This first article deals with definitions of slips, concepts of mixing efficiency, the general effects of chemistry and slip density on rheology, and the rheology of slips used for plastic forming and slip casting.
Most slip properties and behavior characteristics also apply to slurries, but the inclusion of plastic clays in ceramic slips often gives improvements in fluid and forming properties over those of non-plastic slurries. The specific desirable fluid properties depend on the process in which the slip is used, but low viscosity and stability are usually near the top of the wish list. Other desirable properties may be high slip density or specific gravity (SG), thixotropy, or pseudoplasticity.
Finer, higher-surface-area materials should be added first so that they receive the benefits of longer mixing time, because the mechanical energy input necessary to separate particles from each other is proportional to their surface area of contact. Dry components also increase the slip's SG and decrease the inter-particle spacing (IPS), which, if the particle velocity does not change, increases the shear rate between adjacent particles. At a constant mixing speed, this effect drives the necessary power input upward.
With increasing SG, the increasing interactions between particles constrain flow from turbulent to laminar (higher to lower energy), resulting in a lower shear rate locally. Increasing the mixing speed can move the flow behavior toward turbulent again. This increases the scrubbing action between particles and works to physically disperse agglomerates into their component individual particles. At a high energy input, this can lead to the rounding of rough particles and more fines by autogenous grinding, creating a change in particle size distribution. Additional shear is more efficient than additional time for good dispersion.
The language of rheology includes terms such as Newtonian, in which fluid viscosity is constant with time and shear; shear thinning, or pseudoplasticity, in which viscosity decreases with shear; the opposite case of shear thickening, or dilatancy; thixotropy, wherein viscosity decreases with time at constant shear; and its opposite, rheopexy. A related term is syneresis, which refers to a compression of agglomerates (flocs) over time at low or zero shear and under highly flocculated conditions due to minimum repulsive charge conditions. Accompanied by the release of suspending fluid as supernatant, syneresis gives a "thixotropic" but detrimental rheology. Pseudoplasticity or dilatancy are usually shown on graphs of shear stress or viscosity vs. shear rate, while thixotropy or rheopexy are shown on graphs of shear stress or viscosity vs. time (gel curves).
Ceramic slips are complex systems and usually have variable flow behavior as a function of shear rate or time or both, as well as a function of slip density and chemistry. Therefore, all of the above terms may apply in such systems.
As particle size decreases or SG or chemical flocculation increases, attractive forces become more dominant and bonding increases between particles, resulting in higher viscosity. The flocs form a three-dimensional network comprising loose bundles of particles bi-continuously interspersed with a liquid phase. Figure 1 shows a two-dimensional computer modeled sequence of the coagulation of spheres over time in a liquid phase in the absence of repulsive forces.1 On the log-log viscosity vs. shear rate graph, the negatively sloped plot moves upward but in parallel and is linear until the point where, at low shear, the rheology becomes time dependent due to syneretic compression caused by attractive forces. The three-dimensional floc network breaks into individual flocs surrounded by fluid.
Use of this concept will allow an increase in SG for flocculated slips (if sufficient mixing power is available). The rule of thumb is that if supernatant forms above a flocculated slip at rest, then that volume of liquid did not belong in the slip as originally produced. Often in such situations, the settled slip below the supernatant appears cracked, even though it is "under water." The cracks are the major channels by which the supernatant comes to the surface, but they are also an indication of decreased bonding between flocs, and thus lower plastic strength. If the supernatant is decanted and the slip remixed, the viscosity will be very high because the SG is too high and IPS too small. A too-small IPS means that the repulsive forces in the slip are dominated by attractive forces. A further addition of deflocculant is necessary to increase surface charge and repulsion forces, and thereby decrease viscosity into a useful range for mixing, pumping or pouring. Both slip stability and pseudoplasticity will actually increase if the addition is made correctly.
The goal here in terms of attractive and repulsive forces and colloidal theory is to set the IPS (using SG) and the degree of deflocculation or flocculation so that adjacent particles reside in the "secondary energy minimum" with respect to each other. When this is the case, an energy input is necessary to move particles either closer together or farther apart. Without that energy input, the slip is stable, and a gel curve will show neither thixotropy nor rheopexy after the initial recovery from mixing and/or shear rate change.
Figure 4 illustrates this effect for non-deflocculated coal-water slurries ground at three solids contents.3 Here the effect was not achieved by a longer grinding time, but by a lower solids content, which produces a less coarse fraction during milling. The equivalent effect in clays would be the removal of the coarse end of the size distribution, which is usually the non-plastic fraction. More pseudoplasticity in slip means higher plasticity in the formed body, as long as the material is the same and there is no difference in the intrinsic plasticity of the material.
Figures 2 and 3 showed the transition from pseudoplasticity to dilatancy with deflocculation and increasing shear on a Brookfield viscometer for kaolin and ball clay. Dilatancy is caused by the interaction of fast moving particles that are repelling each other but cannot avoid each other due to a small IPS. For a given local surface charge, finer particle size and more rounded particles tend to decrease dilatancy or at least move it to higher deflocculation levels or higher shear rates. Increasing SG decreases IPS and increases the tendency for dilatancy. All fluids become dilatant if the shear rate is high enough. Figure 5 shows a set of theoretical rheological curves extended to high shear.4
If possible, dilatancy should be avoided in all slips by keeping the viscosity high enough to maintain a linear plot on the log-log viscosity vs. shear rate graph. A slip showing dilatancy during pumping or pouring will not only require more energy to move, but must relax after the shear is removed. The slip is unstable, and in slip casting that can cause problems. Dilatancy problems are particularly difficult in situations where very coarse, non-plastic materials such as silicon carbide (SiC) are being formed. Such problems can be partially addressed with particle size distribution and/or slip chemistry changes. In some instances, the deflocculant can be tailored or chosen to provide a high minimum viscosity, above the region where dilatancy will occur. In extreme cases, a plastic forming body can be totally deflocculated but still act flocculated due to the properties of the deflocculant.
The time functions of thixotropy and rheopexy are dependent on the shear rate being used for the measurement and also on the previous shear history. For example, an over-flocculated system in syneresis will decrease in viscosity over time at very low shear, will increase in viscosity over time at low to intermediate shear, will break down to lower viscosity at higher shear (pseudoplastic), and will build up viscosity over time at very high shear (in turbulence). A qualitative check on slip stability can be made by choosing one or more shear rates and measuring the viscosity over time for each, in gel curves.
Future articles in this series will focus on rheology and slip stability as measured by gel curves, the effect of plasticity imparted by clays and associated chemistry, and the relationship to casting properties; simple and more complex deflocculation systems, with a focus on sodium silicate; and other types of deflocculants, as well as the effects of aging on slips.
Editor's note: Neither the author nor Ceramic Industry endorses any of the products mentioned.
For more information about slips and their behaviors, contact Funk at email@example.com .