Ink composition and method for producing the same, ink set, and image forming method

1. An ink composition comprising:

initiator particles comprising a water-insoluble polymerization initiator and a water-soluble polymer and having a volume average particle diameter of 500 nm or less;

a water-soluble polymerizable compound;

a pigment; and

water.

2. The ink composition according to claim 1 , wherein the initiator particles are obtained by mixing an organic solvent solution containing the water-insoluble polymerization initiator with an aqueous solution containing the water-soluble polymer.

3. The ink composition according to claim 1 , wherein the pigment is a water-dispersible pigment wherein at least a portion of the surface of the pigment is coated with a water-insoluble polymer.

4. The ink composition according to claim 1 , wherein the water-soluble polymer is a nonionic water-soluble polymer.

5. The ink composition according to claim 4 , wherein the nonionic water-soluble polymer is a block copolymer having a polyoxyethylene structure and a polyoxypropylene structure.

6. The ink composition according to claim 5 , wherein the initiator particles are obtained by mixing an organic solvent solution containing the water-insoluble polymerization initiator with an aqueous solution containing the water-soluble polymer.

7. The ink composition according to claim 6 , wherein the pigment is a water-dispersible pigment wherein at least a portion of the surface of the pigment is coated with a water-insoluble polymer.

8. An ink set comprising the ink composition of claim 7 and a treatment liquid that is capable of forming an aggregate by contact with the ink composition.

9. The ink set according to claim 8 , wherein the treatment liquid includes at least one selected from the group consisting of an acidic compound, a polyvalent metal salt, and a cationic polymer.

10. An ink set comprising the ink composition of claim 1 and a treatment liquid that is capable of forming an aggregate by contact with the ink composition.

11. The ink set according to claim 10 , wherein the treatment liquid includes at least one selected from the group consisting of an acidic compound, a polyvalent metal salt, and a cationic polymer.

12. An image forming method comprising:

applying the ink composition of claim 1 onto a recording medium by an inkjet method to form an image; and

applying a treatment liquid that is capable of forming an aggregate by contact with the ink composition onto the recording medium.

13. The image forming method according to claim 12 , further comprising irradiating active energy rays to the ink composition applied onto the recording medium.

14. The image forming method according to claim 12 , wherein the recording medium is a coated paper.

15. The image forming method according to claim 12 , wherein the ink composition is applied onto the recording medium onto which the treatment liquid has been applied.

16. A method for producing an ink composition, comprising:

mixing an organic solvent solution containing a water-insoluble polymerization initiator with an aqueous solution containing a water-soluble polymer to obtain initiator particles containing the water-insoluble polymerization initiator and the water-soluble polymer and having a volume average particle diameter of 500 nm or less; and

mixing the initiator particles, a water-soluble polymerizable compound, a pigment, and water to obtain an ink composition.