the solution-polymerization method using an organic solvent is more preferable, from the viewpoint of an aggregation rate and droplet ejecting stability when the self-dispersible poly› mer particles are employed in the ink composition. 

From the viewpoint of an aggregation rate, it is preferable that the self-dispersible polymer particles used in the present invention contain a first polymer, the polymer being synthe› sized in an organic solvent and having a carboxyl group and the acid value of 20 to 100, and the self-dispersible polymer particles being prepared as a polymer dispersion in which the carboxyl group of the first polymer is partially or thoroughly neutralized and water functions as a continuous phase. 

More specifically, the method of producing the self-dis› persible polymer particles used in the present invention pref› erably includes a process of synthesizing the first polymer in an organic solvent and a process of dispersing the first poly› mer to form an aqueous dispersion thereof, at least a part of the carboxyl group of the first polymer being neutralized. 

The dispersing preferably includes the following processes (1) and (2). 

Process (1): Stirring a mixture containing a first polymer (water-insoluble polymer), an organic solvent, a neutral› izer, and an aqueous medium; and 

Process (2): Removing the organic solvent from the mixture. 

The process (1) preferably includes obtaining a dispersion by, at first, dissolving the first polymer (water-insoluble poly› mer) in an organic solvent, and then gradually adding a neu› tralizer and an aqueous medium, and mixing and stirring the mixture. The addition of the neutralizer and the aqueous medium to a solution of the water-insoluble polymer dis- solved in an organic solvent makes it possible to obtain self› dispersible polymer particles having particle diameters capable of imparting higher storage stability without strong shearing force. 

There is no limitation on a stirring method of the mixture, and generally-used mixing and stirring devices or, as required, dispersers such as an ultrasonic disperser or a high voltage homogenizer can be used. 

Preferable examples of the organic solvent include an alco› hol solvent, a ketone solvent, and an ether solvent. 

Examples of the alcohol solvent include isopropyl alcohol, n-butanol, t-butanol, and ethanol. Examples of the ketone solvent include acetone, methyl ethyl ketone, diethyl ketone, and methyl isobutyl ketone. Examples of the ether solvent include dibutyl ether and dioxane. Among these solvents, the ketone solvent such as methyl ethyl ketone or the alcohol solvent such as isopropyl alcohol is preferable. It is also preferable to use isopropyl alcohol and methyl ethyl ketone in combination so that the change in polarity at the time of phase inversion from an oil phase to a water phase can be moder- ated. By using the solvents in combination, self-dispersible polymer particles that can be free from aggregation-precipi› tation or fusion of particles and can have high dispersion stability and fine particle diameters can be obtained. 

The neutralizer is used for forming an emulsion state or a dispersion state in which the dissociative group is partially or thoroughly neutralized and the self-dispersible polymer is stabilized in water. Examples of the neutralizer which can be used when the self-dispersible polymer employed in the invention has an anionic dissociative group ( e.g., a carboxyl group) as the dissociative group include basic compounds such as organic amine compounds, ammonia, or hydroxides of alkali metals. Examples of the organic amine compound include monomethylamine, dimethylamine, trimethylamine, monoethylamine, diethylamine, triethylamine, monopropy› lamine, dipropylamine, monoethanolamine, diethanolamine, triethanolamine, N,N-dimethyl-ethanolamine, N,N-diethyl- 

ethanolamine, 2-dimethylamino-2-methyl-1-propanol, 2-amino-2-methyl-1-propanol, N-methyldiethanolamine, N-ethyldiethanolamine, monoisopropanolamine, diisopro› panolamine, and tri-isopropanolamine. Examples of the hydroxides of alkali metals include lithium hydroxide, sodium hydroxide, and potassium hydroxide. Among the above, from the viewpoint of stabilization of dispersion of the self-dispersible polymer particles employed in the invention in water, sodium hydroxide, potassium hydroxide, triethy- !amine, and triethanolamine are preferable. 

The content of the basic compound is preferably from 5 to 120% by mo!, more preferably from 10 to 110% by mo!, and still more preferably from 15 to 100% by mo!, with respect to 100% by mo! of the dissociative groups. Stabilization of the dispersion of the particles in water can be further demon› strated when the content of the basic compound is adjusted to 15% by mo! or more. Reduction in content of the water› soluble components can be effected when the content of the basic compound is adjusted to 100% by mo! or less. 

In the process (2), an aqueous dispersion of the self-dis- persible polymer particles can be obtained by inverting a phase of the dispersion, which has been obtained in the pro› cess (1 ), to a water phase as a result of distilling off the organic solvent from the dispersion by a common procedure such as vacuum distillation. The thus-obtained aqueous dispersion is substantially free of the organic solvent. Specifically, the amount of the organic solvent contained in the aqueous dis› persion is preferably 0.2% by mass or less, and more prefer- ably 0.1 % by mass or less. 

The average particle diameter of the self-dispersible poly- mer particles is preferably in the range of 10 nm to 400 nm, more preferably in the range of 10 nm to 200 nm, and still more preferably in the range of 10 nm to 100 nm. When the average particle diameter is 10 nm or more, production suit- ability of the polymer particles may be increased. When the average particle diameter is 400 nm or less, the storage sta› bility may be increased. The particle size distribution of the self-dispersible polymer particles is not particularly limited. The polymer particles may have either a broad particle size distribution or a monodisperse particle size distribution. Two or more water-insoluble particles may be used in combination as a mixture. 

The average particle diameter and particle size distribution of the polymer particles can be determined by a light scatter- ing method. 

The self-dispersible polymer particles used in the present invention can be favorably contained in, for example, in a water-based ink composition and may be used solely or in combination of two or more kinds 

Solid Wetting Agent 

The ink composition used in the present invention may contain a solid wetting agent. 

The solid wetting agent used in the present invention refers to a water-soluble compound that has a water-retaining func- tion, and that is solid at 25° C. The solid wetting agent has high water-retaining function whereby an ink can be pre› vented from dryness and solidification which are not desir› able for the ink. As a result, in the present invention, the solid wetting agent can be favorably used in the ink composition. 

As the solid wetting agent usable in the invention, com- pounds commonly used for water-based ink compositions can be used without any modification, and, more specifically, urea, and urea derivatives, sugars, and polyhydric alcohols such as sugar alcohols, hyaluronic acids, trimethylol propane or 1,2,6-hexanetriol are exemplified. 

Examples of the urea derivatives include compounds in which the hydrogen atom on the nitrogen of urea is substi- 

