Complex coacervation behavior between cationic polyamides and anionic epoxy resins endowed improved cold-pressing performance of soy protein adhesive

The weak internal cohesion and initial viscosity of soy protein adhesives lead to the poor cold-pressing bonding performance of plywood preparation. This deficiency restricted the assembly efficiency during the sizing process. In this work, we report that electrostatically induced “coacervation behavior” between anionic and cationic resins can be considered a facile strategy to guide the preparation of a high-temperature soy meal (HSM)-based adhesive with improved cold-pressing bonding performance. A robust internal cohesive framework was aided by multiple interactions (e.g., electrostatic interactions, cation-π interactions and hydrogen bonding) with the combination of polyanionic carboxyl-grafted waterborne epoxy resin (WEU) and cationic polyamides resin (PAE) in HSM matrix. The resultant adhesives exhibited improved initial viscosity with cold-pressing bonding shear strength at 1.17 MPa, which is 200.0% higher than that of the pristine HSM adhesive. Ring-opening reactions occurred in the adhesive system under hot-pressing, and the chemical crosslinking network rendered the excellent water-resistance of the adhesives. The maximum wet shear strength of the plywood reached 1.62 MPa, meeting the fundamental requirements (0.7 MPa) for its industrial application. Given its improved cold-pressing and wet bonding shear strength, this effective and novel coacervation strategy is promising to pave the way for high-performance soy protein-based composites in plywood manufacturing.