Mechanical properties of biodegradable soy-protein plastics
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T. Babcock, S. Wang, and J. Jane Department of Food Science and Human Nutrition, Center for Crops Utilization Research, Iowa State University, Ames, Iowa 50011-3110 (Received 3 November 1994; accepted 22 May 1995)
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 °C, an effect which was pronounced for the glycerin-containing specimens.
I. INTRODUCTION Johnson and co-workers published an excellent review of the early development of soy-protein plastics; the first attempts to process these plastics are reflected in European patents dating back to the beginning of this century.1 In the 1930s and 1940s, Henry Ford significantly increased the research and development of these materials and went so far as to produce an automobile with a soy-plastic exterior and a factory producing a variety of household consumer items. Unfortunately, sustained production of soy-plastics did not result from these efforts because of the high processing costs relative to petroleum-based plastics and the inability to prevent degradation by water absorption in finished parts.1 Today, there is renewed interest in developing new uses and improved properties of soy-protein plastics as a result of recent surpluses in U.S. agricultural commodities and rising petroleum prices. In addition, there is interest from the standpoint of producing environmentally benign, biodegradable plastics from renewable resources. Recent studies by Pateau and co-workers focused on identifying and optimizing the critical processing parameters of compression-molded soy-protein (e.g., the temperature, pressure, and time duration of molding, and the purity, the water content, and the type of acid treatment of the press powder).2-3 The main purpose of this study is to report an extensive survey of the mechanical properties of soy-protein plastics that were prepared by the optimized methods of Pateau J. Mater. Res., Vol. 10, No. 9, Sep 1995 http://journals.cambridge.org
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and co-workers. We examined the effects of glycerin plasticizer on the elastic, tensile, and fra
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