Compositions: coating or plastic – Materials or ingredients – Pigment – filler – or aggregate compositions – e.g. – stone,...
C106S408000, C106S442000, C106S446000, C106S458000, C106S480000, C106S484000
This invention relates to calcined kaolin clay pigments. In a more specific aspect, this invention relates to calcined kaolin clay pigments having improved color. This invention also relates to a process for the manufacture of these pigments.
BACKGROUND OF THE INVENTION
Kaolin is a naturally occurring, relatively fine, white clay mineral which may be generally described as a hydrated aluminum silicate. After beneficiation, kaolin is widely used as a filler and pigment in various materials, such as rubber and resins, and in various coatings, such as paints and coatings for paper.
The use of kaolin in paper coatings serves, for example, to improve brightness, color, gloss, smoothness, opacity, printability and uniformity of appearance of the coated paper. As a filler in paper formulations, kaolin is used to extend fiber and reduce cost, and to improve opacity, brightness and other desirable characteristics of the filled paper product.
Calcined kaolin is a particular type of kaolin and is often used in large quantities for paper manufacture. Calcined kaolin can be obtained by heating beneficiated kaolin clay at temperatures of at least 550° C. The calcination step dehydroxylates and converts the kaolin into a noncrystalline aluminosilicate phase or metakaolin. The term “dehydroxylates” refers to the removal of structural hydroxyl groups from the kaolin in the form of water vapor. The particles of the feed clay are aggregated by calcination. The particle aggregation increases the light scattering characteristics of the kaolin (as compared to non-calcined kaolin) and, therefore, contributes a high degree of opacity to the coated paper. In addition, calcination increases the brightness of the kaolin.
Fanselow et al. U.S. Pat. No. 3,586,523 describes calcined kaolin clays.
Calcined kaolin clay pigments (such as those marketed by Thiele Kaolin Company of Sandersville, Georgia under the trademark KAOCAL) are widely used in the paper industry. The high brightness of the calcined clay is partly due to the removal of organic material at elevated temperatures. The brightness can also be improved through pre-calcination beneficiation processes such as magnetic separation, froth flotation, selective flocculation and chemical leaching.
For example, Forbus et al. U.S. Pat. No. 5,624,488 describes the method of floating, grinding, classifying and calcining Middle Georgia kaolin to produce ultrahigh brightness (96%+) calcined clay pigments. Removal of yellow tinted iron oxides, titania and organic matter helps increase the brightness and whiteness of the kaolin, and decreases the yellowness as indicated by a reduction of the Hunter b-value.
With regard to Hunter b-value, a positive b-value indicates yellow, whereas a negative b-value indicates blue. A lower b-value (near zero or negative) provides an optically enhanced and more desirable pigment color.
Though removal of discoloring impurities from the kaolin decreases the yellowness or Hunter b-value of the calcined kaolin pigment, many paper coating operations still must add optical whiteners to their coating formulations to further enhance the color of the coated paper.
The prior art discloses that the addition of a blue agent to kaolin and other mineral pigments can reduce the yellowness and increase the whiteness of the pigment, which carries over into the coating color formulation. Severinghaus U.S. Pat. No. 4,661,164 discloses that intermixing a blue pigment or dye with a mineral filler (such as calcium carbonate, kaolin, talc, feldspar, etc.) would increase the whiteness of the pigment. The blue agent preferred by Severinghaus is ultramarine blue, although cobalt aluminate, cobalt blue, iron blue, titanium blue, phthalocyanine blue and others are also disclosed as effective. This patent is focused on adding the blue agent directly to the mineral filler. For calcium carbonate, the blue agent addition would preferably occur during the grinding stage.
Young U.S. Pat. No. 5,154,766 blends a small amount of discrete iron oxide, as magnetite, with hydrous kaolin clays to decrease the yellowness of the pigment. The b-value is decreased, but the pigment brightness is significantly reduced as a result of the iron oxide addition.
Marshall et al. U.S. Pat. Nos. 5,587,011 and 5,667,639 disclose that the whiteness of clay materials is increased by adding cationic organic dyes. The dyes are added after the final beneficiation stage of hydrous kaolin processing (leaching/filtration) to make optically whitened pigments.
Von Raven U.S. Pat. No. 5,846,382 teaches the use of photoactivators of sulfonated zinc, manganese or aluminum phthalocyanines. The photosensitizing compounds are added to enhance the whiteness, brightness and chromaticity of inorganic fillers and pigments.
Each of the above prior art methods for improving the whiteness of hydrous kaolin pigments involves the addition of blue pigments or dyes to hydrous kaolin itself, or to paper coating formulations containing kaolin. In most cases, the resultant pigment is a simple physical mixture in which the blue agent can be subject to separation or segregation from the kaolin. In addition, organic pigments would burnout at elevated temperatures. Many inorganic pigments would undergo phase changes at elevated temperatures, and these changes could dramatically alter the color of such pigments.
Huege et al. U.S. Pat. No. 4,578,118, discloses a method for increasing the brightness of a calcined clay through the addition of an alumina source to the clay, then calcining the mixture. The brightness improvement shown by this patent for the calcined kaolin alumina blend is greater than the brightness improvement seen in a process that blends an alumina source and a previously calcined kaolin.
There is no method in the prior art for decreasing the yellowness and improving the whiteness of a calcined kaolin by adding a blue agent that overcomes the limitations of the prior art which are discussed above. The demand for calcined kaolin in paper making applications is increasing, as is the demand for these calcined kaolin products to be less yellow. In view of the foregoing discussion, there is a need in the industry for a calcined kaolin clay pigment having improved color (i.e., reduced yellowness and increased whiteness).
SUMMARY OF THE INVENTION
Briefly described, the present invention provides a calcined kaolin clay pigment which has improved color. The present invention also provides a process by which a blue agent is added to kaolin clay prior to calcination to produce a pigment having improved color.
As used in this application, the term “improved color” refers to reduced yellowness (as indicated by the Hunter b-value) and increased whiteness of the calcined kaolin clay pigment when compared to an equivalent calcined kaolin clay pigment which does not contain a blue agent.
We believe that, because the blue agent is calcined in conjunction with the kaolin clay, the blue agent becomes incorporated within the aggregates of the calcined kaolin clay. As the blue agent is permanently bound to the kaolin clay aggregates, the resulting pigment is not a physical mixture. Consequently, a significant advantage of this invention over the prior art is the reduced possibility for separation or segregation of the blue agent from the calcined kaolin clay pigment.
Accordingly, an object of this invention is to provide a calcined kaolin clay pigment.
Another object of this invention is to provide a calcined kaolin clay pigment having improved color.
Another object of this invention is to provide a calcined kaolin clay pigment having reduced yellowness.
Another object of this invention is to provide a calcined kaolin clay pigment having increased whiteness.
Another object of this invention is to provide a calcined kaolin clay pigment containing blue agent which has been added prior to calcination.
Another object of this invention is to provide a calcined kaolin clay pigment having a blue agent incorporated within the aggregates of the calcined kaolin clay pigment.
Another object of this invention is to
Maxwell Chris B.
Murray Haydn H.
Baker, Donelson Bearman & Caldwell
Thiele Kaolin Company
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