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As a dispersing agent having a crosslinkable functional group, 9.5 parts of a styrene/methacrylic acid copolymer (acid value: 290; molecular weight: 15,000) were dissolved in 80 parts of dimethyl sulfoxide, and 10 parts of an isoindolinone pigment (C.I. Pigment Yellow 109) were suspended in the resultant solution in a flask at 25° C. under an air atmosphere. A 30% methanol solution of potassium hydroxide was then added dropwise little by little to dissolve the isoindolinone pigment. After the pigment solution was stirred for 3 hours, p-phenylene-bis(ethyl)carbodiimide as a crosslinking agent was added to and dissolved in the pigment solution in a proportion of 1 part per 10 parts of the pigment, and the resultant solution was quickly poured into ion-exchanged water (400 parts of ion-exchanged water per 10 parts of the pigment; 0° C.) stirred by an impellor type agitating blade (800 rpm), purged with nitrogen and cooled and temperature-maintained by means of 2 system dispensers (manufactured by Musashi Engineering K.K.; needle internal diameter: 0.57 mm; delivery pressure: 4.0 kgf/cm2) to obtain an aqueous dispersion of isoindolinone-pigment-containing particles having an average particle diameter of 35.6 nm. This aqueous dispersion was transferred to a reaction vessel equipped with a mantle heater, cooling tower, stirrer and thermometer, this system was purged with nitrogen with stirring at 300 rpm, 10% hydrochloric acid was added dropwise to adjust the pH of the reaction mixture to 4.75. After continuing the reaction for additional 5 hours, the resultant aggregates were filtered under reduced pressure through a membrane filter (holding particle diameter: 0.45 am) and washed 3 times with 500 ml of ion-exchanged water to obtain paste of an aqueous dispersion of the isoindolinone-pigment-containing particles desalted and desolvated.

1. stirringagitating blade (800 rpm)
2. custompurged with nitrogen
3. temperaturecooled
4. temperaturetemperature-maintained by means of 2 system dispensers (manufactured by Musashi Engineering K.K.; needle internal diameter: 0.57 mm; delivery pressure: 4.0 kgf/cm2)