Rheological properties of blend melts for starch-based foams in extrusion processing

Blends composed of 49% wheat and cam starch (dry solid basis), 33% syntheti c polymers, 10.5% total water 7% blowing agents, and 0.5% nucleating agent were extruded at different screw speeds (70-130 rpm) using a single-screw C . W. Brabender laboratory-scale extruder with a slit capillary die attached in-line. Shear stress at wall, shear rate at wall, shear viscosity, and ex it pressure of the blends melted at different die temperatures (403, 413 42 3, and 433 K) were determined at various flow rates and pressure gradients within the slit capillary die. Shear stress at wall increased at a given sc rew speed with decreased die temperature; whereas, shear rate at wall decre ased. At each screw speed and die temperature, shear stress at wall and she ar rate at wall of blend melts containing normal wheat starch were higher t han those of blend melts containing hydroxypropylated wheat starch. The ble nd melts transformed from dilatant to pseudoplastic fluid when temperature increased from 403 to 433 K. The blend melts containing normal wheat starch exhibited dilatant behavior at 403 and 413 K and pseudoplastic behavior at 423 and 433 K. The blend melts containing hydroxypropylated wheat starch e xhibited dilatant behavior at 403 K and pseudoplastic behavior at 413 423, and 433 K.