Effects of molding temperature and pressure on properties of soy protein polymers

Because of the worldwide environmental pollution problem with petroleum pol ymers, soy protein polymers have been considered as alternatives for biodeg radable plastics. The objective of this research was to study the curing be havior of soy protein isolates (SPIs) for that application. The molding var iables of temperature, pressure, and time and curing quality factors of ten sile strength, strain, and water resistance were evaluated. The maximum str ess of 42.9 MPa and maximum strain of 4.61% of the specimen were obtained w hen SPI was molded at 150 degrees C and 20 MPa for 5 min. The water absorpt ion of the specimen decreased as molding temperature and time increased. Gl ycerol greatly improved the flexibility of the specimen but decreased its s trength. For SPI with 25% glycerol added, the maximum stress and strain of about 12 MPa and 140%, respectively, were achieved when the specimen was mo lded at 140 degrees C for 5 min. Molding temperature, pressure, and time ar e major parameters influencing the curing quality of soy protein polymers. At fixed pressure, the molding temperature and time had significant interac tive effects on curing quality. At high temperature (e.g., at 150 degrees C ) it took about 3 min to reach optimum curing quality; however, at low temp erature (120 degrees C) it took about 10 min to reach optimum curing qualit y. The maximum strength and strain of the cured protein polymer occurred at the molding temperature close to its phase transition temperature or about 40 degrees C below its exothermic temperature. (C) 1999 John Wiley & Sons, Inc.