Compositions: coating or plastic – Materials or ingredients – Pigment – filler – or aggregate compositions – e.g. – stone,...
Reexamination Certificate
2000-08-02
2002-02-12
Marcheschi, Michael (Department: 1755)
Compositions: coating or plastic
Materials or ingredients
Pigment, filler, or aggregate compositions, e.g., stone,...
C106S468000, C501S146000, C501S147000
Reexamination Certificate
active
06346145
ABSTRACT:
BACKGROUND OF THE INVENTION
It is normal practice to produce kaolin pigments by calcination of purified fine particle size hydrous kaolin clay. Calcination of kaolin at temperatures up to about 1100° C. cements particles together and produces products of improved whiteness and opacity. Such pigments are widely used by the paper, plastics, rubber and paint industries. A seminal patent is commonly assigned U.S. Pat. No. 3,586,523, Fanselow et al, the disclosure of which is incorporated herein by cross-reference. This patent is directed to producing low abrasion fine particle size opacifying calcined kaolin clay, such as the pigments supplied under the registered trademarks ANSILEX and ANSILEX 93. Such pigments have an average particle size of about 0.8 microns. When used in paints, the fine calcined pigments obtained by practice of the Fanselow et al patent also provide tint strength and high sheen.
For flat paints, the low angle sheen as measured by 85 degree gloss is generally less than 10. For better flat paints, the 85 degree sheen is 4 or below. To achieve low sheen the particle size of the calcined clays is increased by changing the particle size distribution of the feed such that the feed is coarser than used by Fanselow et al. While a sheen of 4 is achievable by calcining a coarser feed, the hiding and tint strength properties are compromised (reduced). It is extremely difficult to balance the properties of a paint formulation since 85 degree sheen and opacity/tint strength move in opposite direction to their particle size. Thus, as particle size in the optically efficient range increases, opacification decreases. The following commonly assigned patent, U.S. Pat. No. 4,525,818, Kostanzek, discloses means to produce certain coarse calcined kaolin clay pigments especially useful as primary extenders in paint. The calcined clay pigments have an average particle size of about 3 to 4 microns.
Practice of our invention utilizes fluxing compounds during calcination of the kaolin, the compounds being added to hydrous kaolin before calcination. Reference is made to U.S. Pat. No. 2,307,239, Rowland, which is a pioneer patent in the field of calcined kaolin pigments. This patent broadly discloses addition of various alkali and alkaline earth compounds to clay before calcination. A preferred compound is sodium chloride. U.S. Pat. No. 3,853,573, Ferrigno, discloses pigmentary compositions produced by combining a fluxing agent composed of one or a combination of metal oxides with calcined kaolin, with optional hydrous kaolin, and an inorganic binder. Dry addition of flux is practiced by Rowland and Ferrigno; no attempt is made to coarsen the clay in the optically efficient particle size range. In a thesis, “The Kaolin to Mullite Reaction Series,” Wilfred Anthony Martinez, Rutgers University, 1979, various “mineralizers” were added to both a fine particle size kaolin and a relatively coarse particle size hydrous kaolin, primarily to explore the effect on the crystallinity. Dry addition of mineralizer was practiced in all experimental work. There is no attempt to control the coarsening of kaolin clay during calcination throughout the optically active size. The phrase “controlled coarsening” as used herein refers to increasing the percentage of coarsened particles throughout the particle size range of 0.5 to 2 microns.
So-called “flash” or “shock” calcination is used to produce relatively coarse kaolin derived pigments. Reference is made to U.S. Pat. No. 3,021,195, Podschus et al. Special calciners must be used and multi-stage thermal treatment is used. The rotary calciners or Nichols furnaces generally used by the kaolin industry cannot be readily reconstructed to function as shock calciners. While the calcined pigments are relatively coarse, average size about 1.4 microns, and sheen is low, the pigments tend to be more yellow than other calcined clay pigments.
SUMMARY OF THE INVENTION
Our invention overcomes this difficulty by controlled coarsening through calcination of hydrous fine size kaolin with controlled low dosages of fluxing agents. The controlled coarsening allows the calcined pigments to attain unusually low sheen while other important properties such as hiding and tint are not compromised.
The new products can be made with conventional rotary calcination kilns without new capital. This does not require use of flash calcination process to produce low-sheen pigments. The experimental products made by our invention have higher whiteness and less yellowness than commercial flash calcined low sheen pigments.
An essential feature of the process is that the fluxing agent must be water soluble and is added to the hydrous kaolin with mixing in the presence of water. Dry mixing of flux with clay before calcination, as taught in the prior art, does not result in controlled coarsening throughout the desired range of 0.5 to 2 microns. Further, dry mixing results in large amounts of oversize (grit) which may render the product of limited if any commercial use.
DETAILED DESCRIPTION OF THE INVENTION
The fluxing agent may be mixed with kaolin clay in the presence of water at various points during the processing of hydrous kaolin but is preferably added to a dispersed slurry of hydrous kaolin in solution form just prior to spray drying. The spray dried pigment is pulverized and calcined at temperatures between 500 to 1200° C. but preferably between 800 to 1070° C. See Fanselow et al, supra. The calcined product is pulverized before use in formulating paints, paper coating colors, plastics, rubber and other applications. The dosage of fluxing agent and the type of fluxing agent required are dependent on the hydrous kaolin feed and its particle size. The upper limit of temperature of calcination is dictated by the mullite index. The mullite index measures the amount of mullite (3Al
2
O
3
.SiO
2
) formed from calcination.
As taught in Fanselow et al and other patents in the art, kaolin clay is conventionally dispersed before spray drying to provide fluid slurries of commercially practical solid levels. In typical practice soda ash is added normally as a part of a dispersant package during dispersion of kaolin slurries. As in the illustrative examples herein, two dispersant combinations, SAP and SAC, are generally used. SAP is an 18 to 21% solution of soda ash, partially neutralized polyacrylic acid (C211) and SHMP (sodium hexametaphosphate) at an active ratio of 49/22/29 respectively. SAC is an 18 to 20% solution of soda ash and C2111 at an active ratio of 50/50. Normally the highest level of these dispersants used is 6 #/T or 0.3% by weight (pH of slurry adjusted to 7.5). This translate to 0.15% soda ash added with SAP and 0.15% soda ash added in the case of SAC. The highest level encountered in a typical commercial plant is approximately 8 #/T SAP (pH of slurry adjusted to 8.0) which translates to 0.20% soda ash. It is believed that other kaolin manufacturers also use the same range of soda ash concentrations to disperse their slurries. In the practice of our invention, one of the preferred fluxes is soda ash. The lowest soda ash level (≧0.30%) employed is at least 50% higher than the highest soda ash level used for dispersing kaolin slurries.
Suitable, preferred and especially preferred feeds for the practice of this invention have PSD as follows:
Cumulative Mass Finer (%)
Diameter (um)
Suitable
Preferred
Most Preferred
10.0
100
100
100
5.0
100
100
100
2.0
87-100
95-100
96-100
1.0
70-100
80-99
88-99
0.5
37-95
60-93
70-93
0.3
19-95
35-70
50-70
At a given dosage of flux, the finer the feed, the greater is the degree of coarsening across the whole particle size rage. The degree of coarsening is defined as the change in % cumulative mass finer between the calcined product and the hydrous kaolin feed at a given diameter (equivalent spherical diameter esu). At the same dosage of flux as the hydrous kaolin feed becomes coarser, the degree of coarsening across the whole particle size range is less. A broad window for controlled coarsening i
Ginez Andres
Hen John
Mathur Sharad
Young Ray
Engelhard Corporation
Keller Raymond F.
Marcheschi Michael
LandOfFree
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