In this section, 0.14 wt.% NH4PAA was added into all slurries as the dispersant since this
concentration had shown the best performance in a previous study4. It indicated that at
higher concentration (> 0.14 wt.%), the NH4PAA will remain unabsorbed in the suspension
and the functions as an electrolyte, which reduces the range and extent of the electrostatic
repulsion4.
Figure 4.22 shows the relationship between the viscosity and the shear rate of 20 vol.% alumina slurries with different PVA contents.
0 100 200 300 400 500 600 0.00 0.05 0.10 0.15 0.20 0.25 V is c o si ty (P a s) Shear rate (1/s) 0 wt.% Binder 3 wt.% Binder 6 wt.% Binder 8 wt.% Binder 10 wt.% Binder
Figure 4.22 Viscosity of 20 vol.% solids loading alumina slurries with and without PVA binder (with 0.14 wt.% NH4PAA dispersant).
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It can be seen from Figure 4.22, that the behaviour of each slurry was similar: when the shear rate was low, the viscosity was high, while at higher shear rates, the viscosity became relatively low, and the viscosity decreased with the increase of the shear rate, which represented a typical shear thinning behaviour.
The addition of PVA to dispersant-stabilized alumina slurry could change the viscosity of the slurry significantly. Figure 4.23 shows the viscosity at four different shear rates as a function of the PVA content. The viscosity increased monotonically as the PVA content increased. As the PVA content increased from 0 wt.% to 8 wt.%, the viscosity went up almost linearly, while over 8 wt.%, the viscosity increased abruptly.
-2 0 2 4 6 8 10 12 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 V is co si ty ( P a • s ) PVA content (wt.% ) 50 (1/s) 100 (1/s) 200 (1/s) 400 (1/s)
Figure 4.23 Viscosity of 20 vol.% solids loading alumina slurries as a function of PVA content (with 0.14 wt.% NH4PAA dispersant).
The increase of the viscosity of the NH4PAA stabilized slurries at different shear rates with
increasing of the PVA content indicated that the addition of the PVA had probably caused a flocculation of the alumina stabilized particles with the 0.14 wt.% dispersant. The degree of
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the flocculation increased as the PVA content went up, and this effect of flocculation became more pronounced when the PVA content was more than 8 wt.%.
It has been discussed in the literature review (Chapter 2, section 2.3) that when particles are dispersed in a medium (aqueous or nonaqueous), there are normally four different kinds of interactions between the different compositions in the slurries. Among them, van der Waals attractive forces are the most important. Particularly for particulate materials with high surface area, it usually causes agglomeration of the particles. If the particles have an electrical charge, caused by the adsorption of a charged dispersant, the agglomeration of the particles in a colloidal suspension can be restrained by the formation of the so-called
electrical double layer18. Adsorption (or grafting) of polymer molecules can impede the
agglomeration of the particles through the steric stabilization mechanism as well. The addition of a non-adsorbing polymer to a stabilized colloidal suspension may result in depletion flocculation, whereby the concentration of the non-adsorbing free polymer is effectively reduced between the particles, and the higher concentration outside the particle–
particle approach zone, exerts an osmotic pressure causing the particles to flocculate18.
Figure 4.24 illustrates schematically different influence factors that could cause instability in a stabilized suspension.
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Figure 4.24 Different factors that cause instability in a stabilized suspension. The arrow direction indicates the increasing of the effect (After Khan18).
According to the agglomeration theories described above, the agglomeration in the PVA added stabilized alumina suspension in this section (Figure 4.22) can be explained as follows.
It has been shown by Pasi19 that PVA has a rather low adsorption ability on an Al2O3 surface.
Pasi and his colleagues showed that an acidic condition was better for the adsorption of PVA. Only 5% of PVA (from the 0.5 wt.% PVA solution) was adsorbed on an alumina surface at pH 4.3 and 95% remained in the solution; however, with the increase of PVA concentration (from 2.15 wt.% PVA solution), the adsorbed fraction was even lower, 2.1%. Meanwhile, in Khan’s study, it had been found that PVA did not adsorb in their measurable extent (40 vol.% alumina with 1.5 wt.% PVA). Therefore, the possibility that the flocculation of this alumina
(stabilized by NH4PAA) and PVA system was caused by a bridging mechanism can be
reasonably excluded.
The alumina particles, in the presence of aqueous NH4PAA solutions, are surface charged,
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which can be confirmed by a former study4. This study also indicated that alumina particles
were stabilized largely by a classical electrostatic repulsion mechanism. However, in the case of stabilized particles in the presence of non-adsorbing PVA, the flocculation is unlikely to be due to van der Waals forces because the particles are stable in the absence of the PVA (Figure 4.23, 0 wt.% Binder). It is therefore highly probable that the flocculation of the
NH4PAA stabilized alumina slurries was induced by a depletion flocculation mechanism
when PVA was introduced to these slurries. A schematic explanation is given in Figure 4.25. In the absence of PVA, the alumina particles were in a well dispersed state (Figure 4.25 (a)). When PVA was added to the dispersed colloidal system, a depletion flocculation was caused and some particles agglomerated forming “large” particles as shown in Figure 4.25 (b). With the increase of PVA content, the degree of flocculation increased and the amount of the “large” particles increased as well.
Figure 4.25 Schematic illustration of depletion flocculation.