Influence of Nature and Amount of Dispersant on Rheology of AgedAqueous Alumina Gelcasting Slurries

Santanu Dhara and Parag Bhargavaw

Materials Science Centre, IIT Kharagpur, Kharagpur 721302, India

Alumina slurries with three different levels of ammonium po-lyacrylate (Darvan 821 A) and dibasic ammonium citrate(DBAC) dispersants—lower than optimum, optimum, and high-er than optimum—were chosen for investigation. The nature andamount of dispersants had a significant influence on viscosityand time-dependent behavior of freshly prepared and aged slur-ries. All slurries were subjected to static and dynamic agingtreatments. All slurries were pseudoplastic in behavior, with thenon-Newtonian index (n) being maximum for optimum amountof each of the dispersants. For freshly prepared slurries, hyster-esis was the least for optimum amount and maximum for slurrieswith less than optimum amount of Darvan and DBAC disper-sants. Slurries with below optimum amount of dispersant exhib-ited greatest aging. DBAC-based slurries experienced a greaterpercentage increase in viscosity than Darvan-based slurries dur-ing static aging. Biodegradability of DBAC contributed to agingof the DBAC-based slurries. All slurries, except the one withbelow optimum amount of DBAC, exhibited a decrease in vis-cosity after 48 h of dynamic aging. Static aging influenced thethixotropic behavior of slurries while dynamic aging had a min-imal effect.

I. Introduction

RHEOLOGY of ceramic-loaded slurries is governed by volumepercent of powder particles, nature and amount of process-

ing aids, pH, and the preparation procedure.1–11 Preparation ofhighly loaded (50–60 vol%) aqueous ceramic slurries for castingdense solid bodies in general requires use of additives such as thedispersant for the purpose of achieving low viscosity at highsolids loading, acid or base for pH adjustment, antifoamingagent for obtaining bubble-free slurries, etc. The slurries foraqueous gelcasting12,13 additionally must contain water-solublemonomer and cross linker, which undergoes in situ polymeriza-tion after casting.

There are a number of electrolyte3,4,6–10 or polyelectro-lyte1,2,5,8–15 dispersants that are commonly used to disperse ce-ramic particles in slurries through electrostatic or electrostericmeans. Highly loaded slurries experience inter-particle attractiveforces even in the presence of dispersants. For an optimum dos-age of the dispersant, the attractive force is reduced to a min-imum, while it is much higher for both dispersant amountsbelow and above optimum. Experimentally, these differences ininter-particle network characteristics for different dispersantsand their amounts are clearly manifested in terms of slurry vis-cosity and slurry behavior as a function of time. Hysteresis dur-ing rheological measurements and changes in measured slurryviscosity over longer times (aging) have been commonly used tocharacterize the time-dependent behavior of slurries.1,9,16–18

Millian et al.17 recently reported the aging behavior of alu-mina casting slips. Although they showed that slurries withgreater than about 43 vol% solids loading experienced time-de-pendent changes in viscosity, no attempt was made at systematicanalysis of the observations. Thixotropic behavior in highlyloaded ceramic slurries has been reported often1,3,16,19–21 andis ascribed to the existence of an inter-particle network that un-dergoes changes with time. The thixotropic behavior is a func-tion of solids loading and the dispersant amount. It has beenobserved that thixotropy increases with an increase in ceramicsolids loading while it is minimum at an optimum coverage ofceramic particles by the dispersant.1

Few studies have investigated the effect of the nature andamount of dispersant in highly loaded ceramic slurries on agingbehavior and thixotropy of fresh and aged slurries. Preliminarystudies by the present authors had shown that, as-prepared slur-ries when just left undisturbed appeared to undergo an irrevers-ible change in slurry characteristics. From an applicationstandpoint, it is desirable to study the above irreversible chang-es as well as examine a suitable method of storing the slurriessuch that the slurry characteristics could be retained over a rea-sonable period of time. Thus, systematic experiments wereperformed with use of the two most natural choices of storingthe slurries—one without any disturbance (static aging) and theother with constant agitation (dynamic aging). As part of thestatic aging, a mixing/milling step was introduced to understandwhether the irreversible effect of aging could be diminished andwhether the slurries could be revived by any means that wouldmake them usable for casting.

The changes in slurry characteristics following static anddynamic aging were studied for different amounts of two com-monly used dispersants—one a polyelectrolyte (higher molecu-lar weight) and another a salt (lower molecular weight). The twodispersants used were Darvan 821 A (ammonium polyacrylate)and dibasic ammonium citrate (DBAC), and the alumina solidsloading in the slurries was 55 vol%. Experiments were conduct-ed to investigate the differences in severity of aging as a functionof the dispersant amounts. The effect of aging was seen in termsof viscosity and thixotropy. The thixotropic measurements alsoprovided valuable insight into the changes in the inter-particlenetwork in the slurries.

II. Experimental Procedures

All experiments in the present study involved the use of aluminaas a model ceramic material. Alumina powder (Alcoa CT 3000SG, Alcoa, Pittsburgh, PA) with a mean particle size 0.70 mmand a surface area of 7 m2/gm was used. As per the manufac-turer’s specifications, this alumina powder contains impuritieslike Na2O, Fe2O3, SiO2, CaO, and MgO, etc. In order to ascer-tain whether these ionic impurities would leach into the solventand thus affect the comparison between behavior of freshly pre-pared and aged alumina slurries, conductance measurementswere carried out over the same period over which the slurrieswere aged.

Conductance measurements of suspensions soon after addi-tion of ceramic powders indicated two orders of magnitude

547

JournalJ. Am. Ceram. Soc., 88 [3] 547–552 (2005)

DOI: 10.1111/j.1551-2916.2005.00122.x

J. E. Blendell—contributing editor

wAuthor to whom correspondence should be addressed. e-mail: parag_bhargava@yahoo.com

Manuscript No. 10599. Received October 16, 2003; approved July 20, 2004.

higher conductivity as compared with that of water apparentlybecause of leaching of ionic impurities present in the powder.For example, after addition of 25 vol% alumina powder theconductivity increased from 0.03 m

A

for water to 1.3 m

A

for thealumina suspension. Over a period of 12 h the conductivity in-creased marginally to 1.46 m

A

and the conductivity remainedalmost unchanged over the next 48 h. The above observationssuggest that majority of the leaching must be occurring duringthe first few hours of the 24 h mixing/milling step of slurrypreparation. Thus, it can be concluded that the leaching of ionicimpurities will not contribute to any difference in rheologicalbehavior of fresh and aged slurries.

(1) Slurry Preparation

Slurry preparation began with a 15 wt% premix of methacryl-amide (MAM) and methylene bisacrylamide (MBAM) (E. Me-rck, Darmstadt, GermanyQ3 ) in a 6:1 ratio in water.12,13 Asmentioned earlier, two dispersants—Darvan 821A (40 wt% so-lution of ammonium polyacrylate, average MW 6000)8 (R.T.Vanderbilt and Co. Norwalk, CT)Q4 and DBAC (FW 226.2, Si-gma-AldrichQ5 , Gillingham, U.K.)—were used in this study. Dis-persant was added to the premix contained in a polypropylenebottle in accordance with the intended ceramic solids loading.At this stage, part of the ceramic powder and zirconia-grindingmedia was added (1.4:1, powder to media weight ratio) to thepolypropylene bottle for milling. Remaining powder was addedin steps as the milling progressed until the whole of the intendedamount had been added. All slurries were milled for 24 h. Eachtime, a 60 mL batch of slurry corresponding to 130 gm of thealumina powder (55 vol% solids loading) was prepared.

Initially, the optimum dosage of each of the dispersants for55 vol% alumina slurries was determined based on attainment oflowest viscosity. Three different levels of the dispersants—lowerthan optimum, optimum, and higher than optimum—were cho-sen for investigation of rheological behavior of the gelcastingslurries. The above slurry compositions were subjected to staticand dynamic aging treatments. Rheological measurements of allfresh and aged slurries were carried out to study the change inviscosity, thixotropic behavior as a result of aging.

(2) Slurry Characterization

A co-axial cylinder viscometer (Model RVT/73232, BrookfieldQ6 ,Middleboro, MA) was used with shear rates varying from 0.5 to46.5 s�1 for these measurements. After the slurry was pouredinto the cylindrical cup of the viscometer, a small amount (10drops) of silicone oil was added from the top to inhibit loss ofsolvent (water) from the surface during measurements. Initially,the slurry in the viscometer was subjected to a shear at the rateof 46.5 s�1 for 1 min. Viscosity of the slurries was measured inan upward sweep (Zuw) with shear rates increasing from 0.5 to46.5 s�1, and followed by the same measurements during thedownward sweep (Zdw). The whole cycle of the upward anddownward shear rate sweep was completed in about 20 min.

The thixotropic behavior exhibited by the slurries was seenfrom the viscosity measurements during the upward and down-ward shear rate sweeps. The thixotropy was expressed in termsof the difference in the viscosity during the downward and theupward sweep (Zdw�Zuw).

The measured flow curves for fresh slurries were fitted with apower law model16,21,22

Z ¼ kgn�1

where Z is the viscosity of slurries, g is the applied shear rate,and k and n are the consistency factor and non-Newtonian in-dex, respectively.

z potential of the slurries was measured using a Laser ZeeMeter (Model M-501, Pen Kem IncQ7 ., Bedford Hills, NY) by theelectrophoretic method (Table I). All slurries were diluted to thesame extent, the level of dilution being decided by the ability toimage individual particles during the z potential measurements.

pH of the diluted suspensions was maintained same as of slurriesprior to dilution (pH 9).

(3) Aging Treatments

(A) Static Aging: Following completion of the slurrypreparation step, the slurries along with the milling mediawere kept for 45 h without any disturbance. It was observedthat after this period of static aging, the slurries could not bepoured out of the milling jars. Since one of the objectives of thestudy was to evaluate the effect of the aging treatment on slurryrheology, a milling step was introduced. The statically aged non-pourable slurries were milled for 3 h, which also helped to ho-mogenize the slurries prior to rheological measurements. Theslurry viscosity was measured immediately after the 3 h milling.

(B) Dynamic Aging: The milling media were separatedfrom the freshly prepared slurries using a coarse plastic sieve andthe slurries were rolled for 48 h. In this aging treatment, themedia were removed to prevent degradation of the dispersant ashas been reported by other researchers.18

Both of the above-stated aging treatments were carried outfor slurries with three different amounts of each of the disper-sants. Aging behavior was expressed in terms of the difference inthe viscosity of aged and fresh slurries (ZA�ZF) at correspond-ing shear rates. The pH of all slurries before and after aging was9.170.2. Thus, pH of the slurries did not play any significantrole in influencing the observations during the study.

The influence of possible bacterial degradation of the disper-sants, during the aging period, on slurries prepared with Darvanand DBAC was examined by conducting accelerated degrada-tion studies. This influence of bacterial degradation was exam-ined for Darvan- and DBAC-based slurries with the twoextreme dispersant amounts—below and above the optimumfor each of the dispersants. Bacteria were added and allowed togrow over the 48 h aging period.

III. Results and Discussions

(1) Optimization of Dispersant Amount

The amount that resulted in the lowest viscosity, in the range ofshear rate used (0.5–46.5 s�1), was taken as the optimum. Theoptimum amounts of the dispersants were found to be 4.6 mg ofDarvan 821A (7.6� 10�7 mol, dry weight basis) and 1.9 mgDBAC (8.4� 10�6 mol) per gm of powder for the 55 vol% alu-mina slurries. At the optimum dispersant levels, the slurry vis-cosity at shear rate of 9.3 s�1 was 1.1 Pa � s for Darvan-basedslurries and 0.8 Pa � s for DBAC-based slurries.

(2) Non-Newtonian Index

All slurries considered in the present study were pseudoplastic innature. The extent of pseudoplasticity could be characterizedby the non-Newtonian index, n, as defined by the power lawmodel.16,21,22 The non-Newtonian index, n, is indicative of theinter-particle network strength in the slurries. Absence of inter-particle network results in Newtonian behavior as observed fordilute suspensions and it corresponds to an n value of unity. For

Table I. f Potential of Freshly Prepared and Aged DiluteAlumina Suspensions (pH 9) for Different Amounts of Darvan

821 A and DBAC Dispersant

Slurry

z potential (mV)

Darvan 821 A (mg) DBAC (mg)

3.2 4.6 6.3 1.4 1.9 4.2

Fresh �64.4 �73.6 �69.1 �35.3 �63.4 �52.2Aged �68.8 �77.9 �74.5 �21.8 �65.8 �78.2

DBAC, dibasic ammonium citrate.

548 Journal of the American Ceramic Society—Dhara and Bhargava Vol. 88, No. 3

pseudoplastic slurries, the lower the value of n below unity, thestronger the inter-particle network.

The non-Newtonian index as a function of deviation indispersant amount from the optimum is shown in Fig. 1. Allslurries had n values in the range of 0.11–0.33. It was seen that nwas maximum for the optimum amount of dispersant. It is no-table that the n values decreased rapidly for similar fractionaldeviation below optimum as compared with that above opti-mum. This behavior points to the stronger inter-particle networkat dispersant amounts below optimum than above optimum forboth Darvan- and DBAC-based slurries. The evidence forstronger inter-particle network for below optimum amount ofthe dispersants lies in the significantly higher viscosity of theslurries.19,20,23 For DBAC-based slurries, it is the low z potentialthat drives the formation of strong inter-particle networks. Forpolyelectrolyte (longer molecule) dispersants like Darvan, bridg-ing between the particles can contribute significantly to the inter-particle network formation.1,8,9,24–26 Interestingly, highly loadedslurries even at the optimum dispersant levels start to form anetwork at low shear rates, or when left undisturbed.

(3) Thixotropy of Fresh Slurries

Thixotropy is indicative of the kinetics of the network re-for-mation over the time period of the experiment. In Figs. 2(a) and(b), the restoration of the network structure is indicated by zerodifference in viscosity during downward and upward shear ratesweeps, while negative values indicate delay in restoration of thenetwork to the original state.

(A) Darvan-Based Slurries: It is clear from the graph(Fig. 2(a)) that for slurries with below optimum amount of dis-persant, the inter-particle network is not restored even when thelowest shear rate is reached during the downward sweep. It wasobserved during the experiment that if sufficient time was pro-vided during measurement at the lowest shear rate, the viscosity

values rose to the same level as observed at the beginning of theupward sweep.

For slurries with optimum level of dispersant, the particlenetwork is restored even before the lowest shear rate is reachedas indicated by the zero difference in viscosity between the up-ward and downward sweep. Above the optimum amount, theparticle network attained the original state just at the lowestshear rate, again indicated by the zero difference in viscositybetween upward and downward shear rate sweep.

(B) DBAC-Based Slurries: For DBAC slurries madewith less than optimum of dispersant, it was seen that therewas a delayed response in terms of re-formation of the networkstructure (Fig. 2(b)). Even after reaching the lowest shear rate inthe downward sweep, the slurry viscosity was lower than thecorresponding value during the upward sweep. For slurries withan optimum amount of dispersant, the particle network struc-ture was restored rapidly and the difference in slurry viscositybetween the downward and upward sweep attained zero valuebefore the shear rate reached the lowest value.

For the behavior of slurry with above optimum amount ofDBAC, the behavior was most unique. During the downwardsweep, the particle network appeared to be restored soon afterthe shear rate was lowered from the peak value, as indicated bylack of hysteresis (Fig. 2(b)).

(C) Comparison of Darvan- and DBAC-Based Slur-ries: It is notable that for optimum and above optimumamount of the dispersants, the inter-particle network inDBAC-based slurries recovers faster than the Darvan-basedslurries. The re-formation of the particle network in DBAC-based slurries is expected to be quicker because it is an electro-lyte unlike Darvan, which is a long-chain polyelectrolyte. Thetangling8,16,21 of long polyelectrolyte chains may hinder the re-formation of the particle network, when the Darvan dispersantis present in excess. For slurries with above optimum amount ofthe dispersants, as there exists a long-range attraction3,20,23,27

because of compression of double layer, it is expected that theseslurries will recover more rapidly than even the slurries with anoptimum amount of dispersant where double-layer thickness isgreater.

(4) Effect of Aging on Slurry Viscosity

(A) Static Aging: Both Darvan- and DBAC-based slur-ries with below optimum amount of dispersant exhibited a sig-nificant increase in viscosity over that of fresh slurries despite the3 h of milling (Figs. 3 and 4). When the results were seen interms of percentage increase in viscosity over that of the viscos-ity of fresh slurries, for slurries with below optimum amount of

0

0.07

0.14

0.21

0.28

0.35

−0.5 0 0.5

Non

New

tona

in In

dex

(n)

n (Darvan)

n (DBAC)

Fractional Deviation

Fig. 1. Power law exponent, n, of freshly prepared slurries with differ-ent dispersant amounts expressed in terms of fractional deviation fromthe optimum amount.

− 3500− 3000− 2500− 2000− 1500− 1000

− 5000

500

0.1 1 10 100

1.4 mg1.9 mg4.2 mg

− 1400− 1200− 1000

− 800− 600− 400− 200

0200

0.1 1 10 100

3.2 mg4.6 mg6.3 mg

Shear rate (s−1) Shear rate (s−1)

(ηdw

-ηuw

) m

Pa.

s

(ηdw

-ηuw

) m

Pa.

s

(a) (b)

Fig. 2. Thixotropy in freshly prepared slurries expressed as differencein viscosity during downward (Zdw) and upward (Zuw) shear rate sweepfor slurries with different amounts of (a) Darvan and (b) dibasic am-monium citrate.

− 14000

− 12000

− 10000

− 8000

− 6000

− 4000

− 2000

0

2000

4000

6000

3.2 mg-S

4.6 mg-S

6.3 mg-S

3.2 mg-D

4.6 mg-D

6.3 mg-D

0.1 1 10 100

Shear Rate (s−1)

(ηA-η

F)

mP

a.s

Fig. 3. Change in viscosity as result of static (- - - - - ) and dynamic aging(—) of slurries with different amounts of Darvan.

March 2005 Aged Aqueous Alumina Gelcasting Slurries 549

dispersant, the aged DBAC-based slurries experienced a greaterpercentage increase in viscosity than Darvan-based slurries. Thiseffect was attributed to the lower relative z potential for DBAC-based slurries (Table I) and the non-polymeric nature of thedispersant allowing particles to approach much closer to eachother, thus forming stronger networks. Also, considering thesusceptibility of citric acid to biodegradation as is well knownfrom the Kreb’s TCA cycle,28 the aging behavior of DBAC-based slurries was expected to be influenced by the biodegrad-ability of the dispersant.28–30

Change in viscosity as a result of static aging was insignificantfor slurries with optimum and above optimum amount of Dar-van and DBAC (Figs. 3 and 4). This insignificant change in theviscosity of Darvan- and DBAC-based slurries was because of ahigher z potential (Table I), which prevented formation ofstrong inter-particle network while the slurries were kept undis-turbed. Additionally, for DBAC-based slurries degradation ofthe dispersant may take place but the effect is compensated byfurther adsorption of the undecomposed dispersant from thesolvent at the alumina particle surface. This dual effect of com-pensation of the degraded dispersant and the decrease in salteffect explains the observed insignificant change in viscosity forslurries with optimum and above optimum amount.

(B) Dynamic Aging: The change in viscosity as a resultof dynamic aging for the slurries is shown in Figs. 3 and 4. Theslurry with below optimum amount of DBAC experienced asignificant increase in viscosity while the slurry with below op-timum amount of Darvan experienced a decrease in viscosity.For all slurries with optimum and above optimum amount ofDarvan and DBAC, the viscosity exhibited a slight decrease af-ter 48 h of dynamic aging.

For Darvan-based slurries, the results of dynamic aging canbe viewed in terms of studies reported in research literature,where it has been shown that alumina slurries with pH around9.0 experience a low-affinity Langmuir-type adsorption1,2,24 ofthe dispersant. The adsorption of dispersant molecules onto theparticle surface takes place over a period of time, leading to agradual increase in the particle surface coverage. It is likely that,from the time of the measurement of viscosity of the freshlyprepared slurries, the dispersant coverage of the particle surfaceincreased during the 48 h dynamic aging treatment. Measure-ments confirmed an increase in z potential for Darvan-basedslurries (Table I). Thus, the slurry viscosity decreased during48 h aging treatment. The large decrease in viscosity for less thanoptimum amount in comparison with that for optimum andabove optimum amount of Darvan can be attributed to the de-crease in inter-particle bridging1,8,24–26 with an increase in par-ticle surface coverage by the dispersant. It is important to note

that this increase in adsorption of the dispersant, if allowed tooccur while the slurry is kept at rest (static aging), results information of strong particle networks leading to a net increase inviscosity. The formation of a stronger inter-particle network isevident from the significantly higher thixotropy following staticaging despite the 3 h milling step prior to rheological measure-ment. During dynamic aging, the formation of strong particlenetworks in Darvan-based slurries was prevented by continuousrolling of the slurries as confirmed from least thixotropy.

As mentioned earlier in the previous section, during the 48 hof aging, the dispersant DBAC undergoes degradation becauseof its biodegradable nature.29,30 For slurries with less than op-timum amount of DBAC, the degradation of the dispersant re-sults in a decrease in z potential (Table I), leading toenhancement of inter-particle network. For slurries with opti-mum and above optimum amount of dispersant, the degrada-tion of DBAC was compensated by further adsorption of theundecomposed dispersant from the solvent at the alumina par-ticle surface. This compensation of the degraded dispersant, ac-companying a decrease in salt effect and application of constantmechanical agitation during dynamic aging, which minimizedthe inter-particle network, explains the observed decrease in vis-cosity for slurries.

(C) Accelerated Aging in the Presence of Bacteria: In-dependent experiments on accelerated aging in the presence ofthe bacteria showed a significant increase in viscosity for slurrieswith DBAC (below optimum amount). In contrast, the viscosityof DBAC slurries with above optimum amount (excess) of dis-persant in the presence of bacteria was almost unaffected overthe period of aging (Fig. 5). For slurries with above optimumamount the effect because of bacterial degradation of DBACwas compensated by further adsorption of the undecomposeddispersant from the solvent at the alumina particle surface. TheDarvan-based slurries irrespective of the dispersant amountwere almost unaffected by aging in the presence of bacteria, in-dicating that Darvan did not help in bacterial growth (Fig. 5).

The susceptibility of the dispersants for bacterial degradationwas also evaluated by observing the growth of bacteria in aque-ous solutions of Darvan and DBAC. In aqueous solution ofDBAC, the bacteria grew at such a rapid pace that changescould be observed visually while those in Darvan solution didnot appear to grow significantly. As per reports in the literature,polyacrylic acid (Darvan) and substituted acids like polyalkyla-crylates resist biodegradation as was observed in the presentstudy.31,32.

(5) Thixotropy of Aged Slurries

The thixotropic behavior of the aged slurries was very similar incharacter to that of the freshly prepared slurries (Fig. 2). The

− 4000

− 2000

0

2000

4000

6000

8000

10000

12000 1. 4 m g-S

1. 9 m g-S

4. 2 m g-S

1. 4 m g-D

1. 9 m g-D

4. 2 m g-D

0.1 1 10 100

Shear Rate (s−1)

(ηA-η

F)

mP

a.s

Fig. 4. Change in viscosity as result of static (- - - - - ) and dynamic aging(—) of slurries with different amounts of dibasic ammonium citrate.

0

50

100

150

0.1 1 10 100

1.4 mg DBAC-B1.4 mg DBAC-WB4.2 mg DBAC-B4.2 mg DBAC-WB3.2 mg DARVAN-B3.2 mg DARVAN-WB6.3 mg DARVAN-B6.3 mg DARVAN-WB

Shear Rate (s−1)

∆η (

mP

a.s)

Fig. 5. Change in viscosity of Darvan- and dibasic ammonium citrate(DBAC)-based slurries with different dispersant amounts following ag-ing—without bacteria (WB) and with bacteria (B).

550 Journal of the American Ceramic Society—Dhara and Bhargava Vol. 88, No. 3

influence of static and dynamic aging on thixotropy of agedslurries is discussed below.

(A) Darvan-Based Slurries: For slurries with lower thanoptimum amount of dispersant, the hysteresis was maximum forstatically aged slurry and minimum for dynamically aged slur-ries. The formation of strong inter-particle network that resultedin an increase in slurry viscosity was also responsible for theobserved large thixotropy in statically aged slurries.

For slurries with optimum amount of dispersant, the hyster-esis was maximum for statically aged slurry while much less hys-teresis was observed for both fresh and dynamically aged slurry.

For slurries with more than optimum amount of dispersant,the hysteresis for dynamically and statically aged slurries was lessthan that of fresh slurries. The decreased thixotropy in agedslurries may be explained on the basis of increase in surfacecharge and double-layer thickness because of adsorption of freepolyelectrolyte from the solvent. This reduction in hysteresis fol-lowing aging is less for static aged slurries because of formationof stronger inter-particle network.

(B) DBAC-Based Slurries: For slurries with below opti-mum amount of dispersant, the hysteresis behavior was mostpronounced for statically aged slurries. The above slurry expe-rienced a delay in network reformation during the downwardshear rate sweep.

For slurries with optimum amount of dispersant, the hyster-esis loop was almost similar for fresh and aged slurries. Forslurries with above optimum amount of dispersant, hysteresiswas almost absent for fresh as well as aged slurries. The extent ofstructural rearrangement involved in recovery of the inter-par-ticle network is much less for slurries with optimum or aboveoptimum amount of dispersant as compared with that for thebelow optimum slurries.

All results reported above are for slurries prepared with asreceived alumina powders. The alumina powder used in thepresent study was a commercial grade super ground powder(Alcoa CT 3000 SG), which, as supplied by the manufacturer,contains organic additives (o0.2 wt%). For most practical ap-plications, super ground alumina powders like the one used hereare not calcined prior to use in slurries. Even then in order toverify whether the influence of nature and amount of dispersantwas applicable to alumina slurries in general, rheological meas-urements were carried out for slurries without any binder(MAM-MBAM) made using calcined super ground aluminapowder. The influence of the nature and amount of the twodispersants on thixotropy and aging behavior for slurries pre-pared with calcined powders devoid of MAM-MBAM was sim-ilar to results for MAM-MBAM containing gelcasting slurriesmade using the as received powders.

IV. Summary

Differences in rheological behavior of slurries over the shear raterange considered with varying dispersant amounts could be sys-tematically characterized in terms of the power law exponent.The deviation from Newtonian behavior, reflected in the valuesof n (power law exponent), was maximum for dispersantamount below optimum, indicating strong inter-particle net-work formation in the slurries. The deviation from Newtonainbehavior is lesser for slurries with dispersant amount above op-timum while it is the least for slurries with optimum amount ofthe dispersant.

The extent of thixotropy was greatest for ceramic slurries withdispersant amount below optimum and least for slurries withoptimum amount of dispersant. Slurries with above optimumamount of DBAC exhibited exceptional behavior in thixotropy,being very similar to that for the optimum amount. This was insharp contrast to the behavior of slurries with Darvan where forabove optimum amount of dispersant the entanglement of thepolymeric chains delayed the recovery of inter-particle networkwith a decrease in shear rate and thus exhibited relatively greaterthixotropy than for the optimum amount.

Among all Darvan- and DBAC-based slurries, the ones withbelow optimum amount of dispersant exhibited significantchange in viscosity as a result of both static and dynamic agingtreatments. The increase in viscosity was greater for DBAC-based slurries because of the additional effect of bio-degradationof the dispersant. Roll mixing of slurries in the absence of mill-ing media (dynamic aging) prevented formation of the inter-particle network in the slurries and promoted increase in particlesurface coverage by the dispersant from solution.

Slurries with optimum and above optimum amounts of thetwo dispersants exhibited much lesser change in viscosity as aresult of static or dynamic aging treatments. The change in vis-cosity was minimal as a result of static aging while the viscosityof slurries decreased with dynamic aging. The decrease in vis-cosity as a result of dynamic aging is attributed to the increasein particle coverage by the dispersant leading to an increase indouble-layer thickness.

Acknowledgments

The authors would like to acknowledge Prof. H. Das (Agriculture and FoodEngineering, IIT Kharagpur) and Dr. J. P. Srivastava (Scientist, NML Jams-hedpur) for their kind support.

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