Ink, inkjet recording method, and inkjet recording device

Abstract

Provided is an ultraviolet radiation-curable inkjet-use ink which has a peak in the wavelength range of not less than 250 nm and less than 300 nm in a luminescence spectrum, with the intensity of said peak being 50% or more based on the total intensity in the wavelength range of 200-800 nm, is cured by being irradiated with ultraviolet radiation. The ink is characterized by containing at least one polymerizable compound selected from the group consisting of (A) to (D), and in that the content of a photopolymerization initiator in the ink is 0.40 mass% or less based on the total mass of the ink.

Description

Ink, inkjet recording method, and inkjet recording apparatus

[0001] The present invention relates to an ink, an inkjet recording method, and an inkjet recording apparatus.

[0002] Traditionally, analog printing, in which printing plates are created and then used to print large quantities of printed materials, has been widely accepted in society. However, with analog printing, it is difficult to flexibly change colors, designs, and other aspects to meet customer requests. Furthermore, when printing small runs, the cost of printing plates becomes a significant factor relative to the printed material, resulting in higher overall printing costs. In order to provide printed materials that precisely meet the diverse needs of such customers, digital printing has been on the rise in recent years.

[0003] Digital printing allows for the printing of a wide variety of products in small quantities, and various studies are being conducted to accommodate printing on a wide variety of recording media. For example, when recording an image on a non-absorbent recording medium such as a resin or tile by ejecting ink from an inkjet recording head, one method uses ultraviolet-curable ink. Ultraviolet-curable ink can be cured by ultraviolet light. Therefore, even on a non-absorbent recording medium, the ink can be fixed on the recording medium by irradiating the ink applied to the recording medium with actinic energy rays.

[0004] UV-curable inks generally contain a photopolymerization initiator that can initiate a polymerization reaction by UV light. The photopolymerization initiator generates radicals when irradiated with UV light. These radicals then promote the polymerization or crosslinking reaction of polymerizable monomers or polymerizable oligomers in the ink, thereby curing the ink.

[0005] However, images recorded on recording media using ultraviolet-curable inks may contain unreacted photopolymerization initiators and decomposition products thereof, making the recording media difficult to recycle.

[0006] Therefore, studies have been conducted on recording images using inks that do not contain a photoinitiator. In Patent Document 1, a method has been proposed in which an inkjet ink that does not contain a photoinitiator adhered onto a substrate is polymerized by performing UVC radiation from a first ultraviolet light source.

[0007] Japanese Patent Application Laid-Open No. 2005-509719

[0008] According to the studies by the present inventors, in the method described in Patent Document 1, although the stickiness of the image recorded by the cured ink is suppressed, cracks may occur in the recorded image when the recording medium is folded, and it has been found that the fold resistance of the image is not sufficient.

[0009] Therefore, an object of the present invention is to provide an ink that suppresses the stickiness of a recorded image and has excellent fold resistance even when the content of the photoinitiator is low, an inkjet recording method using the ink, and an inkjet recording apparatus.

[0010] The above object is achieved by the following present invention.

[0011] That is, according to the present invention, there is provided an ultraviolet-curable inkjet ink that is cured by irradiation with ultraviolet light having a peak in a wavelength region of 250 nm or more and less than 300 nm in the emission spectrum, and the intensity of the peak is 50% or more based on the total intensity in a wavelength region of 200 nm or more and 800 nm or less. The ink contains a (meth)acrylate compound having a carbon atom to which all four single bonds are bonded to atoms other than hydrogen atoms, and the content of the photoinitiator in the ink is 0.40% by mass or less based on the total mass of the ink.

[0012] According to the present invention, even when the content of the photoinitiator is low, it is possible to provide an ink that suppresses the stickiness of a recorded image and has excellent fold resistance, an inkjet recording method using the ink, and an inkjet recording apparatus.

[0013] The present invention will be described in further detail below with reference to preferred embodiments. In the present invention, when the compound is a salt, the salt exists in the ink as dissociated ions, but for convenience it will be expressed as "containing a salt." Furthermore, UV-curable inkjet inks may be simply referred to as "ink." Furthermore, inkjet recording methods and inkjet recording apparatuses may be simply referred to as "recording methods" and "recording apparatuses," respectively. Unless otherwise specified, physical property values ​​are values ​​at room temperature (25°C) and normal pressure (1 atmosphere). Furthermore, when "(meth)acrylate," "(meth)acrylic acid," and "(meth)acrylamide" are written, they mean "acrylate or methacrylate," "acrylic acid or methacrylic acid," and "acrylamide or methacrylamide," respectively.

[0014] The present inventors conducted research into inks that suppress stickiness of recorded images and have excellent fold-crack resistance, even when the photopolymerization initiator content is low, such as 0.40% by mass or less based on the total mass of the ink. As a result, they discovered that it is important to irradiate ink containing at least one polymerizable compound selected from the group consisting of the following (A) to (D) with specific ultraviolet light, leading to the present invention. Here, the specific ultraviolet light to be irradiated refers to ultraviolet light that has an emission spectrum peak in the wavelength region of 250 nm or more and less than 300 nm, and whose peak intensity is 50% or more based on the total intensity in the wavelength region of 200 nm or more and 800 nm or less. (A) a (meth)acrylate compound having a carbon atom in which all four single bonds are bonded to atoms other than hydrogen atoms (hereinafter also referred to as polymerizable compound (A)); (B) a (meth)acrylate compound having a dioxane skeleton or a dioxolane skeleton (hereinafter also referred to as polymerizable compound (B)); (C) a (meth)acrylate compound having a cyclic carbonate skeleton or a lactone skeleton (hereinafter also referred to as polymerizable compound (C)); (D) a (meth)acrylamide compound having a carbon atom in which all four single bonds are bonded to atoms other than hydrogen atoms (hereinafter also referred to as polymerizable compound (D));

[0015] Although it is not clear why this ink is able to improve resistance to creases while suppressing stickiness of the printed image, the inventors speculate as follows.

[0016] The inventors speculate that this is because the polymerizable compounds (A) to (D) can efficiently generate polymerizable radicals when irradiated with the above-mentioned specific ultraviolet light. Specifically, such ultraviolet light irradiation causes dissociation of single bonds of carbon atoms in which all four single bonds are bonded to atoms other than hydrogen atoms in the polymerizable compounds (A) and (D). Furthermore, ring-opening of the dioxane skeleton or dioxolane skeleton occurs in the polymerizable compound (B), and ring-opening of the cyclic carbonate skeleton or lactone skeleton occurs in the polymerizable compound (C). The inventors speculate that the radicals generated by these dissociations and ring-openings can react with polymerizable groups, such as vinyl groups, of other polymerizable compounds to grow polymer chains.

[0017] Furthermore, the ink of the present invention can harden images even when the photopolymerization initiator content is as low as 0.40% by mass or less, or even when the ink does not contain a photopolymerization initiator at all, and can improve the fold crack resistance while suppressing stickiness of the recorded image. Therefore, the amount of unreacted photopolymerization initiator and its decomposition products contained in the image recorded using the ink of the present invention can be reduced, making it easier to recycle the recording medium from the recorded image. Therefore, the technology described herein can contribute to the realization of a sustainable society, such as a decarbonized / recycling-based society.

[0018] The ultraviolet-curable inkjet ink of the present invention will be described in detail below.

[0019] <Ink> The UV-curable inkjet ink of the present invention is a UV-curable inkjet ink that cures upon exposure to UV rays of a specific wavelength. Here, the specific UV rays are UV rays whose emission spectrum has a peak in the wavelength region of 250 nm or more and less than 300 nm, and whose peak intensity is 50% or more of the total intensity in the wavelength region of 200 nm or more and 800 nm or less. The ink is characterized by containing at least one polymerizable compound selected from the group consisting of (A) to (D) above, and the content of a photopolymerization initiator in the ink is 0.40% by mass or less, based on the total mass of the ink. Each component contained in the ink is described below.

[0020] [(A) (Meth)acrylate Compound Having a Carbon Atom Wherein All Four Single Bonds Are Bonded to Atoms Other Than Hydrogen Atoms] The (meth)acrylate compound that is the polymerizable compound (A) has a carbon atom Wherein all four single bonds are bonded to atoms other than hydrogen atoms.

[0021] There are no particular limitations on the (meth)acrylate compound as long as it has a carbon atom in which all four single bonds are bonded to atoms other than hydrogen atoms, and a commercially available product may be used, or a product obtained by a known synthesis method may be used. Examples of commercially available (meth)acrylate compounds in which all four single bonds are bonded to atoms other than hydrogen atoms include the following: adamantyl-1-yl (meth)acrylate, tertiary butyl (meth)acrylate, 2-methyl adamantyl-2-yl (meth)acrylate (all manufactured by Tokyo Chemical Industry Co., Ltd.), isobornyl (meth)acrylate (trade name: IBXA; manufactured by Osaka Organic Chemical Industry Co., Ltd.), di(meth)acrylate of polypropylene oxide adduct of bisphenol A (trade name: Light Acrylate BP-4A; manufactured by Kyoeisha Chemical Co., Ltd.), polyethylene oxide adduct of bisphenol A (trade name: Light Acrylate BP-4A; manufactured by Kyoeisha Chemical Co., Ltd.), and the like. The additives included di(meth)acrylate (trade name: Light Acrylate BP-4EAL; manufactured by Kyoeisha Chemical Co., Ltd.), cyclic trimethylolpropane formal (meth)acrylate (trade name: Viscoat #200; manufactured by Osaka Organic Chemical Industry Co., Ltd.), neopentyl glycol-(meth)acrylic acid-benzoic acid ester (trade name: Light Acrylate BA104; manufactured by Kyoeisha Chemical Co., Ltd.), (2-methyl-2-ethyl-1,3-dioxolan-4-yl)methyl (meth)acrylate (trade name: Product name: MEDOL-10; manufactured by Osaka Organic Chemical Industry Co., Ltd.), (2-isobutyl-2-methyl-1,3-dioxolan-4-yl)methyl (meth)acrylate (manufactured by Aldrich Chemical Industry Co., Ltd.), 3,3,5-trimethylcyclohexyl (meth)acrylate (trade name: Viscoat #196; manufactured by Osaka Organic Chemical Industry Co., Ltd.), trimethylolpropane tri(meth)acrylate (trade name: Light Acrylate TMP-A; manufactured by Kyoeisha Chemical Co., Ltd., and product name: Viscoat #295; manufactured by Osaka Yusen Chemical Co., Ltd.), Kikai Chemical Co., Ltd.), polyethylene oxide adduct of trimethylolpropane tri(meth)acrylate (trade names: A-TMPT-9EO and AT-20E; Shin-Nakamura Chemical Co., Ltd.), polyethylene oxide adduct of pentaerythritol tetra(meth)acrylate (trade names: A-TMM-3L and A-TMM-3LM-N; Shin-Nakamura Chemical Co., Ltd.), dipentaerythritol hexa(meth)acrylate (trade name: Light Acrylate DPE-6A;Kyoeisha Chemical Co., Ltd.), pentaerythritol tri(meth)acrylate (trade name: Light Acrylate PE-3A; Kyoeisha Chemical Co., Ltd.), pentaerythritol tetra(meth)acrylate (trade name: Light Acrylate PE-4A; Kyoeisha Chemical Co., Ltd.), 3-O-acryloyl-1,2:5,6-bis-O-isopropylidene-D-glucofuranose (Aldrich Chemical Co., Ltd.), mevalonic acid lactone (meth)acrylate (Tokyo Chemical Industry Co., Ltd.), (manufactured by Tokyo Chemical Industry Co., Ltd.), tert-butyl (meth)acrylate (manufactured by Tokyo Chemical Industry Co., Ltd.), 1-methylcyclopentyl (meth)acrylate (manufactured by Tokyo Chemical Industry Co., Ltd.), 1-ethylcyclopentyl (meth)acrylate (manufactured by Tokyo Chemical Industry Co., Ltd.), (3-ethyloxetan-3-yl)methyl (meth)acrylate (manufactured by Tokyo Chemical Industry Co., Ltd.), 2-(tert-butylamino)ethyl (meth)acrylate (manufactured by Tokyo Chemical Industry Co., Ltd.), and the like;

[0022] Both methacrylate compounds and acrylate compounds can be used, but acrylate compounds are preferred because they are easier to cure.

[0023] Furthermore, the (meth)acrylate compound has a carbon atom in which all four single bonds are bonded to atoms other than hydrogen atoms, and it is preferable that at least one of the atoms bonded to this carbon atom is a heteroatom.

[0024] [(B) (Meth)acrylate Compound Having a Dioxane Skeleton or a Dioxolane Skeleton] The (meth)acrylate compound as the polymerizable compound (B) has a dioxane skeleton or a dioxolane skeleton. In the present invention, the dioxane skeleton refers to a structure represented by the following formula (1), and the dioxolane skeleton refers to a structure represented by the following formula (2).

[0025]

[0026]

[0027] There are no particular limitations on the (meth)acrylate compound as long as it has a dioxane skeleton or a dioxolane skeleton, and commercially available products or compounds obtained by known synthesis methods may be used. Commercially available (meth)acrylate compounds having a dioxane skeleton or a dioxolane skeleton include the following: (2-methyl-2-ethyl-1,3-dioxolan-4-yl)methyl acrylate (trade name: MEDOL-10; manufactured by Osaka Organic Co., Ltd.), cyclic trimethylolpropane formal acrylate (trade name: Biscoat #200; manufactured by Osaka Organic Co., Ltd.), and (2-methyl-2-isobutyl-1,3-dioxolan-4-yl)methyl acrylate (manufactured by Aldrich Co., Ltd.).

[0028] Furthermore, a known method for synthesizing a (meth)acrylate compound having a dioxane skeleton or a dioxolane skeleton is described, for example, in JP 2009-286718 A. Specifically, JP 2009-286718 A describes a method for purifying (1,3-dioxolan-4-yl)alkyl alcohol to a high purity. Furthermore, JP 2009-286718 A describes a method for producing a high-purity dioxolane ring-containing (meth)acrylic acid ester monomer using the (1,3-dioxolan-4-yl)alkyl alcohol obtained by this method.

[0029] Both methacrylate compounds and acrylate compounds can be used, but acrylate compounds are preferred because they are easier to cure.

[0030] [(C) (Meth)acrylate Compound Having a Cyclic Carbonate Skeleton or a Lactone Skeleton] The (meth)acrylate compound as the polymerizable compound (C) has a cyclic carbonate skeleton or a lactone skeleton. In the present invention, the cyclic polycarbonate skeleton refers to a cyclic structure having —O—C(═O)—O—, and the lactone skeleton refers to a cyclic structure having —O—C(═O)—.

[0031] There are no particular limitations on the (meth)acrylate compound as long as it has a cyclic carbonate skeleton or a lactone skeleton, and commercially available products may be used, or compounds obtained by known synthesis methods may be used. Commercially available (meth)acrylate compounds having a cyclic carbonate skeleton or a lactone skeleton include the following: glycerin carbonate acrylate (trade name: M-910; manufactured by Toagosei Co., Ltd.), γ-butyrolactone acrylate (trade name: γ-GBLA; manufactured by Toagosei Co., Ltd.), γ-butyrolactone methacrylate (trade name: γ-GBLMA; manufactured by Toagosei Co., Ltd.), etc.

[0032] Furthermore, known methods for synthesizing (meth)acrylate compounds having a cyclic polycarbonate skeleton or a lactone skeleton include the methods described in, for example, JP-A-10-130181 and JP-A-2009-286718. Specifically, JP-A-10-130181 describes a production method in which ethylene oxide (EO) is added to a desired alcohol. Furthermore, JP-A-2009-286718 describes a method in which a desired (meth)acrylic acid ester monomer is produced using a desired alcohol. Therefore, the (meth)acrylate compound can be synthesized by first modifying the desired alcohol with EO using the method described in JP-A-10-130181, and then carrying out a transesterification reaction using the method described in JP-A-2009-286718.

[0033] Among these, the (meth)acrylate compound is preferably at least one compound selected from the group consisting of glycerin carbonate acrylate, γ-butyrolactone acrylate, and γ-butyrolactone methacrylate.

[0034] Both methacrylate compounds and acrylate compounds can be used, but acrylate compounds are preferred because they are easier to cure.

[0035] [(D) (Meth)acrylamide Compound Having a Carbon Atom Wherein All Four Single Bonds Are Bonded to Atoms Other Than Hydrogen Atoms] The (meth)acrylamide compound that is the polymerizable compound (D) has a carbon atom Wherein all four single bonds are bonded to atoms other than hydrogen atoms.

[0036] There are no particular limitations on the (meth)acrylamide compound as long as it has a carbon atom in which all four single bonds are bonded to atoms other than hydrogen atoms, and a commercially available product or one obtained by a known synthesis method may be used. Examples of commercially available (meth)acrylamide compounds in which all four single bonds are bonded to atoms other than hydrogen atoms include the following: N-tertiary butyl(meth)acrylamide (manufactured by Tokyo Chemical Industry Co., Ltd.), N-[tris(hydroxymethyl](meth)acrylamide (manufactured by Aldrich Chemical Industry Co., Ltd.), acrylamide tertiary butyl sulfonic acid (manufactured by Toagosei Co., Ltd.), diacetone(meth)acrylamide (manufactured by Tokyo Chemical Industry Co., Ltd.), N-(1,1,3,3-tetramethylbutyl)(meth)acrylamide (manufactured by Tokyo Chemical Industry Co., Ltd.), N-[3-(4,4,5,5-tetramethyl-1,3,2-dioxaboran-2-yl)phenyl](meth)acrylamide (manufactured by Tokyo Chemical Industry Co., Ltd.), etc.

[0037] Both methacrylamide and acrylamide can be used, but acrylamide is preferred because it hardens more easily.

[0038] Furthermore, the (meth)acrylamide compound has a carbon atom in which all four single bonds are bonded to atoms other than hydrogen atoms, and it is preferable that at least one of the atoms bonded to this carbon atom is a heteroatom.

[0039] The total content of the polymerizable compounds (A) to (D) is not particularly limited, but is preferably in the following range. That is, the total content of the polymerizable compounds (A) to (D) is preferably 20% by mass or more, more preferably 30% by mass or more, even more preferably 50% by mass or more, and particularly preferably 80% by mass or more, based on the total mass of the ink. When the total content of the polymerizable compounds (A) to (D) is within the above range, curing of the ink when irradiated with ultraviolet light can be further promoted.

[0040] In addition to the polymerizable compounds (A) to (D), the ink may contain other polymerizable compounds as long as the effects of the present invention can be achieved. Examples of such polymerizable compounds include (meth)acrylate compounds, (meth)acrylamide compounds, and N-vinyl compounds. Two or more of these polymerizable compounds may also be used in combination. However, because the polymerizable compound having a maleimide skeleton may affect the color of the ink, it is preferable that the content of the polymerizable compound having a maleimide skeleton in the ink is low. Specifically, the content of the polymerizable compound having a maleimide skeleton is preferably less than 0.1% by mass, based on the total mass of the ink, and more preferably 0% by mass (i.e., the ink does not contain any polymerizable compound having a maleimide skeleton).

[0041] Preferred examples of the polymerizable compound are shown below.

[0042] (Monofunctional (meth)acrylate compound) There are no particular limitations on the monofunctional (meth)acrylate compound, and commercially available products or compounds obtained by known synthesis methods may be used. Examples of commercially available monofunctional (meth)acrylate compounds include the following. Phenoxyethyl acrylate (trade name: Viscoat #192, manufactured by Osaka Organic Chemical Industry Co., Ltd.), ethyl acrylate (manufactured by Tokyo Chemical Industry Co., Ltd.), ethyl carbitol acrylate (trade name: Viscoat #190, manufactured by Osaka Organic Chemical Industry Co., Ltd.), 2-(diethylamino)ethyl acrylate (manufactured by Tokyo Chemical Industry Co., Ltd.), 3-(methoxydimethylsilyl)propyl acrylate (manufactured by Tokyo Chemical Industry Co., Ltd.), 2-(dimethylamino)ethyl acrylate (manufactured by Tokyo Chemical Industry Co., Ltd.), benzyl acrylate (trade name: BZA, manufactured by Osaka Organic Chemical Industry Co., Ltd.), dicyclopentanyl acrylate (trade name: FA-513A, manufactured by Hitachi Chemical Co., Ltd.), cyclohexyl acrylate (trade name: CHA, manufactured by Osaka Organic Chemical Industry Co., Ltd.), α-allyloxymethyl acrylate (trade name: AOMA, manufactured by Nippon Shokubai Co., Ltd.), tetrahydrofurfuryl acrylate (trade name: T HF-A (manufactured by Osaka Organic Chemical Co., Ltd.), tetrahydrofurfuryl methacrylate (manufactured by Tokyo Chemical Industry Co., Ltd.), 2-morpholinoethyl methacrylate (manufactured by Tokyo Chemical Industry Co., Ltd.), 2-oxotetrahydrofuran-3-yl methacrylate (manufactured by Tokyo Chemical Industry Co., Ltd.), 2-ethylhexyl acrylate (manufactured by Tokyo Chemical Industry Co., Ltd.), 2-hydroxy-3-phenoxypropyl acrylate (manufactured by Tokyo Chemical Industry Co., Ltd.), 2-(2-ethoxyethoxy)ethyl acrylate (manufactured by Tokyo Chemical Industry Co., Ltd.), isopentyl acrylate (manufactured by Tokyo Chemical Industry Co., Ltd.), 2-(diisopropylamino)ethyl methacrylate (manufactured by Tokyo Chemical Industry Co., Ltd.), isopropyl methacrylate (manufactured by Tokyo Chemical Industry Co., Ltd.), 5-oxotetrahydrofuran-3-yl methacrylate (manufactured by Tokyo Chemical Industry Co., Ltd.), furfuryl methacrylate (manufactured by Tokyo Chemical Industry Co., Ltd.), and the like.

[0043] (Polyfunctional (meth)acrylate compound) There are no particular limitations on the polyfunctional (meth)acrylate compound, and commercially available products or compounds obtained by known synthesis methods may be used. Examples of polyfunctional (meth)acrylate compounds include pentaerythritol derivatives, isocyanurate derivatives, and trimethylolpropane derivatives, and any of these may be used.

[0044] Examples of pentaerythritol derivatives include pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, ethylene oxide-modified pentaerythritol tri(meth)acrylate, propylene oxide-modified pentaerythritol tri(meth)acrylate, ethylene oxide-modified pentaerythritol tetra(meth)acrylate, propylene oxide-modified pentaerythritol tetra(meth)acrylate, dipentaerythritol penta(meth)acrylate, dipentaerythritol hexa(meth)acrylate, and tripentaerythritol(meth)acrylate. Among these, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol penta(meth)acrylate, dipentaerythritol hexa(meth)acrylate, and tripentaerythritol(meth)acrylate are preferred. Examples of commercially available pentaerythritol derivatives include the following: Trade name: Light Acrylate PE-3A; manufactured by Kyoeisha Chemical Co., Ltd., Trade name: Light Acrylate PE-4A; manufactured by Kyoeisha Chemical Co., Ltd., Trade name: Light Acrylate DPE-6A; manufactured by Kyoeisha Chemical Co., Ltd., Trade name: SR295; manufactured by Sartomer Co., Ltd., Trade name: Viscoat #300; manufactured by Osaka Organic Chemical Industry Co., Ltd., Trade name: MT-3549; manufactured by Toagosei Co., Ltd., Trade name: U-6PLA; manufactured by Shin-Nakamura Chemical Co., Ltd., Trade name: U-15HA; manufactured by Shin-Nakamura Chemical Co., Ltd., Trade name: KAYARAD D-310; manufactured by Nippon Kayaku Co., Ltd., Trade name: KAYARAD D-310; manufactured by Nippon Kayaku Co., Ltd., Trade name: KAYARAD D-330, manufactured by Nippon Kayaku Co., Ltd.; trade name: A-DPH, manufactured by Shin-Nakamura Chemical Co., Ltd.; trade name: Viscoat #300, manufactured by Osaka Organic Chemical Industry Co., Ltd., and the like.

[0045] Examples of isocyanurate derivatives include tris(2-hydroxyethyl)isocyanurate di(meth)acrylate, tris(2-hydroxyethyl)isocyanurate tri(meth)acrylate, polyfunctional pentaerythritol derivatives having a hydroxyl group with an isocyanurate-type polyisocyanate, and urethane acrylate-type isocyanurate derivatives obtained by reacting a polyfunctional trimethylol derivative with the isocyanurate polyisocyanate via a urethane bond. Among these, tris(2-hydroxyethyl)isocyanurate tri(meth)acrylate and urethane acrylate-type isocyanurate derivatives are preferred. Examples of commercially available isocyanurate derivatives include the following: Trade name: FANCRYL FA-731A (manufactured by Hitachi Chemical Co., Ltd.), Trade name: SR368 (manufactured by Sartomer Corporation), Trade name: ARONIX M-315 (manufactured by Toagosei Co., Ltd.), and Trade name: U-15HA (manufactured by Shin-Nakamura Chemical Co., Ltd.).

[0046] Examples of trimethylolpropane derivatives include trimethylolpropane tri(meth)acrylate, propoxylated trimethylolpropane tri(meth)acrylate, ethylene oxide-modified trimethylolpropane triacrylate, propylene oxide-modified trimethylolpropane, and ditrimethylolpropane tetraacrylate. Among these, preferred trimethylolpropane derivatives are trimethylolpropane tri(meth)acrylate and ditrimethylolpropane tetraacrylate. Examples of commercially available trimethylolpropane derivatives include the following: Trade name: Sunester TMP; manufactured by Sanshin Chemical Industry Co., Ltd., trade name: Viscoat #295; manufactured by Osaka Organic Chemical Industry Co., Ltd., trade name: Lightester TMP; manufactured by Kyoeisha Chemical Co., Ltd., trade name: Acryester TMP; manufactured by Mitsubishi Chemical Holdings Corporation, trade name: Miramar M410; manufactured by Toyo Chemicals Co., Ltd., trade name: Miramar M300; manufactured by Toyo Chemicals Co., Ltd., trade name: Miramar M301; manufactured by Toyo Chemicals Co., Ltd., trade name: EBECRYL140; manufactured by Daicel-Okunex Co., Ltd., trade name: EBECRYL1142; manufactured by Daicel-Okunex Co., Ltd., trade name: SR355; manufactured by Sartomer Co., Ltd., trade name: A-TMPT; manufactured by Shin-Nakamura Chemical Co., Ltd., trade name: AD-TMP; manufactured by Shin-Nakamura Chemical Co., Ltd., and the like.

[0047] (Monofunctional (meth)acrylamide compound) There are no particular limitations on the monofunctional (meth)acrylamide compound, and commercially available products may be used, or products obtained by known synthesis methods may be used. Examples of commercially available monofunctional (meth)acrylamide compounds include the following: N,N-dimethylacrylamide (trade name: DMAA; manufactured by KJ Chemicals Co., Ltd.), N-isopropylacrylamide (trade name: NIPAM; manufactured by KJ Chemicals Co., Ltd.), N-propylacrylamide (manufactured by Tokyo Chemical Industry Co., Ltd.), N-diacetacrylamide (manufactured by Tokyo Chemical Industry Co., Ltd.), N-[2-(diethylamino)ethyl]acrylamide (manufactured by Tokyo Chemical Industry Co., Ltd.), N-[3-(dimethylamino)propyl]acrylamide ( (manufactured by Tokyo Chemical Industry Co., Ltd.), N,N-diethylacrylamide (trade name: DEAA; manufactured by KJ Chemicals Co., Ltd.), acryloylmorpholine (trade name: ACMO; manufactured by KJ Chemicals Co., Ltd.), acryloylpiperidine (manufactured by Kokusan Chemical Co., Ltd.), N-(methoxymethyl)acrylamide (manufactured by Tokyo Chemical Industry Co., Ltd.), N-(isobutoxymethyl)acrylamide (manufactured by Tokyo Chemical Industry Co., Ltd.), N-(butoxymethyl)acrylamide (manufactured by Tokyo Chemical Industry Co., Ltd.), (manufactured by Tokyo Chemical Industry Co., Ltd.), 3-acryloyl-2-oxazolidinone (manufactured by Tokyo Chemical Industry Co., Ltd.), N-[2-(dimethylamino)ethyl]acrylamide (manufactured by Tokyo Chemical Industry Co., Ltd.), N-(2-hydroxyethyl)acrylamide (manufactured by Tokyo Chemical Industry Co., Ltd.), N-(hydroxymethyl)acrylamide (manufactured by Tokyo Chemical Industry Co., Ltd.), N,N-dimethylmethacrylamide (manufactured by Tokyo Chemical Industry Co., Ltd.), N-isopropylmethacrylamide (manufactured by Tokyo Chemical Industry Co., Ltd. ), N-methylmethacrylamide (manufactured by Tokyo Chemical Industry Co., Ltd.), N-(methoxymethyl)methacrylamide (manufactured by Tokyo Chemical Industry Co., Ltd.), N-[3-(dimethylamino)propyl]methacrylamide (manufactured by Tokyo Chemical Industry Co., Ltd.), N-(2-hydroxypropyl)methacrylamide (manufactured by Tokyo Chemical Industry Co., Ltd.), N-butylacrylamide (manufactured by Tokyo Chemical Industry Co., Ltd.), N-(3-methoxypropyl)acrylamide (manufactured by Sigma-Aldrich K.K.), and the like.

[0048] (N-Vinyl Compound) There are no particular limitations on the N-vinyl compound, and commercially available products may be used, or compounds obtained by known synthesis methods may be used. Commercially available N-vinyl compounds include the following: N-vinylacetamide (manufactured by Tokyo Chemical Industry Co., Ltd.), 2-methyl-1-vinylimidazole (manufactured by Tokyo Chemical Industry Co., Ltd.), N-vinylcaprolactam (manufactured by Tokyo Chemical Industry Co., Ltd.), 1-vinylimidazole (manufactured by Tokyo Chemical Industry Co., Ltd.), 1-vinyl-2-pyrrolidone (manufactured by Tokyo Chemical Industry Co., Ltd.), and N-vinylmethyloxazolidinone (trade name: VMOX; manufactured by BASF Corporation).

[0049] [Photopolymerization Initiator] Examples of photopolymerization initiators include aromatic ketone compounds, oxime ester compounds, acylphosphine oxide compounds, thioxanthone compounds, benzophenone compounds, benzoate compounds, aromatic onium salt compounds, organic peroxides, thio compounds (such as thiophenyl group-containing compounds), α-aminoalkylphenone compounds, hexaarylbiimidazole compounds, borate compounds, azinium compounds, metallocene compounds, active ester compounds, compounds having a carbon-halogen bond, and alkylamine compounds. Photoradical polymerization initiators described in JP 2018-35369 A, JP 2018-39265 A, and the like can also be used. Among these, α-hydroxyketone compounds, α-aminoalkylphenone compounds, oxime ester compounds, acylphosphine oxide compounds, and benzophenone compounds are preferred, with oxime ester compounds being more preferred. The photopolymerization initiators can be used alone or in combination of two or more. The content of the photopolymerization initiator in the ink is preferably 0.03 mass % or less based on the total mass of the ink, and more preferably the ink does not contain a photopolymerization initiator. If necessary, two or more types of photopolymerization initiators can be used in combination.

[0050] Examples of aromatic ketone compounds include acetophenone, 2,2-diethoxyacetophenone, 2,2-dimethoxy-2-phenylacetophenone, 2,2'-phenyl-p-tert-butyltrichloroacetophenone, p-tert-butyldichloroacetophenone, methyl benzoyl formate, N,N'-tetramethyl-4,4'-diaminobenzophenone (Michler's ketone), 1-hydroxycyclohexyl phenyl ketone, 1-[4-(2-hydroxyethoxy)-phenyl]-2-hydroxy-2-methylpropane, 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1,2-methyl-[4-(methylthio)phenyl]-2-morpholino-1-propane, 2-(dimethylamino)-2-[(4-methylphenyl)methyl]-1-[4-(4-morpholinyl)phenyl]-1-butanone, and the like.

[0051] Examples of the α-hydroxyketone compound include 2-hydroxy-2-methyl-1-phenyl-1-propanone, 2-hydroxy-1-{4-[4-(2-hydroxy-2-methyl-propionyl)-benzyl]-phenyl}-2-methyl-propan-1-one, and 1-hydroxycyclohexyl phenyl ketone.

[0052] Examples of the α-aminoalkylphenone compound include 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1, 2-methyl-[4-(methylthio)phenyl]-2-morpholino-1-propane, and 2-(dimethylamino)-2-[(4-methylphenyl)methyl]-1-[4-(4-morpholinyl)phenyl]-1-butanone.

[0053] Examples of the oxime ester compound include 1,2-octanedione, 1-[4-(phenylthio)phenyl]-, 2-(O-benzoyloxime), ethanone, 1-[9-ethyl-6-(2-methylbenzoyl)-9H-carbazol-3-yl]-, 1-(O-acetyloxime), (9-ethyl-6-nitro-9H-carbazol-3-yl)-(4-((1-methoxypropan-2-yl)oxy)-2-methylphenyl)methanone-o-acetyloxime, and 1-[4-[[4-(2-hydroxyethoxy)phenyl]thio]phenyl-1]-1,2-propanedione-2-(O-acetyloxime).

[0054] Examples of the acylphosphine oxide compound include 2,4,6-trimethylbenzoyldiphenylphosphine oxide and bis(2,4,6-trimethylbenzoyl)-diphenylphosphine oxide.

[0055] Examples of the benzoin alkyl ether compound include benzoin methyl ether, benzoin ethyl ether, benzoin butyl ether, and benzoin isopropyl ether.

[0056] Examples of the benzoin ether compounds include methyl benzoin and ethyl benzoin.

[0057] Examples of the thioxanthone compound include 2-chlorothioxanthone, 2,4-diethylthioxanthone, isopropylthioxanthone, and 2-methylthioxanthone.

[0058] Examples of the benzophenone compound include benzophenone, 4-methylbenzophenone, 4-phenylbenzophenone, 4-(4-methylphenylthio)benzophenone, and 4,4'-bis(diethylamino)benzophenone.

[0059] Examples of the benzoate compound include ethyl-4-(dimethylamino)benzoate, ethylhexyl-4-dimethylaminobenzoate, methyl-o-benzoylbenzoate, and 3-methylbutyl p-(dimethylamino)benzoate.

[0060] [Colorant] The ink of the present invention may contain a colorant. Known dyes and pigments can be used as the colorant, but pigments are preferred from the viewpoint of light resistance. Furthermore, the pigment may be either an inorganic pigment or an organic pigment.

[0061] (Black Pigment) Examples of black pigments include carbon black produced by the furnace method or the channel method.

[0062] (White Pigment) Examples of white pigments that can be used include alkaline earth metal sulfates, alkaline earth metal carbonates, finely powdered silicic acid, silicas such as synthetic silicates, calcium silicate, alumina, alumina hydrate, titanium oxide, zinc oxide, talc, clay, etc. Examples of alkaline earth metal sulfates include barium sulfate, etc. Examples of alkaline earth metal carbonates include calcium carbonate, etc.

[0063] (Yellow Pigment) Examples of yellow pigments include Pig. Yellow pigments, such as Pigment Yellow 1, Pigment Yellow 2, Pigment Yellow 3, Pigment Yellow 12, Pigment Yellow 13, Pigment Yellow 14, Pigment Yellow 16, Pigment Yellow 17, Pigment Yellow 73, Pigment Yellow 74, Pigment Yellow 75, Pigment Yellow 83, Pigment Yellow 93, Pigment Yellow 95, Pigment Yellow 97, Pigment Yellow 98, Pigment Yellow 114, Pigment Yellow 120, Pigment Yellow 128, Pigment Yellow 129, Pigment Yellow 138, Pigment Yellow 150, Pigment Yellow 151, Pigment Yellow 154, Pigment Yellow 155, and Pigment Yellow 180.

[0064] (Magenta Pigment) Examples of magenta pigments include the following: Pigment Red pigments, such as Pigment Red 5, Pigment Red 7, Pigment Red 12, Pigment Red 48 (Ca), Pigment Red 48 (Mn), Pigment Red 57 (Ca), Pigment Red 57:1, Pigment Red 112, Pigment Red 122, Pigment Red 123, Pigment Red 168, Pigment Red 184, Pigment Red 202, and Pigment Violet 19.

[0065] (Cyan Pigment) Examples of cyan pigments include Pig. Blue pigments, such as Pigment Blue 1, Pigment Blue 2, Pigment Blue 3, Pigment Blue 15, Pigment Blue 15:3, Pigment Blue 15:4, Pigment Blue 16, Pigment Blue 22, Pigment Blue 60, Vat Blue 4, and Vat Blue 60.

[0066] In addition, various inorganic pigments and organic pigments can be used as needed, taking into consideration physical properties, etc. The content of the pigment in the ink is preferably 0.5% by mass or more and 10.0% by mass or less, and more preferably 1.0% by mass or more and 7.0% by mass or less, based on the total mass of the ink.

[0067] The ink of the present invention may further contain, if necessary, surfactants such as higher fatty acid-based, silicone-based and fluorine-based surfactants, and polymeric pigment dispersants having polar groups.

[0068] The viscosity of the ink of the present invention at 25°C is preferably 3 mPa·s or more and 200 mPa·s or less, and more preferably 3 mPa·s or more and 100 mPa·s or less. However, in the case of a recording head that can be heated, for example, the viscosity of the ink can be reduced by heating the recording head, so in this case the viscosity of the ink does not need to be within the above range. An example of a commercially available recording head that can be heated is Nitrox (trade name) manufactured by Saar.

[0069] <Recording Method and Recording Apparatus> Next, the recording method and recording apparatus of the present invention will be described.

[0070] The recording apparatus of the present invention includes an inkjet recording head for ejecting ink onto a recording medium and an ultraviolet irradiation device for irradiating the ink applied to the recording medium with specific ultraviolet rays. The recording method of the present invention includes the steps of ejecting ink from the inkjet recording head onto a recording medium and irradiating the ink ejected onto the recording medium with specific ultraviolet rays to cure the ink. The specific ultraviolet rays are ultraviolet rays whose emission spectrum has a peak in the wavelength range of 250 nm or more and less than 300 nm, and whose peak intensity is 50% or more of the total intensity in the wavelength range of 200 nm or more and 800 nm or less. The ink used is the ink described above in the "Ink" section.

[0071] Ink jet recording heads are classified into piezo type and thermal type, and either type can be used, but the piezo type is preferred as it can be used with a wider range of ink types.

[0072] In the emission spectrum, the intensity (irradiation intensity) of ultraviolet light having a peak in the wavelength region of 250 nm or more and less than 300 nm is preferably 70% or more of the total intensity (total irradiation intensity) in the wavelength region of 200 nm or more and less than 800 nm, and more preferably 90% or more.

[0073] Furthermore, it is preferable that the peak intensity in the wavelength region of 250 nm or more and less than 300 nm is the highest among the total intensity in the wavelength region of 200 nm or more and 800 nm or less in the emission spectrum. Examples of ultraviolet light sources that can be used in the ultraviolet irradiation device include metal halide lamps, xenon lamps, carbon arc lamps, chemical lamps, low-pressure mercury lamps, high-pressure mercury lamps, and ultraviolet light-emitting diodes (UV-LEDs). Among these, UV-LEDs are preferable. UV-LEDs are compact and lightweight, which allows for the miniaturization and energy conservation of recording devices. Furthermore, UV-LEDs have excellent variability in exposure conditions, which allows for optimal exposure conditions to be set depending on the ink, enabling images to be formed with high productivity.

[0074] Currently commercially available UV-LEDs with peaks in the wavelength range of 250 nm or more but less than 300 nm include those with single peaks at 255 nm, 265 nm, 275 nm, 280 nm, and 285 nm. These are available, for example, from Taiko Seisakusho Co., Ltd., Nikkiso Co., Ltd., and Stanley Electric Co., Ltd. Commercially available metal halide lamps, xenon lamps, carbon arc lamps, chemical lamps, low-pressure mercury lamps, and high-pressure mercury lamps are available, for example, from Ushio Inc., Iwasaki Electric Co., Ltd., CCS Inc., and Hilux Electric Co., Ltd. High-pressure mercury lamps and metal halide lamps, which are capable of high output, can be used as light sources limited to a specific wavelength range using a bandpass filter. However, high-pressure mercury lamps and metal halide lamps have drawbacks, such as an inability to limit the wavelength range to the same extent as UV-LEDs, a shorter lamp life than UV-LEDs, and the time required for output stabilization.

[0075] The wavelength of the light source can be measured using a commercially available spectrophotometer. For example, the emission spectrum of the light source can be measured in standard mode using a multichannel spectrometer (product name: PMA12; manufactured by HAMAMATSU). Specifically, from this emission spectrum, the integral value of the intensity in the wavelength range of 250 nm to less than 300 nm is calculated, assuming that the integral value of the intensity in the wavelength range of 200 nm to less than 800 nm is 100%. This allows the ratio (%) of the peak intensity in the wavelength range of 250 nm to less than 300 nm to the total intensity in the wavelength range of 200 nm to less than 800 nm to be calculated.

[0076] By replacing the area to be irradiated with ultraviolet light with an inert gas such as nitrogen or argon, it is possible to reduce the termination reaction in the polymerization reaction caused by oxygen, thereby obtaining a good printed matter with a higher degree of polymerization.

[0077] [Recording Medium] Examples of the recording medium include absorbent and non-absorbent recording media. The above-mentioned recording method can be widely applied to recording media with various absorption capabilities, from non-absorbent recording media into which aqueous inks have difficulty penetrating to absorbent recording media into which aqueous inks easily penetrate. However, when the ink is applied to a non-absorbent recording medium, it may be necessary to provide a drying process after curing by irradiating with ultraviolet light.

[0078] The absorbent recording medium is not particularly limited, but examples thereof include plain paper and inkjet paper, which have high permeability to aqueous inks, and art paper, coated paper, cast paper, etc., which are used in general offset printing and have relatively low permeability to aqueous inks. As inkjet paper, those having an ink-receiving layer containing inorganic particles such as silica particles and alumina particles, or hydrophilic polymers such as polyvinyl alcohol (PVA) and polyvinylpyrrolidone (PVP) can be used.

[0079] Non-absorbent recording media include, but are not limited to, films, sheets, and plates of plastics such as polyvinyl chloride (PVC), polyethylene, polypropylene, and polyethylene terephthalate (PET); metal plates such as iron, silver, copper, and aluminum; metal plates and plastic films made by vapor deposition of these metals; and alloy plates such as stainless steel and brass.

[0080] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples as long as the gist of the invention is not exceeded. "Parts" and "%" used to describe the amounts of components are by mass unless otherwise specified.

[0081] <<Ink Using Polymerizable Compound (A)>> <Preparation of Yellow Pigment Dispersion> Yellow pigment: C.I. Pigment Yellow 155 (trade name: NOVOPERM YELLOW 4G-01; manufactured by Clariant) 30 parts Surfactant (trade name: BYK-168; manufactured by BYK-Chemie) 20 parts Polymerizable compound: 50 parts of a 1:1 mass ratio mixture of isobornyl acrylate (trade name: IBXA; manufactured by Osaka Organic Chemical Industry Ltd.) and trimethylolpropane triacrylate (trade name: Biscoat #295; manufactured by Osaka Organic Chemical Industry Ltd.)

[0082] The above components were stirred using a disperser (trade name: Motor Mill M50; manufactured by Eiger) to obtain a yellow pigment dispersion. The stirring using this disperser was carried out using zirconia beads with a diameter of 0.65 mm at a peripheral speed of 9 m / s for 8 hours.

[0083] <Ink Preparation> The components listed in Table 1-1 were mixed and stirred to obtain UV-curable inkjet inks of Examples 1-1 to 1-31 and Comparative Examples 1-1 to 1-8. The numerical values ​​in Table 1-1 represent the blend amount (parts by mass) of each component. The "other (meth)acrylate compounds" listed in Table 1-1 refer to (meth)acrylate compounds other than (meth)acrylate compounds having a carbon atom in which all four single bonds are bonded to atoms other than hydrogen atoms. The details of each component listed in Table 1-1 are as follows:

[0084] ((Meth)acrylate compounds having a carbon atom in which all four single bonds are bonded to atoms other than hydrogen atoms) Mixed monomer 1-A: a mixed solution of isobornyl acrylate (trade name: IBXA; manufactured by Osaka Organic Chemical Industry Co., Ltd.) and trimethylolpropane triacrylate (trade name: Viscoat #295; manufactured by Osaka Organic Chemical Industry Co., Ltd.) in a mass ratio of 1:1 Mixed monomer 1-B: a mixed solution of adamantyl-1-yl acrylate (manufactured by Tokyo Chemical Industry Co., Ltd.) and trimethylolpropane triacrylate (trade name: Viscoat #295; manufactured by Osaka Organic Chemical Industry Co., Ltd.) in a mass ratio of 1.0:2.5 Mixed Monomer 1-C: A mixed solution of neopentyl glycol-acrylic acid-benzoic acid ester (trade name: Light Acrylate BA104; manufactured by Kyoeisha Chemical Co., Ltd.), 2-methyl adamantyl-2-yl acrylate (trade name: Tokyo Chemical Industry Co., Ltd.), and trimethylolpropane triacrylate (trade name: Biscoat #295; manufactured by Osaka Organic Chemical Industry Co., Ltd.) in a mass ratio of 1:1:2. Mixed Monomer 1-D: A mixed solution of isobornyl acrylate (trade name: IBXA; manufactured by Osaka Organic Chemical Industry Co., Ltd.), adamantyl-1-yl acrylate (manufactured by Tokyo Chemical Industry Co., Ltd.), and dipentaerythritol hexaacrylate (trade name: Light Acrylate DPE-6A; manufactured by Kyoeisha Chemical Co., Ltd.) in a mass ratio of 1.5:1.0:2.0. Mixed Monomer 1-E: a mixed solution of trimethylolpropane triacrylate EO adduct (product name: A-TMPT-9EO; manufactured by Shin-Nakamura Chemical Co., Ltd.) and adamantyl-1-yl acrylate (manufactured by Tokyo Chemical Industry Co., Ltd.) in a mass ratio of 1.0:1.0

[0085] ((Meth)acrylate compounds other than those having a carbon atom in which all four single bonds are bonded to atoms other than hydrogen atoms) 1,6-hexanediol diacrylate (trade name: Viscoat #230; manufactured by Osaka Organic Chemical Industry Ltd.)

[0086] (Photopolymerization initiator) Bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide (trade name: Omnirad 819; manufactured by IGM Resins B.V.)

[0087] <Image Recording and Evaluation> (Image Recording) The inks prepared in Examples 1-1 to 1-31 and Comparative Examples 1-1 to 1-8 were filled into the ink tanks of an inkjet recording device equipped with a piezoelectric inkjet recording head, and recording was performed on a recording medium. Then, adjacent to the recording head of this recording device, a UV-LED irradiation device (manufactured by Taiko Seisakusho) or a metal halide lamp (product name: EX-250; manufactured by Schott Nippon Co., Ltd.) was installed as an ultraviolet irradiation device. The UV-LED irradiation device used had maximum peak wavelengths of 265 nm, 365 nm, and 385 nm, respectively. The metal halide lamp was adjusted, if necessary, using a mountable bandpass filter to achieve the irradiation intensity ratios for each wavelength region listed in Table 1-2. The UV-LED irradiation device and the metal halide lamp were adjusted so that the integrated intensity (total irradiation intensity) of the wavelength region of 200 nm to 800 nm was the same. Furthermore, ultraviolet irradiation was performed under two atmospheres: air and nitrogen flow. In this inkjet recording device, the resolution is 600 dpi x 600 dpi, and the recording duty of an image recorded under the conditions of depositing eight droplets of 3.8 ng of ink in a unit area of ​​1 / 600 inch x 1 / 600 inch is defined as 100%. Using this inkjet recording device, a 100% duty solid image was printed in one pass on a white PET film and a transparent PET film. Immediately after that, ultraviolet light was irradiated multiple times at a conveyor speed of 10 m / min. The integrated light amount was measured separately and found to be 10,000 mJ / cm. ２ The white PET film used was a product manufactured by Toray Industries, Inc. under the trade name of Lumirror E20 (thickness 75 μm), and the transparent PET film used was a product manufactured by Toray Industries, Inc. under the trade name of Lumirror T60 (thickness 25 μm). In Comparative Example 1-9, an image was recorded using the same ink and recording method as in Comparative Example 1-4, except that a low-pressure mercury lamp was used instead of a metal halide lamp as the ultraviolet irradiation device. The low-pressure mercury lamp used was a product manufactured by JELIGHT COMPANY under the trade name of UVO-CLEANER MODEL 42. The integrated value of the intensity of the low-pressure mercury lamp in the wavelength region of 200 nm to 800 nm was adjusted to be the same as that of the metal halide lamp.

[0088] Table 1-2 shows the ultraviolet irradiation conditions implemented in Examples 1-1 to 1-31 and Comparative Examples 1-1 to 1-8. In Table 1-2, the percentage of irradiation intensity refers to the percentage of irradiation intensity in each wavelength region when the integrated value of the intensity in the wavelength region of 200 nm or more and 800 nm or less is taken as 100%. The low-pressure mercury lamp used in Comparative Example 1-9 has a peak at 254 nm, but also a peak in the wavelength region of 300 nm or more. Therefore, in the emission spectrum of this low-pressure mercury lamp, the peak intensity in the wavelength region of 250 nm or more and less than 300 nm is less than 50% of the total intensity in the wavelength region of 200 nm or more and 800 nm or less.

[0089] (Evaluation of Stickiness) The image recorded on the white PET film was touched with a hand and evaluated for stickiness of the image according to the following criteria: AAA: No stickiness of the image at all. AA: Very slight stickiness of the image. A: Slight stickiness of the image. B: Stickiness of the image.

[0090] (Evaluation of fold crack resistance) A recorded product with an image recorded on a white PET film was repeatedly folded twice in a mountain fold and a valley fold, and the fold crack resistance was evaluated according to the following criteria. AAA: No scratches are visible on the image. AA: Scratches are visible only on the surface of the image, but not on the white part of the white PET film. A: Scratches on the image are visible, and the white part of the white PET film is slightly visible. B: Scratches on the image are visible, and the white part of the white PET film is clearly visible.

[0091] (Evaluation of Elution of Photopolymerization Initiator) A recorded material in which an image was recorded on a transparent PET film was placed in a one-side elution tester (trade name: MK10 type; manufactured by Maeda Manufacturing Co., Ltd.), and the material was filled to the brim with 100 ml of 98% ethanol. After storage at 80°C for 5 days, the photopolymerization initiator was quantified from 100 ml of the 98% ethanol content using a gas chromatograph mass spectrometer and a liquid chromatograph mass spectrometer, and the elution of the photopolymerization initiator was evaluated according to the following criteria. The gas chromatograph mass spectrometer used was a GC2100A manufactured by Shimadzu Corporation, with a polydimethylsiloxane-based column. The liquid chromatograph mass spectrometer used was a HLC-8220GPC manufactured by Tosoh Corporation, with a Shodex GPCLF-804 column. AAA: No photopolymerization initiator was detected eluted from the image. AA: The amount of photopolymerization initiator eluted from the image was 100 ppb or less. A: The amount of photopolymerization initiator eluted from the image was 1000 ppb or less. B: The amount of photopolymerization initiator eluted from the image was more than 1000 ppb.

[0092] The evaluation results for stickiness, resistance to cracking at folds, and elution of the photopolymerization initiator are shown in Table 1-3. In addition, for Comparative Example 1-9, which is not listed in Table 1-3, the evaluation results were the same as those for Comparative Example 1-4.

[0093]

[0094]

[0095]

[0096] <<Ink Using Polymerizable Compound (B)>> <Preparation of Yellow Pigment Dispersion> Yellow pigment: C.I. Pigment Yellow 155 (trade name: NOVOPERM YELLOW 4G-01; manufactured by Clariant) 30 parts Surfactant (trade name: BYK-168; manufactured by BYK-Chemie) 20 parts Polymerizable compound: trimethylolpropane triacrylate (trade name: Biscoat #295; manufactured by Osaka Organic Chemical Industry Ltd.) 50 parts

[0097] The above components were stirred using a disperser (trade name: Motor Mill M50; manufactured by Eiger) to obtain a yellow pigment dispersion. The stirring using this disperser was carried out using zirconia beads with a diameter of 0.65 mm at a peripheral speed of 9 m / s for 8 hours.

[0098] <Ink Preparation> The components listed in Table 2-1 were mixed and stirred to obtain UV-curable inkjet inks of Examples 2-1 to 2-30 and Comparative Examples 2-1 to 2-8. The numerical values ​​in Table 2-1 represent the blend amount (parts by mass) of each component. Furthermore, the "other (meth)acrylate compounds" listed in Table 2-1 refer to (meth)acrylate compounds other than (meth)acrylate compounds having a dioxane skeleton or a dioxolane skeleton. Furthermore, details of each component listed in Table 2-1 are as follows:

[0099] ((Meth)acrylate compounds having a dioxane skeleton or a dioxolane skeleton) Compound 2-A: (2-methyl-2-ethyl-1,3-dioxolan-4-yl)methyl acrylate (trade name: MEDOL-10; manufactured by Osaka Organic Chemical Industry Ltd.) Compound 2-B: cyclic trimethylolpropane formal acrylate (trade name: Viscoat #200; manufactured by Osaka Organic Chemical Industry Ltd.) Compound 2-C: (2-methyl-2-isobutyl-1,3-dioxolan-4-yl)methyl acrylate (manufactured by Aldrich) Compound 2-D: 1,3-dioxolane-4-methyl acrylate

[0100] Compound 2-D was synthesized by transesterification of 1,3-dioxolane-4-methanol and methyl acrylate according to the method described in JP-A-2009-286718.

[0101] The synthesized 1,3-dioxolane-4-methyl acrylate was analyzed by gas chromatography (hereinafter, gas chromatography will be referred to as GC), and the result was that the content of 1,3-dioxolane-4-methyl acrylate was 99.9 mol % and the content of 1,3-dioxolane-4-methanol was 0.1 mol %.

[0102] (Method of GC Analysis) GC analysis was carried out using a gas chromatography system (trade name: Agilent 6850; manufactured by Agilent Technology) under the following measurement conditions.

[0103] At the injection port, the heater temperature was set to 280°C, the split ratio to 50:1, and the pressure to 50 kPa. An FID was used as the detector, and the heater temperature of the detector was set to 280°C. The GC column used was a trade name: HP-1 (length 30 m, inner diameter 0.32 mm, film thickness 0.25 μm, manufactured by Agilent Technology). The oven was initially kept at 70°C for 5 minutes, and then heated at a rate of 10°C per minute until it reached 280°C and was kept at that temperature for 10 minutes. The injection volume was 0.2 μl.

[0104] ((Meth)acrylates other than (meth)acrylate compounds having a dioxane skeleton or a dioxolane skeleton) 1,6-hexanediol diacrylate (trade name: Viscoat #230, manufactured by Osaka Organic Chemical Industry Ltd.)

[0105] (Photopolymerization initiator) Bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide (trade name: Omnirad 819; manufactured by IGM Resins B.V.)

[0106] <Image Recording and Evaluation> Image recording and evaluation using the inks prepared in Examples 2-1 to 2-30 and Comparative Examples 2-1 to 2-8 were performed in the same manner as in Example 1-1. Furthermore, in Comparative Example 2-9, an image was recorded using the same ink and recording method as in Comparative Example 2-4, except that a low-pressure mercury lamp was used as the ultraviolet irradiation device instead of the metal halide lamp. The low-pressure mercury lamp used was a UVO-CLEANER MODEL 42 manufactured by JELIGHT COMPANY. The integrated value of the intensity of the low-pressure mercury lamp in the wavelength region of 200 nm to 800 nm was adjusted to be the same as that of the metal halide lamp.

[0107] Table 2-2 shows the ultraviolet irradiation conditions implemented in Examples 2-1 to 2-30 and Comparative Examples 2-1 to 2-8. In Table 2-2, the percentage of irradiation intensity refers to the percentage of irradiation intensity in each wavelength region when the integrated value of the intensity in the wavelength region of 200 nm or more and 800 nm or less is taken as 100%. The low-pressure mercury lamp used in Comparative Example 2-9 has a peak at 254 nm, but also a peak in the wavelength region of 300 nm or more. Therefore, in the emission spectrum of this low-pressure mercury lamp, the peak intensity in the wavelength region of 250 nm or more and less than 300 nm is less than 50% of the total intensity in the wavelength region of 200 nm or more and 800 nm or less.

[0108] The evaluation results for stickiness, resistance to cracking at folds, and elution of the photopolymerization initiator are shown in Table 2-3. In addition, for Comparative Example 2-9, which is not listed in Table 2-3, the evaluation results were the same as those for Comparative Example 2-4.

[0109]

[0110]

[0111]

[0112] <<Ink Using Polymerizable Compound (C)>> <Preparation of Yellow Pigment Dispersion> Yellow pigment: C.I. Pigment Yellow 155 (trade name: NOVOPERM YELLOW 4G-01; manufactured by Clariant) 30 parts Surfactant (trade name: BYK-168; manufactured by BYK-Chemie) 20 parts Polymerizable compound: trimethylolpropane triacrylate (trade name: Biscoat #295; manufactured by Osaka Organic Chemical Industry Ltd.) 50 parts

[0113] The above components were stirred using a disperser (trade name: Motor Mill M50; manufactured by Eiger) to obtain a yellow pigment dispersion. The stirring using this disperser was carried out using zirconia beads with a diameter of 0.65 mm at a peripheral speed of 9 m / s for 8 hours.

[0114] <Ink Preparation> The components listed in Table 3-1 were mixed and stirred to obtain UV-curable inkjet inks of Examples 3-1 to 3-30 and Comparative Examples 3-1 to 3-8. The numerical values ​​in Table 3-1 represent the blend amount (parts by mass) of each component. The "other (meth)acrylate compounds" listed in Table 3-1 refer to (meth)acrylate compounds other than (meth)acrylate compounds having a cyclic carbonate skeleton or a lactone skeleton. Details of each component listed in Table 3-1 are as follows:

[0115] ((Meth)acrylate compounds having a cyclic carbonate skeleton or a lactone skeleton) Compound 3-A: glycerin carbonate acrylate (trade name: M-910; manufactured by Toagosei Co., Ltd.) Compound 3-B: γ-butyrolactone acrylate (trade name: GBLA; manufactured by Osaka Organic Chemical Industry Ltd.) Compound 3-C: γ-butyrolactone methacrylate (trade name: GBLMA; manufactured by Osaka Organic Chemical Industry Ltd.) Compound 3-D: EO-modified glycerin carbonate acrylate

[0116] Compound 3-D was prepared by adding an average of 1 mole of ethylene oxide (EO) to the hydroxyl groups of glycerin carbonate according to the method described in JP-A-10-130181, to obtain an EO-modified glycerin carbonate. Then, according to the method described in JP-A-2009-286718, the obtained EO-modified glycerin carbonate was subjected to a transesterification reaction with methyl acrylate to synthesize an EO-modified glycerin carbonate acrylate.

[0117] The synthesized EO-modified glycerin carbonate acrylate was analyzed by gas chromatography (hereinafter, gas chromatography will be referred to as GC). As a result, the content of EO-modified glycerin carbonate acrylate was 98.5 mol %, and the content of EO-modified glycerin carbonate was 1.5 mol %.

[0118] (Method of GC Analysis) GC analysis was carried out using a gas chromatography system (trade name: Agilent 6850; manufactured by Agilent Technology) under the following measurement conditions.

[0119] At the injection port, the heater temperature was set to 280°C, the split ratio to 50:1, and the pressure to 50 kPa. An FID was used as the detector, and the heater temperature of the detector was set to 280°C. The GC column used was a trade name: HP-1 (length 30 m, inner diameter 0.32 mm, film thickness 0.25 μm, manufactured by Agilent Technology). The oven was initially kept at 70°C for 5 minutes, and then heated at a rate of 10°C per minute until it reached 280°C and was kept at that temperature for 10 minutes. The injection volume was 0.2 μl.

[0120] ((Meth)acrylates other than (meth)acrylate compounds having a cyclic carbonate skeleton or a lactone skeleton) 1,6-hexanediol diacrylate (trade name: Viscoat #230; manufactured by Osaka Organic Chemical Industry Ltd.)

[0121] (Photopolymerization initiator) Bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide (trade name: Omnirad 819; manufactured by IGM Resins B.V.)

[0122] <Image Recording and Evaluation> Image recording and evaluation using the inks prepared in Examples 3-1 to 3-30 and Comparative Examples 3-1 to 3-8 were performed in the same manner as in Example 1-1. Furthermore, in Comparative Example 3-9, an image was recorded using the same ink and recording method as in Comparative Example 3-4, except that a low-pressure mercury lamp was used instead of a metal halide lamp as the ultraviolet irradiation device. The low-pressure mercury lamp used was a UVO-CLEANER MODEL 42 manufactured by JELIGHT COMPANY. The integrated value of the intensity of the low-pressure mercury lamp in the wavelength region of 200 nm to 800 nm was adjusted to be the same as that of the metal halide lamp.

[0123] Table 3-2 shows the ultraviolet irradiation conditions implemented in Examples 3-1 to 3-30 and Comparative Examples 3-1 to 3-8. In Table 3-2, the percentage of irradiation intensity refers to the percentage of irradiation intensity in each wavelength region when the integrated value of the intensity in the wavelength region of 200 nm or more and 800 nm or less is taken as 100%. The low-pressure mercury lamp used in Comparative Example 3-9 has a peak at 254 nm, but also a peak in the wavelength region of 300 nm or more. Therefore, in the emission spectrum of this low-pressure mercury lamp, the peak intensity in the wavelength region of 250 nm or more and less than 300 nm is less than 50% of the total intensity in the wavelength region of 200 nm or more and 800 nm or less.

[0124] The evaluation results for stickiness, resistance to cracking at folds, and elution of the photopolymerization initiator are shown in Table 3-3. In addition, for Comparative Example 3-9, which is not listed in Table 3-3, the evaluation results were the same as those for Comparative Example 3-4.

[0125]

[0126]

[0127]

[0128] <<Ink Using Polymerizable Compound (D)>> <Preparation of Yellow Pigment Dispersion> Yellow pigment: C.I. Pigment Yellow 155 (trade name: NOVOPERM YELLOW 4G-01; manufactured by Clariant) 30 parts Surfactant (trade name: BYK-168; manufactured by BYK-Chemie) 20 parts Polymerizable compound: 50 parts of a 1:1 mass ratio mixture of isobornyl acrylate (trade name: IBXA; manufactured by Osaka Organic Chemical Industry Ltd.) and trimethylolpropane triacrylate (trade name: Biscoat #295; manufactured by Osaka Organic Chemical Industry Ltd.)

[0129] The above components were stirred using a disperser (trade name: Motor Mill M50; manufactured by Eiger) to obtain a yellow pigment dispersion. The stirring using this disperser was carried out using zirconia beads with a diameter of 0.65 mm at a peripheral speed of 9 m / s for 8 hours.

[0130] <Ink Preparation> The components listed in Tables 4-1-1 and 4-1-2 were mixed and stirred to obtain UV-curable inkjet inks of Examples 4-1 to 4-30 and Comparative Examples 4-1 to 4-8. The numerical values ​​in Tables 4-1-1 and 4-1-2 represent the blend amount (parts by mass) of each component. The "other polymerizable compounds" listed in Tables 4-1-1 and 4-1-2 refer to polymerizable compounds other than (meth)acrylamide compounds having a carbon atom in which all four single bonds are bonded to atoms other than hydrogen atoms. The other polymerizable compounds used include a first (meth)acrylate compound and a second (meth)acrylate compound. The first (meth)acrylate compound is a (meth)acrylate compound having a carbon atom in which all four single bonds are bonded to atoms other than hydrogen atoms. The second (meth)acrylate compound is a (meth)acrylate compound other than a (meth)acrylate compound having a carbon atom in which all four single bonds are bonded to atoms other than hydrogen atoms. Details of each component listed in Tables 4-1-1 and 4-1-2 are as follows.

[0131] ((Meth)acrylamide compounds having a carbon atom in which all four single bonds are bonded to atoms other than hydrogen atoms) Mixed monomer 4-A: a mixture of diacetone acrylamide (manufactured by Tokyo Chemical Industry Co., Ltd.) and N-(1,1,3,3-tetramethylbutyl)acrylamide (manufactured by Tokyo Chemical Industry Co., Ltd.) in a mass ratio of 1:1 Mixed monomer 4-B: a mixture of diacetone acrylamide (manufactured by Tokyo Chemical Industry Co., Ltd.), N-(1,1,3,3-tetramethylbutyl)acrylamide (manufactured by Tokyo Chemical Industry Co., Ltd.) and N-[tris(hydroxymethyl)(meth)acrylamide (manufactured by Aldrich Chemical Co., Ltd.) in a mass ratio of 1.0:1.0:0.3

[0132] (First (meth)acrylate compound) Trimethylolpropane triacrylate (trade name: Viscoat #295; manufactured by Osaka Organic Chemical Industry Co., Ltd.) Mixed monomer X: a mixture of isobornyl acrylate (trade name: IBXA; manufactured by Osaka Organic Chemical Industry Co., Ltd.) and trimethylolpropane triacrylate (trade name: Viscoat #295; manufactured by Osaka Organic Chemical Industry Co., Ltd.) in a mass ratio of 1:2 Mixed monomer Y: a mixture of isobornyl acrylate (trade name: IBXA; manufactured by Osaka Organic Chemical Industry Co., Ltd.), damantyl-1-yl acrylate (manufactured by Tokyo Chemical Industry Co., Ltd.), and dipentaerythritol hexaacrylate (trade name: Light Acrylate DPE-6A; manufactured by Kyoeisha Chemical Co., Ltd.) in a mass ratio of 1.0:0.5:2.5

[0133] (Second (meth)acrylate compound) 1,6-hexanediol diacrylate (trade name: Viscoat #230; manufactured by Osaka Organic Chemical Industry Ltd.)

[0134] (Photopolymerization initiator) Bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide (trade name: Omnirad 819; manufactured by IGM Resins B.V.)

[0135] <Image Recording and Evaluation> Image recording and evaluation using the inks prepared in Examples 4-1 to 4-30 and Comparative Examples 4-1 to 4-8 were performed in the same manner as in Example 1-1. Furthermore, in Comparative Example 4-9, an image was recorded using the same ink and recording method as in Comparative Example 4-4, except that a low-pressure mercury lamp was used as the ultraviolet irradiation device instead of the metal halide lamp. The low-pressure mercury lamp used was a UVO-CLEANER MODEL 42 manufactured by JELIGHT COMPANY. The integrated intensity of the low-pressure mercury lamp in the wavelength region of 200 nm to 800 nm was adjusted to be the same as that of the metal halide lamp.

[0136] Table 4-2 shows the ultraviolet irradiation conditions implemented in Examples 4-1 to 4-30 and Comparative Examples 4-1 to 4-8. In Table 4-2, the percentage of irradiation intensity refers to the percentage of irradiation intensity in each wavelength region when the integrated value of the intensity in the wavelength region of 200 nm or more and 800 nm or less is taken as 100%. The low-pressure mercury lamp used in Comparative Example 4-9 has a peak at 254 nm, but also a peak in the wavelength region of 300 nm or more. Therefore, in the emission spectrum of this low-pressure mercury lamp, the peak intensity in the wavelength region of 250 nm or more and less than 300 nm is less than 50% of the total intensity in the wavelength region of 200 nm or more and 800 nm or less.

[0137] The evaluation results for stickiness, resistance to cracking at folds, and elution of the photopolymerization initiator are shown in Table 4-3. In addition, for Comparative Example 4-9, which is not listed in Table 4-3, the evaluation results were the same as those for Comparative Example 4-4.

[0138]

[0139]

[0140]

[0141]

[0142] The present invention is not limited to the above-described embodiments, and various modifications and variations can be made without departing from the spirit and scope of the present invention. Therefore, the following claims are appended to apprise the public of the scope of the present invention.

[0143] This application claims priority based on Japanese Patent Application No. 2024-088412, Japanese Patent Application No. 2024-088413, Japanese Patent Application No. 2024-088414, Japanese Patent Application No. 2024-088415, and Japanese Patent Application No. 2025-087393 filed on May 30, 2024, the entire contents of which are incorporated herein by reference.

Claims

1. An ultraviolet-curable inkjet ink that cures when irradiated with ultraviolet light having an emission spectrum with a peak in the wavelength region of 250 nm or more and less than 300 nm, the intensity of which is 50% or more of the total intensity in the wavelength region of 200 nm or more and 800 nm or less, the ink containing at least one polymerizable compound selected from the group consisting of: (A) (meth)acrylate compounds having a carbon atom in which all four single bonds are bonded to atoms other than hydrogen atoms; (B) (meth)acrylate compounds having a dioxane skeleton or a dioxolane skeleton; (C) (meth)acrylate compounds having a cyclic carbonate skeleton or a lactone skeleton; and (D) (meth)acrylamide compounds having a carbon atom in which all four single bonds are bonded to atoms other than hydrogen atoms; and the content of a photopolymerization initiator in the ink is 0.40% by mass or less based on the total mass of the ink.

2. The ink according to claim 1, wherein the content of the photopolymerization initiator is 0.03% by mass or less based on the total mass of the ink.

3. The ink according to claim 1 or 2, which does not contain a photopolymerization initiator.

4. The ink according to any one of claims 1 to 3, wherein the total content of the polymerizable compounds (A) to (D) in the ink is 20 mass % or more based on the total mass of the ink.

5. The ink according to any one of claims 1 to 4, wherein in the polymerizable compound (A), at least one of the atoms to which the carbon atom is bonded is a heteroatom.

6. The ink according to any one of claims 1 to 5, wherein the polymerizable compound (B) includes at least one compound selected from the group consisting of cyclic trimethylolpropane formal acrylate, 2-methyl-2-ethyl-1,3-dioxane-4-methyl acrylate, and 2-methyl-2-isobutyl-1,3-dioxane-4-methyl acrylate.

7. The ink according to any one of claims 1 to 6, wherein the polymerizable compound (C) comprises at least one compound selected from the group consisting of glycerin carbonate acrylate, γ-butyrolactone acrylate, and γ-butyrolactone methacrylate.

8. The ink according to any one of claims 1 to 7, wherein the polymerizable compound (D) comprises at least one compound selected from the group consisting of glycerin carbonate acrylate, γ-butyrolactone acrylate, and γ-butyrolactone methacrylate.

9. An inkjet recording method comprising the steps of: ejecting ink from an inkjet recording head onto a recording medium; and curing the ink by irradiating the ink applied to the recording medium with ultraviolet light having an emission spectrum with a peak in a wavelength region of 250 nm or more and less than 300 nm, the peak intensity being 50% or more of the total intensity in a wavelength region of 200 nm or more and 800 nm or less, wherein the ink contains at least one polymerizable compound selected from the group consisting of: (A) (meth)acrylate compounds having a carbon atom in which all four single bonds are bonded to atoms other than hydrogen atoms; (B) (meth)acrylate compounds having a dioxane skeleton or a dioxolane skeleton; (C) (meth)acrylate compounds having a cyclic carbonate skeleton or a lactone skeleton; and (D) (meth)acrylamide compounds having a carbon atom in which all four single bonds are bonded to atoms other than hydrogen atoms; The inkjet recording method according to claim 1, wherein the content of the photopolymerization initiator in the ink is 0.40% by mass or less based on the total mass of the ink.

10. The ink jet recording method according to claim 9, wherein the ultraviolet light is emitted by an ultraviolet light emitting diode.

11. An inkjet recording apparatus comprising: an inkjet recording head for ejecting ink onto a recording medium; and an ultraviolet irradiation device for irradiating the ink applied to the recording medium with ultraviolet light having a peak in the wavelength region of 250 nm or more and less than 300 nm in its emission spectrum, the intensity of the peak being 50% or more of the total intensity in the wavelength region of 200 nm or more and 800 nm or less, wherein the ink contains at least one polymerizable compound selected from the group consisting of: (A) (meth)acrylate compounds having a carbon atom in which all four single bonds are bonded to atoms other than hydrogen atoms; (B) (meth)acrylate compounds having a dioxane skeleton or a dioxolane skeleton; (C) (meth)acrylate compounds having a cyclic carbonate skeleton or a lactone skeleton; and (D) (meth)acrylamide compounds having a carbon atom in which all four single bonds are bonded to atoms other than hydrogen atoms; and wherein the content of a photopolymerization initiator in the ink is 0.40% by mass or less based on the total mass of the ink.

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