Compositions: coating or plastic – Materials or ingredients – Pigment – filler – or aggregate compositions – e.g. – stone,...
Reexamination Certificate
2000-10-04
2003-09-09
Koslow, C. Melissa (Department: 1755)
Compositions: coating or plastic
Materials or ingredients
Pigment, filler, or aggregate compositions, e.g., stone,...
C106S416000, C106S484000, C423S432000
Reexamination Certificate
active
06616749
ABSTRACT:
The present invention relates to pigment products, and their production and use in coating compositions.
The invention concerns the preparation of improved inorganic pigments for paper coating compositions, and, in particular, pigments for use in compositions for preparing gloss coated paper, especially lightweight and ultra lightweight coated paper.
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 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 &mgr;m 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
31 2
.
A very important white inorganic pigment for use in preparing coating compositions for the manufacture of LWC and ULWC papers 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 consists predominantly of the mineral kaolinite, together with small proportions of various impurities. Kaolinite exists in the form of hydrous aluminosilicate crystals in the shape of thin hexagonal plates, but these plates tend to adhere together face-to-face to form 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 of up to 10 &mgr;m or more. Generally speaking, kaolin clay particles which have an equivalent spherical diameter 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 a centrifuging step, to a grinding 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 colour 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 the Applicants 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 21 &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.
Such commercially available pigment products are not ideal, however. For example, their performance with starch adhesives could be improved. Starches are naturally available from vegetable sources and are the cheapest adhesives for use in coating compositions. However, LWC, ULWC and like papers produced using a coating composition comprising commercially available pigment together with starch suffers from a reduction in gloss compared with similar paper coated with a composition incorporating more expensive synthetic adhesive.
It is an object of the present invention to provide a pigment product showing improved gloss, particularly but not exclusively when used in a paper coating composition incorporating starch as adhesive or binder ingredient to coat paper to produce LWC, ULWC and other gloss paper products.
According to the present invention in a first aspect there is provided a pigment product suitable for use in a coating composition to provide a gloss coating on paper, the pigment product comprising a processed particulate kaolin which has a particle size distribution (psd) such that at least 80% by weight of the particles have an equivalent spherical diameter (esd) less than 2 &mgr;m and not less than 8%, preferably not less than 12% by weight, of the particles have an equivalent spherical diameter (esd) less than 0.25 &mgr;m and whose particles have a shape factor of at least 45.
The pigment product according to the first aspect of the invention unexpectedly and beneficially provides improved gloss when incorporated in coating compositions, especially including starch, for gloss paper coating applications as demonstrated later in this specification.
The pigment product according to the first aspect of the invention has an unusual combination of properties which are not normally obtained by conventional processing of kaolin clays from known sources. The reasons for this are as follows.
A kaolin product of high shape factor is considered to be more “platey” than a kaolin product of low shape factor. “Shape factor” as used herein is a measure of an average 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-2240398/U.S. Pat. No. 5,128,606/EP-A-0528078 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 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 differ from those in the United States of America in that the English deposits are of primary kaolin, whilst those in the USA are of the sedimentary or secondary type. Kaolin was formed in geological times by the hydrothermal decomposition
Husband John Claude
Payton Desmond Charles
Renals Michele Susan
Imerys Pigments, Inc.
Koslow C. Melissa
Manlove Shalie
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