| 摘要 |
PROBLEM TO BE SOLVED: To obtain an aqueous ink composition which is excellent in both setting stability and storage stability by at least using an aqueous medium and a surface-modified pigment dispersible and/or dissolvable in water in the absence of a dispersant and by adjusting the concentration of a free monovalent cation in the ink to a specified value or lower. SOLUTION: The concentration of the free monovalent cation in the ink is adjusted to 2,500 ppm or lower. Alkali metal and ammonium ions are listed as the monovalent cation. Preferable pigments to be surface-modified are carbon black and organic pigments. Especially preferably, the surface-modified pigment is prepared by oxidizing carbon black or an organic pigment with a halogenic acid compound (e.g. an aqueous solution of sodium hypochlorite). The concentration of the free monovalent cation in the ink can be decreased preferably by purifying a dispersion containing the surface-modified pigment oxidized and dispersed therein by reverse osmosis, ultrafiltration, electrodialysis, or the like and then preparing the ink. |
