Rheological and thermal properties of thermoplastic starch with high glycerol content

This work is part of a series of papers devoted to the study of LDPE/thermoplastic starch (TPS) blends. In those papers, a starch/glycerol/water suspension was fed to a twin-screw extruder (TSE) and a water-free TPS was prepared in the first half of the TSE. Molten LDPE was added via a single screw extruder added midway on the TSE and the latter half of the TSE was used to prepare the blend. This approach allows for the preparation of TPS with particularly high loadings of glycerol plasticizer. In this paper, the thermal and rheological properties of water-free TPS (glycerol content 29-40%) prepared in the first half of a TSE using the above approach was analyzed. DSC analysis reveals the presence of a glass transition temperature (T-g) below ambient temperature, which indicates the complete gelatinization of starch granules. The T-g decreases from - 45.6 to - 56.2degreesC as glycerol content increases from 29 to 40%. Rheological properties in the shear and oscillation mode were determined on-line from the TSE and by using two oscillatory rheometers, respectively. Time sweep measurements demonstrate that TPS has excellent thermal stability at 150degreesC, but becomes unstable at temperatures above 180degreesC. The studies indicate, however, that TPS stability is maintained for short time periods at temperatures up to 200degreesC. These results are important since they indicate that the potential exists to prepare stable TPS materials at high temperature by maintaining short residence times during processing. The shear viscosity of TPS (at gamma similar to 130 s(-1)) decreases by 20% when the glycerol content is increased from 36 to 40%. Similarly, G(l) and G(ll) decrease as glycerol content increases, with a particularly dramatic reduction around 30% glycerol. This suggests the percolation threshold of the well-plasticized glycerol-rich soft regions. These results concerning the plasticization of starch are critical for morphology control protocols. The shear and oscillation viscosities of TPS (40% glycerol, TPS40) were compared with that of LDPE. It is shown that TPS does not follow the Cox-Merz relation, whereas LDPE measured under similar conditions does. The viscosity ratio of the system TPS40/LDPE is greater than 1 if measured in the dynamic mode, while it is less than 1 if determined from steady shear viscosity values. These results are highly relevant considering the importance of viscosity ratio on the prediction of the morphology of immiscible polymer blends. (C) 2004 Elsevier Ltd. All rights reserved.