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

Abstract Ammonium polyacrylate (NH 4 PA) was introduced into powdered mixtures consisting of anatase-structured TiO 2 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 ( γ =1–1000 s −1 ) and NH 4 PA concentrations. The alkoxide precursors were mixtures of tetraethyl orthosilicate (TEOS, Si(OC 2 H 5 ) 4 ), ethyl alcohol (C 2 H 5 OH), H 2 O and HCl in a constant [H 2 O]/[TEOS] ratio of 11. The nanoparticle–sol mixtures generally exhibited a pseudoplastic flow behavior over the shear-rate regime examined. The NH 4 PA appeared to serve as an effective surfactant which facilitates the suspension flow by reducing the flow resistance at low NH 4 PA concentrations. At φ =0.10, a viscosity reduction ca. 85% was found at γ =10 s −1 when the NH 4 PA concentration was held at 2.5 wt.% of the solids. As the NH 4 PA exceeded a critical level, e.g., [NH 4 PA]≥3.0 wt.%, the NH 4 PA acted as a catalyst which quickly turned the TiO 2 –silica sol mixtures ( φ =0.10) into a gelled structure, resulted in a pronounced increase of mixture viscosity. The maximum solids concentration ( φ m ) of the mixtures was experimentally determined from a derivative of relative viscosity, i.e., (1− η r −1/2 )– φ dependence. The estimated φ m increased from 0.127 to 0.165 when NH 4 PA of 0.5 wt.% was introduced into the TiO 2 –silica sol mixtures.