Radiation-curable ink jet ink composition

Abstract

To provide a radiation-curable ink jet ink composition or the like that is excellent in odor properties.SOLUTION: A radiation-curable ink jet ink composition includes a nitrogen-containing monofunctional polymerizable compound and a photoinitiator, wherein the nitrogen-containing monofunctional polymerizable compound includes N-vinyl methyl oxazolidinone or acryloyl morpholine, and the photoinitiator includes a high-molecular-weight photoinitiator having a molecular weight of 500 or greater.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a radiation-curable ink-jet ink composition. [Background technology]

[0002] Inkjet recording methods are capable of recording high-resolution images using relatively simple equipment and have been rapidly developing in various fields. Among these methods, various studies have been conducted on printing methods using radiation-curable inkjet ink compositions. For example, Patent Document 1 discloses a photocurable inkjet printing ink composition that contains 5 to 50% by mass of vinylmethyloxazolidinone based on the total ink composition, 10 to 50% by mass of a monofunctional photopolymerizable monomer having a glass transition temperature of 10°C or lower based on the total ink composition, a colorant, and a photopolymerization initiator. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2021 / 199760 Summary of the Invention [Problem to be solved by the invention]

[0004] However, when printing is performed using the photocurable ink jet printing ink composition described in Patent Document 1, it has been found that there is room for improvement in both the odor and curability of the recorded matter. [Means for solving the problem]

[0005] The present invention provides a radiation-curable inkjet ink composition comprising a nitrogen-containing monofunctional polymerizable compound and a photoinitiator, wherein the nitrogen-containing monofunctional polymerizable compound comprises N-vinylmethyloxazolidinone or acryloylmorpholine, and the photoinitiator comprises a high-molecular-weight photoinitiator having a molecular weight of 500 or more. [Brief explanation of the drawings]

[0006] [Figure 1] FIG. 2 is a schematic diagram of a recording apparatus used in the present embodiment. [Figure 2] 1 is a table showing the results of examples. [Figure 3] 1 is a table showing the results of examples. [Figure 4] 1 is a table showing the results of examples. [Figure 5] 1 is a table showing the results of examples. DETAILED DESCRIPTION OF THE INVENTION

[0007] Below, we will explain in detail the embodiment of the present invention (hereinafter referred to as the ``present embodiment''), but the present invention is not limited to this and various modifications are possible within the scope of the gist of the present invention.

[0008] 1. Radiation-curable inkjet ink composition The radiation-curable inkjet ink composition of this embodiment contains a nitrogen-containing monofunctional polymerizable compound and a photoinitiator, wherein the nitrogen-containing monofunctional polymerizable compound contains N-vinylmethyloxazolidinone or acryloylmorpholine, and the photoinitiator contains a high-molecular-weight photoinitiator having a molecular weight of 500 or more.

[0009] It is known that radiation-curable inkjet inks are used to record on non-absorbent recording media such as flexible packaging materials and shrink films. However, there is a problem that recorded matter printed using a radiation-curable inkjet ink composition tends to retain an odor derived from the ink, making it difficult to use in applications requiring low odor, such as food packaging and other flexible packaging applications.

[0010] Therefore, the present inventors have found that the odor of the resulting recorded matter can be reduced by using a high molecular weight photoinitiator in a radiation-curable inkjet ink composition. However, research and development by the inventors has revealed that the high molecular weight photoinitiator does not provide sufficient curability to the ink composition.

[0011] Therefore, in this embodiment, in addition to a high-molecular-weight photoinitiator having a molecular weight of 500 or more, N-vinylmethyloxazolidinone or acryloylmorpholine is used in combination as a nitrogen-containing monofunctional polymerizable compound. By including N-vinylmethyloxazolidinone or acryloylmorpholine, sufficient curability can be obtained even when a high-molecular-weight photoinitiator is used. In other words, the radiation-curable inkjet ink composition of this embodiment not only has the effect of reducing the odor of recorded materials due to the high-molecular-weight photoinitiator, but also has excellent curability.

[0012] Each component of the radiation-curable inkjet ink composition of this embodiment will be described in detail below. Note that, hereinafter, the radiation-curable inkjet ink composition of this embodiment will also be simply referred to as the "ink composition."

[0013] 1.1. Polymerizable compounds In this embodiment, the polymerizable compound conceptually encompasses monofunctional polymerizable compounds having one polymerizable functional group and polyfunctional polymerizable compounds having two or more polymerizable functional groups. When the ink composition of this embodiment is irradiated with radiation, a polymerization reaction of the polymerizable compound contained in the ink composition begins, causing the ink composition to harden. Polymerizable compounds include monomers and oligomers. The polymerizable compound is not particularly limited, but examples include nitrogen-containing monomers, nitrogen-containing oligomers, cyclic ether-containing monomers, cyclic ether-containing oligomers, hydroxyl group-containing monomers, and hydroxyl group-containing oligomers. The polymerizable compounds may be used alone or in combination of two or more.

[0014] In the present embodiment, the number of polymerizations of the monomers in the oligomer is not particularly limited, but is, for example, 2 to 100, 2 to 50, 2 to 25, 2 to 10, or 2 to 5.

[0015] The content of the polymerizable compound is preferably 60 to 95 mass %, 65 to 90 mass %, or 70 to 90 mass % relative to the total amount of the ink composition.

[0016] The weighted average of the glass transition temperatures of the polymerizable compounds contained in the ink composition of this embodiment is preferably 20 to 70°C, 25 to 65°C, 30 to 60°C, or 40 to 50°C. Having a weighted average of the glass transition temperatures of 20°C or higher tends to improve blocking resistance. Furthermore, having a weighted average of the glass transition temperatures of 70°C or lower tends to improve the conformability of the ink composition film on the recording medium when the recording medium is heated and shrunk. In other words, the shrink properties of the ink composition tend to be improved. Therefore, having a weighted average of the glass transition temperatures of the polymerizable compounds contained in the ink composition of this embodiment within the above range tends to achieve both blocking resistance and shrink properties. Note that blocking is a phenomenon in which stacked recordings adhere to each other due to the adhesiveness of the ink. Improved blocking resistance reduces this adhesion.

[0017] The "glass transition temperature of the polymerizable compound" means the glass transition temperature of a homopolymer of the polymerizable compound. The weighted average glass transition temperature of the polymerizable compound can be adjusted by the glass transition temperature of the homopolymer of the polymerizable compound used and the content mass ratio of the polymerizable compound used.

[0018] Here, a method for calculating the weighted average of the glass transition temperatures of the homopolymers in the polymerizable compound will be described. The weighted average of the glass transition temperatures of the homopolymers is called Tg All The glass transition temperature of the homopolymer of each polymerizable compound is Tg N The content mass ratio of the polymerizable compound is X N(wt%). N is a number starting from 1 depending on the type of polymerizable compound contained in the ink composition of this embodiment. For example, when three types of polymerizable compounds are used, Tg1, Tg2, and Tg3 are generated. The weighted average Tg of the glass transition temperatures of homopolymers All is the glass transition temperature Tg of the homopolymer calculated for each polymerizable compound. N and the mass ratio X N Therefore, the following equation (1) holds true: Tg All =Σ(Tg N ×X N ) ···(1)

[0019] The glass transition temperature of a homopolymer of a polymerizable compound can be measured by differential scanning calorimetry (DSC) in accordance with JIS K 7121. For example, a measuring device such as a DSC6220 manufactured by Seiko Electronics Co., Ltd. can be used, and a sample can be prepared by polymerizing the polymerizable compound to such an extent that the glass transition temperature of the homopolymer becomes constant.

[0020] The molecular weight of the polymerizable compound is preferably 50 to 500, 75 to 450, or 100 to 400. When the molecular weight of the polymerizable compound is within the above range, the curability tends to be further improved.

[0021] 1.1.1. Monofunctional polymerizable compounds In the present embodiment, the monofunctional polymerizable compound is not particularly limited, but examples thereof include nitrogen-containing monofunctional polymerizable compounds, cyclic ether-containing monofunctional polymerizable compounds, hydroxyl group-containing monofunctional polymerizable compounds, and other monofunctional polymerizable compounds other than these monofunctional polymerizable compounds.

[0022] 1.1.1.1. Nitrogen-containing monofunctional polymerizable compounds The ink composition of this embodiment contains a nitrogen-containing monofunctional polymerizable compound. This tends to provide sufficient curability, even when the ink composition of this embodiment contains a high-molecular-weight photoinitiator, as described below. The reason for the improved curability is not particularly limited, but is thought to be that the nitrogen-containing monofunctional polymerizable compound is less susceptible to the loss of radicals derived from the photoinitiator due to oxygen.

[0023] The ink composition of this embodiment contains N-vinylmethyloxazolidinone (VMOX) or acryloylmorpholine (ACMO) as a nitrogen-containing monofunctional polymerizable compound. By including VMOX or ACMO in the ink composition of this embodiment, the curability of the ink composition tends to be further improved. The ink composition of this embodiment may also contain a nitrogen-containing monofunctional polymerizable compound other than VMOX or ACMO. Examples of nitrogen-containing monofunctional polymerizable compounds other than VMOX and ACMO include, but are not limited to, nitrogen-containing monofunctional vinyl monomers such as N-vinylcaprolactam, N-vinylformamide, N-vinylcarbazole, N-vinylacetamide, and N-vinylpyrrolidone; nitrogen-containing monofunctional acrylate monomers such as N-(2-hydroxyethyl)acrylamide; nitrogen-containing monofunctional acrylamide monomers such as (meth)acrylamide, N-hydroxymethyl(meth)acrylamide, diacetoneacrylamide, N,N-dimethyl(meth)acrylamide, and (meth)acrylamides such as dimethylaminoethyl acrylate benzyl chloride quaternary salt; and oligomers thereof.

[0024] The ink composition of this embodiment preferably contains a nitrogen-containing monofunctional polymerizable compound having a nitrogen-containing heterocyclic structure. By including a nitrogen-containing monofunctional polymerizable compound having a nitrogen-containing heterocyclic structure in the ink composition of this embodiment, the curability of the ink composition tends to be further improved. VMOX and ACMO are nitrogen-containing monofunctional polymerizable compounds having a nitrogen-containing heterocyclic structure.

[0025] The content of the nitrogen-containing monofunctional polymerizable compound relative to the total amount of the ink composition is preferably 10 to 50 mass%, 15 to 45 mass%, 20 to 40 mass%, or 20 to 35 mass%. When the content of the nitrogen-containing monofunctional polymerizable compound relative to the total amount of the ink composition is within the above range, the curability of the ink composition tends to be further improved.

[0026] The content of the nitrogen-containing monofunctional polymerizable compound relative to the total amount of polymerizable compounds is preferably 15 to 55 mass%, 20 to 50 mass%, 25 to 45 mass%, or 25 to 40 mass%. When the content of the nitrogen-containing monofunctional polymerizable compound relative to the total amount of polymerizable compounds is within the above range, the curability of the ink composition tends to be further improved.

[0027] The content of the nitrogen-containing monofunctional polymerizable compound relative to the total amount of the monofunctional polymerizable compounds is preferably 20 to 65 mass%, 25 to 60 mass%, or 30 to 55 mass%. When the content of the nitrogen-containing monofunctional polymerizable compound relative to the total amount of the monofunctional polymerizable compounds is within the above range, the curability of the ink composition tends to be further improved.

[0028] The molecular weight of the nitrogen-containing monofunctional polymerizable compound is preferably 100 to 200, more preferably 110 to 180. When the molecular weight of the nitrogen-containing monofunctional polymerizable compound is within the above range, the curability of the ink composition tends to be further improved.

[0029] The glass transition temperature of the nitrogen-containing monofunctional polymerizable compound is preferably 50 to 200° C., and more preferably 100 to 175° C. When the glass transition temperature of the nitrogen-containing monofunctional polymerizable compound is within the above range, blocking resistance and shrink properties tend to be further improved.

[0030] 1.1.1.2. Cyclic ether-containing monofunctional polymerizable compound The ink composition of this embodiment may contain a cyclic ether-containing monofunctional polymerizable compound. Examples of the cyclic ether-containing monofunctional polymerizable compound include, but are not limited to, tetrahydrofurfuryl acrylate, cyclic trimethylolpropane formal acrylate, (2-methyl-2-ethyl-1,3-dioxolan-4-yl)methyl acrylate, and oligomers thereof. By including a cyclic ether-containing monofunctional polymerizable compound in the ink composition of this embodiment, the curability of the ink composition and the adhesion of the ink composition to a recording medium tend to be further improved.

[0031] The content of the cyclic ether-containing monofunctional polymerizable compound relative to the total amount of the ink composition is preferably 10 to 50 mass%, 15 to 45 mass%, or 20 to 40 mass%. When the content of the cyclic ether-containing monofunctional polymerizable compound relative to the total amount of the ink composition is within the above range, the curability of the ink composition tends to be further improved.

[0032] The content of the cyclic ether-containing monofunctional polymerizable compound relative to the total amount of polymerizable compounds is preferably 10 to 55 mass%, 15 to 50 mass%, 20 to 45 mass%, or 25 to 40 mass%. When the content of the cyclic ether-containing monofunctional polymerizable compound relative to the total amount of polymerizable compounds is within the above range, the curability of the ink composition tends to be further improved.

[0033] The content of the cyclic ether-containing monofunctional polymerizable compound relative to the total amount of the monofunctional polymerizable compounds is preferably 20 to 60 mass%, 25 to 55 mass%, or 30 to 50 mass%. When the content of the cyclic ether-containing monofunctional polymerizable compound relative to the total amount of the monofunctional polymerizable compounds is within the above range, the curability of the ink composition tends to be further improved.

[0034] The molecular weight of the cyclic ether-containing monofunctional polymerizable compound is preferably 150 to 300, more preferably 175 to 250. When the molecular weight of the cyclic ether-containing monofunctional polymerizable compound is within the above range, the curability of the ink composition tends to be further improved.

[0035] The glass transition temperature of the cyclic ether-containing monofunctional polymerizable compound is preferably −50 to 100° C., and more preferably −25 to 50° C. When the glass transition temperature of the cyclic ether-containing monofunctional polymerizable compound is within the above range, blocking resistance and shrink properties tend to be further improved.

[0036] 1.1.1.3. Hydroxyl group-containing monofunctional polymerizable compounds The ink composition of this embodiment may contain a hydroxyl group-containing monofunctional polymerizable compound. Examples of the hydroxyl group-containing monofunctional polymerizable compound include, but are not limited to, 4-hydroxybutyl(meth)acrylate, 2-hydroxybutyl(meth)acrylate, 2-hydroxyethyl(meth)acrylate, 2-hydroxypropyl(meth)acrylate, 2-hydroxy-3-phenoxypropyl(meth)acrylate, and N-hydroxymethyl(meth)acrylamide, as well as oligomers thereof. By including a hydroxyl group-containing monofunctional polymerizable compound in the ink composition of this embodiment, the curability of the ink composition tends to be further improved.

[0037] The content of the hydroxyl group-containing monofunctional polymerizable compound relative to the total amount of the ink composition is preferably 5.0 to 30.0 mass%, 7.5 to 25.0 mass%, or 10.0 to 20.0 mass%. When the content of the hydroxyl group-containing monofunctional polymerizable compound relative to the total amount of the ink composition is within the above range, the curability of the ink composition tends to be further improved.

[0038] The content of the hydroxyl group-containing monofunctional polymerizable compound relative to the total amount of polymerizable compounds is preferably 7.5 to 40.0 mass%, 10.0 to 35.0 mass%, or 12.5 to 30.0 mass%. When the content of the hydroxyl group-containing monofunctional polymerizable compound relative to the total amount of polymerizable compounds is within the above range, the curability of the ink composition tends to be further improved.

[0039] The content of the hydroxyl group-containing monofunctional polymerizable compound relative to the total amount of the monofunctional polymerizable compounds is preferably 10.0 to 45.0 mass%, 12.5 to 40.0 mass%, or 15.0 to 35.0 mass%. When the content of the hydroxyl group-containing monofunctional polymerizable compound relative to the total amount of the monofunctional polymerizable compounds is within the above range, the curability of the ink composition tends to be further improved.

[0040] The molecular weight of the hydroxyl group-containing monofunctional polymerizable compound is preferably 50 to 250, more preferably 100 to 200. When the molecular weight of the hydroxyl group-containing monofunctional polymerizable compound is within the above range, the curability of the ink composition tends to be further improved.

[0041] The glass transition temperature of the hydroxyl group-containing monofunctional polymerizable compound is preferably −100 to 50° C., and more preferably −50 to 25° C. When the glass transition temperature of the hydroxyl group-containing monofunctional polymerizable compound is within the above range, blocking resistance and shrink properties tend to be further improved.

[0042] 1.1.1.4. Aromatic group-containing monofunctional polymerizable compounds The ink composition of this embodiment may contain an aromatic group-containing monofunctional polymerizable compound. Examples of the aromatic group-containing monofunctional polymerizable compound include, but are not limited to, phenoxyethyl (meth)acrylate, benzyl (meth)acrylate, alkoxylated 2-phenoxyethyl (meth)acrylate, ethoxylated nonylphenyl (meth)acrylate, alkoxylated nonylphenyl (meth)acrylate, p-cumylphenol EO-modified (meth)acrylate, and 2-hydroxy-3-phenoxypropyl (meth)acrylate, as well as oligomers thereof.

[0043] The content of the aromatic group-containing monofunctional polymerizable compound relative to the total amount of the ink composition is preferably 10 to 50 mass%, 15 to 45 mass%, or 20 to 40 mass%. When the content of the aromatic group-containing monofunctional polymerizable compound relative to the total amount of the ink composition is within the above range, the odor reduction and curability of the ink composition tend to be further improved.

[0044] The content of the aromatic group-containing monofunctional polymerizable compound relative to the total amount of polymerizable compounds is preferably 10 to 55 mass%, 15 to 50 mass%, 20 to 45 mass%, or 25 to 40 mass%. When the content of the aromatic group-containing monofunctional polymerizable compound relative to the total amount of polymerizable compounds is within the above range, the odor reduction and curability of the ink composition tend to be further improved.

[0045] The content of the aromatic group-containing monofunctional polymerizable compound relative to the total amount of the monofunctional polymerizable compounds is preferably 20 to 60 mass%, 25 to 55 mass%, or 30 to 50 mass%. When the content of the aromatic group-containing monofunctional polymerizable compound relative to the total amount of the monofunctional polymerizable compounds is within the above range, the odor reduction and curability of the ink composition tend to be further improved.

[0046] 1.1.1.5. Saturated aliphatic group-containing monofunctional polymerizable compounds The ink composition of this embodiment may contain a saturated aliphatic group-containing monofunctional polymerizable compound. The saturated aliphatic group-containing monofunctional polymerizable compound is not particularly limited, and examples thereof include alicyclic group-containing monofunctional monomers such as tert-butylcyclohexyl acrylate (TBCHA), isobornyl acrylate, and 2-(meth)acrylic acid-1,4-dioxaspiro[4,5]dec-2-ylmethyl; isoamyl (meth)acrylate, stearyl (meth)acrylate, lauryl (meth)acrylate, octyl (meth)acrylate, decyl (meth)acrylate, and isodecyl Examples of the monomer include linear or branched aliphatic group-containing monofunctional monomers such as (meth)acrylate, isomyristyl (meth)acrylate, isostearyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, butoxyethyl (meth)acrylate, tetrahydrofurfuryl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, and 2-hydroxypropyl (meth)acrylate; lactone-modified flexible (meth)acrylates; and oligomers thereof.

[0047] The content of the saturated aliphatic group-containing monofunctional polymerizable compound relative to the total amount of the ink composition is preferably 10 to 50 mass%, 15 to 45 mass%, or 20 to 40 mass%. When the content of the saturated aliphatic group-containing monofunctional polymerizable compound relative to the total amount of the ink composition is within the above range, the curability of the ink composition tends to be further improved.

[0048] The content of the saturated aliphatic group-containing monofunctional polymerizable compound relative to the total amount of polymerizable compounds is preferably 10 to 55 mass%, 15 to 50 mass%, 20 to 45 mass%, or 25 to 40 mass%. When the content of the saturated aliphatic group-containing monofunctional polymerizable compound relative to the total amount of polymerizable compounds is within the above range, the curability of the ink composition tends to be further improved.

[0049] The content of the saturated aliphatic group-containing monofunctional polymerizable compound relative to the total amount of the monofunctional polymerizable compounds is preferably 20 to 60 mass%, 25 to 55 mass%, or 30 to 50 mass%. When the content of the saturated aliphatic group-containing monofunctional polymerizable compound relative to the total amount of the monofunctional polymerizable compounds is within the above range, the curability of the ink composition tends to be further improved.

[0050] The molecular weight of the saturated aliphatic group-containing monofunctional polymerizable compound is preferably 100 to 300, more preferably 150 to 250. When the molecular weight of the saturated aliphatic group-containing monofunctional polymerizable compound is within the above range, the curability of the ink composition tends to be further improved.

[0051] The glass transition temperature of the saturated aliphatic group-containing monofunctional polymerizable compound is preferably 0 to 150° C., and more preferably 25 to 100° C. When the glass transition temperature of the saturated aliphatic group-containing monofunctional polymerizable compound is within the above range, blocking resistance and shrink properties tend to be further improved.

[0052] 1.1.1.6. Other monofunctional polymerizable compounds The ink composition of this embodiment may contain other monofunctional polymerizable compounds in addition to the monofunctional polymerizable compounds described above. Examples of other monofunctional polymerizable compounds include unsaturated carboxylic acids such as (meth)acrylic acid, itaconic acid, crotonic acid, isocrotonic acid, and maleic acid; salts of the unsaturated carboxylic acids; esters, urethanes, amides, and anhydrides of unsaturated carboxylic acids; acrylonitrile, styrene, various unsaturated polyesters, unsaturated polyethers, unsaturated polyamides, and unsaturated urethanes.

[0053] The content of the other monofunctional polymerizable compounds relative to the total amount of the ink composition is not particularly limited, but is, for example, 0.1 to 5.0 mass%, 0.1 to 2.5 mass%, or 0.1 to 1.0 mass%, or the ink composition may not contain other monofunctional polymerizable compounds.

[0054] The content of the other monofunctional polymerizable compounds relative to the total amount of the polymerizable compounds is not particularly limited, but is, for example, 0.1 to 5.0 mass%, 0.1 to 2.5 mass%, or 0.1 to 1.0 mass%, or may not contain other monofunctional polymerizable compounds.

[0055] The content of the other monofunctional polymerizable compounds relative to the total amount of the monofunctional polymerizable compounds is not particularly limited, but is, for example, 0.1 to 5.0 mass%, 0.1 to 2.5 mass%, or 0.1 to 1.0 mass%, or may not contain other monofunctional polymerizable compounds.

[0056] The molecular weight of the other monofunctional polymerizable compound is preferably 100-300, more preferably 150-250.

[0057] The glass transition temperature of the other monofunctional polymerizable compound is preferably 0 to 150°C, more preferably 25 to 100°C.

[0058] The total amount of the monofunctional polymerizable compound is preferably 30.0 to 80.0 mass %, 35.0 to 77.5 mass %, 40.0 to 75.0 mass %, or 45.0 to 72.5 mass %, relative to the total amount of the ink composition.

[0059] The total amount of the monofunctional polymerizable compounds is preferably 50% by mass or more, 50 to 95% by mass, 55 to 90% by mass, or 60 to 85% by mass, relative to the total amount of the polymerizable compounds. When the content of the monofunctional polymerizable compounds is within the above range, the flexibility of the ink composition film on the recording medium is improved, and the conformability of the film when the recording medium is heated and shrunk tends to be improved. In other words, the shrinkability of the ink composition tends to be improved.

[0060] 1.1.2. Multifunctional polymerizable compound The ink composition of this embodiment may or may not contain a polyfunctional polymerizable compound. When the ink composition of this embodiment contains a polyfunctional polymerizable compound, the curability tends to be further improved. The polyfunctional polymerizable compound is not particularly limited, but examples thereof include vinyl ether group-containing (meth)acrylates, bifunctional (meth)acrylates, trifunctional or higher functional (meth)acrylates, and oligomers thereof. When the ink composition of this embodiment contains a polyfunctional polymerizable compound, the curability of the ink composition tends to be further improved.

[0061] 1.1.2.1. Vinyl ether group-containing (meth)acrylate The ink composition of this embodiment may or may not contain a vinyl ether group-containing (meth)acrylate. The vinyl ether group-containing (meth)acrylate is not particularly limited, but examples include compounds represented by the following formula (1): A high-molecular-weight photoinitiator tends to improve the viscosity of the ink composition, but the inclusion of such a vinyl ether group-containing (meth)acrylate tends to reduce the viscosity of the ink composition and further improve the ejection stability. Furthermore, the curability of the ink composition is further improved, and the improved curability also enables the recording speed to be increased. CH2=CR1-COOR2-O-CH=CH-R3... (1) (In the formula, R1 is a hydrogen atom or a methyl group, R2 is a divalent organic residue having 2 to 20 carbon atoms, and R3 is a hydrogen atom or a monovalent organic residue having 1 to 11 carbon atoms.)

[0062] In the above formula (1), examples of the divalent organic residue having 2 to 20 carbon atoms and represented by R2 include linear, branched, or cyclic alkylene groups having 2 to 20 carbon atoms, which may be substituted, alkylene groups having 2 to 20 carbon atoms and having an oxygen atom via an ether bond and / or an ester bond in the structure, and divalent aromatic groups having 6 to 11 carbon atoms, which may be substituted. Among these, alkylene groups having 2 to 6 carbon atoms, such as ethylene, n-propylene, isopropylene, and butylene, and alkylene groups having 2 to 9 carbon atoms and having an oxygen atom via an ether bond in the structure, such as oxyethylene, oxy-n-propylene, oxyisopropylene, and oxybutylene, are preferred. Furthermore, from the viewpoint of further reducing the viscosity of the composition and further improving the curability of the composition, a compound having a glycol ether chain in which R2 is an alkylene group having 2 to 9 carbon atoms and having an oxygen atom via an ether bond in the structure, such as an oxyethylene group, an oxy-n-propylene group, an oxyisopropylene group, or an oxybutylene group, is more preferred.

[0063] In the above formula (1), the monovalent organic residue having 1 to 11 carbon atoms represented by R3 is preferably a linear, branched, or cyclic alkyl group having 1 to 10 carbon atoms which may be substituted, or an aromatic group having 6 to 11 carbon atoms which may be substituted. Among these, alkyl groups having 1 to 2 carbon atoms such as a methyl group or an ethyl group, and aromatic groups having 6 to 8 carbon atoms such as a phenyl group and a benzyl group are preferably used.

[0064] When each of the above organic residues is a group that may be substituted, the substituent is divided into a group containing carbon atoms and a group not containing carbon atoms. First, when the above substituent is a group containing carbon atoms, the carbon atom is counted in the number of carbon atoms of the organic residue. Examples of the group containing carbon atoms include, but are not limited to, a carboxyl group and an alkoxy group. Next, examples of the group not containing carbon atoms include, but are not limited to, a hydroxyl group and a halo group.

[0065] Specific examples of the compound of formula (1) include, but are not limited to, 2-vinyloxyethyl (meth)acrylate, 3-vinyloxypropyl (meth)acrylate, 1-methyl-2-vinyloxyethyl (meth)acrylate, 2-vinyloxypropyl (meth)acrylate, 4-vinyloxybutyl (meth)acrylate, 1-methyl-3-vinyloxypropyl (meth)acrylate, 1-vinyloxymethylpropyl (meth)acrylate, 2-methyl-3-vinyloxypropyl (meth)acrylate, 1,1-dimethyl-2-vinyloxyethyl (meth)acrylate, p) 3-vinyloxybutyl acrylate, 1-methyl-2-vinyloxypropyl (meth)acrylate, 2-vinyloxybutyl (meth)acrylate, 4-vinyloxycyclohexyl (meth)acrylate, 6-vinyloxyhexyl (meth)acrylate, 4-vinyloxymethylcyclohexylmethyl (meth)acrylate, 3-vinyloxymethylcyclohexylmethyl (meth)acrylate, 2-vinyloxymethylcyclohexylmethyl (meth)acrylate, p-vinyloxymethylphenylmethyl (meth)acrylate, m-vinyloxymethylphenyl (meth)acrylate methyl, o-vinyloxymethylphenylmethyl (meth)acrylate, 2-(2-vinyloxyethoxy)ethyl methacrylate, 2-(2-vinyloxyethoxy)ethyl acrylate, 2-(vinyloxyisopropoxy)ethyl (meth)acrylate, 2-(vinyloxyethoxy)propyl (meth)acrylate, 2-(vinyloxyethoxy)isopropyl (meth)acrylate, 2-(vinyloxyisopropoxy)propyl (meth)acrylate, 2-(vinyloxyisopropoxy)isopropyl (meth)acrylate, 2-(vinyloxyethoxy)isopropyl (meth)acrylate, 2-(vinyloxyethoxy)ethyl (meth)acrylate (meth)acrylate 2-(vinyloxyethoxyisopropoxy)ethyl, (meth)acrylate 2-(vinyloxyisopropoxyethoxy)ethyl, (meth)acrylate 2-(vinyloxyisopropoxyisopropoxy)ethyl, (meth)acrylate 2-(vinyloxyethoxyethoxy)propyl, (meth)acrylate 2-(vinyloxyethoxyisopropoxy)propyl, (meth)acrylate 2-(vinyloxyisopropoxyethoxy)propyl, (meth)acrylate 2-(vinyloxyisopropoxyisopropoxy)propyl, (meth)acrylate 2-(vinyloxyisopropoxyisopropoxy)propyl,2-(vinyloxyethoxyethoxy)isopropyl (meth)acrylate, 2-(vinyloxyethoxyisopropoxy)isopropyl (meth)acrylate, 2-(vinyloxyisopropoxyethoxy)isopropyl (meth)acrylate, 2-(vinyloxyisopropoxyisopropoxy)isopropyl (meth)acrylate, 2-(vinyloxyethoxyethoxyethoxy)ethyl (meth)acrylate, 2-(vinyloxyethoxyethoxyethoxyethoxy)ethyl (meth)acrylate Examples of suitable acrylates include ethyl (meth)acrylate, 2-(isopropenoxyethoxy)ethyl (meth)acrylate, 2-(isopropenoxyethoxyethoxy)ethyl (meth)acrylate, 2-(isopropenoxyethoxyethoxy)ethyl (meth)acrylate, 2-(isopropenoxyethoxyethoxyethoxy)ethyl (meth)acrylate, polyethylene glycol monovinyl ether (meth)acrylate, and polypropylene glycol monovinyl ether (meth)acrylate. Among these specific examples, 2-(2-vinyloxyethoxy)ethyl acrylate is particularly preferred in that it is easy to balance the curability and viscosity of the composition. In this embodiment, 2-(2-vinyloxyethoxy)ethyl acrylate is also referred to as VEEA.

[0066] The content of the vinyl ether group-containing (meth)acrylate relative to the total amount of the ink composition is preferably 5 to 50 mass %, 10 to 40 mass %, or 10 to 30 mass %. When the content of the vinyl ether group-containing (meth)acrylate relative to the total amount of the ink composition is within the above range, the curability of the ink composition tends to be further improved and the viscosity tends to be further reduced.

[0067] The content of the vinyl ether group-containing (meth)acrylate relative to the total amount of polymerizable compounds is preferably 10.0 to 60.0 mass%, 12.5 to 50.0 mass%, or 15.0 to 40.0 mass%. When the content of the vinyl ether group-containing (meth)acrylate relative to the total amount of polymerizable compounds is within the above range, the curability of the ink composition tends to be further improved and the viscosity tends to be further reduced.

[0068] The molecular weight of the vinyl ether group-containing (meth)acrylate is preferably 100 to 350, and more preferably 150 to 300. When the molecular weight of the vinyl ether group-containing (meth)acrylate is within the above range, the curability of the ink composition tends to be further improved.

[0069] The glass transition temperature of the vinyl ether group-containing (meth)acrylate is preferably 0 to 100° C., and more preferably 25 to 75° C. When the glass transition temperature of the vinyl ether group-containing (meth)acrylate is within the above range, blocking resistance and shrink properties tend to be further improved.

[0070] 1.1.2.2. Difunctional (meth)acrylates The ink composition of this embodiment may or may not contain a bifunctional (meth)acrylate. The bifunctional (meth)acrylate is not particularly limited, and examples thereof include dipropylene glycol diacrylate (DPGDA), tripropylene glycol diacrylate (TPGDA), diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, dipropylene glycol dimethacrylate, tripropylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, and 1,4-butanediol. di(meth)acrylate, 1,6-hexanediol di(meth)acrylate (HDDA), 1,9-nonanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, dimethylol-tricyclodecane di(meth)acrylate, EO (ethylene oxide) adduct di(meth)acrylate of bisphenol A, PO (propylene oxide) adduct di(meth)acrylate of bisphenol A, hydroxypivalic acid neopentyl glycol di(meth)acrylate, and polytetramethylene glycol di(meth)acrylate.

[0071] The content of the bifunctional (meth)acrylate relative to the total amount of the ink composition is preferably 5 to 50 mass%, 10 to 40 mass%, or 10 to 30 mass%. When the content of the bifunctional (meth)acrylate relative to the total amount of the ink composition is within the above range, the curability of the ink composition tends to be further improved and the viscosity tends to be further reduced.

[0072] The content of the bifunctional (meth)acrylate relative to the total amount of the polymerizable compounds is preferably 10.0 to 60.0 mass%, 12.5 to 50.0 mass%, or 15.0 to 40.0 mass%. When the content of the bifunctional (meth)acrylate relative to the total amount of the polymerizable compounds is within the above range, the curability of the ink composition tends to be further improved and the viscosity tends to be further reduced.

[0073] The molecular weight of the bifunctional (meth)acrylate is preferably 150 to 400, and more preferably 200 to 350. When the molecular weight of the bifunctional (meth)acrylate is within the above range, the curability of the ink composition tends to be further improved.

[0074] The glass transition temperature of the bifunctional (meth)acrylate is preferably 0 to 150° C., and more preferably 25 to 125° C. When the glass transition temperature of the bifunctional (meth)acrylate is within the above range, blocking resistance and shrink properties tend to be further improved.

[0075] 1.1.2.3. Tri- or higher functional (meth)acrylates The ink composition of this embodiment may or may not contain a tri- or higher functional (meth)acrylate. The tri- or higher functional (meth)acrylate is not particularly limited, and examples thereof include trimethylolpropane tri(meth)acrylate, EO-modified trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, glycerin propoxy tri(meth)acrylate, caprolactone-modified trimethylolpropane tri(meth)acrylate, pentaerythritol ethoxy tetra(meth)acrylate, and caprolactam-modified dipentaerythritol hexa(meth)acrylate.

[0076] The content of the tri- or higher functional (meth)acrylate is preferably 5.0 to 20.0% by mass, and more preferably 10.0 to 17.5% by mass, relative to the total amount of the ink composition.

[0077] The content of the tri- or higher functional (meth)acrylate is preferably 10.0 to 60.0 mass %, 12.5 to 50.0 mass %, or 15.0 to 40.0 mass % relative to the total amount of the polymerizable compounds.

[0078] The total amount of the polyfunctional polymerizable compound is preferably 7 to 45 mass %, 8 to 40 mass %, 9 to 35 mass %, or 10 to 30 mass % relative to the total amount of the ink composition.

[0079] The total amount of the polyfunctional polymerizable compounds is preferably 8 to 55 mass%, 9 to 50 mass%, 10 to 45 mass%, 11 to 40 mass%, or 12 to 35 mass%, relative to the total amount of the polymerizable compounds.

[0080] 1.2. Photoinitiators The ink composition of this embodiment contains a photoinitiator. When the photoinitiator is irradiated with radiation, the photoinitiator generates active species. The photoinitiator may also contain a compound that not only generates active species upon irradiation with radiation but also functions as a photosensitizer. The function as a photosensitizer refers to the ability to absorb light having a certain wavelength and emit fluorescence having another wavelength. Specifically, the function refers to the ability to absorb light having a wavelength around 300 to 450 nm and emit fluorescence having a wavelength around 400 to 500 nm. The photoinitiator includes high-molecular-weight photoinitiators having a molecular weight of 500 or more and low-molecular-weight photoinitiators having a molecular weight less than 500. Hereinafter, high-molecular-weight photoinitiators having a molecular weight of 500 or more will be referred to simply as high-molecular-weight photoinitiators, and low-molecular-weight photoinitiators having a molecular weight less than 500 will be referred to simply as low-molecular-weight photoinitiators. One type of photoinitiator may be used alone, or two or more types may be used in combination.

[0081] The content of the photoinitiator is preferably 1 to 20% by mass, and more preferably 5 to 15% by mass, relative to the total amount of the ink composition.

[0082] 1.2.1. High molecular weight photoinitiators The ink composition of this embodiment contains a high molecular weight photoinitiator. Since high molecular weight photoinitiators are less likely to volatilize, the ink composition of this embodiment contains a high molecular weight photoinitiator, which reduces the odor of the ink composition.

[0083] In addition, since low-molecular-weight components in ink compositions are easily volatile, it is believed that the low-molecular-weight components cause a deterioration in odor. Therefore, when a low-molecular-weight photoinitiator is used, unreacted low-molecular-weight photoinitiators remain in the coating film, and this volatilization is thought to worsen the odor. In this regard, since high-molecular-weight photoinitiators are less likely to volatilize, it is thought that using a high-molecular-weight photoinitiator reduces the odor of the ink composition. However, the mechanism by which the odor of the ink composition is reduced is not limited to the above.

[0084] The molecular weight of the high molecular weight photoinitiator is 500 or more, preferably 500 to 2000, 750 to 1950, or 1000 to 1900. When the molecular weight of the high molecular weight photoinitiator is within the above range, odor resistance and curability tend to be excellent. The high molecular weight photoinitiator is not particularly limited as long as it has a molecular weight of 500 or more, and examples thereof include thioxanthone-based initiators, α-hydroxyketone-based initiators, ketosulfonic acid-based initiators, and amine-based initiators. Note that some amine-based initiators also function as photosensitizers.

[0085] More specifically, the high molecular weight photoinitiator may be 1-(4-[(4-benzoylphenyl)thio]phenyl)-2-methyl-2-[(4-methylphenyl)sulfonyl]-1-propan-1-one (CAS: 272460-97-6), oligo-[2-hydroxy-2-methyl-1-(4-(1-methylvinyl)phenyl)propanone (CAS: 163702-01-0), alpha-[2-[( 9-oxo-9H-thioxanthenyl)oxy]acetyl]-omega-[[2-[(9-oxo-9H-thioxanthenyl)oxy]acetyl]oxy]poly(oxy-1,4-butanediyl) (CAS: 813452-37-8), 1,3-di({a-[1-chloro-9-oxo-9H-thioxanthen-4-yl)oxy]acetylpoly[oxy(1-methylethylene)]}oxy)-2,2 -bis({a-[1-methylethylene]}oxymethyl)propane (CAS: 1003567-83-6), 1,3-di({-4-(dimethylamino)benzoylpoly[oxy(1-methylethylene)]}oxy)-2,2-bis({-4-(dimethylamino)-benzoylpoly[oxy(1-methylethylene)]}oxymethyl)propane and a mixture of {a-4-(dimethylamino)benzoylpoly(oxyethylene)-poly[oxy(1-methylethylene)]-poly(oxyethylene)}4-dimethylamino)benzoate (CAS: 1003567-84-7 and 1003557-17-2), and polyethylene glycol di(β-4-[4-(2-dimethylamino-2-benzyl)butonylphenyl]piperazine)propionate (CAS: 886463-10-1). Specific products include, but are not limited to, ESACURE 1001M, ESACURE KIP 150, Omnipol TX, SpeedCure 7010, SpeedCure 7040, and Omnipol 910 (all of which are product names manufactured by IGM REGIN).Mixtures of 1,3-di({-4-(dimethylamino)benzoylpoly[oxy(1-methylethylene)]}oxy)-2,2-bis({-4-(dimethylamino)-benzoylpoly[oxy(1-methylethylene)]}oxymethyl)propane and {α-4-(dimethylamino)benzoylpoly(oxyethylene)-poly[oxy(1-methylethylene)]-poly(oxyethylene)}4-dimethylamino)benzoate and polyethylene glycol di(β-4-[4-(2-dimethylamino-2-benzyl)butonylphenyl]piperazine)propionate also function as photosensitizers.

[0086] The content of the high molecular weight photoinitiator relative to the total amount of the ink composition is preferably 1.0 to 10.0 mass%, 1.5 to 9.0 mass%, or 2.0 to 8.0 mass%. When the content of the high molecular weight photoinitiator relative to the total amount of the ink composition is within the above range, the odor of the resulting recorded matter tends to be further reduced.

[0087] The content of the high molecular weight photoinitiator relative to the total amount of photoinitiators is preferably 15 to 100 mass%, 20 to 85 mass%, 25 to 70 mass%, or 30 to 60 mass%. When the content of the high molecular weight photoinitiator relative to the total amount of photoinitiators is within the above range, the odor of the resulting recorded matter tends to be further reduced, and the curability of the ink composition also tends to be maintained.

[0088] 1.2.2. Low molecular weight photoinitiators The ink composition of this embodiment may contain a low-molecular-weight photoinitiator. When the ink composition contains a low-molecular-weight photoinitiator, the curability tends to be further improved. In other words, when the ink composition of this embodiment contains a high-molecular-weight photoinitiator and a low-molecular-weight photoinitiator, it tends to be possible to achieve both odor reduction and curability.

[0089] The molecular weight of the low-molecular-weight photoinitiator is less than 500, preferably 50 or more and less than 500, such as 100 to 450, 200 to 450, 300 to 450, or 350 to 450. The low-molecular-weight photoinitiator is not particularly limited as long as it has a molecular weight of less than 500, and examples thereof include acylphosphine-based photoinitiators, α-hydroxyketone-based photoinitiators, amine-based photoinitiators, and thioxanthone-based photoinitiators.

[0090] More specifically, examples of low molecular weight photoinitiators include phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (CAS: 162881-26-7), 2-hydroxy-1-[4-(4-(2-hydroxy-2-methylpropionyl)phenoxy)-phenyl]-2-methylpropan-1-one (CAS: 71868-15-0), (methylimino)diethane-2,1-diylbis[4-(dimethylamino)-benzoate] (CAS: 925246-00-0), ethyl phenyl(2,4,6-trimethylbenzoyl)phosphinate, and 2,4-diethylthioxanthone. Specific products include, but are not limited to, Omnirad 819, ESACURE KIP 160, ESACURE A198, Speedcure DETX, and Omnirad TPO-L (all of which are product names manufactured by IGM REGIN). (Methylimino)diethane-2,1-diylbis[4-(dimethylamino)-benzoate] also functions as a photosensitizer.

[0091] The content of the low-molecular-weight photoinitiator relative to the total amount of the ink composition is preferably 1.0 to 10.0 mass%, 1.5 to 9.0 mass%, or 2.0 to 8.0 mass%. When the content of the low-molecular-weight photoinitiator relative to the total amount of the ink composition is within the above range, the odor of the resulting recorded matter tends to be further reduced.

[0092] The content of the low-molecular-weight photoinitiator relative to the total amount of photoinitiators is preferably 40 to 99 mass%, 45 to 90 mass%, 50 to 80 mass%, or 55 to 70 mass%. When the content of the low-molecular-weight photoinitiator relative to the total amount of photoinitiators is within the above range, the odor of the resulting recorded matter tends to be further reduced, and the curability of the ink composition also tends to be maintained.

[0093] 1.3.Polymerization inhibitors The ink composition of this embodiment may contain a polymerization inhibitor. The polymerization inhibitor is not particularly limited, but examples thereof include hydroquinones typified by hydroquinone, hydroquinone monomethyl ether (MEHQ), 1-o-2,3,5-trimethylhydroquinone, and 2-tert-butylhydroquinone; catechols typified by catechol, 4-methylcatechol, and 4-tert-butylcatechol; phenol, butylhydroxytoluene, butylhydroxyanisole, p-methoxyphenol, cresol, pyrogallol, 3,5-di-t-butyl-4-hydroxytoluene, 2,2'-methylenebis(4-methyl-6-t-butylphenol), and 2,2'-methylenebis(4-ethyl-6-butyl phenols, such as 4,4'-thiobis(3-methyl-6-t-butylphenol); compounds having a 2,2,6,6-tetramethylpiperidine-N-oxyl skeleton, such as 2,2,6,6-tetramethyl-4-hydroxypiperidine-1-oxyl; compounds having a 2,2,6,6-tetramethylpiperidine skeleton, such as bis(2,2,6,6-tetramethyl-4-piperidyl) sebacate; compounds having a 2,2,6,6-tetramethylpiperidine-N-alkyl skeleton; and hindered amines, such as compounds having a 2,2,6,6-tetramethylpiperidine-N-acyl skeleton. The polymerization inhibitors may be used alone or in combination.

[0094] The content of the polymerization inhibitor relative to the total amount of the ink composition is not particularly limited, but is, for example, 0.1 to 1.0% by mass.

[0095] 1.4.Surfactants The ink composition of this embodiment may contain a surfactant. Examples of the surfactant include silicone-based surfactants, fluorine-based surfactants, and acetylene glycol-based surfactants. Among these, silicone-based surfactants are preferred. The surfactant may be used alone or in combination of two or more types.

[0096] Examples of silicone surfactants include polysiloxane compounds and polyether-modified organosiloxanes. Commercially available silicone surfactants include, but are not limited to, BYK-306, BYK-307, BYK-333, BYK-341, BYK-345, BYK-346, BYK-347, BYK-348, BYK-349, and BYK-UV3500 (all trade names, manufactured by BYK Japan K.K.), KF-351A, KF-352A, KF-353, KF-354L, KF-355A, KF-615A, KF-945, KF-640, KF-642, KF-643, KF-6020, X-22-4515, KF-6011, KF-6012, KF-6015, and KF-6017 (all trade names, manufactured by Shin-Etsu Chemical Co., Ltd.).

[0097] The fluorine-based surfactant is not particularly limited, but examples thereof include perfluoroalkyl sulfonate, perfluoroalkyl carboxylate, perfluoroalkyl phosphate, perfluoroalkyl ethylene oxide adduct, perfluoroalkyl betaine, and perfluoroalkyl amine oxide compound. Commercially available fluorine-based surfactants are not particularly limited, but examples thereof include S-144, S-145 (manufactured by Asahi Glass Co., Ltd.); FC-170C, FC-430, Fluorad-FC4430 (manufactured by Sumitomo 3M Limited); FSO, FSO-100, FSN, FSN-100, FS-300 (manufactured by DuPont); and FT-250, 251 (manufactured by Neos Corporation).

[0098] The acetylene glycol surfactant is not particularly limited, but preferably includes at least one selected from the group consisting of 2,4,7,9-tetramethyl-5-decyne-4,7-diol, alkylene oxide adducts of 2,4,7,9-tetramethyl-5-decyne-4,7-diol, and alkylene oxide adducts of 2,4-dimethyl-5-decyne-4-ol and 2,4-dimethyl-5-decyne-4-ol. Commercially available acetylene glycol surfactants are not particularly limited, but examples thereof include the E series such as Olfin 104 series and Olfin E1010 (trade names manufactured by Air Products Japan, Inc.), Surfynol 465 and Surfynol 61 (trade names manufactured by Nissin Chemical Industry Co., Ltd.).

[0099] The content of the surfactant relative to the total amount of the ink composition is not particularly limited, but is, for example, 0.1 to 5.0% by mass.

[0100] 1.5.Colorants The ink composition of this embodiment may contain a colorant. Examples of colorants include dyes and pigments. The content of the colorant relative to the total amount of the ink composition is not particularly limited, but is, for example, 1 to 10% by mass. The ink composition of this embodiment may be a clear ink that does not contain a colorant, or that contains a colorant to an extent that is not intended to color the ink (for example, 0.1% by mass or less). The colorant may be used alone, or two or more types may be used in combination.

[0101] 1.5.1. Pigments By using a pigment as the coloring material, the light resistance of the ink composition of this embodiment can be improved. Both inorganic pigments and organic pigments can be used as the pigment.

[0102] As inorganic pigments, carbon blacks (CI (Colour Index Generic Name) Pigment Black 7) such as furnace black, lamp black, acetylene black, and channel black, iron oxide, and titanium oxide can be used.

[0103] Examples of organic pigments include azo pigments such as insoluble azo pigments, condensed azo pigments, azo lakes, and chelate azo pigments; polycyclic pigments such as phthalocyanine pigments, perylene and perinone pigments, anthraquinone pigments, quinacridone pigments, dioxane pigments, thioindigo pigments, isoindolinone pigments, and quinophthalone pigments; dye chelates (for example, basic dye chelates, acid dye chelates, etc.); dye lakes (basic dye lakes, acid dye lakes), nitro pigments, nitroso pigments, aniline black, and daylight fluorescent pigments.

[0104] More specifically, carbon blacks used for the black include No. 2300, No. 900, MCF88, No. 33, No. 40, No. 45, No. 52, MA7, MA8, MA100, No. 2200B, etc. (all manufactured by Mitsubishi Chemical Corporation), Raven 5750, Raven 5250, Raven 5000, Raven 3500, Raven 1255, Raven 700, etc. (all manufactured by Carbon Columbia), Rega1 400R, Rega1 330R, Rega1 660R, Mogul L, Monarch 700, Monarch 800, Monarch 880, Monarch 900, Monarch 1000, Monarch 1100, Monarch 1300, Monarch 1400 (manufactured by Cabot Corporation (CABOTJAPAN KK)), Color Black FW1, Color Black FW2, Color Black FW2V, Color Black FW18, Color Black FW200, Color Black S150, Color Black S160, Color Black S170, Printex 35, Printex U, Printex V, Printex 140U, Special Black 6, Special Black 5, Special Black 4A, Special Black 4 (all manufactured by Degussa).

[0105] Pigments used for white include CI Pigment White 6, 18, and 21.

[0106] Pigments used for yellow include CI Pigment Yellow 1, 2, 3, 4, 5, 6, 7, 10, 11, 12, 13, 14, 16, 17, 24, 34, 35, 37, 53, 55, 65, 73, 74, 75, 81, 83, 93, 94, 95, 97, 98, 99, 108, 109, 110, 113, 114, 117, 120, 124, 128, 129, 133, 138, 139, 147, 151, 153, 154, 155, 167, 172, 180, and 185.

[0107] Pigments used for magenta include CI Pigment Red 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 14, 15, 16, 17, 18, 19, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 2, 23, 30, 31, 32, 37, 38, 40, 41, 42, 48(Ca), 48(Mn), 57(Ca), 57:1, 88, 112, 114, 122, 123, 144, 146, 149, 150, 166, 168, 170, 171, 175, 176, 177, 178, 179, 184, 185, 187, 202, 209, 219, 224, 245, or CI Pigment Violet 19, 23, 32, 33, 36, 38, 43, 50.

[0108] Pigments used for cyan include CI Pigment Blue 1, 2, 3, 15, 15:1, 15:2, 15:3, 15:34, 15:4, 16, 18, 22, 25, 60, 65, 66, CI Vat Blue 4, 60.

[0109] Furthermore, examples of pigments other than magenta, cyan, and yellow include CI Pigment Green 7, 10, CI Pigment Brown 3, 5, 25, 26, and CI Pigment Orange 1, 2, 5, 7, 13, 14, 15, 16, 24, 34, 36, 38, 40, 43, and 63.

[0110] The content of the pigment relative to the total amount of the ink composition is not particularly limited, but is, for example, 1 to 10% by mass.

[0111] Dispersants When the ink composition of this embodiment contains a pigment, it may contain a dispersant to improve the dispersibility of the pigment. The dispersant may be used alone or in combination of two or more types.

[0112] The dispersant is not particularly limited, but examples thereof include dispersants commonly used in preparing pigment dispersions, such as polymer dispersants, and specific examples thereof include those containing one or more of polyoxyalkylene polyalkylene polyamines, vinyl polymers and copolymers, acrylic polymers and copolymers, polyesters, polyamides, polyimides, polyurethanes, amino polymers, silicon-containing polymers, sulfur-containing polymers, fluorine-containing polymers, and epoxy resins as the main component.

[0113] Commercially available polymer dispersants include the Ajisper series manufactured by Ajinomoto Fine-Techno Co., Ltd., the Solsperse series (Solsperse 36000, etc.) available from Avecia and Noveon, the Disperbic series manufactured by BYK Additives & Instruments, and the Disparlon series manufactured by Kusumoto Chemicals Co., Ltd.

[0114] The content of the dispersant relative to the total amount of the ink composition is not particularly limited, but is, for example, 0.01 to 1.00% by mass.

[0115] 1.5.3.Dye The dye is not particularly limited, and examples thereof include acid dyes, direct dyes, reactive dyes, and basic dyes. The dye is not particularly limited, and examples thereof include CI Acid Yellow 17, 23, 42, 44, 79, 142, CI Acid Red 52, 80, 82, 249, 254, 289, CI Acid Blue 9, 45, 249, CI Acid Black 1, 2, 24, 94, CI Food Black 1, 2, CI Direct Yellow 1, 12, 24, 33, 50, 55, 58, 86, 132, 14 2, 144, 173, CI Direct Red 1, 4, 9, 80, 81, 225, 227, CI Direct Blue 1, 2, 15, 71, 86, 87, 98, 165, 199, 202, CI Direct Black 19, 38, 51, 71, 154, 168, 171, 195, CI Reactive Red 14, 32, 55, 79, 249, CI Reactive Black 3, 4, 35.

[0116] The content of the dye is not particularly limited to the total amount of the ink composition, but is, for example, 1 to 10% by mass.

[0117] 2. Method for producing radiation-curable inkjet ink composition The radiation-curable inkjet ink composition of this embodiment is not particularly limited, and may be, for example, a mixture of the above-described components. Alternatively, a colorant dispersion may be prepared by dispersing a colorant and a dispersant in a solvent, and the resulting colorant dispersion may be mixed with the other components described above. Note that in the radiation-curable inkjet ink composition, the solvent is not limited to the polymerizable compound described above.

[0118] 3. Recording Media The recording medium used for recording with the ink composition of this embodiment is not particularly limited, but examples thereof include absorbent recording media, low absorbent recording media, and non-absorbent recording media.

[0119] Absorbent recording media are not particularly limited, but examples include plain paper such as electrophotographic paper with high ink permeability, inkjet paper (paper specifically for inkjet printers with an ink absorbing layer made of silica particles or alumina particles, or an ink absorbing layer made of a hydrophilic polymer such as polyvinyl alcohol (PVA) or polyvinylpyrrolidone (PVP)), and fabric.

[0120] The low-absorbency recording medium is not particularly limited, but examples thereof include art paper, coated paper, cast paper, and the like, which are used in general offset printing and have relatively low ink permeability.

[0121] Non-absorbent recording media are not particularly limited, but examples include films and plates of plastics such as polyvinyl chloride, polyethylene, polypropylene, polyethylene terephthalate (PET), polycarbonate, polystyrene, polyurethane, etc.; plates of metals such as iron, silver, copper, aluminum, etc.; metal plates and plastic films manufactured by vapor deposition of these various metals, and alloy plates such as stainless steel and brass; and recording media in which a plastic film such as polyvinyl chloride, polyethylene, polypropylene, polyethylene terephthalate (PET), polycarbonate, polystyrene, polyurethane, etc. is adhered (coated) to a paper substrate.

[0122] The ink composition of this embodiment is suitable for use in printing on low-absorbency recording media and non-absorbency recording media, and is particularly suitable for use in printing on recording media such as flexible packaging and shrink labels, which are non-absorbency recording media that shrink when heated.

[0123] 4. Recording device The recording apparatus of this embodiment is an inkjet recording apparatus used in printing using the ink composition of this embodiment. As an example of an inkjet recording apparatus, FIG. 1 shows a perspective view of a serial printer. As shown in FIG. 1, the serial printer 20 includes a conveying unit 220 and a recording unit 230. The conveying unit 220 conveys the recording medium F fed to the serial printer to the recording unit 230 and ejects the recording medium after recording outside the serial printer. Specifically, the conveying unit 220 has feed rollers and conveys the fed recording medium F in the sub-scanning direction T2.

[0124] The recording unit 230 also includes an inkjet head 231 that ejects ink, etc. onto the recording medium F sent from the conveying unit 220, a light source 232 that irradiates radiation onto the adhering ink, etc., a carriage 234 that carries these, and a carriage movement mechanism 235 that moves the carriage 234 in the main scanning directions S1 and S2 of the recording medium F.

[0125] In the case of a serial printer, an inkjet head 231 having a length smaller than the width of the recording medium is provided, and the head moves to perform recording in multiple passes. In a serial printer, the head 231 is mounted on a carriage 234 that moves in a predetermined direction, and the head moves in conjunction with the movement of the carriage, thereby ejecting the ink composition onto the recording medium F. In this way, recording is performed in two or more passes. A pass is also called a main scan. A sub-scan is performed to transport the recording medium between passes. In other words, main scans and sub-scans are performed alternately.

[0126] Furthermore, while the recording device of this embodiment has been described above with reference to Figure 1 using a serial printer as an example, the recording device of this embodiment may also be a line printer. In a line printer, a line head is fixed and the recording medium is moved in the sub-scanning direction (the longitudinal direction of the recording medium, the transport direction), and ink droplets are ejected from the nozzle openings of the head in conjunction with this movement to record an image on the recording medium. Generally, in a line printer, the head is fixed and does not move, and recording is performed in one pass (single pass), which gives line printers an advantage over serial printers in terms of faster recording speed.

[0127] 5. Inkjet recording method The inkjet recording method of this embodiment includes a discharge step of discharging the ink composition of this embodiment onto a recording medium, and may also include, as necessary, a curing step of curing the ink composition of this embodiment that has been deposited on the recording medium in the discharge step, a transport step of transporting the recording medium, a winding step of winding up the recording medium, etc.

[0128] 5.1.Discharge process In the ejection step of this embodiment, the ink composition of this embodiment is ejected from an inkjet head onto a recording medium. More specifically, a pressure generating means provided in the inkjet head is driven to eject the ink composition filled in the pressure generating chamber of the inkjet head from the nozzle onto the recording medium.

[0129] Inkjet heads used in the ejection step include a line head that performs recording by a line method and a serial head that performs recording by a serial method.

[0130] In the line method using a line head, for example, an inkjet head having a width equal to or greater than the recording width of the recording medium is fixed to the recording device. The recording medium is then moved in the sub-scanning direction (the direction in which the recording medium is transported), and ink droplets are ejected from the nozzles of the inkjet head in conjunction with this movement to record an image on the recording medium.

[0131] In the serial method using a serial head, for example, the inkjet head is mounted on a carriage that can move in the width direction of the recording medium. The carriage is then moved in the main scanning direction (the width direction of the recording medium), and ink droplets are ejected from the nozzles of the inkjet head in conjunction with this movement to record an image on the recording medium.

[0132] The thickness of the ink film formed on the recording medium by the ejection step is preferably 5 μm or less, and more preferably 1 to 5 μm. When the thickness of the ink film is within the above range, the shrinkability of the ink composition, and the adhesion and curability of the ink composition to the recording medium tend to be improved.

[0133] 5.2.Curing process The inkjet recording method of this embodiment may include a curing step of curing the ink composition of this embodiment that has been deposited on the recording medium in the ejection step. In the curing step, the ink composition of this embodiment that has been deposited on the recording medium is cured by irradiating it with radiation. When irradiated with radiation, a polymerization reaction of the polymerizable compound is initiated, curing the composition and forming a coating film. If a photoinitiator is present at this time, it generates active species (initiating species) such as radicals, acids, and bases, and the polymerization reaction of the polymerizable compound is promoted by the function of the initiating species.

[0134] Examples of the radiation include ultraviolet light, infrared light, visible light, and X-rays. The radiation source is installed downstream of the inkjet head and irradiates the composition. The radiation source is not particularly limited, but examples include UV-LEDs. Use of such a radiation source can reduce the size and cost of the device. UV-LEDs as an ultraviolet light source are small and can be installed inside the inkjet recording device.

[0135] 5.3.Transportation process The inkjet recording method of this embodiment may include a transport step of transporting a recording medium. In the transport step, the recording medium is transported in a predetermined direction within the recording device. More specifically, the recording medium is transported from a paper feed section to a paper discharge section of the recording device using a transport roller or a transport belt provided within the recording device. During this transport process, the ink composition ejected from the inkjet head adheres to the recording medium, forming a recorded product.

[0136] 5.4. Winding process The inkjet recording method of this embodiment may include a winding step of winding the recording medium. In the winding step, for example, the recording medium onto which the ink composition of this embodiment has been ejected is wound up.

[0137] By carrying out the winding process, the recorded matter can be stored more efficiently. On the other hand, the winding process causes the recorded matter to overlap with each other under greater pressure. This makes the problem of blocking more likely to occur. In this regard, the ink composition of this embodiment has excellent blocking resistance, making the blocking problem less likely to occur, and the effects of the present invention more pronounced.

[0138] In the inkjet recording method of this embodiment, the order and timing of the ejection step, curing step, transport step, and winding step are not particularly limited, and for example, the four steps may be performed simultaneously or alternately. Also, some of these steps may be performed simultaneously and the remaining steps may be performed separately. [Example]

[0139] The present invention will be described in more detail below using examples and comparative examples. The present invention is not limited to the following examples. Unless otherwise specified, the experiments in the examples and comparative examples were carried out at room temperature (25°C) and 1 atmosphere.

[0140] 1. Preparation of Radiation-Curable Inkjet Ink Composition First, the colorant, dispersant, and a portion of each polymerizable compound were weighed and placed in a pigment dispersion tank. A ceramic bead mill with a diameter of 1 mm was then placed in the tank and stirred to obtain a pigment dispersion in which the colorant was dispersed in the polymerizable compound. Next, the remaining components were placed in a stainless steel mixing tank, and mixed and stirred to completely dissolve the components, resulting in the composition shown in Figures 2 to 5. The pigment dispersion obtained above was then added, and the mixture was further mixed and stirred at room temperature. The resulting mixture was then filtered through a membrane filter to obtain the ink composition of each example. The numerical values ​​for each component shown in each example in the figures represent % by mass unless otherwise specified. Furthermore, the % by mass of substances that are solid at room temperature represents the solids concentration. Furthermore, the components in the tables are as follows:

[0141] MEHQ (product name: p-methoxyphenol, hydroquinone monomethyl ether, manufactured by Kanto Chemical Co., Ltd.) LA-7RD (2,2,6,6-tetramethyl-4-hydroxypiperidine-1-oxyl, manufactured by ADEKA Corporation) Omnirad 819 (acylphosphine photoinitiator, phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, manufactured by IGM Resin) ESACURE KIP 160 (α-hydroxyketone photoinitiator, 2-hydroxy-1-[4-(4-(2-hydroxy-2-methylpropionyl)phenoxy)-phenyl]-2-methylpropan-1-one, manufactured by IGM Resin) ESACURE A198 (amine photoinitiator, (methylimino)diethane-2,1-diylbis[4-(dimethylamino)-benzoate], manufactured by IGM Resin) SpeedCure DETX (thioxanthone-based photoinitiator, 2,4-diethylthioxanthone, manufactured by IGM Resin) ESACURE 1001M (ketosulfonic acid photoinitiator, 1-(4-[(4-benzoylphenyl)thio]phenyl)-2-methyl-2-[(4-methylphenyl)sulfonyl]-1-propan-1-one, manufactured by IGM Resin) ESACURE KIP 150 (α-hydroxyketone photoinitiator, oligo-[2-hydroxy-2-methyl-1-(4-(1-methylvinyl)phenyl]propanone, manufactured by IGM Resin) SpeedCure 7010 (thioxanthone photoinitiator, 1,3-di({a-[1-chloro-9-oxo-9H-thioxanthen-4-yl)oxy]acetylpoly[oxy(1-methylethylene)]}oxy)-2,2-bis({a-[1-methylethylene]}oxymethyl)propane, manufactured by IGM Resin) Omnipol TX (thioxanthone-based photoinitiator, alpha-[2-[(9-oxo-9H-thioxanthenyl)oxy]acetyl]-omega-[[2-[(9-oxo-9H-thioxanthenyl)oxy]acetyl]oxy]poly(oxy-1,4-butanediyl), manufactured by IGM Resin) Omnipol 910 (amine photoinitiator, polyethylene glycol di(β-4-[4-(2-dimethylamino-2-benzyl)butonylphenyl]piperazine)propionate, manufactured by IGM Resin) SpeedCure 7040 (an amine-based photoinitiator, a mixture of 1,3-di({-4-(dimethylamino)benzoylpoly[oxy(1-methylethylene)]}oxy)-2,2-bis({-4-(dimethylamino)-benzoylpoly[oxy(1-methylethylene)]}oxymethyl)propane and {α-4-(dimethylamino)benzoylpoly(oxyethylene)-poly[oxy(1-methylethylene)]-poly(oxyethylene)}4-dimethylamino)benzoate, manufactured by IGM Resin) BYK UV3500 (BYK Additives & Instruments, polyether-modified polydimethylsiloxane with acryloyl groups) Solsphere 36000 (Avecia) Carbon black (product name "MA-100", manufactured by Mitsubishi Chemical Corporation)

[0142] 2. Creation of Records Using an inkjet printer "PX-G5000" (product name, manufactured by Seiko Epson Corporation), the radiation-curable inkjet ink composition of each example was ejected onto a recording medium, PET film "Bonset" (product name, manufactured by Takiron C.I.), at room temperature and 1 atmosphere pressure, at a recording resolution of 600 dpi x 600 dpi and a droplet weight of 10 ng, to record a solid pattern image consisting of an ink film with a thickness of 5 μm. Note that the solid pattern image is an image in which dots are recorded in all pixels, which are the minimum recording unit area defined by the recording resolution, i.e., an image recorded with a dot generation rate of 100%. While printing as described above, ultraviolet light was irradiated from a UV-LED light source mounted on the carriage, resulting in a recorded product in which a 5 μm-thick cured ink film was formed on the recording medium.

[0143] 3. Evaluation Method 3.1. Curability The radiation-curable inkjet ink composition of each example was applied to a PET film using a bar coater so that the thickness of the cured ink film was 5 μm. 2 The irradiation energy [mJ / cm ] was measured until the sample became tack-free. 2 ] is the irradiance of the light emitted from the light source on the irradiated surface [mW / cm 2 ] was measured and calculated by multiplying this by the duration of irradiation [s].

[0144] The tack-free state was judged under the following conditions. That is, it was judged by whether or not the radiation-curable inkjet ink composition adhered to the cotton swab, or whether or not scratches were left on the cured ink film on the recording medium. The cotton swab used was a Johnson cotton swab manufactured by Johnson & Johnson. The swabs were rubbed back and forth 10 times, with a rubbing force of 100 g.

[0145] Based on the irradiation energy at which the coating became tack-free, the curability was evaluated according to the following evaluation criteria. A grade of B or higher was rated as good. The evaluation results are shown in Figures 2 to 5. [Evaluation criteria] AA: Tack-free energy 150mJ / cm 2 less than A: Tack-free energy 150mJ / cm 2 More than 250mJ / cm 2 less than B: Tack-free energy 250mJ / cm 2 More than 350mJ / cm 2 less than C: Tack-free energy 350mJ / cm 2 End

[0146] 3.2. Odor The odor of the ink cured film of the recorded matter obtained by the above-mentioned production method was smelled and evaluated according to the following criteria. A or higher was considered to be a good level. The evaluation results are shown in Figures 2 to 5. [Evaluation criteria] A: No odor or slight odor. B: There is an odor. C: Has a strong odor.

[0147] 3.3.Adhesion The ink cured film obtained in the above curability evaluation was subjected to a cross-cut test according to the method described in JIS K5600-5-6 to evaluate adhesion. Specifically, grid-shaped cuts were made in the ink cured film of the obtained recording material, and transparent adhesive tape was applied to the grid-shaped cuts. The tape was then rubbed thoroughly with a finger so that the ink cured film was visible through the tape, adhering the tape to the ink cured film. Next, within 5 minutes of tape application, the tape was peeled off from the ink cured film at an angle of approximately 60° in 0.5 to 1.0 seconds. The presence or absence of peeling of the ink cured film from the recording medium at this time was evaluated according to the following evaluation criteria. A grade of B or higher was considered good. The evaluation results are shown in Figures 2 to 5. [Evaluation criteria] A: Peeling of the cured ink film was observed in less than 20% of the grid-shaped area. B: Peeling of the cured ink film was observed in 20% or more and less than 30% of the grid-shaped portion. C: Peeling of the cured ink film was observed in 30% or more of the grid-shaped area.

[0148] 3.4.Viscosity Using a rotational viscometer (product name "Rheometer MCR-301", manufactured by Anton Paar), the viscosity of the ink compositions of the examples and comparative examples was measured immediately after preparation in an environment of 20°C according to JIS Z 8803. The evaluation results are shown in Figures 2 to 5. [Evaluation criteria] AA: Viscosity is less than 15 mPa·s. A: Viscosity is 15 mPa·s or more but less than 25 mPa·s. B: Viscosity is 25 mPa·s or more.

[0149] 3.5.Blocking resistance The recorded matter obtained by the above-mentioned production method was rolled up with the ink cured film facing inward and processed into a cylindrical shape. This recorded matter was placed around a glass bottle, which was the packaged object that had been preheated in a thermostatic chamber, and the glass bottle and the cylindrical recorded matter placed around the glass bottle were left to stand in the thermostatic chamber at 90°C for 10 seconds, causing the cylindrical recorded matter to shrink and adhere tightly to the glass bottle.

[0150] The glass bottles to which the printed matter was attached were visually inspected to see if any traces of the cured ink film had been transferred to the side of the glass bottle, and the blocking resistance was evaluated based on the following criteria. The evaluation results are shown in Figures 2 to 5. [Evaluation criteria] A: The cured ink film does not stick to the glass bottle. B: There is a slight amount of ink cured film sticking to the glass bottle. C: The cured ink film is stuck to the glass bottle (the cured ink film is peeled off from the recording medium).

[0151] 3.6.Shrink properties The recorded matter used for the blocking resistance test was visually observed for the occurrence of wrinkles after shrinkage, and the shrink properties were evaluated based on the following evaluation criteria. The evaluation results are shown in Figures 2 to 5. [Evaluation criteria] A: There are no wrinkles in the ink cured film. B: There are some wrinkles in the cured ink film. C: There are large wrinkles in the cured ink film. [Explanation of symbols]

[0152] 20... serial printer, 220... conveying section, 230... recording section, 231... inkjet head, 234... carriage, 235... carriage moving mechanism, F... recording medium, S1, S2... main scanning direction, T2... sub-scanning direction

Claims

1. a nitrogen-containing monofunctional polymerizable compound and a photoinitiator; the nitrogen-containing monofunctional polymerizable compound comprises N-vinylmethyloxazolidinone or acryloylmorpholine; The photoinitiator comprises a high molecular weight photoinitiator having a molecular weight of 500 or more. Radiation-curable inkjet ink compositions.

2. The high molecular weight photoinitiator comprises at least one of a thioxanthone-based initiator, an α-hydroxyketone-based initiator, a ketosulfonic acid-based initiator, and an amine-based initiator; The radiation-curable ink-jet ink composition of claim 1 .

3. the photoinitiator comprises a low molecular weight photoinitiator having a molecular weight of less than 500; The radiation-curable ink-jet ink composition of claim 1 .

4. The content of the high molecular weight photoinitiator is 15 to 100% by mass based on the total amount of the photoinitiator. The radiation-curable ink-jet ink composition of claim 1 .

5. The polymeric photoinitiator may be 1-(4-[(4-benzoylphenyl)thio]phenyl)-2-methyl-2-[(4-methylphenyl)sulfonyl]-1-propan-1-one, oligo-[2-hydroxy-2-methyl-1-(4-(1-methylvinyl)phenyl)propanone, alpha-[2-[(9-oxo-9H-thioxanthenyl)oxy]acetyl]-omega-[[2-[(9-oxo-9H-thioxanthenyl)oxy]acetyl]oxy]poly(oxy-1,4-butanediyl), 1,3-di({a-[1-chloro-9-oxo-9H-thioxanthen-4-yl)oxy]acetylpoly[oxy(1-methylethylene)]}oxy)-2, a mixture of 2-bis({a-[1-methylethylene]}oxymethyl)propane, 1,3-di({-4-(dimethylamino)benzoylpoly[oxy(1-methylethylene)]}oxy)-2,2-bis({-4-(dimethylamino)-benzoylpoly[oxy(1-methylethylene)]}oxymethyl)propane and {a-4-(dimethylamino)benzoylpoly(oxyethylene)-poly[oxy(1-methylethylene)]-poly(oxyethylene)}4-dimethylamino)benzoate, or polyethylene glycol di(β-4-[4-(2-dimethylamino-2-benzyl)butonylphenyl]piperazine)propionate; The radiation-curable ink-jet ink composition of claim 1 .

6. The low molecular weight photoinitiator comprises one or more of phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, 2-hydroxy-1-[4-(4-(2-hydroxy-2-methylpropionyl)phenoxy)-phenyl]-2-methylpropan-1-one, (methylimino)diethane-2,1-diylbis[4-(dimethylamino)-benzoate], ethyl phenyl(2,4,6-trimethylbenzoyl)phosphinate, or 2,4-diethylthioxanthone; The radiation-curable ink-jet ink composition according to claim 3 .

7. a cyclic ether-containing monofunctional polymerizable compound, The radiation-curable ink-jet ink composition of claim 1 .

8. containing a hydroxyl group-containing polymerizable compound, The radiation-curable ink-jet ink composition of claim 1 .

9. the weighted average of the glass transition temperatures of the polymerizable compounds contained in the inkjet ink composition is 20 to 70°C; The radiation-curable ink-jet ink composition of claim 1 .

10. the total amount of monofunctional polymerizable compounds contained in the inkjet ink composition is 50% by mass or more relative to the total amount of polymerizable compounds contained in the inkjet ink composition; The radiation-curable ink-jet ink composition of claim 1 .

11. a step of ejecting the radiation-curable inkjet ink composition according to any one of claims 1 to 10 onto a recording medium; Inkjet recording method.

12. the thickness of the ink film formed on the recording medium by the ejection step is 5 μm or less; The inkjet recording method according to claim 11.

13. a winding step of winding the recording medium, The inkjet recording method according to claim 11.

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