Improving Bond Performance and Reducing Cross-linker Dosage for Soy Flour Adhesives Inspired by Spider Silk

Soy-based adhesives are environmentally friendly alternatives to the harmful formaldehyde-based resins in manufacturing wood-based panels. However, almost all of the modification methods use large amounts of additives that negatively impact the environment. Therefore, using a low-dosage cross-linker to improve both adhesive bond performance and toughness remains a challenge in soy-based adhesive modification, thereby limiting its applicability. Inspired by the microphase- b separated structure giving spider silk high toughness and performance, TEMPO-oxidized cellulose nanofibers (TOGNFs, hard phase) were grafted by the hydroxymethyl melamine prepolymer (HMP, enhance the TOCNFs/SF interface) to form HTs, and then mixed with soy flour (SF, soft phase) to develop a novel soy protein-based adhesive. This microphase structure effectively improved the toughness of the soy-based adhesive, while the HTs reacted with SF to form a cross-linked structure, which enhanced the water resistance of the soy-based adhesive and hindered the formation and expansion of cracks during the breaking process. The results show that by using only 0.7% HTs in the adhesive formulation, the dry and wet shear strengths of the resultant plywood increase by 117 and 322%, respectively, while wood failure reaches 90%, which satisfies the requirements for interior plywood use. Thus, using a low-dosage (72-94% lower compared to those in the literature) nanomaterial to build a microphase-separated structure in a soy-based adhesive is a promising means for plywood production.