Properties of injection moulded starch synthetic polymer blends - IV. Thermal and morphological properties

The morphological properties of blends of corn starch and maleated polyolefins are presented in this paper. Four different starches were used: unmodified waxy maize, industrial corn, common corn and a high amylose starch. The concentration of starch in the blends was kept at 70% by weight. Three different synthetic polymers: ethylene-vinyl acetate, low density polyethylene and high density polyethylene were melt blended with the respective starches in a corotating twin screw extruder. A corresponding maleated synthetic polymer was added as a compatibilizer. Differential scanning calorimeter (DSC), dynamic mechanical analyzer (DMA), scanning (SEM) and transmission electron (TEM) microscope and optical microscopy (OM) were used to evaluate the phase morphology. DSC studies indicate that starch granules underwent melting with each subsequent processing step. This was also confirmed from the optical photomicrographs where the starch granule size decreased after each subsequent processing step. DMA studies showed that the blends exhibited two distinct glass transitions, one for each component and that the glass transition temperature of starch decreased as the amylose content increased in the starch blends. Microscopy studies indicated that the starch phase became finer as the amylopectin content in the starch increased. For blends containing waxy starch, a co-continuous phase with synthetic polymers was evident. (C) 1998 Elsevier Science Ltd. All rights reserved.