Dual hydrogen-bonding network strategy enables fabrication of robust soy protein adhesive capable of excellent bonding at ambient temperature

Abstract

The application of biobased soy protein adhesives is significantly limited by poor initial adhesion during assembly. This deficiency leads to weak interfacial adhesion and cohesion toughness attributed to insufficient intermolecular interactions. In this study, we propose a dual hydrogen bonding system within a waterborne epoxy emulsion (WEU), in which the 2-ureido-4[1 H]-pyrimidinone (UPy) moiety with quadruple hydrogen bonds is grafted onto an epoxy skeleton, and polyethylene glycol (PEG) blocks with regular weak hydrogen bond sites is used to emulsify the WEU by simple mixing. WEU then acts as a crosslinker to improve the mechanical performances of high-temperature soybean meal (HSM) wood adhesives. The introduction of UPy and PEG blocks contributes to the formation of a dense cohesion crosslinking network and an energy dissipation system via multiple hydrogen bonding with peptide chains, enhancing the cold-pressing bond strength of adhesives with a 140% increase, relative to that of a pure HSM adhesive. Thermocuring facilitates interfacial penetration and covalent links via an epoxy ring-opening reaction in the system, creating a secondary crosslinking platform to optimize the water resistance and toughness of the adhesives. Consequently, the cured HSMW-15 adhesive exhibits an improvement in wet bond strength from 0.11 MPa to 0.90 MPa, with optimal toughness at 2.74 MJ/m3. This effective and feasible design provides valuable insights into modification strategies for preparing biobased wood adhesives with superior performance.