Rheological Characterization of Starch Gelatinization: Effects of plasticizer/starch ratio and clay content in corn-star
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Rheological Characterization of Starch Gelatinization: Effects of plasticizer/starch ratio and clay content in corn-starch/glycerol thermoplastic starch (TPS) Authors: Sandra Lara1and Felipe Salcedo1 Affiliations: 1Grupo de Diseño de Productos y Procesos (GDPP), Department of Chemical Engineering, Universidad de los Andes, Bogotá Colombia
ABSTRACT
This work presents a systematic rheological study of the gelatinization process of cornstarch plasticized with glycerol, showing the effects of the glycerol/starch ratio, water/starch ratio, and clay (montmorillonite) content. Gelatinization temperatures at different heating rates in rheological oscillatory temperature-sweep experiments were determined for different cornstarch/glycerol/clay formulations. The influences of the different formulation variables on the gelatinization processes and on the gel properties are analyzed. Some hypotheses postulating how the different intermolecular interactions present in the composites are responsible for these effects are discussed.
INTRODUCTION
Thermoplastic starch (TPS) is an attractive material having potential to replace polymers produced from fossil fuels commonly used in applications such as packaging, agricultural, and biomedical among others. It has advantages such as biodegradability, wide availability and low cost [1]. During TPS production and life cycle, complex phenomena involving the rearrangements of polysaccharide molecules (amylose and amylopectin), such as gelatinization and retrogradation (recrystallization), take place. During the gelatinization process granular starch is transformed into a useful thermoplastic material, TPS. The gelatinization process can be affected by process variables such as the heating rate, the starch botanical source, the starch/plasticizer ratio, and fillers addition, among others [1] [2]. TPS has become one of the most studied bio-based materials but exhibits some drawbacks limiting its current use in commercial applications. In particular, the dependence of starch gelatinization on several formulation and processing variables makes it difficult to produce final goods with consistent properties. Starch retrogradation makes it an unstable material over time and limits its applications. Additionally, low strength and poor water resistance are limiting factors for the use of materials manufactured only from TPS, so that different blending and compounding approaches has been studied. One of them is the addition of clays (like montmorillonite) to the TPS matrix [2][3], trying to take advantage of the fact that melt intercalating polymers into the clay silicate layers has been proven to be an excellent way to greatly increase mechanical, thermal and barrier properties with only a few percent of clay [4]. Expected favorable
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intermolecular interactions between (negatively charged) clay layers and (polar) starch molecules could also be helpful to prevent retrogradation issues in TPS. The gelatinization process is basically a thermomechanical process in which the granules of native st
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