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isometric shape improve fluidity which is, on the contrary, suppressed by
the formation of porous agglomerates. The flocculation degree is given by
the surface properties of particles, particularly those less than 1 צע in
size. Flocculation usually concerns the clay component of the casting
A satisfactory casting slip exhibits a low or almost zero yield point,
a high fluidity at low stress and a low water content. Clay casting slips
usually belong to the group of anomalous liquids showing little
difference from a Newtonian liquid (Fig. 155).
For practical purposes, casting slips are often characterized by so-
called apparent viscosity. This characteristic is defined for any point
on the rheological curve (Fig. 155) as a ratio of shear stress to the
deformation rate at the given point. The inadequacy of this
characteristic is demonstrated by Fig. 155: a dilatant and a
pseudoplastic mix have the same apparent viscosity at a stress
corresponding to the intersection of their rheological curves, despite
their quite diverse rheological behaviours. However, determination of
apparent viscosity may be useful, for example in routine qualitative
Thixotropy of casting slips is characterized by the time dependence of
both viscosity and yield point. This phenomenon is caused by reversible
sol-gel transition in the clay component, which can be afFccted by
mechanical means (stirring, vibration). The effect is much more distinct
with enamel slips than with ceramic casting slips. A more marked yield
point is required with the former, while ceramic casting suspensions are
usually characterized by a pseudoplastic rheological behaviour without a
distinct yield point.
Rheological properties of casting slips are controlled by small
additions of suitable electrolytes which promote deflocculation of
aggregates and thus cause a decrease in yield point and increase in
fluidity, which is desirable with respect to reduction of the required
water content (refer to the section on slip casting). The stability of
the suspension is also improved, i.e. the sedimentation of the solids is
Deflocculation, including its secondary effects, is a result of ion
exchange on the surface of clay particles. The efficacy of this colloid-
chemical adjustment is illustrated by the example reported by Norton
(1952): addition of NaOH solution to mono-disperse fraction of H + -
kaolinite prepared by dialysis gives rise to an (approximately) 200-fold
decrease in apparent viscosity, affected instantaneously in the region of
pH 5-6, where complete exchange of H+ ions for Na+ has taken place.
Deflocculation is therefore a significant technological adjustment of
the colloid--chemical nature of casting slips. The clay mineral crystals
have on their surface either positive or negative charges which are
neutralized by adsorbed ions of opposite charge. For example, particles
of natural kaolinite usually have their negative charges neutralized by
Ca+ and Mg+ ions; their aqueous suspension then coagulates into
* Apparent viscosity is usually measured by a capillary viscometer
calibrated with viscous oils; the pressure affecting the outflowing
liquid is variable within certain limits.
voluminous agglomerates containing water among freely located crystals.
Such a suspension behaves approximately as a Bingham body. Addition of an
alkali salt brings about ion exchange (e.g. Ca2+ ^2Na+). Exchange of Ca2+
for larger monovalent cations has the result that the surface electric
charge is compensated by
FIG. 159. Rheological behaviour of an aqueous suspension of the Sedlec
kaolin, peptized with various amounts of Na2C03 (Vycudilik, 1974).
a thick layer of hydrated Na + ions inhibiting mutual attraction of
particles to the range of coagulating forces. A stable suspension with a
low yield point, approximating the Newtonian liquid, is formed. The
change in rheological behaviour is illustrated by Fig. 159, which has
been obtained by measuring on a rotary viscometer with continuously
increasing or decreasing stress (indicated by arrows). The hysteresis
loops result from thixotropic behaviour which is suppressed with
increasing electrolyte content (designated by the numbers on the curves).
For higher Na2C03 contents, traces of dilatancy appear, these not being
detrimental in slip casting, because of the low deformation rates and
FIG. 160. Apparent viscosity of a-AI203 aqueous suspension in terms of pH
(from Norton, 1952).
In practice, deflocculation is carricd out with salts containing
monovalent cations, e.g. NH4OH, Na2C03, NaAI02, Na2Si03 (water glass),
NaP03, etc. Advantageous properties are exhibited by substances whose
anion forms an insoluble product with Ca2+ or Mg2+ (e.g. Na2C03 ->
CaC03). In this way, the exchanged ions are precipitated from solution
and the exchange proceeds to a higher degree.