Effect of ammonium polyacrylate on rheology of anatase TiO2 nanoparticles dispersed in silicon alkoxide sols

Ammonium polyacrylate (NH4PA) was introduced into powdered mixtures consisting of anatase-structured TiO2 nanoparticles and silicon alkoxide precursors at the sol level, and the rheological behavior of the mixtures was examined under various solid loadings (φ=0.05–0.13 in volumetric ratios), shear rates (γ̇=1–1000 s−1) and NH4PA concentrations. The alkoxide precursors were mixtures of tetraethyl orthosilicate (TEOS, Si(OC2H5)4), ethyl alcohol (C2H5OH), H2O and HCl in a constant [H2O]/[TEOS] ratio of 11. The nanoparticle–sol mixtures generally exhibited a pseudoplastic flow behavior over the shear-rate regime examined. The NH4PA appeared to serve as an effective surfactant which facilitates the suspension flow by reducing the flow resistance at low NH4PA concentrations. At φ=0.10, a viscosity reduction ca. 85% was found at γ̇=10 s−1 when the NH4PA concentration was held at 2.5 wt.% of the solids. As the NH4PA exceeded a critical level, e.g., [NH4PA]≥3.0 wt.%, the NH4PA acted as a catalyst which quickly turned the TiO2–silica sol mixtures (φ=0.10) into a gelled structure, resulted in a pronounced increase of mixture viscosity. The maximum solids concentration (φm) of the mixtures was experimentally determined from a derivative of relative viscosity, i.e., (1−ηr−1/2)–φ dependence. The estimated φm increased from 0.127 to 0.165 when NH4PA of 0.5 wt.% was introduced into the TiO2–silica sol mixtures.