rheological studies on stabilised zirconia aqueous suspensions

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Copyright © 2012 American Scientific PublishersAll rights reservedPrinted in the United States of America

Journal ofColloid Science and Biotechnology

Vol. 1, 175–184, 2012

Rheological Studies on Stabilised ZirconiaAqueous Suspensions

Asad U. Khan1�∗, Nasir M. Ahmad2, and Nasir Mahmood1�31Department of Chemical Engineering, COMSATS Institute of Information Technology,

Defence Road, Off Raiwind Road, 54000 Lahore, Pakistan2Department of Materials Engineering, School of Chemical and Materials Engineering (SCME),

National University of Sciences and Technology, Islamabad-44000, Pakistan3Institut Für Chemie FG Mikro-Und Nanostrukturbasierte Polymerverbundwerkstoffe, Martin-Luther-Universität

Halle-Wittenberg, Heinrich-Damerow-Str. 4, D-06120 Halle (Saale), Germany

The rheological characterisation of the aqueous submicron zirconia particulate suspensionstabilised with two polyvalent organic salt dispersants called Tiron and Aluminon including apoly(vinyl) alcohol (PVA) binder are investigated. It is observed that dispersants adsorbs to thezirconia particles and significantly influenced the rheology of the systems. The addition of PVA incombination with the dispersants causes the flocculation of the particles in the suspension. For agiven amount of the dispersant, increasing the PVA concentration increases the viscosity, storageand loss moduli. At relatively low PVA concentrations, the excess amount of the dispersant in thesystem causes the flocculation by a reduction of the electrostatic (double layer) effect, whereas atrelatively higher PVA concentration depletion flocculation occurs.

Keywords: Binder, Rheology, Processing, Suspensions, ZrO2.

1. INTRODUCTION

The handling of colloidal ceramic particles in suspensionscauses several problems during the steps and operationsinvolved in the formation of the ceramic.1–4 Due tovery high surface area these particles form aggregates oragglomerates under the action of attractive van der Waalsforces. It is of academic as well as industrial interests tohave better understanding about the controlled processesdue to which such agglomerates can be broken downto the individual particles for homogenous microstructuremanufacturing of the green (unfired) compacts. The pres-ence of the agglomerates in the suspensions will invari-ably create large pores and nonuniform microstructuresduring the subsequent sintering because of the local dif-ferential shrinkages. The agglomeration of the ceramicparticles can be eliminated or minimised during the wetprocessing using the principles of the colloidal chemistry.There is also a growing demand, from the ecofriendlyperspective to adopt to ‘green’, i.e., environment friendlyprocesses. Therefore, the use of aqueous based ceramicprocesses is preferred over those where organic solvents

∗Author to whom correspondence should be addressed.

being utilised. The dispersion ‘quality’ of the ceramic sus-pensions prior to the forming process must be controlledreasonably in order to maintain high standards of manufac-turing and to obtain reproducible products. This is a signif-icant challenge in aqueous based systems as the stabilisingforces are relatively sensitive to the ambient conditions.There are different approaches that can be used to charac-terise the dispersion quality of suspensions, and amongstthese include, the rheological, sedimentation, adsorptionand electrophoresis experiments.Organic polyelectrolyte or polyvalent salts are often

used for dispersing the ceramic particles in the aqueousmedia.5–7 There are a number of dispersants which aresuitable for this purpose. The dispersant molecules adsorbon the ceramic particles and thereby create an electricalcharge on it as a result of the dissociation of the chemi-cal functional groups on it. When adsorb onto the ceramicparticles, the electrical charge on it becomes effectivelythe charge on the particles. The presence of these effec-tively charged particles in a polar media especially in theaqueous media makes them well dispersed, as the chargedparticles repel each and prevent aggregation.6�7

Organic binders are used to provide sufficient strengthto the green bodies to prevent breakage or damage duringthe handling. The ceramic forming processes such as slip

J. Colloid Sci. Biotechnol. 2012, Vol. 1, No. 2 2164-9634/2012/1/175/010 doi:10.1166/jcsb.2012.1025 175


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Rheological Studies on Stabilised Zirconia Aqueous Suspensions Khan et al.

casting, tape casting, extrusion, roll forming, dry pressing,thick film printing, compression and injection mouldingetc.8�9 A number of organic substances are reported in theliterature that have been used, or categorised, as potentiallyuseful binders for ceramics and the list of such substancesand descriptions of them have been published.10–17 Materi-als such as poly(vinyl) alcohol, cellulose based materials,natural gums, starches, sodium and ammonium alginatesetc., are used quite often for the processing of ceramics.Polymer latex systems have also been reported in the liter-ature as potential binders.5�18�19 The rheology of the liquidphase as well as the suspension is affected by the additionof organic binders.20�21 The flow characteristics includingthe viscosity change from Newtonian (for water) to shearthinning, are normally the main consequences. The pro-cessing behaviour is also affected by the changes in therheology of the binder/liquid solution upon the addition ofparticulates to the solution.Rheology is perhaps the most efficient and commonly

used probe to determine the quality and the agglom-eration properties of the concentrated ceramic powdersuspensions, which are invariably a precursor to the finalproduct. It is also used as an analytical tool for determiningthe optimum viscosity of a suspension; usually this meansthe minimum viscosity for the maximum solids loading.Also, in industrial processes, rheological measurements areoften used for routine quality control in order to min-imise the expected batch-to-batch variations in the feedsbefore a ceramic suspension is further processed furthersuch as through slip cast or spray dried. In more sophis-ticated usages, the rheological behaviour can be used asa direct process parameter, which should be appropriatelyadjusted in order to obtain optimal green body propertiesafter forming. The general and effective application of thisapproach requires a more fundamental understanding ofthe ceramic forming methods in question in the context ofthe appropriate rheological behaviour associated with eachforming process.Electrophoresis/zeta potential technique is widely

employed to characterise the charge particles e.g., ceramicand mineral particles. Adsorption of dispersants, ions,polymer, etc. onto the charged particles affect the elec-trophoresis behaviour of the particles and therefore it canbe used an analytical tool to calculated the amount of thesesubstance adsorbed on the particles. In recent studies theelectrophoresis experiments have been used to establishthe amount of the adsorbed polymers onto the electricallycharged particles like CaCO3, droplets of emulsion andlipid nancapsules.22–24

Investigations of the rheological behaviour for differentsuspensions of zirconia powders having different particlesizes and surface areas have found that these can be sta-bilised by adjusting only the pH values.2 In another work,the effect of the two dispersants namely Aluminon andTiron was investigated without employing any binder on

the rheological behaviour of the zirconia suspensions.7 Inview of above mentioned studies, present study aims toextend the investigation by incorporation a PVA binderin addition to the dispersants. The rheological effects areinvestigated for two commercial dispersants, Aluminonand Tiron, in the presence of poly(vinyl) alcohol (PVA) asbinder on the rheology of zirconia suspensions. Both ofthese dispersants are polyvalent organic salts. In additionadsorption isotherms and electrophoresis techniques havebeen employed to characterise the zirconia suspensions.The study describes the selected rheological properties ofzirconia suspensions in the presence of a PVA binder andtwo dispersants namely Aluminon and Tiron. The effect ofthe PVA concentrations on the reheological characteristicsof the dispersions and flocculation has also been correlatedto the relative PVA concentrations.

2. EXPERIMENTAL DETAILS

2.1. Materials

A zirconia HSY-8 powder was used in the present studywas 8 mole% doped with yttria and was obtained fromTosoh Europe, Netherlands. It has BET surface area of8.75 m2 g−1 and its mean particle size was 0.51 micron.Some of the characteristics of the zirconia powder used aregiven in Table I. Aluminon and Tiron were used as the dis-persants used were. Aluminon is anaurintricarboxylic acidammonium salt (C22H23N3O9� and Tiron is 4-5-dihydroxy-1, 3-benzenedisulfonic acid disodium salt (C6H4Na2O8S2).The structural formulas of these two dispersants may befound elsewhere.6 The source of these chemicals is FlukaChemicals, UK. The poly(vinyl) alcohol (PVA) used was“Mowiol 10-98” from Halow Chemical Co. Ltd., UK, andhad an average molecular weight 61,000, with a degree ofhydrolysis 98%. The suspensions described in this paperfor rheological characterisation were composed of 40%by volume of zirconia and deionised water was usedto prepare the suspensions. For the electrophoresis (zetapotential measurement) experiments very dilute suspensionwere used.

2.2. Experimental Methods

2.2.1. Rheology

A Bohlin VOR rheometer (Bohlin Rheologi, Lund,Sweden) was used for he rheological experiments. The

Table I. Characteristics of the zirconia powder used. The particle sizesD10, D50 and D90 denote the diameters of the particles which constitute10, 50 and 90% by volume, respectively.

Particle size, �mZirconia (ZrO2� Doping BET surfacepowder Code component area (m2/g) D10 D50 D90

8 mole % HSY-8 8 mole 8.75 0.15 0.51 1.08yttria % Y2O3

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Khan et al. Rheological Studies on Stabilised Zirconia Aqueous Suspensions

suspensions were pre-sheared at a comparatively highshear rate of 960 s−1 for ca. 3–4 minutes prior to theactual rheological tests, followed by at least a 2 minutes“rest” period in an attempt to provide a uniform and con-sistent shear history for the systems. Initially, viscosity atlow shear rates was measured, stepping up to higher shearrates, covering the shear rate range of ca. 10−1 to 103 s−1.The results reported in this study are taken from theseshear rates data. All the rheological experiments were car-ried out at 25 �C. For the dynamic measurements, initially,a “strain sweep” measurement was performed in whichthe imposed strain amplitude was varied while the fre-quency was kept constant at 1 Hz. A “linear viscoelas-tic region” was identified in which the values of the G′

and G′′ moduli remain virtually constant as a function ofthe applied strain amplitude. Within this linear viscoelasticregion (strain range), the imposed strain was fixed withinthis region and the deformation frequency was varied (usu-ally from 0.05 to 10 Hz). These measurements are calledhere the “oscillatory measurement”. The specific resultsreported in the present investigation are for a frequency of1 Hz and are taken from the oscillatory measurements. Allthe rheological experiments were conducted at the naturalpH i.e., without adjusting the pH of the suspensions. Forthe Aluminon suspensions the pH varied from 7.1 to 7.3and for the Tiron suspensions it varied from 7.3–7.5.

2.2.2. Electrophoresis

A stock solution containing ca. 1% (wt/v) particles of thezirconia powder and without any dispersant was preparedin a 10−3 M KNO3 solution. The suspensions were soni-cated using a Vibra-Cell VCX 600 ultrasonicator. A fewdrops of the stock suspension were used to prepare severalmore dilute suspensions (30 ml in volume) in a 10−3 MKNO3 solution. The pH of the suspensions was adjustedusing concentrated HNO3 and the dilute suspensions werestored over night. The pH of suspensions measured againjust prior to the experiments.The zeta potential of the particles was determined with

a Zeta Master microelectrophoresis instrument (ModelPCS, Malvern Instruments, UK). The average mobility ofwas then calculated from at least 15 different experimen-tal runs.The amount of the dispersant and poly(vinyl) alcohol

(PVA) used is expressed on a dry weight of the powderbasis (dwb), which means it is equivalent to the wt./wt.basis of the anhydrous solid.

3. RESULTS AND DISCUSSION

The prepared dispersions were characterized by differenttechniques such as for their zeta potential, effect of disper-sants concentration on viscosity and rheology, and effectof PVA binders on the rheological characteristics.

–50

–30

–10

10

30

50

70

3 4 5 6 7 8 9 10

Zet

a P

oten

tial (

mV

)

pH

Fig. 1. Zeta potential of zirconia HSY-8 as a function of pH values.

3.1. Zeta Potential Characteristics

In Figure 1, the zeta potential of zirconia HSY-8 is shownas a function of pH. From the figure, the isoelectric point(iep), which is the pH value at which zeta potential iszero, was found at ca. pH 6.5. However, when the disper-sants (Aluminon and Tiron) were adsorbed on the powderparticles, the zeta potential of the particles was changed.Figure 2, shows the zeta potential of the zirconia HSY-8as a function of dispersant concentration of Tiron at a pHof 7.4 and Aluminon at a pH of 7.2. It can be observedfrom the figure that addition of the dispersants to the zir-conia powder suspensions changes the zeta potential of theparticles. The change in behaviour is caused by the adsorp-tion of the dispersant molecules onto the zirconia particlesin the suspension. The dispersant molecules at the statedpH values are dissociated and thereby negatively charged.The adsorption of them to the particles makes the particleseffectively negatively charged. The fact that Aluminon andTiron dispersants molecules adsorb to zirconia HSY-8 hasbeen established as discussed elsewhere.7

The Aluminon dispersant is a violet colour substanceand gives a violet colour solution in water. The suspen-sions made using Aluminon having concentration below0.20% dwb, were white in colour i.e., the colour of

–90

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–60

–50

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–20

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0

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Zet

a P

oten

tial (

mV

)

Dispersant Concentration (% dwb)

AluminonTiron

Fig. 2. Zeta potential of zirconia HSY-8 as a function of two dispersant(Aluminon and Tiron) concentration at pH 7�4± 0�1 for Tiron and atpH 7�2±0�1 for Aluminon.

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the zirconia powder. However, for Aluminon concentratedbetween 0.20 and 1.0% dwb, the colour of the suspensionsgradually changed becoming more violet with the increaseof the Aluminon concentration. The change in colour byvisual observation was indicating that the dispersant molesare absorbed.

3.2. Effect of Dispersant Concentration onViscosity and Rheology

Figure 3 represents the viscosity as a function of the Alu-minon and Tiron concentrations, respectively, at a shearrate of 1.46 s−1 for the zirconia (40% (v/v)) suspensions.The pH values for the Aluminon suspensions varied from7.1 to 7.3 and for the Tiron suspensions it varied from7.3–7.5. For the Aluminon dispersant, a concentration ofabout 0.2% dwb (0.23 mg/m2) gives the lowest viscos-ity for the suspension, and for the Tiron system the low-est viscosity is found when the concentration is about0.125% dwb (0.14 mg/m2). The concentrations of the dis-persants which give the lowest viscosity is defined here asthe optimum concentration.Initially, when the dispersant concentration is increased,

the adsorption of the dispersant molecules takes place onthe surface of the zirconia powder particles. After the opti-mum concentration, there is hardly any more adsorptionof the dispersant molecules onto the surface of the zirco-nia particles.7 Aluminon and Tiron dispersants are poly-valent organic salts, and dissociate in aqueous media. Theadsorption of the dispersant molecules (anions) on the zir-conia particles effectively make them negatively charged.In the suspensions, there is repulsion between chargedparticles, they do not aggregate as a result of repulsionand under applied strain they easily pass over each other,and hence the viscosity is decreased. The dissociated non-adsorbing counter ions (cations) are present in the aqueousmedium and surround the negatively charged zirconia par-ticles. These counter ions form a layer surrounding thezirconia particles, called “the diffuse part of the electri-cal double layer”.25 At the concentrations lower than theoptimum concentration, the surface coverage of the zir-conia particles with the dispersant molecules is incom-plete and the particles aggregate due to the significant vander Waals attractive forces, and therefore the viscosity ishigher than the minimum viscosity. Above the optimumconcentration, the dispersant molecules hardly adsorb any-more; however, the dissociation of the unadsorbed disper-sant molecules in the suspension results in an increase ofthe counter ion concentration surrounding the negativelycharged zirconia particles. The increased concentration ofthe counter ions reduces the thickness of the double layer.The reduction of the thickness of the double layer causesthe increase in the viscosity of the suspensions.6�7

These rheological results are consistent with the adsorp-tion of the dispersants. For example, when the Aluminon

0.01

0.1

1

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0 0.25 0.5 0.75 1 1.25 1.5

Vis

cosi

ty (

Pa.

s)

Concentration of Dispersant (% dwb)

TironAluminon

c.c.c of Tiron

c.c.c of Aluminon

Fig. 3. Viscosity versus dispersant concentration of Zirconia 40% v/vsuspensions for Aluminon and Tiron dispersants at a shear rate of1.46 s−1.7

concentration was increased from 0.1 to 0.2% dwb theviscosity of the zirconia suspensions, at a shear rate of1.46 s−1, decreased by a factor of ca. 4–5 and a factor ofca. 2 for Tiron when its concentration was increased from0.05 to 0.125% dwb (Fig. 3). In the adsorption isothermexperiments it has been observed that by increasing theconcentration of the dispersant up to a point in the sus-pensions increases the amount adsorbed on zirconia andthereafter there is hardly any more adsorption.7 Therefore,the maximum amount adsorbed corresponds to the low-est viscosity. In the electrophoresis experiments it appearsthat the adsorbed amount keep on increasing as the moredispersant amount is added to the suspensions. However,it must be noted that the suspensions used in the elec-trophoresis experiments was very dilute and had volumefraction �0.1% where as the volume fraction of the sus-pensions used in the rheological experiments were 40%.Therefore, when the difference of the volume fractionsin the two sets of the experiments is several orders ofmagnitude, it is hard to compare the amount of disper-sants adsorbed quantitatively. The values of the dispersantsquoted in Figure 2 are the amount of the dispersants addedto the suspensions. The important aspect is that the elec-trophoresis experiments do confirm the adsorption of thedispersants. It must be noted that sediment height vs. dis-persant concentration reported in Ref. [7] also validate therheological results. In the electrophoresis experiments ithas been observed that increasing the dispersant amountincreases the value of the zeta potential indicating thathigher amount of the dispersant is adsorbed.The critical coagulation points (c�c�c) for the zirconia

is shown in Figure 4 by the arrow signs in the presence oftwo dispersants. The c�c�c� values were calculated usingthe following Eq. (25)

c�c�c = 9�85×104�3×k5T 5�4

Na · e6×A2z6(1)

where z is the charge of the ion, A is Hamaker’s, e ischarge on an electron, Na is the Avogadro’s constant, k is



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the Boltzmann’s constant, � is permittivity, T is absolutetemperature,. The term � is given by Eq. (2) in which �d

is the surface or zeta potential of the particle

� = exp ze�d/2kT �−1exp ze�d/2kT �+1

(2)

The substitution of the constants into Eq. (1) for theaqueous dispersion leads to Equation (3)25

c�c�c = 3�84×10−39�4

A2 · z2 (3)

The c�c�c� values were calculated using the surfacepotentials 75 mV and 66 mV when Aluminon and Tirondispersant were adsorbed on the zirconia particles, respec-tively. The zeta-potential was taken equal to the surfacepotential and was measured using electrophoresis experi-ment at the suspension pH. A value of 10× 10−20 J wasused for Hamakar’s constant for the system under inves-tigation. The calculated c�c�c� values for Aluminon andTiron were 0.112 and 0.076 mol ·dm−3, respectively. Thesevalues correspond to 0.57% and 0.27% dwb for Alumi-non and Tiron, respectively. The c�c�c� values are abovethe minimum viscosity concentrations (optimum concen-trations) which would correspond to maximum stabilisa-tion by the dispersant. The amount of the dispersants abovethe optimum concentrations increases the electrolyte con-centration of the bulk aqueous continuous phase and wouldbe expected to cause aggregation of the particles. Thusexperimentally we estimate the c�c�c� value to be above0.2% dwb for the Aluminon and above 0.125% dwb forthe Tiron systems. Therefore, the calculated c�c�c� value0.57% dwb for Aluminon and 0.27% dwb for Tiron arein accord with this. Aluminon and Tiron molecules weretaken as 2:1 salts, respectively.In the dynamic measurements, when the Aluminon was

used as a dispersant with the zirconia HSY-8, the dynamicviscosity as a function of Aluminon concentration is com-parable in magnitude to that of shear viscosity at a shearrate of 1.46 s−1 (see Fig. 4). Close to the optimum value of

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amic

Vis

cosi

ty (

Pa.

s) &

G´

and

G´´

(Pa.

s)

Aluminon Concentration (% dwb)

G´G´´Dyn. Viscosity

Fig. 4. Dynamic behaviour of Zirconia HSY-8 40% v/v suspensionagainst Aluminon concentration at a frequency of 1 Hz.

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0 0.2 0.4 0.6 0.8 1 1.2

Dyn

amic

Vis

cosi

ty (

Pa.

s) &

G' a

nd G

''(P

a)

Concentration of Tiron (% dwb)

G'G''Dyn. Viscosity

Fig. 5. Dynamic behaviour of Zirconia HSY-8 40% v/v suspensionagainst Tiron concentration at a frequency of 1 Hz.

the concentration of these dispersants, the storage modulusis less than the loss modulus, however at concentrationmore than the optimum, the storage modulus is signif-icantly higher than the loss modulus. When the Tirondispersant is used with the zirconia HSY-8, the dynamicbehaviour is similar to that of zirconia HSY-8 with Alu-minon; (see Fig. 5). The lowest dynamic viscosity wasfound when the concentration of the Tiron was 0.125%dwb, where the loss modulus is less than the storage mod-uli when the concentration is 0.10 and 0.125% dwb. Whenthe concentration of an electrolyte dispersant exceeds theoptimum amount, the excess amount resides in the contin-uous medium and acts a free electrolyte which screens theeffective charge on the particles and enhances the van derWaals attractive forces and as a result the particles startaggregating. This aggregation effect increases the viscosityas well as the storage and loss moduli.

3.3. Effect of PVA Binders

The effect of a PVA binder on the rheological behaviour ofthe suspensions stabilised with the dispersants was inves-tigated. Two combinations consisting of two dispersants,

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0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

Vis

cosi

ty (

Pa.

S)

Concentration of PVA (% dwb)

Shear Rate 146 /sShear Rate 14.6 /sShear Rate 1.46 /s

Fig. 6. Viscosity against PVA concentration, for Zirconia HSY-840% v/v suspensions stabilised with 0.125% dwb Tiron, at three differentshear rates.



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Aluminon and Tiron, and a binder, PVA, were studied forthe zirconia HSY-8 suspensions.

3.3.1. Tiron - PVA Combination

Increasing the PVA concentration changes the viscosity ofthe Tiron stabilised zirconia HSY-8 suspensions. The vis-cosity at three different shear rates (1.46, 14.6 and 146 s−1)against the PVA concentration for a 0.125% Tiron concen-tration (the optimum amount) is shown in Figure 6.At the highest shear rate, 146 s−1, the viscosity is the

lowest and for the intermediate and the lowest shear rates(14.6 and 1.46 s−1� the viscosities are intermediate andhigher, respectively. The difference in the viscosities atthe different shear rates indicates that the suspensions areshear thinning in their character and the magnitude ofthe differences in the viscosity gives an indication of thedegree of the shear thinning behaviour.Increasing the Tiron concentrations beyond the optimum

amount (ca. 0.125%) influences the viscosity against thePVA concentration response. The results for the variousTiron concentrations, at a shear rate of 1.46 s−1, are shownin Figure 7. In general, for all the Tiron concentrations theviscosity increases almost linearly as the PVA concentra-tion is increased from 0.05 to 1.5% dwb. For the higherconcentrations of the Tiron, the viscosities are higher thanthe viscosities of the lower Tiron concentration zirconiasuspensions, for a fixed PVA concentration.Figure 8 represents the viscoelastic properties, as a func-

tion of the PVA concentration, at a frequency of 1 Hz., ofthe zirconia HSY-8 suspensions containing 0.125% dwbof Tiron. The storage modulus (G′) and loss modulus (G′′)values, as well as the dynamic viscosity, increases gradu-ally as the PVA concentration is increased. These increasesof the moduli and also the dynamic viscosity are similarto those of the increases in the viscosity. The differencebetween the storage modulus (G′) and loss modulus (G′′)

0

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0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

Vis

cosi

ty (

Pa.

s)


Tiron 0.125%Tiron 0.25%Tiron 0.5%Tiron 1%Tiron 2%

Fig. 7. Viscosity against PVA concentration, for Zirconia HSY-840% v/v suspensions stabilised with different concentrations of Tiron, ata shear rate of 1.46 s−1.

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0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

Dyn

amic

Vis

csoi

ty (

Pa.

s) &

G´

and

G´´

(Pa)


G´

G´´

Dyn. Viscosity

Fig. 8. Dynamic behaviour against PVA concentration, for ZirconiaHSY-8 40% v/v suspensions stabilised with 0.125% dwb Tiron, at afrequency of 1 Hz.

values increases with the increasing of the PVA concentra-tion. The storage modulus (G′) and loss modulus (G′′) val-ues for the different concentrations of the Tiron are shownin Figures 9 and 10, respectively.The values of the storage modulus (G′) and loss mod-

ulus (G′′) increase as the Tiron concentration is increasedfor a fixed PVA concentration. Increasing the PVA concen-tration, for all the Tiron concentrations for these zirconiasuspensions, causes an increase in the storage modulus(G′) and loss modulus (G′′) values. However, for the Tironconcentrations which are higher than 0.125% dwb, theincrease in the storage modulus (G′) and loss modulus(G′′) values is less uniform than for the 0.125% Tironconcentration.The zirconia HSY-8 suspensions containing 0.125%

dwb Tiron and above, without any PVA addition, are sta-bilised by an electrostatic mechanism discussed earlier. Inthe presence of PVA, it was found that the addition ofan electrolyte (KNO3� increases the rate of flocculation,and the PVA addition does not change the primary mech-anism of the stabilisation of these suspensions. Therefore,the increase of the relative viscosity at a shear rate of1.46 s−1, and the storage modulus (G′) and loss modulus

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10000

100000

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

Sto

rage

Mod

ulus

, G´

(Pa)



Fig. 9. Storage modulus (G′) against PVA concentration, for ZirconiaHSY-8 40% v/v suspensions stabilised with different concentrations ofTiron, at a frequency of 1 Hz.



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0.1

1

10

100

1000

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

Loss

Mod

ulus

, G´´

(Pa)



Fig. 10. Loss modulus (G′′) against PVA concentration, for ZirconiaHSY-8 40% v/v suspensions stabilised with different concentrations ofTiron, at a frequency of 1 Hz.

(G′′) values at a frequency of 1 Hz. are not likely to bedue to the aggregation of the particles by van der Waalsattractive forces. It is also improbable that the flocculationis caused by a bridging mechanism, as the PVA moleculesare non-adsorbing to the zirconia particles. Therefore, theincrease in the viscosity, and the storage modulus (G′)and loss modulus (G′′) values, with the increasing of thePVA concentration is probably due to the depletion mech-anism. In separate studies it was established that whenalumina suspensions were stabilised with Aluminon andTiron dispersants, addition of the PVA does not adsorb andcaused flocculation by the depletion mechanism.26�27 In thepresent study in some test cases it was found that additionof only PVA without any dispersant in the aqueous zirco-nia suspensions does not affect the rheology and therefore,it can be concluded that PVA does not adsorb onto thepresent zirconia system. It has also been found that addi-tion of Aluminon and Tiron in the PVA water solutiondoes not affect the viscosity of the solution26 and there-fore, there are not any significant magnitude of interactionsbetween the two dispersants and PVA in aqueous solu-tion. The above discussion leads to the possibilities thatthe flocculation by the addition of PVA is by the depletionmechanism.However, for the zirconia suspensions, stabilised with

0.125% Tiron and containing different concentrations ofPVA, the increase in the relative viscosity at a shear rateof 1.46 s−1 and other viscoelastic properties describedabove, is small in comparison to the corresponding zirco-nia suspensions having higher concentration of Tiron. Thissuggests that the extent of the depletion flocculation occur-ring in the zirconia suspensions, stabilised with 0.125%Tiron and incorporating PVA, is weaker than the corre-sponding zirconia suspensions having higher Tiron con-centration. The higher values of the viscosities and theother rheological properties, of the zirconia suspensionsfor the higher concentrations of Tiron (>0.125% dwb), fora given PVA concentration, also signify that main stabilis-ing mechanism is by an electrostatic route; when the Tiron

concentration is increased above the optimum concentra-tion, the excess unadsorbed Tiron molecules function as anelectrolyte which “compresses” the double layer surround-ing the zirconia particles, thereby reducing the range ofelectrostatic repulsion and hence increasing the viscosityand the other rheological properties.

3.3.2. Aluminon - PVA Combination

The addition of PVA to the Aluminon stabilised HSY-8suspensions changes the rheology of the suspensions sig-nificantly. This behaviour is depicted in Figure 11 which isa plot of the viscosity of the zirconia HSY-8 suspensionsagainst PVA concentration at three different shear rates of1.46, 14.6 and 146 s−1. The systems were stabilised with0.2% dwb Aluminon which is the optimum amount in theabsence of a binder. The suspensions are shear thinningwhich is why there is a difference in the viscosity at thedifferent shear rates. The other aspect of the figure is thechange of viscosity with the changing of the PVA concen-tration. When the PVA concentration is increased from 0 to0.05% dwb the viscosity increases by approximately oneorder of magnitude. The increase in the viscosity is com-paratively low when the PVA concentration is increasedfrom 0.05 to 0.2%. However, there is again a large increasein the viscosity as the PVA concentration is increased from0.2 to 0.5%. The trend is almost same for all the shearrates but is somewhat less evident at the higher shear rates.Suspensions containing more than 0.5% dwb PVA concen-tration could not be prepared using the technique adoptedfor the lower PVA concentration suspensions because theresulting viscosity was too high to homogenise the com-ponents. For the suspensions containing Aluminon 0.2%dwb and above, the viscosity at the shear rate of 1.46 s−1

against the PVA concentration is shown in Figure 12.The viscosity against the PVA concentration trend is

same for the all the Aluminon concentrations. The viscos-ity increases with the increasing of Aluminon concentra-tion at a given PVA concentration.

0.01

0.1

1

10

100

0 0.1 0.2 0.3 0.4 0.5

Vis

cosi

ty (

Pa.

S)


Shear Rate 146 /sShear Rate 14.6 /sShear Rate 1.46 /s

Fig. 11. Viscosity against PVA concentration, for Zirconia HSY-8 40%v/v suspensions stabilised with 0.2% dwb Aluminon, at three differentshear rates.



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0.1

1

10

100

1000

0 0.1 0.2 0.3 0.4 0.5 0.6

Vis

cosi

ty (

Pa.

s)

Concentration PVA (% dwb)

Aluminon 0.2%

Aluminon 0.4%

Aluminon 0.6%

Aluminon 1%

Fig. 12. Viscosity against PVA concentration, for Zirconia HSY-8 40%v/v suspensions stabilised with different concentrations of Aluminon, ata shear rate of 1.46 s−1.

For the different concentrations of Aluminon, the stor-age modulus (G′) and loss modulus (G′′) values are shownin Figures 12 and 13, respectively, as a function of thePVA concentration.As the PVA concentration is changed from 0 to 0.05%

the storage (G′) and loss (G′) moduli values increaseconsiderably. The extent of this increase in the two mod-uli is greater for the lower concentrations of the Alumi-non. For the concentration of PVA between 0.05 and 2%dwb the storage modulus (G′) and loss modulus (G′′) val-ues decrease as the PVA concentration is increased andbeyond the 0.2% PVA concentration the storage modu-lus (G′) and loss (G′′) again increase significantly. Thistrend of the variation of the storage modulus (G′) and lossmodulus (G′′) parameters with the changing of the PVAconcentration is similar for all the Aluminon concentra-tion investigated. The two moduli are higher for the largervalues of the Aluminon concentrations for a fixed PVAconcentration.The zirconia HSY-8 suspensions stabilised with the Alu-

minon concentrations of more than 0.2% dwb and contain-ing different concentrations of PVA were predominantlyelectrostatically stabilised. The addition of a salt (KNO3�

0.1

1

10

100

1000

10000

0.00 0.10 0.20 0.30 0.40 0.50 0.60

Sto

rage

Mod

ulud

, G´(

Pa)


Aluminon 0.2%

Aluminon 0.4%

Aluminon 0.6%

Aluminon 1%

Fig. 13. Storage modulus (G′) against PVA concentration, for ZirconiaHSY-8 40% v/v suspensions stabilised with different concentrations ofAluminon, at a frequency of 1 Hz.

to the suspensions containing ∼8% (v/v) zirconia HSY-8 particles and having different concentrations of Alumi-non (>0.2%) and PVA, increased the rate of sedimentationof the zirconia particles. The same conclusion was drawnfrom the electrophoresis experiments and the rheologicalexperiments for the zirconia HSY-8 suspensions stabilisedwith Aluminon and without any PVA. This is also evidentfrom the rheological results described above (Figs. 11 to13), since the viscosity and the storage (G′) and loss (G′′)moduli values increase as the Aluminon concentration isincreased above 0.2% for a fixed PVA concentration. Thisindicates that the excess unadsorbed Aluminon operates asan electrolyte and decreases the range of the electrostaticeffect which brings about the aggregation of the particleshence increases the viscosity as well as the storage (G′)and loss (G′′) moduli values. Therefore, the increase in theviscosity, upon the addition of PVA to the zirconia suspen-sions, is unlikely to be due to the aggregation of particlesby van der Waals attractive forces which was the case withthe other dispersant/PVA combinations in the zirconia andzirconia HSY-8 systems.In a test case with zirconia HSY-8 suspensions contain-

ing ∼8% (v/v) particulate material and containing 0.2%dwb Aluminon, it was found that PVA does not displacethe adsorbed Aluminon, molecules from the zirconia HSY-8 particles. Thus, PVA does not adsorb on the particlesin the zirconia suspensions stabilised with Aluminon dis-persant. This was established by comparing the coloursof the supernatants obtained by separating the particles ofthe different suspensions with and without PVA addition,using a centrifuge. The colour of the supernatant result-ing from the HSY-8 suspension containing PVA was alsocompared with the supernatant of the zirconia suspensionwith a similar PVA concentration. It was also found insome test experiments that the addition of PVA withoutany dispersant does not affect the rheology of the zirconiasuspensions.Because PVA is a non-adsorbing polymer, the increase

in the viscosity and also the storage (G′) and loss (G′′)moduli values induced by the aggregation of the particlesis probably not due to a bridging mechanism. It is possi-ble that, when the PVA concentration is increased (espe-cially from 0 to 0.05% dwb), the aggregation of particles iscaused by the depletion flocculation mechanism as was thecase with the Aluminon/PVA combination in the zirconiasuspensions. For the zirconia suspensions, with differentconcentrations of Aluminon, as the PVA concentration ischanged from 0 to 0.05% dwb the increase in the storagemodulus (G′) value is relatively large for the 0.2% Alumi-non concentration as compared to the other high Aluminonconcentration suspensions. For the Aluminon concentra-tion of 0.2%, without any PVA, the zirconia particles arewell dispersed and when the Aluminon concentration isincreased further (without PVA addition) the suspensionsbecome flocculated due to the reduction in the electro-static potential. Therefore, when the PVA concentration is



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1

10

100

1000

0 0.1 0.2 0.3 0.4 0.5 0.6

Loss

Mod

ulus

, G´´

(Pa)


Aluminon 0.2%Aluminon 0.4%Aluminon 0.6%Aluminon 1%

Fig. 14. Loss modulus (G′′) against PVA concentration, for ZirconiaHSY-8 40% v/v suspensions stabilised with different concentrations ofAluminon, at a frequency of 1 Hz.

increased from 0 to 0.05% the increase in the storage mod-ulus (G’) value, for the 0.2% Aluminon concentration, islarger than those for the other higher Aluminon concen-tration Zirconia suspensions which are already flocculated.Figures 13 and 14 show that, as the PVA concentration isincreased from 0.05 to 0.2% dwb, the storage (G′) and loss(G′′) moduli values decrease. Increasing the PVA concen-tration from 0.2 to 0.5% dwb, produces a large increase inthe storage (G′) and loss (G′′) moduli values. The originof this behaviour is uncertain but it is possible that theremay be some impurities in the system (probably in the zir-conia as was observed in the supernatant colour) and theseimpurities may change the interactions within the systemgiving rise to the observed behaviour.

4. CONCLUSIONS

The present study describes the rheological properties ofzirconia suspensions in the presence of a PVA binderand two dispersants namely Aluminon and Tiron. Thebehaviour of the effect of various concentration combina-tions of the PVA and the dispersants has been discussed.Both the dispersants when used alone without adding anyPVA, at relatively lower concentration first decrease theviscosity, storage modulus (G′) and loss modulus (G′),and above the optimum concentration of the dispersantsthe viscosity, storage modulus (G′) and loss modulus (G′′)increases again. When the PVA is used in the suspen-sions stabilised with the dispersants, the viscosity, storagemodulus (G′) and loss modulus (G′′) increase significantly.The effect is more pronounced with the Aluminon-PVAcombination. As the concentration of the dispersants isincreased above the optimum value, the viscosity of theresulting zirconia suspensions is higher than the viscos-ity of the next lower concentration level of the dispersant,for a given value of the PVA concentration. The dynamicbehaviour, i.e., the storage modulus (G′) and loss modulus(G′′), parameters and dynamic viscosity against the PVAconcentration exhibit similar trends to that of viscosity

against the PVA concentration behaviour. The high valuesof the viscosities and other rheological parameters notedfor the above optimum dispersant concentrations are con-sistent with the action of an electrostatic shielding mech-anism. The nonadsorbed excess amount of the dispersantsremains in the continuous aqueous phase and functions asan electrolyte and its counter ions compress the range ofthe double layer and hence changes the rheological prop-erties. The viscosity increase due to the increasing of thePVA concentration increases the interparticle interactionswithin the system leading to enhanced interactions causean aggregation of the particles by a depletion mechanism.The use of the PVA causes increase in the relative viscos-ity of the dispersant stabilised zirconia suspensions. Theincrease in the relative viscosity is thought to be because ofa depletion flocculation mechanism. The significant effectarises from the electrostatic screening induced by excessdispersant addition but excessive polymer addition alsopromotes depletion flocculation.

References and Notes

1. Y. Wang, J. Liu, C. Wang, L. Fei, Y. Han, and J. Wang, AdvancedMaterials Research 452–453, 35 (2012).

2. A. Khan, U. A. Haq, U. N. Mahmood, and Z. Ali, MaterialsResearch 15, 21 (2012).

3. D. Dong, Y. Huang, X. Zhang, L. He, C.-Z. Li, and H. Wang, Jour-nal of Material Chemistry 19, 7070 (2009).

4. A. U. Khan, B. J. Briscoe, and P. F. Luckham, Colloid and Surfaces161, 243 (2000).

5. R. Greenwood, E. Roncari, and C. Galassi, Journal of EuropeanCeramic Society 17, 1393 (1997).

6. B. J. Briscoe, A. U. Khan, and P. F. Luckham, Journal of EuropeanCeramic Society 18, 2141 (1998).

7. B. J. Briscoe, A. U. Khan, and P. F. Luckham, Journal of EuropeanCeramic Society 18, 2169 (1998).

8. A. Tsetsekou, C. Agrafiotis, I. Leon, and A. Milias, Journal of Euro-pean Ceramic Society 21, 493 (2001).

9. M. R. B. Romdhane, S. J. Baklouti, B. T. Chartier, andJ. F. Baumard, Journal of European Ceramic Society 24, 2723(2004).

10. A. Schrijnemakers, S. André, G. Lumay, N. Vandewalle, F. Boschini,R. Cloots, and B. Vertruyen, Journal of European Ceramic Society29, 2169 (2009).

11. J. S. Park, Journal of Applied Polymer Science 117, 428 (2010).12. M. R. B. Romdhane, S. Boufi, S. Baklouti, T. Chartier, and J. F.

Baumard, Colloid and Surfaces A 12, 271 (2003).13. M. R. B. Romdhane, S. Baklouti, J. Bouaziz, T. Chartier, and J. F.

Baumard, Journal American Ceramic Society 89, 104 (2006).14. M. R. B. Romdhane, T. Chartier, S. Baklouti, J. Bouaziz,

C. Pagnoux, and J. F. Baumard, Journal of European CeramicSociety 27, 2687 (2007).

15. H. Karimian and A. A. Babaluo, Iran Polymer Journal 15, 879(2006).

16. A. Wild, American Ceramic Society Bulletin 33, 368 (1954).17. A. R. Teter, Ceramic Age 82, 30 (1966).18. M. R. B. Romdhane, S. Baklouti, J. Bouaziz, T. Chartier, and J. F.

Baumard, Journal American Ceramic Society 89, 104 (2006).19. S. Begum and M. S. J. Hashmi, Journal of Materials Processing

Technology 167, 542 (2005).20. Y. J. Shin, C. C. Su, and Y. H. Shen, Materials Research Bulletin

41, 1964 (2006).



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21. A. Schrijnemakers, S. André, G. Lumay, N. Vandewalle, F. Boschini,

R. Cloots, and B. Vertruyen, Journal of European Ceramic Society

29, 2169 (2009).

22. A. Jada and S. Erlenmeyer, Journal of Colloid Science and Biotech-

nology 1, 129 (2012).

23. J. S. Nambam and J. Philip, Journal of Colloid Science and Biotech-

nology 1, 51 (2012).

24. M. Gonnet, G. Bastiat, T. Ivanova, and F. Boury, Journal of ColloidScience and Biotechnology 1, 137 (2012).

25. D. J. Shaw, Introduction to Colloid and Surface Chemistry, 4th edn.,Butterworth-Heinemann, Oxford, UK (1992), pp. 225–227.

26. A. U. Khan, N. Mahmood, and P. F. Luckham, Journal of DispersionScience and Technology 33, 1210 (2012).

27. A. U. Khan, P. F. Luckham, S. Manimaaran, and M. Rivenet, Journalof Materials Chemistry 12, 1743 (2012).

Received: 16 July 2012. Accepted: 24 August 2012.


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