Colloid systems and wetting agents; subcombinations thereof; pro – Continuous liquid or supercritical phase: colloid systems;... – Aqueous continuous liquid phase and discontinuous phase...
Reexamination Certificate
2001-04-26
2003-12-02
Lovering, Richard D. (Department: 1712)
Colloid systems and wetting agents; subcombinations thereof; pro
Continuous liquid or supercritical phase: colloid systems;...
Aqueous continuous liquid phase and discontinuous phase...
C516S033000, C524S406000, C524S408000, C524S413000
Reexamination Certificate
active
06656976
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a process of preparing robust, homogeneous, well-dispersed aqueous and ethanolic multicomponent mixtures, such as mixtures of WC+Co (hard metal)-based materials.
DESCRIPTION OF THE RELATED ART
In the description of the background of the present invention that follows reference is made to certain structures and methods, however, such references should not necessarily be construed as an admission that these structures and methods qualify as prior art under the applicable statutory provisions. Applicants reserve the right to demonstrate that any of the referenced subject matter does not constitute prior art with regard to the present invention.
It is of utmost importance to be able to control all of the steps in the manufacturing of reliable products using a powder metallurgy approach. For optimum performance and high reliability, materials produced by a powder metallurgy route should have a microstructure characterised by a small defect size, well dispersed phases and a homogeneous grain boundary composition. One of the problems limiting the development of materials with these characteristics relates to the difficulty of achieving a good mix of two or more particulate materials to obtain homogeneous composite mixtures. Since fine powders are cohesive and thus difficult to mix in the dry state, most mixing is performed in the wet state. Typically, the particulate components are mixed with a liquid, a proper dispersant and possibly further additives so that a well dispersed, non-agglomerated slurry can be made. If this is done right, i.e. a proper dispersant is used to disperse the powder, it is possible to obtain a very homogeneous particulate mixture.
The slurry is then processed further. One of the most common shaping methods involves dry pressing; this requires the production of free flowing granules, usually by spray drying the slurry. This is the most common method of producing hard metal (WC+Co-based) inserts for metal cutting applications. It is clear that spray drying of fine powders in large quantities requires a high degree of process control to reach the desired microstructural characteristics and size distribution of the granules. One of the controlling parameters of the spray drying process is the viscosity of the slurry. It is preferred that the slurry should display a low viscosity at the appropriate shear rates. Shear thickening has to be avoided as a sudden increase in viscosity at high shear rates may cause clogging or serious damage to the spraying nozzle.
The importance of the suspensions in reliable processing has resulted in a substantial interest in developing technologies and methods for preparing well dispersed, homogeneous particulate slurries displaying a low viscosity. Well-established recipes exist today for several materials in both aqueous and non-aqueous media. A commercial dispersant, Hypermer KD3, produced by ICI Chemicals has proved to disperse a wide range of ceramic powders in non-polar media, e.g. silicon nitride (L. Bergström, “Rheological properties of concentrated, non-aqueous silicon nitride suspensions”, J. Am. Ceram. Soc., 79, 3033, 1996), and alumina (L. Bergström, “Rheological properties of Al
2
O
3
and SiC-whisker composite suspensions”, J. Mater. Sci., 31, 5257, 1996).
Thickeners represent a different group of polymeric additives used for adjustment and control of the rheological properties. They are commonly used to increase the viscosity of the liquid to reduce the settling and thus prevent segregation. In WO 98/00257 it is shown that by the addition of suitable thickeners it is possible to prevent settling of cemented carbide slurries while still producing suitable rheological characteristics for spray drying.
Polyelectrolytes are usually the dispersant of choice when preparing aqueous inorganic powder suspensions. The popularity of polyelectrolytes stems from their low cost and high efficiency in dispersing many different kinds of powders in aqueous media. The effect of polyelectrolyte addition on the colloidal stability and theological behaviour is determined by a complex interplay between the polyelectrolyte, the powder surface and the solution phase. In order to understand the adsorption behaviour of polyeletrolytes and the nature of the induced interparticle forces, one has to consider the surface chemistry of the solid phase as well as the solution properties of the polyelectrolyte. Polyelectrolytes acquire a charge in aqueous solutions due to the dissociation of functional groups; i.e., both the conformation and charged fraction of the polyelectrolyte is strongly dependent on pH and ionic strength. The surface charge density of the solid phase is also controlled by the solution conditions. Fundamentally, the surface charge density is dependent on the number and density of surface groups, the pKa values of the surface reactions and the ionic strength of the solution.
Previous studies have shown that pH is a very important parameter in controlling polyelectrolyte adsorption. It is useful to distinguish between pH regimes where the particle surface and the polyelectrolyte carry net charges of either the same or opposite sign. If the segment-surface interaction is purely electrostatic, adsorption will only take place if the polyelectrolyte bears a net charge of the opposite sign. This is the basis of the general rule that an acidic powder, which displays a negative surface charge over most of the pH-range, can be dispersed using a positively charged, cationic polyelectrolyte. Oppositely, a basic powder, which carries a positive charge over most of the pH-range, can be dispersed using a negatively charged, anionic polyelectrolyte.
For example, well-dispersed, highly concentrated alumina suspensions have been prepared by Novich et al (U.S. Pat. No. 4,904,411) using low amounts (0.5-2 wt %) of a polyacrylate polyelectrolyte. They used the same type of dispersant for dispersing steel powder and zirconia. Novich et al were also able to disperse acidic powders like silica using a cationic polyelectrolyte called CORCAT P-12 and P-600.
Although these principles have been most useful for finding suitable dispersants for simple ceramic systems, the situation rapidly becomes more complex when the number of particulate constituents in a slurry is increased. When the suspension contains mixtures of acidic and basic powders it is usually necessary to resort to trial and error to find a suitable dispersant for a specific system.
Hard metals, such as mixtures of WC and Co, together with additional particulate constituents, are commercially important systems which have to be dispersed and spray dried for subsequent mass production of, for example, inserts for metal cutting tools. However, the slightly soluble and widely different acid/base properties of the two main particulate constituents (WC or rather the surface oxide WO
3
is acidic and CoO is basic) make this system difficult to disperse in polar media. The current process technology typically involves dispersing the powders in an ethanol-rich medium under strong agitation prior to spray drying. The solids content in the mixture must be relatively low, around 20 vol %, to keep the viscosity at a sufficiently low level. With robust, well dispersed suspensions of WC+Co-based particulate mixtures there is a possibility to increase the solids content and thus reduce the energy consumption during spray drying. There is also a large interest in developing well dispersed aqueous suspensions of WC+Co-based particulate mixtures to eliminate the explosion hazards and reduce the environmental impact of ethanol-based suspensions.
SUMMARY OF THE INVENTION
The present invention addresses the above-referenced problems of the conventional art.
The present invention provides a procedure for making well-dispersed suspensions.
The present invention provides a procedure for making well dispersed suspensions comprising mixtures of WC and Co powders displaying a low viscosity, which remain stable over an extended period of time. According
Bergström Lennart
Laarz Eric
Burns Doane , Swecker, Mathis LLP
Sandvik Aktiebolag
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