Kaolin clay pigments, their preparation and use

Details Kaolin clay pigments, their preparation and use Kaolin clay pigments, their preparation and use

2001-12-20

2003-05-13

Koslow, C. Melissa (Department: 1755)

Data processing: artificial intelligence

Neural network

Structure

C106S416000, C106S484000, C106S487000

Reexamination Certificate

active

06564199

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to kaolin raw materials to produce novel kaolin clay pigments especially useful for coating lightweight and ultra-lightweight paper for rotogravure and offset printing. More particularly, the invention relates to the use of kaolin crude having a desired amount of non-kaolin phyllosilicate clay minerals to produce mechanically delaminated kaolin coating pigments. These pigments possess a unique combination of desirable properties not available in other kaolin pigments. When used in certain coating compositions, these pigments yield superior gloss in coated and printed papers compared to other kaolin pigments.
BACKGROUND OF THE INVENTION
Paper coating compositions are generally prepared by forming a fluid aqueous suspension of pigment material together with a hydrophilic adhesive and other optional ingredients.
Lightweight coated, or LWC, paper is generally coated to a weight of from bout 5 g.m
−2
to about 13 g.m
−2
on each side, and the total grammage, or weight per unit area of the coated paper is generally in the range of from about 49 g.m
−2
to about a5 g.m
−2
. LWC paper is generally used for printing magazines, catalogues and advertising or promotional material. The coated paper is required to meet certain standards of surface gloss and smoothness. For example, the paper is generally required to have a gloss value of at least about 32, and up to about 50, TAPPI units, and a Parker Print Surf value in the range of from about 0.5 to about 1.6 &mgr;m.
Ultra lightweight coated, or ULWC, paper is sometimes otherwise known as light lightweight coated, or LLWC, paper and is used for catalogues and for advertising and promotional material sent through the mail to reduce mailing costs. The coating weight is generally in the range of from 5 g.m
−2
to 7 g.m
−2
per side. The grammage is generally in the range of from about 35 g.m
−2
to about 48 g.m
−2
.
A very important white inorganic pigment for use in preparing coating compositions for the manufacture of LWC and ULWC papers for rotogravure or offset printing is kaolin obtained from kaolin clay. Large deposits of kaolin clay exist in Devon and Cornwall, England and in the States of Georgia and South Carolina, United States of America. Important deposits also occur in Brazil, Australia, and in several other countries.
Kaolin clay, also referred to as china clay or hydrous kaolin, consists predominantly of mineral kaolinite (Al
2
Si
2
0
5
(OH)
4
), an hydrous aluminum silicate, together with small concentrations of various other minerals.
Some of these other minerals in kaolin, such as fine ferruginous or titaniferous impurities impart undesirable color to the clay. Many processing techniques are known to remove significant amounts of these impurity minerals.
Additional minerals in kaolin, which fall into a classification of silicates and referred to as phyllosilicates, comprise mica, smectite, vermiculite, hydrobiotite, mixed or layered illite-smectite or mixed layers of other clay minerals. These latter minerals are known to have higher water adsorption capability than kaolinite, and therefore, generally have an undesirable effect on the rheology of high solids kaolin-water suspensions. That is, these minerals cause the suspensions or slurries to thicken and become more viscous at high and low shear rates. The kaolin industry generally avoids mining clays with micaceous or smectitic minerals. Alternatively, the kaolin industry removes micaceous and smectitic minerals with processes, such as desliming, or the industry blends such clays with relatively pure kaolin ores to reduce the concentration of micaceous and smectitic minerals to a level that does not affect clay-water suspension rheology. Examples of such beneficiation techniques include U.S. Pat. Nos. 4,182,785; 4,334,985; 4,477,422; and 5,593,490.
Kaolinite is an hydrous aluminosilicate that exists in the form of clay-sized crystals in the shape of thin hexagonal plates or in booklets of platelets called “stacks”. Kaolinite stacks may be subjected to a grinding action to easily separate or delaminate them into individual plates.
As long ago as 1939, Maloney disclosed in U.S. Pat. No. 2,158,987 that the finish, or gloss, of a clay coated paper is greatly improved if the clay, before incorporation in the coating composition, is treated so that a large percentage, for example 80% by weight or more, of the clay particles have a size in the range of 0.1 &mgr;m to 2 &mgr;m. In order to increase the proportion of fine particles in the raw kaolin, the raw kaolin may, according to the disclosure in U.S. Pat. No. 2,158,987 be subjected, before the centrifuging step, to a grinding or delamination operation in which a suspension containing from about 50% to about 75% by dry weight of kaolin and a dispersing gent is subjected to pebble milling. When the kaolin from the finer fraction is recovered, mixed with a suitable paper coating binder, and applied to the surface of a base paper, a coating of good gloss and color is obtained.
Various pigment products which are made using the principles described by Maloney in U.S. Pat. No. 2,158,987 are commercially available and provide good gloss and smoothness in coated papers, especially for LWC and ULWC paper. For example, a pigment product available from Imerys Minerals Ltd., formerly ECC International Ltd., and recommended for gloss coatings of LWC consists of a refined English kaolin product having a particle size distribution, “psd”, such that 89% by weight of the particles have an esd less than 2 &mgr;m, 74% by weight of the particles have an esd less than 1 &mgr;m and 25% by weight of the particles have an esd less than 0.25 &mgr;m.
Generally, the commercially available pigment products used for gloss coatings of LWC and ULWC paper are delaminated kaolin products, that is, they are subjected to a grinding process in order to break the stacks into plates. For a typical delaminated product, a kaolin clay slurry may be subjected to magnetic separation, grinding or delaminating, classifying, leaching, and filtering. The kaolin may be spray dried or formed into slurry containing 65% solids. Delaminated kaolin pigments can be made into flowable slurry at 65% solids as compared to kaolin pigments at 70% solids. Solids concentration lower than 65% has been considered undesirable.
A kaolin product of high shape factor is considered to be more “platy” than a kaolin product of low shape factor. “Shape factor”, as used herein, is a number that represents an average aspect ratio value (on a weight average basis) of particles, that is the ratio of particle diameter to particle thickness for a population of particles of varying size and shape as measured using the electrical conductivity method and apparatus described in GB-A-2240398/U.S. Pat. No. 5,128,606/EP-A-0528078 and in U.S. Pat. No. 5516617 and using the equations derived in these patent specifications. “Mean particle diameter” is defined as the diameter of a circle which has the same area as the largest face of the particle.
In the measurement method described in GB-A-2240398/U.S. Pat. No. 5,128,606/EP-A-0528078 the electrical conductivity of a fully dispersed aqueous suspension of the particles under test is caused to flow through an elongated tube. Measurements of the electrical conductivity are taken between (a) a pair of electrodes separated from one another along the longitudinal axis of the tube, and (b) a pair of electrodes separated from one another across the transverse width of the tube, and using the difference between the two conductivity measurements the shape factor of the particulate material under test is determined.
The kaolin deposits in England are of primary kaolin, whilst those in the USA are of the primary and secondary types. Kaolin was formed in geological times by the hydrothermal decomposition or by the weathering of the feldspar and mica components of granite and feldspathic metamorphic rocks, and primary kaolin is that which is obtained directly from the crystalli

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