Langmuir 1991, 7, 134Q-1343
1340
Rheological Properties of Silica Suspensions in Aqueous Cellulose Derivative Solutions. 1. Viscosity Measurements Masami Kawaguchi,' Yoshitaka Ryo, and Takehiro Hada Department of Chemistry for Materials, Faculty of Engineering, Mie University, 1515 Kamihama-cho, Tsu Mie 514, Japan Received June 25, 1990. In Final Form: November 2, 1990 Viscosities of nonporous silica suspensionsin aqueous hydroxypropylmethyl cellulose (HPMC)solutions have been investigated by using a coaxial cylinder rheometer as functions of both HPMC and silica concentrations and molecular weight of HPMC. The viscosities of silica suspensions strongly depend on the compositions of the mixtures. Newtonian flows are not observed even at the low shear rate. For higher concentrations of the high molecular weight HPMC, the silica suspensions show rheopexy behavior and their viscosities cross that of the HPMC solutions at a critical shear rate where the silica suspensions reinforced by adsorption of HPMC are broken. Above the critical shear rate, the viscosities are lower than that of the HPMC solution. The magnitude,of the critical shear rate increases with an increase in the HPMC and silica concentrations. In contrast, for low molecular weight HPMC, the viscosities of the silica suspensions exceed that of the HPMC solution over the entire shear rate range.
Introduction
out some rheological measurements of silica suepensions mixed with poly(ethy1ene oxide)s of molecular weights Fumed silica such as Aerosil is well characterized and (6-40) X lo3 and Heath and Tadrose have performed the widely used as an adsorbent for polymer adsorption measurements with poly(viny1alcohol) of molecular weight e~periments.l-~ When Aerosil silica is mixed with water, 45 X lo3. Most studies were focused on the flocculated the resulting silica slurry is known to show thickening and silicas by polymer bridging, and the rheological results gelation due to aggregation of silica particles. The gelashowed hysteresis behavior in the stress-shear rate curves, tion mechanism for the silica slurry is generally a ~ c e p t e d . ~ indicative of thixotropy. Recently, Otsubo and If such a silica slurry is mixing with aqueous polymer c o - ~ o r k e r s " extensively ~~ measured the rheological besolutions, polymer chains adsorb at the silicas and the havior of silica suspensions in aqueous solutions of polypolymer adsorption would lead to the flocculation of the acrylamides with molecular weights (2-5) X lo6. They silicas or the sterically stabilized silica suspensions. These have observed the irreversible increase in viscosity beyond phenomena are based on two contradictory conceptions, the critical shear rate and proposed an idea of shearbecause the flocculation arises from attractive forces induced bridging in which the flocculate-flocculate bond between silicas, whereas the steric stabilization occurs is formed by adsorption of polymer extending from one particle to a particle in the other flocculate under shear under the conditions where the interaction forces are flow. Moreover, they showed that the shear-induced repulsive. The steric interaction should strongly depend bridging was interpreted in terms of the percolation theory. on the adsorbed amounts of polymer chains and the Most of their data were somewhat inclined toward using polymer's molecular p eight.^ For example, long chains the relatively lower or higher molecular weight polymers are able to adsorb more easily on the surfaces of some as a flocculant. However, data of the rheologicalproperties colloidal particles than small ones. When the surface of the silica suspensions in aqueous solutions of polymers coverage with polymers is low, the long chains act as crosswith a molecular weight of less than 1 X lo6 do not seem linking agents. In contrast, at the high surface coverage to be sufficient. they play a role in stabilizing the silica particles due to The aim of this study is to relate the viscosity of the steric stabilization. silica suspensions in aqueous solutions, of hydroxypropyl In order to understand the stability of gellike silica methyl cellulose (HPMC) to the structures of silica aggregates by adding polymer chains, it is necessary to suspensions adsorbed by HPMC chains, in which the added investigate the rheological behavior of various contents of HPMC chains take the roll as a flocculant or a stabilizer, the disperse phase by changing the added amounts of silica as functions of concentrations of silica and HPMC samples. and polymer and the molecular weight of polymer. Some HPMC samples have molecular weights of (100-500)X systematic investigations of the rheology of aqueous Aerolo3. The viscosity measurements of the silica suspensions si1 suspensions have been reported in the 1iterature.&l3 were carried out with a coaxial cylinder rheometer. For example, Eisenlauer, Killmann, and Korn'have carried Experimental Section (1) Takahashi, A.; Kawaguchi, M. Adu. Polym. Sci. 1982,46, 1. Materials. Two HPMC samples of 65SH-4000and 65SH-50 (2) Cohen Stuart,M. A.; Ccegrove, T.; Vincent, B.Adu. Colloidlnterjace were kindly supplied from Shin-Etu Chemical Co., Ltd. They Sci. 1986, 24, 143. (3) Howard, G. J. In Interfacial Phenomena in Apolar Media; Eicke, were purified by precipitation of their aqueous solutions conH.-F., Parfitt, G. D., Eds.; Dekker: New York, 1987; p 281. taining 0.02 wt % NaN3into acetone and freeze-driedfrom their (4) Iler, R. The Chemistry of Silica; John Wiley & Sons: New York, aqueous solutions. The purified samples were stored in a 1978. desiccator with PzOs. The molecular weights of the samples were (5) Napper, D. H. Polymeric Stabilization of Colloidal Dispersions; Academic Press: London, 1983. (6)Eisenlauer, J.; Killmann, E. J.Colloid. Interface Sci. 1980,74,108. (7) Eisenlauer, J.; Killmann, E.; Korn, M. J. Colloid Interface Sci. 1980, 74, 120.
(8)Heath, D.; Tadros, Th. F. J. Colloid Interface Sci. 1983,93,320. (9) Otaubo, Y.; Umeya, K. J. Colloid Interface Sci. 1983, 95,279.
0743-7463191/ 2407-1340$02.50/0
(10) Otaubo,Y.; Watanabe,K.NipponReorojiGakkaishi 1986,14,82. Otaubo, Y. J. Colloid Interface Sci. 1986, 112, 380. (11) Otaubo, Y.; Watanabe, K. J. ColloidInterface Sei. 1988,122,346. (12) Otaubo, Y.; Watanabe, K. J. ColloidInterface Sei. 1989,127,214. (13) Otaubo, Y.; Watanabe, K. J. ColloidInterface Sci. 1989,133,491.
0 1991 American Chemical Society
5v
Table I. Molecular Characteristics of HPMC codes 65SH-4000 65SH-50
MI,x 10-8 403 107
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Langmuir, Vol. 7, No. 7, 1991 1341
Viscosity of Silica Suspensions
DS 1.8
MS 0.15
1.8
0.15
determined from the intrinsic viscosity measurements in aqueous 0.1 N NaCl solution at 25.0 f 0.05 OC using an Ubbelohde viscometer." The molecular characteristics, such as molecular degree of substitution (DS) of methyl group, and weight (MI,), molar substitution (MS) of hydroxypropyl group are summarized in Table I. The nonporous Aerosil 130 silica (Degussa A. G., West Germany) with a surface area of 141 m2/g, the particle diameter of 16 nm, and the silanol density of 3/100 A2 was used as recieved. It was dried in a vacuum oven at 200 OC.ls Water purified by a Millipore Q-TM system was used. Puregrade quality NaNs was used as a preservative for HPMC samples. Silica slurry was prepared by dispersing the silica powder in water by mechanically shaking and ultrasonic irradiation. An aqueous HPMC solution with known concentration was added to the resulting silica slurry and the mixtures, hereafter called as the silica suspension were mixed well by mechanically shaking for 1-2 weeks and by further ultrasonic irradiation to obtain a homogeneous mixture. The amounts of added silicasand HPMC are expressed as weight percent in the finalmixtures. To prevent the degradation of HPMC in aqueous solutions by bacteria, a preservative, NaNs, is added to keep its concentration at ca. 0.02
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Figure 1. Double-logarithmic plot of viscosity of the 7.5% silica slurry in water as a function of shear rate.
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Since the silica suspensions are turbid, their stabilities were roughly estimated from only the eye-observation of how long the silica suspensions are suspended without any sedimentation of silica particles after homogeneous mixing. From such an observation, we classified the silica suspensions into three cases: (1)a superstable suspension in which no sedimentation of silica is observed over 2 months; (2) a stable suspension in which no sedimentation of silica is observed for less than 2 months and more than 1 month; (3) a semistable suspension, which is separated into sedimented silicas and supernatant solution less than 1month; (4) no suspension in which silicas are sedimented soon after mixing. Thus, we used the first, second, and third categories for the rheological measurements. Rheometer. The rheometer apparatus was a MR-3 Soliquid Meter produced by Rheology Co., Ltd.(Kyoto Japan). Viscosity measurements of the silica suspensions were performed using a coaxial cylinder geometry. The outer and inner diameters of the cylinder are 39.90 and 36.97 mm, respectively, and the immersion length is 8.97 mm. The steady-flow measurements were carried out in the range of 3 X 10" to 1.48 X lo2s-l. The temperature of the sample chamber was maintained at 27 f 1OC. As far as known, the silica suspensions were subjected to preshearing at the highest shear rate of 1.48 X lo2s-l for 5 min to destroy some structures of the silica suspensions before the measurements were carried out. Viscosities were obtained as the ratio of shear stress to shear rate.
0
10
20 T (min)
30
Figure 2. Semilogarithmic plots of shear stress of the 5.0% silica suspension in aqueous 1.5%65SH-4000solution at various shear rates suddenly depressed after preshearing of the highest shear rate of 1.48 x 102 s-l as a function of time: 0,1.48 X 10-1 S-'; Q7.4 X 10-l S-l; A, 1.48 S-'; 0 , 7.4 8-l; m,74 S-l; A, 1.48 X loa S-1.
(14) Kato, T.; Tokuya, T.; Takahashi, A.KoubunshiRonbunshu 1982,
obtained for other silica-water systems.B-sJ6On the other hand, the viscosities of 2.5% and 5% silica slurry in water were small and their reproducibility was not good. Figure 2 shows plots of the shear stress for a superstable suspension of 5.0% silica in aqueous 1.5% 65SH-4000 solution as a function of time when the shear rate is changed suddenly from a high value of 1.48 X lo2 s-l to a considerably low value. As seen from the figure, the shear stress increases sigmoidally with an increase in time and finally reaches a plateau value. The lower the shear rate is, the longer time dependence the shear stress shows. This behavior is called rheopexy. Since the preshearing at the high shear rate should assure breakage of some structures such as the sterically stabilized silica suspensions induced by adsorption of HPMC, the gradual increase in viscosities is probably attributed to the recovery or re-formation of the aggregated structures of the silica suspensions by shear force. Time dependence of the shear stress became weaker by decreasing both the silica contents and initially added concentrations of HPMC. Rheopoxy was also observed for the suspensions of 5 and 7.5% silicas in aqueous 1.0 and 1.5% 65SH-50 solutions, but the time interval of an increase in shear stress was much shorter than those of the silica suspensions in aqueous 65SH-4000 solutions. The difference in the time dependence of the shear stress
(15) Kawaguchi, M.; Chikazawa, M.; Takahashi, A. Macromolecules 1989,22, 2195.
(16) Umeya, K.; Isoda, T.; Ishii, T.; Sawamura, K. Powder Technol. 1969/1970,3, 259.
Results and Discussion Viscosity of the silica slurry of 7.5% in water is shown as a function of the shear rate in Figure 1. The viscosity decreases with an increase in the shear rate and it does not attain a constant value in the shear rate ranges measured. Therefore, the silica slurry does not behave as a Newtonian flow. This flow behavior should be related to the gelation character of fumed silicas suspended in water. The pH value of the silica slurry was determined to be ca. 7.0, and this high pH value is enough to induce the formation of gel structure by aggregation of silica particles in concentrated silica di~persions.~J3 Similar results were 39, 293.
1342 Langmuir, Vol. 7, No. 7, 1991
Kawaguchi et al.
5
L
0
c
10' 1
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lo2
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Figure 3. Double-logarithmic plote of viscosities of the silica suspensions in aqueous 0.5% 65SH-4000solution as a function of shear rate: [3,5.0%silicasuspension;0,7.5% silica suspension. Filled circle indicates the viscosity of the aqueous 0.5% 65SH4000 solution. between the silica suspensions containing 65SH-4000 and 65SH-50 will be discussed later by taking into account the polymer adsorption behavior. The rheopoxy behavior was observed for other suspensions in polymer solutions,where the suspended solids were not spherical but elliptical in their shape.17J8 The viscosities of the silica suspensions in aqueous 0.5% 65SH-4000 solution at different silica concentrations, which are semistable, are plotted versus the shear rate in Figure 3. The viscosity at each shear rate in the figure correspondsto the plateau value, which reaches a constant value independent of time after the preshearing of the highest shear rate of 1.48 X lo2 s-l. For comparison, the viscosity of the disperse medium, i.e., the aqueous 0.5% 65SH-4000 solution, is also displayed; the HPMC concentration is more than 3 times higher than the threshold concentrati~nl~ at which 65SH-4000 chains begin to penetrate each other. The curves of the viscosities for the 5%and 7.5 % silica suspensions in the aqueous 0.5 % 65SH4000 solutions are similar in shape to that of the silica slurry. The magnitude of the viscosity for the silica suspension is larger than that for the silica slurry due to the formation of cross-linkage by adsorption of HPMC. In Figure 4, the viscosites of silica suspensions with different silica concentrations in aqueous 1.0% 65SH-4000 solution, which are superstable, are plotted as a function of the shear rate. The viscosity curves for the 5% and 7.5% silica suspensions cross the viscosity curve of the corresponding dosage HPMC solution at around a shear rate of 7 s-l. In contrast with the 5% and 7.5% silica suspensions, the viscosities of the 2.5 % silica suspension are somewhat lower than those of the disperse medium in the entire ranges of the shear rate. These lower viscosities may result from both the nonformation of gellike structures of the silica suspension because of the low silica contents and the decrease in viscosity of the disperse medium due to the adsorption of HPMC. Above a shear rate of 20 s-l, all viscosities of three silica suspensions at a constant shear rate decrease with an increase in silica concentration. In contrast, below a shear rate of 7 s-l silica concentration dependenceof the viscosity is quite the reverse. Similar silica concentration dependence was observed for the silica suspensions in aqueous (17) Mikami, Y.; Mataumoto, T.; Onogi, S. Nihon Reoroji Cakkaishi
1976, 4, 86.
(18)Onogi, S.; Mikami, Y.; Mataumoto, T. Polym. Eng. Sei. 1977,17, I.
(19) De Gennes, P.-G. Scaling Concepts in Polymer Physics; Cornell University Press: Ithaca, N.Y., 1979.
-
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Figure 5. Double-logarithmic plots of viscosities of the silica suspensions in aqueous 1.5% 65SH-4000solution as a function of shear rate. Filled circle indicates the viscosity of the aqueous 1.5% 65SH-4000solution. Symbols are the same as in Figure 4. 1.5%65SH-4000 solution, which also are extremelystable, as shown in Figure 5. As seen from Figure 5, all viscosity curves cross that of the aqueous 1.5'% 65SH-4000 solution at higher shear rates than the case of the 1.0% concentration of 65SH-4000. Regardless of the lowest silica contents, the viscosity of the 2.5 '% silica suspension in the 1.5 % 65SH-4000 is larger than that of the corresponding disperse medium below the shear rate of 20 5-l. Thisshould stem from the large adsorbed amounts of 66SH-4000. However, the adsorbed amounts of 65SH-4000 on the silica surfaces could not be determined since the silica suspensions were not sedimented by centrifugation. This supporta that the silica suspensions are stabilized by 65SH4000 chains adsorbed. While for the fourth category suspension, which corresponds to almost flocculated suspensions, we could determine the adsorbed amounts of HPMC.20 The shear rate at which the silica concentration dependence of the viscosity becomes the reverse increaees with increasing the HPMC concentration. Such a shear rate may correspond to a critical shear rate where the stabilized silica suspensions will be broken. Above the critical shear rate, the structural transition of the silica suspensions from a solidlike structure, i.e., and aggregated silica structure to a structure without aggregation of silica or a structure with less aggregation of silica, will occur if the shear rates were increased. In other words, above the critical shear rate the viscosities of the silica suspensions (20) Kawaguchi, M.; Ryo, Y. Unpublished data.
Viscosity of Silica Suspensions
5 n
.-% l
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Figure 6. Double-logarithmic plots of viscosities of the silica suspensions in aqueous 1.0% 65SH-50 solution as a function of shear rate. Filled circle indicates the viscosity of the aqueous 1.0% 65SH-50 solution. Symbols are the same aa in Figure 4. may be governed by the viscosity of the medium after the adsorption of HPMC since they decrease with an increase in the silica amounts added. In contrast, below the critical shear rate the aggregated silicas stabilized by adsorption of HPMC will be preserved even if some deformations of the silica suspensions should be induced by shear flow. In fact, above the critical shear rate the rheopexy behavior was not observed. This may mean that the aggregated silica structure in the silica suspensionwas once destroyed and could not be recovered. In Figure 6, the viscosities of the silica suspensions in aqueous 1.0% 65SH-50solution, which belong to the stable suspension class, are plotted double logarithmically versus the shear rate as a function of amounts of silica. For comparison, the viscosity of the corresponding 65SH-50 solution of disperse medium is also displayed and it is smaller than the viscosities of the silica suspensions throughout the entire shear rate range. Namely, there is not any critical shear rates as observed for the silica suspensions in the 65SH-4000 solutions described above. The viscosity of the 7.5% silica suspension is almost the same as for the corresponding slurry. The enhanced increase in the viscosities of the silica suspensions can be realized by higher dosage concentrations of 65SH-50, as shown in Figure 7. The silica suspensions in aqueous 1.5% SH65-50solution are extremely stable and this means that polymer adsorption is more effective for the stabilization of the silica suspensions than that for the 1.0% solution. In the figures, except for the 7.5% silica suspensions, the viscosities reach a constant value at the higher shear rate. Similar results were obtained by Otsubo et al., who have observed that the silica concentration at which the viscosity of the silica suspension exceeds that of the disperse medium becomes lower with a decrease in the molecular weight of polyacrylamide; the viscosities of the silica suspensions have been almost constant at the higher shear rate, while at higher silica concentrations they suddenly showed dilatant behavioral0
Figure 7. Double-logarithmic plots of viscosities of the silica
suspensions in aqueous 1.5% 65SH-50 solution aa a function of shear rate. Filled circle indicates the viscosity of the aqueous 1.5% 65SH-50 solution. Symbols are the same as in Figure 4.
The reason why the critical shear rate was not observed for the silica suspensions in aqueous 65SH-50 solutions may be attributed to the difference in the mechanical strength of the silica suspensions for preshearing: since adsorption of a low molecular weight HPMC leads to a thin and tight thickness on the adsorbing surface, which mainly serves for the steric stabilization of the silica suspensions, and the aggregated structures of the silica suspensions in aqueous 65SH-50 solutions are not sufficiently destroyed by preshearing. This fact shows that 65SH-50 does behave more effectively as a steric stabilizer of silica suspensions than 65SH-4000, regardless of the low molecular weight. However, the silica suspensions mixed with aqueous 65SH-50 solutions of concentrations less than 1.0% were unstable and the silicas were sedimented.
Conclusions HPMC samples with molecular weights of (1OO-5OO)X 103 change the rheological properties of silica suspensions by their adsorption. For the high molecular weight HPMC sample, the transition in structure of the silica suspension from a solidlike to a liquidlike substance by shear flow was observed and the critical shear rate at which such a transition occurs increases with an increase in HPMC concentration. This should be attributed to the large adsorbed amounts of HPMC. Therefore, the solidlike behavior of the silica suspensions in the aqueous HPMC solutions substantially stems from the gellike matter of the silica slurry reinforced by adsorption of HPMC. On the other hand, the low molecular weight HPMC sample, there is not a critical shear rate and the viscosities of the silica suspensions increase with increasing concentrations of added silica and polymer. Registry No. HPMC, 9004-65-3; silica, 7631-86-9.
