Inkjet ink and image forming method

Abstract

To provide an active energy ray-curable inkjet ink enabling improvement of fold-crack resistance, cut-peel resistance, gloss uniformity, surface curability, and scratch resistance of an image formed therewith.SOLUTION: An active energy ray-curable inkjet ink comprises a polymerization initiator, a polymerizable compound including a multifunctional monomer and a monofunctional monomer, and a gelling agent, wherein the polymerization initiator includes a polymerization initiator having a molecular weight of 360 or more, and the ratio of the content of the monofunctional monomer to the multifunctional monomer is more than 0 and 0.29 or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to inkjet inks and imaging methods. [Background technology]

[0002] Inkjet recording methods are used in various printing fields because they can form images easily and inexpensively. One type of inkjet ink is known to be an ink containing a polymerizable compound that polymerizes and crosslinks when irradiated with actinic rays and hardens (hereinafter simply referred to as "actinic ray-curable ink") (see, for example, Patent Document 1). Actinic ray-curable inks have been attracting attention in recent years because they can form images with high adhesion even on recording media with low water absorption.

[0003] Patent Document 1 describes an actinic ray-curable inkjet ink composition (inkjet ink) containing a polyfunctional (meth)acrylate and a polymerization initiator having a molecular weight of 340 or more. The actinic ray-curable inkjet ink composition described in Patent Document 1 becomes a cured film when irradiated with actinic rays after being ejected onto a recording medium. Because the actinic ray-curable inkjet ink composition described in Patent Document 1 contains a polyfunctional (meth)acrylate, a crosslinked structure is formed during the formation of a cured film, thereby suppressing migration. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-209518 Summary of the Invention [Problem to be solved by the invention]

[0005] However, a cured film using the actinic ray-curable ink composition of Patent Document 1 has a crosslinked structure, and therefore becomes hard, which may result in reduced resistance to cracking after folding and peeling after cutting. Actinic ray-curable inks such as the actinic ray-curable ink composition of Patent Document 1 may be required to have various properties depending on the printing field in which the inkjet recording method is used. For example, in order to form images with desirable image quality in various fields, actinic ray-curable inks are required to have gloss uniformity, surface hardening properties, and scratch resistance in addition to the resistance to cracking after folding and peeling after cutting of the formed images.

[0006] An object of the present invention is to provide an actinic ray-curable inkjet ink that can further improve the fold crack resistance, cut-peel resistance, gloss uniformity, surface hardening property, and scratch resistance of a formed image, and to provide an image forming method using such an actinic ray-curable inkjet ink. [Means for solving the problem]

[0007] The first aspect of the present invention relates to the following ink-jet ink. [1] An actinic radiation-curable inkjet ink comprising a polymerization initiator, a polymerizable compound containing a polyfunctional monomer and a monofunctional monomer, and a gelling agent, The polymerization initiator includes a polymerization initiator having a molecular weight of 360 or more, the ratio of the content of the monofunctional monomer to the content of the polyfunctional monomer is greater than 0 and not more than 0.29; Inkjet ink. [2] The inkjet ink according to [1], wherein the polymerization initiator having a molecular weight of 360 or more includes a radical polymerization initiator having an acylphosphine structure. [3] The inkjet ink according to [1] or [2], wherein the content of the polymerization initiator having a molecular weight of 360 or more is 3.0% by mass or more and less than 8.0% by mass. [4] The inkjet ink according to any one of [1] to [3], wherein the polymerizable compound contains phenoxy polyethylene glycol acrylate. [5] The ink-jet ink according to any one of [1] to [4], wherein the content of the gelling agent is 1.0% by mass or more and less than 6.0% by mass.

[0008] The second aspect of the present invention relates to the following image forming method. [6] A step of ejecting the inkjet ink according to any one of [1] to [5] from a nozzle of an inkjet head and causing it to land on a recording medium; a step of irradiating the inkjet ink that has landed on the recording medium with actinic radiation to cure the inkjet ink; An image forming method comprising: [7] The image forming method according to [6], further comprising a step of controlling the surface temperature of the recording medium on the side where the inkjet ink lands to be 20°C or higher and 40°C or lower before or simultaneously with the step of landing the ink on the recording medium. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide an actinic ray-curable inkjet ink that can further improve the fold crack resistance, cut-and-peel resistance, gloss uniformity, surface hardening property, and scratch resistance of a formed image, and an image forming method using such an actinic ray-curable inkjet ink. DETAILED DESCRIPTION OF THE INVENTION

[0010] 1. Inkjet ink The inkjet ink of this embodiment is an actinic ray curable inkjet ink containing a polymerization initiator, a polymerizable compound, and a gelling agent. When the inkjet ink that has landed on a recording medium is irradiated with actinic rays, the inkjet ink is cured to form a cured film.

[0011] The inkjet ink according to the present invention can further improve the fold crack resistance, cut-peel resistance, gloss uniformity, and surface hardening properties of the formed image, for the following reasons.

[0012] When an actinic ray-curable inkjet ink containing a polymerization initiator, a polymerizable compound, and a gelling agent is cured by irradiation with actinic rays, polymerization begins at the polymerization initiator, but at this time, a portion of the polymerization initiator remains unreacted. The unreacted polymerization initiator, uncured photopolymerizable compound, and gelling agent precipitate on the surface of the image, which can cause a decrease in the gloss of the image and a decrease in the surface curing property of the inkjet ink.

[0013] In contrast, the inkjet ink of the present invention contains a polymerization initiator with a relatively large molecular weight, so that unreacted polymerization initiator is less likely to move in the inkjet ink. Furthermore, the inkjet ink of the present invention uses a polyfunctional monomer and a monofunctional monomer in combination, which reduces the degree of polymerization (fewer crosslinking points) and softens the cured film, which is thought to improve fold crack resistance and cut peel resistance while maintaining scratch resistance.

[0014] If the amount of monofunctional monomer is too high compared to the polyfunctional monomer, the amount of residual monomer increases and it becomes more likely to precipitate on the surface, so it was necessary to adjust the ratio of monofunctional monomer to polyfunctional monomer.By combining a polymerization initiator with a molecular weight of 360 or more and a monofunctional monomer, and the ratio of monofunctional monomer to polyfunctional monomer is 0.29 or less, it was possible to achieve appropriate curing while reducing the amount of polymerization initiator added.

[0015] Furthermore, by using a gelling agent in combination, the gel covers the surface of the inkjet ink image, and the gel and polymerization initiator incorporate the monofunctional monomer, making it less likely that unreacted substances or decomposition products will appear on the surface of the image, thereby further improving the gloss uniformity and surface hardening properties of the formed image.

[0016] 1-1. Polymerization initiator The polymerization initiator (hereinafter also simply referred to as "initiator") has the effect of initiating polymerization and crosslinking of a polymerizable compound, which will be described later, upon irradiation with actinic rays. The inkjet ink may contain only one type of polymerization initiator, or two or more types of polymerization initiators.

[0017] The polymerization initiator includes a polymerization initiator having a molecular weight of 360 or more. A polymerization initiator having a molecular weight of 360 or more is less likely to move in the cured film, and its movement is further restricted by a network structure formed by polymerization and crosslinking of the polymerizable compound, which will be described later, and therefore is less likely to precipitate on the image surface. Therefore, it is thought that in an image obtained by curing the inkjet ink, there is less decrease in the gloss of the image surface and less decrease in surface hardening properties.

[0018] The content of the polymerization initiator is preferably 1.0% by mass or more and 9.0% by mass or less, based on the total mass of the inkjet ink. The content of the polymerization initiator having a molecular weight of 360 or more is preferably 3.0% by mass or more and less than 8.0% by mass, based on the total mass of the inkjet ink. By ensuring that the content of the polymerization initiator having a molecular weight of 360 or more is 3.0% by mass or more, the polymerizable compound can be sufficiently polymerized and crosslinked, thereby sufficiently curing the inkjet ink. In addition, the gloss uniformity and surface hardening properties of the cured film can be improved. Note that, by ensuring that the content of the polymerization initiator, particularly a polymerization initiator having a molecular weight of 360 or more, is less than 8.0% by mass, the cured film can be made moderately flexible, thereby improving its resistance to fold cracking and cutting peeling.

[0019] The polymerization initiator may optionally contain a polymerization initiator having a molecular weight of less than 360, as long as the effects of the present invention are achieved. However, from the viewpoint of suppressing a decrease in gloss of the image surface and a decrease in surface hardening properties due to precipitation of the decomposed polymerization initiator, the content of the polymerization initiator having a molecular weight of less than 360 is preferably 3.0% by mass or less relative to the total mass of the inkjet ink.

[0020] Examples of the polymerization initiator include a radical polymerization initiator and a cationic polymerization initiator.

[0021] Examples of the radical polymerization initiator include an initiator having a bisacylphosphine structure, an initiator having an alkylphenone structure, an initiator having a thioxanthone structure, an initiator having an acylphosphine structure, and an initiator having a benzophenone structure. From the viewpoint of further enhancing the effects of the present invention, particularly scratch resistance, the radical polymerization initiator is preferably an initiator having an acylphosphine structure.

[0022] A radical polymerization initiator having an acylphosphine structure and a molecular weight of 360 or more is highly reactive and easily develops polymerization and crosslinking. Therefore, when a radical polymerization initiator having an acylphosphine structure and a molecular weight of 360 or more is added, the movement of the decomposed polymerization initiator is more restricted, and therefore, the decrease in gloss of the image surface and the decrease in surface hardening properties are less.

[0023] Examples of radical polymerization initiators having an acylphosphine structure and a molecular weight of 360 or more are as follows. Examples of such polymerization initiators include 2,4,6-trimethylbenzoylethoxyphenylphosphine oxide, 2,6-dimethylbenzoylethoxyphenylphosphine oxide, 2,4,6-trimethylbenzoylmethoxyphenylphosphine oxide, 2,6-dimethylbenzoylmethoxyphenylphosphine oxide, 2,4,6-trimethylbenzoyl(4-pentyloxyphenyl)phenylphosphine oxide, and 2,6-dimethylbenzoyl(4-pentyloxyphenyl)phenylphosphine oxide. Radical polymerization initiators having an acylphosphine structure and a molecular weight of 360 or more can provide a cured film with appropriate flexibility, thereby improving its resistance to fold cracking and cut peeling.

[0024] Commercially available examples of radical polymerization initiators having an acylphosphine structure and a molecular weight of 360 or more include IRGACURE 819 manufactured by BASF ("IRGACURE" is a registered trademark of the company), OMNIRAD 380 manufactured by IGM Resins BV, and A-TPO-N (2,4,6-trimethylbenzoylphosphine oxide derivative) manufactured by Gyroid Materials, Inc.

[0025] Examples of radical polymerization initiators having a bisacylphosphine structure and a molecular weight of 360 or greater are as follows: Examples of the polymerization initiators include bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide, bis(2,6-dimethylbenzoyl)phenylphosphine oxide, bis(2,4,6-trimethylbenzoyl)-2-methoxyphenylphosphine oxide, bis(2,6-dimethylbenzoyl)-2-methoxyphenylphosphine oxide, bis(2,4,6-trimethylbenzoyl)-2,4-dimethoxyphenylphosphine oxide, bis(2,6-dimethylbenzoyl)-2,4-dimethoxyphenylphosphine oxide, bis(2,4,6-trimethylbenzoyl)-2,4-dipentyloxyphenylphosphine oxide, and bis(2,6-dimethylbenzoyl)-2,4-dipentyloxyphenylphosphine oxide.

[0026] Examples of commercially available radical polymerization initiators having a bisacylphosphine structure and a molecular weight of 360 or more include IRGACURE 819, manufactured by BASF.

[0027] Examples of commercially available radical polymerization initiators having an alkylphenone structure and a molecular weight of 360 or more are as follows: IRGACURE 369 and IRGACURE 379, manufactured by BASF, and ESACURE 1001M and ESACURE KIP150, manufactured by Lamberti (ESACURE is a registered trademark of the company).

[0028] Commercially available examples of radical polymerization initiators having a thioxanthone structure and a molecular weight of 360 or more include GENOPOL TX-1 manufactured by Rahn and SPEEDCURE 7010 manufactured by Lambson.

[0029] Commercially available examples of radical polymerization initiators having a benzophenone structure and a molecular weight of 360 or more include SPEEDCURE 7005 manufactured by Lambson, and Omnipol BP manufactured by Rahn AG.

[0030] Examples of commercially available radical polymerization initiators having a bisacylphosphine structure and a molecular weight of less than 360 include LUCIRIN TPO, manufactured by BASF ("LUCIRIN" is a registered trademark of the company).

[0031] Commercially available examples of radical polymerization initiators having an alkylphenone structure and a molecular weight of less than 360 include IRGACURE 184 and IRGACURE 907, both manufactured by BASF.

[0032] Examples of commercially available radical polymerization initiators having a thioxanthone structure and a molecular weight of less than 360 include SPEEDCURE ITX, manufactured by BASF.

[0033] Examples of the cationic polymerization initiator include salts of aromatic onium compounds, sulfonates that generate sulfonic acid, halides that photogenerate hydrogen halide, and iron-allene complexes.

[0034] Examples of oniums include diazonium, ammonium, iodonium, sulfonium, and phosphonium. Examples of counterions that form salts of aromatic onium compounds include B(C6F5) 4- , P.F. 6- , AsF 6- , SbF 6- and CF3SO 3- Includes:

[0035] Examples of aromatic onium compounds include the compound described in paragraph 0134 of JP 2005-255821 A. Examples of sulfonated compounds that generate sulfonic acid include the compound described in paragraph 0136 of JP 2005-255821 A. Examples of sulfonated compounds that generate sulfonic acid include the compound described in claim 1 of JP 2004-91698 A and the compound described in claim 1 of JP 2006-518332 A. Examples of halides that photogenerate hydrogen halide include the compound described in paragraph 0138 of JP 2005-255821 A. Examples of iron allene complexes include the compound described in paragraph 0140 of JP 2005-255821 A.

[0036] Commercially available examples of cationic polymerization initiators having a molecular weight of 360 or greater include CPI-100P manufactured by San-Apro Co., Ltd. and IRGACURE 250 manufactured by BASF.

[0037] Examples of cationic polymerization initiators having a molecular weight of less than 360 include triphenylsulfonium bromide.

[0038] 1-2.Polymerizable compound Examples of the polymerizable compound include a radically polymerizable compound and a cationically polymerizable compound. The polymerizable compound crosslinks or polymerizes when irradiated with actinic rays, thereby curing the inkjet ink. The polymerizable compound may be a monomer, a polymerizable oligomer, a prepolymer, or a mixture thereof. The inkjet ink may contain only one type of polymerizable compound, or two or more types of polymerizable compounds.

[0039] Examples of actinic rays include ultraviolet rays, electron beams, α rays, γ rays, and X-rays. From the viewpoint of safety and the ability to cause polymerization and crosslinking with a lower energy dose, ultraviolet rays or electron beams are preferred as actinic rays.

[0040] The polymerizable compound includes polyfunctional polymerizable compounds (hereinafter simply referred to as "polyfunctional monomers") having two or more polymerizable functional groups within the molecule that can be polymerized by exposure to actinic rays, and monofunctional polymerizable compounds (hereinafter simply referred to as "monofunctional monomers"). Polyfunctional monomers are easily polymerized and crosslinked by exposure to actinic rays to form a network-like polymer with a high crosslink density. It is believed that this network-like polymer restricts the movement of each component during curing of the inkjet ink, thereby making it difficult for the polymerization initiator, gelling agent, and uncured polymerizable compound to precipitate on the image surface. Therefore, it is believed that polyfunctional monomers can suppress a decrease in gloss and surface curability of the image surface.

[0041] The content of the monofunctional monomer is preferably more than 0% by mass and not more than 25% by mass, more preferably from 5% to 23% by mass, and even more preferably from 10% to 21% by mass, relative to the total mass of the inkjet ink. If the content of the monofunctional monomer is within the above range, the crosslink density can be increased, making it more difficult for the polymerization initiator, gelling agent, and uncured polymerizable compound to precipitate on the image surface, and the crosslink density can be increased to improve scratch resistance.

[0042] The ratio of the content of the monofunctional monomer to the polyfunctional monomer is greater than 0 and equal to or less than 0.29, and preferably equal to or greater than 0.1 and equal to or less than 0.29. When the ratio of the monofunctional monomer to the polyfunctional monomer is 0.29 or less, the amount of polymerization initiator added can be reduced, while the fold crack resistance and cut peel resistance can be improved.

[0043] The content of the polymerizable compound may be in any range that allows the effects of the present invention to be obtained, and may be, for example, from 1% by mass to 97% by mass, both inclusive, based on the total mass of the inkjet ink. The content of the polymerizable compound is preferably from 30% by mass to 95% by mass, both inclusive, based on the total mass of the inkjet ink.

[0044] Examples of radical polymerizable compounds include unsaturated carboxylic acids and their salts, unsaturated carboxylic acid ester compounds, unsaturated carboxylic acid urethane compounds, unsaturated carboxylic acid amide compounds and their anhydrides. Other examples of radical polymerizable compounds include acrylonitrile, styrene, unsaturated polyesters, unsaturated polyethers, unsaturated polyamides, and unsaturated urethanes. Examples of unsaturated carboxylic acids include (meth)acrylic acid, itaconic acid, crotonic acid, isocrotonic acid, and maleic acid.

[0045] The radical polymerizable compound is preferably an unsaturated carboxylic acid ester compound, more preferably a (meth)acrylate. In the present invention, "(meth)acrylate" means acrylate or methacrylate, "(meth)acryloyl group" means acryloyl group or methacryloyl group, and "(meth)acrylic" means acrylic or methacrylic.

[0046] Examples of polyfunctional (meth)acrylates include triethylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, 1,9-nonanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, dimethylol-tricyclodecane di(meth)acrylate, bisphenol A PO adduct di(meth)acrylate, and hydroxypivalic acid neopentyl glycol di(meth)acrylate. Examples of the (meth)acrylates include difunctional (meth)acrylates such as polyol di(meth)acrylate, polytetramethylene glycol di(meth)acrylate, polyethylene glycol diacrylate, and tripropylene glycol diacrylate, and trifunctional or higher (meth)acrylates such as 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, and pentaerythritol ethoxy tetra(meth)acrylate.

[0047] Examples of monofunctional (meth)acrylates include isoamyl (meth)acrylate, stearyl (meth)acrylate, lauryl (meth)acrylate, octyl (meth)acrylate, decyl (meth)acrylate, isomylstyryl (meth)acrylate, isostearyl (meth)acrylate, 2-ethylhexyl-diglycol (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 2-(meth)acryloyloxyethyl hexahydrophthalic acid, butoxyethyl (meth)acrylate, ethoxydiethylene glycol (meth)acrylate, methoxydiethylene glycol (meth)acrylate, methoxypolyethylene glycol (meth)acrylate, methyl ... Examples of suitable acrylates include ethylene glycol (meth)acrylate, methoxypropylene glycol (meth)acrylate, phenoxyethyl (meth)acrylate, tetrahydrofurfuryl (meth)acrylate, isobornyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 2-hydroxy-3-phenoxypropyl (meth)acrylate, 2-(meth)acryloyloxyethyl succinate, 2-(meth)acryloyloxyethyl phthalate, 2-(meth)acryloyloxyethyl-2-hydroxyethyl-phthalate, and t-butylcyclohexyl (meth)acrylate.

[0048] The radical polymerizable compound preferably contains a (meth)acrylate modified with ethylene oxide or propylene oxide (hereinafter simply referred to as a "modified (meth)acrylate"). That is, the radical polymerizable compound preferably contains an ethylene oxide group (hereinafter simply referred to as an "EO group") or a propylene oxide group (hereinafter simply referred to as a "PO group"). Modified (meth)acrylates have higher photosensitivity. Furthermore, modified (meth)acrylates are more likely to be encapsulated in a card-house structure when the ink gels at low temperatures, and are more compatible with other ink components even at high temperatures. Furthermore, modified (meth)acrylates have less shrinkage upon curing, making printed matter less likely to curl during image formation.

[0049] When the weighted average (mass basis) of the number of EO groups per molecule in the polymerizable compound is X and the weighted average (mass basis) of the number of PO groups per molecule in the polymerizable compound is Y, it is preferable that formula (1) is satisfied. 3.5≦X+Y≦9 (1)

[0050] In formula (1), X+Y is preferably 3.5 or more and 8 or less, more preferably 3.5 or more and 7 or less. When X+Y is 3.5 or more, the flexibility of the cured film can be increased, making it more difficult for the image to crack when folded. Furthermore, when X+Y is 8 or less, the hardness of the cured film is less likely to decrease, making it more difficult for the image to have abrasion resistance. Furthermore, when X+Y is 8 or less, the ink viscosity is less likely to increase.

[0051] In formula (1), X is preferably 1.5 or more and 7 or less, more preferably 1.5 or more and 6 or less. EO groups have higher molecular mobility than PO groups, so a larger number of EO groups can further increase the flexibility of the cured film. Therefore, when X is 1.5 or more, the image is less likely to crack when folded. Furthermore, when X is 7 or less, the hardness of the cured film is less likely to decrease, and the abrasion resistance of the image is further increased.

[0052] In the polymerizable compound, the weighted average (by mass) number of repetitions of EO groups and PO groups per molecule is preferably 2 or more and 10 or less, and more preferably 2 or more and 8 or less.

[0053] Here, "the number of repeats of EO groups and PO groups per molecule" refers to the number of consecutive EO or PO groups in a molecular chain. When a molecule contains multiple units of consecutive EO or PO groups, the number of repeats is calculated by dividing the total number of consecutive EO or PO groups by the number of units. The weighted average number of repeats refers to the total value for each polymerizable compound calculated by multiplying the number of repeats per molecule of polymerizable compound contained in the polymerizable compound by the mass ratio of the polymerizable compound in the polymerizable compound.

[0054] When the weighted average number of repetitions is 2 or more, the EO or PO groups are continuous in the polymer chain, lengthening the length of the region with high molecular mobility and increasing the flexibility of the cured product. This further reduces image cracking. Furthermore, when the weighted average number of repetitions is 10 or less, the hardness of the cured film is less likely to decrease, further increasing the abrasion resistance of the image.

[0055] Examples of (meth)acrylates containing EO groups include polyethylene glycol diacrylate, EO-modified 1,6-hexanediol di(meth)acrylate, EO-modified pentaerythritol tetraacrylate, EO-modified dipentaerythritol pentaacrylate, EO-modified dipentaerythritol hexaacrylate, EO-modified trimethylolpropane tri(meth)acrylate, phenoxy polyethylene glycol acrylate, EO-modified cresol (meth)acrylate, and EO-modified bisphenol A diacrylate. Examples of phenoxy polyethylene glycol acrylate include EO-modified phenol acrylate and EO-modified nonylphenol (meth)acrylate. Phenoxy polyethylene glycol acrylate can improve gloss uniformity and scratch resistance of the cured product.

[0056] Examples of (meth)acrylates having a PO group include tripropylene glycol diacrylate, PO-modified neopentyl glycol diacrylate, and PO-modified trimethylolpropane triacrylate. Other examples of (meth)acrylates having a PO group include PO-modified glyceryl triacrylate, PO-modified bisphenol A diacrylate, and PO-modified nonylphenol acrylate.

[0057] Commercially available examples of polyfunctional modified (meth)acrylates include CD561, SR454, SR499, and SR494 manufactured by Sartomer Co., Ltd. Other commercially available examples include NK Ester A-400, NK Ester A-600, NK Ester 9G, NK Ester 14G, NK Ester DOD-N, NK Ester A-DCP, and NK Ester DCP manufactured by Shin-Nakamura Chemical Co., Ltd.

[0058] Examples of the cationically polymerizable compound include an epoxy compound, a vinyl ether compound, and an oxetane compound.

[0059] From the viewpoint of suppressing shrinkage of the recording medium when the inkjet ink is cured, the polymerizable compound preferably contains at least one type of oxetane compound and at least one type of compound selected from epoxy compounds and vinyl ether compounds.

[0060] From the viewpoint of accelerating curing, the polymerizable compound is preferably an aromatic epoxide or an alicyclic epoxide, and more preferably an alicyclic epoxide.

[0061] Examples of aromatic epoxides include di- or polyglycidyl ethers prepared by reacting a polyhydric phenol having at least one aromatic nucleus or its alkylene oxide adduct with epichlorohydrin. Examples of aromatic epoxides include di- or polyglycidyl ethers of bisphenol A and its alkylene oxide adduct, hydrogenated bisphenol A and its alkylene oxide adduct, and novolac-type epoxy resins. Examples of the alkylene oxide include ethylene oxide and propylene oxide.

[0062] Examples of alicyclic epoxides include cyclohexene oxide- or cyclopentene oxide-containing compounds, which can be obtained by epoxidizing a compound having at least one cycloalkane ring, such as a cyclohexene or cyclopentene ring, with a suitable oxidizing agent, such as hydrogen peroxide or a peracid.

[0063] Examples of aliphatic epoxides include di- or polyglycidyl ethers of aliphatic polyhydric alcohols or their alkylene oxide adducts. Examples of such aliphatic epoxides include diglycidyl ethers of alkylene glycols, polyglycidyl ethers of polyhydric alcohols, and diglycidyl ethers of polyalkylene glycols. Examples of alkylene glycols include diglycidyl ethers of ethylene glycol, diglycidyl ethers of propylene glycol, and diglycidyl ethers of 1,6-hexanediol. Examples of polyhydric alcohols include glycerin and its alkylene oxide adducts. Examples of polyalkylene glycols include polyethylene glycol and its alkylene oxide adducts, and polypropylene glycol and its alkylene oxide adducts. Examples of alkylene oxides include ethylene oxide and propylene oxide.

[0064] Examples of vinyl ether compounds include monovinyl ether compounds, divinyl ether compounds, and trivinyl ether compounds. Examples of monovinyl ether compounds include ethyl vinyl ether, n-butyl vinyl ether, isobutyl vinyl ether, octadecyl vinyl ether, cyclohexyl vinyl ether, hydroxybutyl vinyl ether, 2-ethylhexyl vinyl ether, cyclohexanedimethanol monovinyl ether, n-propyl vinyl ether, isopropyl vinyl ether, isopropenyl ether-o-propylene carbonate, dodecyl vinyl ether, diethylene glycol monovinyl ether, and octadecyl vinyl ether. Examples of divinyl ether compounds include ethylene glycol divinyl ether, diethylene glycol divinyl ether, triethylene glycol divinyl ether, propylene glycol divinyl ether, dipropylene glycol divinyl ether, butanediol divinyl ether, hexanediol divinyl ether, and cyclohexanedimethanol divinyl ether. Examples of trivinyl ether compounds include trimethylolpropane trivinyl ether.

[0065] From the viewpoints of curability, adhesion, and surface hardness, the vinyl ether compound is preferably a divinyl ether compound or a trivinyl ether compound, and more preferably a divinyl ether compound.

[0066] The oxetane compound is a compound having an oxetane ring. Examples of the oxetane compound include the compounds having an oxetane ring described in JP-A Nos. 2001-220526 and 2001-310937.

[0067] The oxetane compound preferably has 1 to 4 oxetane rings, from the viewpoints of not increasing the viscosity of the inkjet ink more than necessary and of imparting sufficient adhesion to the cured product obtained by appropriately adjusting the glass transition temperature of the inkjet ink.

[0068] Examples of compounds having 1 to 4 oxetane rings include compounds represented by general formula (1) in paragraph number 0089, general formula (2) in paragraph number 0092, general formula (7) in paragraph number 0107, general formula (8) in paragraph number 0109, and general formula (9) in paragraph number 0116 of JP-A-2005-255821.

[0069] 1-3. Gelling agents The gelling agent can gelatinize the inkjet ink droplets that have landed on the recording medium, thereby temporarily fixing them (pinning them). When the inkjet ink is pinned in a gel state, the inkjet ink is prevented from spreading and adjacent dots are less likely to mix, allowing for the formation of higher-resolution images. Furthermore, when the inkjet ink is in a gel state, oxygen from the environment is prevented from entering the inkjet ink droplets, making it less likely that oxygen will inhibit curing, allowing for the formation of higher-resolution images at higher speeds. The inkjet ink may contain only one type of gelling agent, or two or more types.

[0070] The gelling agent content is preferably 1.0% by mass or more and less than 5.0% by mass relative to the total mass of the inkjet ink. When the gelling agent content is 1.0% by mass or more, the wetting and spreading of each inkjet ink droplet on the recording medium can be controlled to the same extent, making it possible to reduce the occurrence of gloss differences within an image. By reducing the gelling agent content to less than 5.0% by mass, it is possible to reduce the occurrence of gloss differences within an image, and to increase the strength of the cured film and improve scratch resistance. From the above perspectives, the gelling agent content in the inkjet ink is more preferably 1.5% by mass or more and less than 5.0% by mass, and even more preferably 2.0% by mass or more and less than 4.0% by mass.

[0071] Furthermore, it is preferable that the gelling agent is substantially free of photopolymerizable functional groups. "Substantially free" means that the amount of photopolymerizable functional groups per mole of gelling agent is 0.1 molar equivalents or less. By being substantially free of photopolymerizable functional groups, it is thought that the functional groups in the gelling agent are less likely to inhibit the polymerization and crosslinking of the photopolymerizable compound, thereby enabling a sufficient increase in crosslink density.

[0072] For the following reasons, it is preferable that the gelling agent crystallizes in the ink at a temperature equal to or lower than the gelling temperature of the ink. The gelling temperature is the temperature at which the gelling agent undergoes a phase transition from sol to gel when ink that has been solated or liquefied by heating is cooled, causing a sudden change in the viscosity of the ink. Specifically, the solated or liquefied ink is cooled while its viscosity is measured using a viscoelasticity measuring device (e.g., MCR300, manufactured by Physica), and the temperature at which the viscosity suddenly increases can be determined to be the gelling temperature of the ink.

[0073] When the gelling agent crystallizes in the ink, a structure (hereinafter simply referred to as a "house of cards structure") may be formed in which the photopolymerizable compound is encapsulated in a three-dimensional space formed by the gelling agent crystallized into plates. When a house of cards structure is formed, the liquid polymerizable compound is retained within the space, making it more difficult for the inkjet ink droplets to wet and spread, and improving the pinning ability of the inkjet ink. When the pinning ability of the inkjet ink is improved, it becomes more difficult for the inkjet ink droplets that land on the recording medium to mix with each other, allowing for the formation of higher-resolution images.

[0074] To form a house-of-cards structure, it is preferable that the polymerizable compound and gelling agent dissolved in the inkjet ink are compatible with each other. On the other hand, if the polymerizable compound and gelling agent dissolved in the inkjet ink are phase-separated, it may be difficult to form a house-of-cards structure.

[0075] Examples of gelling agents suitable for forming a house-of-cards structure by crystallization include ketone waxes, ester waxes, petroleum-based waxes, vegetable waxes, animal waxes, mineral waxes, hydrogenated castor oil, modified waxes, higher fatty acids, higher alcohols, hydroxystearic acid, fatty acid amides including N-substituted fatty acid amides and special fatty acid amides, higher amines, esters of sucrose fatty acids, synthetic waxes, dibenzylidene sorbitol, dimer acid and dimer diol.

[0076] Examples of ketone waxes include dilignoceryl ketone, dibehenyl ketone, distearyl ketone, dieicosyl ketone, dipalmityl ketone, dilauryl ketone, dimyristyl ketone, myristyl palmityl ketone and palmityl stearyl ketone.

[0077] Examples of ester waxes include behenyl behenate, icosyl icosanoate, stearyl stearate, palmityl stearate, cetyl palmitate, myristyl myristate, cetyl myristate, myricyl cerotate, stearyl stearate, oleyl palmitate, glycerin fatty acid esters, sorbitan fatty acid esters, propylene glycol fatty acid esters, ethylene glycol fatty acid esters, and polyoxyethylene fatty acid esters.

[0078] Examples of commercially available ester waxes include the EMALEX series manufactured by Nippon Emulsion Co., Ltd. ("EMALEX" is a registered trademark of the company), and the Rikemal series and Poem series manufactured by Riken Vitamin Co., Ltd. ("Rikemal" and "Poem" are both registered trademarks of the company).

[0079] Examples of petroleum-based waxes include paraffin wax, microcrystalline wax, and petroleum-based waxes containing petrolatum.

[0080] Examples of vegetable waxes include candelilla wax, carnauba wax, rice wax, Japan wax, jojoba oil, jojoba solid wax, and jojoba esters.

[0081] Examples of animal-based waxes include beeswax, lanolin and spermaceti.

[0082] Examples of mineral waxes include montan wax and hydrogenated wax.

[0083] Examples of modified waxes include montan wax derivatives, paraffin wax derivatives, microcrystalline wax derivatives, 12-hydroxystearic acid derivatives and polyethylene wax derivatives.

[0084] Examples of higher fatty acids include behenic acid, arachidic acid, stearic acid, palmitic acid, myristic acid, lauric acid, oleic acid, and erucic acid.

[0085] Examples of higher alcohols include stearyl alcohol and behenyl alcohol.

[0086] Examples of hydroxystearic acids include 12-hydroxystearic acid.

[0087] Examples of fatty acid amides include lauric acid amide, stearic acid amide, behenic acid amide, oleic acid amide, erucic acid amide, ricinoleic acid amide and 12-hydroxystearic acid amide.

[0088] Examples of commercially available fatty acid amides include the Nikkaamide series manufactured by Nippon Kasei Co., Ltd. ("Nikkaamide" is a registered trademark of the company), the ITOWAX series manufactured by Ito Oil Mills, and the FATTYAMID series manufactured by Kao Corporation.

[0089] Examples of N-substituted fatty acid amides include N-stearyl stearic acid amide and N-oleyl palmitic acid amide.

[0090] Examples of the special fatty acid amides include N,N'-ethylenebisstearylamide, N,N'-ethylenebis-12-hydroxystearylamide, and N,N'-xylylenebisstearylamide.

[0091] Examples of higher amines include dodecylamine, tetradecylamine and octadecylamine.

[0092] Examples of esters of sugar fatty acids include sucrose stearate and sucrose palmitate.

[0093] Examples of commercially available sucrose fatty acid esters include the Ryoto Sugar Ester series manufactured by Mitsubishi Chemical Foods Corporation (Ryoto is a registered trademark of the company).

[0094] Examples of synthetic waxes include polyethylene wax and α-olefin maleic anhydride copolymer wax.

[0095] Commercially available examples of synthetic waxes include the UNILIN series manufactured by Baker-Petrolite ("UNILIN" is a registered trademark of the company).

[0096] Examples of dibenzylidene sorbitols include 1,3:2,4-bis-O-benzylidene-D-glucitol.

[0097] An example of a commercially available dibenzylidene sorbitol product is Gelall D, manufactured by New Japan Chemical Co., Ltd. ("Gelall" is a registered trademark of the company).

[0098] Examples of commercially available dimer diols include the PRIPOR series manufactured by CRODA ("PRIPOR" is a registered trademark of the company).

[0099] From the viewpoint of improving pinning ability, the gelling agent is preferably a ketone wax, an ester wax, a higher fatty acid, a higher alcohol, or a fatty acid amide, and more preferably a ketone wax represented by general formula (G1) or an ester wax represented by general formula (G2). The inkjet ink may contain only one type of the ketone wax represented by general formula (G1) and the ester wax represented by general formula (G2). The inkjet ink may contain two or more types of the ketone wax represented by general formula (G1) and the ester wax represented by general formula (G2). The inkjet ink may contain either one or both of the ketone wax represented by general formula (G1) and the ester wax represented by general formula (G2).

[0100] General formula (G1): R1-CO-R2 In general formula (G1), R1 and R2 are both linear or branched hydrocarbon groups having 9 or more and 25 or less carbon atoms.

[0101] General formula (G2): R3-COO-R4 In general formula (G2), R3 and R4 are both linear or branched hydrocarbon groups having 9 or more and 25 or less carbon atoms.

[0102] The ketone wax represented by general formula (G1) or the ester wax represented by general formula (G2) has a linear or branched hydrocarbon group with 9 or more carbon atoms, which enhances the crystallinity of the gelling agent and creates more space in the house-of-card structure. This makes it easier for the polymerizable compound to be fully enclosed within the space, improving the pinning ability of the inkjet ink. Furthermore, since the linear or branched hydrocarbon group has 25 or less carbon atoms, the melting point of the gelling agent does not increase excessively, eliminating the need to excessively heat the inkjet ink when ejecting it. From the above perspective, it is particularly preferable that R1 and R2 are linear hydrocarbon groups with 11 or more but less than 23 carbon atoms.

[0103] Furthermore, from the viewpoint of increasing the gelling temperature of the ink and gelling the inkjet ink more rapidly after landing, it is preferable that either R1 or R2, or either R3 or R4, be a saturated hydrocarbon group having 11 or more but less than 23 carbon atoms. From the viewpoint above, it is more preferable that both R1 and R2, or both R3 and R4, be saturated hydrocarbon groups having 11 or more but less than 23 carbon atoms.

[0104] Examples of the ketone wax represented by the general formula (G1) include dilignoceryl ketone (number of carbon atoms: 23 to 24), dibehenyl ketone (number of carbon atoms: 21 to 22), distearyl ketone (number of carbon atoms: 17 to 18), dieicosyl ketone (number of carbon atoms: 19 to 20), dipalmityl ketone (number of carbon atoms: 15 to 16), dimyristyl ketone (number of carbon atoms: 13 to 14), dilauryl ketone (number of carbon atoms: 11 to 12), lauryl ketone (number of carbon atoms: 13 to 14 ... These include lauryl myristyl ketone (carbon number: 11-14), lauryl palmityl ketone (11-16), myristyl palmityl ketone (13-16), myristyl stearyl ketone (13-18), myristyl behenyl ketone (13-22), palmityl stearyl ketone (15-18), palmityl behenyl ketone (15-22), and stearyl behenyl ketone (17-22). The number of carbon atoms in the parentheses indicates the number of carbon atoms in each of the two hydrocarbon groups separated by the carbonyl group.

[0105] Commercially available examples of ketone waxes represented by general formula (G1) include 18-Pentatriacontanone manufactured by Alfa Aeser, Hentriacontan-16-one manufactured by Alfa Aeser, and Kaowax T1 manufactured by Kao Corporation.

[0106] Examples of fatty acids or ester waxes represented by general formula (G2) include behenyl behenate (number of carbon atoms: 21 to 22), icosanoic acid icosyl (number of carbon atoms: 19 to 20), stearyl stearate (number of carbon atoms: 17 to 18), palmityl stearate (number of carbon atoms: 17 to 16), lauryl stearate (number of carbon atoms: 17 to 12), cetyl palmitate (number of carbon atoms: 15 to 16), stearyl palmitate (number of carbon atoms: 15 to 18), These include myristyl myristate (carbon number: 13-14), cetyl myristate (carbon number: 13-16), octyldodecyl myristate (carbon number: 13-20), stearyl oleate (carbon number: 17-18), stearyl erucate (carbon number: 21-18), stearyl linoleate (carbon number: 17-18), behenyl oleate (carbon number: 18-22), and arachidyl linoleate (carbon number: 17-20). The number of carbon atoms in the parentheses above indicates the number of carbon atoms in each of the two hydrocarbon groups separated by the ester group.

[0107] Commercially available examples of the ester wax represented by general formula (G2) include Unistar M-2222SL, Sperm Acetate, Nissan Elector WEP3, and Nissan Elector WEP2, manufactured by NOF Corporation ("Unistar" and "Nissan Elector" are registered trademarks). Other examples of such commercially available products include Exepar SS and Exepar MY-M, manufactured by Kao Corporation ("Exepar" is a registered trademark), and EMALEX CC-18 and EMALEX CC-10, manufactured by Nippon Emulsion Co., Ltd. ("EMALEX" is a registered trademark). Other examples of such commercially available products include Amureps PC, manufactured by Kokyu Alcohol Kogyo Co., Ltd. ("Amureps" is a registered trademark). These commercially available products are often mixtures of two or more types, and may be separated and purified as necessary before being incorporated into the inkjet ink.

[0108] 1-4.Other ingredients The inkjet ink may further contain other components, including a colorant, a dispersant, a polymerization initiator aid, a polymerization inhibitor, a sensitizer, and a surfactant, as long as the effects of the present invention are achieved. The inkjet ink may contain only one type of these components, or two or more types of these components.

[0109] The coloring material includes dyes and pigments. The coloring material is preferably a pigment from the viewpoint of obtaining an image with good weather resistance. The pigment can be selected from, for example, a yellow pigment, a red or magenta pigment, a blue or cyan pigment, and a black pigment depending on the color of the image to be formed.

[0110] Examples of yellow pigments include CI Pigment Yellow (hereinafter, "CI Pigment Yellow" may be simply referred to as "PY") 1, PY3, PY12, PY13, PY14, PY17, PY34, and PY35. Other examples of yellow pigments include PY37, PY55, PY74, PY81, PY83, PY93, PY94, PY95, PY97, PY108, PY109, and PY110. Examples of yellow pigments include PY137, PY138, PY139, PY153, PY154, PY155, PY157, PY166, PY167, PY168, PY180, PY185, and PY193.

[0111] Examples of red or magenta pigments include CI Pigment Red (hereinafter, "CI Pigment Red" may also be simply referred to as "PR") 3, PR5, PR19, PR22, PR31, PR38, PR43, PR48:1, and PR48:2. Other examples of red or magenta pigments include PR48:3, PR48:4, PR48:5, PR49:1, PR53:1, PR57:1, PR57:2, PR58:4, PR63:1, PR81, PR81:1, and PR81:2. Other examples of red or magenta pigments include PR81:3, PR81:4, PR88, PR104, PR108, PR112, PR122, PR123, PR144, PR146, PR149, PR166, PR168, and PR169. Other examples of red or magenta pigments include PR170, PR177, PR178, PR179, PR184, PR185, PR208, PR216, and PR226. Other examples of red or magenta pigments include PR257, CI Pigment Violet (hereinafter, "CI Pigment Violet" may also be referred to simply as "PV") 3, PV19, PV23, PV29, PV30, PV37, PV50, and PV88. Other examples of red or magenta pigments include CI Pigment Orange (hereinafter, "CI Pigment Orange" may also be referred to simply as "PO") 13, PO16, PO20, and PO36.

[0112] Examples of blue or cyan pigments include CI Pigment Blue (hereinafter, "CI Pigment Blue" may also be simply referred to as "PB") 1, PB15, PB15:1, PB15:2, PB15:3, PB15:4, PB15:6, PB16, PB17-1, PB22, PB27, PB28, PB29, PB36, and PB60.

[0113] Examples of black pigments include CI Pigment Black (hereinafter, "CI Pigment Black" may also be simply referred to as "PBk") 7, PBk28, and PBk26.

[0114] The volume average particle size of the pigment is preferably 0.08 μm or more and 0.5 μm or less from the viewpoint of further improving the storage stability and ejection stability of the inkjet ink. The maximum particle size of the pigment is preferably 0.3 μm or more and 10 μm or less, and more preferably 0.3 μm or more and 3 μm or less.

[0115] The average particle size of pigment particles refers to the value determined by dynamic light scattering using a Datasizer Nano ZSP (Malvern). Note that inks containing colorants are highly concentrated, and light does not pass through this measuring device, so the inkjet ink is diluted 200 times before measurement. The measurement temperature is room temperature (25°C).

[0116] The pigment content is preferably 0.1% by mass or more and 20.0% by mass or less, and more preferably 0.4% by mass or more and 10.0% by mass or less, based on the total mass of the inkjet ink. By setting the pigment content to 0.1% by mass or more, the formed image can have sufficient color development. By setting the pigment content to 20.0% by mass or more, the ejection stability of the inkjet ink can be further improved.

[0117] Examples of dispersants include hydroxyl group-containing carboxylic acid esters, salts of long-chain polyaminoamides and high-molecular-weight acid esters, salts of high-molecular-weight polycarboxylic acids, salts of long-chain polyaminoamides and polar acid esters, and high-molecular-weight unsaturated acid esters. Other examples of dispersants include polymer copolymers, modified polyurethanes, modified polyacrylates, polyether ester-type anionic surfactants, and naphthalene sulfonic acid formalin condensate salts. Other examples of dispersants include aromatic sulfonic acid formalin condensate salts, polyoxyethylene alkyl phosphate esters, polyoxyethylene nonylphenyl ether, and stearylamine acetate.

[0118] Commercially available examples of dispersants include BYK-2164, BYK-168, BYK N-22024, BYK JET-9150, and BYK JET-9151 manufactured by BYK-Chemie ("BYK" is a registered trademark). Other commercially available examples of dispersants include EFKA 7701, EFKA 4310, EFKA 4320, and EFKA 4401 manufactured by BASF. Commercially available examples of dispersants include SOLSPERSE 24000GR and SOLSPERSE 39000 manufactured by Lubrizol ("SOLSPERSE" is a registered trademark). Commercially available examples of dispersants include Ajisper PB821 and Ajisper PB824 manufactured by Ajinomoto Fine-Techno Co., Ltd. ("Ajisper" is a registered trademark).

[0119] The content of the dispersant can be, for example, 20% by mass or more and 70% by mass or less relative to the total mass of the pigment. By making the content of the dispersant 20% by mass or more relative to the total mass of the pigment, the dispersant covers the pigment surface, making it possible to further reduce the occurrence of aggregation of pigment particles during storage of the ink. By making the content of the dispersant 70% by mass or less relative to the total mass of the pigment, association between the dispersant and the gelling agent is further reduced, allowing the inkjet ink that has landed on a recording medium to be more fully gelled and pinned.

[0120] Examples of the polymerization initiator coagent include tertiary amine compounds, including aromatic tertiary amine compounds.

[0121] Examples of aromatic tertiary amine compounds include N,N-dimethylaniline, N,N-diethylaniline, N,N-dimethyl-p-toluidine, and N,N-dimethylamino-p-benzoic acid ethyl ester. Other examples of aromatic tertiary amine compounds include N,N-dimethylamino-p-benzoic acid isoamyl ethyl ester, N,N-dihydroxyethylaniline, triethylamine, and N,N-dimethylhexylamine.

[0122] Examples of polymerization inhibitors include (alkyl)phenols, hydroquinone, catechol, resorcinol, p-methoxyphenol, t-butylcatechol, t-butylhydroquinone, pyrogallol, and 1,1-picrylhydrazyl. Other examples include phenothiazine, p-benzoquinone, nitrosobenzene, 2,5-di-t-butyl-p-benzoquinone, dithiobenzoyl disulfide, picric acid, cupferron, and aluminum N-nitrosophenylhydroxylamine. Other examples of polymerization inhibitors include tri-p-nitrophenylmethyl, N-(3-oxyanilino-1,3-dimethylbutylidene)aniline oxide, dibutyl cresol, cyclohexanone oxime cresol, and guaiacol. Other examples of polymerization inhibitors include o-isopropylphenol, butyraldoxime, methyl ethyl ketoxime, and cyclohexanone oxime.

[0123] Examples of sensitizers include polycyclic aromatic compounds, carbazole derivatives, thioxanthone derivatives, and anthracene derivatives, all of which have at least one hydroxyl group, optionally substituted aralkyloxy group, or optionally substituted alkoxy group as a substituent. From the viewpoint of increasing the curing rate, it is preferable that the sensitizer has ultraviolet spectrum absorption at wavelengths longer than 300 nm.

[0124] Examples of surfactants include anionic surfactants such as dialkyl sulfosuccinates, alkyl naphthalene sulfonates, and fatty acid salts. Other examples of surfactants include nonionic surfactants such as polyoxyethylene alkyl ethers, polyoxyethylene alkyl allyl ethers, acetylene glycols, and polyoxyethylene-polyoxypropylene block copolymers. Other examples of surfactants include cationic surfactants such as alkylamine salts and quaternary ammonium salts, as well as silicone-based and fluorine-based surfactants.

[0125] Commercially available examples of silicone surfactants include KF-351A, KF-352A, KF-642, and X-22-4272 manufactured by Shin-Etsu Chemical Co., Ltd. Other commercially available examples include BYK307, BYK345, BYK347, and BYK348 manufactured by BYK Chemie ("BYK" is a registered trademark), and TSF4452 manufactured by Toshiba Silicones Co., Ltd.

[0126] The content of the surfactant is preferably 0.001% by mass or more and less than 1.0% by mass based on the total mass of the primer.

[0127] 1-5.Physical properties The viscosity of the inkjet ink at 80°C is preferably 3 mPa·s or more and 20 mPa·s or less from the viewpoint of improving ejection properties from an inkjet head, and the viscosity of the inkjet ink at 25°C is preferably 1000 mPa·s or more from the viewpoint of sufficiently gelling the inkjet ink when it lands and is cooled to room temperature.

[0128] The gelation temperature of the inkjet ink is preferably 40° C. or higher and 70° C. or lower. When the gelation temperature of the inkjet ink is 40° C. or higher, the inkjet ink quickly gels after landing on a recording medium, resulting in higher pinning properties. When the gelation temperature of the inkjet ink is 70° C. or lower, the inkjet ink is less likely to gel when ejected from an inkjet head, where the ink temperature is usually around 80° C., allowing for more stable ejection of the inkjet ink.

[0129] The viscosity of an inkjet ink at 80°C, the viscosity at 25°C, and the gelation temperature can be determined by measuring the temperature change of the dynamic viscoelasticity of the inkjet ink using a rheometer. In the present invention, these viscosity and gelation temperature values ​​are obtained by the following method. The inkjet ink is heated to 100°C, and the viscosity is measured using a stress-controlled rheometer, Physica MCR301 (cone-plate diameter: 75 mm, cone angle: 1.0°), manufactured by Anton Paar. While the above measurement is being performed, the ink is cooled to 20°C under conditions of a shear rate of 11.7 (1 / s) and a cooling rate of 0.1°C / s, to obtain a viscosity temperature curve. The viscosity at 80°C and the viscosity at 25°C are determined by reading the viscosities at 80°C and 25°C, respectively, on the viscosity temperature curve. The gelation temperature is determined as the temperature at which the viscosity reaches 200 mPa·s on the viscosity temperature curve.

[0130] 2. Inkjet Ink Preparation The inkjet ink can be obtained, for example, by mixing a polymerization initiator, a polymerizable compound, a gelling agent, and any other optional components under heating. The resulting mixture is preferably filtered through a predetermined filter. When the inkjet ink contains a pigment and a dispersant, a pigment dispersion in which the pigment and dispersant are dispersed in a solvent may be prepared in advance, and the remaining components may be added to this and mixed under heating.

[0131] The pigment and dispersant can be dispersed using, for example, a ball mill, a sand mill, an attritor, a roll mill, an agitator, a Henschel mixer, a colloid mill, an ultrasonic homogenizer, a pearl mill, a wet jet mill, or a paint shaker.

[0132] 3.Image forming method The image forming method of the present invention can be carried out in the same manner as a known image forming method in which ink-jet ink is ejected from an ink-jet head, landed on a recording medium, and cured, except that the ink-jet ink described above is used.

[0133] For example, the image forming method of the present invention includes a step of ejecting inkjet ink from the nozzles of an inkjet head to land on a recording medium, and a step of irradiating the landed inkjet ink with actinic rays to cure the inkjet ink.

[0134] 3-1. The process of impacting the ink droplets onto the recording medium In the step of impacting the ink on the recording medium, droplets of inkjet ink are ejected from an inkjet head and made to impact the recording medium at positions corresponding to the image to be formed.

[0135] Examples of inkjet head ejection methods include on-demand and continuous methods. Examples of inkjet heads compatible with on-demand ejection methods include electro-mechanical conversion inkjet heads and electro-thermal conversion inkjet heads. Electro-mechanical conversion inkjet heads include single-cavity, double-cavity, bender, piston, shear-mode, and shared-wall types. Electro-thermal conversion inkjet heads include thermal inkjet and bubble-jet (Bubble Jet is a registered trademark of Canon Inc.) types.

[0136] From the viewpoint of further improving droplet ejection stability, the inkjet ink is preferably ejected from the inkjet head in a heated state. From the viewpoint above, the temperature of the ink when ejected is preferably 35°C or higher and 100°C or lower, and more preferably 35°C or higher and 80°C or lower. Furthermore, from the viewpoint of improving ejection stability, the viscosity of the inkjet ink ejected from the inkjet head is preferably 7 mPa·s or higher and 15 mPa·s or lower, and more preferably 8 mPa·s or higher and 13 mPa·s or lower.

[0137] The inkjet ink is preferably heated to a temperature at least 10°C higher than the gelation temperature of the inkjet ink and not more than 30°C higher than the gelation temperature. The temperature of the inkjet ink when filled into the inkjet head improves the ejection properties from the inkjet head. When the temperature of the inkjet ink inside the inkjet head is at least 10°C higher than the gelation temperature, gelation of the inkjet ink is less likely to occur inside the inkjet head or on the nozzle surface, thereby further improving ejection stability. When the temperature of the inkjet ink inside the inkjet head is not more than 30°C higher than the gelation temperature, the inkjet ink components are less likely to deteriorate during image formation.

[0138] The method for heating the inkjet ink is not particularly limited. For example, at least one of an ink supply system including an ink tank, a supply pipe, and a front chamber ink tank immediately before the head, piping with a filter, and a piezo head can be heated to a predetermined temperature by a panel heater, a ribbon heater, or warm water.

[0139] From the viewpoint of facilitating faster image formation and further improving image quality, the volume of the ejected inkjet ink droplets is preferably 2 pL or more and 20 pL or less.

[0140] 3-2. Hardening process In the curing step, the inkjet ink that has landed on a recording medium in the landing step is irradiated with actinic radiation to form an image by curing the inkjet ink. The actinic radiation is preferably irradiated for 0.001 to 1.0 seconds after the ink has landed, and more preferably for 0.001 to 0.5 seconds to form a high-definition image.

[0141] Examples of actinic radiation include electron beams, ultraviolet rays, α rays, γ rays, and X-rays. From the viewpoint of safety and the ability to cure inkjet inks with a lower amount of energy, the actinic radiation is preferably ultraviolet rays or electron beams, and more preferably ultraviolet rays. Using an LED as the light source makes it less likely that poor curing of the inkjet ink will occur due to the inkjet ink melting due to the radiant heat of the light source, so the light source is preferably a light-emitting diode (LED). Examples of LED light sources that emit ultraviolet rays include a 395 nm water-cooled LED manufactured by Phoseon Technology.

[0142] From the viewpoint of sufficiently curing the inkjet ink, the LED light source should have a peak irradiance of 0.5 W / cm on the image surface of ultraviolet light having a wavelength of 370 nm or more and 410 nm or less. 2 More than 10W / cm 2 Less than 1 W / cm, preferably 2 More than 5W / cm 2 The amount of light irradiated onto the image is set to 350 mJ / cm2 in order to prevent radiant heat from being irradiated onto the ink. 2 Less than is preferred.

[0143] The actinic radiation irradiation may be divided into two stages, in which the inkjet ink is first pre-cured by irradiating it with actinic radiation in the manner described above for 0.001 seconds to 2.0 seconds after landing, and then the inkjet ink is fully cured by irradiating it with actinic radiation again after all printing is completed. By dividing the actinic radiation irradiation into two stages, shrinkage of the recording material that occurs when the ink is cured is less likely to occur.

[0144] From the viewpoint of more suitably achieving the effects of the present invention, it is preferable to control the surface temperature of the recording medium on the side where the inkjet ink lands to 20°C or higher and 40°C or lower before or simultaneously with the curing step.

[0145] By setting the total ink film thickness after the inkjet ink that has landed on a recording medium and cured by irradiating it with actinic rays to between 2 μm and 20 μm, it is possible to more efficiently prevent curling and wrinkling of the recording medium, as well as changes in the texture of the recording medium. The term "total ink film thickness" refers to the total film thickness of all inkjet inks applied or printed on the recording medium. The total ink film thickness can be, for example, the average value of the film thicknesses measured at multiple points where a large amount of inkjet ink is expected to land.

[0146] 3-3. Recording Media The recording medium used in the image forming method of the present invention may be any recording medium on which an image is formed using an inkjet ink. Examples of recording media include non-absorbent recording media (plastic substrates) made of plastic, non-absorbent inorganic recording media, and absorbent papers (e.g., coated printing paper and coated printing paper). Examples of plastics include polyester, polyvinyl chloride, polyethylene, polyurethane, polypropylene, acrylic resin, and polycarbonate. Other examples of plastics include polystyrene, acrylonitrile-butadiene-styrene copolymer, polyethylene terephthalate, and polybutadiene terephthalate. Examples of inorganic recording media include metal-laminated or metal-deposited paper or plastic film, metals, and glass. When using the ink composition of the present invention for package printing, it is preferable to use a recording medium with a relatively thin film thickness. In this case, the film thickness is preferably 150 μm or less, more preferably 120 μm or less, even more preferably 90 μm or less, and particularly preferably 50 μm or less. [Example]

[0147] The present invention will be described in more detail below with reference to examples, which should not be construed as limiting the scope of the present invention. 1. Inkjet Ink Preparation An inkjet ink was prepared using the following ingredients:

[0148] [Multifunctional Monomer] Multifunctional monomer 1: Tripropylene glycol diacrylate Multifunctional monomer 2: Polyethylene glycol #600 diacrylate Multifunctional Monomer 3:3PO Modified Trimethylolpropane Triacrylate Multifunctional Monomer 4:3EO Modified Trimethylolpropane Triacrylate

[0149] [Monofunctional monomer] Monofunctional Monomer 1:4EO Modified Phenol Acrylate Monofunctional monomer 2: Nonylphenol 4EO modified acrylate Monofunctional Monomer 3: Lauryl Acrylate

[0150] [Polymerization initiator] (Initiators with molecular weights less than 360) Polymerization initiator 1: diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, molecular weight 348 Polymerization initiator 2: ethylphenyl(2,4,6-trimethylbenzoyl)phosphinate (TPO-L), molecular weight 316 (Initiators with molecular weights of 360 or more) Polymerization initiator 3: bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide (IRGACURE 819), molecular weight 418 Polymerization initiator 4: Chemical formula (1) below, molecular weight 488 Polymerization initiator 5: Chemical formula (2) below, molecular weight 583 Polymerization initiator 6: Chemical formula (3) below, molecular weight 483

[0151] [ka]

[0152] [ka]

[0153] [ka]

[0154] [Gelling agent] Distearyl ketone was used as the gelling agent.

[0155] 1-2. Preparation of pigment dispersion Nine parts by weight of a pigment dispersant (EFKA-7701, BASF) and 71 parts by weight of tripropylene glycol diacrylate were placed in a stainless steel beaker and heated to 65°C on a hot plate while stirring for 1 hour. The mixture was then cooled to room temperature. After stirring, 20 parts by weight of a cyan pigment (Pigment Blue 15:4, Chromofine Blue 6332JC, Dainichiseika Color & Chemicals Mfg. Co., Ltd.) was added to the stainless steel beaker, and the mixture was then placed in a glass bottle together with 200 g of zirconia beads (0.3 mm diameter, Nikkato Corporation) and sealed. This pigment-containing liquid was dispersed using a paint shaker, and the zirconia beads were removed to obtain a pigment dispersion. The dispersion time was 4 hours when using cyan and black pigments, and 6 hours when using magenta and yellow pigments. Pigment Black 7 (#52, Mitsubishi Chemical Corporation) was used as the black pigment. Pigment Violet 19 (Fastogen Super Red BRZ, manufactured by DIC Corporation) was used as the magenta pigment, and Pigment Yellow 185 (D1155, manufactured by BASF Corporation) was used as the yellow pigment.

[0156] 1-3. Preparation of inkjet ink The following components were added to the pigment dispersions of each color in the following proportions and stirred at 80°C to obtain ink solutions. The ink solutions were then filtered through a 3 μm Teflon (registered trademark) membrane filter manufactured by Advantec Corporation to obtain inkjet ink 1. Pigment dispersion: 2.0% by mass Multifunctional monomer 1: 20.0 mass% Multifunctional monomer 2: 10.0 mass% Multifunctional monomer 3: 20.0 mass% Multifunctional monomer 4: 33.0% by mass Monofunctional monomer 1: 12.0% by mass Polymerization initiator 4: 1.0% by mass Gelling agent: 4.0% by mass

[0157] Inkjet inks 2 to 20 were obtained in the same manner as inkjet ink 1, except that the above components and ratios were changed as shown in Table 1.

[0158] [Table 1]

[0159] 2. Imaging and Evaluation 2-1. Bending crack resistance Using a paper folding machine (AFV-564FKT, Horizon Co., Ltd.), an image formed on a recording medium using inkjet inks (cyan ink) 1 to 20 was folded in half. The state of the image was then observed, and the percentage of the area of ​​the recording medium that was not exposed due to cracks (unexposed area) was determined by image analysis. The fold crack resistance was evaluated according to the following criteria. 5: No cracks were observed in the image 4: Cracks were visible in the image, and only a small amount of white was exposed. 3: Cracks were visible in the image, and less than half of the area was exposed white. 2: Cracks were visible in the image, and more than half of the area was exposed white. 1: Cracks were visible in the image and white was exposed in the entire area.

[0160] 2-2. Cutting and peeling resistance (image abrasion resistance) Using a desktop paper cutter (PC-P430, Horizon Co., Ltd.), the image-formed portion of the recording medium was cut using inkjet inks (cyan ink) 1 to 20. The cut portion was rubbed with a finger, and the degree of peeling of the coating film (image) was evaluated for cut peel resistance according to the following criteria. 5: No peeling of the coating occurred 4: Peeling was observed but not visible to the naked eye. 3: There was peeling, but it was difficult to see with the naked eye. 2: Peeling was visible to the naked eye 1: Large peeling was clearly visible

[0161] 2-3. Gloss uniformity The recording device used was a Konica Minolta KM1800i inkjet recording device (number of nozzles: 1776). The ink tank was filled with inkjet ink (cyan ink), and the recording was performed on printing art paper (Tokubishi Art, basis weight 104.7 g / m²) in an environment of 23°C and 55% RH. 2 The ink was applied at a rate of 9 g / m2 to a 2 cm x 2 cm section set on a paper (Mitsubishi Paper Mills, Ltd.). 2 The inkjet ink was then ejected so that the ink was irradiated with ultraviolet light having a wavelength of 395 nm from an LED (Kyocera Corporation) to form a solid image. The ultraviolet light irradiation conditions were: irradiation width: 68 mm, irradiation distance: 70 mm, illuminance on the recording medium: 2.50 W / cm 2 The temperature of the substrate surface during ejection was adjusted to 30° C., and the printing speed was adjusted to 60 m / min. Images were formed in the same manner for inkjet inks 2 to 20.

[0162] The density gradation patch images formed in an area of ​​2 cm x 2 cm were varied in dot ratio to 20%, 50%, 70%, and 100%, and the gloss and gloss difference between the formed image area and the unprinted area (white area) of the recording medium were visually observed. The cut-peel resistance was evaluated according to the following criteria. 5: In images with all dot rates, no difference in gloss was observed between the density gradation patch image and the unprinted area of ​​the recording medium. 4: For all dot ratio images, there was almost no difference in gloss between the density gradation patch image and the unprinted area of ​​the recording medium. 3: In some dot rate images, there was a slight difference in gloss between the density gradation patch image and the unprinted part of the recording medium, but the quality was within the range of practical acceptability. 2: In images with a dot rate of more than half, a difference in gloss was observed between the density gradation patch image and the unprinted part of the recording medium, creating an unnatural image. 1: In all images with all dot rates, there was a difference in gloss between the density gradation patch image and the unprinted part of the recording medium, creating an unnatural image.

[0163] 2-4.Surface hardening In the same manner as in 2-1 above, except that inkjet inks (cyan inks) 1 to 20 were filled into the ink tanks of the inkjet recording device, printing art paper (Tokubishi Art, basis weight 104.7 g / m) was printed. 2 , Mitsubishi Paper Mills) with an ink application rate of 10 g / m 2 A solid image of 1000 ppm was formed.

[0164] The solid image was rubbed with a finger to evaluate the stickiness of the surface. 2 Cut the printable art paper (Tokubishi Art, basis weight 104.7 g / m) to the desired size. 2 A sheet of paper (Mitsubishi Paper Mills, Ltd.) was placed on the solid image and rubbed with a load of 800 g. 2 The degree of color transfer to art paper for printing cut into pieces of the same size was visually observed. Surface curability was evaluated according to the following criteria. 5: The surface was not sticky and no color transfer was observed. 4: The surface is not sticky, but slight color transfer is observed. 3: The surface was slightly sticky and some color transfer was faintly visible. 2: The surface was sticky and color transfer was visible 1: The surface was sticky and it was clearly visible that the color had transferred to the entire rubbed paper.

[0165] 2-5.Scratch resistance Using the same procedure as in 2-1 above, print art paper (Tokubishi Art, basis weight 104.7 g / m 2 , Mitsubishi Paper Mills) with inkjet ink (cyan ink) 1-20 with a coating amount of 9g / m 2The formed image was rubbed three times with a fingernail from the top to the bottom, and the scratch resistance was evaluated based on the presence or absence of scratches on the image surface and the size of white spots according to the following criteria. 5: No peeling was observed 4: Small peeling occurred in some areas 3: Peeled off, but most of the rubbed area remained. 2: Large areas of the film peeled off 1: Almost all of the rubbed area peeled off

[0166] The inkjet inks of the other colors obtained were also evaluated for resistance to cracking when folded, resistance to peeling when cut, gloss uniformity, surface hardening, and scratch resistance, but there were no differences in the evaluation results depending on the color.

[0167] 2-6.Evaluation Results The evaluation results are shown in Table 2.

[0168] [Table 2]

[0169] The images formed using inkjet inks Nos. 1 to 16 were excellent in all respects: resistance to cracking when folded, resistance to peeling when cut, uniform gloss, surface hardening, and scratch resistance.

[0170] The images formed using inkjet inks Nos. 2, 7, 8, 9, and 12, which contained 3 to 8% by mass of polymerization initiators with molecular weights of 360 or greater, had better surface curability and scratch resistance than the image formed using inkjet ink No. 1. This is thought to be because the content of polymerization initiators with molecular weights of 360 or greater was within the appropriate range, allowing the cured films to cure sufficiently. Furthermore, the images formed with inkjet inks Nos. 7 and 8, which contain a polymerization initiator with a molecular weight of less than 360, had equivalent surface hardening properties, but slightly inferior scratch resistance, to the images formed with inkjet inks Nos. 3 to 6. This is thought to be because the inclusion of a polymerization initiator with a molecular weight of less than 360 prevented the deterioration of gloss uniformity and surface hardening properties from decreasing.

[0171] Images formed using inkjet inks 6, 11, and 14, which had an increased monofunctional monomer content and a higher ratio of monofunctional monomer to polyfunctional monomer, also exhibited good resistance to cracking when folded, resistance to peeling when cut, uniform gloss, surface hardening, and scratch resistance.

[0172] Furthermore, the images formed using inkjet inks Nos. 7 and 8, which further contained a polymerization initiator with a molecular weight of less than 360, were inferior in gloss uniformity and surface curability compared to the images formed using inkjet inks Nos. 1 to 4. This is thought to be because the high content of polymerization initiator with a molecular weight of less than 360 reduced the flexibility of the cured film.

[0173] On the other hand, inkjet ink No. 17, which had a monofunctional monomer to polyfunctional monomer ratio of more than 0.29, exhibited poor gloss uniformity and scratch resistance. This is thought to be because the crosslink density in the cured ink was low, causing decomposed initiator, gelling agent, and uncured photopolymerizable compound to precipitate on the image surface, resulting in low gloss.

[0174] Furthermore, the image formed using inkjet ink No. 18, which does not contain a monofunctional monomer, exhibited poor resistance to cracking after folding and peeling after cutting. This is thought to be because the cured film did not soften due to the absence of a monofunctional monomer.

[0175] Images formed with inkjet ink No. 19, which does not contain a polymerization initiator with a molecular weight of 360 or more, exhibited poor gloss uniformity and surface curing properties. This is thought to be due to the deposition of small molecular weight polymerization initiators on the surface of the image.

[0176] The image formed with inkjet ink No. 20, which does not contain a gelling agent, was evaluated as low in gloss uniformity, surface hardening, and scratch resistance. This is thought to be because the ink does not contain a gelling agent, so the image surface does not have a glossy appearance, and hardening is inhibited by oxygen, resulting in low surface hardening. [Industrial Applicability]

[0177] The inkjet ink according to the present invention can form an image with low gloss unevenness within the image and high image robustness. Therefore, the present invention can be used to form images on food packaging, for example, and is expected to contribute to the further spread of inkjet recording methods in this field.

Claims

1. An actinic ray-curable inkjet ink comprising a polymerization initiator, a polymerizable compound containing a polyfunctional monomer and a monofunctional monomer, and a gelling agent, The polymerization initiator includes a polymerization initiator having a molecular weight of 360 or more, the ratio of the content of the monofunctional monomer to the content of the polyfunctional monomer is greater than 0 and not more than 0.29; Inkjet ink.

2. The ink-jet ink according to claim 1 , wherein the polymerization initiator having a molecular weight of 360 or greater comprises a radical polymerization initiator having an acylphosphine structure.

3. The ink-jet ink according to claim 1 , wherein the content of the polymerization initiator having a molecular weight of 360 or more is 3.0% by mass or more and less than 8.0% by mass.

4. The ink-jet ink of claim 1 , wherein the polymerizable compound comprises phenoxy polyethylene glycol acrylate.

5. The ink-jet ink according to claim 1 , wherein the content of the gelling agent is 1.0% by mass or more and less than 6.0% by mass.

6. a step of ejecting the inkjet ink according to any one of claims 1 to 5 from a nozzle of an inkjet head and causing it to land on a recording medium; a step of irradiating the deposited inkjet ink with actinic rays to cure the inkjet ink; Including, Image forming method.

7. 7. The image forming method according to claim 6, further comprising a step of controlling the surface temperature of the recording medium on the side where the inkjet ink lands to 20° C. or higher and 40° C. or lower before or simultaneously with the step of landing the ink on the recording medium.

Images