Fully Biobased Soy Protein Adhesives with Integrated High-Strength, Waterproof, Mildew-Resistant, and Flame-Retardant Properties

The low comprehensive properties of soy protein-based wood adhesives (SPWAs), such as weak bonding strength, poor water and mildew resistance, etc., are the main issues that severely restrict their wide application in the wood manufacturing industry. In this study, fully biobased SPWAs with excellent bonding strength, water and mildew resistance, and flame retardancy were developed via a facile cross-linking reaction between soy protein isolate (SPI), furfuryl alcohol (FA), and phytic acid (PA). Because SPI, FA, and PA were all derived from green and sustainable biomass resources, the components of this kind of SPWAs achieved 100% biomass content. Response surface methodology (RSM) was utilized to optimize the preparation of SPWAs, and the predicted values were highly consistent with the experimental results. The optimized SPWA with a pH of 4.0 and the content of FA addition of 16.7% exhibited high aged and wet shear strengths of 1.07 and 1.47 MPa, respectively, which are superior to those for other reported soy protein-based adhesives. Meanwhile, this kind of SPWAs also displayed excellent mildew resistance, thermal stability, and flame retardancy because of the dense multiple chemical/physical cross-linking network and high P/N content. Therefore, this work provides a green and effective strategy for the development of multifunctional and high-performance fully biobased soy protein wood adhesives.