Pigment dispersion and ink composition and ink set both containing the same

1. A pigment dispersion, comprising at least a pigment, an aqueous medium, a copolymer resin between a hydrophobic monomer and a hydrophilic monomer, and a urethane resin, wherein said copolymer resin has an acid value in a range of 50 to 320, and

wherein said urethane resin has an acid value in a range of 10 to 300.

2. The pigment dispersion according to claim 1 , wherein said copolymer resin and said urethane resin have a weight ratio (former/latter) in a range of 1/2 to 2/1.

3. The pigment dispersion according to claim 1 , wherein the content of said copolymer resin is in a range of 10 to 50 parts by weight per 100 parts by weight of said pigment, and the content of said urethane resin is in a range of 10 to 40 parts by weight per 100 parts by weight of said pigment.

4. The pigment dispersion according to claim 3 , wherein said copolymer resin and said urethane resin have a weight ratio (former/latter) in a range of 1/2 to 2/1.

5. The pigment dispersion according to claim 1 , wherein said copolymer resin is at least one of a styrene-(meth)acrylic acid copolymer resin, a styrene-methylstyrene-(meth)acrylic acid copolymer resin, a styrene-maleic acid copolymer resin, a (meth)acrylic acid-(meth)acrylic acid ester copolymer resin, and a styrene-(meth)acrylic acid-(meth)acrylic acid ester copolymer resin.

6. The pigment dispersion according to claim 1 , wherein said urethane resin has urethane linkages and/or amide linkages, and acidic groups.

7. The pigment dispersion according to claim 1 , wherein the weight ratio between a solid component of said pigment and non-pigment solid components (former/latter) is in a range of 100/20 to 100/80.

8. The pigment dispersion according to claim 2 , wherein the content of said copolymer resin is in a range of 10 to 50 parts by weight per 100 parts by weight of said pigment, and the content of said urethane resin is in a range of 10 to 40 parts by weight per 100 parts by weight of said pigment.

9. The pigment dispersion according to claim 2 , wherein said copolymer resin has an acid value in a range of 50 to 320, and said urethane resin has an acid value in a range of 10 to 300.

10. The pigment dispersion according to claim 1 , wherein said copolymer resin has a weight average molecular weight (Mw) in a range of 2,000 to 30,000, and said urethane resin has a weight average molecular weight (Mw) in a range of 100 to 200,000.

11. The pigment dispersion according to claim 1 , wherein said copolymer resin has a glass transition temperature (Tg; measured in accordance with JIS K6900) of at least 30° C., and said urethane resin has a glass transition temperature (Tg; measured in accordance with JIS K6900) in a range of −50 to 200° C.

12. The pigment dispersion according claim 1 , wherein said copolymer resin has a maximum particle diameter of not more than 0.3 μm, and said urethane resin has a maximum particle diameter of not more than 0.3 μm.

13. The pigment dispersion according to claim 1 , wherein said pigment is an organic pigment.

14. The pigment dispersion according to claim 1 , wherein said pigment is a pigment that has been subjected to kneading treatment.

15. The pigment dispersion according to claim 1 , wherein said pigment is a pigment that has been prepared using a bead mill or an impact jet mill.

16. The pigment dispersion according to claim 1 , wherein the pigment dispersion has been subjected to ion exchange treatment or ultra-filtration.

17. The pigment dispersion according to claim 1 , wherein an epoxy resin having a glycidyl ether as a backbone thereof, or a resin having oxazoline groups has been added to the pigment dispersion as a crosslinking agent.

18. The pigment dispersion according to claim 17 , wherein said crosslinking agent is a resin that reacts with carboxyl groups.

19. The pigment dispersion according to claim 17 , wherein the amount added of said crosslinking agent is in a range of 1 to 50 wt% relative to said urethane resin.

20. The pigment dispersion according to claim 19 , wherein said urethane resin has a weight average molecular weight (Mw) after reaction with said crosslinking agent of at least 10,000.

21. The pigment dispersion according to claim 17 , wherein the amount added of said crosslinking agent is such that an effective solid component weight ratio (weight of crosslinking agent/(total weight of copolymer resin between hydrophobic monomer and hydrophilic monomer, and urethane resin)) is in a range of 0.5/100 to 50/100.

22. An ink composition, containing at least the pigment dispersion according to claim 1 , and an aqueous medium.

23. The ink composition according to claim 22 , wherein said aqueous medium contains a penetrating solvent, a wetting solvent, and/or a surfactant.

24. The ink composition according to claim 23 , wherein said penetrating solvent is an alkanediol and/or a glycol ether.

25. The ink composition according to claim 23 , wherein said wetting solvent is a polyhydric alcohol.

26. The ink composition according to claim 23 , wherein said surfactant is acetylenic glycol and/or a polysiloxane.

27. The ink composition according to claim 22 , characterized by further containing an alkanolamine or an alkylamine as a pH adjuster.

28. The ink composition according to claim 22 , wherein the ink composition has a pH of at least 8.0, a pigment maximum particle diameter of not more than 0.3 μm, and a pigment 50% cumulative dispersed diameter of not more than 0.15 μm.

29. The ink composition according to claim 22 , wherein the content of said pigment is not more than 10 wt% of the ink composition.

30. An ink set, comprising at least the ink composition according to claim 22 .

31. A method of manufacturing a pigment dispersion according to claim 1 , comprising:

a pretreatment step of making a particle diameter of the pigment minute and uniform through kneading treatment or chipping treatment;

a dispersion step of adding the copolymer resin between the hydrophobic monomer and the hydrophilic monomer, and then dispersing the pigment using a bead mill or an impact jet mill; and

an after-treatment step of further adding the urethane resin and a crosslinking agent and carrying out crosslinking treatment.

32. A pigment dispersion, comprising at least a pigment, an aqueous medium, a copolymer resin between a hydrophobic monomer and a hydrophilic monomer, and a urethane resin,

wherein said urethane resin has urethane linkages and/or amide linkages, and acidic groups.

33. A pigment dispersion, comprising at least a pigment, an aqueous medium, a copolymer resin between a hydrophobic monomer and a hydrophilic monomer, and a urethane resin,

wherein an epoxy resin having a glycidyl ether as a backbone thereof, or a resin having oxazoline groups has been added to the pigment dispersion as a crosslinking agent, and

wherein said crosslinking agent is a resin that reacts with carboxyl groups.