COMPOSITION AND METHOD OF MAKING IMPROVED HIGH BULKING CLAYS

Abstract: A delaminated and deslimed pigment is bulked with small and controlled quantities of a cationic polyelectrolyte such as polydimethyldiallyl ammonium chloride to produce a highly bulked pigment ideal for improving printability, opacity and sheet gloss in coated paper applications, particularly for LWC rotogravure papers.

Description: COMPOSITION AND METHOD OF MAKING IMPROVED HIGH BULKING CLAYS BACKGROUND OF THE INVENTION 1.1 Field of the Invention The present invention is directed toward compositions and methods of producing high bulking clay pigments. The pigments are demonstrated to significantly improve paper properties such as printability, opacity, and sheet gloss in paper coating applications, particularly for rotogravure papers including lightweight coating (LWC) and ultra lightweight coating (ULWC) papers. The pigment can also be used as a filler for paper webs. 1.2 Related Art It has been recognized that a relative narrowing of the particle size distribution of mechanically delaminated as well as non-delaminated kaolin particles results in pigments providing improved opacity in filling applications. Such pigments are disclosed as being especially advantageous when used in the manufacture of lightweight coated paper for rotogravure printing (see U.S. 4,948,664, Brociner et al.). It is well known to remove ultrafine kaolin particle, e.g., particles finer than about 0.3 micrometers, e.s.d. after delamination. This contributes to the production of a delaminated pigment product having a narrower particle size distribution than it would have if the ultrafines were not removed. U.S. 4,948,664, supra, shows that in cases where very narrow particle size distribution was required, delamination was followed by a coarse fractionation and secondary fine removal steps. Patentees did not remove the primary fines prior to delamination. In illustrative examples, there was a significant amount of secondary fines that were generated during delamination which had to be removed latter. Removal of fines is referred to as "defining" in U.S. 4,943,324 Bundy et al., and U.S. 5,085,707, also Bundy et al. In these patents, defining is always conducted after what appears to be a mild or slight delamination step. Sometimes removal of the fines is termed "desliming" as is used in the present patent application. The Bundy patents also disclose surface treatment of the clay with such materials as aluminum sulfate and hexamethylenediamine. It has also been recognized that "bulking" undelaminated kaolin pigments is an effective way of increasing the opacity of the pigment. U.S. 4,738,726, for example, discloses a highly bulked kaolin pigment useful for in making aqueous coating colors suitable for manufacturing coating lightweight publication papers or as filler for paper webs. The pigment is prepared by mixing a small but effective amount of a water-soluble cationic polyelectrolyte flocculant with a kaolin clay pigment in the presence of water to prepare the bulked clay pigment. The base kaolin clay is selected to have a particle size distribution prior to flocculation wherein less than 35 percent weight are finer than 0.3 micrometers. While the foregoing pigments possess particular characteristics, there is a continuing search for new pigments able to impart improved characteristics to paper. In particular, rotogravure paper applications have particular characteristics needing to be satisfied such as compressibility, printability (helio), opacity, sheet gloss, smoothness, and low coating weight. Applicants have found a new method and pigment compositions which surprisingly provide markedly improved paper characteristics such as those discussed above particularly in rotogravure paper printing applications. SUMMARY OF THE INVENTION The present invention relates to compositions and methods of producing kaolin-based pigments which impart markedly improved characteristics to paper, particularly paper used in rotogravure printing. One embodiment of the invention relates to a method for making a kaolin- based pigment comprising the steps of: (a) delaminating a kaolin clay, wherein the degree of delamination of the kaolin clay is done to the extent that increases the particle size distribution less than 2 μm in the range of about 5 to 40 percent over the particle size distribution less than 2 μm before delamination; (b) removing a portion of particles smaller than 0.3 μm from the delaminated clay; and (c) adding a bulking agent; wherein the degrees of delamination, removal of particles smaller than 0.3 μm, and addition of bulking agent are adjusted to an extent sufficient to provide a pigment containing particles in the range of 5 to 30 percent equal or less than 0.3 μm. Advantages of the compositions of this invention include improved properties to coated paper. In particular for coated paper used in rotogravure applications, significant improvements in smoothness, rotoprintability, brightness, opacity are obtained while maintaining desired sheet gloss. The improved properties may allow the reduction in use of expensive calcined clay or other more expensive additives without sacrificing optical and printing properties while achieving increased sheet gloss. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION The compositions and methods of this invention relate to kaolin-based pigments which impart improved properties to paper, particularly paper used in rotogravure applications. The present invention will become more apparent from the following definitions and accompanying discussion. Particle size distribution, as herein reported, is based on equivalent spherical diameter (e.s.d.) on a weight basis as measured by conventional sedimentation techniques using the SEDIGRAPH ® particle size analyzer supplied by Micromeretics Inc. It should be understood that the measurements of the size of clay particles for an undelaminated kaolin pigment that are 0.3 micrometer or finer are of limited reproducibility. Thus, when a SEDIGRAPH ® analyzer is employed, the value for weight percent may be -5% when tested by another operator or a different SEDIGRAPH ® analyzer is employed. The limited reproducibility extends to kaolin clay particle sizes above 0.3 micrometer for a delaminated kaolin pigment. This is stressed here because delaminated pigment is one of several essential features of this invention. Another essential feature of the invention is bulking. Bulking produces structuring of the pigment in a concentrated aqueous slurry of greater than 55% solids as observed in the increased value for Brookfield viscosity (or low shear viscosity). However, in preparation of a pigment sample for SEDIGRAPH ® analysis, the clay slurry is diluted to 6% solids and such a dilution for a bulked pigment renders the effect of bulking or structuring between particles not observable. Delamination as used herein refers to the operation of subjecting the naturally occurring kaolin particle "stacks" or "booklets" in the aqueous clay slurry to shearing force thereby reducing the kaolin stacks to thin platelets. Delamination may be carried out by subjecting an aqueous slurry of stacked kaolin particles to shearing action in a sand grinder, ball or pebble mills, extruders or rotor-stator colloid mills, or other suitable devices. Reference may be made to commonly assigned U.S. Pat. No. 5,645,635, the disclosure of which is hereby incorporated by reference, for a thorough discussion of the process of delamination of kaolin clay. The term "desliming" as used herein refers to the operation of separating and discarding a percentage of the fine fraction of the kaolin suspension. In each example presented herein, the defining operation was carried out in a centrifuge. The kaolin suspension to be "deslimed" was supplied to the centrifuge and processed therein to separate the suspension into a coarse fraction and a fine fraction. The fine fraction may be discarded in its entirety or only a selected percentage by volume of the fine fraction may be discarded, while the remainder of the fine fraction may be admixed with the coarse fraction for further processing. When discarding the selected percentage of the fine fraction, it is conventional that the percent defining level expressed refers to the volume percentage of the fine fraction which is discarded. For example, defining to a level of 40 percent means that 40 percent of the fine fraction from the centrifuge was discarded and that the remaining 60 percent of the fine fraction from the centrifuge was admixed with the coarse fraction from the centrifuge for further processing. The term "bulking " refers to a process by which clay pigments are modified to improve light scatter, which is a property quantifiable as a scattering coefficient, and generally provides a measure of the opacifying power of the pigment. Reference is made to commonly assigned U.S. Patent Numbers 4,640,716 and 4,738,726, the disclosures of which is incorporated by reference, which includes a more complete discussion of bulking and methods and materials ("bulking agents") useful in preparing bulked pigments. The term "hydrous" is intended to describe a clay which has not been subjected to calcination, i.e., a temperature at which the basic crystalline structure of the clay becomes altered. In the case of kaolin, maintaining the clay at temperatures under 450°C will not alter the kaolin's crystalline structure. Clays suitable for use in this invention include a wide variety of hydrous kaolin clays. While the examples of this invention may exhibit particular particle size distributions, the present invention is not intended to be limited to any particular particle size distribution. Conventional kaolin clay crudes used as sources of pigment grades of kaolin usually contain about 40 percent to 75 percent by weight of particles finer than 2 micrometers (μm) after removal of grit and coarse impurities. In conventional kaolin processing, the crude is fed into a blunger to separate the kaolin into small particles, that are mixed with water and a primary dispersant to form a clay-water slip or slurry. The primary dispersant can be sodium silicate, sodium polyphosphate or sodium polyacrylate and those known in the art. The amount of dispersing agent used will generally be in the range of from about 0.025 to 0.3 % by weight based on the weight of the dry clay. The clay particles treated with primary dispersant has a negative electric charge, that cause them to repel each other when the particles are suspended in water. The clay-water slurry is pumped from the blunger to rake classifiers or hydrocyclones and screens to remove most of the grits and very coarse impurities. The degritted slurry is collected into large, agitated storage tanks and pumped to the processing plant. At the processing plant, the kaolin slurry is collected in large storage tanks at the plant before it is processed. Generally the kaolin slurry is first scalped which separate the kaolin particles into a coarse and fine fraction through continuous centrifuges. The purpose of this step is to remove remaining grits and very coarse booklets. The fine fraction is the desired intermediate that is subjected to further processing. The degree of scalping is influenced by the desired particle size distribution, rheology and pigment optical properties of the final product. In general, the kaolin slurry is scalped to 70 to 95 % finer than 2 μm, preferably to 80 to 90 % at 2 μm. Scalping may be performed after other downstream beneficiation or processing steps such as after delamination. Delamination of the clay is conducted by conventional means such as those described in the above definition of delamination. The degree of delamination in order to obtain the benefits of the present invention will vary to some extent based on such variables as crude clay particle size distribution, source of crude, amounts of fines in the crude and smoothness of the kaolin surface. However, for a crude clay having a particle size distribution of 50% less than 2 μm to 70% less than 2 μm, good results were achieved by delaminating to a 5 to 40% delta at 2 μm, preferably to a 10 to 20% delta at 2 μm. In other words, a "delta" of 5 to 40% refers to increasing the particle size distribution at the 2 μm level by absolute percentage points of 5 to 40% over the undelaminated 2 μm particle size distribution level. At delamination levels below 5% delta, particle size distribution determinations are of limited reproducibility particularly when using conventional sedimentation particle size distribution techniques. For example, typical sedigraphs have a repeatability of about 4 to 5% at 2 μm so using this technique to measure particle size distribution deltas below 5% is of limited accuracy and value. At delamination levels above 40%, the rate of particle size distribution delta for all practical purposes levels off and no further delamination benefit is obtained. Therefore it is not practical to aim for a delamination delta above 40%. In the practice of this invention, scalping is optionally performed before or after delamination, or further downstream to remove the remaining grits and excessively coarse particles. In general, the kaolin slurry is scalped to 70 to 95 % finer than 2 μm, preferably to 80 to 90 % at 2 μm. The present invention is not intended to be limited to any particular scalping condition. Desliming of the delaminated clay can be done by any of a number of conventional desliming devices or methods such as those listed in the foregoing definition of desliming. Desliming refers to particle size separation leading to removal of the finest particles in the distribution of particle sizes. The finest particles are generally considered as particles finer than 0.3 μm. Desliming is customarilly accomplished by mechanical means. While chemical desliming is an emerging technology, the practice of this invention includes any means of effective removal of very fine particles in a kaolin slurry. An example of chemical removal of fine particles is disclosed in commonly assigned U.S. Patent Application 08/891,666, filed July 1 1, 1997, the disclosure of which is incorporated by reference. Mechanical desliming of a deflocculated aqueous slurry of hydrous kaolin may be performed by using a centrifuge such as a nozzle discharge disc centrifuge or a scroll discharge centrifuge. An example of a commercial unit is a horizontal three-phase centrifuge from Alfa Laval Co. (Greenwood, Indiana). The Alfa Laval centrifuges apply greater much greater "g" forces (in the range of about 3,000 to 10,000 g-forces) than conventional Bird centrifuges. The high speed Alfa Laval centrifuge can effect a sharp separation of kaolin particles finer than about 0.3 microns from larger kaolin particles. In the lab, desliming was accomplished with lower speed centrifuge, the Damon/IEC CU-5000 centrifuge at 2800 rpm and for about 7 to 15 minutes. In general, desliming is carried out to achieve particle size distribution at the 0.3 μm level of 5 to 30%, preferably in the 5 to 20 % range. It is believed that a wide variety of bulking agents may be used in accordance with this invention and meet the desired application particularly regarding rotogravure printing properties. Such bulking agents are referred to as water soluble cationic polyelectrolyte flocculants described, for example, in commonly assigned U.S. Patent 4,738,726, the disclosure of which is incorporated by reference. Cationic polyelectrolytes refer to substances containing macromolecules carrying a large number of cationic changes at the pH of application. Suitable polyelectrolytes include quaternary ammonium salt polymers, copolymers of aliphatic secondary amines with epichlorohydrin, poly(quaternary ammonium) polyester salts that contain quaternary nitrogen, polyamines and polyimines such as polyethyleneimines and polyampholytes having a plurality of cationic groups. One particularly useful group of polyelectrolytes described as bulking agents in copending, commonly assigned U.S. Patent Application 08/936,702, filed September 24, 1997, the disclosure of which is incorporated by reference, are the polyquaternary amine polymers derived from (i) reaction of secondary amines, such as dialkylamines, and difunctional epoxide compounds or precursors thereof or (ii) reaction of a lower dialkylamine (C,-C 3 ), a difunctional epoxy type reactant (the same as (i)) and a third reactant selected from the group consisting of ammonia, primary amines, alkylenediamines of from 2-6 carbon atoms, and polyamines. The group (i) polymers are disclosed in U.S. Pat.No. Re. 28,807 (Panzer, et. al.). The entire disclosure of this reissue patent is hereby incorporated by reference herein. As is stated in that reissue patent, the polyquaternary polymers of group (i) are derived from reaction of secondary amines, such as dialkylamines, and difunctional epoxide compounds or precursors thereof. In accordance with the reissue patent disclosure, the water soluble or water dispersible polyquaternary polymers, used as the second component in the present invention, consist essentially of the repeat units of wherein R and R, are independently selected from the group consisting of lower alkyl (1 -3 carbon atoms). E is the residue obtained after bifunctional reaction of a compound selected from the group consisting of epihalohydrins, diepoxides, precursors for epihalohydrins and diepoxides, and mixtures thereof, m and n are integers of substantially equal value. X θ represents the anion forming a portion of the polyquaternary compound; m and n are integers both representing the molar quantities of amine reactants and bifunctional reactant compound, respectively. In summary, the polymers of group (i) involve only two reactants: a lower dialkylamine, and a difunctional epoxy type reactant. As to the epoxy reactant, epihalohydrins, such as epichlorohydrin and epibromohydrin are especially useful. Epichlorohydrin is preferred. Diepoxides such as 1 ,4-butanediol-diglycidyl ethers are also useful. Precursors for epihalohydrins and diepoxides are also useful. Exemplary precursors include: l,3-dichloropropanol-2 and l,4-dichloro-2,3-dihydroxybutane. As to the secondary amines which may be used as reactants, these include dimethylamine, diethylamine, dipropylamine, and secondary amines containing mixtures of alkyl groups having 1 to 3 carbon atoms. Suitable reaction parameters may be found in U.S. Pat. No. Re. 28,807 and are not be repeated here. The preferred polymer of group (i) is formed from dimethylamine and epichlorohydrin reaction. Such reaction is detailed in Example 1 of the reissue patent. The preferred polyquaternary polymers of group (i) are thought to have the structure: CH 3 N θ - CH 2 CHCH 2 Cl θ CH 3 OH Suitable commercially available polymers of the group(i) type are sold under the trade names SHARPFLOC ® 22, SHARPFLOC ® 23, and SHARPFLOC ® 24. The molecular weight of these polymers are estimated to be in the range of approximately 2,000-10,000 atomic mass units (amu). The particular molecular weights of these polymers are not critical as long as the polymers remains water soluble or water dispersible. The group (ii) polymers which may be used in accordance with the invention, may be generically characterized as branched polyquaternary ammonium polymers and are described in detail in U.S. Patent No. Re. 28,808 (Panzer, et al.). The entire disclosure of this reissue patent is hereby incorporated by reference. As is stated in the 28,808 reissue patent, the group (ii) water dispersible polyquaternary polymer consists essentially of repeating units of wherein R, R,, E, m, and n are the same as given above for the group (i) polymers. A is the residue obtained after bifunctional reaction of a polyfunctional polyamine selected from the group consisting of ammonia, primary amines, alkylene diamines of 2 to 6 carbon atoms, polyalkylpolyamines of the structure wherein y represents an integer of about 1 to 5, R 3 is an alkylene radical of about 2 to 6 carbon atoms, and R 4 is selected from the group consisting of hydrogen, alkyl of about 1 to 3 carbon atoms, and omega -aminoalkyls of about 2 to 6 carbon atoms, a polyglycolamine of a structure such as CH 3 H 2 N-[-CH 2 CH-O -]- a CH 2 CH-NH 2 wherein a is an integer of about 1 to 5, piperazine heteroaromatic diamines, and polyamine-polybasic acid condensation products of molecular weight up to about 10,000; X θ is an ion forming the anionic portion of said polyquaternary compound; m and p are integers which represent molar quantities of amine reactants, the ratio of m to p being from about 99:1 to 85:15; n represents the molar quantity of E forming the principal chain of said polyquaternary, the molar quantity represented by n being substantially equal to the sum of the molar quantities of m and p; said polyfunctional amine containing in addition to the amount of E required for difunctional reaction therewith an amount of E which is from zero to about the full functional equivalency remaining in said A; the sum of m, n and p being such as to provide a polyquaternary compound which as a 37% aqueous solution, by weight, based on the total weight of the cationic portion of said polyquaternary has a viscosity at 25° C. of at least = 100 centistokes and Z is an integer such as to satisfy anion requirements of the polyquaternary compound. In summary, the group (ii) polymers are formed from three reactants: a lower dialkylamine (C,-C 3 ), a difunctional epoxy type reactant (the same as in the group (i) polymers) and a third reactant selected from the group consisting of ammonia, primary amines, alkylenediamines of from 2-6 carbon atoms, and polyamines as defined hereinabove for A. The preferred type of third reactant is ammonia or at least a trifunctional amine, i.e., an amine capable of reacting at no fewer than 3 sites on the amine or amines. Examples of such amines are all primary amines and/or polyfunctional amines such as ethylene diamine and diethylene triamine. Exact reaction parameters for the group (ii) cationic polyelectrolytes are specified in aforementioned U.S. Pat. No. Re. 28,808. A preferred group (ii) polymer is a cross-linked polyquaternary polymer formed from ethylenediamine, dimethylamine and epichlorohydrin (see for instance Example 2 of U.S. Pat. No. Re. 28,808). The preferred group (ii) polymer is thought to have the structure: CH, OH OH -N Θ -CH 7 -CH-CH,- N ffi -CH,- CH-CH,- CH; ! CH 2 I 1 -CH 2 -CH-CH 2 - - N θ -CHy -CH-CH2- - N φ - 1 CH 3 Suitable commercially available polymers of the group (ii) type are sold under the trade names of SHARPFLOC ® 25, SHARPFLOC ® 26, SHARPFLOC ® 27, SHARPFLOC ® 28, SHARPFLOC ® 29, SHARPFLOC ® 30, SHARPFLOC ® 31 , SHARPFLOC ® 32, and SHARPFLOC ® 33. The molecular weight of these polymers are estimated to range from approximately 20,000 to 500.000 amu. The particular molecular weights of these polymers are not critical as long as the polymers remain water soluble or water dispersible. Thus, the foregoing molecular weight range and the molecular weight ranges of other polyelectrolytes disclosed in this invention should not be interpreted as limitative in the present invention. Yet another particularly useful group of polyelectrolytes are quaternary ammonium salts. Most preferred are dialkyl, diallyl quaternary ammonium salt polymers which contain alkyl groups of about 1 to 4 carbon atoms, preferably methyl. A dimethyl diallyl quaternary ammonium chloride polymer commercially available under the trademark designation Polymer 261 LV from the Calgon Corporation having a molecular weight estimated to be between 50,000-250,000 has been found particularly useful in the practice of the present invention and has FDA approval (Code 176-170) for aqueous and fatty food use. Many reagents heretofore proposed to bulk clay do not have FDA approval. However, the invention is not limited to Polymer 261 LV since other cationic flocculants appear to provide equivalent, if not superior results. The amount of polyelectrolyte employed is carefully controlled to be sufficient to improve the opacity of the clay as a result of forming a bulked (aggregated) structure in which the aggregates are sufficiently strong to survive mechanical forces exerted during manufacture and end use but is carefully limited so as to assure that the product can be formed into a clay-water slurry that has a solids content of at least 55 percent or higher, which slurry has acceptable rheology. As discussed above, while bulking has a strong influence on rheology for a concentrated kaolin slurry, the particle size distribution measured by SEDIGRAPH ® analysis does not normally show a significant change from that prior to bulking. The amount of the cationic polyelectrolyte salt used to treat the kaolin clay may vary with characteristics of the polyelectrolyte including charge density of the polyelectrolyte, the particle size distribution of the clay and solids content of the clay slurry to which the polyelectrolyte is added. Using the presently preferred dimethyldiallyl ammonium salt polyelectrolyte with clay having a medium size in the range of about 0.4 to 0.9 micrometers, preferably 0.5 to 0.7 μm, and having less than 25 percent finer than 0.3 micrometers and adding polyelectrolyte to a previously deflocculated clay-water suspension having a clay solids content of about 15-40 percent by weight, useful amounts range from about 0.02 to about 0.20 percent by weight of the moisture free weight of the clay, most preferably about 0.06 to about 0.12 percent by weight. When insufficient polyelectrolyte is used, the effect on opacity and printability in coating applications may be less than desired. On the other hand, an excessive amount of the polyelectrolyte may impair other desired properties of the clay, especially rheology. The polyelectrolyte, which is water soluble, is added to the slurry as a dilute aqueous solution, e.g., 1/4 -2 percent concentration on a weight basis, with agitation to achieve good distribution in the slurry. Ambient temperature can be used. It may be advantageous to heat the slurry of clay, solution of polyelectrolyte, or both to about 150° to 180°F. Satisfactory results have been realized when the cationic polyelectrolyte was added to deflocculated clay suspensions having pH values in the range of 6 to 9. After addition of polyelectrolyte, the suspension is substantially thickened as a result of flocculation. The resulting thickened system is then acidified, typically to a pH below 5, usually pH 3-4, and bleached using conventional clay bleach (hydrosulfite salt such as sodium hydrosulfite) and aged. The bleaches used are usually reductants which reduce any color forming ferric ion constituents to a more water soluble and therefore more easily removeable ferrous state. The bleaching agents are added to the clay mineral slurry in an amount in the range of 1 to 15 lb of bleaching agent per ton of dry clay. The such treated clay suspension is dewatered by filtering to a moist filter cake having a solids content of between about 50 to about 60% by weight. The filter cake is then washed to remove soluble material and then fluidized by addition of a secondary dispersing agent, such as tetrasodium pyrophosphate or sodium polyacrylate or a mixture of the two. To remedy possible problems encountered when slurries of this invention are stored or exposed to high temperature during storage, J) shipment or subjected to moderate shear conditions, additives such as those disclosed in U.S. Patents 4,772,332 and 4,767,466 may be beneficially used instead of conventional secondary dispersants like tetrasodium pyrophosphate or sodium polyacrylate. The products of this invention may be shipped in slurry or in dry form. Desirably the slurry will have a total solids content ranging from at least 55% solids. Types of additives that may be used with the present invention include those described in U.S. Patents 4,772,332 and 4,767,466 the disclosures of which are hereby incorporated by reference. These additives are particularly useful for remedying problems encountered when aqueous slurries containing pigments of this invention are stored or exposed to high temperature during storage, shipment, or use for example when slurries are prepared into coating colors while providing acceptable rheology. The rheological requirements of pigments of this invention are concerned both with acceptable high solids slurry rheology and coating color rheology. The viscosity of the high solids suspension of the clay coating pigment must be sufficiently low to permit mixing and pumping. After the binder is incorporated, the resulting coating color must also have suitable viscosity for handling and application to the paper sheet. In addition, it is highly desirable to obtain a coated calendered sheet which has good opacity, gloss, brightness and printability. Generally, paper makers seek to use clay coating pigments capable of forming high solids clay -water slurries which have a low shear viscosity below 1200 cp, preferably below 800 cp, when measured by the Brookfield viscometer at 20 r.p.m. High shear viscosity for these slurries should be such that they are no more viscous than a slurry having a Hercules endpoint viscosity of 150 r.p.m., preferably 800 r.p.m., using the "A" bob at 16 x 10 5 dyne-cm. Those skilled in the art are aware that when using the Hercules viscometer and measuring endpoints of 1100 r.p.m. or higher, endpoint viscosities are reported in units of 10 5 dyne-cm at 1 100 r.p.m.; apparent viscosity decreases as the value for dyne-cm increases. It is conventional to use the abbreviated term "dyne" in place of 10 5 dyne-cm. Thus, a "2 dyne" clay slurry is less viscous than a "9 dyne clay" slurry. As used hereinafter the expressions 150 r.p.m. or higher, or 800 r.p.m. or higher, are intended to include lower viscosities such that endpoint measurements are at 1100 r.p.m. and the values are reported as dynes. Another requirement of pigments of this invention is that of durability to survive the various stages of production and end-use while possessing the capability of being dispersed to form high solids clay- water slurries having acceptable viscosity. The general wet processing scheme typically employed in making pigments of this invention is by adding a bulking agent before filtration, and therefore the filtered pigment is in the filter cake containing the bulked assemblages when the filter cake is "made down" into a fluid slurry. The expressions "make down" and "made down" are conventional in the industry and refer to the preparation of dispersed pigment-water slurries. In some cases, it may be necessary to apply mechanical work to the filter cake to reduce the viscosity to usable values. The pigment must be sufficiently tenacious to survive the mechanical forces during such treatment. Bulking pigments must also be sufficiently stable under the influence of shear to maintain the bulked structure under the high shear rates, such as the high shear rates encountered in pumping high solids clay water slurries in centrifugal pumps. Moreover, the pigment must be capable of being retained when the deflocculated clay water slurry is formed into a coating color using standard makedown equipment. Also, the pigment must survive during the coating application and subsequent calendering. The fragility of the bulked structures obtained by prior art chemical treatments of hydrous clay has limited their commercial use. Generally, a criterion for durability of a bulked structure is the retention of improved opacification after the above-described handling. In preparing coating colors, conventional binders or mixtures of binders are used with the deflocculated clay slip. For example, useful coating color compositions are obtained by thoroughly mixing with the clay slip from about 5 to about 20 parts by weight binder per 100 parts by weight of polyelectrolyte treated clay. Such a coating color, when used for coating lightweight publication paper, produces a product which has excellent printability, smoothness, opacity, brightness and desired level of sheet gloss. The term "binder" as used herein refers to those materials known for use in connection with paper pigments, which aid in holding the pigment particles together and. in turn, holding the coating to the paper surface. Such materials include, for example, casein, soybean proteins, starches (dextrins, oxidized starches, enzyme-converted starches, hydroxylated starches), animal glue, polyvinyl alcohol, rubber latices, styrene-butadiene copolymer latex and synthetic polymeric resin emulsions such as derived from acrylic and vinyl acetates. When the binder comprises a starch which is jet cooked in the presence of added bulking pigment, it may be desirable to heat the slurry of clay into which the polyelectrolyte is added during preparation of the bulking pigment in order to avoid the development of extremely viscous, unworkable coating colors. Temperatures in the range of about 150° -200°F. are recommended. A temperature of about 180°F. has been used with success. However, use of heat during preparation may decrease the scattering ability of the pigment. The coating color compositions prepared in accordance with the present invention can be applied to paper sheets in a conventional manner. The bulked pigment may be used alone or blended with a known coating clay or other pigments to improve optical and printing properties of the coated paper sheet. The binders and additives used in the coating color are those typically used in the industry and are known to those skilled in the art. The bulked pigment could also be used as filler in paper web. EXAMPLES Example 1 Properties of four (4) kaolin pigments were compared. The first pigment, P-l, was a standard fraction delaminated coating grade kaolin available under the tradename NUCLAY supplied by Engelhard Corporation. A slurry of NUCLAY was prepared according to the PL-1 and PL-3 procedures of Engelhard Corporation described in U.S. Patent 4,738,726 the disclosure of which having already been incorporated by reference. The dispersant used to re-disperse the P-l pigment during the make down procedure was a 18 to 21 weight % aqueous solution of soda ash, partially neutralized polyacrylic acid, and sodium hexametaphosphate ("SAP" dispersant) at an active weight ratio of 45.5/24.5/30, respectively. The second pigment, P-2, was prepared from a deflocculated aqueous suspension of Georgia kaolin clay. The deflocculating agent was sodium silicate. Solids content was about 29%. The particle size distribution of the clay in the deflocculated aqeous suspensions was 68% less than 2.0 micrometers, 0.76 micrometers median diameter and 22% less than 0.3 micrometers diameter. The suspension was further deflocculated with 2 lbs./ton of sodium polyacrylate and delaminated to a particle size distribution of 15-20 percent delta at 2 μm in a 5-gallon wet grinder using a 1 to 1 volume ratio of glass beads to pigment suspension. The particle size distribution of the delaminated suspension was 85.8% less than 2 micrometers, 0.55 micrometers median diameter and 25.9% less than 0.3 micrometers diameter. The delaminated suspension was then fractionated in a centrifuge Damon/TEC CU-5000 Centrifuge to yield an overflow suspension with particle size distribution of 87.9% less than 2.0 micrometers, 0.50 micrometers median diameter and 28.3% less than 0.3 micrometers diameter. The resulting suspension was passed through a magnetic separator magnet (Carpco-CC WHIMS 3x4L) to achieve above 83 brightness, actual brightness was 83.4. The suspension was deslimed by centrifugation with a Damon TEC CU-5000 centrifuge to obtain an underflow suspension with particle size distribution of 84.0% less than 2.0 micrometers, 0.66 micrometers median diameter and 16.2% less than 0.3 micrometers diameter. The resulting suspension was flocced using 8 lbs./ton of aluminum sulfate and followed by lowering in pH to 2.8 with sulfuric acid and addition of 10 lbs./ton of sodium hydrosulfite (commercially available as KBrite). The flocced pigment suspension was pan-filtered and rinsed with at least an equal volume of clean water to remove water soluble salts. The rinsed filter cake was redispersed with about 5 to 7 lbs./ton of SAP dispersant. Portions of this redispersed suspension was spray dried and then added back to the suspension to raise the solids to 67.0% total solids. The third pigment, P-3, was a deslimed and bulked kaolin coating grade pigment (i.e, undelaminated) available under the tradename EXSILON supplied by Engelhard Corporation and made down according to Engelhard's PL-1 and PL-3 methods as described above for the make down of the P-l pigment using the C-235 dispersant hereinafter described. The fourth pigment, E-l, was a bulked version of P-2 pigment but with additional differences in the redispersion package. To prepare E-l, the deslimed intermediate from P-2 was diluted to 20% solids and bulked with 1.6 lbs./ton (0.08%) of polydimethyldiallyl ammonium chloride (polyDADMAC). Addition of polyDADMAC flocced the suspension. Further floccing was accomplished by lowering the pH to about 3.5 with sulfuric acid. In addition. 10 lbs./ton of sodium hydrosulfite was added as bleach. The resulting dilute suspension was aged overnight, pan-filtered and rinsed with at least an equal volume of clean water to remove water soluble salts. The rinsed filter cake was redispersed with a special dispersant stabilizer additive package sufficient to raise the pH to about 6.5 to 7.0. The special dispersant package used is a mixture of sodium ligno-sulfonate, partially neutralized polyacrylic acid, pentasodium salt of aminotri(methylenephosphonic acid) and caustic and commercially available under the tradename Colloid 235 ("C-235") from Vinings Industries, Inc. Variations of the special dispersant package is disclosed in U.S. Patent 4,772,332 the disclosure being incorporated by reference. Portions of this redispersed suspension was spray dried and then added back to the suspension to raise the solids to about 61 to 62% total solids. While pigments P-l and P-2 used a different dispersant package than that of pigments P-3 and E-l, such differences are not expected to affect the coating color composition properties hereinafter reported except that the C-235 package might give slightly lower sheet brightness than the SAP package. In any event, since C-235 was used with the inventive composition of this invention, the use of C-235 certainly would not provide a brightness benefit in the coating color compositions compared to color compositions using the SAP dispersant package. Table 1. Hydrous Kaolin Pigment Properties subsequent examples. 2 - Hercules viscosity as measured in U.S. Patent No. 4,738,726 in this and subsequent examples. The physical properties of the four pigments and rheology of their pigment slurries are given in Table 1. Within experimental error as discussed before, the particle size distribution for P-2 and E-l are not substantially differently despite the fact that E-l is a bulked version of P-2. Both P-2 and E-l have lower quantities of kaolin particle fraction below 0.3 μm compared to P-l and P-3. The influence of bulking in E-l vs the unbulked P-2 was evident in their low shear rheology and the solids level of their suspension. P*-2 can be made into 67% solids suspension while E-l was only made into a 61.4% solids suspension. Brookfield rheology showed that structuring from bulking resulted in much higher low shear viscosity in E-l even at a significantly lower solids level compared to P-2. The four pigments were made into coating colors at 58 percent total solids in a generic LWC coating formulation containing 88 parts of one of the hydrous kaolin pigments, 12 parts of calcined clay and 6.0 parts of a styrene/butadiene (SBR) latex binder, 0.8 parts of calcium stearate. The pH of the coating colors were adjusted to 8.2. The colors were coated on the wire side of a 28 lb lightweight paper basestock using the laboratory Dow Coater. The coat weight was applied at 5.5 lb/R (R=ream=3300 sq.ft.) The sheets were conditioned at 50 percent relative humidity and 72°F and were calendered on a lab soft-nip calender . This condition was arrived at that which the P-l (NUCLAY) control achieved a gloss target of 57-58. The following tests were performed on the coated papers: TAPPI 75° gloss (T480 om-85), TAPPI Opacity (T425 om-91), ISO Brightness (ISO Method #2469). Parker Print Surf (at 10 kgF/cm 2 ) (ISO 8791/4 and TAPPI T555), Compressibility and Heliotest (rotoprintability test). Compressibility of the coated sheet was determined by an Engelhard in-house test based in part on the Parker Print Surf (PPS) test. The principle of the PPS test is based on determining the smoothness (or roughness) of the surface of a sheet of paper or board as a function of the rate at which air will pass between the surface and a flat circular land pressed against it at a specified pressure. Compressibility is gauged in this invention by testing the relative air flow at two different pressures. The air flow rates are measured by doing the PPS test at 5 kg/cm 2 and 10 kg/cm 2 . Compressibility is then calculated as Compressibility = 100 [1-[(PPS 5kg/cm2 - PPS .o^ PPS Αg/Cm2 ]] By this method the lower the number, the higher the compressibility and vice versa. It is generally accepted that good quality gravure printing requires a compressible paper capable of reliably contacting the printing cylinder over its entire area thereby ensuring that the maximum possible ink pick-up from the cylinder is achieved. In the Helio test, the coated sheet is printed with a gravure cylinder, which has a pattern of ink holding cavities that decrease in diameter from one end to the other. Thus, the test print has large dots at one end and small ones at the other. Skipped dots are counted starting at the large-dot end, and the print quality is reported as the distance in millimeters from the start of the test print to the 20th missing dot. For a given coat weight, the longer the distance in millimeters the better the printability of the coated paper. The results are summarized in Table 2. Table 2. Properties of Sheet Coated with Coating Color Containing Polymer Bulked, Deslimed and Delaminated Clay and PPS (smoothness) Data in Table 2 indicate that while the delaminated and deslimed pigment P-2 gave significantly improved opacity and brightness over the P-l (NUCLAY) control, no appreciable improvement in helio printability was observed. However, by bulking P-2 with a small amount of polyDADMAC (i.e., to make E-l), E-l exhibited similar advantages in opacity and brightness but with surprisingly marked improvements in helio printability, smoothness (from the lower PPS readings) and compressibility. Further Helio printability, compressibility, and smoothness of coated sheets treated with E-l are significantly better than P-3, the undelaminated but bulked pigment. Example 2 New batches of P-2 and E-l were prepared and are referred to P-2' and E-l', respectively. The pigment P-2' was prepared from a deflocculated aqueous suspension of Georgia kaolin clay. The particle size distribution of the clay in the deflocculated aqeous suspensions was 64% less than 2 micrometers and 15% less than 0.3 micrometers diameter. The 30.0% solids suspension was further deflocculated with 2 lbs./ton of sodium polyacrylate and delaminated to a particle size distribution of 15-20 percent delta at 2 μm in a 5-gallon wet grinder using a 1 to 1 volume ratio of glass beads to pigment suspension. The particle size distribution of the delaminated suspension was 82% less than 2 micrometers and 20% less than 0.3 micrometers diameter. The delaminated suspension was then fractionated in a centrifuge Damon/TEC CU-5000 Centrifuge to yield an overflow suspension with particle size distribution of 89% less than 2 micrometers and 22% less than 0.3 micrometers diameter. The resulting suspension gave a pigment brightness of 83.5 and was not passed thru a magnet separator. The suspension was deslimed by centrifugation with a Damon TEC CU-5000 centrifuge to obtain an underflow suspension with particle size distribution of 87% less than 2 micrometers, 0.65 micrometers median diameter and 15% less than 0.3 micrometers diameter. The resulting suspension was flocced using 8 lbs./ton of aluminum sulfate and followed by lowering in pH to 2.8 with sulfuric acid and addition of 10 lbs./ton of sodium hydrosulfite (commercially available as KBrite). The flocced pigment suspension was pan-filtered and rinsed with at least an equal volume of clean water to remove water soluble salts. The rinsed filter cake was redispersed with about 5 to 7 lbs./ton of SAP dispersant as described and used in Example 1. Portions of this redispersed suspension was spray dried and then added back to the suspension to raise the solids to 67.1% total solids. The pigment, E-l' was a bulked version of P-2' pigment but with additional differences in redispersion package. To prepare E-l', the deslimed intermediate from P-2' was diluted to 20% solids and bulked with 1.6 lbs./ton (0.08%) of polydimethyldiallyl ammonium chloride (polyDADMAC). Addition of polyDADMAC flocced the suspension. Further floccing was accomplished by lowering the pH to about 3.5 with sulfuric acid. In addition, 10 lbs./ton of sodium hydrosulfite was added as bleach. The resulting dilute suspension was aged overnight, pan-filtered and rinsed with at least an equal volume of clean water to remove water soluble salts. The rinsed filter cake was redispersed with the C-235 dispersant additive package as described and used in Example 1 and in an amount sufficient to raise the pH to about 6.5 to 7.0. Portions of this redispersed suspension was spray dried and then added back to the suspension to raise the solids to about 61 to 62% total solids. Table 3. Properties of Improved Bulked Pigment The physical properties of the three (3) pigments including the same P-l (NUCLAY) control sample as used in Example 1 and rheology of their pigment slurries are given in Table 3. Within experimental error as discussed before, the particle size distribution for P-2' and E-l' are not substantially different despite the fact that E-l' is a bulked version of P-2'. Both P-2' and E-l' have lower quantities of kaolin particle fraction below 0.3 μm compared to P-l . The influence of bulking in E-l' versus the unbulked P-2' was again evident in the solids level achievable for their suspension. P-2' can be made into 67.1% solids suspension while E-l' was only made into a 61.5% solids suspension. If P-2' and E-l' suspensions were compared at 61.5% solids, the low shear viscosity of P-2' is expected to be extremely low compared to E-l'. This expectation is based on the structuring effects from bulking. The three (3) pigments were formulated into 57 percent total solids coating colors using substantially the same formulation as in Example 1. The coating color was applied to a 28 lb., weight basestock using the CLC coater (a high speed pilot batch coater). Coat weight was 5.0 lb/3300 sq.ft. Conditioning of the coated sheets was identical to that in Example 1. Calendering conditions were selected to achieve a gloss target of 56 for the Nuclay control. The properties of the coated sheet are given in Table 4 based on an equal calendering conditions as noted below in Table 4. Table 4. Coated Sheet Properties Equal Calendering P-l (NUCLAY) 88 P-2' 88 E-r 88 94 Calcined Clay 12 12 12 6 % Sheet Gloss 56 63 58 63 ISO Brightness 71.1 72.0 71.9 71.8 Tappi Opacity 84.7 85.8 85.8 85.8 Helio Printability 74 79 89 86 PPS 10 Kgf/cm2 0.99 0.92 0.88 0.89 Compressibility 72.2 72.0 70.8 72.5 CLC Runnability Better Better Good Better Calendering conditions: Equal Calendering Pli (lbs./linear inch) 1093 Temp (F) 175 Speed (ft/min) 37 Passes 2 At equal calendering conditions, E-l' gave better to much better coating and print properties than the control P-l at equal calcined clay content (12 parts). E-l' had a significant advantage over the unbulked version, P-2' as well. Helio printability, smoothness (PPS), opacity, brightness and compressibility were significantly improved over the P-l control. Also at equal parts of calcined clay (12 parts), E-l' outperformed P-2' with marked improvements in helio printability, smoothness (PPS) and compressibility. Sheet gloss for E-l' can be improved dramatically by reducing calcined clay from 12 parts to 6 parts while maintaining helio printability, smoothness, brightness and opacity. By reducing the amount of calcined clay, CLC runnability was also improved. Example 3 This example demonstrates the use of another bulking agent described as a copolymer of aliphatic secondary amines with epichlorhydrin commercially available under the tradename SHARPFLOC ® 26 ("SF-26") available from Sharpe Speciality Chemical Company and compares results against the bulking agent polyDADMAC used in making E-l and E-l'. This time, another deslimed and delaminated kaolin clay intermediate was used from that used in Examples 1 and 2 and identified as P-4 in Table 5 below. The intermediate was derived from a crude having a particle size distribution range of approximately 50 to 75 % less than 2 μm. P-4 was also redispersed with the SAP dispersant of Example 1 after having been filtered and rinsed. Pigment E-2 was bulked with 1.6 lbs/ton of poly(DADMAC) while E-3 was bulked with 2.0 lbs/ton of SF-26 polymer. After bulking, both pigment suspensions were filtered and rinsed with clean water of remove soluble salts. The E-l and E-2 pigment suspensions were redispersed with 6 lbs/ton of SAP dispersant as described and used in Example 1 and passed through a 325 mesh screen and spray dried. Thus in this example, pigments P-l, P-4, E-2, and E-3 all used the SAP dispersant. The pigment and slurry properties of P-4, E-2, E-3 and the previous control P-l (NUCLAY) pigment are given in Table 5. Both E-2 and E-3 have slightly lower kaolin particle fraction below 0.3 μm than P-l and P-4. Both pigments were made down to significantly lower solids than the P-l control and P-4 pigments. Table 5. Hydrous Kaolin Pigment Properties Pigments P-l, E-2, and E-3 were made into coating colors at 57 percent total solids in the same generic LWC coating formulation discussed in Example 1. The coating color was applied to a 28 lb., weight basestock using the CLC coater (a high speed pilot batch coater). Coat weight was 5.5 lb/3300 sq.ft. Conditioning of the coated sheets was identical to that in Example 1. Calendering conditions were selected to achieve a gloss target of 54 for the Nuclay control. The properties of the coated sheet are given in Table 6 based on equal calendering conditions as noted below in Table 6. Table 6. Coated Sheet Properties Calendering conditions: Equal calendering Pli (lbs/linear inch) 1407 Temp (F) 150 Speed (ft/min) 36.5 Passes 2 E-2, the poly(DADMAC) bulked pigment, gave better print properties and smoothness than the control P-l . E-3, the copolymer of aliphatic secondary amines with epichlorhydrin, gave significantly better print properties, smoothness and opacity than E-2, the poly(DADMAC) bulked pigment. Separately, P-l and P-4 were made into coating colors at 58.0 percent total solids in a generic LWC coating formulation containing 100% hydrous kaolin clay (i.e., without calcined clays), 6.0 parts of a styrene/butadiene (SBR) latex and 0.5 parts of calcium stearate. The colors were coated unto the wire side of a commercial LWC basestock at 5.5 lb/3300 ft 2 . The properties of the coated sheet are given in Table 7. Table 7 While P-4 gave better smoothness, brightness and slightly improved opacity over P-l , helioprintability was not improved within variability of the helio test in contrast with the significant improvement shown by E-2 and E-3 over P-l in Table 6. The principles, preferred embodiments, and modes of operating of this invention have been described in the foregoing specification. However, the invention which is intended to be protected herein is not to be construed as limited to the particular forms disclosed, since they are to be regarded as illustrative rather than restrictive. Variations and changes may be made by those skilled in the art without departing from the spirit of the invention.

Claims: What is claimed is: 1. A method for making a kaolin-based pigment comprising the steps of: (a) delaminating a kaolin clay, wherein the degree of delamination of the kaolin clay is done to the extent that increases the particle size distribution less than 2 μm in the range of about 5 to 40 percent over the particle size distribution less than 2 μm before delamination; (b) removing a portion of particles smaller than 0.3 μm from the delaminated clay; and (c) adding a bulking agent; wherein the degrees of delamination, removal of particles smaller than 0.3 μm, and addition of bulking agent are adjusted to an extent sufficient to provide a pigment containing particles in the range of 5 to 30 percent equal or less than 0.3 μm. 2. The method of claim 1, wherein the degree of delamination of the kaolin clay is done to the extent that increases the particle size distribution less than 2 μm in the range of about 10 to 20 percent over the particle size distribution less than 2 μm before delamination. 3. The method of claim 2, wherein the bulking agent is a cationic polyelectrolyte. 4. The method of claim 3, wherein the cationic polyelectrolyte is added in amount ranging from about 0.02 to about 0.20 percent by weight of the moisture-free weight of the clay. 5. The method of claim 4, wherein the polyelectrolyte is selected from the group consisting of quaternary ammonium salts, copolymers of aliphatic secondary amines with epichlorohydrin, poly(quaternary ammonium)polyester salts that contain quaternary nitrogen, polyamines and polyimines. 6. The method of claim 5, wherein the polyelectrolyte is a quaternary ammonium salt. 7. The method of claim 6, wherein the polyelectrolyte is a dialkyl, diallyl quaternary ammonium salt polymer containing alkyl groups of about 1 to 4 carbon atoms. 8. The method of claim 7, wherein the alkyl groups are methyl. 9. The method of claim 4, wherein the polyelectrolyte is a reaction product of secondary amines and difunctional epoxide compounds or precursors thereof. 10. The method of claim 4, wherein the polyelectrolyte is a reaction product of a dialkylamine containing from 1 to 3 carbon atoms, a difunctional epoxy compound or precursors thereof, and a third reactant selected from the group consisting of ammonia, primary amines, alkylenediamines of from 2 to 6 carbon atoms, and polyamines. 1 1. The method of claim 9, wherein said difunctional epoxide compound is an epihalohydrin. 12. The method of claim 10, wherein said difunctional epoxide compound is an epihalohydrin. 13. The method of claim 9, wherein said difunctional epoxide is an epichlorohydrin. 14. The method of claim 10, wherein said difunctional epoxide is an epichlorohydrin. 15. The method of claim 9, wherein said secondary amines are dimethylamine. diethylamine, or dipropylamine and said difunctional epoxide is an epihalohyrin. 16. The method of claim 15, wherein said secondary amine is dimethyl amine and said epihalohydrin is epichlorohydrin. 17. The method of claim 10, wherein said third reactant is ammonia or at least a trifunctional amine selected from the group consisting of a primary amine, ethylene diamine and diethylene triamine and said difunctional epoxide is an epihalohydrin. 18. The method of claim 17, wherein said third reactant is ethylene diamine and said epihalohydrin is epichlorohydrin. 19. The method of claim 1, wherein the particles less than 0.3 μm are in the range of 5 to 20 percent. 20. The method of claims 1 to 19, further comprising the step of removing grits and very coarse booklets from the clay to achieve a clay slurry having a particle size distribution of 70 to 95% less than 2 μm prior to the step of adding the bulking agent. 21. The product produced by the method of any of the claims 1 to 19. 22. The products produced by the method of claim 20. 23. A paper containing any of the products of claim 21, said products being present as a paper coating, paper filler, or both. 24. A paper containing any of the products of claim 22, said products being present as a paper coating, paper filler, or both.

Inventors: BERBERICH JOSEPH P, HEN JOHN, BERBERICH, JOSEPH, P., HEN, JOHN
Applicants: ENGELHARD CORP, ENGELHARD CORPORATION
Application year: 1999
Main classification: C09C1
IPC codes: C09C1, C09C3, D21H17, D21H19, D21H21