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 an d the resulting morphology of thermoplastic starch (TPS)/poly(hydroxy ester ether) blends is studied. The viscosity ratio (lambda) was found to vary o ver two orders of magnitude (0.1-10) with changes in the TPS moisture conte nt (15-30%) and temperature (120-160 degrees C). This had a substantial eff ect on the level of mixing, nature of dispersion and the onset and nature o f co-continuity. Deformation (in the flow direction) of the dispersed starc h phase was possible under high moisture conditions (higher lambda), leadin g to fibrillar and laminar types of morphologies at 50-80% starch level, wh ereas processing at a low moisture level (lower lambda) produced a more dis persed morphology. When the viscosities of the two phases were significantl y different, the low-viscosity polymer migrated to the surface. Surprisingl y lambda increased with temperature at 20% moisture TPS but decreased with temperature at 30% moisture TPS. Hence, moisture content and temperature gr eatly affected the surface enrichment of the lower viscosity polymer. Water partitioning took place between TPS and PHEE during extrusion, dependent o n 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.