Kaolin pigments, their preparation and use

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

2001-12-20

2003-03-25

Bell, Mark L. (Department: 1755)

Compositions: coating or plastic

Materials or ingredients

Pigment, filler, or aggregate compositions, e.g., stone,...

C106S484000, C106S485000, C106S487000, C106S488000, C106S416000, C423S432000

Reexamination Certificate

active

06537363

ABSTRACT:

1. Field of the Invention
The present invention relates to pigment products, and their production and use in coating compositions for preparing gloss coated paper, especially lightweight and ultra-lightweight coated paper. More particularly, the invention relates to paper coating pigments made from a crude feed comprising a naturally platy clay having a particle size distribution such that greater than 50% by weight of the particles have an esd less than 2 microns and less than 25% by weight of the particles have an esd less than 0.25 microns and a shape factor of 15 or more.
2. 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 about 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 65 g·m
−2
. The coating may conveniently be applied by means of a coating machine including a short dwell time coating head, which is a device in which a captive pond of coating composition under a slightly elevated pressure is held in contact with a moving paper web for a time in the range of from 0.0004 second to 0.01 second, before excess coating composition is removed by means of a trailing blade. However, other types of coating apparatus may also be used for preparing lightweight coated paper. LWC paper is generally used for printing magazines, catalogues and used for preparing lightweight coated paper. 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 60, 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
.
Rotogravure printing generally involves the use of an engraved or etched cylinder as an image carrier. Image areas are etched or engraved below non-image areas in the form of tiny sunken cells. The cylinder is immersed in ink, and the excess ink is scraped off by a blade. When the substrate contacts the printing cylinder, ink transfers, forming the image.
Offset printing is an indirect printing method in which the inked image on a press plate is first transferred to a rubber blanket that, in turn, “offsets” the inked impression to a press sheet.
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
O
5
(OH)
4
), an hydrous aluminum silicate, together with small proportions of various impurities.
Some of these impurities, such as fine ferruginous or titaniferous impurities impart undesirable color to the clay. Other impurities, such as mica, smectite, vermiculite, hydrobiotite, mixed or layered illite-smectite or mixed layers of clay minerals generally have an undesirable effect on the rheology of the kaolin clay-water viscosity. In order to eliminate or to reduce these impurities, the kaolin crude is subject to one or several beneficiation steps, most of which are well known to the mineral processing industry.
Kaolinite exists in the form of hydrous aluminosilicate crystals in the shape of thin hexagonal plates or booklets of platelets called “stacks”. The individual plates may have mean diameters of 1 &mgr;m or less, but kaolinite particles in the form of stacks of plates may have an equivalent spherical diameter (“esd”) of 10 &mgr;m or more. Generally speaking, kaolin clay particles which have an esd of 2 &mgr;m or more are in the form of stacks of kaolinite plates, rather than 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 delaminating operation in which a suspension containing from about 50% to about 75% by dry weight of kaolin and a dispersing agent 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., a British corporation, 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
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 measure of an average aspect ratio value (on a weight average basis) of the ratio of mean 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-2,240,398/U.S. Pat. No. 5,1286,06/EP-A-0,528,078 and in U.S. Pat. No. 5,516,617 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 both the primary and the sedimentary (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 granite matrix in which it was originally formed. On the other hand, secondary kaolin, also known as sedimentary kaolin, has been was

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