Extrusion tests have been carried out for non-aqueous dispersions of clay with adsorbed water under a variety of solid concentrations as well as the amounts of water adsorbed. The extrusion pressure can be divided into two components; one is the pressure drop in nozzle cone, which is independent on the extrusion speed, and the other is that in nozzle. From the rheological analysis of the dependence of the pressure drop in nozzle on the extrusion speed, flow properties of these dispersions were found to be represented approximately by the Bingham plastic model.
With increasing a solid concentration, the yield stress increased exponentially, whereas the plastic viscosity remained unchanged above a critical solid concentration. This can be interpreted by the saturation of the effective volume fraction of particles including a part of dispersion medium immobilized within flocs, which may be resulted from a decrease in a void fraction between the particles. A further increase in the yield stress above the critical solid concentration can be attributed to an increase in a packing fraction of primary particles in flocs.
The yield stress and the plastic viscosity were also dependent on the amount of water adsorbed, and showed maximum values at different amounts of water, respectively. These can be interpreted by considering the changes not only in a volume fraction of flocs but also in a packing fraction of primary particles in flocs, which may be resulted from the changes in the particle interactions associated with the cohesive force of water bridge as well as the repulsive force of the diffuse double layers between adjacent particles with water film. In addition, it is necessary to take into account the London-van der Waals' attractive force in order to interpret the rheological properties for the dispersions of dry powders.
