Study on adhesive properties of soybean meal-based adhesives modified by ultrasonic-chemical treatment

In order to enlarge the industrial applications of wood products made with soybean meal-based adhesive (SMA), ultrasonic and chemical treatment of soybean protein derived from soybean meal (SM) to improve the wet shear strength of the plywood panels bonded was carried out. Chemical modifiers zinc oxide (ZnO) and polyamide epichlorohydrin resin (PAE) were added to SM to enhance the effect of ultrasonic modification. The purpose was to optimize chemical and ultrasonic modification factors, in order to maximize the water resistance under optimal modification conditions. Results indicated that under the optimal conditions, the wet shear strength of an SMA reached as high as 1.21 MPa, which was higher than the China National Standard for interior use of plywood (≥0.70 MPa) of 72.86%, and was higher than by chemical modification alone by 19%. The adhesion mechanisms involved were analyzed using scanning electron microscopy (SEM), Fourier infrared spectroscopy (FTIR) and X-ray diffraction (XRD). The results obtained reflected the synergy between ultrasonic treatment and chemical modification, revealing that the sonication process opens the spherical structure of the protein, allowing a more thorough chemical modification.