Compositions: coating or plastic – Materials or ingredients – Pigment – filler – or aggregate compositions – e.g. – stone,...
Reexamination Certificate
1999-04-01
2001-04-24
Brunsman, David (Department: 1755)
Compositions: coating or plastic
Materials or ingredients
Pigment, filler, or aggregate compositions, e.g., stone,...
C106S464000, C106S287350, C423S432000
Reexamination Certificate
active
06221146
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the production of precipitated calcium carbonate (pcc) having a selected crystal form and products produced thereby. In particular, the invention relates to the preparation of pcc products having a coarse crystalline form. Such products are useful in matte paper grades, i.e. grades in which the paper surface has a dull, low-gloss finish.
Manufacture of pcc products and the use of such products in compositions to make or coat paper are well known. However, there can be considerable variation in the properties of the pcc product depending upon the process conditions employed to prepare the product.
PCC products which are coarse i.e. have a mean particle size greater than 1.5 &mgr;m are known in the prior art, e.g. as described in U.S. Pat. No. 3,320,026, U.S. Pat. No. 4,882,310 wherein the particles are prismatic form and use of such coarse products in compositions to provide matte paper grades is also well known, e.g. from TAPPI Proceedings of 1982 Coating Conference, pages 143-146, “The influence of particle pigment shape on the performance of a paper coating, by Crenshaw; “Pigments for Matt Coated Papers” by Dr Billy Allwood, paper presented at PIRA Coated Paper Development Seminar held Sep. 13, 1990, Bolton, United Kingdom, U.S. Pat. No. 4,714,603 and U.S. Pat. No. 5,643,415 (WO9221613).
U.S. Pat. No. 4,714,603 describes a process for making such a coarse pcc product. The well known carbonation of slaked lime slurry using carbon dioxide is employed in the process described. A polyphosphate additive is included in the lime slurry to facilitate formation of coarse pcc particles. Carbonation is begun at a temperature of at least 15° C., preferably from 30° C. to 35° C. The particle form obtained is a spherical cluster or aggregate of fine nodules, e.g. containing up to 5,000 such nodules, the nodules having a height of from 0.02 &mgr;m to 1.0 &mgr;m.
U.S. Pat. No. 5,643,415 describes production of a coarse pcc and use of the same in coating paper to give a dull (matte) finish in the manner described in the earlier prior art specified above. The pcc crystal form is described as prismatic but the form illustrated in the Figures of the patent is roughly spheroidal rather than what is understood in the art to be prismatic. Polyphosphate, sodium hexametaphosphate, is added to a basic calcium carbonate (lime/pcc) slurry to assist crystal nucleation.
It is an object of the present invention to provide an improved process for producing a coarse pcc product for use in coating matte grade papers.
It is a further object of the present invention to provide an improved, selected pcc product form for use in coating matte grade papers and to provide a method of producing such a product form.
These and further objects which will become apparent from the following description are met by the present invention.
SUMMARY OF THE INVENTION
According to the present invention in a first aspect there is provided a method of producing a precipitated calcium carbonate (pcc) product which has a coarse crystalline form which comprises the steps of:
(a) preparing a suspension in an aqueous medium of calcium hydroxide particles which partially dissolve as calcium ions in the aqueous medium, the aqueous medium also incorporating an agent which promotes release into solution in the aqueous medium of calcium ions from the calcium hydroxide;
(b) adding carbon dioxide to the aqueous medium to react with the calcium ions therein to produce a suspension in the aqueous medium of a precipitate of calcium carbonate crystals;
(c) concentrating the suspension of the precipitate by a dewatering process; and
(d) adding to the suspension of the precipitate a dispersing agent for the precipitate;
wherein in step (a) the calcium hydroxide particles of the suspension of calcium hydroxide particles have a specific surface area measured by the BET nitrogen absorption method of less than 33 m
2
/g and the said agent comprises a water soluble organic compound present in an amount of from about 0.01% to about 0.20% by weight based on the dry weight of calcium carbonate to be produced from the calcium hydroxide in the aqueous medium, and in step (b) the carbon dioxide is first added to the aqueous medium when the temperature of the aqueous medium is in the range of from about 5° C. to about 12° C. and is added at a rate such that the average rate of reaction of carbon dioxide and calcium ions is in the range of from about 0.4 g/min/L to about 2.0 g/min/L.
According to the present invention in a second aspect there is provided a pcc product having a coarse, blocky particulate form as described further later in this specification, the product being produced by the method of the first aspect.
According to the present invention in a third aspect there is provided a coating composition for use in producing a matte grade coating on paper and other substrates which composition comprises an aqueous suspension of a particulate pigment together with a hydrophilic adhesive or binder, wherein the particulate pigment comprises the pcc product according to the second aspect of the invention.
DESCRIPTION OF THE INVENTION
The present invention provides a novel method of producing a pcc product having a coarse crystalline form in the manner defined earlier, a pcc product produced thereby, and a composition, for coating paper and the like, incorporating such a pcc product. The particles of the pcc product obtained may themselves comprise individual, coarse, crystals.
The particles of the pcc product after steps (c) and (d) of the method of the invention have been applied may have a median particle size (size at which 50% by weight of the particles are present in the particle size distribution—this size is known as “d
50
”) in the range of from about 1.8 &mgr;m to about 2.7 &mgr;m, especially about 2.0 &mgr;m to about 2.3 &mgr;m.
The median particle size and other particle size properties of the pcc product may be measured by the well known procedure of sedimentation, e.g. by use of a SEDIGRAPH™ particle size analysis instrument, e.g. a SEDIGRAPH™ 5100 instrument, available from Micromeritics Corporation. The “size” measured by this instrument is the diameter of an equivalent sphere which sediments at the same rate as the measured particle, the so called “equivalent spherical diameter”.
The median particle size of the particles in the suspension produced immediately following step (b) may be from about 0.2 &mgr;m to about 0.6 &mgr;m greater than that of the particles of the pcc product following steps (c) and (d).
The particles obtained following steps (c) and (d) may beneficially comprise individual crystals which have a rhombic, prismatic or blocky form. Such a form is generally the rhombohedral habit of the calcite form of calcium carbonate.
At least 60%, preferably at least 65% by weight of the pcc product comprising the particles obtained following steps (c) and (d) may have a particle size of from 5 &mgr;m to 2 &mgr;m. Preferably, the pcc product obtained following steps (c) and (d) has less than 12% by weight of its particles, desirably less than 6% by weight of its particles having a size less than 1&mgr;m.
Preferably the specific surface area measured by the BET nitrogen absorption method of the particles of the pcc product obtained following steps (c) and (d) is not greater than 5.5 m
2
.g
−1
, desirably not greater than 4.5 m
2
.g
−1
. Desirably, the Blaine surface area of the particles of the pcc product is in the range 10,000 to 20,000 cm
2
.g
−1
. Blaine surface area is a measure of particle coarseness as determined by air permeability. Preferably the Blaine surface area is from 11,000 to 13,000 cm
2
.g
−1
.
The method according to the first aspect of the invention may be carried out without seeding, i.e. without the use of seed crystals of the required crystal form in the aqueous medium at the start of the reaction between the carbon dioxide and calcium ions to provide growth sites for the pcc crystals. Such use is time consuming and therefore costly t
Bryan David P.
Fortier Steven M.
Jackson William B.
O'Rorke Peter B.
Perez Ricardo
Brunsman David
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Imerys Pigments, Inc.
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