Anais do 44º Congresso Brasileiro de Cerâmica
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31 de maio a 4 de junho de 2000 - São Pedro – S.P.
EFFECT
OF RHEOLOGY ON GREEN DENSITY OF ZIRCON-ALUMINA CASTS.
L.B. Garrido* and E.F. Aglietti**
CETMIC. Centro de Tecnología de Recursos Minerales y Cerámica (CIC-CONICETUNLP). Cno. Centenario y 506. C.C.49 (1897) M.B. Gonnet. Buenos Aires.
ARGENTINA.
E-mail:garridol@netverk.com.ar
ABSTRACT
Rheological properties of aqueous suspensions (48 vol%) of zircon-alumina
mixtures containing different amounts of polyelectrolyte as dispersant were studied. Slip
casting in a plaster mould and pressure filtration at 8 MPa experiments were performed
to correlate the rheological properties of the suspensions with the relative density of
green casts. Flow curves fitted satisfactorily to the Casson model. The Casson viscosity
values remained nearly constant whereas Casson yield stress parameter decreased to
a minimum and then increased with increasing the amount of dispersant added.
Although green densities decreased as yield stress Casson parameter increased, some
suspensions having low apparent viscosity and small yield stress produced casts with
slightly higher relative density values. This is attributed to a slight higher viscosity at low
shear rates of these suspensions in which segregation of the particles and /or
migration of the fines can not occur.
Key words: Zircon, alumina, rheology, slip casting, pressure filtration.
INTRODUCTION
Reaction sintering of alumina and zircon is a common route to obtain a mullitezirconia composite. Several studies showed that materials composed of mullite and
zirconia may be obtained using alumina and zircon powders by the traditional method
that consisted in: premix milling, dry-pressing operations to produce the compacts and
sintering. However, as the mean diameter of these powders are large, the achieved
* CONICET Research member
** CONICET-UNLP Research member
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density was relatively low
(1).
To improve densification very fine powders must be
chosen as densification must occur before mullitization
(2).
A high solid concentration of fines particles that are necessary in processes such
as slip casting or pressure filtration often produced slips which are difficult to cast.
In concentrated suspensions, the rheological behavior is mainly dominated by
hydrodynamic effects, Brownian motion, electrostatic and steric repulsion and Van der
Waal’s attraction. With large particles (or hard-sphere systems) the colloidal
interparticle forces may be ignored and the contribution to rheology of the
hydrodynamic effects depend on their volume concentration, shape, size, relative
orientation and position
(3-5)
. The particle size distribution is an important parameter for
these concentrated suspensions, acting mainly on the maximum volume fraction. The
small particles fit the interstices between large ones. Thus to achieve a high particle
packing efficiency and to minimize the viscosity bimodal or a continuous broad particle
size distribution are used.
As the particle size decreases to the colloidal range, flow behavior is strongly
influenced by interparticle colloidal interactions as repulsion and Van der Waal’s
attraction that control spatial arrangement (network structure) and dynamics of the
suspended particles. Thus, an extra increase in viscosity and a complex rheological
behavior are observed as a result of the formation of flocs due to relatively high van der
Waals attraction as compared with the repulsion force. Certainly, flocculation has a
considerable effect on size and shape of the flocs. By enclosing and thus immobilizing
some liquid in the flocs have the effect of the increase the effective solid volume and
though the viscosity of the suspension
(3-5).
Particle flocculation in the suspension leads
to a low packing density because the flocs can not be close packed. However, welldispersed particles in the suspension optimize the rheological properties and the
packing density of the cast.
In this work, a polyacrylate based dispersant was used to control the degree of
dispersion (to improve the rheology) of concentrated aqueous suspensions of aluminazircon mixtures. The effects of the amount of dispersant added and the different powder
compositions on the rheological properties and on the green density of compacts
obtained by slip casting and pressure filtration at 8 MPa were studied.
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MATERIALS AND METHODS
Starting powders used in this study were commercially available -Al2O3 and
ZrSiO4. The mean particle size were 0.5 and 1.8 m, respectively. The specific surface
area (BET) was 9.5 m2/g for -Al2O3 (6) and 4.1 m2/g for zircon.
Powder compositions of different alumina/ zircon (wt% ) ratios in the mixtures (
45.5/54.5, 51.7/48.3 and 35.1/64.9) were studied. These proportions correspond to: a)
stoichiometric, b) with a 6 wt% excess of alumina, and c) with a 10 wt% of zircon
excess, respectively. The stoichiometric alumina-zircon ratio is which after a theoretical
complete reaction at high temperature a mullite-zirconia composite is obtained.
Aqueous 48 vol% suspensions were prepared by adding the powder to aqueous
solutions with different polyacrylate content (Dolapix CE64, Zschimmers and Schwartz)
at pH 9.1-9.2. After mixing, the suspensions were dispersed by ultrasonic process for
30 min.
Flow curves of the suspensions were obtained using a rotational viscometer
Haake vt550 of coaxial cylinders with NV measure system at 25°C.
The consolidation of suspensions by slip casting was carried out in a plaster
mould to produce samples in a form of bars (12x12x60 mm 3).
Pressure filtration was made on a metallic sintered porous filter with a wet filter
paper placed on it and uniaxial pressure was employed. A pressure of 8 MPa was
applied from the top and the liquid moved through the porous filter. Pressure was
maintained constant for 30 min. Cylindrical cast samples (25 mm diameter and 10-15
mm thick) were obtained.
Bars and cylindrical compacts were dried 24 h at room temperature and then at
110 C up to a constant weight. Relative density (% of the theoretical) was measured
by Hg immersion.
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RESULTS AND DISCUSION
Effect of the amount of dispersant on the rheological properties
The isoelectric point (iep) of alumina used has been determined to be at pH near
9
(6)
. Previous works on the casting behavior of a commercial zircon powder
(7,8)
showed the electrophoretic mobility measurements as a function of the pH . They
reported that the iep was at pH 5 and the zeta potential became strongly negative with
further increase in pH. According to this result, the zircon particles had a high negative
zeta potential in the basic pH range. Thus, at pH close to 9, an electrostatic attraction
between the neutral alumina particle and the high negatively charged zircon particle can
occur.
Figure 1 shows the flow curves of the aqueous 48 vol% suspensions of: the
stoichiometric, with a 6 wt% excess of alumina, and with a 10 wt% of zircon excess
powder mixtures for different amounts of polyacrylate added.
Both shear stress and apparent viscosity which is the shear stress to shear rate
ratio at any shear rate decreased to a minimum and then gradually increased with
increasing the amount of dispersant added. For these suspensions, the maximum
degree of dispersion of the particles is achieved when the suspension contained a 0.220.24 wt/wt % of dispersant that produced a minimum of viscosity. All the suspensions
having a minimum viscosity showed a Newtonian behavior.
Like others anionic polyelectrolytes, adsorption of polyacrylate on the powder
surface increases the negative surface charge resulting in a strongly repulsive double
layer force and produces, in addition, some steric repulsion that overcome the van der
Waal’s attraction. Then, polyacrylate adsorption on both powders in the basic pH range,
generated high negative zeta potentials and therefore, electrostatic interaction between
zircon and alumina particles changed to repulsive at pH 9. Then, a minimum in viscosity
was obtained as the particles had a high surface charge at low electrolyte concentration
(high zeta potential).
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30
S h e a r s t re s s , P a
25
1.2
20
0.97
15
0.13
0.65
10
0.49
0.22-0.36
5
0
0
100
200
300
400
500
600
Shear rate, s-1
a
30
0.13
S h e a r s t re s s , P a
25
1.2
20
0.97
15
0.65
0.49
10
0.24
5
0
0
100
200
300
400
500
600
Shear rate, s-1
b
30
0.065
S h e a r s t re s s , P a
25
1.2
20
0.97
15
0.65
0.13
10
0.22
5
0
0
100
200
300
400
500
600
Shear rate, s-1
c
Figure 1: Shear stress vs. shear rate curves for alumina-zircon suspensions (48
vol%) with different content (wt/wt %) of dispersant. a: stoichiometric, b: with a 6 wt%
excess of alumina, c: with a 10 wt% of zircon excess powder mixtures.
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High apparent viscosities and non Newtonian behavior were obtained at lower
polyacrylate concentrations. The flow behavior changed to shear thinning or plastic
depending on the powder composition. Flow curves exhibited a slight hysteresis loop
indicating that the suspension was not fully deflocculated. This may be explained by an
insufficient adsorption of polyacrylate anion (mainly on the alumina surface) due to low
amount of available dispersant. Incomplete adsorption decreased the net negative
charge of the mixed powders and consequently repulsion was low. Moreover,
heterocoagulation between the uncoated alumina and zircon particles produced a
significant increase in viscosity. As the shear rate increased, the breakdown of the flocs
released some immobilized liquid increasing the volume of free liquid and consequently
the suspension viscosity decreased.
For additions higher than 0.65 wt% the apparent viscosity and the shear thinning
behavior gradually increased probably due to the presence of an excess of
polyelectrolyte in solution.
The differences between the three compositions studied were small. Suspensions
of the three alumina-zircon mixtures at pH 9.1-9.2 containing 0.22-0.24 wt% of
polyacrylate showed a minimum viscosity. The composition with the zircon excess in
the mixture had the lower viscosity values. The viscosity of the suspension decreased
close to the minimum by addition of 0.13 wt% of dispersant due to low specific surface
area of this mixture. The minimum viscosity were obtained at lower dispersant
concentration than for the other compositions.
At low polyelectrolyte concentration, the suspension of the mixture which
contained the highest amount of alumina showed higher viscosity values than the ones
of the other compositions due to its higher surface area. Also, higher the content of
alumina in the mixture, lower zeta potential value of the powders. This fact was
because of negative surface charge of the uncoated alumina powder at pH near 9.2
was lower than the one of the zircon. Therefore, the electrostatic repulsion decreased.
For polyacrylate concentrations higher than 0.65 wt%, the viscosity of all the
alumina/zircon ratio mixtures increased to similar values.
Flow curves were fitted with the Casson model:
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 1/2= c 1/2+ (c D)1/2
where  is the shear stress [Pa], D is the shear rate [s -1] and c and c are the
Casson yield stress and viscosity parameters, respectively. Except for the suspensions
with 0.13 wt% of
dispersant which showed a slight time dependency, the Casson
model showed a good fit of the experimental data.
Figure 2 shows the viscosity and yield stress (Casson model parameters that fit
best flow curves of Fig 1 a, b, c) as a function of the dispersant content. Casson
viscosity remained unchanged whereas yield stress Casson parameter decreased to a
minimum and then increased with increasing the amount of dispersant added. Guo et
al.(9) related a c parameter with the magnitude of the attractive interparticle interactions,
thus a high c parameter indicates strongly attractive interactions whereas c increases
with increasing solid volume concentration.
The large variation in the yield stress Casson parameter (Figure 2b) indicated that
there was significant differences in the network structure of the suspensions as a result
of strong changes in the repulsion between particles which is a function the added
amount of dispersant.
C a s s o n v is c o s it y , P a s
0.1
0.08
0.06
0.04
0.02
0
0
0.2
0.4
0.6
0.8
1
1.2
1.4
Dispersant added , wt%
a
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C a s s o n y ie ld s t re s s , P a
4
3
2
1
0
0
0.2
0.4
0.6
0.8
1
1.2
1.4
Dispersant added , wt%
b
Figure 2: a: viscosity, b: yield stress Casson model parameters (best fit of flow
curves of Figure 1 a, b, and c) as a function of the dispersant content for alumina-zircon
suspensions of:  stoichiometric,  with a 6 wt% excess of alumina and  with a 10 wt%
of zircon excess powder mixtures.
Effect of the rheological parameters on the density of green cast
Figure 3 and 4 show the relative density of green compacts obtained from 48 vol%
suspensions of the three compositions studied by slip casting and pressure filtration,
respectively as a function of the yield stress Casson parameter.
D rie d d e n s it y , g / c m 3
3.2
3
2.8
2.6
2.4
0
0.5
1
1.5
2
2.5
3
Casson yield stress, Pa
3.5
4
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Figure 3: Relative density of green compacts prepared from 48 vol% suspensions
of:  stoichiometric,  with a 6 wt% excess of alumina and  with a 10 wt% of zircon
excess powder mixtures by slip casting.
Density was nearly constant up to a certain c value and then decreased with
increasing c. Compacts resulting from well dispersed Newtonian suspensions or with
low c parameter in which repulsion between particles was comparatively large (at a
constant Casson viscosity) give compacts with the highest green density. Relative
density values of 69.9, 70.3, 71 and 70.4, 70.7, 72 % were found for green compacts
prepared by slip casting and pressure filtration, respectively. A high c parameter
indicating strongly attractive interparticle interactions was related to low electrostatic
repulsion due to insufficient adsorption of polyacrylate. Previous studies on alumina
(10)
showed that, the network that result from a strongly attractive interactions resisted
consolidation and, although it was compressible, a poor packing density was obtained.
For suspensions with large excess of dispersant in solution, intermediate c values were
obtained. Due to reduced electrostatic repulsion between particles the influence of van
der Waals attraction became dominant but density values were close to those from well
deflocculated suspension. These suspensions were weakly flocculated as they
exhibited higher viscosity than the minimum but smaller c and lower shear thinning
character than those resulting from high attractive forces. Weakly flocculated
suspensions showed a different packing behavior. As the strength of these network are
lower than that formed by high attractive interactions a comparative high density can be
obtained
added.
(10).
A decrease of 3% in density was found when 1.2% of dispersant was
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D rie d d e n s it y , g / c m 3
3.2
3
2.8
2.6
2.4
0
0.5
1
1.5
2
2.5
3
3.5
4
Casson y ield stress, Pa
Figure 4: Relative density of green compacts prepared from 48 vol% suspensions
of:  stoichiometric,  with a 6 wt% excess of alumina and with a 10 wt% of zircon
excess powder mixtures by pressure filtration.
Figures 3 and 4 show that compacts prepared from the zircon excess composition
showed a maximum in density that is reached at a c value somewhat higher than that
corresponding to the other compositions. Since this powder contained a higher
proportion of coarse and dense zircon particles, weakly flocculation may be favorable.
Slight higher viscosity at low shear rates prevent settling of zircon particles and
segregation of components may be reduced if it took place.
CONCLUSIONS
Rheological properties of aqueous suspensions (48 vol%) of zircon-alumina
mixtures containing different amounts of polyacrylate based dispersant were studied. At
low dispersant concentrations the viscosity was dependent on the shear rate and time.
These characteristics were attenuated with increasing the amount of dispersant added
and then gradually increased.
The composition with an excess of zircon in the mixture had the lower viscosity
values and the amount of dispersant required to reach the minimum viscosity
decreased.
Flow curves fitted satisfactorily to the Casson model. The Casson viscosity values
remained nearly constant whereas the Casson yield stress parameter showed a similar
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behavior to that observed for the apparent viscosity variation with increasing the
dispersant concentration.
Compacts resulting from suspension in which repulsion between particles was
comparatively large showing low yield stress Casson parameter (at a constant Casson
viscosity) gave compacts with the highest green density. Maximum relative density
values of 69.9, 70.3, 71 and 70.4, 70.7, 72 % were found for green compacts resulting
from well deflocculated suspensions by slip casting and pressure filtration, respectively.
These values were obtained using 0.24 to 0.49 wt% of dispersant concentration.
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