Effective dispersants for concentrated, nonaqueous suspensions
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With the aim of identifying effective dispersants that would yield stable, high solids loading (i£60 vol. %) suspensions of oxides, carbides, or nitrides in nonaqueous carriers such as paraffinic oils, a number of dispersants were evaluated, using in most cases A16SG grade alumina from Alcoa as the filler. Among those evaluated were some common dispersants, such as menhaden fish oil and oleic acid, and commercial dispersants not commonly used in ceramic processing, such as polymeric fatty esters and petroleum sulfonates. More importantly, a few dispersants were synthesized and evaluated. The latter dispersants contained straight or cyclic (benzenic) side chains located far from the head group on 18 carbon main-chain fatty acid molecules. Among these, the dispersants with a 5-10 carbon side chain or with a benzenic side chain yielded very fluid suspensions (^60 vol. %) compared to those with long polymeric or oligomeric side chains, or with no side chains, or the commercial dispersants; in some cases, for the same solid loading, the suspension viscosities were an order of magnitude lower with the synthesized side chain dispersants. These results indicate that molecules with an optimum side chain length located sufficiently far from the head group and an optimum backbone (main chain) constitute the most effective dispersants for concentrated suspensions. By combining the advantages provided by wider particle size distributions and by these effective dispersants, suspensions highly concentrated (up to 74 vol. %), and yet processable and "flowing" paste-like have been prepared.
I. INTRODUCTION In a number of application areas of solid-liquid dispersions, such as electronics, ceramics, pharmaceuticals, and paints and coating industries, it is desirable to obtain stable dispersions of high solid content in aqueous or nonaqueous media, such as organic solvents, paraffinic oils, etc. However, the viscosities should not be too large for ease of processing. For example, in the case of thermally conducting pastes, the thermal conductivity of the concentrated suspension increases with increasing solid content. However, the viscosity of a concentrated suspension also increases significantly with minor increases in solid content. It is therefore desirable to be able to increase the solid content while not drastically increasing the suspension viscosity. The keys in achieving these goals are effective dispersants and a proper choice of the solid particles (their nature, size, and size distribution) and the liquid carrier. It has been shown1 that for the same loading (carrier and dispersant remaining the same), the viscosity of a suspension made by mixing two grades of particles (one submicron and the other a few microns in average a
'l. Sushumna is currently associated with IBM, East Fishkill, Z-40 E, Route 52, Hopewell Junction, New York 12533. b) R. K. Gupta is with the Department of Chemical Engineering, West Virginia University, Morgantown, West Virginia. c )Address correspondence to E. Ruckenstein. 2884
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