Ink composition and image forming method

1. An ink composition comprising:

(a) a polymer comprising a repeating unit (a1) having a group represented by the following Formula (1′) and a repeating unit (a2) having a hydrophilic group;

represented by the following Formula (2);

(b) a radical polymerizable compound;

(c) water; and

(d) a colorant;

wherein, in Formula(1′), each of R a and R b independently represents an alkyl group having from 1 to 4 carbon atoms; R a and R b may be bound to each other to form a 4- to 6-membered alicyclic structure; R c represents a hydrogen atom or a methyl group; Z represents a single bond, —COO—*, or —CONR d —*; R d represents a hydrogen atom or an alkyl group having from 1 to 4 carbon atoms; * represents a site bonding to X; and X represents a divalent organic group, and

wherein, in Formula (2), R cy represents a hydrogen atom or a methyl group; Z y represents —COO—*, —CONR dy —*, or a single bond; R dy represents a hydrogen atom or an alkyl group having from 1 to 4 carbon atoms; R y represents a group selected from the group consisting of a single bond, an alkylene group, an arylene group, and an aralkylene group; A represents a hydrophilic group; and * represents a site bonding to R.

2. The ink composition according to claim 1 , wherein the (b) radical polymerizable compound is a compound having a (meth)acryloyloxy group or a (meth)acrylamide group.

3. The ink composition according to claim 1 , wherein a content of the (a) polymer is from 0.5% by mass to 20% by mass, and a content of the (b) radical polymerizable compound is from 1% by mass to 20% by mass, with respect to the ink composition.

4. The ink composition according to claim 1 , wherein the divalent organic group represented by X is an alkylene group having from 2 to 20 carbon atoms, the alkylene group optionally comprising at least one selected from the group consisting of an ether bond, an ester bond, an amide bond, a urethane bond, and an arylene group.

5. The ink composition according to claim 1 , wherein the hydrophilic group in the repeating unit (a2) is at least one group selected from the group consisting of an amide group, a carbamoyl group, an alkyl-substituted carbamoyl group, an alcoholic hydroxyl group, a group having a polyalkyleneoxy structure, and a residue obtained by removing one hydrogen atom from a heterocyclic compound that includes a nitrogen atom or an oxygen atom.

6. The ink composition according to claim 1 , wherein the hydrophilic group in the repeating unit (a2) is at least one group selected from the group consisting of a carboxyl group, a sulfo group, a phosphoric acid group, a phosphonic acid group, a phenolic hydroxyl group and salts thereof, and a quaternary ammonium salt.

7. The ink composition according to claim 1 , wherein a content of the (c) water is from 10% by mass to 97% by mass with respect to the ink composition.

8. The ink composition according to claim 1 , further comprising (e) a water-soluble organic solvent.

9. The ink composition according to claim 8 , wherein the (e) water-soluble organic solvent is a compound represented by the following Formula (E1);

wherein, in Formula (E1), each of R e1 and R e2 independently represents —CH 2 —, NR e4 —, or —O—; R e1 and R e2 do not simultaneously represent —CH 2 —; R e4 represents an alkyl group having from 1 to 3 carbon atoms, a hydroxyalkyl group having from 1 to 3 carbon atoms, a hydroxyl group, or a hydrogen atom; R e3 represents a hydrocarbon group represented by —C m H 2m —, —C m H 2m-2 —, or C m H 2m-4 —; and m represents an integer of from 2 to 8.

10. The ink composition according to claim 9 , wherein the (e) water-soluble organic solvent is 2-pyrrolidone or γ-butyrolactone.

11. An inkjet recording ink comprising the ink composition according to claim 1 .

12. An image forming method comprising:

applying the ink composition according to claim 1 onto a recording medium; and

irradiating the ink composition with active energy rays.

13. The ink composition according to claim 1 , wherein (d) the colorant is a pigment.

14. The ink composition according to claim 1 , wherein the copolymerization ratio of the repeating unit represented by Formula (1′) to the repeating unit represented by Formula (2) is 30:70 to 90:10 by mass.