Acidic Biopolymers as Dispersants for Ceramic Processing
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ACIDIC BIOPOLYMERS AS DISPERSANTS FOR CERAMIC PROCESSING N. PELLERIN,* J. T. STALEY,* T. REN,* G. L. GRAFF, D. R. TREADWELL, and I. A. AKSAY Department of Microbiology,* and Department of Materials Science and Engineering; and Advanced Materials Technology Center, Washington Technology Center, University of Washington, Seattle, WA 98195
Some acidic biopolymers serve as dispersants for colloidal processing of ceramics. One biopolymer we tested was alginate, a heteropolysaccharidecontaining two carboxylic sugar acids, D-mannuronic and D-guluronic. Kelp alginate was a suitable dispersant,provided that its viscosity was reduced by partial acid hydrolysis. Low molecular weight polymers rich in guluronic acid proved to be better dispersants than those rich in mannuronic acid, perhaps due to their greater charge density caused by their buckled molecular configuration. In situ processing of ceramic materials was tested by growing the alginate-producingbacterium, Azotobacter vinelandii, in the presence of alumina particles. Growth occurred at 15 vol% alumina in medium. Alumina particles which were exposed to such treatment showed a high packing density comparable to that with purifiedpolymer. We also tested polypeptide polymers of the dicarboxylic amino acids, glutamate and aspartate, which also served as excellent dispersantsfor small alumina particles.
INTRODUCTION Colloidal processing of submicron-size ceramic powders is impeded by interparticle attractions from van der Waals forces which cause aggregations that effectively increase the particle size and leave undesired voids in the finished product.) To achieve high density in a green compact it is desirable to disperse particles in a liquid medium to prevent the formation of 24 strong agglomerates. Polyelectrolytes have been used in aqueous solvents to overcome agglomeration and achieve dispersion by coating the particles. In such a system, the pH-lis adjusted so that the polyelectrolyte 2 and particles have opposite charges. Further, the cake densities and viscosities of such systems are dependent not only on the solids loading of the suspension but also upon the concentration of polymer relative to the solids. Too little polymer results in incomplete coating of the particles and an agglomerated suspension. Excess polymer in the system causes depletion flocculation 3 and loss of stability. We have investigated the use of several naturally occurring polyelectrolytes as dispersants for high purity, submicron-size A120 3 powders. Naturally occurring polymers are nontoxic and biodegradable and, therefore, do not the present problems of handling and disposal that may be encountered with some of their synthetic counterparts. Since it has been known for many Mat. Res. Soc. Symp. Proc. Vol. 218. @1991Materials Research Society
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