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United States Department of Agriculture

Agricultural Research Service

Title: Screw Design Effects in Single Screw Extruder Processing of Thermoplastic Starch

Authors
item Willett, Julious
item Jasberg, Brian

Submitted to: Proc American Chemical Soc Div Of Polymeric Materials Science & Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 17, 1994
Publication Date: N/A

Interpretive Summary: There is considerable interest in the use of starch as a biodegradable, annually renewable substitute for resource-depleting, nonbiodegradable plastics. Starch is converted into plastic materials by extrusion processing. The design of the screw is important in determining the properties of the extruded starch. This work shows that screw design affects the output and quality of extruded starches. The results are important in understanding the effects of extrusion processing on starch properties, and will be of use to those working in the area of starch-based plastic materials.

Technical Abstract: Unmodified cornstarch with 22% moisture content was extruded in a single screw extruder with three screw designs: a 1:1 no-compression screw, a 3:1 compression screw, and a high shear mixing screw. Extrusion conditions were 25 rpm and 100 rpm at 120 deg C and 160 deg C. At 120 deg C/25 rpm, the starch melts were dilatant and contained remnants of the original granule structure. At 160 deg C/100 rpm, the 1:1 and 3:1 screw extrudates contained clumps of intact starch granules; the mixing screw extrudates had no intact starch granules. The melts extruded at 160 deg C exhibited shear thinning behavior. X-ray diffraction showed that virtually all crystalline structure of the native granules were destroyed by extrusion. Extrudates from the high shear screw had the highest intrinsic viscosity, despite having the greatest mechanical energy input. The no compression screw had the smallest pressure/output ratio, but the high shear screw gave the most uniform extrudates.

Last Modified: 10/25/2014
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