Thermally Induced Reversible Coagulation in Ceramic Powder-Polymer Liquid Suspensions

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<ul><li><p>Thermally Induced Reversible Coagulation in Ceramic Powder-Polymer Liquid Suspensions </p><p>Jeffrey A. Horn*.+ and Burton R. Patterson* Department of Materials and Mechanical Engineering, The University of Alabama, Birmingham, Alabama 35294 </p><p>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. </p><p>I. Introduction </p><p>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. </p><p>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. </p><p>D. J. Shanefielb-contributing editor. </p><p>Manuscript No. 192084. Received January 17, 1996; approved January 16, 1997. Presented in part at the 97th Annual Meeting of the American Ceramic Society, </p><p>Cincinnati, OH, May 1, 1995 (Paper No SXX-16-95). Based on work performed by J. A. Horn in partial fulfillment of the requirements </p><p>for the Ph.D. degree in materials engineering. The Universitv of Alabama. Birming- ham, AL, 1995.- </p><p>I - - Supported by the Alabama DOUEPSCoR Traineeship Program. Member, American Ceramic Society. </p><p>Present address: The Pennsylvania State University, 147 Research Building West, University Park, Pennsylvania 16802-6809. </p><p>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 </p><p>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. </p><p>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. </p><p>11. Experimental Procedure </p><p>(1) Mateh l s The materials used in this study, along with the properties </p><p>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, </p><p>1789 </p></li><li><p>1790 Journal of the American Ceramic Society-Horn and Patterson Vol. 80, No. 7 </p><p>Table I. ExDerimental Materials Ceramic powder or Median powder size Supplier </p><p>polymer liquid or Polymer M. and trade name </p><p>TiO, 3 pm Alfa-Aesar 41203 0.5 pm Alcoa, XAlOOO A1203 1.5 p n Alcoa, XA1520 </p><p>3.5 km Alcoa, XA4OOO 1 krn Magnesium Elektron, </p><p>A1203 zfl2 </p><p>SC105 As-received 660 Amoco, H35 </p><p>Oxidized polybutene 660 Amoco, H35 Panalane 320 Amoco, L14E Panalane 1340 Amoco, H300E </p><p>polybutene </p><p>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 </p><p>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. </p><p>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 </p><p>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. </p><p>2000 1750 1500 1250 1 000 Wavenumber (l/cm) </p><p>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. </p><p>Table II. GPC Results for Amoco H35 Polybutene Day at 100C M 3 1040 1.69 5 1050 1.68 </p><p>10 1090 1.64 </p><p>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 </p><p>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: </p><p>(1) The sample was loaded onto the rheometer plates at room temperature. </p><p>(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. </p><p>(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. </p><p>(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. </p><p>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. </p><p>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. </p><p>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 </p><p>III. Results and Discussion </p><p>( I ) Unfilled Polybutene The rheological data for the unfilled polybutene (Amoco </p><p>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 </p></li><li><p>July 1997 Thermally Induced Reversible Coagulation in Ceramic Powder-Polymer Liquid Suspensions 1791 </p><p>85.00 - </p><p>- h </p><p>0 Q </p><p>Q, </p><p>75.00 - - 5" a </p><p>n </p><p>~ </p><p>8 65.00 - c </p><p>55.00 </p><p>- </p><p>&gt; </p><p>0.10 -A- 70.C </p><p>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 </p><p>0.01 0.10 1 .oo 10.00 100.00 Shear Rate (l/s) </p><p>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. </p><p>95.00 I </p><p>Ternperalum </p><p>+- 20C + 5o*c </p><p>70C </p><p>I I I I I l l 1 1 I I I I - r r r r </p><p>10.00 100.00 I 45.00 ~- </p><p>1 .00 Frequency (rads) </p><p>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. </p><p>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.,~ </p><p>(2) TiO, Slurries in As-received Polybutene Figures 4 and 5 show the rheological data for a 30 vol% </p><p>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&amp; 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...</p></li></ul>


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