JP7637606B2 - Pigment dispersion for ultraviolet-curable inkjet ink, method for producing same, and ultraviolet-curable inkjet ink - Google Patents

Description

The present invention relates to a pigment dispersion for ultraviolet-curable inkjet ink, a method for producing the same, and an ultraviolet-curable inkjet ink.

Inkjet printing does not require a plate and can print images on demand by outputting data from a computer. Inkjet printing uses water-based inkjet ink, solvent-based inkjet ink, and UV-curable inkjet ink. Of these, UV-curable inkjet ink, which does not require a drying process and can produce prints with excellent durability, is attracting attention for industrial applications such as sign displays, packaging, and printing on plastic containers.

Inkjet printing methods are shifting from single-pass printing to high-speed printing, where ink is ejected from a line head in one go to print a large-area image in one go. There is a need to develop ultraviolet-curable inkjet (hereinafter also referred to as "UVIJ ink") that can handle such high-speed printing.

UVIJ ink is prepared, for example, by using a pigment dispersion liquid in which an ultraviolet-curable monomer is used as the liquid medium, and blending an oligomer, a photoinitiator, and the like. UVIJ inks usually include the three primary color inks of cyan ink, magenta ink, and yellow ink, as well as black ink and white ink. In addition, complementary color inks such as green ink, orange ink, and blue ink are also used. When preparing each of these color inks, a pigment dispersion liquid in which the pigment of each color is dispersed in a liquid medium is used.

Pigment dispersions and inks prepared using them are required to have finely dispersed pigment particles and low viscosity so that they can be stably ejected from inkjet recording heads. In addition, they are required to be resistant to aggregation and have stable viscosity even when stored for long periods of time, meaning that the pigment dispersion state is resistant to fluctuations and must be maintained at a constant state. However, finely dispersing pigments increases the surface energy, making them more susceptible to aggregation. As a result, there are issues such as increased viscosity and increased particle size during storage.

The liquid medium used in the pigment dispersion for UVIJ ink is an ultraviolet-curable monomer, which is an oil-based organic compound. The yellow pigment, which is an azo-based pigment used in each color UVIJ ink, and the orange pigment, which is also used as a vat dye, have lower solvent resistance than blue pigments such as copper phthalocyanine, which is a highly crystalline metal salt, and magenta pigments such as quinacridone pigments, which are highly crystalline due to the influence of high hydrogen bonds. In addition, highly polar monomers such as ether-based monomers such as 2-(2-vinyloxyethoxy)ethyl acrylate; and monomers having amide bonds such as acryloylmorpholine, N-vinylpyrrolidone, and N-vinylcaprolactam; are used as ultraviolet-curable monomers in recent UVIJ inks. These ultraviolet-curable monomers have strong solubility for organic compounds. Therefore, when a pigment is dispersed in these ultraviolet-curable monomers, the dispersibility of the pigment is likely to decrease because crystals tend to grow. Therefore, the viscosity increases during dispersion, and the dispersibility decreases during storage, which makes it easy for pigment particles to aggregate and increase in viscosity. Therefore, there is a demand for pigment dispersions for UVIJ inks that use yellow and orange pigments and have excellent pigment dispersibility as well as excellent storage stability.

Various pigment dispersions using yellow or orange pigments, for example, have been proposed as pigment dispersions for UVIJ inks with improved pigment dispersibility (Patent Documents 1 to 3).

JP 2006-282758 A JP 2004-2528 A JP 2014-145053 A

However, even the pigment dispersions proposed in Patent Documents 1 to 3 are still not sufficient in terms of pigment dispersibility and storage stability as pigment dispersions for preparing UVIJ inks that are more suitable for high-speed printing, and there is still room for further improvement.

The present invention has been made in consideration of the problems associated with the conventional technology, and has as its object the provision of a pigment dispersion liquid used to prepare an ultraviolet-curable inkjet ink that has excellent dispersibility and storage stability for yellow and orange pigments. Another object of the present invention is to provide a method for producing this pigment dispersion liquid, and an ultraviolet-curable inkjet ink that uses this pigment dispersion liquid.

That is, according to the present invention, there is provided a pigment dispersion liquid as shown below.
[1] A pigment dispersion for use in an ultraviolet-curable inkjet ink, comprising a pigment including at least one of a yellow pigment and an orange pigment, a pigment derivative, a polymer dispersant having an amino group, and an ultraviolet-curable monomer, wherein the pigment derivative has a structure represented by the following general formula (1):

（前記一般式（１）中、ＸはＯＨ又はＯＣＨ_３を示し、４つのＸのうち２以上がＯＨである）

(In the general formula (1), X represents OH or _OCH3 , and two or more of the four Xs are OH.)

[2] The pigment dispersion according to [1], wherein the content of the pigment derivative is 3 to 20 parts by mass relative to 100 parts by mass of the pigment.
[3] The pigment dispersion liquid according to [1] or [2], wherein the polymer dispersant is a polymer having an amine value of 0.5 to 100 mgKOH/g and a number average molecular weight of 5,000 to 20,000, the polymer having 3 to 50 mass% of structural units (1) derived from at least one selected from the group consisting of dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate, 2-vinylpyridine, 4-vinylpyridine, and vinylimidazole, some amino groups of which may be converted to quaternary ammonium salts, 40 to 90 mass% of structural units (2) derived from a macromonomer represented by the following general formula (2), and 0.5 to 20 mass% of structural units (3) derived from other monomers:

（前記一般式（２）中、Ｒ_１は水素原子又はメチル基を示し、ＡはＯ又はＮＨを示し、Ｑはエチレン基又はメチルエチレン基を示し、ＹはＯ、ＮＨＣＯＯ、又はＮＨＣＯＮＨを示し、ｍ及びｎは、相互に独立に、０以上の平均繰り返し単位数を示すとともに、ｍ＋ｎ＝２０～１００であり、Ｒ_２は水素原子、炭素数１～１８のアルキル基、アリール基、又はアルキルアリール基を示す）

(In the general formula (2), _R1 represents a hydrogen atom or a methyl group, A represents O or NH, Q represents an ethylene group or a methylethylene group, Y represents O, NHCOO, or NHCONH, m and n each independently represent an average number of repeating units of 0 or more, and m+n=20 to 100, and _R2 represents a hydrogen atom, an alkyl group having 1 to 18 carbon atoms, an aryl group, or an alkylaryl group.)

[4] The pigment dispersion according to any one of [1] to [3], wherein the yellow pigment is at least one selected from the group consisting of C.I. Pigment Yellow 74, 128, 138, 139, 150, 155, 180, and 185, and the orange pigment is at least one selected from the group consisting of C.I. Pigment Orange 16, 34, 43, and 64.

According to the present invention, there is also provided a method for producing a pigment dispersion liquid as described below.
[5] A method for producing a pigment dispersion liquid according to any one of [1] to [4] above, comprising the steps of mixing the pigment with an aqueous solution of an alkali neutralization product of the pigment derivative, and then adding an acid to precipitate the pigment derivative, thereby obtaining a pigment-derivative-treated pigment.

Furthermore, according to the present invention, there is provided the following ultraviolet-curable ink-jet ink.
[6] An ultraviolet-curable inkjet ink containing the pigment dispersion liquid according to any one of [1] to [4] above.

According to the present invention, it is possible to provide a pigment dispersion liquid used for preparing an ultraviolet-curable inkjet ink that has excellent dispersibility and storage stability for yellow and orange pigments. In addition, according to the present invention, it is possible to provide a method for producing this pigment dispersion liquid, and an ultraviolet-curable inkjet ink that uses this pigment dispersion liquid.

<Pigment Dispersion>
Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to the following embodiments. The pigment dispersion of the present invention is a pigment dispersion used in an ultraviolet-curable inkjet ink, which contains a pigment including at least one of a yellow pigment and an orange pigment, a pigment derivative, a polymer dispersant having an amino group, and an ultraviolet-curable monomer. The pigment derivative has a structure represented by the following general formula (1). Hereinafter, the pigment dispersion of the present invention will be described in detail.

（前記一般式（１）中、ＸはＯＨ又はＯＣＨ_３を示し、４つのＸのうち２以上がＯＨである）

(In the general formula (1), X represents OH or _OCH3 , and two or more of the four Xs are OH.)

(Pigment)
The pigment includes at least one of a yellow pigment and an orange pigment. The pigment is preferably either a yellow pigment or an orange pigment. The hue (yellow) of the pigment derivative does not significantly interfere with the hue of the pigment used. Examples of yellow pigments include C.I. Pigment Yellow 1, 2, 3, 4, 5, 6, 7, 10, 11, 12, 13, 14, 16, 17, 24, 34, 35, 37, 53, 55, 65, 73, 74, 75, 81, 83, 93, 94, 95, 97, 98, 99, 108, 109, 110, 113, 114, 117, 120, 124, 128, 129, 133, 138, 139, 147, 150, 151, 153, 154, 155, 167, 172, 180, and 185. Examples of orange pigments include C.I. Pigment Orange 1, 2, 5, 7, 13, 14, 15, 16, 24, 34, 36, 38, 40, 43, 63, and 64.

Among them, the yellow pigment is preferably at least one selected from the group consisting of C.I. Pigment Yellow 74, 128, 138, 139, 150, 155, 180, and 185, because of its good color development and light resistance. The orange pigment is preferably at least one selected from the group consisting of C.I. Pigment Orange 16, 34, 43, and 64. In particular, the yellow pigment is preferably at least one selected from the group consisting of C.I. Pigment Yellow 138, 150, 155, 180, and 185, because of its wide hue range. The orange pigment is preferably at least one selected from the group consisting of C.I. Pigment Orange 43 and 64.

(Pigment derivatives)
The pigment derivative has a structure represented by the following general formula (1).

（前記一般式（１）中、ＸはＯＨ又はＯＣＨ_３を示し、４つのＸのうち２以上がＯＨである）

(In the general formula (1), X represents OH or _OCH3 , and two or more of the four Xs are OH.)

The pigment derivative is a compound having a skeleton similar to that of the pigment or a structure that is adsorbed to the pigment. The pigment dispersion of the present invention contains an ultraviolet-curable monomer, which is an oil-based compound, as a liquid medium. The pigment is an oil-based organic compound, and the polymer dispersant used to finely disperse the pigment in the liquid medium and increase the storage stability of the pigment dispersion is also oil-based. The polymer dispersant needs to be adsorbed to the pigment without dissolving in the liquid medium. The pigment derivative is a component that mediates the adsorption of the polymer dispersant to the pigment. Since the liquid medium is an organic compound, the polymer dispersant is adsorbed to the pigment by ionic bonding to suppress the detachment of the polymer dispersant. For this reason, the pigment derivative has an acidic carboxy group as an active group in its molecular structure. In addition, a basic polymer having an amino group is used as the polymer dispersant. For this reason, the polymer dispersant adsorbed to the pigment imparts a carboxy group, which is an active group, to the surface of the pigment particles. Then, the carboxy group imparted to the surface of the pigment particles and the amino group in the polymer dispersant are ionic bonded, and the pigment is stably adsorbed to the polymer dispersant. This allows the pigment to be finely dispersed in the UV-curable monomer liquid medium, creating a pigment dispersion with excellent storage stability.

Since the pigment derivative has an aromatic ring and an amide bond in its molecular structure, it strongly adsorbs to the yellow pigment and the orange pigment. In addition, since the pigment derivative has two or more carboxy groups in its molecular structure, it can impart carboxy groups to the pigment particle surface by adsorbing to the pigment. In the general formula (1), the number of "OH" groups represented by "X" may be expressed as an average value. That is, the pigment derivative represented by the general formula (1) may be a mixture of multiple compounds having different numbers of "OH" groups represented by "X". In the general formula (1), the number of "OH" groups represented by "X" is preferably "4". That is, by using a pigment dispersant represented by the general formula (1) in which the number of carboxy groups in one molecule is 4, it is possible to effectively impart carboxy groups to the pigment particle surface. In addition, since the number of carboxy groups imparted is increased, the polarity can be further increased and the pigment becomes more difficult to dissolve in the ultraviolet curable monomer used as the liquid medium.

The pigment derivative represented by general formula (1) can be produced according to a conventional method. For example, the pigment derivative can be produced by azo coupling. In particular, the pigment derivative represented by general formula (1) can be easily produced by using C.I. Pigment Yellow 155 (hereinafter also referred to as "PY-155") having the structure represented by the following formula (A) as a raw material and hydrolyzing the methyl ester in the molecular structure under alkaline conditions, for example.

After dispersing PY-155 in water, an alkali metal hydroxide such as sodium hydroxide or potassium hydroxide is added. If all methyl esters are to be hydrolyzed, an alkali metal hydroxide in an amount equal to or greater than the moles of the methyl esters may be used. The ester bonds are hydrolyzed by heating at room temperature, preferably to around 80°C, to form the alkali metal salt of the carboxy group. The progress of the hydrolysis reaction can be tracked by IR analysis using an infrared spectrophotometer to confirm the decrease in the absorption peak derived from the ester bonds and the increase in the absorption peak derived from the carboxy group, or by confirming the pH change.

After hydrolysis, an acidic substance such as hydrochloric acid or acetic acid is added to neutralize the solution, preferably until the pH is 3 or less. This allows the pigment derivative represented by general formula (1) to be precipitated, which is now water-insoluble. The precipitated pigment derivative can be extracted as a solid by filtering and drying. However, if the number of carboxy groups in one molecule is "4", it may be very hard and difficult to dissolve in a solvent, making it difficult to handle. For this reason, it is preferable to extract the precipitated pigment derivative in the form of a water paste without drying it.

In the pigment dispersion, the content of the pigment derivative relative to 100 parts by mass of pigment is preferably 3 to 20 parts by mass, and more preferably 5 to 15 parts by mass. If the content of the pigment derivative is less than 3 parts by mass relative to 100 parts by mass of pigment, the amount of active groups derived from the pigment derivative imparted to the pigment surface may be insufficient. On the other hand, if the content of the pigment derivative is more than 20 parts by mass relative to 100 parts by mass of pigment, the hue derived from the pigment derivative is more likely to be reflected, and the desired hue may not be obtained.

(Polymer Dispersant)
Polymer dispersant is a basic polymer dispersant having amino group. Examples of polymer dispersant include long-chain alkyl monoamine; polyalkylene glycol adduct of long-chain alkyl amine; amide amine salt of polyester such as poly12-hydroxysaric acid and polyethyleneimine, polyvinylamine, polyallylamine; reaction product of compound having tertiary and primary amino groups such as polyethylene glycol, polyester and diethylaminopropylamine and isocyanate polynuclear; vinyl polymer having vinylimidazole, vinylpyridine, etc. as constituent; polymer having tertiary amino groups such as AB block copolymer having (meth)acrylic acid monomer as constituent, A chain soluble in solvent, B chain being constituent of amino group-containing (meth)acrylate such as dimethylaminoethyl methacrylate, etc., with quaternary ammonium salt by quaternizing agent such as methyl chloride, benzyl chloride, etc.; and the like.

As the polymer dispersant, it is preferable to use a polymer having a structural unit (1) derived from at least one selected from the group consisting of dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate, 2-vinylpyridine, 4-vinylpyridine, vinylimidazole, and quaternary ammonium salts thereof, in which some amino groups may be converted to quaternary ammonium salts, a structural unit (2) derived from a macromonomer represented by the following general formula (2), and a structural unit (3) derived from other monomers.

（前記一般式（２）中、Ｒ_１は水素原子又はメチル基を示し、ＡはＯ又はＮＨを示し、Ｑはエチレン基又はメチルエチレン基を示し、ＹはＯ、ＮＨＣＯＯ、又はＮＨＣＯＮＨを示し、ｍ及びｎは、相互に独立に、０以上の平均繰り返し単位数を示すとともに、ｍ＋ｎ＝２０～１００であり、Ｒ_２は水素原子、炭素数１～１８のアルキル基、アリール基、又はアルキルアリール基を示す）

(In the general formula (2), _R1 represents a hydrogen atom or a methyl group, A represents O or NH, Q represents an ethylene group or a methylethylene group, Y represents O, NHCOO, or NHCONH, m and n each independently represent an average number of repeating units of 0 or more, and m+n=20 to 100, and _R2 represents a hydrogen atom, an alkyl group having 1 to 18 carbon atoms, an aryl group, or an alkylaryl group.)

[Structural unit (1)]
The structural unit (1) is a structural unit derived from at least one member selected from the group consisting of dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate, 2-vinylpyridine, 4-vinylpyridine, and vinylimidazole, in which some of the amino groups may be converted to quaternary ammonium salts.

A part of the amino group in the structural unit (1) may be converted to a quaternary ammonium salt in order to adjust the solubility in the ultraviolet-curable monomer. That is, the structural unit (1) may have a quaternary ammonium base. The quaternary ammonium base can be formed by reacting the amino group with a quaternizing agent. Examples of the quaternary chlorinating agent include methyl chloride, benzyl chloride, benzyl bromide, dimethyl sulfate, diethyl sulfate, methyl p-toluenesulfonate, methyl methylsulfonate, and methyl trifluoromethylsulfonate. Examples of the counter ion of the quaternary ammonium base include chloride ion, bromide ion, and sulfate ester ion. The counter ion may be changed by reacting with a metal cation. Examples of the metal cation include bis(fluorosulfonyl)imide lithium salt, (fluorosulfonyl)(trifluoromethylsulfone)imide lithium salt, bis(trifluoromethylsulfone)imide lithium salt, sodium tetrafluoroborate, and sodium hexafluorophosphate.

The content of structural unit (1) in the polymer dispersant (polymer) is preferably 3 to 50% by mass, and more preferably 10 to 30% by mass, based on the total of structural units (1) to (3). If the content of structural unit (1) in the polymer is less than 3% by mass, the amount of amino groups will be insufficient, and the adsorption to the pigment will tend to decrease, and the dispersibility of the pigment may decrease slightly. On the other hand, if the content of structural unit (1) in the polymer is more than 50% by mass, the amount of amino groups may be too high. This may make it easier for the polymerization reaction during production to be inhibited, and the pigment may tend to aggregate.

[Structural unit (2)]
The structural unit (2) is a structural unit derived from a macromonomer represented by the following general formula (2).

（前記一般式（２）中、Ｒ_１は水素原子又はメチル基を示し、ＡはＯ又はＮＨを示し、Ｑはエチレン基又はメチルエチレン基を示し、ＹはＯ、ＮＨＣＯＯ、又はＮＨＣＯＮＨを示し、ｍ及びｎは、相互に独立に、０以上の平均繰り返し単位数を示すとともに、ｍ＋ｎ＝２０～１００であり、Ｒ_２は水素原子、炭素数１～１８のアルキル基、アリール基、又はアルキルアリール基を示す）

(In the general formula (2), _R1 represents a hydrogen atom or a methyl group, A represents O or NH, Q represents an ethylene group or a methylethylene group, Y represents O, NHCOO, or NHCONH, m and n each independently represent an average number of repeating units of 0 or more, and m+n=20 to 100, and _R2 represents a hydrogen atom, an alkyl group having 1 to 18 carbon atoms, an aryl group, or an alkylaryl group.)

The structural unit (2) is a structural unit having a polyalkylene glycol chain. The polymer dispersant containing this structural unit (2) has a structure in which the polyalkylene glycol chain is grafted. The polyalkylene glycol chain is a molecular chain that can be dissolved in the ultraviolet-curable monomer, which is a liquid medium (dispersion medium). When the polymer dispersant is adsorbed to the pigment, the polyalkylene glycol chain has steric repulsion to suppress the aggregation of the pigment, and the pigment is less likely to aggregate even after a long period of time has passed, thereby improving storage stability. In addition, O (oxygen atom), NHCOO (urethane bond), and NHCONH (urea bond) represented by Y in general formula (2) have the function of adsorbing to the pigment by hydrogen bonding with hydrogen atoms constituting carboxyl groups, hydroxyl groups, etc. on the surface of the pigment. This allows the pigment to be well and stably dispersed in the liquid medium for a long period of time.

In general formula (2), m is the average number of repeating units of propyleneoxy groups (-CH(CH ₃ )CH ₂ O-), and n is the average number of repeating units of ethyleneoxy groups (-CH ₂ CH ₂ O-). m and n each independently represent the average number of repeating units of 0 or more, where m+n=20 to 100, and preferably m+n=35 to 100. That is, the molecular weight of the polyalkylene glycol chain is preferably 880 to 5,800, and more preferably 1,540 to 5,800. In addition, in general formula (2), the alkyl group having 1 to 18 carbon atoms represented by R ₂ is preferably a methyl group, an ethyl group, a propyl group, a butyl group, a dodecyl group, a stearyl group, a phenyl group, a naphthyl group, or a nonylphenyl group.

Examples of the macromonomer represented by general formula (2) include a macromonomer in which Y is a urethane bond (NHCOO) obtained by reacting (meth)acryloyloxyethyl isocyanate with a glycol ether monoalkyl ether such as polyethylene glycol monomethyl ether, polyethylene glycol polypropylene glycol monobutyl ether, polypropylene glycol monomethyl ether, or polyethylene glycol monododecyl ether; a macromonomer in which Y is a urea bond (NHCONH) obtained by reacting (meth)acryloyloxyethyl isocyanate with a monoether monoamine such as polyethylene glycol polypropylene glycol monoamine; and the like.

In the general formula (2), Y is preferably a urea bond (NHCONH) that does not require a catalyst when reacting an isocyanate with an amine. Also, the polyalkylene glycol chain in the general formula (2) is preferably a random copolymer of propylene oxide and ethylene oxide. Furthermore, in the general formula (2), _R2 is preferably a methyl group. The molecular weight of the polyalkylene glycol chain is preferably 2,000 to 4,000, and m+n=36 to 90.

The content of structural unit (2) in the polymer dispersant (polymer) is preferably 40 to 90% by mass, and more preferably 45 to 85% by mass, based on the total of structural units (1) to (3). Structural unit (2) is a structural unit contained in a relatively large amount in the polymer. By using a polymer containing a large amount of such structural unit (2) as a polymer dispersant, the densely arranged polyalkylene glycol chains act as steric hindrance, preventing the pigments from approaching each other, and the pigments can be stably dispersed.

[Structural unit (3)]
The structural unit (3) is a structural unit derived from another monomer capable of copolymerizing with each of the monomers constituting the structural units (1) and (2). As the other monomer, a conventionally known vinyl-based monomer or the like can be used. As the other monomer, there can be mentioned (meth)acrylic acid-based monomers such as (meth)acrylic acid and (meth)acrylic acid esters; vinyl monomers such as styrene, vinyl toluene, vinyl pyridine, vinyl caprolactone, vinyl imidazole, α-methyl styrene, and vinyl acetate; and the like. Among them, α-methyl styrene is preferred from the viewpoint of easy control of the molecular weight.

The content of structural unit (3) in the polymer dispersant (polymer) is preferably 0.5 to 20 mass% based on the total of structural units (1) to (3), and more preferably 0.6 to 16 mass%. If the content of structural unit (3) in the polymer exceeds 20 mass%, the content of other structural units decreases relatively, and the function as a dispersant may be slightly reduced.

[polymer]
The amine value of the polymer used as the polymer dispersant is 0.5 to 100 mgKOH/g, preferably 10 to 90 mgKOH/g, and more preferably 20 to 80 mgKOH/g. The number average molecular weight (Mw) of the polymer is 5,000 to 20,000, and preferably 10,000 to 15,000. If the number average molecular weight of the polymer is less than 5,000, the content of the structural unit (2) is somewhat low, and therefore the dispersion stability may be insufficient. On the other hand, if the number average molecular weight of the polymer is more than 20,000, the amount of the polymer dispersant required to disperse the pigment may be too large, and the viscosity of the pigment dispersion may be excessively high. In this specification, the number average molecular weight (Mn) and the weight average molecular weight (Mw) are both values calculated in terms of polystyrene measured by gel permeation chromatography (GPC).

(Method of synthesizing polymer dispersant)
The polymer used as the polymer dispersant can be synthesized by a conventionally known method. For example, it can be synthesized by a conventionally known radical polymerization method; a polymerization method for adjusting the molecular weight using a chain transfer agent such as thiol, an atom transfer radical polymerization method (ATRP method), a reversible addition-fragmentation chain transfer polymerization method (RAFT method), a nitroxide method (NMP method), an organotellurium method (TERP method), an iodine transfer polymerization method (ITP method), a reversible transfer catalyst polymerization method (RTCP method), a reversible catalyst-mediated polymerization method (RCMP method), and other living radical polymerization methods. Among them, the living radical polymerization method is preferred because it is possible to make the molecular weight of the main chain more uniform and to make an AB block copolymer by the addition method.

The polymerization may be either thermal polymerization or photopolymerization, and an azo-based radical generator, a peroxide-based radical generator, a photosensitizer, etc. may be added to the polymerization reaction system. The polymerization method may be any of solventless polymerization, solution polymerization, and emulsion polymerization, among which solution polymerization is preferred.

The UVIJ ink prepared using the pigment dispersion of the present invention is an environmentally friendly ink that contains substantially no volatile components, in which the components ejected and applied to the substrate are cured by ultraviolet light to form an image. For this reason, it is preferable that the solution containing the polymer dispersant obtained by the above-mentioned synthesis method or the like contains substantially no volatile components. For example, after solution polymerization, it is preferable to precipitate the solution in a poor solvent or volatilize the solvent used. In order to obtain a polymer dispersant that contains substantially no volatile components, the polymer dispersant may be synthesized in a non-volatile solvent.

(UV-curable monomer)
The ultraviolet curable monomer is a component that can also function as a liquid medium for dispersing a pigment. The ultraviolet curable monomer may be a monofunctional monomer having one ultraviolet curable reactive group, a polyfunctional monomer having two or more reactive groups, or a mixture of these. Furthermore, an oligomer (photocurable oligomer) or a polymer (photocurable polymer) having an ultraviolet curable reactive group may be coexisted.

Among the UV-curable monomers, monofunctional monomers include methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, hexyl (meth)acrylate, cyclohexyl (meth)acrylate, trimethylcyclohexyl (meth)acrylate, t-butylcyclohexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, isodecyl (meth)acrylate, lauryl (meth)acrylate, stearyl (meth)acrylate, isobornyl (meth)acrylate, dicyclopentenyl (meth)acrylate, adamantyl (meth)acrylate, adamantyl methyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, Examples of radical polymerizable monomers include propyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, glycidyl (meth)acrylate, dimethylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate, polyethylene glycol mono(meth)acrylate, polyethylene glycol monomethyl ether (meth)acrylate, 2-ethoxyethyl (meth)acrylate, tetrahydrofurfuryl (meth)acrylate, allyl (meth)acrylate, benzyl (meth)acrylate, phenoxyethyl (meth)acrylate, (3-ethyloxetan-3-yl)methyl (meth)acrylate, an adduct of phthalic anhydride and 2-hydroxyethyl (meth)acrylate, acryloylmorpholine, N-vinylpyrrolidone, and N-vinylcaprolactam. Among these, (meth)acrylate-based compounds with good ultraviolet curing properties are preferred.

Polyfunctional monomers and photocurable oligomers include vinyloxyethoxyethyl (meth)acrylate, neopentyl glycol di(meth)acrylate, ethylene glycol di(meth)acrylate, poly(n=2 or more) ethylene glycol di(meth)acrylate, polypropylene glycol (n=2 or more) di(meth)acrylate, polybutylene glycol (n=2 or more) di(meth)acrylate, 2,2-bis(4-(meth)acryloxyethoxyphenyl)propane, 2,2-bis(4-(meth)acryloxydiethoxyphenyl)propane, trimethylolpropane di(meth)acrylate, Acrylate, bis(2-(meth)acryloxyethyl)-hydroxyethyl-isocyanurate, trimethylolpropane tri(meth)acrylate, tris(2-(meth)acryloxyethyl)isocyanurate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol tetra(meth)acrylate, dipentaerythritol penta(meth)acrylate, dipentaerythritol hexa(meth)acrylate, epoxy di(meth)acrylate obtained by reacting bisphenol A diepoxy with (meth)acrylic acid, urethane tri(meth)acrylate obtained by reacting 2-hydroxyethyl(meth)acrylate with a trimer of 1,6-hexamethylene diisocyanate; urethane di(meth)acrylate obtained by reacting isophorone diisocyanate with 2-hydroxypropyl(meth)acrylate; urethane hexa(meth)acrylate obtained by reacting isophorone diisocyanate with pentaerythritol tri(meth)acrylate; urethane di(meth)acrylate obtained by reacting dicyclohexyl diisocyanate with 2-hydroxyethyl(meth)acrylate. ) acrylate, urethane poly(meth)acrylate such as urethane di(meth)acrylate obtained by reacting 2-hydroxyethyl (meth)acrylate with a urethane reaction product of dicyclohexyl diisocyanate and poly(n=6-15) tetramethylene glycol, polyester poly(meth)acrylate such as polyester (meth)acrylate obtained by reacting trimethylolethane with succinic acid and (meth)acrylic acid, and polyester (meth)acrylate obtained by reacting trimethylolpropane with succinic acid, ethylene glycol, and (meth)acrylic acid. Among these, (meth)acrylate-based multifunctional monomers and photocurable oligomers with good ultraviolet curing properties are preferred.

Examples of photocurable polymers include polymers having multiple radically polymerizable (meth)acryloyloxy groups at the ends or side chains of polymers such as poly(meth)acrylate, polyurethane, polyester, polyamide, polyimide, and polyepoxy resin.

(Additives)
The pigment dispersion may further contain various additives other than the above-mentioned components. Examples of additives include polymerization inhibitors, surfactants, antioxidants, UV absorbers, light stabilizers, leveling agents, thickeners, viscosity adjusters, preservatives, antifungal agents, dyes, waxes, fillers, acid generators, alkali generators, etc. In addition, since the pigment dispersion of the present invention is used to produce UVIJ ink, it may contain a photopolymerization initiator in advance.

The polymerization inhibitor is a component that prevents the ultraviolet-curable monomer from polymerizing and solidifying during dispersion or storage. Examples of polymerization inhibitors include phenolic compounds such as p-methoxyphenol, t-butylcatechol, di-t-butyl-para-cresol, hydroquinone monomethyl ether, 3,5-di-t-butyl-4-hydroxytoluene, and 2,2'-methylenebis(4-methyl-6-t-butylphenol); quinone compounds such as p-benzoquinone; amine compounds such as p-benzylaminophenol, di-β-naphthyl-para-phenylenediamine, phenylhydroxylamine, and diethylhydroxylamine; oxime compounds such as quinonedioxime and cyclohexanoneoxime; and sulfur compounds such as phenothiazine.

(Pigment Dispersion)
The preferred contents of each component in the pigment dispersion are as follows. That is, the total content of the pigment and pigment derivative (content of pigment treated with a pigment derivative) in the pigment dispersion is preferably 5 to 30 mass%, and more preferably 10 to 25 mass%. The content of the polymer dispersant having an amino group in the pigment dispersion is preferably 0.5 to 30 mass%, and more preferably 1 to 25 mass%. Furthermore, the content of the ultraviolet-curable monomer in the pigment dispersion is preferably 40 to 94.5 mass%, and more preferably 60 to 90 mass%.

The viscosity of the pigment dispersion may be adjusted to a viscosity suitable for inkjet printing, taking into consideration the properties of the pigment. The viscosity of the pigment dispersion at 25°C is preferably 3 to 100 mPa·s. The surface tension of the pigment dispersion at 25°C is preferably 15 to 45 mN/m, and more preferably 20 to 40 mN/m.

<Method of producing pigment dispersion>
The pigment dispersion can be produced according to a conventional method. For example, a mixture is obtained by mixing an ultraviolet-curable monomer, a pigment, a pigment derivative, and a polymer dispersant, and adding an organic solvent as necessary. The mixture is finely dispersed in the pigment using a paint shaker, a ball mill, an attritor, a sand mill, a horizontal media mill, a colloid mill, a roll mill, or the like to prepare a dispersion. Next, the ultraviolet-curable monomer is added to the prepared dispersion to adjust the concentration, and other additives are further added as necessary, to obtain a pigment dispersion. It is preferable to remove coarse particles using a centrifuge or a filter as necessary.

To bring the number average particle size (particle size distribution) of the pigment into the desired range, techniques such as reducing the size of the grinding media used during dispersion, increasing the packing rate of the grinding media, lengthening the processing time, slowing down the discharge speed, and classifying the pigment using a filter or centrifuge after grinding can be used.

When producing a pigment dispersion, it is preferable to mix a pigment with an aqueous solution of an alkali neutralized pigment derivative, add an acid, and precipitate the pigment derivative to obtain a pigment derivative-treated pigment. That is, the method for producing a pigment dispersion of the present invention preferably includes a step of mixing a pigment with an aqueous solution of an alkali neutralized pigment derivative, adding an acid, and precipitating the pigment derivative to obtain a pigment derivative-treated pigment. In addition, it is preferable that the pigment dispersion of the present invention is obtained by a production method including a step of mixing a pigment with an aqueous solution of an alkali neutralized pigment derivative, adding an acid, and precipitating the pigment derivative to obtain a pigment derivative-treated pigment. In this way, by going through the step of obtaining a pigment derivative-treated pigment, a pigment dispersion having excellent pigment dispersibility and storage stability can be obtained. In addition, since the pigment derivative is hard and poorly soluble in organic solvents, it is preferable to handle it as described above.

Specifically, first, the pigment is peptized with water. The pigment may be in the form of a water paste immediately after synthesis, or in the form of a water paste of finely divided pigment. The amount of pigment is preferably 10% by mass or less of the amount of water, and more preferably 5% by mass or less. To peptize the pigment with water, a stirrer such as a disperser or homogenizer, or a dispersing machine such as a bead mill can be used. Next, the water paste of the pigment derivative is diluted with water so that the content of the pigment derivative is 10% by mass or less, preferably 3% by mass or less, and then an alkali component is added to neutralize the carboxyl group of the pigment derivative to prepare an aqueous solution of the alkali neutralized pigment derivative. Examples of the alkali component that can be used include ammonia, organic amines, and alkali metal hydroxides. The amount of the alkali component may be an amount that allows the pigment derivative to dissolve in water, and it is preferable to use an alkali component with a molar number equal to or greater than the number required to neutralize all the carboxyl groups of the pigment derivative.

After mixing the water slurry in which the pigment has been fully peptized with an aqueous solution of an alkali neutralized pigment derivative, an acid is gradually added to precipitate the pigment derivative. As the acid, an acid component such as hydrochloric acid or acetic acid can be used. These acid components are preferably used in the form of an aqueous solution of 10% by mass or less. By adding the acid, the pigment derivative is precipitated on the surface of the pigment in units of one molecule or several molecules, and a pigment derivative-treated pigment can be obtained in which the pigment derivative is adsorbed on at least a portion of the surface of the pigment. The obtained pigment derivative-treated pigment (hereinafter also simply referred to as "treated pigment") can be filtered and washed, and then further dried and pulverized for use.

The resulting treated pigment is dispersed in an ultraviolet-curable monomer, which is a dispersion medium, using a polymeric dispersant having an amino group. By using the polymeric dispersant, the pigment particles can be finely dispersed. In addition, the amino groups of the polymeric dispersant ionically bond to the carboxy groups derived from the pigment derivative present on the surface of the pigment, and the polymeric dispersant is adsorbed to the pigment. This suppresses pigment aggregation, making it possible to produce a pigment dispersion with excellent storage stability.

The treated pigment may be prepared by a method other than the above-mentioned method. For example, the treated pigment can be obtained by mixing an aqueous paste of the pigment and an aqueous paste of the pigment derivative, followed by filtering and drying. Alternatively, when the pigment is milled in the presence of a salt and a water-soluble organic solvent to be finely milled, an aqueous paste of the pigment derivative is allowed to coexist, or an aqueous paste of the pigment derivative is added after fine milling. The treated pigment can then be obtained by washing with water, desalting, removing the solvent, and drying.

<UV-curable inkjet ink>
By using the above-mentioned pigment dispersion, it is possible to produce an ultraviolet-curable inkjet ink that has excellent dispersibility and storage stability for a yellow or orange pigment, even when the pigment is used. That is, the ultraviolet-curable inkjet ink of the present invention is an ultraviolet-curable inkjet ink that contains the above-mentioned pigment dispersion.

The UVIJ ink contains, as components other than the pigment dispersion liquid described above, for example, a photopolymerization initiator, an ultraviolet-curable polymer, and the above-mentioned additives. The photopolymerization initiator is a component for photopolymerizing the ultraviolet-curable monomer by irradiating ultraviolet light onto the ink applied to the substrate by the inkjet method, thereby forming a cured image. The photopolymerization initiator may be any that generates active species such as radicals or cations and initiates polymerization of the ultraviolet-polymerizable monomer. As the photopolymerization initiator, a photoradical polymerization initiator or a photocationic polymerization initiator can be used. Among these, it is preferable to use a photoradical polymerization initiator.

Examples of photopolymerization initiators include benzophenone-based photopolymerization initiators, thioxanthone-based photopolymerization initiators, ketoxime ester-based photopolymerization initiators, and acylphosphine oxide-based photopolymerization initiators. The content of photopolymerization initiator in the UVIJ ink is preferably 3 to 12% by mass based on the entire UVIJ ink. If the content of photopolymerization initiator is less than 3% by mass, the curing properties may be somewhat insufficient. On the other hand, if the content of photopolymerization initiator is more than 12% by mass, the photopolymerization initiator may remain or the cured film may become discolored.

After mixing and thoroughly stirring each component, the mixture can be filtered as necessary to remove coarse particles and foreign matter to obtain a UVIJ ink. Using the obtained UVIJ ink, images can be recorded (printed) on various printing substrates by inkjet recording. Printing substrates include paper, photographic paper, glossy photographic paper, plastic film, fabric, ceramics, metal, etc. By using the above UVIJ ink, an image can be formed that is a printing film with high saturation and high color development as well as excellent durability such as adhesion and abrasion resistance.

The present invention will be described in detail below based on examples, but the present invention is not limited to these examples. Note that "parts" and "%" in the examples and comparative examples are based on mass unless otherwise specified.

<Synthesis of pigment derivatives>
(Synthesis Example 1)
50 parts of C.I. Pigment Yellow 155 (PY-155, trade name "Ingjet Yellow 4GC", manufactured by Clariant) and 1,000 parts of ion-exchanged water were placed in a flask and stirred to obtain a mixed solution. When a drop of the obtained mixed solution was spotted on a filter paper (No. 2), it remained on the surface of the filter paper without striking through. 500 parts of a 5% aqueous sodium hydroxide solution was added to the obtained mixed solution, which was then heated to 80°C and reacted for 4 hours to obtain a yellow reaction solution with a slight greenish tinge. When a drop of the obtained reaction solution was spotted on a filter paper, it struck through. This confirmed that the obtained reaction solution was in a dissolved state.

After cooling to room temperature, 5% hydrochloric acid was gradually added dropwise while measuring the pH. Viscosity increased at around pH 3.5. Further hydrochloric acid was added dropwise to make the reaction liquid pH 2.8. When a drop of the resulting reaction liquid was spotted on filter paper, there was no bleed-through, and reddish yellow powder remained on the surface of the filter paper. After filtering the reaction liquid, it was thoroughly washed with ion-exchanged water to obtain an aqueous paste with a solid content of 11.0%.

A portion of the resulting aqueous paste was dried in a dryer at 100° C. for 24 hours and then pulverized to obtain a hard powder with a strong reddish color. Analysis using an infrared spectrophotometer confirmed that the absorption peak at 1,713 cm ⁻¹ derived from the methyl ester in the structure of PY-155 had almost completely disappeared, and an absorption peak at about 1,695 cm ⁻¹ derived from the carboxy group had appeared. This confirmed that all of the methyl ester in the structure of PY-155 had been hydrolyzed to form a carboxy group, and the desired pigment derivative (hereinafter also referred to as “SYN”) had been obtained.

0.1 g of SYN ground using a grinder was added to 10 mL each of toluene, methyl ethyl ketone, tetrahydrofuran, dimethylformamide, phenoxyethyl acrylate (PEA), and isobornyl acrylate (IBXA) and stirred. As a result, it was confirmed that SYN was hardly soluble in any of the liquid media.

<Production of Treated Pigment>
(Production Example 1)
2.5 parts of sodium hydroxide and 900 parts of water were put into a flask and stirred, and then 90.9 parts of the water paste of SYN obtained in Synthesis Example 1 was added to obtain an aqueous solution of SYN. When a part of the aqueous solution of SYN was spotted on a filter paper, it showed bleed-through. This confirmed that SYN had dissolved. 100 parts of PY-155 and 2,000 parts of water were put into a flask, and while stirring at 2,000 rpm using a disper, an aqueous solution of SYN was added, and the mixture was stirred for 2 hours to obtain a pigment slurry with a pH of 79. When a drop of the obtained pigment slurry was spotted on a filter paper, the pigment remained on the surface of the filter paper, and SYN bled yellow and showed bleed-through. 5% hydrochloric acid was gradually added to the obtained pigment slurry to make it pH 2.8. The mixture was slightly thickened around pH 3.0. When a drop of the pigment slurry was spotted on a filter paper, SYN that had become water-insoluble and precipitated remained on the surface of the filter paper. The pigment slurry was filtered and washed, then dried at 100° C. for 24 hours, and further pulverized in a pulverizer to obtain a treated yellow pigment (treated Y-1). The non-volatile content (constant weight at 120° C.) was 99.4%.

(Production Examples 2 to 5)
Treated yellow pigments (Treatment Y-2 and Treatment Y-3) and treated orange pigments (Treatment O-1 and Treatment O-2) were obtained in the same manner as in Production Example 1 above, except that the formulations were as shown in Table 1. The meanings of the abbreviations in Table 1 are as follows.
PY-180: C.I. Pigment Yellow 180 (manufactured by Dainichiseika Color & Chemicals Co., Ltd.)
PO-43: C.I. Pigment Orange 43 (manufactured by Dainichiseika Color & Chemicals Co., Ltd.)

<Synthesis of polymer dispersant>
(Synthesis Example 2)
One-terminated aminated polypropylene glycol polyethylene glycol monomethyl ether (M2005, trade name "Jeffamine M2005", Huntsman, m + n = 35 (m = 29, n = 6), actual amine value 28.05 mg KOH / g) 100 parts (0.05 mol) was placed in a reactor and placed in a water bath at 10 ° C. Methacryloyloxyethyl isocyanate (MOI, trade name "Kalends MOI", Showa Denko) 7.75 parts (0.05 mol) was dropped into the reactor over 30 minutes and reacted. A part of the reaction solution was sampled and subjected to IR measurement to confirm the disappearance of the isocyanate group derived from MOI and the formation of urea bonds. In addition, the amine value of the product measured using a 0.1 mol / L 2-propanol hydrochloric acid solution using a potentiometric automatic titrator was 0.1 mg KOH / g. This confirmed that the reaction between the amino group and the isocyanate group was almost complete. The resulting product was a macromonomer having a methacryloyl group bonded to one end of polypropylene glycol polyethylene glycol monomethyl ether (PPG/PEG). The number average molecular weight (Mn) of the macromonomer in terms of polystyrene, measured by gel permeation chromatography (GPC) using tetrahydrofuran (THF) as a developing solution, was 3,400.

Next, 86.7 parts of polyalkylene glycol monoalkyl ether (Unilube, product name "Unilube 50MB-26", NOF Corporation, average molecular weight 2,000), 3.0 parts of α-methylstyrene, 10 parts of methyl methacrylate (MMA), and 35 parts of 2-(N,N-dimethylamino)ethyl methacrylate (DMAEMA) were added and heated while bubbling nitrogen. When the internal temperature reached 75°C, 5.0 parts of 2,2'-azobis(isobutyric acid)dimethyl (V-601, product name "V-601", Fujifilm Wako Pure Chemical Industries) were added, and the mixture was heated to 75°C and polymerized for 4 hours. 0.5 parts of V-601 was further added and polymerized at 75°C for 4 hours. The product's Mn was measured by sampling a portion of the reaction solution and was found to be 15,600, and the polydispersity (PDI = weight average molecular weight (Mw) / number average molecular weight (Mn)) was 1.96. The polymer concentration in the reaction solution calculated from the peak area ratio of the polymer and the solvent (Unilube) in the GPC chart was 64.3%. The amine value (polymer equivalent) of the polymer in the reaction solution was 77.4 mg KOH/g. From the above, a solution (D-1S) was obtained that contained dispersant (D-1), a polymer dispersant (polymer) having a main chain with amino groups and a graft chain in which a polypropylene glycol polyethylene glycol monomethyl ether chain is grafted to this main chain.

(Synthesis Example 3)
In the same manner as in Synthesis Example 2, a Unilube solution of a polymer having a main chain with an amino group and a graft chain in which a polypropylene glycol polyethylene glycol monomethyl ether chain was grafted to the main chain was obtained. After the obtained solution was cooled to room temperature, a mixture of 14.1 parts of benzyl chloride and 14.1 parts of Unilube was dropped over 1 hour. The mixture was heated to 80°C and reacted for 5 hours to convert the amino group into a quaternary ammonium salt. A part of the reaction solution was sampled, and the Mn of the polymer measured by GPC using 0.1 mol/L lithium chloride-dimethylformamide (Li-DMF) solvent as the developing solvent was 18,600, and the PDI was 2.36. The content of the polymer in the reaction solution was 63.0%, and the amine value of the polymer (polymer equivalent) was 35.1 mgKOH/g. As a result of the above, a solution (D-2S) containing a dispersant (D-2), which is a polymer dispersant (polymer) having a main chain having an amino group and a quaternary ammonium base and a graft chain in which a polypropylene glycol polyethylene glycol monomethyl ether chain is grafted to this main chain, was obtained.

(Synthesis Example 4)
341 parts of diethylene glycol monobutyl ether (BDG), 3.0 parts of iodine, 9 parts of 2,2'-azobis (2,4-dimethylvaleronitrile) (V-65, trade name "V-65", Fujifilm Wako Pure Chemical Industries, Ltd.), 52 parts of MMA, 52.6 parts of butyl methacrylate, 52 parts of 2-ethylhexyl methacrylate, 94.2 parts of tetrahydrofurfuryl methacrylate, and 0.1 parts of N-iodosuccinimide were placed in a reactor and stirred, and polymerized at 62 ° C. for 4 hours while flowing nitrogen. The Mn of the polymer measured by sampling a part of the reaction solution was 9,500, and the PDI was 1.21. The nonvolatile content that reached a constant weight in a 150 ° C. dryer was 42.9%, and it was confirmed that the polymerization was almost 100%.

108.3 parts of DMAEMA and 3 parts of V-65 were added, and polymerization was carried out at 62°C for 5 hours. After cooling to room temperature, a mixture of 21.8 parts of benzyl chloride and 157.7 parts of BDG was added dropwise over 1 hour, and the mixture was reacted at 80°C for 5 hours to obtain a polymerization solution containing a polymer. The polymerization rate and reaction rate calculated from the non-volatile content were both approximately 100%, and the non-volatile content was 43.2%. The Mn of the obtained polymer was 14,600, the PDI was 1.36, and the amine value (polymer equivalent) was 75.6 mgKOH/g. The obtained polymer was an AB block copolymer having a polymer block A that is compatible with the solvent and a polymer block B that has an amino group and a quaternary ammonium base.

1,178 parts of ion-exchanged water was placed in a flask and stirred at 2,000 rpm using a disperser. When 149.6 parts of the polymerization solution were added, the polymer dissolved in the water, resulting in a slightly blue translucent polymer aqueous solution. When a mixture of 6.7 parts of bis(trifluoromethanesulfonyl)imide lithium (TFSI lithium salt) and 20 parts of water was gradually added dropwise, the polymer precipitated. It is believed that the chloride ions of the quaternary ammonium base and the TFSI ions exchanged ions, resulting in the polymer becoming insoluble in water and precipitating. After filtering the polymerization solution, it was thoroughly washed with ion-exchanged water and dried in a dryer at 100°C to obtain dispersant (D-3), a solid polymer dispersant (polymer). The Mn of the obtained polymer was 17,600, the PDI was 1.44, the non-volatile content was 99.8%, and the amine value was 68.7 mgKOH/g. The resulting polymer is an AB block copolymer having a polymer block A that dissolves in a solvent and a polymer block B that has an amino group and a quaternary ammonium base. The quaternary ammonium base of this polymer block B has chloride ions and TFSI ions as counter ions.

<Production of Pigment Dispersion>
Example 1
20 parts of treatment Y-1, 6.2 parts of D-1S (4 parts of dispersant), 73.8 parts of phenoxyethyl acrylate (PEA), and 0.25 parts of p-methoxyphenol (HP) were placed in a resin container and thoroughly stirred using a disperser to obtain a pigment slurry. 300 parts of zirconia beads with a diameter of 0.5 mm were added and sealed, and a dispersion was performed for 60 minutes using a Scandex to obtain a dispersion. After filtering through a 0.5 μm filter to remove coarse particles, PEA was added to adjust the concentration, and a yellow pigment dispersion liquid-1 (pigment content 15%) was obtained. The viscosity (initial viscosity) of the yellow pigment dispersion liquid-1 measured using an E-type viscometer at 60 rpm and 25 ° C. was 29.3 mPa · s. In addition, the number average particle size (initial number average particle size) of the pigment particles measured using a dynamic light scattering particle size distribution measuring device was 111.0 nm. A storage test was carried out in which the yellow pigment dispersion-1 was left to stand for one week at 60° C. The viscosity of the yellow pigment dispersion-1 after the storage test (viscosity after storage) was 28.4 mPa·s, and the number average particle diameter of the pigment particles (number average particle diameter after storage) was 110 nm.

(Comparative Example 1)
18.2 parts of PY-155, 1.8 parts of a pigment derivative (product name "Solsperse 22000", manufactured by Lubrizol Corporation, pigment-similar structure), 6.2 parts of D-1S (4 parts of dispersant), 73.8 parts of PEA, and 0.25 parts of HP were placed in a resin container. The pigment derivative used does not have a structure represented by general formula (1). Thereafter, a yellow pigment dispersion was obtained in the same manner as in Example 1 described above. The initial viscosity of the obtained pigment dispersion was 69.3 mPa·s, and the initial number average particle size of the pigment particles was 201 nm. Furthermore, the pigment dispersion had no fluidity after the storage test and did not maintain a dispersed state.

Example 2
In a resin container, 18.2 parts of PY-155, 1.8 parts of the powder of the pigment derivative (SYN) obtained in Synthesis Example 1, 6.2 parts of D-1S (4 parts of dispersant), 73.8 parts of PEA, and 0.25 parts of HP were placed in a resin container. Thereafter, in the same manner as in Example 1 described above, a yellow pigment dispersion was obtained. The initial viscosity of the obtained pigment dispersion was 36.4 mPa·s, and the initial number average particle diameter of the pigment particles was 156 nm. In addition, red coarse particles were confirmed in the residue of the filter. The viscosity of the pigment dispersion after storage was 49.3 mPa·s, and the number average particle diameter of the pigment particles after storage was 186 nm. Although the viscosity and the particle diameter of the pigment particles increased due to storage, the fluidity of the pigment dispersion was maintained to a certain extent.

(Examples 3 to 15)
Each pigment dispersion was obtained in the same manner as in Example 1, except that the blending formulations shown in Tables 2 to 4 were used. The meanings of the abbreviations in Table 4 are as follows.
BzA: benzyl acrylate IBXA: isobornyl acrylate ACMO: acryloylmorpholine VEEA: vinyloxyethoxyethyl acrylate

<Production of UVIJ ink>
(Example 16)
18 parts of the yellow pigment dispersion-1 obtained in Example 1, 53.5 parts of PEA, 19 parts of VEEA, 5 parts of photopolymerization initiator 1 (trade name "Lucirin TPO", manufactured by BASF), and 4.5 parts of photopolymerization initiator 2 (trade name "Irgacure 819", manufactured by BASF) were mixed and thoroughly stirred, and then filtered through a 5 μm filter to obtain a UVIJ ink. The number average particle diameter of the pigment particles in the obtained UVIJ ink was 108 nm, and the viscosity of the UVIJ ink was 4.9 mPa·s. In addition, the number average particle diameter of the pigment particles in the UVIJ ink after the storage test was 107.1 nm, and the viscosity of the UVIJ ink after storage was 4.8 mPa·s.

The obtained UVIJ ink was filled into an ink cartridge and loaded into an inkjet printer (product name "EB100", manufactured by Konica Minolta). Using this inkjet printer, solid images were recorded continuously for one hour on polyethylene terephthalate (PET) film. As a result, images were recorded smoothly without clogging of the recording head. Furthermore, the recorded images were free of streaks or wrinkles, and the ink ejection stability was good.

The image recorded using a UV lamp was irradiated with ultraviolet light, hardening the image to obtain a print sample. The print sample obtained was cut into a checkerboard pattern using a cutter, and then cellophane tape was applied and then peeled off in one go in an adhesion (fixation) test. As a result, the hardened image did not peel off at all, and was in good adhesion to the film. Furthermore, instead of the PET film, images were recorded using polyvinyl chloride film and surface-treated polypropylene film, and a similar adhesion (fixation) test was performed. As a result, the images recorded on either film did not peel off, and were in good adhesion to the film.

In addition, when UVIJ inks were prepared in the same manner using the pigment dispersions obtained in Examples 3 to 15, the fine dispersion, storage stability, and ejection stability of all UVIJ inks were good.

The pigment dispersion of the present invention is particularly useful as a raw material for preparing UVIJ inks for printing on substrates such as transparent plastic films.

Claims (5)

A pigment dispersion liquid for use in an ultraviolet-curable inkjet ink, the pigment dispersion liquid containing a pigment including at least one of a yellow pigment and an orange pigment, a pigment derivative, a polymer dispersant having an amino group, and an ultraviolet-curable monomer,
The pigment derivative has a structure represented by the following general formula (1):
The pigment dispersion liquid is a polymer having an amine value of 0.5 to 100 mgKOH/g and a number average molecular weight of 5,000 to 20,000, the polymer comprising 3 to 50 mass% of structural units (1) derived from at least one member selected from the group consisting of dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate, 2-vinylpyridine, 4-vinylpyridine, and vinylimidazole, some amino groups of which may be converted to quaternary ammonium salts, 40 to 90 mass% of structural units (2) derived from a macromonomer represented by the following general formula (2), and 0.5 to 20 mass% of structural units (3) derived from other monomers :

(In the general formula (1), X represents OH or _OCH3 , and two or more of the four Xs are OH.)

(In the general formula (2), R1 _represents a hydrogen atom or a methyl group, A represents O or NH, Q represents an ethylene group or a methylethylene group, Y represents O, NHCOO, or NHCONH, m and n each independently represent an average number of repeating units of 0 or more, and m+n=20 to 100, and R2 _represents a hydrogen atom, an alkyl group having 1 to 18 carbon atoms, an aryl group, or an alkylaryl group.) The pigment dispersion according to claim 1, wherein the content of the pigment derivative per 100 parts by mass of the pigment is 3 to 20 parts by mass. The yellow pigment is at least one selected from the group consisting of C.I. Pigment Yellow 74, 128, 138, 139, 150, 155, 180, and 185;
3. The pigment dispersion according to claim 1 , wherein the orange pigment is at least one selected from the group consisting of C.I. Pigment Orange 16, 34, 43, and 64. A method for producing the pigment dispersion liquid according to any one of claims 1 to 3 ,
a step of mixing the pigment with an aqueous solution of an alkali neutralized product of the pigment derivative, and then adding an acid to precipitate the pigment derivative, thereby obtaining a pigment-derivative-treated pigment. 4. An ultraviolet-curable ink-jet ink comprising the pigment dispersion liquid according to claim 1 .