JP2013199508A - Ink composition - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a photocurable ink composition suitable for exposure by a UV-LED light source.SOLUTION: An ink composition includes a photopolymerization initiator represented by general formula (1) (wherein R-Rare each independently a hydrogen atom or a substituent; Xand Xare each independently an oxygen atom, a sulfur atom, an alkylene group or a single bond and at least one of Xand Xis a sulfur atom), a polymerizable compound, and a sensitizing dye having an absorption peak in a wavelength range of 330-400 nm.

Description

  The present invention relates to a photocurable ink composition. The present invention also relates to an ink set and an image forming method using the ink composition.

As an image recording method for forming an image on a recording medium such as paper based on an image data signal, there are an electrophotographic method, a sublimation type and a melt type thermal transfer method, an ink jet method and the like.
The ink jet method is an inexpensive printing apparatus and does not require a plate at the time of printing, and the ink composition is ejected only on the required image portion to form an image directly on a recording medium. It can be used efficiently, and the running cost is low especially for small lot production. Furthermore, it has low noise and is excellent as an image recording method, and has attracted attention in recent years.
In particular, an ink composition for ink jet recording (a radiation curable ink composition for ink jet recording) that can be cured by irradiation with radiation such as ultraviolet rays is cured by irradiation with radiation such as ultraviolet rays, and most of the components of the ink composition are cured. It is an excellent method in that it can be printed on various recording media because it is excellent in drying property and difficult to bleed out as compared with the solvent-based ink composition.

In recent years, from the viewpoint of energy saving and space saving, it is desired to use a UV light source as an LED instead of a conventional mercury lamp or the like. However, generally used α-hydroxyacetophenone-based photopolymerization initiators (for example, Patent Document 1) have a short absorption wavelength (around 280 nm), and are polymerized at a UV-LED light source (around 365 to 385 nm). Can't start. Also, benzyl ketal photopolymerization initiators have been reported (for example, Patent Documents 2 to 4), but even this polymerization initiator has a small absorption at around 365 nm, and the polymerization efficiency is not satisfactory.
In addition, in order to increase the absorption efficiency in the UV-LED region, it has been proposed to add a sensitizer to the ink composition and thereby perform sensitization. However, since the above-mentioned benzyl ketal photopolymerization initiator has a high energy level in the triplet excited state (T ₁ ), the sensitizer suitable for the wavelength of the UV-LED such as a thioxanthone sensitizer does not increase. The feeling was difficult.

JP-A-6-228218 JP 2008-247939 A JP 2008-247940 A JP 2009-138150 A

  An object of the present invention is to provide a photocurable (photopolymerizable) ink composition having high sensitivity to light in the vicinity of 365 to 385 nm and suitable for photosensitivity with a UV-LED light source. Another object of the present invention is to provide an image forming method capable of forming an image excellent in curability and blocking resistance using the ink composition.

  The present inventors have conducted intensive studies in view of the above problems. As a result, the photopolymerization initiator having a benzylthioketal structure is sensitized by a sensitizing dye having excellent absorption with respect to light in the vicinity of 365 to 385 nm, and radicals are generated from the photopolymerization initiator by sensitization. And found that the polymerizable compound polymerizes quickly. The ink composition containing these sensitizing dye, polymerization initiator and polymerizable compound exhibits a good sensitivity to the UV-LED light source and is cured quickly, and is an image having excellent curability and blocking resistance. It was found that can be obtained. The present invention has been completed based on these findings.

Specific means for solving the above problems are as follows.
<1> An ink composition comprising a photopolymerization initiator represented by the following general formula (1), a polymerizable compound, and a sensitizing dye having an absorption peak in a wavelength region of 330 nm to 400 nm.

(In General Formula (1), R ^{1 to} R ¹² each independently represents a hydrogen atom or a substituent. X ¹ and X ² each independently represent an oxygen atom, a sulfur atom, an alkylene group, or a single bond, (At least one of X ¹ and X ² is a sulfur atom.)
<2> The ink composition according to <1>, wherein in the general formula (1), at least one of R ^{1 to} R ⁵ is a group represented by the following general formula (2).

(In general formula (2), X ³ represents an oxygen atom or a sulfur atom. R ¹³ represents a substituent. * Represents a bonding site with a benzene ring.)
<3> The ink composition according to <1> or <2>, wherein the sensitizing dye is a sensitizing dye having substantially no absorption in a wavelength region of more than 400 nm and not more than 700 nm.
<4> The ink composition according to any one of <1> to <3>, wherein the sensitizing dye is a sensitizing dye represented by the following general formula (I).

(In the general formula ^{(I), R} 21 ~R ²⁶ independently denotes a hydrogen atom or a substituent, two substituents adjacent may ^{.X 21} may form a ring bonded to each is Represents an oxygen atom, a sulfur atom or NR ^27. R ²⁷ represents a hydrogen atom or a substituent.)
<5> The ink composition according to any one of <1> to <4>, wherein the sensitizing dye is a sensitizing dye represented by the following general formula (I-1).

(In general formula (I-1), R ^{28 to} R ³⁵ each independently represent a hydrogen atom or a substituent, and two adjacent substituents may be bonded to each other to form a ring.)
<6> The ink composition according to any one of <1> to <3>, wherein the sensitizing dye is a sensitizing dye represented by the following general formula (II).

(In the general formula ^{(II), R} 41 ~R ⁴⁸ independently denotes a hydrogen atom or a substituent, two substituents adjacent may ^{.X 41} may form a ring bonded to each is Represents an oxygen atom, a sulfur atom or NR ^27. R ²⁷ represents a hydrogen atom or a substituent.)
<7> The ink composition according to <6>, wherein in the general formula (II), X ⁴¹ is a sulfur atom.
<8> The content ratio of the photopolymerization initiator and the sensitizing dye according to any one of <1> to <7>, wherein the molar ratio is in the range of 1 to 5 to 5 to 1. Ink composition.
<9> The ink composition according to any one of <1> to <8>, wherein the polymerizable compound is at least one selected from a (meth) acrylamide compound, a (meth) acrylate compound, and a vinyl compound. object.
<10> The ink composition according to any one of <1> to <9>, wherein the polymerizable compound is a (meth) acrylamide compound.
<11> The ink composition according to any one of <1> to <10>, which contains a colorant.
<12> The ink composition according to any one of <1> to <11>, which contains an aqueous medium.
<13> The ink composition according to any one of <1> to <12>, wherein in the general formula (1), X ¹ and X ² are sulfur atoms.
<14> The ink composition according to any one of <2> to <13>, wherein in the general formula (1), R ³ is a group represented by the general formula (2).
<15> The ink composition according to any one of <1> to <14>, wherein in the general formula (1), at least one of R ¹¹ and R ¹² is a substituted or unsubstituted alkyl group.
<16> An ink set comprising the ink composition according to any one of <1> to <15>, and a treatment liquid that aggregates the aqueous ink composition.
<17> A treatment liquid application step of applying the treatment liquid described in <16> onto a recording medium, and the ink composition described in any one of <1> to <15> on a recording medium. An image forming method comprising: applying an ink to form an image.
In the present invention, “(meth) acrylate” represents both and / or acrylate and methacrylate, and “(meth) acrylamide” represents both and / or acrylamide and methacrylamide.
In the present invention, a numerical range represented by using “to” means a range including numerical values described before and after “to” as a lower limit value and an upper limit value.

  ADVANTAGE OF THE INVENTION According to this invention, the photocurable ink composition which shows favorable sclerosis | hardenability with respect to 365-385 nm light, such as UV-LED, can be provided. Moreover, according to this invention, the image forming method which can form the image excellent in sclerosis | hardenability and blocking resistance using the said ink composition can be provided.

The ink composition of the present invention comprises a photopolymerization initiator represented by the following general formula (1), a polymerizable compound, and a sensitizing dye having an absorption peak in a wavelength region of 330 nm to 400 nm.
Hereinafter, each component of the ink composition of the present invention will be described.

[Photopolymerization initiator]
The photopolymerization initiator used in the ink composition of the present invention is represented by the following general formula (1).
In the present invention, when the substituent is referred to as xxx group, the xxx group may have an arbitrary substituent. Moreover, when there are a plurality of groups indicated by the same symbol, they may be the same or different.

In general formula (1), R < ¹ > -R < ¹² > represents a hydrogen atom or a substituent each independently. R ¹¹ and R ¹² may combine to form a ring together with X ¹ and X ² .
Examples of the substituent for R ^{1 to} R ¹² include a halogen atom, an alkyl group (linear, branched, cyclic (including cycloalkyl group, bicycloalkyl group, tricycloalkyl group, etc.)), and an alkenyl group (linear, branched, Cyclic (including cycloalkenyl groups, bicycloalkenyl groups, etc.), alkynyl groups, aryl groups, heterocyclic groups (heterocyclic groups), cyano groups, hydroxy groups, nitro groups, carboxy groups (or salts thereof), alkoxy groups, Aryloxy group, silyloxy group, heterocyclic oxy group, acyloxy group, carbamoyloxy group, alkoxycarbonyl group, aryloxycarbonyl group, amino group (including anilino group), ammonio group, acylamino group, aminocarbonylamino group, alkoxycarbonyl Amino group, aryloxycarbonylamino group, sulfamoyl Mino group, alkyl and arylsulfonylamino group, mercapto group, alkylthio group, arylthio group, heterocyclic thio group, sulfamoyl group, sulfo group, alkyl and arylsulfinyl group, alkyl and arylsulfonyl group, acyl group, aryloxycarbonyl group, Alkoxycarbonyl group, carbamoyl group, aryl and heterocyclic azo group, imide group, phosphino group, phosphinyl group, phosphinyloxy group, phosphinylamino group, phosphono group, silyl group, hydrazino group, ureido group, boronic acid group (-B _(OH) 2), phosphato group (-OPO _{(OH) 2,} or a salt thereof), a sulfato group (-OSO ₃ H, or a salt thereof), and other known substituents. Hereinafter, these substituents are collectively referred to as “substituent R”.

R ^{1 to} R ¹⁰ are preferably a hydrogen atom, an aliphatic hydrocarbon group (more preferably an alkyl group or alkenyl group that may be substituted), an aryl group (more preferably a phenyl group that may be substituted), or a heterocyclic group. It is. More preferably, it is a hydrogen atom or an alkyl group, particularly preferably a hydrogen atom or an unsubstituted alkyl group (preferably having 1 to 10 carbon atoms, more preferably 1 to 5 carbon atoms, and particularly preferably carbon atoms. Equations 1-3).

R ¹¹ and R ¹² are preferably an aliphatic hydrocarbon group, more preferably an alkyl group or an alkenyl group that may be substituted, and still more preferably an alkyl group that may be substituted.
R ¹¹ and R ¹² are particularly preferably an alkyl group (an unsubstituted alkyl group is preferable, and the number of carbon atoms is preferably 1 to 10, more preferably 1 to 5, particularly preferably 1 to 3), or — It is a group represented by Y ¹ -Z ¹ . Here, Y ¹ represents a divalent linking group having 1 to 10 carbon atoms, and Z ¹ represents a water-soluble group.
Y ¹ is preferably a divalent linking group having 1 to 6 carbon atoms, more preferably 1 to 3 carbon atoms. The linking group is preferably an alkylene group or a group represented by —R ^1a OR ^2a —. Here, R ^1a and R ^2a each independently represent an alkylene group.
Examples of the water-soluble group of Z ¹ include a hydroxyl group, a carboxylic acid (salt) group, a sulfonic acid (salt) group, a quaternary ammonium (salt) group, and an ammonium (salt) group. These water-soluble groups may be in the form of salts. Examples of counter ions that form salts with hydroxyl groups, carboxylic acid groups, sulfonic acid groups, etc. include alkali metal ions (eg, lithium ions, sodium ions, potassium ions). Examples of counter ions that form a salt with a quaternary ammonium group or ammonium group include halide ions (fluoride ions, chloride ions, bromide ions, iodide ions, etc.). Hereinafter, these water-soluble groups and salts thereof are referred to as “water-soluble groups W”. Among these, Z ¹ is preferably a hydroxyl group, a carboxylic acid (salt) group, a sulfonic acid (salt) group, or an ammonium (salt) group.
The plurality of Y ¹ and Z ¹ may be the same or different and are preferably the same.

A combination of Y ¹ and Z ¹ in the group represented by —Y ¹ —Z ¹ is a combination in which Y ¹ is an alkylene group and Z ¹ is a hydroxyl group, Y ¹ is an alkylene group, and Z ¹ is a carboxyl group. A combination that is an acid (salt) group, a combination in which Y ¹ is an alkylene group and Z ¹ is a sulfonic acid (salt) group, and a combination in which Y ¹ is an alkylene group and Z ¹ is an ammonium (salt) group are preferable.

In General Formula (1), X ¹ and X ² each independently represent an oxygen atom, a sulfur atom, an alkylene group, or a single bond, and at least one of X ¹ and X ² is a sulfur atom. The alkylene group of X ¹ and X ² is preferably an alkylene group having 1 to 5 carbon atoms, and more preferably an alkylene group having 1 to 3 carbon atoms.
X ¹ and X ² are preferably sulfur atoms or oxygen atoms, and more preferably X ¹ and X ² are sulfur atoms.

In the general formula (1), at least one of R ^{1 to} R ⁵ is preferably a group represented by the following general formula (2).

In General Formula (2), X ³ represents an oxygen atom or a sulfur atom, and R ¹³ represents a substituent. * Represents a binding site with a benzene ring.
Examples of the substituent for R ¹³ include the substituent R described above. Preferably, it is an aliphatic hydrocarbon group (more preferably an alkyl group or alkenyl group that may be substituted), an aryl group (more preferably a phenyl group that may be substituted), or a heterocyclic group. More preferred is an alkyl group or alkenyl group which may be substituted, and further preferred is an alkyl group which may be substituted. R ¹³ is particularly preferably an alkyl group (an unsubstituted alkyl group is preferred, and the number of carbon atoms is preferably 1 to 10, more preferably 1 to 5, particularly preferably 1 to 3), or —Y ³ —. is a group represented by Z ^3.
Y ³ is synonymous with Y ¹ and represents a divalent linking group having 1 to 10 carbon atoms. Further, the preferred range is the same as Y ¹ , preferably a divalent linking group having 1 to 6 carbon atoms, more preferably 1 to 3 carbon atoms. The linking group is preferably an alkylene group or a group represented by —R ^1a OR ^2a — (R ^1a and R ^2a each independently represents an alkylene group).
Z ³ is synonymous with Z ¹ and represents a water-soluble group, and examples of the water-soluble group include the aforementioned “water-soluble group W”. Further, preferred ranges are also the same as that of Z ^1, hydroxyl group, carboxylic acid (salt) group, a sulfonic acid (salt) group, or an ammonium (salt) group are preferred.
A preferable combination of Y ³ and Z ³ is the same as the combination of Y ¹ and Z ¹ .

The compound represented by the general formula (1) of the present invention is preferably a compound in which the group represented by the general formula (2) is substituted at the para position of the benzene ring. That is, in the general formula (1), it is preferable that at least R ³ is a group represented by the general formula (2). Furthermore, at least one of R ^{6 to} R ¹⁰ is preferably a group represented by the general formula (2), and at least R ⁸ is more preferably a group represented by the general formula (2). Particularly preferred is a compound in which R ³ and R ⁸ are at least a group represented by the general formula (2).
In the case where a plurality of groups represented by the general formula (2) are contained in the general formula (1), the plurality of X ³ and R ¹³ may be the same or different, and are preferably the same.

  Specific examples of the photopolymerization initiator represented by the general formula (1) are shown below, but the present invention is not limited thereto.

The photopolymerization initiator represented by the general formula (1) can be synthesized by reacting a diaryl diketone compound and a mercapto compound (a compound having a thiol group) in a solvent. You may use a catalyst suitably for reaction.
The diaryl diketone compound and the mercapto compound are substituted with a corresponding substituent (for example, the above-described R ^{1 to} R ¹¹ , a group represented by the general formula (2), etc.) according to the structure of the target compound. A thing can be used suitably. Such a diaryl diketone compound is commercially available or can be synthesized by an ordinary synthesis method. Examples of the method for synthesizing the diaryl diketone compound include the method described in JP-A-2008-247940 and the like, or a method analogous thereto. Mercapto compounds can also be obtained commercially or synthesized by conventional synthesis methods.
Also, Tetrahedron Letters, 1977, 695. , Journal of the American Chemical Society, 1981, 103, 10, 2757. Bulletin of the Chemical Society of Japan, 1971, 44, 828. Bulletin of the Chemical Society of Japan, 1971, 44, 832. Etc., or a method analogous thereto.

In the ink composition of the present invention, the photopolymerization initiator represented by the general formula (1) may be used alone or in combination of two or more.
The content of the photopolymerization initiator represented by the general formula (1) in the ink composition is preferably 0.1 to 40% by mass, and 1 to 30% by mass with respect to the total amount of the solid components. More preferably, it is more preferably 5 to 20% by mass.
In the present specification, “total amount of solid components” or “total solid content” refers to the total mass of components excluding the solvent component from the total composition of the composition.
Moreover, it is preferable to use 0.1-30 mass parts of photoinitiators with respect to 100 mass parts of polymeric compounds, It is more preferable to use 1-20 mass parts, It is more preferable to use 5-15 mass parts. .
The photopolymerization initiator represented by the general formula (1) exhibits high sensitivity particularly to light in the ultraviolet region, and generates radicals. The generated radicals accelerate the polymerization reaction of the polymerizable compound in the ink composition.

[Sensitizing dye]
A sensitizing dye (sensitizer) absorbs light by itself and passes energy of a wavelength or wavelength that cannot be absorbed by the photopolymerization initiator alone to the photopolymerization initiator. Plays a role in promoting outbreaks. The ink composition of the present invention contains a specific sensitizing dye having an absorption peak in the wavelength region of 330 nm to 400 nm, and thereby efficiently emits light having a wavelength of around 365 to 385 nm irradiated from a UV-LED light source. Can be absorbed. In the present invention, the absorption peak of the sensitizing dye means an absorption spectrum peak seen in an acetonitrile solution.
The sensitizing dye used in the present invention is not particularly limited as long as it has an absorption peak in the wavelength range of 330 nm to 400 nm. For example, azo dyes, quinacridone dyes, anthraquinone dyes, phthalocyanine dyes, indigo dyes. , Diketopyrrolopyrrole dye, cyanine dye, merocyanine dye, benzylidene dye, coumarin dye, xanthone dye, xanthene dye, thiochromanone dye, thioxanthone dye, thioxanthene dye, anthracene dye, pyrene And dyes, thiazine dyes, azine dyes, acridine dyes, porphyrin dyes, squarylium dyes, aminostyryl dyes, thiopyrylium salt dyes, and the like.

The sensitizing dye used in the present invention preferably has an absorption peak in the region of 330 nm to 400 nm, and has substantially no absorption in the region exceeding 400 nm which is the visible region and not more than 700 nm. . Here, “substantially no absorption” means that the absorption of the sensitizing dye has little effect on the color purity due to the absorption of the color material of the ink component, specifically 0.001 mass. It means that the absorbance in a% acetonitrile solution is 0.01 or less. The absorbance of the sensitizing dye may be measured by an ordinary method / apparatus and is not limited. For example, an ultraviolet / visible / near infrared spectrophotometer (UV-3600) manufactured by Shimadzu Corporation can be used. It can be measured using the device.
More preferably, the sensitizing dye has an absorption peak in a wavelength region of 360 nm to 390 nm and substantially no absorption in a wavelength region of more than 400 nm and not more than 700 nm. By using such a sensitizing dye, it is possible to obtain an ink composition that can be exposed to light by a UV-LED light source and that does not affect the color of the ink and is excellent in storage stability. From these viewpoints, among the dyes described above, xanthone dyes, xanthene dyes, thiochromanone dyes, thioxanthone dyes, thioxanthene dyes, and anthracene dyes are preferable, and thiochromanone dyes and thioxanthone dyes are more preferable.

  The sensitizing dye used in the present invention is more preferably a sensitizing dye represented by the following general formula (I) or (II), and more preferably a sensitizing dye represented by the following general formula (I). It is.

In the general formula (I), R ^{21 to} R ²⁶ each independently represent a hydrogen atom or a substituent, and two adjacent substituents may be bonded to each other to form a ring. Examples of the substituent of R ^{21 to} R ²⁶ include the substituent R described above. R ^{21 to} R ²⁶ are preferably a hydrogen atom, a halogen atom (for example, fluorine atom, bromine atom, chlorine atom, iodine atom), amino group, alkyl group, hydroxy group, alkoxy group, aryl group, aryloxy group, hetero A cyclic group, a heterocyclic oxy group, a mercapto group, an alkylthio group, an arylthio group, and a heterocyclic thio group are preferable, and a halogen atom (preferably a fluorine atom and a chlorine atom) alkyl group, an alkoxy group, and an alkylthio group are more preferable. The number of carbon atoms in the alkyl moiety in the alkyl group, alkoxy group, and alkylthio group is preferably 1-20, more preferably 1-10, and particularly preferably 1-5. These substituents are preferably further substituted with the water-soluble group W described above.

In the general formula (I), X ²¹ represents an oxygen atom, a sulfur atom or NR ²⁷ . R ²⁷ represents a hydrogen atom or a substituent. Examples of the substituent for R ²⁷ include the substituent R described above. Preferably, a hydrogen atom, an alkyl group (preferably having 1 to 20 carbon atoms, more preferably 1 to 10 carbon atoms, particularly preferably 1 to 5 carbon atoms), an aryl group (preferably a phenyl group, an alkyl group). Substituted phenyl group, naphthyl group).
X ²¹ is preferably a sulfur atom.

  The sensitizing dye represented by the general formula (I) is preferably a sensitizing dye represented by the following general formula (I-1).

In general formula (I-1), R < ²⁸ > -R < ³⁵ > is synonymous with R < ²¹ > -R < ²⁶ > of general formula (I), each independently represents a hydrogen atom or a substituent, and 2 adjacent to R < ²⁸ > -R < ³⁵ > Two substituents may be bonded to each other to form a ring. Moreover, the preferable range of R < ²⁸ > -R < ³⁵ > is the same as that of R < ²¹ > -R < ²⁶ > of general formula (I).

  Although the preferable specific example of the sensitizing dye represented by general formula (I) or (I-1) below is shown, this invention is not limited to these.

  The sensitizing dye used in the present invention is also preferably a thiochromanone compound represented by the following general formula (II).

In formula ^{(II), R} 41 ~R ⁴⁸ independently represents a hydrogen atom or a substituent. It is preferable that at least one of R ^{41 to} R ⁴⁸ is a substituent, at least one of R ^{41 to} R ⁴⁴ is a substituent, and at least one of R ^{45 to} R ⁴⁸ is a substituent. More preferred.
Examples of the substituent include an alkyl group, a halogen atom (for example, fluorine atom, bromine atom, chlorine atom, iodine atom), hydroxy group, cyano group, nitro group, amino group, alkylthio group, alkylamino group, alkoxy group, alkoxy group. Examples thereof include a carbonyl group, an acyloxy group, an acyl group, a carboxy group, a sulfo group, and a carboxyalkyl group (hydroxycarbonylalkyl group). The number of carbon atoms of the alkyl moiety in the alkyl group, alkylthio group, alkylamino group, alkoxy group, alkoxycarbonyl group, acyloxy group, acyl group, and carboxyalkyl group is preferably 1-20, and 1-8 More preferably, it is more preferably 1-4. The acyloxy group may be an aryloxycarbonyl group, and the acyl group may be an arylcarbonyl group. In this case, the number of carbon atoms in the aryl moiety is preferably 6 to 14, and more preferably 6 to 10.
Among the above substituents, an alkyl group, a halogen atom, a hydroxy group, an alkylthio group, an alkylamino group, an alkoxy group and an acyloxy group are preferable, an alkyl group having 1 to 20 carbon atoms and a halogen atom are more preferable, and a carbon number of 1 to More preferred are an alkyl group of 4 and a halogen atom.

In R ^{41 to} R ⁴⁸ , two adjacent substituents may be bonded to each other to form a ring. Among them, in R ⁴¹ , R ⁴² , R ⁴³ and R ⁴⁴ , it is preferable that two adjacent groups are connected to each other to form a ring.
Examples of the ring structure in the case where these form a ring include 5-membered or 6-membered aliphatic rings and aromatic rings, and may be a heterocycle containing atoms other than carbon atoms. Examples of the hetero atom of the heterocyclic ring include a nitrogen atom, an oxygen atom, and a sulfur atom. Further, the formed rings may be further combined to form a ring having two or more nuclei (for example, a condensed ring). These ring structures may further have a substituent. Examples of the substituent include the substituents of R ^{41 to} R ⁴⁸ described above.

In the general formula (II), X ⁴¹ represents an oxygen atom, a sulfur atom or NR ²⁷ . R ²⁷ has the same meaning as in formula (I) and represents a hydrogen atom or a substituent, and the preferred range is also the same.
X ⁴¹ is preferably a sulfur atom.

  Preferred specific examples of the sensitizing dye represented by the general formula (II) are shown below, but the present invention is not limited thereto.

  Among the sensitizing dyes shown above, (B-2) and (B-9) are more preferable as the sensitizing dye represented by the general formula (I). Further, as the sensitizing dye represented by the general formula (II), (C-14), (C-17) and (C-19) are more preferable, and (C-14) is particularly preferable.

In the ink composition of the present invention, sensitizing dyes may be used alone or in combination of two or more.
The content of the sensitizing dye in the ink composition is preferably 0.01 to 30% by mass, more preferably 0.1 to 20% by mass, based on the total amount of the solid components. More preferably, it is 5-15 mass%.
The content ratio of the photopolymerization initiator to the sensitizing dye in the ink composition is preferably in the range of 1 to 5 to 5 to 1, and in the range of 1 to 4 to 4 to 1. More preferably, it is in the range of 1 to 3 to 3 to 1.

In the present invention, a sensitizing dye having an absorption peak in the wavelength region of 330 nm to 400 nm and a benzylthioketal compound having a specific structure represented by the general formula (1) as a photopolymerization initiator are used in combination. An ink composition exhibiting good curability is obtained with a UV-LED light source. The excellent curing characteristics of the ink composition of the present invention with respect to the UV-LED light source are presumed to be due to the following mechanism, but the present invention is not limited thereto.
As described above, conventionally used α-hydroxyacetophenone-based photopolymerization initiators and benzyl ketal-based photopolymerization initiators (see Patent Documents 1 to 4) are UV rays that irradiate light in the vicinity of 365 to 385 nm. -Polymerization efficiency was very low because polymerization could not be initiated with an LED light source or the sensitivity was poor. On the other hand, the photopolymerization initiator represented by the general formula (1) used in the present invention has high sensitivity in the wavelength region of the UV-LED light source.
In order for sensitization by the sensitizing dye to occur, the energy level (hereinafter also referred to as “T ₁ level”) of the triplet excited state (T ₁ ) of the photopolymerization initiator is changed to the T of the sensitizing dye. _Although it is essential that the level is lower than ₁ level, the conventional photopolymerization initiator as described above has a high T ₁ level and may be sensitized by a sensitizing dye that can absorb light from a UV-LED light source. could not. In contrast, the photopolymerization initiator represented by the general formula (1) has a T ₁ level higher than that of a sensitizing dye having an absorption peak in a wavelength region of 330 nm to 400 nm that can absorb light from a UV-LED light source. Since it is low, sensitization with the sensitizing dye becomes possible. Although it is speculated, it is thought that a low T ₁ level can be obtained by a structure in which either or both of X ¹ and X ² are sulfur atoms in the general formula (1).

[Polymerizable compound]
The polymerizable compound used in the ink composition of the present invention is not particularly limited as long as it is a compound that can initiate a polymerization reaction by the photopolymerization initiator represented by the general formula (1), and can be radically polymerized in the molecule. A compound having at least one ethylenically unsaturated bond can be used. The polymerizable compound may be any of a monomer, an oligomer, a polymer and the like.
The polymerizable compound is preferably a water-soluble compound from the viewpoint of ejection stability of the ink composition. The solubility of the polymerizable compound is not particularly limited, but the solubility in water at 25 ° C. is preferably 2% by mass or more, more preferably 5% by mass or more, and further preferably 10% by mass or more. It is particularly preferably 20% by mass or more, and most preferably uniformly mixed with water at an arbitrary ratio.
Specific examples of such polymerizable compounds include (meth) acrylamide compounds, (meth) acrylate compounds, vinyl compounds, maleimide compounds, vinylsulfone compounds, N-vinylamide compounds, and derivatives thereof. These compounds are more preferably bifunctional or more. More preferred are (meth) acrylamide compounds, (meth) acrylate compounds and vinyl compounds, and particularly preferred are bifunctional or higher functional (meth) acrylamide compounds. In the ink composition of the present invention, these polymerizable compounds may be used alone or in combination of two or more. When two or more types are used in combination, two or more types selected from (meth) acrylamide compounds, (meth) acrylate compounds, vinyl compounds, maleimide compounds, vinylsulfone compounds, and N-vinylamide compounds may be used in combination. Preferably, at least one of them is a (meth) acrylamide compound.
The polymerizable compound has a poly (ethyleneoxy) chain, a poly (propyleneoxy) chain, an ionic group (for example, a carboxyl group, a sulfo group, etc.), a hydroxyl group, etc. in the molecule from the viewpoint of improving water solubility. It may be.

  As the (meth) acrylamide compound, any of a monofunctional (meth) acrylamide compound and a polyfunctional (meth) acrylamide compound can be used, and a polyfunctional (meth) acrylamide compound is preferable. Specific examples of the monofunctional (meth) acrylamide compound and the polyfunctional (meth) acrylamide compound are shown below, but the present invention is not limited thereto.

Among the monomers exemplified above, monomers 21, 27, 28, 41, 43, and 48 are preferable from the viewpoint of curability and solubility.
These (meth) acrylamide compounds can be synthesized using a general method for synthesizing acrylamide compounds (for example, Journal of the American Chemical Society, 1979, 101, 5383 as a reference).

As the (meth) acrylate compound, any of a monofunctional (meth) acrylate compound (a compound having one (meth) acrylate group) and a polyfunctional (meth) acrylate compound can be used, preferably a polyfunctional (meth). It is an acrylate compound.
Specifically, as a monofunctional (meth) acrylate compound, isoamyl (meth) acrylate, stearyl (meth) acrylate, lauryl (meth) acrylate, octyl (meth) acrylate, isooctyl (meth) acrylate, decyl (meth) acrylate, Isoamylstil (meth) acrylate, isostearyl (meth) acrylate, 2-ethylhexyl diglycol (meth) acrylate, 2-hydroxybutyl (meth) acrylate, butoxyethyl (meth) acrylate, methoxydiethylene glycol (meth) acrylate, methoxypolyethylene glycol (Meth) acrylate, methoxypropylene glycol (meth) acrylate, tetrahydrofurfuryl (meth) acrylate, isobornyl (meth) acrylate 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, 2- (meth) acryloxyethyl succinic acid, 2- (meth) acryloxyethyl-2-hydroxyethylphthalic acid, lactone Examples include modified flexible (meth) acrylate, t-butylcyclohexyl (meth) acrylate, 2- (2-ethoxyethoxy) ethyl acrylate, cyclopentenyl acrylate, cyclopentenyloxyethyl acrylate, and dicyclopentanyl acrylate. Polyfunctional (meth) acrylates include bis (4-acryloxypolyethoxyphenyl) propane, neopentyl glycol di (meth) acrylate, ethoxylated (2) neopentyl glycol di (meth) acrylate (neopentyl glycol ethylene oxide 2) Compound obtained by diacrylated molar adduct), propoxylated (2) neopentyl glycol di (meth) acrylate (compound obtained by diacrylated neopentylglycol propylene oxide 2 mol adduct), 1,6-hexanediol di ( (Meth) acrylate, 1,9-nonanediol di (meth) acrylate, ethylene glycol di (meth) acrylate, diethylene glycol di (meth) acrylate, triethylene glycol di (meth) acrylate, tetrae Lenglycol di (meth) acrylate, polyethylene glycol di (meth) acrylate, dipropylene glycol di (meth) acrylate, tripropylene glycol di (meth) acrylate, tetrapropylene glycol di (meth) acrylate, polypropylene glycol di (meth) acrylate , Pentaerythritol tri (meth) acrylate, pentaerythritol tetra (meth) acrylate, dipentaerythritol tetra (meth) acrylate, trimethylolpropane tri (meth) acrylate, tetramethylol methane tetra (meth) acrylate, tetramethylol methane tri (meth) ) Acrylate, dimethylol tricyclodecane di (meth) acrylate, modified glycerin tri (meth) acrylate, modified vinyl Phenol A di (meth) acrylate, propylene oxide (PO) adduct di (meth) acrylate of bisphenol A, ethylene oxide (EO) adduct di (meth) acrylate of bisphenol A, dipentaerythritol hexa (meth) acrylate, caprolactone modified Examples include dipentaerythritol hexa (meth) acrylate.

  Specific examples of vinyl compounds include divinylbenzene, N-vinyl-ε-caprolactam, styrene, N-vinylpyrrolidone, N-vinylcarbazole, N-vinylimidazole, N-vinylformamide, vinyl alcohol, acrolein, and 2-vinylcroton. Aldehyde, 2-vinyl furan, 3-vinyl furan, methyl vinyl ether, ethyl vinyl ether, decyl vinyl ether, isobutyl vinyl ether, butyl vinyl ether, divinyl ether, phenyl vinyl ether, 3-methylphenyl ether, vinylamine, acrylonitrile, vinylbenzyl alcohol, vinylbenzylamine , Vinyl acetate, vinyl sulfonic acid, vinyl propionate, vinyl butyrate, vinyl stearate, vinyl laurate, cyclohexanecarboxylic acid Vinyl, vinyl pivalate, vinyl monochloroacetate, vinyl adipate, vinyl methacrylate and vinyl sorbate and the like.

  The content of the polymerizable compound in the ink composition is preferably 1 to 50% by mass, more preferably 1 to 40% by mass, and 1 to 30% by mass with respect to the total amount of the solid components. More preferably it is.

[Aqueous medium]
The ink composition of the present invention may contain a medium, and preferably contains an aqueous medium. The aqueous medium contains at least water, and optionally contains at least one organic solvent.
The water used for the aqueous medium is preferably water that does not contain ionic impurities such as ion-exchanged water and distilled water.
The water content in the ink composition can be appropriately selected according to the purpose, but is usually preferably 10 to 95% by mass, and preferably 30 to 90% by mass with respect to the total mass of the ink composition. % Is more preferable.

-Organic solvent-
The aqueous medium preferably contains a water-soluble organic solvent. By containing the water-soluble organic solvent, the effect of preventing drying, wetting or promoting penetration can be obtained. In order to prevent drying, the ink adheres to and drys at the ink discharge port of the ejection nozzle and is used as an anti-drying agent to prevent clogging. For drying prevention and wetting, the vapor pressure is lower than that of water. A water-soluble organic solvent is preferred. Further, for penetration promotion, it can be used as a penetration enhancer that enhances ink permeability to paper.
A water-soluble organic solvent may be used individually by 1 type, or may be used in mixture of 2 or more types.
The content of the water-soluble organic solvent in the ink composition is preferably 1 to 60% by mass and more preferably 5 to 40% by mass with respect to the total mass of the ink composition.

  Examples of water-soluble organic solvents include, for example, glycerin, 1,2,6-hexanetriol, trimethylolpropane, ethylene glycol, propylene glycol and other alkanediols (polyhydric alcohols); sugar alcohols; ethanol, methanol, C1-C4 alkyl alcohols such as butanol, propanol, isopropanol; ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, ethylene glycol monomethyl ether acetate, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol mono -N-propyl ether, ethylene glycol mono-isopropyl ether, diethylene glycol No-isopropyl ether, ethylene glycol mono-n-butyl ether, ethylene glycol mono-t-butyl ether, diethylene glycol mono-t-butyl ether, triethylene glycol monoethyl ether, 1-methyl-1-methoxybutanol, propylene glycol monomethyl ether, propylene Glycol monoethyl ether, propylene glycol mono-t-butyl ether, propylene glycol mono-n-propyl ether, propylene glycol mono-isopropyl ether, dipropylene glycol, dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether, dipropylene glycol mono N-propyl ether, dipropylene glycol mono-isopropyl ether , Glycol ethers such as tripropylene glycol monomethyl ether, and the like. These can be used individually by 1 type or in combination of 2 or more types.

  For the purpose of preventing drying and wetting, polyhydric alcohols are useful. For example, glycerin, ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, 1,3-butanediol, 2 , 3-butanediol and the like. These may be used individually by 1 type and may use 2 or more types together.

  For the purpose of promoting penetration, polyol compounds are preferred, and aliphatic diols are preferred. Examples of the aliphatic diol include 2-ethyl-2-methyl-1,3-propanediol, 3,3-dimethyl-1,2-butanediol, 2,2-diethyl-1,3-propanediol, -Ethyl-1,3-hexanediol, 2,2,4-trimethyl-1,3-pentanediol and the like. Among these, preferred examples include 2-ethyl-1,3-hexanediol and 2,2,4-trimethyl-1,3-pentanediol.

  The water-soluble organic solvent used in the present invention preferably contains at least one compound represented by the following structural formula (A) from the viewpoint of curling generation in the recording medium.

In the structural formula (A), l, m, and n each independently represent an integer of 1 or more, satisfying l + m + n = 3-15, and l + m + n is preferably in the range of 3-12, and in the range of 3-10. More preferred. When the value of l + m + n is 3 or more, a good curling suppression force is shown, and when it is 15 or less, a good discharge property is obtained. In the structural formula (A), AO represents ethyleneoxy (EO) and / or propyleneoxy (PO), and among them, a propyleneoxy group is preferable. Each AO in (AO) ₁ , (AO) _m , and (AO) _n may be the same or different.

  Specific examples of the compound represented by the structural formula (A) are shown below. However, the present invention is not limited to this. In the exemplary compounds, the description “POP (3) glyceryl ether” means glyceryl ether in which a total of three propyleneoxy groups are bonded to glycerin, and the same applies to other descriptions.

In addition, the water-soluble organic solvent used in the ink composition of the present invention is preferably a water-soluble organic solvent exemplified below from the viewpoint of curling generation in the recording medium.

_{· N-C 4 H 9 O} (AO) 4 -H ( in AO = EO or PO, the ratio of EO: PO = 1: 1)
In _{· n-C 4 H 9 O} (AO) 10 -H AO = EO or PO, the ratio of EO: PO = 1: 1)
HO (AO) ₄₀ -H (AO = EO or PO, the ratio is EO: PO = 1: 3)
HO (AO) ₅₅ -H (AO = EO or PO, the ratio is EO: PO = 5: 6)
・ HO (PO) ₃ -H
・ HO (PO) ₇ -H
・ 1,2-Hexanediol

  The content of the water-soluble organic solvent represented by the structural formula (A) and the water-soluble organic solvent exemplified above in the total water-soluble organic solvent is preferably 3% by mass or more, more preferably 4% by mass or more, 5 mass% or more is more preferable. By setting it within this range, it is preferable because curling can be suppressed without deteriorating the stability and ejection performance of the ink.

[Colorant]
The ink composition of the present invention can be a clear ink (colorless ink) composition that does not contain a colorant (coloring material) or an ink composition that contains a colorant, but contains a colorant. It is preferable.
The ink composition of the present invention (hereinafter sometimes simply referred to as “ink”) can be used not only for the formation of a single color image but also for the formation of a multicolor image (for example, a full color image). Two or more colors can be selected to form an image. When forming a full-color image, the ink composition can be used as, for example, a magenta color ink, a cyan color ink, and a yellow color ink. Further, in order to adjust the color tone, it may be used as a black color tone ink.

In addition, the ink composition of the present invention includes inks of red (R), green (G), blue (B), and white (W) colors other than yellow (Y), magenta (M), and cyan (C). It can be used as a composition or an ink composition having a special color in the so-called printing field.
The ink composition of each color tone can be prepared by changing the hue of a colorant (for example, pigment) as desired.

  In the ink composition of the present invention, known dyes, pigments and the like can be used without particular limitation as the colorant. From the viewpoint of ink colorability, a colorant that is almost insoluble or hardly soluble in water is preferable. Specifically, various pigments, disperse dyes, oil-soluble dyes, dyes forming J aggregates, and the like can be mentioned. In consideration of light resistance, pigments are more preferable.

There is no restriction | limiting in particular in the kind of pigment used for the ink composition of this invention, A normal organic and inorganic pigment can be used.
Examples of the organic pigment include azo pigments, polycyclic pigments, dye chelates, nitro pigments, nitroso pigments, and aniline black. Among these, azo pigments and polycyclic pigments are more preferable. Examples of the azo pigments include azo lakes, insoluble azo pigments, condensed azo pigments, and chelate azo pigments. Examples of the polycyclic pigment include phthalocyanine pigments, perylene pigments, perinone pigments, anthraquinone pigments, quinacridone pigments, dioxazine pigments, indigo pigments, thioindigo pigments, isoindolinone pigments, and quinophthalone pigments. Examples of the dye chelates include basic dye chelates and acidic dye chelates.

  Examples of the inorganic pigment include titanium oxide, iron oxide, calcium carbonate, barium sulfate, aluminum hydroxide, barium yellow, cadmium red, chrome yellow, and carbon black. Among these, carbon black is particularly preferable. In addition, as said carbon black, what was manufactured by well-known methods, such as a contact method, a furnace method, and a thermal method, is mentioned, for example.

  Specific examples of the pigment that can be used in the present invention include pigments described in paragraph numbers [0142] to [0145] of JP2007-100071A.

Moreover, when using a dye as a coloring component in this invention, what hold | maintained the dye to the water-insoluble support | carrier can be used as a coloring agent. As the dye, known dyes can be used without particular limitation. For example, the dyes described in JP-A-2001-115066, JP-A-2001-335714, JP-A-2002-249677 and the like can be used in the present invention. It can be used suitably. The carrier is not particularly limited as long as it is insoluble in water or hardly soluble in water, and inorganic materials, organic materials, and composite materials thereof can be used. Specifically, the carriers described in JP-A-2001-181549, JP-A-2007-169418, etc. can be suitably used in the present invention.
The carrier holding the dye (colorant) can be used as it is or in combination with a dispersant as required. As the dispersant, a dispersant described later can be suitably used.

The above colorants may be used alone or in combination of the above plural types.
The content of the colorant (particularly the pigment) in the ink composition is preferably 1 to 25% by mass with respect to the total mass of the ink composition from the viewpoint of color density, granularity, ink stability, and ejection reliability. 5 to 20% by mass is more preferable.

[Dispersant]
When the colorant used in the ink composition of the present invention is a pigment, it is preferable to form colored particles (hereinafter simply referred to as “colored particles”) dispersed in an aqueous solvent by a dispersant.
The dispersant may be a polymer dispersant or a low molecular surfactant type dispersant. The polymer dispersant may be either a water-soluble polymer dispersant or a water-insoluble polymer dispersant. In the present invention, a water-insoluble polymer dispersant is preferable from the viewpoint of dispersion stability and dischargeability when applied to an ink jet system.

-Water-insoluble polymer dispersant-
The water-insoluble polymer dispersant in the present invention (hereinafter sometimes simply referred to as “dispersant”) is a water-insoluble polymer and is not particularly limited as long as the pigment can be dispersed. A dispersant can be used. The water-insoluble polymer dispersant can be constituted by including, for example, both a hydrophobic structural unit and a hydrophilic structural unit.

Examples of the monomer constituting the hydrophobic structural unit include styrene monomers, alkyl (meth) acrylates, aromatic group-containing (meth) acrylates, and the like.
Moreover, as a monomer which comprises the said hydrophilic structural unit, if it is a monomer containing a hydrophilic group, there will be no restriction | limiting in particular. Examples of the hydrophilic group include a nonionic group, a carboxyl group, a sulfonic acid group, and a phosphoric acid group. Examples of the nonionic group include a hydroxyl group, an amide group (nitrogen atom is unsubstituted), a group derived from an alkylene oxide polymer (for example, polyethylene oxide, polypropylene oxide, etc.), a group derived from a sugar alcohol, and the like. .
From the viewpoint of dispersion stability, the hydrophilic structural unit in the present invention preferably contains at least a carboxyl group, and preferably includes a nonionic group and a carboxyl group.

Specific examples of the water-insoluble polymer dispersant in the present invention include a styrene- (meth) acrylic acid copolymer, a styrene- (meth) acrylic acid- (meth) acrylic acid ester copolymer, and a (meth) acrylic acid ester. -(Meth) acrylic acid copolymer, polyethylene glycol (meth) acrylate- (meth) acrylic acid copolymer, styrene-maleic acid copolymer and the like.
Here, “(meth) acrylic acid” means acrylic acid or methacrylic acid.

  The water-insoluble polymer dispersant used in the present invention is preferably a vinyl polymer containing a carboxyl group from the viewpoint of dispersion stability of the pigment, and a structural unit derived from at least an aromatic group-containing monomer as a hydrophobic structural unit. And a vinyl polymer having a structural unit containing a carboxyl group as a hydrophilic structural unit.

  The weight-average molecular weight of the water-insoluble polymer dispersant is preferably 3,000 to 200,000, more preferably 5,000 to 100,000, and still more preferably 5, from the viewpoint of pigment dispersion stability. 000 to 80,000, particularly preferably 10,000 to 60,000.

The content of the dispersant in the colored particles in the present invention is preferably 10 to 90% by mass of the dispersant with respect to the pigment, from the viewpoint of the dispersibility of the pigment, the ink colorability, and the dispersion stability. % By mass is more preferable, and 30 to 50% by mass is particularly preferable.
When the content of the dispersant in the colored particles is within the above range, the pigment is preferably coated with an appropriate amount of the dispersant, and it tends to be easy to obtain colored particles having a small particle size and excellent stability over time.

  The colored particles in the present invention may contain other dispersants in addition to the water-insoluble polymer dispersant. For example, a conventionally known water-soluble low-molecular dispersant, water-soluble polymer, or the like can be used. The content of the dispersant other than the water-insoluble polymer dispersant can be used within the range of the content of the dispersant.

  The colored particles in the present invention are preferably configured to contain the pigment and the water-insoluble polymer dispersant from the viewpoint of dispersion stability and dischargeability, and at least a part of the surface of the pigment is a water-insoluble polymer dispersant. It is preferable that it is configured to be coated with. Such colored particles can be obtained as a colored particle dispersion by, for example, dispersing a mixture containing a pigment, a dispersant, and, if necessary, a solvent (preferably an organic solvent) with a disperser.

  The colored particle dispersion is, for example, a step of adding an aqueous solution containing a basic substance to a mixture of the pigment, the water-insoluble polymer dispersant, and an organic solvent that dissolves or disperses the dispersant (mixing / hydration step). Then, the process (solvent removal process) except the said organic solvent can be provided, and it can manufacture as a dispersion. Thereby, the colorant is finely dispersed, and a dispersion of colored particles having excellent storage stability can be produced.

  The organic solvent needs to be able to dissolve or disperse the dispersant, but in addition to this, it is preferable that the organic solvent has a certain degree of affinity for water. Specifically, those having a solubility in water at 20 ° C. of 10 to 50% by mass or less are preferable.

More specifically, the colored particle dispersion can be produced by a production method including the following step (1) and step (2), but is not limited thereto.
Step (1): A step of dispersing a mixture containing a basic substance and a solution containing water as a main component together with a pigment, a dispersant, and an organic solvent in which the dispersant is dissolved or dispersed Step (2): Removing at least a portion of the organic solvent from the mixture after the dispersion treatment;

  In the step (1), first, the dispersant is dissolved or dispersed in an organic solvent to obtain a mixture (mixing step). Next, a solution containing a colorant and a basic substance and containing water as a main component, water, and, if necessary, a surfactant and the like are added to the mixture and mixed and dispersed to obtain an oil-in-water dispersion. Get.

  The basic substance is used for neutralization of an anionic group (preferably a carboxyl group) that the polymer may have. There is no particular limitation on the degree of neutralization of the anionic group. Usually, the liquid property of the finally obtained dispersion of colorant particles is preferably, for example, a pH of 4.5 to 10. The pH can also be determined by the desired degree of neutralization of the polymer.

  Preferable examples of the organic solvent include alcohol solvents, ketone solvents, and ether solvents. Among these, examples of alcohol solvents include ethanol, isopropanol, n-butanol, tertiary butanol, isobutanol, and diacetone alcohol. Examples of the ketone solvent include acetone, methyl ethyl ketone, diethyl ketone, and methyl isobutyl ketone. Examples of the ether solvent include dibutyl ether, tetrahydrofuran, dioxane and the like. Among these solvents, isopropanol, acetone and methyl ethyl ketone are preferable, and methyl ethyl ketone is particularly preferable. The organic solvent may be used alone or in combination.

In the production of the dispersion of the colored particles, a strong shearing force is applied using a two-roll, three-roll, ball mill, tron mill, disper, kneader, kneader, homogenizer, blender, single-screw or twin-screw extruder, etc. The kneading and dispersing process can be performed while feeding. Details of kneading and dispersion are described in “Paint Flow and Pigment Dispersion” by TC Patton (published by John Wiley and Sons, 1964).
If necessary, use a vertical or horizontal sand grinder, pin mill, slit mill, ultrasonic disperser, etc., and fine dispersion using beads made of glass having a particle diameter of 0.01 to 1 mm, zirconia, etc. It can be obtained by performing processing.

  The removal of the organic solvent in the method for producing a colored particle dispersion in the present invention is not particularly limited, and can be removed by a known method such as vacuum distillation.

The colored particles in the colored particle dispersion thus obtained maintain a good dispersion state, and the obtained colored particle dispersion has excellent temporal stability.
In the ink composition of the present invention, the colored particles may be used singly or in combination of two or more.

In the present invention, the volume average particle diameter of the colorant (or colored particles) is preferably 10 to 200 nm, more preferably 10 to 150 nm, and still more preferably 10 to 100 nm. When the volume average particle size is 200 nm or less, the color reproducibility is good, and in the case of the ink jet method, the droplet ejection characteristics are good. Moreover, light resistance becomes favorable because a volume average particle diameter is 10 nm or more.
Moreover, there is no restriction | limiting in particular regarding the particle size distribution of a coloring agent (or colored particle | grains), Any of a wide particle size distribution or a monodispersed particle size distribution may be sufficient. Further, two or more colorants having a monodispersed particle size distribution may be mixed and used.
The volume average particle size and particle size distribution of the colorant (or colored particles) can be measured using, for example, a light scattering method.

[Other polymerization initiators]
The ink composition of the present invention may contain an arbitrary amount of a polymerization initiator other than the polymerization initiator represented by the general formula (1) (hereinafter referred to as “other polymerization initiator”).
As another polymerization initiator, a radical polymerization initiator is preferable. Specific examples of the radical polymerization initiator include (a) aromatic ketones, (b) acylphosphine compounds, (c) aromatic onium salt compounds, (d) organic oxides, (e) thio compounds, ( f) hexaarylbiimidazole compound, (g) ketoxime ester compound, (h) borate compound, (i) azinium compound, (j) metallocene compound, (k) active ester compound, (l) compound having carbon halogen bond And (m) alkylamine compounds and the like. More specifically, polymerization initiators described in pages 65 to 148 of “Ultraviolet curing system” written by Kiyosuke Kato (published by General Technology Center Co., Ltd .: 1989) can be given.

The other polymerization initiator may be either a water-insoluble initiator dispersed in water or a water-soluble initiator, but is preferably a water-soluble polymerization initiator. The term “water-soluble” as used herein means that 0.5% by mass or more dissolves in water at 25 ° C. The water-soluble initiator is preferably dissolved in water at 1% by mass or more at 25 ° C., more preferably 3% by mass or more.
Another polymerization initiator may be used individually by 1 type, and may use 2 or more types together.

[Resin particles]
The ink composition of the present invention preferably contains resin particles. By including the resin particles, it is possible to effectively improve the fixability of the ink composition to the recording medium, the abrasion resistance of the image, and the blocking resistance.
In addition, the resin particles have a function of fixing the ink composition, that is, the image by aggregating or destabilizing the ink and increasing the viscosity of the ink when contacted with a treatment liquid described later or a paper region where the resin liquid is dried. It is preferable to have. Such resin particles are preferably dispersed in at least one of water and an organic solvent.

  The resin particles used in the present invention include acrylic resins, vinyl acetate resins, styrene-butadiene resins, vinyl chloride resins, acrylic-styrene resins, butadiene resins, styrene resins, crosslinked acrylic resins, and crosslinked styrene resins. Resins, benzoguanamine resins, phenol resins, silicone resins, epoxy resins, urethane resins, paraffin resins, fluorine resins, etc., or latex thereof can be used. Preferred examples include acrylic resins, acrylic-styrene resins, styrene resins, cross-linked acrylic resins, and cross-linked styrene resins. The resin particles can also be used in the form of latex.

In the ink composition of the present invention, the resin particles may be used alone or in combination of two or more.
The weight average molecular weight of the resin particles is preferably 10,000 or more and 200,000 or less, more preferably 100,000 or more and 200,000 or less.
The average particle size of the resin particles is preferably in the range of 10 nm to 1 μm, more preferably in the range of 10 to 200 nm, still more preferably in the range of 20 to 100 nm, and particularly preferably in the range of 20 to 50 nm.
The glass transition temperature Tg of the resin particles is preferably 30 ° C. or higher, more preferably 40 ° C. or higher, and further preferably 50 ° C. or higher.

The content of the resin particles in the ink composition is preferably 0.1 to 20% by mass, more preferably 0.1 to 20% by mass, and 0.1 to 15% by mass with respect to the total mass of the ink composition. Is more preferable.
Moreover, there is no restriction | limiting in particular regarding the particle size distribution of a resin microparticle, What has a wide particle size distribution or a thing with a monodispersed particle size distribution may be sufficient.

[Surfactant]
The ink composition of the present invention may contain a surfactant as necessary. The surfactant can be used as a surface tension adjusting agent.
As a surface tension modifier, a compound having a structure having both a hydrophilic part and a hydrophobic part in the molecule can be used effectively. Anionic surfactant, cationic surfactant, amphoteric surfactant, nonionic surfactant Either a surfactant or a betaine surfactant can be used. Further, the above dispersant (polymer dispersant) may be used as a surfactant.
In the present invention, a nonionic surfactant is preferable from the viewpoint of suppression of ink droplet ejection interference, and an acetylene glycol derivative is more preferable.

When the ink composition of the present invention is used in an ink jet recording system, a surfactant (surface tension adjusting agent) is used so that the surface tension of the ink composition is 20 to 60 mN / m from the viewpoint of favorably discharging the ink composition. The amount is preferably adjusted to 20 to 45 mN / m, more preferably 25 to 40 mN / m.
The content of the surfactant in the ink composition is not particularly limited except that the surface tension is preferably within the range, and is preferably 1% by mass or more, more preferably 1%, based on the total mass of the ink composition. It is 10 mass%, More preferably, it is 1-3 mass%.

[Other ingredients]
In addition to the above components, the ink composition may further contain various additives as other components as necessary.
Examples of the various additives include, for example, ultraviolet absorbers, antifading agents, antifungal agents, pH adjusters, rust inhibitors, antioxidants, emulsion stabilizers, preservatives, antifoaming agents, viscosity modifiers, and dispersions. Known additives such as stabilizers, chelating agents, solid wetting agents and the like can be mentioned.

  Examples of the UV absorber include benzophenone UV absorbers, benzotriazole UV absorbers, salicylate UV absorbers, cyanoacrylate UV absorbers, nickel complex salt UV absorbers, and the like.

  As the anti-fading agent, various organic and metal complex anti-fading agents can be used. Examples of organic anti-fading agents include hydroquinones, alkoxyphenols, dialkoxyphenols, phenols, anilines, amines, indanes, chromans, alkoxyanilines, heterocycles, and metal complexes. There are nickel complex, zinc complex and the like.

Examples of the antifungal agent include, for example, sodium dehydroacetate, sodium benzoate, sodium pyridinethione-1-oxide, ethyl p-hydroxybenzoate, 1,2-benzisothiazolin-3-one, sodium sorbate, pentachlorophenol Sodium etc. are mentioned.
The content of the antifungal agent in the ink composition is preferably in the range of 0.02 to 1.00% by mass with respect to the total mass of the ink composition.

  The pH adjuster is not particularly limited as long as it can adjust the pH to a desired value without adversely affecting the ink composition to be prepared, and can be appropriately selected according to the purpose. For example, alcohol amines (for example, diethanolamine, triethanolamine, 2-amino-2-ethyl-1,3-propanediol, etc.), alkali metal hydroxides (for example, lithium hydroxide, sodium hydroxide, potassium hydroxide) Etc.), ammonium hydroxide (for example, ammonium hydroxide, quaternary ammonium hydroxide, etc.), phosphonium hydroxide, alkali metal carbonate and the like.

  Examples of the rust inhibitor include acidic sulfite, sodium thiosulfate, ammonium thiodiglycolate, diisopropylammonium nitrite, pentaerythritol tetranitrate, and dicyclohexylammonium nitrite.

  Examples of the antioxidant include phenol antioxidants (including hindered phenol antioxidants), amine antioxidants, sulfur antioxidants, phosphorus antioxidants, and the like.

  Examples of the chelating agent include sodium ethylenediaminetetraacetate, sodium nitrilotriacetate, sodium hydroxyethylethylenediaminetriacetate, sodium diethylenetriaminepentaacetate, sodium uramil diacetate and the like.

[Physical properties of ink composition]
The surface tension (25 ° C.) of the ink composition of the present invention is preferably 20 mN / m or more and 60 mN / m or less. More preferably, it is 20 mN or more and 45 mN / m or less, More preferably, it is 25 mN / m or more and 40 mN / m or less.
The surface tension is measured under conditions of 25 ° C. using an Automatic Surface Tensiometer CBVP-Z (manufactured by Kyowa Interface Science Co., Ltd.).

The viscosity at 25 ° C. of the ink composition of the present invention is preferably 1.2 mPa · s or more and 15.0 mPa · s or less, more preferably 2 mPa · s or more and less than 13 mPa · s, and still more preferably. Is 2.5 mPa · s or more and less than 10 mPa · s.
The viscosity is measured using a VISCOMETER TV-22 (manufactured by TOKI SANGYO CO. LTD) under conditions of 25 ° C.

  The pH of the ink composition is preferably pH 6 to 11 from the viewpoint of the stability of the ink composition. When the ink set described later is used, it is preferable that the ink composition aggregates at a high speed by contact with the treatment liquid, and thus pH 7 to 10 is more preferable, and pH 7 to 9 is further preferable.

<Ink set>
The ink set of the present invention includes the above-described ink composition of the present invention and a treatment liquid containing a flocculant capable of forming an aggregate upon contact with the ink composition.
By forming an image using an ink composition containing a polymerizable compound and the polymerization initiator of the present invention and a processing liquid containing an aggregating agent, the image has good image quality, high curing sensitivity, and high blocking resistance. An excellent image can be formed.
Hereinafter, the treatment liquid of the ink set will be described.

[Treatment solution]
The treatment liquid of the ink set includes at least an aggregating agent that aggregates the components in the ink composition, and includes other components as necessary.
The flocculant used in the treatment liquid is capable of aggregating (fixing) the ink composition by contacting the ink composition on the recording medium, and functions as a fixing agent. For example, when the treatment liquid is applied to a recording medium (preferably coated paper), the ink composition further drops and comes into contact with the aggregating agent in the state where the aggregating agent is present on the recording medium. In addition, the components in the ink composition can be aggregated to fix the ink composition on the recording medium.
Examples of the flocculant include acidic compounds, polyvalent metal salts, and cationic polymers. These can be used alone or in combination of two or more.

[Acid compounds]
Examples of acidic compounds include sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, polyacrylic acid, acetic acid, glycolic acid, malonic acid, malic acid, maleic acid, ascorbic acid, succinic acid, glutaric acid, fumaric acid, citric acid, and tartaric acid. , Lactic acid, sulfonic acid, orthophosphoric acid, metaphosphoric acid, pyrrolidone carboxylic acid, pyrone carboxylic acid, pyrrole carboxylic acid, furan carboxylic acid, pyridine carboxylic acid, coumaric acid, thiophene carboxylic acid, nicotinic acid, and derivatives of these compounds, and These salts etc. are mentioned suitably.

Among these, acidic compounds with high water solubility are preferable. Further, from the viewpoint of fixing the whole ink by reacting with the ink composition, an acidic compound having a valence of 3 or less is preferable, and an acidic compound having a valence of 2 to 3 is particularly preferable.
An acidic compound may be used individually by 1 type, and may use 2 or more types together.

  When the treatment liquid contains an acidic compound, the pH (25 ° C.) of the treatment liquid is preferably 0.1 to 6.8, more preferably 0.5 to 6.0, 0.8 More preferably, it is -5.0.

  The content of the acidic compound is preferably 40% by mass or less, more preferably 15 to 40% by mass, and 15 to 35% by mass with respect to the total mass of the treatment liquid. More preferably, it is particularly preferably 20% by mass to 30% by mass. By setting the content of the acidic compound to 15 to 40% by mass, the components in the ink composition can be more efficiently fixed.

The amount of the acidic compound applied to the recording medium is not particularly limited as long as it is an amount sufficient to aggregate the ink composition, but 0.5 g / m ² to 4 from the viewpoint that the ink composition can be easily fixed. is preferably .0g / ^{m 2,} more preferably 0.9g / ^{m 2} ~3.75g / m 2

[Multivalent metal salt]
The polyvalent metal salt is a compound containing a divalent or higher metal such as an alkaline earth metal or a zinc group metal, and examples thereof include acetates and oxides of metal ions such as Ca ²⁺ , Cu ²⁺ and Al ^3+. it can.
In the present invention, the aggregation reaction of the ink composition when the ink composition is ejected onto a recording medium (preferably coated paper) to which the treatment liquid containing the polyvalent metal salt is applied is dispersed in the ink composition. This can be achieved by reducing the dispersion stability of the particles, for example, colorants typified by pigments and resin particles, and increasing the viscosity of the entire ink composition. For example, when particles such as pigments and resin particles in the ink composition have weakly acidic functional groups such as carboxyl groups, the particles are dispersed and stabilized by the action of the weakly acidic functional groups. Can be reduced by interacting with the polyvalent metal salt to lower the dispersion stability. Therefore, the polyvalent metal salt as the immobilizing agent contained in the treatment liquid needs to have a valence of 2 or more, that is, multivalent from the viewpoint of the aggregation reaction, and from the viewpoint of the aggregation reactivity, 3 A polyvalent metal salt composed of a polyvalent metal ion having a valence higher than that is preferable.

  From the above viewpoint, the polyvalent metal salt that can be used in the treatment liquid is at least one of the following salts of polyvalent metal ions and anions, polyaluminum hydroxide, and polyaluminum chloride. preferable.

Examples of the polyvalent metal ions include Ca ²⁺ , Cu ²⁺ , Ni ²⁺ , Mg ²⁺ , Sr ²⁺ , Zn ²⁺ , Ba ²⁺ , Al ³⁺ , Fe ³⁺ , Cr ³⁺ , Co ³⁺ , Fe ²⁺ , La ³⁺ , Nd ³⁺ , Y ³⁺ , Zr ^{4+ and the} like. In order to contain these polyvalent metal ions in the treatment liquid, the polyvalent metal salt may be used.

  The salt is a metal salt composed of the above polyvalent metal ions and an anion that binds to these ions, but is preferably soluble in a solvent. Here, the solvent is a medium that constitutes the treatment liquid together with the polyvalent metal salt, and examples thereof include water and the aforementioned water-soluble organic solvent.

Preferred anions for forming a salt with the polyvalent metal ion include, for example, Cl ⁻ , NO ₃ ⁻ , I ⁻ , Br ⁻ , ClO ₃ ⁻ , CH ₃ COO ⁻ , SO ₄ ^{2− and the} like. .
A polyvalent metal ion and an anion can form a salt of a polyvalent metal ion and an anion, respectively, using a single species or a plurality of species.

  Examples of other polyvalent metal salts include polyaluminum hydroxide and polyaluminum chloride.

In the present invention, it is preferable to use a salt of a polyvalent metal ion and an anion from the viewpoints of reactivity, colorability, and ease of handling, and examples of the polyvalent metal ion include Ca ²⁺ and Mg ^2+. , Sr ²⁺ , Al ³⁺ and Y ³⁺ are preferable, and Ca ²⁺ is more preferable.
Further, as the anion, NO ₃ ⁻ is particularly preferable from the viewpoint of solubility and the like.
The said polyvalent metal salt can be used individually by 1 type or in mixture of 2 or more types.

  Content of the said polyvalent metal salt is 15 mass% or more with respect to the total mass of the said process liquid. By setting the content of the polyvalent metal salt to 15% by mass or more, the components in the ink composition can be more effectively fixed. More preferably, they are 15 mass%-35 mass% with respect to the total mass of a process liquid, and it is still more preferable that they are 20 mass%-30 mass%.

The amount of the polyvalent metal salt applied to the recording medium is not particularly limited as long as it is an amount sufficient to aggregate the ink composition, but 0.5 g / m ² from the viewpoint that the ink composition can be easily fixed. is preferably to 4.0 g / ^{m 2,} more preferably 0.9g / ^{m 2} ~3.75g / m 2

[Cationic polymer]
The cationic polymer is at least one cationic polymer selected from poly (vinyl pyridine) salt, polyalkylaminoethyl acrylate, polyalkylaminoethyl methacrylate, poly (vinyl imidazole), polyethyleneimine, polybiguanide, and polyguanide. is there.
A cationic polymer may be used individually by 1 type, and may use 2 or more types together.
Among the cationic polymers, polyguanides (preferably poly (hexamethyleneguanidine) acetate, polymonoguanide, polymeric biguanide), polyethyleneimine, and poly (vinylpyridine), which are advantageous from the viewpoint of aggregation rate, are preferable.

  The weight average molecular weight of the cationic polymer is preferably smaller in terms of the viscosity of the treatment liquid. When the treatment liquid is applied to the recording medium by an inkjet method, a range of 500 to 500,000 is preferable, a range of 700 to 200,000 is more preferable, and a range of 1,000 to 100,000 is more preferable. . When the weight average molecular weight is 500 or more, it is advantageous from the viewpoint of aggregation speed, and when it is 500,000 or less, it is advantageous from the viewpoint of ejection reliability. However, this is not the case when the treatment liquid is applied to the recording medium by a method other than inkjet.

  The treatment liquid contains a cationic polymer, but the pH (25 ° C.) of the treatment liquid is preferably 1.0 to 10.0, more preferably 2.0 to 9.0. More preferably, it is 0-7.0.

The content of the cationic polymer is preferably 1% by mass to 35% by mass and more preferably 5% by mass to 25% by mass with respect to the total mass of the treatment liquid.
The amount of the cationic polymer applied to the coated paper is not particularly limited as long as it is an amount sufficient to stabilize the ink composition, but 0.5 g / m from the viewpoint that the ink composition is easily fixed. it is preferably ^{2 ~4.0g} / ^{m 2,} more preferably ^{0.9g / m 2 ~3.75g / m 2} .

<Image forming method>
The image forming method of the present invention includes a treatment liquid application step for applying a treatment liquid contained in the ink set onto a recording medium, and an ink composition contained in the ink set on the recording medium to form an image. An ink application process to be formed, and further includes other processes as necessary.

[recoding media]
The recording medium used in the image forming method of the present invention is not particularly limited, but general printing paper mainly composed of cellulose, such as so-called high-quality paper, coated paper, and art paper, used for general offset printing or the like is used. Can do. General printing paper mainly composed of cellulose is relatively slow in ink absorption and drying in image recording by a general ink jet method using water-based ink, and color material movement is likely to occur after droplet ejection, and image quality is likely to deteriorate. However, according to the ink jet recording method of the present invention, it is possible to record a high-quality image excellent in color density and hue by suppressing the movement of the color material.

  As the recording medium, commercially available media can be used. For example, “OK Prince fine quality” manufactured by Oji Paper Co., Ltd., “Shiorai” manufactured by Nippon Paper Industries Co., Ltd., and Nippon Paper Industries ( High-quality paper (A) such as “New NPI fine quality” manufactured by Nippon Paper Industries Co., Ltd., high-quality coated paper such as “Silver Diamond” manufactured by Nippon Paper Industries Co., Ltd., “OK Everlight Coat” manufactured by Oji Paper Co., Ltd. and Nippon Paper Industries Light coated paper (A3), such as “Aurora S” manufactured by Co., Ltd., “OK Coat L” manufactured by Oji Paper Co., Ltd., and “Aurora L” manufactured by Nippon Paper Industries Co., Ltd., Oji Coated paper (A2, B2) such as “OK Top Coat +” manufactured by Paper Industries Co., Ltd. and “Aurora Coat” manufactured by Nippon Paper Industries Co., Ltd. “OK Kanto +” manufactured by Oji Paper Co., Ltd. and Mitsubishi Paper Industries ( Art paper (A1) such as "Tokuhishi Art" made by Nippon Paper Industries Raito, and the like. It is also possible to use various photographic papers for ink jet recording.

Among these, from the viewpoint of obtaining a high-quality image having a large color material movement suppression effect and better color density and hue than before, the water absorption coefficient Ka is preferably 0.05 to 0.5. a recording medium mL ^/ m 2 ^{· ms 1/2,} more preferably recording medium 0.1~0.4mL ^/ m 2 ^{· ms 1/2,} more preferably 0.2-0.3 ml / It is a recording medium of m ² · ms ^1/2 .

  The water absorption coefficient Ka is synonymous with that described in JAPAN TAPPI paper pulp test method No. 51: 2000 (issued by Japan Paper Pulp Technology Association). Specifically, the absorption coefficient Ka is an automatic scanning absorption meter. It is calculated from the difference in the amount of water transferred between the contact time of 100 ms and the contact time of 900 ms using KM500Win (manufactured by Kumagai Riki Co., Ltd.).

  Among the recording media, so-called coated paper used for general offset printing is preferable. The coated paper is obtained by applying a coating material to the surface of high-quality paper, neutral paper, or the like that is mainly surface-treated with cellulose as a main component and is not surface-treated. The coated paper tends to cause quality problems such as image gloss and scratch resistance in image formation by ordinary aqueous inkjet, but in the inkjet recording method of the present invention, gloss unevenness is suppressed and gloss and resistance are reduced. An image having good rubbing properties can be obtained. In particular, it is preferable to use a coated paper having a base paper and a coat layer containing kaolin and / or calcium bicarbonate. More specifically, art paper, coated paper, lightweight coated paper, or finely coated paper is more preferable.

[Processing liquid application process]
In the treatment liquid application step, a treatment liquid containing a flocculant contained in the ink set is applied on the recording medium. For applying the treatment liquid to the recording medium, a known liquid application method can be used without particular limitation, and any method such as spray application, application using an application roller, application by an inkjet method, immersion, etc. can be selected. .

  Specifically, for example, a size press method represented by a horizontal size press method, a roll coater method, a calendar size press method, etc .; a size press method represented by an air knife coater method, etc .; Knife coater method; transfer roll coater method such as gate roll coater method, direct roll coater method, reverse roll coater method, roll coater method represented by squeeze roll coater method; bill blade coater method, short duel coater method; Blade coater method typified by stream coater method, etc .; Bar coater method typified by rod bar coater method, etc .; Bar coater method typified by rod bar coater method, etc .; Cast coater method; Gravure coater method; Coater method; die coater method; brush coater method; and the like transfer method.

  Moreover, the method of apply | coating by controlling a coating amount by using the coating device provided with the liquid quantity limiting member like the coating device of Unexamined-Japanese-Patent No. 10-230201 may be used.

  The region to which the treatment liquid is applied may be the entire surface application to be applied to the entire recording medium or may be the partial application to be partially applied to the region to which ink is applied in the ink application process. In the present invention, the application amount of the treatment liquid is uniformly adjusted, and fine lines and fine image portions are uniformly recorded, and from the viewpoint of suppressing density unevenness such as image unevenness, coating is performed by application using an application roller or the like. It is preferable to apply the entire surface to the entire paper.

  Examples of the method of applying the treatment liquid by controlling the application amount of the treatment liquid within the above range include a method using an anilox roller. An anilox roller is a roller with a ceramic-sprayed roller surface processed with a laser to give it a pyramid shape, a diagonal line, a turtle shell shape, or the like. The treatment liquid enters the dent portion formed on the roller surface, is transferred when it comes into contact with the paper surface, and is applied in an application amount controlled by the dent of the anilox roller.

[Ink application process]
In the ink application process, the ink composition contained in the ink set is applied onto the recording medium. The ink composition application method is not particularly limited as long as the ink composition can be applied in a desired image-like manner, and a known ink application method can be used. For example, a method of applying an ink composition onto a recording medium by means of an ink jet method, a copying method, a printing method, or the like can be given. Among these, the step of applying the ink composition by an ink jet method is preferable from the viewpoint of compactness of the recording apparatus and high-speed recording properties.

[Inkjet method]
In image formation by an ink jet method, by supplying energy, an ink composition is discharged onto a recording medium to form a colored image. As a preferable ink jet recording method for the present invention, the method described in paragraph Nos. 0093 to 0105 of JP-A No. 2003-306623 can be applied.

The inkjet method is not particularly limited, and is a known method, for example, a charge control method that discharges ink using electrostatic attraction, a drop-on-demand method (pressure pulse method) that uses vibration pressure of a piezoelectric element, Any of an acoustic ink jet method in which an electric signal is converted into an acoustic beam, irradiated onto ink, and ink is ejected using radiation pressure may be used.
The ink jet head used in the ink jet method may be an on-demand method or a continuous method. Furthermore, there are no particular limitations on the ink nozzles used when recording by the ink jet method, and they can be appropriately selected according to the purpose.
The ink jet method includes a method of ejecting a large number of low-density inks called photo inks in a small volume, a method of improving image quality using a plurality of inks having substantially the same hue and different concentrations, and colorless and transparent inks. The method using is included.

  Also, as an ink jet system, a short serial head is used, a shuttle system that performs recording while scanning the head in the width direction of the recording medium, and a line head in which recording elements are arranged corresponding to the entire area of one side of the recording medium There is a line system using. In the line method, an image can be recorded on the entire surface of the recording medium by scanning the recording medium in a direction orthogonal to the arrangement direction of the recording elements, and a carriage system such as a carriage for scanning a short head is not necessary. Further, since complicated scanning control of the carriage movement and the recording medium is not required, and only the recording medium is moved, the recording speed can be increased as compared with the shuttle system.

  In the present invention, the order in which the treatment liquid application step and the ink application step are performed is not particularly limited, but from the viewpoint of image quality, it is preferable that the ink application step is performed after the treatment liquid application step. That is, the ink application step is preferably a step of applying the ink composition onto the recording medium to which the treatment liquid has been applied.

[Active energy ray irradiation process]
The image forming method of the present invention preferably includes a step of irradiating the ink composition applied on the recording medium with active energy rays. By irradiating active energy rays, the polymerizable compound contained in the ink composition is polymerized to form a cured film containing a colorant. Thereby, the abrasion resistance and blocking resistance of the image are more effectively improved.

The ink composition applied on the recording medium is cured by irradiation with active energy rays. This is because the polymerization initiator of the present invention contained in the ink composition is decomposed by irradiation with active energy rays to generate radicals, whereby the polymerization reaction of the polymerizable compound is initiated and accelerated, and the ink composition is cured. It is to do.
When an acidic compound is contained in the processing solution, the ink composition is further aggregated (fixed) by the acid supplied from the compound during irradiation with the active energy ray, and the image quality (rubbing resistance / blocking resistance) is increased. Improved).

In the present invention, α-rays, γ-rays, electron beams, X-rays, ultraviolet rays, visible light, infrared light, and the like can be used as active energy rays. As described above, the photopolymerization initiator represented by the general formula (1) has a particularly high absorption with respect to light in the ultraviolet region. 600 nm is preferable, 300 to 450 nm is more preferable, and 350 to 420 nm is still more preferable.
The output of the actinic energy ray is preferably at 5000 mJ / cm ² or less, more preferably 10~4000mJ / cm ^2, further preferably 20~3000mJ / cm ^2.

Mercury lamps and gas / solid lasers are mainly used as active energy ray sources, and mercury lamps and metal halide lamps are widely known as light sources used for curing UV photocurable ink jet recording inks. ing. However, from the viewpoint of environmental protection, mercury-free is strongly desired, and replacement with a GaN-based semiconductor ultraviolet light-emitting device is very useful industrially and environmentally. In addition, LEDs and LDs are small, have a long life, have high efficiency, and are low in cost, and are expected as light sources for photocurable ink jets.
In the present invention, a light emitting diode (LED) and a laser diode (LD) can be used as an active energy ray source. In particular, an ultraviolet LED (UV-LED) and an ultraviolet LD (UV-LD) can be used as an ultraviolet ray source. For example, Nichia Corporation has introduced a purple LED whose main emission spectrum has a wavelength between 365 nm and 420 nm.
A particularly preferable active energy ray source in the present invention is a UV-LED, and a UV-LED having a peak wavelength of 350 to 420 nm is particularly preferable.

[Ink drying process]
The image forming method of the present invention may include an ink drying step for drying and removing a solvent (for example, water, the above-described aqueous medium) in the ink composition applied onto the recording medium, if necessary. Good. The ink drying step is not particularly limited as long as at least a part of the ink solvent can be removed, and a commonly used method can be applied.

  EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. Unless otherwise specified, “part” and “%” are based on mass.

Example I.1. Preparation of clear ink composition and evaluation of sensitivity Polymerizable compounds M-1 to M-6 shown below were synthesized.

[Synthesis of Polymerizable Compound M-1]
Ethylenediamine 40.0 g (666 mmol), sodium hydroxide 90.0 g (2250 mmol), water 200 g, and acetonitrile 600 mL were added to a 1 L three-necked flask equipped with a stirrer, and 132.6 g (1470 mmol) of acrylic acid chloride in an ice bath. Was added dropwise over 1 hour. After dropping, the mixture was stirred at room temperature for 5 hours, and then acetonitrile was distilled off from the resulting reaction mixture under reduced pressure. Next, the aqueous layer was extracted three times with 400 ml of chloroform, and the obtained organic layer was dried over magnesium sulfate, filtered, and the solvent was distilled off under reduced pressure to obtain 45 white solids of the target polymerizable compound M-1. 0.0 g (266 mmol, 40% yield) was obtained.

[Synthesis of Polymerizable Compound M-2]
To a 1 L three-necked flask equipped with a stirrer was added 40.0 g (182 mmol) of 4,7,10-trioxa-1,13-tridecanediamine, 37.8 g (450 mmol) of sodium bicarbonate, 100 g of water, and 200 g of tetrahydrofuran. In an ice bath, 35.2 g (389 mmol) of acrylic acid chloride was added dropwise over 20 minutes. After dropping, the mixture was stirred at room temperature for 5 hours, and then tetrahydrofuran was distilled off from the obtained reaction mixture under reduced pressure. Next, the aqueous layer was extracted four times with 200 ml of ethyl acetate, and the obtained organic layer was dried over magnesium sulfate, filtered, and the solvent was distilled off under reduced pressure to obtain 35 of the desired polymerizable compound M-2 solid. 0.0 g (107 mmol, 59% yield) was obtained.

[Synthesis of Polymerizable Compound M-3]
In the synthesis of the polymerizable compound 3, a polymerizable compound M-3 was synthesized in the same manner as the synthesis of the polymerizable compound M-1, except that equimolar amount of diethylenetriamine was used instead of ethylenediamine.

[Polymerizable compounds M-4 to M-6]
Polymeric compounds M-4 and M-5 were purchased from Tokyo Chemical Industry Co., Ltd., and polymerizable compound M-6 was purchased from Wako Pure Chemical Industries, Ltd.

  Specific examples of the photopolymerization initiator represented by the general formula (1) include the following compounds A-4, A-7, A-11, A-18, A-19, A-26, A-29, A- 30, A-31 were synthesized by the following method.

[Synthesis of Water-Soluble Photopolymerization Initiator A-4]
As shown in Scheme 1 below, Chemische Berichte, 1935, 68, 1588. Compound A-4 was synthesized with reference to the method described in Tetrahedron, 1970, 26, 1747.
1.0 g (3.70 mmol) of p-anisyl and 1.5 ml (22.20 mmol) of thioglycolic acid are stirred in 6 ml of toluene, and 0.42 g (2.22 mmol) of p-toluenesulfonic acid monohydrate is added. And heated at an internal temperature of 90 ° C. for 5 hours. The mixture was allowed to cool to room temperature, and the precipitate was suction filtered and washed with pure water. This was vacuum dried to obtain 0.83 g (1.90 mmol, yield 51%) of a carboxylic acid form. This carboxylic acid compound was stirred in 6 ml of pure water, and 76 mg (1.90 mmol) of sodium hydroxide was added. After stirring at room temperature for 15 minutes, 0.91 g of compound A-4 was obtained by distilling off pure water with an evaporator.

[Synthesis of Water-Soluble Photopolymerization Initiator A-7]
Compound A-7 was synthesized in the same manner except that the thioglycolic acid in the synthesis of Compound A-4 was changed to equimolar sodium 2-mercaptoethanesulfonate.

[Synthesis of Water-Soluble Photopolymerization Initiator A-11]
As shown in Scheme 2 below, after synthesizing a diaryl diketone with reference to the methods described in Journal of Medicinal Chemistry, 1996, 39, 3418, and Japanese Patent Application Laid-Open No. 2008-247940, a compound using 2-mercaptoethanol is synthesized. A-11 was synthesized.

[Water-soluble photopolymerization initiator A-18]
As shown in the following scheme 3, a diaryl diketone is synthesized from 2- (phenylthio) ethanol and oxalyl chloride by the method described in JP-A-2008-247940, and using this, a compound is synthesized in the same manner as the synthesis of compound A-4. A-18 was synthesized.

[Water-soluble photopolymerization initiator A-19]
A diaryl diketone was synthesized in the same manner as Compound A-18, and using this, Compound A-19 was synthesized in the same manner as Compound A-7.

[Synthesis of water-soluble photopolymerization initiator A-26]
As shown in Scheme 4 below, dimethylphenoxyethanol was synthesized from 2,6-dimethylphenol and 2-bromoethanol with reference to the method described in Journal of Medicinal Chemistry, 1996, 39, 3418, and then compound A- A diaryl diketone was synthesized in the same manner as in No.18. Using the obtained diaryl diketone as a starting material, A-26 was synthesized in the same manner as the synthesis of A-4, except that the thioglycolic acid in the synthesis of A-4 was changed to an equimolar amount of 3-mercaptopropionic acid.

[Synthesis of Water-Soluble Photopolymerization Initiator A-29]
Tetrahedron Letters, 1977, 695. Or Journal of the American Chemical Society, 1981, 103, 10, 2757. The photopolymerization initiator A-29 was synthesized by reacting arylbutane-1,3-dione with α-chlorosulfenyl-α-chloro-1,2-diarylethanone with reference to the method described in 1. . α-Chlorosulfenyl-α-chloro-1,2-diarylethanone was synthesized by reacting 1,2-diarylethanone with thionyl chloride.

[Synthesis of Water-Soluble Photopolymerization Initiator A-30, 31]
Bulletin of the Chemical Society of Japan, 1971, 44, 828. , And Bulletin of the Chemical Society of Japan, 1971, 44, 832. The photopolymerization initiator A-30 is hydrogenated with 1,2-diaryl-2-mercaptoethanone by the reaction of 2-chloro-1,2-diarylethanone with a mercapto compound with reference to the method described in 1. Photoinitiator A-31 was each synthesize | combined by reaction of sodium and methyl iodide.

The following dyes B-2, B-6, B-9, B-21, B-27, and C-1 were used as sensitizing dyes.
These sensitizing dyes had an absorption peak in the wavelength region of 330 nm to 400 nm and substantially no absorption (absorbance of 0.01 or less) in the region exceeding 400 nm and 700 nm or less.

As sensitizing dyes B-2 and B-21, those manufactured by Tokyo Chemical Industry Co., Ltd. were used.
Sensitizing dyes B-6, B-9, and B-27 were synthesized by the following method.
The sensitizing dye C-1 manufactured by Wako Pure Chemical Industries, Ltd. was used.

[Synthesis of Sensitizing Dye B-6]
As shown in the following scheme 5, 2-hydroxythioxanthone was synthesized with reference to the method described in Journal of the Chemical Society, 1910, 97, 1296, and this was combined with sodium 2-bromoethanesulfonate described in scheme 4 above. A sensitizing dye B-6 was synthesized by a method of reaction.

[Synthesis of Sensitizing Dye B-9]
Sensitizing dye B-9 was synthesized with reference to the synthesis method described in US Pat. No. 4,791,213.

[Synthesis of Sensitizing Dye B-27]
Sensitizing dye B-27 was synthesized with reference to the method described in Chemische Berichte, 1907, 40, 2452.

[Preparation of monomer solutions 1 to 19]
Polymeric compound M-1 (500 mg), water-soluble photopolymerization initiator A-4 (25 mg), and sensitizing dye B-2 (25 mg) were dissolved in methanol (2 ml) to prepare monomer solution 1.

  In the monomer solution 1, A-7, A-11, A-18, A-19, A-26, A-29, A-30, and A-31 are used in place of the water-soluble photopolymerization initiator A-4. Monomer solutions 2 to 9 were prepared in the same manner except that the mass was used.

  In the monomer solution 1, in the same manner except that the same mass of B-6, B-9, B-21, B-27, C-1 was used instead of the sensitizing dye B-2, 14 was prepared.

  In the monomer solution 4, monomer solutions 15 to 19 were prepared in the same manner except that the same mass of M-2 to M-6 was used instead of the polymerizable compound M-1.

[Preparation of Comparative Monomer Solutions 20-36]
In the monomer solution 1, in place of the polymerization initiator A-4 or the sensitizing dye B-2, the same amount of the polymerization initiator or the sensitizing dye shown in Table 1 was used. ~ 36 were prepared. The amount of the polymerization initiator was adjusted to 50 mg for the monomer solution not containing the sensitizing dye, and the amount of the sensitizing dye was adjusted to 50 mg for the monomer solution not containing the polymerization initiator.
For comparison photopolymerization initiators D-1 to D-3, the following compounds were used.

Comparative photopolymerization initiator D-1: IRGACURE 651 (trade name, manufactured by BASF Japan Ltd.)
Comparative photopolymerization initiator D-2: IRGACURE 2959 (trade name, manufactured by BASF Japan)
Comparative photopolymerization initiator D-3: synthesized by the method described in JP-A-2008-247940 and used.

Examples 1-19, Comparative Examples 1-17
Monomer solutions 1 to 36 (10 μl) were each dropped onto a copper thin film with a microsyringe, and this was exposed to light of 600 mJ / cm ² using a 385 nm UV-LED irradiation device, and FT-IR (manufactured by VARIAN, The polymerization rate was determined by Varian 3100 FT-IR), and the relative sensitivity (molar conversion) was derived from this value (sensitivity when using monomer solution 1 was 1.0). The results are shown in Table 1.

  As is apparent from the results in Table 1, Examples 1 to 19 using a combination of the polymerization initiator represented by the general formula (1) and a sensitizing dye are based on a 385 nm UV-LED light source. High sensitivity was shown. In contrast, Comparative Examples 1 to 9 using no sensitizing dye, Comparative Example 10 using no polymerization initiator, and Comparative Examples 11 to 17 using a comparative photopolymerization initiator were all inferior in sensitivity.

Example II. Preparation of colorant-containing ink composition and evaluation of sensitivity [synthesis of polymer dispersant P-1]
To a 1000 ml three-necked flask equipped with a stirrer and a condenser tube, 88 g of methyl ethyl ketone was added and heated to 72 ° C. under a nitrogen atmosphere. Here, 50 g of methyl ethyl ketone was mixed with 0.85 g of dimethyl 2,2′-azobisisobutyrate and benzyl methacrylate. A solution in which 60 g, 10 g of methacrylic acid and 30 g of methyl methacrylate were dissolved was dropped over 3 hours. After completion of the dropwise addition, the reaction was further continued for 1 hour, and then a solution of 0.42 g of dimethyl 2,2′-azobisisobutyrate in 2 g of methyl ethyl ketone was added, heated to 78 ° C. and heated for 4 hours. The obtained reaction solution was reprecipitated twice in a large excess amount of hexane, and the precipitated resin was dried to obtain 96 g of a polymer dispersant P-1.
The composition of the obtained resin was confirmed by ¹ H-NMR, and the weight average molecular weight (Mw) determined by GPC was 44,600. Furthermore, when the acid value was calculated | required by the method as described in JIS specification (JISK0070: 1992), it was 65.2 mgKOH / g.

[Preparation of dispersion of resin-coated pigment]
-Resin-coated cyan pigment dispersion-
Pigment Blue 15: 3 (phthalocyanine blue A220, manufactured by Dainichi Seika Co., Ltd.), 5 parts of the above polymer dispersant P-1, 42 parts of methyl ethyl ketone, 5.5 parts of 1 mol / L NaOH aqueous solution, Then, 87.2 parts of ion-exchanged water was mixed and dispersed with a bead mill using 0.1 mmφ zirconia beads for 2 to 6 hours.
Methyl ethyl ketone was removed from the obtained dispersion at 55 ° C. under reduced pressure, and a part of water was further removed to obtain a dispersion (colored particles) of a resin-coated cyan pigment having a pigment concentration of 10.2% by mass. It was.

-Resin-coated magenta pigment dispersion-
In the preparation of the resin-coated cyan pigment dispersion, the resin coating was carried out in the same manner as above except that Chromathal Jet Magenta DMQ (Pigment Red 122, manufactured by BASF Japan Ltd.) was used instead of phthalocyanine blue A220 used as the pigment. A magenta pigment dispersion (colored particles) was obtained.

-Resin-coated yellow pigment dispersion-
Resin-coated yellow was prepared in the same manner as described above except that Irgalite Yellow GS (Pigment Yellow 74, manufactured by BASF Japan Ltd.) was used instead of phthalocyanine blue A220 used as a pigment in the preparation of the resin-coated cyan pigment dispersion. A pigment dispersion (colored particles) was obtained.

-Resin-coated black pigment dispersion-
In the preparation of the resin-coated cyan pigment dispersion, a resin was obtained in the same manner as above except that the pigment dispersion CAB-O-JETTM 200 (carbon black, manufactured by CABOT) was used instead of the phthalocyanine blue A220 used as the pigment. A dispersion (colored particles) of the coated black pigment was obtained.

Example 2-1 Preparation of Ink Set 1 As described below, cyan ink (C1), magenta ink (M1), yellow ink (Y1), black ink (K1), and treatment liquid 1 of ink formulation 1 Were prepared, and an ink set 1 comprising these ink compositions and the treatment liquid 1 was obtained.

[Ink Formula 1]
(Preparation of cyan ink C1)
Using the above resin-coated cyan pigment dispersion, the resin-coated cyan pigment dispersion, ion-exchanged water, polymerization initiator, sensitizing dye, polymerizable compound, and surfactant are mixed so that the following ink formulation 1 is obtained. Thereafter, the mixture was filtered through a 5 μm membrane filter to prepare cyan ink C1 of ink formulation 1.

-Ink formula 1
・ Resin-coated cyan pigment dispersion: 6%
Compound A-4 (water-soluble photopolymerization initiator): 2.0%
Compound B-9 (water-soluble sensitizing dye): 2.0%
・ Polymerizable compound M-2 10%
・ Orphine E1010 ... 1%
(Nissin Chemical Co., Ltd .; surfactant)
・ Ion-exchanged water: Added to 100% overall.

  When the pH (25 ° C.) of the cyan pigment ink C1 was measured using a pH meter WM-50EG (manufactured by Toa DKK), the pH value was 8.5.

(Preparation of magenta ink (M1))
In the preparation of the cyan ink (C1), a magenta ink (M1) of the ink formulation 1 was prepared in the same manner as above except that a dispersion of the resin-coated magenta pigment was used instead of the dispersion of the resin-coated cyan pigment. did. The pH value was 8.5.

(Preparation of yellow ink (Y1))
In the preparation of the cyan ink (C1), a yellow ink (Y1) of ink formulation 1 was prepared in the same manner as described above except that a resin-coated yellow pigment dispersion was used instead of the resin-coated cyan pigment dispersion. did. The pH value was 8.5.

(Preparation of black ink (K1))
In the preparation of the cyan ink (C1), instead of the resin-coated cyan pigment dispersion, a pigment dispersion CAB-O-JETTM200 (carbon black dispersion, manufactured by CABOT) was used in the same manner as above, A black ink (K1) of ink formulation 1 was prepared. The pH value was 8.5.

[Preparation of Treatment Solution 1]
The following materials were mixed to prepare a treatment liquid 1. It was 1.0 when the pH (25 degreeC) of the processing liquid 1 was measured with the pH meter WM-50EG by Toa DKK Co., Ltd.
-Treatment solution composition-
・ Malonic acid 25.0%
・ Tripropylene glycol monomethyl ether (water-soluble organic solvent)
5%
・ Ion exchange water 70.0%

Examples 2-2 to 2-6: Preparation of ink sets 2 to 6 In ink set 1, instead of the water-soluble photopolymerization initiator A-4, A-7, A-11, A-18, A-19, Ink compositions C2 to C6, ink compositions M2 to M6, ink compositions Y2 to Y6, and ink compositions K2 to K6 were prepared in the same manner except that the same amount of A-26 was used. Here, an ink set composed of the ink compositions C1, M1, Y1, and K1 is referred to as an ink set 1, and similarly, an ink set composed of the ink compositions C2 to C6, M2 to M6, Y2 to Y6, and K2 to K6. Were set to ink sets 2 to 6, respectively.

Examples 2-7 and 2-8: Preparation of Ink Sets 7 and 8 Ink sets 1 and 3 were prepared in the same manner except that M-3 was used in the same mass in place of the polymerizable compound M-2. Sets 7 and 8 were obtained.

Comparative Examples 2-1 to 2-6: Comparative ink sets 9 to 14
Ink sets 1 to 6 were respectively obtained in the same manner except that the sensitizing dye B-9 was not used and the content of the photopolymerization initiator was changed to 4% by mass.

Comparative Examples 2-7 and 2-8: Comparative ink sets 15 and 16
Ink sets 15 and 16 were obtained in the same manner except that the same amount of comparative photopolymerization initiator D-1 or D-2 was used instead of photopolymerization initiator A-4 in ink set 1. .

[Inkjet recording]
As a recording medium (coated paper), Tokuhishi Art Double Sided N (Mitsubishi Paper, Basis Weight: 104.7 g / m ² ) was prepared, and an image was formed as shown below. Was evaluated.

Using the ink sets 1 to 15 prepared above, line images and solid images were formed by four-color single-pass recording.
At this time, a line image was formed by ejecting a 1200-dot line of 1 dot width, a 2-dot width line, and a 4-dot width line in a single pass in the main scanning direction.
The solid image was formed by discharging the ink composition over the entire surface of a sample obtained by cutting the recording medium into A5 size. The conditions for recording are as follows.

(1) Treatment liquid application process The entire surface of the recording medium is processed with a roll coater whose application amount is controlled by an anilox roller (number of lines: 100 to 300 / inch) so that the application amount is 1.4 g / m ^2. The liquid was applied.

(2) Processing Step Next, a drying treatment and a permeation treatment were performed on the recording medium coated with the treatment liquid under the following conditions.
・ Wind speed: 10m / s
-Temperature: Heated by a contact type flat heater from the opposite side (back side) of the recording surface of the recording medium so that the surface temperature of the recording surface of the recording medium was 60 ° C.

(3) Ink application step Thereafter, the ink composition was ejected by an inkjet method on the application surface of the recording medium coated with the treatment liquid under the following conditions to form a line image and a solid image, respectively.
・ Head: 1,200 dpi / 20 inch wide piezo full line head for 4 colors ・ Discharged droplet volume: 2.0 pL
・ Drive frequency: 30kHz

(4) Ink drying step Next, the recording medium provided with the ink composition was dried under the following conditions.
・ Drying method: blow drying ・ wind speed: 15m / s
-Temperature: Heated by a contact type flat heater from the opposite side (back side) of the recording surface of the recording medium so that the surface temperature of the recording surface of the recording medium was 60 ° C.

(5) Immobilization step Next, using an ultraviolet light-emitting diode (UV-LED (NC4U134 manufactured by Nichia Corporation), peak wavelength: 385 nm), the recording image has an energy of 1000 mJ / cm ² , Irradiated with ultraviolet rays as actinic radiation, an image recorded matter was obtained.

[Evaluation]
The obtained image sample was subjected to an ink curability test and an image anti-blocking test as follows. The results are shown in Table 2.

[Curing sensitivity]
Unprinted special diamond art both sides N (Mitsubishi Paper Co., Ltd.) is wrapped around a paperweight (weight 470 g, size 15 mm x 30 mm x 120 mm) (the area where the unprinted special diamond art and the evaluation sample contact is 150 mm ² ) The sample was rubbed three times (corresponding to a load of 260 kg / m ² ). The marking screen after rubbing was visually observed and evaluated according to the following evaluation criteria.
(Evaluation criteria)
A: No peeling of the image (coloring material) was visible on the printing surface.
B: Slight peeling of the image (coloring material) was observed on the printed surface.
C: The peeling of the image (coloring material) can be visually recognized on the printing surface, which is a level causing a practical problem.

[Blocking resistance]
Immediately after printing a 2 cm square solid portion of a recording medium on which a solid image was recorded, an unrecorded recording medium (Tokuhishi Art Double Side N (Mitsubishi Paper)) (hereinafter referred to as an unused sample in this evaluation) is superimposed. Then, a load of 350 kg / m ² was applied and the sample was left under environmental conditions of 60 ° C. and 30% RH for 24 hours. The degree of ink transfer to a white background portion of an unused sample was visually observed and evaluated according to the following evaluation criteria.
(Evaluation criteria)
A: There was no transfer of ink.
B: Ink transfer was hardly noticeable.
C: Some transfer of ink was observed, which was a practically acceptable limit level.
D: Ink transfer was remarkable.

  As is apparent from the results in Table 2, the images formed by the ink sets of Examples 2-1 to 2-8 using the polymerization initiator represented by the general formula (1) together with the sensitizing dye, Both curing sensitivity and blocking resistance were good. On the other hand, in the image formed by the ink set of Comparative Examples 2-1 to 2-8 using no sensitizing dye or using a comparative polymerization initiator, both the sensitivity and the blocking resistance were inferior. It was.

Examples 2-10, 2-11
An ink set was prepared and an image was formed in the same manner as the ink set 2-1, except that the treatment liquid 2 or the treatment liquid 3 having the following composition was used instead of the treatment liquid 1.

[Preparation of treatment liquid 2]
The following materials were mixed to prepare treatment liquid 2. It was 4.0 when pH (25 degreeC) of the processing liquid 2 after pH adjustment was measured with the pH meter WM-50EG by Toa DKK Co., Ltd ..
-Treatment solution composition-
・ Polyethyleneimine (cationic polymer) 13.0%
・ Ion exchange water 87.0%

[Preparation of treatment liquid 3]
The following materials were mixed to prepare a treatment liquid 3. The pH (25 ° C.) of the treatment solution 3 was measured with a pH meter WM-50EG manufactured by Toa DKK Co., Ltd., and was 4.0.
-Treatment solution composition-
・ Magnesium nitrate: 15%
・ Diethylene glycol monoethyl ether: 4%
・ Orphine E1010 (manufactured by Nissin Chemical Industry Co., Ltd.): 1%
・ Ion exchange water: 80%

  Evaluation similar to Example 2-1 was performed about the obtained image. As a result, when the treatment liquid 2 or the treatment liquid 3 was used, the same good results as in Example 2-1 were obtained.

Claims (17)

An ink composition comprising a photopolymerization initiator represented by the following general formula (1), a polymerizable compound, and a sensitizing dye having an absorption peak in a wavelength region of 330 nm to 400 nm.

(In General Formula (1), R ^{1 to} R ¹² each independently represents a hydrogen atom or a substituent. X ¹ and X ² each independently represent an oxygen atom, a sulfur atom, an alkylene group, or a single bond, (At least one of X ¹ and X ² is a sulfur atom.) The ink composition according to claim 1, wherein in the general formula (1), at least one of R ^{1 to} R ⁵ is a group represented by the following general formula (2).

(In general formula (2), X ³ represents an oxygen atom or a sulfur atom. R ¹³ represents a substituent. * Represents a bonding site with a benzene ring.)   The ink composition according to claim 1, wherein the sensitizing dye is a sensitizing dye that substantially does not absorb in a wavelength region of more than 400 nm and not more than 700 nm. The ink composition according to claim 1, wherein the sensitizing dye is a sensitizing dye represented by the following general formula (I).

(In the general formula ^{(I), R} 21 ~R ²⁶ independently denotes a hydrogen atom or a substituent, two substituents adjacent may ^{.X 21} may form a ring bonded to each is Represents an oxygen atom, a sulfur atom or NR ^27. R ²⁷ represents a hydrogen atom or a substituent.) The ink composition according to claim 1, wherein the sensitizing dye is a sensitizing dye represented by the following general formula (I-1).

(In general formula (I-1), R ^{28 to} R ³⁵ each independently represent a hydrogen atom or a substituent, and two adjacent substituents may be bonded to each other to form a ring.) The ink composition according to claim 1, wherein the sensitizing dye is a sensitizing dye represented by the following general formula (II).

(In the general formula ^{(II), R} 41 ~R ⁴⁸ independently denotes a hydrogen atom or a substituent, two substituents adjacent may ^{.X 41} may form a ring bonded to each is Represents an oxygen atom, a sulfur atom or NR ^27. R ²⁷ represents a hydrogen atom or a substituent.) The ink composition according to claim 6, wherein in the general formula (II), X ⁴¹ is a sulfur atom.   The ink composition according to any one of claims 1 to 7, wherein a content ratio of the photopolymerization initiator and the sensitizing dye is in a range of 1 to 5 to 5 to 1 in terms of molar ratio.   The ink composition according to claim 1, wherein the polymerizable compound is at least one selected from a (meth) acrylamide compound, a (meth) acrylate compound, and a vinyl compound.   The ink composition according to claim 1, wherein the polymerizable compound is a (meth) acrylamide compound.   The ink composition according to claim 1, comprising a colorant.   The ink composition according to claim 1, comprising an aqueous medium. In Formula (1), the ink composition according to any one of claims 1 to 12 ^{X 1} and ^{X 2} is a sulfur atom. 14. The ink composition according to claim 2, wherein R ³ in the general formula (1) is a group represented by the general formula (2). The ink composition according to claim 1, wherein in the general formula (1), at least one of R ¹¹ and R ¹² is a substituted or unsubstituted alkyl group.   An ink set comprising the ink composition according to any one of claims 1 to 15 and a treatment liquid for aggregating the aqueous ink composition.   A treatment liquid application step for applying the treatment liquid according to claim 16 onto a recording medium, and an ink composition according to any one of claims 1 to 15 to be applied on the recording medium to form an image. An image forming method including an ink application step.