Inkjet recording method and inkjet recording apparatus

Abstract

To provide an inkjet recording method that can achieve excellent printing coat-film color reproducibility (coat-film color tone) and reduce the heat damage to the recording medium.SOLUTION: Provided is an inkjet recording method that comprises an ejection step of ejecting a radiation-curable inkjet composition from an inkjet head onto a recording medium, and a curing step of irradiating the inkjet composition ejected onto the recording medium with radiation ray from an LED irradiator having a peak wavelength at 250 to 310 nm to form a cured coat-film, and in which the inkjet composition contains a polymerizable compound and a α-hydroxyketone-based initiator.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an inkjet recording method and an inkjet recording apparatus. [Background technology]

[0002] Conventionally, various methods have been used as a recording method for forming an image on a recording medium such as paper based on image data signals. Among these, the inkjet method is an inexpensive device that ejects ink only onto the required image area to form an image directly on the recording medium, making efficient use of ink and low running costs. Furthermore, the inkjet method is excellent as a recording method because it produces little noise.

[0003] In recent years, in inkjet recording methods, radiation-curable ink compositions (inkjet compositions) that cure upon exposure to radiation have been used as ink compositions that can provide good water resistance, solvent resistance, abrasion resistance, and the like.

[0004] For example, Patent Document 1 discloses an ultraviolet-curable ink composition for inkjet recording that contains an acylphosphine oxide (APO)-based photopolymerization initiator. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2007-231082 A Summary of the Invention [Problem to be solved by the invention]

[0006] However, although an inkjet composition containing an APO-based initiator as a photopolymerization initiator can be cured favorably using an LED irradiator with a peak wavelength of 365 nm to 405 nm, there is a problem in that the printed coating film is prone to yellowing and the color reproducibility (coating film color tone) is poor.

[0007] On the other hand, when an α-hydroxyketone-based initiator is used as the photopolymerization initiator, yellowing of the printed coating film is unlikely to occur, but it is necessary to irradiate radiation with a peak wavelength of about 280 nm. Therefore, a large lamp with a high heat output, such as a metal halide lamp or a high-pressure mercury lamp, is required as the radiation irradiator, which causes a problem of thermal damage to the recording medium.

[0008] Therefore, there is a demand for a printed coating film having excellent color reproducibility (coating film color tone) and for thermal damage to be reduced. [Means for solving the problem]

[0009] One aspect of the inkjet recording method according to the present invention comprises: a step of discharging the radiation curable ink jet composition from an ink jet head onto a recording medium; a curing step of irradiating the ink jet composition discharged onto the recording medium with radiation from an LED irradiator having a peak wavelength of 250 to 310 nm to form a cured coating film, The inkjet composition contains a polymerizable compound and an α-hydroxyketone-based initiator.

[0010] One aspect of the inkjet recording apparatus according to the present invention is to an inkjet head that ejects a radiation-curable inkjet composition containing a polymerizable compound and an α-hydroxyketone-based initiator onto a recording medium; It is equipped with an LED irradiator with a peak wavelength of 250 to 310 nm. [Brief description of the drawings]

[0011] [Figure 1] 1 is a perspective view of an inkjet recording apparatus that can be used in an inkjet recording method according to an embodiment. [Diagram 2] FIG. 2 is a front view of the radiation irradiation device shown in FIG. [Diagram 3]AA arrow view of Figure 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, an embodiment of the present invention will be described. The embodiment described below is an example of the present invention. The present invention is not limited to the following embodiment, and includes various modified forms that are implemented within the scope that does not change the gist of the present invention. Note that not all of the configurations described below are necessarily essential configurations of the present invention.

[0013] 1. Inkjet recording method An inkjet recording method according to one embodiment of the present invention includes a discharge step of discharging a radiation-curable inkjet composition from an inkjet head onto a recording medium, and a curing step of irradiating the inkjet composition discharged onto the recording medium with radiation from an LED irradiator having a peak wavelength of 250 to 310 nm to form a cured coating film, wherein the inkjet composition contains a polymerizable compound and an α-hydroxyketone-based initiator.

[0014] In the inkjet recording method according to the present embodiment, an LED irradiator having a peak wavelength of 250 to 310 nm is used in the curing step, and since the irradiator is energy-saving, it can irradiate radiation having a peak wavelength of 250 to 310 nm while reducing thermal damage to the recording medium. In addition, the radiation-curable inkjet composition discharged in the discharging step contains an α-hydroxyketone initiator, which is less likely to yellow the coating film and is easy to absorb radiation having the above peak wavelength. Therefore, according to the inkjet recording method according to the present embodiment, the printed coating film can be sufficiently cured, thermal damage to the recording medium can be reduced, and yellowing of the printed coating film can be prevented.

[0015] Each step in the inkjet recording method according to this embodiment will be described below.

[0016] 1.1.Discharge process The inkjet recording method according to this embodiment includes a discharge step of discharging a radiation curable inkjet composition from an inkjet head onto a recording medium.

[0017] 1.1.1. Radiation-curable inkjet composition The radiation-curable inkjet composition (hereinafter also referred to as "inkjet composition" or simply "ink") used in the inkjet recording method according to this embodiment contains a polymerizable compound and an α-hydroxyketone initiator. Each component contained in the inkjet composition will be described below.

[0018] 1.1.1.1. Polymerizable compounds The radiation-curable inkjet composition used in the inkjet recording method according to this embodiment contains a polymerizable compound. The polymerizable compound can be polymerized when irradiated with radiation, either alone or by the action of a photopolymerization initiator, to cure the ink on the recording medium. The polymerizable compound is not particularly limited, but specifically, conventionally known monofunctional, bifunctional, and trifunctional or higher polyfunctional monomers and oligomers can be used. The polymerizable compound may be used alone or in combination of two or more types. Examples of these polymerizable compounds are given below.

[0019] Here, the polymerizable compound contains a radical polymerizable compound from the viewpoint of further enhancing the curability of the inkjet composition, and from the viewpoint of seeking high versatility and high simplicity. In addition to or instead of that, the polymerizable compound preferably contains a polymerizable compound having a vinyl ether group and a (meth)acrylate group from the viewpoint of enhancing the curability, further reducing the viscosity of the inkjet composition, and enhancing the solubility of the polymerization initiator when a polymerization initiator is used. As the polymerizable compound having a vinyl ether group and a (meth)acrylate group, a radical polymerizable compound having a vinyl ether group and a (meth)acrylate group is preferable. As such a polymerizable compound, for example, a (meth)acrylate having a monofunctional or polyfunctional vinyl ether group can be mentioned, and from the same viewpoint as above, these are preferable.

[0020] In this specification, "(meth)acrylic acid" means both acrylic acid and the corresponding methacrylic acid, "(meth)acrylate" means both acrylate and the corresponding methacrylate, and "(meth)acrylic" means both acrylic and the corresponding methacrylic.

[0021] The (meth)acrylate having a vinyl ether group (hereinafter also referred to as "vinyl group-containing (meth)acrylate") is not particularly limited, but from the viewpoints of lowering the viscosity of the composition, increasing the flash point, and further enhancing the curability of the composition, it is preferable that the (meth)acrylate contains a compound represented by the following general formula (I). H2C=CR 1 -CO-OR 2 -O-CH=CH-R 3 (I) (In the formula, R 1 is a hydrogen atom or a methyl group, R 2 is a divalent organic residue having 2 to 20 carbon atoms, and R 3 is a hydrogen atom or a monovalent organic residue having 1 to 11 carbon atoms. Hereinafter, the vinyl group-containing (meth)acrylate represented by general formula (I) may be simply referred to as "a compound of formula (I)".

[0022] The inkjet composition used in the inkjet recording method according to this embodiment contains the compound of formula (I), which tends to improve the curability of the inkjet composition. In addition, the inkjet composition tends to have a low viscosity due to the compound of formula (I). Furthermore, in order to improve the curability of the inkjet composition, it is preferable to use a compound having both a vinyl ether group and a (meth)acrylic group in one molecule, rather than using a compound having a vinyl ether group and a compound having a (meth)acrylic group separately.

[0023] In the above general formula (I), R 2 Preferred examples of the divalent organic residue having 2 to 20 carbon atoms and represented by the formula (I) include linear, branched or cyclic alkylene groups having 2 to 20 carbon atoms which may be substituted, alkylene groups having 2 to 20 carbon atoms which may have an oxygen atom due to an ether bond and / or an ester bond in the structure, and divalent aromatic groups having 6 to 11 carbon atoms which may be substituted. Among these, alkylene groups having 2 to 6 carbon atoms such as ethylene group, n-propylene group, isopropylene group and butylene group, and alkylene groups having 2 to 9 carbon atoms which have an oxygen atom due to an ether bond in the structure such as oxyethylene group, oxyn-propylene group, oxyisopropylene group and oxybutylene group are preferably used. Furthermore, from the viewpoint of further reducing the viscosity of the radiation-curable ink jet composition and further improving the curability, R 2 is an alkylene group having 2 to 9 carbon atoms and having an oxygen atom by an ether bond in the structure, such as an oxyethylene group, an oxyn-propylene group, an oxyisopropylene group, or an oxybutylene group, is more preferred.

[0024] In the above general formula (I), R 3As the monovalent organic residue having 1 to 11 carbon atoms represented by the formula (1), a linear, branched or cyclic alkyl group having 1 to 10 carbon atoms which may be substituted, or an aromatic group having 6 to 11 carbon atoms which may be substituted, is preferred. Among these, an alkyl group having 1 to 2 carbon atoms, such as a methyl group or an ethyl group, and an aromatic group having 6 to 8 carbon atoms, such as a phenyl group or a benzyl group, are preferably used.

[0025] When each of the organic residues is a group that may be substituted, the substituent is divided into a group that contains carbon atoms and a group that does not contain carbon atoms.First, when the substituent is a group that contains carbon atoms, the carbon atom is counted in the number of carbon atoms of the organic residue.As the group that contains carbon atoms, but is not limited to the following, for example, a carboxyl group and an alkoxy group can be mentioned.Next, as the group that does not contain carbon atoms, but is not limited to the following, for example, a hydroxyl group and a halo group can be mentioned.

[0026] The content of the compound of formula (I) is preferably 1% by mass or more and 50% by mass or less, more preferably 10% by mass or more and 50% by mass or less, even more preferably 10% by mass or more and 35% by mass or less, and particularly preferably 10% by mass or more and 25% by mass or less, based on the total amount (100% by mass) of the inkjet composition. When the content of the compound of formula (I) is 1% by mass or more, particularly preferably 10% by mass or more, the inkjet composition can be made low in viscosity and the curability of the inkjet composition tends to be further improved. On the other hand, when the content is 50% by mass or less, the storage stability of the ink tends to be maintained in an excellent state.

[0027] Specific examples of the compound of formula (I) include, but are not limited to, 2-vinyloxyethyl (meth)acrylate, 3-vinyloxypropyl (meth)acrylate, 1-methyl-2-vinyloxyethyl (meth)acrylate, 2-vinyloxypropyl (meth)acrylate, 4-vinyloxybutyl (meth)acrylate, 1-methyl-3-vinyloxypropyl (meth)acrylate, 1-vinyloxymethylpropyl (meth)acrylate, 2-methyl-3-vinyloxypropyl (meth)acrylate, 1,1-dimethyl-2-vinyloxyethyl (meth)acrylate, 3-vinyloxybutyl acrylate, 1-methyl-2-vinyloxypropyl (meth)acrylate, 2-vinyloxybutyl (meth)acrylate, 4-vinyloxycyclohexyl (meth)acrylate, 6-vinyloxyhexyl (meth)acrylate, 4-vinyloxymethylcyclohexylmethyl (meth)acrylate, 3-vinyloxymethylcyclohexylmethyl (meth)acrylate, 2-vinyloxymethylcyclohexylmethyl (meth)acrylate, p-vinyloxymethylphenylmethyl (meth)acrylate, m-vinyloxymethylphenylmethyl (meth)acrylate 2-(vinyloxymethylphenylmethyl) (meth)acrylate, 2-(2-vinyloxyethoxy)ethyl methacrylate, 2-(2-vinyloxyethoxy)ethyl acrylate (VEEA), 2-(vinyloxyisopropoxy)ethyl (meth)acrylate, 2-(vinyloxyethoxy)propyl (meth)acrylate, 2-(vinyloxyethoxy)isopropyl (meth)acrylate, 2-(vinyloxyisopropoxy)propyl (meth)acrylate, 2-(vinyloxyisopropoxy)isopropyl (meth)acrylate, 2-(vinyloxyisopropoxy)isopropyl (meth)acrylate, 2-(vinyloxyethyl)acrylate 2-(vinyloxyethoxy)ethyl (meth)acrylate, 2-(vinyloxyethoxyisopropoxy)ethyl (meth)acrylate, 2-(vinyloxyisopropoxyethoxy)ethyl (meth)acrylate, 2-(vinyloxyisopropoxyisopropoxy)ethyl (meth)acrylate, 2-(vinyloxyethoxyethoxy)propyl (meth)acrylate, 2-(vinyloxyethoxyisopropoxy)propyl (meth)acrylate, 2-(vinyloxyisopropoxyethoxy)propyl (meth)acrylate, 2-(vinyloxyisopropoxyisopropoxy)propyl (meth)acrylate,2-(vinyloxyethoxyethoxy)isopropyl (meth)acrylate, 2-(vinyloxyethoxyisopropoxy)isopropyl (meth)acrylate, 2-(vinyloxyisopropoxyethoxy)isopropyl (meth)acrylate, 2-(vinyloxyisopropoxyisopropoxy)isopropyl (meth)acrylate, 2-(vinyloxyethoxyethoxyethoxy)ethyl (meth)acrylate, 2-(vinyloxyethoxyethoxyethoxyethoxy)ethyl (meth)acrylate ethyl, 2-(isopropenoxyethoxy)ethyl (meth)acrylate, 2-(isopropenoxyethoxyethoxy)ethyl (meth)acrylate, 2-(isopropenoxyethoxyethoxy)ethyl (meth)acrylate, 2-(isopropenoxyethoxyethoxyethoxy)ethyl (meth)acrylate, 2-(isopropenoxyethoxyethoxyethoxy)ethyl (meth)acrylate, polyethylene glycol monovinyl ether (meth)acrylate, and polypropylene glycol monovinyl ether (meth)acrylate.

[0028] Among these, 2-(vinyloxyethoxy)ethyl (meth)acrylate, i.e., at least one of 2-(vinyloxyethoxy)ethyl acrylate and 2-(vinyloxyethoxy)ethyl methacrylate, is preferred because it can reduce the viscosity of the radiation-curable inkjet composition, has a high flash point, and provides excellent curability of the inkjet composition, and 2-(vinyloxyethoxy)ethyl acrylate is more preferred. Both 2-(vinyloxyethoxy)ethyl acrylate and 2-(vinyloxyethoxy)ethyl methacrylate have simple structures and small molecular weights, and therefore can significantly reduce the viscosity of the radiation-curable inkjet composition. Examples of 2-(vinyloxyethoxy)ethyl (meth)acrylate include 2-(2-vinyloxyethoxy)ethyl (meth)acrylate and 2-(1-vinyloxyethoxy)ethyl (meth)acrylate, and examples of 2-(vinyloxyethoxy)ethyl acrylate include 2-(2-vinyloxyethoxy)ethyl acrylate and 2-(1-vinyloxyethoxy)ethyl acrylate. Note that 2-(vinyloxyethoxy)ethyl acrylate is superior in curability to 2-(vinyloxyethoxy)ethyl methacrylate.

[0029] The content of the vinyl ether group-containing (meth)acrylic esters, particularly 2-(vinyloxyethoxy)ethyl (meth)acrylate, is preferably 10% by mass or more and 70% by mass or less, and more preferably 20% by mass or more and 50% by mass or less, based on the total amount (100% by mass) of the inkjet composition. When the content is 10% by mass or more, the viscosity of the radiation-curable inkjet composition can be reduced, and the curability of the radiation-curable inkjet ink composition may be superior. On the other hand, when the content is 70% by mass or less, the storage stability of the inkjet composition may be superior, and the surface gloss of the recorded matter may be superior.

[0030] In the present embodiment, the inkjet composition may contain one or more monofunctional, bifunctional, and trifunctional or higher polyfunctional monomers other than those exemplified above. Examples of such monomers include, but are not limited to, unsaturated carboxylic acids such as (meth)acrylic acid, itaconic acid, crotonic acid, isocrotonic acid, and maleic acid; salts of the unsaturated carboxylic acids; esters, urethanes, amides, and anhydrides of unsaturated carboxylic acids; acrylonitrile, styrene, various unsaturated polyesters, unsaturated polyethers, unsaturated polyamides, and unsaturated urethanes.

[0031] In addition, the monomer may contain an N-vinyl compound as another monofunctional monomer or polyfunctional monomer. The N-vinyl compound is not particularly limited, but examples thereof include N-vinylformamide, N-vinylcarbazole, N-vinylacetamide, N-vinylpyrrolidone, N-vinylcaprolactam, and acryloylmorpholine, and derivatives thereof.

[0032] The inkjet composition may contain a monofunctional (meth)acrylate as a monofunctional monomer. In this case, the inkjet composition has a low viscosity, excellent solubility of the photopolymerization initiator and other additives, and ejection stability during inkjet recording is easily achieved. The monofunctional (meth)acrylate is not particularly limited, but examples thereof include isoamyl (meth)acrylate, stearyl (meth)acrylate, lauryl (meth)acrylate, octyl (meth)acrylate, decyl (meth)acrylate, isomyristyl (meth)acrylate, isostearyl (meth)acrylate, 2-ethylhexyl-diglycol (meth)acrylate, 2-hydroxybutyl (meth)acrylate, butoxyethyl (meth)acrylate, ethoxydiethylene glycol (meth)acrylate, methoxydiethylene glycol (meth)acrylate, methoxypolyethylene glycol (meth)acrylate, methoxypropylene glycol (meth)acrylate, phenoxyethyl (meth)acrylate, tetrahydrofurfuryl (meth)acrylate, isobornyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl ... propyl (meth)acrylate, 2-hydroxy-3-phenoxypropyl (meth)acrylate, 4-hydroxybutyl acrylate, lactone modified flexible (meth)acrylate, t-butylcyclohexyl (meth)acrylate, dicyclopentanyl (meth)acrylate, dicyclopentenyloxyethyl (meth)acrylate, 2-(isopropenoxyethoxy)ethyl (meth)acrylate, 2-(isopropenoxyethoxyethoxy)ethyl (meth)acrylate, 2-(isopropenoxyethoxyethoxyethoxy)ethyl (meth)acrylate, 2-(isopropenoxyethoxyethoxyethoxy)ethyl (meth)acrylate, 2-(isopropenoxyethoxyethoxyethoxy)ethyl (meth)acrylate, polyethylene glycol monovinyl ether (meth)acrylate, polypropylene glycol monovinyl ether (meth)acrylate, and cyclic trimethylolpropane formal (meth)acrylate. Of these, phenoxyethyl (meth)acrylate, isobornyl (meth)acrylate and cyclic trimethylolpropane formal (meth)acrylate are preferred.When these compounds are used as monofunctional monomers, the curability tends to be good, the amount of photopolymerization initiator that can cause coloring can be reduced, and the coating film color tone can be improved.

[0033] The content of the monofunctional monomer is preferably 10% by mass or more and 80% by mass or less, more preferably 20% by mass or more and 60% by mass or less, based on the total amount (100% by mass) of the inkjet composition. By setting the content within the above preferred range, the curability, initiator solubility, storage stability, and ejection stability tend to be more excellent.

[0034] The inkjet composition may contain a polyfunctional (meth)acrylate as a polyfunctional monomer. Among them, the bifunctional (meth)acrylate is not particularly limited, but examples thereof include triethylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, dipropylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, 1,9-nonanediol di(meth)acrylate, and the like. acrylate, neopentyl glycol di(meth)acrylate, dimethylol-tricyclodecane di(meth)acrylate, EO (ethylene oxide) adduct di(meth)acrylate of bisphenol A, PO (propylene oxide) adduct di(meth)acrylate of bisphenol A, hydroxypivalic acid neopentyl glycol di(meth)acrylate, polytetramethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, and triethylene glycol di(meth)acrylate.

[0035] Examples of trifunctional or higher (meth)acrylates include trimethylolpropane tri(meth)acrylate, EO-modified trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, glycerin propoxy tri(meth)acrylate, cowprolactone-modified trimethylolpropane tri(meth)acrylate, pentaerythritol ethoxy tetra(meth)acrylate, and caprolactam-modified dipentaerythritol hexa(meth)acrylate.

[0036] Among these, dipropylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, pentaerythritol tri(meth)acrylate and dipentaerythritol hexa(meth)acrylate are preferred, and dipropylene glycol di(meth)acrylate and dipentaerythritol hexa(meth)acrylate are more preferred.

[0037] The content of the polyfunctional monomer is preferably 60% by mass or less, more preferably 50% by mass or less, and even more preferably 40% by mass or less, based on the total amount (100% by mass) of the inkjet composition. By setting the content within the above range, the curability, storage stability, ejection stability, and surface gloss of the printed matter tend to be more excellent.

[0038] In addition, since the toughness, heat resistance, and chemical resistance of the cured film are increased, it is also preferable to use monofunctional (meth)acrylates and polyfunctional (meth)acrylates in combination, and among these, it is more preferable to use phenoxyethyl (meth)acrylate, isobornyl (meth)acrylate, dipropylene glycol di(meth)acrylate, and dipentaerythritol hexa(meth)acrylate in combination.

[0039] In addition, in order to improve the adhesion and toughness of the cured film, monofunctional or polyfunctional oligomers having two or more functionalities may be contained in addition to the above monomers. The type of oligomer is not particularly limited, but includes various oligomers such as acrylic oligomers formed from acrylic monomers, styrene-acrylic oligomers formed from styrene and acrylic monomers, urethane acrylate oligomers such as aliphatic, alicyclic, and aromatic oligomers, epoxy acrylate oligomers, and polyester acrylate oligomers, which are collectively referred to as acrylate oligomers. The content of the acrylate oligomer is preferably 3% by mass or more and less than 30% by mass, more preferably 5% by mass or more and less than 25% by mass. By the content being equal to or more than the lower limit, the adhesion and toughness of the cured film are good, and by the content being equal to or less than the upper limit, the ink viscosity is suitably low, and the dischargeability may be good.

[0040] The content of the polymerizable compound is preferably 35% by mass or more and 95% by mass or less, and more preferably 45% by mass or more and 90% by mass or less, based on the total amount (100% by mass) of the inkjet composition. By having the content of the polymerizable compound within the above range, it is possible to reduce viscosity and odor, and also to improve the solubility and reactivity of the photopolymerization initiator and the surface gloss of the printed matter.

[0041] 1.1.1.2. Photoinitiator The radiation-curable inkjet composition used in the inkjet recording method according to this embodiment contains an α-hydroxyketone-based photopolymerization initiator. By using an α-hydroxyketone-based photopolymerization initiator, yellowing of the recorded matter can be prevented, and recording with excellent coating color tone can be achieved.

[0042] The α-hydroxyketone initiator refers to a photopolymerization initiator having an α-hydroxyketone structure in the molecule. Examples of the α-hydroxyketone initiator include 2-hydroxy-1-(4-(4-(2-hydroxy-2-methyl-propionyl)-benzyl)-phenyl)-2-methyl-propane, 1-(4-(2-hydroxyethoxyl)-phenyl)-2-hydroxy-methylpropanone, oligo(2-hydroxy-2-methyl-1-(4-(1-methylvinyl)phenyl)propanone, 2-hydroxy-1-[4-{4-(2-hydroxy-2-methylpropanoyl)phenoxy}phenyl]-2-methylpropan-1-one, 1-hydroxycyclohexylphenylketone, 2,2-dimethoxy-2-phenylacetophenone, and 2-hydroxy-2-methoxypropiophenone.

[0043] Among these, α-hydroxyketone-based initiators having a molecular weight of 210 or more are preferred. That is, examples of α-hydroxyketone-based initiators include 2-hydroxy-1-(4-(4-(2-hydroxy-2-methyl-propionyl)-benzyl)-phenyl)-2-methyl-propane (molecular weight: 340), 1-(4-(2-hydroxyethoxyl)-phenyl)-2-hydroxy-methylpropanone (molecular weight: 224), oligo(2-hydroxy-2-methyl-1-(4-(1-methylvinyl)phenyl)propanone (molecular weight: 483), and 2-hydroxy-1-[4-{4-(2-hydroxy-2-methyl propanoyl)phenoxy}phenyl]-2-methylpropan-1-one (molecular weight: 342) and the like are preferred. The molecular weight of the α-hydroxyketone initiator is more preferably 300 or more, even more preferably 400 or more, and particularly preferably 450 or more. When the molecular weight of the α-hydroxyketone initiator is within the above range, there is a tendency to be able to reduce the generation of odor from the printed coating film. There is also a tendency to be able to reduce the migration of the photopolymerization initiator remaining in the printed coating film.

[0044] Commercially available α-hydroxyketone initiators include, for example, Omnirad 127 (trade name, manufactured by IGM Resins BV, 2-hydroxy-1-(4-(4-(2-hydroxy-2-methyl-propionyl)-benzyl)-phenyl)-2-methyl-propane), Omnirad 2959 (trade name, manufactured by IGM Resins BV, 1-(4-(2-hydroxyethoxyl)-phenyl)-2-hydroxy-methylpropanone), ESACURE KIP 150 (trade name, manufactured by Lamberti, oligo(2-hydroxy-2-methyl-1-(4-(1-methylvinyl)phenyl)propanone), ESACURE KIP 160 (trade name, manufactured by Lamberti, 2-hydroxy-1-[4-{4-(2-hydroxy-2-methylpropanoyl)phenoxy}phenyl]-2-methylpropan-1-one), Omnirad Examples of suitable amines include Omnirad 184 (trade name, manufactured by IGM Resins BV, 1-hydroxycyclohexyl phenyl ketone), Omnirad 651 (trade name, manufactured by IGM Resins BV, 2,2-dimethoxy-2-phenylacetophenone), and Omnirad 1173 (trade name, manufactured by IGM Resins BV, 2-hydroxy-2-methoxypropiophenone).

[0045] The lower limit of the content of the α-hydroxyketone initiator is preferably 4% by mass or more, more preferably 6% by mass or more, and even more preferably 7.5% by mass or more, based on the total amount (100% by mass) of the inkjet composition. The upper limit of the content of the photopolymerization initiator is preferably 20% by mass or less, more preferably 15% by mass or less, and even more preferably 10% by mass or less, based on the total amount (100% by mass) of the inkjet composition. By having the content of the α-hydroxyketone initiator within the above range, the curing speed is sufficiently high, and there is little residual dissolution of the α-hydroxyketone initiator, which may reduce the generation of odor.

[0046] <Other photopolymerization initiators> The radiation-curable inkjet composition used in the inkjet recording method according to this embodiment may contain a photopolymerization initiator other than the above-mentioned α-hydroxyketone-based initiator. Such a photopolymerization initiator is not particularly limited as long as it generates active species such as radicals and cations by irradiation with active radiation and initiates a polymerization reaction of the above-mentioned polymerizable compound. As the other photopolymerization initiator, a photoradical polymerization initiator or a photocationic polymerization initiator can be used, but it is preferable to use a photoradical polymerization initiator.

[0047] Among radiations, ultraviolet rays (UV) are used, which is safe and allows the cost of the irradiation unit to be reduced. Therefore, it is preferable that the other photopolymerization initiators have an absorption peak in the ultraviolet region.

[0048] Examples of the photoradical polymerization initiator include aromatic ketones, acylphosphine oxide compounds, aromatic onium salt compounds, organic peroxides, thio compounds (thioxanthone compounds, thiophenyl group-containing compounds, etc.), hexaarylbiimidazole compounds, ketoxime ester compounds, borate compounds, azinium compounds, metallocene compounds, active ester compounds, compounds having a carbon-halogen bond, and alkylamine compounds.

[0049] Among these, at least one selected from an acylphosphine oxide compound and a thioxanthone compound is preferred, and an acylphosphine oxide compound and a thioxanthone compound may be used in combination.

[0050] Among the acylphosphine oxide compounds, 2,4,6-trimethylbenzoyldiphenylphosphine oxide and bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide are more preferable. These photopolymerization initiators may be used in combination.

[0051] 2,4,6-trimethylbenzoyldiphenylphosphine oxide is preferred because it has excellent compatibility with the above-mentioned polymerizable compound. Specifically, an example of 2,4,6-trimethylbenzoyldiphenylphosphine oxide is "SpeedCure TPO" (trade name, manufactured by BASF Japan). Bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide is preferred because it has a wide range of absorption characteristics. Specifically, an example of bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide is "IRGACURE 819" (trade name, manufactured by BASF Japan).

[0052] Specific examples of the photoradical polymerization initiator include acetophenone, acetophenone benzyl ketal, xanthone, fluorenone, benzaldehyde, fluorene, anthraquinone, triphenylamine, carbazole, 3-methylacetophenone, 4-chlorobenzophenone, 4,4'-dimethoxybenzophenone, 4,4'-diaminobenzophenone, Michler's ketone, benzoin propyl ether, benzoin ethyl ether, benzil dimethyl ketal, 1-(4-isopropylphenyl)-2-hydroxy-2-methylpropan-1-one, 2-hydroxy-2-methylpropan-1-one, and 2-hydroxy-2-methylpropan-1-one. 4-(methylthio)phenyl]-2-morpholino-propan-1-one, bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide, 2,4,6-trimethylbenzoyl-diphenyl-phosphine oxide, 2,4-diethylthioxanthone, and bis-(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide.

[0053] Commercially available photoradical polymerization initiators include, for example, IRGACURE 907 (2-methyl-1-(4-methylthiophenyl)-2-morpholinopropan-1-one), IRGACURE 369 (2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1), Omnirad 379 (2-(dimethylamino)-2-[(4-methylphenyl)methyl]-1-[4-(4-morpholinyl)phenyl]-1-butanone), DAROCUR TPO (2,4,6-trimethylbenzoyl-diphenyl-phosphine oxide), IRGACURE 819 (bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide), IRGACURE 820 (bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide), IRGACURE 830 (bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide), IRGACURE 840 (bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide), IRGACURE 850 (bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide), IRGACURE 860 (bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide), IRGACURE 870 (bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide), IRGACURE 880 (bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide), IRGACURE 89 ... 784 (Bis(η5-2,4-cyclopentadien-1-yl)-bis(2,6-difluoro-3-(1H-pyrrol-1-yl)-phenyl)titanium), IRGACURE OXE 01 (1,2-octanedione, 1-[4-(phenylthio)-, 2-(O-benzoyloxime)]), IRGACURE OXE 02 (Ethanone, 1-[9-ethyl-6-(2-methylbenzoyl)-9H-carbazol-3-yl]-, 1-(O-acetyloxime)), IRGACURE 754 (Mixture of oxyphenylacetic acid, 2-[2-oxo-2-phenylacetoxyethoxy]ethyl ester and oxyphenylacetic acid, 2-(2-hydroxyethoxy)ethyl ester), Lucirin TPO, LR8893, LR8970 (all manufactured by BASF Japan), KAYACURE Examples of such compounds include DETX-S (2,4-diethylthioxanthone) (manufactured by Nippon Kayaku Co., Ltd.), Ubecryl P36 (manufactured by UCB), Speedcure TPO (diphenyl-2,4,6-trimethylbenzoylphosphine oxide), and Speedcure TPO (diphenyl-(2,4,6-trimethylbenzoyl)phosphine oxide) (all manufactured by Lambson).

[0054] The photopolymerization initiators may be used alone or in combination of two or more.

[0055] The upper limit of the content of the photopolymerization initiator other than the α-hydroxyketone initiator is preferably 5% by mass or less, more preferably 3% by mass or less, even more preferably 1% by mass or less, and particularly preferably 0% by mass, relative to the total amount (100% by mass) of the inkjet composition. By having the content of the α-hydroxyketone initiator within the above range, yellowing of the printed coating film is prevented, and excellent coating film color tone tends to be obtained.

[0056] Colorants The radiation curable ink jet composition used in the ink jet recording method according to this embodiment may contain a coloring material. The coloring material may be at least one of a pigment and a dye.

[0057] As the pigment, both inorganic and organic pigments can be used.

[0058] As the inorganic pigment, carbon blacks such as furnace black, lamp black, acetylene black, and channel black (CI Pigment Black 7), iron oxide, and titanium oxide can be used.

[0059] Examples of organic pigments include azo pigments such as insoluble azo pigments, condensed azo pigments, azo lakes, and chelate azo pigments; polycyclic pigments such as phthalocyanine pigments, perylene and perinone pigments, anthraquinone pigments, quinacridone pigments, dioxane pigments, thioindigo pigments, isoindolinone pigments, and quinophthalone pigments; dye chelates (e.g., basic dye chelates, acid dye chelates, etc.); dye lakes (basic dye lakes, acid dye lakes), nitro pigments, nitroso pigments, aniline black, and daylight fluorescent pigments.

[0060] More specifically, examples of carbon black used in black ink include No. 2300, No. 900, MCF88, No. 33, No. 40, No. 45, No. 52, MA7, MA8, MA100, No. 2200B, etc. (all manufactured by Mitsubishi Chemical Corporation), Raven 5750, Raven 5250, Raven 5000, Raven 3500, Raven 1255, Raven 700, etc. (all manufactured by Carbon Columbia), Rega1 400R, Rega1 330R, Rega1 660R, Mogul L, Monarch 700, Monarch 800, Monarch 880, Monarch 900, Monarch 1000, Monarch 1100, Monarch 1300, Monarch 1400, etc. (manufactured by CABOT Corporation). Examples of such inks include Color Black FW1, Color Black FW2, Color Black FW2V, Color Black FW18, Color Black FW200, Color Black S150, Color Black S160, Color Black S170, Printex 35, Printex U, Printex V, Printex 140U, Special Black 6, Special Black 5, Special Black 4A, and Special Black 4 (all manufactured by Degussa).

[0061] Pigments used in white inks include CI Pigment White 6, 18, and 21.

[0062] Pigments used in the yellow inks include CI Pigment Yellow 1, 2, 3, 4, 5, 6, 7, 10, 11, 12, 13, 14, 16, 17, 24, 34, 35, 37, 53, 55, 65, 73, 74, 75, 81, 83, 93, 94, 95, 97, 98, 99, 108, 109, 110, 113, 114, 117, 120, 124, 128, 129, 133, 138, 139, 147, 151, 153, 154, 155, 167, 172, and 180.

[0063] The pigments used in the magenta inks are CI Pigment Red 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 14, 15, 16, 17, 18, 19, 21, 22, 23, 30, 31, 32, 37, 38, 40, 41, 42, 48(Ca), 48(Mn), 57(Ca), 57:1, 88, 112, 114, 122, 123, 144, 146, 149, 150, 166, 168, 170, 171, 175, 176, 177, 178, 179, 184, 185, 187, 202, 209, 219, 224, 245, or CI Pigment Violet. 19, 23, 32, 33, 36, 38, 43, and 50.

[0064] Pigments used in cyan inks include CI Pigment Blue 1, 2, 3, 15, 15:1, 15:2, 15:3, 15:34, 15:4, 16, 18, 22, 25, 60, 65, 66, CI Vat Blue 4, 60.

[0065] Examples of pigments other than magenta, cyan, and yellow include CI Pigment Green 7, 10, CI Pigment Brown 3, 5, 25, and 26, and CI Pigment Orange 1, 2, 5, 7, 13, 14, 15, 16, 24, 34, 36, 38, 40, 43, and 63.

[0066] The above pigments may be used alone or in combination of two or more, but the radiation-curable inkjet composition used in the inkjet recording method according to this embodiment is preferably a white ink containing the pigment used in the white ink. Because the white ink is white, yellowing of the printed coating film is easily noticeable and color reproducibility is easily poor, but according to the inkjet recording method according to this embodiment, even such a white ink can provide an excellent coating film color tone.

[0067] In this specification, the term "white" when referring to white ink does not only refer to perfect white, but also includes chromatic or achromatic colors and glossy colors as long as they are visible as white. White inks include inks that are sold under names that suggest that they are white inks. More quantitatively, "white" can be expressed as, for example, L in CIELAB. * Not only colors with L = 100, * is between 80 and 100, and a * and b * The above range also includes colors where the difference between the ink levels is within ±10. More specifically, the above range is reached when a sufficient amount of color ink is applied to a transparent recording medium and the applied area is measured in reflection mode using a colorimeter conforming to CIELAB. A sufficient amount of ink is, for example, 15 mg / cm 2 Such an ink is called a white ink.

[0068] When the above pigment is used, the average particle size is preferably 500 nm or less, more preferably 50 to 300 nm. When the average particle size is within the above range, the inkjet composition has better reliability in terms of ejection stability and dispersion stability, and can form an image with excellent image quality. Here, the average particle size in this specification is measured by a dynamic light scattering method.

[0069] The dye is not particularly limited, and acid dyes, direct dyes, reactive dyes, and basic dyes can be used. Examples of the dyes include CI Acid Yellow 17, 23, 42, 44, 79, 142, CI Acid Red 52, 80, 82, 249, 254, 289, CI Acid Blue 9, 45, 249, CI Acid Black 1, 2, 24, 94, CI Food Black 1, 2, CI Direct Yellow 1, 12, 24, 33, 50, 55, 58, 86, 132, 142, 144 , 173, CI Direct Red 1, 4, 9, 80, 81, 225, 227, CI Direct Blue 1, 2, 15, 71, 86, 87, 98, 165, 199, 202, CI Direct Black 19, 38, 51, 71, 154, 168, 171, 195, CI Reactive Red 14, 32, 55, 79, 249, and CI Reactive Black 3, 4, and 35.

[0070] The above dyes may be used alone or in combination of two or more.

[0071] The content of the coloring material is preferably 1.2% by mass or less, more preferably 1.0% by mass or less, and even more preferably 0.8% by mass or less, based on the total amount (100% by mass) of the inkjet composition. It also includes 0% by mass, which does not include any coloring material. If the content of the coloring material is within the above range, yellowing of the printed coating film is easily noticeable and color reproducibility is easily deteriorated, but according to the inkjet recording method of this embodiment, even with such an ink, excellent coating film color tone can be obtained.

[0072] <Dispersant> When the inkjet composition contains a pigment, it may further contain a dispersant to improve the pigment dispersibility. The dispersant is not particularly limited, but examples thereof include dispersants commonly used for preparing pigment dispersions, such as polymer dispersants. Specific examples thereof include those mainly composed of one or more of polyoxyalkylene polyalkylene polyamines, vinyl polymers and copolymers, acrylic polymers and copolymers, polyesters, polyamides, polyimides, polyurethanes, amino polymers, silicon-containing polymers, sulfur-containing polymers, fluorine-containing polymers, and epoxy resins. Commercially available polymer dispersants include the Ajisper series manufactured by Ajinomoto Fine-Techno Co., Ltd., the Solsperse series (Solsperse36000, etc.) available from Avecia and Noveon, the Disperbyk series (BYK180, etc.) manufactured by BYK Additives & Instruments, and the Disparlon series manufactured by Kusumoto Chemicals Co., Ltd.

[0073] Other ingredients The radiation curable ink jet composition used in the ink jet recording method according to this embodiment may contain a surfactant and a polymerization inhibitor as other components.

[0074] <Surfactant> The surfactant is preferably a silicone surfactant, more preferably a polyester-modified silicone or a polyether-modified silicone. Examples of polyester-modified silicone include BYK-347, 348, BYK-UV3500, 3510, and 3530 (all manufactured by BYK Additives & Instruments), and examples of polyether-modified silicone include BYK-3570 (manufactured by BYK Additives & Instruments). The surfactant may be used alone or in combination of two or more.

[0075] <Polymerization inhibitor> As the polymerization inhibitor, a hindered amine compound is preferred. Other polymerization inhibitors include, but are not limited to, p-methoxyphenol, hydroquinone monomethyl ether (MEHQ), hydroquinone, cresol, t-butylcatechol, 3,5-di-t-butyl-4-hydroxytoluene, 2,2'-methylenebis(4-methyl-6-t-butylphenol), 2,2'-methylenebis(4-ethyl-6-butylphenol), 4,4'-thiobis(3-methyl-6-t-butylphenol), and 2,2,6,6-tetramethyl-4-hydroxypiperidine-1-oxyl (trade name "LA-7RD", manufactured by ADEKA Corporation). The polymerization inhibitor may be used alone or in combination of two or more.

[0076] The content of the polymerization inhibitor is preferably 0.05% by mass or more and 0.5% by mass or less, and more preferably 0.1% by mass or more and 0.5% by mass or less, relative to the total amount (100% by mass) of the inkjet composition.

[0077] Physical Properties The viscosity at 20°C of the radiation curable inkjet composition used in the inkjet recording method according to this embodiment is preferably 25 mPa·s or less, more preferably 5 to 20 mPa·s. When the viscosity at 20°C of the inkjet composition is within the above range, an appropriate amount of the inkjet composition is ejected from the nozzle, and deflection and scattering of the inkjet composition can be further reduced, making it suitable for use in inkjet recording devices. The viscosity can be measured using a viscoelasticity tester MCR-300 (manufactured by Pysica) in an environment of 20°C by increasing the shear rate from 10 to 1000 and reading the viscosity at a shear rate of 200.

[0078] The surface tension at 20°C of the radiation curable inkjet composition used in the inkjet recording method according to this embodiment is preferably 20 mN / m or more and 30 mN / m or less. When the surface tension at 20°C of the inkjet composition is within the above range, the inkjet composition is less likely to wet a nozzle that has been treated for liquid repellency. This allows an appropriate amount of the inkjet composition to be ejected from the nozzle, and the inkjet composition can be further prevented from deflecting or scattering, making it suitable for use in inkjet recording devices. The surface tension can be measured by checking the surface tension when a platinum plate is wetted with ink in an environment of 20°C using an automatic surface tensiometer CBVP-Z (manufactured by Kyowa Interface Science Co., Ltd.).

[0079] 1.1.1.6. Method for ejecting ink-jet composition The radiation curable ink jet composition used in the ink jet recording method according to this embodiment is ejected onto a recording medium by an ink jet head. An ink jet recording apparatus equipped with an ink jet head for ejecting the ink jet composition will be described later.

[0080] 1.1.2. Recording medium The recording medium used in the inkjet recording method according to this embodiment may have a recording surface that absorbs liquid such as ink, or may not have a recording surface that absorbs liquid. Therefore, the recording medium is not particularly limited, and examples thereof include absorbent recording media such as paper, film, and cloth, low absorbent recording media such as printing paper, and non-absorbent recording media such as polymers.

[0081] Here, the term "low-absorbency recording medium" or "non-absorbency recording medium" refers to a recording medium that does not absorb liquid such as ink at all or absorbs very little of it. Quantitatively, a low-absorbency recording medium or a non-absorbency recording medium is one that "is capable of absorbing ink within 30 msec from the start of contact in the Bristow method." 1 / 2 Water absorption up to 10mL / m 2This refers to a recording medium that is "absorbent or less than 100%." ​​The Bristow method is the most widely used method for measuring the amount of liquid absorbed in a short period of time, and is also adopted by the Japan Pulp and Paper Technical Association (JAPAN TAPPI). Details of the test method are described in Standard No. 51 "Paper and paperboard - Liquid absorbency test method - Bristow method" of the "JAPAN TAPPI Paper and Pulp Test Method 2000 Edition." In contrast, liquid absorbent recording media refers to recording media that do not fall under the category of low absorbency recording media or non-absorbency recording media.

[0082] Examples of non-absorbent recording media include those in which a plastic is coated on a substrate such as paper, those in which a plastic film is adhered to a substrate such as paper, plastic films that do not have an absorption layer (receptive layer), etc. Examples of plastics include films and plates of polymers such as polyvinyl chloride, polyethylene terephthalate, polycarbonate, polystyrene, polyurethane, polyethylene, polypropylene, polyvinyl acetal, and blend compositions of two or more of these, as well as films and plates of cellulose diacetate, cellulose triacetate, cellulose propionate, cellulose butyrate, cellulose acetate butyrate, cellulose nitrate, etc.

[0083] The recording medium may be colorless and transparent, semi-transparent, colored and transparent, chromatic and opaque, achromatic and opaque, etc. Furthermore, the recording medium itself may be colored, semi-transparent, or transparent.

[0084] The recording medium used in the inkjet recording method according to the present embodiment is preferably a non-absorbent recording medium. Non-absorbent recording media containing plastics, particularly non-absorbent recording media containing polyethylene terephthalate or polypropylene, are susceptible to heat damage and shrinkage. However, according to the inkjet recording method according to the present embodiment, even if the recording medium is non-absorbent, it is possible to prevent the yellowing of the printing coating while preventing deterioration of the recording medium due to heat.

[0085] 1.2.Curing process The inkjet recording method according to the present embodiment includes a curing step of irradiating the inkjet composition ejected onto a recording medium with radiation from an LED irradiator having a peak wavelength of 250 to 310 nm to form a cured coating film. Hereinafter, an inkjet recording device equipped with an LED irradiator having a peak wavelength of 250 to 310 nm and an inkjet head that can be used in the inkjet recording method according to the present embodiment will be described.

[0086] 1.2.1. Configuration of Inkjet Printing Apparatus An embodiment of an inkjet recording apparatus that can be used in the inkjet recording method according to the present embodiment will be described below. The inkjet recording apparatus that can be used in the inkjet recording method according to the present embodiment includes an inkjet head and an LED irradiator having a peak wavelength of 250 to 310 nm.

[0087] An example of an inkjet recording apparatus that can be used in the inkjet recording method according to this embodiment is shown in Fig. 1. Fig. 1 is a perspective view of an inkjet recording apparatus that can be used in the inkjet recording method according to this embodiment. Fig. 2 is a front view of radiation irradiation apparatuses 90A (corresponding to 190A in Fig. 2) and 90B (corresponding to 190B in Fig. 2) shown in Fig. 1. Fig. 3 is a view taken along the arrow AA in Fig. 2.

[0088] The inkjet recording device 20 shown in FIG. 1 comprises a motor 30 that feeds the recording medium P in the sub-scanning direction SS, a platen 40, an inkjet head 52 that forms radiation curable ink into fine droplets and ejects them from head nozzles onto the recording medium P, a carriage 50 on which the inkjet head 52 is mounted, a carriage motor 60 that moves the carriage 50 in the main scanning direction MS, and a pair of radiation irradiation devices 90A, 90B that irradiate radiation onto the droplets of radiation curable ink ejected from the inkjet head 52 and deposited on the recording medium P.

[0089] The carriage 50 is pulled by a traction belt 62 driven by a carriage motor 60 and moves along a guide rail 64 .

[0090] The inkjet head 52 is mounted on a carriage 50, and moves in the main scanning direction MS (hereinafter also referred to as the "main scanning direction") as the carriage 50 moves in that direction.

[0091] In addition, the inkjet head 52 is capable of ejecting a radiation-curable inkjet composition. In the example of Fig. 1, the inkjet head 52 is a serial type head for full-color printing that ejects four colors of ink, and each color is provided with a large number of head nozzles. In addition to the inkjet head 52, the carriage 50 on which the inkjet head 52 is mounted also mounts a black cartridge 54 as a black ink container that contains black ink to be supplied to the inkjet head 52, and a color ink cartridge 56 as a color ink that contains color ink to be supplied to the inkjet head 52. The ink contained in each cartridge 54, 56 is a radiation-curable inkjet composition.

[0092] In the ejection step of this embodiment, the amount of droplets ejected from the inkjet head 52 is preferably 1 pl or more and 20 pl or less. By having the amount of droplets within the above range, ejection stability is good, and a high-quality image can be obtained.

[0093] A capping device 80 is provided at the home position of the carriage 50 (the position on the right side in FIG. 1) for sealing the nozzle surface 52a of the inkjet head 52 when the carriage 50 is stopped. When the print job ends and the carriage 50 reaches above the capping device 80, the capping device 80 automatically rises by a mechanism (not shown) to seal the nozzle surface 52a of the inkjet head 52. This capping prevents the ink in the nozzles from drying out or deteriorating.

[0094] Although not shown, a wipe unit for wiping the nozzle surface 52a of the inkjet head 52 when the carriage 50 is stopped may be provided at the home position (the position on the right side in FIG. 1) of the carriage 50. The wipe unit is provided at a position that does not interfere with the capping device 80, and can remove droplets, mist, and / or hardened products thereof adhering to the nozzle surface 52a by scrubbing the nozzle surface 52a of the inkjet head 52, on which the nozzles are formed, with a squeegee-like wiper blade.

[0095] Droplets of a radiation curable ink jet composition can be discharged and deposited on a recording medium by using such an ink jet recording apparatus 20. Furthermore, the ink jet recording apparatus 20 makes it possible to continuously perform the discharge step and the curing step in one apparatus, without performing the discharge step and the curing step in separate apparatuses.

[0096] The radiation is not particularly limited as long as it has a peak wavelength of 250 nm or more and 310 nm or less. Examples of the irradiation means capable of irradiating radiation include the radiation irradiation devices shown in Figs.

[0097] As shown in FIGS. 1 to 3, the radiation irradiation devices 190A and 190B are attached to both ends of the carriage 50 along the direction of movement thereof.

[0098] 2, the radiation irradiation device 190A attached to the left side of the inkjet head 52 irradiates droplets ejected onto the recording medium P with radiation during a right scan in which the carriage 50 moves to the right (in the direction of arrow B in FIG. 2). On the other hand, the radiation irradiation device 190B attached to the right side of the inkjet head 52 irradiates droplets ejected onto the recording medium P with radiation during a left scan in which the carriage 50 moves to the left (in the direction of arrow C in FIG. 2).

[0099] Each of the radiation irradiation devices 190A, 190B is attached to the carriage 50 and includes a housing 194 that supports aligned LED irradiators 192, and a light source control circuit (not shown) that controls the turning on and off of the LED irradiators 192. As shown in Figures 2 and 3, each of the radiation irradiation devices 190A, 190B is provided with one LED irradiator 192, but two or more LED irradiators may be provided.

[0100] The LED irradiator 192 has a peak wavelength of 250 nm or more and 310 nm or less. More preferably, it has a peak wavelength of 260 nm or more and 300 nm or less, and even more preferably, it has a peak wavelength of 270 nm or more and 290 nm or less. If the LED irradiator 192 has a peak wavelength within the above range, radicals are generated more effectively in the α-hydroxyketone initiator, and therefore the curing property may be superior. In addition, since curing can be performed with less energy, thermal damage to the recording medium may be reduced. Note that, when the radiation irradiated from the LED irradiator has multiple peak wavelengths, it is sufficient that at least one peak wavelength is within the above range.

[0101] Commercially available products of such LED irradiators include Deep Ultraviolet LED Irradiator (trade name, manufactured by iGraphics, peak wavelength: 280 nm), Deep Ultraviolet LED Irradiator (trade name, manufactured by Micro Square, peak wavelengths: 285 nm and 300 nm), VM0815W-171 (trade name, manufactured by Nikkiso Giken, peak wavelength: 285 nm), Evaluation Surface Irradiation 7x7 (trade name, manufactured by Nikkiso Giken, peak wavelengths: 265 nm, 280 nm, 285 nm, 300 nm, and 310 nm), and the like.

[0102] Furthermore, the LED irradiators 192 may emit light of the same wavelength or different wavelengths.

[0103] According to the radiation irradiation devices 190A and 190B, as shown in FIG. 2, droplets deposited on the recording medium P by ejection from the inkjet head 52 are irradiated with radiation 192a by an LED irradiator 192 that irradiates the recording medium P near the inkjet head 52, thereby hardening at least the surface of the droplets and forming an image on the recording medium.

[0104] Hereinafter, a method of forming an image in a desired area by repeating the ejection step and the curing step multiple times in this embodiment will be described in detail.

[0105] First, while moving the carriage 50 to the right (the direction of arrow B in FIG. 2), droplets of a radiation curable ink jet composition of one or more colors are ejected onto the recording medium P, and the ejected ink is irradiated with radiation by the radiation irradiation device 190A. Next, a sub-scan is performed to move the recording medium P in the sub-scanning direction SS. Note that in this specification, one main scan in which droplets are ejected while moving the carriage 50 in one direction in the main scanning direction MS, and the droplets are irradiated with radiation, is referred to as one pass.

[0106] Thereafter, while moving the carriage 50 in the left direction (the direction of the arrow C in FIG. 2), droplets of one or more colors are discharged onto the recording medium P by the method shown in the above-mentioned discharge step, and one main scan (one pass) is further performed in which the discharged ink is irradiated with radiation by the radiation irradiation device 190B. At this time, the ink jet composition on the recording medium is irradiated with radiation by the radiation irradiation device 190A and the radiation irradiation device 190B. Next, a sub-scan is further performed in which the recording medium P is moved in the sub-scanning direction SS.

[0107] Through the above operations, the coating film formed in the first pass is irradiated with radiation in two passes, once in the first pass and twice in the second pass, for a total of three irradiations. Also, the coating film formed in the second pass is irradiated with radiation in one pass, for a single irradiation.

[0108] By repeating this operation, an image consisting of a collection of coating films can be formed in a predetermined area.

[0109] Furthermore, the inkjet recording apparatus 20 of this embodiment may further include a separate radiation irradiation means (not shown) downstream in the sub-scanning direction SS, which is the moving direction of the recording medium P. This allows the coating film on the recording medium P to be sufficiently cured to the inside after all passes have been completed and the formation of an image on the recording medium has been completed.

[0110] For example, if the accumulated irradiation energy irradiated by the radiation irradiation device 190A or 190B is insufficient and the inside of the droplet on the recording medium P is not hardened, the inside of the droplet can be reliably hardened by using a radiation irradiation means in the sub-scanning direction SS.

[0111] The radiation irradiating means in the sub-scanning direction SS only needs to be provided at a position where it can irradiate droplets on the recording medium P sent in the sub-scanning direction SS with radiation, and can be installed, for example, on the carriage 50 and downstream of the inkjet head 52 (in the sub-scanning direction SS which is the direction of movement of the recording medium P). Furthermore, as the radiation irradiating means in the sub-scanning direction SS, a device similar to the radiation irradiating device 190A (190B) can be used.

[0112] 2. Inkjet recording device An inkjet recording apparatus according to one embodiment of the present invention includes an inkjet head that ejects a radiation-curable inkjet composition containing a polymerizable compound and an α-hydroxyketone initiator onto a recording medium, and an LED irradiator having a peak wavelength in the range of 250 to 310 nm.

[0113] The inkjet recording apparatus according to the present embodiment is equipped with an LED irradiator having a peak wavelength of 250 to 310 nm, and since the irradiator is energy-saving, it can irradiate radiation having a peak wavelength of 250 to 310 nm while reducing thermal damage to the recording medium. In addition, the radiation-curable inkjet composition used in the recording apparatus according to the present embodiment contains an α-hydroxyketone initiator, which is less likely to cause yellowing of the coating film and is easy to absorb radiation having the above-mentioned peak wavelength. Therefore, according to the inkjet recording apparatus according to the present embodiment, the printed coating film can be sufficiently cured, thermal damage to the recording medium can be reduced, and yellowing of the printed coating film can be prevented. In addition, since the inkjet recording apparatus according to the present embodiment is equipped with an LED irradiator, the size of the radiation irradiator can be made compact compared to the case where a metal halide lamp or the like is used, and therefore the size of the entire inkjet recording apparatus can be made compact.

[0114] As the inkjet recording apparatus according to this embodiment, it is possible to use the inkjet recording apparatus as shown in the above-mentioned Figures 1 to 3. As the radiation curable inkjet composition used in the inkjet recording apparatus according to this embodiment, it is possible to use the above-mentioned radiation curable inkjet composition.

[0115] 3. Working Example The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. In the following, "%" is based on mass unless otherwise specified.

[0116] 3.1. Preparation of Inkjet Composition First, the pigment, dispersant, and base monomer (PEA) were weighed and placed in a pigment dispersion tank so as to obtain the composition shown in Table 1 below, and a ceramic bead mill with a diameter of 1 mm was placed in the tank and stirred to obtain each dispersion liquid in which the pigment was dispersed in the base monomer. Next, a polymerizable compound, an initiator, a polymerization inhibitor, and a surfactant were placed in a mixing tank, which is a stainless steel container, so as to obtain the composition shown in Table 1 below, and after mixing and stirring to completely dissolve, the dispersion liquid obtained above was added, and further mixed and stirred at room temperature for 1 hour, and further filtered through a 5 μm membrane filter to obtain an inkjet composition of each example. The numerical value of each component shown in each example in the table represents mass %.

[0117] [Table 1]

[0118] The following provides additional explanations for each component shown in Table 1 above. <Pigments> PB15:3 (CI Pigment Blue 15:3) Titanium dioxide (CI Pigment White 6) <Dispersant> Dispersion A: Solsperse 36000 (product name, Lubrizol, polymer dispersant) Dispersion B: BYK180 (product name, manufactured by BYK Japan, polymer dispersant) <Polymerizable compound> VEEA (trade name, manufactured by Nippon Shokubai Co., Ltd., 2-(2-vinyloxyethoxy)ethyl acrylate) PEA (product name "Viscoat #192", manufactured by Osaka Organic Chemical Industry Co., Ltd., phenoxyethyl acrylate) IBXA (product name, Osaka Organic Chemical Industry Co., Ltd., isobornyl acrylate) · CTFA (product name "SR531", manufactured by SARTOMER, cyclic trimethylolpropane formal acrylate) DPGDA (product name "APG-100", Shin-Nakamura Chemical Co., Ltd., dipropylene glycol diacrylate) TPGDA (product name "APG-200", Shin-Nakamura Chemical Co., Ltd., tripropylene glycol diacrylate) A-DHP (product name, Shin-Nakamura Kogyo Co., Ltd., dipentaerythritol hexaacrylate) <Initiator> Omnirad 127 (trade name, IGM Resins BV, 2-hydroxy-1-(4-(4-(2-hydroxy-2-methyl-propionyl)-benzyl)-phenyl)-2-methyl-propane) Omnirad 2959 (trade name, IGM Resins BV, 1-(4-(2-hydroxyethoxyl)-phenyl)-2-hydroxymethylpropanone) ESACURE KIP 150 (trade name, manufactured by Lamberti, oligo(2-hydroxy-2-methyl-1-(4-(1-methylvinyl)phenyl)propanone) ESACURE KIP 160 (trade name, manufactured by Lamberti, 2-hydroxy-1-[4-{4-(2-hydroxy-2-methylpropanoyl)phenoxy}phenyl]-2-methylpropan-1-one) Omnirad 184 (product name, IGM Resins BV, 1-hydroxycyclohexyl phenyl ketone) Speedcure TPO (trade name, Lambson, 2,4,6-trimethylbenzoyl-diphenylphosphine oxide) Omnirad 819 (trade name, manufactured by IGM Resins BV, bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide) Omnirad 379 (trade name, IGM Resins BV, 2-dimethylamino-2-(4-methyl-benzyl)-1-(4-morpholin-4-yl-phenyl)butan-1-one) <Surfactant> BKY-UV3500 (product name, manufactured by BYK Japan, polyether-modified polydimethylsiloxane) <Polymerization inhibitor> LA-7RD (trade name, manufactured by ADEKA, 2,2,6,6-tetramethyl-4-hydroxypiperidine-1-oxyl) MEHQ (product name, manufactured by Kanto Chemical Co., Ltd., hydroquinone monomethyl ether)

[0119] 3.2.Evaluation Methodology 3.2.1. Recording test Each of the inkjet compositions obtained above was filled into each nozzle row using a modified inkjet printer "PX-G5000" (product name, manufactured by Seiko Epson Corporation) modified so as to perform recording under the curing conditions described in Tables 2 and 3 below. A solid pattern image was printed on a thin PET film (product name "PET50A PL Shin", manufactured by Lintec Corporation) at room temperature and normal pressure, with the ink dots having a medium diameter and a film thickness of 10 μm, and the solid pattern image was cured under the curing conditions described in Tables 2 and 3 below. In this manner, a recorded product in which a solid pattern image was printed on a thin PET film was produced.

[0120] It should be noted that a "solid pattern image" is an image in which dots are recorded for all pixels, which are the minimum recording unit area defined by the recording resolution.

[0121] Details of the curing conditions shown in Tables 2 and 3 below are as follows. <Curing conditions: LED-1> The inkjet recording device has a radiation light source in a radiation irradiation device with a peak wavelength of 280 nm and an irradiation energy of 100 mW / cm. 2 The "Deep-UV LED Irradiator" (product name, manufactured by iGraphics Co., Ltd.) was installed on both sides of the carriage with an irradiation range of 210 mm wide. The carriage speed was set to 1300 mm / s, and a modified inkjet printer "PX-G5000" (product name, manufactured by Seiko Epson Corporation) was modified to the conditions of approximately 33 mJ per pass. Printing was performed in 8 passes so that the cumulative light amount was 260 mJ.

[0122] <Curing conditions: LED-2> The inkjet recording device has a radiation light source in a radiation irradiation device with a peak wavelength of 310 nm and irradiation energy of 50 mW / cm. 2 The "Evaluation Surface Irradiation 7x7" (product name, manufactured by Nikkiso Giken Co., Ltd.) was installed on both sides of the carriage with an irradiation range of 210 mm wide. The carriage speed was set to 1300 mm / s, and a modified inkjet printer "PX-G5000" (product name, manufactured by Seiko Epson Corporation) was modified to the conditions of approximately 17 mJ per pass. Printing was performed in 16 passes so that the cumulative light amount was 260 mJ.

[0123] <Curing conditions: LED-11> The inkjet recording device has a radiation light source in a radiation irradiation device with a peak wavelength of 385 nm and irradiation energy of 300 mW / cm. 2 A modified inkjet printer "PX-G5000" (Seiko Epson Corporation product name) was used, in which the size of the LED panel and the distance between the LED panel and the recording medium were adjusted so that the cumulative energy per pass was 65 mJ. An image was recorded in 4 passes so that the cumulative light amount was 260 mJ.

[0124] <Curing conditions: Metal halide-1> The inkjet recording device used was a modified inkjet printer "PX-G5000" (product name manufactured by Seiko Epson Corporation) in which the radiation light source in the radiation irradiation device mounted next to the carriage was a metal halide lamp (manufactured by Eye Graphics Co., Ltd.) and the distance between the metal halide lamp and the recording medium was adjusted so that the cumulative energy per pass was 150 mJ. An image was recorded in 4 passes so that the cumulative light amount was 600 mJ.

[0125] Note that a "PASS" refers to one operation in which the inkjet head scans in the main scanning direction to form an image and irradiate radiation. The cumulative energy per PASS [mJ] is the radiation energy [mW / cm2] on the irradiated surface irradiated from the light source.2 ], irradiation duration per PASS [s], and irradiated area per PASS [cm 2 The irradiation energy was measured using an integrating illuminometer corresponding to each peak wavelength.

[0126] [Table 2]

[0127] [Table 3]

[0128] 3.2.2. Evaluation of curability The printed surface of each of the recorded materials obtained above was rubbed with a cotton swab and evaluated according to the following criteria. The cotton swab used was a Johnson cotton swab manufactured by Johnson & Johnson (registered trademark). The swabs were rubbed back and forth 10 times, with a rubbing force of 100 g. (Evaluation Criteria) A: There is no mark at all. B: Slightly scratched. C: Scratch marks are clearly visible.

[0129] 3.2.3.Evaluation of coating color Each of the recorded materials obtained above was visually observed and judged according to the following evaluation criteria. (Evaluation Criteria) A: I can't see any yellowness. B: Yellowish tinge can be seen.

[0130] 3.2.4. Evaluation of heat damage Each of the recorded materials obtained above was visually observed and judged according to the following evaluation criteria. (Evaluation Criteria) A: Smooth and free of defects such as wrinkles. B: Wrinkles can be seen.

[0131] 3.2.5. Evaluation of odor from printed matter Each of the recorded materials obtained above was smelled and evaluated according to the following criteria. (Evaluation Criteria) A: There is no odor. B: There is a slight odor. C: There is an odor.

[0132] 3.3.Evaluation Results The results of the evaluation tests are shown in Tables 2 and 3 above.

[0133] From the above evaluation results, in each Example, an LED irradiator having a peak wavelength of 250 to 310 nm and an inkjet composition containing an α-hydroxyketone-based initiator were used, which prevented the coating film from yellowing, achieved excellent color reproducibility (coating film color tone), and prevented thermal damage to the recording medium.

[0134] In contrast, in Comparative Examples 1 and 2, an inkjet composition containing an α-hydroxyketone initiator was used, but since the peak wavelength of the LED irradiator was 385 nm, the ink did not harden in the first place and the recorded matter was not usable. Also, in Comparative Examples 3 to 6, either the coating film color tone or heat damage resistance was poor.

[0135] The following can be derived from the above-described embodiment.

[0136] One aspect of the inkjet recording method is a step of discharging the radiation curable ink jet composition from an ink jet head onto a recording medium; a curing step of irradiating the ink jet composition discharged onto the recording medium with radiation from an LED irradiator having a peak wavelength of 250 to 310 nm to form a cured coating film, The inkjet composition contains a polymerizable compound and an α-hydroxyketone-based initiator.

[0137] In the above-mentioned inkjet recording method, The inkjet composition may be a white ink.

[0138] In the above-mentioned inkjet recording method, The inkjet composition may have a coloring material content of 1.2% by mass or less based on the total mass of the composition.

[0139] In the above-mentioned inkjet recording method, The molecular weight of the α-hydroxyketone initiator may be 210 or more.

[0140] The polymerizable compound may contain a vinyl group-containing (meth)acrylate represented by the following general formula (I) in an amount of 10% by mass or more and 50% by mass or less based on the total amount of the composition. H2C=CR 1 -CO-OR 2 -O-CH=CH-R 3 (I) (In the formula, R 1 is a hydrogen atom or a methyl group, R 2 is a divalent organic residue having 2 to 20 carbon atoms, and R 3 is a hydrogen atom or a monovalent organic residue having 1 to 11 carbon atoms.

[0141] The recording medium may be a non-absorbent recording medium.

[0142] One aspect of the inkjet recording apparatus is an inkjet head that ejects a radiation-curable inkjet composition containing a polymerizable compound and an α-hydroxyketone-based initiator onto a recording medium; It is equipped with an LED irradiator with a peak wavelength of 250 to 310 nm.

[0143] The present invention is not limited to the above-described embodiment, and various modifications are possible. For example, the present invention includes a configuration that is substantially the same as the configuration described in the embodiment, for example, a configuration that has the same function, method, and result, or a configuration that has the same purpose and effect. The present invention also includes a configuration in which non-essential parts of the configuration described in the embodiment are replaced. The present invention also includes a configuration that has the same action and effect as the configuration described in the embodiment, or a configuration that can achieve the same purpose. The present invention also includes a configuration in which publicly known technology is added to the configuration described in the embodiment. [Explanation of symbols]

[0144] 20...inkjet recording device, 30...motor, 40...platen, 50...carriage, 52...inkjet head, 52a...nozzle surface, 54...black ink cartridge, 56...color ink cartridge, 60...carriage motor, 62...traction belt, 64...guide rail, 80...capping device, 90A (190A), 90B (190B)...radiation irradiation device, 192...LED irradiator, 192a...radiation, 194...housing, P...recording medium, MS...main scanning direction, SS...sub-scanning direction

Claims

1. A radiation curable ink jet composition is ejected from an ink jet head onto a recording medium. The output process and The ink jet composition discharged onto the recording medium has a peak wavelength of 250 to 310 nm. and a curing step of irradiating radiation from an LED irradiator having a long length to form a cured coating film. A method for recording a dummy image, comprising: The inkjet composition includes a polymerizable compound and an α-hydroxyketone initiator. fruit, The content of the polymerizable compound is 35% by mass or more based on the total amount of the inkjet composition. the law of nature, The molecular weight of the α-hydroxyketone initiator is 210 or more; As the α-hydroxyketone initiator, oligo(2-hydroxy-2-methyl-1- (4-(1-methylvinyl)phenyl)propanone and 2-hydroxy-1-[4-{4 -(2-hydroxy-2-methylpropanoyl)phenoxy}phenyl]-2-methylprop propan-1-one, As the polymerizable compound, a vinyl group-containing (meth)acrylate represented by the following general formula (I) Monofunctional polymerizable compounds, bifunctional polymerizable compounds, and trifunctional or higher polyfunctional polymerizable compounds (However, the radiation has an acceleration voltage of 130 kV or more.) (Excluding the electron beam recording method below.) H 2 C=CR 1 -CO-OR 2 -O-CH=CH-R 3 ... (I) (wherein R 1 is a hydrogen atom or a methyl group, and R 2 is a divalent organic residue having 2 to 20 carbon atoms. and R 3 is a hydrogen atom or a monovalent organic residue having 1 to 11 carbon atoms.

2. The inkjet ink of claim 1 , wherein the inkjet composition is a white ink. Recording method.

3. The inkjet composition has a coloring material content of 1.2% by mass or less based on the total mass of the composition. The ink-jet recording method according to claim 1 or 2,

4. As the polymerizable compound, a vinyl group-containing (meth)acrylate represented by the following general formula (I) The composition according to any one of claims 1 to 5, wherein the content of the ester is 10% by mass or more and 50% by mass or less based on the total amount of the composition.

4. The ink jet recording method according to any one of claims 3 to 3. H 2 C=CR 1 -CO-OR 2 -O-CH=CH-R 3 ・・・ (I) (In the formula, R 1 is a hydrogen atom or a methyl group, R 2 is a divalent organic residue having 2 to 20 carbon atoms. R 3 is a hydrogen atom or a monovalent organic residue having 1 to 11 carbon atoms.

5. 5. The recording medium according to claim 1, wherein the recording medium is a non-absorbent recording medium. An inkjet recording method.

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