Polyelectrolyte Complex/MMT Hybrid for SPI/WE Adhesives with Balanced Flame Retardancy and Shear Strength

Soybean protein isolate (SPI) adhesives are eco-friendly but suffer from poor water resistance and bonding strength. Waterborne epoxy (WE) can effectively improve these properties; however, WE-modified SPI adhesives can hardly satisfy the growing stringent flame-retardant demands for plywood. Herein, we report a composite adhesive (SPI/20WE/THPS@CS/MMT) that simultaneously exhibits favorable flame retardancy and dry and wet shear strengths. In this system, a phosphorus-, nitrogen-, and sulfur-containing polyelectrolyte complex (THPS@CS) is fabricated by vacuum-drying an ionically crosslinked polyelectrolyte hydrogel derived from bis[tetrakis(hydroxymethyl)phosphonium] sulfate (THPS) and chitosan (CS) and subsequently incorporated into the SPI/WE matrix (SPI/20WE) together with montmorillonite (MMT). The plywood bonded with SPI/20WE/THPS@CS/MMT achieves a limiting oxygen index (LOI) of 28.0% and shows significantly reduced heat release in cone calorimeter tests. Meanwhile, the resultant plywood possesses high dry and wet shear strengths that fulfill the interior-use plywood criteria specified in GB/T 9846–2015. This work offers new insights into the design and fabrication of high-performance eco-friendly plywood adhesives.