Effects of Particle Alignment on the Flow Properties of an Expandable Mica in Na5P3O10 and K4P2O7 Solutions

Abstract

The mode of particle alignment in suspensions of an expandable mica (EM) has been investigated in the presence of sodium tripolyphosphate (STP) and potassium pyrophosphate (PPP) solutions. X-ray diffraction patterns of the suspensions at a STP to solids weight ratio between 0.3 and 0.7 were very sharp and analysis of these higher order reflections showed d-spacings between oriented clay mineral plates varying from 7.0 to 4.9 nm. The sharpness and high-order nature of the reflections indicate that the clay mineral particles arrange very regularly along the line perpendicular to the sheet to produce stacks with three-dimensional order. On the contrary, X-ray diffraction patterns of the clay mineral in PPP solutions did not show regular Bragg reflections, indicating no regular structure along a line perpendicular to the sheet. The flow behavior of the clay mineral in STP and PPP solutions was also radically different. The viscosity of suspensions in the presence of STP showed a decrease in viscosity consistent with particle alignment and three-dimensional stacking such that the apparent volume fraction of the suspension had reduced. In PPP solutions, the steady shear viscosity decreased initially but then showed rheopectic or shear thickening behavior at higher dose rates. The data are rationalized in terms of both the X-ray data and the interparticle force based on the DLVO theory which indicates that the clay mineral is present in a secondary minimum in STP solutions but in a primary minimum in PPP solutions. The rheology and X-ray data indicate that the clay mineral is forming stacks with two-dimensional order at high PPP concentrations in which the particles are linked together by a partial overlap of clay mineral plates to form a flat sheet. The role of the phosphate anion in the control of the alignment of clay mineral plates is thus seen to be highly dependent on the anion size and valency.