Thermoplastic starch/layered silicate composites: structure, interaction, properties

Composites were prepared from thermoplastic starch and layered silicates organophilized with different surfactants. Sodium montmorillonite and silicates treated with aminododecanoic acid, stearyl dihydroxyethyl ammonium chloride and distearyl dimethyl ammonium chloride surfactants, respectively, were added to plasticized starch in amounts of 0-7 vol%. Changes in the structure of the composites were followed by X-ray diffraction. Dynamic mechanical analysis and tensile tests were carried out to obtain information about the mechanical properties of the composites. The degradation of starch was followed by the measurement of colour, and water absorption was also determined on selected samples. The results revealed that, in spite of the wide variety of organophilization, no or only limited exfoliation took place during the homogenization of the composites. On the other hand, practically all components of the system entered into interaction with each other. Competitive dissolution and adsorption of the surfactant and the plasticizer took place in the composites. The occurrence of chemical reactions cannot be excluded either in the case of the silicate modified with the amino acid. The type and strength of interaction determine the properties of the composites. The reinforcing effect of the silicates varies in a wide range: the amino acid modified clay improves strength almost ten times as much as the silicate treated with the aliphatic amine surfactant. Changes in all other properties including viscosity, colour and water adsorption correspond to those observed in strength and are dominated by interactions.