US 8,393,724 B2  36  35  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- Examples of the urea derivatives include compounds in  which the hydrogen atom on the nitrogen of urea is substi- 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.  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.  The ink composition used in the present invention may  contain a solid wetting agent.  Solid Wetting Agent  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  The average particle diameter and particle size distribution  of the polymer particles can be determined by a light scatter- ing method.  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.  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 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.  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. 