WO2009009553A1

WO2009009553A1 - Fine, steep aragonite for coating

Info

Publication number: WO2009009553A1
Application number: PCT/US2008/069457
Authority: WO
Inventors: Anthony V. Lyons; Jun Yuan
Original assignee: Imerys Pigments, Inc.
Priority date: 2007-07-12
Filing date: 2008-07-09
Publication date: 2009-01-15

Abstract

Disclosed herein are precipitated calcium carbonate compositions for coating exhibiting at least one of improved opacity, sheet gloss, print gloss, and brightness. The precipitated calcium carbonate compositions may be characterized by having a crystalline aragonite content of greater than or equal to about 30% by weight relative to the total weight of the composition, less than or equal to about 10% by weight of particles having a particle size less than or equal to about 0.25 microns, less than or equal to about 4% by weight of particles having a particle size greater than or equal to about 2.0 microns, and a particle size distribution steepness of greater than or equal to about 50. Also disclosed herein are coating compositions comprising at least one precipitated calcium carbonate composition of the present disclosure.

Description

FINE, STEEP ARAGONITE FOR COATING

CLAIM OF PRIORITY

[001] This international PCT application claims the benefits of priority to, and incorporates by reference herein in its entirety, U.S. Provisional Patent Application No. 60/949,343 filed July 12, 2007.

FIELD OF THE INVENTION

[002] Disclosed herein are precipitated calcium carbonate compositions for use in coating, for example, in coating paper and paperboard products. The compositions may exhibit at least one property chosen from improved opacity, sheet gloss, print gloss, and brightness.

BACKGROUND OF THE INVENTION

[003] Coated paper and paperboard products are used in a wide range of products, for instance, packaging, brochures, magazines, and leaflets. Such coated products ideally exhibit at least one beneficial property, such as brightness, opacity, sheet gloss, and print gloss.

[004] Common coating formulations used in the production of coated paper and paperboard products may comprise at least one additive, for example, delaminated and calcined clays, talc, and titanium dioxide. In general, those additives strongly scatter light, thus resulting in a coated paper product having suitable opacity and brightness; however, those additives also typically result in a higher production cost for the coated paper products. In addition, titanium dioxide, while exhibiting good light scattering properties, tends to have a smaller particle size, which may make it difficult to adhere to the paper surface. [005] The use of precipitated calcium carbonate (PCC) to completely or partially replace conventional light-scattering additives has been recognized in the prior art, for example, in lshley et al., 1992 TAPPI Coating Conference Proceedings, pp. 335-348. However, depending on the particular application, the use of PCC in coating formulations may result in coated paper products with less coating uniformity, increased roughness, and lower gloss, as compared to products coated with conventional additives. Thus, the prior art, for instance, Crawshaw et al., "The Influence of Pigment Particle Shape on the Performance of a Paper Coating," 1982 TAPPI Coating Conference Proceedings, pp.143-164, has studied the different properties, such as particle shape, of PCC and their effects on coating performance. In addition, U.S. Patent Nos. 5,120,365, 5,478,388, and 5,861 ,209, British Patent No. 2 139 606, and Japanese Patent No. 6-73695 have described various combinations of PCC properties, such as particle shape, particle size distribution, average particle size, and surface area, in an effort to improve at least one aspect of coating performance.

[006] Calcium carbonate can be precipitated from aqueous solution in three different principal crystal forms: vaterite, calcite, and aragonite. The vaterite form is uncommon because it is generally thermodynamically unstable. The calcite form is the most stable form and the most abundant in nature. The aragonite form is metastable under ambient temperature and pressure, but converts to calcite at elevated temperatures and pressures. All three forms of precipitated calcium carbonate may be prepared by carbonation of milk of lime (Ca(OH)₂) slurry or slake by suitable variation of the process conditions. A non-limiting example of such a production process is described in U.S. Patent No. RE 38,301. [007] The calcite form may have one or more of several different shapes, for example, rhombic and scalenohedral shapes. The rhombic shape is the most common and may be characterized by crystals having approximately equal lengths and diameters, which may be aggregated or unaggregated. Scalenohedral crystals are similar to double, two-pointed pyramids and are generally aggregated.

[008] The aragonite crystalline form may be characterized by acicular, needle- or spindle-shaped crystals, which are generally aggregated and which typically exhibit high length-to-width or aspect ratios. For instance, aragonite may have an aspect ratio ranging from about 3:1 to about 15:1. Aragonite may be produced, for example, by the reaction of carbon dioxide with slaked lime. However, due to the aggregation and needle-shape of the aragonite crystals, the aragonite form of PCC often results in poor rheological behavior and poor paper coating performance, for instance, poor sheet and print gloss properties. On the other hand, because aragonite has a higher refractive index than calcite, it may exhibit improved optical performance over calcite. Thus, it would be desirable to employ aragonite in coating formulations and to optimize the formulation parameters so as to improve at least one of opacity, brightness, sheet gloss, and print gloss in a coated paper or paperboard product.

BRIEF DESCRIPTION OF THE FIGURES

[009] Figure 1 is a diagram illustrating sheet gloss properties for various coating compositions.

[010] Figure 2 is a diagram illustrating print gloss and quarter-micron particle size content trends as a function of particle surface area for various coating compositions. [011] Figure 3 is a diagram illustrating corrected opacity trends as a function of the particle size steepness for various coating compositions.

[012] Figure 4 is a diagram illustrating corrected opacity trends as a function of the aspect ratio for various coating compositions.

SUMMARY OF THE INVENTION

[013] Disclosed herein are precipitated calcium carbonate (PCC) compositions for coating exhibiting at least one property chosen from improved opacity, improved sheet gloss, improved print gloss, and improved brightness. The PCC compositions are characterized by their crystalline aragonite content of greater than or equal to about 30% by weight relative to the total weight of the composition.

[014] The crystalline aragonite content of a composition may be readily determined through visual inspection by use of, for example, a scanning electron microscope or by x-ray diffraction. Such determination may be based upon the identification of the aragonite crystalline form and is well known to those of skill in the art.

[015] The PCC compositions may also be characterized by their particle size distribution (psd). In one embodiment of the present disclosure, less than or equal to about 4% by weight of the PCC composition particles have a size greater than or equal to about 2.0 microns. In another embodiment, less than or equal to about 10% by weight of the PCC composition particles have a size less than or equal to about 0.25 microns. In another embodiment, the PCC compositions comprise a particle size distribution steepness of greater than or equal to about 50.

[016] The PCC compositions may also be characterized by their average particle size. In one embodiment, the PCC compositions comprise particles with an average particle size of less than or equal to about 0.7 microns. [017] The PCC compositions may be further characterized by their aspect ratio. In one embodiment, the PCC composition particles have an aspect ratio of greater than or equal to about 4.

[018] The PCC compositions may additionally be characterized by their BET surface area. In one embodiment, the PCC composition particles have a BET surface area of less than or equal to about 15 m²/g.

[019] Also disclosed herein are PCC compositions for coating having an aragonite content of greater than or equal to about 30% by weight relative to the total weight of the composition, wherein at least about 70% by weight of the PCC particles have a particle size ranging from about 0.25 to about 0.5 microns.

[020] Further disclosed herein are coating compositions comprising at least one precipitated calcium carbonate composition of the present disclosure. The coating composition may further comprise at least one optional additive, such as those conventionally used in coating compositions.

[021] Additionally disclosed herein are processes for coating paper or paperboard comprising applying to paper or paperboard products a coating composition of the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION Precipitated Calcium Carbonate Compositions

[022] The PCC compositions of the present disclosure comprise PCC in the form of aragonite. Aragonite may be manufactured by processes now known in the art or hereafter discovered. For example, aragonite may be produced by the carbonation of an aqueous lime (Ca(OH)₂) slurry. The lime slurry may be prepared by adding water to calcium oxide, then agitating the mixture. A small amount of aragonite may then be added to the slurry as seed crystals, followed by the addition of a stream of carbon dioxide gas. The resulting PCC slurry typically undergoes at least one further treatment, for example, at least one of dewatehng, grinding, screening, and dispersion.

[023] The PCC compositions of the present disclosure are characterized by an aragonite content of greater than or equal to about 30% by weight relative to the total weight of the composition. In one embodiment, the PCC compositions have an aragonite content of greater than or equal to about 40% by weight. In another embodiment, the PCC compositions have an aragonite content greater than or equal to about 60% by weight. In a further embodiment, the PCC compositions have an aragonite content greater than or equal to about 80% by weight. In yet another embodiment, the PCC compositions have an aragonite content greater than or equal to about 90% by weight.

[024] The PCC compositions of the present disclosure may optionally comprise, or may optionally be available as, at least one commercially available PCC product. In one embodiment, the PCC product has undergone at least one screening to obtain the desired average particle size and/or particle size distribution steepness.

[025] Although a PCC production process may aim to produce only one form of PCC, it is unlikely to produce a product that is 100% in the selected form. Thus, it is common to employ PCC compositions comprising mixtures of crystalline forms (e.g., aragonite and calcite) in coating formulations. Even in the case of PCC compositions predominantly comprising one form (e.g., predominately aragonite), the compositions are likely to contain a small amount of at least one other crystal PCC structure (e.g., calcite). As a result, the PCC compositions of the present disclosure may optionally comprise at least one second PCC form, which is not aragonite. In one embodiment, the at least one second PCC form is calcite. In another embodiment, the at least one second PCC form is calcite in a substantially rhombic shape. In a further embodiment, the at least one second PCC form is calcite in a substantially scalenohedral shape.

[026] The PCC compositions of the present disclosure may optionally comprise at least one added pigment. Suitable pigments are those now known or that may be hereafter discovered. Exemplary pigments include, but are not limited to, titanium dioxide, calcined clays, delaminated clays, talc, calcium sulphate, other calcium carbonate, kaolin clays, calcined kaolin, satin white, plastic pigments, aluminum thhydrate, and mica.

[027] The optional at least one second PCC form and the optional at least one pigment may be separately present in the PCC compositions of the present disclosure in an amount less than about 70% by weight relative to the total weight of the composition. It is to be understood that the skilled artisan will select any amounts of the optional at least one second PCC form and the optional at least one pigment in such a way so as to obtain various desired properties without affecting, or without substantially affecting, the advantageous properties of the PCC compositions disclosed herein.

[028] The PCC compositions may be characterized by their particle size distribution (psd), as measured by a SEDIGRAPH particle size analyzer. As used herein and as generally defined in the art, the average particle size (also called d₅o) is defined as the size at which 50 percent of the particle volume is accounted for by particles having a diameter less than or equal to the specified value. In one embodiment, the PCC compositions comprise particles with an average particle size of less than or equal to about 0.7 microns. In another embodiment, the PCC compositions comprise particles with an average particle size of less than or equal to about 0.5 microns. In yet another embodiment, the average particle size of the PCC compositions ranges from about 0.3 to about 0.6 microns.

[029] As a general trend, print gloss and light scatter improve with a decrease in the number of particles having a small particle size. Therefore, it may be advantageous to limit the amount of particles having a particle size less than or equal to about 0.25 microns. In one embodiment, the PCC composition comprises less than or equal to about 15% by weight of particles having a particle size less than or equal to about 0.25 microns. In another embodiment, the PCC composition comprises less than or equal to about 10% by weight of particles having a particle size less than or equal to about 0.25 microns. In yet another embodiment, the PCC composition comprises less than or equal to about 5% by weight of particles having a particle size less than or equal to about 0.25 microns.

[030] As a general trend, sheet gloss and opacity improve with a decrease in the number of particles having a large particle size. Therefore, it may be advantageous to limit the amount of particles having a particle size greater than or equal to about 2.0 microns. In one embodiment, the PCC composition comprises less than or equal to about 4% by weight of particles having a particle size greater than or equal to about 2.0 microns. In another embodiment, the PCC composition comprises less than or equal to about 2% by weight of particles having a particle size greater than or equal to about 2.0 microns.

[031] In order to limit the amount of particles having a particle size greater than or equal to about 2.0 microns, one may, for example, accomplish centhfugation of the PCC composition to remove course particles from the distribution. [032] The PCC compositions may also be characterized by their psd steepness, as calculated from measurements from a SEDIGRAPH particle size analyzer. The steepness of the particle size distribution is determined by the following formula:

As a general trend, print gloss and light scatter improve with an increase in steepness. In one embodiment, the psd steepness is greater than or equal to about 50. In another embodiment, the psd steepness is greater than or equal to about 55. In a further embodiment, the psd steepness is greater than or equal to about 65. In yet another embodiment, the psd steepness ranges from about 55 to about 65.

[033] The particles of the PCC compositions may be further characterized by their aspect ratio. The aspect ratio of the particles of a PCC composition may be determined by first depositing a pigment slurry on a standard SEM stage and coating the slurry with platinum. Images are then obtained and the particle dimensions are determined, using a computer based analysis in which it is assumed that the thickness and width of the particles are equal. The aspect ratio may then be determined by averaging fifty calculations of individual particle length-to-width aspect ratios. In one embodiment, the PCC composition particles have an aspect ratio of at least about 4. In another embodiment, the PCC composition particles have an aspect ratio of at least about 7. In still another embodiment, the PCC composition particles have an aspect ratio ranging from about 4 to about 15. In a further embodiment, the PCC composition particles have an aspect ratio ranging from about 7 to about 10. [034] The particles of the PCC compositions may additionally be characterized by their BET surface area. Generally, print gloss tends to improve with a decrease in surface area. In one embodiment, the PCC composition particles exhibit a BET surface area of less than or equal to about 15 m²/g. In another embodiment, the PCC composition particles exhibit a BET surface area of less than or equal to about 12 m²/g. In a further embodiment, the PCC composition particles exhibit a BET surface area of less than or equal to about 10 m²/g. In yet another embodiment, the PCC composition particles exhibit a BET surface area ranging from about 10 m²/g to about 15 m²/g.

[035] The PCC compositions of the present disclosure may be in dispersed form, i.e., the PCC compositions may be dispersed in an aqueous medium. In one embodiment, the dispersed PCC composition comprises at least about 60% PCC by weight relative to the total weight of the dispersion. In another embodiment, the dispersed PCC composition comprises at least about 70% PCC by weight. The dispersed PCC composition may comprise at least one dispersing agent, which may be chosen from dispersing agents now known in the art or hereafter discovered for the dispersion of PCC. Examples of suitable dispersing agents include, but are not limited to: polycarboxylate homopolymers, polycarboxylate copolymers comprising at least one monomer chosen from vinyl and olefinic groups substituted with at least one carboxylic acid group, and water soluble salts thereof. Example of suitable monomers include, but are not limited to, acrylic acid, methacrylic acid, itaconic acid, crotonic acid, fumaric acid, maleic acid, maleic anhydride, isocrotonic acid, undecylenic acid, angelic acid, and hydroxyacrylic acid. The at least one dispersing agent may be present in the dispersed PCC composition in an amount ranging from about 0.01 % to about 2%. In one embodiment, the at least one dispersing agent is present in an amount ranging from about 0.02% to about 1.5% by weight relative to the total weight of the dispersion.

[036] It is believed that, by achieving at least one attribute selected from a low number of particles with a particle size over 2 microns, a low number of particles with a particle size under 0.25 microns, and a high particle size distribution steepness, a good balance of paper properties (including at least one of improved opacity, sheet gloss, print gloss, and brightness) may be achieved. In addition, one may further control the aspect ratio, average particle size, particle size distribution, and surface area to further refine and improve at least one or more of the aforementioned properties. In particular, it is believed that the balance of one or more of those attributes acts to create a relatively or substantially uniform or unimodal distribution of the particles of the PCC composition such that at least one of opacity, sheet gloss, print gloss, and brightness may be improved. This is in contrast to the teaching of the prior art, such as U.S. Patent No. 5,861 ,209, which teaches that a multimodal particle size distribution is to be preferred.

Coating Composition

[037] Also disclosed herein are coating compositions comprising at least one PCC composition of the present disclosure. In one embodiment, the coating composition comprises a PCC composition having an aragonite content of greater than or equal to about 30% by weight relative to the total weight of the PCC composition, less than or equal to about 10% by weight of particles having a particle size less than or equal to about 0.25 microns, less than or equal to about 4% by weight of particles having a particle size greater than or equal to about 2.0 microns, and a particle size distribution steepness of greater than or equal to about 50. [038] The coating composition may further comprise at least one optional additive. The at least one optional additive may be chosen from those conventionally used in coating compositions. Non-limiting examples of suitable additives include, but are not limited to, pigments, binders, thickeners, and lubricants.

[039] The at least one optional additive may be at least one pigment. Exemplary pigments include, but are not limited to, titanium dioxide, calcined clays, delaminated clays, talc, calcium sulphate, kaolin clays, calcined kaolin, satin white, plastic pigments, aluminum trihydrate, and mica. The at least one pigment may be present in the coating composition in an amount less than about 70% by weight relative to the total weight of the composition.

[040] The at least one optional additive may be at least one binder. Exemplary binders include, but are not limited to, starches, proteins such as casein, and latex binders such as styrene butadiene rubbers and acrylic polymers. The at least one binder may be present in the coating composition in an amount ranging from about 2% to about 10% by weight relative to the total weight of the coating composition.

[041] The at least one optional additive may be at least one thickener. Exemplary thickeners include, but are not limited to, carboxymethyl cellulose, hydroxyethyl cellulose, and polyacrylates. The at least one thickener may be present in the coating composition in an amount ranging from about 0.1 % to about 2% by weight relative to the total weight of the coating composition.

[042] The at least one optional additive may be at least one lubricant. Exemplary lubricants include, but are not limited to, calcium stearate, ureic acid, and polystyrene. The optional lubricant may be present in the coating composition in an amount ranging from about 0.5% to about 1.5% by weight relative to the total weight of the coating composition.

[043] It is to be understood that the skilled artisan will select any at least one optional additive and amount thereof in such a way so as to obtain various desired properties without affecting, or without substantially affecting, the advantageous properties of the PCC compositions of the present disclosure.

Coating Process

[044] Further disclosed herein are processes for coating paper or paperboard comprising applying to paper or paperboard products a coating composition of the present disclosure. The particular application process may be accomplished by any known or hereinafter discovered process.

Coating Properties

[045] Sheet gloss is a measure of the amount of reflected light and may be determined, for example, by using a 75 degree Hunter Gloss meter.

[046] Print gloss is a measure of the amount of light reflected from a printed piece of paper and may be determined, for example, by using a 75 degree Hunter Gloss meter.

[047] Opacity (or corrected opacity) is a measure of the transparency of paper and may be determined, for example, by measuring the reflectance of a diffuse source of light with paper and black backgrounds.

[048] Brightness is a measure of the directional reflectance from paper of light at a wavelength of 457 nm and may be determined, for example, by a directional geometry "brightness tester." [049] Other than in the examples, or where otherwise indicated, all numbers and numeric identifiers used in the specification and claims are to be understood as being modified in all instances by the term "about." Accordingly, unless indicated to the contrary, the numerical parameters set forth in the specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the present disclosure. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should be construed in light of the number of significant digits and ordinary rounding approaches.

[050] Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, unless otherwise indicated the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical values, however, inherently contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements. The section headings used in this disclosure are provided merely for the convenience of the reader and are not intended to limit the scope of the inventions described herein.

[051] By way of non-limiting illustration, examples of certain embodiments of the present disclosure are given below.

EXAMPLES

Example 1

[052] Various coating compositions were prepared by combining a variety of PCC samples with Capim DG (a kaolin pigment commercially available from lmerys Paper Group) in a weight ratio of 40:60. The PCC samples were in various forms, chosen from calcite (rhombic and scalenohedric shapes) and aragonite forms, and had average particle sizes ranging from about 0.4 to about 0.6 microns. As used herein, the terms "aragonite form" or "aragonitic PCC composition" mean a PCC composition having an aragonite content of greater than or equal to about 30% by weight relative to the total weight of the composition. As used herein, the terms "calcite form" and "calcite PCC composition" mean a PCC composition having an aragonite content of less than about 30% by weight relative to the total weight of the composition and a calcite content greater than any vatehte content. PCC particle properties and coating properties were determined according to the measurement protocols discussed above. The properties of the aragonitic PCC compositions are illustrated below in Table I and the properties for the calcitic PCC compositions are illustrated in Table II.

Table I

Sample PCC 2 is an aragonite PCC product available from lmerys Paper Group. Sample PCC 3 is an available aragonite PCC product.

Table Il

¹ Sample 7 is a rhombic calcite PCC product.

Sample 8 is a rhombic calcite PCC product.

¹ Sample 9 is a rhombic calcite PCC product available from lmerys Paper Group. 1 Sample 10 is a rhombic calcite PCC product.

Sample 11 is a rhombic calcite PCC product.

¹ Sample 12 is a rhombic calcite PCC product available from lmerys Paper Group. 1 Sample 13 is a scalenohedral calcite PCC product.

Example 2

[053] Various coating compositions were prepared as in Example 1 and applied to sheets with a wood free base, a basis weight of 51 g/m², and an ISO brightness of 81.7 at a target coat weight of 10 g/m². The properties of the coated paper were measured. Three coated sheets were produced and measured for each coating composition sample and the average was taken. The results for the aragonitic PCC compositions are illustrated in Table III, and the results for the calcitic PCC compositions are illustrated in Table IV.

Table

Table IV

"10 Sample PCC 14 is a rhombic calcite PCC product with a d₅₀ of about 0.6 μm. "11 Sample PCC 15 is a rhombic calcite PCC product with a d₅₀ of about 0.6 μm.

"12 Sample PCC 16 is a rhombic calcite PCC product with a d₅₀ of about 0.6 μm. "13 Sample PCC 17 is a rhombic calcite PCC product with a d₅₀ of about 0.6 μm.

"14 Sample PCC 18 is a scalenohedral calcite PCC product with a d₅₀ of about 1.0 microns.

[054] Various trends for sheet gloss, print gloss, opacity, and brightness as a function of particle shape, average particle size, particle size distribution steepness, aspect ratio, and BET surface area were determined and are discussed in more detail. Sheet Gloss

[055] Generally, sheet gloss tends to improve with a decrease in the number of particles having a large particle size. Figure 1 illustrates the measured sheet gloss values for various aragonitic and calcitic PCC coating compositions. Notably, compositions comprising aragonitic PCC exhibited improved sheet gloss values over the other samples at the same particle size. Without wishing to be bound by theory, it is hypothesized that the aragonite crystal structure contributed to the improved sheet gloss because the long, thin needles may have laid flatter on the paper surface.

Print Gloss

[056] As shown in Figure 2, print gloss tended to improve as BET surface area increased and as the amount of fine particles (e.g., particles smaller than about 0.25 μm) decreased. Thus, print gloss tended to be improved with larger, coarser particles. However, aragonitic PCC compositions with lower d₅o values were shown to exhibit comparable values if tailored to reduce the amount of particles smaller than about 0.25 μm.

Opacity

[057] Figure 3 suggests that, in the case of aragonite PCC compositions, opacity tended to improve as particle size distribution steepness increased. In addition, Figure 4 suggests that, in the case of compositions with high steepness values, opacity tended to improve as the aspect ratio increased.

Brightness

[058] As with opacity, and not wishing to be bound by theory, it is hypothesized that brightness tended to improve as particle size distribution steepness increased. Conclusions

[059] Because of the opposite trends for sheet gloss and print gloss with respect to particle size, it is hypothesized that one compromise for optimizing both gloss properties would be a PCC composition with a large number of particles falling within the range of from about 0.25 μm to about 0.5 μm. For example, a PCC composition having a d₅o value of about 0.5 μm and a steep particle size distribution may be suitable. In addition, as shown in at least the case of aragonitic PCC compositions, the steep particle size distribution may serve to improve the opacity and brightness of the composition. Opacity may further be improved by using aragonitic PCC compositions with higher aspect ratios.

Claims

WHAT IS CLAIMED IS:

Claim 1 : A composition comprising precipitated calcium carbonate, wherein the composition has:

(a) an aragonite content of greater than or equal to about 30% by weight relative to the total weight of the composition;

(b) less than or equal to about 4% by weight of particles having a particle size greater than or equal to about 2.0 microns;

(c) less than or equal to about 10% by weight of particles having a particle size less than or equal to about 0.25 microns; and

(d) a particle size distribution steepness of greater than or equal to about 50.

Claim 2: The composition of claim 1 , wherein the aragonite content is greater than or equal to about 40% by weight relative to the total weight of the composition.

Claim 3: The composition of claim 2, wherein the aragonite content is greater than or equal to about 60% by weight relative to the total weight of the composition.

Claim 4: The composition of claim 3, wherein the aragonite content is greater than or equal to about 90% by weight relative to the total weight of the composition.

Claim 5: The composition of claim 1 , further comprising at least one pigment.

Claim 6: The composition of claim 5, wherein the at least one pigment is chosen from precipitated calcium carbonate in the calcite form. Claim 7: The composition of claim 6, wherein the calcite has a shape chosen from the group consisting of rhombic and scalenohedral.

Claim 8: The composition of claim 5, wherein the at least one pigment is chosen from the group consisting of titanium dioxide, calcined clays, delaminated clays, talc, calcium sulphate, kaolin clays, calcined kaolin, satin white, plastic pigments, aluminum trihydrate, and mica.

Claim 9: The composition of claim 1 , wherein the average particle size is less than or equal to about 0.7 microns.

Claim 10: The composition of claim 9, wherein the average particle size is less than or equal to about 0.5 microns

Claim 11 : The composition of claim 10, wherein the average particle size ranges from about 0.3 to about 0.5 microns.

Claim 12: The composition of claim 1 , wherein the composition comprises less than or equal to about 5% by weight of particles having a particle size less than or equal to about 0.25 microns.

Claim 13: The composition of claim 1 , wherein the composition comprises less than or equal to about 2% by weight of particles having a particle size less than or equal to about 2.0 microns. Claim 14: The composition of claim 1 , wherein the particle size distribution steepness is greater than or equal to about 55.

Claim 15: The composition of claim 14, wherein the particles size distribution steepness is greater than or equal to about 60.

Claim 16: The composition of claim 14, wherein the particle size distribution steepness ranges from about 55 to about 65.

Claim 17: The composition of claim 1 , wherein the composition has an aspect ratio of at least about 4.

Claim 18: The composition of claim 17, wherein the composition has an aspect ratio of at least about 7.

Claim 19: The composition of claim 17, wherein the composition has an aspect ratio ranging from about 4 to about 15.

Claim 20: The composition of claim 18, wherein the composition has an aspect ratio ranging from about 7 to about 10.

Claim 21 : The composition of claim 1 , wherein the composition has a BET surface area of less than or equal to about 15 m²/g. Claim 22: The composition of claim 21 , wherein the composition has a BET surface area of less than or equal to about 10 m²/g.

Claim 23: The composition of claim 1 , wherein the composition has a BET surface area ranging from about 10 m²/g to about 15 m²/g.

Claim 24: The composition of claim 1 , wherein the composition is dispersed in an aqueous medium.

Claim 25: The composition of claim 24, wherein the precipitated calcium carbonate is present in the composition in an amount greater than or equal to 60% by weight relative to the total weight of the composition.

Claim 26: The composition of claim 25, wherein the precipitated calcium carbonate is present in the composition in an amount greater than or equal to 70% by weight relative to the total weight of the composition.

Claim 27: The composition of claim 24, further comprising at least one dispersing agent.

Claim 28: The composition of claim 27, wherein the at least one dispersing agent is chosen from the group consisting of polycarboxylate homopolymers, polycarboxylate copolymers comprising at least one monomer chosen from vinyl and olefinic groups substituted with at least one carboxylic acid group, and water soluble salts thereof. Claim 29: The composition of claim 27, wherein the at least one dispersing agent is present in an amount ranging from about 0.01 % to about 2% by weight relative to the total weight of the composition.

Claim 30: The composition of claim 1 , wherein at least about 70% by weight of the aragonite particles have a size ranging from about 0.25 microns to about 0.5 microns.

Claim 31 : A coating composition comprising at least one precipitated calcium carbonate composition having:

(a) an aragonite content of greater than or equal to about 30% by weight relative to the total weight of the composition,

Claim 32: The composition of claim 31 , further comprising at least one optional additive chosen from pigments, binders, thickeners, and lubricants.

Claim 33: The composition of claim 32, wherein the at least one additional pigment is chosen from precipitated calcium carbonate in the calcite form. Claim 34: The composition of claim 33, wherein the calcite has a shape chosen from the group consisting of rhombic and scalenohedral.

Claim 35: The composition of claim 32, wherein the pigment is chosen from titanium dioxide, calcined clays, delaminated clays, talc, calcium sulphate, kaolin clays, calcined kaolin, satin white, plastic pigments, aluminum trihydrate, and mica.

Claim 36: The composition of claim 32, wherein the binder is chosen from starches, proteins, and latex binders.

Claim 37: The composition of claim 36, wherein the binder is present in the composition in an amount ranging from about 2% to about 21 % by weight relative to the total weight of the composition.

Claim 38: The composition of claim 32, wherein the thickener is chosen from carboxymethyl cellulose, hydroxyethyl cellulose, and polyacrylates.

Claim 39: The composition of claim 38, wherein the thickener is present in the composition in an amount ranging from about 0.1 % to about 2% by weight relative to the total weight of the composition.

Claim 40: The composition of claim 38, wherein the lubricant is calcium stearate. Claim 41 : The composition of claim 40, wherein the lubricant is present in the composition in an amount ranging from about 0.5% to about 1.5% by weight relative to the total weight of the composition.

Claim 42: A composition comprising precipitated calcium carbonate, wherein the composition has:

(d) a particle size distribution steepness of greater than or equal to about 50;

(d) an aspect ratio of greater than or equal to 4; and,

(e) a BET surface area of greater than or equal to 15 m²/g; wherein the composition comprises an average particle size of less than or equal to about 0.7 microns.

Claim 43: A method of making a precipitated calcium carbonate composition, wherein the composition has an aragonite content of greater than or equal to about 30% by weight relative to the total weight of the composition; including the step of performing centrifugation of the composition to remove coarse particles such that less than or equal to about 4% by weight of the particles have a particle size greater than or equal to about 2.0 microns. Claim 44: The method of claim 43, including the step of performing centrifugation of the composition to remove coarse particles such that less than or equal to about 2% by weight of the particles have a particle size greater than or equal to about 2.0 microns.

Claim 45: A composition comprising precipitated calcium carbonate, wherein the composition has:

(d) a substantially unimodal particle size distribution.