MECHANICAL-PROPERTIES OF BIODEGRADABLE SOY-PROTEIN PLASTICS

Experiments were performed to evaluate the room-temperature mechanical properties of soy-protein that were compression-molded with varying concentrations of glycerin plasticizer. Specimens exhibited stiff and brittle behavior with good tensile strength reliability based on Weibull statistics analysis. Raising the glycerin concentration from 0 to 20% progressively increased the tensile strain-to-failure from 1.1 to 1.8% and reduced the tensile strength from 42.1 to 23.6 MPa, the tangent modulus from 4.56 to 1.79 GPa, and the Rockwell hardness from R118.4 to R75.7. Ultrasonic measurements indicated that raising the glycerin concentration from 0 to 20% increased Poisson's ratio from 0.348 to 0.409 and reduced Young's modulus from 7.01 to 5.4 GPa and the shear modulus from 2.5 to 1.8 GPa. Significant increases in the tensile strength and the strength reliability resulted from eliminating Griffith's flaws by sieving the press powder before compression molding. Rockwell hardness rapidly decreased upon immersing these plastics in water at 25 degrees C, an effect which was pronounced for the glycerin-containing specimens.