Thermally Induced Reversible Coagulation in Ceramic Powder-Polymer Liquid Suspensions

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  • Thermally Induced Reversible Coagulation in Ceramic Powder-Polymer Liquid Suspensions

    Jeffrey A. Horn*.+ and Burton R. Patterson* Department of Materials and Mechanical Engineering, The University of Alabama, Birmingham, Alabama 35294

    It was observed that slurries of oxide powders in oxidized polybutene fluids can be caused to change reversibly be- tween fluid, nearly Newtonian behavior and plastic behav- ior by modest changes in temperature. This phenomenon was believed to result from changes in the dispersion vs association among the particles. The rheological effects of temperature, polymer oxidation, and particle size were ob- served for 30 vol% slurries of Ti02, A1203, and ZrO, pow- ders in polybutene fluids. Elasticity (in oscillation) and low- shear-rate viscosity (in steady shear) were observed to increase with increasing temperature for TiO, and A1203 particles in oxidized polybutene fluids. This behavior was attributed to the creation of interparticle structures. The attainment of this structure on heating was observed to be inhibited by increased oxidation of the polymer and in- creased particle size. It was concluded that the adsorption of oxidized molecules from the polymer liquid, along with the high viscosity of the bulk polymer, resulted in suspen- sions that were metastable against coagulation. Increased temperature resulted in lower viscosities of the liquid, al- lowing coagulation on a short time scale. The presence of the adsorbed polymer, however, prevented intimate contact among the particles so that the coagulated structure was easily destroyed upon subsequent cooling and shearing.

    I. Introduction

    HE control of interparticle interactions in powder-liquid T suspensions is central to the use of these materials in a vast number of important technologies. The unique properties of inks and paints, precursor materials for certain manufacturing processes, and some intelligent materials systems are due in large part to the nature of the interactions among dispersed particles in liquid media.

    Much recent work has concerned the control of short-range interparticle potentials in slurries of ceramic powder for use in novel processing techniques. It has been shown that the prop- erties of slurries having a short-range barrier to flocculation make them amenable to certain forming operations.- A re- pulsive barrier due to adsorbed species such as water, polymer chains, or fatty acids creates the situation in which the particles are loosely coagulated in a continuous, nontouching network. The plastic nature of these systems could be described as more solidlike than fluidlike, enhancing shapability.

    D. J. Shanefielb-contributing editor.

    Manuscript No. 192084. Received January 17, 1996; approved January 16, 1997. Presented in part at the 97th Annual Meeting of the American Ceramic Society,

    Cincinnati, OH, May 1, 1995 (Paper No SXX-16-95). Based on work performed by J. A. Horn in partial fulfillment of the requirements

    for the Ph.D. degree in materials engineering. The Universitv of Alabama. Birming- ham, AL, 1995.-

    I - - Supported by the Alabama DOUEPSCoR Traineeship Program. Member, American Ceramic Society.

    Present address: The Pennsylvania State University, 147 Research Building West, University Park, Pennsylvania 16802-6809.

    It has also been found that suspensions that can be changed reversibly between fluidlike and solidlike states by external stimuli are useful in intelligent materials systems. In electro- and magnetorheological suspensions, applied potentials or magnetic fields are used to alternate the properties of the ma- terial between the viscous behavior of a fluid and the elasticity of a solid.8-1 The changes in properties result from changes in the extent and/or nature of interparticle association. These kinds of materials are being used in applications such as clutches, valves,8 shock absorbers,2 robotics, and variable- stiffness catheter^.'^ They have been highlighted recently in the popular scientific literature as being important inkey tech- nologies of the future.14

    The purpose of this study was to investigate a novel phe- nomenon observed recently in certain powder slurries. The rheological properties of titanium oxide, aluminum oxide, and zirconium oxide powders in polybutene fluids were observed to change in an anomalous and reversible manner with changes in temperature. The low-temperature properties were found to be characteristic of a well-dispersed, fluid system, while the high- temperature properties were much more plastic, resisting flow at low shear rates (or low shear stresses). With increasing tem- perature the following observations were made: an increase in the low-shear-rate viscosity; the establishment of a yield point; and a decrease in the low-strain oscillatory phase angle. Each of these changes was found to be reversible with a subsequent decrease in temperature and continued shear. These observa- tions indicate the reversible induction of a stress-bearing struc- ture in the slurries, controlled by temperature. This phenom- enon is analogous to the effects discussed above in which interparticle structures are controlled by chemical and electro- magnetic means. It was desired to identify the fundamental mechanisms responsible for the thermally controlled proper- ties, as well as the variables that might be used to control the magnitude of these effects.

    The best applications for the thermally controlled properties of these slurries have not yet been identified. Since the general properties of these systems are comparable to those of materi- als used in powder processing techniques and intelligent ma- terials systems, it is believed that there are potential applica- tions in these areas.

    11. Experimental Procedure

    (1) Mateh l s The materials used in this study, along with the properties

    reported by the respective suppliers, are shown in Table I. The Ti02 and A1203 particles were mostly angular in shape and likely contained some agglomerates. The ZrO, particles were slightly smoother, and appeared to be aggregates of polygo- nally shaped primary units. While particle shape and size dis- tibution are known to have profound effects on suspension rheology,IJ those effects were not the focus of the present study. Particle sizes were only compared among the Al,03 powders, all prepared by similar means and having similar properties except for size. The polybutene fluids in which the powders were suspended were low molecular weight liquids,

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  • 1790 Journal of the American Ceramic Society-Horn and Patterson Vol. 80, No. 7

    Table I. ExDerimental Materials Ceramic powder or Median powder size Supplier

    polymer liquid or Polymer M. and trade name

    TiO, 3 pm Alfa-Aesar 41203 0.5 pm Alcoa, XAlOOO A1203 1.5 p n Alcoa, XA1520

    3.5 km Alcoa, XA4OOO 1 krn Magnesium Elektron,

    A1203 zfl2

    SC105 As-received 660 Amoco, H35

    Oxidized polybutene 660 Amoco, H35 Panalane 320 Amoco, L14E Panalane 1340 Amoco, H300E

    polybutene

    with a small percentage of the bonds on each chain being unsaturated.I6 Each molecule is expected to have an unsaturat- ed double bond at the chain end. A few (less than about 2%) of the other groups along each molecule are expected to be con- jugated dienes. Panalene is an Amoco polymer product that is similar in structure to polybutene except that it is hydrogenated so that virtually no double bonds remain.18

    The term oxidized is used here for polybutenes that were kept in a recirculating air oven for 5 days at 100C. Oxidation was recognized by a change in color from transparent to bright yellow. Absorbance of visible light in the yellow fluid was measured at wavelengths near 300 nm using a UV-visible spectrophotometer. FTIR analyses confirmed the presence of carbonyl (C=O) groups in the oxidized polymers by a broad absorption band that peaked at 1735 cm-l.19 The infrared spec- tra of as-received and oxidized polybutenes are shown in Fig. 1.

    Molecular weight distributions of the polybutene were mea- sured using gel permeation chromatography. Table II contains a summary of the GPC data, including the effects of the oxi- dizing heat treatment. The average molecular weight of the H35 was observed to increase slightly, with the M JM,, disper- sity slightly decreasing, for increasing times at 100C. This effect is believed to be due to the volatilization of low molecu- lar weight constituents, confirmed by weight loss during long- term heating. (2) Sample Preparation

    Slurries were prepared by hand stirring the powder (always in the as-received condition) into the polymer and allowing the mixture to stand overnight at room temperature. The samples were mixed with a spatula in 30 mL amounts in 100 mL plastic cups. They were stirred for approximately 5 min for the initial mixing, then for 1 min immediately prior to rheological testing.

    2000 1750 1500 1250 1 000 Wavenumber (l/cm)

    Fig. 1. lTIR spectra for H35 polybutene in the as-received and oxi- dized conditions. The spectrum for the oxidized polybutene shows a broad absorption peak near 1735 cm-I.

    Table II. GPC Results for Amoco H35 Polybutene Day at 100C M 3 1040 1.69 5 1050 1.68

    10 1090 1.64

    The chosen powder loading for most of the slurries was 30 ~01%. This concentration was found to provide the range of rheological behavior desired for this study with modest changes in temperature. (3) Rheohgical Testing

    Rheological measurements were made on a controlled-strain rheometer fitted with parallel plates (RFSII, Rheometrics, Pis- cataway, NJ). The standard testing procedure was as follows:

    (1) The sample was loaded onto the rheometer plates at room temperature.

    (2) An insulating cover was placed around the plates and the temperature was lowered to 5OC. During this time, dry N, was passed across the sample at a low flow rate to displace moist air and minimize water condensation.

    (3) Data were obtained under the following conditions (in order): (1) increasing oscillatory frequencies from 0.2 to 100 rads at 1% strain; (2) increasing strain amplitude from 1% strain to 100% strain at 10 rads; and (3) decreasing rates of steady shear from 12.5 s-l (or the highest rate possible within the torque specifications of the rheometer transducer) to 0.05 s-l.

    (4) The temperature was increased, allowed to equilibrate at the next value (constant within approximately 0.2C), and the above sequence of tests was repeated.

    The order of the rheological tests was designed so that any structure within the samples was maintained during the initial measurements and progressively disrupted by increasing shear strains. The limits of torque sensitivity and strain resolution in the equipment precluded testing within the linear strain limit of the samples. All of the rheological parameters reported are nonequilibrium values, intended to demonstrate general trends in the structure of the slurries.

    For the steady shear measurements, each shear rate was maintained for 10 s before data acquisition and for 10 s while data were acquired and averaged. This equilibration time was enough for the response of the samples to reach nearly steady state for the time scale of each rheological measurement (10 s) at each shear rate and temperature.

    The rheometer plates were covered with 120 grit, adhesive- backed grinding paper in order to inhibit slip near these sur- faces. It has been observed that the particle concentration in a suspension is often lower near flat testing surfaces than in the bulk of the sample.20 This situation causes an apparent slip due to low torque transfer to the measuring surface. Also, for the high-viscosity samples in this study, actual macroscopic slip ping can occur between the sample and the plates. The pres- ence of the grinding paper inhibits both of these effects by providing an irregularly shaped surface adjacent to the sample.? The particle concentration is more uniform across the thickness of the sample and the sample is gripped more se- curely than if the testing surfaces were smooth. This testing apparatus was observed to provide significant protection against slip in a previous study of coagulated slurries of ce- ramic powders in water.22

    III. Results and Discussion

    ( I ) Unfilled Polybutene The rheological data for the unfilled polybutene (Amoco

    H35) at 20, 50, and 70C are shown in Figs. 2 and 3. The polybutene, whether oxidized or not, exhibited essentially Newtonian behavior with no yield point (Fig. 2). Oxidation of the polymer caused slight increases in viscosity at each tem- perature due to the loss of low molecular weight constituents

  • July 1997 Thermally Induced Reversible Coagulation in Ceramic Powder-Polymer Liquid Suspensions 1791

    85.00 -

    - h

    0 Q

    Q,

    75.00 - - 5" a

    n

    ~

    8 65.00 - c

    55.00

    -

    >

    0.10 -A- 70.C

    I I I I I ~ I I I I I I I I I I I I I I I I I I I I I I

    0.01 0.10 1 .oo 10.00 100.00 Shear Rate (l/s)

    Fig. 2. Viscosity as a function of shear rate for H35 polybutene at three temperatures. Closed and open symbols represent as-received and oxidized polybutene, respectively.

    95.00 I

    Ternperalum

    +- 20C + 5o*c

    70C

    I I I I I l l 1 1 I I I I - r r r r

    10.00 100.00 I 45.00 ~-

    1 .00 Frequency (rads)

    Fig. 3. Phase angle as a function of frequency for H35 polybutene at three temperatures. The data shown are for as-received material. Data for oxidized polybutene coincide.

    during prolonged heating, but the Newtonian nature of the material was unaltered. The viscosity of the polymer decreased with increasing temperature, but remained essentially Newto- nian at all temperatures. A slight degree of shear thinning was observed at low shear rates and high temperatures, although the reason for this is not yet known. The magnitude of the viscosity change in this region, however, is very small in comparison to the changes observed in the powder-filled slurries described in Sections III(2) and III(3). The phase angles, measures of the viscous modulus relative to the elastic modulus, were observed to be near 90" for the unfilled polymer, indicating viscous rather than elastic behavior. This behavior indicates that there was no significant association among the molecules (or "flow units") in the polymer at any tem~erature.,~

    (2) TiO, Slurries in As-received Polybutene Figures 4 and 5 show the rheological data for a 30 vol%

    slurry of TiO, powder in as-received H35 polybutene at tem- perature intervals between 5" and 70C. The slurry was ob- served to be shear-thinning with a yield point at each tempera- ture (Fig. 4). This behavior is much different from that of the unfilled polybutene. The data suggest that there was a sign& cant amount of association among the flow units of the slurry, forming a structural network. It is noted that the maximum attainable shear rate for this slurry without overloading the torque capacity of the rheometer was approximately 2 s-'. It should also be kept in mind that while measurement errors caused by slip were minimized by the use of abrasive paper on the rheometer plates, there is still the likelihood of a nonlinear velocity profile across this viscoelastic sample.21 The steady shear data are best viewed as illustrating qualitative trends in the behavior of the slurry, rather than quantitative changes in viscosity.

    On heating, the viscosity of this slurry at each shear rate was observed to decrease slightly, though not in an entirely sys- tematic manner. The slopes of the curves in the viscosity plot (Fig. 6) are all in the range -0.9 to -1.0, indicating a similar degree of shear-thinning at all of the temperatures. A slope of -1.0 also indicates that flow occurred at a constant stress in- dependent of the shear rate, characteristic of a material with a yield point.24

    The phase angles measured in oscillatory testing confirm that the flow units of this slurry were strongly associated. Since a phase angle of 90" indicates completely viscous flow, and 0" completely elastic response, these frequency sweeps reveal be- havior that is intermediate between viscous and elastic but tending to be more elastic (

  • 1792

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    E 20.00

    s al cn -

    3

    0.00 I I I I l l l l

    Journal of the American Ceramic Society--Horn and Patterson Vol. 80, No. 7

    I I I I l l l l I I I I l l l l l

    Tempamtun

    6'C

    Fig. 5. Phase angle as a function of frequency for a 30 vol% TiO, suspension in as-received H35 polybutene at several temperatures.

    intervals from 5" to 70C are shown in Figs. 6 and 7. For this slurry, the nature of the rheological behavior was observed to depend strongly on temperature. At 5C the slurry exhibited nearly Newtonian behavior, having no yield point (Fig. 6), viscosity only slightly decreasing with increasing shear rate and phase angles near 90" (Fig. 7). With increasing tempera- ture, however, the behavior was observed to become more plastic. A yield point was apparent above 50C. and the slope of the viscosity-shear rate curve in Fig. 6 was observed to change from -0.1 at 5C to -0.9 at 50" and 70C. Accordingly, the phase angles obtained from the frequency sweep (Fig. 7) were observed to decrease with increasing temperature, indi- cating increased elasticity. At 70C the phase angles were found to approach 45" at some frequencies.

    10000.00 r----lI

    1000.00 h

    '? h .- 0 8 8

    v

    100.00

    10.00 1 4 0.01 0.10 1 .oo 10.00 100.00

    Shear Rate (lls)

    Fig. 6. Viscosity as a function of shear rate for a 30 ~ 0 1 % TiO, suspension in oxidized H35 polybutene at increasing temperatures. Open circles represent 5C data after heating to 70C and cooling.

    100.00

    3 80.00 s al al ul - 2 3

    60.00

    40.00

    /p +- 10% I I I I I I I I I I I I I I l l 1

    0.10 1 .oo 10.00 100.00 Frequency (rads)

    Fig. 7. Phase angle as a function of frequency for a 30 vol% TiO, suspension in oxidized H35 polybutene at increasing temperatures. Open circles represent 5C after heating to 70C and cooling.

    It is especially notable that, while the high-shear-rate vis- cosity of this slurry was found to decrease with increasing temperature, the low-shear-rate regime displayed an increase in viscosity. This increase in the low-shear-rate viscosity was in accord with the establishing of a yield point in the material. In other words, the slurry was observed to stiffen upon heating, undergoing a transition from fluidlike to solidlike behavior at low shear rates.

    The heat-induced stiffening was observed to be reversed by a decrease in temperature and continued steady shear. As shown in Fig. 6, the viscosity was the same after heating and cooling as before heating. With repeated thermal cycling and shear, the high-temperature properties were observed to be- come more Newtonian than plastic (i.e., the low-shear viscosi- ties were reduced). In this sense, the reversibility of the viscous to plastic transformation was dependent on the history of the sample.

    The "reversible" stiffening behavior was observed macro- scopically in the 100 mL cup in which the slurry was mixed. At room temperature the slurry was pourable from the cup or from a lifted spatula. The general appearance of the material at this temperature was similar to that of latex paint. After heating in an oven, however, the slurry was no longer pourable and could be molded into shapes independent of the walls of the con- tainer. In this condition, the material had an appearance similar to that of whipped cream. The stiffened condition was observed to remain as the slurry cooled until it was subsequently sheared, under which action it regained its fluid properties.

    (B) Probable Mechanism for the Observed Effects: The changes in the rheological properties on heating the slurry in oxidized polybutene are consistent with a change in structure from discrete, dispersed flow units suspended in the liquid to a continuous network of associated units throughout the material. Similar to the unfilled polymer, the slurry in oxidized polybu- tene at 5C was found to be nearly Newtonian, indicating very little association among the flow units. At higher temperatures, however, the behavior was more like that of the slurry in as- received polybutene, showing a yield point, shear-thinning, and significant elasticity. Since the rheology of the unfilled poly- butenes, oxidized or not, did not indicate intermolecular asso- ciation on heating, it was assumed that the structure created in

  • July 1997 Thermally Induced Reversible Coagulation in Ceramic Powder-Polymer Liquid Suspensions 1793

    the slurry of powder and oxidized polybutene resulted from either polymer-particle or particle-particle interactions. It is believed that the latter were the primary cause for structure formation, due to van der Waals attraction between the oxide particles. As discussed below, polymer-particle interactions were enhanced by oxidation of the polymer, and resulted in stabilization against an elastic network. The conclusion is that structure formation was enhanced by particle-particle interac- tions, and inhibited by polymer-particle interactions.

    The most striking difference in the behavior of the slurries in oxidized vs as-received polybutenes was observed at the lower temperatures, i.e., the Newtonian flow of the oxidized systems in contrast to the strong shear-thinning of the as-received. This difference can be attributed to the adsorption of the oxidized polymer. Prior to oxidation, the polybutene molecules were nonpolar and had no specific affinity for the surfaces of the oxide particles. They did not readily adsorb, leaving the par- ticles to agglomerate in intimate contact. The creation of car- bony1 groups (C=O) on the polymer chains, however, intro- duced points of polarity with the oxygen end of each group having a partial negative charge and the carbon a partial posi- tive charge. The use of oxidation to create polar groups on stabilizer molecules is well known in surfactant and dispersant technology, e.g., fish to enhance adsorption onto pow- ders through acid-base interactions.26 The most probable type of interaction for the oxidized polybutenes and oxide powders would be one in which the carbonyl groups of the polymer behave as bases (or electron donors), associating with acidic sites on the surface of the oxide particles. Carbonyl groups, which are the primary functional groups in these polymers, are expected to behave as bases.27 The surfaces of oxide particles usually contain both acidic and basic sites,28 so the adsorption of basic groups depends on the number concentration of acidic sites on the powders as well as the acid strength of those sites.29 The adsorbed layer of oxidized polymer provided a barrier to interparticle coagulation that was not present in the slurries containing as-received polymer. The Newtonian behavior of the slurries in oxidized polybutenes at lower temperatures re- sulted from an apparent stabilization achieved by adsorption of the polymer, providing a barrier to the close approach of the particles.

    The attachment of polymer chains to particle surfaces has been shown to provide elastic steric stabilization when the medium is a melt of the same material as the stabilizing spe- c i e ~ . ~ ~ . ~ In the present case, it is believed that oxidized poly- butene molecules adsorbed onto the ceramic particles when the powder and polymer were mixed, creating the necessary con- ditions for elastic steric stabilization in siru.

    Polymer adsorption was not directly measured in this study, but experimental observations were made that are consistent with enhanced suspension stability due to polymer adsorption. The slurries in oxidized polybutene were easier to stir after sitting overnight, and often changed from unpourable to pour- able at room temperature overnight. This observation indicates polymer adsorption with time. Further softening was ob- served at elevated temperatures by a decrease in the non- Newtonian viscosity at low shear rates with continued testing, indicating that as the material was sheared and heated, in- creased polymer adsorption enhanced the stability of the par- ticles against association. A more dense or thicker layer of the polymer on the particles surfaces should more successfully prevent association.

    The initial response of the slurry containing oxidized poly- butene to increased temperature (Figs. 6 and 7) indicates en- hanced structure formation. That is, the effect of increased temperature is to decrease the stability of the suspensions. This behavior is not readily explained by the theories of steric sta- bilization, which attribute instability to a decrease in the solu- bility of the stabilizing chains in the suspending medium.

    Reversible coagulation in sterically stabilized suspensions has been observed with changes in temperature for systems of latex particles dispersed in a ~ o l v e n t . ~ ~ ~ The particles in those

    systems were covered with stabilizing chains having at least one moiety well-solvated by the suspending medium (a ther- modynamic state that is alterable by thermal and chemical changes). Colloidal stability was observed to be reversibly lost when the conditions were such that the solvation of the stabi- lizers was diminished. Both upper and lower temperature limits to colloidal stability have been observed in sterically stabilized systems.34

    Since no solvent as distinct from a solute existed in the systems of the present study, poor stabilizer solubility cannot account for the loss of stability. Changes in the stability of the present systems must be due to physical rather than chemical effects.

    The most dramatic change in the properties of the polymer with heating was the reduction in viscosity. Without the solids, the viscosity of the H35 polybutene was observed to decrease by about 40 times (-10 to -0.25 Pas, see Fig. 2) for the temperature change 20 to 70C. Since the viscous forces ex- erted by the polymer fluid oppose the formation of interparticle structures in the powder slurries, increasing the temperature tends to enhance interparticle association by decreasing the opposing forces. That is, the ever present van der Waals at- traction between the particles is more effective at higher tem- peratures when the viscous resistance of the polymer is dimin- ished.

    At the lower temperatures, then, the suspensions were meta- stable. The polymer-coated particles remained attractive and, given enough time, would coagulate. However, the elastic steric repulsion of the adsorbed polymer (independent of tem- perature for a constant amount of adsorbate) and the viscous resistance of the bulk polymer (enhanced at lower tempera- tures) increased the time required for coagulation. With in- creasing temperature and decreasing polymer viscosity, coagu- lation occurs more rapidly. Still, even at 50C (see Fig. 8), the time dependence of coagulation was indicated by increasing viscosities at low shear rates with increasing equilibration times. Figure 8 shows plots of viscosity vs shear rate for vari- ous equilibration times between measurements at 50C. At low shear rates, longer equilibration times resulted in higher vis- cosities, indicating enhanced structure formation.

    ( C ) Efects of Long-Term Shearing and Heating: The ef- fect of time and shear on viscosity at 95C is illustrated in Fig. 9. Viscosity vs shear rate data are shown for a 30 vol% slurry

    a .g 100.00 fn 0 8 5

    Y

    10.00 1 0.10 1 .OO 10.00

    Shear Rate (11s)

    Fig. 8. Viscosity as a function of shear rate for a 30 v018 TiO, suspension in oxidized H35 polybutene at 5OoC, sheared for different times at each rate before data collection.

  • 1794 Journal of the American Ceramic Society-Horn and Patterson Vol. 80, No. 7

    I I -A- 23hr

    0.01 0.10 1 .oo 10.00 100.00 Shear Rate (l/s)

    Fig. 9. Viscosity as a function of shear rate for a 30 ~01% TiO, suspension in H35 polybutene (9:l. as receivd.oxidized) at 95C. tested at various times over 23 h.

    of TiO, in a slightly oxidized sample of polybutene (i.e., a 9: 1 ratio of as-received polybutene to oxidized polybutene). The initial curve illustrates strongly shear-thinning behavior. At low shear rates the viscosity was observed to decrease with time, indicating the continual breakdown of the interparticle structure as the polymer layer builds. At high shear rates the viscosity, approaching the high-shear-rate Newtonian regime, was observed to increase during this process. This increase is consistent with the effective growing of the dispersed par- ticles as polymer molecules attach. Increases in the apparent particle size (particle + adsorbed layer) have the effect of in- creasing the volume fraction of the dispersed phase, which increases Newtonian visc0sity.3~ It should be noted that the presence of the oxide particles, the oxide groups on the poly- butene, and the relatively high temperature are all factors that encourage further oxidation in the polymer. The observed ef- fects could be due to an increase in the concentration of ad- sorbable carbonyl groups in the polymer, as well as increased adsorption of the carbonyl groups already present. Whatever the specific chemical process, the rheological effects indicate increased adsorption with time and temperature.

    It is reasonable that adsorption was enhanced at increased temperature. Diffusion of the adsorbing species to the particle surfaces should be accelerated with increased temperature, and attachment might also be encouraged by increased temperature. According to Sat0 and R u c ~ , ~ ~ the temperature dependence of adsorption is a function of the heat of adsorption, which is specific to the system.

    (4) TiOz Slumes in Polymers of Controlled Oxygen Content

    In order to observe the rheological effects of limited polymer adsorption onto the powder, polymer mixtures were prepared in which some of the molecules were highly resistant to oxi- dation. Amoco Panalane is similar in structure to the polybu- tenes, except virtually all of the double bonds are removed by hydrogenation. Adsorption of this polymer is not expected be- cause oxidation, even in the presence of other oxidized mate- rial, is unlikely.

    Two grades of Panalane, L14E and H300E, were mixed in the ratio 37:63 (L14E:H300E) by weight to obtain viscosities very near to those of H35 polybutene at all of the temperatures used. Oxidized H35 polybutene was then added to batches of

    the Panalane mixture in concentrations corresponding to 0.5%, 1.0% and 2.0% of the dry weight of TiO, used to make the 30 ~01% slurries. In other words, the oxidized polybutene was treated as a dispersant or surfactant in slurries of TiO, powder in Panalane fluid.

    The phase angles are shown in Fig. 10 for 30 vol% TiO, slurries at 1 rads and 1% strain. The viscosity at a shear rate of 0.05 s-l is shown in Fig. 1 1. This shear rate, the lowest tested, was chosen to illustrate the steady shear response of the slurry under conditions which least disrupt the microstructure.

    The data indicate that the oxidized polymer indeed behaved as a dispersant in the slurries, attaching to the particle surfaces and providing a barrier to coagulation. In the absence of any oxidized polybutene, the slurries were stiff and pasty at all of the temperatures used. The data for slurries in Panalane mixed with 0.5% and 1.0% oxidized polybutene show strong shear- thinning under steady shear and significant elasticity in oscil- lation. For these compositions, the phase angles and viscosities were not strongly affected by changes in temperature (Figs. 10 and 11). With increasing amounts of oxidized polybutene, however, the rheological behavior was less elastic and more Newtonian at the lower temperatures, undergoing the viscous- to-elastic transition with increasing temperature. For 2.0% and greater concentrations of the oxidized polymer, elasticity and low-shear viscosities were observed to be lowered at the lower temperatures, but increased by heating. These results c o n F i adsorption of the oxidized polymer as the effective stabilization mechanism.

    (5) Slum*es of Other Powders in Oxidized Polybutenes The same type of temperature-dependent behavior observed

    for the slurry of TiO, powder in oxidized polybutene was also observed for 30 vol% slurries of A1,0, and ZrO, powders. Table III shows phase angles for these slurries at three tem- peratures for oscillation at 1 rads and 1% strain. Data for the unfilled polybutene are included as a reference point, demon- strating Newtonian behavior at each temperature. No value is shown for the unfilled polybutene at 70C because the torque was below the sensitivity of the rheometer transducer.

    As discussed in Section III(3)(A), the phase angles for the TiO, slurries in oxidized polybutene were observed to decrease

    90.00

    70.00 h U J 0) 0

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    Y - P a 50.00

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    a, In m r

    30.00

    10.00

    0.00 40.00 80.00 Temperature (deg C)

    Flg. 10. Phase angle at 1 rad/s and 1% strain as a function of tem- perature for 30 v018 TiO, suspensions in Panalane containing various amounts of oxidized H35 polybutene.

  • July 1997 Thermally Induced Reversible Coagulation in Ceramic Powder-Polymer Liquid Suspensions 1795

    i 100OOO.00

    A /

    100.00,

    0.00 40.00 80.00 Temperature ("C)

    Fig. 11. Viscosity at 0.05 s-I as a function of temperature for 30 ~01% TiO, suspensions in Panalane containing various amounts of oxidized H35 polybutene.

    Table III. Phase Angles for 30 vol% Powder Slurries in Oxidized H35 Polybutene

    Phase angle at 1 d s , 1% strain (deg) Powder 20C 50C 70C

    Unfilled 90 90 3 p,m TiO, 85 73 42 3.5 pm A1203 90 88 85 1.5 p,m Al,03 88 59 31 0.5 p,m Al,03 87 30 31 1 F Z l f 3 2 84 63 44

    with increasing temperature. The data shown in Table III in- dicate a change from nearly Newtonian behavior with very little interparticle association at 2OoC to elastic behavior and strongly associated particles at 70C.

    Similar effects were observed for slumes of Al,03 in oxi- dized polybutene. For the 3.5 pm Al,O3, the temperature de- pendence of the phase angle was not as strong as for the 3 pm TiO,. This difference is consistent with the fact that the value of the Hamaker constant is much higher for TiO, (rutile) than for Al,0,.37 The strength of van der Waals-driven associations should be weaker for the Al,O,-filled systems, resulting in less elastic behavior.

    It was also observed that the phase angle for the A1203-filled slumes decreased with decreasing particle size (see Fig. 12). Finer powders resulted in more elastic slumes, indicating a greater tendency for interparticle association. The low-shear- rate viscosity at each temperature was observed to increase with decreasing particle size (Fig. 13). The viscosity of the 0.5 and 1.5 pm powder slurries also showed an initial increase in viscosity with increasing temperature, while that of the 3.5 pm powder sluny decreased until approximately 60C. The de- crease in viscosity for the 1.5 pm powder slurry above 30C is believed to result from increased polymer adsorption with time. The steady shear data confirm the trends of the dynamic pa- rameters, showing decreased stabilization against interparticle association for decreasing particles size, In other words, it was observed that slumes of finer powder tended to associate or coagulate more readily.

    The above dependence of viscosity on particle size is con- sistent with interparticle van der Waals association. It has been

    10.00

    0.00 40.00 80.00 Temperature ("C)

    Fig. 12. Phase angle at 1 rad/s and 1% strain as a function of tem- perature for 30 vol% suspensions of three sizes of Al,03 powder in oxidized H35 polybutene.

    100oO0.00 c h fn & 10000.00 8 .- E 8 5: 5 1000.00 9! !! z Y al c

    g 100.00

    '1 I

    10.00

    0.00 40.00 80.00 Temperature ("C)

    Fig. 13. Viscosity at 0.05 s-' as a function of temperature for 30 vol% suspensions of three sizes of Al,03 powder in oxidized H35 polybutene.

    observed for unstable, coagulated that finer particles tend to result in higher viscosities and yield points, indicating increased energy dissipation during flow and interparticle re- arrangement. While the magnitude of van der Waals attractive forces at a given interparticle spacing is lesser for smaller par- ticles (scaling inversely with particle size), the number of as- sociated particles per unit volume of material is much greater (scaling inversely with size cubed), and interparticle spacing is less (scaling inversely with size). Thus, the interparticle struc- ture among finer particles is more resistant to destruction due to viscous forces.

    Smitham and NappeP reported that elastic-steric stabiliza- tion was only effective for (stabilizer length: particle size) ra-

  • 1796 Journal of the American Ceramic Society-Horn and Patterson Vol. 80, No. 7

    tios greater than a minimum value. For a single polymer mo- lecular weight there was a maximum particle size for which stabilization was effective. That is, smaller particles tended to be more stable. In the present case, however, stability was enhanced by increasing the particle size of the Al,O, powders. This effect is opposite to that reported by Smitham and Napper, but is explainable in terms of the viscous forces resisting co- agulation.

    According to the Navier-Stokes equation, larger particles experience greater viscous resistance in a moving fluid and should be more inhibited from forming associations than are smaller particles. The formation of interparticle associations requires the removal of the intervening fluid, the resistance of which increases with particle size. Also, the viscous stresses that act to disrupt existing associations are greater on larger particles. This effect is similar to the effect of the Peclet num- ber on suspension viscosity. The Peclet number, a ratio of the times for diffusion and convection, increases with increasing particle size? At high values of the Peclet number, which occur at lower shear rates and liquid viscosities for larger par- ticles, viscous forces dominate the rheology. Similarly, in the present case viscous forces tend to overwhelm interparticle attractions more effectively for larger particles.

    The viscosity of the ZrO, slurry was not observed to increase on heating over the entire temperature range used, but the systematic decrease in phase angle indicates increased elastic- ity. Since the Hamaker constant for Zro, is intermediate be- tween that of A1,0, and Ti0, in the same medium? it might be expected that the phase angle of the 1 pm ZrO, system would be consistently lower than that of the 1.5 pm Al,03 system. It is not yet understood why that is not the case, al- though differences in particle shape could be an important factor. As described in Section 11( l), the TiO, and Al,O, par- ticles were angular in shape (from grinding), while the ZrO, particles were slightly smoother.

    A quantitative comparison among the different types of pow- ders requires precise knowledge of the extent of polymer ad- sorption, which tends to stabilize the suspension, and the mag- nitude of the van der Waals forces, which contribute to association. The stiffenability of each system depends upon the balance of these opposing processes. The thickness and struc- ture of the adsorbed layer most likely depends on the concen- tration and strength of acidic sites on the surface of the pow- ders, as well as the size and morphology of the particles. Because accurate measurements of these properties have not been made, such an analysis is not presented here.

    IV. Other Observations The following observations, not discussed above, were made

    during the course of this work. Though rigorous analyses have not yet been performed to identify the reasons for and impli- cations of these observed effects, they should prove useful in directing future work.

    (1) Drying the TiO, powder (2 h at 100C under a vacuum of 90 kPa below ambient) before mixing with the oxidized polybutene fluid produced a pasty, apparently coagulated slurry which was non-Newtonian at any temperature. The most probable reason for the instability of this system was a lack of acid-base interactions between the polymer and the powder. Solomon and Hawthorne describe changes in the acidity of TiO, surfaces with dehydration.28 The removal of dissociated water from the powder must have weakened the acidity of the surface with respect to the basic groups of the oxidized poly- butene.

    (2) Humidifying the TiO, powder, by placing an open con- tainer of powder near boiling water for 2 h, before mixing with the oxidized polybutene liquid also produced pasty slurries which could not be made Newtonian with decreased tempera- ture. Even slurries containing as-received powders were ob- served to become pasty if water condensation was allowed to form on the rheology samples at low temperatures. This loss of

    stability is similar to that reported by Kramer and Lange for Si,N, slurries stabilized in nonaqueous media by physadsorbed alcohol^.^ Coagulation in these cases must result from the dis- placement of the adsorbed molecules (oxidized polybutene in the present work) by water.

    (3) The phase angles for slurries of the TiO, powder in oxidized L50 polybutene (a low-viscosity Amoco polybutene, M,, 590) were consistently lower than those for TiO, powder in oxidized H35 and HlOO polybutenes (M,, 1050 and M,, 1190, respectively) at any temperature. It is believed that the greater elasticity of the slurries in the low molecular weight polymer is due both to the lower liquid viscosity in the L50 polybutene, producing less disruptive stresses during shear, and to the shorter adsorbed molecules, allowing closer approach of the particles into deeper attractive potential wells.

    (4) Commercial antioxidants added to the polybutene after slight oxidation did not affect the rheological properties of slurries prepared with the polybutene. The slumes containing antioxidants behaved similarly to those containing only oxi- dized polybutene. Progressive oxidation of the polymer could only be avoided by mixing the hydrogenated Amoco Panalane with the polybutene.

    V. Summary of Results

    It was observed that slurries of TiO,, A1203, and ZrOz in oxidized polybutene liquids could be made to change revers- ibly between a dispersed, fluid state and a coagulated, pasty state by changes in temperature. At lower temperatures the slurries were nearly Newtonian, characteristic of a system of dispersed, unassociated flow units. Increases in temperature induced shear-thinning behavior, a yield point and enhanced elasticity, indicative of an associated interparticle structure. The low-shear-rate viscosity was observed to increase with increasing temperature for most of the slumes observed.

    Stabilization against interparticle association was observed to be enhanced by increased concentration of oxidized groups (C = 0) in the polybutene, increased viscosity of the suspend- ing medium, increased time (especially at higher temperatures), and increased particle size.

    VI. Conclusions

    The following conclusions were made based upon the above results:

    (1) The presence of oxidized groups in polybutene fluids enhances adsorption of the polymer materials onto the surface of oxide powders.

    (2) Oxide powders may be dispersed in a metastable fash- ion in oxidized polybutene fluids due to the elastic steric forces of the adsorbed polymer chains and the viscous forces of the bulk polymer medium.

    (3) With increased temperature, increased viscosity at low shear rates and increased elasticity occur due to interparticle association, enhanced by the decreased viscosity of the poly- mer.

    (4) Reestablishment of nearly Newtonian behavior on cooling is due to increased polymer viscosity, opposing inter- particle association.

    (5 ) Stability is enhanced at a constant temperature by those factors which tend to increase viscous stresses and the separa- tion distance among the particles. These factors include in- creased polymer viscosity, increased particle size, and in- creased polymer adsorption.

    (6) Increased time and temperature enhance the further ad- sorption of polymer chains such that the stability of the sus- pensions is enhanced and interparticle association with increas- ing temperature is inhibited.

    Acknowledgments: We gratefully acknowledge numerous helpful dis- cussions with h.. Jimmy Mays in the Department of Chemistry at the University of Alabama, Birmingham, and Dr. Peter Clark in the Department of Chemical

  • July 1997 Thermally Induced Reversible Coagulation in Ceramic Powder-Polymer Liquid Suspensions 1797

    Engineering at the University of Alabama, Tuscaloosa. We also acknowledge the assistance of graduate students J. Zhou, for molecular weight analyses of the polybutenes, and J. Haman, for FTIR analyses.

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