Aqueous ink composition and image forming apparatus

1. An aqueous ink composition, comprising:

an aqueous solvent;

a coloring agent dissolved or dispersed in the aqueous solvent;

an ABA-type amphipathic polymer comprising a hydrophobic segment A and a hydrophilic segment B; and

an anionic surfactant configured to dissolve or disperse the ABA-type amphipathic polymer in the aqueous solvent,

wherein a viscosity of the aqueous ink composition is changed depending on a pH thereof, and

wherein the ABA-type amphipathic polymer is a hydrophobic-modified polyetherurethane.

2. The aqueous ink composition as claimed in claim 1 , wherein the anionic surfactant dissolved in the aqueous solvent has a buffering function.

3. The aqueous ink composition as claimed in claim 1 , wherein the ABA-type amphipathic polymer is an A n B-type amphipathic polymer with 3 or more hydrophobic segments A.

4. The aqueous ink composition as claimed in claim 1 , wherein the hydrophobic segment A comprises at least one selected from the group consisting of a straight chain alkyl group, a branched alkyl group, a cyclic alkyl group, and a phenyl group.

5. The aqueous ink composition as claimed in claim 1 , wherein the hydrophilic segment B comprises at least one selected from the group consisting of a straight chain polyethylene oxide and a multi-branched polyethylene oxide.

6. The aqueous ink composition as claimed in claim 1 , wherein the anionic surfactant is a carboxylic-acid-type surfactant.

7. The aqueous ink composition as claimed in claim 6 , wherein the carboxylic-acid-type surfactant comprises at least one selected from the group consisting of a salt of aliphatic carboxylic acid, a salt of polyoxyethylene alkyl ether carboxylic acid, a salt of N-acylsarcosine acid, a salt of N-acylglutamic acid, a salt of alpha-sulfo-aliphatic acid ester, and a salt of multi-chain- and multi-hydrophilic-group-type carboxylic acid.

8. The aqueous ink composition as claimed in claim 1 , further comprising an electrolyte configured to improve an electrical conductivity of the aqueous ink composition.

9. An image forming apparatus, comprising:

an image forming device configured to apply the aqueous ink composition as claimed in claim 1 onto a recording medium depending on an image signal to form an image.

10. The image forming apparatus as claimed in claim 9 , further comprising a pH control device configured to conduct a pH treatment changing a viscosity of the aqueous ink composition applied on the recording medium.

11. The image forming apparatus as claimed in claim 9 , further comprising a pressure application device configured to apply a pressure to the recording medium with the aqueous ink composition to improve a fixation property of the aqueous ink composition on the recording medium.

12. The image forming apparatus as claimed in claim 9 , further comprising:

an intermediate transfer body, the aqueous ink composition being applied to the intermediate transfer body by the image forming device;

a pH control device configured to conduct a pH treatment changing a viscosity of the aqueous ink composition applied to the intermediate transfer body by the image forming device, and

a transfer device configured to transfer the aqueous ink composition with a viscosity changed by the pH treatment from the intermediate transfer body to the recording medium.

13. The image forming apparatus as claimed in claim 12 , wherein the image forming device is located to oppose the intermediate transfer body at a distance capable of forming a bridge of the aqueous ink composition together with the intermediate transfer body when the aqueous ink composition is applied to the intermediate transfer body, and the pH control device comprises a voltage application device configured to apply a voltage between the intermediate transfer body and the image forming device on a condition that the bridge is formed, so that water contained in the aqueous ink composition is electrolyzed to conduct the pH treatment.