Paper coating pigments
A coating composition for paper and other substrates, particularly mechanical papers such as lightweight coated (LWC) paper, comprises an aqueous suspension of a particulate pigment together with a binder, wherein the particulate pigment comprises: (a) a first component which is a precipitated calcium carbonate consisting predominantly of aragonitic or rhombohedral particle shapes or of aragonitic and rhombohedral particle shapes in a weight ratio of between around 40:60 and about 60:40 (e.g. about 50:50) aragonitic:rhombohedral, and a second component which is a processed particulate hydrous kaolin clay having a shape factor greater than or equal to about 25 and a steepness greater than or equal to about 20; or (b) a first component which is a fine particulate calcium carbonate consisting predominantly of particles having a generally spherical particle shape, and a second component which is a processed particulate hydrous kaolin clay having a shape factor greater than or equal to about 45 and a mean equivalent particle diameter (d 50 ) less than about 0.5 μm; or (c) a first component which is a precipitated calcium carbonate consisting predominantly of aragonitic and rhombohedral particle shapes in a weight ratio of between about 40:60 and about 60:40 (e.g. about 50:50) aragonitic:rhombohedral, and a second component which is a processed particulate hydrous kaolin clay having a shape factor less than about 25.
1. A pigment composition comprising:
(a) a precipitated calcium carbonate comprising a mixture of aragonitic particles and rhombohedral particles in a weight ratio ranging from 40:60 to 60:40 based on aragonitic:rhombohedral, and
(b) a kaolin clay with a shape factor greater than 25 and a steepness greater than or equal to 20.
2. The composition of claim 1 , wherein the precipitated calcium carbonate comprises a predominantly rhombohedral precipitated calcium carbonate.
3. The composition of claim 2 , wherein the predominantly rhombohedral precipitated calcium carbonate has a d 50 of less than about 0.8 microns.
4. The composition of claim 2 , wherein the predominantly rhombohedral precipitated calcium carbonate has a d 50 of less than about 0.7 microns.
5. The composition of claim 2 , wherein the predominantly rhombohedral precipitated calcium carbonate has a d 50 of at least about 0.2 microns.
6. The composition of claim 2 , wherein the predominantly rhombohedral precipitated calcium carbonate has a d 50 ranging from about 0.25 microns to about 0.45 microns.
7. The composition of claim 2 , wherein the predominantly rhombohedral precipitated calcium carbonate has a d 50 ranging from about 0.4 microns to about 0.6 microns.
8. The composition of claim 2 , wherein the predominantly rhombohedral precipitated calcium carbonate has a particle size distribution such that at least about 93% by weight of the particles have an equivalent spherical diameter less than 2 microns.
9. The composition of claim 2 , wherein the predominantly rhombohedral precipitated calcium carbonate has a particle size distribution such that at least about 86% by weight of the particles have an equivalent spherical diameter less than 1 micron.
10. The composition of claim 2 , wherein the predominantly rhombohedral precipitated calcium carbonate has a particle size distribution such that at least about 22% by weight of the particles have an equivalent spherical diameter less than 0.5 microns.
11. The composition of claim 2 , wherein the predominantly rhombohedral precipitated calcium carbonate has a particle size distribution ranging from 5% to 25% by weight of the particles have an equivalent spherical diameter less than 0.25 microns.
12. The composition of claim 2 , wherein the predominantly rhombohedral precipitated calcium carbonate has a GE brightness of at least 90.
13. The composition of claim 2 , wherein the predominantly rhombohedral precipitated calcium carbonate has a GE brightness of at least 92.
14. The composition of claim 2 , wherein the kaolin clay has a shape factor greater than 30.
15. The composition of claim 2 , wherein the predominantly rhombohedral precipitated calcium carbonate has a particle size distribution such that:
at least 93% by weight of the particles have an equivalent spherical diameter of less than 2 microns;
at least 86% by weight of the particles have an equivalent spherical diameter of less than 1 micron;
at least 22% by weight of the particles have an equivalent spherical diameter of less than 0.5 microns; and
from 5% to 25% by weight of the particles have an equivalent spherical diameter less than 0.25 microns.
16. The composition of claim 1 , wherein the precipitated calcium carbonate comprises a predominantly aragonitic precipitated calcium carbonate.
17. The composition of claim 16 , wherein the predominantly aragonitic precipitated calcium carbonate has a d 50 of less than about 0.8 microns.
18. The composition of claim 16 , wherein the predominantly aragonitic precipitated calcium carbonate has a d 50 of less than about 0.7 microns.
19. The composition of claim 16 , wherein the predominantly aragonitic precipitated calcium carbonate has a d 50 of at least about 0.2 microns.
20. The composition of claim 16 , wherein the predominantly aragonitic precipitated calcium carbonate has a d 50 ranging from 0.25 microns to about 0.45 microns.
21. The composition of claim 16 , wherein the predominantly aragonitic precipitated calcium carbonate has a particle size distribution such that at least about 90% by weight of the particles have an equivalent spherical diameter less than 2 microns.
22. The composition of claim 16 , wherein the predominantly aragonitic precipitated calcium carbonate has a particle size distribution such that at least about 75% by weight of the particles have an equivalent spherical diameter less than 1 micron.
23. The composition of claim 16 , wherein the predominantly aragonitic precipitated calcium carbonate has a particle size distribution such that at least about 60% by weight of the particles have an equivalent spherical diameter less than 0.5 microns.
24. The composition of claim 16 , wherein the predominantly aragonitic precipitated calcium carbonate has a particle size distribution ranging from 15% to 40% by weight of the particles have an equivalent spherical diameter less than 0.25 microns.
25. The composition of claim 16 , wherein the predominantly aragonitic precipitated calcium carbonate has a GE brightness of at least 90.
26. The composition of claim 16 , wherein the predominantly aragonitic precipitated calcium carbonate has a GE brightness of at least 92.
27. The composition of claim 16 , wherein the kaolin clay has a shape factor greater than 25.
28. The composition of claim 16 , wherein the predominantly aragonitic precipitated calcium carbonate has a particle size distribution such that:
at least 90% by weight of the particles have an equivalent spherical diameter of less than 2 microns;
at least 75% by weight of the particles have an equivalent spherical diameter of less than 1 micron;
at least 60% by weight of the particles have an equivalent spherical diameter of less than 0.5 microns; and
from 15% to 40% by weight of the particles have an equivalent spherical diameter less than 0.25 microns.
29. The composition of claim 16 , wherein the predominantly aragonitic precipitated calcium carbonate has a particle size distribution such that:
at least 95% by weight of the particles have an equivalent spherical diameter of less than 2 microns;
at least 82% by weight of the particles have an equivalent spherical diameter of less than 1 micron;
at least 66% by weight of the particles have an equivalent spherical diameter of less than 0.5 microns; and
from 23% to 33% by weight of the particles have an equivalent spherical diameter less than 0.25 microns.
30. The composition of claim 1 , wherein the kaolin clay has a shape factor greater than about 30.
31. The composition of claim 1 , wherein the kaolin clay has a shape factor greater than about 35.
32. The composition of claim 1 , wherein the kaolin clay has a shape factor greater than about 45.
33. The composition of claim 1 , wherein the kaolin clay has a d 50 of less than about 0.5 microns.
34. The composition of claim 1 , wherein the kaolin clay has a d 50 ranging from about 0.1 microns to about 0.5 microns.
35. The composition of claim 1 , wherein the kaolin clay has a d 50 of greater than about 0.5 microns.
36. The composition of claim 1 , wherein the kaolin clay has a d 50 ranging from about 0.5 microns to about 1.5 microns.
37. The composition of claim 1 , wherein the kaolin clay has a steepness ranging from about 25 to about 45.
38. The composition of claim 1 , wherein the kaolin clay has a steepness ranging from about 35 to about 45.
39. The composition of claim 1 , wherein the kaolin clay comprises at least 50% by weight kaolinite.
40. The composition of claim 1 , wherein the kaolin clay comprises greater than 75% by weight kaolinite.
41. The composition of claim 1 , wherein the kaolin clay comprises greater than 90% by weight kaolinite.
42. The composition of claim 1 , wherein the kaolin clay has a GE brightness of at least 85.
43. The composition of claim 1 , wherein the kaolin clay has a GE brightness of at least 90.
44. The composition of claim 1 , wherein the precipitated calcium carbonate comprises at least about 40% by weight relative to the total composition.
45. The composition of claim 1 , wherein the precipitated calcium carbonate comprises at least about 70% by weight relative to the total composition.
46. The composition of claim 1 , wherein the precipitated calcium carbonate comprises not more than about 75% by weight relative to the composition.
47. A coating composition for paper and other substrates, the composition comprising an aqueous suspension of a particulate pigment according to claim 1 and a binder.
48. The composition according to claim 47 , wherein the binder comprises a modified starch.
49. The composition according to claim 47 , further comprising at least one additional component chosen from: cross linkers; water retention aids; viscosity modifiers and thickeners; lubricity/calendering aids; dispersants; antifoamers/defoamers; dry and wet pick improvement additives; dry and wet rub improvement and/or abrasion resistance additives; gloss-ink hold-out additives; optical brightening agents (OBA) and/or fluorescent whitening agents (FWA); dyes; biocides/spoilage control agents; levelling and evening aids; grease and oil resistance additives; water resistance additives; additional pigments; and mixtures thereof.
50. The composition according to claim 49 , consisting essentially of the aqueous suspension of the particulate pigment, the binder, and the at least one additional component, with less than about 10% by weight of the at least one additional component.
51. A method for preparing a coating composition comprising an aqueous suspension of a particulate pigment according to claim 1 and a binder, comprising: mixing the particulate pigment and the binder into an aqueous liquid medium to prepare a suspension of the solid components therein.
52. A method for preparing a coated gloss paper comprising: applying to the paper a composition comprising an aqueous suspension of a particulate pigment according to claim 1 and a binder and calendering the paper to form a gloss coating thereon.
53. A paper coated with a gloss coating comprising a dry residue of a composition comprising an aqueous suspension of a particulate pigment according to claim 1 and a binder.
54. The paper according to claim 53 , which is a coated mechanical paper.
55. The paper according to claim 53 , which is a coated lightweight coated paper (LWC).