Effects of polyhydric alcohols on the mechanical properties of soy protein plastics

Polyhydric alcohols, i.e., ethylene glycol, glycerol, propylene glycol, 1,3-propanediol (trimethylene glycol), and polyethylene glycol (200 and 400), were investigated for their plasticizing effects on soy protein plastics. Tensile strength and percentage elongation at break of compression-molded soy protein specimens containing each of the polyhydric alcohols were measured, and Young's modulus was calculated. Ethylene glycol and glycerol demonstrated the greatest effects on the tensile properties. Differential scanning calorimetric (DSC) thermograms of soy protein with glycerol showed a major exothermic transition. Dynamic mechanical spectra showed that dry plastic specimens had a shear-storage modulus (G' = 1.76 GPa) about 50% larger than those of the high performance synthetic polymers tested. The shear-storage modulus was little dependent on temperature change up to ca. 130 degrees C. After the specimens were moisturized at 50% relative humidity, the G' was reduced to 0.22 GPa. The shear-storage modulus of specimens containing 30% glycerol decreased when the temperature increased above -60 degrees C. Rheological properties of soy protein with and without glycerol were evaluated by using a torque rheometer.