1. Field of the Invention
The present invention relates to an aqueous pigment dispersion containing a surface-modified carbon black, and a water-base ink composition comprising this aqueous pigment dispersion.
2. Background Art
Water-soluble dyes have hitherto been used in recording liquids for water-base writing utensils and ink jet printers. On the other hand, in recent years, pigments, such as carbon blacks, have become used as colorants from the viewpoint of improving image quality and fastness properties of records. For example, Japanese Patent Laid-Open Nos. 6074/1989 and 31881/1989 describe water-base pigment inks wherein carbon black was dispersed with the aid of a surfactant or a polymeric dispersant.
In these inks, however, increasing the content of the colorant in the ink to enhance the print density of records has sometimes resulted in gretely increased viscosity of the ink. The use of an excessive amount of a surfactant or a polymeric dispersant is necessary to stably disperse carbon black in the ink. The presence of the surfactant and the polymeric dispersant, however, bubbles or deteriorated antifoaming properties, and, particularly in ink jet recording, may deteriorate the printing stability.
In order to solve these problems, Japanese Patent Laid-Open Nos. 3498/1996 and 120958/1998 describe surface-modified carbon black dispersions wherein surface active hydrogen or a salt thereof has been introduced in a given amount or larger into carbon black to permit the carbon black to be solely dispersed in an aqueous solvent without any surfactant or polymeric dispersant. Japanese Patent Laid-Open No. 110127/1998 describes a method for introducing sulfone groups into carbon black. Further, Japanese Patent Laid-Open No. 95941/1998 proposes an ink for ink jet recording, comprising the above surface-modified carbon black and a glycol ether.
The present inventors have now found that a surface-modified carbon black having a hydrophilic functional group content/specific surface area value in a specific range can realize printing at higher print density and can provide ink compositions which yield records having higher image quality. The present invention has been made based on such finding.
Accordingly, it is an object of the present invention to provide an aqueous pigment dispersion and a water-base ink composition which can realize-printing at high print density and can yield records having higher image quality.
According to one aspect of the present invention, there is provided an aqueous pigment dispersion comprising at least water and a surface-modified carbon black, the surface-modified carbon black being dispersible and/or dissolvable in water without any dispersant and having a hydrophilic functional group content/specific surface area value of 5 to 30 xcexcmol/m2.
According to another aspect of the present invention, there is provided a water-base ink composition comprising at least the above aqueous pigment dispersion.