Tough sustainable bio-based soybean protein adhesive with salt-bridge hydrogen bonds

Soy protein adhesives have great potential in replacing traditional petrochemical adhesives to produce eco-friendly and formaldehyde-free plywood. However, weak toughness, poor water resistance and low mold resistance during their usage have severely limited their application in practical production. In this study, we introduced bio-based sodium carboxymethyl cellulose (CMCNa) and lauroyl arginine ethyl ester hydrochloride (LAE) into the soybean meal (SM) system to increase the network interweaving density through the electrostatic interactions and multiple hydrogen bonds between CMCNa, LAE and SM. Both SM and CMCNa can form highly specific guanidinium-carboxylate salt-bridge hydrogen bonds with LAE, leading to the formation of a non-homogeneous phase structure in the system, which can improve the internal energy dissipation and further enhance the toughness and water resistance of adhesive. The results show that the modified adhesive has good toughness, and the wet shear strength is significantly increased by 128 % to 1.07 MPa, and can be stored for >8 days. The tough, waterproofing and mildew resistant bio-adhesive offers a eco-friendly alternative to the traditional chemical adhesive.