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During my years of experience in production facilities and research laboratories in several ceramic industries, I have found a number of concepts useful when dealing with ceramic slips, their chemistry and their properties. This is the first in a series of four articles that, while not intended to be comprehensive, provides a review of the basics of ceramic slips along with information that is less well known, especially by those newly employed in the ceramic industry. In addition to my observations, the articles are a compilation of general experience and some of the information available in industry literature.
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.
TerminologyWe use the term slip to denote a mixture of largely insoluble ceramic raw materials, suspended or dispersed in a fluid phase that is normally water, with perhaps an addition of a soluble modifying chemistry to provide desirable fluid properties. Slurry differs from slip in that it normally comprises only one raw material in addition to the fluid phase, although it usually includes some or all of the dissolved chemical additives. Slurries can be made of many materials, of which ceramic raw materials are just a subset.
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.
Efficient MixingSlips are blended from the component materials, which may be added dry or in pre-blended slurry form. In the interest of good slip dispersion, the fluid phase is added to (or the initial slurry is made in) the mixing vessel first, followed by any other slurry components. Dry components are added last. Mixing should begin as early as possible, and always before the addition of dry components, while applying the maximum shear available.
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 Role of AdditivesDispersants or deflocculants are commonly added during dispersion (blunging) to create chemically coated and charged part