Orient Chemical Industries, Ltd.

Nobuyuki Murai

Tatsuya Yagyu

Sohko Itoh

Toshiyuki Nagasawa

Kazuo Goda

Makiko Matsui

It is provided an aqueous pigment dispersion having improved dispersing stability in which carbon black shows appropriate initial dispersing property and long-term dispersing property to water, and carbon black particles are less susceptible to aggregation. Moreover, it is provided a raw material for ink-jet ink having improved printing density when alkali metal cation is substituted by potassium ion. The present invention is summarized as an aqueous pigment dispersion, which contains surface modified carbon black that is obtained by wet-oxidizing carbon black by using hypohalous acid and/or a salt thereof, and alkali metal cation, characterized in that concentration of the alkali metal cation is not more than 2% by weight.

Aqueous pigment dispersion and process for producing the same

A writing board ink composition comprises a medium formed of a lower alcohol solvent which contains an alcohol-soluble polymer, a scraping (erasing) agent, a surfactant and a diketopyrrolopyrrole pigment having a median Stokes diameter (ultrafine particle size) of not more than 0.2 .mu.m and represented by the following formula ##STR1## R.sup.1 and R.sup.2 independently represent an unsubstituted phenyl or naphthyl group of a non-water-solubilizing substituent substituted phenyl or naphthyl group, and R.sup.3 and R.sup.4 independently represent hydrogen or a non-water-solubilizing substituent. The writing board ink composition remains stable for a long time while in pens even when the resin content (alcohol-soluble polymer) is such that unclear writing and scrapability reduction (loss of scrapability or erasability) can be avoided, since the pigment of ultrafine particle size is fast to lower alcohol solvents.

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Osaka, Japan

Nobuyuki Murai