Effect of moisture level on the morphology and melt flow behavior of thermoplastic starch/poly(hydroxy ester ether) blends

The effect of moisture content and temperature on the melt flow behavior and the resulting morphology of thermoplastic starch (TPS)/poly(hydroxy ester ether) blends is studied. The viscosity ratio (lambda) was found to vary over two orders of magnitude (0.1-10) with changes in the TPS moisture content (15-30%) and temperature (120-160 degrees C). This had a substantial effect on the level of mixing, nature of dispersion and the onset and nature of co-continuity. Deformation (in the flow direction) of the dispersed starch phase was possible under high moisture conditions (higher lambda), leading to fibrillar and laminar types of morphologies at 50-80% starch level, whereas processing at a low moisture level (lower lambda) produced a more dispersed morphology. When the viscosities of the two phases were significantly different, the low-viscosity polymer migrated to the surface. Surprisingly lambda increased with temperature at 20% moisture TPS but decreased with temperature at 30% moisture TPS. Hence, moisture content and temperature greatly affected the surface enrichment of the lower viscosity polymer. Water partitioning took place between TPS and PHEE during extrusion, dependent on the pre-extrusion moisture content of TPS but independent of the relative concentrations of the two polymers. (C) 2000 Published by Elsevier Science Ltd. All rights reserved.