Author
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Willett, Julious |
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Submitted to: Cereal Chemistry
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 9/8/2000 Publication Date: N/A Citation: N/A Interpretive Summary: Starch is a low cost, renewable material with potential applications as a filler in biodegradable materials. Recent advances have led to starch-filled materials with starch contents of 50 percent or more by weight which retain useful properties such as strength and stiffness. In order to process these materials by conventional plastics processing methods, the materials must be melted and forced to flow under pressure. At these high starch contents, the resistance to flow, or viscosity, is strongly dependent on manner in which starch particles pack together. The packing characteristics of various starch types were measured under different conditions. It was found that potato, wheat, and corn starches packed most efficiently, while rice and amaranth packed less efficiently. The behavior of these two starches was due to the agglomeration, or clumping, of individual starch particles. These data are important for researchers characterizing the flow properties of starch-filled materials and indicate how combinations of different starches may be blended to vary the packing efficiency. This effect is important in determining bulk properties of starch powders in various applications. Technical Abstract: The rheological properties of granular materials and dispersions of solid particles in fluids are dependent on the packing characteristics of the particles. Maximum packing fractions (phim) have been measured for corn, wheat, rice, potato, and amaranth starches, in the dry state and dispersed in either ethanol or hexane, using a tapping method. The observed maximum packing fraction increases with tapping time to a constant value. Values measured for dry starches were lower than those measured in liquids, and reflect the effects of granule shape and inter-granular friction. Values measured in fluids for potato, corn, and wheat starches were all similar, and close to the value for random close packing of monodisperse spheres (0.63). Values for amaranth and rice starches were significantly lower, due to agglomeration and clumping of individual granules. Blends of corn and potato starches show a slight enhancement of packing, with some phim values greater than potato starch, consistent with data for bimodal blends of spheres. Blends of rice and potato starches displayed enhanced packing above ideal mixing but did not exceed the packing fraction of the potato starch. Knowledge of starch packing fractions is required for fundamental understanding of the rheological properties of granular starch-filled materials and important for predicting their processing characteristics. |
