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 plastics 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 temperatures. Hydrochloric acid, sulfuric acid, acetic acid, and propionic acid were examined for their suitability for treating soy protein with regard to final properties. The molded specimens were tested and calculated for their tensile and yield strength, percent elongation, Young's modulus, and water absorption. The pH of the molding material was crucial with respect to the water absorption of the plastic, with an optimum pH around the isoelectric point of the protein (pH 4.5). The plastics 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 values 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. Plastic specimens made from soy concentrate displayed similar tensile strength but greater water absorption compared with plastics made from soy isolate.