Finely divided calcium carbonate has long been used by the paper/board industry, alone or more often with other materials, in high solids slurries or dispersions as an opacifying filler or pigment, and as a coating for paper. Many processes for grinding or comminuting calcium carbonate to give slurries or dispersions for these and other end uses, and the finely divided calcium carbonate products obtained thereby, have been described in the prior art. Typically such processes involve either horizontal wet micromedia milling, e.g., sand milling, usually carried out with the use of dispersants and followed by centrifuging, when necessary, to remove the coarsest particles, or vertical wet micromedia milling, usually but not invariably in the presence of dispersants, using microballs, beads or sand. Processes involving hammer milling or the use of jaw, cone or gyratory crushers have also been used to comminute naturally-occurring calcium carbonate. Exemplary of such comminution processes are those disclosed in U.S. Pat. Nos. 3,989,195; 4,166,582; 4,278,208 and 4,325,514 and British Patent No. 1,482,258, all assigned to English Clays Lovering Pochin & Company Limited, and Canadian Patent No. 1,161,010, assigned to Pleuss-Stauffer.
It has now been discovered that certain finely divided, wet-ground, dispersant-free calcium carbonate pigment compositions in which the particle size distribution of calcium carbonate is within limits as set out in detail hereinbelow are particularly suitable for use in aqueous slurry or dry form as opacifying fillers for paper, and that when such pigment compositions are so used they provide a balance of filler retention, opacity, brightness and strength characteristics hitherto unobtainable using available ground calcium carbonate fillers, particularly those containing dispersants, and approaching or in some cases equalling that obtainable using typical precipitated calcium carbonate compositions. It has been discovered in particular that the omission of dispersant greatly enhances the filler retention of papers made using the novel calcium carbonate fillers of this invention. At the same time, these novel fillers increase opacity.
It is, therefore, an object of this invention to provide novel, aqueous or dried, finely divided calcium carbonate compositions.
It is also an object of this invention to provide novel, aqueous or dried, finely divided, wet-ground, dispersant-free calcium carbonate filler or pigment compositions.
A further object of this invention is to provide novel, aqueous or dried, finely divided, wet-ground, dispersant-free calcium carbonate filler or pigment compositions which, when used as fillers for paper, have a balance of filler retention, opacity, brightness and strength hitherto unobtainable using available ground calcium carbonate fillers, particularly those containing dispersants, and approaching or in some cases equalling that obtainable using typical precipitated calcium carbonate compositions.
These and other objects, as well as the nature, scope and utilization of the invention, will become readily apparent to those skilled in the art from the following description, the drawings and the appended claims.