Ind. Eng. Chem. Res. 1994,33, 2437-2442
2437
Influence of Processing on the Rheology of Titanium Dioxide Pigment Suspensions Petra V. Liddell and David V. Boger* Department of Chemical Engineering, University of Melbourne, Parkville, Victoria 3052, Australia
A knowledge of the rheology of titanium dioxide pigment suspensions and the variation with processing conditions is essential for optimum plant operation and in ultimate paint production. In this paper the influence of pH, solids concentration, and particle size on the rheological properties of four commercial Ti02 pigment suspensions is reported. The suspensions exhibit pseudoplastic behavior, conforming to the power-law model in the shear rate range investigated. Extreme sensitivity with pH was observed, whereby the suspension viscosity can be changed by as much as an order of magnitude with pH variation at constant volume fraction. Also, the pH at maximum viscosity coincides with the isoelectric point, independent of solids concentration. The maximum viscosity (viscosity a t the isoelectric point) increases exponentially with solids concentration and increases with decreasing particle size. The important outcome from the investigation is the dependence of rheology on the compressional history used for dewatering the suspensions. Two identical pigments from two different filtration devices exhibited a large difference in their viscosity, as a consequence of being subjected to different compressional forces. Finally, the presence of a monolayer coating of organic on the pigment surface illustrates how to lower the observed suspension viscosity with steric stabilization.
Introduction Titanium dioxide pigments form the basis for the world's paint industry. Since the pigments are produced in a wet suspension stage before ultimate drying and milling, and are redispersed in paints, the rheological properties of Ti02 suspensions are important in the production process and also in the redispersed pigment. Not only are the normal variables such as solids concentration (volume fraction), particle size, and pH (colloidal forces) important to determine the basic rheology of Ti02 pigment suspensions but so are the conditions used to dewater the pigment in the process, as will be demonstrated in this paper. The work first examines how the rheology of commercial Ti02 paint pigments is influenced by solids concentration, particle size, and pH, and then how processing conditions and additives can markedly change the rheology of the final product. The topic is particularly suitable for this special issue of Industrial & Engineering Chemistry Research to recognize the contributions of Professor Arthur B. Metzner in chemical engineering. Metzner (1958) coauthored an early paper on the rheology of Ti02 suspensions where dilatant behavior was demonstrated but also has had a profound impact in bringing nonNewtonian fluid mechanics to the chemical industry. A Ti02 pigment is a Ti02 particle with an inorganic coating of alumina and/or silica which usually represents 3-5 wt % of the pigment. Although considerable work has been published on the rheology of pure Ti02 (Metzner, 1958;Rao, 1987),and even more work on the rheology of paint formulated with the Ti02 pigments (Patton, 1968; Kuge, 1983),there has been little work doneon therheology of the Ti02 pigment in aqueous systems in the absence of organic dispersants. It is well established that colloidal forces control the rheological properties of fine particle suspensions (Russel, 1980). Colloidal forces arise from interaction between the suspended particles, and are significant for particle sizes less than 1pm. The Ti02 pigment particles used in this work are approximately 0.2 pm, and hence colloidal forces will be significant. The two main colloidal forces are the van der Waals attractive forces which originate from fluctuating dipoles as a result of the motions of outer
FILTER Plate and Frame
Coated Ti02
Press Belt
U 175 deg 40 min
MICRONIZER (mill) Steam Temp. 180 deg 30 sec
Final Product Dry Powder 15 m2/g
Figure 1. A section of the process diagram for the manufacture of Ti02 pigmenta.
electrons on the interacting particles and the electrostatic repulsive forces due to the presence of like charges on the particles and a dielectric medium (Hunter, 1987). For a given system the van der Waals forces are essentially constant. The electrostatic forces, however, will vary with the surface charge density of the suspended particles and the ionic strength of the suspension. The surface charge on metal oxides is established by the amphoteric reaction of the surface hydroxyl groups with H+ or OH- (Yates, 1975). Hence the surface charge is a function of pH. A high surface charge density results in strong electrostatic repulsion forces between particles, so that the suspension favors the dispersed state, exhibiting a low viscosity. Conversely a low surface charge density results in weak electrostatic forces between particles, so that the suspension favors the flocculated state,exhibitinga high viscosity. Leong et al. (1990,1991,1993a)have demonstrated these principles with reference to zirconia suspensions. Of significancein this work is the effect of process history on the rheology. The final stage of the process for the production of the Ti02 pigment is illustrated in Figure 1. The Ti02 particles from the sulfate process are dry milled and wet milled, after which they are coated with inorganic oxides. These coated Ti02 particles are then filtered,
OSSS-5SS5/94/2633-2437~Q4.50/0 0 1994 American Chemical Society
2438 Ind. Eng. Chem. Res., Vol. 33, No. 10, 1994
washed, dried, and micronized (milled) to give the final product. The influence of filtering and milling on the rheology of the Ti02 pigment is examined, with particular attention to the effects of compression during filtration. Earlier studies have demonstrated that the shear history has a significant effect on the compressibility of bauxite residue suspensions. De Guingand (1986) showed that a bauxite residue sample left stagnant produced a less compressible sediment than one which was sheared. He proposed that the increased resistance to compression was due to the formation of interfloc bonds in the sediment during the stagnation period. Landman et al. (1991) have demonstrated that quite different dewatering efficiencies can be obtained from a strongly flocculated suspension depending on how the applied pressure is administered to the particle network. This link between shear history and compressibility is well documented. It is an objective of this work to demonstrate the link between compression history and the shear rheology of the Ti02 product, a link which has important implications in the industry; particularly in determining the appropriate dewatering device and its operation for a particular situation. The addition of small molecules which adsorb onto the particle surface can dramatically alter the rheological properties of a suspension (Leong et al., 1993a-d). The adsorption of various additives on the Ti02 pigment, and the effect on the rheology, has been studied in great detail (e.g., Strauss et al., 1993; Hulden and Sjoblom, 1990; Saarnak and Hansen, 1984; Crow1 and Malati, 1966) because of its importance in industry. The function of an additive in a pigment suspension is to ensure thorough dispersion, which is an essential requirement for many of its end uses (e.g., opacity, tinting strength, gloss development, brightness, durability, and economic usage of pigment in paints all depend on maximum dispersion of the pigment). The effect of an organic additive on the rheology of the Ti02 pigment suspension is also investigated in this study.
Materials and Methods The four Ti02 pigment samples used were supplied by Tioxide Chemicals Pty. Ltd. The pigments differ in their crystal size, particle shape, and coating composition. The first three pigments were obtained from the Australian Tioxide plant. One of the pigments is coated with alumina, and the other two are coated with a blend of alumina and silica. Their crystal sizes are reported (by Tioxide Chemicals) to be 180, 210, and 230 nm. The pigments have a density of 4 g/cm3 and an aspect ratio of approximately 1. Samples of the pigments were obtained from two stages of production: at the filter discharge and as the final product (see Figure 1). The fourth pigment was obtained from the UK Tioxide plant. This is an ultrafine pigment which is also coated with a blend of alumina and silica. Transmission electron microscopy was used to estimate the particle size and aspect ratio, which were found to be approximately 40 nm and 5, respectively. The density of this pigment is 3 gIcm3. Suspensions of various solids concentrations were prepared by adding triple distilled millipore filtered water to the pigment sample. Sonication was performed on the suspensions for 2 min with a high intensity sonic probe (Sonifier B30, 20 kHz, 350 W), creating a smooth flocculated suspension with a pH of around 7. Sonication helps to break up any large lumps or granules in the suspension. Once this is achieved (usually within the first 30-60 s of sonicating), there is little effect on the rheology
I\
1 0.1
\\\
1
100
1000
v pH=lO.l 10
1
Shear Rate (l/s) Figure 2. Viscosity versus shear rate for a 50 wt % solids pigment
suspension at several pH's (n = 0.2, 5 < K
< 30).
with any further sonication. After sonication the suspensions were dispersed with either concentrated HNO3 or KOH, followed by an additional 1min of sonication. All suspensions were rested for a t least a day prior to any measurements. The rheology of the suspensions was measured at increments in pH from 5 to 10. Suspension pH was adjusted by additions of either 5 M KOH or "03. After each addition the suspension was rested for 2 h and then stirred with a spatula before conducting rheological measurements. The limits for the pH range were determined from atomic absorption spectroscopy, which revealed that dissolution of alumina from the pigment coating would occur at pH < 4 and pH > 10. The steady shear viscosity was measured as a function of shear rate with a constant stress Bohlin Rheometer using the cone and plate geometry. A Matec Instrument MBS-8000 system was used for electroacoustic measurements. The Matec Instrument generated dynamic mobility data for each of the samples, from which the isoelectric point (IEP) was determined as the pH where the dynamic mobility is zero. Nitrogen absorption analysis was performed with a Micromeritics ASAP-2000 system. The results were used to evaluate a bulk particle density from the computed pore sizes. Particle size distributions of diluted dispersed samples of the pigments were measured with a Coulter counter. The Coulter counter uses laser diffraction in combination with polarization intensity differential scattering (PIDS) to measure the particle size (Allen, 1990).
Results and Discussion Figure 2 shows the viscosity as a function of shear rate for a typical pigment suspension as a function of pH. The suspension exhibits pseudoplastic or shear thinning behavior, which conforms to the Ostwald-de Waele power law model for a large shear rate range (Barnes et al., 19891, i.e., 7
= K+"
(1)
Figure 2 demonstrates that the suspension pH has a significant effect on the rheology. For the pH range studied it is observed that the consistency factor, K, exhibits sensitivity to the pH, while the flow index, n, remains essentially constant. This suggests a flocculated structure with varying strength throughout the pH range. Complete dispersion, which is generally indicated by Newtonian-
Ind. Eng. Chem. Res., Vol. 33, No. 10, 1994 2439
-z
1.6
1.4
a
1.2
? .0.
1.0
c,
0.8
v
u3 (d
3 8
0.6
.H
rJl
0.4
rJl
5
2
0.2
5
7
6
8
9
10
11
PH Figure 3. Viscosity at 50 s-l versus pH for a pigment at several solid concentrations (IEP = 7.9).
-
like behavior (n l),is not observed in most of the suspensions studied. The inability to achieve complete dispersion may be attributed to the relatively high ionic strength of the suspensions (Hunter, 1987). The average ionic strength of these suspensions has been calculated to be around 0.05 M; however, since the pigment samples come directly from the plant, it is expected that they contain impurities in the form of salts, which will lead to a considerably higher ionic strength than that calculated. Figure 3 is a graph of viscosity at 50 s-1 versus pH for solids concentrations ranging from 45 to 55 w t 5% solids. The results highlight the effect of pH illustrated in Figure 2 and illustrate the transition from a state of weak flocculation at low pH, to maximum flocculation a t a pH of 7.9, and then back to a state of weak flocculation a t high pH. This change in the degree of flocculation is attributed to a positive surface charge at low pH which decreases in magnitude as the pH is increased, until the point where there is zero net surface charge a t a pH of 7.9, after which a negative surface charge is developed, which increases in magnitude as the pH is further increased (Hunter, 1987; Yates, 1975). It has been demonstrated that the pH of maximum flocculation correlates with the isoelectric point, IEP (Leong et al., l990,1991,1993a,b). At the IEP the net surface charge density on the particle is zero so that there are no electrostatic repulsive forces to oppose the attractive van der Waals forces; hence the net force is a maximum attraction. Figure 3 therefore indicates that the suspension has an IEP of 7.9 which is independent of solids concentration, as theory predicts (Marlow et al., 1988). This value compares well with the IEP of 8.1 determined from electroacoustic measurements on the Matec instrument, as shown in Figure 4. Figure 4 is a graph of dynamic mobility versus pH, from which the IEP is determined as the pH at which the dynamic mobility is zero. Figure 5 shows the relationship between suspension viscosity and solids concentration. In this graph the maximum viscosity a t 50 s-1 is plotted against volume fraction for several pigments of similar coating compositions but different sizes. The reported crystal sizes of the pigments are indicated on the graph. This graph represents the condition of maximum flocculation where only the van der Waals attractive forces are present. Theory states that the van der Waals forces decrease with decreasing particle size (Hunter, 1987;Israelachvili, 1992); however, the results in Figure 5 suggest that the suspension viscosity increases with decreasing particle size. This can
#
-o.041 5
'
'
'
6I
'
'
' ' ' 7 ' '
'
' 8I :
* '
'
' 9I '
'
'
1
' 10
PH Figure 4. Dynamic mobility versus pH for the same pigment a~ in Figure 3 (IEP = 8.1).
- 1
i
z 2 a v In
e
1
(d
u VI
5
3
22 0.1
" " " " ' ~ " " " " ' ~ " " " " ' ~
2440 Ind. Eng. Chem. Res., Vol. 33, No. 10, 1994
a2
0 P l a t e a n d F r a m e Filter Discharge V P r e s s Belt Filter Discharge
h
0 Filter Discharge
2 a
v Final P r o d u c t
v
v
m
\
c,
d
0
m
s
'I
x
24 o . 1 " " ' " " " ' " " " ~ " ' ' " ' ' ' 0.10
0.15
0.20
0.25
10
0
R d e , 6
2
a, 0
s
4
VI
2
0 0.1
10
1
Particle Diameter ( u m ) Figure 7. Particle size distributions for the 210-nm pigment.
unexpected in that they revealed a significant difference in suspension viscosity for the same pigment at different stages of production. For example, at a volume fraction of 0.15 the viscosity of the filter discharge suspension is 0.56 Pa s compared to 0.30 Pa s for the final product suspension, a difference of 46 % In Figure 7 the particle size distributions for the plate and frame filter discharge and the final product are compared. Any differences between the particle size distributions can be largely attributed to the micronization process. It is important to note that the measurement of a "primary" particle size with the Coulter counter technique is virtually impossible because these particles exist predominantly as aggregatesrather than as single particles. An aggregate is defined as a compact cluster of several primary particles (Nelson, 1988; Parfitt, 1981). Transmission electron micrographs of some of the pigment samples confirmed the existence of aggregates. The size of these aggregates will vary quite dramatically with suspension conditions, which means that reproducibility can only be attained under identical conditions. Hence the particle size distributions are really only a qualitative description of aggregates rather than an accurate measure of primary particle size. Figure 7 shows that the final product has a much finer particlelaggregate size than the filter discharge, as expected. This would have the effect of increasing the suspension viscosity, according to the general trend depicted in Figure 5. However, Figure 7
.
0.20
0.25
Volume Fraction
Volume Fraction Figure 6. Maximum viscosity at 50 s-l versus volume fraction for the 210-nm pigment (alumina coating).
2
o.1~""""'"''"'"'""""'1 0.10 0.15
Figure 8. Maximum viscosity at 50 s-l versus volume fraction for the 230-nm pigment.
also shows that the final product has a much greater degree of polydispersity, which would have the opposite effect (Cheng et al., 1990). The lower suspension viscosity exhibited by the final product can therefore be partly attributed to ita greater degree of polydispersity. Nitrogen adsorption analysis revealed a minimal difference in porosity between the filter discharge and the final product samples (cf. 19% porosity for the filter discharge and 17% porosity for the final product). The average pore size was determined to be 10 nm, which is an indication that the pores are voids between primary particles within an aggregate, rather than microcavities on the surface of an individual particle (Gregg and Sing, 1982). Therefore the porosity results suggest that the aggregates formed in the final product suspension have a more compact structure than in the filter discharge suspension. This can be attributed to the greater degree of polydispersity in the final product sample, as well as the presence of more uniformly shaped particles (Le., irregularly shaped sintered particles and aggregates in the filter discharge would be broken down in the micronizer). Since a greater quantity of liquid is immobilized within the pores of the aggregates in the filter discharge suspension, the viscosity at a given volume fraction would be greater than that of the final product. By removing the porosity variable (i.e., using the bulk density instead of the true density to calculate volume fractions),it was found that the small difference in porosity could not account for the large difference in viscosity between the filter discharge and the final product suspensions. Figure 8 adds another perspective to the picture. In this graph the effect of the filtering device on the rheology of a second pigment is shown. The results illustrate that the product from the plate and frame filter discharge has a significantly greater suspension viscosity than the press belt filter discharge. For example, at a volume fraction of 0.15 the viscosity of the plate and frame filter discharge suspension is 0.53 P a s compared to 0.30 P a s for the press belt filter discharge suspension, a difference of 43 % It is important to note that both samples exhibit similar porosities (21 5% for the plate and frame filter discharge and 19% ' for the press belt filter discharge); and have very similar particle size distributions (see Figure 10). The only difference between these samples is the amount of compression they have experienced. This is direct evidence that the compression history will influence the shear rheology. Previous experience with both bauxite residue suspensions and coal tailing suspensions confirm this link
.
Ind. Eng. Chem. Res., Vol. 33, No. 10, 1994 2441 10
I
I
I
Final Product:V
@ via the Plate and Frame Filter
a
1
I
I
I
I
I
,
I
I
I
/
,
I
I
I
I
~
,
/
,
,
,
,
,
,
,
Final Product Final Product with TMP
v via the Press Belt Filter
v
o.l"'""""'""'"'"''~ 0.10 0.15
0.20
0.25
Volume Fraction Figure 11. Maximum viscosity at 50 s-1 versus volume fraction for the 180-nm pigment. 0 0
6\"
a
Plate and Frame Filter Discharge Press Belt Filter Discharge
t
i
e, P)
0
a
u
s
[II
0.1
1
10
Particle Diameter (um) Figure 10. Particle size distributions for the 230-nm pigment.
between compression history and shear rheology [Nguyen, 1983; de Kretser, 1993 (private communication)l. Both suspensions showed an increased resistance to flow after they had been compressed. The effect of compression can be understood from a consideration of the suspension structure. The main structural elements in a concentrated suspension are flocs, which are composed of a collection of aggregates (Nelson, 1988; Parfitt, 1981). In the flocculated state the flocs are interconnected to form a continuous three-dimensional network structure. It is proposed that subjecting the suspension to compressional forces will induce the formation of stronger and more extensive interfloc bonds, thus resulting in an increased viscosity. This explains the higher suspension viscosity of the plate and frame filter discharge with ita increased compression, relative to the press belt filter discharge. It is interesting to find that after drying and micronization both the plate and frame filter discharge and the press belt filter discharge exhibit about the same suspension viscosity. Shown in Figure 9 is the suspension viscosity of the final product, which is a combination of the results of a sample which has undergone plate and frame filtration followed by drying and micronization and one which has undergone press belt filtration followed by drying and micronization. The large difference in viscosity between the two samples at the filter discharge stage has now vanished, implying similar suspension structures in the final product. One really wonders why the plate and frame
filter is used since the structure induced needs to be broken down, which is not the case with the press belt filter. A comparison of Figures 8 and 9 reveals that the suspension viscosity of the final product is similar to that of the press belt filter discharge suggesting that the compressional forces applied to the pigment during press belt filtration are too small to impart any considerable additional structure. This'observation may provide the key to understanding the results in Figure 6. It is reasonable to propose that the difference in suspension viscosity between the plate and frame filter discharge and the final product for the first pigment illustrated in Figure 6 is primarily due to the high compressional forces exerted by the plate and frame filter, rather than the effect of any of the other processes following filtration. Additive type and concentration are also important process variables used in the industry to alter the rheological properties of the process material. In the manufacture of the Ti02 pigment a small amount of trimethylolpropane (TMP) is added to the system during the micronization stage to enhance the dispersibility of the pigment. Figure 11illustrates the effect of this additive on the rheology. Addition of 0.4% dwb (dry weight basis) of TMP reduces the viscosity by 39 % at a volume fraction of 0.15; or the volume fraction could be increased from 0.158 to 0.180 while maintaining the viscosity constant at 1.0 Pa s. The reduction in viscosity is due to steric hindrance, whereby close approach between pigment particles is prevented due to the layer of TMP molecules on the particle surface (Israelachvili, 1992). The increased distance between particles results in a reduction of the van der Waals forces, and consequently a weakened flocculated structure.
Conclusion The rheological properties of titanium dioxide pigment suspensions have been shown to be affected by pH, solids concentration, and particle size. The suspensions exhibit pseudoplastic behavior over the pH range from 4 to 10, indicating a flocculated stpcture. A graph of viscosity at 50 s-1 versus pH reveals a maximum viscosity which can be correlated with the isoelectric point. Electroacoustic data confirm the existence of the viscosity maximum at the IEP. The suspension viscosityat the IEP varies exponentially with solids concentration. A graph of maximum viscosity (viscosity at the IEP) at 50 s-l versus volume fraction for three pigments of different particle sizes demonstrates
2442 Ind. Eng. Chem. Res., Vol. 33,No. 10, 1994
that the suspension viscosity increases with decreasing particle size, an effectthat can be explained by the presence of a greater number of bonds per unit volume for the smaller particle sizes. The work demonstrates conclusively that compression history is an important variable in suspension rheology. Two identical pigments from different filtration devices were compared at the same concentration. The samples were obtained directly after the filtration process. It was found that the discharge from a plate and frame filter exhibited a significantly greater suspension viscosity than the discharge from a press belt filter at the same concentration expressed as either weight percent or volume percent. The difference in viscosity can be attributed to the higher compressional forces exerted on the pigment in the plate and frame filter relative to the press belt filter. Compression induces the formation of a stronger, more extensive flocculated structure which does not readily break down. In the pigment industry agglomeration of the particles is undesirable, and hence micronization is performed to disrupt this flocculated structure as a final stage in the process. It is demonstrated that the suspension viscosity of the final product sample is independent of the type of filter used in its production. The important conclusion is that the amount of work in micronization required to break down the suspension structure can be significantly reduced by the use of smaller compressional forces in the dewatering stage. The effect of a monolayer coating of organicon a pigment was shown to lower the suspension viscosity. This lowering of suspension viscosity is achieved through a reduction of the van der Waals forces by steric hindrance.
Acknowledgment Our work in suspension mechanics is supported by the Advanced Mineral Products Research Centre, a special research center funded by the Australian Research Council. Special thanks to Tioxide Chemicals Pty. Ltd. for their cooperation and support. Very special thanks to Professor T. W. Healy and Dr. P. J. Scales for their interest and help with this work. Literature Cited Allen, T. Particle Size Measurement, 4th ed.; Chapman and Hall: New York, 1990. Barnes, H. A,;Hutton,J. F.; Walters, K. AnZntroduction toRheology; Elsevier: Amsterdam, 1989. Cheng, D. C.-H.; Kruszewski, A. P.; Senior, J. R.; Roberts, T. A. The Effect of Particle SizeDistribution on the Rheologyof an Industrial Suspension, J.Mater. Sci. 1990,25,353. Crowl, V. T.; Malati, M. A. Adsorption of Polymers and the Stability of Pigment Dispersions. Discuss. Faraday SOC. 1966,142,301. De Guingand, N. J. The Behaviour of Flocculated Suspensions in Compression. M.S. Thesis, University of Melbourne, 1986.
Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press: London, 1982. Hulden, M.; SjBblom, E. Adsorption of Some Common Surfactants and Polymers on Ti02 Pigments. Prog. Colloid Polym. Sci. 1990, 82,28. Hunter, R. J. Foundations of Colloid Science I; Clarendon Press: Oxford, 1987. Israelachvili,J. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1992. Kuge, Y. Rheological Characterization of Paints. J. Coat. Technol. 1983,55, 59. Landman, K. A.; Sirakoff, C.; White, L. R. Dewatering of Flocculated Suspensions by Pressure Filtration. Phys. Fluids A 1991,3,1495. Leong, Y. K.;Boger, D. V. Surface Chemistry Effects on Concentrated Suspension Rheology. J. Colloid Interface Sci. 1990,136,249. Leong, Y. K.; Boger, D. V. Surface and Rheological Properties of Zirconia Suspensions. Trans. Znst. Chem. Eng. 1991,69,381. Leong, Y. K.;Katiforis, N.; Harding, D. B.; Healy, T. W.; Boger, D. V. Role of Rheology in Colloidal Processing of Zirconia. J.Mater. Process. Manuf. Sci. 1993a,I, 445. Leong, Y. K.; Scales, P. J.; Healy, T. W.; Boger, D. V. Effect of Particle Size on ColloidalZirconiaRheologyat the IsoelectricPoint. J. Am. Ceram. SOC.1993b,in press. Leong, Y. K.; Scales, P. J.; Healy, T. W.; Boger, D. V. Rheological Evidence of Adsorbate-Mediated Short-Range Forces in Concentrated Dispersions.J. Chem. SOC.,Faraday Trans. 1993c,89,2473. Leong, Y. K.;Boger, D. V.; Scales, P. J.; Healy, T. W.; Buscall, R. Control of the Rheology of Concentrated Aqueous Colloidal Systems by Steric and Hydrophobic Forces. J.Chem. SOC.,Chem. Commun. 1993d,7,639. Marlow,B. J.; Fairhurst, D.; Pendse, H. P. Colloid Vibration Potential and the Electrokinetic Characterization of Concentrated Colloids. Langmuir 1988,4,611. Metzner, A. B.; Whitlock, M. Flow Behaviour of Concentrated (Dilatant) Suspensions. Trans. SOC. Rheol. 1958,2,239. Nelson, R. D. Dispersing Powders in Liquids; Elsevier: Amsterdam: 1988. Nguyen, Q.D. Rheology of Concentrated Bauxite Residue. Ph.D. Thesis, Monash University, 1983. Patton, T. C. Fundamentals of Paint Rheology. J. Paint Technol. 1968,40,301. Parfitt, G. D. Dispersion of Powders in Liquids with Special Reference t o Pigments, 3rd ed.; Applied Science: Amsterdam: 1981. Rao, A. S.Rheology of Aqueous Dispersions of Alumina, Titania and a Mixture of Alumina and Titania Powders. J. Dispersion Sci. Technol. 1987,8, 457. Russel, W. B. Review of the Role of Colloidal Forces in the Rheology of Suspensions. J.Rheol. 1980,24,287. Saarnak, A,; Hansen, C. M. Adsorption on Pigment Surfaces. Polym. Mater. Sci. Eng. 1984,51,698. Strauss, H.; Heegn, H.; Stienitz, I. Effect of PAA Adsorption on Stability and Rheology of Ti02 Dispersions. Chem. Eng. Sci. 1993, 48,323. Yates, D. E. The Structure of the Oxide/Aqueous Electrolyte Interface. Ph.D. Thesis, University of Melbourne, 1975. Received for review March 7, 1994 Accepted June 22,1994 @
* Abstract published in Advance ACS Abstracts, August 15, 1994.
