JP7721968B2 - UV-curable water-based ink for inkjet printing, UV-curable water-based composition for inkjet printing, and printed matter - Google Patents

Description

The present invention relates to a UV-curable water-based ink for ink-jet printing.
The present invention also relates to a printed matter using this UV-curable water-based ink for ink-jet printing, and further relates to a UV-curable water-based composition for ink-jet printing that is suitable for this UV-curable water-based ink for ink-jet printing.

Inkjet printers have a number of advantages, including the ease of full-color printing, low noise, ability to produce high-resolution images at low cost, high-speed printing, the ability to print on curved surfaces as well as flat surfaces, and ease of printing on large areas. For these reasons, inkjet printers are not limited to personal use; in recent years, they have rapidly become popular as commercial inkjet printers for signage, window film, posters, car wrapping, wallpaper, and more.

Commercial inkjet printers sometimes print large areas or items for outdoor use, so the long-term durability of the printed film and productivity are important. Therefore, the printing inks used in commercial inkjet printers must have the following properties:
(1) High image quality: No bleeding of the image (including printed characters; the same applies hereinafter), a thin printed film (hereinafter referred to as "ink thickness"), and excellent surface smoothness of the printed film.
(2) High coating film performance: The printed film formed has high coating film strength and is excellent in water resistance, solvent resistance such as alcohol resistance, light resistance, and weather resistance.
(3) High-speed printing: Excellent ink ejection and quick-drying properties (tackiness).
(4) Substrate versatility: Printing can be performed on a variety of recording media (substrates), including polyethylene terephthalate (PET), polyvinyl chloride (PVC), polypropylene (PP), paper, textiles (cloth and fabrics), and other materials.
(5) Environmental and safety: Low odor and low VOC (organic solvents, unreacted monomers) content.

Conventionally, the following types of inks have been provided as printing inks for commercial inkjet printers: Hereinafter, "UV ink" refers to ultraviolet-curable ink.
Water-based ink in which a pigment is dispersed in a water-based medium. Solvent UV ink in which a pigment and a UV-curable monomer are dispersed or dissolved in an organic solvent. Solventless UV ink in which a pigment is dispersed in a solvent-free UV-curable monomer. Water-based latex ink in which a pigment and a resin are dispersed in a water-based medium. UV-curable water-based ink in which a pigment and a UV-curable oligomer are dispersed in a water-based medium (for example, Patent Documents 1 to 3).

Because they are water-based, water-based inks are environmentally friendly and safe. They also produce thin ink layers and have excellent surface smoothness. However, water-based inks have poor coating performance, high-speed printing capabilities, and substrate versatility, and can also pose problems with image bleeding.

Solvent-based UV inks have good coating film performance, substrate versatility, ink thickness, and printed film surface smoothness. However, solvent-based UV inks have problems with image bleeding, are not suitable for high-speed printing, and are solvent-based, which makes them less environmentally friendly and safer.

Solvent-free UV inks do not bleed and offer excellent coating performance, high-speed printing, and substrate versatility. However, solvent-free UV inks are environmentally and safety hazardous, and their high viscosity results in thick ink layers and poor surface smoothness for printed films.

Because they are water-based, water-based latex inks are environmentally friendly and safe, have excellent substrate versatility, and have other relatively good properties. However, water-based latex inks are not fully satisfactory in terms of coating performance and high-speed printing, and there is inconsistency in image quality bleeding.

Of these, UV-curable water-based inks offer a relatively good balance of all properties, but further improvement is desired.

JP 2015-48435 A JP 2014-189715 A JP 2011-201973 A

One aspect of the present invention is to provide a UV-curable water-based ink for inkjet printing that has superior properties to conventional UV-curable water-based inks as a printing ink for inkjet printers, particularly excellent alcohol resistance, abrasion resistance, and elution resistance, a UV-curable water-based composition for inkjet printing therefor, and a printed item using this UV-curable water-based ink for inkjet printing. However, it is not essential for the present invention to simultaneously possess all of the above performance characteristics (1) to (5).

The present inventors have found that by using an ultraviolet-curable oligomer having a specific structure, it is possible to realize an ink that is particularly excellent in alcohol resistance, abrasion resistance, and elution resistance.
The present invention is summarized as follows [1] to [22].

[1] An ultraviolet-curable water-based ink for inkjet printing containing at least an ultraviolet-curable oligomer and a colorant,
The ultraviolet-curable water-based ink for inkjet printing, wherein the ultraviolet-curable oligomer is an ultraviolet-curable oligomer having a structural unit derived from a polyisocyanate compound (A), a structural unit derived from a compound (B') shown below, and a structural unit derived from a compound (C') shown below.
Compound (B'): A compound containing two or more polymerizable unsaturated bonds and capable of bonding with the polyisocyanate compound (A). Compound (C'): A water-soluble compound capable of bonding with the polyisocyanate compound (A) and having a (meth)acryloyl group.

[2] The ultraviolet-curable water-based ink for inkjet printing according to [1], wherein the compound (C') is a compound containing one terminal hydroxyl group.

[3] The ultraviolet-curable water-based ink for inkjet printing according to [1] or [2], wherein the compound (C') is a polyalkylene glycol (C) having a (meth)acryloyl group.

[4] The ultraviolet-curable water-based ink for inkjet printing according to [3], wherein the compound (C') is a polyalkylene glycol mono-substituted ether having a (meth)acryloyl group.

[5] The ultraviolet-curable water-based ink for inkjet printing according to [4], wherein the compound (C') is polyethylene glycol mono(meth)acrylate.

[6] The ultraviolet-curable water-based ink for inkjet printing according to any one of [1] to [5], wherein the compound (B') is a hydroxyl group-containing polyfunctional (meth)acrylate (B).

[7] The ultraviolet-curable water-based ink for inkjet printing described in [6], wherein the ultraviolet-curable oligomer is formed by bonding a structural unit derived from the polyisocyanate compound (A) to a structural unit derived from the hydroxyl group-containing polyfunctional (meth)acrylate (B) and a structural unit derived from the polyalkylene glycol (C) having a (meth)acryloyl group via a urethane bond.

[8] The ultraviolet-curable water-based ink for inkjet printing according to any one of [1] to [7], wherein the polyisocyanate compound (A) has three or more isocyanate groups.

[9] The ultraviolet-curable water-based ink for inkjet printing according to any one of [1] to [8], wherein the ultraviolet-curable oligomer is dispersed as particles in an aqueous medium.

[10] The UV-curable water-based ink for inkjet printing according to any one of [1] to [9], wherein the average particle size of the UV-curable oligomer is 10 nm or more and 200 nm or less.

[11] The UV-curable water-based ink for inkjet printing according to any one of [1] to [10], having a viscosity at 25°C of 1 mPa·sec or more and 25 mPa·sec or less.

[12] The ultraviolet-curable water-based ink for inkjet printing according to any one of [1] to [11], wherein the content of the colorant is 0.1% by mass or more and 8% by mass or less, and the content of the ultraviolet-curable oligomer is 3% by mass or more and 20% by mass or less.

[13] The ultraviolet-curable water-based ink for inkjet printing according to any one of [1] to [12], further comprising one or more of a polymerization initiator, a sensitizer, and a surfactant.

[14] The UV-curable water-based ink for inkjet printing according to [13], which contains a polymerization initiator and/or a sensitizer, and at least a portion of the polymerization initiator and/or sensitizer is encapsulated in the UV-curable oligomer.

[15] A printed matter having, on a recording medium, a cured product formed from the UV-curable water-based ink for inkjet printing described in any one of [1] to [14].

[16] A UV-curable aqueous composition for inkjet printing, comprising a UV-curable oligomer and at least one of a polymerization initiator, a sensitizer, and a surfactant,
The ultraviolet-curable aqueous composition for inkjet printing is an ultraviolet-curable oligomer having a structural unit derived from a polyisocyanate compound (A), a structural unit derived from a compound (B') shown below, and a structural unit derived from a compound (C') shown below.
Compound (B'): A compound containing two or more polymerizable unsaturated bonds and capable of bonding with the polyisocyanate compound (A). Compound (C'): A water-soluble compound capable of bonding with the polyisocyanate compound (A) and having a (meth)acryloyl group.

[17] The ultraviolet-curable aqueous composition for inkjet printing according to [16], wherein the compound (C') is a compound containing one terminal hydroxyl group.

[18] The ultraviolet-curable aqueous composition for inkjet printing according to [16] or [17], wherein the compound (C') is a polyalkylene glycol (C) having a (meth)acryloyl group.

[19] The ultraviolet-curable aqueous composition for inkjet printing according to [18], wherein the compound (C') is a polyalkylene glycol mono-substituted ether having a (meth)acryloyl group.

[20] The ultraviolet-curable aqueous composition for inkjet printing according to [19], wherein the compound (C') is polyethylene glycol mono(meth)acrylate.

[21] The UV-curable aqueous composition for inkjet printing according to any one of [16] to [20], which contains at least one polymerization initiator and a sensitizer and/or surfactant.

[22] The ultraviolet-curable aqueous composition for inkjet printing according to any one of [16] to [21], which contains a polymerization initiator and/or a sensitizer, and at least a portion of the polymerization initiator and/or sensitizer is encapsulated in the ultraviolet-curable oligomer.

The UV-curable water-based ink for inkjet printing of the present invention satisfies the required properties of an ink for inkjet printers in a well-balanced manner, and is particularly excellent in alcohol resistance, abrasion resistance, and elution resistance. The water-based composition of the present invention can provide a UV-curable water-based ink for inkjet printing that is excellent in the required properties.

Embodiments of the present invention are described in detail below.

The following description of the constituent elements is one example (representative example) of an embodiment of the present invention, and the present invention is not limited to these contents as long as it does not exceed the gist of the present invention.

In this invention, when the expression "X to Y" (X and Y are arbitrary numbers) is used, it includes the meaning of "X or more and Y or less," as well as "preferably greater than X" and "preferably smaller than Y," unless otherwise specified.

In the present invention, "(meth)acryloyl group" means an acryloyl group or a methacryloyl group, and "(meth)acrylate" means an acrylate or a methacrylate.

The ultraviolet-curable water-based ink for inkjet printing of the present invention (hereinafter sometimes referred to as "the ink of the present invention") is ultraviolet-curable, but the active energy rays used for curing are not limited to ultraviolet light.
When curing the ink of the present invention, it is not limited to curing by actinic energy rays, but may also be cured by, for example, heat.

In the present invention, the term "structural unit derived from X" refers to a structural unit that is incorporated into the molecular structure of an ultraviolet-curable oligomer by reacting compound X with another compound, using compound X as a raw material.
The "structural unit derived from X" is not necessarily limited to a unit formed from compound X. Even if the unit is formed from a raw material other than X, it is considered to be a "structural unit derived from X" as long as the chemical structure is the same.

1: UV-curable water-based ink for inkjet printing The ink of the present invention is a UV-curable water-based ink for inkjet printing containing at least a UV-curable oligomer and a colorant, characterized in that the UV-curable oligomer is a UV-curable oligomer having a structural unit derived from polyisocyanate compound (A), a structural unit derived from compound (B') shown below, and a structural unit derived from compound (C') shown below (hereinafter, sometimes referred to as the "UV-curable oligomer of the present invention").
Compound (B'): A compound containing two or more polymerizable unsaturated bonds and capable of bonding with the polyisocyanate compound (A). Compound (C'): A water-soluble compound capable of bonding with the polyisocyanate compound (A) and having a (meth)acryloyl group.

1-1: UV-Curable Oligomer The UV-curable oligomer of the present invention has a structural unit derived from polyisocyanate compound (A), a structural unit derived from compound (B') shown below, and a structural unit derived from compound (C') shown below, and is usually produced by reacting polyisocyanate compound (A), compound (B'), and compound (C').
Compound (B'): A compound containing two or more polymerizable unsaturated bonds and capable of bonding with the polyisocyanate compound (A). Compound (C'): A water-soluble compound capable of bonding with the polyisocyanate compound (A) and having a (meth)acryloyl group.

In the ultraviolet-curable oligomer of the present invention, the structural unit moiety derived from the polyisocyanate compound (A) contributes to the adhesion of the printed film to the recording medium.
The structural unit portion derived from the compound (B') contributes to the ultraviolet curability. When the compound (B') is a hydroxyl group-containing polyfunctional (meth)acrylate (B), the (meth)acryloyl group contained in the structural unit derived from the hydroxyl group-containing polyfunctional (meth)acrylate (B) contributes to the ultraviolet curability.
The structural unit portion derived from compound (C') contributes to the aqueous dispersibility of the oligomer in the ink, and the presence of a (meth)acryloyl group contributes to coating film strength such as alcohol resistance, abrasion resistance, and elution resistance. Furthermore, when compound (C') is a polyalkylene glycol (C) having a (meth)acryloyl group, the polyalkylene glycol chain of the structural unit derived from the polyalkylene glycol (C) having a (meth)acryloyl group contributes to the aqueous dispersibility of the oligomer in the ink.
The present invention is characterized by the use of an oligomer having all of these structural units in the same molecule.
From the viewpoint of reactivity, the compound (B') is preferably a hydroxyl group-containing polyfunctional (meth)acrylate (B). From the viewpoint of water dispersibility, the compound (C') is preferably a polyalkylene glycol (C) having a (meth)acryloyl group. Hereinafter, the ultraviolet-curable oligomer of the present invention in this case may be referred to as an "ultraviolet-curable (meth)acrylate oligomer."

Furthermore, since excellent resistance to solvents such as alcohol can be achieved, it is preferable that the structural units derived from the polyisocyanate compound (A) are bonded to the structural units derived from the hydroxyl group-containing polyfunctional (meth)acrylate (B) and the structural units derived from the polyalkylene glycol (C) having a (meth)acryloyl group via urethane bonds.

As a result, the UV-curable oligomer of the present invention has excellent UV curability, coating film performance, substrate adhesion, and water dispersibility.

There are no particular restrictions on whether the UV-curable oligomer used in the present invention is ionic or nonionic; it may be nonionic or ionic (anionic, cationic, or amphoteric). However, nonionicity can suppress pigment aggregation and improve the storage stability of the ink. Here, nonionicity means, for example, that the hydrophilic groups of the UV-curable oligomer are composed of ether bonds or hydroxyl groups that do not undergo ionic dissociation in water.

As described above, a preferred embodiment of the compound (B') is the hydroxyl group-containing polyfunctional (meth)acrylate (B). However, a preferred embodiment of the compound (B') may also be "a compound (B'') containing a hydroxyl group and two or more polymerizable unsaturated bonds."
The "compound capable of bonding to the polyisocyanate compound (A)" in the compound (B') is one in which the hydroxyl group can be substituted with a carboxyl group, an amino group, or the like. Examples of the polymerizable unsaturated bond include a carbon-carbon double bond and a carbon-carbon triple bond, with a carbon-carbon double bond being preferred. More specific examples include carbon-carbon double bonds derived from a vinyl group, a (meth)acryloyl group, or the like.

The water-soluble compound in the compound (C') includes a water-soluble polymer, and specifically includes polyglycerin, polyhydroxy(meth)acrylate, polyamine, quaternary aminated polystyrene, sulfonated polystyrene, polyether, polyalkylene glycol, etc. Among them, nonionic water-soluble compounds such as polyglycerin, polyhydroxy(meth)acrylate, and polyalkylene glycol are preferred, and polyalkylene glycol is particularly preferred. Each of these water-soluble compounds may be a copolymer.
Compound (C') is such a water-soluble compound that has a (meth)acryloyl group and further has the structure of a "compound that can bond to polyisocyanate compound (A)". Here, the "compound that can bond to polyisocyanate compound (A)" can be selected from structures similar to those exemplified as compound (B') above.

The structural units derived from the polyisocyanate compound (A) bond with the structural units derived from the hydroxyl group-containing polyfunctional (meth)acrylate (B) and the structural units derived from the polyalkylene glycol (C) having a (meth)acryloyl group to form urethane bonds, and these urethane bonds may be substituted with urea bonds or amide bonds. To form urea bonds, the hydroxyl groups in the hydroxyl group-containing polyfunctional (meth)acrylate (B) may be substituted with amino groups as compound (B'), or the terminal hydroxyl groups in the (meth)acryloyl group-containing polyalkylene glycol (C) may be substituted with amino groups as compound (C'). To form amide bonds, the hydroxyl groups in the hydroxyl group-containing polyfunctional (meth)acrylate (B) may be substituted with carboxyl groups as compound (B'), or the terminal hydroxyl groups in the (meth)acryloyl group-containing polyalkylene glycol (C) may be substituted with carboxyl groups as compound (C'). Instead of converting the hydroxyl group in this way, a compound capable of forming a urea bond or an amide bond may be used as it is to obtain a substantially similar chemical structure.
As described above, the structural units derived from the polyisocyanate compound (A) contribute to the adhesion of the printed film to the recording medium, and this is thought to be because, as described above, the isocyanate groups in the polyisocyanate compound (A) form polar moieties such as urethane bonds, urea bonds, or amide bonds in the ultraviolet-curable oligomer.

In the present invention, when the hydroxyl group-containing polyfunctional (meth)acrylate (B) is converted into compound (B') or compound (B") as described above, or when the polyalkylene glycol (C) having a (meth)acryloyl group is converted into compound (C') as described above, the preferred aspects or specific aspects when using the hydroxyl group-containing polyfunctional (meth)acrylate (B) or polyalkylene glycol (C) having a (meth)acryloyl group, described below, can be similarly applied to preferred or specific aspects.

The following describes each compound that makes up the UV-curable oligomer of the present invention. In the present invention, the term "oligomer" is not limited to a specific molecular weight or average molecular weight range, but rather refers to any compound having the structure shown below.

The ink of the present invention may contain only one type of UV-curable oligomer, or may contain two or more types.

1-1-1: Polyisocyanate compound (A)
The polyisocyanate compound (A) is a compound having a total of two or more isocyanate groups in one molecule.

The type of polyisocyanate compound (A) is not particularly limited, and examples include linear aliphatic polyisocyanates, aromatic polyisocyanates, and alicyclic polyisocyanates. Among these, it is preferable that the polyisocyanate compound (A) contains a polyisocyanate trimer compound from the viewpoints of weather resistance and hardness.

A chain aliphatic polyisocyanate is a compound having a chain aliphatic structure and two or more isocyanate groups. A chain aliphatic polyisocyanate is preferred from the viewpoints of weather resistance and stretchability. The chain aliphatic structure in the chain aliphatic polyisocyanate is not particularly limited, but a linear or branched alkylene group having 1 to 12 carbon atoms, preferably 1 to 6 carbon atoms, is preferred. Examples of chain aliphatic polyisocyanates include aliphatic diisocyanates such as tetramethylene diisocyanate, hexamethylene diisocyanate, trimethylhexamethylene diisocyanate, and dimer acid diisocyanate, as well as trimer compounds of these polyisocyanates.

Aromatic polyisocyanates are compounds that have an aromatic structure and two or more isocyanate groups. Aromatic polyisocyanates are preferred from the standpoint of coating film strength. The aromatic structure in aromatic polyisocyanates is not particularly limited, but aromatic structures with 6 to 13 carbon atoms are preferred. Examples of aromatic polyisocyanates include aromatic diisocyanates such as tolylene diisocyanate, xylylene diisocyanate, diphenylmethane diisocyanate, m-phenylene diisocyanate, and naphthalene diisocyanate, as well as trimer compounds of these polyisocyanates.

Alicyclic polyisocyanates are compounds having an alicyclic structure and two or more isocyanate groups. The alicyclic structure in alicyclic polyisocyanates is not particularly limited, but typically has 5 or more carbon atoms, preferably 6 or more, and typically 15 or less, preferably 14 or less, and more preferably 13 or less. A cycloalkylene group is particularly preferred for the alicyclic structure. Examples of alicyclic polyisocyanates include diisocyanates having an alicyclic structure, such as bis(isocyanatomethyl)cyclohexane, cyclohexane diisocyanate, bis(isocyanatocyclohexyl)methane, and isophorone diisocyanate, as well as trimer compounds of these polyisocyanates.

The ultraviolet-curable oligomer of the present invention may use only one of these polyisocyanate compounds (A) or two or more of them in combination. Furthermore, polyisocyanates having two or more of the following structures may also be used as the polyisocyanate compound (A): a chain aliphatic structure, an aromatic structure, and an alicyclic structure.

As the polyisocyanate compound (A), those having three or more isocyanate groups are preferred, particularly from the viewpoint of adhesion to the substrate, while those having six or fewer are preferred. Furthermore, as the polyisocyanate compound (A), trimers obtained by trimerization of hexamethylene diisocyanate, trimethylhexamethylene diisocyanate, phenylene diisocyanate, etc. are preferred, and trimers of hexamethylene diisocyanate are particularly preferred.

1-1-2: Compound (B')
The compound (B') is a compound that contains two or more polymerizable unsaturated bonds and can bond with the polyisocyanate compound (A).
Examples of the compound (B') include compounds having a hydroxyl group, an amino group, or a carboxyl group. Examples of the compound (B') include polyfunctional vinyl monomers and polyfunctional (meth)acrylates. Among these, the compound (B') is preferably a hydroxyl group-containing polyfunctional (meth)acrylate (B).

The hydroxyl group-containing polyfunctional (meth)acrylate (B) has one or more hydroxyl groups and two or more (meth)acryloyl groups. Specific examples include (meth)acrylic acid partial esters of polyhydric alcohols. The polyfunctional (meth)acrylate with hydroxyl groups forms a good crosslinked structure due to the involvement of multiple (meth)acryloyl groups in the curing reaction, which can improve physical properties such as stain resistance and abrasion resistance.

The number of hydroxyl groups in the hydroxyl group-containing polyfunctional (meth)acrylate (B) is preferably 3 or less, more preferably 2 or less, and even more preferably 1. The number of (meth)acryloyl groups in the hydroxyl group-containing polyfunctional (meth)acrylate (B) is preferably 8 or less, and more preferably 6 or less.

Examples of hydroxyl group-containing polyfunctional (meth)acrylates (B) include pentaerythritol tri(meth)acrylate, dipentaerythritol penta(meth)acrylate, caprolactone-modified dipentaerythritol penta(meth)acrylate, caprolactone-modified pentaerythritol tri(meth)acrylate, ethylene oxide-modified dipentaerythritol penta(meth)acrylate, ethylene oxide-modified pentaerythritol tri(meth)acrylate, 2-hydroxy-1,3-dimethacryloxypropane, and 2-hydroxy-3-acryloyloxypropyl methacrylate.

To produce the UV-curable oligomer of the present invention, these hydroxyl group-containing polyfunctional (meth)acrylates (B) may be used alone or in combination of two or more.

As the hydroxyl group-containing polyfunctional (meth)acrylate (B), particularly from the viewpoint of the coating film strength of the resulting cured film, those having one hydroxyl group and 3 to 5 (meth)acryloyl groups, such as dipentaerythritol penta(meth)acrylate and pentaerythritol tri(meth)acrylate, are preferred. Dipentaerythritol penta(meth)acrylate is particularly preferred because it forms a good crosslinked structure and increases the mechanical strength of the cured film.

1-1-3: Compound (C')
The compound (C') is a water-soluble compound that can bond to the polyisocyanate compound (A) and has a (meth)acryloyl group.
The compound (C') has a (meth)acryloyl group, which can improve the alcohol resistance, abrasion resistance, and elution resistance.The reason for this is considered to be that the compound (C') has a relatively highly reactive (meth)acryloyl group, which increases the polymerizable group possessed by the entire ultraviolet-curable oligomer, and allows the polymerization reaction to occur not only at the terminal derived from the compound (B') but also at the (meth)acryloyl group derived from the compound (C'), thereby increasing the crosslinking density between particles and increasing the coating film strength, thereby improving the alcohol resistance, abrasion resistance, and elution resistance.
In addition, the compound (C') is preferably a compound containing one terminal hydroxyl group, as this provides good water dispersibility.

As mentioned above, examples of the water-soluble compound in compound (C') include water-soluble polymers, and among these, polyalkylene glycols are particularly preferred. In other words, polyalkylene glycols (C) having a (meth)acryloyl group are particularly preferred as compound (C').

The molecular weight (meaning the number average molecular weight if not a single molecular weight) of the polyalkylene glycol (C) having a (meth)acryloyl group is not limited, but is preferably 100 or more, more preferably 200 or more, even more preferably 600 or more, and particularly preferably 700 or more, and is preferably 5000 or less, more preferably 2000 or less.

The polyalkylene glycol (C) having a (meth)acryloyl group is not limited, but preferably has a structure in which one end of the polyalkylene glycol is substituted with a (meth)acryloyl group, i.e., the polyalkylene glycol preferably has a mono-substituted structure, and the substituent of the mono-substituted structure is preferably a (meth)acryloyl group.
The polyalkylene glycol (C) having a (meth)acryloyl group may be a mixture of a compound having the above-mentioned preferred structure and a compound having a structure other than the above-mentioned preferred structure.

When the polyalkylene glycol (C) having a (meth)acryloyl group has the preferred structure described above, the mono-substituted structure of the polyalkylene glycol is not limited, but polyalkylene glycol mono-substituted ethers are preferred. Among these, polyethylene glycol mono-substituted ethers, polytrimethylene glycol mono-substituted ethers, and polypropylene glycol mono-substituted ethers are more preferred, and polyethylene glycol mono-substituted ethers are even more preferred.

Among the above polyalkylene glycol mono-substituted ethers, polyalkylene glycol mono-substituted ethers that do not contain ionic substituents in the ether moiety are more preferred. In other words, polyethylene glycol mono-substituted ethers having a (meth)acryloyl group are more preferably those represented by the following general formula (1):

(In formula (1), X is an alkylene group, Y is a (meth)acryloyl group, and n is an integer of 2 or greater.)

Specific examples of polyalkylene glycol mono-substituted ethers represented by the above general formula (1) include polyethylene glycol mono(meth)acrylate, polypropylene glycol mono(meth)acrylate, poly(ethylene glycol-propylene glycol) mono(meth)acrylate, poly(ethylene glycol-tetramethylene glycol) mono(meth)acrylate, and poly(propylene glycol-tetramethylene glycol) mono(meth)acrylate. Of these, polyethylene glycol mono(meth)acrylate is preferred.

Among these, X in general formula (1) is preferably an alkylene group having 1 to 3 carbon atoms, more preferably an ethylene group, a trimethylene group, or a propylene group, and even more preferably an ethylene group from the viewpoint of pigment dispersion stability or storage stability at high temperatures.

In general formula (1), n is typically 2 or more, preferably 5 or more, and more preferably 6 or more, from the viewpoint of the coating film strength of the resulting cured film, and is typically 500 or less, preferably 100 or less, and more preferably 50 or less.

To produce the UV-curable oligomer of the present invention, one of these polyalkylene glycols (C) having a (meth)acryloyl group may be used alone, or two or more may be used in combination. The polyalkylene glycol (C) having a (meth)acryloyl group may also be a mixture of compounds having different molecular weights (compounds having different n values in general formula (1)).

1-1-4: Method for Producing Ultraviolet-Curable Oligomer [0043] The method for producing the ultraviolet-curable oligomer of the present invention is not particularly limited, but it is preferable to produce it by reacting the above-mentioned polyisocyanate compound (A), compound (B'), and compound (C') to form chemical bonds therebetween.

When compound (B') is a hydroxyl group-containing polyfunctional (meth)acrylate (B) and compound (C') is a polyalkylene glycol (C) having a (meth)acryloyl group, it is preferable to produce the polymer by reacting the above-mentioned polyisocyanate compound (A), hydroxyl group-containing polyfunctional (meth)acrylate (B), and (meth)acryloyl group-containing polyalkylene glycol (C) to form urethane bonds between the isocyanate groups of polyisocyanate compound (A), the hydroxyl groups of the hydroxyl group-containing polyfunctional (meth)acrylate (B), and the hydroxyl groups of the (meth)acryloyl group-containing polyalkylene glycol (C), respectively.

In the present invention, from the viewpoint of pigment dispersion stability and the coating film strength of the obtained cured film, it is preferable to use an oligomer in which two hydroxyl group-containing polyfunctional (meth)acrylates (B) and one polyalkylene glycol (C) having a (meth)acryloyl group form a urethane bond with respect to the polyisocyanate compound (A).
The ultraviolet-curable oligomer may additionally have other structures as long as it has a structural unit derived from the polyisocyanate compound (A), a structural unit derived from the compound (B′), and a structural unit derived from the compound (C′).

1-1-5: Weight-average molecular weight [0043] From the viewpoint of the performance of the coating film to be formed and ease of handling, the ultraviolet-curable oligomer of the present invention has a weight-average molecular weight, as calculated on a polystyrene basis by gel permeation chromatography (GPC), of usually 1,000 or more, preferably 2,000 or more, and usually 100,000 or less, preferably 50,000 or less.

1-1-6: Average particle size In the ink of the present invention, the ultraviolet-curable oligomer is present in the aqueous medium as particles having an average particle size of preferably 10 nm or more and 200 nm or less, and more preferably 20 nm or more and 150 nm or less. When the average particle size of the ultraviolet-curable oligomer of the present invention is within the above range, the dispersion stability is good.

The average particle size of the ultraviolet-curable oligomer is, for example, the volume average particle size (D ₅₀ ) measured by a particle size measuring device using a dynamic light scattering method.
In the examples described later, the average particle size of ultraviolet-curable oligomer particles in an ultraviolet-curable oligomer aqueous dispersion is measured, and the average particle size of the oligomer particles in this aqueous dispersion is almost the same as the average particle size of the oligomer particles in the ink.
In the ink of the present invention, as long as the ultraviolet-curable oligomer exists as particles, it is included in the above-mentioned state of "existing as particles" even if there is aggregation or even if other substances are contained within the particles.
The average particle size of the ultraviolet-curable oligomer means the particle size (primary particle size) of the ultraviolet-curable oligomer particles.

The components contained in the ink of the present invention other than the UV-curable oligomer are described below.

1-2: Colorant The ink of the present invention contains a colorant.
As the colorant used in the ink of the present invention, various dyes or pigments known as colorants used in inkjet inks can be used. However, from the viewpoints of exposure to ultraviolet light and the long-term storage durability of printed images, it is preferable to use a pigment.

1-2-1: Dyes Dyes that can be used in the present invention are not particularly limited, and examples thereof include water-soluble dyes such as acid dyes, direct dyes, and reactive dyes, disperse dyes, etc. Among these, anionic dyes are preferred.

Examples of water-soluble dyes include azo dyes, methine dyes, azomethine dyes, xanthene dyes, quinone dyes, phthalocyanine dyes, triphenylmethane dyes, and diphenylmethane dyes. Specific examples of such dyes are listed below, but the dyes are not limited to these examples.

<C.I. Acid Yellow>
1, 3, 11, 17, 18, 19, 23, 25, 36, 38, 40, 42, 44, 49, 59, 61, 65, 67, 72, 73, 79, 99, 104, 110, 114, 116, 118, 121, 127, 129, 135, 137, 141, 143, 151, 155, 158, 159, 169, 176, 184, 193, 200, 204, 207, 215, 219, 220, 230, 232, 235, 241, 242, 246
<C.I. Acid Orange>
3, 7, 8, 10, 19, 24, 51, 56, 67, 74, 80, 86, 87, 88, 89, 94, 95, 107, 108, 116, 122, 127, 140, 142, 144, 149, 152, 156, 162, 166, 168
<C.I. Acid Red>
88, 97, 106, 111, 114, 118, 119, 127, 131, 138, 143, 145, 151, 183, 195, 198, 211, 215, 217, 225, 226, 249, 251, 254, 256, 257, 260, 261, 265, 266, 274, 276, 277, 289, 296, 299, 315, 318, 336, 337, 357, 359, 361, 362, 364, 366, 399, 407, 415
<C.I. Acid Violet>
17, 19, 21, 42, 43, 47, 48, 49, 54, 66, 78, 90, 97, 102, 109, 126

<C.I. Acid Blue>
1, 7, 9, 15, 23, 25, 40, 62, 72, 74, 80, 83, 90, 92, 103, 104, 112, 113, 114, 120, 127, 128, 129, 138, 140, 142, 156, 158, 171, 182, 185, 193, 199, 201, 203, 204, 205, 207, 209, 220, 221, 224, 225, 229, 230, 239, 249, 258, 260, 264, 278, 279, 280, 284, 290, 296, 298, 300, 317, 324, 333, 335, 338, 342, 350
<C.I. Acid Green>
9, 12, 16, 19, 20, 25, 27, 28, 40, 43, 56, 73, 81, 84, 104, 108, 109
<C.I. Acid Brown>
2, 4, 13, 14, 19, 28, 44, 123, 224, 226, 227, 248, 282, 283, 289, 294, 297, 298, 301, 355, 357, 413
<C.I. Acid Black>
1, 2, 3, 24, 26, 31, 50, 52, 58, 60, 63, 107, 109, 112, 119, 132, 140, 155, 172, 187, 188, 194, 207, 222

<C.I. Direct Yellow>
8, 9, 10, 11, 12, 22, 27, 28, 39, 44, 50, 58, 79, 86, 87, 98, 105, 106, 130, 132, 137, 142, 147, 153
<C.I. Direct Orange>
6, 26, 27, 34, 39, 40, 46, 102, 105, 107, 118
<C.I. Direct Red>
2, 4, 9, 23, 24, 31, 54, 62, 69, 79, 80, 81, 83, 84, 89, 95, 212, 224, 225, 226, 227, 239, 242, 243, 254
<C.I. Direct Violet>
9, 35, 51, 66, 94, 95
<C.I. Direct Blue>
1, 15, 71, 76, 77, 78, 80, 86, 87, 90, 98, 106, 108, 160, 168, 189, 192, 193, 199, 200, 201, 202, 203, 218, 225, 229, 237, 244, 248, 251, 270, 273, 274, 290, 291
<C.I. Direct Green>
26, 28, 59, 80, 85
<C.I. Direct Brown>
44, 106, 115, 195, 209, 210, 222, 223
<C.I. Direct Black>
17, 19, 22, 32, 51, 62, 108, 112, 113, 117, 118, 132, 146, 154, 159, 169

<C.I. Basic Yellow>
1, 2, 11, 13, 15, 19, 21, 28, 29, 32, 36, 40, 41, 45, 51, 63, 67, 70, 73, 91
<C.I. Basic Orange>
2, 21, 22
<C.I. Basic Red>
1, 2, 12, 13, 14, 15, 18, 23, 24, 27, 29, 35, 36, 39, 46, 51, 52, 69, 70, 73, 82, 109
<C.I. Basic Violet>
1, 3, 7, 10, 11, 15, 16, 21, 27, 39
<C.I. Basic Blue>
1, 3, 7, 9, 21, 22, 26, 41, 45, 47, 52, 54, 65, 69, 75, 77, 92, 100, 105, 117, 124, 129, 147, 151
<C.I. Basic Green>
1, 4
<C.I. Basic Brown>
1

<C.I. Reactive Yellow>
2, 3, 7, 15, 17, 18, 22, 23, 24, 25, 27, 37, 39, 42, 57, 69, 76, 81, 84, 85, 86, 87, 92, 95, 102, 105, 111, 125, 135, 136, 137, 142, 143, 145, 151, 160, 161, 165, 167, 168, 175, 176
<C.I. Reactive Orange>
1, 4, 5, 7, 11, 12, 13, 15, 16, 20, 30, 35, 56, 64, 67, 69, 70, 72, 74, 82, 84, 86, 87, 91, 92, 93, 95, 107
<C.I. Reactive Red>
2, 3, 5, 8, 11, 21, 22, 23, 24, 28, 29, 31, 33, 35, 43, 45, 49, 55, 56, 58, 65, 66, 78, 83, 84, 106, 111, 112, 113, 114, 116, 120, 123, 124, 128, 130, 136, 141, 147, 158, 159, 171, 174, 180, 183, 184, 187, 190, 193, 194, 195, 198, 218, 220, 222, 223, 228, 235
<C.I. Reactive Violet>
1, 2, 4, 5, 6, 22, 23, 33, 36, 38
C.I. Reactive Blue
2, 3, 4, 5, 7, 13, 14, 15, 19, 21, 25, 27, 28, 29, 38, 39, 41, 49, 50, 52, 63, 69, 71, 72, 77, 79, 89, 104, 109, 112, 113, 114, 116, 119, 120, 122, 137, 140, 143, 147, 160, 161, 162, 163, 168, 171, 176, 182, 184, 191, 194, 195, 198, 203, 204, 207, 209, 211, 214, 220, 221, 222, 231, 235, 236
<C.I. Reactive Green>
8, 12, 15, 19, 21
C.I. Reactive Brown
2, 7, 9, 10, 11, 17, 18, 19, 21, 23, 31, 37, 43, 46
C.I. Reactive Black
5, 8, 13, 14, 31, 34, 39
<C.I. Hood Black>
1, 2

1-2-2: Pigment As the pigment, conventionally known organic and inorganic pigments can be used. Examples include azo pigments such as azo lakes, insoluble azo pigments, condensed azo pigments, and chelate azo pigments; polycyclic pigments such as phthalocyanine pigments, perylene and perylene pigments, anthraquinone pigments, quinacridone pigments, dioxandine pigments, thioindigo pigments, isoindolinone pigments, and quinophthaloni pigments; dye lakes such as basic dye lakes and acid dye lakes; organic pigments such as nitro pigments, nitroso pigments, aniline black, and daylight fluorescent pigments; and inorganic pigments such as carbon black, titanium oxide, and iron oxide pigments, with anionic pigments being preferred.

Specific examples of organic pigments are listed below.

<Magenta or red pigments>
C.I. Pigment Red 2, C.I. Pigment Red 3, C.I. Pigment Red 5, C.I. Pigment Red 6, C.I. Pigment Red 7, C.I. Pigment Red 15, C.I. Pigment Red 16, C.I. Pigment Red 48:1, C.I. Pigment Red 53:1, C.I. Pigment Red 57:1, C.I. Pigment Red 122, C.I. Pigment Red 123, C.I. Pigment Red 139, C.I. Pigment Red 144, C.I. Pigment Red 149, C.I. Pigment Red 150, C.I. Pigment Red 166, C.I. Pigment Red 177, C.I. C.I. Pigment Red 178, C.I. Pigment Red 222, etc.

<Orange or yellow pigment>
C.I. Pigment Orange 31, C.I. Pigment Orange 43, C.I. Pigment Yellow 12, C.I. Pigment Yellow 13, C.I. Pigment Yellow 14, C.I. Pigment Yellow 15, C.I. Pigment Yellow 17, C.I. Pigment Yellow 74, C.I. Pigment Yellow 93, C.I. Pigment Yellow 94, C.I. Pigment Yellow 128, C.I. Pigment Yellow 138, C.I. Pigment Yellow 155, etc.

<Pigments for green or cyan>
C.I. Pigment Blue 15, C.I. Pigment Blue 15:2, C.I. Pigment Blue 15:3, C.I. Pigment Blue 16, C.I. Pigment Blue 60, C.I. Pigment Green 7, etc.

These dyes and pigments may be used alone or in combination of two or more.

1-3: Aqueous Medium The ink of the present invention is an aqueous ink, and the term "aqueous ink" means an ink containing an aqueous medium. The aqueous medium is water and/or a water-soluble organic solvent.
The aqueous medium used in the present invention is preferably water or a mixture of water and a water-soluble organic solvent.

Water-soluble organic solvents include those that function as moisturizing solvents to increase the ink's moisture retention and wettability, and those that are used as aqueous media to adjust the ink's viscosity and improve its handling and ejection properties. However, the two are not clearly distinguished, and water-soluble organic solvents used as moisturizing solvents also function as aqueous media.

In the present invention, a water-soluble organic solvent refers to a compound that is soluble in water. There are no restrictions on the solubility in water, but compounds that can dissolve in water at any ratio are preferred. Furthermore, even compounds that are difficult to exhibit solvent properties on their own (for example, compounds that are solid or highly viscous at room temperature) are included in the category of water-soluble organic solvents as long as they can be used as a solvent when mixed uniformly with water.

Examples of water-soluble organic solvents include polyhydric alcohols, ethers such as polyhydric alcohol alkyl ethers and polyhydric alcohol aryl ethers, nitrogen-containing heterocyclic compounds, amides, amines, and sulfur-containing compounds.

Specific examples of water-soluble organic solvents include ethylene glycol, diethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 3-methyl-1,3-butanediol, triethylene glycol, polyethylene glycol, polypropylene glycol, 1,2-pentanediol, 1,3-pentanediol, 1,4-pentanediol, 2,4-pentanediol, and 1,5-pentanediol. polyhydric alcohols such as 1,2-hexanediol, 1,6-hexanediol, 1,3-hexanediol, 2,5-hexanediol, 1,5-hexanediol, glycerin, 1,2,6-hexanetriol, 2-ethyl-1,3-hexanediol, ethyl-1,2,4-butanetriol, 1,2,3-butanetriol, 2,2,4-trimethyl-1,3-pentanediol, and petriol; ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, and diethylene glycol Examples of suitable alkyl ethers include polyhydric alcohols such as ethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, tetraethylene glycol monomethyl ether, propylene glycol monomethyl ether, and propylene glycol monoethyl ether; polyhydric alcohol aryl ethers such as ethylene glycol monophenyl ether and ethylene glycol monobenzyl ether; nitrogen-containing heterocyclic compounds such as 2-pyrrolidone, N-methyl-2-pyrrolidone, N-hydroxyethyl-2-pyrrolidone, 1,3-dimethyl-2-imidazolidinone, ε-caprolactam, and γ-butyrolactone; amides such as formamide, N-methylformamide, N,N-dimethylformamide, 3-methoxy-N,N-dimethylpropionamide, and 3-butoxy-N,N-dimethylpropionamide; amines such as monoethanolamine, diethanolamine, and triethylamine; sulfur-containing compounds such as dimethyl sulfoxide, sulfolane, and thiodiethanol; propylene carbonate, and ethylene carbonate.

It is preferable to use a water-soluble organic solvent with a boiling point of 250°C or less, as this not only functions as a moisturizing solvent but also provides good drying properties.

As the water-soluble organic solvent, polyol compounds having 8 or more carbon atoms and glycol ether compounds are also preferably used.
Specific examples of polyol compounds having 8 or more carbon atoms include 2-ethyl-1,3-hexanediol and 2,2,4-trimethyl-1,3-pentanediol.
Specific examples of glycol ether compounds include polyhydric alcohol alkyl ethers such as ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, diethylene glycol monoisobutyl ether, tetraethylene glycol monomethyl ether, and propylene glycol monoethyl ether; and polyhydric alcohol aryl ethers such as ethylene glycol monophenyl ether and ethylene glycol monobenzyl ether.

These water-soluble organic solvents may be used alone or in combination of two or more.

1-4: Polymerization Initiator The ink of the present invention preferably contains a polymerization initiator.

The polymerization initiator is a photoradical polymerization initiator that generates active radicals by receiving the energy of light (ultraviolet rays) when irradiated with ultraviolet rays, and initiates the photopolymerization of the UV-curable oligomer. This causes the ink present on the surface of the recording medium to harden, forming an image.

The polymerization initiator may be contained in the ink in a state where it is not encapsulated in the UV-curable oligomer, or in a state where it is encapsulated in UV-curable oligomer particles. Furthermore, it may be encapsulated in both of these states.

The polymerization initiator may be a fat-soluble polymerization initiator (hereinafter may be referred to as a "fat-soluble initiator") or a water-soluble polymerization initiator (hereinafter may be referred to as a "water-soluble initiator").
The term "fat-soluble initiator" refers to a polymerization initiator that is compatible with the ultraviolet-curable oligomer or that dissolves in an organic solvent. The term "water-soluble initiator" refers to an initiator that dissolves in water at a concentration of 1% by mass or more. The same applies to the "fat-soluble sensitizer" and "water-soluble sensitizer" described below.

Polymerization initiators used in the present invention include, but are not limited to, aromatic ketones, acylphosphine oxide compounds, aromatic onium salt compounds, organic peroxides, thio compounds (thioxanthone compounds, thiophenyl group-containing compounds), α-aminoalkylphenone compounds, hexaarylbiimidazole compounds, ketoxime ester compounds, borate compounds, azinium compounds, metallocene compounds, active ester compounds, compounds having a carbon-halogen bond, and alkylamine compounds.

Among these, it is preferable that the polymerization initiator contains at least one of an acylphosphine oxide compound and a thioxanthone compound. Use of such a polymerization initiator tends to improve the curability of the ink.

Examples of fat-soluble polymerization initiators include, but are not limited to, acetophenone, 2,2-diethoxyacetophenone, p-dimethylaminoacetophenone, benzophenone, 2-chlorobenzophenone, p,p'-dichlorobenzophenone, p,p'-bisdiethylaminobenzophenone, Michler's ketone, benzil, benzoin, benzoin methyl ether, benzoin ethyl ether, benzoin isopropyl ether, benzoin-n-propyl ether, benzoin isobutyl ether, benzoin-n-butyl ether, benzil methyl ketal, 2,2-dimethoxy-1,2-diphenylethan-1-one, 1-hydroxycyclohexyl phenyl ketone, 2-hydroxy-1-{4-[4-(2-hydroxy-2-methylpropionyl)benzyl]phenyl}2-methylpropan-1-one, and 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)butanone-1,2-di Methylamino-2-(4-methylbenzyl)-1-(4-morpholin-4-yl-phenyl)butan-1-one, 2,4,6-trimethylbenzoyldiphenylphosphine oxide, bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide, bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide, 2-methyl-1-[4-(methylthio)phenyl]2-morpholinopropan-1-one, thioxanthone, 2-chlorothioxanthone, 2-methylthioxanthone, 2-isopropylthioxanthone, 4-isopropylthioxanthone, 2-hydroxy-2-methyl-1-phenyl-1-one, 1-(4-isopropylphenyl)-2-hydroxy-2-methylpropan-1-one, methylbenzofilformate, azobisisobutyronitrile, benzoyl peroxide, and di-tert-butyl peroxide.

Water-soluble polymerization initiators include, but are not limited to, 1-[4-(2-hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1-propan-1-one, sodium phenyl(2,4,6-trimethylbenzoyl)phosphinate, and 2-(3-dimethylamino-2-hydroxypropoxy)-3,4-dimethyl-9H-thioxanthone-9-one methchloride.

Commercially available polymerization initiators include, for example, GENOPOL TX-2 manufactured by RAHN Corporation, and Irgacure 369, Irgacure 500, and Irgacure 2959 manufactured by Ciba Specialty Chemicals.

The polymerization initiator may be used alone or in combination of two or more. For example, a fat-soluble initiator and a water-soluble initiator may be used in combination, with the fat-soluble initiator being encapsulated in particles of the ultraviolet-curable oligomer and the water-soluble initiator being dissolved in an aqueous medium.
As the polymerization initiator, a thermal radical polymerization initiator may be used in combination with the above-mentioned photoradical polymerization initiator.

1-5: Surfactant The ink of the present invention preferably contains a surfactant to improve the flatness of the coating film formed and the wettability with the substrate.

Surfactants that can be used include silicone surfactants, fluorine-based surfactants, amphoteric surfactants, nonionic surfactants, and anionic surfactants.

There are no particular restrictions on silicone surfactants and they can be selected appropriately depending on the purpose. Among these, those that do not decompose even at high pH are preferred, such as side-chain-modified polydimethylsiloxane, both-end-modified polydimethylsiloxane, one-end-modified polydimethylsiloxane, and both-end-modified polydimethylsiloxane. Those with polyoxyethylene groups or polyoxyethylene-polyoxypropylene groups as modifying groups are particularly preferred, as they exhibit good properties as aqueous surfactants. Polyether-modified silicone surfactants can also be used as silicone surfactants, such as compounds in which a polyalkylene oxide structure is introduced into the Si side chain of dimethylsiloxane.

The fluorine-based surfactant is preferably a compound having 2 to 16 fluorine-substituted carbon atoms, more preferably a compound having 4 to 16 fluorine-substituted carbon atoms.
As fluorine-based surfactants, for example, perfluoroalkyl sulfonic acid compounds, perfluoroalkyl carboxylic acid compounds, perfluoroalkyl phosphate ester compounds, perfluoroalkyl alkylene oxide adducts, and polyoxyalkylene ether polymer compounds having perfluoroalkyl ether groups on the side chain are preferred because of their low foaming properties. Examples of perfluoroalkyl sulfonic acid compounds include perfluoroalkyl sulfonic acid and perfluoroalkyl sulfonate salts. Examples of perfluoroalkyl carboxylic acid compounds include perfluoroalkyl carboxylic acids and perfluoroalkyl carboxylate salts. Examples of perfluoroalkyl phosphate ester compounds include perfluoroalkyl phosphate esters and perfluoroalkyl phosphate ester salts. Examples of perfluoroalkyl alkylene oxide adducts include perfluoroalkyl ethylene oxide adducts. Examples of polyoxyalkylene ether polymer compounds having perfluoroalkyl ether groups on the side chain include sulfate ester salts of polyoxyalkylene ether polymers having perfluoroalkyl ether groups on the side chains, and salts of polyoxyalkylene ether polymers having perfluoroalkyl ether groups on the side chains. Counter ions of the salts in these fluorine-based surfactants include Li, _Na , K, _NH4 , _NH3CH2CH2OH , _NH2 ( _{CH2CH2OH ) 2} , _NH ( _CH2CH2OH ) ₃ , and _the like.

Among these, polyoxyalkylene ether polymer compounds having perfluoroalkyl ether groups in the side chains are more preferred because they have particularly low foaming properties, and fluorine-based surfactants represented by the following general formulas (3A) and (3B) are particularly preferred.

_{CF3CF2 ( CF2CF2 ) s} - _{CH2CH2O ( CH2CH2O ) tH} ...(3A)
In the compound represented by formula (3A), s is preferably an integer of 0 or more and 10 or less, and t is preferably an integer of 0 or more and 40 or less, in order to impart water solubility.

C _r F _2r+1 -CH ₂ CH(OH)CH ₂ -O-(CH ₂ CH ₂ O) _c -Z...(3B)
In the compound represented by general formula (3B), Z is H, or C _d F _2d+1 where d is an integer of 1 or more and 6 or less, or CH ₂ CH(OH)CH ₂ -C _e F _2e+1 where e is an integer of 4 or more and 6 or less, or C _f H _2f+1 where f is an integer of 1 or more and 19 or less. In addition, r is an integer of 1 or more and 6 or less, and c is an integer of 4 or more and 14 or less.

Commercially available fluorine-based surfactants can be used. Examples of commercially available products include Surflon S-111, S-112, S-113, S-121, S-131, S-132, S-141, and S-145 (all manufactured by Asahi Glass Co., Ltd.); Fullard FC-93, FC-95, FC-98, FC-129, FC-135, FC-170C, FC-430, and FC-431 (all manufactured by Sumitomo 3M Limited); Megafac F-470, F-1405, and F-474 (all manufactured by DIC Corporation); Zonyl TBS, FSP, FSA, FSN-100, FSN, FSO-100, and F SO, FS-300, UR (all manufactured by DuPont); FT-110, FT-250, FT-251, FT-400S, FT-150, FT-400SW (all manufactured by Neos Co., Ltd.), Polyfox PF-136A, PF-156A, PF-151N, PF-154, PF-159 (manufactured by Omnova), Noigen FN-1287 (manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.), Unidyne DSN-403N (manufactured by Daikin Industries, Ltd.), LE-604, LE-605, LE-606, LE-607 (manufactured by Kyoeisha Chemical Co., Ltd.), and others.

Examples of amphoteric surfactants include lauryl aminopropionate, lauryl dimethyl betaine, stearyl dimethyl betaine, and lauryl dihydroxyethyl betaine.

Examples of nonionic surfactants include polyoxyethylene alkylphenyl ethers, polyoxyethylene alkyl esters, polyoxyethylene alkylamines, polyoxyethylene alkylamides, polyoxyethylene propylene block polymers, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, acetylene alcohol derivatives, and acetylene glycol derivatives.

Examples of anionic surfactants include polyoxyethylene alkyl ether acetates, dodecylbenzenesulfonates, laurates, and salts of polyoxyethylene alkyl ether sulfates.

These may be used alone or in combination of two or more types.

As mentioned above, there are no particular restrictions on the silicone surfactants and they can be selected appropriately depending on the purpose, but polyether-modified silicone surfactants having polyoxyethylene groups or polyoxyethylene polyoxypropylene groups as modifying groups are particularly preferred as they exhibit good properties as aqueous surfactants.

Such surfactants may be synthesized as appropriate or commercially available products may be used. Commercially available products are available from, for example, BYK Corporation, Shin-Etsu Chemical Co., Ltd., Dow Corning Toray Silicone Co., Ltd., Nippon Emulsion Co., Ltd., and Kyoeisha Chemical Co., Ltd.

There are no particular restrictions on the polyether-modified silicone surfactant, and it can be selected appropriately depending on the purpose. For example, it can be one in which a polyalkylene oxide structure is introduced into the Si moiety side chain of dimethylpolysiloxane, as represented by the following general formula (2):

(In formula (2), p, q, a, and b represent integers. R and R' represent hydrocarbon groups.)

Commercially available polyether-modified silicone surfactants can be used. Commercially available products include, for example, KF-618, KF-642, and KF-643 (manufactured by Shin-Etsu Chemical Co., Ltd.), SAG001, SAG002, SAG003, SAG005, SAG503, and SAG008 (manufactured by Nissin Chemical Industry Co., Ltd.), EMALEX-SS-5602 and SS-1906EX (manufactured by Nippon Emulsion Co., Ltd.), FZ-2105, FZ-2118, FZ-2154, FZ-2161, FZ-2162, FZ-2163, and FZ-2164 (manufactured by Dow Corning Toray Silicone Co., Ltd.), BYK-33 and BYK-387 (manufactured by BYK-Chemie Co., Ltd.), and TSF4440, TSF4452, and TSF4453 (manufactured by Toshiba Silicon Co., Ltd.).

1-6: Sensitizer The ink of the present invention may contain a sensitizer. When a sensitizer is present in the ink together with a polymerization initiator, the sensitizer in the system absorbs actinic energy rays and becomes excited, and upon contact with the polymerization initiator, promotes decomposition of the polymerization initiator, allowing for a more sensitive curing reaction.

The sensitizer, like the polymerization initiator, may be fat-soluble or water-soluble. If the sensitizer is fat-soluble, it can be encapsulated within the particles of the UV-curable oligomer.

Sensitizers that can be used include aliphatic amines, amines having aromatic groups, and cyclic amine compounds such as piperidine; thioxanthone compounds, alkoxyanthracene compounds, urea compounds such as o-tolylthiourea; sulfur compounds such as sodium diethylthiophosphate or soluble salts of aromatic sulfinic acids; nitrile compounds such as N,N'-disubstituted-p-aminobenzonitrile; phosphorus compounds such as tri-n-butylphosphine or sodium diethyldithiophosphate; Michler's ketone, N-nitrosohydroxylamine derivatives, oxazolidine compounds, tetrahydro-1,3-oxazine compounds; and nitrogen compounds such as condensates of formaldehyde or acetaldehyde with diamines.

These sensitizers may be used alone or in combination of two or more.

1-7: Other Oligomers, Resins, and Monomers In addition to the components described above, the ink of the present invention may contain, as necessary, oligomer components other than the UV-curable oligomer, any resin component, or any monomer component (collectively referred to as "other resin components"). The other resin components may be encapsulated in particles of the UV-curable oligomer, may be dissolved in the aqueous medium, or may be dispersed alone in the ink or may be in a composite state with other components.

1-8: Other Additives In addition to the above components, the ink of the present invention may contain other additives, if necessary.
Examples of other additives include known additives such as anti-fading agents, emulsion stabilizers, penetration enhancers, ultraviolet absorbers, preservatives, antifungal agents, rust inhibitors, pH adjusters, viscosity adjusters, dispersants, dispersion stabilizers, antifoaming agents, solid wetting agents, chelating agents, etc. These various additives may be added directly after preparing the ink, or may be added during the preparation of the ink.
For other additives, reference can be made to the descriptions in paragraphs 0088 to 0096 of JP-A No. 2010-65205 and paragraphs 0083 to 0090 of JP-A No. 2010-70669, as appropriate.

1-9: Content of each component The content of water in the ink of the present invention is not particularly limited and can be appropriately selected depending on the purpose. From the viewpoint of the drying properties and ejection reliability of the ink, however, it is usually 10% by mass or more, preferably 20% by mass or more, and usually 90% by mass or less, preferably 80% by mass or less.

When the ink of the present invention contains a water-soluble organic solvent, its content (the total content of the water-soluble organic solvent that also serves as a moisturizing solvent and the water-soluble organic solvent that serves as an aqueous medium) is not particularly limited and can be selected appropriately depending on the type of water-soluble organic solvent used and the purpose. However, from the viewpoints of drying properties, ejection reliability, wettability with the substrate, etc., it is typically 10% by mass or more and typically 50% by mass or less, preferably 40% by mass or less.

From the viewpoint of drying properties and ejection reliability, the ink of the present invention is adjusted so that the total solids concentration, which is the concentration of components other than the aqueous medium, i.e., water and/or water-soluble organic solvent, is typically 5% by mass or more, preferably 7% by mass or more, more preferably 9% by mass or more, and typically 30% by mass or less, preferably 25% by mass or less, more preferably 20% by mass or less, and even more preferably 15% by mass or less.

When a mixture of water and a water-soluble organic solvent is used as the aqueous medium, the ratio of water to water-soluble organic solvent (the total of the water-soluble organic solvent that also serves as a moisturizing solvent and the water-soluble organic solvent used as the aqueous medium) is usually 1:0.05 to 1:1.5 (by mass), preferably 1:0.1 to 1:1.2 (by mass), and more preferably 1:0.15 to 1:1.1 (by mass), from the perspective of improving drying properties and ejection properties.

The content of the ultraviolet-curable oligomer in the ink of the present invention is usually 3% by mass or more, preferably 5% by mass or more, and more preferably 7% by mass or more, from the viewpoint of the performance of the resulting printed coating film and ultraviolet curability, and on the other hand, is usually 20% by mass or less, preferably 15% by mass or less, and more preferably 12% by mass or less, from the viewpoint of ejection stability.
From the same viewpoint, the content of the ultraviolet-curable oligomer in the total solid content of the ink of the present invention is usually 30% by mass or more, preferably 50% by mass or more, and more preferably 60% by mass or more, and is usually 90% by mass or less, preferably 85% by mass or less, and more preferably 80% by mass or less.

Here, the total solids content in the ink of the present invention can be rephrased as the components constituting the cured film (printed film) formed by the ink of the present invention, and the above-mentioned numerical ranges can be similarly adopted. The content of each component in the total solids content is approximately equal to the content of that component in the cured film (printed film) formed by the ink of the present invention.
Therefore, the content of the ultraviolet-curable oligomer in the total solids content of the ink of the present invention is usually 30% by mass or more, preferably 50% by mass or more, and more preferably 60% by mass or more, and usually 90% by mass or less, preferably 85% by mass or less, and more preferably 80% by mass or less, can be treated as being equivalent to the content of components derived from the ultraviolet-curable oligomer in the cured film (printed film) formed from the ink of the present invention being usually 30% by mass or more, preferably 50% by mass or more, and more preferably 60% by mass or more, and usually 90% by mass or less, preferably 85% by mass or less, and more preferably 80% by mass or less.
The same applies to the contents of the colorants and other components described below.

The content of the colorant in the ink of the present invention is usually 0.1% by mass or more, preferably 1% by mass or more, and usually 8% by mass or less, preferably 6% by mass or less, from the viewpoints of improving image density, good fixability, and ejection stability.
From the same viewpoint, the content of the colorant in the total solid content of the ink of the present invention is usually 1% by mass or more, preferably 5% by mass or more, and usually 40% by mass or less, preferably 30% by mass or less.

[0044] When the ink of the present invention contains a polymerization initiator, the content thereof is usually 0.05% by mass or more, preferably 0.1% by mass or more, more preferably 0.3% by mass or more, and even more preferably 0.4% by mass or more, and usually 8% by mass or less, preferably 5% by mass or less, more preferably 3% by mass or less, even more preferably 2% by mass or less, and particularly preferably 1% by mass or less. By having the content of the polymerization initiator within this range, the curing rate can be sufficiently improved, and residual polymerization initiator and coloration resulting from the polymerization initiator can be avoided.
From the same viewpoint, the content of the polymerization initiator in the total solid content of the ink of the present invention is usually 0.5% by mass or more, preferably 1% by mass or more, more preferably 2% by mass or more, and even more preferably 3% by mass or more, and is usually 20% by mass or less, more preferably 15% by mass or less, even more preferably 10% by mass or less, and particularly preferably 8% by mass or less.

When the ink of the present invention contains a surfactant, the content thereof is not particularly limited and can be selected appropriately depending on the purpose. However, from the viewpoint of achieving excellent wettability and ejection stability and improving image quality, the content of the surfactant in the ink is usually 0.001% by mass or more, preferably 0.01% by mass or more, and more preferably 0.03% by mass or more, and is usually 5% by mass or less, preferably 3% by mass or less, and more preferably 1% by mass or less.
From the same viewpoint, the content of the surfactant in the total solid content of the ink of the present invention is usually 0.01% by mass or more, preferably 0.1% by mass or more, more preferably 0.2% by mass or more, and is usually 10% by mass or less, preferably 5% by mass or less, more preferably 3% by mass or less.

When the ink of the present invention contains a sensitizer, the content thereof is usually 0.01% by mass or more, preferably 0.03% by mass or more, more preferably 0.05% by mass or more, and usually 4% by mass or less, preferably 3% by mass or less, more preferably 1% by mass or less, and even more preferably 0.7% by mass or less. When the content of the sensitizer is within the above range, the effect of the sensitizer can be sufficiently obtained.
From the same viewpoint, the content of the sensitizer in the total solid content of the ink of the present invention is usually 0.05% by mass or more, preferably 0.1% by mass or more, more preferably 0.3% by mass or more, and even more preferably 0.5% by mass or more, and is usually 8% by mass or less, preferably 6% by mass or less, and more preferably 5% by mass or less.

2: UV-curable aqueous composition for inkjet printing The UV-curable aqueous composition for inkjet printing of the present invention (hereinafter may be referred to as "the aqueous composition of the present invention") is an aqueous composition containing a UV-curable oligomer and one or more of a polymerization initiator, a sensitizer, and a surfactant, wherein the UV-curable oligomer is the UV-curable oligomer described above.

The UV-curable aqueous composition for inkjet printing of the present invention is an aqueous composition and, like the ink of the present invention, contains an aqueous medium. In other words, the aqueous composition of the present invention corresponds to an embodiment of the ink of the present invention that does not contain a colorant, and the aqueous medium, polymerization initiator, sensitizer, surfactant, and other additives contained in the aqueous composition are the same as those described above in the section regarding the ink of the present invention, and the contents thereof are also the same as the contents of each component of the ink of the present invention, excluding the colorant.

The use form of the aqueous composition of the present invention is not particularly limited, but a preferred use form is to prepare the ink of the present invention by adding the above-mentioned colorant to the aqueous composition of the present invention. The colorant to be added may be one color or two or more colors. By optionally adding two or more colorants, the color of the ink can be adjusted to a desired color.
In addition, the water-based composition of the present invention can be used as a clear ink.

3: Ink Manufacturing Method There are no particular limitations on the method for manufacturing the ink of the present invention. Examples include a method in which a dispersion (hereinafter sometimes referred to as an "oligomer dispersion") in which particles of ultraviolet-curable oligomer (hereinafter sometimes referred to as "oligomer particles") are dispersed in an aqueous medium, and a dispersion (hereinafter sometimes referred to as a "pigment dispersion") in which a colorant such as a pigment is dispersed in an aqueous medium are separately prepared, and then the oligomer dispersion and the pigment dispersion are mixed with a polymerization initiator (water-soluble initiator), other additives, and an organic solvent. Another example is a method in which a colorant is added to the aqueous composition of the present invention. Either method can produce inks with similar performance.

By adding and mixing a fat-soluble initiator or fat-soluble sensitizer during preparation of the oligomer dispersion, these can be encapsulated in the oligomer particles. Alternatively, the fat-soluble initiator or fat-soluble sensitizer can be dissolved separately in an organic solvent at a concentration of approximately 0.1% by mass to 10% by mass and then mixed with other components.

3-1: Preparation of Oligomer Dispersion The oligomer dispersion can be prepared by mixing an ultraviolet-curable oligomer with an aqueous medium (preferably water).
By adjusting the temperature and stirring speed at this time, the average particle size of the resulting oligomer particles can be adjusted.
The oligomer particle concentration of the oligomer dispersion thus prepared is preferably about 10% by mass or more and 30% by mass or less from the viewpoint of ease of handling.

By further adding and mixing a fat-soluble initiator or fat-soluble sensitizer during preparation of the oligomer dispersion, a dispersion of oligomer particles encapsulating these can be obtained. In this case, to prevent the formation of particles consisting of only the initiator or sensitizer without the oligomer, it is preferable to add the fat-soluble initiator and/or fat-soluble sensitizer to the UV-curable oligomer and then mix with an aqueous medium such as water. When producing oligomer particles encapsulating a fat-soluble initiator and/or fat-soluble sensitizer, from the perspective of production stability, the content of the fat-soluble initiator and/or fat-soluble sensitizer in the oligomer particles is preferably approximately 0.1% by mass or more and 8% by mass or less relative to the UV-curable oligomer.

3-2: Preparation of Pigment Dispersion Liquid The pigment dispersion liquid can be prepared by adding a colorant such as a pigment to an aqueous medium such as water and mixing them.
The concentration of the colorant such as the pigment in the pigment dispersion is usually 5% by mass or more, preferably 10% by mass or more, and usually 40% by mass or less, preferably 35% by mass or less, from the viewpoints of handling and storage stability.

Commercially available pigment dispersions may be used as is.

4: Method for Producing Water-Based Composition The method for producing the water-based composition of the present invention is not particularly limited, but examples include a method in which an oligomer dispersion is prepared in the same manner as in the above-described method for producing the ink of the present invention, and the prepared oligomer dispersion is mixed with one or more of a polymerization initiator (water-soluble initiator), a sensitizer, and a surfactant, as well as other additives and an organic solvent that are used as needed.

As mentioned above, by adding and mixing a fat-soluble initiator or fat-soluble sensitizer during preparation of the oligomer dispersion, these can be encapsulated in the oligomer particles. The fat-soluble initiator or fat-soluble sensitizer can also be dissolved separately in an organic solvent at a concentration of approximately 0.1% by mass to 10% by mass and then mixed with other components as a solution.

The method for preparing the oligomer dispersion and the dispersion of oligomer particles encapsulating the fat-soluble initiator and fat-soluble sensitizer is the same as in the method for producing the ink of the present invention described above.

5: Ink Viscosity The ink of the present invention has excellent pigment dispersion stability, so the viscosity of the ink can be kept low. As a result, the ink ejection properties are good even during high-speed printing. The viscosity of the ink of the present invention at 25°C is preferably 25 mPa·sec or less, more preferably 20 mPa·sec or less, and even more preferably 10 mPa·sec or less. There are no particular restrictions on the lower limit of the ink viscosity, but it is preferably 1 mPa·sec or more, and more preferably 2 mPa·sec or more.

6: Viscosity of the aqueous composition The viscosity of the aqueous composition of the present invention at 25° C. is preferably 50 mPa sec or less, more preferably 30 mPa sec or less, and even more preferably 20 mPa sec or less. The lower limit of the viscosity of the aqueous composition is not particularly limited, but is preferably 0.5 mPa sec or more, and more preferably 1 mPa sec or more.

The ink of the present invention can be stored in an ink cartridge or ink bottle, which eliminates the need to directly touch the ink during ink transport, ink replacement, and other operations, preventing staining of fingers and clothing, and preventing the inclusion of foreign matter such as dust into the ink.

The shape, size, material, etc. of the ink container itself are not particularly limited as long as they are suitable for the inkjet printer or other device to which they are applied. It is desirable that the material of the ink container be a light-blocking material that does not transmit light, or that the container be covered with a light-blocking sheet or the like.

8: Inkjet Recording Method An inkjet recording method using the ink of the present invention preferably comprises a step of ejecting the ink of the present invention from the ejection nozzle of an inkjet printer to deposit it on a recording medium, a heating step of heating the recording medium to which the ink has been deposited, and an irradiation step of irradiating the ink deposited on the recording medium with active energy rays.
The step of applying the ink of the present invention to a recording medium is not necessarily limited to a method using an inkjet printer, as long as the ink is applied to the recording medium in a mist (mist or spray) form.

There are no particular restrictions on the recording medium to which the ink of the present invention can be applied. Since the ink of the present invention is an ultraviolet-curable water-based ink that is highly versatile for various substrates, it is possible to form high-quality printed images with good adhesion on a variety of substrates, including plastic materials such as polyesters such as polyethylene terephthalate (PET), polyvinyl chloride (PVC), polyethylene (PE), and polypropylene (PP), paper, textiles (cloth and fabrics), leather, glass, ceramics, wood, metal, and composites of these.

8-2: Heating Step In the heating step, the recording medium to which the ink of the present invention is attached is preferably heated to 40°C or higher. The heating temperature is more preferably 45°C or higher, and even more preferably 50°C or higher. By carrying out the heating, volatile components such as water in the ink can be dried, which tends to further enhance curing properties. There is no particular upper limit to the heating temperature, but since the presence of a heating means generally tends to cause ink on the nozzle surface to dry out, resulting in ejection defects, a temperature of 120°C or lower is preferred, and 100°C or lower is more preferred. Here, the heating temperature refers to the surface temperature of the recording surface of the recording medium.

The heating means is not particularly limited, but examples include ceramic heaters, halogen heaters, and quartz tube heaters.

The timing of heating may be before, during, or after the ink of the present invention is applied to the recording medium, but it is more preferable to continue heating throughout the entire process, before, during, and after application.

8-3: Irradiation Step In the irradiation step, the polymerization reaction of the UV-curable oligomer is initiated by irradiation with active energy rays. Furthermore, the polymerization initiator contained in the ink decomposes upon irradiation with active energy rays to generate initiating species such as radicals, acids, and bases, and the polymerization reaction of the UV-curable oligomer is accelerated by the function of these initiating species. If a sensitizer is present in the ink together with the polymerization initiator, the sensitizer in the system absorbs the active energy rays and enters an excited state. This accelerates the decomposition of the polymerization initiator upon contact with the polymerization initiator, enabling a more sensitive curing reaction.

Widely known light sources (active energy ray sources) include mercury lamps, metal halide lamps, and gas/solid-state lasers. However, there is currently a strong demand for mercury-free devices from an environmental perspective, and replacement with GaN-based semiconductor ultraviolet light-emitting devices would be extremely beneficial from both an industrial and environmental perspective. Furthermore, ultraviolet light-emitting diodes (UV-LEDs) and ultraviolet laser diodes (UV-LDs) are small, have a long lifespan, are highly efficient, and are low-cost, and are expected to be used as light sources for UV-curable inkjet printers. Of these, UV-LEDs are preferred.

The emission peak wavelength of the active energy ray source to be irradiated is preferably in the range of 350 to 450 nm, and the irradiation energy is preferably 20 J/ ^cm2 or less, for example, 0.5 to 10 J/ ^cm2 .

The emission peak wavelength may be one or more within the above wavelength range.

The irradiation step is not limited to the above-mentioned intentional step, but may also be, for example, outdoor exposure to sunlight.
If the reactivity (curability) of the ultraviolet-curable oligomer is high, the heating step alone is sufficient, and the irradiation step is not required. In other words, the ink of the present invention is not limited to being used in a printing method that includes an irradiation step, as long as it has ultraviolet curability.

The ink of the present invention is water-based and therefore environmentally friendly and safe, and satisfies in a balanced manner the required properties of an ink for inkjet printers, such as high image quality, high coating performance, high-speed printing, and substrate versatility, and is particularly excellent in alcohol resistance, abrasion resistance, and elution resistance. Moreover, the ink of the present invention also has excellent pigment dispersion stability and storage stability at high temperatures.
Therefore, the ink of the present invention has the advantage of being able to print images with high image quality and excellent coating film performance on various recording media with high productivity, and can be used in a variety of applications, such as posters, road signs, signboards, billboards, various outdoor and indoor display boards, building materials (surface materials for exteriors, interiors, walls, floors, ceilings, windows, etc.), exteriors of vehicles (automobiles, trains, aircraft, etc.), surface materials for furniture and office automation equipment, and printed paper. In particular, because of its excellent alcohol resistance, abrasion resistance, and elution resistance, it can be suitably used in food packaging or textile applications.

The present invention will be explained in more detail below with reference to the following examples.

[Preparation of UV-Curable Oligomer A]
An ultraviolet-curable oligomer was produced by reacting 0.4 mol of a trimer of hexamethylene diisocyanate, 0.8 mol of dipentaerythritol pentaacrylate, and 0.4 mol of polyethylene glycol monoacrylate (n=30 to 40 in the general formula (1)).

[Preparation of UV-Curable Oligomer B]
An ultraviolet-curable oligomer was produced by reacting 0.4 mol of hexamethylene diisocyanate trimer, 0.8 mol of dipentaerythritol pentaacrylate, and 0.4 mol of polyethylene glycol monoallyl ether (a structure in which n=30 to 40 in the general formula (1) and Y is changed to an allyl group).

[Preparation of UV-Curable Oligomer Aqueous Dispersion α]
To 20 parts by mass of the UV-curable oligomer A obtained above, 0.2 parts by mass of GENOPOL TX-2 manufactured by RAHN Corporation was added as a fat-soluble initiator, and the mixture was maintained at 60°C. While stirring, ion-exchanged water preheated to 60°C was added dropwise to obtain a UV-curable oligomer aqueous dispersion α (UV-curable oligomer concentration 20% by mass) containing a fat-soluble initiator. The average particle size ( _D50 ) of the UV-curable oligomer particles in this aqueous dispersion was measured using a particle size distribution analyzer MICROTRAC UPA (MODEL: 9340-UPA, manufactured by Nikkiso Co., Ltd.) and found to be 33 nm.

[Preparation of UV-Curable Oligomer Aqueous Dispersion β]
An ultraviolet-curable oligomer aqueous dispersion β (ultraviolet-curable oligomer concentration 20% by mass) was obtained in the same manner as ultraviolet-curable oligomer aqueous dispersion α, except that no fat-soluble initiator was used. The average particle size (D ₅₀ ) of the ultraviolet-curable oligomer particles in this aqueous dispersion was measured using a particle size distribution analyzer MICROTRAC UPA (MODEL: 9340-UPA, manufactured by Nikkiso Co., Ltd.) and was found to be 36 nm.

[Preparation of UV-Curable Oligomer Aqueous Dispersion γ]
An ultraviolet-curable oligomer aqueous dispersion γ (ultraviolet-curable oligomer concentration 20% by mass) containing a fat-soluble initiator was obtained in the same manner as for the ultraviolet-curable oligomer aqueous dispersion α, except that ultraviolet-curable oligomer B was used instead of ultraviolet-curable oligomer A. The average particle size (D ₅₀ ) of the ultraviolet-curable oligomer particles in this aqueous dispersion was measured using a particle size distribution analyzer MICROTRAC UPA (MODEL: 9340-UPA, manufactured by Nikkiso Co., Ltd.) and was found to be 32 nm.

[Preparation of UV-curable oligomer aqueous dispersion δ]
An ultraviolet-curable oligomer aqueous dispersion δ (ultraviolet-curable oligomer concentration: 20% by mass) was obtained in the same manner as for the ultraviolet-curable oligomer aqueous dispersion β, except that ultraviolet-curable oligomer B was used instead of ultraviolet-curable oligomer A. The average particle size (D ₅₀ ) of the ultraviolet-curable oligomer particles in this aqueous dispersion was measured using a particle size distribution analyzer MICROTRAC UPA (MODEL: 9340-UPA, manufactured by Nikkiso Co., Ltd.) and was found to be 29 nm.

[Preparation of fat-soluble initiator solution]
To 100 parts by mass of diethylene glycol monoisobutyl ether as a water-soluble organic solvent, 1 part by mass of GENOPOL TX-2 manufactured by RAHN was added as a fat-soluble initiator, and the mixture was stirred at 40°C to completely dissolve the mixture, thereby obtaining a fat-soluble initiator solution.

[Water-soluble organic solvent]
The following compounds were used as the water-soluble organic solvents.
a: Propylene glycol b: Diethylene glycol monoisobutyl ether

[Surfactant]
The following commercially available surfactants were used:
[1]: Noigen FN1287 manufactured by Daiichi Kogyo Seiyaku
[2]: LE-605 manufactured by Kyoeisha Chemical

[Example 1]
Ion-exchanged water, ultraviolet-curable oligomer aqueous dispersion α, 1,2-butanediol as a moisturizing solvent, water-soluble organic solvent a, a water-soluble initiator, a water-soluble sensitizer, a surfactant, and EMACOL SF CYAN AE2034F manufactured by Sanyo Dye Co., Ltd. as a pigment dispersion were added and mixed to obtain the composition ratios shown in Table 1, to obtain ink composition 1. The viscosity of ink composition 1 at 25°C was measured using a digital viscometer (DV-I+ manufactured by BROOKFIELD Corporation) and was found to be 7 mPa sec.

[Example 2]
An ink composition liquid 2 was obtained in the same manner as in Example 1, except that the ultraviolet-curable oligomer aqueous dispersion liquid α was changed to the ultraviolet-curable oligomer aqueous dispersion liquid β, the water-soluble organic solvent a was changed to the water-soluble organic solvent b, and an oil-soluble initiator solution was added to obtain the composition ratio shown in Table 1.

[Example 3]
An ink composition liquid 3 was obtained in the same manner as in Example 1, except that the ultraviolet-curable oligomer aqueous dispersion liquid α was changed to the ultraviolet-curable oligomer aqueous dispersion liquid β, and the water-soluble organic solvent a was changed to the water-soluble organic solvent b, so that the composition ratio was as shown in Table 1.

[Comparative Example 1]
Ink composition liquid 4 was obtained in the same manner as in Example 1, except that the ultraviolet-curable oligomer aqueous dispersion liquid α was replaced with the ultraviolet-curable oligomer aqueous dispersion liquid γ. The viscosity of ink composition liquid 4 at 25°C was measured using a digital viscometer (DV-I+ manufactured by BROOKFIELD Corporation) and was found to be 8 mPa sec.

[Comparative Example 2]
An ink composition liquid 5 was obtained in the same manner as in Example 2, except that the ultraviolet-curable oligomer aqueous dispersion liquid β was changed to the ultraviolet-curable oligomer aqueous dispersion liquid δ.

[Formation of cured film]
The ink composition liquids of Examples 1 to 3 and Comparative Examples 1 and 2 were applied to a PVC film using a bar coater to a film thickness of 15 μm, and after heating at 80°C for 10 minutes, the coating was irradiated with ultraviolet light from an LED having a peak wavelength of 385 nm at an irradiation energy of 7 J/ ^cm2 to form a cured film.

[Evaluation of alcohol resistance of cured film]
The cured films of Examples 1 to 3 and Comparative Examples 1 and 2 were used to evaluate alcohol resistance.
The evaluation was carried out by rubbing the surface of the cured film with absorbent cotton soaked in absolute ethanol with either no load or a load of 200 g, and visually observing whether or not color transfer to the absorbent cotton occurred. The evaluation was carried out according to the following criteria. The results are shown in Table 2.
○: No color transfer to absorbent cotton in any test.
×: No color transfer to absorbent cotton when no load was applied, but color transfer to absorbent cotton when a load of 200 g was applied.
××: Color transfer to absorbent cotton was observed in all tests.

[Evaluation of Scratch Resistance of Cured Film]
The cured films of Examples 1 and 2 and Comparative Example 1 were used to evaluate scratch resistance.
The evaluation was performed using a friction and wear tester, Tribogear HEIDON-14DR (manufactured by Shinto Scientific Co., Ltd.), with a #1000 grain size wrapping film sheet (manufactured by Trusco Nakayama Corporation) set on a circular terminal with a contact surface of 27 mm diameter. The cured film surface was rubbed with a 400 g load, 10 times over a test distance of 50 mm, with a test time of 10 seconds. The peeling of the cured film was binarized using image analysis software, and the percentage (%) of the peeled area of the cured film was determined. The results are shown in Table 2. Note that abrasion resistance was not evaluated for Example 3 and Comparative Example 2.

[Example 4]
Ion-exchanged water, ultraviolet-curable oligomer aqueous dispersion α, 1,2-butanediol as a moisturizing solvent, a water-soluble initiator, a water-soluble sensitizer, a surfactant, and EMACOL SF CYAN AE2034F manufactured by Sanyo Dye Co., Ltd. as a pigment dispersion were added and mixed to obtain the composition ratio shown in Table 3, thereby obtaining ink composition 6.

[Comparative Example 3]
An ink composition liquid 7 was obtained in the same manner as in Example 4, except that the ultraviolet-curable oligomer aqueous dispersion liquid α was changed to the ultraviolet-curable oligomer aqueous dispersion liquid γ and the composition ratio was set as shown in Table 3.

[Formation of cured film]
The ink composition liquids of Example 4 and Comparative Example 3 were applied to a PET film using a bar coater to a film thickness of 15 μm, and the applied film was heated at 80° C. for 10 minutes. After that, the applied film was irradiated with ultraviolet light from an LED having a peak wavelength of 385 nm at an irradiation energy of 7 J/ ^cm2 to form a cured film.

[Evaluation of elution resistance of cured film]
The cured films of Example 4 and Comparative Example 3 were used to evaluate the elution resistance.
The cured film was cut to a size that would fit into a vial, and an amount equivalent to a total area of 125 ^cm2 was placed in the vial. 3 mL of 50% aqueous ethanol solution was added to the vial, and heated at 70°C for 2 hours. After heating, the solution alone was transferred from the vial to an aluminum cup, and the solution was dried to obtain the weight of the dried product. As a blank, the same procedure was performed on a PET film without a cured film, and the weight A of the cured film eluate was determined by subtracting the weight of the dried product from the weight of the dried product.
Next, a cured film having a thickness of 15 μm was formed on a glass plate, and the cured film was then scraped off. From the area and weight, the weight B of the cured film per 1 cm ² was calculated.
The value obtained from the following formula was taken as the elution resistance (%). The results are shown in Table 3.

The above evaluation results (Tables 2 and 3) demonstrate that the ink of the present invention, which uses an ultraviolet-curable oligomer having a specific structure in which compound (C') is a water-soluble compound capable of bonding with polyisocyanate compound (A) and has a (meth)acryloyl group, has excellent alcohol resistance, abrasion resistance, and elution resistance.

Although the present invention has been described in detail with reference to specific embodiments, it will be apparent to those skilled in the art that various modifications can be made without departing from the spirit and scope of the invention.
This application includes the contents of Japanese Patent Application Nos. 2019-216741 filed on November 29, 2019, 2019-216742 filed on November 29, 2019, 2020-033702 filed on February 28, 2020, and 2020-033703 filed on February 28, 2020, all of which are incorporated by reference in their entirety.

Claims (22)

An ultraviolet-curable water-based ink for ink-jet printing, comprising at least an ultraviolet-curable oligomer and a colorant,
The ultraviolet-curable water-based ink for inkjet printing, wherein the ultraviolet-curable oligomer is an ultraviolet-curable oligomer having a structural unit derived from a polyisocyanate compound (A), a structural unit derived from a compound (B') shown below, and a structural unit derived from a compound (C') shown below.
Compound (B'): A compound containing two or more polymerizable unsaturated bonds and capable of chemically bonding with the isocyanate group of the polyisocyanate compound (A). Compound (C'): A water-soluble compound capable of chemically bonding with the isocyanate group of the polyisocyanate compound (A) and having a (meth)acryloyl group. The ultraviolet-curable water-based ink for inkjet printing according to claim 1, wherein compound (C') is a compound containing one terminal hydroxyl group. The ultraviolet-curable water-based ink for inkjet printing according to claim 1 or 2, wherein the compound (C') is a polyalkylene glycol (C) having a (meth)acryloyl group. The ultraviolet-curable water-based ink for inkjet printing according to claim 3, wherein the compound (C') is a polyalkylene glycol mono-substituted ether having a (meth)acryloyl group. The ultraviolet-curable water-based ink for inkjet printing according to claim 4, wherein the compound (C') is polyethylene glycol mono(meth)acrylate. The ultraviolet-curable water-based ink for inkjet printing according to any one of claims 1 to 5, wherein the compound (B') is a hydroxyl group-containing polyfunctional (meth)acrylate (B). 7. The ultraviolet-curable water-based ink for inkjet printing according to claim 6, wherein the ultraviolet-curable oligomer is formed by chemically bonding a structural unit derived from the polyisocyanate compound (A) to a structural unit derived from the hydroxyl group-containing polyfunctional (meth)acrylate (B) and a structural unit derived from the polyalkylene glycol (C) having a (meth)acryloyl group via a urethane bond. The ultraviolet-curable water-based ink for inkjet printing according to any one of claims 1 to 7, wherein the polyisocyanate compound (A) has three or more isocyanate groups. The UV-curable water-based ink for inkjet printing according to any one of claims 1 to 8, wherein the UV-curable oligomer is dispersed as particles in an aqueous medium. The UV-curable water-based ink for inkjet printing according to any one of claims 1 to 9, wherein the average particle size of the UV-curable oligomer is 10 nm or more and 200 nm or less. The UV-curable water-based ink for inkjet printing according to any one of claims 1 to 10, having a viscosity at 25°C of 1 mPa·sec or more and 25 mPa·sec or less. The ultraviolet-curable water-based ink for inkjet printing according to any one of claims 1 to 11, wherein the content of the colorant is 0.1% by mass or more and 8% by mass or less, and the content of the ultraviolet-curable oligomer is 3% by mass or more and 20% by mass or less. The ultraviolet-curable water-based ink for inkjet printing according to any one of claims 1 to 12, further comprising one or more of a polymerization initiator, a sensitizer, and a surfactant. The UV-curable water-based ink for inkjet printing according to claim 13, further comprising a polymerization initiator and/or a sensitizer, at least a portion of which is encapsulated in the UV-curable oligomer. A printed material having a cured product formed on a recording medium from the UV-curable water-based ink for inkjet printing described in any one of claims 1 to 14. A UV-curable aqueous composition for inkjet printing, comprising a UV-curable oligomer and at least one of a polymerization initiator, a sensitizer, and a surfactant,
The ultraviolet-curable aqueous composition for inkjet printing is an ultraviolet-curable oligomer having a structural unit derived from a polyisocyanate compound (A), a structural unit derived from a compound (B') shown below, and a structural unit derived from a compound (C') shown below.
Compound (B'): A compound containing two or more polymerizable unsaturated bonds and capable of chemically bonding with the isocyanate group of the polyisocyanate compound (A). Compound (C'): A water-soluble compound capable of chemically bonding with the isocyanate group of the polyisocyanate compound (A) and having a (meth)acryloyl group. The ultraviolet-curable aqueous composition for inkjet printing according to claim 16, wherein the compound (C') is a compound containing one terminal hydroxyl group. The ultraviolet-curable aqueous composition for inkjet printing according to claim 16 or 17, wherein the compound (C') is a polyalkylene glycol (C) having a (meth)acryloyl group. The ultraviolet-curable aqueous composition for inkjet printing according to claim 18, wherein the compound (C') is a polyalkylene glycol mono-substituted ether having a (meth)acryloyl group. The ultraviolet-curable aqueous composition for inkjet printing according to claim 19, wherein the compound (C') is polyethylene glycol mono(meth)acrylate. The UV-curable aqueous composition for inkjet printing according to any one of claims 16 to 20, containing at least one polymerization initiator, a sensitizer, and/or a surfactant. The UV-curable aqueous composition for inkjet printing according to any one of claims 16 to 21, further comprising a polymerization initiator and/or a sensitizer, at least a portion of which is encapsulated in the UV-curable oligomer.