This investigation examined the significance of particle packing factors for size distributions on the transient shear thickening (antithixotropic) behavior of suspensions of ground calcium carbonate (GCC), such as those used in paper coatings. The rhe-ological properties of GCC with different particle size distribution were examined using Couette flow geometry. Three size fractions were isolated from commercial coating pigment slurries. Fractions were blended to vary the particle size distribution. In the low shear rate region, shear thinning behavior was observed and was enhanced by increasing the finer size fraction. At higher shear rates where thickening occurs, additional finer size fraction reduced viscosity and mitigated the onset of shear thickening. For blends of particles, a minimum in relative high shear rate limiting viscosity was observed for 30-40 vol% of finer size fraction. When subjected to a constant rate of shear, a reversible time-dependent antithixotropic phenomenon (shear stress increase with time) was observed. This was evident for solid contents and shear rates that fall within the shear thicken ing region. These effects contribute to rheograms recorded from conventional ramp tests where shear rate is continually increased. Transient curves that exhibited antithixotropy were divided into three regions and described by empirical parameters. Quantitative relationships were found between these parameters and the rate of constant shear, solid contents, particle size distribution and packing. It was hypothesized that the increase of stress with time suggests the time evolution of flow-induced structures in the shear thickening transition within the shear zone. The results from transient rheological experiments favor the transient cluster formation mechanism of the existing shear thickening theory.
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