RHEOLOGY AND MORPHOLOGY OF STARCH SYNTHETIC-POLYMER BLENDS

Corn starch and maleic anhydride functionalized synthetic polymers wer e melt blended in a Haake twin-screw extruder. The amount of starch in the blends was 60 and 70% by weight. The synthetic polymer used was e ither styrene maleic anhydride (SMA) or ethylene propylene maleic anhy dride copolymer (EPMA). The blends did not exhibit normal thermoplasti c behavior; and hence, rheological data was obtained by extrusion feed ing the material through a slit die or cylindrical tube viscometer. Th e starch/SMA blends were extruded through a slit viscometer with a 45- degrees half entry angle, while the starch/EPMA blends were extruded t hrough a cylindrical tube viscometer with a half entry angle of 37.5-d egrees. For the blends, data could be obtained at low to moderate shea r rates (10 < gamma(app) < 200 s-1). At higher shear rates, blends exh ibited slip and/or degradation of starch. The viscosity of the blends exhibited shear-thinning properties. Regrinding and re-extruding throu gh the viscometer a second time showed a significant reduction in shea r viscosity for starch/SMA blends. Gel permeation chromatography data indicated that starch macromolecules degraded upon successive extrusio n. Extensional viscosity, as estimated from entrance pressure drop met hod for starch/EPMA blends showed stretch thinning properties. Regrind ing and re-extruding showed that the samples were more sensitive to ch anges in extensional viscosity as observed from the Trouton ratio vers us extension rate plot. Optical microscopy showed the presence of star ch granules after melt blending, the size of which was related to the torque (or stress) generated during extrusion. The higher the torque, the smaller the size of the starch granules. Successive extrusion runs reduced the number of unmelted granules.