A High Solid Content Bioadhesive Derived from Soybean Meal and Egg White: Preparation and Properties

Soy protein-based adhesives (SPAs) inherently possess low solid content and poor water resistance. Chemical crosslinking can effectively improve the water resistance, but will also contribute to the low solid content. In this study, egg white (EW) was introduced to soybean meal (SM) adhesive as a substitute for water, followed by crosslinking with triglycidylamine (TGA) to develop a high-performance bioadhesive with a high solid content. The solid content, rheology, water resistance, functional groups, crystallinity, thermal degradation behavior, and fracture surface micrographs of the resulting adhesives were investigated. Three-ply plywood was fabricated and its wet shear strength was tested. The results indicated that 57.1 % EW increased the solid content of SM adhesive up to 43.32 %, while maintaining an appropriate viscosity for application. The water resistance of SM adhesive was improved by 36.0 % as optimal EW addition amount of 57.1 % was introduced. The results were attributed to the following observations: (1) EW further increased the crosslinking density of cured adhesive by involving in the crosslinking reaction. (2) EW promoted the transformation from small molecule element to skeleton structure of the cured system. (3) The cured SM/EW possessed a dense and smooth crosslinking structure, successfully preventing the water intrusion. The optimal adhesive formulation provided the plywood with the highest wet shear strength of 1.14 MPa which was improved by 153.3 and 80.9 % compared with that bonded by the SM and SM/TGA adhesive. The improvement can be explained by: (1) high solid content of SM/EW adhesive reduced the water evaporation, effectively limiting the damage to the mechanical properties of plywood. (2) The cured SM/EW adhesive possessed higher water resistance. (3) The appropriate viscosity of adhesive formed more interlockering with wood. The results of wet shear strength also indicated that the performance of new SM/EW bioadhesive was superior to that of conventional urea–formaldehyde (UF) resin and PAE cross-linked SM adhesive, and was comparable to that of melamine modified UF resin. The high solid content of the SM/EW bioadhesive combined two typical vegetable and animal proteins and pushed the performance limit of low solid content SPAs.