large-scale application of soy protein adhesive and the sustainable development of the wood-based panel industry.