US 8,393,724 B2  10  9  about 100,000, and particularly preferably in the range of  about 3,000 to about 50,000. The resin is preferably a resin  that can dissolve in an organic solvent to form a solution.  When the number average molecular weight of a resin is  within the above ranges, the resin can exhibit sufficient func- tion as a cover layer on pigment particles or as a coated layer  of an ink composition. The resin is preferably used in the form  of an alkali metal salt or an organic amine salt.  The resin used for the encapsulated pigment may be, for  example, a material having an anionic group, and examples  thereofinclude thermoplastic, thermosetting, or modified res› ins that are composed of the following types of resin: a poly› mer compound such as an acrylic resin, an epoxy resin, a  polyurethane resin, a polyether resin, a polyamide resin, an  unsaturated polyester resin, a phenol resin, a silicone resin, or  a fluorine resin; a polyvinyl resin such as polyvinyl chloride,  polyvinyl acetate, polyvinyl alcohol or polyvinyl butyral; a  polyester resin such as an alkyd resin or a phthalic acid resin;  an amino resin such as a melamine resin, a melamine-form› aldehyde resin, an amino alkid co-condensed resin, a urea  formaldehyde resin, or a urea resin; and copolymers or mix› tures of two or more of these resins.  Of the above resins, an anionic acrylic resin can be  obtained, for example, by polymerizing, in a solvent, an  acrylic monomer having an anionic group (hereinafter,  referred to as an anionic group-containing acrylic monomer)  and, optionally, one or more other monomers copolymeriz› able with the anionic group-containing acrylic monomer.  Examples of the anionic group-containing acrylic monomer  include an acrylic monomer having one or more anionic  groups selected from the group consisting of a carboxylic  group, a sulfonic acid group and a phosphonic group. Among  these acrylic monomers, an acrylic monomer having a car› boxyl group is especially preferable.  Examples of the acrylic monomer having a carboxyl group  include acrylic acid, methacrylic acid,  crotonic acid,  ethacrylic acid, propylacrylic acid, isopropylacrylic acid, ita› conic acid and fumaric acid. Among these monomers, acrylic  acid and methacrylic acid are preferable.  An encapsulated pigment can be manufactured by a con› ventional physical and/or chemical method by using the  above components. According to a preferable embodiment of  the present invention, the encapsulated pigment can be manu› factured by the methods described in JP-A Nos. 9-151342,  10-140065, 11-209672, 11-172180, 10-25440, or 11-43636.  Specifically, examples of the method for manufacturing the  encapsulated pigment include a phase-inversion emulsifica› tion method and an acid precipitation method described in  JP-A Nos. 9-151342 and 10-140065.  The phase-inversion emulsification method is a self-dis› persion (phase-inversion emulsification) method, which may  basically include a process of dispersing a fused mixture of a  self-dispersible or water-soluble resin and a pigment in water.  The fused mixture may contain the above-described curing  agent or polymer compound as a component thereof. The  "fused mixture" refers to a state in which undissolved com› ponents are mixed, or a state in which dissolved components  are mixed, or a state including both of the above states. Spe› cific examples of the phase-inversion method include that  described in JP-A No. 10-140065.  For more detailed information about the phase-inversion  emulsification method and the acid precipitation method,  JP-A Nos. 9-151342 and 10-140065 can be referred to.  Water-Insoluble Resin  The water-insoluble resin used as a pigment dispersant in  the present invention is preferably a hydrophilic and water› insoluble resin having a hydrophilic structural unit (a) and a  Examples of the hydrophilic structural unit (a) include  hydrophilic functional group-containing vinyl monomers  such as hydrophilic functional group-containing (meth)acry- !ates, hydrophilic functional group-containing (meth)acryla› mides, or hydrophilic functional group-containing vinyl  esters.  Examples of the hydrophilic structural unit (a) further  include a structural unit derived from a monomer having a  nonionic hydrophilic group. The monomer forming a struc- tural unit having a nonionic hydrophilic group is not particu› larly limited as long as it has both a nonionic hydrophilic  functional group and a functional group (for example, an  ethylenically unsaturated bond) that can form a polymer, and  may be selected from monomers known in the art. Vinyl  monomers are preferable from the viewpoints of availability,  handleability and general versatility.  Among these dissociative group-containing monomers,  the unsaturated carboxylic acid monomer is preferable, and  acrylic acid and methacrylic acid are more preferable, from  the viewpoints of dispersion stability and ejection stability.  Accordingly, the hydrophilic structural unit (a) preferably  includes a structural unit derived from (meth)acrylic acid.  Specific examples of the unsaturated carboxylic acid  monomer include acrylic acid, methacrylic acid, crotonic  acid, itaconic acid, maleic acid, fumaric acid, citraconic acid,  and  2-methacryloyloxy  methylsuccinic  acid.  Specific  examples of the unsaturated sulfonic acid monomer include  styrene sulfonic acid, 2-acrylamido-2-methyl propane sul› fonic acid, 3-sulfopropyl(meth)acrylate, and bis-(3-sulfopro- pyl)-itaconic acid ester. Specific examples of the unsaturated  phosphoric acid monomer include vinyl phosphoric acid,  vinyl phosphate, bis(methacryloxyethyl)phosphate, diphe› nyl-2-acryloyloxyethyl phosphate, diphenyl-2-methacryloy› loxyethyl phosphate, and dibutyl-2-acryloyloxyethyl phos- phate.  The hydrophilic group-containing monomer is preferably a  dissociative group-containing monomer, and specifically, the  hydrophilic group-containing monomer is preferably a disso› ciative group-containing monomer having a dissociative  group and an ethylenically unsaturated bond. Examples of the  dissociative group-containing monomer include an unsatur› ated carboxylic acid monomer, an unsaturated sulfonic acid  monomer, and an unsaturated phosphoric acid monomer.  The dissociative group may be preferable from the view› points of stabilizing the emulsion state or the dispersion state.  Examples of the dissociative group include a carboxyl group,  a phosphoric acid group, and a sulfonic acid group. Among  these groups, the carboxyl group is preferable from the view› point of the dispersion stabilizing property when the ink  composition is formed therewith.  The dissociative group and/or a nonionic hydrophilic  group may be incorporated into the water-insoluble resin used  in the present invention, by using a monomer having a disso› ciative group and/or a monomer having a nonionic hydro› philic group.  hydrophobic structural unit (b ). The water-insoluble resin  may further have another structural unit which is different  from the hydrophilic structural unit (a) and the hydrophobic  structural unit (b) in accordance with necessity.  Hydrophilic Structural Unit (a)  There is no particular limitation on the hydrophilic struc› tural unit (a) insofar as it is derived from a hydrophilic group› containing monomer, and may be derived from one hydro› philic group-containing monomer or may be derived from  two or more hydrophilic group-containing monomers. The  hydrophilic group is not particularly limited and may be a  dissociative group or a nonionic hydrophilic group. 