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United States Department of Agriculture

Agricultural Research Service

Title: The Effect of Density on Melt Viscosity Calculations of Thermoplastic Starch

Author
item Willett, Julious

Submitted to: Starch
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 1, 1996
Publication Date: N/A

Interpretive Summary: There is considerable interest in using extrusion processing of starch to produce biodegradable plastics and modified starches. An understanding of the flow properties of starch in extruders is needed to produce acceptable materials by extrusion. One method of characterizing the flow properties is to measure the viscosity, or resistance to flow. Knowledge of the density of starch is required to accurately measure the viscosity. Previous studies of starch extrusion assumed a constant density. This report utilizes recently published data for starch density to show that the assumption of constant density introduces errors of only a few percent. These results are of use to scientists studying starch extrusion, and show that time-consuming measurements of starch density are not necessary.

Technical Abstract: Previous studies of the viscosity of starch using extruders have been made without benefit of detailed data on the density of starch/water systems at elevated temperatures and pressures common to extrusion, which are necessary to convert the mass flow rates to volumetric flow rates for the calculation of shear rates. Prior studies have assumed constant starch density. Using recently published data for starch density, it is shown that density corrections have minimal impact on the results compared to the assumption of constant density. The power law index remains essentially unchanged, and changes in the consistency are on the order of a few percent. The choice of the reference density has the major impact on shear rate calculations. Similar results are obtained for low density polyethylene, and by extension, to other polymer melts.

Last Modified: 10/25/2014
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