Effect of ultrasonic technique on properties of soy meal-based wood adhesive

Soybean meal-based (SM) wood adhesives have regained wide attention due to environmental problems and shortage of oil resources. However, SM wood adhesives have not been widely used because of poor shear strength and low water resistance. Consequently, various modification methods have been considered for improving the properties of SM-based adhesives. The objective of this research was to test the adhesive properties of an SM wood adhesive which had been modified by ultrasound, with the effects of SM mass concentration, ultrasonic time, ultrasonic amplitude and ultrasonic pulse ratio considered. The results indicated that the viscosity, solids content and bonding performance of the modified adhesives all decreased and then increased with increasing ultrasonic intensity. Meanwhile, the dry bond strength, viscosity and the solids content reached the lowest when the ultrasonic time was 1 min, ultrasonic pulse ratio was 20% and the ultrasonic amplitude was 30%. Fourier transform infrared spectroscopy (FTIR) indicated that ultrasonic treatment can only open the spatial structure of the protein molecule, and that new compounds were not formed, with the sonication time increasing, the opened molecular chains may be cross-linked again. The crystallinity and fracture surfaces of the adhesives were also analyzed using X-Ray diffraction (XRD) and scanning electron microscopy (SEM). Based on the above results, after ultrasonic treatment the viscosity of SM wood adhesives were more suitable for bonding wood, and the spherical structure of soy protein was opened, resulting in a change in the spatial structure of the protein, which provided favorable conditions for further modification.