Image formation methods

Abstract

To provide an image forming method that allows for appropriate post-processing regardless of the transport speed of the recording medium. [Solution] The image forming method of the present invention includes the steps of ejecting inkjet ink onto a recording medium and curing the inkjet ink ejected onto the transported recording medium by irradiating it with active light in an active light irradiation area, wherein in the step of curing the inkjet ink, when the active light irradiation area is divided into two parts in the center of the transport direction of the recording medium, the active light is irradiated such that the accumulated amount of active light in the downstream part is greater than the accumulated amount of active light in the upstream part.

Description

[Technical Field]

[0001] The present invention relates to an image forming method. [Background technology]

[0002] Inkjet image formation methods are used in various fields because they can form images simply and inexpensively. One known inkjet image formation method involves depositing inkjet ink onto a recording medium and then curing the inkjet ink by irradiating it with active light (see, for example, Patent Document 1). In recent years, in the field of digital commercial printing using light-curing inkjet inks, increasing the transport speed of the recording medium is being considered to improve productivity. In inkjet printing, as the transport speed of the recording medium increases, the time from when the inkjet ink hits the medium until the light irradiation is completed is shortened, thus requiring high pinning performance.

[0003] Patent Document 1 describes an image forming method in which an active light-curable inkjet ink is deposited onto a recording medium and then cured by irradiating it with ultraviolet light. In the image forming method described in Patent Document 1, the pinning performance is enhanced by adding a gelling agent to the active light-curable inkjet ink. Furthermore, in the transport direction of the recording medium, the irradiated ultraviolet light has a unimodal irradiation distribution. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] International Publication No. 2013 / 161328 [Overview of the project] [Problems that the invention aims to solve]

[0005] However, in the image forming method described in Patent Document 1, the amount of gelling agent added to the active light-curable inkjet ink limits the effectiveness of the gelling agent due to its compatibility with other materials and the injection viscosity of the inkjet ink. Furthermore, in the image forming method described in Patent Document 1, there is room for consideration regarding the pinning performance of the active light-curable inkjet ink when the transport speed of the recording medium becomes faster, as has been the case in recent years. Thus, the image formed on the recording medium may not have a fully cured interior due to the inkjet ink, making it impossible to perform post-processing such as cutting after image formation.

[0006] Therefore, the object of the present invention is to provide an image forming method that allows for appropriate post-processing regardless of the transport speed of the recording medium. [Means for solving the problem]

[0007] An image forming method according to one embodiment of the present invention is an image forming method for forming an image on a recording medium being transported from upstream to downstream using an active light-curable inkjet ink containing a polymerizable compound, a photopolymerization initiator, and a gelling agent, which undergoes a phase transition with temperature, and comprises the steps of: ejecting the inkjet ink onto the recording medium; and curing the inkjet ink ejected onto the transported recording medium by irradiating it with active light in an active light irradiation area, wherein in the step of curing the inkjet ink, when the active light irradiation area is divided into two parts in the center of the transport direction of the recording medium, the active light is irradiated such that the integrated amount of active light in the downstream part is greater than the integrated amount of active light in the upstream part. [Effects of the Invention]

[0008] According to the present invention, an image forming method is provided that allows for appropriate post-processing regardless of the transport speed of the recording medium. [Brief explanation of the drawing]

[0009] [Figure 1] FIG. 1 is a flowchart of an image forming method according to an embodiment of the present invention. [Figure 2] FIGS. 2A to 2C are graphs schematically showing the illuminance distribution in the step of curing the inkjet ink of the image forming method according to an embodiment of the present invention. [Figure 3] FIG. 3 is a schematic diagram showing an example of an image forming apparatus.

MODE FOR CARRYING OUT THE INVENTION

[0010] Hereinafter, an embodiment of the present invention will be described in detail with reference to the accompanying drawings. Here, first, the inkjet ink will be described, and then an image forming method and an image forming apparatus using the inkjet ink will be described.

[0011] (Configuration of Inkjet Ink) The inkjet ink is an active energy ray-curable inkjet ink and contains a photopolymerizable compound, a photoinitiator, and a gelling agent.

[0012] The photopolymerizable compound is a compound that crosslinks or polymerizes by irradiation with active energy rays. The photopolymerizable compound may be used alone or in combination of two or more kinds. The photopolymerizable compound is a radical-polymerizable compound or a cationic-polymerizable compound, and a radical-polymerizable compound is preferable.

[0013] The radical-polymerizable compound is a compound (monomer, oligomer, polymer or a mixture thereof) having a radically polymerizable ethylenically unsaturated bond. The radical-polymerizable compound may be used alone or in combination of two or more kinds.

[0014] Examples of compounds having ethylenically unsaturated bonds that can be radically polymerized include unsaturated carboxylic acids and their salts, unsaturated carboxylic acid ester compounds, unsaturated carboxylic acid urethane compounds, unsaturated carboxylic acid amide compounds, acrylonitrile, styrene, unsaturated polyesters, unsaturated polyethers, unsaturated polyamides, and unsaturated urethanes. Examples of unsaturated carboxylic acids include (meth)acrylic acid, itaconic acid, crotonic acid, isocrotonic acid, and maleic acid.

[0015] The radical polymerizable compound is preferably an unsaturated carboxylic acid ester compound, and more preferably a (meth)acrylate compound. The (meth)acrylate compound may be a monomer, oligomer, a mixture of monomer and oligomer, a modified product, or an oligomer having polymerizable functional groups. The (meth)acrylate compound may be used alone or two or more in combination. From the viewpoint of improving curability, it is preferable that the (meth)acrylate compound has two or more (meth)acrylate groups in one molecule.

[0016] From the viewpoint of improving compatibility with highly hydrophobic gelling agents, it is preferable that the (meth)acrylate compound contains a (meth)acrylate compound with a ClogP value of 4.0 to 7.0. A (meth)acrylate compound with a ClogP value of 4.0 to 7.0 contains (-C(CH3)H-CH2-O-) in its molecule. m (Meth)acrylate compounds having a structure represented by (m is an integer from 3 to 14) with three or more functions, or (meth)acrylate compounds having a cyclic structure within the molecule with two or more functions, are preferred. (Meth)acrylate compounds have high photocurability and are less prone to curing shrinkage, thus improving the reproducibility of the sol-gel phase transition.

[0017] (-C(CH3)H-CH2-O-) is present in the molecule. m(Meth)acrylate compounds having a structure represented by (m being an integer between 3 and 14) are obtained by modifying the hydroxyl groups of a compound having three or more hydroxyl groups with propylene oxide, and then esterifying the resulting modified product with (meth)acrylic acid. Examples of such compounds include 3PO-modified trimethylolpropane triacrylate Photomer 4072 (molecular weight 471, ClogP 4.90, manufactured by Cognis). Another example of such a compound is 3PO-modified trimethylolpropane triacrylate Miramer M360 (molecular weight 471, ClogP 4.90, manufactured by Miwon).

[0018] Examples of (meth)acrylate compounds with a cyclic structure within the molecule include those in which the hydroxyl groups of a compound having two or more hydroxyl groups and a tricycloalkane are esterified with (meth)acrylic acid. Examples of such compounds include tricyclodecanedimethanol diacrylate NK ester A-DCP (molecular weight 304, ClogP 4.69) and tricyclodecanedimethanol dimethacrylate NK ester DCP (molecular weight 332, ClogP 5.12). Another example of such a compound is 1,10-decanediol dimethacrylate NK ester DOD-N (molecular weight 310, ClogP 5.75, manufactured by Shin-Nakamura Chemical Co., Ltd.). Furthermore, another example of such a compound is nonylphenol 8EO modified acrylate Miramer M166 (ClogP 6.42, manufactured by Miwon).

[0019] The ClogP values ​​of these (meth)acrylate compounds are preferably in the range of 4.0 to 7.0, and more preferably in the range of 4.5 to 6.0. If the ClogP value is less than 4.0, the inkjet ink containing it will be hydrophilic, making it difficult for the hydrophobic gelling agent to dissolve. Inkjet inks containing such compounds may not completely dissolve the gelling agent even when heated. On the other hand, if the ClogP value is greater than 7.0, the solubility of photopolymerization initiators and initiator aids in the inkjet ink tends to decrease, which can lead to reduced curability and reduced ejection stability.

[0020] The content of (meth)acrylate compounds with a ClogP value in the range of 4.0 to 7.0 in inkjet ink is preferably in the range of 10 to 40% by mass, and more preferably in the range of 15 to 35% by mass. If the content of the (meth)acrylate compound is less than 10% by mass, the inkjet ink will exhibit hydrophilicity, which tends to reduce the solubility of hydrophobic gelling agents. On the other hand, if the content of the (meth)acrylate compound exceeds 40% by mass, hardening shrinkage of ink droplets is likely to occur, and printed materials tend to curl. Therefore, images are more likely to crack when folded.

[0021] The (meth)acrylate compound may include other (meth)acrylate compounds. Examples of other (meth)acrylate compounds include ethylene oxide-modified (meth)acrylate compounds. Ethylene oxide-modified (meth)acrylate compounds are highly photosensitive and readily form the cardhouse structure described later when gelled at low temperatures.

[0022] Examples of ethylene oxide-modified (meth)acrylate compounds include 4EO-modified hexanediol diacrylate CD561 (molecular weight 358) and 3EO-modified trimethylolpropane triacrylate SR454 (molecular weight 429), both manufactured by Sartomer. Other examples include 6EO-modified trimethylolpropane triacrylate SR499 (molecular weight 560) and 4EO-modified pentaerythritol tetraacrylate SR494 (molecular weight 528). Furthermore, examples include polyethylene glycol diacrylate NK ester A-400 (molecular weight 508) and NK ester A-600 (molecular weight 742), both manufactured by Shin-Nakamura Chemical Co., Ltd. Finally, examples include polyethylene glycol dimethacrylate NK ester 9G and NK ester 14G, and tetraethylene glycol diacrylate V#335HP, manufactured by Osaka Organic Chemical Co., Ltd.

[0023] The molecular weight of the photopolymerizable compound contained in the active light-curable inkjet ink is preferably in the range of 280 to 1500, and more preferably in the range of 300 to 800. This is because inkjet inks containing a photopolymerizable compound with a molecular weight of less than 280 and a gelling agent exhibit too large a change in viscosity before and after the ejection temperature. On the other hand, inkjet inks containing a photopolymerizable compound with a molecular weight exceeding 1500 and a gelling agent are unsuitable as inkjet inks because their sol viscosity is too high.

[0024] The total content of photopolymerizable compounds in inkjet inks is preferably in the range of 1 to 97% by mass, and more preferably in the range of 30 to 95% by mass.

[0025] The gelling agent has the function of reversibly inducing a sol-gel phase transition in inkjet ink by temperature. The gelling agent must dissolve in a photopolymerizable compound or the like at a temperature at least higher than the gelation temperature, and crystallize in the inkjet ink at a temperature below the gelation temperature.

[0026] When the gelling agent crystallizes in the inkjet ink, it is preferable that the plate-like crystals, which are the crystalline products of the gelling agent, form a three-dimensionally enclosed space, and that the photopolymerizable compound is contained within this space. This structure, in which the photopolymerizable compound is contained within a three-dimensionally enclosed space of plate-like crystals, is sometimes called a "card house structure." When a card house structure is formed, the liquid photopolymerizable compound can be held, and the inkjet ink can be pinned. This suppresses the coalescence of the liquid droplets.

[0027] For a cardhouse structure to form, it is preferable that the photopolymerizable compound and gelling agent dissolved in the sol-like inkjet ink are compatible. Conversely, if the photopolymerizable compound and gelling agent dissolved in the sol-like inkjet ink are in phase separation, it may be difficult to form a cardhouse structure.

[0028] Examples of gelling agents include aliphatic ketone compounds; aliphatic ester compounds; petroleum waxes such as paraffin wax, microcrystalline wax, and petrolactam; plant waxes such as candelilla wax, carnauba wax, rice wax, wood wax, jojoba oil, jojoba solid wax, and jojoba esters; animal waxes such as beeswax, lanolin, and whale wax; mineral waxes such as montan wax and hydrogenated wax; hydrogenated castor oil or hydrogenated castor oil derivatives; montan wax derivatives, paraffin wax derivatives, and microcrystalline wax derivatives. Modified waxes such as body or polyethylene wax derivatives; higher fatty acids such as behenic acid, arachidic acid, stearic acid, palmitic acid, myristic acid, lauric acid, oleic acid, and erucic acid; higher alcohols such as stearyl alcohol and behenyl alcohol; hydroxystearic acid such as 12-hydroxystearic acid; 12-hydroxystearic acid derivatives; fatty acid amides such as lauric acid amide, stearic acid amide, behenic acid amide, oleic acid amide, erucic acid amide, ricinoleic acid amide, and 12-hydroxystearic acid amide (e.g., manufactured by Nippon Chemical Corporation). Nikka Amid series, ITOWAX series from Ito Oil Co., Ltd., FATTYAMID series from Kao Corporation, etc.; N-substituted fatty acid amides such as N-stearyl stearate amide and N-oleyl palmitate amide; special fatty acid amides such as N,N'-ethylenebisstearylamide, N,N'-ethylenebis-12-hydroxystearylamide, and N,N'-xylylenebisstearylamide; higher amines such as dodecylamine, tetradecylamine, or octadecylamine; fatty acid ester compounds such as stearyl stearate, oleyl palmitic acid, glycerin fatty acid ester, sorbitan fatty acid ester, propylene glycol fatty acid ester, ethylene glycol fatty acid ester, and polyoxyethylene fatty acid ester (e.g., EMALLEX series from Nippon Emulsion Co., Ltd., Rikemar series from Riken Vitamin Co., Ltd., Poem series from Riken Vitamin Co., Ltd.); esters of sucrose fatty acids such as sucrose stearic acid and sucrose palmitic acid (e.g., Ryoto Sugar Ester series from Mitsubishi Chemical Foods Co., Ltd.);Synthetic waxes such as polyethylene wax and α-olefin maleic anhydride copolymer wax (e.g., Baker-Petrolite's UNILIN series); dimer acids; dimer diols (e.g., CRODA's PRIPOR series); fatty acid inulins such as inulin stearate; fatty acid dextrins such as dextrin palmitate and dextrin myristate (e.g., Chiba Flour Milling Co., Ltd.'s Leopal series); glyceryl eicosanedioate behenate; polyglyceryl eicosanedioate behenate (e.g., Nisshin Oillio's Nomcoat series); amide compounds such as N-lauroyl-L-glutamic acid dibutylamide and N-(2-ethylhexanoyl)-L-glutamic acid dibutylamide (available from Ajinomoto Fine Techno); 1,3:2,4-bis-O-benzylidene-D-glucitol (Gelol D This includes dibenzylidenesorbitol derivatives (available from Shin Nippon Rika), and low-molecular-weight oil gelling agents described in Japanese Patent Publication No. 2005-126507, Japanese Patent Publication No. 2005-255821, and Japanese Patent Publication No. 2010-111790.

[0029] From the viewpoint of facilitating the formation of a cardhouse structure, the gelling agent is preferably a compound containing an alkyl chain with 12 or more carbon atoms. The alkyl chain in the compound may be branched.

[0030] Compounds containing a linear alkyl group with 12 or more carbon atoms include aliphatic ketone compounds, aliphatic ester compounds, higher fatty acids, higher alcohols, or fatty acid amides, which have a linear alkyl group with 12 or more carbon atoms. However, higher fatty acids, higher alcohols, and fatty acid amides have polar groups such as -OH and -COOH at the end of the alkyl chain, so they have low stability in sol-like inks and are prone to precipitation or layer separation. In addition, gelling agents may leach from the cured film of inkjet ink. For this reason, aliphatic ketone compounds or aliphatic ester compounds are preferred as gelling agents. In other words, compounds represented by the following general formulas (G1) or (G2) are preferred as gelling agents. General formula (G1): R1-CO-R2 General formula (G2): R3-COO-R4

[0031] In general formulas (G1) and (G2), R1 to R4 each independently represent a hydrocarbon group having a linear portion with 12 or more carbon atoms. R1 to R4 may also have branched portions. In general formula (G1), the hydrocarbon groups represented by R1 and R2 are preferably aliphatic hydrocarbon groups containing a linear portion with 12 to 25 carbon atoms. If the number of carbon atoms in the linear portion of the aliphatic hydrocarbon group represented by R1 and R2 is less than 12, it will not function as a gelling agent because it does not have sufficient crystallinity, and there is a risk that it will not be able to form sufficient space in the cardhouse structure to encapsulate the photopolymerizable compound. On the other hand, if the number of carbon atoms in the linear portion of the aliphatic hydrocarbon group exceeds 25, the melting point will be too high, and there is a risk that it will not dissolve in the inkjet ink unless the ejection temperature of the inkjet ink is increased.

[0032] Examples of aliphatic ketone compounds represented by general formula (G1) include dilignoseryl ketone (C24-C24), dibehenyl ketone (C22-C22, melting point 88°C), distearyl ketone (C18-C18, melting point 84°C), dieicosyl ketone (C20-C20), dipalmyl ketone (C16-C16, melting point 80°C), dimyristyl ketone (C14-C14), dilauryl ketone (C12-C12, melting point 68°C), and This includes uryl myristyl ketone (C12-C14), lauryl palmityl ketone (C12-C16), myristyl palmityl ketone (C14-C16), myristyl stearyl ketone (C14-C18), myristyl behenyl ketone (C14-C22), palmityl stearyl ketone (C16-C18), palmityl behenyl ketone (C16-C22), and stearyl behenyl ketone (C18-C22).

[0033] Examples of commercially available aliphatic ketone compounds represented by general formula (G1) include 18-Pentatriacontanon (Alfa Aeser), Hentriacontan-16-one (Alfa Aeser), and Kao Wax T1 (Kao Corporation). Aliphatic ketone compounds may be used individually or in combination of two or more.

[0034] In general formula (G2), the hydrocarbon groups represented by R3 and R4 are not particularly limited, but aliphatic hydrocarbon groups containing a linear portion with 12 to 26 carbon atoms are preferred. When the linear portion of the aliphatic hydrocarbon groups represented by R3 and R4 has 12 to 26 carbon atoms, it is possible to form the aforementioned cardhouse structure while possessing the crystallinity necessary for a gelling agent, similar to the compound represented by general formula (G1), and the melting point does not become too high.

[0035] Examples of aliphatic ester compounds represented by general formula (G2) include behenyl behenate (C21-C22, melting point 70°C), eicosyl eicosanoate (C19-C20), and stearyl stearate (C17-C18, melting point 60°C). Other examples of these compounds include palmityl stearate (C17-C16), lauryl stearate (C17-C12), and cetyl palmitate (C15-C16, melting point 54°C). Other examples of these compounds include stearyl palmitate (C15-C18), myristyl myristate (C13-C14, melting point 43°C), cetyl myristate (C13-C16, melting point 50°C), and octyldodecyl myristate (C13-C20). Examples of these compounds include stearyl oleate (C17-C18), stearyl erucate (C21-C18), stearyl linoleate (C17-C18), behenyl oleate (C18-C22), and myricyl cerotate (C25-C16). Furthermore, examples of these compounds include stearyl montanate (C27-C18), behenyl montanate (C27-C22), arachidyl linoleate (C17-C20), and palmityl triacontanoate (C29-C16).

[0036] Examples of commercially available aliphatic ester compounds represented by general formula (G2) include Unistar M-2222SL (manufactured by NOF Corporation), Excepearl SS (manufactured by Kao Corporation, melting point 60°C), and EMALEX CC-18 (manufactured by Nippon Emulsion Co., Ltd.). Other examples of this compound include Amlepus PC (manufactured by Higher Alcohol Industry Co., Ltd.), Excepearl MY-M (manufactured by Kao Corporation), Sperm Acetate (manufactured by NOF Corporation), and EMALEX CC-10 (manufactured by Nippon Emulsion Co., Ltd.). Since many of these commercially available products are mixtures of two or more compounds, separation and purification may be performed as needed.

[0037] The aliphatic ester compounds contained in the active light-curing inkjet ink may be used individually or in combination of two or more types.

[0038] The gelling agent content in active light-curable inkjet inks is preferably in the range of 0.5 to 7.0% by mass, and more preferably in the range of 1.0 to 5.0% by mass, relative to the total amount of inkjet ink. If the gelling agent content is less than 0.5% by mass, the inkjet ink may not be sufficiently gelled (sol-gel phase transition due to temperature). On the other hand, if the gelling agent content exceeds 7.0% by mass, the resulting cured film may have insufficient hardness, making the image surface susceptible to scratches.

[0039] Photopolymerization initiators are classified into two types: intramolecular bond cleavage type and intramolecular hydrogen abstraction type.

[0040] Examples of intramolecular bond cleavage type photopolymerization initiators include diethoxyacetophenone, 2-hydroxy-2-methyl-1-phenylpropan-1-one, and benzyldimethylketal. Examples of such compounds include 1-(4-isopropylphenyl)-2-hydroxy-2-methylpropan-1-one and 4-(2-hydroxyethoxy)phenyl-(2-hydroxy-2-propyl)ketone. Other examples of such compounds include acetophenone derivatives such as 1-hydroxycyclohexylphenyl ketone, 2-methyl-2-morpholino(4-thiomethylphenyl)propan-1-one, and 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone; benzoin derivatives such as benzoin, benzoin methyl ether, and benzoin isopropyl ether; acylphosphine oxide derivatives such as 2,4,6-trimethylbenzoindiphenylphosphine oxide; and benzyl and methylphenylglyoxyesters.

[0041] Examples of intramolecular hydrogen abstraction type photopolymerization initiators include benzophenone derivatives such as benzophenone, o-benzoylmethyl-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 derivatives such as 2-isopropylthioxanthone, 2,4-dimethylthioxanthone, 2,4-diethylthioxanthone, and 2,4-dichlorothioxanthone; aminobenzophenone derivatives such as Mihila-ketone and 4,4'-diethylaminobenzophenone; and 10-butyl-2-chloroacridone, 2-ethylanthraquinone, 9,10-phenanthrenequinone, and camphorquinone.

[0042] From the viewpoint of high photosensitivity, acylphosphine oxides and acylphosphonates are preferred as photopolymerization initiators, and specifically, bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide and bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide are more preferred.

[0043] The content of the photopolymerization initiator is preferably in the range of 0.1 to 10.0% by mass of the total inkjet ink, and more preferably in the range of 2 to 8% by mass.

[0044] Inkjet inks may contain photoacid generators as photopolymerization initiators. Examples of photoacid generators include compounds used in chemically amplified photoresists and photocationic polymerization. Inkjet inks may further contain photopolymerization initiator aids and polymerization inhibitors as needed.

[0045] The photopolymerization initiator auxiliaries may be tertiary amine compounds, and aromatic tertiary amine compounds are preferred. Examples of aromatic tertiary amine compounds include N,N-dimethylaniline, N,N-diethylaniline, N,N-dimethyl-p-toluidine, and N,N-dimethylamino-p-benzoate ethyl ester. Other examples of such compounds include N,N-dimethylamino-p-benzoate isoamyl ethyl ester, N,N-dihydroxyethylaniline, triethylamine, and N,N-dimethylhexylamine. N,N-dimethylamino-p-benzoate ethyl ester and N,N-dimethylamino-p-benzoate isoamyl ethyl ester are preferred as photopolymerization initiator auxiliaries. One type of photopolymerization initiator auxiliary may be used alone, or two or more types may be used in combination.

[0046] Examples of polymerization inhibitors include (alkyl)phenol, hydroquinone, catechol, resorcinol, p-methoxyphenol, t-butylcatechol, t-butylhydroquinone, pyrogallol, 1,1-picrylhydrazyl, and phenothiazine. Examples of these compounds also include p-benzoquinone, nitrosobenzene, 2,5-di-tert-butyl-p-benzoquinone, dithiobenzoyl disulfide, picric acid, cuperone, and aluminum N-nitrosophenylhydroxylamine. Furthermore, examples of these compounds include tri-p-nitrophenylmethyl, N-(3-oxyanilino-1,3-dimethylbutylidene)aniline oxide, dibutylcresol, cyclohexanone oxime cresol, guaiacol, and o-isopropylphenol. Finally, examples of these compounds include butyraldoxime, methyl ethyl ketoxime, and cyclohexanone oxime.

[0047] Inkjet ink may further contain a colorant. The colorant may be a dye or a pigment, with pigments being preferred.

[0048] Examples of pigments include CIPigment Yellow 1,2,3,12,13,14,16,17,73,74,75,81,83,87,93,95,97,98,109,114,120,128,129,138,150,151,154,155,180,185,213; CIPigment Red 5,7,12,22,38,48:1,48:2,48:4,49:1,53:1,57:1,63:1,101,112,122,123,144,146,168,184,185,202; CIPigment Violet 19,23; and CIPigment Blue. Includes 1, 2, 3, 15:1, 15:2, 15:3, 15:4, 18, 22, 27, 29, 60, CIPigment Green 7, 36, CIPigment White 6, 18, 21, and CIPigment Black 7.

[0049] The average particle size of the pigment is preferably in the range of 0.08 to 0.50 μm, and the maximum particle size of the pigment is preferably in the range of 0.3 to 10.0 μm, with a preference of 0.3 to 3.0 μm. By adjusting the particle size of the pigment, clogging of the inkjet head nozzles can be suppressed, and the storage stability, transparency, and curing sensitivity of the inkjet ink can be maintained.

[0050] On the other hand, the dyes contained in reactive light-curing inkjet inks are oil-soluble dyes, etc. Oil-soluble dyes include the following types of dyes:

[0051] Examples of magenta dyes include MS Magenta VP, MS Magenta HM-1450, MS Magenta HSo-147 (all manufactured by Mitsui Toatsu Co., Ltd.), AIZENSOT Red-1, AIZEN SOT Red-2, AIZEN SOTRed-3, AIZEN SOT Pink-1, SPIRON Red GEH SPECIAL (all manufactured by Hodogaya Chemical Co., Ltd.), RESOLIN Red FB 200%, MACROLEX Red Violet R, MACROLEX ROT5B (all manufactured by Bayer Japan Co., Ltd.), KAYASET Red B, KAYASET Red 130, KAYASET Red 802 (all manufactured by Nippon Kayaku Co., Ltd.), PHLOXIN, ROSE BENGAL, ACID Red (all manufactured by Daiwa Chemical Co., Ltd.), HSR-31, DIARESIN Red K (all manufactured by Mitsubishi Chemical Corporation), and Oil Red (manufactured by BASF Japan Co., Ltd.).

[0052] Examples of cyan dyes include MS Cyan HM-1238, MS Cyan HSo-16, Cyan HSo-144, MS Cyan VPG (all manufactured by Mitsui Toatsu Co., Ltd.), AIZEN SOT Blue-4 (manufactured by Hodogaya Chemical Co., Ltd.), RESOLIN BR.Blue BGLN 200%, MACROLEX Blue RR, CERES Blue GN, SIRIUS SUPRATURQ.Blue Z-BGL, SIRIUS SUPRA TURQ.Blue FB-LL 330% (all manufactured by Bayer Japan Co., Ltd.), KAYASET Blue FR, KAYASET Blue N, KAYASET Blue 814, Turq.Blue GL-5 200, Light Blue BGL-5 200 (all manufactured by Nippon Kayaku Co., Ltd.), DAIWA Blue 7000, Oleosol Fast Blue GL (all manufactured by Daiwa Chemical Co., Ltd.), DIARESIN Blue P (manufactured by Mitsubishi Chemical Corporation), and SUDAN Blue. This includes the 670, NEOPEN Blue 808, and ZAPON Blue 806 (all manufactured by BASF Japan).

[0053] Examples of yellow dyes include MS Yellow HSm-41, Yellow KX-7, Yellow EX-27 (Mitsui Toatsu), AIZEN SOT Yellow-1, AIZEN SOT Yellow-3, AIZEN SOT Yellow-6 (all manufactured by Hodogaya Chemical Co., Ltd.), MACROLEX Yellow 6G, MACROLEX FLUOR.Yellow 10GN (both manufactured by Bayer Japan), KAYASET Yellow SF-G, KAYASET Yellow 2G, KAYASET Yellow AG, KAYASET Yellow EG (all manufactured by Nippon Kayaku Co., Ltd.), DAIWA Yellow 330HB (Daiwa Chemical Co., Ltd.), HSY-68 (Mitsubishi Chemical Corporation), SUDAN Yellow 146, and NEOPEN Yellow 075 (all manufactured by BASF Japan).

[0054] Examples of black dyes include MS Black VPC (manufactured by Mitsui Toatsu Co., Ltd.), AIZEN SOT Black-1, AIZEN SOT Black-5 (both manufactured by Hodogaya Chemical Co., Ltd.), RESORIN Black GSN 200%, RESOLIN BlackBS (both manufactured by Bayer Japan), KAYASET Black AN (manufactured by Nippon Kayaku Co., Ltd.), DAIWA Black MSC (manufactured by Daiwa Chemical Co., Ltd.), HSB-202 (manufactured by Mitsubishi Chemical Corporation), NEPTUNE Black X60, and NEOPEN Black X58 (both manufactured by BASF Japan).

[0055] The colorant content is preferably in the range of 0.1 to 20.0% by mass, and more preferably in the range of 0.4 to 10.0% by mass. If the colorant content is too low, the resulting image will not have sufficient color development, and if it is too high, the viscosity of the inkjet ink will increase, and the ejection stability will tend to decrease.

[0056] Inkjet inks may further contain synergists as dispersing aids, depending on the type of pigment. The total amount of dispersant and dispersing aids is preferably in the range of 1 to 50% by mass relative to the pigment.

[0057] Inkjet ink is preferably obtained by mixing a pigment dispersion with other ink components, from the viewpoint of facilitating the dispersion of pigments.

[0058] Pigment dispersions can be obtained by dispersing pigments in a dispersion medium. Pigment dispersion can be performed using, for example, a ball mill, sand mill, attritor, roll mill, agitator, Henschel mixer, colloid mill, ultrasonic homogenizer, pearl mill, wet jet mill, or paint shaker. Dispersants may also be added when dispersing pigments. Polymeric dispersants are preferred, and examples of polymeric dispersants include Avecia's Solsperse series and Ajinomoto Fine Techno's PB series. The dispersion medium for pigment dispersions is a solvent or a photopolymerizable compound. To facilitate gelation immediately after impact with a recording medium, suppress the decrease in solvent resistance of the cured product, and mitigate problems such as residual volatile organic compounds from the solvent, it is preferable that the pigment dispersion does not contain a solvent. Therefore, a photopolymerizable compound, particularly a monomer with low viscosity, is preferred as the dispersion medium for pigment dispersions due to its dispersion suitability.

[0059] Inkjet inks may contain other components as needed. Examples of other components include additives and other resins. Examples of additives include surfactants, leveling additives, matting agents, UV absorbers, infrared absorbers, antimicrobial agents, and basic compounds to improve the storage stability of the ink. Examples of basic compounds include basic alkali metal compounds, basic alkaline earth metal compounds, and basic organic compounds such as amines. Examples of other resins include resins to adjust the physical properties of the cured film, such as polyester resins, polyurethane resins, vinyl resins, acrylic resins, rubber resins, and waxes.

[0060] Because inkjet inks contain a gelling agent, they undergo a reversible sol-gel phase transition depending on the temperature. At high temperatures (for example, around 80°C), inkjet inks undergoing a sol-gel phase transition are liquid (sol) and can be ejected from the inkjet head. After being ejected at high temperatures, droplets of inkjet ink land on the recording medium and then naturally cool, causing them to gel. This suppresses the coalescence of adjacent dots and improves image quality.

[0061] From the viewpoint of improving the ejection stability of inkjet ink, it is preferable that the viscosity of the inkjet ink at high temperatures be below a certain level. Specifically, it is preferable that the viscosity of the inkjet ink at 80°C be in the range of 3 to 20 mPa·s. On the other hand, from the viewpoint of suppressing the coalescence of adjacent dots, it is preferable that the viscosity of the inkjet ink at room temperature after impact be above a certain level. Specifically, it is preferable that the viscosity of the inkjet ink at 25°C be 1000 mPa·s or higher.

[0062] The gelation temperature of inkjet ink is preferably in the range of 40°C to 70°C, and more preferably in the range of 50°C to 65°C. When the ejection temperature is near 80°C, if the gelation temperature of the inkjet ink exceeds 70°C, gelation is likely to occur during ejection, which tends to reduce ejection stability. On the other hand, if the gelation temperature is below 40°C, the inkjet ink does not gel quickly after landing on the recording medium. The gelation temperature is the temperature at which inkjet ink in a sol state gels and its fluidity decreases during the cooling process.

[0063] The viscosity at 80°C, the viscosity at 25°C, and the gelation temperature of inkjet ink can be determined by measuring the temperature dependence of the dynamic viscoelasticity of the inkjet ink using a rheometer. Specifically, the inkjet ink is heated to 100°C and cooled to 20°C under conditions of a shear rate of 11.7 (1 / s) and a cooling rate of 0.1°C / s, and a temperature dependence curve of viscosity is obtained. The viscosity at 80°C and 25°C can then be determined by reading the viscosity values ​​at 80°C and 25°C, respectively, from the temperature dependence curve. The gelation temperature can be determined from the temperature dependence curve of viscosity, as the temperature at which the viscosity becomes 200 mPa·s.

[0064] A stress-controlled rheometer from Anton Paar's PhysicaMCR series can be used. The cone plate diameter can be set to 75 mm and the cone angle to 1.0°.

[0065] Active light-curable inkjet inks can be obtained by preparing a pigment dispersion solution in which a colorant (especially a pigment) is dispersed in a certain photopolymerizable compound, and then mixing this pigment dispersion solution with a photopolymerizable compound, a photopolymerization initiator, a gelling agent, and the like.

[0066] (Image forming method) Figure 1 is a flowchart of an image forming method according to one embodiment of the present invention. As shown in Figure 1, the image forming method of this embodiment includes the steps of ejecting inkjet ink onto a recording medium (S110) and curing the inkjet ink (S120). The image forming method of the present invention can be performed, for example, by an image forming apparatus described later. In the image forming apparatus, inkjet ink is ejected by an inkjet head onto a recording medium that is transported from the upstream side to the downstream side, and then the inkjet ink is cured by irradiating it with active light.

[0067] In the step of ejecting inkjet ink onto a recording medium (S110), the inkjet ink is ejected from the inkjet head of the inkjet device onto the recording medium. From the viewpoint of improving the ejection stability of the inkjet ink, it is preferable to set the temperature of the inkjet ink inside the inkjet head to a temperature 10 to 30°C higher than its gelation temperature.

[0068] If the temperature of the inkjet ink inside the inkjet head is less than 10°C above its gelation temperature, the inkjet ink may gel inside the inkjet head or on the nozzle surface, which can easily reduce the ejection stability of the inkjet ink. On the other hand, if the temperature of the inkjet ink inside the inkjet head exceeds 30°C above its gelation temperature, the inkjet ink becomes too hot, which can cause the components of the inkjet ink to deteriorate.

[0069] The amount of ink ejected per drop from each nozzle of the inkjet head is preferably in the range of 0.5 to 10.0 pL, depending on the image resolution, and more preferably in the range of 0.5 to 2.5 pL to form a high-definition image.

[0070] Ink droplets applied to a recording medium are cooled and rapidly gelled through a sol-gel phase transition. This prevents the ink droplets from diffusing, allowing for pinning. Furthermore, because oxygen does not easily diffuse into the gelled inkjet ink, oxygen inhibition of photopolymerizable compounds can be reduced.

[0071] The recording medium may be paper or a resin film. Examples of paper include coated paper for printing, art paper for printing, fine paper, and plain paper. Examples of resin films include polyethylene terephthalate film and polyvinyl chloride film. The temperature of the recording medium when the inkjet ink lands is preferably set to 10 to 20°C lower than the gelation temperature of the inkjet ink. If the temperature of the recording medium is too low, the inkjet ink may gel before it has sufficiently leveled, which can reduce the gloss of the image. On the other hand, if the temperature of the recording medium is too high, the inkjet ink will not gel easily, and adjacent dots will mix together easily (making pinning difficult). By appropriately adjusting the temperature of the recording medium, appropriate leveling that prevents adjacent dots from mixing together and proper pinning can be achieved. The temperature of the recording medium can be adjusted by the temperature control means of the image forming apparatus described later.

[0072] The speed at which the recording medium is transported from upstream to downstream is preferably 50 m / min or more, and more preferably 60 m / min or more. Furthermore, the transport speed of the recording medium is preferably 120 m / min or less. If the transport speed of the recording medium is less than 50 m / min, the inkjet recording speed will be reduced. On the other hand, if the transport speed of the recording medium exceeds 120 m / min, there is a risk that the inkjet ink will not cure sufficiently.

[0073] In the process of curing the inkjet ink (S120), the inkjet ink that has landed on the recording medium is irradiated with active light to cure the photopolymerizable compounds contained in the inkjet ink. Irradiation with active light can be performed by an image forming apparatus described later. It is preferable to irradiate with active light after all ink droplets have been ejected from the inkjet heads. In order to suppress the coalescence of adjacent ink droplets, it is preferable to irradiate with active light within 10 seconds, preferably within 0.001 to 5.000 seconds, and more preferably within 0.01 to 2.00 seconds, after the ink droplets have landed on the recording medium.

[0074] In the process of curing the inkjet ink (S120), when the active light irradiation area in the image forming apparatus is divided into two parts in the center of the transport direction of the recording medium, the active light is irradiated in such a way that the integrated amount of active light in the downstream part is greater than the integrated amount of active light in the upstream part. The active light irradiation area refers to the area in the image forming apparatus where the active light irradiation unit irradiates with active light.

[0075] Figure 2A shows a schematic illuminance distribution to illustrate an example of integrated light quantity, while Figures 2B and 2C show schematic illuminance distributions to illustrate other integrated light quantities. In Figures 2A-C, the horizontal axis represents the transport distance of the recording medium from the upstream end of the active light irradiation area, and the vertical axis represents illuminance. The dashed lines in Figures 2A-C indicate the center position when the active light irradiation area is bisected in the direction of recording medium transport. In Figures 2A-C, the left half represents the upstream side, and the right half represents the downstream side.

[0076] As shown in Fig. 2A, the illuminance distribution may be a smooth bimodal distribution having one peak in the upstream portion and one peak in the downstream portion. Also, as shown in Fig. 2B, the illuminance distribution may be a distribution having a peak only in the downstream portion. Further, as shown in Fig. 2C, the illuminance distribution may be a stepwise changing distribution. Here, the integrated light quantity can be calculated by multiplying the illuminance by the irradiation time of the active light. In the present embodiment, since the conveyance speed of the recording medium is constant, the value obtained by multiplying the illuminance by the conveyance distance can be treated as a value corresponding to the integrated light quantity. That is, in Figs. 2A to 2C, the integrated light quantity on the upstream side and the integrated light quantity on the downstream side can be compared by comparing the area of the shaded portion in the left half and the area of the shaded portion in the right half. In the present embodiment, the area of the shaded portion in the right half (downstream side) is larger than the area of the shaded portion in the left half (upstream side).

[0077] It is preferable to irradiate the active light so that the ratio of the integrated light quantity of the active light in the upstream portion to the integrated light quantity of the active light in the downstream portion is 20:80 to 40:60.

[0078] The maximum illuminance of the active light in the upstream portion is preferably in the range of 0.01 W / cm 2 or more and 1.5 W / cm 2 or less. The maximum illuminance of the active light in the downstream portion is preferably in the range of 4 W / cm 2 or more and 8 W / cm 2 or less. When the maximum illuminance of the active light in the upstream portion is less than 0.01 W / cm 2 , there is a risk that the inkjet ink may not be sufficiently cured. On the other hand, when the maximum illuminance of the active light in the upstream portion is 1.5 W / cm 2 or more, there is a risk that the inkjet ink may melt. When the maximum illuminance of the active light in the downstream portion is less than 4 W / cm 2 , a sufficient integrated light quantity may not be obtained, and there is a risk that the adhesion to the recording medium may deteriorate. On the other hand, when the maximum illuminance of the active light in the downstream portion is 8 W / cm 2 or more, the curability becomes too high, and there is a risk of warpage depending on the type of the recording medium.

[0079] In the process of curing inkjet ink, the total integrated amount of active light irradiated in the active light irradiation area is 200 mJ / cm². 2 More than 500mJ / cm 2 A range of less than is preferable. Here, the total integrated light amount refers to the integrated light amount of the active light irradiated from the upstream end to the downstream end of the active light irradiation area. In Figures 2A to C, the total integrated light amount is the sum of the area of ​​the shaded part on the left half and the area of ​​the shaded part on the right half. Integrated light amount of 200 mJ / cm 2 If the value is less than 500 mJ / cm², the inkjet ink may not cure completely. 2 In the above cases, the gelled inkjet ink on the recording medium may melt due to heat.

[0080] The method for adjusting the integrated light intensity on the recording medium is not particularly limited. Here, we will describe the case where the recording medium is transported at a constant speed. An example of a method for adjusting the integrated light intensity is to adjust the distance between the recording medium and the light source on the upstream and downstream sides of the active light irradiation area. Specifically, the light source is positioned so that the distance between the recording medium and the light source is long on the upstream side of the active light irradiation area, and so that the distance between the recording medium and the light source is short on the downstream side of the active light irradiation area. Another example of a method for adjusting the integrated light intensity is to adjust the density of the light sources positioned on the upstream and downstream sides of the active light irradiation area. In this case, the distance between the recording medium and the light source is the same on the upstream and downstream sides of the active light irradiation area. Specifically, the light sources are positioned so that the density is low on the upstream side of the active light irradiation area, and so that the density is high on the downstream side of the active light irradiation area.

[0081] Here, we will explain the mechanism by which the image forming method of this embodiment can appropriately cure active light-curable inkjet ink regardless of the transport speed of the recording medium. The gelling agent of the inkjet ink ejected from the inkjet head and deposited on the recording medium precipitates after deposition. The precipitated gelling agent of the inkjet ink is irradiated with active light with a low cumulative light intensity. This suppresses the melting of the ink coating due to an excessive temperature rise on the surface of the recording medium and causes the gelling agent to disperse unevenly within the ink coating (forming a three-dimensional network structure). Next, the inkjet ink is irradiated with active light with a high cumulative light intensity. This causes the gelling agent in the ink coating to be uniformly fixed within the network. In the image formed in this way, the gelling agent also functions as a filler. Therefore, it is presumed that the uniform arrangement of the gelling agent in the cured inkjet ink (image) equalizes (disperses) the force applied during cutting, resulting in good cutting performance. The image forming method of this embodiment is particularly effective when printing thick films on highly permeable recording media such as high-quality paper.

[0082] (Configuration of an image forming apparatus) The image forming apparatus used in the image forming method of this embodiment may be a line recording type (single-pass recording type) or a serial recording type. From the viewpoint of increasing the recording speed, an image forming apparatus using a line recording type is preferred.

[0083] Figure 3 is a schematic diagram showing the configuration of an image forming apparatus.

[0084] As shown in Figure 3, the image forming apparatus 10 includes a head carriage 11 for housing a plurality of inkjet heads 21, an ink supply unit 12 for supplying inkjet ink to the head carriage 11, a temperature control means 13 located on the lower surface of the recording medium W, and an active light irradiation unit 14 located downstream of the head carriage 11 (in the direction of transport of the recording medium).

[0085] The head carriage 11 is positioned to cover the entire width of the recording medium W and houses a plurality of inkjet heads 21, one for each color. The number of head carriages 11 is not particularly limited. In this embodiment, the number of head carriages 11 is the same as the number of inkjet ink colors.

[0086] The inkjet head 21 ejects inkjet ink supplied from the ink supply unit 12 onto the recording medium W. Multiple inkjet heads 21 are arranged in the transport direction of the recording medium W for each color. The number of inkjet heads 21 arranged in the transport direction of the recording medium W is determined by the nozzle density of the inkjet heads 21 and the resolution of the printed image. For example, to form an image with a resolution of 1440 dpi using an inkjet head 21 with a droplet volume of 2 pL and a nozzle density of 360 dpi, four inkjet heads 21 should be arranged offset from each other in the transport direction of the recording medium W. Also, to form an image with a resolution of 720 × 720 dpi using an inkjet head 21 with a droplet volume of 6 pL and a nozzle density of 360 dpi, two inkjet heads 21 should be arranged offset from each other. dpi represents the number of ink droplets (dots) per 2.54 cm.

[0087] The ink supply unit 12 supplies inkjet ink to the inkjet head 21. The ink supply unit 12 has a tank 31 in which inkjet ink is stored, and an ink channel 32 connecting the tank 31 and the inkjet head 21.

[0088] The tank 31 is connected to the head carriage 11 via an ink channel 32. From the viewpoint of stably ejecting ink droplets, a heating mechanism is provided (not shown) for heating the ink in the tank 31, ink channel 32, head carriage 11, and inkjet head 21 to a predetermined temperature.

[0089] The active light irradiation unit 14 irradiates the recording medium W, onto which the inkjet ink ejected from the inkjet head 21 has landed, with active light. The active light irradiation unit 14 is located downstream of the recording medium W in the transport direction. The active light irradiation unit 14 includes a housing 41 for defining the active light irradiation area and a light source 42 for irradiating with active light.

[0090] The housing 41 is positioned to cover the light source 42 and is designed to prevent active light from leaking out of the active light irradiation area. The housing 41 is made of, for example, aluminum, and is designed so that the active light emitted from the light source 42 is reflected from its inner surface toward the recording medium W.

[0091] The light source 42 irradiates the recording medium W with active light. The number of light sources 42 is not particularly limited as long as there are multiple light sources. In this embodiment, multiple light sources are arranged on both the upstream and downstream sides of the active light irradiation area. The type of active light emitted from the light sources 42 is appropriately set according to the type of inkjet ink. The type of active light is, for example, ultraviolet light. The peak wavelength of the active light emitted by the light source 42 is preferably in the range of 360 to 420 nm, and more preferably in the range of 380 to 410 nm. The light source 42 is not particularly limited, but a surface-emitting LED is preferred, and a surface-emitting UV-LED is more preferred. A surface-emitting UV-LED includes a substrate and a plurality of light-emitting elements arranged thereon, and may further include lenses or diffusers in front of the light-emitting elements as needed to adjust the focusing and diffusion of light.

[0092] The temperature control means 13 is located on the underside of the recording medium W and maintains the recording medium W at a predetermined temperature. The temperature control means 13 is, for example, various heaters.

[0093] (effect) As described above, in the present invention, since the activated light is irradiated in such a way that the integrated amount of activated light in the downstream section is greater than the integrated amount of activated light in the upstream section, the cuttable properties of the formed image are good regardless of the transport speed of the recording medium W. [Examples]

[0094] The present invention will be described in more detail below with reference to examples. These examples are not intended to limit the scope of the present invention.

[0095] Inkjet ink materials As photopolymerizable compounds, 25% by mass of TPGDA (tripropylene glycol diacrylate), 30% by mass of 2PO-NPGDA (2PO-modified neopentyl glycol diacrylate), and 27.3% by mass of NPGDA (neopentyl glycol diacrylate) were used. As a polymerization initiator, 3.0% by mass of IRGACURE 819 (BASF) was used. As a polymerization inhibitor, 0.15% by mass of Irgastub UV10 (BASF) was used. As a surfactant, 0.05% by mass of TSF-4452 (Momentive Performance Materials Japan LLC) was used. As a gelling agent, 4.0% by mass of Emalex CC-18 (stearate ester; Nippon Emulsion Co., Ltd.) was used. As a pigment dispersion, a 10% by mass pigment dispersion prepared by the following method was used.

[0096] Preparation of pigment dispersion 9% by mass of Azispar PB824 (manufactured by Ajinomoto Fine Techno Co., Ltd.) and 71% by mass of tripropylene glycol diacrylate (APG-200, manufactured by Shin Nakamura Chemical Co., Ltd.) were placed in a stainless steel beaker and heated on a 65°C hot plate for 1 hour while stirring to dissolve. After the resulting solution was cooled to room temperature, 20 parts by mass of Pigment Black 7 (manufactured by Mitsubishi Chemical Corporation, #52) were added, and the mixture was placed in a glass bottle with 200 g of 0.5 mm diameter zirconia beads and sealed tightly. This was dispersed in a paint shaker for 5 hours, after which the zirconia beads were removed to obtain a pigment dispersion.

[0097] Preparation of inkjet inks According to the aforementioned component composition, each material was mixed and heated to 80°C while being stirred. Then, while heating the mixture, it was filtered through a 3μm Teflon® membrane filter (manufactured by ADVANTEC®) to obtain inkjet ink 1. In the method for preparing inkjet ink 1, inkjet ink 2 was obtained in the same manner except that, instead of a gelling agent, the amount of each monomer was increased while maintaining the ratio of the monomer composition.

[0098] Image formation The obtained inkjet ink 1 or inkjet ink 2 was loaded into the image forming apparatus described above. The inkjet head of the image forming apparatus had a piezo-type inkjet nozzle. Using the image forming apparatus loaded with inkjet ink 1 or inkjet ink 2, A4 size coated paper (OK Kinto, basis weight 104.7 g / m²) was processed. 2 Inkjet ink 1 or inkjet ink 2 was ejected onto a material (manufactured by Oji Paper Co., Ltd.) to form a solid image (process of ejecting inkjet ink onto the recording medium). Next, the material was irradiated with active light (ultraviolet light) under the conditions shown in Table 1 (process of curing the inkjet ink). The transport speed of the recording medium was set to 50 m / min.

[0099] evaluation Evaluation of suitability for getting behind the defense The ink density that bled through to the back of each solid image was measured using a fluorescence spectrophotometer FD-7 (Konica Minolta). The results were evaluated according to the following criteria. A higher value in the criteria below indicates a better result, with 3 and 4 being considered passing grades. 4. The ink density was between 0.00 and 0.05. 3. The ink density was between 0.06 and 0.10. 2: The ink density was between 0.11 and 0.15. 1: The ink density was between 0.16 and 0.20.

[0100] Evaluation of cut-processability Each obtained solid image was cut using an automatic cutting machine (Horizon PC-P430) to remove the printed area. 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 of the cutting blade (edge ​​of the paper) to the ink-printed surface. The results were evaluated according to the following criteria. A shorter peeling distance indicates a better result, with scores of 3 and 4 being considered passing grades. 4. Peeling distance is less than 10 μm (peeling is not visible, no practical problems) 3: Peeling distance is between 10 μm and 80 μm (peeling is not visible, no practical problems) 2: Peeling distance is 80 μm or more but less than 200 μm (peeling is visible) 1: Peeling distance is 200 μm or more (peeling is visible)

[0101] Table 1 shows the conditions for the inkjet ink curing process and the evaluation results.

[0102] [Table 1]

[0103] Evaluation results As shown in Table 1, in the image forming methods of Examples 1 to 11, where the integrated amount of active light in the downstream section was greater than the integrated amount of active light in the upstream section, good back-penetration suitability and cutting processability were observed. Furthermore, the ratio of the integrated amount of active light in the upstream section to the integrated amount of active light in the downstream section is 20:80 to 40:60, or the total integrated amount is 200 mJ / cm². 2 More than 500mJ / cm 2 It is within the range of less than 0.01 W / cm², or the maximum illuminance of active light in the upstream area is 0.01 W / cm². 2 More than 1.5W / cm 2 It is within the range of less than 4 W / cm², or the maximum illuminance of the active light in the downstream section is 4 W / cm². 2 More than 8W / cm 2In the image forming methods of Examples 7 to 11, which were within the range of less than 6, the transparency and cut-through properties were even better compared to Example 6. Furthermore, the ratio of the integrated light intensity of active light in the upstream section to the integrated light intensity of active light in the downstream section was 20:80 to 40:60, and the total integrated light intensity was 200 mJ / cm². 2 More than 500mJ / cm 2 It is within the range of less than 0.01 W / cm², and the maximum illuminance of active light in the upstream area is 0.01 W / cm². 2 More than 1.5W / cm 2 It is within the range of less than 4 W / cm², and the maximum illuminance of the active light in the downstream section is 4 W / cm². 2 More than 8W / cm 2 In the image forming methods of Examples 1 to 5, which were within the range of less than 6, the transparency and cut-through properties were even better compared to Example 6.

[0104] On the other hand, in the image forming methods of Comparative Examples 1 to 6, where the integrated amount of active light in the downstream section was less than the integrated amount of active light in the upstream section, the transparency and cutability were poor. [Industrial applicability]

[0105] The image forming method according to the present invention allows for high-precision ejection of ink from the inkjet head. Therefore, the accuracy of the image formed by the image forming method can be improved, making it useful, for example, in the field of image formation. [Explanation of Symbols]

[0106] 10 Image forming apparatus 11 Head Carriage 12. Ink supply unit 13 Temperature control means 14 Active ray irradiation section 21 Inkjet Heads 31 tanks 32 Ink channels 41 cabinets 42 Light source W recording medium

Claims

1. An image forming method for forming an image on a recording medium transported from upstream to downstream, using an active light-curable inkjet ink comprising a polymerizable compound, a photopolymerization initiator, and a gelling agent, which undergoes a phase transition with temperature, A step of ejecting the inkjet ink onto the recording medium, A step of curing the inkjet ink ejected onto the transported recording medium by irradiating it with active light in an active light irradiation area, Includes, In the process of curing the inkjet ink, when the active light irradiation area is divided into two parts in the center of the transport direction of the recording medium, the active light is irradiated such that the integrated amount of active light in the downstream part is greater than the integrated amount of active light in the upstream part. Image forming method.

2. The image forming method according to claim 1, wherein in the step of curing the inkjet ink, the active light is irradiated such that the ratio of the integrated amount of active light in the upstream section to the integrated amount of active light in the downstream section is 20:80 to 40:

60.

3. In the process of curing the inkjet ink, the total integrated amount of active light irradiated in the active light irradiation area is 200 mJ / cm². ２ More than 500mJ / cm ２ The image forming method according to claim 1, wherein the range is less than [amount].

4. In the process of curing the inkjet ink, the maximum illuminance of the active light in the upstream section is 0.01 W / cm². ２ 1.5W / cm or more ２ It is within the range of less than 4 W / cm², and the maximum illuminance of the active light in the downstream section is 4 W / cm². ２ More than 8W / cm ２ The image forming method according to claim 1, wherein the range is less than [amount].

5. The image forming method according to claim 1, wherein the speed at which the recording medium is transported from upstream to downstream is 50 m / min or more.

Images