Rheology of silica dispersions stabilized by polymers
2019
Potanin, Andrei
Silica dispersions form the backbone of many consumer products, such as toothpaste. They are typically used in combination with polymeric binders/thickeners which stabilize these dispersions. In this paper rheological properties and stability, defined in terms of viscosity loss in real-time aging, of silica dispersions in the presence of the two most commonly used polymers, Xanthan gum and carboxymethyl cellulose (CMC), are considered. These two polymers represent two different types of rheological effects. Xanthan gum imparts stable viscosity to dispersions of silica particles, whereas the dispersion of the same particles with CMC results in slow viscosity loss during aging. The difference between the two polymers is also characterized by oscillatory strain sweeps. Silica dispersions without polymers tend to yield via a two-step process with two distinct plateaus of the storage modulus, G’. While the origin of such behavior is not completely understood, one possible explanation attributes it to the two-tier structure of these dispersions which can be thought of as a network of interconnected aggregates. As strain amplitude increases, the network breaks first, followed by breakup of aggregates. With Xanthan gum added, the two yield points converge indicating integration of the particles into a uniform space-filling network. CMC, on the other hand, while thickening the dispersion, retains the two-step yielding, presumably due to failure to interconnect the particles. Such two-step yielding is indeed rather common for toothpastes.
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