JP7491353B2 - Photocurable inkjet ink set and inkjet recording method using same - Google Patents

Description

The present invention relates to a photocurable inkjet ink set and an inkjet recording method using the same.

Traditionally, 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 uses inexpensive equipment to eject ink only onto the required image areas to form an image directly on the recording medium, allowing for efficient use of ink and low running costs. Furthermore, the inkjet method is excellent as a recording method because it produces little noise.

In recent years, photocurable inks that harden when exposed to light (ultraviolet rays) have been used in inkjet recording methods to form prints on the surface of recording media that have excellent water resistance, solvent resistance, and abrasion resistance.

On the other hand, a recording method in which various photocurable inks are successively applied over one another is widely used. For example, a color image may be formed by applying overlays of inks of different colors, such as cyan (C), magenta (M), yellow (Y), black (K), and white (W), or a colorless, transparent clear ink may be applied over a color image to protect the image or add gloss. In such cases, technology is required to obtain images of the desired image quality.

For example, Patent Document 1 discloses a method of overcoating energy beam-curable ink compositions, in which a printed coating is formed on a recording medium using an energy beam-curable ink composition for inkjet recording (Ia), and then a clear coating is formed on the coating using an energy beam-curable clear ink composition for inkjet recording (Ib), and the relationship between 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 satisfies 31.5≦Sa<35.0 or 27.0≦Sa<31.5 and Sb<Sa (paragraph 0009 of the specification of Document 1).

JP 2006-181801 A

However, with the overcoating method disclosed in Patent Document 1, when applying the clear ink for the upper layer on top of the color ink film applied for the lower layer, the line width of the clear ink is very small, which causes a problem of poor wetting and spreading of the ink.

Therefore, one of the objects of the present invention is to provide a photocurable inkjet ink set that has excellent ink wetting and spreading properties.

The present inventors have conducted extensive research to solve the above problems. As a result, they have found that there is a correlation between the composition of the ink, particularly the composition of the underlayer photocurable inkjet ink, and the relationship in surface tension between the ink and the overlayer photocurable inkjet ink. More specifically, they have found that even when the surface tensions of the underlayer photocurable inkjet ink and the overlayer photocurable inkjet ink are in the relationship disclosed in Patent Document 1, the overlayer photocurable inkjet ink has excellent spreadability when the underlayer photocurable inkjet ink contains a specific vinyl ether group-containing (meth)acrylic acid ester.

As a result of further investigations, the inventors of the present application have found that the above problems can be solved by a photocurable inkjet ink set (hereinafter also simply referred to as an "ink set") that includes an underlayer photocurable inkjet ink (hereinafter also simply referred to as an "underlayer ink") that contains a specific vinyl ether group-containing (meth)acrylic acid ester, and an overlayer photocurable inkjet ink (hereinafter also simply referred to as an "overlayer ink"), and in which the surface tension of the underlayer ink is greater than that of the overlayer ink, and thus completed the present invention.

That is, the present invention is as follows.
[1]
The following general formula (I):
CH ₂ ═CR ¹ -COOR ² -O-CH═CH-R ³ (I)
(wherein 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), and an underlayer photocurable inkjet ink, the underlayer photocurable inkjet ink having a surface tension greater than that of the overlayer photocurable inkjet ink.
[2]
[0023] The photocurable inkjet ink set according to [1], wherein a difference in surface tension between the underlayer photocurable inkjet ink and the overlayer photocurable inkjet ink is 5 or more.
[3]
The photocurable inkjet ink set according to [1] or [2], wherein the content of the vinyl ether group-containing (meth)acrylic acid ester represented by general formula (I) contained in the photocurable inkjet ink for underlayer is 20 to 80 mass % relative to the total mass of the photocurable inkjet ink for underlayer.
[4]
The photo-curable inkjet ink set according to any one of [1] to [3], wherein the photo-curable inkjet ink for an overcoat layer contains a vinyl ether group-containing (meth)acrylic acid ester represented by general formula (I), and the content of the vinyl ether group-containing (meth)acrylic acid ester in the photo-curable inkjet ink for an undercoat layer is greater than that in the photo-curable inkjet ink for an overcoat layer.
[5]
The photocurable inkjet ink set according to any one of [1] to [4], wherein the underlayer photocurable inkjet ink contains an acrylic surfactant.
[6]
The photocurable inkjet ink set according to any one of [1] to [5], wherein the photocurable inkjet ink for the upper layer contains a silicone-based surfactant.
[7]
An inkjet recording method comprising: forming a coating film of the underlayer photocurable inkjet ink according to any one of [1] to [6] on a recording medium; and forming a coating film of the overlayer photocurable inkjet ink according to any one of [1] to [6] on the coating film.

FIG. 2 is a conceptual diagram for explaining an example of a head unit and a transport unit of a recording apparatus that can be used in one embodiment of the present invention.

The following describes in detail the embodiments for carrying out the present invention. Note that the present invention is not limited to the following embodiments, and can be modified in various ways within the scope of the gist of the present invention.

In this specification, "ink wetting and spreading" 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 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 the recorded matter is viewed from the side where the cured matter (image) is formed. "Ejection stability" refers to the property of ejecting stable ink droplets from the nozzle without clogging the nozzle. "Storage stability" refers to the property of the ink not changing viscosity before and after storage when stored.

In this specification, the terms "lower layer" and "upper layer" simply refer to their relative positional relationship in the image obtained by overlaying inks, and can be rephrased as "undercoat" and "topcoat," respectively.

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.

In this specification, the term "recorded matter" refers to a cured product formed by recording ink on a recording medium. In addition, in this specification, the term "cured product" refers to a cured substance, and the term "ink coating film" refers to a film formed by applying and curing ink, i.e., a cured coating film. When a coating film of the underlayer photocurable inkjet ink described below is formed and a coating film of the upper layer photocurable inkjet ink is formed on the coating film, the curing of the coating film of the underlayer photocurable inkjet ink refers to a state in which the coating film is determined to be cured in a curability test described below. The coating film may be a solid pattern image or a design pattern image such as a picture, character, or design.
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.

[Photocurable inkjet ink set]
A photocurable inkjet ink set according to one embodiment of the present invention includes an underlayer photocurable inkjet ink containing a vinyl ether group-containing (meth)acrylic acid ester represented by the general formula (I) described below, and an overlayer photocurable inkjet ink. In addition, the ink set satisfies the relationship that the surface tension of the underlayer ink is greater than the surface tension of the overlayer ink.
The ink set may further include inks other than the underlayer photocurable inkjet ink and the overlayer photocurable inkjet ink.

First, the above surface tension will be explained. The ink set of this embodiment needs to satisfy the above relationship, and in addition, the difference between the surface tension of the underlayer ink and the surface tension of the overlayer ink ("surface tension of the underlayer ink" - "surface tension of the overlayer ink") is preferably 5 or more, and more preferably 10 or more. If the difference is within the above range, the ink will have even better wetting and spreading properties.

It can be said that the surface tension of each of the underlayer ink and the overlayer ink is suitable when it is within the following range, provided that the above relationship is satisfied. In other words, it is preferable that the surface tension of both the overlayer ink and the underlayer ink is in the range of 20 to 40 mN/m, and it is more preferable that the surface tension of the overlayer ink is 20 to 30 mN/m and the surface tension of the underlayer ink is 25 to 40 mN/m. When the surface tensions of the overlayer ink and the underlayer ink are each within the above ranges, the ejection stability from the inkjet head is excellent.

The surface tension of ink is determined by its composition (the type and content of each component), but the main factor is the type and content of the surfactant contained in the ink. The surfactant will be described later. Note that the surface tension used in this specification is the value obtained by the measurement method performed in the Examples section below.

The additives (components) that are or may be included in the ink (composition) of this embodiment are described below. Note that below, the overcoat ink and undercoat ink may be collectively referred to as "ink."

[Polymerizable compound]
The polymerizable compound contained in the overcoat ink and undercoat ink in this embodiment is polymerized when irradiated with radiation by the action of a photopolymerization initiator described below, and can cure the printed ink.

(1. Vinyl Ether Group-Containing (Meth)Acrylic Acid Esters)
At least the underlayer ink of the inks in this embodiment contains a polymerizable compound represented by the following general formula (I):
CH ₂ ═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.)

By including the vinyl ether group-containing (meth)acrylic acid ester in the ink, the ink can have excellent curing properties and can also have a low viscosity. Furthermore, in terms of improving the ink curing properties, 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.

In particular, by containing the vinyl ether group-containing (meth)acrylic acid ester in the underlayer ink, the overlayer ink has excellent wetting and spreading properties, and the underlayer ink also has excellent curing properties and embedding properties on the recording medium.

On the other hand, the overcoat ink may or may not contain the vinyl ether group-containing (meth)acrylic acid esters, but it is preferable that it does, since the following effects can be obtained. When the overcoat ink also contains vinyl ether group-containing (meth)acrylic acid esters, the undercoat ink and the overcoat ink may contain the same compound or different compounds as the vinyl ether group-containing (meth)acrylic acid esters.

In the above general formula (I), the divalent organic residue having 2 to 20 carbon atoms represented by R ² is preferably a linear, branched or cyclic alkylene group having 2 to 20 carbon atoms which may be substituted, an alkylene group having 2 to 20 carbon atoms which may be substituted and which has an oxygen atom due to an ether bond and/or an ester bond in the structure, or a divalent aromatic group having 6 to 11 carbon atoms which may be substituted. Among these, alkylene groups having 2 to 6 carbon atoms such as an ethylene group, an n-propylene group, an isopropylene group, and a 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 an oxyethylene group, an oxy n-propylene group, an oxyisopropylene group, and an oxybutylene group are preferably used.

In the above general formula (I), the monovalent organic residue having 1 to 11 carbon atoms represented by ^R3 is preferably 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. 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.

When each of the above organic residues is a group that may be substituted, the substituent is divided into a group that contains a carbon atom and a group that does not contain a carbon atom. First, when the above substituent is a group that contains a carbon atom, the carbon atom is counted in the number of carbon atoms of the organic residue. Examples of groups that contain a carbon atom include, but are not limited to, a carboxyl group and an alkoxy group. Next, examples of groups that do not contain a carbon atom include, but are not limited to, a hydroxyl group and a halo group.

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-dimethyl-2-vinyloxyethyl (meth)acrylate, 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)propyl (meth)acrylate, 2-(vinyloxyisopropoxy)isopropyl, (meth)acrylate 2-(vinyloxyethoxyethoxy)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-(vinyloxyisopropoxyethoxy)propyl, (meth)acrylate 2-(vinyloxyisopropoxyisopropoxy)propyl, (meth)acrylate 2-(vinyloxyethoxyethoxy)isopropyl, (meth)acrylate 2-(vinyloxyisopropoxyisopropoxy)propyl, (meth)acrylate 2-(vinyloxyethoxyethoxy)isopropyl, (meth)acrylate 2-(vinyloxyethoxyisopropoxy)isopropyl, (meth)acrylate 2-(vinyloxyethoxyisopropoxy)isopropyl, (meth)acrylate 2-(vinyloxyethoxyisopropoxy)isopropyl Examples of such compounds include 2-(vinyloxyisopropoxyethoxy)isopropyl acrylate, 2-(vinyloxyisopropoxyisopropoxy)isopropyl (meth)acrylate, 2-(vinyloxyethoxyethoxyethoxy)ethyl (meth)acrylate, 2-(vinyloxyethoxyethoxyethoxyethoxy)ethyl (meth)acrylate, 2-(vinyloxyethoxyethoxyethoxyethoxy)ethyl (meth)acrylate, 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.

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 has excellent ink curing properties, with 2-(vinyloxyethoxy)ethyl acrylate being more preferred. In particular, 2-(vinyloxyethoxy)ethyl acrylate and 2-(vinyloxyethoxy)ethyl methacrylate both have simple structures and small molecular weights, making it possible to 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 terms of curability to 2-(vinyloxyethoxy)ethyl methacrylate.

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

The content of the vinyl ether group-containing (meth)acrylic acid esters contained in the underlayer ink is preferably 20 to 80% by mass, and more preferably 30 to 80% by mass, relative to the total mass (100% by mass) of the underlayer ink. If the content is 20% by mass or more, the viscosity of the underlayer ink can be reduced, the curing properties of the underlayer ink can be improved, and the wetting and spreading properties of the overlayer ink can be further improved. On the other hand, if the content is 80% by mass or less, the storage stability of the underlayer ink can be improved.

In particular, when not only the undercoat ink but also the overcoat ink contains the vinyl ether group-containing (meth)acrylic ester, the content of the vinyl ether group-containing (meth)acrylic ester in the undercoat ink is preferably greater than that in the overcoat ink, and more preferably 10% by mass or more greater than that in the overcoat ink, thereby making it possible to provide an ink set that has superior wettability and various other properties.
In addition, the content of the ink for the overlayer in the above case is not particularly limited, since it is preferable to determine the content in consideration of properties other than curability. In this embodiment, when the surface tension of the ink for the underlayer is greater than that of the ink for the underlayer, and the ink for the underlayer contains the vinyl ether group-containing (meth)acrylic acid ester, the reason why the ink for the overlayer has excellent wettability and spreadability on the coating film of the ink for the underlayer is presumably because the coating film derived from the vinyl ether group of the vinyl ether group-containing (meth)acrylic acid ester has some effect on the wettability and spreadability of the ink for the overlayer. However, the reason is not limited to this.

The above-mentioned vinyl ether group-containing (meth)acrylic acid esters can be produced by, but are not limited to, esterifying (meth)acrylic acid with a hydroxyl group-containing vinyl ether (Production Method B), esterifying (meth)acrylic acid halide with a hydroxyl group-containing vinyl ether (Production Method C), esterifying (meth)acrylic anhydride with a hydroxyl group-containing vinyl ether (Production Method D), transesterifying (meth)acrylic acid with a hydroxyl group-containing vinyl ether (Production Method E), esterifying (meth)acrylic acid with a halogen-containing vinyl ether (Production Method F), esterifying an alkali (earth) metal salt of (meth)acrylic acid with a halogen-containing vinyl ether (Production Method G), transvinylating a hydroxyl group-containing (meth)acrylic acid ester with a vinyl carboxylate (Production Method H), or transetherifying a hydroxyl group-containing (meth)acrylic acid ester with an alkyl vinyl ether (Production Method I).

Among these, manufacturing method E is preferred because it can more effectively achieve the desired effects of this embodiment.

(2. Polymerizable compounds other than those mentioned above)
As the polymerizable compound other than the above (hereinafter referred to as "other polymerizable compound"), various conventionally known monofunctional, bifunctional, and trifunctional or more polyfunctional monomers and oligomers can be used. 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, acrylonitrile, styrene, various unsaturated polyesters, unsaturated polyethers, unsaturated polyamides, and unsaturated urethanes. Examples of the oligomers include oligomers formed from the above monomers, such as linear acrylic oligomers, epoxy (meth)acrylates, oxetane (meth)acrylates, aliphatic urethane (meth)acrylates, aromatic urethane (meth)acrylates, and polyester (meth)acrylates.

In addition, N-vinyl compounds may be included as other monofunctional or polyfunctional monomers. Examples of N-vinyl compounds include N-vinylformamide, N-vinylcarbazole, N-vinylacetamide, N-vinylpyrrolidone, N-vinylcaprolactam, and acryloylmorpholine, as well as derivatives thereof.

Among other polymerizable compounds, esters of (meth)acrylic acid, i.e., (meth)acrylates, are preferred.

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, butoxyethyl (meth)acrylate, ethoxydiethylene glycol (meth)acrylate, methoxydiethylene glycol (meth)acrylate, methoxypolyethylene glycol (meth)acrylate, and methoxypolyethylene glycol (meth)acrylate. Examples of suitable acrylates include ethylene glycol (meth)acrylate, methoxypropylene glycol (meth)acrylate, phenoxyethyl (meth)acrylate, tetrahydrofurfuryl (meth)acrylate, isobornyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 2-hydroxy-3-phenoxypropyl (meth)acrylate, lactone-modified flexible (meth)acrylate, t-butylcyclohexyl (meth)acrylate, dicyclopentanyl (meth)acrylate, and dicyclopentenyloxyethyl (meth)acrylate. Among these, phenoxyethyl (meth)acrylate is preferred.

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 is preferred.

Among the above (meth)acrylates, examples of polyfunctional (meth)acrylates having three or more functional groups 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.

Among these, it is preferable that the ink contains a monofunctional (meth)acrylate as another polymerizable compound. In this case, the ink has a low viscosity, has excellent solubility of the photopolymerization initiator and other additives, and is likely to have good ejection stability during inkjet recording. Furthermore, in order to increase the toughness, heat resistance, and chemical resistance of the coating film, it is more preferable to use a monofunctional (meth)acrylate and a bifunctional (meth)acrylate in combination, and among these, it is even more preferable to use phenoxyethyl (meth)acrylate and dipropylene glycol di(meth)acrylate in combination.

The above other polymerizable compounds may be used alone or in combination of two or more.

The content of the other polymerizable compounds is preferably 10 to 90% by mass, more preferably 30 to 90% by mass, and even more preferably 60 to 90% by mass, relative to the total mass of the ink (100% by mass). If the content is within the above range, the viscosity and odor can be reduced, and the solubility and reactivity of the photopolymerization initiator can be improved.

[Surfactant]
In the present embodiment, the ink preferably contains a surfactant. In particular, by making the overcoat ink contain a surfactant, the relationship that the surface tension of the undercoat ink is greater than the surface tension of the overcoat ink can be more reliably satisfied. Furthermore, by making the undercoat ink contain a surfactant as well as the overcoat ink, the undercoat ink also has excellent embedding properties on the recording medium.

Examples of the surfactant include, but are not limited to, silicone-based surfactants, acrylic-based surfactants, cationic surfactants, anionic surfactants, nonionic surfactants, amphoteric surfactants, and fluorine-based surfactants. Among these, at least one of silicone-based surfactants and acrylic-based surfactants is preferred, since they can significantly reduce the surface tension and prevent damage to members that come into contact with the ink, such as the head and the ink supply device.

The silicone surfactants mentioned above are not limited to the following, but for example, modified silicones such as polyester modified silicones and polyether modified silicones can be used. Among these, polyether modified silicones are preferred because they can significantly reduce the surface tension, which further improves the wetting and spreading properties of the ink on the recording medium.

As the polyether-modified silicone, polyether-modified polydimethylsiloxane is preferably used, and as the polyester-modified silicone, polyester-modified polydimethylsiloxane is preferably used.

Commercially available silicone surfactants include, for example, BYK-347, BYK-348, BYK-UV3500, BYK-UV3510, BYK-UV3530, and BYK-UV3570 (all trade names manufactured by BYK Corporation). Among these, BYK-UV3500 is preferred because it provides superior wetting and spreading of the ink for the upper layer.

In addition, the above-mentioned acrylic surfactant is not limited to the following, but a preferred example is a (meth)acrylic acid-based copolymer.

Examples of commercially available acrylic surfactants include 1970, 230, LF-1980, LF-1982(-50), LF-1983(-50), LF-1984(-50), LHP-95, LHP-96, UVX-35, UVX-36, UVX-270, UVX-271, UVX-272, AQ-7120, AQ-7130 (all of which are product names manufactured by Kusumoto Chemical Industries), BYK-350, BYK-352, BYK-354, BYK-355, BYK-358, BYK-380, BYK-381, BYK-392 (all of which are product names manufactured by BYK Corporation), and Polyflow No. 57 and 95 (all of which are product names manufactured by Kyoeisha Yushi Kagaku Kogyo Co., Ltd.). Among these, BYK-350 is preferred because it provides excellent ejection stability for the underlayer ink when the underlayer ink contains an acrylic surfactant.

As for the combination of surfactants that are or may be contained in the overcoat ink and undercoat ink, any of the first to third cases below is preferred, as this more reliably satisfies the above-mentioned surface tension relationship, with the third case being more preferred.

First, it is preferable that the overcoat ink and the undercoat ink contain the same surfactant, and that the surfactant content in the overcoat ink is greater than that in the undercoat ink, with the difference in content being preferably 0.2% by mass or more, and more preferably 0.3% by mass or more.
In the first case described above, the content of each surfactant contained in the overcoat ink and the undercoat ink is preferably 0.1 to 0.5 mass% relative to the total mass (100 mass%) of each ink, and the content in the undercoat ink is more preferably 0.1 to 0.3 mass%.
In the first case, the wetting and spreading properties of the overcoat ink are further improved, and the cost of ink materials can be reduced by using a common surfactant between the overcoat ink and the undercoat ink.

Secondly, it is preferable that both the overcoat ink and the undercoat ink contain a modified silicone surfactant (however, the compounds may be the same or different), and that the surfactant content in the overcoat ink is greater than that in the undercoat ink. The difference in content is preferably 0.2% by mass or more, and more preferably 0.3% by mass or more.

In the second case, the modified silicone surfactant for the upper layer ink and the lower layer ink is preferably at least one of polyether modified silicone and polyester modified silicone, and polyether modified silicone is more preferable. In addition, using the same modified silicone surfactant is preferable because it reduces the cost of ink materials by sharing the same surfactant.

In the second case, the content of each surfactant contained in the overcoat ink and the undercoat ink is preferably 0.1 to 0.5 mass% relative to the total mass (100 mass%) of each ink, and the content in the undercoat ink is more preferably 0.1 to 0.3 mass%.
In the second case, the ink for the undercoat layer has excellent embedding properties on the recording medium.

Thirdly, the ink for the upper layer preferably contains a modified silicone-based surfactant. On the other hand, the ink for the lower layer may or may not contain a surfactant. However, if the ink for the lower layer contains a surfactant, the surfactant is preferably an acrylic surfactant. By containing a modified silicone-based surfactant in the ink for the upper layer, the ink for the upper layer has better wetting and spreading properties. Examples of the modified silicone-based surfactant include, but are not limited to, polyether-modified silicone and polyester-modified silicone. Among them, polyether-modified silicone is preferred because it is easy to obtain and provides excellent embedding of the ink for the lower layer on the recording medium.

In the third case described above, the content of each surfactant contained in the overcoat ink and the undercoat ink is preferably 0.1 to 0.5 mass% relative to the total mass (100 mass%) of each ink, and the content in the undercoat ink is more preferably 0.1 to 0.3 mass%.
In the third case, the ink wetting and spreading properties are further improved, and this is advantageous in that the difference in surface tension between the overcoat ink and the undercoat ink can be stably secured.

[Photopolymerization initiator]
The photopolymerization initiator that may be contained in the ink in this embodiment is used to form a print by curing the ink present on the surface of the recording medium through photopolymerization due to irradiation with radiation. By using ultraviolet light (UV) among the radiation (light), it is possible to achieve excellent safety and reduce the cost of the light source lamp. There are no limitations on the type of initiator as long as it generates active species such as radicals and cations by the energy of light and initiates polymerization of the polymerizable compound, but a photoradical polymerization initiator or a photocationic polymerization initiator can be used, and it is preferable to use a photoradical polymerization initiator.

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.

Among these, it is preferable to use at least an acylphosphine oxide compound, since this can further improve the curing properties of the ink. It is also preferable to use a thioxanthone compound in addition to the acylphosphine oxide compound.

Specific examples of photoradical polymerization initiators include acetophenone, acetophenone benzyl ketal, 1-hydroxycyclohexyl phenyl ketone, 2,2-dimethoxy-2-phenylacetophenone, 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-methyl-1-phenylpropan-1-one, thioxanthone, diethylthioxanthone, 2-isopropylthioxanthone, 2-chlorothioxanthone, 2-methyl-1-[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.

Commercially available photoradical polymerization initiators include, for example, IRGACURE 651 (2,2-dimethoxy-1,2-diphenylethane-1-one), IRGACURE 184 (1-hydroxy-cyclohexyl-phenyl-ketone), DAROCUR 1173 (2-hydroxy-2-methyl-1-phenyl-propan-1-one), IRGACURE 2959 (1-[4-(2-hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1-propan-1-one), IRGACURE 127 (2-hydroxy-1-{4-[4-(2-hydroxy-2-methyl-propionyl)-benzyl]phenyl]-2-methyl-propan-1-one}, IRGACURE 907 (2-methyl-1-(4-methylthiophenyl)-2-morpholinopropan-1-one), IRGACURE 369 (2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1), IRGACURE 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 IRGACURE 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) (all manufactured by BASF), Speedcure TPO, Speedcure Examples include DETX (2,4-diethylthioxanthone), Speedcure ITX (2-isopropylthioxanthone) (all manufactured by Lambson), KAYACURE DETX-S (2,4-diethylthioxanthone) (manufactured by Nippon Kayaku Co., Ltd.), Lucirin TPO, LR8893, LR8970 (all manufactured by BASF), and Ubecryl P36 (manufactured by UCB).

The photopolymerization initiator may be used alone or in combination of two or more.

The content of the photopolymerization initiator is preferably 3 to 20% by mass relative to the total mass of the ink (100% by mass), in order to improve the UV curing speed and provide excellent curing properties, and to avoid residual photopolymerization initiator or coloring resulting from the photopolymerization initiator.

[Polymerization inhibitor]
The ink in this embodiment may further contain a polymerization inhibitor. By containing a polymerization inhibitor in the ink, a polymerization reaction of the polymerizable compound before curing can be prevented, and the storage stability of the ink can be improved.

The above-mentioned polymerization inhibitors include, but are not limited to, phenol-based polymerization inhibitors such as p-methoxyphenol, 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), as well as hindered amines, hydroquinone monomethyl ether (MEHQ), and hydroquinone.

The polymerization inhibitor may be used alone or in combination of two or more kinds. The content of the polymerization inhibitor is not particularly limited, since it is determined in relation to the content of other components.

[Coloring materials]
The ink in this embodiment may contain a coloring material. The coloring material may be at least one of a pigment and a dye.

(Pigment)
In this embodiment, the light resistance of the ink can be improved by using a pigment as the coloring material. The pigment can be either an inorganic pigment or an organic pigment.

Inorganic pigments that can be used include carbon blacks (C.I. Pigment Black 7) such as furnace black, lamp black, acetylene black, and channel black, as well as iron oxide and titanium oxide.

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.

More specifically, the carbon black used as black ink includes No. 2300, No. 900, MCF88, No. 33, No. 40, No. 45, No. 52, MA7, MA8, MA100, and No. 2200B, etc. (the above are product names manufactured by Mitsubishi Chemical Corporation), Raven 5750, Raven 5250, Raven 5000, Raven 3500, Raven 1255, Raven 700, etc. (the above are manufactured by Columbia Carbon), 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. (the above are product names of Cabot Japan K.K.), 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, Special Black 4 etc. (all trade names manufactured by Degussa)

Pigments used in white ink include C.I. Pigment White 6, 18, and 21.

Pigments used in yellow ink include C.I. 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, 167, 172, and 180.

Pigments used in magenta ink include C.I. 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 C.I. Pigment Violet 19, 23, 32, 33, 36, 38, 43, 50.

Pigments used in cyan inks include C.I. Pigment Blue 1, 2, 3, 15, 15:1, 15:2, 15:3, 15:34, 15:4, 16, 18, 22, 25, 60, 65, 66, and C.I. Vat Blue 4, 60.

In addition, examples of pigments other than magenta, cyan, and yellow include C.I. Pigment Green 7, 10, C.I. Pigment Brown 3, 5, 25, 26, and C.I. Pigment Orange 1, 2, 5, 7, 13, 14, 15, 16, 24, 34, 36, 38, 40, 43, and 63.

The above pigments may be used alone or in combination of two or more.

When the above pigments are used, the average particle size is preferably 300 nm or less, and more preferably 50 to 200 nm. If the average particle size is within the above range, the ink has better reliability in terms of ejection stability and dispersion stability, and can form images with excellent image quality. Here, the average particle size in this specification is measured by dynamic light scattering.

(dye)
In the present embodiment, a dye can be used as the coloring material. The dye is not particularly limited, and acid dyes, direct dyes, reactive dyes, and basic dyes can be used.
Examples of the dyes include C.I. Acid Yellow 17, 23, 42, 44, 79, 142, C.I. Acid Red 52, 80, 82, 249, 254, 289, C.I. Acid Blue 9, 45, 249, C.I. Acid Black 1, 2, 24, 94, C.I. Food Black 1, 2, C.I. Direct Yellow 1, 12, 24, 33, 50, 55, 58, 86, 132, 142, 144, 173, C.I. Direct Red 1, 4, 9, 80, 81, 225, 227, C.I. Direct Blue 1, 2, 15, 71, 86, 87, 98, 165, 199, 202, C.I. Directed Black 19, 38, 51, 71, 154, 168, 171, 195, C.I. Reactive Red 14, 32, 55, 79, 249, C.I. Reactive Black 3, 4, 35.
The dyes may be used alone or in combination of two or more.

The content of colorant is preferably 0.5 to 20% by mass relative to the total mass of the ink (100% by mass) to obtain excellent hiding power and color reproducibility.

[Other additives]
The ink of this embodiment may contain additives (components) other than the additives listed above. Such components are not particularly limited, and may include, for example, conventionally known dispersants, polymerization promoters, penetration promoters, and wetting agents (moisturizers), as well as other additives. Examples of the above-mentioned other additives include conventionally known fixing agents, antifungal agents, preservatives, antioxidants, ultraviolet absorbers, chelating agents, and thickeners.

As described above, according to this embodiment, since the line width of the overcoat ink applied on the coating film of the undercoat ink does not become narrow, a photocurable inkjet ink set having excellent wetting and spreading properties of the overcoat ink can be provided. Furthermore, according to this embodiment, a photocurable inkjet ink set having excellent curing properties and ejection stability of both the overcoat ink and the undercoat ink, as well as excellent embedding properties of the undercoat ink on the recording medium can be provided. Thus, in this photocurable inkjet ink set, the undercoat ink and the overcoat ink are in a relationship in which they affect each other in terms of composition and physical properties. In other words, when determining the composition and physical properties of the undercoat ink and the overcoat ink, it is necessary to take into account the composition and physical properties of the overcoat ink and the undercoat ink, respectively.

[Recording medium]
The photocurable inkjet ink set of this embodiment is ejected onto a recording medium using an inkjet recording method described below, to obtain a recorded matter. Examples of such recording media 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 capabilities, from non-absorbent recording media into which ink is difficult to penetrate, to absorbent recording media into which ink can easily penetrate.

Absorbent recording media include, but are not limited to, ordinary paper such as electrophotographic paper, which has high ink permeability, inkjet paper (paper specifically for inkjet printing with an ink absorbing layer made of silica particles or alumina particles, or an ink absorbing layer made of hydrophilic polymers such as polyvinyl alcohol (PVA) or polyvinylpyrrolidone (PVP)), as well as art paper, coated paper, cast paper, and other papers used in general offset printing, which have relatively low ink permeability.

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

[Inkjet recording method]
An embodiment of the present invention relates to an inkjet recording method. The ink set of the above embodiment can be suitably used in the inkjet recording method of the present embodiment.

[Inkjet recording apparatus for carrying out the inkjet recording method]
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 above embodiment will be described below. The recording apparatus has a first recording mode, a second recording mode, and a recording control unit.

The first recording mode involves forming a coating film by overcoating an upper layer ink on top of a coating film formed with an underlayer ink.

The second recording mode involves applying an underlayer ink over a coating film formed with an overlayer ink to form a coating film.

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

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 the upstream roller 12A, the transport drum 12, and the downstream roller 12B. The head units include a first white head unit 30A (on the left side of the paper in 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 the paper in FIG. 1) that ejects white ink W. The irradiation unit 40 includes irradiation units 42a, 41a, 41b, 41c, 41d, and 42b provided downstream of the head units 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 Figure 2 of JP 2010-208218 A.

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

On the other hand, when the second recording mode is performed, the recording medium S is transported to a position facing each head unit on the transport drum, and in the order in which the recording medium S is transported, first, the upper layer ink is ejected 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 ejected from the second white head unit 30A onto 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.

When the coating of the underlayer ink is to be a solid pattern, the above-mentioned recording control unit is adjusted so that the ink amount per dot of the underlayer ink in the second recording mode is preferably 10% or more greater, and more preferably 10 to 50% greater, than in the first recording mode. 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 the ink droplet that is deposited per pixel, which is the smallest recording unit defined by the recording resolution.

Here, the second recording mode does not necessarily have to be provided, and in this case, the second white head unit 30A and the irradiation unit 42b do not have to be provided. Also, the ink that is coated later, i.e., the upper layer ink in the first recording mode and the lower layer ink in the second recording mode, only needs to be cured by at least one of irradiation by the irradiation unit (41a-41d, 42b) provided downstream of the head unit and irradiation by the irradiation unit 43. If it is cured by an irradiation unit provided downstream of the head unit, the irradiation unit 43 may not be required.

[Overcoat ink and undercoat ink]
In this embodiment, the underlayer ink is an ink that forms a coating film on the recording medium prior to the overlayer ink in the first recording mode, and the overlayer ink is an ink that forms a coating film on top of the coating film formed by the underlayer ink on the recording medium 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, for example, a color ink or other ink that is used to visually confirm the coating film, and the auxiliary ink may be, for example, a special color ink such as a white ink, a clear ink, or a metallic ink, or a functional ink that improves the performance of the main ink such as adhesion, wetting spreadability, and abrasion resistance.
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.

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 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, it is possible to improve the filling ability of the underlayer ink.

[Inkjet recording method using an inkjet recording device]
The inkjet recording method of the present 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.

The first recording process and the second recording process each include an ejection step in which ink is ejected onto the recording medium, and a curing step in which the ejected ink is irradiated with ultraviolet light to cure the ink. In this way, an image, i.e., a cured ink coating, is formed on the recording medium. These steps are described below.

[Discharge stage]
In the ejection stage, the ink is ejected toward the recording medium, and the underlayer ink adheres to the recording medium, and the overlayer ink adheres to the coating of the underlayer ink. The viscosity of the ink during ejection is preferably 15 mPa·s or less, and 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 ink temperature during ejection is preferably 20 to 30° C. On the other hand, the ink may be ejected with a preferred viscosity by heating it to a predetermined temperature. In this way, good ejection stability is achieved.

The ink for the underlayer and the ink for the overlayer in this embodiment are both photocurable inks, which have a higher viscosity than normal water-based inks, and therefore undergo large viscosity fluctuations due to temperature fluctuations during ejection. Such ink viscosity fluctuations have a significant effect on changes in droplet size and droplet ejection speed, which can ultimately cause deterioration in image quality. Therefore, it is preferable to keep the temperature of the ink as constant as possible during ejection.

[Curing stage]
Next, in the curing step, the ink that has been ejected toward the recording medium and adhered (landed) thereon is cured by irradiation with ultraviolet light. This is because the photopolymerization initiator that may be contained in the ink is decomposed by irradiation with light (ultraviolet light) 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 ultraviolet light. At this time, if a sensitizing dye is present in the ink together with the photopolymerization initiator, the sensitizing dye in the system absorbs ultraviolet light and becomes excited, and by coming into contact with the photopolymerization initiator, the decomposition of the photopolymerization initiator is promoted, and a more sensitive curing reaction can be achieved.

Mercury lamps and gas/solid-state lasers are mainly used as sources of ultraviolet light, and mercury lamps and metal halide lamps are widely known as light sources used to cure photocurable inkjet inks. On the other hand, there is currently a strong desire to go mercury-free 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 expected to be used as light sources for photocurable inkjet inks. Of these, UV-LEDs are preferred.

Here, since it is easy to increase the output of the LED, it is preferable to use a UV-LED having an emission peak wavelength preferably in the range of 350 to 420 nm, and to use a set of photocurable inkjet inks that can be cured with an irradiation energy of preferably 200 mJ/ ^cm2 or less 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 photopolymerization 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.

As described above, according to this embodiment, it is possible to provide an inkjet recording method using a photocurable inkjet ink set that is excellent in the wetting and spreading properties of the overcoat ink, the curing properties and ejection stability of both the overcoat ink and the undercoat ink, and the embedding properties of the undercoat ink on the recording medium.

The following describes the embodiments of the present invention in more detail with reference to examples, but the present embodiments are not limited to these examples.

[Materials used]
The materials used in the following examples and comparative examples are as follows.
[Pigments]
C.I. Pigment Blue 15:3 (Phthalocyanine Blue, manufactured by DIC Corporation, hereinafter abbreviated as "PB15:3")
C.I. Pigment White 6 (titanium oxide, manufactured by Teika Co., Ltd., hereinafter abbreviated as "PW6")
[Polymerizable compound]
(1. Vinyl Ether Group-Containing (Meth)Acrylic Acid Esters)
VEEA (2-(2-vinyloxyethoxy)ethyl acrylate, trade name of Nippon Shokubai Co., Ltd., hereinafter abbreviated as "VEEA")
(2. Other Polymerizable Compounds)
Viscoat #192 (phenoxyethyl acrylate, product name of Osaka Organic Chemical Industry Ltd., hereinafter abbreviated as "PEA")
NK Ester APG-100 (product name of SHIN-NAKAMURA CHEMICAL CO., LTD., dipropylene glycol diacrylate, hereinafter abbreviated as "DPGDA")
[Polymerization inhibitor]
MEHQ (hydroquinone monomethyl ether, manufactured by Kanto Chemical Co., Ltd.)
[Surfactant]
・BYK-UV3500 (product name manufactured by BYK, silicone-based, hereinafter abbreviated as “UV3500”)
BYK-UV3570 (product name manufactured by BYK, silicone-based, hereinafter abbreviated as "UV3570")
・ BYK-350 (product name of BYK Corporation, acrylic type, hereinafter abbreviated as "BYK350")
・ BYK-381 (product name manufactured by BYK Corporation, acrylic type, hereinafter abbreviated as "BYK381")
[Photopolymerization initiator]
IRGACURE 819 (product name manufactured by BASF, solid content 100%, hereinafter abbreviated as "819")
DAROCUR TPO (product name manufactured by BASF, solid content 100%, hereinafter abbreviated as "TPO")
KAYACURE DETX-S (product name manufactured by Nippon Kayaku Co., Ltd., solid content 100%, hereinafter abbreviated as "DETX")

[Preparation of Overcoat Ink and Undercoat Ink]
Each material was added so as to obtain the composition (unit: mass %) shown in Tables 1 and 2 below, and this was stirred with a high-speed water-cooled stirrer to prepare a cyan upper layer ink and a white lower layer ink.
Table 1 shows the composition of each overcoat ink, and overcoat inks 1, 2, 3, and 4 are hereinafter abbreviated as overcoat inks 1, 2, 3, and 4. Table 2 shows the composition of each undercoat ink, and undercoat inks 1, 2, 3, 4, 5, 6, and 7 are hereinafter abbreviated as undercoat inks 1, 2, 3, 4, 5, 6, and 7.

[Examples 1 to 7, Comparative Examples 1 to 5]
Ink sets were prepared using the combinations of overcoat inks and undercoat inks shown in the upper rows of each of Tables 3 and 4. The surface tensions γ (mN/m) of the overcoat inks and undercoat inks in each of the examples and comparative examples were values measured at room temperature and normal pressure by the Wilhelmy method using a surface tensiometer CBVP-Z (trade name, manufactured by Kyowa Interface Science Co., Ltd.), and are shown in Tables 3 and 4 below.

[Evaluation item]
The ink sets prepared in the respective Examples and Comparative Examples were evaluated for wetting and spreading properties, filling properties, ejection stability, and curing properties by the following methods.

(1. Wetting and Spreading Properties)
The ink for the underlayer was ejected toward a recording medium (PET50A, manufactured by Lintec Corporation) under conditions of 720×720 dpi and 15 ng/dot, and then cured to form a cured product. Then, one line (one dot row) was formed on the cured product using the ink for the overlayer under conditions of 720 dpi and 10 ng/dot.

The ink was ejected after adjusting the temperature so that the viscosity of each ink was 10 mPa·s. The evaluation was performed by measuring the line width (X) of the cured overcoat ink on the cured product (cured film of the undercoat ink).
The evaluation criteria were as follows: The evaluation results are shown in Tables 3 and 4 below.
A: 70 μm≦X
B: 65 μm≦X<70 μm
C: 60 μm≦X<65 μm
D: X<60 μm

(2. Burying ability)
The ink for the underlayer was ejected onto a recording medium (PET50A) under conditions of 720×720 dpi and 10 ng/dot, and then cured.

The ink was discharged after adjusting the temperature so that the viscosity of each ink was 10 mPa·s. The obtained cured product was visually observed for its embedding property on the ink for the underlayer, thereby carrying out this evaluation.
The evaluation criteria are as follows. The evaluation results are shown in Tables 3 and 4 below.
A: Filled with ink when visually observed from a height of 30 cm from the recording medium (no streaks of the base were visible).
B: Stripes on the background were visible when visually observed from a height of 30 cm from the recording medium.

(3. Discharge Stability)
The ejection stability of each of the overcoat ink and the undercoat ink was evaluated. Each of the overcoat ink and the undercoat ink was ejected continuously for 60 minutes from a head having 180 nozzles. In this manner, the ejection stability of the head when each of the overcoat ink and the undercoat ink was ejected was evaluated.
The evaluation criteria are as follows. The evaluation results are shown in Tables 3 and 4 below.
A: No missing nozzles occurred during continuous ejection.
B: Nozzle missing occurred during continuous ejection.

(4. Curability)
The curing properties of the overcoat ink and the undercoat ink were evaluated. Using an inkjet recording device equipped with a piezoelectric inkjet nozzle, the overcoat ink and the undercoat ink in each Example and Comparative Example were filled into the respective nozzle rows. The overcoat ink and the undercoat ink were ejected toward a recording medium (PET50A) under normal temperature and pressure conditions at 720 x 720 dpi and 10 ng/dot, and a solid pattern image was printed so that the film thickness of the printed matter (recorded matter) was 7 to 8 μm.

Thereafter, ultraviolet light having an irradiation intensity of 1 W/ ^cm2 and a peak wavelength of 395 nm was irradiated from a UV-LED in an ultraviolet irradiation device mounted next to the carriage toward the solid pattern image, gradually increasing the amount of irradiation energy up to a maximum of 200 mJ/ ^cm2 per pass. In this manner, a recorded product was produced. During this irradiation, when the tackiness of the coating film surface on the recorded product disappeared, it was deemed to have cured at that point.
The absence of tackiness was judged under the following conditions. That is, it was judged based on whether ink adhered to the cotton swab or whether scratches were left on the solid pattern image on the recording medium. If ink did not adhere to the cotton swab and scratches were not left on the solid pattern image on the recording medium, it was considered that the tackiness was absent. The cotton swab used was a Johnson cotton swab manufactured by Johnson &amp; Johnson. The number of times of rubbing was 10 times back and forth, and the rubbing force was 100 g load.
The "number of passes" refers to the number of times the head moved relative to the recorded material and irradiated the coating with ultraviolet light from the ultraviolet irradiation device mounted on the head. The irradiation energy [mJ/ ^cm2 ] was calculated by measuring the irradiation intensity [mW/ ^cm2 ] on the irradiated surface irradiated from the light source and multiplying this by the duration of irradiation [s]. The irradiation intensity was measured using a UV intensity meter UM-10 and a light receiving unit UM-400 (both manufactured by KONICA MINOLTA SENSING, INC.).
The evaluation criteria are as follows. The evaluation results are shown in Tables 3 and 4 below.
A: Cured with irradiation energy of 200 mJ/ ^cm2 or less.
B: Not cured with an irradiation energy of 200 mJ/cm ² .

The above results show that ink sets (each Example) that contain an underlayer ink containing a vinyl ether group-containing (meth)acrylic acid ester represented by general formula (I) and an overlayer ink, and in which the surface tension of the underlayer ink is greater than the surface tension of the overlayer ink, are superior in wetting and spreading of the overlayer ink compared to ink sets (each Comparative Example) that do not, and are also superior in curing and ejection stability of both the overlayer ink and the underlayer ink, as well as in embedding of the underlayer ink on the recording medium.

Although not shown in the above table, the ink was prepared in the same manner as the underlayer ink 3, except that the content of DPGDA in the underlayer ink 3 was set to 26.75% by mass and 0.05% by mass of UV3500 was added as a surfactant. The inks were evaluated in the same manner as in the above examples, using overlayer inks 1 and 4 as the overlayer inks and the underlayer inks as the underlayer inks. As a result, the embedding property of the underlayer ink on the recording medium was B, the ejection stability of the underlayer ink was A, and the curing property of the underlayer ink was A, while the overlayer inks both had a underlayer ink γ-upper layer ink γ of 5 [mN/m] and the wetting and spreading property of the overlayer ink was B.

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 (7)

The following general formula (I):
CH ₂ ═CR ¹ -COOR ² -O-CH═CH-R ³ (I)
(wherein 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), and a photocurable ink-jet ink for an upper layer,
a surface tension of the underlayer photocurable inkjet ink is greater than a surface tension of the overlayer photocurable inkjet ink,
the photocurable ink-jet ink for the upper layer contains a silicone-based surfactant,
the underlayer photocurable inkjet ink contains an acrylic surfactant ,
the underlayer photocurable inkjet ink is a white ink, and the overlayer photocurable inkjet ink is any one of a yellow ink, a magenta ink, a cyan ink, and a black ink;
the underlayer photocurable inkjet ink and the overlayer photocurable inkjet ink are radically polymerizable inks containing an acylphosphine oxide compound as a photoradical polymerization initiator . 2. The photocurable ink-jet ink set according to claim 1, wherein the underlayer photocurable ink-jet ink and the overlayer photocurable ink-jet ink each contain a photoradical polymerization initiator in an amount of 3 to 20% by mass relative to the total mass of the ink. The photocurable inkjet ink set according to claim 1 or 2, wherein the content of the vinyl ether group-containing (meth)acrylic acid ester represented by the general formula (I) contained in the underlayer photocurable inkjet ink is 20 to 80% by mass relative to the total mass of the underlayer photocurable inkjet ink. the photocurable inkjet ink for the upper layer contains a vinyl ether group-containing (meth)acrylic acid ester represented by the general formula (I),
4. The photocurable ink-jet ink set according to claim 1, wherein a content of the vinyl ether group-containing (meth)acrylic acid ester in the underlayer photocurable ink-jet ink is greater than a content of the vinyl ether group-containing (meth)acrylic acid ester in the overlayer photocurable ink-jet ink. 5. The photocurable ink-jet ink set according to claim 1, wherein the underlayer photocurable ink-jet ink contains an acrylic surfactant in an amount of 0.1 to 0.3 mass % relative to the total mass of the underlayer photocurable ink-jet ink . The photocurable inkjet ink set according to any one of claims 1 to 5, wherein the upper layer photocurable inkjet ink contains 0.1 to 0.5 mass % of a silicone surfactant. An inkjet recording method comprising forming a coating film of the undercoat photocurable inkjet ink according to any one of claims 1 to 6 on a recording medium, and forming a coating film of the overcoat photocurable inkjet ink according to any one of claims 1 to 6 on the coating film.

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