JP2022180413A - Photocurable inkjet ink set and inkjet recording method using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a photocurable inkjet ink set excellent in wet spreadability of an ink.

SOLUTION: The photocurable inkjet ink set includes: a photocurable inkjet ink for a lower layer which contains vinyl ether group-containing (meth)acrylates represented by general formula (I); and a photocurable inkjet ink for an upper layer. The surface tension of the photocurable inkjet ink for a lower layer is larger than that of the photocurable inkjet ink for an upper layer.

SELECTED DRAWING: Figure 1

COPYRIGHT: (C)2023,JPO&INPIT

Description

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

2. Description of the Related Art 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 methods, the ink jet method uses an inexpensive device to directly form an image on a recording medium by ejecting ink only to a required image portion, so that the ink can be used efficiently and the running cost is low. Further, the ink jet system is excellent as a recording method because of its low noise.

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

On the other hand, a recording method is widely used in which various photocurable inks are successively overcoated. For example, different color inks such as cyan (C), magenta (M), yellow (Y), black (K), and white (W) are overcoated to form a color image, protect the image, or In some cases, colorless and transparent clear ink is overcoated on a color image for the purpose of imparting gloss. In such a case, there is a demand for a technique for obtaining an image of desired image quality.

For example, in Patent Document 1, an energy ray-curable inkjet recording color ink composition (Ia) is used to form a printed coating film on a recording member, and then an energy ray-curable inkjet recording clear is applied on the coating film. The ink composition (Ib) is used to form a clear coating film, and between the surface tension Sa (mN/m) of Ia applied as the lower layer and the surface tension Sb (mN/m) of Ib applied as the upper layer discloses a method for overcoating an energy ray-curable ink composition that satisfies the relationship of 31.5≦Sa<35.0 or 27.0≦Sa<31.5 and Sb<Sa (Document 1 paragraph 0009) of the specification.

Japanese Patent Application Laid-Open No. 2006-181801

However, according to the overcoating method disclosed in Patent Document 1, when the clear ink for the upper layer is applied on the coating film of the color ink applied for the lower layer, the line width of this clear ink is very small. , there arises a problem that the wetting and spreading property of the ink is inferior.

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

The inventors of the present application have made intensive studies to solve the above problems. As a result, it was found that the composition of the ink, particularly the composition of the photocurable inkjet ink for the lower layer, and the surface tension relationship between the ink and the photocurable inkjet ink for the upper layer are correlated. More specifically, even when the surface tensions of the photocurable inkjet ink for the lower layer and the photocurable inkjet ink for the upper layer are in the relationship as disclosed in Patent Document 1, the photocurable inkjet ink for the lower layer has a predetermined It has been found that the photocurable inkjet ink for the upper layer exhibits excellent spreadability by containing the vinyl ether group-containing (meth)acrylic acid esters.

Accordingly, the inventors of the present application conducted further studies, and found that a photocurable inkjet ink for the lower layer containing a predetermined vinyl ether group-containing (meth)acrylic acid ester (hereinafter, also simply referred to as "ink for the lower layer"). , and a photocurable inkjet ink for the upper layer (hereinafter also simply referred to as "ink for the upper layer"), and the surface tension of the ink for the lower layer is higher than the surface tension of the ink for the upper layer. (hereinafter also simply referred to as "ink set").

That is, the present invention is as follows.
[1]
The following general formula (I):
_CH2 = ^CR1 -COOR2- ^O -CH=CH- ^R3 (I)
(In the formula, R ¹ is a hydrogen atom or a methyl group, R ² is a divalent organic residue having 2 to 20 carbon atoms, R ³ is a hydrogen atom or a monovalent organic residue having 1 to 11 carbon atoms. A photocurable inkjet ink set including a lower layer photocurable inkjet ink containing a vinyl ether group-containing (meth)acrylic acid ester represented by and a photocurable upper layer photocurable inkjet ink. and a photocurable inkjet ink set, wherein the surface tension of the photocurable inkjet ink for the lower layer is higher than the surface tension of the photocurable inkjet ink for the upper layer.
[2]
The photocurable inkjet ink set according to [1], wherein the difference between the surface tension of the photocurable inkjet ink for the lower layer and the surface tension of the photocurable inkjet ink for the upper layer is 5 or more.
[3]
The content of the vinyl ether group-containing (meth)acrylic acid ester represented by the general formula (I) contained in the photocurable inkjet ink for the lower layer is , 20 to 80% by mass, the photocurable inkjet ink set according to [1] or [2].
[4]
The upper layer photocurable inkjet ink contains the vinyl ether group-containing (meth)acrylic acid ester represented by the general formula (I), and the content of the vinyl ether group-containing (meth)acrylic acid ester is , the photocurable inkjet ink set according to any one of [1] to [3], wherein the photocurable inkjet ink for the lower layer is larger than the photocurable inkjet ink for the upper layer.
[5]
The photocurable inkjet ink set according to any one of [1] to [4], wherein the lower layer photocurable inkjet ink contains an acrylic surfactant.
[6]
The photocurable inkjet ink set according to any one of [1] to [5], wherein the upper layer photocurable inkjet ink contains a silicone surfactant.
[7]
A coating film of the lower layer photocurable inkjet ink according to any one of [1] to [6] is formed on a recording medium, and the coating film according to any one of [1] to [6] is formed on the coating film. An inkjet recording method, comprising forming a coating film of a photocurable inkjet ink for the upper layer.

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

DETAILED DESCRIPTION OF THE INVENTION Embodiments for carrying out the present invention will be described in detail below. It should be noted that the present invention is not limited to the following embodiments, and various modifications can be made within the scope of the gist of the present invention.

In the present specification, the term "wetting and spreadability of ink" refers to the property of relatively broadening the line width when the upper layer ink is linearly applied over the coating film of the lower layer ink. “Curing” means that when an ink containing a polymerizable compound is irradiated with radiation, the polymerizable compound is polymerized and the ink is solidified. “Curability” refers to the property of curing upon exposure to light. The “embedding property” is also referred to as filling property, and refers to the property that the underlying recording medium cannot be seen when the recorded matter is viewed from the side on which the cured product (image) is formed. “Ejection stability” refers to the property of constantly ejecting ink droplets stably from nozzles without clogging the nozzles. The term “storage stability” refers to the property that the viscosity of the ink does not easily change before and after storage.

In the present specification, the terms "lower layer" and "upper layer" merely represent the relative positional relationship in the image obtained by overcoating the inks, and can be rephrased as "undercoat" and "topcoat", respectively. can.

As used herein, "(meth)acrylate" means at least one of acrylate and its corresponding methacrylate, "(meth)acryloyl" means at least one of acryloyl and its corresponding methacryloyl, (Meth)acrylic" means acrylic and/or its corresponding methacrylic.

As used herein, the term "recorded material" refers to a material formed by recording ink on a recording medium to form a cured product. Further, in this specification, a cured product means a cured substance, and an ink film means a film formed by applying ink and curing, that is, a cured film. Curing of the coating film of the photocurable inkjet ink for the lower layer when forming a coating film of the photocurable inkjet ink for the lower layer described later and forming a coating film of the photocurable inkjet ink for the upper layer on the coating film means a state judged to be cured in the curing test described later. The coating film may be a solid pattern image or a design pattern image such as a picture, letter, or pattern.
In this specification, a "solid pattern image" means an image in which dots are printed on all pixels that are the minimum printing unit area defined by the printing resolution.

[Photo-curing inkjet ink set]
A photocurable inkjet ink set according to an embodiment of the present invention comprises a photocurable inkjet ink for a lower layer containing a vinyl ether group-containing (meth)acrylic acid ester represented by the general formula (I) described below; and a photocurable inkjet ink. In addition to this, the ink set satisfies the relationship that the surface tension of the ink for the lower layer is higher than that of the ink for the upper layer.
The ink set may further include inks other than the photocurable inkjet ink for the lower layer and the photocurable inkjet ink for the upper layer.

First, the above surface tension will be explained. The ink set of the present embodiment must satisfy the above relationship, and in addition, the difference between the surface tension of the lower layer ink and the surface tension of the upper layer ink ("surface tension of lower layer ink" - "upper layer ink The surface tension of ink for printing") is preferably 5 or more, more preferably 10 or more. When the difference is within the above range, the wetting and spreading properties of the ink are further improved.

It can be said that the surface tensions of the lower layer ink and the upper layer ink preferably fall within the following ranges on the premise that the above relationship is satisfied. That is, it is preferable that the surface tension of both the upper layer ink and the lower layer ink be in the range of 20 to 40 mN/m. More preferably, it is 25 to 40 mN/m. When the surface tensions of the upper layer ink and the lower layer ink are within the above ranges, the ejection stability from the inkjet head is excellent.

Here, the surface tension of the ink is determined by its composition (the type and content of each component), and the main factors include the type and content of the surfactant contained in the ink. The surfactant will be described later. In addition, the surface tension in this specification adopts the value obtained by the measuring method performed in the later-described Examples section.

Additives (components) that are contained or can be contained in the ink (composition) according to the present embodiment are described below. In the following description, the upper layer ink and the lower layer ink may be collectively referred to as "ink".

[Polymerizable compound]
The polymerizable compound contained in the upper layer ink and the lower layer ink in the present embodiment can be polymerized by the action of a photopolymerization initiator, which will be described later, upon exposure to radiation, and the printed ink can be cured.

(1. Vinyl ether group-containing (meth)acrylic esters)
At least the ink for the lower layer among the inks in the present embodiment has the following general formula (I) as a polymerizable compound:
_CH2 = ^CR1 -COOR2- ^O -CH=CH- ^R3 (I)
(In formula (I) above, 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 1 to 11 carbon atoms. It contains a vinyl ether group-containing (meth)acrylic acid ester represented by the following.

By containing the vinyl ether group-containing (meth)acrylic acid ester in the ink, it is possible to improve the curability of the ink, and furthermore, it is possible to reduce the viscosity of the ink. Furthermore, using a compound having both a vinyl ether group and a (meth)acrylic group in one molecule is more effective than using a compound having a vinyl ether group and a compound having a (meth)acrylic group separately. It is preferable for improving curability.

In particular, when the lower layer ink contains the vinyl ether group-containing (meth)acrylic acid ester, the upper layer ink has excellent wetting and spreading properties, and the curability of the lower layer ink and the lower layer ink on the recording medium The filling property of the ink is also excellent.

On the other hand, the upper layer ink may or may not contain the vinyl ether group-containing (meth)acrylic acid ester, but it is preferable to contain it because the following effects can be obtained. When the upper layer ink also contains a vinyl ether group-containing (meth)acrylic acid ester, the lower layer ink and the upper layer ink contain the same compound as the vinyl ether group-containing (meth)acrylic acid ester. may also contain different compounds.

In the general formula (I) above, the divalent organic residue having 2 to 20 carbon atoms represented by R ² is a linear, branched or cyclic substituted group having 2 to 20 carbon atoms. an alkylene group that may be substituted, an optionally substituted alkylene group having 2 to 20 carbon atoms having an oxygen atom formed by an ether bond and/or an ester bond in the structure, an optionally substituted divalent alkylene group having 6 to 11 carbon atoms, Aromatic groups are preferred. Among these, an alkylene group having 2 to 6 carbon atoms such as an ethylene group, an n-propylene group, an isopropylene group, and a butylene group, an oxyethylene group, an oxyn-propylene group, an oxyisopropylene group, an oxybutylene group, and the like An alkylene group having 2 to 9 carbon atoms having an oxygen atom formed by an ether bond in the structure of is preferably used.

In the general formula (I) above, the monovalent organic residue having 1 to 11 carbon atoms represented by R ³ is a linear, branched or cyclic substituted group having 1 to 10 carbon atoms. Alkyl groups having 6 to 11 carbon atoms and optionally substituted aromatic groups are 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 and a benzyl group are preferably used.

When each of the above organic residues is an optionally substituted group, the substituent is divided into a group containing a carbon atom and a group not containing a carbon atom. First, when the above substituent is a group containing a carbon atom, the carbon atom is counted as the number of carbon atoms in the organic residue. Examples of groups containing carbon atoms include, but are not limited to, carboxyl groups and alkoxy groups. Examples of groups containing no carbon atoms include, but are not limited to, hydroxyl groups and halo groups.

Examples of the vinyl ether group-containing (meth)acrylic esters include, but are not limited to, 2-vinyloxyethyl (meth)acrylate, 3-vinyloxypropyl (meth)acrylate, and 1-methyl (meth)acrylate. -2-vinyloxyethyl, 2-vinyloxypropyl (meth)acrylate, 4-vinyloxybutyl (meth)acrylate, 1-methyl-3-vinyloxypropyl (meth)acrylate, 1-vinyloxymethyl (meth)acrylate Propyl, 2-methyl-3-vinyloxypropyl (meth)acrylate, 1,1-dimethyl-2-vinyloxyethyl (meth)acrylate, 3-vinyloxybutyl (meth)acrylate, 1-methyl-(meth)acrylate 2-vinyloxypropyl, 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-vinyloxymethylphenyl (meth)acrylate Methyl, o-vinyloxymethylphenylmethyl (meth)acrylate, 2-(vinyloxyethoxy)ethyl (meth)acrylate, 2-(vinyloxyisopropoxy)ethyl (meth)acrylate, (meth)acrylic acid 2 -(vinyloxyethoxy)propyl, 2-(vinyloxyethoxy)isopropyl (meth)acrylate, 2-(vinyloxyisopropoxy)propyl (meth)acrylate, 2-(vinyloxyisopropoxy) (meth)acrylate Isopropyl, 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, (meth)acrylate 2-(vinyloxyisopropoxyethoxy)propyl acrylate, 2-(vinyloxyisopropoxyisopropoxy)propyl (meth)acrylate, 2-(vinyloxy)acrylate (meth) Xyethoxyethoxy)isopropyl, 2-(vinyloxyethoxyisopropoxy)isopropyl (meth)acrylate, 2-(vinyloxyisopropoxyethoxy)isopropyl (meth)acrylate, 2-(vinyloxyisopropoxy)acrylate (meth) Isopropoxy)isopropyl, 2-(vinyloxyethoxyethoxyethoxy)ethyl (meth)acrylate, 2-(vinyloxyethoxyethoxyethoxyethoxy)ethyl (meth)acrylate, 2-(isopropenoxyethoxy)acrylate (meth) ) ethyl (meth)acrylate, 2-(isopropenoxyethoxyethoxy)ethyl (meth)acrylate, 2-(isopropenoxyethoxyethoxyethoxy)ethyl (meth)acrylate, 2-(isopropenoxyethoxyethoxyethoxy)acrylate (meth) ethoxy)ethyl, polyethylene glycol monovinyl ether (meth)acrylate, and polypropylene glycol monovinyl ether (meth)acrylate.

Among these, 2-(vinyloxyethoxy)ethyl (meth)acrylate, that is, 2-(vinyloxy)acrylate, can be used to lower the viscosity of the ink, have a high flash point, and have excellent curability of the ink. At least one of ethyl methacrylate and 2-(vinyloxyethoxy)ethyl methacrylate is preferred, and 2-(vinyloxyethoxy)ethyl acrylate is more preferred. In particular, 2-(vinyloxyethoxy)ethyl acrylate and 2-(vinyloxyethoxy)ethyl methacrylate both have simple structures and small molecular weights, so that they can significantly reduce 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. , 2-(vinyloxyethoxy)ethyl acrylate includes 2-(2-vinyloxyethoxy)ethyl acrylate and 2-(1-vinyloxyethoxy)ethyl acrylate. 2-(Vinyloxyethoxy)ethyl acrylate is superior to 2-(vinyloxyethoxy)ethyl methacrylate in curability.

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

The content of the vinyl ether group-containing (meth)acrylic acid ester contained in the lower layer ink is preferably 20 to 80% by mass, preferably 30 to 80% by mass, relative to the total mass (100% by mass) of the lower layer ink. is more preferred. When the content is 20% by mass or more, the viscosity of the lower layer ink can be reduced, the curability of the lower layer ink can be improved, and the wet spreadability of the upper layer ink can be further improved. can be On the other hand, when the content is 80% by mass or less, the storage stability of the lower layer ink can be improved.

In particular, when not only the lower layer ink but also the upper layer ink contains the vinyl ether group-containing (meth)acrylic acid ester, the content of the vinyl ether group-containing (meth)acrylic acid ester is higher in the lower layer ink. It is preferably larger than the ink for the upper layer, and more preferably the ink for the lower layer is larger than the ink for the upper layer by 10 mass % or more. As a result, the wetting and spreading properties of the ink for the upper layer can be further improved, and an ink set having excellent various performances can be obtained.
Note that the content of the upper layer ink in the above case is not particularly limited because it is preferably determined in consideration of performance other than curability. In the present embodiment, when the surface tension of the lower layer ink is higher than the surface tension of the lower layer ink and the lower layer ink contains the vinyl ether group-containing (meth)acrylic acid ester, the lower layer ink is applied. The reason why the upper layer ink spreads well on the film is that the coating film derived from the vinyl ether group of the vinyl ether group-containing (meth)acrylic acid ester has some effect on the wetting and spreading property of the upper layer ink. It is speculated that However, the cause is not limited to this.

The method for producing the vinyl ether group-containing (meth)acrylic acid ester is not limited to the following, but is a method of esterifying (meth)acrylic acid and a hydroxyl group-containing vinyl ether (manufacturing method B), (meth)acrylic acid halide and a hydroxyl group-containing vinyl ether (manufacturing method C), a method of esterifying a (meth)acrylic anhydride and a hydroxyl group-containing vinyl ether (manufacturing method D), a (meth)acrylic acid ester and a hydroxyl group-containing vinyl ether esterified A method of exchanging (manufacturing method E), a method of esterifying (meth)acrylic acid and a halogen-containing vinyl ether (manufacturing method F), a method of esterifying an alkali (earth) metal (meth)acrylate and a halogen-containing vinyl ether (Manufacturing method G), a method of vinyl-exchanging a hydroxyl group-containing (meth)acrylic acid ester and a vinyl carboxylate (manufacturing method H), and a method of ether-exchanging a hydroxyl group-containing (meth)acrylic acid ester and an alkyl vinyl ether (manufacturing method I). mentioned.

Among these, manufacturing method E is preferable because the desired effect can be further exhibited in the present embodiment.

(2. Polymerizable compounds other than the above)
As polymerizable compounds other than the above (hereinafter referred to as "other polymerizable compounds"), conventionally known various monomers and oligomers such as monofunctional, difunctional, and trifunctional or higher polyfunctional can be used. . Examples of the above monomers include unsaturated carboxylic acids such as (meth)acrylic acid, itaconic acid, crotonic acid, isocrotonic acid and maleic acid, salts or esters thereof, urethane, amides and anhydrides thereof, acrylonitrile, styrene, various unsaturated polyesters, unsaturated polyethers, unsaturated polyamides, and unsaturated urethanes. Examples of the above 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 aromatic urethane (meth)acrylates. Acrylates and polyester (meth)acrylates may be mentioned.

Also, an N-vinyl compound may be contained as another monofunctional monomer or polyfunctional monomer. N-vinyl compounds include N-vinylformamide, N-vinylcarbazole, N-vinylacetamide, N-vinylpyrrolidone, N-vinylcaprolactam, acryloylmorpholine, derivatives thereof, and the like.

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

Among the (meth)acrylates, monofunctional (meth)acrylates include, for example, isoamyl (meth)acrylate, stearyl (meth)acrylate, lauryl (meth)acrylate, octyl (meth)acrylate, decyl (meth)acrylate, iso myristyl (meth) acrylate, isostearyl (meth) acrylate, 2-ethylhexyl-diglycol (meth) acrylate, 2-hydroxybutyl (meth) acrylate, butoxyethyl (meth) acrylate, ethoxydiethylene glycol (meth) acrylate, methoxydiethylene glycol ( meth)acrylate, methoxypolyethylene glycol (meth)acrylate, methoxypropylene glycol (meth)acrylate, phenoxyethyl (meth)acrylate, tetrahydrofurfuryl (meth)acrylate, isobornyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (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, bifunctional (meth)acrylates include, for example, 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-nonane Diol di(meth)acrylate, neopentylglycol di(meth)acrylate, dimethylol-tricyclodecane di(meth)acrylate, EO (ethylene oxide) adduct di(meth)acrylate of bisphenol A, PO (propylene oxide) of bisphenol A ) adducts di(meth)acrylate, neopentylglycol hydroxypivalate di(meth)acrylate, and polytetramethylene glycol di(meth)acrylate. Among these, dipropylene glycol di(meth)acrylate is preferred.

Among the above (meth)acrylates, tri- or more functional polyfunctional (meth)acrylates include, for example, trimethylolpropane tri(meth)acrylate, EO-modified trimethylolpropane tri(meth)acrylate, and pentaerythritol tri(meth)acrylate. , pentaerythritol tetra (meth) acrylate, dipentaerythritol hexa (meth) acrylate, ditrimethylolpropane tetra (meth) acrylate, glycerin propoxy tri (meth) acrylate, cowprolactone-modified trimethylol propane tri (meth) acrylate, pentaerythritol ethoxytetra(meth)acrylate and caprolactam-modified dipentaerythritol hexa(meth)acrylate.

Among these, the ink preferably contains a monofunctional (meth)acrylate as another polymerizable compound. In this case, the viscosity of the ink is low, the solubility of the photopolymerization initiator and other additives is excellent, and good ejection stability during ink jet recording can be easily obtained. Furthermore, since the toughness, heat resistance, and chemical resistance of the coating film are increased, it is more preferable to use a monofunctional (meth)acrylate and a difunctional (meth)acrylate together, and among them, phenoxyethyl (meth)acrylate and dipropylene glycol. It is more preferable to use a di(meth)acrylate together.

These other polymerizable compounds may be used singly or in combination of two or more.

The content of the other polymerizable compound 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 (100% by mass) of the ink. When 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]
The ink in this embodiment preferably contains a surfactant. In particular, when the upper layer ink contains a surfactant, it is possible to more reliably satisfy the relationship that the surface tension of the lower layer ink is higher than the surface tension of the upper layer ink. In addition, since the lower layer ink as well as the upper layer ink contains a surfactant, the lower layer ink has excellent embedding properties on the recording medium.

Examples of the surfactant include, but are not limited to, silicone surfactants, acrylic surfactants, cationic surfactants, anionic surfactants, nonionic surfactants, amphoteric surfactants, and fluorosurfactants. Among these, silicone surfactants and acrylic surfactants can significantly reduce surface tension and prevent damage to members that come into contact with ink, such as heads and ink supply devices. At least one of the agents is preferred.

Examples of the silicone-based surfactant include, but are not limited to, modified silicones such as polyester-modified silicones and polyether-modified silicones. Among these, polyether-modified silicone is preferable because it can greatly reduce the surface tension, thereby further improving the wetting and spreading properties of the ink on the recording medium.

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

Examples of commercially available silicone-based surfactants include BYK-347, BYK-348, BYK-UV3500, BYK-UV3510, BYK-UV3530, and BYK-UV3570 (all trade names manufactured by BYK). be done. Among these, BYK-UV3500 is preferable because the ink for the upper layer has better wetting and spreading properties.

Moreover, the acrylic surfactant is not limited to the following, but preferably includes, for example, 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 (the above are trade names manufactured by Kusumoto Kasei Co., Ltd.), BYK-350, BYK-352 , BYK-354, BYK-355, BYK-358, BYK-380, BYK-381, BYK-392 (these are trade names manufactured by BYK), Polyflow No. 57, 95 (these are trade names manufactured by Kyoeisha Yushi Kagaku Kogyo Co., Ltd.). Among these, BYK-350 is preferable because when the lower layer ink contains an acrylic surfactant, the lower layer ink has excellent ejection stability.

As for the combination of surfactants that are contained or can be contained in the upper layer ink and the lower layer ink, the above-described surface tension relationship can be more reliably satisfied. is preferred, and the following third case is more preferred.

First, it is preferable that the upper layer ink and the lower layer ink contain the same surfactant, and that the upper layer ink has a higher surfactant content than the lower layer ink. The content difference is preferably 0.2% by mass or more, more preferably 0.3% by mass or more.
The content of each surfactant contained in the upper layer ink and the lower layer ink in the first case is preferably 0.1 to 0.5% by mass with respect to the total mass (100% by mass) of each ink, Further, the content in the lower layer ink is more preferably 0.1 to 0.3% by mass.
In the first case, the wetting and spreading property of the upper layer ink is further improved, and the cost of the ink material can be reduced by sharing the surfactant between the upper layer ink and the lower layer ink.

Second, both the upper layer ink and the lower layer ink contain a modified silicone surfactant (the compounds may be the same or different), and the content of the surfactant is the same as that of the upper layer ink. is preferably larger than the lower layer ink. The content difference is preferably 0.2% by mass or more, more preferably 0.3% by mass or more.

As the modified silicone-based surfactant in the second case, the upper layer ink and the lower layer ink are each independently preferably at least one of polyether-modified silicone and polyester-modified silicone, and polyether-modified silicone is preferable. more preferred. In addition, it is preferable to use the same modified silicone-based surfactant because the cost of the ink material can be reduced due to common use of the surfactant.

The content of each surfactant contained in the upper layer ink and the lower layer ink in the second case is preferably 0.1 to 0.5% by mass with respect to the total mass (100% by mass) of each ink, Further, the content in the lower layer ink is more preferably 0.1 to 0.3% by mass.
In the second case, the lower layer ink fills the recording medium well.

Third, the upper layer ink preferably contains a modified silicone surfactant. On the other hand, the underlayer ink may or may not contain a surfactant. However, when the lower layer ink contains a surfactant, the surfactant is preferably an acrylic surfactant. When the upper layer ink contains the modified silicone-based surfactant, the wet spreadability of the upper layer ink is further improved. Examples of the modified silicone-based surfactant include, but are not limited to, polyether-modified silicone and polyester-modified silicone. Polyether-modified silicones are preferred because they are superior.

The content of each surfactant contained in the upper layer ink and the lower layer ink in the third case is preferably 0.1 to 0.5% by mass with respect to the total mass (100% by mass) of each ink, Further, the content in the lower layer ink is more preferably 0.1 to 0.3% by mass.
In the third case, the wetting and spreading properties of the ink are further improved, which is advantageous in that the difference in surface tension between the upper layer ink and the lower layer ink is stably ensured.

[Photopolymerization initiator]
The photopolymerization initiator that can be contained in the ink in the present embodiment is used to form a print by curing the ink present on the surface of the recording medium through photopolymerization by irradiation with radiation. By using ultraviolet rays (UV) among radiations (lights), the safety is excellent and the cost of the light source lamp can be suppressed. There is no limitation as long as it generates active species such as radicals and cations by light energy and initiates the polymerization of the polymerizable compound, but a radical photopolymerization initiator or a cationic photopolymerization initiator may be used. Among them, 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.), hexaarylbi imidazole compounds, ketoxime ester compounds, borate compounds, azinium compounds, metallocene compounds, active ester compounds, compounds having a carbon-halogen bond, and alkylamine compounds.

Among these, at least an acylphosphine oxide compound is preferably used because it can further improve the curability 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-hydroxycyclohexylphenyl ketone, 2,2-dimethoxy-2-phenylacetophenone, xanthone, fluorenone, benzaldehyde, fluorene, anthraquinone, and triphenylamine. , carbazole, 3-methylacetophenone, 4-chlorobenzophenone, 4,4'-dimethoxybenzophenone, 4,4'-diaminobenzophenone, Michler's ketone, benzoin propyl ether, benzoin ethyl ether, benzyl 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.

Examples of commercially available radical photopolymerization initiators include IRGACURE 651 (2,2-dimethoxy-1,2-diphenylethan-1-one), IRGACURE 184 (1-hydroxy-cyclohexyl-phenyl-ketone), and DAROCUR 1173. (2-hydroxy-2-methyl-1-phenyl-propan-1-one), IRGACURE 2959 (1-[4-(2-hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1-propane- 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 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) (manufactured by BASF), Speedcure TPO , Speedcure DETX (2,4-diethylthioxanthone), Speedcure ITX (2-isopropylthioxanthone) (above, Lambson), KAYACURE DETX-S (2,4-diethylthioxanthone) (Nippon Kayaku Co., Ltd.), Lucirin TPO, LR8893, LR8970 (manufactured by BASF), and ubecryl P36 (manufactured by UCB) and the like can be mentioned.

A photoinitiator may be used individually by 1 type, and may be used in combination of 2 or more type.

The content of the photopolymerization initiator can improve the ultraviolet curing speed and make the curability excellent, and in order to avoid dissolution of the photopolymerization initiator and coloring derived from the photopolymerization initiator, It is preferably 3 to 20% by mass with respect to the total mass (100% by mass) of the ink.

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

Examples of the above polymerization inhibitor include, but are not limited to, 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), etc. Inhibitors and hindered amines, hydroquinone monomethyl ether (MEHQ), and hydroquinone.

A polymerization inhibitor may be used individually by 1 type, and may be used in combination of 2 or more type. The content of the polymerization inhibitor is not particularly limited because it is determined in relation to the content of other components.

[Color material]
The ink in this embodiment may contain a coloring material. At least one of pigments and dyes can be used as the coloring material.

(pigment)
In this embodiment, the light resistance of the ink can be improved by using a pigment as the colorant. Both inorganic pigments and organic pigments can be used.

Examples of inorganic pigments that can be used include carbon black (CI Pigment Black 7) such as furnace black, lamp black, acetylene black, and channel black, iron oxide, and titanium oxide.

Organic pigments include azo pigments such as insoluble azo pigments, condensed azo pigments, azo lakes and chelate azo pigments, phthalocyanine pigments, perylene and perinone pigments, anthraquinone pigments, quinacridone pigments, dioxane pigments, thioindigo pigments, isoindolinone pigments, quinophthalone pigments, and the like. polycyclic 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, daylight Fluorescent pigments may be mentioned.

More specifically, as carbon black used as black ink, No. 2300, No. 900, MCF88, no. 33, No. 40, No. 45, No. 52, MA7, MA8, MA100, No. 2200B, etc. (manufactured by Mitsubishi Chemical Corporation), Raven 5750, Raven 5250, Raven 5000, Raven 3500, Raven 1255, Raven 700, etc. (manufactured by Carbon Columbia), Reg. 400R, Rega1 330R, Rega1 660R, Mogul L, Monarch 700, Monarch 800, Monarch 880, Monarch 900, Monarch 1000, Monarch 1100, Monarch 1300, Monarch 1400, etc.名）、Ｃｏｌｏｒ Ｂｌａｃｋ ＦＷ１、Ｃｏｌｏｒ Ｂｌａｃｋ ＦＷ２、Ｃｏｌｏｒ Ｂｌａｃｋ ＦＷ２Ｖ、Ｃｏｌｏｒ Ｂｌａｃｋ ＦＷ１８、Ｃｏｌｏｒ Ｂｌａｃｋ ＦＷ２００、Ｃｏｌｏｒ Ｂ１ａｃｋ 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. (the above are trade names of Degussa).

Pigments used in white ink include C.I. I. Pigment White 6, 18, 21 can be mentioned.

Pigments used in yellow ink include C.I. 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, 180.

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

Pigments used in cyan ink include C.I. 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. I. bat blue 4,60.

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

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

When the above pigments are used, their average particle size is preferably 300 nm or less, more preferably 50 to 200 nm. When the average particle size is within the above range, the reliability of ink ejection stability and dispersion stability is further improved, and an image with excellent image quality can be formed. Here, the average particle size in this specification is measured by a dynamic light scattering method.

(dye)
In this embodiment, a dye can be used as the coloring material. Acid dyes, direct dyes, reactive dyes, and basic dyes can be used as the dye without any particular limitation.
Examples of the dye include C.I. I. Acid Yellow 17, 23, 42, 44, 79, 142, C.I. I. Acid Red 52, 80, 82, 249, 254, 289, C.I. I. Acid Blue 9,45,249, C.I. I. Acid Black 1, 2, 24, 94, C.I. I. Food Black 1, 2, C.I. I. Direct Yellow 1, 12, 24, 33, 50, 55, 58, 86, 132, 142, 144, 173, C.I. I. Direct Red 1, 4, 9, 80, 81, 225, 227, C.I. I. Direct Blue 1, 2, 15, 71, 86, 87, 98, 165, 199, 202, C.I. I. Directed Black 19, 38, 51, 71, 154, 168, 171, 195, C.I. I. Reactive Red 14, 32, 55, 79, 249, C.I. I. Reactive Black 3, 4, 35 can be mentioned.
The above dyes may be used singly or in combination of two or more.

The content of the coloring material is preferably 0.5 to 20% by mass with respect to the total mass of the ink (100% by mass) in order to obtain excellent hiding power and color reproducibility.

[Other additives]
The ink in the present embodiment may contain additives (components) other than the additives listed above. Such components are not particularly limited, but may include, for example, conventionally known dispersants, polymerization accelerators, penetration enhancers, wetting agents (humectants), and other additives. Examples of the above-mentioned other additives include fixing agents, antifungal agents, preservatives, antioxidants, ultraviolet absorbers, chelating agents, and thickening agents that are conventionally known.

As described above, according to the present embodiment, since the line width of the upper layer ink applied over the coating film of the lower layer ink does not become narrow, a photocurable inkjet ink set having excellent wetting and spreading properties of the upper layer ink is provided. can provide. Furthermore, according to the present embodiment, a photocurable inkjet ink set is provided that is excellent in the curability and ejection stability of both the upper layer ink and the lower layer ink, and the embedding property of the lower layer ink on the recording medium. can do. As described above, in the photocurable inkjet ink set, the lower layer ink and the upper layer ink have a relationship of exerting influence on each other in terms of composition and physical properties. In other words, when determining the composition and physical properties of the lower layer ink and the upper layer ink, it is necessary to consider the composition and physical properties of the upper layer ink and the lower layer ink, respectively.

[Recording medium]
The photocurable inkjet ink set of the present embodiment is ejected onto a recording medium using an inkjet recording method to be described later to obtain a recorded matter. Examples of the recording medium include absorbent and non-absorbent recording media. The inkjet recording method of the embodiments described later can be widely applied to recording media having various absorption properties, from non-absorbent recording media in which ink permeation is difficult to absorbent recording media in which ink easily permeates. .

The absorbent recording medium is not particularly limited, but examples include plain paper such as electrophotographic paper having high ink permeability, inkjet paper (an ink absorbing layer composed of silica particles or alumina particles, or polyvinyl alcohol ( Inkjet-only paper with an ink-absorbing layer made of a hydrophilic polymer such as PVA) or polyvinylpyrrolidone (PVP)), art paper, coated paper, and general offset printing with relatively low ink permeability. cast paper and the like.

Examples of non-absorbent recording media include, but are not limited to, films, sheets, and plates of plastics such as polyvinyl chloride (PVC), polyethylene, polypropylene, polyethylene terephthalate (PET), iron, silver, copper, Plates of metals such as aluminum, metal plates produced by vapor deposition of these various metals, films made of plastic, and plates of alloys such as stainless steel and brass can be used.

[Inkjet recording method]
One embodiment of the present invention relates to an inkjet recording method. The ink set of the above embodiment can be suitably used for 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 also 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. The recording apparatus includes a first recording mode, a second recording mode, and a recording control section.

In the first recording mode, a coating film is formed by overcoating the coating film formed with the lower layer ink with the upper layer ink.

In the second recording mode, a coating film is formed by overcoating the coating film formed with the upper layer ink with the lower layer ink.

The recording control section controls the amount of ink per dot of the solid pattern image formed with the ink for the lower layer in the first recording mode and the second recording mode.
An example of the above recording apparatus will be described in detail below.

FIG. 1 is a conceptual diagram for explaining an example of a head unit and a transport unit of a printing apparatus that can be used in this embodiment. The continuous sheet-like recording medium S is conveyed from the upstream side to the downstream side in the conveying direction as the upstream roller 12A, the conveying drum 12, and the downstream roller 12B rotate. The head units consist of 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, and a magenta head unit that ejects magenta ink M. 30C, 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 that ejects white ink W (the one on the right side of FIG. 1 on the page). Prepare. The irradiation unit 40 includes irradiation sections 42a, 41a, 41b, 41c, 41d, and 42b provided downstream in the transport direction for each head unit, and an irradiation section 43 provided last in the transport direction. there is Such a recording apparatus can be configured, for example, as shown in FIG. 2 in JP-A-2010-208218.

Here, when the first recording mode is performed, the recording medium S conveyed to the position facing each head unit on the conveying drum is first conveyed in the following order: The lower layer ink is ejected from the first white head unit 30A, and irradiation is performed from the irradiation section 42a to form a coating film of the lower layer ink. Next, on the coating film, 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 ejects the ink for the upper layer. Irradiation is performed from the irradiated irradiation unit. Finally, irradiation is performed from the irradiation unit 43 to form a coating film. The second white head unit 30A and irradiation section 42b are not used.

On the other hand, when the second recording mode is performed, the recording medium S conveyed to the position facing each head unit on the conveying drum is conveyed in the order in which the recording medium S is conveyed. At least one of the head unit 30B, the magenta head unit 30C, the cyan head unit 30D, and the black head unit 30E ejects the ink for the upper layer, and the irradiation unit provided downstream of the head emits the ink for the upper layer. A coating of ink is formed. Next, the lower layer ink is discharged from the second white head unit 30A onto the coating film, and irradiation is performed from the irradiation section 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 section 42a are not used.

When the coating film of the lower layer ink is a solid pattern, the recording control unit preferably has a lower ink amount per dot in the second recording mode than in the first recording mode. It should be adjusted to be 10% or more, more preferably 10 to 50%. As a result, even in the second recording mode, the wetting and spreading property of the lower layer ink equivalent to that in the first recording mode is ensured. Note that one dot means an ink droplet deposited per pixel, which is the minimum recording unit defined by the recording resolution.

Here, the second recording mode may not necessarily be provided, and in this case, the second white head unit 30A and irradiation section 42b may not be provided. Among the inks, the inks to be coated later, that is, the inks for the upper layer in the first recording mode and the inks for the lower layer in the second recording mode are used in the irradiation section (41a) provided downstream of the head unit. 41d, 42b) or irradiation by the irradiation section 43. When curing is performed by the irradiation section provided downstream of the head unit, the irradiation section 43 is not necessary. good.

[Upper layer ink and lower layer ink]
In the present embodiment, the lower layer ink is ink that forms a coating film on the recording medium before the upper layer ink in the first recording mode, and the upper layer ink forms a coating film on the recording medium in the first recording mode. This ink forms a coating film on top of the coating film of the lower layer ink. For example, one of the upper layer ink and the lower layer ink may be used as the main ink and the other as the auxiliary ink.
The main ink may be an ink such as a color ink that is used by visually recognizing the coating film. In addition, the auxiliary ink is, for example, a special color ink such as white ink, clear ink, and metallic ink, or a functional ink that improves the performance of the main ink such as adhesion, wetting spreadability, and abrasion resistance. Just do it.
Here, both the upper layer ink and the lower layer ink may be used as main inks, or both may be used as auxiliary inks. Moreover, it is preferable because the performance of the main ink is improved. When one ink is used as the main ink and the other ink is used as the auxiliary ink, which of the upper layer ink and the lower layer ink should be used as the main ink may be determined in consideration of the properties of each of the main ink and the auxiliary ink described above. Therefore, in FIG. 1, the first white head unit 30A is the head that ejects the lower layer ink in the first recording mode, but the lower layer ink is not limited to white ink.

The reason why the amount of ink is different between the first recording mode and the second recording mode is that in the second recording mode, the line width of the lower layer ink on the coating film of the upper layer ink becomes narrower. This is because the fillability of the solid pattern image with the ink for the lower layer deteriorates. Therefore, by increasing the amount of ink as described above, it is possible to improve the fillability of the ink for the lower layer.

[Inkjet recording method using inkjet recording apparatus]
The inkjet recording method of the present embodiment comprises a first recording step of forming a coating film of the lower layer ink of the above embodiment on a recording medium, and overcoating the coating film with the upper layer ink of the above embodiment. and a second recording step of forming an image by forming a coating film of the ink.

Each of the first recording process and the second recording process includes an ejection step of ejecting ink onto a recording medium, and a curing step of irradiating the ejected ink with ultraviolet rays to cure the ink. It is a thing. In this manner, the cured ink on the recording medium forms an image, ie, a cured ink coating. These stages are described below.

[Discharge stage]
In the ejection step, the ink is ejected toward the recording medium, and the lower layer ink adheres to the recording medium, and the upper layer ink adheres to the coating film of the lower layer ink. The viscosity of the ink during ejection is preferably 15 mPa·s or less, more preferably 12 mPa·s or less. If the viscosity of the ink is the above at room temperature or without heating, the ink may be discharged at room temperature or without heating. At that time, the temperature of the ink during ejection is preferably 20 to 30.degree. On the other hand, the ink may be ejected with a preferable viscosity by heating the ink to a predetermined temperature. In this way, good ejection stability is achieved.

Both the ink for the lower layer and the ink for the upper layer in the present embodiment are photocurable inks, and have a higher viscosity than normal water-based inks. Such ink viscosity fluctuations have a large effect on changes in droplet size and droplet ejection speed, and may eventually cause image quality deterioration. Therefore, it is preferable to keep the temperature of the ink during ejection as constant as possible.

[Curing stage]
Next, in the curing stage, the ink that has been ejected and adhered (landed) to the recording medium is cured by being irradiated with ultraviolet rays. This is because the photopolymerization initiator that can 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 initiated by the initiating species. This is because it is facilitated by function. Alternatively, this is because the irradiation of ultraviolet rays initiates the polymerization reaction of the polymerizable compound. At this time, if the sensitizing dye is present together with the photopolymerization initiator in the ink, the sensitizing dye in the system absorbs the ultraviolet light and becomes excited, and contacts with the photopolymerization initiator to accelerate the decomposition of the photopolymerization initiator. to achieve a more sensitive curing reaction.

Mercury lamps, gas/solid lasers, and the like are mainly used as ultraviolet light sources, and mercury lamps and metal halide lamps are widely known as light sources for curing photocurable inkjet inks. On the other hand, from the viewpoint of environmental protection, there is currently a strong desire to eliminate mercury, and replacement with GaN-based semiconductor ultraviolet light emitting devices is very 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 in cost, and are expected to serve as light sources for photocurable inkjets. Among these, UV-LED is preferable.

Here, since it is easy to increase the output of the LED, a photocurable inkjet ink that can be cured with an irradiation energy of preferably 200 mJ/cm ² or less using a UV-LED whose emission peak wavelength is preferably in the range of 350 to 420 nm. is preferably used in the ink jet recording method. In this case, low-cost printing and high printing speed can be achieved. Such an ink 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 the present embodiment, the wettability of the upper layer ink, the curability and ejection stability of both the upper layer ink and the lower layer ink, and the embedding property of the lower layer ink on the recording medium It is possible to provide an inkjet recording method using a photocurable inkjet ink set, which is also excellent.

EXAMPLES Hereinafter, embodiments of the present invention will be described in more detail with reference to examples, but the present embodiments are not limited only to these examples.

[Materials used]
Materials used in the following examples and comparative examples are as follows.
[Pigment]
・C. I. Pigment Blue 15:3 (phthalocyanine blue, manufactured by DIC, hereinafter abbreviated as "PB15:3")
・C. I. Pigment White 6 (titanium oxide, manufactured by Tayca, hereinafter abbreviated as "PW6")
[Polymerizable compound]
(1. Vinyl ether group-containing (meth)acrylic esters)
・VEEA (2-(2-vinyloxyethoxy)ethyl acrylate, trade name manufactured by 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 (SHIN-NAKAMURA CHEMICAL CO., LTD. trade name, dipropylene glycol diacrylate, hereinafter abbreviated as "DPGDA")
[Polymerization inhibitor]
・MEHQ (hydroquinone monomethyl ether, manufactured by Kanto Chemical Co., Ltd.)
[Surfactant]
· BYK-UV3500 (trade name manufactured by BYK, silicone type, hereinafter abbreviated as "UV3500")
· BYK-UV3570 (trade name manufactured by BYK, silicone type, hereinafter abbreviated as "UV3570".)
・BYK-350 (trade name manufactured by BYK, acrylic, hereinafter abbreviated as “BYK350”)
・BYK-381 (trade name manufactured by BYK, acrylic, hereinafter abbreviated as “BYK381”)
[Photopolymerization initiator]
・ IRGACURE 819 (BASF trade name, solid content 100%, abbreviated as “819” below.)
・DAROCUR TPO (trade name manufactured by BASF, solid content 100%, hereinafter abbreviated as “TPO”)
・ KAYACURE DETX-S (trade name manufactured by Nippon Kayaku Co., Ltd., solid content 100%, hereinafter abbreviated as “DETX”)

[Preparation of upper layer ink and lower layer ink]
Each material was added so that the composition (unit: mass %) shown in Tables 1 and 2 below was obtained, and the mixture was stirred with a high-speed water-cooled stirrer to obtain a cyan upper layer ink and a white lower layer ink. prepared ink for
Table 1 shows the composition of each upper layer ink, and upper layer inks 1, 2, 3, and 4 are abbreviated as upper layer 1, 2, 3, and 4 below, respectively. Table 2 shows the composition of each lower layer ink. , and 7.

[Examples 1 to 7, Comparative Examples 1 to 5]
An ink set was prepared using a combination of the upper layer ink and the lower layer ink shown in the upper part of each of Tables 3 and 4 below. In each example and each comparative example, the surface tension γ (mN/m) of the upper layer ink and the lower layer ink was measured by the Wilhelmy method using a surface tension meter CBVP-Z (manufactured by Kyowa Interface Science Co., Ltd.). are values measured under normal temperature and normal pressure, and are shown in Tables 3 and 4 below.

[Evaluation item]
The wet spreadability, filling property, ejection stability, and curability of the ink sets prepared in each example and each comparative example were evaluated by the following methods.

(1. Wetting and spreading property)
The lower layer ink was ejected under conditions of 720×720 dpi and 15 ng/dot toward a recording medium (PET50A, manufactured by Lintec Corporation), and then cured to form a cured product. After that, using the upper layer ink, one line (one dot row) was formed on the cured product under conditions of 720 dpi and 10 ng/dot.

Each ink was discharged 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 upper layer ink on the cured product (cured film of the lower layer ink).
Evaluation criteria are 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. Embedability)
The lower layer ink was discharged toward the recording medium (PET50A) under the conditions of 720×720 dpi and 10 ng/dot and cured.

Each ink was ejected after adjusting the temperature so that the viscosity of each ink was 10 mPa·s. This evaluation was carried out by visually observing the fillability of the resulting cured product on top of the ink for the lower layer.
Evaluation criteria are as follows. The evaluation results are shown in Tables 3 and 4 below.
A: Buried when visually observed from a height of 30 cm above the recording medium (no underlying streaks were visible).
B: Underlying streaks were visible when viewed from a height of 30 cm above the recording medium.

(3. Ejection stability)
The ejection stability of each of the upper layer ink and the lower layer ink was evaluated. Each of the upper layer ink and the lower layer ink was discharged continuously for 60 minutes from a head having 180 nozzles. In this way, the ejection stability in the head was evaluated when each of the upper layer ink and the lower layer ink was ejected.
Evaluation criteria are as follows. The evaluation results are shown in Tables 3 and 4 below.
A: No nozzle missing occurred during continuous ejection.
B: Nozzle missing occurred during continuous ejection.

(4. Curability)
Curability of each of the upper layer ink and the lower layer ink was evaluated. Using an inkjet recording apparatus equipped with piezo-type inkjet nozzles, each nozzle row was filled with the upper layer ink and the lower layer ink in each example and each comparative example. Each of the upper layer ink and the lower layer ink is discharged under the conditions of 720 × 720 dpi and 10 ng / dot toward the recording medium (PET50A) at normal temperature and pressure, and the film thickness of the printed matter (recorded matter) is 7 to 8 μm. A solid pattern image was printed so that

After that, from the UV-LED in the ultraviolet irradiation device mounted on the side of the carriage, ultraviolet rays with an irradiation intensity of 1 W/cm ² and a peak wavelength of 395 nm are gradually irradiated up to a maximum of 200 mJ/cm ² per pass. By increasing the amount, the solid pattern image was irradiated. A recorded matter was produced in this manner. When the tackiness of the coating film surface on the recorded matter disappeared during the irradiation, it was considered to be cured at that point.
Whether or not the tacky feeling disappeared was judged under the following conditions. That is, it is judged whether or not ink adheres to the cotton swab, or whether or not the solid pattern image on the recording medium is scratched. The tack feeling was lost when no scratches were found. The cotton swabs used at that time were Johnson &amp; Johnson swabs. The number of times of rubbing was 10 reciprocations, and the strength of rubbing was 100 g load.
Further, the "number of passes" means the number of times the head moves with respect to the recorded matter and the coating film is irradiated with ultraviolet rays from the ultraviolet irradiation device mounted on the head. Also, the irradiation energy [mJ/cm ² ] was obtained by measuring the irradiation intensity [mW/cm ² ] on the irradiated surface irradiated from the light source and multiplying it by the irradiation duration time [s]. The irradiation intensity was measured using an ultraviolet intensity meter UM-10 and a light receiving part UM-400 (both manufactured by KONICA MINOLTA SENSING, INC.).
Evaluation criteria are as follows. The evaluation results are shown in Tables 3 and 4 below.
A: Cured with irradiation energy of 200 mJ/cm ² or less.
B: Not cured with irradiation energy of 200 mJ/cm ² .

From the above results, the lower layer ink containing the vinyl ether group-containing (meth)acrylic acid ester represented by the general formula (I) and the upper layer ink are included, and the surface tension of the lower layer ink is The ink set (each example) that satisfies the relationship greater than the surface tension of the upper layer ink is superior in the wetting and spreadability of the upper layer ink compared to the ink set (each comparative example) that does not satisfy the relationship. It has been found that both the ink for the upper layer and the ink for the lower layer are excellent in curability and ejection stability, and the embedding property of the ink for the lower layer on the recording medium is also excellent.

Although not shown in the above table, the content of DPGDA in the lower layer ink 3 was set to 26.75% by mass, and 0.05% by mass of UV3500 was added as a surfactant. Evaluation was performed in the same manner as in the above example, using inks prepared in the same manner as for the upper layer ink 3, using the upper layer inks 1 and 4 as the upper layer inks, and using the lower layer inks as the lower layer inks. . As a result, the embedding property of the lower layer ink on the recording medium was B, the ejection stability of the lower layer ink was A, and the curability of the lower layer ink was A. - The upper layer ink γ was 5 [mN/m], and the wetting and spreading property of the upper layer ink was B.

12 Conveying 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 Units 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):
_CH2 = ^CR1 -COOR2- ^O -CH=CH- ^R3 (I)
(In the formula, R ¹ is a hydrogen atom or a methyl group, R ² is a divalent organic residue having 2 to 20 carbon atoms, R ³ is a hydrogen atom or a monovalent organic residue having 1 to 11 carbon atoms. A photocurable inkjet ink set including a lower layer photocurable inkjet ink containing a vinyl ether group-containing (meth)acrylic acid ester represented by and a photocurable upper layer photocurable inkjet ink. hand,
A photocurable inkjet ink set, wherein the photocurable inkjet ink for the lower layer has a higher surface tension than the photocurable inkjet ink for the upper layer. 2. The photocurable inkjet ink set according to claim 1, wherein the difference between the surface tension of the photocurable inkjet ink for the lower layer and the surface tension of the photocurable inkjet ink for the upper layer is 5 or more. The content of the vinyl ether group-containing (meth)acrylic acid ester represented by the general formula (I) contained in the photocurable inkjet ink for the lower layer is , 20 to 80 mass %, the photocurable inkjet ink set according to claim 1 or 2. The upper layer photocurable inkjet ink contains a vinyl ether group-containing (meth)acrylic acid ester represented by the general formula (I), and
The content of the vinyl ether group-containing (meth)acrylic acid ester according to any one of claims 1 to 3, wherein the photocurable inkjet ink for the lower layer has a higher content than the photocurable inkjet ink for the upper layer. Photo-curable inkjet ink set. The photocurable inkjet ink set according to any one of claims 1 to 4, wherein the photocurable inkjet ink for lower layer contains an acrylic surfactant. The photocurable inkjet ink set according to any one of claims 1 to 5, wherein the upper layer photocurable inkjet ink contains a silicone surfactant. A coating film of the lower layer photocurable inkjet ink according to any one of claims 1 to 6 is formed on a recording medium, and the coating film according to any one of claims 1 to 6 is formed on the coating film. An inkjet recording method, comprising forming a coating film of a photocurable inkjet ink for the upper layer.

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