Rheological Behavior of Nanoclay Containing Nanofluids
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Rheological Behavior of Nanoclay Containing Nanofluids Jeffrey C. Munro and YuanQiao Rao Core R&D Labs, The Dow Chemical Company, 2301 N. Brazosport Blvd., Freeport, TX 77541, U.S.A. ABSTRACT A synthetic hectorite nanoclay, Laponite®, with a disc shape (20 – 30 nm in diameter and 1 nm in thickness) was used as a model nanoparticle to prepare dispersions in water and different organic solvents. Although up to 20 wt% of nanoclay can be dispersed in water to form a lowviscosity stable colloidal sol, dispersions in organic solvents behave differently. The effect of the dielectric constant of the medium on the viscosity of the dispersion was studied systematically using two series of water-organic solvent miscible blends. Changes in the rheological behavior of the dispersions suggest that the dielectric constant of the organic solvent is a determining factor in the sol-gel transition of a nanoclay-containing nanofluid. INTRODUCTION The incorporation of high aspect ratio nanofillers into organic media is desirable due to the potential for creating high performance materials. One important aspect of these dispersions is the rheological behavior of the nanofluid. In the work described in this paper a synthetic hectorite, Laponite®, with a disc shape (20 – 30 nm in diameter and 1 nm in thickness) was used as a model nanoparticle. Dispersion of this nanofiller in water and other organic solvents were explored. While the rheology of clay containing nanocomposites has been reported exhaustively in the literature, most of the work has centered on melt rheology and steady high-shear viscosity of nanoclays in polymers.1,2 Rheological studies and low-shear viscosity of nanoclays in liquid media at room temperature, other than water, are fairly limited.3,4 In general, rheology has been used, in combination with other techniques such as x-ray scattering, as a means to determine the state of dispersion of clay in a polymer.5 For clay fillers (aspect ratio ~ 200) in an epoxy monomer, low-viscosity liquids were reported for systems with only 0.7 wt% clay, but a gel state was observed at 5 wt% clay loading.6 Meanwhile, in rheological studies of dispersions of Laponite RD in polyethylene oxide (Mw ~ 10,000 g/mol) the critical clay concentration at the liquid-to-gel transition varied from 0.5 to 1 wt% depending on the method used to prepare the dispersion.5 A transition at lower clay concentration was associated with improved exfoliation of the clay. It is noted that this critical concentration is well below the maximum packing density of clay spheres (where it is assumed each Laponite platelet occupies a spherical volume with a diameter equal to that of the Laponite platelet) of 3.7 wt%.7 Note that if the maximum packing density is assumed to be the point at which the rotation of the platelets becomes geometrically constrained, as suggested in another study, the weight fraction of Laponite at the concentrated regime threshold is approximately 25 wt%.4
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EXPERIMENT Materials The following materials were used: Laponite S482 was obtaine
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