Constructing a triple network structure to prepare strong, tough, and mildew resistant soy protein adhesive

Abstract Traditional formaldehyde-based adhesives release toxic substances such as formaldehyde and phenol, which pollutes the environment, endangers human health, and relies on petrochemical resources excessively. Developing a strong soy protein adhesive with reasonable viscosity, toughness, and mildew resistance to replace formaldehyde-based adhesive is important and also a challenge for the industry of wood adhesive. In this study, a triple network structure including covalent bonds, hydrogen bonds and ionic bonds was built in soy protein adhesive. The results showed that compared with SPI adhesive, the viscosity of resultant adhesive decreased by 94.3% to 9952 mPa s, the dry and wet shear strength of resultant plywood increased by 33.9% (2.57 MPa) and 116% (1.36 MPa), respectively. The fracture strain of plywood increased by 36.4%, and the mildew resistance of adhesive increased from 1 day to more than 15 days. This improvement resulted from the adhesive's water resistance improvement by constructing the covalent network and the toughness increasing by the synergistic effect of hydrogen bonds and ionic bonds. This work provides a new method for the preparation of eco-friendly and high-performance adhesives and promotes the development of the green adhesive.