Polyphenol-Metal Ion Redox-Induced Gelation System for Constructing Plant Protein Adhesives with Excellent Fluidity and Cold-Pressing Adhesion.

Soy protein (SP) adhesives can resolve several problems with aldehyde-based adhesives, including formaldehyde release and excessive dependence on petroleum-based materials. Nevertheless, their development is hindered by the lack of balance between fluidity and high cold-pressing adhesive strength. A dynamically cross-linked SP adhesive with excellent fluidity and cold-pressing adhesion was developed in this study based on the polyphenol-metal ion redox-induced gelation system. SP was blended with acrylamide (AM), ammonium persulfate (APS), and the tannic acid (TA)-Fe complex to prepare an adhesive gel precursor with good fluidity. In situ gelation of SP adhesive was then achieved via AM polymerization, as initiated by redox between TA and Fe. As expected, the prepared adhesive gel exhibited outstanding cold-pressing bonding strength (650 kPa) to the veneers compared to the neat SP adhesive, which has almost no cold-pressing bonding strength to the veneers. The TA-Fe complex induced an in situ gelation system, which endowed the SP adhesive with strong cohesion; the topological entanglement of the adhesive gel in the veneers contributed to tight interfacial combinations. The TA-Fe complex served not only as an accelerator of SP adhesive gelation but also as a "cross-linking core" for the cross-link SP adhesive system. The prepared SP-based adhesive also exhibited outstanding hot-pressing bonding strength and mildew resistance. The proposed polyphenol-metal ion-induced in situ gelation strategy may provide a new approach for developing advanced vegetable protein adhesives to replace aldehyde adhesives.