Thermal modification of soy proteins in the vacuum chamber and wood adhesion

Abstract

Adhesives based on renewable resources have inferior efficiency compared to the synthetic ones. To improve the adhesive properties of soy protein isolate (SPI) adhesives, a new method of thermally modifying SPI was developed. The commercial SPI powder was thermally modified in a vacuum chamber at different temperatures (50, 100, 150, and 200 °C). Thermally modified and unmodified SPI powders were dispersed in distilled water and stirred at 24, 50 or 90 °C. Dispersions were prepared without and with pH adjustment to 10.0. The viscosity of the resulting adhesives was measured. Effective penetration and bond strength in longitudinal tensile shear were tested with bonded beech specimens. Regardless of the pH value and dispersion preparation temperature, SPI thermally modified at 150 and 200 °C exhibited no adhesive properties, while adhesive bonding was achieved with SPI modified at 50 and 100 °C. Adhesives without pH adjustment had viscosity below 28 mPas and exhibited no adhesive penetration. pH adjustment increased the viscosity and adhesive penetration, which improved the adhesive bond strength. Thermal modification at 50 °C resulted in improved wet shear strength, while the temperature of 100 °C deteriorated it. Increased dispersion preparation temperature to 50 °C increased wet shear strength, while temperature of 90 °C decreased it. The combination of thermal modification at 50 °C, dispersion preparation at 50 °C, and pH increase resulted in an adhesive with the highest shear strength.