BIODEGRADABLE PLASTIC MADE FROM SOYBEAN PRODUCTS .1. EFFECT OF PREPARATION AND PROCESSING ON MECHANICAL-PROPERTIES AND WATER-ABSORPTION

Preparation and processing conditions for making biodegradable plastic s from soy isolate and soy concentrate were explored. Soy isolate and concentrate, as well as acid-treated soy isolate and soy concentrate, were compression molded at various moisture levels and molding tempera tures. Hydrochloric acid, sulfuric acid, acetic acid, and propionic ac id were examined for their suitability for treating soy protein with r egard to final properties. The molded specimens were tested and calcul ated for their tensile and yield strength, percent elongation, Young's modulus, and water absorption. The pH of the molding material was cru cial with respect to the water absorption of the plastic, with an opti mum pH around the isoelectric point of the protein (pH 4.5). The plast ics obtained were rigid and brittle with tensile strength values from 10 to 40 MPa, yield strength values from 1.0 to 5.9 MPa, elongation va lues from 1.3 % to 4.8 %, and water absorption values from 30 % to 167 % weight after 26-h submersion in distilled water at 25-degrees-C. Pl astic specimens made from soy concentrate displayed similar tensile st rength but greater water absorption compared with plastics made from s oy isolate.