US 8,393,724 B2  26  25  aromatic substituent, a phenyl acrylate, an alkyl methacrylate  which may have an aromatic substituent, a phenyl methacry› late, a cycloalkyl methacrylate, an alkyl crotonate, a dialkyl  itaconate, a dialkyl maleate, vinyl alcohol, and derivatives of  the above compounds.  Homopolymers or copolymers of monomers having the  a,~-ethylenically unsaturated group may be used as a poly› mer dispersant.  Examples ofthepolymerdispersants include an alkyl acry› late-acrylic acid copolymer, an alkyl methacrylate-meth- acrylic acid copolymer, a styrene-alkyl acrylate-acrylic acid  copolymer,  styrene-phenylmethacrylate-methacrylic acid  copolymer,  styrene-cyclohexyl  methacrylate-methacrylic  acid copolymer, styrene-styrene sulfonic acid copolymer,  styrene-maleic acid copolymer, styrene-methacrylic acid  copolymer, styrene-acrylic acid copolymer, vinyl naphtha› lene-maleic acid copolymer, vinyl naphthalene-methacrylic  acid copolymer, vinyl naphthalene-acrylic acid copolymer,  polystyrene, polyester and polyvinyl alcohol.  Water-Insoluble Volatile Solvent  When the pigment dispersion is prepared, at least one  water-insoluble volatile solvent may be used. Since the water› insoluble volatile solvent has less influence on the dispers› ibility, the water-insoluble volatile solvent can be removed  finally, while maintaining a good dispersibility in the disper› sion step, so that the dispersion can be thickened while main› taining a good dispersed state, and the pigment dispersion  with an excellent storability over a long period of time can be  obtained. Moreover, when an ink composition is prepared and  used for recording, an image recording with a high ink ejec- tion stability and suppressed curl of a recording medium can  be attained.  The term "water-insolubility" refers to the characteristic  such that when a solvent is mixed with pure water in the same  amount of the solvent, and the mixture is gently stirred at one  atmospheric pressure and 20°  C., the mixture does not appear  to be homogeneous, even after the flow due to stirring is  ceased. The solubility with respect to water at 20°  C. is  preferably 80 g/100 ml or less, and more preferably 50 g/100  ml or less.  The "volatility" of a solvent means that the solvent has a  boiling point of200°  C. or less, and preferably 150°  C. or less.  The water-insoluble volatile solvent may be appropriately  selected from organic solvents with water-insolubility and  volatility. Specific examples of the water-insoluble volatile  solvents include ketone-based solvents (for example, methyl  ethyl ketone, diethyl ketone and the like), and ether-based  solvents (for example, dibutyl ether and the like). In particu› lar, from the viewpoint of the dispersion stability, ketone› based solvents are preferable, and methyl ethyl ketone is  particularly preferable.  The use amount of the water-insoluble volatile solvent is  preferably from 10% by mass to 1,000% by mass, more  preferably from 50% by mass to 800% by mass, and still more  preferably from 100% by mass to 500% by mass with respect  to the use amount of the alkyleneoxide adduct of glycerol, in  view of good dispersibility and stability after dispersion, and  the ink ejection stability when the water-insoluble volatile  solvent is used for an ink composition for recording, and  suppression of curl of a recorded medium.  The water-insoluble volatile solvent as described in the  above is preferably removed from the liquid after the pigment  is dispersed. In such a way, the amount of the water-insoluble  volatile solvent which becomes urmecessary finally is  reduced, and a thickened pigment dispersion can be obtained,  while the dispersibility of pigment and storability of the dis› persion can be maintained over a long period of time. Further,  The polymer particles are not particularly limited.  Examples of polymer particles include polymer particles  composed of a resin, such as a thermoplastic resin, for  The polymer particles are used preferably as polymer latex  in which the particles are dispersed in water.  The ink composition used in the present invention contains  at least one kind of polymer particles.  Although the content of the "pigment covered with water› insoluble resin" in the ink composition of the present inven- tion is not specifically restricted, the content is preferably  from 0.05% by mass to 30% by mass, more preferably from  0.1 % by mass to 20% by mass, and particularly preferably  from 0.15% by mass to 15% by mass. When the content is  0.05% by mass or more, the phenomena that ink coloration  (coloring properties) becomes insufficient can be effectively  prevented. Further, when the content is 30% by mass or less,  an increase in the viscosity of ink can be effectively sup› pressed and deterioration of the ink ejection stability and the  like can be effectively prevented.  Further, the average particle diameter and of the particle  size distribution of pigment particles can be obtained by  measuring the volume average particle diameter by a  dynamic light scattering method using NANOTRAC particle  size distribution measuring device UPA-EXl 50 (trade name,  manufactured by Nikkiso Co., Ltd.).  The average particle diameter of pigment particles dis- persed in the pigment dispersion is preferably in the range of  from 30 nm to 200 nm, and more preferably in the range of  from 50 nm to 150 nm. When the average particle diameter of  pigment particles is 30 nm or more, the production suitability  is improved, and when the average particle diameter of pig› ment particles is 200 nm or less, the storability of the disper› sion becomes good. The size distribution of the pigment  particles covered with resin is not specifically restricted, and  either particles having a broad particle diameter distribution  or particles having a mono-dispersed particle diameter distri› bution may be used.  It is preferable that, after the removal process of the water› insoluble volatile solvent, the water-insoluble volatile solvent  is substantially removed from the prepared pigment disper› sion. Specifically, the remaining amount of the water-in- soluble volatile solvent in the pigment dispersion is prefer› ably 5% by mass or less with respect to the addition amount  of the water-insoluble volatile solvent at the time of dispers› ing, from the viewpoint of the thickening of the pigment  dispersion, the ejection stability when an ink composition  using the water-insoluble volatile solvent is used, and the  suppression of occurrence of curl of a recording medium. The  remaining amount of the water-insoluble volatile solvent in  the pigment dispersion is preferably 1 % by mass or less, and  more preferably 0.1 % by mass or less.  The removal of the water-insoluble volatile solvent can be  performed by conventional methods including a drying  method such as heating or air blowing, or distillation under  reduced pressure, and the water-insoluble volatile solvent is  distilled away from the dispersion obtained by the dispersion  process, so that the dispersion is thickened and phase-inverted  to an aqueous system. In this case, when the water-insoluble  resin is used as a dispersant for pigment, a dispersion of  pigment particles, in which the surface of the pigment par› ticles is covered with the water-insoluble resin, can be  obtained.  when the water-insoluble volatile solvent is used for prepa› ration of pigment ink, and the ink is used for recording an  image, the ink ejection stability can be attained, and occur› rence of curl of a recording medium can be prevented.  Polymer Particles 