Ink set and inkjet recording method

Abstract

To provide an ink set whereby: when an image is formed by overcoating, it is possible to improve the adhesion of a coating film to a recording medium and ensure the wet spreadability of an over layer ink.SOLUTION: An ink set of radiation curable ink jet compositions, includes an under layer ink and an over layer ink. The under layer ink contains, as a polymerizable compound, 80.0 mass% or more of a monofunctional polymerizable compound with respect to a total mass of polymerizable compounds and contains 55.0 mass% or more of a polymerizable compound having a nitrogen heterocyclic structure and / or a polymerizable compound having a hydroxy group in total with respect to the total mass of the polymerizable compounds.SELECTED DRAWING: None

Description

Technical Field

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

Background Art

[0002] Conventionally, various methods have been used as a recording method for forming an image on a recording medium such as paper based on an image data signal. Among these, the inkjet method uses an inexpensive device to eject ink only onto the required image area and directly form an image on the recording medium. Therefore, ink can be used efficiently and the running cost is low. Furthermore, since the inkjet method produces little noise, it is an excellent recording method.

[0003] In recent years, in order to form printing with excellent water resistance, solvent resistance, and abrasion resistance on the surface of a recording medium, in an inkjet recording method, a radiation-curable ink that cures when irradiated with radiation (light, ultraviolet rays) has been used.

[0004] On the other hand, a recording method of sequentially overcoating various radiation-curable inks is widely practiced. For example, inks of different colors such as cyan (C), magenta (M), yellow (Y), black (K), and white (W) are overcoated to form a color image, or a colorless and transparent underlayer ink is overcoated on the color image for purposes such as image protection and gloss imparting.

[0005] For example, Patent Document 1 discloses a method of forming a clear coating film on a printed coating film using an energy-ray curable inkjet recording color ink composition (Ia) and then using an energy-ray curable inkjet recording underlayer ink composition (Ib) on the coating film, and setting the surface tension Sa (mN / m) of Ia applied to the lower layer and the surface tension Sb (mN / m) of Ib applied to the upper layer to satisfy the relationship Sb < Sa. The document also describes that the repellency of the upper layer composition applied on the printed coating film is reduced.

Prior Art Documents

[0006] [Patent Document 1] JP 2006-181801 A Summary of the Invention [Problem to be solved by the invention]

[0007] In general, when an image is formed using a curable composition, a large amount of a monofunctional polymerizable compound that is likely to form a relatively soft cured product may be blended with the intention of improving the adhesion of the image to a recording medium, etc. The same is true when an image is formed by overcoating, and it is believed that the adhesion of the coating film to a recording medium can be improved by using a large amount of a monofunctional polymerizable compound in the composition that forms the cured coating film.

[0008] However, it has been found that when a large amount of monofunctional polymerizable compounds is blended into the composition forming the lower layer in a multilayer coating, it is difficult to ensure the wetting and spreading of the upper layer composition simply by adjusting the relationship between the surface tensions of the lower layer composition and the upper layer composition. [Means for solving the problem]

[0009] One aspect of the ink set according to the present invention is An ink set of a radiation curable ink jet composition comprising an undercoat ink and an overcoat ink, The underlayer ink contains, as polymerizable compounds, a monofunctional polymerizable compound in an amount of 80.0 mass% or more relative to the total amount of the polymerizable compounds, and a polymerizable compound having a nitrogen heterocyclic structure and / or a polymerizable compound having a hydroxyl group in a total amount of 55.0 mass% or more relative to the total amount of the polymerizable compounds.

[0010] One aspect of the inkjet recording method according to the present invention comprises: An inkjet recording method using the ink set described above, a first application step of applying the underlayer ink to a recording medium; a first curing step of irradiating the ink for an underlayer on the recording medium with radiation to form an underlayer cured film; a second adhesion step of adhering the upper layer ink to the lower layer cured film; a second curing step of irradiating the ink for an upper layer on the cured film for the lower layer with radiation to form a cured film for an upper layer; has. [Brief description of the drawings]

[0011] [Figure 1] FIG. 2 is a conceptual diagram of an example of a head unit and a transport unit of a recording apparatus that can be used in the embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Several embodiments of the present invention will be described below. The embodiments described below are merely examples of the present invention. The present invention is not limited to the following embodiments, and includes various modified forms that are implemented within the scope of the present invention. Note that not all of the configurations described below are necessarily essential configurations of the present invention.

[0013] 1. Ink set The ink set according to this embodiment is an ink set of a radiation curable ink jet composition, and includes an underlayer ink and an overlayer ink. The radiation curable ink jet composition is cured by irradiation with radiation. Examples of the radiation include ultraviolet light, electron beams, infrared light, visible light, and X-rays. As the radiation, ultraviolet light is preferred, since radiation sources are easily available and widely used, and materials suitable for curing by ultraviolet radiation are easily available and widely used.

[0014] 1.1.Ink for the undercoat The underlayer ink is a radiation-curable inkjet composition. In this specification, the terms "underlayer" and "upper layer" simply indicate the relative positional relationship in an image obtained by overcoating the inks, and can be rephrased as "undercoat" and "topcoat", respectively. The underlayer ink contains a polymerizable compound.

[0015] The underlayer ink contains, as polymerizable compounds, a monofunctional polymerizable compound in an amount of 80.0 mass% or more relative to the total amount of polymerizable compounds, and a polymerizable compound having a nitrogen heterocyclic structure and / or a polymerizable compound having a hydroxyl group in a total amount of 55.0 mass% or more relative to the total amount of polymerizable compounds.

[0016] 1.1.1. Polymerizable compounds The underlayer ink contains, as the polymerizable compound, a monofunctional polymerizable compound, a polymerizable compound having a nitrogen heterocyclic structure and / or a polymerizable compound having a hydroxyl group. The underlayer ink may contain other polymerizable compounds. Furthermore, each polymerizable compound may be used alone or in combination of two or more. In this specification, among polymerizable compounds, those having a molecular weight of 1000 or less may be referred to as "monomers", and "monofunctional monomers" and "polyfunctional monomers" may be referred to as "monofunctional monomers" and "polyfunctional monomers".

[0017] Below, examples of polymerizable compounds that can be contained in the underlayer ink will be given, and then a description will be given of a monofunctional polymerizable compound, a polymerizable compound having a nitrogen-containing heterocyclic structure, and a polymerizable compound having a hydroxyl group.

[0018] 1.1.1.(1) Polymerizable compound The underlayer ink may contain a vinyl ether group-containing (meth)acrylic acid ester. That is, the underlayer ink contains a polymerizable compound represented by the following general formula (I): CH2=CR 1 -COOR 2 -O-CH=CH-R 3 (I) (In the above formula (I), 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. The vinyl ether group-containing (meth)acrylic ester may contain a vinyl ether group-containing (meth)acrylic ester represented by the following formula: In this specification, the vinyl ether group-containing (meth)acrylic ester corresponds to a polyfunctional monomer.

[0019] In this specification, "(meth)acrylate" means at least one of acrylate and its corresponding methacrylate, "(meth)acryloyl" means at least one of acryloyl and its corresponding methacryloyl, and "(meth)acrylic" means at least one of acrylic and its corresponding methacrylic.

[0020] When the ink for the undercoat layer contains the vinyl ether group-containing (meth)acrylic acid ester, the curability can be improved and the viscosity can be reduced. In addition, in order to improve the curability of the ink, it is more preferable to use a compound having both a vinyl ether group and a (meth)acrylic group in one molecule than to use a compound having a vinyl ether group and a compound having a (meth)acrylic group separately.

[0021] 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 (1) 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, oxy n-propylene group, oxyisopropylene group and oxybutylene group are preferably used.

[0022] 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.

[0023] 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.

[0024] Examples of the vinyl ether group-containing (meth)acrylic acid esters 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-di(meth)acrylate, Methyl-2-vinyloxyethyl, 3-vinyloxybutyl (meth)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, o-vinyloxymethylphenylmethyl (meth)acrylate, 2-(vinyloxyethoxy)ethyl (meth)acrylate, 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-(vinyloxyethoxy)acrylic acid ethoxy)ethyl, (meth)acrylic acid 2-(vinyloxyethoxyisopropoxy)ethyl, (meth)acrylic acid 2-(vinyloxyisopropoxyethoxy)ethyl, (meth)acrylic acid 2-(vinyloxyisopropoxyisopropoxy)ethyl, (meth)acrylic acid 2-(vinyloxyethoxyethoxy)propyl, (meth)acrylic acid 2-(vinyloxyethoxyisopropoxy)propyl, (meth)acrylic acid 2-(vinyloxyisopropoxyethoxy)propyl, (meth)acrylic acid 2-(vinyloxyisopropoxyisopropoxy)propyl, (meth)acrylic acid 2-(vinyloxyisopropoxyisopropoxy)propyl,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.

[0025] 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 lower the viscosity of the ink, has a high flash point, and is excellent in curing properties, and 2-(vinyloxyethoxy)ethyl acrylate (VEEA) is more preferred. In particular, 2-(vinyloxyethoxy)ethyl acrylate and 2-(vinyloxyethoxy)ethyl methacrylate both have simple structures and small molecular weights, and therefore can significantly lower the viscosity of the ink. 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.

[0026] The vinyl ether group-containing (meth)acrylic esters may be used alone or in combination of two or more.

[0027] The ink for the underlayer may contain various monomers and oligomers, such as monofunctional, bifunctional, and trifunctional or more polyfunctional monomers and oligomers. Examples of the monomers include unsaturated carboxylic acids such as (meth)acrylic acid, itaconic acid, crotonic acid, isocrotonic acid, and maleic acid, and their salts or esters, urethane, amides and their anhydrides, vinyl compounds such as acrylonitrile and styrene, unsaturated polyesters, unsaturated polyethers, unsaturated polyamides, and unsaturated urethanes. Examples of the monomers include oligomers formed from monomers such as linear acrylic oligomers, epoxy (meth)acrylates, oxetane (meth)acrylates, aliphatic urethane (meth)acrylates, and aromatic urethane (meth)acrylates.

[0028] In addition, other monofunctional or polyfunctional monomers may include N-vinyl compounds, such as N-vinylformamide, N-vinylcarbazole, N-vinylacetamide, N-vinylpyrrolidone, N-vinylcaprolactam, and acryloylmorpholine, as well as derivatives thereof.

[0029] Among the above (meth)acrylates, examples of monofunctional (meth)acrylates 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, 4-hydroxybutyl (meth)acrylate (4-HBA), butoxyethyl (meth)acrylate, ethoxydiethylene glycol (meth)acrylate, methoxydiethylene glycol (meth)acrylate, and the like. Examples of the acrylate include methoxypolyethylene glycol (meth)acrylate, methoxypropylene glycol (meth)acrylate, phenoxyethyl (meth)acrylate, tetrahydrofurfuryl (meth)acrylate, isobornyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 2-hydroxy-3-phenoxypropyl (meth)acrylate, lactone-modified flexible (meth)acrylate, t-butylcyclohexyl (meth)acrylate, dicyclopentanyl (meth)acrylate, and dicyclopentenyloxyethyl (meth)acrylate. Among these, phenoxyethyl (meth)acrylate is preferred.

[0030] Among the above (meth)acrylates, examples of bifunctional (meth)acrylates 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, 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, and polytetramethylene glycol di(meth)acrylate. Of these, dipropylene glycol di(meth)acrylate (DPGDA) is preferred.

[0031] Among the above (meth)acrylates, examples of the trifunctional or higher polyfunctional (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.

[0032] The above polymerizable compounds may be used alone or in combination of two or more kinds.

[0033] The ink for the lower layer may contain a nitrogen-containing monofunctional monomer. The nitrogen-containing monofunctional monomer is not particularly limited, but examples thereof include nitrogen-containing monofunctional vinyl monomers such as N-vinylcaprolactam, N-vinylformamide, N-vinylcarbazole, N-vinylacetamide, vinylmethyloxazolidinone (VMOX) and N-vinylpyrrolidone; nitrogen-containing monofunctional acrylate monomers such as acryloylmorpholine; and nitrogen-containing monofunctional acrylamide monomers such as (meth)acrylamide, N-hydroxymethyl(meth)acrylamide, diacetoneacrylamide, N,N-dimethyl(meth)acrylamide, dimethylaminoethyl acrylate benzyl chloride quaternary salt.

[0034] Among these, it is more preferable to select either a nitrogen-containing monofunctional vinyl monomer or a nitrogen-containing monofunctional acrylate monomer, and more preferably a monomer having a nitrogen-containing heterocyclic structure such as N-vinylcaprolactam, N-vinylcarbazole, N-vinylpyrrolidone, or acryloylmorpholine (ACMO), and further preferably containing acryloylmorpholine.

[0035] The use of such nitrogen-containing monofunctional monomers tends to further improve the scratch resistance of the coating film. Furthermore, nitrogen-containing monofunctional acrylate monomers having a nitrogen-containing heterocyclic structure such as acryloylmorpholine tend to further improve the stretchability and adhesion of the coating film.

[0036] When the nitrogen-containing monofunctional monomer is used in the undercoat ink, the content is preferably 2.0 to 15.0 mass %, more preferably 3.0 to 13.0 mass %, and even more preferably 4.0 to 12.0 mass %, relative to the total amount of the polymerizable compounds. When the content of the nitrogen-containing monofunctional monomer relative to the total amount of the polymerizable compounds is within the above range, the abrasion resistance and adhesion of the coating film tend to be further improved.

[0037] The ink for the underlayer may contain a monofunctional (meth)acrylate having a bridged condensed ring structure. Examples of the monofunctional (meth)acrylate having a bridged condensed ring structure include dicyclopentenyl (meth)acrylate, dicyclopentenyloxyethyl (meth)acrylate, and dicyclopentanyl (meth)acrylate. The bridged condensed ring structure means a structure in which two or more ring structures share a side in a one-to-one relationship, and two or more atoms of the same ring structure or different ring structures that are not adjacent to each other are linked.

[0038] Among these, it is more preferable to include dicyclopentenyl (meth)acrylate (DCPA). By using such a monofunctional (meth)acrylate having a crosslinked condensed ring structure, the abrasion resistance and the stretchability and adhesion of the coating film tend to be improved, and the decrease in gloss of the coating film of the undercoat ink tends to be suppressed.

[0039] When the underlayer ink contains a monofunctional (meth)acrylate having a crosslinked condensed ring structure, the content is preferably 5.0 to 65.0% by mass, more preferably 10.0 to 63.0% by mass, and even more preferably 15.0 to 60.0% by mass, based on the total amount of polymerizable compounds. When the content of the monofunctional (meth)acrylate having a crosslinked condensed ring structure based on the total amount of polymerizable compounds is within the above range, the adhesion and abrasion resistance of the coating film tend to be further improved.

[0040] The ink for the underlayer may contain an aromatic group-containing monofunctional monomer. The aromatic group-containing monofunctional monomer is not particularly limited, but examples thereof include phenoxyethyl (meth)acrylate (PEA), benzyl (meth)acrylate (BZA), alkoxylated 2-phenoxyethyl (meth)acrylate, ethoxylated nonylphenyl (meth)acrylate, alkoxylated nonylphenyl (meth)acrylate, p-cumylphenol EO-modified (meth)acrylate, and 2-hydroxy-3-phenoxypropyl (meth)acrylate.

[0041] Among these, phenoxyethyl (meth)acrylate and benzyl (meth)acrylate are preferred, and phenoxyethyl acrylate (PEA) is more preferred. By using such aromatic group-containing monofunctional monomers, the solubility of the polymerization initiator is improved, and the curability of the ink for the underlayer tends to be improved. In particular, when an acylphosphine oxide-based polymerization initiator or a thioxanthone-based polymerization initiator is used, the solubility tends to be good.

[0042] The ink for the underlayer may contain a saturated aliphatic group-containing monofunctional monomer. The saturated aliphatic group-containing monofunctional monomer is not particularly limited, but may be, for example, alicyclic group-containing (meth)acrylates such as 3,3,5-trimethylcyclohexyl (meth)acrylate, isobornyl (meth)acrylate, tert-butylcyclohexanol (meth)acrylate, and 2-(meth)acrylic acid-1,4-dioxaspiro[4,5]dec-2-ylmethyl; isoamyl (meth)acrylate, stearyl (meth)acrylate, lauryl (meth)acrylate, and octyl (meth)acrylate. Examples of the saturated aliphatic group-containing monofunctional monomer include linear or branched aliphatic group-containing (meth)acrylates such as decyl (meth)acrylate, isodecyl (meth)acrylate, isomyristyl (meth)acrylate, isostearyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, butoxyethyl (meth)acrylate, tetrahydrofurfuryl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, and 2-hydroxypropyl (meth)acrylate; and lactone-modified flexible (meth)acrylates. The saturated aliphatic group-containing monofunctional monomer is not a compound having a crosslinked condensed ring structure.

[0043] Among these, 3,3,5-trimethylcyclohexyl acrylate (TMCHA), isobornyl acrylate (IBXA), tert-butylcyclohexanol acrylate (TBCHA), and lauryl acrylate (LA) are preferred. By using such a saturated aliphatic group-containing monofunctional monomer, the curability and abrasion resistance of the underlayer ink tend to be improved, and the decrease in gloss of the coating film of the underlayer ink tends to be suppressed.

[0044] 1.1.1.(2) Composition of polymerizable compound The underlayer ink contains, as polymerizable compounds, a monofunctional polymerizable compound in an amount of 80.0 mass% or more relative to the total amount of polymerizable compounds, and a polymerizable compound having a nitrogen heterocyclic structure and / or a polymerizable compound having a hydroxyl group in a total amount of 55.0 mass% or more relative to the total amount of polymerizable compounds.

[0045] (Monofunctional polymerizable compound) A monofunctional polymerizable compound is a compound having one polymerizable functional group in the molecule of the polymerizable compound. The monofunctional polymerizable compound is not particularly limited, but includes a conventionally known monofunctional monomer having a polymerizable functional group, such as a (meth)acrylic group, a vinyl group, or the like, based on a carbon-carbon unsaturated double bond. The monofunctional polymerizable compound can be selected from the polymerizable compounds exemplified above.

[0046] As the monofunctional polymerizable compound, it is preferable to use a monofunctional monomer. By using a monofunctional monomer with a relatively small molecular weight, the viscosity of the ink for the underlayer can be further reduced. Specific examples of the monofunctional monomer include phenoxyethyl (meth)acrylate (PEA), (meth)acryloylmorpholine (ACMO), 4-hydroxybutyl (meth)acrylate (4-HBA), dicyclopentenyl (meth)acrylate (DCPA), isobornyl (meth)acrylate (IBXA), vinylmethyloxazolidinone (VMOX), 3,3,5-trimethylcyclohexyl (meth)acrylate (TMCHA), lauryl acrylate (LA), and tert-butylcyclohexanol acrylate (TBCHA).

[0047] The content of the monofunctional polymerizable compound contained in the underlayer ink is 80.0% by mass or more relative to the total amount of the polymerizable compounds. The content of the monofunctional polymerizable compound contained in the underlayer ink is preferably 85.0% by mass or more, more preferably 90.0% by mass or more, relative to the total amount of the polymerizable compounds. When the content of the monofunctional polymerizable compound is 80.0% by mass or more relative to the total amount of the polymerizable compounds, the stretchability of the coating film is further improved. The upper limit of the content of the monofunctional polymerizable compound in the underlayer ink is not particularly limited, but is preferably 99.0% by mass or less, more preferably 98.0% by mass or less, and even more preferably 97.0% by mass or less, relative to the total amount of the polymerizable compounds. When the content of the monofunctional polymerizable compound is 99.0% by mass or less relative to the total amount of the polymerizable compounds, the odor of the ink is suppressed, the gloss is further improved, and the curing property and abrasion resistance of the coating film tend to be further improved.

[0048] The content of the monofunctional polymerizable compound contained in the underlayer ink is preferably 70.0% by mass or more, more preferably 75.0% by mass or more, and even more preferably 80.0% by mass or more, based on the total amount of the underlayer ink. When the content of the monofunctional polymerizable compound is 70.0% by mass or more, based on the total amount of the underlayer ink, the stretchability of the coating film tends to be improved. The upper limit of the content of the monofunctional polymerizable compound is preferably 95.0% by mass or less, more preferably 92.0% by mass or less, and even more preferably 90.0% by mass or less, based on the total amount of the underlayer ink. When the content of the monofunctional polymerizable compound is 95.0% by mass or less, based on the total amount of the underlayer ink, the odor of the ink is suppressed, the glossiness is improved, and the curing property and abrasion resistance of the coating film tend to be improved.

[0049] (Polymerizable compound having a nitrogen heterocyclic structure and / or a polymerizable compound having a hydroxyl group) The polymerizable compound having a nitrogen heterocyclic structure is not particularly limited, but examples thereof include (meth)acrylates or vinyl compounds having a skeleton derived from a conventionally known nitrogen composite ring, such as pyrrolidine, piperidine, pyridine, pyrimidine, imidazole, oxazoline, oxazolidine, oxazole, pyridine, pyrimidine, morpholine, oxazine, etc. The polymerizable compound having a nitrogen heterocyclic structure can be selected from the above-mentioned polymerizable compounds. Specific examples of the polymerizable compound having a nitrogen heterocyclic structure include (meth)acryloylmorpholine (ACMO), vinylmethyloxazolidinone (VMOX), etc.

[0050] The polymerizable compound having a hydroxyl group is not particularly limited, but examples thereof include conventionally known (meth)acrylates or vinyl compounds containing a hydroxyl group. The polymerizable compound having a hydroxyl group can be selected from the polymerizable compounds exemplified above. Specific examples of the polymerizable compound having a hydroxyl group include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 2-hydroxy-3-phenoxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate (4-HBA), hydroxypivalic acid neopentyl glycol di(meth)acrylate, and the like.

[0051] The underlayer ink of this embodiment contains at least one of a polymerizable compound having a nitrogen-containing heterocyclic structure and a polymerizable compound having a hydroxyl group, and the underlayer ink contains the polymerizable compound having a nitrogen-containing heterocyclic structure and / or the polymerizable compound having a hydroxyl group in a total amount of 55.0 mass % or more based on the total amount of the polymerizable compounds.

[0052] Furthermore, the total content of the polymerizable compound having a nitrogen heterocyclic structure and / or the polymerizable compound having a hydroxyl group contained in the underlayer ink is preferably 60.0 mass % or more, and more preferably 65.0 mass % or more, based on the total amount of the polymerizable compounds.

[0053] In addition, the polymerizable compound having a nitrogen heterocyclic structure and the polymerizable compound having a hydroxyl group may both be the monofunctional polymerizable compound described above. Therefore, when calculating that the monofunctional monomer is 80.0% by mass or more relative to the total amount of the polymerizable compounds in the ink for the undercoat layer, and that the polymerizable compound having a nitrogen heterocyclic structure and / or the polymerizable compound having a hydroxyl group is 55.0% by mass or more relative to the total amount of the polymerizable compounds, the contents of each are calculated independently, and a specific monomer may be included in the calculation of each content in duplicate.

[0054] By making the content of the polymerizable compound having a nitrogen heterocyclic structure and / or the polymerizable compound having a hydroxyl group 55.0% by mass or more based on the total amount of the polymerizable compounds, the adhesion of the overcoat ink to the coating film can be further improved. The upper limit of the content of the polymerizable compound having a nitrogen heterocyclic structure and / or the polymerizable compound having a hydroxyl group in the undercoat ink is not particularly limited, but is preferably 90.0% by mass or less, more preferably 85.0% by mass or less, based on the total amount of the polymerizable compounds.

[0055] 1.1.2. Polymerization initiator The ink for the underlayer may contain a polymerization initiator. The polymerization initiator is not particularly limited as long as it generates active species by irradiation with radiation, and examples thereof include known polymerization initiators such as acylphosphine oxide-based polymerization initiators, alkylphenone-based polymerization initiators, titanocene-based polymerization initiators, and thioxanthone-based polymerization initiators. Among these, acylphosphine oxide-based polymerization initiators are preferred. By using such a polymerization initiator, the curing property of the ink for the underlayer is further improved, and in particular, the curing property by the curing process using UV-LED light tends to be further improved. The polymerization initiator may be used alone or in combination of two or more kinds.

[0056] The acylphosphine oxide polymerization initiator is not particularly limited, but examples thereof include 2,4,6-trimethylbenzoyldiphenylphosphine oxide, bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide, bis-(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide, and the like.

[0057] Commercially available examples of such acylphosphine oxide polymerization initiators include IRGACURE 819 (bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide), IRGACURE 1800 (a mixture of bis-(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide and 1-hydroxy-cyclohexyl-phenyl ketone in a mass ratio of 25:75), and IRGACURE TPO (2,4,6-trimethylbenzoyldiphenylphosphine oxide) (all manufactured by BASF).

[0058] The content of the polymerization initiator in the underlayer ink is preferably 3.0 to 12.0 mass %, more preferably 5.0 to 10.0 mass %, and even more preferably 7.0 to 9.0 mass %, relative to the total amount of the underlayer ink. When the content of the polymerization initiator is within the above range, the curability of the underlayer ink and the solubility of the polymerization initiator tend to be further improved.

[0059] 1.1.3.Colorants The underlayer ink may or may not contain a coloring material. As the coloring material, at least one of a pigment and a dye can be used.

[0060] By using a pigment as a coloring material, the light resistance of the ink for the underlayer can be improved. Either an inorganic pigment or an organic pigment can be used as the pigment. The pigment may be used alone or in combination of two or more kinds.

[0061] As the inorganic pigment, carbon blacks such as furnace black, lamp black, acetylene black, and channel black (CI (Colour Index Generic Name) Pigment Black 7), iron oxide, and titanium oxide can be used.

[0062] 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.

[0063] More specifically, examples of carbon black used for the black 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 (CABOTJAPAN KK)), Color Black FW1, Color Black FW2, Color Black FW2V, Color Black FW18, Color Black FW200, Color Black S150, ColorBlack S160, Color Black S170, Printex 35, Printex U, Printex V, Printex 140U, SpecialBlack 6, Special Black 5, Special Black 4A, Special Black 4 (all manufactured by Degussa (manufactured by Degussa).

[0064] White pigments used for white include CI Pigment White 6, 18, and 21.

[0065] Pigments used for yellow 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.

[0066] Pigments used for magenta include 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, and CI Pigment Violet. 19, 23, 32, 33, 36, 38, 43, and 50.

[0067] Pigments used for cyan 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.

[0068] 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.

[0069] A dye may be used as a coloring material. The dye is not particularly limited, and acid dyes, direct dyes, reactive dyes, and basic dyes can be used. The dye may be used alone or in combination of two or more kinds.

[0070] The dye is not particularly limited, and examples thereof 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, 14 2, 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, CI Reactive Black 3, 4, 35.

[0071] Among these, it is preferable to contain a white pigment as the coloring material. When the underlayer ink contains a white pigment, it is possible to obtain an ink set that can form an image with better background hiding ability and good visibility.

[0072] When the underlayer ink does not contain a white pigment as a colorant, the total content of the colorant is preferably 0.2 to 20.0% by mass, more preferably 0.5 to 15.0% by mass, and even more preferably 1.0 to 10.0% by mass, based on the total amount of the underlayer ink. On the other hand, when the underlayer ink contains a white pigment as a colorant, the total content of the colorant is preferably 5.0 to 25.0% by mass, more preferably 10.0 to 20.0% by mass, based on the total amount of the underlayer ink. By having the white pigment content within the above range, the background concealment ability of the underlayer ink is improved.

[0073] 1.1.4. Other additives The ink for the underlayer may further contain additives such as a dispersant, a polymerization inhibitor, and a slip agent, if necessary.

[0074] (Dispersant) The dispersant is not particularly limited, but may be, for example, a dispersant commonly used for preparing a pigment dispersion, such as a polymer dispersant. Specific examples thereof include 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 as the main component. The dispersant may be used alone or in combination of two or more.

[0075] Commercially available polymer dispersants include the Ajisper series manufactured by Ajinomoto Fine-Techno Co., Ltd., the Solsperse series (Solsperse 36000, etc.) available from Avecia and Noveon, the Disperbyk series manufactured by BYK Additives & Instruments, and the Disparlon series manufactured by Kusumoto Chemicals Co., Ltd.

[0076] The content of the dispersant in the underlayer ink is preferably 0.1 to 2.0 mass %, more preferably 0.1 to 1.0 mass %, and even more preferably 0.1 to 0.5 mass %, relative to the total amount of the underlayer ink.

[0077] (Polymerization inhibitor) The underlayer ink may further contain a polymerization inhibitor. The polymerization inhibitor may be used alone or in combination of two or more kinds.

[0078] Examples of the polymerization inhibitor include, but are not limited to, p-methoxyphenol, hydroquinone monomethyl ether (MEHQ), 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl, 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), and 4,4'-thiobis(3-methyl-6-t-butylphenol), and hindered amine compounds.

[0079] The content of the polymerization inhibitor is preferably 0.05 to 1.0 mass %, and more preferably 0.05 to 0.5 mass %, relative to the total amount of the underlayer ink.

[0080] (slip agent) The undercoat ink may further contain a slipping agent. The slipping agent may be used alone or in combination of two or more kinds.

[0081] The slip agent is preferably a silicone surfactant, more preferably a polyester-modified silicone or a polyether-modified silicone. Examples of the polyester-modified silicone include BYK-347, 348, 350, BYK-UV3500, 3510, and 3530 (all manufactured by BYK Additives & Instruments), and examples of the polyether-modified silicone include BYK-3570 (manufactured by BYK Additives & Instruments).

[0082] The content of the slip agent is preferably 0.01 to 2.0% by mass, and more preferably 0.05 to 1.0% by mass, based on the total amount of the underlayer ink.

[0083] 1.2.Ink for top layer The overcoat ink is a radiation curable ink jet composition. The overcoat ink contains a polymerizable compound. The overcoat ink may contain a polymerization initiator, a coloring material, and other additives, as necessary.

[0084] The polymerizable compound, polymerization initiator, colorant, and other additives that can be used in the overcoat ink are the same as those described in the sections "1.1.1.(1) Polymerizable compound," "1.1.2. Polymerization initiator," "1.1.3. Colorant," and "1.1.4. Other additives" of "1.1. Undercoat ink" above, and by replacing "undercoat ink" in these sections with "overcoat ink," the explanation will be omitted.

[0085] 1.2.1. Composition of polymerizable compound The overcoat ink preferably contains, as a polymerizable compound, 95.0% by mass or more, more preferably 98.0% by mass or more of a monofunctional polymerizable compound relative to the total amount of the polymerizable compound. If the content of the monofunctional polymerizable compound is within this range, the image formed by the overcoat ink is more unlikely to crack. That is, in this way, the flexibility of the cured film formed by the overcoat ink is further increased, and when the cured film of the overcoat ink is distorted due to the stretching or bending of the recording medium, the cured film easily follows and deforms, so that cracks are less likely to occur and the image is easily maintained in good condition.

[0086] In addition, the overcoat ink preferably contains a polymerizable compound that satisfies the following conditions A and B in an amount of 80.0% by mass or more, preferably 85.0% by mass or more, and more preferably 90.0% by mass or more, based on the total amount of polymerizable compounds. Condition A: The volume defined by the van der Waals radius is 0.26 nm 3 That's all. Condition B: The area in the height direction relative to the long side is 0.25 nm 2 That's all.

[0087] When the overcoat ink contains a large amount of a polymerizable compound having a bulkiness as specified by Condition A and Condition B, the glossiness of the image formed by the overcoat ink is improved.

[0088] The volume defined by the van der Waals radius in condition A is preferably 0.27 nm 3 More preferably, it is 0.28 nm or more. 3 The upper limit of the volume defined by the van der Waals radius is not particularly limited, but is preferably 0.60 nm 3 More preferably, it is 0.55 nm or less. 3 or less, and more preferably 0.50 nm or less. 3 The following is the result.

[0089] The area in the height direction relative to the long side defined by the van der Waals radius in condition B is preferably 0.27 nm 2 More preferably, it is 0.29 nm or more. 2 The upper limit of the area in the height direction relative to the long side defined by the van der Waals radius is not particularly limited, but is preferably 0.50 nm 2 More preferably, it is 0.45 nm or less. 2 More preferably, it is 0.40 nm or less. 2 The following is the result.

[0090] The volume and the area in the height direction relative to the long side defined by the van der Waals radius are determined as the volume and the area in the height direction relative to the long side of the molecular structure with the lowest energy among the structural isomers of the molecule. Known software, such as thermodynamic property prediction software, can be used to specify the three-dimensional shape defined by the van der Waals radius of the molecule and to calculate the volume and the area in the height direction relative to the long side based on the specified three-dimensional shape.

[0091] "Volume" is the volume of the cavity created by approximating the molecular state in a vacuum from the chemical formula and using the van der Waals radius. The "long side" is the longest side in the three-dimensional shape defined by the van der Waals radius, and is found by calculating the distance between the farthest terminal skeletal atoms (C, O, N, etc.) when modeling the structure in which the molecule is most stable. "Area in the height direction relative to the long side" is the value obtained by dividing the volume by the long side, and is an index of the area on the plane perpendicular to the long side.

[0092] Examples of polymerizable compounds that satisfy the above conditions include monofunctional monomers having one polymerizable functional group, such as dicyclopentenyl acrylate (DCPA), isobornyl acrylate (IBXA), 3,3,5-trimethylcyclohexyl acrylate (TMCHA), tert-butylcyclohexanol acrylate (TBCHA), isononyl acrylate (INAA), and lauryl acrylate (LA); and polyfunctional monomers having multiple polymerizable functional groups, such as dipropylene glycol diacrylate (DPGDA).

[0093] 1.2.2.Colorants The overcoat ink may or may not contain a coloring material. If it contains a coloring material, the overcoat ink will be a color ink for forming an image. On the other hand, if it does not contain a coloring material, the overcoat ink will be a clear ink for protecting the image recorded by the undercoat ink.

[0094] The overcoat ink preferably contains a pigment other than white. In particular, when the undercoat ink contains a white pigment, if the overcoat ink is a color ink containing a pigment other than white, an image can be formed with the coating film of the undercoat ink as a base, and the visibility of the image can be improved. In addition, if the base layer is white, the image formed by the overcoat ink on top of the base layer is likely to have a high graininess, but in the ink set of this embodiment, the above-mentioned overcoat ink has good wet spreadability, so that the appearance of graininess can be suppressed. In other words, by the overcoat ink containing a pigment other than white, the effect of the good wet spreadability of the overcoat ink becomes more pronounced.

[0095] 1.3. Relationship between polymerizable compounds In the ink set of this embodiment, it is more preferable that the content of the monofunctional polymerizable compound in the overcoat ink is greater than that in the undercoat ink. In this way, the content ratio of the monofunctional polymerizable compound in the undercoat ink is smaller than that of the monofunctional polymerizable compound in the overcoat ink, so that the overcoat ink spreads more easily than the undercoat ink, and when the recording medium is bent or stretched in the image forming section, the image formed by the overcoat ink is less likely to crack.

[0096] 1.4. Surface tension relationships In the ink set of this embodiment, it is preferable that the surface tension of the undercoat ink is greater than the surface tension of the overcoat ink, which can improve the wetting and spreading of the overcoat ink.

[0097] In the ink set of this embodiment, it is more preferable that the surface tension of the undercoat ink is at least 8.0 mN / m higher than the surface tension of the overcoat ink, which can improve the wetting and spreading of the overcoat ink.

[0098] In this specification, "ink wetting and spreading properties" refers to the property of a line width becoming relatively wide when an upper layer ink is applied in a line shape on a coating film of an underlayer ink. "Curing" refers to the ink solidifying when it is irradiated with radiation and the polymerizable compound is polymerized. "Curing property" refers to the property of curing in response to light. "Filling property" is also called filling property, and refers to the property of not being able to see the underlying recording medium when a recorded matter is viewed from the side on which the cured matter (image) is formed. "Ejection stability" refers to the property of ejecting droplets without clogging the nozzle. "Storage stability" refers to the property of the ink being unlikely to change viscosity before and after storage when stored.

[0099] 1.5. Other ink compositions The ink set of the present embodiment may contain other ink compositions, so long as it contains the above-described underlayer ink and overlayer ink compositions.

[0100] 1.6.Effects According to the ink set of the present embodiment, the underlayer ink contains a large amount of monofunctional polymerizable compounds, so that the flexibility of the coating film is good, and the formed image has good adhesion to the recording medium. In addition, since the underlayer ink contains a polymerizable compound having a polar group, the overlayer ink also spreads well on the coating film, and an image with good filling can be formed. When the underlayer ink contains a large amount of monofunctional polymerizable compounds, the overlayer ink cannot be ensured to spread well even if the surface tension of the overlayer ink and the underlayer ink is simply made different. However, in this ink set, the underlayer ink contains a polymerizable compound having a nitrogen heterocyclic structure with a polar moiety in the molecule and / or a polymerizable compound having a hydroxyl group, so that the moiety comes out to the surface of the cured coating film of the underlayer ink when cured, improving the surface free energy of the underlayer ink and improving the overlayer ink to spread well.

[0101] 2. Inkjet recording method The inkjet recording method according to this embodiment is an inkjet recording method that uses the above-mentioned ink set, and includes a first adhesion step of adhering the underlayer ink to the recording medium, a first curing step of irradiating the underlayer ink on the recording medium with radiation to form a underlayer cured film, a second adhesion step of adhering the overlayer ink to the underlayer cured film, and a second curing step of irradiating the overlayer ink on the underlayer cured film with radiation to form an overlayer cured film.

[0102] 2.1. Recording medium The ink set of the present embodiment is ejected onto a recording medium using an inkjet recording method to obtain a recorded matter. Examples of the recording medium include absorbent and non-absorbent recording media. The inkjet recording method of the embodiment described below can be widely applied to recording media with various absorption properties, from non-absorbent recording media into which ink is difficult to penetrate, to absorbent recording media into which ink is easily penetrated.

[0103] Examples of absorbent recording media include, but are not limited to, plain paper such as electrophotographic paper, which has high ink permeability, inkjet paper (paper specifically for inkjet printing having an ink absorbing layer composed of silica particles or alumina particles, or an ink absorbing layer composed of a hydrophilic polymer such as polyvinyl alcohol (PVA) or polyvinylpyrrolidone (PVP)), and art paper, coated paper, cast paper, and the like used in general offset printing, which has relatively low ink permeability.

[0104] Non-absorbent recording media include, but are not limited to, plastic films, sheets, and plates such as polyvinyl chloride (PVC), polyethylene, polypropylene, and polyethylene terephthalate (PET); metal plates such as iron, silver, copper, and aluminum; or metal plates and plastic films produced by vapor deposition of these various metals; and alloy plates such as stainless steel and brass.

[0105] 2.2. Inkjet recording device An example of an inkjet recording apparatus (hereinafter, simply referred to as a "recording apparatus") for carrying out the inkjet recording method using the ink set of the embodiment will be described below. The recording apparatus has a first recording mode, a second recording mode, and a recording control unit.

[0106] The first recording mode is a mode in which an overcoat is formed by overcoating an overcoat formed with an undercoat ink.

[0107] The second recording mode is a mode in which a coating film is formed by overcoating an underlayer ink on a coating film formed by an overlayer ink.

[0108] The recording control section controls the amount of ink per dot of a solid pattern image formed with the underlayer ink in the first recording mode and the second recording mode. An example of the above-mentioned recording device will now be described in detail.

[0109] FIG. 1 is a conceptual diagram for explaining an example of a head unit and a transport unit of a recording device that can be used in this embodiment. A continuous sheet-like recording medium S is transported from the upstream side to the downstream side in the transport direction with the rotation of an upstream roller 12A, a transport drum 12, and a downstream roller 12B. The head unit includes a first white head unit 30A (on the left side of FIG. 1) that ejects white ink W, a yellow head unit 30B that ejects yellow ink Y, a magenta head unit 30C that ejects magenta ink M, a cyan head unit 30D that ejects cyan ink C, a black head unit 30E that ejects black ink K, and a second white head unit 30A (on the right side of FIG. 1) that ejects white ink W. The irradiation unit 40 includes irradiation units 42a, 41a, 41b, 41c, 41d, and 42b provided downstream of each head unit in the transport direction, and an irradiation unit 43 provided at the end of the transport direction. Such a recording device can be configured, for example, as shown in FIG. 2 of JP-A-2010-208218.

[0110] Here, when the first recording mode is performed, the recording medium S is conveyed to a position facing each head unit on the conveying drum, and in the order in which the recording medium S is conveyed, the first white head unit 30A ejects the underlayer ink, and the irradiation unit 42a irradiates the underlayer ink to form a coating film. Next, the overlayer ink is ejected onto the coating film from at least one of the yellow head unit 30B, the magenta head unit 30C, the cyan head unit 30D, and the black head unit 30E, and the irradiation unit provided downstream of the head performs irradiation. Finally, the irradiation unit 43 irradiates the coating film. The second white head unit 30A and the irradiation unit 42b are not used.

[0111] On the other hand, when the second recording mode is performed, the recording medium S is conveyed to a position facing each head unit on the conveying drum, and in the order in which the recording medium S is conveyed, first, the upper layer ink is discharged from at least one of the yellow head unit 30B, the magenta head unit 30C, the cyan head unit 30D, and the black head unit 30E, and irradiation is performed from the irradiation unit provided downstream of the head to form a coating film of the upper layer ink. Next, the lower layer ink is discharged from the second white head unit 30A on the coating film, and irradiation is performed from the irradiation unit 42b. Finally, irradiation is performed from the irradiation unit 43 to form a coating film. Note that the first white head unit 30A and the irradiation unit 42a are not used.

[0112] The above-mentioned recording control unit is preferably adjusted so that the ink amount per dot of the underlayer ink in the second recording mode is preferably 10% or more larger, more preferably 10 to 50% larger, than that in the first recording mode when the coating film of the underlayer ink is to be a solid pattern. This ensures that the wetting and spreading properties of the underlayer ink are equivalent to those of the first recording mode in the second recording mode. Note that one dot refers to an ink droplet that is deposited per pixel, which is the minimum recording unit defined by the recording resolution.

[0113] Here, the second recording mode is not necessarily provided, and in this case, the second white head unit 30A and the irradiation unit 42b may not be provided. Also, the ink that is coated later among the inks, i.e., the overcoat ink in the first recording mode and the undercoat ink in the second recording mode, may be cured by at least one of irradiation by the irradiation unit (41a to 41d, 42b) provided downstream of the head unit and irradiation by the irradiation unit 43, and when it is cured by the irradiation unit provided downstream of the head unit, the irradiation unit 43 may not be provided.

[0114] In this specification, the term "solid pattern image" refers to an image in which dots are recorded for all pixels, which are the minimum recording unit area defined by the recording resolution.

[0115] 2.3. Inkjet recording method In this embodiment, the underlayer ink is an ink that forms a coating film on the recording medium before the overlayer ink in the first recording mode, and the overlayer ink is an ink that forms a coating film on the recording medium on the coating film of the underlayer ink in the first recording mode. For example, one of the overlayer ink and the underlayer ink may be a main ink and the other an auxiliary ink. The main ink may be an ink used to visually recognize the coating film, such as a color ink. The auxiliary ink may be, for example, a special color ink such as a white ink, an underlayer ink, or a metallic ink, or a functional ink that improves the performance of the main ink, such as adhesion, wet spreadability, and abrasion resistance.

[0116] Here, both the overcoat ink and the undercoat ink may be main inks or both may be auxiliary inks, but using one as the main ink and the other as the auxiliary ink is preferable because it broadens the applications of the recorded material and improves the performance of the main ink. When using one as the main ink and the other as the auxiliary ink, the choice of which of the overcoat ink and the undercoat ink to use as the main ink can be determined by taking into consideration the respective properties of the main ink and the auxiliary ink described above. Therefore, in FIG. 1, the head that ejects the undercoat ink in the first recording mode is the first white head unit 30A, but the undercoat ink is not limited to white ink.

[0117] The reason for making the ink amount different between the first and second recording modes is that in the second recording mode, the line width of the underlayer ink on the coating film of the overlayer ink becomes narrower, which deteriorates the filling ability of the solid pattern image of the underlayer ink. Therefore, by increasing the ink amount as described above, the filling ability of the underlayer ink can be improved.

[0118] The inkjet recording method of this embodiment includes a first recording step of forming a coating film of the underlayer ink of the above embodiment on a recording medium, and a second recording step of overcoating the coating film with the overlayer ink of the above embodiment to form a coating film of the ink, thereby forming an image.

[0119] The first recording process and the second recording process each include a first (second) ejection process of ejecting ink onto a recording medium, and a first (second) curing process of irradiating the ejected ink with radiation to cure the ink. In this way, an image, i.e., a cured ink coating, is formed on the recording medium by the cured ink. These steps will be described below.

[0120] [Discharge process] In the first ejection step, the ink is ejected toward the recording medium, and the ink for the lower layer adheres to the recording medium. Meanwhile, in the second ejection step, the ink for the upper layer adheres to the lower layer cured film. The viscosity of the ink at the time of ejection is preferably 15 mPa·s or less, more preferably 12 mPa·s or less. If the viscosity of the ink is as described above when the ink temperature is at room temperature or when the ink is not heated, the ink may be ejected at room temperature or without heating. In this case, the temperature of the ink at the time of ejection is preferably 20 to 30°C. Meanwhile, the ink may be ejected with a preferred viscosity by heating it to a predetermined temperature. In this way, good ejection stability is realized.

[0121] In this embodiment, both the undercoat ink and the overcoat ink are radiation curable inks, which have a higher viscosity than normal water-based inks, and therefore the viscosity varies greatly with temperature fluctuations during ejection. Such ink viscosity fluctuations have a large effect on the changes in droplet size and droplet ejection speed, which can lead to deterioration of image quality. Therefore, it is preferable to keep the ink temperature as constant as possible during ejection.

[0122] [Curing process] Next, in the curing step, the ink discharged toward the recording medium and attached (landed) is cured by irradiation with radiation. The curing step includes a first curing step in which the ink for the lower layer attached to the recording medium in the first discharge step described above is irradiated with radiation to form a coating film for the lower layer, and a second curing step in which the ink for the upper layer attached on the lower layer cured film in the second discharge step described above is irradiated with radiation to form a coating film for the upper layer. This is because the polymerization initiator that may be contained in the ink is decomposed by irradiation with light (ultraviolet rays) to generate initiating species such as radicals, acids, and bases, and the polymerization reaction of the polymerizable compound is promoted by the function of the initiating species. Alternatively, the polymerization reaction of the polymerizable compound is initiated by irradiation with radiation. At this time, if a sensitizing dye is present in the ink together with the polymerization initiator, the sensitizing dye in the system absorbs radiation and becomes excited, and by contacting with the polymerization initiator, the decomposition of the polymerization initiator is promoted, and a curing reaction with higher sensitivity can be achieved.

[0123] As a radiation source, mercury lamps and gas / solid-state lasers that irradiate ultraviolet light are mainly used, and mercury lamps and metal halide lamps are widely known as light sources used for curing radiation-curable inkjet inks. On the other hand, there is currently a strong desire to eliminate mercury from the perspective of environmental protection, and replacement with GaN-based semiconductor ultraviolet light-emitting devices is extremely useful both industrially and environmentally. Furthermore, ultraviolet light-emitting diodes (UV-LEDs) and ultraviolet laser diodes (UV-LDs) are small, have a long life, are highly efficient, and are low cost, and are preferably used as light sources for radiation-curable inkjet inks. Among these, UV-LEDs are preferred.

[0124] Here, since it is easy to increase the output of the LED, a UV-LED having an emission peak wavelength preferably in the range of 350 to 420 nm is used, and the output is preferably 200 mJ / cm 2It is preferable to use a set of radiation-curable inkjet inks that can be cured with the following irradiation energies in the inkjet recording method. In this case, low-cost printing and high printing speeds can be realized. Such inks can be obtained by containing at least one of a polymerization initiator that decomposes when irradiated with ultraviolet light in the above wavelength range and a polymerizable compound that initiates polymerization when irradiated with ultraviolet light in the above wavelength range.

[0125] As described above, according to the present embodiment, it is possible to provide an inkjet recording method using an ink set that is excellent in all respects: the wetting and spreading properties of the overlying ink, the curing properties and ejection stability of both the overlying ink and the underlying ink, and the embedding properties of the underlying ink on the recording medium.

[0126] According to this inkjet recording method, since the undercoat ink contains a large amount of monofunctional polymerizable compounds, the flexibility of the coating film is good, and the adhesion of the formed image to the recording medium is good. Moreover, since the undercoat ink contains a polymerizable compound having a polar group, the wetting and spreading of the overcoat ink is good on the coating film, and an image with good filling can be formed.

[0127] 3. Examples and Comparative Examples The present invention will be further described below with reference to examples and comparative examples, but the present invention is not limited to the following examples in any way.

[0128] 3.1. Preparation of undercoat ink and overcoat ink First, the colorant, dispersant, and a portion of each monomer shown in Tables 1 and 2 were weighed and placed in a pigment dispersion tank, and a ceramic bead mill with a diameter of 1 mm was placed in the tank and stirred to obtain a pigment dispersion in which the colorant was dispersed in the monomer. Next, the remaining monomers, polymerization initiator, and polymerization inhibitor were placed in a stainless steel container for mixing, and mixed and stirred to completely dissolve the monomers, so as to obtain the composition shown in Tables 1 and 2. The pigment dispersion obtained above was then added, and further mixed and stirred at room temperature for 1 hour, and further filtered through a 5 μm membrane filter to obtain an ink for the lower layer and an ink for the upper layer. The numerical values ​​of each component in the tables represent mass % unless otherwise specified.

[0129] [Table 1]

[0130] [Table 2]

[0131] The abbreviations in Tables 1 and 2 are as follows. PEA: Product name "Viscoat #192", manufactured by Osaka Organic Chemical Industry Ltd., phenoxyethyl acrylate. ·ACMO: acryloylmorpholine. "(〇)" indicates that the compound corresponds to a polymerizable compound having a nitrogen heterocyclic structure and / or a polymerizable compound having a hydroxyl group. · 4-HBA: 4-hydroxybutyl acrylate. "(〇)" indicates that the compound corresponds to a polymerizable compound having a nitrogen heterocyclic structure and / or a polymerizable compound having a hydroxyl group. DCPA: dicyclopentenyl acrylate. "(△)" means "Condition A: The volume defined by the van der Waals radius is 0.26 nm 3 or more.” and “Condition B: The area in the height direction relative to the long side is 0.25 nm 2 This indicates that the monomer satisfies the above requirement. ·IBXA: Isobornyl acrylate. "(△)" indicates that the monomer satisfies "Condition A" and "Condition B". ·VMOX: Vinylmethyloxazolidinone. "(〇)" indicates that the compound corresponds to a polymerizable compound having a nitrogen heterocyclic structure and / or a polymerizable compound having a hydroxyl group. ·VEEA: 2-(2-vinyloxyethoxy)ethyl acrylate. ·DPGDA: Product name "SR508", dipropylene glycol diacrylate manufactured by Sartomer Co., Ltd. "(△)" indicates that the monomer satisfies "Condition A" and "Condition B". 819: Product name "IRGACURE 819" manufactured by BASF, bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide. TPO: Product name "IRGACURE TPO", manufactured by BASF, 2,4,6-trimethylbenzoyldiphenylphosphine oxide. MEHQ: Product name "p-Methoxyphenol", manufactured by Kanto Chemical Co., Ltd., hydroquinone monomethyl ether. BYK-350: A product name manufactured by BYK, an acrylic surfactant, abbreviated as "BYK 350". PW6: CI Pigment White 6 (titanium oxide, manufactured by Teika) · Solsperse 36000: Polymeric dispersant manufactured by Lubrizol. TMCHA: Product name "Viscoat #196", 3,3,5-trimethylcyclohexyl acrylate, manufactured by Osaka Organic Chemical Industry Co., Ltd. "(△)" indicates that the monomer satisfies "Condition A" and "Condition B". · LA: Product name "Light Acrylate LA", lauryl acrylate manufactured by Kyoeisha Chemical Co., Ltd. "(△)" indicates that the monomer satisfies "Condition A" and "Condition B". TBCHA: Product name "SR217", tert-butyl cyclohexanol acrylate, manufactured by Sartomer Corporation. "(△)" indicates that the monomer satisfies "Condition A" and "Condition B". ·BYK-UV3500: BYK Additives & Instruments, polyether-modified polydimethylsiloxane with acryloyl groups. PB15:3:CI Pigment Blue 15:3.

[0132] Tables 1 and 2 show the ratio of monofunctional monomers to the total amount of polymerizable compounds in each ink, and the surface tension (mN / m). Table 1 also shows the ratio of polar monomers to the total amount of polymerizable compounds in each ink. Table 2 also shows the ratio of monomers that satisfy "Condition A" and "Condition B" to the total amount of polymerizable compounds in each ink.

[0133] In Table 2, "Volume" is the volume of the cavity of the molecule formed by the Van-der-Waals radii of each atom constituting the molecule when the molecule is suspended in a vacuum, as calculated using the software "COSMOtherm" (MOLSIS).

[0134] The "long side" in Table 2 represents the longest side when calculating the "volume" calculated using the software "COSMOtherm" (MOLSIS). More specifically, it was determined by calculating the distance between the farthest terminal backbone atoms (C, O, N, etc.) when modeling the structure in which the molecule is most stable.

[0135] "Volume / long side" in Table 2 represents the value obtained by dividing the "volume" by the "long side". This value means the area of ​​the cross section perpendicular to the long side.

[0136] 3.2. Evaluation Content The ink sets of the examples and comparative examples shown in Table 3 were evaluated as follows.

[0137] 3.2.1. Evaluation of wetting and spreading properties The contact angle of the upper layer ink on the underlayer ink coating film prepared using a bar coater was evaluated according to the following criteria, and the results are shown in Table 3. A: 25° or less B: More than 25° and less than 28° C: More than 28° and less than 30° D: More than 30°

[0138] 3.2.2. Evaluation of filling ability The underlayer ink was discharged at 20 ng / dot onto a recording medium (PET E20) and cured. The overlayer ink was then placed on top of the underlayer ink under the same conditions, and the filling ability was visually checked. Evaluation was based on the following criteria, and the results are shown in Table 3. A: The recording medium was buried even when visually inspected from a distance of less than 30 cm. B: When visually inspected from a distance of 30 cm to less than 1 m from the recording medium, it was buried. C: Not buried even when visually inspected from a distance of 1m or more from the recording medium.

[0139] 3.2.3. Evaluation of adhesion A coating film was discharged onto a PVC (polyvinyl chloride) film at 20 ng / dot, cured, and cut at one point, then tape peeled off. The coating was evaluated according to the following criteria, and the results are shown in Table 3. A: No peeling. B: Part of the coating peeled off. C: The coating film was completely peeled off.

[0140] 3.2.4. Evaluation of curability For each of the undercoat ink and overcoat ink, a coating film was discharged onto a PVC (polyvinyl chloride) film at 20 ng / dot, cured, and subjected to a cotton swab rubbing test to evaluate the irradiation energy at which the ink became tack-free. The evaluation was based on the following criteria, and the results are shown in Table 3. A: 200mJ / cm 2 It hardened in less than 100°C. B: 200mJ / cm 2 It did not harden beyond this.

[0141] [Table 3]

[0142] 3.3.Evaluation Results The ink sets of the examples in which the underlayer ink contained 80.0% or more by mass of a monofunctional polymerizable compound relative to the total amount of polymerizable compounds, and contained 55.0% or more by mass of a polymerizable compound having a nitrogen heterocyclic structure and / or a polymerizable compound having a hydroxyl group relative to the total amount of polymerizable compounds, showed good results in terms of wetting and spreading, filling, and adhesion. In contrast, the ink sets of the comparative examples in which the underlayer ink contained less than 80.0% by mass of a monofunctional polymerizable compound relative to the total amount of polymerizable compounds, or contained less than 55.0% by mass of a polymerizable compound having a nitrogen heterocyclic structure and / or a polymerizable compound having a hydroxyl group relative to the total amount of polymerizable compounds, showed poor wetting and adhesion.

[0143] According to the ink set of the embodiment, since the underlayer ink contains a large amount of monofunctional polymerizable compounds, the flexibility of the coating film is good, and it is considered that the adhesion of the formed image to the recording medium is good. Furthermore, since the underlayer ink contains a polymerizable compound having a polar group, the overlayer ink also spreads well on the coating film, and it was found to form an image with good filling. In other words, when the underlayer ink contains a large amount of monofunctional polymerizable compounds, the overlayer ink cannot be ensured to spread well even if the surface tension of the overlayer ink and the underlayer ink is simply made different. In this ink set, since the underlayer ink contains a polymerizable compound having a nitrogen heterocyclic structure with a polar portion in the molecule and / or a polymerizable compound having a hydroxyl group, it is considered that the portion comes out to the surface of the cured coating film of the underlayer ink when cured, thereby improving the overlayer ink's spreadability.

[0144] The present invention includes configurations that are substantially the same as the configurations described in the embodiments, for example, configurations with the same functions, methods, and results, or configurations with the same purpose and effects. The present invention also includes configurations in which non-essential parts of the configurations described in the embodiments are replaced. The present invention also includes configurations that achieve the same effects as the configurations described in the embodiments, or configurations that can achieve the same purpose. The present invention also includes configurations in which publicly known technology is added to the configurations described in the embodiments.

[0145] The following can be derived from the above-described embodiment and modified examples.

[0146] The ink set is An ink set of a radiation curable ink jet composition comprising an undercoat ink and an overcoat ink, The underlayer ink contains, as polymerizable compounds, a monofunctional polymerizable compound in an amount of 80.0 mass% or more relative to the total amount of the polymerizable compounds, and a polymerizable compound having a nitrogen heterocyclic structure and / or a polymerizable compound having a hydroxyl group in a total amount of 55.0 mass% or more relative to the total amount of the polymerizable compounds.

[0147] According to this ink set, the underlayer ink contains a large amount of monofunctional polymerizable compounds, so that the flexibility of the coating film is good, and the formed image has good adhesion to the recording medium. In addition, since the underlayer ink contains a polymerizable compound having a polar group, the overlayer ink also spreads well on the coating film, and an image with good filling can be formed. When the underlayer ink contains a large amount of monofunctional polymerizable compounds, the overlayer ink cannot be ensured to spread well even by simply making a difference in surface tension between the overlayer ink and the underlayer ink. However, in this ink set, the underlayer ink contains a polymerizable compound having a nitrogen heterocyclic structure with a polar moiety in the molecule and / or a polymerizable compound having a hydroxyl group, so that the moiety comes out to the surface of the cured coating film of the underlayer ink when cured, thereby improving the surface free energy of the underlayer ink and improving the overlayer ink to spread well.

[0148] In the above ink set, The surface tension of the underlayer ink may be greater than the surface tension of the overlayer ink.

[0149] This ink set provides better wetting and spreading of the overcoat ink.

[0150] In the above ink set, The surface tension of the underlayer ink may be at least 8.0 mN / m higher than the surface tension of the overlayer ink.

[0151] This ink set provides better wetting and spreading of the overcoat ink.

[0152] In the above ink set, The underlayer ink may contain a white pigment.

[0153] This ink set allows the formation of images with better background hiding properties and good visibility.

[0154] In the above ink set, The overcoat ink may contain a pigment other than white.

[0155] With this ink set, the effect of the wetting and spreading of the overcoat ink becomes more pronounced.

[0156] In the above ink set, The underlayer ink may contain, as a polymerizable compound, a vinyl group-containing (meth)acrylate represented by the following general formula (I). H2C=CR 1 -CO-OR 2 -O-CH=CH-R 3 (I) (In formula (I), R 1 represents a hydrogen atom or a methyl group, R 2 represents a divalent organic group having 2 to 20 carbon atoms, and R 3 represents a hydrogen atom or a monovalent organic group having 1 to 11 carbon atoms.

[0157] According to this ink set, the viscosity of the underlayer ink can be further reduced and the curing property can be excellent.

[0158] In the above ink set, The content of the monofunctional polymerizable compound in the overcoat ink may be greater than the content of the monofunctional polymerizable compound in the undercoat ink.

[0159] According to this ink set, the content ratio of the monofunctional polymerizable compound in the underlayer ink is smaller than the content ratio of the monofunctional polymerizable compound in the overlayer ink, so that the overlayer ink spreads more easily than the underlayer ink, and when the recording medium is bent or stretched in the image forming section, the image formed by the overlayer ink is less likely to crack.

[0160] In the above ink set, The content of the monofunctional polymerizable compound in the overcoat ink may be 95.0% by mass or more based on the total amount of the polymerizable compounds.

[0161] This ink set makes it even more difficult for cracks to occur in the image formed with the overcoat ink.

[0162] In the above ink set, The overcoat ink may contain, as a polymerizable compound, a polymerizable compound that satisfies the following condition A and condition B in an amount of 80.0% by mass or more relative to the total amount of the polymerizable compounds. Condition A: The volume defined by the van der Waals radius is 0.26 nm 3 That's all. Condition B: The area in the height direction relative to the long side is 0.25 nm 2 That's all.

[0163] According to this ink set, the overcoat ink contains a large amount of polymerizable compound having the bulkiness as defined by conditions A and B, and thus the glossiness of the image produced by the overcoat ink is improved.

[0164] The inkjet recording method includes An inkjet recording method using the ink set described above, a first application step of applying the underlayer ink to a recording medium; a first curing step of irradiating the ink for an underlayer on the recording medium with radiation to form an underlayer cured film; a second adhesion step of adhering the upper layer ink to the lower layer cured film; a second curing step of irradiating the ink for an upper layer on the cured film for the lower layer with radiation to form a cured film for an upper layer; has.

[0165] According to this inkjet recording method, since the undercoat ink contains a large amount of monofunctional polymerizable compounds, the flexibility of the coating film is good, and the adhesion of the formed image to the recording medium is good. Moreover, since the undercoat ink contains a polymerizable compound having a polar group, the wetting and spreading of the overcoat ink is good on the coating film, and an image with good filling can be formed. [Explanation of symbols]

[0166] 12...conveyor drum, 12A...upstream roller, 12B...downstream roller, 30A...white head unit, 30B...yellow head unit, 30C...magenta head unit, 30D...cyan head unit, 30E...black head unit, 40...irradiation unit, 41a, 41b, 41c, 41d, 42a, 42b, 43...irradiation section, S...recording medium, W...white ink, Y...yellow ink, M...magenta ink, C...cyan ink, K...black ink

Claims

1. An ink set of a radiation curable ink jet composition comprising an undercoat ink and an overcoat ink, the underlayer ink contains, as a polymerizable compound, a monofunctional polymerizable compound in an amount of 80.0% by mass or more based on a total amount of the polymerizable compounds; A polymerizable compound having a nitrogen heterocyclic structure, the polymerizable compound having a nitrogen-containing heterocyclic structure includes vinylmethyloxazolidinone, and the polymerizable compound having a nitrogen-containing heterocyclic structure and / or the polymerizable compound having a hydroxyl group are contained in a total amount of 55.0 mass% or more based on the total amount of the polymerizable compounds.

2. In claim 1, The ink set, wherein the surface tension of the underlayer ink is greater than the surface tension of the overlayer ink.

3. 3. The ink set according to claim 1, wherein the surface tension of the underlayer ink is at least 8.0 mN / m higher than the surface tension of the overlayer ink.

4. 4. The ink set according to claim 1, wherein the underlayer ink contains a white pigment.

5. 5. The ink set according to claim 1, wherein the overcoat ink contains a pigment other than white.

6. 6. The ink set according to claim 1, wherein the underlayer ink contains a vinyl group-containing (meth)acrylate represented by the following general formula (I) as a polymerizable compound: H 2 C=CR 1 -CO-OR 2 -O-CH=CH-R 3 ... (I) (In formula (I), R 1 represents a hydrogen atom or a methyl group, R 2 represents a divalent organic group having 2 to 20 carbon atoms; R 3represents a hydrogen atom or a monovalent organic group having 1 to 11 carbon atoms.

7. 7. The ink set according to claim 1, wherein a content of the monofunctional polymerizable compound in the overcoat ink is greater than a content of the monofunctional polymerizable compound in the undercoat ink.

8. 8. The ink set according to claim 1, wherein a content of the monofunctional polymerizable compound in the overcoat ink is 95.0 mass % or more with respect to a total amount of the polymerizable compound.

9. 9. The ink set according to claim 1, wherein the overcoat ink contains, as a polymerizable compound, a polymerizable compound satisfying the following condition A and condition B in an amount of 80.0 mass% or more relative to the total amount of the polymerizable compound: Condition A: The volume defined by the van der Waals radius is 0.26 nm3 or more. Condition B: The area in the height direction relative to the long side is 0.25 nm2 or more.

10. An inkjet recording method using the ink set according to any one of claims 1 to 9, a first application step of applying the underlayer ink to a recording medium; a first curing step of irradiating the ink for an underlayer on the recording medium with radiation to form an underlayer cured film; a second adhesion step of adhering the upper layer ink to the lower layer cured film; a second curing step of irradiating the ink for overcoating on the cured lower layer film with radiation to form a cured upper layer film.

Images