PROPERTIES OF BLENDS OF STARCH AND SYNTHETIC-POLYMERS CONTAINING ANHYDRIDE GROUPS .2. EFFECT OF AMYLOPECTIN AMYLOSE RATIO IN STARCH

Corn starch with different amylopectin to amylose ratios was blended w ith styrene maleic anhydride copolymer (SMA) and ethylene-propylene-g- maleic anhydride copolymer (EPMA). The starch had an amylose content o f approximately 0, 50, and 70%. The concentration of starch in the ble nd was kept constant at 60% by weight. The samples were melt blended i n a corotating twin screw extruder. Scanning electron micrographs show ed that the amount of starch granules remaining in the samples varied with the torque. Optical micrographs showed that starch/EPMA blends fo rmed a cocontinuous phase in all blends irrespective of starch variety . For starch/SMA blends, the starch granules remained dispersed in the SMA phase. The torque during blending, tensile strength, water absorp tion, storage and loss modulus, and data on biodegradability of the bl ends are presented. Tensile strength and water absorption correlated w ell with the torque generated during blending: the higher the torque, the lower the tensile strength and the higher the water absorption. Th e tensile strength of blends containing SMA decreased when the humidit y increased. Fractured surfaces of starch/SMA blends exhibited brittle failure; for the ductile starch/EPMA blends, shear tearing appeared t o be the major failure mechanism. For blends containing EPMA, the perc entage elongation increased with increased humidity. Dynamic mechanica l analysis of the blends showed two sharp peaks for tan delta vs. temp erature plot for starch/EPMA plots, but showed a single peak for starc h/SMA blends. Starch/EPMA blends had a higher percentage of water abor ption that became constant after 20 days. Using the ASTM test method D 5902, the starch content in the samples was found to degrade. (C) 1995 John Wiley and Sons, Inc.