Specific examples of the pigments are listed below by each representative hue.

Yellow pigments include, for example, C. I. Pigment Yellow 1, 2, 3, 12, 13, 14, 16, 17, 42, 73, 74, 75, 81, 83, 87, 93, 95, 97, 98, 108, 109, 114, 120, 128, 129, 138, 139, 150, 151, 155, 166, 180, 184, 185, 213, etc.

Magenta pigments include, for example, C. I. Pigment Red 5, 7, 12, 22, 38, 48:1, 48:2, 48:4, 49:1, 53:1, 57, 57:1, 63:1, 101, 102, 112, 122, 123, 144, 146, 149, 168, 177, 178, 179, 180, 184, 185, 190, 202, 209, 224, 242, 254, 255, 270, C. I. Pigment Violet 19, etc.

Cyan pigments include, for example, C. I. Pigment Blue 1, 2, 3, 15, 15:1, 15:2, 15:3, 15:4, 15:6, 16, 18, 22, 27, 29, 60, etc.

Black pigments include, for example, carbon black (C. I. Pigment Black 7), etc.

White pigments include, for example, titanium oxide, aluminum oxide, etc., whose surface may have been treated with alumina, silica, or various other materials.

In the ink composition, the percentage of the pigment is preferably 1.0% by mass or more, or more preferably 1.2% by mass or more, from the viewpoint of improving image printing density on printed matters, and it is preferably 5.0% by mass or less, or more preferably 4.0% by mass or less, from the viewpoint of improving discharge stability. It should be noted that, if the pigment is a white pigment, the percentage of the white pigment in the ink composition is preferably 3.0% by mass or more, or more preferably 5.0% by mass or more, and preferably 15.0% by mass or less, or more preferably 10.0% by mass or less.

If the aforementioned pigment is used in the UV-curable aqueous inkjet printing ink composition proposed by the present invention, preferably a pigment dispersant is used.

The pigment dispersant is used to improve dispersibility of the pigment and storage stability of the ink composition, and although any conventionally used pigment dispersant can be used without specific limitations, preferably a polymer pigment dispersant is used, for example. For the pigment dispersant, any one pigment dispersant can be used alone, or two or more types can be combined.

The polymer pigment dispersant may be, for example, a carbodiimide-based dispersant, polyester amine-based dispersant, fatty acid amine-based dispersant, modified polyacrylate-based dispersant, modified polyurethane-based dispersant, multi-chain polymer nonionic dispersant, polymer ion activator, etc.

Such polymer pigment dispersant is preferably soluble in water, such as BYKJET-9150, BYKJET-9151, BYKJET-9170, DISPERBYK-168, DISPERBYK-190, DISPERBYK-198, DISPERBYK-2010, DISPERBYK-2012, and DISPERBYK-2015 by BYK-Chemie GmbH, SMA 1440, SMA 2625, SMA 17352, SMA 3840, SMA 1000, SMA 2000, and SMA 3000 by Cray Valley SA, JONCRYL 67, JONCRYL 678, JONCRYL 586, JONCRYL 611, JONCRYL 680, JONCRYL 682, JONCRYL 690, JONCRYL 819, JONCRYL-JDX 5050, EFKA 4550, EFKA 4560, EFKA 4585, EFKA 5220, and EFKA 6230 by BASF Japan Ltd., and SOLSPERSE 20000, SOLSPERSE 27000, SOLSPERSE 41000, SOLSPERSE 41090, SOLSPERSE 43000, SOLSPERSE 44000, SOLSPERSE 46000, SOLSPERSE 47000, SOLSPERSE 54000 by The Lubrizol Corporation, and the like.

The content of the pigment dispersant is preferably 1.0 part by mass or more, or more preferably 5.0 parts by mass or more, and preferably 100 parts by mass or less, or more preferably 60.0 parts by mass or less, relative to 100 parts by mass of the pigment, from the viewpoint of increasing dispersibility of the pigment and storage stability of the ink composition.

(Water)

Water is blended in as a solvent for dissolving and dispersing a coloring agent, etc. Water includes purified water, ion-exchanged water, etc.

Preferably the content of water is 10% by mass or more in the ink composition. Also, preferably the content of water is 40% by mass or less in the ink composition. Keeping the content of water within the aforementioned range provides an advantage in that the ink composition will have good dispersion stability.

(Optional Components)

The ink composition proposed by the present invention may contain surface conditioners, water-soluble organic solvents, storability improving agents, UV absorbents, antioxidants, defoaming agents, antifungal agents, rustproofing agents, thickening agents, humectant, pH adjusting agents and various other additives as optional components as deemed appropriate.

Surface Conditioners

Surface conditioners that may be contained in the ink composition proposed by the present invention are not specifically limited. Silicone-based surface conditioners, fluorine-based surface conditioners, acetylene-based surface conditioners, etc., are preferred. The acetylene-based surface conditioners include DYNOL 607, DYNOL 609, EXP-4001, EXP-4300, and OLFINE E1010 (Nissin Chemical Industry Co., Ltd.), for example. The silicone-based surface conditioners include BYK-307, 333, 347, 348, 349, 345, 378, and 3455 (BYK-Chemie GmbH), for example. The fluorine-based surface conditioners include F-410, 444 and 553 (DIC Corporation), as well as FS-65, 34, 35, 31, and 30 (DuPont de Nemours, Inc.), for example.

If a surface conditioner is contained, the content of the surface conditioner is not specifically limited; however, preferably the content is such that the surface tension of the ink composition will become 25 to 40 mN/m, or more preferably the content is 0.1 to 1.5% by mass in the ink composition.

Water-Soluble Organic Solvents

The ink composition proposed by the present invention may contain a water-soluble organic solvent to adjust its storage stability and discharge stability, jetting property of the ink, and so on. The water-soluble organic solvent is not specifically limited, but preferably it is an alcohol, polyalcohol, lower alkyl ether of polyalcohol, ketone, ether, ester, nitrogen-containing compound, amide, etc. Also, the content of the water-soluble organic solvent is not specifically limited.

The alcohol may be methanol, ethanol, n-propanol, n-butanol, n-pentanol, n-hexanol, n-heptanol, n-octanol, n-nonyl alcohol, n-decanol, or isomer thereof, cyclopentanol, cyclohexanol, etc.

The polyalcohol may be glycerin, ethylene glycol, propylene glycol, 1,3-butylene glycol, 1,4-butylene glycol, 1,2-pentanediol, 4-methyl-1,2-pentanediol, 3,3-dimethyl-1,2-butanediol, 1,5-pentanediol, neopentyl glycol, 1,2-hexanediol, 2-ethyl-1,3-hexanediol, 2-methyl-1,3-butanediol, 5-methyl-1,2-hexanediol, 3-methyl-1,3-butanediol, 3-methyl-1,5-butanediol, 1,6-hexanediol, 1,2-cyclohexanediol, 2,2,4-trimethyl-1,3-pentanediol, 2,5-dimethyl-2,5-hexanediol, 1,2-octanediol, 1,8-octanediol, 1,9-nonanediol, 1,10-decanediol, pentaerythritol, diethylene glycol, dipropylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, polypropylene glycol, thiodiglycol, etc.

The lower alkyl ether of polyalcohol may be ethylene glycol monomethyl ether, ethylene glycol dimethyl ether, ethylene glycol monoethyl ether, ethylene glycol diethyl ether, ethylene glycol monopropyl ether, ethylene glycol isopropyl ether, ethylene glycol monobutyl ether, ethylene glycol isobutyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol monopropyl ether, propylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, dipropylene glycol mono-n-propyl ether, dipropylene glycol mono-n-butyl ether, etc.

The ketone may be acetone, methyl ethyl ketone, methyl butyl ketone, methyl isobutyl ketone, diisopropyl ketone, cyclopentanone, cyclohexanone, etc.

The ether may be diethyl ether, isopropyl ether, n-butyl ether, tetrahydrofuran, tetrahydropyran, 1,4-dioxane, etc.

The ester may be ethyl acetate, propylene carbonate, methyl acetate, ethyl acetate, propyl acetate, isopropyl acetate, butyl acetate, isobutyl acetate, amyl acetate, ester lactate, ester butyrate, dibutyl phthalate, dioctyl phthalate, or ε-caprolactone, ε-caprolactam, or other cyclic ester, etc.

The nitrogen-containing compound may be 2-pyrrolidone, N-methylpyrrolidone, N-ethylpyrrolidone, N-methyloxazolidinone, N-ethyloxazolidinone, etc.

The amide may be β-alkoxypropionamide, etc.

Storability Improving Agent

As a storability improving agent, a hindered amine of N—CH₃type, N—H type, N—OR type, etc., can be used.

UV Absorbents

UV absorbents include benzophenone-based UV absorbents, benzotriazole-based UV absorbents, salicylate-based UV absorbents, hydroxyphenyl triazine-based UV absorbents, cyanoacrylate-based UV absorbents, nickel complex salt-based UV absorbents, etc.

Antioxidants

Antioxidants include phenol-based antioxidants, amine-based antioxidants, sulfur-based antioxidants, phosphorus-based antioxidants, etc.

Defoaming Agents

Defoaming agents include silicone-based defoaming agents, Pluronic-based defoaming agents, etc.

The UV-curable aqueous inkjet printing ink composition proposed by the present invention has a viscosity at 25° C. of preferably 100 mPa·s or lower, or more preferably 30 mPa·s or lower. The ink composition is blended with a viscosity-adjusting agent, etc., as necessary. It should be noted that the viscosity stated in the Specification pursuant to the present application for patent represents a value measured at 25° C. using a type-E viscometer (Viscometer Model RE100L, Toki Sangyo Co., Ltd.).

The method for preparing the UV-curable aqueous inkjet printing ink composition proposed by the present invention is not specifically limited. As an example, the ink composition can be prepared by mixing the materials contained therein using a bead mill, triple-roll mill, etc. It should be noted that, with regard to the pigment, the pigment may be mixed with the aforementioned pigment dispersants and photopolymerizable monomers to obtain a base ink composition beforehand, with the remainder of the aforementioned components added thereto to a desired composition, to prepare the ink composition.

The method for printing using the UV-curable aqueous inkjet printing ink composition proposed by the present invention is explained.

The inkjet printing device used is not specifically limited so long as it is a device capable of printing UV-curable ink compositions.

Also, the base materials to be printed on are not specifically limited, either, and various resin base materials, paper, capsule, gel, metal foil, glass, wood, fabric, etc., can be adopted as base materials.

The area printed with the UV-curable aqueous inkjet printing ink composition proposed by the present invention, obtained by discharging the ink composition onto the base material surface, is irradiated with ultraviolet ray for partial curing (pinning). Next, ultraviolet ray is irradiated for full curing, to cure the ink composition in the printed area.

Here, “partial curing (pinning)” means curing the ink composition, which has been discharged onto the base material surface, to the extent that the ink composition loses fluidity, and the pinned ink composition contains the uncured components and solvent.

During this partial curing (pinning), the surface of the printed area is irradiated with a UV-LED light having a peak wavelength between 365 and 405 nm, to a cumulative light quantity of 100 mJ/cm²or less.

The full curing step that follows the partial curing is a step to irradiate ultraviolet ray to a cumulative light quantity sufficient to cure the ink composition completely, and preferably ultraviolet ray is irradiated so that a cumulative light quantity of 100 to 300 mJ/cm²is achieved in the full curing step alone.

EXAMPLES

Ink compositions 1 to 14 specified in Table 1 below were prepared and pinned under the conditions in Table 2.

- EO-modified TMPTA: Trimethylolpropane EO-modified triacrylate
- CN131B: Epoxy (meth)acrylate oligomer (product name “Sartomer CN131B”, Sartomer Company, Inc.)
- CN371: Amine-modified (meth)acrylate oligomer (product name “Sartomer CN371”, Sartomer Company, Inc.)
- TPO-L: (2,4,6-trimethylbenzoyl)-ethoxyphenylphosphine oxide
- BYK-333: Silicone-based surface conditioner (BYK-Chemie GmbH)
- P. B. 15:4: Pigment Blue 15:4
- P. R. 122: Pigment Red 122
- P. Y. 138: Pigment Yellow 138
- P. Bk. 7: Carbon black
- BYKJET-9151: Water-soluble polymer dispersant (BYK-Chemie GmbH) Methods for Evaluation
  
  (Compatibility)

Each ink was put in a brown bottle and kept stationary for 24 hours at room temperature, and then visually inspected for presence or absence of separation.

- ◯: No separation
- x: Separated into two layers.

Subsequent evaluations were performed on the inks other than those that were given “x” in the compatibility evaluation.

(Storage Stability)

Each ink was sampled into a brown bottle and then sealed and stored for 2 weeks at 60° C., after which the ink was measured for viscosity using a type-E viscometer (Viscometer Model RE100L, Toki Sangyo Co., Ltd.) under the conditions of 25° C. in temperature and 20 rpm in rotor revolution speed, to calculate the rate of increase in viscosity according to Formula (1), thereby evaluating storage stability thereof according to the evaluation criteria below.

((Viscosity after 2 weeks at 60° C.−Viscosity before storage)/Viscosity before storage)×100 Formula (1)

- ◯: The rate of increase in viscosity is under 10%.
- x: The rate of increase in viscosity is 10% or higher.
  
  (Grainy Feel)

Each ink was printed on OK Top Coat paper (Oji Paper Co., Ltd.) using an inkjet evaluation machine, pinned by UV-LED irradiation to the cumulative light quantity for pinning shown in Table 2, and then cured by irradiating 200 mJ/cm²of ultraviolet ray in cumulative light quantity. The obtained cured coating film was visually observed to evaluate the grainy feel according to the evaluation criteria below.

- ◯: There is no grainy feel.
- Δ: There is a slight grainy feel.
- x: There is a significant grainy feel.
  
  (Curability)

Each ink was printed on OK Top Coat paper (Oji Paper Co., Ltd.) using an inkjet evaluation machine, pinned by UV-LED irradiation to the cumulative light quantity for pinning shown in Table 2, and then cured by irradiating 200 mJ/cm²of ultraviolet ray in cumulative light quantity. The obtained cured coating film was rubbed with a cotton swab to evaluate curability according to the evaluation criteria below.

- ◯: No marks were left where rubbed with the cotton swab.
- Δ: Slight rubbing marks were left.
- x: The coating film was removed by the cotton swab.
  
  (Water Resistance)

Each ink was printed on OK Top Coat paper (Oji Paper Co., Ltd.) using an inkjet evaluation machine, pinned by UV-LED irradiation to the cumulative light quantity for pinning shown in Table 2, and then cured by irradiating 200 mJ/cm²of ultraviolet ray in cumulative light quantity. The obtained cured coating film was rubbed with a cotton swab moistened with water to evaluate water resistance according to the evaluation criteria below.

- ◯: Not removed by 40 back-and-forth rubs.
- Δ: Removed by 11 to 39 back-and-forth rubs.
- x: Removed by no more than 10 back-and-forth rubs.

TABLE 1

Ink compositions
1
2
3
4
5
6
7

Water-soluble
Acryloyl
25.00
40.00
10.00
24.30
24.70
24.50
28.25

acrylamide-based
morpholine

monomers
Diacetone
5.00
5.00
5.00
5.00
5.00
5.00
5.00

acrylamide

Water-soluble hydroxyl
4-hydroxybutyl
25.00
10.00
40.00
24.30
24.70
24.50
28.25

group-containing
acrylate

monofunctional monomer

Water-insoluble
EO-modified
5.00
5.00
5.00
5.00
5.00
5.00
2.00

TMPTA

polymerizable
CN131B
6.00
6.00
6.00
6.00
6.00
6.00
3.00

compounds
CN371
2.50
2.50
2.50
2.50
2.50
2.50
1.00

Purified water
23.80
23.80
23.80
23.80
23.80
23.80
23.80

Photopolymerization
TPO-L
5.00
5.00
5.00
5.00
5.00
6.00
6.00

initiator

Surface conditioner
BYK-333
0.20
0.20
0.20
0.20
0.20
0.20
0.20

Pigments
P. B. 15:4
2.00
2.00
2.00

2.00

P. R. 122

3.00

P. Y. 138

2.50

P. Bk. 7

2.00

Pigment dispersant
BYKJET-9151
0.50
0.50
0.50
0.90
0.60
0.50
0.50

Total
100.00
100.00
100.00
100.00
100.00
100.00
100.00

Evaluations
Compatibility
○
○
○
○
○
○
○

Storage stability
○
○
○
○
○
○
○

Ink compositions
8
9
10
11
12
13
14

Water-soluble
Acryloyl
20.25
36.90
45.00
5.00
21.50
30.00
19.00

acrylamide-based
morpholine

monomers
Diacetone
5.00
5.00
5.00
5.00
5.00
5.00
5.00

acrylamide

Water-soluble hydroxyl
4-hydroxybuty1
20.25
36.90
5.00
45.00
21.50
30.00
19.00

group-containing
acrylate

monofunctional

monomer

Water-insoluble
EO-modified
8.00
5.00
5.00
5.00
5.00
1.00
10.00

TMPTA

polymerizable
CN131B
9.00
6.00
6.00
6.00
6.00
2.00
11.00

compounds
CN371
5.00
2.50
2.50
2.50
2.50
1.00
5.00

Purified water
23.80

23.80
23.80
23.80
23.30
23.30

Photopolymerization
TPO-L
6.00
5.00
5.00
5.00
12.00
5.00
5.00

initiator

Surface conditioner
BYK-333
0.20
0.20
0.20
0.20
0.20
0.20
0.20

Pigments
P. B. 15:4
2.00
2.00
2.00
2.00
2.00
2.00
2.00

P. R. 122

P. Y. 138

P. Bk. 7