Synthesis of soy-protein adhesives with excellent cohesion and cold-pressing bonding strength through multiple noncovalent crosslinking network of waterborne epoxy resin

Abstract

In order to achieve the purpose of reducing cost and increasing efficiency of plywood fabrication, development of high-temperature soy meal (HSM)-based adhesives with high cold-pressing bonding strength without compromising its internal cohesion and interfacial adhesion is a meaningful goal in production. However, due to the agglomerated intrinsic structure, weak intermolecular interactions and strong hydrophilicity, the application of HSM adhesive was limited by its poor cohesion and unstable bonding performances. Taking consideration of the abundant hydrogen bonds sites and toughened microphase separation structure, in this work, polyurethane (PU) fragments served to provide bonding strength during prepressing stage. Sulfonate groups were grafted to prepare a stable waterborne epoxy emulsion (WEU) and then used as a crosslinker for HSM adhesives. Besides the introduced carbamate and polyether segments that provided regular hydrogen bonding platforms within HSM matrix, the soft polymer long-chain coils overlapped to increase the initial viscosity of the adhesives. This enhanced cohesive network strengthened the cold-pressing bonding strength of HSM-WPA2–15, reaching 0.76 MPa, which is enough for actual factory requirements. Pressure and thermal condition can stimulate resin penetrating into wood voids and cure it, opening active epoxy groups to covalent bonding within HSM chains. The secondary crosslinking network optimized the water resistance of cured adhesives, achieving 1.00 MPa of the adhesives under wet shear strength testing. This novel and feasible method demonstrates a foundational design, being able to be utilized for developing other plant-based wood adhesives with improved performances.