Compositions: coating or plastic – Materials or ingredients – Pigment – filler – or aggregate compositions – e.g. – stone,...
Reexamination Certificate
2000-03-24
2002-11-05
Marcheschi, Michael (Department: 1755)
Compositions: coating or plastic
Materials or ingredients
Pigment, filler, or aggregate compositions, e.g., stone,...
C106S416000, C106S484000
Reexamination Certificate
active
06475274
ABSTRACT:
TECHNICAL FIELD
This invention relates to a process for modifying particle size distribution. In a more specific aspect, this invention relates to a process for modifying the particle size distribution of fine-grained Tertiary kaolin clays. This invention also relates to the kaolin clay pigments which are produced by this process and to paper products which are filled and/or coated with these pigments.
For ease of reference, this invention will be described with regard to fine-grained Tertiary kaolin clays from the State of Georgia, U.S.A. However, this invention will be understood as applicable to fine-grained Tertiary kaolin clays from other geographical regions.
BACKGROUND OF THE INVENTION
Kaolin is a naturally occurring, relatively fine, white clay which may be generally described as a hydrated aluminum silicate. Kaolin clay, after beneficiation, 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 kaolin clays found in the State of Georgia can be generally classified into two types: hard kaolins and soft kaolins. The hard kaolins are fine-grained (usually finer than 80%<2 microns e.s.d.) and typically found in Tertiary age sediments. The soft kaolins are relatively coarse-grained (usually coarser than 65%<2 microns e.s.d.) and typically found in Cretaceous age sediments.
The Cretaceous soft kaolin clays of Georgia contain abundant stacks and books of kaolinite, together with individual platelets of kaolinite. The coarse-grained nature of these clays is due to the recrystallization of kaolinite during weathering and/or diagenesis of the kaolin clay deposit. Tertiary hard kaolin clays of Georgia contain individual kaolinite platelets with a face-to-face association characteristic of a sedimentational fabric.
Kaolin clay filler and coating pigments are conventionally produced by dispersing a kaolin clay in water with the appropriate dispersing agent (such as sodium hexametaphosphate or sodium silicate) and a pH modifier (such as sodium carbonate or sodium hydroxide). After dispersion, the kaolin slurry is degritted using hydroseparators and screens to remove very coarse (>325 mesh) particles which are predominately silica and mica impurities. Following degritting, modification of the particle size distribution is made using centrifugation to remove coarse kaolinite particles. This centrifugation classification is generally used to produce pigments which conform to industry standards for paper coating applications. For example: a No. 1 coating clay has approximately 90%<2 microns e.s.d., and a No. 2 coating clay has approximately 80%<2 microns e.s.d.).
Another particle size modification technique used in the production of kaolin pigments is delamination. The Cretaceous aged coarse-grained kaolin clays contain coarse stacks and books of kaolinite which, when delaminated, cleave upon the basal planes of the kaolinite particle to form very platy particles with a high aspect ratio (aspect ratio=particle length/particle thickness).
After classification, the kaolin slurry can be treated using a reducing agent such as sodium hydrosulfite, an acid such as sulfuric acid and alum such as aluminum sulfate. This reductive leaching process improves the brightness of the pigment by reducing the ferric iron phases to soluble ferrous iron. In some instances, other processing techniques are conducted prior to leaching to produce higher brightness products. These techniques include flotation, selective flocculation and magnetic separation.
Following reduction of the iron, the clay slurry can be dewatered using filtration to remove the soluble iron along with other soluble salts. The dewatered kaolin clay is then redispersed using a dispersing agent such as sodium polyacrylate and a pH modifier such as sodium carbonate. After redispersion, the clay slurry may be spray dried or made into a high-solids slurry (65-72% solids by weight) for shipping.
One characteristic that kaolin clay pigments impart to paper coatings is sheet gloss. The relative fineness and particle shape of the kaolinite particles in the coating pigment control the sheet gloss, with the finer kaolinite forming a smoother and glossier paper surface. Furthermore, during the paper coating process, calendering may be used to further modify sheet gloss by passing the paper through a heated nip and compressing the coating. The advantage of using fine-grained clays over coarser clays is that fine-grained clays do not require a large amount of calendering (pressure or heat) which leads to a weaker sheet, reduced brightness, reduced opacity, slower production rates and overall higher paper processing costs.
The effect of fine kaolinite (i.e., particles<0.2 microns e.s.d.) on the printing characteristics of paper coatings is well known. The variation in the amount of fine kaolinite in paper coating pigments greatly affects the interaction of printing inks with the paper coating. In general, as fines increase, the print gloss of a coated paper decreases due to the effect of the fines on ink adsorption. Similarly, as fines decrease, the print gloss of a coated paper increases. Prior art has shown that the reduction of fines improves print gloss.
Modification of the particle size distribution of kaolin pigments used in paper and filler applications has been shown to alter the opacity of kaolin pigments. In general, a narrow particle size distribution improves the opacity of paper by increasing the light scattering ability of the pigment. Furthermore, delamination of coarse-grained Cretaceous kaolins which contain books and stacks of kaolinite can also modify the light scattering ability of the pigment by modifying particle shape and packing.
Many patents have described the use of kaolin pigments to provide improved sheet gloss, print gloss and opacity in paper coatings. In these patents, particle size modification, specifically by the removal of fine kaolinite (i.e., <0.2 microns e.s.d.), improves the paper coating and printing characteristics.
U.S. Pat. No. 2,992,936 describes a coating clay with improved brightness, gloss and opacity by producing a pigment with a narrow particle size distribution. U.S. Pat. No. 3,085,894 describes the same clay; however, this patent describes blends of the kaolin clay and calcium carbonate.
In general, the particle size distribution providing these improved characteristics in the prior art is as follows:
88-100% by wt. less than 1.7 microns e.s.d.
85-97% by wt. less than 1.5 microns e.s.d.
70-84% by wt. less than 1.0 microns e.s.d.
25-37% by wt. less than 0.5 microns e.s.d.
10-15% by wt. less than 0.3 microns e.s.d.
U.S. Pat. No. 4,279,661 describes grinding and/or classification to produce a mineral filler or coating composition with the particle size distribution having at most 15% less than 0.2 microns e.s.d. and 80-95% less than 1.0 micron e.s.d. This pigment is stated to provide a higher sheet and print gloss over those pigments with particle size distributions outside the claimed distribution.
U.S. Pat. No. 4,943,324 describes a >89 G.E. brightness pigment with a particle size distribution in which at most 15% of the particles are less than 0.3 microns e.s.d. The process uses fines removal (i.e., “defining”) and delamination to produce this pigment. Furthermore, the defined and delaminated clay is chemically treated to produce the pigment with improved opacifying properties.
U.S. Pat. No. 4,948,664 describes a coating composition comprising a clay with not more than 5% of the particles less than 0.25 microns e.s.d. and at least 5% of the particles having a particle size not less than 10 microns e.s.d. This patent further discloses that at least 40% of the particles have a particle size not less than 3 microns e.s.d. This composition is disclosed as useful in providing an improved pigment for use in gravure printing.
U.S. Pat. No. 5,085,707 describes paper coating compositions utilizing a defined and delaminated pigment, and these compositions cover a variety of combi
Black Don R.
Lowe R. Andrew
Malla Prakash B.
McClarin III William M.
Baker, Donelson Bearman & Caldwell
Marcheschi Michael
Thiele Kaolin Company
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