Image forming system

Abstract

The object of the present invention is to provide an image forming system that exhibits good surface and internal curing properties of the coating film, prevents peeling of the coating film from the substrate during cutting, expands the range of usable substrates, and provides good inkjet ejection properties. [Solution] The present invention provides an image forming system in which an ink is applied to a recording medium, and then ultraviolet light is irradiated from an ultraviolet irradiation means to cure the ink and form an image, wherein the ultraviolet irradiation means has a lamp output of 150 W / cm or more per arc length, and the conversion efficiency of the input power to UV-C radiation in the wavelength range of 180 to 280 nm is 10% or more, the ink contains a polymerizable curing compound, and the viscosity of the ink at 80°C is 100 mPa·s or less.

Description

Technical Field

[0001] The present invention relates to an image forming system. In particular, the present invention relates to an image forming system having good surface curability and internal curability of a coating film, preventing peeling of the coating film from a substrate during cutting, expanding the types of substrates that can be used, and having good inkjet injectability.

Background Art

[0002] The initiator contained in UV ink causes odors, and regulations have been strengthened recently. In contrast, for example, Patent Document 1 discloses a lamp with increased output of wavelengths in the UV-C region. According to Patent Document 1, by irradiating with the wavelengths in the UV-C region at the described output, curing can be achieved even in a formulation without an initiator. Also, since it is not necessary to seal the light irradiation path and create anaerobic conditions for irradiating ultraviolet rays, the running cost can be kept low. However, the wavelengths in the UV-C region do not easily reach the deep part of the coating film, so internal curing tends to be insufficient. As a result, there is a problem that the coating film peels off from the substrate during cutting. Also, when the ink has a high viscosity, its permeability to the substrate is poor and its adhesion is inferior, so the types of substrates that can be used are limited.

Prior Art Documents

Patent Documents

[0005] In order to solve the above problems, the inventors investigated the causes of the problems and found that the viscosity of the ink at 80°C was set to 100 mPa·s or less. As a result, even when using wavelengths in the UV-C region, both the surface curability and internal curability of the coating film are good, peeling of the coating film from the substrate during cutting is prevented, the range of usable substrates is expanded, and inkjet extrusion performance is improved. In other words, the above-mentioned problems according to the present invention are solved by the following means.

[0006] 1. An image forming system that forms an image by applying ink to a recording medium and then curing the ink by irradiating it with ultraviolet light from an ultraviolet irradiation means, The ultraviolet irradiation means has a lamp output of 150 W / cm or more per arc length, and a conversion efficiency of 10% or more of the input power to UV-C radiation in the wavelength range of 180-280 nm. The ink contains a polymerizable curing compound, The viscosity of the ink at 80°C is 100 mPa·s or less. An image forming system characterized by the following features.

[0007] 2. The total amount of polymerization initiator in the ink having absorption at a wavelength of 350 nm or less is 0.01% by mass or less. The image forming system according to the first paragraph, characterized in that

[0008] 3. The crosslinking density of the ink is 1.0 or higher. The image forming system according to the first paragraph, characterized in that

[0009] 4. The ink contains a monofunctional monomer, The content of the monofunctional monomer in the ink is 50% by mass or less. The image forming system according to the first paragraph, characterized in that...

[0010] 5. The ink contains wax, The wax content is in the range of 0.1 to 10% by mass. The image forming system according to the first paragraph, characterized in that... [Effects of the Invention]

[0011] The above-described means of the present invention provide an image forming system that exhibits good surface and internal curing properties of the coating film, prevents peeling of the coating film from the substrate during cutting, expands the range of usable substrates, and offers good inkjet extrusion performance. Although the mechanism of action or mechanism of the present invention is not yet clear, it is speculated as follows. It was found that by designing the ink viscosity at 80°C to be 100 mPa·s or less, sufficient levels of both surface and internal curing of the coating film can be obtained. This is presumed to be because lowering the ink viscosity promotes molecular diffusion, increasing the number of collisions between polymerizable groups. Furthermore, obtaining sufficient internal curing of the coating film prevents peeling during cutting. In addition, lowering the ink viscosity improves substrate penetration, expands the types of substrates that can be used, and improves injection properties. [Brief explanation of the drawing]

[0012] [Figure 1] A schematic diagram showing an exemplary configuration of the image forming apparatus used in the present invention. [Modes for carrying out the invention]

[0013] The present invention is an image forming system that forms an image by applying ink to a recording medium, then curing the ink by irradiating it with ultraviolet light from an ultraviolet irradiation means, The ultraviolet irradiation means has a lamp output of 150 W / cm or more per arc length, and a conversion efficiency of 10% or more of the input power to UV-C radiation in the wavelength range of 180-280 nm. The ink contains a polymerizable curing compound, The viscosity of the ink at 80°C is 100 mPa·s or less. This feature is a technical feature common to or corresponding to each of the following embodiments.

[0014] As an embodiment of the present invention, it is preferable that the total amount of polymerization initiators having absorption at a wavelength of 350 nm or less in the ink is 0.01 mass% or less in terms of reducing odor.

[0015] It is preferable that the crosslink density of the ink is 1.0 or more from the viewpoint of the curability of the coating film.

[0016] It is preferable that the ink contains a monofunctional monomer and the content of the monofunctional monomer is 50 mass% or less in the ink from the viewpoint of the curability of the coating film.

[0017] It is preferable that the ink contains wax and the content of the wax is in the range of 0.1 to 10 mass%. Thereby, the droplets of the ink landing on the recording medium can be fixed (pinned), and by suppressing the coalescence of the droplets, a high-definition image can be formed.

[0018] Hereinafter, the present invention, its components, and the forms and embodiments for carrying out the present invention will be described. In the present application, "~" is used in the sense of including the numerical values described before and after it as the lower limit value and the upper limit value.

[0019] [Outline of the Image Forming System of the Present Invention] The image forming system of the present invention is an image forming system that forms an image by applying ink to a recording medium and then irradiating the ink with ultraviolet rays from ultraviolet irradiation means to cure the ink, wherein the ultraviolet irradiation means has a lamp output of 150 W / cm or more per arc length, and the conversion efficiency of the input power to UV-C radiation in the wavelength range of 180 to 280 nm is 10% or more, the ink contains a polymerizable curing compound, and the viscosity of the ink at 80°C is 100 mPa·s or less.

[0020] [Conversion Efficiency] "Conversion efficiency of input power to UV-C radiation in the wavelength range of 180-280 nm" refers to the percentage of input electrical energy (input power) that is converted into UV-C radiation. Therefore, since the lamp output is 150 W / cm or more, it means that UV-C radiation of 15 W / cm or more is irradiated. The aforementioned input power refers to the power input from the power source.

[0021] The conversion efficiency is obtained by calculating the ratio of the irradiation intensity of UV-C radiation to the input power (irradiation intensity / input power). The irradiation intensity can be measured using a measuring instrument such as the "UIT-250" manufactured by Ushio Inc. In order to achieve a conversion efficiency of 10% or more, it is preferable, for example, that the jacket tube of the mercury lamp be made of a material that has high transparency to UV-C radiation. In particular, it is preferable that the material be quartz.

[0022] <Ink viscosity> The viscosity of the ink at 80°C is 100 mPa·s or less, and preferably within the range of 1 to 100 mPa·s. The viscosity of the ink at 80°C can be determined by measuring the temperature dependence of the ink's dynamic viscoelasticity using a rheometer. For example, the ink is heated to 100°C, and while measuring the viscosity with a stress-controlled rheometer, 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 temperature dependence curve of viscosity. As a stress-controlled rheometer, for example, a "Physica MCR301" (cone plate diameter: 75 mm, cone angle: 1.0°, manufactured by Anton Paar) can be used. The viscosity at 80°C is determined by reading the viscosity at 80°C from the viscosity temperature dependence curve.

[0023] The means for achieving a viscosity of 100 mPa·s or less are not particularly limited, but include appropriately adjusting the combination of monomers, pigments, waxes, etc., which are components of the ink.

[0024] <Ink crosslinking density> The crosslinking density of the ink is preferably 1.0 or higher, more preferably 1.5 or higher, even more preferably 2.0 or higher, and particularly preferably 2.5 or higher, from the viewpoint of the curability of the coating film. There is no particular upper limit to the crosslinking density as long as the curability and coating film properties meet the required quality, but it is preferably 10 or lower. In this invention, the crosslinking density of the ink refers to the ratio (percentage) of crosslinking components in the ink. The aforementioned crosslink density is a value obtained by multiplying the double bond density of the polymerizable compound by the double bond consumption rate, and is defined as follows. (Crosslinking density) = (Number of functional groups × Double bond consumption rate (%) / Weight-average molecular weight of polymerizable compound) × (Percentage of polymerizable compound in total solids) The double bond consumption rate is 400 mJ / cm². 2 The coating film before and after exposure was measured using Nicolet 6700 (Thermo Fisher SCIENTIFIC), at 810 cm². -1 It is calculated from the change in peak area corresponding to the nearby C=C double bond. The coating film was formed by applying ink to a thickness of approximately 10 μm, which is similar to the film thickness of actual printed materials.

[0025] The means for achieving a crosslinking density of 1.0 or higher in the ink are not particularly limited and include appropriately adjusting the combination of monomers, pigments, waxes, etc., which are components of the ink.

[0026] [Ink components] The constituent components of the ink used in this invention will be described below. The ink contains a polymerizable curing compound. The ink may also contain wax, colorants, surfactants, and other additives as needed. Furthermore, the ink may contain a polymerization initiator, but the total amount of photopolymerization initiator having absorption at a wavelength of 350 nm or less is preferably 0.01% by mass or less, and more preferably 0% by mass.

[0027] <Polymerizable curing compound> In this invention, "polymerizable curable compound" refers to a compound that crosslinks or polymerizes upon irradiation with UV-C radiation, either in the presence or absence of a polymerization initiator. Therefore, when an ink containing the polymerizable curable compound as a liquid component is irradiated with UV-C radiation, the ink hardens. UV-C radiation is ultraviolet light in the wavelength range of 100 to 280 nm.

[0028] Examples of polymerizable curable compounds include cationic polymerizable compounds, radical polymerizable compounds, or mixtures thereof. In particular, from the viewpoint of polymerization rate and degree of polymerization, radical polymerizable compounds are preferred as the polymerizable curable compound. The polymerizable curable compound may be a monomer, a polymerizable oligomer, a prepolymer, or a mixture thereof.

[0029] A "radical polymerizable compound" is a compound that has an ethylenically unsaturated double bond in its molecule. Radical polymerizable compounds may be monofunctional or polyfunctional compounds.

[0030] Examples of radical polymerizable compounds include, for example, (meth)acrylates, which are unsaturated carboxylic acid ester compounds. In this invention, "(meth)acrylate" refers to acrylate or methacrylate. "(meth)acryloyl group" refers to acryloyl group or methacryloyl group. "(meth)acrylic" refers to acrylic or methacrylic.

[0031] Examples of monofunctional (meth)acrylates include isoamyl (meth)acrylate, stearyl (meth)acrylate, lauryl (meth)acrylate, octyl (meth)acrylate, decyl (meth)acrylate, isomirsutyl (meth)acrylate, isostearyl (meth)acrylate, 2-ethylhexyl-diglycol (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 2-(meth)acryloyloxyethylhexahydrophthalic acid, butoxyethyl (meth)acrylate, ethoxydiethylene glycol (meth)acrylate, methoxydiethylene glycol (meth)acrylate, and methoxypoly Examples 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 succinic acid, 2-(meth)acryloyloxyethyl phthalic acid, 2-(meth)acryloyloxyethyl-2-hydroxyethyl phthalic acid, and t-butylcyclohexyl (meth)acrylate.

[0032] 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, hydroxypivalate neopentyl glycol di(meth)acrylate, and polytetramethylene glycol di(meth)acrylate. Examples include bifunctional (meth)acrylates such as acrylate, polyethylene glycol diacrylate and tripropylene glycol diacrylate; trifunctional (meth)acrylates such as trimethylolpropane tri(meth)acrylate and pentaerythritol tri(meth)acrylate; trifunctional or more (meth)acrylates such as pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, glycerin propoxytri(meth)acrylate and pentaerythritol ethoxytetra(meth)acrylate; oligomers having a (meth)acryloyl group, including polyester acrylate oligomers, and modified products thereof. Examples of the above-mentioned modified products include ethylene oxide-modified (EO-modified) (meth)acrylates with an ethylene oxide group inserted, and propylene oxide-modified (PO-modified) (meth)acrylates with a propylene oxide group inserted.

[0033] Furthermore, a "cationically polymerizable compound" refers to a compound that has a cationic polymerizable group in its molecule. Examples of cationically polymerizable compounds include epoxy compounds, vinyl ether compounds, and oxetane compounds.

[0034] Examples of the epoxy compounds mentioned above include 3,4-epoxycyclohexylmethyl-3′,4′-epoxycyclohexanecarboxylate, bis(3,4-epoxycyclohexylmethyl)adipate, vinylcyclohexene monoepoxide, ε-caprolactone-modified 3,4-epoxycyclohexylmethyl-3′,4′-epoxycyclohexanecarboxylate, 1-methyl-4-(2-methyloxyranyl)-7-oxabicyclo[4,1,0]heptane, 2-(3,4-epoxycyclohexyl-5,5-spiro-3,4-epoxy)cyclohexanone-meth-dioxane and alicyclic epoxy resins such as bis(2,3-epoxycyclopentyl) ether, diglycidyl ether of 1,4-butanediol, diglycidyl ether of 1,6-hexanediol, and glycerin. Examples include aliphatic epoxy compounds such as polyglycidyl ethers of polyether polyols obtained by adding one or more alkylene oxides (such as ethylene oxide and propylene oxide) to aliphatic polyhydric alcohols such as triglycidyl ether of bisphenol A or its alkylene oxide adducts, diglycidyl ethers of hydrogenated bisphenol A or its alkylene oxide adducts, and novolac-type epoxy resins.

[0035] Examples of the vinyl ether compounds mentioned above include monovinyl ether compounds containing 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, as well as di or trivinyl ether compounds containing 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, cyclohexanedimethanol divinyl ether, and trimethylolpropane trivinyl ether.

[0036] Examples of the above oxetane compounds include 3-hydroxymethyl-3-methyloxetane, 3-hydroxymethyl-3-ethyloxetane, 3-hydroxymethyl-3-propyloxetane, 3-hydroxymethyl-3-n-butyloxetane, 3-hydroxymethyl-3-phenyloxetane, 3-hydroxymethyl-3-benzyloxetane, 3-hydroxyethyl-3-methyloxetane, 3-hydroxyethyl-3-ethyloxetane, 3-hydroxyethyl-3-propyloxetane, and 3-hydroxyethyl Examples include tyl-3-phenyloxetane, 3-hydroxypropyl-3-methyloxetane, 3-hydroxypropyl-3-ethyloxetane, 3-hydroxypropyl-3-propyloxetane, 3-hydroxypropyl-3-phenyloxetane, 3-hydroxybutyl-3-methyloxetane, 1,4-bis{[(3-ethyl-3-oxetanyl)methoxy]methyl}benzene, 3-ethyl-3-(2-ethylhexyloxymethyl)oxetane, and di[1-ethyl(3-oxetanyl)]methyl ether.

[0037] The content of the polymerizable curing compound is not particularly limited as long as the viscosity of the ink is 100 mPa·s or less, but it is preferably in the range of 1 to 97% by mass, and more preferably in the range of 30 to 90% by mass, relative to the total mass of the ink. Furthermore, when using monofunctional monomers, from the viewpoint of curability, it is preferable that they make up 50% by mass or less of the total polymerizable curable compound, more preferably 40% by mass or less, and particularly preferable 30% by mass or less.

[0038] <Polymerization initiator> The polymerization initiator can be any agent capable of initiating the polymerization of the polymerizable curable compound mentioned above. However, if the ink absorbs UV-C radiation, becomes excited, initiates the polymerization reaction, and can sufficiently polymerize and cure without a polymerization initiator, then the polymerization initiator is unnecessary.

[0039] When a polymerization initiator is used, for example, if the ink contains a radical polymerizable compound, the polymerization initiator shall be a photoradical initiator. If the ink contains a cationic polymerizable compound, the polymerization initiator shall be a photocationic initiator (photoacid generator).

[0040] Furthermore, polymerization initiators may be used individually or in combination of two or more types. Additionally, both radical polymerization initiators and photoacid generators may be used in combination.

[0041] Examples of radical polymerization initiators include intramolecular bond cleavage type radical polymerization initiators and intramolecular hydrogen abstraction type radical polymerization initiators.

[0042] Examples of intramolecular bond cleavage type radical polymerization initiators include acetophenone-based initiators such as diethoxyacetophenone, 2-hydroxy-2-methyl-1-phenylpropan-1-one, benzyldimethylketal, 1-(4-isopropylphenyl)-2-hydroxy-2-methylpropan-1-one, 4-(2-hydroxyethoxy)phenyl-(2-hydroxy-2-propyl)ketone, 1-hydroxycyclohexylphenyl ketone, 2-methyl-2-morpholino(4-methylthiophenyl)propan-1-one, and 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone; benzoin-based initiators such as benzoin, benzoin methyl ether, and benzoin isopropyl ether; acylphosphine oxide-based initiators; and benzyl and methylphenylglyoxyesters.

[0043] In particular, from the viewpoint of further improving the curability of polymerizable curable compounds, acyl phosphine oxide-based polymerization initiators are preferred.

[0044] Acylphosphine oxide polymerization initiators are not particularly limited and include, for example, 2,4,6-trimethylbenzoyldiphenylphosphine oxide, bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide, and bis-(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide.

[0045] Examples of commercially available acylphosphine oxide polymerization initiators include IRGACURE® 819 (bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide), IRGACURE® 1800 (a mixture of bis-(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide and 1-hydroxycyclohexyl-phenyl ketone in a mass ratio of 25:75), IRGACURE® TPO (2,4,6-trimethylbenzoyldiphenylphosphine oxide), and SpeedCure TPO-L (ethylphenyl(2,4,6-trimethylbenzoyl)phosphine).

[0046] Examples of intramolecular hydrogen abstraction type radical polymerization initiators include benzophenone-based initiators such as benzophenone, o-benzoylmethyl benzoate, 4-phenylbenzophenone, 4,4'-dichlorobenzophenone, hydroxybenzophenone, 4-benzoyl-4'-methyl-diphenyl sulfide, acrylic benzophenone, 3,3',4,4'-tetra(t-butylperoxycarbonyl)benzophenone, and 3,3'-dimethyl-4-methoxybenzophenone; thioxanthone-based initiators such as 2-isopropylthioxanthone, 2,4-dimethylthioxanthone, 2,4-diethylthioxanthone, and 2,4-dichlorothioxanthone; aminobenzophenone-based initiators such as Michler's ketone and 4,4'-diethylaminobenzophenone; 10-butyl-2-chloroacridone, 2-ethylanthraquinone, 9,10-phenanthrenequinone, and camphorquinone.

[0047] Examples of cationic polymerization initiators include photoacid generators. Examples of photoacid generators include sulfonates that generate sulfonic acid, such as B(C6F5)4-, PF6-, AsF6-, SbF6-, and CF3SO3- salts of aromatic onium compounds including diazonium, ammonium, iodonium, sulfonium, and phosphonium; halides that photogenerate hydrogen halides; and iron allene complexes.

[0048] The amount of polymerization initiator can be set arbitrarily, as long as the ink is sufficiently cured by UV-C radiation and the viscosity of the ink is within the range described above. For example, the amount of polymerization initiator can be 0 to 100% of the total mass of the ink. 3% by mass A range within this range is preferred. In particular, in the present invention, the total amount of polymerization initiator having absorption at a wavelength of 350 nm or less is preferably 0.01% by mass or less, and more preferably 0% by mass.

[0049] In addition to the polymerization initiators mentioned above, polymerization accelerators (sensitizers) can also be used in combination. The polymerization accelerator is not particularly limited and includes, for example, trimethylamine, methyldimethanolamine, triethanolamine, p-diethylaminoacetophenone, and ethyl p-dimethylaminobenzoate. Other examples of polymerization accelerators include amine compounds such as p-dimethylaminobenzoate-2-ethylhexyl, N,N-dimethylbenzylamine, and 4,4'-bis(diethylamino)benzophenone. The amount of polymerization accelerator is set appropriately depending on the polymerization initiator used and the amount thereof.

[0050] <Surfactants> The ink may preferably further contain a surfactant as needed. By further containing a surfactant, the surface tension of the ink can be reduced, and the repelling of the ink when it lands on the recording medium can be suppressed.

[0051] As surfactants, any of the following may be used: anionic surfactants such as dialkyl sulfosuccinates, alkylnaphthalene sulfonates, and fatty acid salts; nonionic surfactants such as polyoxyethylene alkyl ethers, polyoxyethylene alkyl allyl ethers, acetylene glycols, and polyoxyethylene-polyoxypropylene block copolymers; cationic surfactants such as alkylamine salts and quaternary ammonium salts; and silicone-based surfactants or fluorine-based surfactants. Among these, silicone-based surfactants or fluorine-based surfactants are preferred from the viewpoint of suppressing ink mixing when printing on recording media made of hydrophobic resins or slow-absorbing recording media such as printing paper.

[0052] Examples of silicone-based surfactants include polyether-modified polysiloxane compounds. Examples of such polyether-modified polysiloxane compounds include KF-351A, KF-352A, KF-642, and X-22-4272 manufactured by Shin-Etsu Chemical Co., Ltd. Other examples of such polyether-modified polysiloxane compounds include BYK307, BYK345, BYK347, and BYK348 manufactured by Big Chemie ("BYK" is a registered trademark of the company), and TSF4452 manufactured by Toshiba Silicone Co., Ltd.

[0053] Fluorine-based surfactants are those in which some or all of the hydrogen atoms bonded to the carbon atoms of the hydrophobic groups in ordinary surfactants are replaced with fluorine. Examples of fluorinated surfactants include Megafac® F from DIC Corporation, Surflon® from AGC Seimi Chemical Corporation, Fluorad® FC from 3M Corporation, Monflor from Imperial Chemical Industries, Zonyls from E.I. DuPont Nemeras & Company, Licowet VPF from Rubewerke Hoechst, and FTERGENT® from Neos Corporation.

[0054] The surfactant content can be set arbitrarily, as long as the viscosity of the ink is within the range described above. Preferably, the surfactant content is 0.001% by mass or more and less than 1.0% by mass, relative to the total mass of the ink.

[0055] <Colorants> The aforementioned ink may preferably further contain a colorant, if necessary. The colorant is not particularly limited and includes pigments and dyes. From the viewpoint of obtaining dispersion stability and weather resistance of the ink, the colorant is preferably a pigment. From the viewpoint of not reducing the sensitivity of the curing reaction by UV-C radiation irradiation, it is preferable to select a compound that does not function as a polymerization inhibitor. Furthermore, colorants may be used individually or in combination of two or more types.

[0056] The pigments are not particularly limited and include, for example, known organic and inorganic pigments. Other examples include resin particles dyed with dyes, commercially available pigment dispersions, and surface-treated pigments (for example, pigments dispersed in an insoluble resin using a dispersion medium, or pigments grafted with resin on the surface). Examples of the pigments mentioned above include those listed in "Dictionary of Pigments" edited by Seijiro Ito (published in 2000) and "Industrial Organic Pigments" by W. Herbst and K. Hunger. Other examples of the pigments mentioned above include those listed in Japanese Patent Publication No. 2002-12607, Japanese Patent Publication No. 2002-188025, Japanese Patent Publication No. 2003-26978, and Japanese Patent Publication No. 2003-342503.

[0057] Examples of red or magenta pigments include: Pigment Red 3, 5, 19, 22, 31, 38, 43, 48:1, 48:2, 48:3, 48:4, 48:5, 49:1, 53:1, 57:1, 57:2, 58:4, 63:1, 81, 81:1, 81:2, 81:3, 81:4, 88, 104, 108, 112, 122, 123, 144, 146, 149, 166, 168, 169, 170, 177, 178, 179, 184, 185, 208, 216, 226, 257; Pigment Violet 3, 19, 23, 29, 30, 37, 50, 88; Pigment Orange Examples include pigments or mixtures thereof selected from 13, 16, 20, and 36. Examples of blue or cyan pigments include pigments selected from Pigment Blue 1, 15, 15:1, 15:2, 15:3, 15:4, 15:6, 16, 17-1, 22, 27, 28, 29, 36, and 60, or mixtures thereof. Examples of green pigments include pigments or mixtures thereof selected from Pigment Green 7, 26, 36, and 50. Examples of yellow pigments include pigments selected from Pigment Yellow 1, 3, 12, 13, 14, 17, 34, 35, 37, 55, 74, 81, 83, 93, 94, 95, 97, 108, 109, 110, 137, 138, 139, 153, 154, 155, 157, 166, 167, 168, 180, 185, and 193, or mixtures thereof. Examples of black pigments include pigments selected from Pigment Black 7, 28, and 26, or mixtures thereof. Titanium dioxide or hollow particles may be used as white pigments.

[0058] Examples of commercially available pigments include Chromofine Yellow 2080, 5900, 5930, AF-1300, 2700L, Chromofine Orange 3700L, 6730, Chromofine Scarlet 6750, Chromofine Magenta 6880, 6886, 6891N, 6790, 6887, Chromofine Violet RE, Chromofine Red 6820, 6830, Chromofine Blue HS-3, 5187, 5108, 5197, 5085N, SR-5020, 5026, 5050, 4920, 4927, 4937, 4824, 4933GN-EP, 4940, 4973, 5205, 5208, 5214, 5221, and 5000P. Chromofine Green 2GN, 2GO, 2G-550D, 5310, 5370, 6830, Chromofine Black A-1103, Seika Fast Yellow 10GH, A-3, 2035, 2054, 2200, 2270, 2300, 2400(B), 2500, 2600, ZAY-260, 2700(B), 2770 Seika Fast Red 8040, C405(F), CA120, LR-116, 1531B, 8060R, 1547, ZAW-262, 1537B, GY, 4R-4016, 3820, 3891, ZA-215, Seika Fast Carmine 6B1476T-7, 1483LT, 3840, 3870, Seika Fast Bordeaux 10B-430, Seika Light Rose R40, Seika Light Violet B800, 7805, Seika Fast Maroon 460N, Seika Fast Orange 900, 2900, Seika Light Blue C718, A612, Cyanine Blue 4933M, 4933GN-EP, 4940, 4973 (manufactured by Dainichi Seika Kogyo Co., Ltd.); KET Yellow 401, 402, 403, 404, 405, 406, 416, 424; KET Orange 501; KET Red 301, 302, 303, 304, 305, 306, 307, 308, 309, 310, 336, 337, 338, 346; KET Blue 101, 102, 103, 104, 105, 106, 111, 118, 124; KET Green 201 (manufactured by Dainippon Ink and Chemicals Co., Ltd.);Colortex Yellow 301, 314, 315, 316, P-624, 314, U10GN, U3GN, UNN, UA-414, U263, Finecol Yellow T-13, T-05, Pigment Yellow1705, Colortex Orange 202, Colortex Red101, 103, 115, 116, D3B, P-625, 102, H-1024, 105C, UFN, UCN, UBN, U3BN, URN, UGN, UG276, U456, U457, 105C, USN, Colortex Maroon601, Colortex BrownB610N, Colortex Violet600, Pigment Red 122, Colortex Blue516, 517, 518, 519, A818, P-908, 510, Colortex Green 402, 403, Colortex Black 702, U905 (manufactured by Sanyo Pigment Co., Ltd.); Lionol Yellow 1405G, Lionol Blue FG7330, FG7350, FG7400G, FG7405G, ES, ESP-S (manufactured by Toyo Ink Co., Ltd.), Toner Magenta E02, Permanent Rubin F6B, Toner Yellow HG, Permanent Yellow GG-02, Hostapean Blue B2G (manufactured by Hoechst Industries, Ltd.); Novoperm P-HG, Hostapean Pink E, Hostapean Blue Examples include B2G (Clariant); Carbon Black #2600, #2400, #2350, #2200, #1000, #990, #980, #970, #960, #950, #850, MCF88, #750, #650, MA600, MA7, MA8, MA11, MA100, MA100R, MA77, #52, #50, #47, #45, #45L, #40, #33, #32, #30, #25, #20, #10, #5, #44, and CF9 (Mitsubishi Chemical Corporation).

[0059] The average particle size of the pigment is not particularly limited, but since finer particles result in better color development, it is preferably in the range of 0.01 to 0.4 μm, and more preferably in the range of 0.02 to 0.2 μm. Furthermore, the maximum particle size of the pigment is preferably about 3 μm, and more preferably about 1 μm. The particle size of the pigment can be adjusted by selecting the type of pigment, dispersant, dispersion medium, dispersion conditions, and filtration conditions. By controlling the particle size of the pigment, clogging of the print head nozzle can be suppressed, and the storage stability, transparency, and curing sensitivity of the ink can be maintained.

[0060] The particle size of pigments can be measured using known measurement methods. Specifically, it can be measured by centrifugal sedimentation light transmission, X-ray transmission, laser diffraction scattering, and dynamic light scattering.

[0061] The pigment or dye content can be set as appropriate, as long as the viscosity of the ink is within the range described above. The content is preferably in the range of 0.1 to 20% by mass, and more preferably in the range of 0.4 to 10% by mass, relative to the total mass of the ink. If the pigment or dye content is 0.1% by mass or more, good color development can be obtained, and if it is 20% by mass or less, an appropriate viscosity of the ink can be obtained.

[0062] <Pigment dispersion> The aforementioned ink is preferably dispersed using a dispersant as needed. When preparing an ink containing pigment, it is preferable to prepare a pigment dispersion containing the pigment and a polymerizable curable compound, and then mix the pigment dispersion with the other components.

[0063] Examples of dispersants include surfactants and polymeric dispersants, with polymeric dispersants being preferred.

[0064] Examples of polymer dispersants include (meth)acrylic resins, styrene-(meth)acrylic resins, 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, high molecular weight unsaturated acid esters, modified polyurethanes, modified polyacrylates, polyether ester-type anionic surfactants, naphthalene sulfonic acid formalin condensate salts, aromatic sulfonic acid formalin condensate salts, polyoxyethylene alkyl phosphate esters, polyoxyethylene nonylphenyl ethers, stearylamine acetate, and pigment derivatives.

[0065] From the viewpoint of improving dispersibility, the pigment may be further used with a dispersion aid as needed.

[0066] The dispersant content is preferably in the range of 10 to 200% by mass relative to the total mass of the pigment. A dispersant content of 10% by mass or more improves the dispersion stability of the pigment. A dispersant content of 200% by mass or less improves the ink ejection stability from the inkjet head.

[0067] When dispersing pigments in polymerizable curable compounds, it is preferable to use a polymerizable curable compound with relatively low viscosity and low molecular weight as the dispersion medium, from the viewpoint of dispersibility and handling with the polymerizable curable compound.

[0068] Pigment dispersions can be prepared by dispersing pigments in polymerizable curable compounds. Pigment dispersion can be carried out using, for example, a ball mill, sand mill, attritor, roll mill, agitator, Henschel mixer, colloid mill, ultrasonic homogenizer, pearl mill, wet jet mill, paint shaker, etc. The above-mentioned dispersant may be added at this time.

[0069] The viscosity of the pigment dispersion at 80°C is preferably in the range of 25 to 500 mPa·s, more preferably in the range of 50 to 300 mPa·s, and particularly preferably in the range of 75 to 200 mPa·s. By setting the viscosity of the pigment dispersion at 80°C to within the range of 25 to 500 mPa·s, the viscosity of the ink at 80°C can be set to 100 mPa·s or less.

[0070] <wax> The wax can function, for example, as a gelling agent. Wax is typically a compound that can gel and temporarily fix (pin) ink droplets that have landed on a recording medium. When ink that has landed on a recording medium gels and is pinned, the wetting and spreading of the ink is suppressed, making it more difficult for adjacent dots to become identical, thus enabling the formation of higher-resolution images. In addition, when the ink is in a gel state, the intrusion of oxygen from the environment into the ink droplets is suppressed, making it less likely for oxygen to inhibit curing, thus enabling the formation of high-resolution images at a faster rate. The inkjet ink may contain only one type of wax, or it may contain two or more types.

[0071] The wax content is preferably in the range of 0.1 to 10.0% by mass relative to the total mass of the ink, more preferably in the range of 0.1 to 5.0% by mass, and particularly preferably in the range of 0.1 to 3.0% by mass. By setting the wax content to 0.5% by mass or more, the pinning properties of the ink are sufficiently enhanced, and a higher-resolution image can be formed. By setting the wax content to 10.0% by mass or less, wax is less likely to precipitate on the surface of the formed image, and the gloss of the image can be made closer to the gloss of an image with other inks. In addition, the ink ejection performance from the inkjet head can be further enhanced.

[0072] From the following perspectives, it is preferable for the wax to crystallize in the ink at a temperature below the ink's gelation temperature. The gelation temperature is the temperature at which, when ink that has been solified or liquefied by heating is cooled, the ink undergoes a phase transition from sol to gel, and the viscosity of the ink changes abruptly. Specifically, the gelation temperature of the ink can be determined by cooling the solified or liquefied ink while measuring its viscosity with a rheometer (for example, Physica MCR300), and the temperature at which the viscosity rapidly increases.

[0073] When wax crystallizes in ink, a structure can be formed in which a polymerizable curing compound is encapsulated within a three-dimensional space created by the plate-like crystallized wax. This structure will be referred to below as the "cardhouse structure." When a cardhouse structure is formed, the liquid polymerizable curing compound is retained within the space, making it more difficult for ink droplets to spread and improving the pinning properties of the ink. Improved pinning properties make it less likely for ink droplets that have landed on the recording medium to coalesce, allowing for the formation of higher-resolution images.

[0074] From the viewpoint of facilitating the formation of a cardhouse structure, it is preferable that the polymerizable cured compound dissolved in the ink and the wax are compatible. Conversely, if the polymerizable cured compound dissolved in the ink and the wax are phase-separated, it may be difficult to form a cardhouse structure.

[0075] The type of wax used in the aforementioned ink is not particularly limited, but examples of waxes suitable for forming a cardhouse structure by crystallization include ketone waxes, ester waxes, petroleum waxes, plant 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 acids, and dimer ols.

[0076] Examples of ketone waxes include dilignoseryl ketone, dibehenyl ketone, distearyl ketone, dieicosyl ketone, dipalmysyl ketone, dilauryl ketone, dimyristyl ketone, myristylpalmysyl ketone, and palmitylstearyl ketone.

[0077] Examples of ester waxes include behenyl behenate, eicosyl eicosanoate, stearyl stearate, palmityl palmitate, cetyl palmitate, myristyl myristate, cetyl myristate, myricyl cerotate, and stearyl stearate. Other examples of ester waxes include 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. Examples of commercially available ester waxes include the EMALEX series, manufactured by Nippon Emulsion Co., Ltd. ("EMALEX" is a registered trademark of the company), the Rikemar series and Poem series, manufactured by Riken Vitamin Co., Ltd. ("Rikemar" and "Poem" are both registered trademarks of the company).

[0078] Examples of petroleum-based waxes include paraffin wax, microcrystalline wax, and petrolactam-based waxes. Examples of plant-based waxes include candelilla wax, carnauba wax, rice wax, wood wax, jojoba oil, jojoba solid wax, and jojoba esters. Examples of animal-derived waxes include beeswax, lanolin, and whale wax. Examples of mineral waxes include montan wax and hydrogenated wax. Examples of modified waxes include montan wax derivatives, paraffin wax derivatives, microcrystalline wax derivatives, 12-hydroxystearic acid derivatives, and polyethylene wax derivatives.

[0079] Examples of higher fatty acids include behenic acid, arachidic acid, stearic acid, palmitic acid, myristic acid, lauric acid, oleic acid, and erucic acid. Examples of higher alcohols include stearyl alcohol and behenyl alcohol. Examples of hydroxystearic acid include 12-hydroxystearic acid.

[0080] 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. Examples of commercially available fatty acid amides include the Nikka Amid series, manufactured by Nippon Chemical Corporation ("Nikka Amid" is a registered trademark of the company), the ITOWAX series, manufactured by Ito Oil Co., Ltd., and the FATTYAMID series, manufactured by Kao Corporation.

[0081] Examples of N-substituted fatty acid amides include N-stearyl stearate and N-oleyl palmitate. Examples of special fatty acid amides include N,N'-ethylenebisstearylamide, N,N'-ethylenebis-12-hydroxystearylamide, and N,N'-xylylenebisstearylamide. Examples of higher amines include dodecylamine, tetradecylamine, and octadecylamine.

[0082] Examples of sucrose fatty acid esters include sucrose stearic acid and sucrose palmitic acid. 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).

[0083] Examples of synthetic waxes include polyethylene wax and α-olefin maleic anhydride copolymer wax. Examples of commercially available synthetic waxes include the UNILIN series, manufactured by Baker-Petrolite ("UNILIN" is a registered trademark of the company).

[0084] Examples of dibenzylidene sorbitol include 1,3:2,4-bis-O-benzylidene-D-glucitol. Examples of commercially available dibenzylidenesorbitol products include Gelol D, manufactured by Shin Nippon Rika Co., Ltd. ("Gelol" is a registered trademark of the company). Examples of commercially available dimer ol products include the PRIPOR series, manufactured by CRODA ("PRIPOR" is a registered trademark of the company).

[0085] Of these waxes, ketone waxes, ester waxes, higher fatty acids, higher alcohols, and fatty acid amides are preferred from the viewpoint of improving pinning properties. From the above viewpoint, compounds represented by the following general formula (G1) and compounds represented by the following general formula (G2) are even more preferred. The compound represented by general formula (G1) will also be referred to below as ketone wax (G1). The compound represented by general formula (G2) will also be referred to below as ester wax (G2).

[0086] The ink may contain only one type of ketone wax (G1) and one type of ester wax (G2), or two or more types. Furthermore, the ink may contain only one of either ketone wax (G1) or ester wax (G2), or both. The inclusion of both ketone wax (G1) and ester wax (G2) is preferable in that it increases the strength of the cardhouse structure.

[0087] General formula (G1): R1-CO-R2 In general formula (G1), R1 and R2 are both linear or branched hydrocarbon groups having 9 to 25 carbon atoms.

[0088] General formula (G2): R3-COO-R4 In general formula (G2), R3 and R4 are both linear or branched hydrocarbon groups having 9 to 25 carbon atoms.

[0089] The ketone wax represented by the above general formula (G1) or the ester wax represented by the above general formula (G2) has a linear or branched hydrocarbon group with 9 or more carbon atoms. Therefore, the crystallinity of the wax is increased, and more space is created in the cardhouse structure. Consequently, polymerizable curable compounds are more easily encapsulated within the space, and the pinning properties of the ink are improved. Furthermore, since the linear or branched hydrocarbon group has 25 or fewer carbon atoms, the solification temperature of the ink does not rise excessively. Therefore, there is no need to excessively heat the ink when ejecting it. From the above viewpoint, it is particularly preferable that R1 and R2 are linear hydrocarbon groups with 11 or more carbon atoms and less than 23 carbon atoms.

[0090] Furthermore, from the viewpoint of raising the gelation temperature of the ink and allowing the ink to gel more rapidly after impact, it is preferable that either R1 or R2, or either R3 or R4, be saturated hydrocarbon groups having 11 to less than 23 carbon atoms. From the above viewpoint, it is even more preferable that both R1 and R2, or both R3 and R4, are saturated hydrocarbon groups having 11 to less than 23 carbon atoms.

[0091] Examples of ketone waxes represented by the above general formula (G1) include dilignoseryl ketone (23-23 carbon atoms), dibehenyl ketone (21-21 carbon atoms), distearyl ketone (17-17 carbon atoms), dieicosyl ketone (19-19 carbon atoms), dipalmityl ketone (15-15 carbon atoms), dimyristyl ketone (13-13 carbon atoms), dilauryl ketone (11-11 carbon atoms), and lauryl myristyl ketone (carbon atoms). This includes prime numbers (11-13), lauryl palmityl ketone (11-15 carbon atoms), myristyl palmityl ketone (13-15 carbon atoms), myristyl stearyl ketone (13-17 carbon atoms), myristyl behenyl ketone (13-21 carbon atoms), palmityl stearyl ketone (15-17 carbon atoms), palmityl behenyl ketone (15-21 carbon atoms), and stearyl behenyl ketone (17-21 carbon atoms). The carbon numbers in parentheses above represent the number of carbon atoms in each of the two hydrocarbon groups separated by the carbonyl group.

[0092] Examples of commercially available ketone waxes represented by general formula (G1) include 18-Pentatriacontanon (distearyl ketone), manufactured by Alfa Aeser; Hentriacontan-16-on (dipalmethyl ketone), manufactured by Alfa Aeser; and Kao Wax T1 (distearyl ketone), manufactured by Kao Corporation.

[0093] Examples of ester waxes represented by general formula (G2) include behenyl behenate (21-22 carbon atoms), eicosyl eicosanoate (19-20 carbon atoms), stearyl stearate (17-18 carbon atoms), palmityl stearate (17-16 carbon atoms), lauryl stearate (17-12 carbon atoms), cetyl palmitate (15-16 carbon atoms), stearyl palmitate (15-18 carbon atoms), and myristyl myristate ( This includes cetyl myristate (13-14 carbon atoms), cetyl myristate (13-16 carbon atoms), octyldodecyl myristate (13-20 carbon atoms), cetyl caprate (9-16 carbon atoms), stearyl oleate (17-18 carbon atoms), stearyl erucate (21-18 carbon atoms), stearyl linoleate (17-18 carbon atoms), behenyl oleate (18-22 carbon atoms), and arachidyl linoleate (17-20 carbon atoms). The carbon numbers in parentheses above represent the carbon numbers of each of the two hydrocarbon groups separated by the ester group.

[0094] Examples of commercially available ester waxes represented by general formula (G2) include Unistar M-2222SL, Nissan Electrol WEP-3 (behenyl behenate) and Sperm Acetate (cetyl myristate), manufactured by NOF Corporation ("Unistar" is a registered trademark of the company), Excepearl SS (stearyl stearate) and Excepearl MY-M (myristyl myristate), manufactured by Kao Corporation ("Excepearl" is a registered trademark of the company), EMALEX CC-18 (stearyl stearate) and EMALEX CC-10 (cetyl caprate), manufactured by Nippon Emulsion Co., Ltd., and Amlepus PC (cetyl palmitate), manufactured by Higher Alcohol Industry Co., Ltd. ("Amlepus" is a registered trademark of the company). Since these commercially available products are often mixtures of two or more types, they may be separated and purified as needed before being included in the ink.

[0095] <Fixing resin> The ink may preferably further contain a fixing resin as needed. By including a fixing resin, the pigment adheres more easily to the recording medium, and the ink coating has improved abrasion resistance and blocking resistance.

[0096] Examples of fixing resins include (meth)acrylic resins, epoxy resins, polysiloxane resins, maleic acid resins, vinyl resins, and polyamide resins. Furthermore, examples of the aforementioned fixing resins include nitrocellulose, cellulose acetate, ethylcellulose, ethylene-vinyl acetate copolymers, urethane resins, polyester resins, and alkyd resins.

[0097] The fixing resin content is preferably in the range of 1 to 20% by mass, and more preferably in the range of 1 to 10% by mass, based on the solid content mass of the total ink. A fixing resin content of 1% by mass or more provides abrasion resistance and blocking resistance of the ink coating. A fixing resin content of 20% by mass or less improves the ink ejection stability from the inkjet head.

[0098] <Other additives> In addition to the above components, the ink preferably contains, as needed, polymerization inhibitors, polysaccharides, viscosity modifiers, resistivity modifiers, film-forming agents, ultraviolet absorbers, antioxidants, fade inhibitors, mold inhibitors, rust inhibitors, and the like.

[0099] [Ink properties] The ink has a viscosity of 100 mPa·s or less at 80°C, as mentioned above.

[0100] The ink, by containing the aforementioned wax, has a phase transition temperature that undergoes a sol-gel phase transition within the range of 40 to 70°C. Because the ink's phase transition temperature is above 40°C, it quickly thickens after landing on the recording medium, making it easily immobilized. Furthermore, because the ink's phase transition temperature is below 70°C, ink ejection stability is achieved when the ejection temperature is set to the commonly used 80°C.

[0101] The viscosity and phase transition temperature of an ink at 80°C can be determined by measuring the temperature dependence of the ink's dynamic viscoelasticity using a rheometer. For example, the ink is heated to 100°C, and while measuring the viscosity with a stress-controlled rheometer (Anton Paar, Physica MCR301 (cone plate diameter: 75 mm, cone angle: 1.0°)), 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 dependence curve. The viscosity at 80°C is determined by reading the viscosity at 80°C on the viscosity temperature dependence curve. The phase transition temperature is determined as the temperature at which the viscosity becomes 200 mPa·s on the viscosity temperature dependence curve.

[0102] [How to prepare ink] The ink according to the present invention is not particularly limited in its preparation means or conditions, and can be prepared by mixing the polymerizable curable compound and any other components under heating. In this case, it is preferable to filter the resulting mixture with a predetermined filter. Furthermore, when preparing an ink containing a pigment and a dispersant, a pigment dispersion in which the pigment and dispersant are dispersed in a polymerizable curable compound or the like may be prepared in advance, and the remaining components may be added to this and mixed while heating.

[0103] [Image forming method] The image forming method according to the present invention is an image forming method using an ink having a viscosity of 100 mPa·s or less at 80°C. Specifically, the image forming method comprises the steps of: ejecting the ink from an inkjet head; directly depositing the ink onto a recording medium; and irradiating the ink deposited on the recording medium with UV-C radiation to cure the ink and form a printed layer. Furthermore, the image forming method preferably includes a step of performing a surface modification process on the printed layer. Inkjet heads will also be referred to simply as "heads" below.

[0104] The "process of forming a printed layer" refers to all processes for forming a printed layer on a recording medium. Specifically, it includes the process of ejecting the ink from the inkjet head and the process of directly depositing the ink onto the recording medium, as well as the process of curing the ink by irradiating it with UV-C radiation, and the process of drying the ink as needed. The term "printed layer" refers to an ink image layer, such as characters, pictures, and photographs, reproduced on a recording medium using the aforementioned ink based on image data.

[0105] "Using ink with a viscosity of 100 mPa·s or less" means that when dispensing and landing droplets of multiple types of ink with different compositions (e.g., types or amounts of colorants) to form a multi-colored image, all of the inks among the multiple types of ink must have a viscosity of 100 mPa·s or less. Furthermore, from the viewpoint of preventing color mixing, it is preferable that all of the dispensed inks are the inks according to the present invention.

[0106] The image forming method according to the present invention uses an inkjet printer. Generally, inkjet printers are classified into on-demand and continuous types based on their ink ejection method. The inkjet printer used in this invention may be of either type. Examples of on-demand inkjet printers include electromechanical conversion methods such as single-cavity, double-cavity, bender, piston, shear-mode, and shared-wall types, as well as electro-thermal conversion methods such as thermal inkjet and bubble jet types. Bubble Jet is a registered trademark of Canon Inc.

[0107] Inkjet printers are classified into scanning type and line type depending on the method of scanning the print head. The inkjet printer used in this invention may be of either type. Either type may be selected depending on the resolution of the recorded material (image) and the recording speed. The ink according to the present invention can be quickly fixed, so high-quality recorded material can be obtained even in high-speed recording using the line type.

[0108] The recording medium can be any recording medium capable of forming an image using the ink according to the present invention. Examples of recording media include non-absorbent recording media composed of plastics including polyester, polyvinyl chloride, polyethylene, polyurethane, polypropylene, acrylic resin, polycarbonate, polystyrene, acrylonitrile-butadiene-styrene copolymer, polyethylene terephthalate, and polybutadiene terephthalate; non-absorbent inorganic recording media such as metals and glass; and papers. Examples of such papers include coated paper for printing and coated paper B for printing. In particular, the ink according to the present invention is preferably used with OK Topcoat or Maricoat for paper, from the viewpoint that varnish processing is preferred as a surface modification process.

[0109] (1) The process of ejecting ink from the print head. In the process of ejecting ink from the print head, ink droplets are ejected from the nozzles of the print head. As mentioned above, the ejection method is not particularly limited.

[0110] The ink temperature inside the print head is preferably in the range of 40 to 100°C, and more preferably in the range of 50 to 85°C. A temperature of 40°C or higher ensures stable ink ejection from the print head. Furthermore, a temperature of 100°C or lower suppresses the evaporation of ink components, thereby reducing the thermal load on the print head.

[0111] Furthermore, when the ink contains wax, it is preferable to raise the temperature of the ink inside the print head to a range of 10°C or more and less than 40°C above the ink's gelation temperature. By raising the ink temperature inside the print head to 10°C or higher above the gelation temperature, the ink will not gel inside the print head or on the nozzle surface, allowing for smooth ink ejection. Additionally, by keeping the ink temperature inside the print head below 40°C above the gelation temperature, the thermal load on the print head can be reduced. In particular, print heads using piezoelectric elements are prone to performance degradation due to thermal load, so it is preferable to keep the ink temperature inside the print head within the above range.

[0112] The method of heating the ink is not particularly limited. For example, the ink can be heated by heating at least one of the following components of the head carriage: the ink tank, supply pipe, and pre-chamber ink tank immediately before the head, the filtered piping, and the piezo head, using a panel heater, ribbon heater, and warm water.

[0113] From the viewpoint of recording speed and image quality, the amount of ink droplets ejected is preferably in the range of 2 to 20 pL.

[0114] (2) The process of directly depositing ink onto the recording medium In the process of directly depositing ink onto the recording medium, the ink ejected in the previous process is deposited onto the recording medium. More specifically, the ink ejected from the nozzle of the print head directly deposits onto the recording medium without passing through an intermediate transfer medium.

[0115] The recording medium can be any medium on which an image can be formed by an inkjet method. Examples of such recording media include absorbent media including coated and uncoated papers, such as art paper, coated paper, lightweight coated paper, lightly coated paper, and cast paper; non-absorbent recording media composed of plastics, such as polyester, polyvinyl chloride, polyethylene, polyurethane, polypropylene, acrylic resin, polycarbonate, polystyrene, acrylonitrile-butadiene-styrene copolymer, polyethylene terephthalate, and polybutadiene terephthalate; and non-absorbent inorganic recording media such as metals and glass.

[0116] Examples of plastic films include PP film, PET film, OPS film, OPP film, ONy film, PVC film, PE film, and TAC film. Other plastics include polycarbonate, acrylic resin, ABS, polyacetal, PVA, and rubbers.

[0117] From the viewpoint of ink immobilization, it is preferable that the temperature of the recording medium be lower than the temperature at which the ink is ejected, and 60°C or lower. Because the temperature of the recording medium is below 60°C, the ink that lands on the surface quickly thickens and becomes fixed.

[0118] (3) A process of curing the ink by irradiating it with UV-C radiation. In the process of curing the ink by irradiating it with UV-C radiation, UV-C radiation is irradiated onto the surface of the recording medium to which the ink has landed.

[0119] The UV-C radiation light source is preferably a mercury lamp or a metal halide lamp. The aforementioned light source has a lamp output of 150 W / cm or more per arc length, and a conversion efficiency of 10% or more of the input power to UV-C radiation in the wavelength range of 180 to 280 nm.

[0120] The speed at which the recording medium is transported when irradiated with a light source (linear velocity) is preferably within the range of 15 to 30 m / min. By setting the linear velocity within this range, the polymerizable curable compound can be sufficiently polymerized and crosslinked, and it is also superior in terms of high speed. The cumulative dose of UV-C radiation irradiated onto the ink is not particularly limited as long as the ink's curability and other quality requirements are met, but is generally between 100 and 2000 mJ / cm². 2 It is preferable that it be within this range. The cumulative irradiation dose can be calculated by the product of the irradiation time per unit area irradiated to the ink and the irradiation dose per unit area irradiated to the ink. Furthermore, it is preferable to set the irradiation time of UV-C radiation applied to the ink so as to satisfy the cumulative irradiation dose.

[0121] (4) The process of drying the ink In the ink drying process, the ink is irradiated with UV-C radiation, and then dried as needed. The drying method may be air drying or heat drying, but heat drying is preferred.

[0122] (5) Process of performing surface modification on the printed layer In the process of performing surface modification on the printed layer, after the process of forming the printed layer, which includes steps (1) to (4) above, surface modification is performed on the printed layer for aesthetic and protective purposes. Since some inks do not require drying, surface modification can be performed immediately after recording.

[0123] Surface modification processes are not particularly limited and include varnishing, lamination, and press coating.

[0124] Varnishing is a technique that involves coating the image surface of a recording with varnish. Varnishing is performed to give the image surface a glossy finish, thereby enhancing its premium appearance, and to improve its scratch resistance and chemical resistance.

[0125] Commercially available varnishes can be used, including PL-LV varnish for digital printing, KM-2 and KM-3 varnishes specifically for KM-EP, UV roll coat varnishes RI-13, RI-13-K2, RI-16, RI-FX-3, RI-XG33, CX-1, CX-2, and CX-3, UV coat varnish AT-B, UV VECTA coat varnish PC-3KW2 (manufactured by T&K TOKA), UV gloss varnish ULTRASHEEN UV-9021A (manufactured by ACTEGA), UV matte varnish 5070E, soft touch ULTRASHEEN UV-XT3037 (manufactured by KUSTOM&GROU), Plus Size (registered trademark) OP-5267 and OP-5275 (manufactured by Go-O Chemical Co., Ltd.), FD Clear Coat PC and C-YS (manufactured by Toyo Ink Co., Ltd.), Brightone (registered trademark) TUV (manufactured by Sakata Inx Co., Ltd.), and others.

[0126] The method of applying the varnish is not particularly limited and may be by inkjet or other methods. Other methods besides inkjet printing include bar coating, spray coating, curtain coating, roll coating, screen printing, offset printing, gravure printing, methods using plates such as letterpress and intaglio plates, and other methods that do not use plates. Among these, screen printing, offset printing, gravure printing, or bar coating are preferred from the viewpoint of ease of operation and uniform application.

[0127] From the viewpoint of simplifying the apparatus configuration and reducing the cost of image formation, the method for applying varnish is particularly preferably an inkjet method.

[0128] Furthermore, if the varnish is an active-ray curing type varnish, after applying the varnish to the printing layer, the varnish is irradiated with active rays to cure it. From the viewpoint of simplifying the setup of the apparatus and efficiently forming images, it is preferable that the conditions for irradiating the varnish with active rays be the same as the conditions for irradiating the ink with UV-C radiation.

[0129] When the process of forming the printed layer and the process of performing surface modification on the printed layer are performed by different machines, or when these processes are performed at different locations on the same machine, the process of transporting the recording medium on which the printed layer has been formed may be included between the process of forming the printed layer and the process of performing surface modification on the printed layer. From the viewpoint of increasing speed, the transport speed of the recording medium on which the printed layer is formed is preferably in the range of 30 to 120 m / min.

[0130] <Image forming apparatus> Next, an image forming apparatus will be described. While the following description focuses on an apparatus that performs image forming using a single-pass method, the ink according to the present invention can also be used in an apparatus that performs image forming using a scanning method.

[0131] Figure 1 is a schematic diagram showing an exemplary configuration of an image forming apparatus 100 used in the present invention. As shown in Figure 1, the image forming apparatus 100 includes an inkjet head 110, a transport path 120, a UV-C radiation irradiation unit (ultraviolet irradiation means) 130, and a temperature control unit 140. In Figure 1, arrow A indicates the transport direction of the recording medium 150. The inkjet head 110 and the UV-C radiation irradiation unit 130 are arranged in this order in contact with the transport path 120 from the upstream side to the downstream side in the transport direction of the recording medium. The image forming apparatus 100 may also have an oxygen concentration adjustment unit (not shown) for adjusting the oxygen concentration when irradiating the ink with UV-C radiation.

[0132] As shown in Figure 1, the image forming apparatus 100 further includes an ink channel 170 and an ink tank 170. The ink channel 170 is connected to a head carriage 160 that houses an inkjet head 110. The ink tank 170 stores the ink supplied through the ink channel 170.

[0133] The head carriage 160 houses each of the inkjet heads 110. The head carriage 160 includes inkjet heads for yellow (Y), magenta (M), cyan (C), and black (K). The head carriage 160 is fixedly positioned, for example, to cover the entire width of the recording medium 150.

[0134] The ink tank 180 contains the inks of each color according to the present invention. Ink is supplied to the inkjet head 110 from the ink tank 180.

[0135] The inkjet head 110 ejects ink supplied from the ink tank 180 to the head carriage 160 via the ink channel 170. At this time, the ink is heated via the ink tank 180, ink channel 170, head carriage 160, and inkjet head 110. As mentioned above, the temperature of the ink ejected by heating is preferably in the range of 40 to 100°C, and more preferably in the range of 50 to 85°C.

[0136] The UV-C radiation irradiation unit 130 covers the entire width of the recording medium 150 and is positioned downstream of the head carriage 160 in the transport direction A of the recording medium 150. The UV-C radiation irradiation unit 130 is ejected by the inkjet head 110 and irradiates the ink droplets that land on the recording medium 150 with UV-C radiation, thereby curing the droplets.

[0137] The temperature control unit 140 is located on the underside of the recording medium 150. As described above, it adjusts the surface temperature of the recording medium 150 to be lower than the temperature at which the ink is ejected and to 60°C or lower. The temperature control unit 140 can be, for example, various heaters. As a result, the ink according to the present invention quickly fixes after landing on the recording medium 150, resulting in high pinning properties and suppression of the leakage of polymerizable curing compound from the formed dots.

[0138] Furthermore, the oxygen concentration adjustment unit (not shown) adjusts the oxygen concentration of the atmosphere surrounding the ink surface that has landed on the recording medium 150 when UV-C radiation is irradiated by the UV-C radiation irradiation unit 130. [Examples]

[0139] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these. In the following examples, unless otherwise specified, the operations were carried out at room temperature (25°C). Unless otherwise specified, "%" and "parts" mean "mass%" and "parts by mass," respectively.

[0140] [Ink materials] The following materials were used as ink ingredients. The viscosity of the polymerizable curable compounds listed below was determined using the same method as that used for the viscosity of the ink at 80°C, as described later.

[0141] [Table 1]

[0142] Wax: EMALEX CC-18 (manufactured by Nippon Emulsion Co., Ltd.) (viscosity at 80°C is 0 mPa·s) Polymerization inhibitor: Irgastab (registered trademark) UV-10 (manufactured by BASF Japan) Surfactant: TSF-4452 (manufactured by Momentive Performance Materials Japan LLC) (viscosity at 80°C is 0 mPa·s) Polymerization initiator: SpeedCure TPO-L (manufactured by Sartomer)

[0143] (Preparation of pigment dispersion) The following ingredients were placed in a stainless steel beaker and heated and stirred for 1 hour on a 65°C hot plate. Pigment dispersant (Adisper® PB824, manufactured by Ajinomoto Fine Techno Co., Ltd.) 9.0 parts by mass Polymerizable curing compound (dipropylene glycol diacrylate) 70.0 parts by mass Polymerization inhibitor (Irgastab® UV-10, manufactured by BASF Japan) 0.02 parts by mass

[0144] After the above mixture was cooled to room temperature, 21.0 parts by mass of Pigment Red 122 (manufactured by Dainichi Seika Kogyo Co., Ltd., Chromofine Red 6112JC) was added. Next, the mixture was placed in a glass bottle with 200 g of 0.5 mm diameter zirconia beads, sealed tightly, and dispersed in a paint shaker for 8 hours. After that, the zirconia beads were removed to prepare pigment dispersion 1. The viscosity of pigment dispersion 1 at 80°C was 20 mPa·s. The viscosity of pigment dispersion 1 was determined using the same method as the viscosity of the ink described later.

[0145] [Preparation of Ink 1] The following ingredients were placed in a stainless steel beaker and stirred for 1 hour while being heated on a hot plate at 80°C. Ink 1 was obtained by filtering the resulting solution through an ADVATEC Teflon® 3μm membrane filter while heating it.

[0146] Polymerizable curing compound: Miramer M142 50.0 parts by mass Polymerizable curing compound: Miramer M220 10.0 parts by mass Polymerizable curing compound: Miramer M300 29.0 parts by mass Wax: EMALEX CC-18 8.0 parts by mass Polymerization inhibitor: Irgastab (registered trademark) UV-10 2.0 parts by mass Surfactant: TSF-4452 1.0 parts by mass

[0147] [Preparation of inks 2-24] Inks 2-24 were prepared in the same manner as ink 1, except that the types and proportions of each component were changed as shown in the table below. A "-" indicates that no component was added.

[0148] [Viscosity of ink at 80°C] The viscosity of the ink prepared above at 80°C was measured using the method described above. Specifically, it was determined by measuring the temperature dependence of the dynamic viscoelasticity of the ink using a rheometer. The ink was heated to 100°C, and while measuring the viscosity with a stress-controlled rheometer, the ink was 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 temperature dependence curve of viscosity. The stress-controlled rheometer used was a "Physica MCR301" (cone plate diameter: 75 mm, cone angle: 1.0°, manufactured by Anton Paar). The viscosity at 80°C was determined by reading the viscosity at 80°C from the viscosity temperature dependence curve.

[0149] [Crosslink density] The crosslinking density of the ink prepared above was calculated using the method described above.

[0150] [Table 2]

[0151] [Table 3]

[0152] [Table 4]

[0153] [Image Formation] The ink prepared as described above was introduced into an inkjet head (HA1024 model, manufactured by Konica Minolta, Inc.). The ink was ejected and deposited onto a recording medium under the following conditions: print width 100mm x 100mm, resolution 720 x 720 dpi, voltage 16V, ambient temperature 25°C, and ambient humidity 55%. Afterward, the deposited ink droplets were irradiated with a mercury lamp. The lamp output (W / cm), the linear velocity for transporting the recording medium (m / min), and the conversion efficiency to UV-C radiation were as shown in Tables II to IV. A solid image with 100% print density was then formed. Printing paper (OK Topcoat®, manufactured by Oji Paper Co., Ltd.) was used as the recording medium. The conversion efficiency to UV-C radiation was obtained by calculating the ratio of the UV-C radiation irradiation intensity to the input power from the power supply (irradiation intensity / input power). The irradiation intensity was measured using a measuring instrument such as the "UIT-250" manufactured by Ushio Inc.

[0154] [evaluation] <Surface hardening> The solid color images obtained above were rubbed with an abrasion resistance tester (Fukuda Kikai Co., Ltd. RT-300, 500g load) and evaluated according to the following criteria based on the degree of color fading. (standard) Rank 5: No color fading even after 30 rubs (no practical problems) Rank 4: No color fading even after 20 rubs (no practical problems) Rank 3: No color fading even after 10 rubs (no practical problems) Rank 2: Color faded after 10 rubs. Rank 1: Color faded after 5 rubs.

[0155] <Cutting and peeling> The solid color image obtained above was cut using an automatic cutting machine (Horizon PC-P430). The cutting edge on the sharp side of the cutting blade was rubbed 10 times with a finger, and the peeling distance was measured using an optical microscope (NIKON LV-100D). The peeling distance refers to the distance from the edge (edge ​​of the paper) where the cutting blade made contact to the ink-printed surface. Furthermore, less peeling indicates better internal curing properties of the ink. (standard) Rank 5: No peeling Rank 4: Peeling distance is less than 10 μm (peeling is not visible, no practical problems) Rank 3: Peeling distance between 10 μm and 80 μm (peeling is not visible, no practical problems) Rank 2: Peeling distance is between 80 μm and 200 μm (peeling is visible) Rank 1: Peeling distance of 200 μm or more (peeling is visible)

[0156] <Applicable base material type> For coated paper, matte paper, high-quality paper, plastic substrate, and coated cardboard, an inkjet recording device can handle 10 g / m² of paper. 2 A solid color image was created. Then, a peel test was performed on the obtained solid color image using cellophane tape (registered trademark). Specifically, cellophane tape was adhered to the printed surface and then peeled off. Adhesion was considered good (OK) if peeling occurred from the inside of each sheet of paper, or if there was no peeling at all. Adhesion was considered poor (NG) if peeling occurred at the interface between the ink and each sheet of paper. For the inkjet recording device, an inkjet head (HA1024 model, manufactured by Konica Minolta, Inc.) was used. Details of each form are as follows. Coated paper (Aurora Coat, manufactured by Nippon Paper Industries Co., Ltd.) Matte paper (Shira-oi Matte, manufactured by Nippon Paper Industries Co., Ltd.) High-quality paper (Shira-oi, manufactured by Nippon Paper Industries Co., Ltd.) Plastic substrate (Peach Coat, manufactured by Daio Paper Products Co., Ltd.) Coated cardboard (OK Ball, manufactured by Oji Materia Co., Ltd.) (standard) Rank 3: Adhesion is OK for all 5 types of paper: coated paper, matte paper, high-quality paper, plastic substrate, and coated cardboard (no practical problems). Rank 2: Adhesion is OK for all four types of paper: coated paper, matte paper, high-quality paper, and plastic substrate (no practical problems). Rank 1: Adhesion is good with three types of paper: coated paper, matte paper, and high-quality paper.

[0157] <Injection properties> Ink was ejected using an inkjet recording device, and the presence or absence of nozzle defects and misaligned ejection was visually observed. Each inkjet ink was then evaluated according to the following criteria. (standard) Rank 4: No nozzle defects or misaligned discharge were observed. (No practical problems) Rank 3: Out of 1024 nozzles, 1 to 5 nozzles showed defects such as missing nozzles or bent discharge. (No practical problems) Rank 2: Out of 1024 nozzles, 6 to 9 nozzles showed defects such as missing nozzles or bent discharge. (No practical problems) Rank 1: Out of 1024 nozzles in total, 10 or more nozzles showed defects such as missing nozzles or bent discharge.

[0158] [Table 5]

[0159] As shown in the results above, when using the ink of the present invention, the surface hardening and internal hardening properties of the coating film are better than those of the comparative example ink, and peeling of the coating film from the substrate during cutting can be prevented. Furthermore, it can be seen that the ink of the present invention can expand the range of usable substrates and exhibits good inkjet ejection properties. [Explanation of symbols]

[0160] 100 Image forming apparatus 110 Inkjet Heads 120 Conveyor paths 130 Active ray irradiation section 140 Temperature Control Unit 150 recording media 160 Head Carriage 170 Ink channel 180 Ink Tanks

Claims

1. An image forming system that forms an image by applying ink to a recording medium and then curing the ink by irradiating it with ultraviolet light from an ultraviolet irradiation means, The ultraviolet irradiation means has a lamp output of 150 W / cm or more per arc length, and the input power has a wavelength range of 180 to 280 nm. The conversion efficiency to UV-C radiation is 10% or more. The ink contains a polymerizable curing compound, The viscosity of the ink at 80°C is 100 mPa·s or less. An image forming system characterized by the following features.

2. The total amount of polymerization initiator in the ink having absorption at a wavelength of 350 nm or less is 0.01% by mass or less. The image forming system according to claim 1.

3. The crosslinking density of the ink is 1.0 or higher. The image forming system according to claim 1.

4. The aforementioned ink contains a monofunctional monomer, The content of the monofunctional monomer in the ink is 50% by mass or less. The image forming system according to claim 1.

5. The aforementioned ink contains wax, The wax content is within the range of 0.1 to 10% by mass. The image forming system according to claim 1.

Images