Radiation-curable inkjet ink composition

Abstract

The purpose of the present disclosure is to provide a radiation-curable inkjet ink composition having excellent odor properties. The radiation-curable inkjet ink composition contains a nitrogen-containing monofunctional polymerizable compound and a photoinitiator, the nitrogen-containing monofunctional polymerizable compound contains N-vinyl methyl oxazolidinone or acryloylmorpholine, and the photoinitiator contains a high molecular weight photoinitiator having a molecular weight of 500 or more.

Description

Technical Field

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

[0002] Inkjet recording methods, which enable the recording of high-definition images using relatively simple equipment, have seen rapid development in various fields. Among these, various research efforts have been conducted on printing methods using radiation-curable inkjet ink compositions. For example, Patent Document 1 discloses a photocurable inkjet printing ink composition comprising 5 to 50% by mass of vinylmethyloxazolidinone, 10 to 50% by mass of a monofunctional photopolymerizable monomer having a glass transition temperature of 10°C or less, a colorant, and a photopolymerization initiator.

[0003] Prior art literature

[0004] Patent Literature

[0005] Patent document 1: International Publication No. 2021 / 199760.

[0006] However, it is known that when printing is performed using the photocurable inkjet printing ink composition described in Patent Document 1, both the odor and curability of the recorded matter can be improved. Summary of the Invention

[0007] The present invention is 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 DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 This is a schematic diagram of a recording device used in this embodiment.

[0009] Figure 2 This is a table showing the results of Examples.

[0010] Figure 3 This is a table showing the results of Examples.

[0011] Figure 4 This is a table showing the results of Examples.

[0012] Figure 5 This is a table showing the results of Examples.

[0013] Description of Reference Numerals

[0014] 20. Serial printer; 220. Transport unit; 230. Recording unit; 231. Inkjet head; 234. Slide; 235. Slide moving mechanism; F. Recording medium; S1, S2, main scanning direction; T1, T2, sub-scanning direction. DETAILED DESCRIPTION

[0015] Hereinafter, an embodiment of the present invention (hereinafter referred to as “this embodiment”) will be described in detail, but the present invention is not limited thereto and various modifications are possible without departing from the spirit and scope of the present invention.

[0016] 1. Radiation-curable inkjet ink composition

[0017] The radiation-curable inkjet ink composition of this embodiment contains a nitrogen-containing monofunctional polymerizable compound and a photoinitiator. 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.

[0018] It is known to use radiation-curable inkjet inks for recording on non-absorbent recording media such as flexible packaging materials and shrink films. However, recorded materials printed using radiation-curable inkjet ink compositions tend to retain an odor derived from the ink, making them difficult to use in applications requiring low odor, such as food packaging and other flexible packaging.

[0019] Therefore, the present inventors have discovered that using a high molecular weight photoinitiator in a radiation-curable inkjet ink composition can reduce the odor of the resulting recorded material. However, the inventors' research and development have shown that ink compositions containing high molecular weight photoinitiators do not have sufficient curing properties.

[0020] Therefore, in this embodiment, in addition to a high molecular weight photoinitiator with a molecular weight of 500 or greater, N-vinylmethyloxazolidinone or acryloylmorpholine is used in combination as a nitrogen-containing monofunctional polymerizable compound. The inclusion of N-vinylmethyloxazolidinone or acryloylmorpholine allows for sufficient curability even when a high molecular weight photoinitiator is used. In other words, the radiation-curable inkjet ink composition of this embodiment exhibits excellent curability in addition to the odor-reducing effect of the high molecular weight photoinitiator on recorded materials.

[0021] Hereinafter, each component of the radiation-curable inkjet ink composition of this embodiment will be described in detail. It should be noted that the radiation-curable inkjet ink composition of this embodiment will also be simply referred to as an "ink composition" hereinafter.

[0022] 1.1. Polymeric compounds

[0023] In this embodiment, the term "polymerizable compound" 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 cure. Polymerizable compounds include monomers and oligomers. Examples of polymerizable compounds include, but are not limited to, nitrogen-containing monomers, nitrogen-containing oligomers, cyclic ether-containing monomers, cyclic ether-containing oligomers, hydroxyl-containing monomers, and hydroxyl-containing oligomers. These polymerizable compounds may be used alone or in combination of two or more.

[0024] In addition, in this embodiment, the number of polymerization of monomers in the oligomer is not particularly limited, and is, for example, 2 to 100, 2 to 50, 2 to 25, 2 to 10, or 2 to 5.

[0025] The content of the polymerizable compound is preferably 60 to 95% by mass, 65 to 90% by mass, or 70 to 90% by mass relative to the total amount of the ink composition.

[0026] The weighted average glass transition temperature of each polymerizable compound contained in the ink composition of this embodiment is preferably 20-70°C, 25-65°C, 30-60°C, or 40-50°C. A weighted average glass transition temperature of 20°C or higher tends to improve blocking resistance. Furthermore, a weighted average glass transition temperature 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 shrinkage characteristics of the ink composition tend to be improved. Therefore, by setting the weighted average glass transition temperature of the polymerizable compounds contained in the ink composition of this embodiment within the above range, both blocking resistance and shrinkage characteristics tend to be achieved. Blocking refers to the phenomenon in which overlapping recorded materials adhere to each other due to the tackiness of the ink. Improved blocking resistance reduces this tendency.

[0027] It should be noted that the "glass transition temperature of a polymerizable compound" refers to the glass transition temperature of a homopolymer of the polymerizable compound. The weighted average of the glass transition temperatures of the polymerizable compound can be adjusted by adjusting the glass transition temperature of the homopolymer of the polymerizable compound used and the mass ratio of the polymerizable compound used.

[0028] Here, the method for calculating the weighted average glass transition temperature of the homopolymer in the polymerizable compound is described. The weighted average value of the glass transition temperature of the homopolymer is set as Tg All The glass transition temperature of the homopolymer of each polymerizable compound is set as Tg NThe mass ratio of the polymerizable compound is set to X N (wt%). N is a number that is added sequentially starting from 1 according to the type of polymerizable compound contained in the ink composition of this embodiment. For example, when three polymerizable compounds are used, Tg1, Tg2, and Tg3 are generated. The weighted average Tg of the glass transition temperature of the homopolymer is All is the glass transition temperature Tg of the homopolymer calculated from each polymerizable compound N and content mass ratio X N Therefore, the following formula (1) holds.

[0029]

[0030] The glass transition temperature of a homopolymer of a polymerizable compound can be measured by differential scanning calorimetry (DSC) in accordance with JIS K7121. For example, the measuring apparatus used is the "DSC6220" manufactured by Seiko Instruments Inc. As a sample, a polymerizable compound can be polymerized to a point where the glass transition temperature of the homopolymer becomes constant.

[0031] 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, curability tends to be further improved.

[0032] 1.1.1. Monofunctional polymerizable compounds

[0033] In this embodiment, the monofunctional polymerizable compound is not particularly limited, and examples thereof include nitrogen-containing monofunctional polymerizable compounds, cyclic ether-containing monofunctional polymerizable compounds, hydroxyl-containing monofunctional polymerizable compounds, and other monofunctional polymerizable compounds other than these monofunctional polymerizable compounds.

[0034] 1.1.1.1. Nitrogen-containing monofunctional polymerizable compounds

[0035] The ink composition of this embodiment contains a nitrogen-containing monofunctional polymerizable compound. Consequently, even when the ink composition of this embodiment contains a high molecular weight photoinitiator, described below, sufficient curability tends to be achieved. While the main reason for the improved curability is not particularly limited, it is believed that the nitrogen-containing monofunctional polymerizable compound is less susceptible to the quenching of free radicals from the photoinitiator caused by oxygen.

[0036] The ink composition of this embodiment contains N-vinylmethyloxazolidinone (VMOX) or acryloylmorpholine (ACMO) as a nitrogen-containing monofunctional polymerizable compound. The inclusion of VMOX or ACMO in the ink composition of this embodiment tends to further improve the curability of the ink composition. Furthermore, the ink composition of this embodiment may also contain nitrogen-containing monofunctional polymerizable compounds other than VMOX and 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, diacetone acrylamide, N,N-dimethyl (meth)acrylamide, and dimethylaminoethyl acrylate benzyl chloride quaternary salt; and oligomers thereof.

[0037] The ink composition of this embodiment preferably contains a nitrogen-containing monofunctional polymerizable compound having a nitrogen-containing heterocyclic structure. The ink composition of this embodiment tends to have improved curability by containing a nitrogen-containing monofunctional polymerizable compound having a nitrogen-containing heterocyclic structure. VMOX and ACMO are nitrogen-containing monofunctional polymerizable compounds having a nitrogen-containing heterocyclic structure.

[0038] The content of the nitrogen-containing monofunctional polymerizable compound relative to the total mass of the ink composition is preferably 10-50 mass%, 15-45 mass%, 20-40 mass%, or 20-35 mass%. When the content of the nitrogen-containing monofunctional polymerizable compound relative to the total mass of the ink composition is within these ranges, the curability of the ink composition tends to be further improved.

[0039] The content of the nitrogen-containing monofunctional polymerizable compound relative to the total amount of the polymerizable compound is preferably 15-55% by mass, 20-50% by mass, 25-45% by mass, or 25-40% by mass. When the content of the nitrogen-containing monofunctional polymerizable compound relative to the total amount of the polymerizable compound is within this range, the curability of the ink composition tends to be further improved.

[0040] The content of the nitrogen-containing monofunctional polymerizable compound relative to the total amount of the monofunctional polymerizable compounds is preferably 20 to 65% by mass, 25 to 60% by mass, or 30 to 55% by mass. When the content of the nitrogen-containing monofunctional polymerizable compound relative to the total amount of the monofunctional polymerizable compounds is within these ranges, the curability of the ink composition tends to be further improved.

[0041] The molecular weight of the nitrogen-containing monofunctional polymerizable compound is preferably 100 to 200, or 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.

[0042] The glass transition temperature of the nitrogen-containing monofunctional polymerizable compound is preferably 50 to 200° C., or 100 to 175° C. When the glass transition temperature of the nitrogen-containing monofunctional polymerizable compound is within the above range, anti-blocking properties and shrinkage characteristics tend to be further improved.

[0043] 1.1.1.2. Cyclic ether-containing monofunctional polymerizable compounds

[0044] The ink composition of this embodiment may contain a cyclic ether-containing monofunctional polymerizable compound. The cyclic ether-containing monofunctional polymerizable compound is not particularly limited; examples include tetrahydrofuran acrylate, cyclic trimethylolpropane formaldehyde acrylate, (2-methyl-2-ethyl-1,3-dioxolan-4-yl) methacrylate, and oligomers thereof. The inclusion of a cyclic ether-containing monofunctional polymerizable compound in the ink composition of this embodiment tends to improve the curability of the ink composition and the adhesion of the ink composition to recording media.

[0045] The content of the cyclic ether-containing monofunctional polymerizable compound is preferably 10-50% by mass, 15-45% by mass, or 20-40% by mass relative to the total amount of the ink composition. When the content of the cyclic ether-containing monofunctional polymerizable compound is within these ranges relative to the total amount of the ink composition, the curability of the ink composition tends to be further improved.

[0046] The content of the cyclic ether-containing monofunctional polymerizable compound relative to the total amount of the polymerizable compounds is preferably 10-55% by mass, 15-50% by mass, 20-45% by mass, or 25-40% by mass. When the content of the cyclic ether-containing monofunctional polymerizable compound relative to the total amount of the polymerizable compounds is within this range, the curability of the ink composition tends to be further improved.

[0047] 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% by mass, 25 to 55% by mass, or 30 to 50% by mass. When the content of the cyclic ether-containing monofunctional polymerizable compound relative to the total amount of the monofunctional polymerizable compounds is within this range, the curability of the ink composition tends to be further improved.

[0048] The molecular weight of the cyclic ether-containing monofunctional polymerizable compound is preferably 150 to 300, or 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.

[0049] The glass transition temperature of the cyclic ether-containing monofunctional polymerizable compound is preferably -50 to 100° C. or -25 to 50° C. When the glass transition temperature of the cyclic ether-containing monofunctional polymerizable compound is within the above range, anti-blocking properties and shrinkage characteristics tend to be further improved.

[0050] 1.1.1.3. Hydroxyl-containing monofunctional polymerizable compounds

[0051] The ink composition of this embodiment may contain a hydroxyl-containing monofunctional polymerizable compound. The hydroxyl-containing monofunctional polymerizable compound is not particularly limited, and examples thereof include 4-hydroxybutyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 2-hydroxy-3-phenoxypropyl (meth)acrylate, N-hydroxymethyl (meth)acrylamide, and oligomers thereof. The inclusion of a hydroxyl-containing monofunctional polymerizable compound in the ink composition of this embodiment tends to further improve the curability of the ink composition.

[0052] The content of the hydroxyl-containing monofunctional polymerizable compound is preferably 5.0 to 30.0% by mass, 7.5 to 25.0% by mass, or 10.0 to 20.0% by mass relative to the total amount of the ink composition. When the content of the hydroxyl-containing monofunctional polymerizable compound is within these ranges relative to the total amount of the ink composition, the curability of the ink composition tends to be further improved.

[0053] The content of the hydroxyl-containing monofunctional polymerizable compound relative to the total amount of the polymerizable compounds is preferably 7.5 to 40.0% by mass, 10.0 to 35.0% by mass, or 12.5 to 30.0% by mass. When the content of the hydroxyl-containing monofunctional polymerizable compound relative to the total amount of the polymerizable compounds is within these ranges, the curability of the ink composition tends to be further improved.

[0054] The content of the hydroxyl-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-containing monofunctional polymerizable compound relative to the total amount of the monofunctional polymerizable compounds is within these ranges, the curability of the ink composition tends to be further improved.

[0055] The molecular weight of the hydroxyl group-containing monofunctional polymerizable compound is preferably 50 to 250, or 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.

[0056] The glass transition temperature of the hydroxyl group-containing monofunctional polymerizable compound is preferably -100 to 50° C., or -50 to 25° C. When the glass transition temperature of the hydroxyl group-containing monofunctional polymerizable compound is within the above range, anti-blocking properties and shrinkage characteristics tend to be further improved.

[0057] 1.1.1.4. Monofunctional polymerizable compounds containing aromatic groups

[0058] 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 particularly 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, 2-hydroxy-3-phenoxypropyl (meth)acrylate, and oligomers thereof.

[0059] The content of the aromatic group-containing monofunctional polymerizable compound is preferably 10-50% by mass, 15-45% by mass, or 20-40% by mass relative to the total amount of the ink composition. When the content of the aromatic group-containing monofunctional polymerizable compound is within these ranges relative to the total amount of the ink composition, the odor resistance and curability of the ink composition tend to be further improved.

[0060] The content of the aromatic group-containing monofunctional polymerizable compound relative to the total amount of the polymerizable compound is preferably 10-55% by mass, 15-50% by mass, 20-45% by mass, or 25-40% by mass. When the content of the aromatic group-containing monofunctional polymerizable compound relative to the total amount of the polymerizable compound is within this range, the odor resistance and curability of the ink composition tend to be further improved.

[0061] The content of the aromatic group-containing monofunctional polymerizable compound relative to the total amount of the monofunctional polymerizable compounds is preferably 20-60% by mass, 25-55% by mass, or 30-50% by mass. When the content of the aromatic group-containing monofunctional polymerizable compound relative to the total amount of the monofunctional polymerizable compounds is within these ranges, the odor resistance and curability of the ink composition tend to be further improved.

[0062] 1.1.1.5. Monofunctional polymerizable compounds containing saturated aliphatic groups

[0063] 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 1,4-dioxaspiro[4,5]dec-2-ylmethyl 2-(meth)acrylate; linear or branched aliphatic group-containing monofunctional monomers such as isoamyl (meth)acrylate, stearyl (meth)acrylate, lauryl (meth)acrylate, octyl (meth)acrylate, decyl (meth)acrylate, isodecyl (meth)acrylate, isomyristyl (meth)acrylate, isostearyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, butoxyethyl (meth)acrylate, tetrahydrofurfuryl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, and 2-(2-ethoxyethoxy)ethyl acrylate; lactone-modified flexible (meth)acrylates; and oligomers thereof.

[0064] The content of the saturated aliphatic group-containing monofunctional polymerizable compound is preferably 10-50% by mass, 15-45% by mass, or 20-40% by mass relative to the total amount of the ink composition. When the content of the saturated aliphatic group-containing monofunctional polymerizable compound is within these ranges, the curability of the ink composition tends to be further improved.

[0065] The content of the saturated aliphatic group-containing monofunctional polymerizable compound relative to the total amount of the polymerizable compound is preferably 10-55% by mass, 15-50% by mass, 20-45% by mass, or 25-40% by mass. When the content of the saturated aliphatic group-containing monofunctional polymerizable compound relative to the total amount of the polymerizable compound is within this range, the curability of the ink composition tends to be further improved.

[0066] The content of the saturated aliphatic group-containing monofunctional polymerizable compound relative to the total amount of the monofunctional polymerizable compounds is preferably 20-60% by mass, 25-55% by mass, or 30-50% by 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 this range, the curability of the ink composition tends to be further improved.

[0067] The molecular weight of the saturated aliphatic group-containing monofunctional polymerizable compound is preferably 100 to 300, or 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.

[0068] The glass transition temperature of the saturated aliphatic group-containing monofunctional polymerizable compound is preferably 0 to 150° C. or 25 to 100° C. When the glass transition temperature of the saturated aliphatic group-containing monofunctional polymerizable compound is within the above range, anti-blocking properties and shrinkage characteristics tend to be further improved.

[0069] 1.1.1.6. Other monofunctional polymerizable compounds

[0070] The ink composition of this embodiment may also contain other monofunctional polymerizable compounds in addition to the above-mentioned monofunctional polymerizable compounds. 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 such unsaturated carboxylic acids; esters, urethanes, amides, and anhydrides of such unsaturated carboxylic acids; acrylonitrile, styrene, various unsaturated polyesters, unsaturated polyethers, unsaturated polyamides, and unsaturated urethanes.

[0071] The content of other monofunctional polymerizable compounds is not particularly limited, and is, for example, 0.1 to 5.0 mass %, 0.1 to 2.5 mass %, or 0.1 to 1.0 mass % relative to the total amount of the ink composition. Alternatively, no other monofunctional polymerizable compounds may be contained.

[0072] The content of other monofunctional polymerizable compounds is not particularly limited, and is, for example, 0.1 to 5.0 mass %, 0.1 to 2.5 mass %, or 0.1 to 1.0 mass % relative to the total amount of the polymerizable compounds. Alternatively, no other monofunctional polymerizable compounds may be contained.

[0073] The content of other monofunctional polymerizable compounds is not particularly limited, and is, for example, 0.1 to 5.0 mass %, 0.1 to 2.5 mass %, or 0.1 to 1.0 mass % relative to the total amount of the monofunctional polymerizable compounds. Alternatively, no other monofunctional polymerizable compounds may be contained.

[0074] The molecular weight of other monofunctional polymerizable compounds is preferably 100-300 or 150-250.

[0075] The glass transition temperature of other monofunctional polymerizable compounds is preferably 0 to 150°C or 25 to 100°C.

[0076] 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.

[0077] The total amount of the monofunctional polymerizable compound is preferably 50% by mass or greater, 50 to 95% by mass, 55 to 90% by mass, or 60 to 85% by mass, relative to the total amount of the polymerizable compound. By adjusting the content of the monofunctional polymerizable compound within this range, the flexibility of the ink composition film on the recording medium tends to be improved, and the film's ability to conform to shrinkage caused by heating the recording medium tends to be improved. In other words, the shrinkage characteristics of the ink composition tend to be improved.

[0078] 1.1.2. Multifunctional polymerizable compounds

[0079] The ink composition of this embodiment may or may not contain a polyfunctional polymerizable compound. The inclusion of a polyfunctional polymerizable compound in the ink composition of this embodiment tends to further enhance curability. The polyfunctional polymerizable compound is not particularly limited; examples include vinyl ether group-containing (meth)acrylates, bifunctional (meth)acrylates, trifunctional or higher-functional (meth)acrylates, and oligomers thereof. The inclusion of a polyfunctional polymerizable compound in the ink composition of this embodiment tends to further enhance curability.

[0080] 1.1.2.1. Vinyl ether (meth)acrylates

[0081] 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, and examples thereof include compounds represented by the following formula (1). High molecular weight photoinitiators tend to increase 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 ejection stability. In addition, the curability of the ink composition can be further improved, and the improved curability can increase the recording speed.

[0082]

[0083] (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.)

[0084] In the above formula (1), examples of the divalent organic residue having 2 to 20 carbon atoms represented by R2 include linear, branched, or cyclic alkylene groups having 2 to 20 carbon atoms that may be substituted, alkylene groups having 2 to 20 carbon atoms that have an oxygen atom formed by an ether bond and / or an ester bond in their structure that may be substituted, and divalent aromatic groups having 6 to 11 carbon atoms that 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, such as oxyethylene, oxy-n-propylene, oxyisopropylene, and oxybutylene, that have an oxygen atom formed by an ether bond in their structure, are preferred. In addition, from the viewpoint of being able to lower the viscosity of the composition and further improve the curability of the composition, it is more preferred that R2 is a compound having a glycol ether chain, such as an alkylene group with 2 to 9 carbon atoms having an oxygen atom formed by an ether bond in its structure, such as oxyethylene, oxy-n-propylene, oxy-isopropylene, and oxy-butylene.

[0085] 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 that may be substituted, or an aromatic group having 6 to 11 carbon atoms that may be substituted. Among them, an alkyl group having 1 to 2 carbon atoms such as a methyl group or an ethyl group, an aromatic group having 6 to 8 carbon atoms such as a phenyl group, and a benzyl group are preferably used.

[0086] When each of the above-mentioned organic residues is a substituted group, the substituent group is divided into a group containing carbon atoms and a group not containing carbon atoms. First, when the above-mentioned substituent group is a group containing carbon atoms, the carbon atom is counted in the number of carbon atoms of the organic residue. Examples of groups containing carbon atoms include, but are not limited to, carboxyl groups and alkoxy groups. Next, examples of groups not containing carbon atoms include, but are not limited to, hydroxyl groups and halogen groups.

[0087] Specific examples of the compound of formula (1) are not particularly limited, and examples thereof include 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, 3-vinyloxybutyl (meth)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-vinyloxymethylphenyl methyl (meth)acrylate, m-vinyloxymethylphenyl methyl (meth)acrylate, o-vinyloxymethylphenyl (meth)acrylate Methyl methacrylate, 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-(vinyloxyethoxyethoxy)ethyl (meth)acrylate, 2-(vinyloxyethoxyisopropoxy)ethyl (meth)acrylate, 2-(vinyloxyisopropoxy)ethyl (meth)acrylate 2-(vinyloxyethoxy)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-(vinyloxyethoxyethoxy)isopropyl (meth)acrylate, 2-(vinyloxyethoxyisopropoxy)isopropyl (meth)acrylate, 2-(vinyloxyethoxyisopropoxy)isopropyl (meth)acrylate, acrylate, 2-(vinyloxyisopropoxyisopropoxy)isopropyl acrylate, 2-(vinyloxyethoxyethoxyethoxy)ethyl (meth)acrylate, 2-(vinyloxyethoxyethoxyethoxy)ethyl (meth)acrylate, 2-(isopropenyloxyethoxy)ethyl (meth)acrylate, 2-(isopropenyloxyethoxyethoxy)ethyl (meth)acrylate, 2-(isopropenyloxyethoxyethoxy)ethyl (meth)acrylate, 2-(isopropenyloxyethoxyethoxyethoxy)ethyl (meth)acrylate, polyethylene glycol monovinyl ether, and polypropylene glycol monovinyl ether (meth)acrylate. Among these specific examples, 2-(2-vinyloxyethoxy)ethyl acrylate is particularly preferred from the viewpoint of easily achieving a balance between the curability and viscosity of the composition. It should be noted that in the present embodiment, 2-(2-vinyloxyethoxy)ethyl acrylate is sometimes also referred to as VEEA.

[0088] The content of the vinyl ether group-containing (meth)acrylate is preferably 5-50% by mass, 10-40% by mass, or 10-30% by mass relative to the total amount of the ink composition. When the content of the vinyl ether group-containing (meth)acrylate is within these ranges, the curability of the ink composition tends to be further improved, and the viscosity tends to be further reduced.

[0089] The content of the vinyl ether group-containing (meth)acrylate relative to the total amount of the polymerizable compound is preferably 10.0 to 60.0% by mass, 12.5 to 50.0% by mass, or 15.0 to 40.0% by mass. When the content of the vinyl ether group-containing (meth)acrylate relative to the total amount of the polymerizable compound is within this range, the curability of the ink composition tends to be further improved and the viscosity tends to be further reduced.

[0090] The molecular weight of the vinyl ether group-containing (meth)acrylate is preferably 100 to 350, or 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.

[0091] The glass transition temperature of the vinyl ether group-containing (meth)acrylate is preferably 0 to 100° C. or 25 to 75° C. When the glass transition temperature of the vinyl ether group-containing (meth)acrylate is within the above range, anti-blocking properties and shrinkage characteristics tend to be further improved.

[0092] 1.1.2.2.2 Functional (meth)acrylates

[0093] 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 di(meth)acrylate, tripropylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, 1,4-butanediol diacrylate, and 1,4-butanediol diacrylate. (Meth)acrylates, 1,6-hexanediol di(meth)acrylate (HDDA), 1,9-nonanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, dihydroxymethyl-tricyclodecane di(meth)acrylate, EO (ethylene oxide) adduct of bisphenol A di(meth)acrylate, PO (propylene oxide) adduct of bisphenol A di(meth)acrylate, hydroxypivalic acid neopentyl glycol di(meth)acrylate, and polytetramethylene glycol di(meth)acrylate.

[0094] The content of the bifunctional (meth)acrylate is preferably 5-50% by mass, 10-40% by mass, or 10-30% by mass relative to the total amount of the ink composition. When the content of the bifunctional (meth)acrylate is within these ranges, the curability of the ink composition tends to be further improved, and the viscosity tends to be further reduced.

[0095] The content of the bifunctional (meth)acrylate relative to the total amount of the polymerizable compound 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 compound is within these ranges, the curability of the ink composition tends to be further improved and the viscosity further reduced.

[0096] The molecular weight of the bifunctional (meth)acrylate is preferably 150 to 400, or 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.

[0097] The glass transition temperature of the bifunctional (meth)acrylate is preferably 0 to 150° C. or 25 to 125° C. When the glass transition temperature of the bifunctional (meth)acrylate is within the above range, anti-blocking properties and shrinkage characteristics tend to be further improved.

[0098] 1.1.2.3.3 Multifunctional (meth)acrylates

[0099] The ink composition of this embodiment may or may not contain a trifunctional or higher polyfunctional (meth)acrylate. The trifunctional or higher polyfunctional (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, glycerol propoxy tri(meth)acrylate, caprolactone-modified trimethylolpropane tri(meth)acrylate, pentaerythritol ethoxy tetra(meth)acrylate, caprolactam-modified dipentaerythritol hexa(meth)acrylate, and dipentaerythritol polyacrylate (A-DPH).

[0100] The content of the trifunctional or higher polyfunctional (meth)acrylate is preferably 5.0 to 20.0% by mass, or 10.0 to 17.5% by mass, relative to the total amount of the ink composition.

[0101] The content of the trifunctional or higher polyfunctional (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 compound.

[0102] The total amount of the multifunctional 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.

[0103] The total amount of the polyfunctional polymerizable compound is preferably 8 to 55 mass %, 9 to 50 mass %, 10 to 45 mass %, 11 to 40 mass %, or 12 to 35 mass % based on the total amount of the polymerizable compound.

[0104] 1.2. Photoinitiator

[0105] The ink composition of this embodiment contains a photoinitiator. When the photoinitiator is irradiated with radiation, the photoinitiator generates active species. In addition to generating active species through irradiation with radiation, a compound that functions as a photosensitizer may also be included in the photoinitiator. It should be noted that the function of a photosensitizer refers to the ability to absorb light having a certain wavelength and emit fluorescence having another wavelength. Specifically, it refers to the ability to absorb light having a wavelength of approximately 300 to 450 nm and emit fluorescence having a wavelength of approximately 400 to 500 nm. The photoinitiator includes a high molecular weight photoinitiator with a molecular weight of 500 or greater and a low molecular weight photoinitiator with a molecular weight of less than 500. Hereinafter, a high molecular weight photoinitiator with a molecular weight of 500 or greater will be referred to as a high molecular weight photoinitiator, and a low molecular weight photoinitiator with a molecular weight of less than 500 will be referred to as a low molecular weight photoinitiator. One type of photoinitiator may be used alone, or two or more types may be used in combination.

[0106] The content of the photoinitiator is preferably 1 to 20% by mass, or 5 to 15% by mass, relative to the total amount of the ink composition.

[0107] 1.2.1. High molecular weight photoinitiators

[0108] The ink composition of this embodiment contains a high molecular weight photoinitiator. Since a high molecular weight photoinitiator is difficult to volatilize, the odor of the ink composition of this embodiment is reduced by including a high molecular weight photoinitiator.

[0109] It should be noted that low-molecular-weight components in ink compositions are easily volatilized, and therefore are believed to contribute to the deterioration of odor. Therefore, it is believed that when a low-molecular-weight photoinitiator is used, unreacted substances in the low-molecular-weight photoinitiator remain in the coating film, and their volatilization worsens odor. In this regard, since high-molecular-weight photoinitiators are less likely to volatilize, it is believed that the use of high-molecular-weight photoinitiators reduces the odor of the ink composition. The mechanism for reducing the odor of the ink composition is not limited to the mechanism described above.

[0110] The molecular weight of the high molecular weight photoinitiator is 500 or greater, preferably 500-2000, 750-1950, or 1000-1900. When the molecular weight of the high molecular weight photoinitiator is within the above range, it tends to have excellent odor resistance and curability. The high molecular weight photoinitiator is not particularly limited as long as the molecular weight is 500 or greater. Examples include thioxanthone initiators, α-hydroxyketone initiators, ketonesulfonic acid initiators, and amine initiators. It should be noted that some amine initiators function as photosensitizers.

[0111] More specifically, examples of the high molecular weight photoinitiator include 1-[4-(4-benzoylphenyl(p-aminophenylsulfonyl))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), α- [2-[(9-oxo-9H-thioxanthenyl)oxy]acetyl]-ω-[[2-[(9-oxo-9H-thioxanthenyl)oxy]acetyl]oxy]poly(oxy-1,4-butanediyl) (CAS: 813452-37-8), 1,3-bis({α-[(1-chloro-9-oxo-9H-thioxanthen-4-yl)oxy]acetylpoly[oxy(1-methylethylene)]}oxy)-2,2-bis({α-[( 1-chloro-9-oxo-9H-thioxanthen-4-yl)oxy]acetylpoly[oxy(1-methylethylene)]}oxymethyl)propane (CAS: 1003567-83-6), 1,3-bis({-4-(dimethylamino)benzoylpoly[oxy(1-methylethylene)]}oxy)-2,2-bis({-4-(dimethylamino)benzoylpoly[oxy(1-methylethylene)]}oxymethyl)propane and {α A mixture of 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)butyrylphenyl]piperazine) propionate (CAS: 886463-10-1). Among them, from the viewpoints of odor resistance and curability, preferably, α-[2-[(9-oxo-9H-thioxanthenyl)oxy]acetyl]-ω-[[2-[(9-oxo-9H-thioxanthenyl)oxy]acetyl]oxy]poly(oxy-1,4-butanediyl) (CAS: 813452-37-8) and 1,3-bis({α-[(1-chloro-9-oxo-9H-thioxanthen-4-yl)oxy]acetylpoly[oxy(1-methylethylene)]}oxy)-2,2-bis({α-[(1-chloro-9-oxo-9H-thioxanthen-4-yl)oxy]acetylpoly[oxy(1-methylethylene)]}oxymethyl)propane are contained. Specific products are not particularly limited, and examples thereof include ESACURE 1001M, ESACURE KIP 150, Omnipol TX, SpeedCure 7010, SpeedCure 7040, and Omnipol 910 (all manufactured by IGM REGIN Co., Ltd.).1,3-bis({-4-(dimethylamino)benzoylpoly[oxy(1-methylethylene)]}oxy)-2,2-bis({-4-(dimethylamino)-benzoylpoly[oxy(1-methylethylene)]}oxymethyl)propane, a mixture of {α-4-(dimethylamino)benzoylpoly(oxyethylene)-poly[oxy(1-methylethylene)]-poly(oxyethylene)}4-(dimethylamino)benzoate, and polyethylene glycol bis(β-4-[4-(2-dimethylamino-2-benzyl)butyrylphenyl]piperazine)propionate also function as photosensitizers.

[0112] The content of the high molecular weight photoinitiator relative to the total mass of the ink composition is preferably 1.0 to 10.0%, 1.5 to 9.0%, or 2.0 to 8.0% by mass. When the content of the high molecular weight photoinitiator relative to the total mass of the ink composition is within these ranges, the resulting recorded material tends to have less odor.

[0113] The content of the high molecular weight photoinitiator relative to the total amount of the photoinitiator is preferably 15-100 mass%, 20-85 mass%, 25-70 mass%, or 30-60 mass%. By adjusting the content of the high molecular weight photoinitiator relative to the total amount of the photoinitiator within these ranges, the resulting recorded material tends to have a reduced odor while maintaining the curability of the ink composition.

[0114] 1.2.2. Low molecular weight photoinitiators

[0115] The ink composition of this embodiment may also contain a low molecular weight photoinitiator. The inclusion of a low molecular weight photoinitiator in the ink composition tends to further improve curability. Specifically, the ink composition of this embodiment, by containing both a high molecular weight photoinitiator and a low molecular weight photoinitiator, tends to achieve a balance between odor resistance and curability.

[0116] The molecular weight of the low molecular weight photoinitiator is less than 500, preferably 50 or more and less than 500, 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 the molecular weight is less than 500, and examples thereof include acylphosphine-based photoinitiators, α-hydroxyketone-based photoinitiators, amine-based photoinitiators, and thioxanthone-based photoinitiators.

[0117] More specific 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)phosphonate, and 2,4-diethylthioxanthone. Specific products are not particularly limited, but examples include Omnirad 819, ESACURE KIP 160, ESACURE A 198, Speedcure DETX, and Omnirad TPO-L (all manufactured by IGMREGIN). (Methylimino)diethane-2,1-diylbis[4-(dimethylamino)-benzoate] also functions as a photosensitizer.

[0118] The content of the low molecular weight photoinitiator relative to the total mass of the ink composition is preferably 1.0 to 10.0%, 1.5 to 9.0%, or 2.0 to 8.0% by mass. By adjusting the content of the low molecular weight photoinitiator relative to the total mass of the ink composition within these ranges, the resulting recorded material tends to have a reduced odor.

[0119] The content of the low molecular weight photoinitiator relative to the total amount of the photoinitiator is preferably 40-99% by mass, 45-90% by mass, 50-80% by mass, or 55-70% by mass. When the content of the low molecular weight photoinitiator relative to the total amount of the photoinitiator is within these ranges, the resulting recorded material tends to have less odor while maintaining the curability of the ink composition.

[0120] 1.3. Inhibitors

[0121] The ink composition of this embodiment may also contain a polymerization inhibitor. The polymerization inhibitor is not particularly limited, and examples thereof include hydroquinones represented by hydroquinone, hydroquinone monomethyl ether (MEHQ), 1-o-2,3,5-trimethylhydroquinone, and 2-tert-butylhydroquinone; catechols represented by catechol, 4-methylcatechol, and 4-tert-butylcatechol; and phenol, butylhydroxytoluene, butylhydroxyanisole, p-methoxyphenol, cresol, pyrogallol, 3,5-di-tert-butyl-4-hydroxytoluene, 2,2'-methylenebis(4-methyl-6-tert-butylphenol), 2,2'-methylenebis(4-tert-butylphenol), and 4-tert-butylphenol. Phenols represented by bis(2,2,6,6-tetramethyl-4-piperidin-1-yl)phenol and 4,4'-thiobis(3-methyl-6-tert-butylphenol); compounds having a 2,2,6,6-tetramethylpiperidin-N-oxy skeleton represented by 2,2,6,6-tetramethyl-4-hydroxypiperidin-1-oxyl, compounds having a 2,2,6,6-tetramethylpiperidin skeleton represented by bis(2,2,6,6-tetramethyl-4-piperidinyl)sebacate, compounds having a 2,2,6,6-tetramethylpiperidin-N-alkyl skeleton, and hindered amines represented by compounds having a 2,2,6,6-tetramethylpiperidin-N-acyl skeleton. The polymerization inhibitor may be used alone or in combination of two or more.

[0122] The content of the polymerization inhibitor is not particularly limited, but is, for example, 0.1 to 1.0% by mass relative to the total amount of the ink composition.

[0123] 1.4. Surfactants

[0124] The ink composition of this embodiment may contain a surfactant. Examples of surfactants include silicone surfactants, fluorine-based surfactants, and acetylene glycol surfactants. Silicone surfactants are preferred. Surfactants may be used alone or in combination of two or more.

[0125] Examples of silicone surfactants include polysiloxane compounds and polyether-modified silicones. Commercially available silicone surfactants are not particularly limited, but specific examples include 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-Chemie Japan Co., Ltd.). , 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, KF-6017 (all are trade names, manufactured by Shin-Etsu Chemical Co., Ltd.), etc.

[0126] Fluorochemical surfactants are not particularly limited, and examples thereof include perfluoroalkyl sulfonates, perfluoroalkyl carboxylates, perfluoroalkyl phosphates, perfluoroalkyl ethylene oxide adducts, perfluoroalkyl betaines, and perfluoroalkyl amine oxide compounds. Commercially available fluorochemical surfactants are not particularly limited, and examples thereof include S-144 and S-145 (manufactured by Asahi Glass Co., Ltd.); FC-170C, FC-430, and Fluorad-FC4430 (manufactured by Sumitomo 3M Co., Ltd.); FSO, FSO-100, FSN, FSN-100, and FS-300 (manufactured by Dupont); and FT-250 and 251 (manufactured by NEOS Corporation).

[0127] The acetylene glycol surfactant is not particularly limited, but is preferably selected from, for example, alkylene oxide adducts of 2,4,7,9-tetramethyl-5-decyn-4,7-diol and 2,4,7,9-tetramethyl-5-decyn-4,7-diol, and alkylene oxide adducts of 2,4-dimethyl-5-decyn-4-ol and 2,4-dimethyl-5-decyn-4-ol. Commercially available acetylene glycol surfactants are not particularly limited, but include, for example, the E series such as OLFINE 104 series and OLFINE 1010 (trade names of Air Products Japan, Inc.), and SURFYNOL 465 and SURFYNOL 61 (trade names of Nissin Chemical Industry Co., Ltd.).

[0128] The content of the surfactant is not particularly limited, but is, for example, 0.1 to 5.0% by mass relative to the total amount of the ink composition.

[0129] 1.5. Pigment

[0130] The ink composition of this embodiment may contain a colorant. Examples of colorants include dyes and pigments. The colorant content relative to the total amount of the ink composition is not particularly limited, but is, for example, 1 to 10% by mass. It should be noted that the ink composition of this embodiment may also be a transparent ink that contains no colorant or contains a colorant to an extent not intended for coloring (e.g., 0.1% by mass or less). A single colorant may be used alone, or two or more may be used in combination.

[0131] 1.5.1. Pigments

[0132] By using a pigment as a colorant, the light resistance of the ink composition of this embodiment can be improved. As the pigment, either an inorganic pigment or an organic pigment can be used.

[0133] As the inorganic pigment, carbon black (CI (Colour Index Generic Name: Color 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.

[0134] Examples of the organic pigments include azo pigments such as insoluble azo pigments, condensed azo pigments, azo lakes, and chelated 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 (e.g., basic dye type chelates, acid dye type chelates, etc.); dye lakes (basic dye type lakes, acid dye type lakes); nitro pigments, nitroso pigments, aniline black, and daylight fluorescent pigments.

[0135] More specifically, examples of carbon black used in 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), Rega 1400R, Rega 1330R, Rega 1400R, Rega 15 ... 660R, Mogul L, Monarch 700, Monarch 800, Monarch 880, Monarch 900, Monarch 1000, Monarch 1100, Monarch 1300, Monarch 1400, etc. (manufactured by CABOT JAPAN K.K.), Pigment Black FW1, Pigment Black FW2, Pigment Black FW2V, Pigment Black FW18, Pigment Black FW200, Pigment Black S150, Pigment Black S160, Pigment Black S170, Printex 35, Printex U, Printex V, Printex 140U, Extra Black 6, Extra Black 5, Extra Black 4A, Extra Black 4 (all manufactured by Degussa).

[0136] Examples of white pigments include CI Pigment White 6, 18, and 21.

[0137] Examples of the yellow pigment 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.

[0138] Examples of the pigment 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, 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, and 245; and CI Pigment Violet 19, 23, 32, 33, 36, 38, 43, and 50.

[0139] Examples of the cyan pigment 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, and CI Vat Blue 4 and 60.

[0140] In addition, 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.

[0141] The content of the pigment is not particularly limited, but is, for example, 1 to 10% by mass relative to the total amount of the ink composition.

[0142] 1.5.2. Dispersants

[0143] When the ink composition of this embodiment contains a pigment, it may contain a dispersant to improve the dispersibility of the pigment. The dispersants may be used alone or in combination of two or more.

[0144] The dispersant is not particularly limited, and examples thereof include polymer dispersants commonly used in preparing pigment dispersions. Specific examples thereof include dispersants primarily composed of one or more of polyoxyalkylene 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.

[0145] Commercially available polymer dispersants include the Ajisper series manufactured by Ajinomoto Fine-Techno Co., Ltd., the Solsperse series (such as Solsperse 36000) available from Avecia and Noveon, the Disperbyk series manufactured by BYK Additives & Instruments, and the Disparlon series manufactured by Kusumoto Chemicals Co., Ltd.

[0146] The content of the dispersant is not particularly limited, but is, for example, 0.01 to 1.00% by mass relative to the total amount of the ink composition.

[0147] 1.5.3. Dyes

[0148] There are no particular limitations on the dyes, and acid dyes, direct dyes, reactive dyes, and basic dyes can be used. Examples of the dyes include CI Acid Yellow 17, 23, 42, 44, 79, and 142; CI Acid Red 52, 80, 82, 249, 254, and 289; CI Acid Blue 9, 45, and 249; CI Acid Black 1, 2, 24, and 94; CI Food Black 1 and 2; CI Direct Yellow 1, 12, 24, 33, 50, 55, 58, 86, and 132; 142, 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.

[0149] The content of the dye is not particularly limited, but is, for example, 1 to 10% by mass relative to the total amount of the ink composition.

[0150] 2. Method for producing radiation-curable inkjet ink composition

[0151] The radiation-curable inkjet ink composition of this embodiment is not particularly limited; for example, the aforementioned components may be mixed. Alternatively, a pigment dispersion may be prepared by dispersing a pigment and a dispersant in a solvent, and the resulting pigment dispersion may be mixed with the other aforementioned components. It should be noted that the solvent in the radiation-curable inkjet ink composition is not limited to the aforementioned polymerizable compound.

[0152] 3. Recording medium

[0153] The recording medium used for recording with the ink composition of this embodiment is not particularly limited, and examples thereof include absorptive recording media, low-absorptive recording media, and non-absorptive recording media.

[0154] There are no particular limitations on the absorptive recording medium, and examples thereof include ordinary paper such as electrophotographic paper having high ink permeability, inkjet paper (inkjet paper having an ink absorptive layer composed of silica particles or alumina particles, or an ink absorptive layer composed of a hydrophilic polymer such as polyvinyl alcohol (PVA) or polyvinyl pyrrolidone (PVP), and cloth.

[0155] The low-absorption recording medium is not particularly limited, and examples thereof include coated paper, coated paper, and cast-coated paper used in ordinary offset printing, which have relatively low ink permeability.

[0156] The non-absorbent recording medium is not particularly limited, and examples thereof include films or plates of plastics such as polyvinyl chloride, polyethylene, polypropylene, polyethylene terephthalate (PET), polycarbonate, polystyrene, and polyurethane; plates of metals such as iron, silver, copper, and aluminum; or metal plates or plastic films produced by vapor deposition of these various metals, or plates of alloys such as stainless steel and brass; and recording media obtained by bonding (coating) a film of a plastic such as polyvinyl chloride, polyethylene, polypropylene, polyethylene terephthalate (PET), polycarbonate, polystyrene, and polyurethane onto a paper substrate.

[0157] The ink composition of this embodiment is suitable for printing on low-absorption recording media or non-absorption recording media. The ink composition of this embodiment is particularly suitable for printing on recording media such as flexible packaging and shrink labels, which are non-absorption recording media that shrink when heated.

[0158] 4. Recording device

[0159] The recording device of this embodiment is an inkjet recording device used in printing using the ink composition of this embodiment. As an example of an inkjet recording device, Figure 1 A perspective view of a serial printer is shown. Figure 1 As shown, the serial printer 20 includes a conveyor unit 220 and a recording unit 230. The conveyor unit 220 conveys the recording medium F supplied to the serial printer toward the recording unit 230 and discharges the recorded recording medium out of the serial printer. Specifically, the conveyor unit 220 includes various conveyor rollers that convey the conveyed recording medium F in the secondary scanning directions T1 and T2.

[0160] In addition, the recording unit 230 includes: an inkjet head 231 for ejecting ink, etc. onto the recording medium F transported from the transport unit 220; a light source 232 for irradiating radiation onto the adhered ink, etc.; a carriage 234 for mounting the above-mentioned inkjet head 231 and the light source 232; and a carriage moving mechanism 235 for moving the carriage 234 in the main scanning directions S1 and S2 of the recording medium F.

[0161] In the case of a serial printer, the inkjet head 231 includes a nozzle whose length is smaller than the width of the recording medium. The nozzle moves to perform recording in multiple strokes. Furthermore, in a serial printer, the inkjet head 231 is mounted on a carriage 234 that moves in a predetermined direction. The nozzle moves with the movement of the carriage, thereby ejecting the ink composition onto the recording medium F. Thus, recording is performed in two or more strokes. It should be noted that a stroke is also referred to as a main scan. A secondary scan is performed between strokes to transport the recording medium. In other words, the main scan and secondary scan are performed alternately.

[0162] In addition, the above reference Figure 1 While the recording device of this embodiment is described using a serial printer as an example, the recording device of this embodiment may also be a line printer. In a line printer, a fixed line head moves the recording medium in the sub-scanning direction (the longitudinal direction of the recording medium, or the transport direction). Ink droplets are ejected from the nozzle openings of the printhead in conjunction with this movement, thereby recording an image on the recording medium. Generally speaking, since the printhead is fixed and does not move, recording is performed in a single stroke (single pass), resulting in a faster recording speed than a serial printer.

[0163] 5. Inkjet recording method

[0164] The inkjet recording method of this embodiment includes a discharge step for discharging the ink composition of this embodiment onto a recording medium. Furthermore, as needed, the method may further include a curing step for curing the ink composition of this embodiment adhered to the recording medium by the discharge step, a transport step for transporting the recording medium, a winding step for winding the recording medium, and the like.

[0165] 5.1. Spraying process

[0166] In the ejection step of this embodiment, the ink composition of this embodiment is ejected from the inkjet head onto the recording medium. More specifically, a pressure generating mechanism provided within the inkjet head is driven to eject the ink composition within the pressure generating chamber of the inkjet head from the nozzle onto the recording medium.

[0167] Examples of the inkjet head used in the discharge step include a line head that performs recording by a line method and a serial head that performs recording by a serial method.

[0168] In a line-type method using a line head, for example, an inkjet head having a width 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, thereby recording an image on the recording medium.

[0169] In a serial method using a serial head, for example, an inkjet head is mounted on a carriage that is movable across the width of the recording medium. The carriage moves in the main scanning direction (the width of the recording medium), and ink droplets are ejected from the nozzles of the inkjet head in conjunction with this movement, thereby recording an image on the recording medium.

[0170] The thickness of the ink film formed on the recording medium during the ejection process is preferably 5 μm or less, and preferably 1 to 5 μm. By setting the ink film thickness within this range, the shrinkage characteristics of the ink composition, as well as the adhesion and curability of the ink composition to the recording medium, tend to be improved.

[0171] 5.2. Curing process

[0172] The inkjet recording method of this embodiment may also include a curing step for 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, deposited on the recording medium, is cured by irradiating the ink composition of this embodiment with radiation. Irradiation with radiation initiates a polymerization reaction of the polymerizable compound, causing the composition to cure and form a coating film. The presence of a photoinitiator generates reactive species (initiating species) such as free radicals, acids, and bases, and the polymerization reaction of the polymerizable compound is accelerated by the function of these initiating species.

[0173] Examples of radiation include ultraviolet rays, infrared rays, visible rays, and X-rays. The composition is irradiated with radiation from a radiation source located downstream of the inkjet head. While the radiation source is not particularly limited, examples include UV-LEDs. Using such radiation sources can minimize the size of the device and reduce costs. UV-LEDs, as ultraviolet light sources, are compact and can therefore be installed within the inkjet recording device.

[0174] 5.3. Transport process

[0175] The inkjet recording method of this embodiment may also include a conveying step for conveying the recording medium. During this conveying step, the recording medium is conveyed in a predetermined direction within the recording apparatus. More specifically, a conveying roller or conveyor belt provided within the recording apparatus is used to convey the recording medium from the paper feed section to the paper discharge section. During this conveying process, the ink composition ejected from the inkjet head adheres to the recording medium, forming a recorded object.

[0176] 5.4. Winding process

[0177] The inkjet recording method of this embodiment may also include a winding step of winding the recording medium. In the winding step, for example, the recording medium on which the ink composition of this embodiment has been ejected is wound.

[0178] The winding process allows for more efficient storage of recorded materials. On the other hand, the winding process causes recorded materials to overlap under greater pressure, which can easily lead to blocking. Regarding this point, the ink composition of this embodiment exhibits excellent blocking resistance, making it less susceptible to blocking, further enhancing the benefits of the present invention.

[0179] In the inkjet recording method of this embodiment, the order and timing of performing the ejection step, curing step, conveying step, and winding step are not particularly limited. For example, the four steps may be performed simultaneously or alternately. Alternatively, some of these steps may be performed simultaneously while the remaining steps are performed separately.

[0180] Example

[0181] The present invention is described in more detail below using Examples and Comparative Examples. The present invention is not limited in any way by the following Examples. Unless otherwise specified, the experiments in the Examples and Comparative Examples were conducted at room temperature (25°C) and 1 atm.

[0182] 1. Preparation of radiation-curable inkjet ink composition

[0183] First, the colorant, dispersant, and a portion of each polymerizable compound are weighed and placed in a pigment dispersion tank. A ceramic bead mill with a diameter of 1 mm is placed in the tank and stirred to obtain a pigment dispersion in which the colorant is dispersed in the polymerizable compound. Figures 2 to 5 The remaining ingredients were added to a stainless steel mixing tank according to the composition described in [1]. After mixing and stirring until completely dissolved, the pigment dispersion obtained above was added, further mixed and stirred at room temperature, and filtered using a membrane filter to obtain the ink composition of each example. It should be noted that the numerical values ​​of each component shown in each example in the figures are expressed in mass % unless otherwise specified. Furthermore, the mass % of substances that are solid at room temperature represents the solids concentration. The components in the table are shown below.

[0184] • MEHQ (product name: p-methoxyphenol, hydroquinone monomethyl ether, manufactured by Kanto Chemical Co., Ltd.)

[0185] •LA-7RD (2,2,6,6-tetramethyl-4-hydroxypiperidin-1-yloxy, manufactured by ADEKA)

[0186] • Omnirad819 (acylphosphine-based photoinitiator, phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, manufactured by IGM Resin)

[0187] •ESACUREKIP160 (α-hydroxyketone photoinitiator, 2-hydroxy-1-[4-(4-(2-hydroxy-2-methylpropionyl)phenoxy)phenyl]-2-methylpropan-1-one, manufactured by IGM Resin)

[0188] •ESACUREA 198 (amine-based photoinitiator, (methylimino)diethane-2,1-diylbis[4-(dimethylamino)-benzoate], manufactured by IGM Resin Co., Ltd.)

[0189] •SpeedCureDETX (thioxanthone-based photoinitiator, 2,4-diethylthioxanthone, manufactured by IGM Resin)

[0190] •ESACURE1001M (ketosulfonic acid-based photoinitiator, 1-[4-(4-benzoylphenyl(p-aminophenylsulfonyl))phenyl]-2-methyl-2-[(4-methylphenyl)sulfonyl]-1-propan-1-one, manufactured by IGM Resin Co., Ltd.)

[0191] •ESACUREKIP150 (α-hydroxyketone photoinitiator, oligo-[2-hydroxy-2-methyl-1-(4-(1-methylvinyl)phenyl)propanone], manufactured by IGM Resin)

[0192] • SpeedCure 7010 (thioxanthone-based photoinitiator, 1,3-bis({α-[(1-chloro-9-oxo-9H-thioxanthen-4-yl)oxy]acetylpoly[oxy(1-methylethylene)]}oxy)-2,2-bis({α-[(1-chloro-9-oxo-9H-thioxanthen-4-yl)oxy]acetylpoly[oxy(1-methylethylene)]}oxymethyl)propane, manufactured by IGM Resin)

[0193] • Omnipol TX (thioxanthone-based photoinitiator, α-[2-[(9-oxo-9H-thioxanthenyl)oxy]acetyl]-ω-[[2-[(9-oxo-9H-thioxanthenyl)oxy]acetyl]oxy]poly(oxy-1,4-butanediyl), manufactured by IGM Resin Co., Ltd.)

[0194] • Omnipol 910 (amine-based photoinitiator, polyethylene glycol di(β-4-[4-(2-dimethylamino-2-benzyl)butyrylphenyl]piperazine) propionate, manufactured by IGM Resin Co., Ltd.)

[0195] • SpeedCure 7040 (amine-based photoinitiator, a mixture of 1,3-bis({-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 Co., Ltd.)

[0196] BYK UV3500 (manufactured by BYK Additives & Instruments, polyether-modified polydimethylsiloxane with an acryloyl group)

[0197] •Solsphere 36000 (made by Avecia)

[0198] •Carbon black (trade name "MA-100", manufactured by Mitsubishi Chemical Corporation)

[0199] 2. Production of Records

[0200] Using a "PX-G5000" inkjet printer (manufactured by Seiko Epson Corporation), each radiation-curable inkjet ink composition was ejected onto a PET film "Bonset" (manufactured by TAKIRON CI Co., Ltd.) as a recording medium at room temperature and 1 atm, with a recording resolution of 600 dpi x 600 dpi and a droplet weight of 10 ng. A solid pattern image consisting of a 5 μm thick ink film was recorded. A solid pattern image is an image in which dots are recorded for all pixels in the minimum recording unit area defined by the recording resolution, that is, an image recorded at a dot generation rate of 100%. Following this printing process, ultraviolet light was irradiated from a UV-LED light source mounted on a carriage, resulting in a recorded object with a 5 μm thick cured ink film formed on the recording medium.

[0201] 3. Evaluation Method

[0202] 3.1. Curing

[0203] 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 ink cured film was 5 μm. The UV-LED (peak wavelength 395 nm or 365 nm, irradiation intensity 1000 mW / cm 2 ) and irradiate to obtain the irradiation energy until the non-stick state is achieved. Irradiation energy [mJ / cm 2 ] is a measure of the intensity of light emitted from a light source on the irradiated surface [mW / cm 2] and is calculated by the product of it and the irradiation duration [s].

[0204] The non-stick state was also determined under the following conditions. Specifically, whether the radiation-curable inkjet ink composition adhered to the cotton swab or whether the cured ink film on the recording medium was scratched was determined. The cotton swab used was a Johnson & Johnson cotton swab. The rubbing frequency was 10 reciprocating strokes, and the rubbing intensity was a load of 100 g.

[0205] The curability was evaluated based on the irradiation energy when the film became non-sticky according to the following evaluation criteria. B or above is a good level. Figures 2 to 5 shown.

[0206] Evaluation Benchmarks

[0207] AA: non-stick energy less than 150mJ / cm 2

[0208] A: Non-stick energy is 150mJ / cm 2 Above and less than 250mJ / cm 2

[0209] B: Non-stick energy is 250mJ / cm 2 Above and less than 350mJ / cm 2

[0210] C: Non-stick energy is 350mJ / cm 2 above

[0211] 3.2. Odor

[0212] The odor of the ink cured film of the recorded material obtained by the above-mentioned production method was smelled and evaluated according to the following criteria. A or above is a good level. Figures 2 to 5 shown.

[0213] Evaluation Benchmarks

[0214] A: There is no odor. Or there is a very small amount of odor.

[0215] B: It smells bad.

[0216] C: There is a strong odor.

[0217] 3.3. Adhesion

[0218] The ink cured film obtained in the evaluation of the curability described above was subjected to a cross-hatch test according to the method described in JIS K5600-5-6 to evaluate the adhesion. Specifically, a grid-like incision was formed on the ink cured film of the recorded object, and a transparent tape was pasted on the grid-like portion where the incision was formed. The tape was rubbed thoroughly with the fingers to make the tape adhere tightly to the ink cured film so that the ink cured film could be seen through the tape. Then, within 5 minutes after the tape was attached, the tape was peeled off from the ink cured film at an angle of about 60° for 0.5 to 1.0 seconds. Whether the ink cured film at this time was peeled off from the recording medium was evaluated based on the following evaluation criteria. B and above are good levels. The evaluation results are as follows Figures 2 to 5 shown.

[0219] Evaluation Benchmarks

[0220] A: Peeling of the ink cured film was observed in less than 20% of the mesh-shaped portions.

[0221] B: Peeling of the ink cured film was observed in 20% or more and less than 30% of the lattice-shaped portions.

[0222] C: Peeling of the ink cured film was observed in 30% or more of the grid-like portions.

[0223] 3.4. Viscosity

[0224] The viscosity of the ink compositions of Examples and Comparative Examples immediately after preparation was measured using a rotational viscometer (product name: Rheometer MCR-301, manufactured by Anton Paar) at 20°C in accordance with JIS Z8803. Figures 2 to 5 shown.

[0225] Evaluation Benchmarks

[0226] AA: Viscosity less than 15mPa•s.

[0227] A: The viscosity is 15 mPa·s or more and less than 25 mPa·s.

[0228] B: Viscosity is 25 mPa•s or more.

[0229] 3.5. Anti-adhesion

[0230] The recorded material obtained by the above-described production method was rounded into a cylindrical shape with the cured ink film facing inward. This recorded material was placed around a glass bottle, which had been preheated in a thermostatic bath, to serve as the packaging. The glass bottle and the cylindrical recorded material placed around the bottle were then left to stand in the thermostatic bath at 90°C for 10 seconds to shrink the cylindrical recorded material and secure it to the glass bottle.

[0231] In the glass bottle with the recorded material in close contact, the trace of the ink cured film adhering to the side of the glass bottle was visually observed to see if it was transferred, and the anti-blocking property was evaluated based on the following evaluation criteria. Figures 2 to 5 shown.

[0232] Evaluation Benchmarks

[0233] A: There is no adhesion of the ink cured film to the glass bottle.

[0234] B: There is a very small amount of adhesion of the ink cured film to the glass bottle.

[0235] C: There is adhesion of the ink cured film to the glass bottle (the ink cured film peels off from the recording medium).

[0236] 3.6. Shrinkage characteristics

[0237] In the recorded material used for the anti-blocking test, the occurrence of wrinkles after shrinkage was visually observed, and the shrinkage characteristics were evaluated based on the following evaluation criteria. Figures 2 to 5 shown.

[0238] Evaluation Benchmarks

[0239] A: There are no wrinkles on the ink cured film.

[0240] B: There are slight wrinkles on the ink cured film.

[0241] C: There are large wrinkles on the ink cured film.

Claims

1. A radiation-curable inkjet ink composition, characterized in that: The radiation-curable inkjet ink composition contains a nitrogen-containing monofunctional polymerizable compound and a photoinitiator. The nitrogen-containing monofunctional polymerizable compound contains N-vinylmethyloxazolidinone or acryloylmorpholine, The photoinitiator contains a high molecular weight photoinitiator with a molecular weight of 500 or more.

2. The radiation-curable inkjet ink composition according to claim 1, wherein The high molecular weight photoinitiator contains at least one of a thioxanthone-based initiator, an α-hydroxyketone-based initiator, a ketosulfonic acid-based initiator, and an amine-based initiator.

3. The radiation-curable inkjet ink composition according to claim 1, wherein The photoinitiator contains a low molecular weight photoinitiator with a molecular weight of less than 500.

4. The radiation-curable inkjet ink composition according to claim 1, wherein The content of the high molecular weight photoinitiator is 15% by mass to 100% by mass relative to the total amount of the photoinitiator.

5. The radiation-curable inkjet ink composition according to claim 1, wherein The high molecular weight photoinitiator contains 1-[4-(4-benzoylphenyl(p-aminophenylsulfonyl))phenyl]-2-methyl-2-[(4-methylphenyl)sulfonyl]-1-propan-1-one, oligo-[2-hydroxy-2-methyl-1-(4-(1-methylvinyl)phenyl)acetone], α-[2-[(9-oxo-9H-thioxanthenyl)oxy]acetyl]-ω-[[2-[(9-oxo-9H-thioxanthenyl)oxy]acetyl]oxy]poly(oxy-1,4-butanediyl), 1,3-bis({α-[(1-chloro-9-oxo-9H-thioxanthen-4-yl)oxy]acetylpoly[oxy(1-methylvinyl)]}oxy)-2,2-bis({α-[(1 any one or more of: a mixture of {α-4-(dimethylamino)benzoylpoly(oxyethylene)-poly[oxy(1-methylethylene)]-poly(oxyethylene)}4-(dimethylamino)benzoate; and polyethylene glycol bis(β-4-[4-(2-dimethylamino-2-benzyl)butyrylphenyl]piperazine)propionate.

6. The radiation-curable inkjet ink composition according to claim 3, wherein The low molecular weight photoinitiator contains any one or more of phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, 2-hydroxy-1-[4-(4-(2-hydroxy-2-methylpropionyl)phenoxy)-phenyl]-2-methylpropane-1-one, (methylimino)diethane-2,1-diylbis[4-(dimethylamino)-benzoate], ethyl phenyl(2,4,6-trimethylbenzoyl)phosphonate and 2,4-diethylthioxanthone.

7. The radiation-curable inkjet ink composition according to claim 1, wherein It also contains a cyclic ether-containing monofunctional polymerizable compound.

8. The radiation-curable inkjet ink composition according to claim 1, wherein It also contains a hydroxyl group-containing polymerizable compound.

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

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

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