Compositions: coating or plastic – Materials or ingredients – Pigment – filler – or aggregate compositions – e.g. – stone,...
Reexamination Certificate
1999-11-30
2002-06-11
Marcheschi, Michael (Department: 1755)
Compositions: coating or plastic
Materials or ingredients
Pigment, filler, or aggregate compositions, e.g., stone,...
C106S416000, C106S484000, C162S181800, C241S024100, C241S016000
Reexamination Certificate
active
06402826
ABSTRACT:
BACKGROUND
1. Field of the Invention
This invention relates generally to kaolin clays and, in particular, to a kaolin clay pigment comprising a coarse kaolin clay or a blend of a coarse kaolin clay and a fine kaolin clay having specially designed particle size, particle shape, and crystal structure. The new pigment possesses improved rheological properties, i.e., Hercules viscosity that is greater than about 250 rpm at 18 dynes in a high solids clay-water slurry, i.e., about 65% to about 75% solids. When used in a coating formulation for coating groundwood or free sheet papers, the pigment provides improved physical and optical properties for the papers.
2. Background of the Invention
Kaolin clay pigments can be used to coat paper. The purpose of paper coating is to cover an irregular paper surface comprised of cellulose wood fiber with a pigment-binder formulation, that, when dry, leaves a smoother and brighter surface ready for printing. It is common practice to use kaolin clay along with other mineral pigments, such as titanium dioxide and calcium carbonates, as a coating in a coating formulation comprising starch and/or latex.
Kaolin clay pigments are obtained from kaolin which is also called china clay or hydrous kaolin. Kaolin comes from weathering, hydrothermal, and sedimentary geological deposits. Major kaolin deposits that are suitable for paper-making applications are found in the United States, Brazil, Australia, and the United Kingdom.
Kaolin is a type of rock formed through weathering or hydrothermal alteration of feldspar or mica minerals to kaolin minerals, or a sedimentary rock containing a high concentration of kaolinite particles or grains. Sedimentary kaolin rocks contain mostly clay or silt sized particles of kaolin minerals and fine and coarse particle size impurities. While the primary mineral in kaolin is kaolinite, a hydrated aluminum silicate (Al
2
Si
2
O
5
(OH)
4
), kaolin also contains impurities. These impurities are undesirable in many industrial applications. Some of the impurities (e.g., fine ferruginous or titaniferous impurities) impart undesirable color to the clay. Other impurities have an undesirable effect on the rheology of the kaolin. Still other impurities are coarse particles called “grit” that are generally above 45 microns which may cause scratching and/or abrasion if used in most applications. Kaolins from different deposits, or even from different parts of the same deposit, can vary widely in the amount and type of impurities as well as particle size distribution and the shape of the kaolin particles.
Kaolin particles occur over a range of sizes and aspect ratios. The particle size of kaolin is conventionally determined by sedimentation using Stokes Law and assuming a spherical particle shape for the kaolin particles. The conventional term “equivalent spherical diameter” (e.s.d.) is used to designate particle size. Aspect ratio is defined as the diameter of a kaolin particle divided by its thickness. Thus, unrefined (crude) kaolin will not contain particles of a single size, such as, for example, particles all of which are 2 microns. Typically, after degriting removes the majority of particles larger than 45 micron particles, the degritted kaolin will contain particles ranging in size from submicron (colloidal) to particles 20 microns or larger. Kaolin particles finer than about 1 micrometer are generally composed of individual platelets, but these platelets generally conglomerate into particles larger than about 1 micrometer. These conglomerate particles are usually composed of stacks or booklets of several platelets mixed with discrete platelets.
Kaolin clay pigments are widely used to coat and/or fill paper products. They can reduce the cost of the paper products and improve their quality by making the paper brighter, smoother, glossier, and easier to print ink onto. By altering the particle size and particle shape of kaolin pigment, one can change the impact of a kaolin clay pigment on the paper products. For use in paper coating applications, kaolin pigments generally need to have 80% or more particles that are less than 2 micrometers (e.s.d.).
Changing Clay Particle Size and Shape by Delamination
One technique to change clay particle size and shape is mechanical delamination of the kaolin particles. Delamination is the process of splitting apart kaolinite stacks or booklets into largely individual platelets. One type of delamination operation involves subjecting the naturally occurring kaolin stacks to shearing forces in an extruder, thereby reducing the kaolin stacks to discrete platelets. An alternative delamination process involves subjecting the naturally occurring kaolin stacks in an aqueous clay slurry to the cleaving or shearing action of an attrition mill or a sand grinder. Reference may be made to U.S. Pat. No. 3,615,806 of Andrew Torock and Thomas F. Walsh for a thorough discussion of the process of delamination of kaolin clay. Kaolin pigments which are delaminated can be used in paper coating to improve the opacity as well as enhance the smoothness of the paper surface. See, for example, U.S. Pat. No. 3,171,718, to Gunn et al and U.S. Pat. No. 4,241,142, to Kaliski et al.
However, delamination is generally detrimental to the fluidity of kaolin pigments in high solids clay-water slurries. Good fluidity at high solids concentration is very desirable and often necessary for kaolin pigments in their paper-coating applications. Superior rheology in coating formulations permits the paper coating equipment to run at higher speeds (which directly increase the productivity of existing coating equipment) or permits the use of coating formulations at higher solids (thus reducing drying time and hence increasing the efficiency of drying equipment). Delaminated kaolin pigments have substantially poorer fluidity (i.e., higher viscosity) at high-shear rate than undelaminated pigments in a clay-water slurry or in paper coating formulatons. Typically, delaminiated kaolinpigments are fluid at 65-68% solids concentration compared with undelaminated kaolin pigments which are typically fluid at 70% solids concentration or higher. Small changes in water content, such as the larger water content in a 65-68% solids-concentration slurry rather than the 70% solids-concentration slurry, may have a significant impact on paper manufacturing. This is because 2% or greater change in water content will vary the balance in coating formulations, detrimentally effecting the coloring process and thereby reducing the final paper product's quality (opacification as well as other desired quality traits). Also, higher water contents increase the cost of the final colored paper because of higher drying costs. Lastly, if any adjustments need to be made to the color formulation these adjustments can be more expensive with a lower solids-concentration slurry because, while it is inexpensive to later increase the water content of the slurry, decreasing it can be difficult and expensive, especially with high solids-content slurries. Thus, with the use of conventional delaminated pigments, papermakers must balance the tradeoff between paper opacification (quality) and efficiency of production (productivity).
It is well known in the art that kaolin clay pigments must have certain rheological and optical properties to be suitable for use in paper manufacture as paper coatings or paper fillers. The kaolin pigment must be available as a high solids suspension typically having a clay solids content of about 50% to 70% by weight, but still exhibiting a viscosity low enough to permit efficient and economical pumping, mixability with other filler or coating components, and application to the paper. Additionally, it is of utmost importance that the kaolin pigment exhibit certain optical. properties, namely high brightness, high gloss, and high opacity.
U.S. Pat. No. 5,411,587, to Willis et al., disclosed a novel mechanically delaminated kaolin clay pigment useful for light weight coated paper which possesses the opacification, smoothness and printability character
Garforth William L.
Golley Christopher R. L.
Iyer Rajan R.
Lyons Anthony V.
Nutbeem Christopher
Finnegan Henderson Farabow Garrett & Dunner
Imerys Pigments, Inc.
Marcheschi Michael
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