Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Particulate matter
Patent
1997-07-30
2000-12-05
Le, H. Thi
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Particulate matter
427220, 428404, B32B 516
Patent
active
061564303
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
It is known that metal oxides usually have hydroxyl groups on their surface. This is particularly the case if the oxides have been produced in an aqueous medium through precipitation of the metal ions as hydroxides or hydrated oxides, and subsequent dehydration. Depending on the respective metal, the metal oxides can have a basic, acidic or amphoteric character, i.e., the hydroxide groups can gain or lose protons. Measuring the pH of an aqueous suspension of the oxide determines the basic, acidic or amphoteric character of an oxide. The quantity of hydroxide groups on the surface can be determined through acid/base titration.
The hydroxide groups lend the oxides a more or less pronounced hydrophilic character. In the production of dispersions comprising fine-particle oxides, the hydroxide groups appear on the surface of a certain oxide particle in attractive interaction with hydroxide groups on the surface of adjacent particles, thus effecting a certain cohesion of the particles that leads to agglomerations and reduces the dispersibility of the oxide particles. It is known that this effect can be counteracted by the addition of cationic or anionic dispersion agents. Depending on the respective metal, these agents generate positive or negative charges on the particle surface that counteract the agglomeration in a desired manner, but, at least in very fine-particle dispersions, lead to spacing between particles that can approach or even exceed the order of magnitude of the particle diameter due to Coulomb repulsion. It is therefore difficult or impossible to produce the commonly-desired dispersions with very small particle diameters, and yet high solids contents.
SUMMARY OF THE INVENTION
With the invention, ceramic powders are created whose surfaces have been rendered hydrophobic and are particularly suited for producing stable dispersions (so-called ceramic slips) that have very small average particle sizes and a high solids content, as are required for high-quality molded bodies produced in accordance with a sintering method. The reason for this is that, because of the steric stabilization effected by the hydrophobing process of the invention, the particles require less space in comparison to those subjected to electrostatic stabilization by means of cationic or anionic dispersion agents. Therefore, the ceramic powders that have been rendered hydrophobic in accordance with the invention can be used to produce stable dispersions having a higher solids content and the same viscosity, or a lower viscosity with the same solids content, in comparison to electrostatically-stabilized dispersions. Extremely-fine ceramic powders can be processed only into reasonably processable dispersions if they have been rendered hydrophobic in accordance with the invention.
Rendering ceramic powders of different origins hydrophobic results in an evening-out of the properties, that is, masking or suppression of certain characteristics stipulated by origin that would otherwise cause a different behavior. Thus, ceramic powders of different origins and having different properties can be processed in the same manner. The course of further processing is therefore independent of the individual properties of the raw materials.
In the dried ceramic green bodies, rendering the surface of the powder particles hydrophobic results in an increased flexibility that is desirable for different applications.
Moreover, ceramic slips produced from ceramic powders that have been rendered hydrophobic according to the invention, and are therefore sterically stabilized, have no yield point, or a considerably lower yield point, in comparison to electrostatically-stabilized slips under otherwise identical conditions, which is advantageous for further processing of the slips.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates the IR spectrum of a zirconium dioxide that has been rendered hydrophobic with palmitic acid.
FIG. 2 shows flow curves of ceramic slips produced from aluminum oxide that has been rendered hydroph
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Eisele Ulrich
Nass Ruediger
Weber Lothar
Le H. Thi
Robert & Bosch GmbH
Spencer George H.
Wells Ashley J.
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