Toughening improvement to a soybean meal-based bioadhesive using an interpenetrating acrylic emulsion network

The goal of this investigation was to improve the water resistance of soybean meal-based (SM) bioadhesive and enhance the mechanical properties of the bonded plywood by combining triglycidylamine (TGA) cross-linking with an acrylic emulsion interpenetrating network (IPN). The solid content, functional groups, crystallinity, mass loss after hydrolyzing, fracture surface micrographs, and thermal stability of the resulting adhesives were characterized. Three-ply plywood was fabricated and its dry and wet shear strength was determined. The experimental results indicated that the introduction of 8 % TGA produced an improvement of 15.1 % in the water resistance of SM adhesive and 86.8 % in the wet shear strength of plywood as a result of the chemical cross-linking between epoxy groups and protein molecules. However, this reaction increased the brittleness of adhesive and caused an insufficient dry bond strength. Incorporating 8 % AE into the SM/TGA adhesive resulted in a water resistance that was improved by 24.6 %, a dry shear strength that was increased by 44.0 % to 1.80 MPa and a wet shear strength that was increased by 47.9 % to 1.05 MPa. The improvements were attributed to the formation of IPN which increased the solid content, improved the toughness of the adhesive, and promoted an uniform and compact cured structure. All results suggested that chemical cross-linking can effectively improve the water resistance of SM adhesive, but will aggravate the brittleness. Toughening improvement is an effective approach to enhance the performance of the SPAs and solve the low dry strength issue of resulting composites.