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Stability of ,Aqueous a =Al203 Suspensions with Poly(Methacry1ic

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Stability of ,Aqueous a =Al203 Suspensions with Poly(Methacry1ic J. Am. Cerum. SOC., 71 14) 250-55 (1988) Stability of ,Aqueous a =Al203Suspensions with Poly(methacry1ic acid) Polyelectrolyte JOSEPH CESARANO III* and ILHAN A. AKSAY* Department of Materials Science and Engiineering, College of Engineering, University of Washington, Seattle, Washington 98 195 ALAN BLEIER* Metals and Ceramics Division, Oak Ridge National Laboratory,* Oak Ridge, Tennessee 3783 1 Stability of aqueous a-A1,O3 suspensions with Na+ salt of have a substantial surface charge of the same sign so that irre- poly(methacry1ic acid) (PMAA-Na) polyelectrolyte was studied versible agglomeration is prevented.' In general, ceramic sus- as a function of pH. At a given pH, the transition from the pensions can be stabilized electrostatically, but improvement of the flocculated to the dispersed state corresponded to the ad- suspensions to better meet the requirements necessary for ceramic sorption saturation limit of the powders by the PMAA. As the processing is possible by incorporating polymeric additives. pH was decreased, the adsorption saturation limit increased Industrial experience shows, for instance, that in highly concen- until insolubility and charge neutralizatioin of the PMAA was trated oxide suspensions, problems related to high viscosity, aging, approached. The critical amount of PMAA required to achieve and processing of multiphase systems can be drastically reduced by stability is outlined in a stability map. using polyelectrolytes as dispersants or deflocc~lants.~.~However, in spite of these advantages in using polyelectrolytes to stabilize suspensions, a great deal of misunderstanding exists in the general ceramic community as to the fundamental roles of these polymeric I. Introduction additives. Thus, this investigation was designed to elucidate the OR many applications in ceramic processing it is desirable to mechanisms of polyelectrolyte stabilization and to relate them to F sinter at relatively low temperatures and to obtain fully dense the chemistry of the powder surface and the polymer additive. An and fine-grain microstructures. One method by which these goals aqueous a-AI2O3 system with sodium poly(methacry1ic acid) can be accomplished is by using submicrometer particles. In order (PMAA-Na) was chosen as the model. to achieve a uniformly consolidated compact, it is also advan- tageous to use dispersed colloidal suspensioins which contain up- wards of 50 vol% solids while still maintaining relatively low 11. Materials and Experimental Methods viscosities of less than 1 Pass. In general, suspensions can be dispersed by electrostatic, steric, (1) Materials and Chemicals or electrosteric stabilization mechanisms.' Electrostatic sta- The ceramic used in this study was a very-high-purity bilization is accomplished by generating a common surface charge (>99.995%), submicrometer-size, and relatively monodispersed on the particles. Steric stabilization, on the other hand, is achieved (0.2 to I .O pm) a-A120s powder.' The average diameter measured by adsorption of polymeric additives which serve to form protec- by an X-ray absorptionisedimentation technique* was 0.37 pm. tive colloids. Electrosteric stabilization requires the presence of The average surface area as measured by standard BET N2 adsorbed polymer or polyelectrolyte and a significant electrical adsorptions5 was 5.9 m'/g. double-layer repulsion. The polyelectrolyte studied was the Nai salt of oly(methacry1ic The main problems, related to ceramic processing, found to exist acid) with an average molecular weight of 15 000.e The PMAA-Na in electrostatically stabilized systems derive from aging effects and structure and dissociation reaction shown below illustrate that the complications of processing multiphase systems.' The aging ef- functional groups are carboxylic acid groups that can be COOH or fects are commonly due to the solubility of particles in the sus- dissociated to COO-. The PMAA-Na structure is pending medium. For instance, in the case of oxide suspensions prepared with H20 under acidic or basic conditions, the pH changes with time because of the solubility of the particles.* These pH changes can drastically influence rheolog,y and suspension be- havior, sometimes within a relatively short time frame of minutes I coo- coo- coo- In to hours.' A second problem with electrostatically stabilizing sus- Na+ Na' Na' pensions which have more than one solid phase is that it is difficult, if not impossible, to find a pH range in whilch all of the particles For each acid group A-COOH + HzO ~----fA-COO- + H,O' (A-COO-) (H,O') ' = (A-COOH) Manuscript No. 199730. Received March 27, 1987: approved September 25, 1987. Presented at the 87th Annual Meeting of the American Ceramic Society, Cincinnati, OH, May 7, 1985 (Basic Science Division. Paper No. 95-B-85). Supported by the US.Air Force Office of Scientific ]Research (AFOSR) and the Defense Advanced Research Projects Agency of the Department of Defense, and monitored by AFOSR under Grant No. AFOSR-83-0375. Also supported by the where a is the fraction of dissociated acid groups. Division of Materials Sciences, US. Department of Energy, under contract No. DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc. The water used was distilled and deionized. The pH was adjusted 'Member, the American Ceramic Society. with standardized analytical-grade HCl and NaOH solutions (0. IN *Operated by Martin Marietta Energy Systems, Inc., for the U.S. Department to ].ON). Analytical-grade NaCl was used to adjust the ionic of Energy. 'AKP-30, Sumitomo Chemical America, Inc., New York. NY. strength as desired. %edigraph 5000, Micromeritics, Norcross, GA (2) Experimental Methods 'Qudntasorb, Quantachrome Co., Syosset. NY. 'Polysciences, Inc., Pittshureh, PA (A) P otentiometric Titrations: To measure both the fraction 250 April 1988 Stability of Aqueous a-AI,03 Suspensions with Poly(methacrylic acid) Polyelectrolyte 25 1 30~ 0 0.00M NoCf Y 0 0. IOM - 0 E L Is'derivotive plot a 2 I 10- < Q 2 DH [L Fig. 2. Fraction of acid groups dissociated versus pH as a function of salt /ON0 I I rneq of titrant concentration for PMAA-Na salt (molecular weight of 15 000). rneq of titront Fig. 1. (A) Typical titration curve for PMAA-Na and its corresponding remaining in solution after adsorption: A known volume of the first-derivativeplot. (B)Linear relationship between the amount of PMAA- Na titrated and the mole equivalent of titrant corresponding to the distance supernatant was diluted with water to approximately 40 mL. This between the peaks in (A). solution was then sealed in the titration cell and the pH was ad- justed to 9.8, while the sample chamber was subjected to N2 bubbling. The pH adjustment to 9.8 ensured that any PMAA-Na would be fully dissociated. This is discussed fully below. The of dissociated COOH groups (a)versus pH for PMAA-Na and first-derivative titrations were then conducted with 0.SN HCI and the surface charge versus pH for Al2O3,potentiometric titrations of the distance between peaks recorded. Standards were prepared to the control electrolyte solutions were compared to titrations of the establish Fig. 1(B) and this calibration was used to determine ad- polymer solutions and oxide suspensions. ** sorption behavior of PMAA-Na. Background salt did not induce This procedure of using potentiometric titrations to determine significant deviation in this calibration. polymer behavior and solid-surface chemistry is clearly outlined by (C) Settling and Zeta-Potential Experiments: Settling ex- Arnold and Overbeek' and by Hunter.6 Titrations were completed periments were done using 2 vol% A1203 suspensions. Various on a blank electrolyte solution and a corresponding polyelectrolyte amounts of PMAA-Na were added to the suspensions which solution or suspension containing a known amount of sample. The were then ultrasonicated and stirred for at least 4 h. Seventy milli- difference between the amounts of titrant added to the blank solu- liters of each suspension was then poured into graduated cylinders, tion and the sample at a specified pH corresponds to the amount of and, after several days, the final sedimentation cake heights titrant that reacted with the sample. With this information, both the were recorded. fraction of functional groups dissociated on polyelectrolytes and Electrophoretic mobility was measured" and used to calculate the surface charge on ceramic powders can be easily calculated.'.' zeta potential according to Henry's equation.8 These measure- The titrations were completed in a N2 atmosphere on 40-mL ments can only be completed on very dilute suspensions. Therefore, samples containing known but small volumes of PMAA-Na (<O. 1 g) the 2 vol% A1203 samples were centrifuged and the supernatant and A1203(<2 ~01%).When desired, NaCl solutions were used to carefully decanted into a beaker; subsequently, a very minute adjust the ionic strength. Prior to titration, the Al2O3 powder was amount of the sediment was remixed with the supernatant. The cleaned to remove soluble ions using Soxhlet extraction' with samples were ultrasonicated and stirred for 1.5 min prior to the freshly distilled water. measurements to ensure that only the mobility of the single par- (B) Adsorption Isotherms: The adsorption of PMAA-Na on ticles was measured. At least 10 measurements were completed in the alumina was evaluated using the automatic titration unit in the each direction for each sample to ensure accuracy. first-derivative titration mode. Suspensions of 20 vol% Alz03were (0) Viscosity Measurements: Viscosity of the suspensions prepared with various amounts of PMAA-Na at various pH values. was measured using a rotary viscometer" equipped with a recorder While the Al2o3was being added, the pH was constantly moni- and a small sample chamber so that only 8-mL samples were tored and adjusted so that it was always within 0.2 pH unit of the necessary for each measurement. desired pH value. After mixing, the samples were ultrasonicated for approximately 1 min at SO W and the pH was measured and adjusted again if necessary. The samples were then put into a gentle 111.
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