Preparation of a superior soy protein adhesive with high solid content by enzymatic hydrolysis combined with cross-linking modification

The production of traditional wood adhesives relies on petrochemical resources and puts enormous pressure on the ecosystem. Therefore, it is important to use bio-based soy protein adhesives to replace traditional adhesives. However, soy protein adhesives have the problems of low solid content, high viscosity and poor water resistance. In this study, we combined enzymatic hydrolysis with cross-linking modification to reduce the molecular mass of soy protein and increase the solid content of adhesive, constructed a uniform and tight covalent bond cross-linking network in the adhesive. Through this modification strategy, we prepared a soy protein adhesive with high solid content, suitable viscosity and excellent bonding strength. The results showed that compared with 0-SPI adhesive, the viscosity of 4-SPI-18/PA/TA adhesive decreased by 61.6%, reached 21899 mPa·s. Meanwhile, the residual rate of the adhesive increased by 18% (59%), the moisture absorption rate reduced by 7.8% to reach 3.8%. The dry/wet shear strength of modified adhesive increased by 56.1% (1.28 MPa) and 44.3% (0.88 MPa), respectively, and the tensile strain at break increased by 58.1% (14.84%). This is because enzymolysis improved its coating performance, and formed mechanical interlocking with wood. In addition, the amount of SPI increased, and more protein molecules participated in the cross-linking reaction, forming a more uniform and compact cross-linking network structure in the adhesive, effectively improving the bonding strength and toughness. This study provides a novel approach for the preparation of cleaning and tenacious adhesives, aiming to explore a green development model.