EFFECTS OF PARTICLE ALIGNMENT ON THE FLOW PROPERTIES OF AN EXPANDABLEMICA IN NA5P3O10 AND K4P2O7 SOLUTIONS

The mode of particle alignment in suspensions of an expandable mica (E M) has been investigated in the presence of sodium tripolyphosphate (S TP) and potassium pyrophosphate (PPP) solutions. X-ray diffraction pat terns of the suspensions at a STP to solids weight ratio between 0.3 a nd 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 in dicate that the clay mineral particles arrange very regularly along th e line perpendicular to the sheet to produce stacks with three-dimensi onal order. On the contrary, X-ray diffraction patterns of the clay mi neral in PPP solutions did not show regular Bragg reflections, indicat ing no regular structure along a line perpendicular to the sheet. The now behavior of the clay mineral in STP and PPP solutions was also rad ically 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 t he suspension had reduced. In PPP solutions, the steady shear viscosit y decreased initially but then showed rheopectic or shear thickening b ehavior at higher dose rates. The data are rationalized in terms of bo th the X-ray data and the interparticle force based on the DLVO theory which indicates that the clay mineral is present in a secondary minim um in STP solutions but in a primary minimum in PPP solutions. The rhe ology and X-ray data indicate that the clay mineral is forming stacks with two-dimensional order at high PPP concentrations in which the par ticles 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 depe ndent on the anion size and valency.